6.2 Absorption model for the production of diluted nitric acid. 89. 6.3 Methods to ..... the concentrated nitric acid is distilled to produce 90-100% nitric acid. The.
ABSORPTION OF NITROGEN OXIDES INTO DILUTED AND CONCENTRATED NITRIC ACID
J. B. Lefers
Delft University Press
I I
ABSORPTION OF NITROGEN OXIDES INTO DILUTED AND CONCENTRATED NITRIC ACID
T3
m o
o
o
U1 U l
BIBLIOTHEEK TU Delft P 1608 4302
C
456614
í
ABSORPTION OF NITROGEN OXIDES INTO DILUTED AND CONCENTRATED NITRIC ACID
PROEFSCHRIFT ter verkrijging van de graad van doctor in de technische wetenschappen aan deTechnische Hogeschool Delft, op gezag van de rector magnificus Prof. dr. ir. F. J.Kievits, v o o r e e n commissie aangewezen door het college van dekanen te verdedigen op woensdag 12 maart 1980 te 16.00 u u r d o o r
Jan Bernard Lef ers scheikundig ingenieur geboren te Enschede
£0 02250 Delft University Press/1980
% D0ELEHSIR.101 £ ~
Dit proefschrift is goedgekeurd door de promotor PROF. DRS. R J. VAN DEN BERG
I
!
Aan mijn ouders
i
i
/
i
I
VOORWOORD
Dit
proefschrift
personen.
i s t o t s t a n d gekomen met
Langs deze weg
de medewerking van een g r o o t a a n t a l
w i l i k hen voor hun e n t h o u s i a s t e i n z e t h a r t e l i j k
en
o p r e c h t danken. In het b i j z o n d e r gaat m i j n dank u i t naar
de a f s t u d e e r d e r s : Okke de Boks, Arend Bos en Jan Laverman, d i e aan d i t onderzoek hebben meegewerkt;
m i j n c o l l e g a Cock van den B l e e k voor de p r e t t i g e samenwerking b i n n e n de onderzoekgroep
en voor z i j n
s t i m u l e r e n d e b i j d r a g e i n de
discussies;
a l l e medewerkers van v e r s c h i l l e n d e s e r v i c e g r o e p e n voor de v e r v a a r d i g i n g , stelling
NO^-
her-
en v e r b e t e r i n g van de a p p a r a t u u r en de u i t v o e r i n g van de t a l r i j k e
l y s e s , zonder w i e r werkzaamheden d i t p r o e f s c h r i f t
zeker n i e t
ana-
t o t s t a n d gekomen
was;
Wim
J o n g e l e e n en Koos Kamps voor het tekenen en v e r k l e i n e n van de
figuren;
S a u l Lemkowitz voor het c o r r i g e r e n van de e n g e l s e t e k s t ;
Marian Wijnen manuscript.
voor het z o r g v u l d i g u i t g e v o e r d e typewerk
en de l a y - o u t van het
C O N T E N T S
SUMMARY
1
SAMENVATTING
3
1.
INTRODUCTION
5
1.1 G e n e r a l remarks
5
1.2 Aim o f t h i s work
9
1.3 O u t l i n e o f t h e t h e s i s
9
References
10
2. THE ABSORPTION APPARATUS
11
2.1 I n t r o d u c t i o n
11
2.2 S e l e c t i o n o f l a b o r a t o r y a b s o r b e r
11
2.3 D e s c r i p t i o n o f the a b s o r p t i o n a p p a r a t u s
14
2.4 Mass t r a n s f e r i n a l a m i n a r f a l l i n g l i q u i d 2.4.1
film
16
Theory
16
2.4.2 E x p e r i m e n t a l
18
2.4.3
Results
2.5 Gas phase mass t r a n s f e r i n l a m i n a r p l u g flow gas streams 2.5.1
Introduction
18 19 19
2.5.2 Theory
20
2.5.3
22
Experimental
2.5.4 R e s u l t s 2.6 C o n c l u s i o n s References
23 27 28
3. SPECTR0PH0T0METRIC DETERMINATION OF NITROGEN OXIDES AND NITRIC ACID VAPOUR
30
3.1 I n t r o d u c t i o n
30
3.2 E x p e r i m e n t a l
30
3.3 R e s u l t s
33
3.4 C o n c l u s i o n s
39
References
41
4. THE ABSORPTION OF NO„/N„0_ INTO DILUTED AND 2 2 4 NITRIC ACID
CONCENTRATED 42
4.1 I n t r o d u c t i o n
42
4.2 Review o f l i t e r a t u r e
42
4.2.1 A b s o r p t i o n o f NO^/I^O^ i n t o aqueous s o l u t i o n s 4.2.2 NO^/N^O^ a b s o r p t i o n i n t o c o n c e n t r a t e d n i t r i c
42 acid solutions
4.3 E x p e r i m e n t a l
48 53
4.4 R e s u l t s
55
4.4.1 The a b s o r p t i o n o f NgO^ i n t o d i l u t e d n i t r i c 4.4.2 The a b s o r p t i o n o f ^ ^4
i
n
t
o
2
acid solutions
concentrated n i t r i c
55
acid
solutions
59
4.5 C o n c l u s i o n s
65
References
65
5. THE OXIDATION AND ABSORPTION OF NO BY NITRIC ACID
67
5.1 I n t r o d u c t i o n
67
5.2 Proposed mechanism
67
5.3 E x p e r i m e n t a l
70
5.4 M a t h e m a t i c a l model and r e s u l t s
70
5.5 D i s c u s s i o n
86
5.6 C o n c l u s i o n s
87
References
6. AN ABSORPTION MODEL FOR THE DESIGN OF A DILUTED NITRIC ACID ABSORBER AND METHODS TO DECREASE THE NO -CONTENT IN TAIL GASES x 6.1 I n t r o d u c t i o n
89
89
6.2 A b s o r p t i o n model f o r t h e p r o d u c t i o n o f d i l u t e d n i t r i c 6.3 Methods t o d e c r e a s e
t h e NO^-content i n t a i l
acid
89
gases o f n i t r i c
acid plants
94
6.3.1 Wet p r o c e s s e s
95
6.3.1.1 Extended a b s o r p t i o n 6.3.1.2 H °2
s
c
r
u
b
b
2
6.3.1.3 N i t r i c
i
n
S
process
acid scrubbing
6.3.2 Dry p r o c e s s e s
95 95 97 100
6.3.2.1 A d s o r p t i o n
100
6.3.2.2 N o n - s e l e c t i v e r e d u c t i o n p r o c e s s e s
100
A
6.3.2.3 S e l e c t i v e r e d u c t i o n p r o c e s s e s References
101 102
APPENDIX I. THE ADDITIVITY OF RESISTANCES FOR MASS TRANSFER IN A WETTED WALL COLUMN
105
1. I n t r o d u c t i o n and g e n e r a l t h e o r y
105
2. R e s u l t s
107
3. C o n c l u s i o n s
113
References
NOMENCLATURE
113
114
SUMMARY
The
subject of t h i s thesis
into n i t r i c absorbers
i s d e a l i n g with the a b s o r p t i o n o f n i t r o g e n oxides
a c i d s o l u t i o n s to o b t a i n data f o r the design o f i n d u s t r i a l
f o r t h e p r o d u c t i o n o f d i l u t e d and c o n c e n t r a t e d n i t r i c a c i d . From t h e
l i t e r a t u r e c o n c e r n i n g t h e a b s o r p t i o n o f n i t r o g e n o x i d e s i n t o aqueous s o l u t i o n s it
i s known t h a t NO, NO^, N^O^
a b s o r p t i o n . Moreover, n i t r i c
and N^O^
a l l p l a y an important
r o l e during the
a c i d and n i t r o u s a c i d can be formed i n t h e gas
phase and i n t h e l i q u i d phase. T h i s complex a b s o r p t i o n mechanism was investigated i n a specially
d e s i g n e d wetted
w a l l column w i t h a known
facial
a r e a between t h e gas phase and t h e l i q u i d phase. A l a m i n a r
liquid
film
and a l a m i n a r p l u g flow o f t h e gas phase w i t h o u t
g r a d i e n t p e r p e n d i c u l a r t o the g a s - l i q u i d wetted
falling
a velocity
i n t e r f a c e c o u l d be r e a l i z e d
w a l l column. The mass t r a n s f e r i n t h e l a m i n a r f a l l i n g
i n v e s t i g a t e d by a b s o r b i n g pure carbon o
inter-
liquid
i n the f i l m was
d i o x i d e i n t o water at a temperature
20 C and at a p r e s s u r e o f 1 b a r . I t was found
of
t h a t w i t h i n t h e measured
c o n d i t i o n s t h e l i q u i d phase mass t r a n s f e r c o u l d be d e s c r i b e d by t h e p e n e t r a t i o n theory.
The gas phase mass t r a n s f e r was i n v e s t i g a t e d by a b s o r b i n g ammonia
a n i t r o g e n gas stream
into 2 N s u l f u r i c
from
a c i d s o l u t i o n s . The e x p e r i m e n t a l
r e s u l t s showed a good agreement w i t h t h e t h e o r e t i c a l l y p r e d i c t e d v a l u e s d e r i v e d from t h e s o l u t i o n o f t h e G r a e t z The nitric
problem.
a b s o r p t i o n o f NOg/N^O^ gas m i x t u r e s
from
a c i d was c a r r i e d out at a temperature
o f about 1 b a r . The e x p e r i m e n t a l
a n i t r o g e n gas stream o o
into
o f 20 C and 30 C and a t a p r e s s u r e
r e s u l t s c o u l d be i n t e r p r e t e d w i t h t h e f o l l o w i n g
model: a) NOg and N^O^,
which a r e i n c o n t i n u o u s
from t h e gas phase t o the g a s - l i q u i d b) N^O^
i s t h e o n l y s p e c i e s which d i f f u s e s i n t o t h e l i q u i d
c) In t h e experiments N^O^
e q u i l i b r i u m w i t h each o t h e r ,
diffuse
interface.
with d i l u t e d n i t r i c
acid
i s accompanied by a r a p i d pseudo f i r s t
phase.
(25% and 40%) t h e d i f f u s i o n o f order r e a c t i o n i n the l i q u i d
phase between N 0^ and water. I t was found t h a t t h e a b s o r p t i o n r a t e o f N^O 2
into diluted n i t r i c In t h e experiments NO
a c i d decreases with i n c r e a s i n g a c i d s t r e n g t h . with concentrated n i t r i c
acid
(63%-80%) t h e r e a c t i o n o f
w i t h water can be n e g l e c t e d and N O dissolves physically a 4 ^4 l i q u i d phase.
i n the
1
The
s o l u b i l i t y o f N^O^ i n c o n c e n t r a t e d n i t r i c
a c i d s o l u t i o n s was c a l c u l a t e d
from t h e t o t a l vapour p r e s s u r e d a t a o f t h e system NgO^-HgO-HNOg. I t was found t h a t w i t h i n t h e c o n d i t i o n s s t u d i e d Henry's law i s v a l i d . s o l u b i l i t y o f NgO^ i n c o n c e n t r a t e d n i t r i c c r e a s i n g a c i d s t r e n g t h and d e c r e a s i n g The
Furthermore, t h e
acid increases strongly with i n -
temperature.
o x i d a t i o n o f NO i n a n i t r o g e n gas stream
by 40%-80% n i t r i c
acid o
s o l u t i o n s was i n v e s t i g a t e d
i n t h e wetted
and 30°C. In t h e experiments experimental
w a l l column a t a temperature
w i t h 63% and 80% n i t r i c
o f 20 C
a c i d s o l u t i o n s the
r e s u l t s were i n t e r p r e t e d w i t h t h e f o l l o w i n g model:
a) The o x i d a t i o n r e a c t i o n t a k e s p l a c e i n t h e gas phase between NO and n i t r i c a c i d vapour and can be c o n s i d e r e d t o be i n f i n i t e l y vapour a r e t r a n s f e r r e d by m o l e c u l a r d i f f u s i o n b u l k and t h e g a s - l i q u i d
f a s t . NO and n i t r i c
from, r e s p e c t i v e l y , t h e gas
i n t e r f a c e t o t h e r e a c t i o n zone o r p l a n e .
found t h a t Danckwerts' s o l u t i o n
acid
f o r instantaneous
I t was
irreversible reactions i n
the l i q u i d phase can a l s o be a p p l i e d t o gas phase r e a c t i o n s . b) The NO^ and N^O^ produced, which a r e i n continuous, e q u i l i b r i u m w i t h each o t h e r , d i f f u s e from
t h e r e a c t i o n p l a n e o r zone t o t h e gas b u l k and t o t h e
gas-liquid
i n t e r f a c e . At t h e g a s - l i q u i d
i n t e r f a c e o n l y Ng0
physically
into the concentrated n i t r i c
acid.
Experiments partially
w i t h 57% n i t r i c
i n t h e l i q u i d phase. Under t h e s e c i r c u m s t a n c e s
t h e gas phase r e a c t i o n
a c i d vapour p r e s s u r e . In t h e experiments
the f i n a l
proceeds
f a s t , a f a c t which may be caused by
a c i d i t was found t h a t t h e o x i d a t i o n t a k e s p l a c e c o m p l e t e l y Under t h e s e c i r c u m s t a n c e s
dissolves
a c i d showed t h a t t h e o x i d a t i o n o f NO a l s o
i s t o o slow t o be c o n s i d e r e d t o be i n f i n i t e l y the r a t h e r low n i t r i c
4
o x i d a t i o n product
w i t h 40% n i t r i c
i n t h e l i q u i d phase.
i s mainly n i t r o u s a c i d .
Based on g e n e r a l c h e m i c a l r e a c t i o n e n g i n e e r i n g c o n s i d e r a t i o n s a mathematical model was developed
t o d e s c r i b e t h e a b s o r p t i o n mechanism which o c c u r s i n the
absorber
f o r the production of d i l u t e d n i t r i c
decrease
t h e amounts o f n i t r o g e n o x i d e s c o n t e n t
p l a n t s were b r i e f l y
2
discussed.
acid. F i n a l l y in tail
v a r i o u s methods t o
gases o f n i t r i c
acid
S A M E N V A T T I N G
Dit
p r o e f s c h r i f t h e e f t a l s onderwerp de a b s o r p t i e van s t i k s t o f o x i d e n
t e r z u u r , hetgeen
van b e l a n g i s b i j het ontwerp van industriële a b s o r b e r s
de p r o d u k t i e van verdund derzoek
i n salpevoor
en g e c o n c e n t r e e r d s a l p e t e r z u u r . U i t een l i t e r a t u u r o n -
i s g e b l e k e n d a t de a b s o r p t i e van s t i k s t o f o x i d e n
i n waterige oplossingen
z e e r g e c o m p l i c e e r d i s , w a a r b i j NO, N^O^, NO2 and N^O^ een b e l a n g r i j k e r o l spelen.
Bovendien
kunnen s a l p e t e r z u u r en s a l p e t e r i g z u u r i n de v l o e i s t o f f a s e en i n
de g a s f a s e worden gevormd. Om een d e r g e l i j k g e c o m p l i c e e r d a b s o r p t i e p r o c e s t e bestuderen
i s een n a t t e wand kolom o n t w i k k e l d waarin het c o n t a c t o p p e r v l a k t u s -
sen g a s - en v l o e i s t o f f a s e goed bekend i s . In deze kolom kon een l a m i n a i r e s t r o ming van de v a l l e n d e v l o e i s t o f f i l m zonder
en een l a m i n a i r e p r o p s t r o m i n g van de g a s f a s e
snelheidsgradiënt l o o d r e c h t op het g a s - v l o e i s t o f c o n t a c t o p p e r v l a k worden
v e r k r e g e n . Het s t o f t r a n s p o r t
i n de l a m i n a i r v a l l e n d e v l o e i s t o f f i l m werd
ondero
z o c h t door z u i v e r CO^ t e a b s o r b e r e n
i n water b i j een temperatuur
een druk van 1 b a r . U i t de experimenten transport Het
kon worden g e c o n c l u d e e r d dat het s t o f -
i n de v l o e i s t o f f a s e b e s c h r e v e n kon worden door de p e n e t r a t i e t h e o r i e .
stoftransport
i n de g a s f a s e werd onderzocht
s t i k s t o f gasstroom overeen
van 20 C en
i n 2 N zwavelzuur.
door NH^ t e a b s o r b e r e n van een
De e x p e r i m e n t e l e r e s u l t a t e n b l e k e n goed
t e komen met de t h e o r e t i s c h v o o r s p e l d e waarden u i t het Graetz-model.
De a b s o r p t i e van NOg/NgO^ gasmengsels van een s t i k s t o f gasstroom o o t e r z u u r werd u i t g e v o e r d b i j een temperatuur ongeveer
en 30 C en een druk van
1 b a r . De e x p e r i m e n t e l e r e s u l t a t e n konden worden beschreven met het
volgende
model.
a) N 0
en NgO^,
2
van 20
i n salpe-
w
e
^
k
e
v o o r t d u r e n d i n evenwicht
zijn,
d i f f u n d e r e n van de gas-
f a s e naar het f a s e g r e n s v l a k . b) N 0 2
4
i s de a c t i e v e component d i e i n de v l o e i s t o f f a s e
c) In de experimenten
met verdund
diffundeert.
s a l p e t e r z u u r (25% en 40%) gaat de d i f f u s i e
van N^O^ gepaard met een s n e l l e pseudo I e o r d e r e a c t i e i n de v l o e i s t o f f a s e t u s s e n N^O^ en water. van NgO^ i n verdund In
de experimenten
U i t de experimenten
b l e e k dat de a b s o r p t i e s n e l h e i d
zuur afnam b i j toenemende z u u r s t e r k t e . met g e c o n c e n t r e e r d s a l p e t e r z u u r (63%-80%) b l e e k dat de
r e a c t i e t u s s e n water en ^ 0 ^ v e r w a a r l o o s d kon worden. In dat g e v a l NO
lost
s l e c h t s f y s i s c h op i n de v l o e i s t o f f a s e .
3
De o p l o s b a a r h e i d van N^O^ i n g e c o n c e n t r e e r d
s a l p e t e r z u u r werd berekend u i t
l i t e r a t u u r g e g e v e n s b e t r e f f e n d e de t o t a l e dampdruk van h e t systeem N^O^-HgOHNOg. Binnen de beschouwde c o n d i t i e s b l i j k t Verder b l i j k t
de o p l o s b a a r h e i d van N 0 2
4
d a t de wet van Henry g e l d i g i s .
i n geconcentreerd
toe t e nemen met toenemende z u u r s t e r k t e en dalende
salpeterzuur sterk
temperatuur.
De o x y d a t i e van NO i n een s t i k s t o f gasstroom door 40%-80% s a l p e t e r z u u r werd onderzocht
i n de n a t t e wand kolom b i j een temperatuur van 20°C en 30°C. De ex-
perimentele
r e s u l t a t e n met 63% en 80% s a l p e t e r z u u r konden worden beschreven
met
het volgende model.
a) De o x y d a t i e r e a c t i e v i n d t p l a a t s i n de g a s f a s e
t u s s e n NO en s a l p e t e r z u u r -
damp en kan a l s o n e i n d i g s n e l worden beschouwd. Salpeterzuurdamp en NO d i f funderen
r e s p e c t i e v e l i j k van h e t g a s - v l o e i s t o f c o n t a c t o p p e r v l a k
b u l k n a a r h e t r e a c t i e v l a k o f de reactiezöne. E x p e r i m e n t e e l
en de gas-
b l e e k dat
Danckwerts' o p l o s s i n g e n voor i n s t a n t a n e i r r e v e r s i b e l e r e a c t i e s i n de v l o e i s t o f f a s e tevens kunnen worden t o e g e p a s t
op i n s t a n t a n e g a s f a s e
b) Het gevormde NOg en N^O^, welke v o o r t d u r e n d
i n evenwicht z i j n ,
reacties. diffunderen
van h e t r e a c t i e v l a k o f de reactiezöne n a a r de b u l k van de g a s f a s e en n a a r het
f a s e g r e n s v l a k . Op h e t f a s e g r e n s v l a k l o s t
alleen
N 2
°
4
f y s i s c h op i n de
vloeistoffase.
Experimenten met 57% s a l p e t e r z u u r toonden aan d a t de o x y d a t i e van NO ook gedeeltelijk niet
v e r l o o p t i n de v l o e i s t o f f a s e .
In d i t g e v a l kan de g a s f a s e
a l s o n e i n d i g s n e l worden beschouwd, hetgeen wordt v e r o o r z a a k t
salpeterzuurdampdruk.
reactie
door de l a g e
B i j 40% s a l p e t e r z u u r v i n d t de o x y d a t i e van NO v o l l e d i g
i n de v l o e i s t o f f a s e p l a a t s , w a a r b i j
s a l p e t e r i g zuur h e t u i t e i n d e l i j k
gevormde
produkt i s . Toepassing
van de algemene b e g i n s e l e n van de chemische reactorkunde
op de
a b s o r p t i e van s t i k s t o f o x i d e n i n s a l p e t e r z u u r r e s u l t e e r d e i n een w i s k u n d i g
mo-
d e l voor h e t ontwerpen van industriële a b s o r b e r s b i j de p r o d u k t i e van verdund s a l p e t e r z u u r . T e n s l o t t e werden de v e r s c h i l l e n d e m o g e l i j k h e d e n te i n afgassen
4
van s a l p e t e r z u u r p l a n t s t e v e r l a g e n met e l k a a r
om h e t N O ~ g e h a l x
vergeleken.
1. INTRODUCTION
1.1 GENERAL REMARKS
Nitric
a c i d i s one o f t h e most important
production of f e r t i l i z e r s ,
t i o n s are s t a i n l e s s s t e e l p i c k l i n g the n i t r i c
nitric
acid with
and metal e t c h i n g . About t h r e e - f o u r t h s o f
a c i d produced i s used i n t h e f e r t i l i z e r
p r o d u c t i o n o f ammonium n i t r a t e , The
i n o r g a n i c a c i d s and i t i s used i n t h e
d y e s t u f f s , r e s i n s and e x p l o s i v e s . F u r t h e r a p p l i c a -
i n d u s t r y , mainly
ammonium phosphates and compound
a c i d needed i n t h e f e r t i l i z e r
f o r the
fertilizers.
industry i s usually diluted
nitric
a c o n c e n t r a t i o n o f 50-70%. F o r most o t h e r a p p l i c a t i o n s , such as
n i t r a t i o n r e a c t i o n s , 90-100% n i t r i c
acid
i s used.
S i n c e t h e development o f t h e Haber-Bosch ammonia s y n t h e s i s i n 1913 n e a r l y all
nitric
a c i d p l a n t s a r e based on t h e o x i d a t i o n o f ammonia and t h e subsequent
absorption of n i t r o g e n oxides
Diluted
An
nitric
acid
production
example o f a flow sheet
the D.S.M. n i t r i c
f o r the production of d i l u t e d n i t r i c
a c i d process
Ammonia mixed w i t h passed over
i n t o water.
i s g i v e n i n F i g . 1 (mono p r e s s u r e
a i r enters a converter
a platinum
gauze c a t a l y s t
a c i d based on system).
(B) i n which t h e gas m i x t u r e i s
a t a temperature o f 850-920°C. The
ammonia i s o x i d i z e d t o NO a c c o r d i n g t o
4NH
The
+
3
50
-»• 4N0
2
+
c o n v e r t e r can be o p e r a t e d
(7-10
6H 0
(1)
2
at atmospheric,
medium (3-5 b a r ) o r h i g h
pressure
b a r ) . The hot gases l e a v i n g t h e c o n v e r t e r a r e c o o l e d i n a waste heat
b o i l e r t o generate cooler-condenser
2N0
+
0
2
steam. The temperature i s f u r t h e r reduced
t o 20°-40°C i n a
(D), and at t h e same time t h e NO formed i s o x i d i z e d t o N 0
X
2N0
2
2
(2)
5
7
12 -H 0 2
NH 3
0-
B
T
11 10
8
60°/. Fig.
1
The A:
D.S.M. n i t r i c acid air
compressor,
condenser,
process
B: converter,
E: absorption
(mono-pressure C: tail
column,
gas
system).
heater,
F: bleaching
HNO3
D:
column,
cooler-
G:
expansion
turbine. 1: air, gas
2: NH , 3: 10% NH
containing
10: bleached
in air,
200-2000 ppm 60% n i t r i c
4: NO,
5: N0 ,
8: unbleached
acid,
11: air,
12:
absorber
The
The n i t r i c
60% n i t r i c acid,
9:
NO^,
some weak a c i d i s
H0 2
•*
2HN0
3
+
NO
(3)
a c i d f o r m a t i o n i s accompanied by NO
i s subsequently
absorbed
e v o l u t i o n which i s r e - o x i d i z e d
i n t o the l i q u i d
a b s o r b e r the r e - o x i d a t i o n r a t e o f NO may tail
i s v e r y slow,
( 2 ) . The
N0
2
phase. In t h e top o f the and
as a f i r s t
approach
be assumed t o be t h e r a t e d e t e r m i n i n g s t e p i n the a b s o r p t i o n p r o c e s s . gas,
c o n t a i n i n g about
200-2000 ppm
Energy
i s subsequently
stream
i s vented t o t h e atmosphere.
The
60% n i t r i c
a b s o r b e r and
6
acid
phase at ambient
i n the gas phase by m o l e c u l a r oxygen a c c o r d i n g t o r e a c t i o n produced
e n t e r s the
and at the o p e r a t i n g p r e s s u r e i n the c o n v e r t e r :
+
2
tail
10 volume % NOg,
(E) where i t r e a c t s w i t h water i n the l i q u i d
temperature
3N0
gas m i x t u r e , c o n t a i n i n g about
7:
water.
The water formed condenses i n the c o o l e r - c o n d e n s o r and produced.
6: weak acid,
2
N0 , x
r e c o v e r e d by expansion
l e a v e s the a b s o r b e r and
acid this The
i s heated.
i n a t u r b i n e a f t e r which the
gas
a c i d c o n t a i n i n g some d i s s o l v e d n i t r o g e n o x i d e s l e a v e s the
i s s t r i p p e d w i t h a i r i n a b l e a c h i n g column
(F) .
A v a r i a n t o f the mono-pressure p r o c e s s
i s the d u a l p r e s s u r e p r o c e s s , at
which the c o n v e r s i o n t a k e s p l a c e at a lower p r e s s u r e than the a b s o r p t i o n . In a d u a l p r e s s u r e system a n i t r o u s gas compressor i s needed. The main advantage o f f e r e d by a low p r e s s u r e i n the c o n v e r t e r i s the d e c r e a s e platinum c a t a l y s t
l o s s e s . On
o f the ammonia
the o t h e r hand the i n v e s t m e n t s
and
r e q u i r e d are
higher. S i m i l a r processes
such as the D.S.M. n i t r i c
a c i d p r o c e s s can be found i n
the l i t e r a t u r e [ 1 ] .
Concentrated
nitric
acid
production
There i s a s u b s t a n t i a l need f o r s t r o n g e r a c i d , p a r t i c u l a r l y
f o r acid with
a
c o n c e n t r a t i o n i n the range o f 90-100%. Such a c i d i s , f o r example, used i n n i t r a t i o n r e a c t i o n s . However, c o n c e n t r a t e d n i t r i c p r e p a r e d by form
distillation
o f d i l u t e d a c i d o f 60%,
a constant b o i l i n g mixture
( a z e o t r o p e ) between an a c i d
E x t r a c t i v e d i s t i l l a t i o n with s u l f u r i c c o m p o s i t i o n w i t h magnesium n i t r a t e and mixture
are r a t h e r e x p e n s i v e
R e c e n t l y some new nitric Fig.
a c i d o f 80%,
a c i d process
which can be d i r e c t l y
NO
produced
i n a waste heat
(B). In the c o o l e r - c o n d e n s e r
(1) i s condensed and
10-12
gas m i x t u r e
bar. The
gas m i x t u r e
2N 0 2
NOg.
+
2
column.
+
0
2
-
4HN0
3
(C)
a c i d at 0°C-10°C and at a p r e s s u r e o f
80%-85% n i t r i c
i s converted to n i t r i c
2H 0
produced
i s produced. The weak a c i d
a b s o r b e r c o n t a i n s some n i t r i c
T h i s can be d e c r e a s e d
c o n t a i n s about 15-30% d i s s o l v e d
4
at a
column (D) where the excess o f water i s r e -
l e a v i n g the p h y s i c a l
about 2000 ppm
i n which the NgO^
( 2 ) . Moreover, the water
l e a v e s the bottom o f the d i s t i l l a t i o n
i n 80%-85% n i t r i c
s c r u b b i n g w i t h water. The absorber
further cooled i n a
i s o x i d i z e d t o NO
from the c o o l e r condenser i s f e d t o a p h y s i c a l a b s o r b e r
i s dissolved
a c i d vapour and
boiler,
t h e NO
some weak a c i d
e n t e r s the weak a c i d d i s t i l l a t i o n
2
n i t r i c acid.
by o x i d a t i o n o f ammonia i n a c o n v e r t e r
o f 15°-70°C a c c o r d i n g t o r e a c t i o n
The
resultant
t o produce c o n c e n t r a t e d
d i s t i l l e d t o produce 100%
temperature
where N0
of the
flow sheet o f the Du Pont de Nemours c o n c e n t r a t e d
[ 2 ] . The
a c i d o f 68%
s t r e n g t h o f 68-69%.
subsequent d i s t i l l a t i o n
by r e a c t i o n
moved. N i t r i c
water
methods.
(A) i s , a f t e r r e c o v e r y o f energy cooler-condenser
directly
a c i d and
a c i d o r m o d i f i c a t i o n o f the a z e o t r o p i c
p r o c e s s e s have been developed
2 gives a s i m p l i f i e d
nitric
a c i d can not be because n i t r i c
t o 200
ppm
a c i d l e a v i n g the bottom o f N 2
°4•
by the
T h i s s o l u t i o n e n t e r s r e a c t o r (E)
a c i d a t 40°-100°C w i t h a i r .
(4)
7
NH
3
2~A
B
6
10 a b s o r p t i o n r a t e was
liquid
f e d by
saturated with
and then s u p p l i e d t o the
wetted
measured from the d e c r e a s e o f the
C0
2
volume at c o n s t a n t p r e s s u r e w i t h a soap f i l m i n a c a l i b r a t e d tube a c c o r d i n g t o N i j s i n g [5].
2.4.3
Results
In o r d e r to check the hydrodynamic b e h a v i o u r
o f the l i q u i d
f i l m the e x p e r i m e n t a l
a b s o r p t i o n r a t e s were compared w i t h the r a t e s p r e d i c t e d by the p e n e t r a t i o n t h e o r y . In our experiments the c o l l e c t i n g l i q u i d
the h e i g h t o f the stagnant
r e s e r v o i r was
found t o be 1.6
d a t a found by Lynn et a l [8-10] and N i j s i n g
Regression
l i n e through
f i l m formed above
cm which agrees w i t h
the
[ 5 ] . In t h i s p a r t o f the f i l m
a b s o r p t i o n r a t e can be n e g l e c t e d . A p l o t o f m ( T ) give a s t r a i g h t
liquid
the
v e r s u s ^ ( h - AlO/v^ s h o u l d
the o r i g i n w i t h a s l o p e o f 2C^
iy^jj/^"
a n a l y s i s shows t h a t the d e v i a t i o n o f the e x p e r i m e n t a l
3
)•
points i s
always l e s s than 2%
and the upper bound and the lower bound o f the 95% -5 2 c o n f i d e n c e i n t e r v a l f o r the i n t e r c e p t was r e s p e c t i v e l y 0.155 x 10 kg/m and -5 2 -0.102 x 10 kg/m . The upper bound o f the 95% c o n f i d e n c e i n t e r v a l o f the "™™ —5 2 4 s l o p e 2C„ WD./l was found t o be, r e s p e c t i v e l y , 7.97 x 10 kg/m .sec and 2 4 -5 2 4 7.68 x 10~ kg/m .sec which agrees w i t h the v a l u e o f 8.04 x 10 kg/m .sec 5
found by N i j s i n g Hence i t was
[5]. concluded
t h a t the a b s o r p t i o n r a t e i n the l i q u i d phase i s w e l l
p r e d i c t e d by the p e n e t r a t i o n t h e o r y and liquid
18
t h a t the hydrodynamic b e h a v i o u r
f i l m agrees w e l l w i t h the t h e o r e t i c a l model.
of
the
12
Fig.
3 Absorption
rate
of CO^ into
water as a function
of the contact
time
(P = 1.013 bar, t = 20°C). Symbol
h (cm)
A
15.9
0
34.9
V
51.9
2.5 GAS PHASE MASS TRANSFER IN LAMINAR PLUG FLOW GAS STREAMS
2.5.1
The
Introduction
effect
laminar
o f gas and l i q u i d
flow
r a t e s on t h e gas phase mass t r a n s f e r i n
gas streams was t h e o r e t i c a l l y and e x p e r i m e n t a l l y
columns and r e c t u a n g u l a r
g a s - l i q u i d flow
I f however t h e gas v e l o c i t y i s u n i f o r m
s t u d i e d i n wetted w a l l
[7,20,22,25,26]. and i s equal
t o the surface v e l o c i t y
o f t h e l i q u i d , no i n f l u e n c e o f t h e moving i n t e r f a c e on t h e gas phase mass t r a n s f e r can be e x p e c t e d . S t r i c t l y
s p e a k i n g t h i s i s t r u e when t h e r e
i s zero
19
drag at t h e i n t e r f a c e . H i k i t a and I s h i m i situation
i n wetted w a l l
numbers. Dekker
[22] s u p e r f i c i a l l y
columns, but no i n f o r m a t i o n
[7] i n v e s t i g a t e d t h e a b s o r p t i o n
studied
i s given
this
at h i g h
Graetz-numbers from 135 up t o 245 i n a w e t t e d w a l l column which was in
a s p e c i a l way t o c r e a t e a f l a t
his
measured a b s o r p t i o n
rates with the t h e o r e t i c a l l y p r e d i c t e d values
t r a n s f e r which was not e x p e r i m e n t a l l y In t h i s p a r t
constructed
v e l o c i t y p r o f i l e o f t h e gas. Comparison o f
however, based upon t h e assumption o f a s m a l l
described
Graetz-
o f ammonia i n t o water at
confirmed.
t h e s e gaps o f i n f o r m a t i o n
i.
0
E
3r
) '
should
be p a i d
C
=C g g>o 3C /3r = 0
g,o
i n which J
-
-c
C
.
6 , 1
g,i
and J
o
that
=
2
E
,
the i n t e r f a c e c o n c e n t r a t i o n
n=l
r
ITjTÜT n 1
are Bessel
n
(14) g. i
C
o
(13)
=C g
to the fact
(12)
g
C
. along the f i l m . According g. i the s o l u t i o n of these equations i s :
C (h,r)
value
(11)
conditions:
r = R - 0
Attention
flow
a l l the
t h e d i f f u s i o n e q u a t i o n can be w r i t t e n as:
3C v
20
f i l m . The
i n F i g . 1. Assuming t h a t
phase t o t h e l i q u i d phase t a k e s p l a c e by m o l e c u l a r d i f f u s i o n o n l y
direction,
[32]
v e l o c i t y o f the l i q u i d
system a r e g i v e n
through-
should
t o Carslaw and J a e g e r
. -D
2 h a
.
V \ i r r ' - p p - 7 f
functions of the f i r s t
v^R-ö,)
'
k i n d and, r e s p e c t i v e l y , o f
the z e r o and f i r s t
W The
=
first
order, while a
are the roots of the equation
0
( 1 6 )
e i g h t r o o t s a r e g i v e n i n T a b l e 2.
a
1
=
2.4048
=
5.5201
a
3
=
8.6537
a
4
= 11.7915
g
= 14.9309
a
a
a . = 18.0711 o a
= 21.2116
?
a
= 24.3525 o
Table
2
First
eight
eigenvalues
The b u l k average
value of C (h,r) g
H-S C (h) =
From e q u a t i o n s
g
(
h
)
_
C
-S C
(15) and (17) t h e v a l u e o f
g i
§ ^
S,o
r.C ( h , r ) d r
- C g,i
(17)
J
2
f
C
f
» (R-6 )
on a g i v e n h e i g h t h i s :
I
1
»
= 4Z n=l
— a n
a
n
v
c g
(
n
) i
s
obtained.
\
exp ( - - 5 — J V
Gz
(18)
'
i n which Gz i s t h e G r a e t z number d e f i n e d as
G
The
z
(
= ^DTE g
average mass t r a n s f e r c o e f f i c i e n t k^ between t h e i n l e t
1
9
)
and o u t l e t o f t h e
mass t r a n s f e r s e c t i o n may be d e f i n e d i n terms o f a l o g a r i t h m i c mean d r i v i n g f o r c e as f o l l o w s :
-ir(R-Ô.) i
_ _ (C -C . ) - ( C (h)-C v (C -C ( h ) ) = k 2Tf(R-6" ) h x — ' ° s g.o g ft i r— *" -.(h) - C g
8
,
1
g
.) (20)
- c o
g,i 21
A f t e r s u b s t i t u t i o n o f e q u a t i o n (18) i n e q u a t i o n (20) the average number can be w r i t t e n
G z o o = - In Z IT
Sh
=1
Sherwood
as:
4 a n
2 , a TT . ( - — Gz
exp
(2D
r I f the G r a e t z number i s l a r g e r than about t o be i n f i n i t e l y
150
the gas phase may
deep, and t h i s s i t u a t i o n c o r r e s p o n d s w i t h the p e n e t r a t i o n
t h e o r y . Under t h i s c o n d i t i o n t h e l o g a r i t h m i c average represented Sh
g
Sherwood number may
be
as: Gz / — In I
=
be c o n s i d e r e d
e)"*
J
The
d e v i a t i o n o f e q u a t i o n (21) w i t h e q u a t i o n (22) at G r a e t z numbers l a r g e r
150
i s s m a l l e r than
2.5.3
The
6%.
Experimental
absorbing l i q u i d
c o n t a i n i n g 0.05% same way
( d i s t i l l e d water,
by weight
IN s u l f u r i c
" T e e p o l " was
as d e s c r i b e d f o r the CO^
a c i d and 2N
sulfuric
meted
a b s o r p t i o n experiments.
w i t h f l o w c o n t r o l l e r s . The
were determined capillary
The
experimental from
cylinders
gas f l o w r a t e s o f ammonia and n i t r o g e n
from the p r e s s u r e drop a c r o s s a c a l i b r a t e d s t a i n l e s s
steel
t u b i n g immersed i n a t h e r m o s t a t i c a l l y c o n t r o l l e d water b a t h
m a i n t a i n e d at 20°C. A f t e r m i x i n g the ammonia and n i t r o g e n gas m i x t u r e was
streams
the
l e d i n c o - c u r r e n t flow through the wetted w a l l column. The
r a t e o f the gas m i x t u r e was was
acid)
f e d t o the wetted w a l l column i n the
equipment i s shown i n F i g . 4. Ammonia and n i t r o g e n were s u p p l i e d and
chosen
i n such a way
e q u a l t o the s u r f a c e v e l o c i t y o f the l i q u i d
t h a t the average gas film.
c o n c e n t r a t i o n o f ammonia i n the i n g o i n g gas stream was
flow
In our experiments varied
determined +
4
by adding 0.1N
sulfuric
a n a l y s e d i n the same way. s u l f u r i c a c i d was
d e v i a t i o n was
22
c o n t e n t o f the i n - and o u t g o i n g l i q u i d
In the experiments
was
reaction was
w i t h water a known amount o f 0.1
sample t o p r e v e n t d e s o r p t i o n o f ammonia.
the ammonia a b s o r p t i o n c o u l d be e s t a b l i s h e d , and
found t o be l e s s than
by
a c i d t o a gas sample and then a n a l y s i n g on
added t o the l i q u i d
A mass b a l a n c e around
the
from 2% t o 10%
c o n t e n t by means o f a c o l o r i m e t r i c method based on the B e r t h e l o t
i n an A u t o - A n a l y z e r . The NH^
gas
velocity
volume. The c o n c e n t r a t i o n o f ammonia i n the i n - and o u t g o i n g gas streams
NH
than
1.5%.
the
N
rfn.T.
- g ,
Fig'. 4
""Vo-c
Expérimental set-up for ammonia absorption
experiments
in a wetted
wall
column.
2.5.4
The
Results
a b s o r p t i o n o f ammonia i n t o s u l f u r i c a c i d s o l u t i o n s i s accompanied w i t h
fast chemical
r e a c t i o n i n the l i q u i d phase. The
ammonia i s e q u a l does not
to zero
absorbing
i n t e r f a c e concentration of
i f an i n c r e a s e o f the a c i d i t y o f t h e a b s o r b i n g
f u r t h e r i n c r e a s e the a b s o r p t i o n r a t e . E x p e r i m e n t s w i t h water,
s u l f u r i c a c i d and
2N
s u l f u r i c a c i d showed no
concluded
t h a t d u r i n g the a b s o r p t i o n o f ammonia i n t o 2N
(18)
and
liquid,
and
i s p o s s i b l e . The
the r e s u l t s were compared w i t h
c o n t a c t time v a l u e s between gas seconds and
i n the i n g o i n g gas
stream from 2.3%
up t o 10.5%
a
time
the
l i q u i d were v a r i e d from 0.25
t o 1.2
and
a b s o r p t i o n r a t e s i n 2N
t h e o r e t i c a l l y d e r i v e d e x p r e s s i o n s . The up
the
sulfuric
s u l f u r i c a c i d s o l u t i o n s were i n v e s t i g a t e d as a f u n c t i o n o f the c o n t a c t v a l u e s between gas
IN
3).
a c i d s o l u t i o n s the i n t e r f a c e c o n c e n t r a t i o n o f ammonia i s e q u a l t o z e r o , f u r t h e r s i m p l i f i c a t i o n of equation
liquid
i n f l u e n c e o f the a c i d i t y o f
l i q u i d on the a b s o r p t i o n r a t e (see T a b l e
Hence i t was
a
and
the c o n c e n t r a t i o n o f ammonia
by volume.
23
C
total
g,o % vol
water
bar
s m/sec
C (h) g C S,o
2 72
1 184
0 543
0.516
2 71
1 191
0 539
0.479
2NH S0„ 2 4
2 76
1 201
0 537
0.505
water
4 85
1 095
0 399
0.424
4 84
1 096
0 400
0.430
4 93
1 111
0 401
0.415
1NH S0 2
4
o
1NH S0 2
4
2NH-S0. 2 4
Table
V
pressure
3
Influence
of the acidity
ammonia into wall
of the liquid
water and sulfuric
column with a height
acid
phase on the absorption solutions
rate of
at 20°C in a wetted
of 51.9 cm and with an inner
diameter of
3.45 cm
Due t o t h e a b s o r p t i o n o f ammonia from t h e gas phase i n t o t h e l i q u i d the gas v e l o c i t y d e c r e a s e s .
Therefore
at Graetz
e x c e s s i v e l y h i g h ammonia c o n c e n t r a t i o n s the maximum ammonia c o n c e n t r a t i o n by
phase
numbers s m a l l e r t h a n 100
i n t h e gas phase s h o u l d be a v o i d e d ,
i n these
and
experiments was t h e r e f o r e about 5%
volume. In t h e experiments w i t h 2N s u l f u r i c a c i d t h e stagnant
liquid
f i l m above t h e
r e c e i v i n g l i q u i d was o b s e r v e d t o be about 2.0 cm. F o r a h i g h l y s o l u b l e gas such as ammonia i n s u l f u r i c the stagnant absorption
liquid
acid solutions, i t i s doubtful
f i l m may be n e g l e c t e d
dioxide
experiments.
From l i t e r a t u r e d a t a stagnant
i f the absorption rate i n
as was done i n t h e carbon
[27,28] i t i s known t h a t t h e e v a p o r a t i o n
water s u r f a c e i n t o d r i e d a i r f l o w i n g a c r o s s
this surface
rate of a i s strongly
reduced by t h e a d d i t i o n o f s m a l l amounts o f s u r f a c e a c t i v e agents. Long s t r a i g h t - c h a i n a l c o h o l s , f o r example, may reduce t h e e v a p o r a t i o n 12%
o f t h e r a t e a t a c l e a n water s u r f a c e . D.W.
Thompson [29] found t h a t
amounts o f s u r f a c e a c t i v e agents d e c r e a s e t h e a b s o r p t i o n water i n an u n s t i r r e d c o n t a i n e r . E x p e r i m e n t s w i t h decanol
24
r a t e t o only small
r a t e o f ammonia i n t o
1 - o c t a d e c a n o l and 1-hexa-
showed a r e d u c t i o n o f r e s p e c t i v e l y 41% and 51% i n t h e a b s o r p t i o n
rate.
c
P
g.o % by volume
bar
0 .782
3.77
1 295
0 0134
0 .772
2 31
1 276
0 0136
0 .780
1 193
0 0167
0 739
5 35
1 189
0 0167
0 .731
2 45
1 208
0 0161
0 744
1 113
0 0194
0 700
1 143
0 0196
0 694
1 113
0 0234
0 683
4 72
1 107
0 0237
0 688
5 34
1 117
0 0225
0 690
5. 20
1 092
0 0262
0 653
5. 49
1 092
0 0256
0 658
4. 05
1 215
0 0391
0 593
5. 30
1 117
0 0567
0 541
4. 53
1 120
0 0571
0 527
2. 74
1 200
0 0621
0 503
2. 78
1 201
0 0626
0 507
4. 98
1. 111
0 0891
0 419
4. 88
1. 111
0 0908
0 411
Absorption
experiments
of NH„ into
acid
acid
solution
coefficient
at 20 C
liquid film
f o r the a b s o r p t i o n o f ammonia
(Ah = 1.5 cm), and t h e G r a e t z numbers were c o r r e c t e d
For the c a l c u l a t i o n
diffusion
2N sulfuric
from the above-mentioned we assumed 75% o f t h e stagnant
i n the wetted w a l l column t o be i n a c t i v e sulfuric
g.o
0 0131
10 5
effect.
C
1 296
5 83
Reasoning
Gz
(h)
g
3 67
10 7
4
C
red
10 4
Table
TT
into
f o r t h i s en
o f t h e Sherwood and G r a e t z numbers t h e gas phase
o f ammonia
i n n i t r o g e n was taken from t h e d a t a g i v e n by
Mason and Monchick [ 3 0 ] .
(D
= 2.3 x 10 3
-5
2 o m / s e c at 25 C and 1.01325 b a r ) .
2 25
Corrections
f o r temperature and p r e s s u r e
r e l a t i o n o f Wilke-Lee The TT/Gz
red
The
d e v i a t i o n s were c a r r i e d o u t u s i n g t h e
[31]. The e x p e r i m e n t a l r e s u l t s a r e g i v e n
r e l a t i v e concentration
i n T a b l e 4.
o f ammonia i n t h e gas phase as a f u n c t i o n o f
i s p l o t t e d i n F i g . 5. l o g a r i t h m i c mean v a l u e
o f t h e Sherwood number as a f u n c t i o n o f t h e
G r a e t z number (Gz .) i s p l o t t e d i n F i g . 6. These f i g u r e s show t h a t t h e red d e v i a t i o n o f t h e measured p o i n t s
from t h e t h e o r e t i c a l l y p r e d i c t e d v a l u e s i s
s m a l l . At G r a e t z numbers o f more than 150 t h e d e v i a t i o n from t h e p e n e t r a t i o n theory
Fig.
5
i s s m a l l e r than 6%.
NH
absorption
3
experiments
20°C; comparison theoretical Symbol
26
between
lines). h (cm)
0
14.9
V
34.9
A
51.9
in a wetted experimental
wall results
column into and theory
2N F.^S0^ at (
Fig.
6
Mean Sherwood number as a function (21),
assymptotic
Symbol
of the Graetz
number (
equation
solution).
h (cm)
0
14.9
A
34.9
V
51.9
2.6 CONCLUSIONS
A wetted
w a l l column was developed
i n which gas a b s o r p t i o n w i t h
simultaneously
o c c u r r i n g gas phase r e a c t i o n s such as t h e a b s o r p t i o n o f n i t r o g e n o x i d e s i n nitric
a c i d may be i n v e s t i g a t e d .
liquid
film
In t h i s wetted
w a l l column a l a m i n a r
and a l a m i n a r p l u g flow o f t h e gas phase w i t h o u t
falling
a velocity
g r a d i e n t p e r p e n d i c u l a r t o t h e g a s - l i q u i d i n t e r f a c e c o u l d be r e a l i z e d . The hydrodynamic b e h a v i o u r
o f t h e l i q u i d f i l m was checked
by a b s o r b i n g COg i n t o
water. I t was found t h a t t h e a b s o r p t i o n r a t e was w e l l p r e d i c t e d by t h e p e n e t r a t i o n t h e o r y . Gas phase mass t r a n s f e r was i n v e s t i g a t e d by a b s o r b i n g 27
ammonia from a n i t r o g e n gas stream i n t o s u l f u r i c
a c i d s o l u t i o n . The e x p e r i m e n t a l
r e s u l t s show a good agreement w i t h t h e G r a e t z model.
REFERENCES
1. Andrew, S.P.S. and Hanson, D., Chem. Eng. Sci., 1961, 14, 105. 2. H o f t i j z e r , P . J . and Kwanten, F.J.G., A b s o r p t i o n o f n i t r o u s gases, from Nonhebel, G., Gas P u r i f i c a t i o n P r o c e s s e s f o r A i r P o l l u t i o n C o n t r o l ,
Newnes-
B u t t e r w o r t h s , London, 1972. 3. Detournay, J.P. and J a d o t , R.H. , Chem. Eng. Sci., 1973, 2_8, 2099. 4. Danckwerts, P.V., G a s - L i q u i d R e a c t i o n s , M c G r a w - H i l l , London, 1970. 5. N i j s i n g ,
R.A.T.O., PhD T h e s i s , D e l f t U n i v e r s i t y o f T e c h n o l o g y , D e l f t , The
N e t h e r l a n d s , 1957. 6. Kramers, H., B l i n d , M.P.P. and Snoeck, E., Chem. Eng. Sci., 1961, 14, 115. 7. Dekker, W.A., PhD T h e s i s , D e l f t U n i v e r s i t y o f T e c h n o l o g y , D e l f t , The N e t h e r l a n d s , 1958. 8. Lynn, S., S t r a a t e m e i e r , J.R. and Kramers, H. , Chem. Eng. Sci., 1955, 4_, 49. 9. Lynn, S., S t r a a t e m e i e r , J.R. and Kramers, H. , Chem. Eng. Sci., 1955, 4_,
58.
10. Lynn, S., S t r a a t e m e i e r , J.R. and Kramers, H., Chem. Eng. Sci., 1955, 4, 63. 11. W i l d , J.D. and P o t t e r , O.E., J . Chem. E. Symposium Series,
1968, 28, 30.
12. A l p e r , E. and Danckwerts, P.V., Chem. Eng. Sci., 1976, 31,
599.
13. Kameoka, Y. and P i g f o r d , R.L., Ind. Eng. Chem. Fundam., 1977, 16, 163. 14. Govindan, T.S. and Quinn, I.A., AIChE J., 1964, 10, 35. 15. Danckwerts, P.V. and Kennedy,
A.M., Chem. Eng. Sci., 1958, 8, 201.
16. G o d f r e y , J.H., PhD T h e s i s , Oregon S t a t e U n i v e r s i t y , U.S.A., 1973. 17. Sada, E., Kumazawa, H., Yamanaka, Y., Kudo, I. and Kondo, T., J. Chem. Eng.
Japan,
1978, 11, 276.
18. H i k i t a , H., A s a i , S., Ishikawa, H. and S a i t o , Y., Chem. Eng. Sci., 1975, 30, 607. 19. Dekker, W.A. Snoeck, E. and Kramers, H. , Chem. Eng. Sci., 1959, 11^, 61. 20. A i h a r a , K., Ukawa, N., Hozawa, M. and T a d a k i , T., Int. Chem. Eng., 1976, 16, 494. 21. H i k i t a , H. and I s h i m i , K., J. Chem. Eng. Japan, 22. ' H i k i t a , H. and I s h i m i , K. , J. Chem. Eng. Japan,
1976, 9_, 1976, 9^,
357. 362.
23. P o r t a l s k i , S. and C l e g g , A . J . , Chem. Eng. Sci., 1971, 26, 773. 24. B a n e r j e e , S., Rhodes, E. and S c o t t , D.S., Chem. Eng. Sci., 1967, 22, 43. 25. B y e r s , H.C. and K i n g , J.C., AIChE J., 1967, l j ! , 628. 26. B y e r s , H.C. and K i n g , J.C., AIChE J., 1967, 13^, 637. 28
27. Sherwood, T.K., P i g f o r d , R.L. and W i l k e , C.R., Mass T r a n s f e r ,
McGraw-Hill,
1975. 28. D a v i e s , J.T.
and R i d e a l , T.K., I n t e r f a c i a l Phenomena, Academic P r e s s , New
York, 1963. 29. Thompson, D.W.,
Ind. Eng. Chem. Fundam. , 1970, j}, 243.
30. Mason, E.A. and Monchick,
L. , J. Chem. Phys. , 1962, 36, 2746.
31. R e i d , R.C., P r a u s n i t z , J.M. and Sherwood, T.K., The P r o p e r t i e s o f Gases and L i q u i d s , M c G r a w - H i l l , 1977. 32. C a r s l a w , H.S.
and J a e g e r , J.C. C o n d u c t i o n o f Heat
i n Solids,
Oxford
U n i v e r s i t y P r e s s , 1959. 33. L e f e r s , J.B.,
Van den B l e e k , C M . , Bos, A.S. and Van den Berg, P.J.,
paper
p r e s e n t e d at t h e 6 t h I n t e r n a t i o n a l Congress o f C h e m i c a l E n g i n e e r i n g , Chemical Equipment Design and Automation,
Prague,
August 1978.
34. K a i s e r , E.W. and Wu, C.H., J. Phys.
Chem., 1977, 81, 1701.
35. K a i s e r , E.W. and Wu, C.H., J. Phys.
Chem., 1977, 81,
36. S t r e i t , Phys.,
187.
G.E. and W e l l s , J.S., F e h s e n f e l d , F.C., Howard, C.J., J. Chem. 1979, 70, 3439.
37. McKinnon, I.R., M a t h i e s o n , J.G. and W i l s o n , I.R., J. Phys.
Chem., 1979, 83,
779.
29
3. SPECTROPHOTOMETRY DETERMINATION OF NITROGEN OXIDES AND NITRIC ACID VAPOUR
3.1 INTRODUCTION
F o r p o l l u t i o n c o n t r o l purposes much a t t e n t i o n has been p a i d t o the of n i t r o g e n oxides
and s e v e r a l a n a l y s i s methods have been d e v e l o p e d
d i s a d v a n t a g e o f most methods i s t h a t they range which o c c u r s
spectroscopy,
however, can a l s o be used f o r t h e d e t e r m i n a t i o n
for
at higher
concentrations.
i n t h e manufacture o f n i t r i c
Infrared absorption
acid. Infrared of nitrogen
c o e f f i c i e n t s o f NOg and NO
p o l l u t i o n c o n t r o l have been measured as a f u n c t i o n o f t h e o p t i c a l
l e n g t h and t h e temperature
[ 6 ] . Guttman
i n t e n s i t i e s o f pure N0„ and N O . pressures
[7] i n v e s t i g a t e d i n t e g r a t e d
i n gas m i x t u r e s c o n t a i n i n g n i t r o g e n
and/or n i t r o u s a c i d vapour can be formed concentrations
nitric
absorption Fontanella 1
number o f 1915 cm , o f N 0
a c i d and p o l l u t i o n c o n t r o l purposes.
a t 1606 cm * and n i t r i c
2
concentrations
the N0
2
absorption
of N0
2
NgO^
Using
a c i d vapour at 1326 cm
1
in
T h i s method i s n o t a p p l i c a b l e at
due t o t h e s t r o n g o v e r l a p o f n i t r i c
a c i d vapour
band.
In t h i s Chapter a method i s d e v e l o p e d f o r t h e d e t e r m i n a t i o n NgO^ and n i t r i c
2 >
Such i n f o r m a t i o n may be o f import-
[10] s t u d i e d t h e c o n c e n t r a t i o n o f NO a t a wave
the s t r a t o s p h e r e u s i n g t h e sun as s o u r c e .
and
i n f o r m a t i o n , however, can be
t h e q u a n t i t a t i v e a n a l y s i s o f NO, N 0
a c i d vapour i n such gas m i x t u r e s .
ance f o r t h e manufacture o f n i t r i c
oxides.
a c i d vapour
[8,9], e s p e c i a l l y at higher
o f n i t r o g e n o x i d e s . Very l i t t l e
found i n t h e l i t e r a t u r e c o n c e r n i n g
higher
absorption
up t o 2 MPa. The r e s u l t s o f Guttman i n d i c a t e t h a t Beer's law i s v a l i d .
Due t o t h e r e a c t i o n o f n i t r o g e n o x i d e s w i t h water vapour n i t r i c
infrared
path
a t temperatures o f 50°C up t o 100°C and a t
O f t e n water vapour i s a l s o p r e s e n t
and
[ 1 - 5 ] . The
a r e not a p p l i c a b l e i n the higher
concentration
oxides
determination
a c i d vapour i n gas m i x t u r e s a t c o n c e n t r a t i o n s
the manufacture o f n i t r i c
o f NO, NOg,
which o c c u r i n
acid.
3.2 EXPERIMENTAL
All 30
s p e c t r a l measurements were c a r r i e d o u t on a P e r k i n - E l m e r
Model 117 i n f r a r e d
spectrophotometer.
The
w i t h an i n n e r diameter
i n f r a r e d a b s o r p t i o n gas c e l l was o f 3.5
cm
and a p a t h l e n g t h o f 10.0
windows were cemented on the gas c e l l which was o
kept
c o n s t a n t temperature
The
o f 25.0
C by a t h e r m o s t a t .
the sample p r e p a r a t i o n o f n i t r i c mixtures
i s g i v e n i n F i g . 1.
experimental to
apparatus,
o x i d e gas m i x t u r e s
3 and
cm.
Silver
chloride
i n a l l experiments
experimental
at a
apparatus
for
and n i t r o g e n d i o x i d e gas
In the f i r s t s t e p o f sample p r e p a r a t i o n the
i n c l u d i n g t h e gas c e l l , was
remove the oxygen and then evacuated.
always checked
constructed of glass
f l u s h e d with dry n i t r o g e n
In e v e r y experiment
the e v a c u a t i o n
was
by means o f a mercury vacuum gauge. A f t e r e v a c u a t i o n v a l v e s 1,
5 were c l o s e d and n i t r i c
made t o p u r i f y the n i t r i c
o x i d e was
oxide.
l e d i n t o the system. No
(Matheson Gas
Products,
attempts
2
were
p u r i t y : 99,2%.) The
s m a l l amounts o f n i t r o u s o x i d e and n i t r o g e n d i o x i d e which a r e p r e s e n t i n commercial n i t r i c
o x i d e were s m a l l enough t o be n e g l e c t e d . A f t e r f i l l i n g
system w i t h n i t r i c
o x i d e the p a r t i a l
pressure of n i t r i c
o x i d e was
measured w i t h
a d i f f e r e n t i a l manometer f i l l e d w i t h bromo-naphthalene, i n which the of
n i t r o g e n o x i d e s i s v e r y low. The
sufficiently The
low t h a t no o p t i c a l
gas c e l l was
The
as d e s c r i b e d f o r NO
temperature
c o n t a i n e r and
apparatus
2
p r e s s u r e s o f NO^ 4
was
o x i d a t i o n o f NO
o f N0
2
and NgO^
i n T a b l e 1. kept
o x i d e i n the
p r e s s u r i s e d w i t h dry n i t r o g e n t i l l
2
which i s i n e q u i l i b r i u m w i t h NgO^
found t o be v e r y i n a c c u r a t e . Due
i n the
t o the s t r o n g
[11] a l l temperatures
such as i n t h e gas-sample c e l l , the
sample Small
cause l a r g e e r r o r s i n the c a l c u l a t i o n o f the
and NgO^
w i t h the e q u i l i b r i u m c o n s t a n t . T h e r e f o r e
NO^
o x i d e s u p p l i e d as d e s c r i b e d above
sample c e l l at an a b s o l u t e p r e s s u r e o f 0.1067
i s complete w i t h i n a few minutes and the p a r t i a l
MPa.
pressures
were then c a l c u l a t e d by means o f the e q u i l i b r i u m c o n s t a n t
In t h i s way
o n l y the temperature
a c c u r a t e l y c o n s t a n t . The
p r e s s u r i s i n g the gas
an
obtained.
p r e p a r e d by o x i d i z i n g n i t r i c
w i t h dry oxygen i n the gas The
i t s vapour.
i n t h e d i f f e r e n t i a l manometer s h o u l d be kept v e r y c o n s t a n t .
d e v i a t i o n s i n t h e temperature partial
was
from
dependence o f t h e above mentioned e q u i l i b r i u m
i n the e x p e r i m e n t a l
and N 0
was
p r e p a r a t i o n o f samples o f N0
same way
found
and a f t e r removing the n i t r i c
sample c e l l was
a b s o l u t e p r e s s u r e o f 0.1067 MPa
solubility
vapour p r e s s u r e o f bromo-naphthalene i s
i n t e r f e r e n c e was
c l o s e d by v a l v e 4,
sample c o n t a i n e r the gas
the
o f the gas
sample c e l l
given
should
be
use o f oxygen i n p l a c e o f n i t r o g e n f o r
sample c e l l had no
i n f l u e n c e on the i n f r a r e d a b s o r p t i o n
measurements. Nitric
a c i d vapours were p r e p a r e d by b u b b l i n g d r i e d n i t r o g e n gas
concentrated n i t r i c
acid solutions
a c o n s t a n t temperature
o f 20°C. To prevent c o n d e n s a t i o n
vapour and t h e water vapour the gas
through
(Merck a n a l y t i c a l grade) which were kept
stream was
o f the n i t r i c
then heated
t o 25.0°C
at
acid and 31
continuously not
l e d t h r o u g h the gas
i n f l u e n c e the i n f r a r e d
tion
i n the n i t r o g e n gas
nitric was
The
c o n c e n t r a t i o n of n i t r i c
adding a known amount o f 0.1
In t h i s method the n i t r a t e
to
nitrite
content
with by
t o a gas
concentrations
with
azo
stream and
reductor conditions
N-l-naphthylethylene-
dye
[12]. In t h e
s m a l l compared t o the
of the n i t r a t e
the
a c o l o r i m e t r i c method.
a copper-cadmium
coupled
were always v e r y
( l e s s than 0.1%
concentra-
sample
i o n then r e a c t s w i t h s u l f a n i l a m i d e under a c i d i c
diamine d i h y d r o c h l o r i d e t o form a r e d d i s h - p u r p l e
concentration
acid
a c i d vapour i n t h e gas
N alkali
i s reduced to n i t r i t e
form a d i a z o compound. T h i s compound was
the n i t r i t e
nitric
changed by v a r y i n g t h e c o n c e n t r a t i o n o f
then a n a l y s i n g on the n i t r a t e and n i t r i t e
column. The
p r e s e n c e o f some water vapour d i d
a b s o r p t i o n measurements. The
stream was
a c i d s o l u t i o n . The
determined by
cell.
samples
nitrate
content).
O,
Np
to vacuum pump
UJ NO-
to
Fig.
1
Experimental A:
sample
set-up
container;
f i l l e d with
for
sample
preparation.
B: molecular
bromo^naphthalene;
manometer; F:
32
thermostat
vacuum gauge;
D: 1, 2,
sieves;
C: differential
infrared
sample
3, 4,
valves.
5
gas
manometer cell;
E:
mercury
3.3
RESULTS
I n f r a r e d a b s o r p t i o n c o e f f i c i e n t s can be determined F o r our purposes
A = log —
at c o n s t a n t
temperature
by u s i n g Lambert-Beer's
law.
the absorbance (A) can be w r i t t e n as:
= a.b.P
(1)
o A
= Absorbance
1,1
= the r e s u l t a n t
o
and
incident
intensities
a
= absorptivity
MPa
.cm
b
= path l e n g t h
cm
P
= partial
MPa
bP
= o p t i c a l path length
Of each component were p r e p a r e d found. The
pressure
(NO,
N0 ,
and n i t r i c
4
at wave numbers at which no
a c i d vapour) c a l i b r a t i o n
o p t i c a l p a t h l e n g t h s and
f o r NO
determined
low o p t i c a l path l e n g t h s
law
w i t h the b a s e - l i n e
at a wave number o f 1908
i n F i g . 2 and F i g . 3. L e a s t square
i n d i c a t e t h a t Beer's
i s only v a l i d
curves
absorbance o f o t h e r components were
absorbance o f each component was
method. C a l i b r a t i o n c u r v e s
presented
Ng0
2
MPa.cm
(< 0.015
fits
1
cm
for high
MPa.cm) a r e
t o the e x p e r i m e n t a l
points
at low o p t i c a l path l e n g t h s (
490
absorption
H o f t i j z e r and Kwanten [1] Gerstacker D i s c u s s i o n at r e f . [15] M o l l [20]
267
l i q u i d N0 i n j e c t i o n i n t o water 2
T r e i n i n and Hayon [21]
300+100
Komiyama and Inoue [16]
f l a s h photolysis
194
—9 D
2 25°C "» m / s at
= 1.41 X 10 2°4,1
H
3
=1.29
kmol/m
.bar at 25°C
desorption
Kramers e t a l [15]
J
2 4
Table
2
Comparison
of literature
data concerning
the absorption
of N 0. p
into
water
diffusion coefficient
and Henry c o e f f i c i e n t
found by Kramers e t a l [ 1 5 ] . The
agreement between t h e r e a c t i o n r a t e c o n s t a n t d e r i v e d from t h e a b s o r p t i o n measurements i s r a t h e r poor. Moll
[20] i n j e c t e d l i q u i d N^O^ i n t o water and t h e r e a c t i o n r a t e c o n s t a n t k
agreed w e l l w i t h t h e v a l u e measured by Kramers e t a l [15] w i t h l a m i n a r j e t experiments.
T r e i n i n and Hanson [21] found r o u g h l y t h e same v a l u e s by means o f
flash photolysis.
In i n d u s t r i a l a b s o r b e r
d e s i g n t h e v a l u e o f H„ „ i/kD. i s much N 0 V 2
more important
4
I
than t h e r e a c t i o n r a t e c o n s t a n t . H o f t i j z e r and Kwanten [1] found
the f o l l o w i n g e q u a t i o n f o r water: 760 l
It
0
g
V o . l / 2 4
5
*
(3° - 75°C)
= " 0.53 -
(13)
i s known t h a t t h e a b s o r p t i o n r a t e o f N^O^ d e c r e a s e s w i t h i n c r e a s i n g
acid
s t r e n g t h . T h i s may be a t t r i b u t e d t o t h e d e c r e a s e o f t h e Henry c o e f f i c i e n t increasing
" f r e e " water becomes r e l a t i v e l y
s m a l l i n more c o n c e n t r a t e d n i t r i c
No r e l i a b l e d a t a can be found the n i t r i c
a c i d s t r e n g t h on t h e H
L o n g s t a f f and S i n g e r nitric
acid [1].
i n the l i t e r a t u r e concerning the i n f l u e n c e of i / kD. v a l u e s . N2O4V Jo
[24] found t h a t i f N 0
2
gas i s i n c o n t a c t w i t h 60%
a c i d u n r e a c t e d N^O^ may be p r e s e n t i n t h e l i q u i d phase and t h a t t h e
n i t r o u s a c i d c o n c e n t r a t i o n may be n e g l e c t e d . The r a t i o C
/(C N
in
with
i o n i c s t r e n g t h and a d e c r e a s e o f k as t h e molar c o n c e n t r a t i o n o f
t h e l i q u i d phase as a f u n c t i o n o f t h e n i t r i c
2°4,J> acid strength
H N 0
+C ) 2,Jl 20 ,Jo given i n F i g . N
4
10
0 5
100
50 1
Fig.
1
The distribution of the nitric
of ^2^4 aoid
m
N0
^
3
n
^
^trie
acid
solutions
as a
function
strength. 47
From t h i s "2^4 ^
a
s
m
x
l
f i g u r e i t can be c o n c l u d e d u
*
r
e
s
into n i t r i c
that the p h y s i c a l absorption o f N0 / 2
a c i d s o l u t i o n s becomes important
fornitric
acid
s o l u t i o n s above 55% and t h a t t h e r e a c t i o n o f N „ 0 . w i t h water may be n e g l e c t e d . z 4 4.2.2 NCvj/NgO^ a b s o r p t i o n i n t o c o n c e n t r a t e d n i t r i c
The
a b s o r p t i o n o f NO„/N 0 i n t o n i t r i c £ £ 4
important of
found c o n c e r n i n g nitric
a c i d s o l u t i o n s o f more than
f o r the p r o d u c t i o n o f d i l u t e d n i t r i c
concentrated n i t r i c
acid.
acid solutions
a c i d as w e l l as t h e p r o d u c t i o n
In t h e l i t e r a t u r e v e r y l i t t l e
the absorption o f N0 /N 0 2
a c i d . Atroshchenko and Kaut
2
gas m i x t u r e s
4
i n f o r m a t i o n can be
into
concentrated
[26] i n v e s t i g a t e d t h e a b s o r p t i o n o f NOg/NgO
i n t o 70 - 98% HNOg and found t h a t t h e a b s o r p t i o n proceeds p u r e l y T h i s f i n d i n g was a l s o c o n f i r m e d
55% i s
by Karavaev and V i s l o g u z o v a
l i t e r a t u r e no i n f o r m a t i o n can be found
concerning
physically.
[25]. In t h e
t h e a b s o r p t i o n mechanism.
Equilibrium
The
s o l u b i l i t y o f NgO^ i n t o c o n c e n t r a t e d n i t r i c
for
the design of i n d u s t r i a l
absorbers.
a c i d s o l u t i o n s i s v e r y important
In t h e l i t e r a t u r e o n l y v a l u e s o f t h e
t o t a l vapour p r e s s u r e o f t h e system N^-HNOg-HgO were found o r d e r t o study
t h e s o l u b i l i t y o f NgO^ i n t o n i t r i c
[27,28,29,30]. In
a c i d s o l u t i o n s i t was assumed
t h a t t h i s t o t a l vapour p r e s s u r e can be d e s c r i b e d a s :
P
= tot
P
HNO3
+
P + H 0
P
2
+ N0
P
N 0
2
2
4
+ P „ + P N 0 NO 2
(14)
3
P„„ and P „ _ a r e v e r y low above c o n c e n t r a t e d n i t r i c a c i d s o l u t i o n s and t h e r e NO N 0 * f o r e they may be n e g l e c t e d . The vapour p r e s s u r e s o f HNO^ and HgO were taken 2
3
from t h e d a t a o f t h e b i n a r y system HNO^-HgO measured by Vandoni and Laudy [ 8 ] . 2 With a i d o f t h e e q u i l i b r i u m c o n s t a n t (K = P / H ' equilibrium 2 2 4 2 P
o
f
t
h
e
n
N
2N0
2
X N 0 2
the vapour p r e s s u r e s o f N 0 If
(15)
4
N
2
and
i t i s assumed t h a t m a i n l y
2
0
4
2
0
N
were c a l c u l a t e d . 4
i s present
i n t h e l i q u i d phase
31,32,33] t h e Henry c o e f f i c i e n t H „ „ s h o u l d be d e f i n e d a s : 2°4 N
N
° 2°4 I
48
[27,29,30,
Henry c o e f f i c i e n t s
(H„ _ ) were c a l c u l a t e d 2°4
from
t h e t o t a l vapour p r e s s u r e
data
N
o f Klemenc and Rupp [28] and t h e r e s u l t s a r e g i v e n i n F i g . 2 and F i g . 3. From t h e s e f i g u r e s i t can be c o n c l u d e d
t h a t t h e Henry c o e f f i c i e n t
i s independent o f
t h e amount o f NgO^ i n t h e l i q u i d phase. A t s m a l l NgO^ c o n t e n t s some d e v i a t i o n s
20
15
10
—v—
V
o
u
o
a.
û
o
CM
~~ A.
A
Q
as a function
o A
A
5 °/o N
Hjy
0
°C
o
S
2
V
o
o 6
Fig.
V
A
10 2
0
4
ri
A.
A
12 5 °C
25
15
b y w e i g h t in
of the FS^O^ content
°C
20
HNO3
in 16 N HNO^.
(V: 0°C; 0: 12.5°C; A: 25°C) [28].
From t h e t o t a l vapour p r e s s u r e d a t a measured by W e i n r e i c h Karavaev and Yarkovaya the temperature
[27]
and
[29] Henry c o e f f i c i e n t s were c a l c u l a t e d as a f u n c t i o n o f
and t h e a c i d s t r e n g t h (see F i g . 4 ) . Some own experiments
c a r r i e d out t o i n v e s t i g a t e t h e Henry c o e f f i c i e n t more d i r e c t l y . N i t r i c
were
acid of
75% was s a t u r a t e d w i t h n i t r o g e n gas c o n t a i n i n g 5-20 volume % o f N 0 by means o f a 2
s a t u r a t o r a t a p r e s s u r e o f 1.04 b a r . The gas phase was a n a l y s e d f o r i t s content w i t h i n f r a r e d phase was determined
spectroscopy
0 2
4
_
(Chapter 3 ) . The NgO^-content i n t h e l i q u i d
by i n j e c t i n g a l i q u i d
NaOH s o l u t i o n and then method
N
sample o f 50 y l i n t o a 10 ml 0.8 N
a n a l y s i n g f o r n i t r i t e content with a c o l o r i m e t r i c
[ 3 4 ] . I t was found t h a t t h e measured Henry c o e f f i c i e n t
was
independent
49
o f t h e p a r t i a l p r e s s u r e o f N^O^ i n the gas phase. The e x p e r i m e n t a l summarized
results are
i n T a b l e 3.
30
25 o
°c
20 -
15
-o
o
o
o
12 • 5 °C
oo o
!
io
o
A
E
A
ft-
C
°C
25
3
5
10
%> N 0 2
Fig.
3
H
n
as a function
4
of the N„0. content
"24 (V;
50
15
b y weight in HN0
0°C; 0: 12.5°C; A: 25°C) [28].
20
25
3
in 19 N nitric
acid.
0-30
Fig.
4
H
- as a function 2 4
of the temperature
and n i t r i c acid
concentration.
a
a
Weinreich
[27]
25% by weight
of N 0
4
in 75% HN0
ff 0^
in 75% HN0
g
+
Weinreich
[27]
20% by weight
of
0
Weinreich
[27]
10% by weight
of N 0
A
Klemenc and Rupp [28]
•
Karavaev
0
This
and Yarkovaya
g
2
4
3
3
in 75% HNO^
[29]
work
From F i g . 4 i t can be seen t h a t t h e r e i s a r a t h e r good agreement w i t h t h e r e s u l t s d e r i v e d from t h e vapour p r e s s u r e d a t a . I t s h o u l d be noted t h a t at a r a t h e r h i g h N^O^ c o n t e n t i n t h e l i q u i d and low temperatures
liquid
i m m i s c i b i l i t y may o c c u r
The heat o f s o l u t i o n o f N O
phase
[31,32,33].
i n t o n i t r i c a c i d s o l u t i o n s can be c a l c u l a t e d
51
from:
d i n (H
_
)
AH
g
_*JL = d
( 1 7 )
(^)
R
i n which T i s t h e a b s o l u t e temperature, R t h e gas c o n s t a n t and A H s o l u t i o n a t t h e temperature c o n s i d e r e d
( t a k e n as n e g a t i v e ) .
g
t h e heat o f
In a f i r s t
a p p r o x i m a t i o n AHg may be assumed t o be c o n s t a n t over a s m a l l range o f temperature. The r e s u l t s
for different n i t r i c
acid strengths are given i n
T a b l e 4. From t h i s t a b l e i t can be seen t h a t t h e heat o f s o l u t i o n o f NgO^ i n t o nitric
acid solutions
i s rather constant f o r d i f f e r e n t
Temperature (°C)
H ™24 kmol/m .bar
7 0
0 0106
- 0 0611
25.2
6 3
0 008
- 0 05
35.2
4 9
0 006
- 0 04
45.3
3 4
0 003
- 0 007
3
Henry coefficient of the
% HN0
52
bar P
20.3
Table
4
strengths.
Measured range
3
Table
acid
Heats
of N 0 2
4
in 75% n i t r i c acid
as a
function
temperature
AH
3
g
(kJ/kmol N 0 ) 2
65
- 23 2 X i o
3
70
- 25 3 X i o
3
75
- 27 2 X i o
3
19 N
- 25 3 X i o
3
of solution
of N 0. p
into
concentrated
4
n i t r i c acid
solutions
4.3
EXPERIMENTAL
The
experimental
apparatus
i s schematically presented
s o l u t i o n s were pumped t o an overhead wetted
2. About 0.05% by weight o f an a l k y l
i n order t o e l i m i n a t e r i p p l e s o f the l i q u i c
f i l m . The f i l m h e i g h t h was c o r r e c t e d f o r t h e end e f f e c t o f t h e s u r f a c e a c t i v e agent experiments
acid
r e s e r v o i r and then f e d by g r a v i t y t o t h e
w a l l column d e s c r i b e d i n Chapter
s u l p h o n a t e was added t o t h e l i q u i d
i n F i g . 5. N i t r i c
caused by t h e p r e s e n c e
and t h e e f f e c t i v e f i l m h e i g h t h' was i n t h e s e
13.7 cm and 34.6 cm. N i t r o g e n d i o x i d e ( b o i l i n g p o i n t : 21.2°C) was
s u p p l i e d from condensation
a cylinder
immersed i n a water b a t h a t 47.5°C. To a v o i d
t h e p i p e s c o n t a i n i n g pure n i t r o g e n d i o x i d e were heated
r e s i s t a n c e heating wire
(Pyrotenax
i n s u l a t e d w i t h g l a s s wool. The N 0
2
with
L t d . , Hebburn-on-Tyne, England) and gas stream
was meted w i t h a n e e d l e
valve
immersed i n a t h e r m o s t a t i c a l l y c o n t r o l l e d water b a t h o f 50°C and then mixed w i t h a n i t r o g e n gas stream. the wetted mixture
The gas m i x t u r e was l e d i n c o - c u r r e n t f l o w
w a l l column i n t h e same way as was d e s c r i b e d i n Chapter
l e a v i n g t h e wetted
w a l l column was scrubbed
through
2. The gas
w i t h an a l k a l i n e hydrogen
p e r o x i d e s o l u t i o n t h e remove t h e n i t r o g e n o x i d e s b e f o r e v e n t i n g i t t o t h e atmosphere. The n i t r i c
a c i d l e a v i n g t h e wetted
stainless steel vessel.
In t h e experiments
w a l l column was s t o r e d i n a
w i t h 63% and 78% t h e n i t r i c
s o l u t i o n was s t r i p p e d w i t h n i t r o g e n t o remove t h e d i s s o l v e d N„0^ nitric
a c i d . The s t r i p p e d n i t r i c
w i t h 25% and 40% n i t r i c was
acid
from t h e
a c i d was then r e c y c l e d . In t h e experiments
acid, the n i t r i c
a c i d l e a v i n g t h e wetted
w a l l column
drained o f f . The
i n - and o u t g o i n g
l i q u i d were a n a l y s e d f o r t h e i r NgO^ and/or HNOg
c o n t e n t by i n j e c t i n g a l i q u i d
sample (50 - 250 y l ) i n t o a 10 mol 0.8 N NaOH
s o l u t i o n . A f t e r reaction the n i t r i t e method
c o n t e n t was determined
[34]. Note t h a t w i t h t h i s method HN0
2
with a c o l o r i m e t r i c
and N 0 ^ i n t h e n i t r i c 2
acid
samples can not be d i s t i n g u i s h e d . The o u t g o i n g gas streams were a n a l y s e d f o r t h e i r NOg,
N 2
0
4
and NO c o n t e n t w i t h i n f r a r e d s p e c t r o s c o p y
concentration o f N0
N
2
and ° 4 2
e s t a b l i s h i n g a mass b a l a n c e
i
n
t
n
e
i n g o i n g gas stream
around t h e wetted
(Chapter 3 ) . The
was c a l c u l a t e d by
w a l l column.
53
to
Fig.
5
air
The
experimental
nitric
acid
for
8: rotameter; less bath;
54
column;
5: stainless
6: vessel
steel
for
the NO^
and
NO absorption
experiments
into
solutions.
1: wetted wall vessel;
set-up
2: stripper;
steel
alkaline
9: overhead
filter;
14: needle
vessel
hydrogen
for
stripped
peroxide;
reservoir;
12: cyclone; valve;
3: scrubber;
13:
15: infrared
4: stainless nitric
10: flow
gas
acid
7: calibrated
sample
solution;
glass
controllers;
thermostatically
steel
11:
controlled cell.
pipe; stainwater
4.4 RESULTS
4.4.1 The a b s o r p t i o n o f N 0 2
4
into diluted n i t r i c
acid
A c c o r d i n g t o t h e model NO^ and NgO^ a r e t r a n s f e r r e d , w i t h each o t h e r , of a N 0 / N 0 2
2
solutions
i n continuous
from t h e gas phase t o t h e g a s - l i q u i d
equilibrium
i n t e r f a c e . The d i f f u s i o n
m i x t u r e i n the gas phase was r e g a r d e d as t h e d i f f u s i o n o f one
4
f i c t i t i o u s component Q d e f i n e d as:
C
X 2,g
Q =
+
2 C
N
(
n 2 4,g
The gas phase d i f f u s i o n can under o u r measured G r a e t z model
8
)
c o n d i t i o n s be d e s c r i b e d by t h e
(Chapter 2) and t h e d i f f u s i o n r a t e o f N 0
the g a s - l i q u i d
1
4
2
from the gas phase t o
i n t e r f a c e p e r u n i t o f s u r f a c e a r e a can be w r i t t e n as:
n
Q
The d i f f u s i o n c o e f f i c i e n t o f t h e f i c t i t i o u s component Q as a f u n c t i o n o f D 2 and D
N
D Q
^
was d e r i v e d by Dekker [ 3 5 ] .
= D 2°4
_ Jl l ++ 8K 8K P„ P„ .. ++ ./ ,/" 1 + 8K
+
N
D„„ N0
/
P
= 1.36 x 10
-5
(20)
P 2
Q,o
2 o m /s a t 20 C and 1.0132 b a r
2
5
D
2
= 0.96 x 1 0 ~ m /s
a t 20°C and 1.0132 b a r
2 4 i n which P_ . i s t h e p a r t i a l Q,i tis
the p a r t i a l
p r e s s u r e o f N0„ + 2N„0„ a t t h e i n t e r f a c e and P„ t2 2 4 Q,o
p r e s s u r e i n t h e m i d d l e o f t h e wetted w a l l column. The
N 2
°4
r e a c t s w i t h water:
N
2°4
+
H
H N 0
2 ° ~*
+
3
H N 0
(
2
2
1
)
Decomposition o f t h e n i t r o u s a c i d produced a c c o r d i n g t o
4HN0
2
•* 2N0 + N 0 2
4
+ H 0 2
(22) 55
does not take p l a c e r a p i d l y i f i t s c o n c e n t r a t i o n
i s low. NO i s very
poorly
s o l u b l e i n aqueous s o l u t i o n s and i t can e a s i l y d i f f u s e i n t o t h e gas phase. Under o u r e x p e r i m e n t a l
c o n d i t i o n s no NO c o u l d be d e t e c t e d
i n f r a r e d spectroscopy.
Therefore
HNOg c a n be n e g l e c t e d with
i t may be assumed t h a t t h e d e c o m p o s i t i o n o f
[1,13,14,15,16,35]. A p p l y i n g
a r a p i d pseudo f i r s t
order
i n t h e gas phase w i t h
t h e theory
o f mass t r a n s f e r
r e a c t i o n i n t h e l i q u i d phase t h e a b s o r p t i o n
r a t e p e r u n i t o f s u r f a c e a r e a based on t h e p e n e t r a t i o n t h e o r y
can be w r i t t e n
as:
T h i s equation was
holds only
The
1 [36]. In accordance w i t h
t h i s equation i t
temperature r i s e n e a r t h e i n t e r f a c e as a r e s u l t o f t h e a b s o r p t i o n was
c a l c u l a t e d with
(-AH
A,
T
= _ 2
The
i f kx »
assumed t h a t N^O^ i s t h e a c t i v e s p e c i e s d u r i n g t h e a b s o r p t i o n .
[36]:
-
2 P
AH )
S_ C
H N
p
2°4
V ——
P N
(24)
2°4,i
heat o f s o l u t i o n o f NgO^ i n t o aqueous s o l u t i o n s and t h e heat o f r e a c t i o n
were t a k e n from t h e d a t a o f M o l l
[20]. W i t h i n
t h e measured c o n d i t i o n s i t can
e a s i l y be shown t h a t t h e temperature r i s e was s m a l l enough t o be n e g l e c t e d (< 0 . 2 ° C ) . The p a r t i a l
pressures
of N 0 2
4
c a l c u l a t e d from t h e measured a b s o r p t i o n
on t h e g a s - l i q u i d i n t e r f a c e were r a t e s , equation
(19) and e q u a t i o n (20)
by means o f an i t e r a t i o n p r o c e d u r e . The measured a b s o r p t i o n of P
was p l o t t e d f o r 25% n i t r i c
r a t e as a f u n c t i o n
a c i d i n F i g . 6 and f o r 40% n i t r i c
acid i n
N
2°4,i F i g . 7. T h i s s h o u l d g i v e a s t r a i g h t
l i n e through the o r i g i n with
a slope of
i s the a c t i v e H„N2O4V „\/kD . From these f i g u r e s i t c a n be c o n c l u d e d t h a t N 0 s p e c i e s d u r i n g t h e a b s o r p t i o n . A l e a s t - s q u a r e method gave t h e s l o p e o f t h e 2
straight
line
and t h e s e v a l u e s
are given
From t h i s t a b l e i t can be seen t h a t nitric
a c i d s t r e n g t h . F o r low n i t r i c
4
i n T a b l e 5.
h N 2
q
decreases with i n c r e a s i n g
a c i d c o n c e n t r a t i o n t h i s i s m a i n l y caused
by t h e d e c r e a s e o f H, „ w i t h i n c r e a s i n g i o n i c s t r e n g t h . A c c o r d i n g t o H o f t i j z e r N2O4 and Kwanten [ l ] t h e i n f l u e n c e o f t h e i o n i c s t r e n g t h on H can be d e s c r i b e d 2°4 with: (H
) 2 4 nitric
= (H acid
) exp (- 0.075 I) 2 4 water
NQ
where I i s t h e i o n i c s t r e n g t h d e f i n e d by
56
(25)
n
2
I = i I . -
(Z.
1
From F i g . 8 i t can
be
Kwanten [1]
is valid
nitric
large
acid
pseudo f i r s t
(26)
C.)
1
seen t h a t for n i t r i c
deviations
order reaction
acid strength since
the
a p p r o x i m a t i o n proposed by
a c i d c o n c e n t r a t i o n s up
o c c u r . T h i s may rate
o f NgO^
caused by
the molar c o n c e n t r a t i o n o f the
into n i t r i c
the
and
Above
fact that
constant k decreases with i n c r e a s i n g
r e l a t i v e l y s m a l l i n more c o n c e n t r a t e d n i t r i c coefficient
be
Hoftijzer
to about 25%.
25%
the
nitric
" f r e e " water t e n d s to become
a c i d . Moreover the
a c i d decreases with i n c r e a s i n g
diffusion nitric
acid
strength.
57
•
bar P
Pig.
N
7 The absorption driving
2
0
4
, i
rate
*
1
,
2
P
N 0
of N 0 2
4
2
, i (a)
into
40% HNC> at 20° C as a function 3
of the
force.
(h> = 0.346 m; x = 0. 722 sec; 0: P
Q
l
; A: P
U
H
m N
n V 2
k D
o
+ P
Q
lV
h 4,i
u
2 4,i
f t ) . Z,%
m au
Reference
4
x 10^ kmol/m .bar
Kramers e t a l [15]
water
0 76
25% HN0
3
0 49 + 0 03
this
work
40% HNOg
0 16 + 0 02
this
work
H o f t i j z e r and Kwanten [ l ]
Table
58
5
Q
ykD^ values
as a function
of the nitric
acid
strength
at 20 C
1 0
H
n
n V kDo 2 4
(A;
values
Kramers
et al
as a function [15]j
approximation
Some a u t h o r s experiments
of
Hoftijzer
the and
proposeu
[10,13,14,35] observed
nitric
r e l a t i v e l y h i g h molar c o n c e n t r a t i o n s o f N^O^. a c i d m i s t was
observed.
Kwanten
by Hoftijzer
c o n c e r n i n g the a b s o r p t i o n o f NgO^
here no n i t r i c
nitric
strength.
[1];
0:
and
a c i d mist
this
worl
Kwanten
[1]).
during t h e i r
i n t o water e s p e c i a l l y D u r i n g t h e experiments
The n i t r i c
phase does not seem t o be v e r y important
acid
as was
acid
at presented
formation i n the
found by Detournay and
gas Jadot
[4]-
4.4.2
The
Previously
a b s o r p t i o n o f N„0„ i n t o c o n c e n t r a t e d n i t r i c 2 4 i t was
l y and t h a t NgO^
acid
solutions
d e r i v e d t h a t t h i s a b s o r p t i o n p r o c e s s proceeds may
be assumed t o be the a c t i v e
d i f f u s i o n o f N0„ and N O
s p e c i e s . The
purely physical-
gas
phase
can be d e s c r i b e d by the Graetz-model a c c o r d i n g t o :
59
The
p h y s i c a l absorption process o f N 0 2
experimental
J N
The
2°4
i n t o t h e l i q u i d phase can under o u r
4
c o n d i t i o n s be w r i t t e n as:
= 2(H 2°4 N
temperature
P 2°4,i
- C 2°4,*,o
N
—
)V
N
T
(28)
T
r i s e near t h e i n t e r f a c e as a r e s u l t o f t h e p h y s i c a l a b s o r p t i o n
was c a l c u l a t e d w i t h [ 3 6 ] : -AH
AT =
PC
H p
N 0„ 24 o
V —
P
N.O^ 24,i
< > 29
a v
I t was found t h a t w i t h i n t h e measured c o n d i t i o n s t h i s temperature o neglected
(< 0.2 C ) . T a b l e 6 and T a b l e 7 g i v e t h e e x p e r i m e n t a l
results.
p r e s s u r e s o f NgO^ on t h e i n t e r f a c e were c a l c u l a t e d from e q u a t i o n equation
(20) w i t h an i t e r a t i o n p r o c e d u r e .
function of the d r i v i n g
f o r c e (H„ „ P„ _ N 04 N 0 2
straight
l i n e through
2
r i s e may be Partial
(27) and
The measured a b s o r p t i o n r a t e as a
4
i
- C„ _ N 0 2
) s h o u l d be g i v e n a 4 )
£
) 0
the o r i g i n .
From F i g s . 9, 10, 11 and 12 i t can be c o n c l u d e d
that N 0 2
4
i s the active
s p e c i e s d u r i n g t h e a b s o r p t i o n . With e q u a t i o n (28) t h e t h e o r e t i c a l a b s o r p t i o n r a t e s were c a l c u l a t e d and compared w i t h t h e measured a b s o r p t i o n r a t e s . The d i f f u s i o n c o e f f i c i e n t o f N_0„ i n t o n i t r i c a c i d s o l u t i o n s was c a l c u l a t e d w i t h 2 4 t h e r e l a t i o n o f Wilke and Chang [ 3 6 ] .
D„ „ = 0.88 x 1 0 ~ 2°4,£
9
2
m /s
f o r 78% HN0„ a t 20°C
N
D N
„ = 0.77 x 1 0 2°4,il
- 9
3
2
m /s
f o r 63% HNO, a t 20°C 3
From T a b l e 6 and T a b l e 7 i t can be c o n c l u d e d d e s c r i b e s t h e experiments
60
fairly
well.
t h a t t h e proposed
a b s o r p t i o n model
Exp.
P
T
c
N
2°4,£,o 3 3 xlO kmol/m
bar
35, 1
1. 093
20
35 2
1 ,093
20
0. 346
1 .58
85
3. 05
0. 346
1. 58
85
3 05
NO
2,o
2 4,o
2 4,i
N
N
2°4 6 2 xlO kmol/m .sec
2°4 6 2 x l O kmol/m . s e c
measured
penetration theory
bar
bar
bar
0 ,0458
0 ,0214
0, 0148
1.11
1, 21
0. 0673
0, 0463
0. 0297
2.39
2 50 0 11
37. 1
1 040
20
0. 346
2 ,40
85
2. 61
0 ,0165
0 ,00279
0. 00231
0.11
37. 2
1 040
20
0, 346
2 40
85
2 61
0 ,0379
0 ,0147
0 0107
0.58
0 70
37 3
1, 040
20
0. 346
2 40
85
2, 61
0 ,0556
0 ,0316
0 0212
1.31
1 44
37 4
1 040
20
0, 346
2 40
85
2 ,61
0 .0664
0 ,0451
0 0296
1.80
2 03
37 5
1 040
20
0. 346
2 40
85
2 61
0 ,0759
0 ,0590
0 0376
2.34
2 60
3 33
0 ,0712
0 ,0241
0 0180
2.13
2 12
3 33
0 ,0931
0 ,0411
0 ,0281
3.92
3 .41 0 .85 2 .85
31 1
1 070
30
0, 137
0 .513
65
31 .2
1 ,070
30
0. 137
0 .513
65
34 1
1 .110
30
0 346
0 .986
65
3 ,10
0 .0538
0 .0137
0 0105
0.88
34 .2
1 .110
30
0 346
0 .986
65
3 , 10
0 .0975
0 .0450
0 ,0323
2.64
36 .1
1 .064
30
0 .346
1 .77
65
2 , 82
0 .0190
0 .00172
0 ,00164
0.03
0 .04
36 ,2
1 .064
30
0 ,346
1 .77
65
2 .82
0 .0436
0 .0090
0 .00745
0.35
0 ,44
36 . 3 1 .064
30
0 .346
1 .77
65
2 ,82
0 .0723
0 .0248
0 ,0191
0.98
1 .23
36 .4
1 ,064
30
0 ,346
1 . 77
65
2 .82
0 .0915
0 .0397
0 .0289
1.69
1 .90
36 .5
1 .064
30
0 346
1 . 77
65
2 ,82
0 .111
0 .0582
0 ,0413
2.49
2 .75
Table 6
N0„ absorption
experiments
into
63% HNO^
Exp.
P
c
T
h'
T
m
C
P N0„ 2,o
K~0. .
2 4,10,0 3
3
P n
2°4,o
P n
2°4,1
N
2°4
N
6
2 xlO kmol/m .sec measured
2°4 6 2 xlO kmol/m .sec p e n e t r a t i o n theory
bar
bar
bar
3 9
0 0676
0 0468
0 0187
4 10
4 05
210
3 9
0 1006
0 1034
0 0377
8 47
8 24
2 15
210
3 6
0
0644
0 0424
0 0194
3 02
3 74
0 346
2 15
210
3 6
0 0740
0 0561
0 0236
4 02
4 58
0 346
2 15
210
3 6
0 0847
0 0734
0 0269
5 52
5 24
0 346
2 15
210
2 6
0 0323
0 0107
0 00570
0 84
1 07
0 00648
1 11
1 37
0 00107
0 13
0 15
xlO
bar
C
m
sec
12 1
1 075
20
0 346
1 71
210
12 2
1 075
20
0 346
1 71
13 1
1 051
20
0 346
13 2
1 051
20
13 3
1 051
20
14 1
1 051
20
kmol/m
14 2
1 079
20
0 346
1 71
210
2 6
0 0348
0
15 1
1 067
20
0 346
2 15
210
2 75
0 0122
0 00151
0124
15 2
1 087
20
0 346
1 71
210
2 75
0 0126
0 00163
0 00110
0 17
0 17
19 1
1 063
20
0 137
0 43
210
3 35
0 0541
0 0299
0 0136
5 47
5 85
19 2
1 063
20
0 137
0 43
210
3 35
0 0785
0 0631
0 0223
146
2 55
0 0746
0 0264
0 0138
4 28
146
2 55
0 0981
0 0456
0 0217
7 06
5 99
2 35
2 49 5 48
18 1
1 039
30
0 137
0 61
18 2
1 039
30
0 137
0 61
12 0
9 67 3 76
17 1
1 014
30
0 137
0 91
146
2 55
0 0638
0 0193
0 01127
17 2
1 014
30
0 137
0 91
146
2 55
0
0 0469
0 0242
5 41
16 1
1 053
30
0 346
2 15
146
3 64
0 0106
0 000533
0 00040
0 08
16 2
1 053
30
0 346
2 15
146
3 64
0
0
0 00335
0 31
0 41
16 3
1 053
30
0 346
2 15
146
3 64
0 0786
0 0293
0 0170
1 93
2 45
146
3 64
0 0582
0 0161
0 0104
1 02
1 47
146
3 64
0
0 0473
0 .0246
3 17
3 59
16 4
1 053
30
0 346
2 15
16 5
1 053
30
0 346
2 15
Table
7 NO^-absorption
experiments
into
78% nitric
0994
0310
0999
acid
00456
-—
N 0
,i
"
N 0
N 0 ,i
*
'2 NO
2
4
2
. l,o /
4
n
N 0 2
(o) 4
bar 2
Fig.
9
The absorption driving
rate
4
,i
K
z
of N^O^ into
N0 2,i NU
C
^
, l.o '
4
H
N 0 2
4
63% HNO^ at 20 C as a function
of the
N„0„ . 2 4,i
P
N 0
The absorption driving
rate
4
,i
N 0 ,i 2
4
"
C
N
A :
P
+
N„0, 2 4,i
05 2
0
,l,o/ N 0 2
* '2 N0 ,i 1
(o)
H
4
-
P
2
of N^O^ into
C
4
N 0 .l,o/ 2
4
H
N 0 2
(
i
)
4
63% HN0 at 30 C as a function
of the
3
force.
(h> = 0.346 m; T = 1.77 sec; 0: P m
2,1
/H
U
0
p
°N„0 N„0j 2 4,l,o 2^4
H
N
2
10
2
~ N 0 ^N 0 ' 2 4,l,o 2 4
P
Fig.
0
L
force.
(h' = 0.346 m; T = 2.40 sec; 0: P F
~ N
_
2 4,%,o
-C 2
/n
r
4
>
l
; A :P
/H 2
4
> * > °
2
4
2
+ 4
>
l
2 4 63
0 02 N 0
P
2
P
N 0 .i 2
Fig.
11 The absorption driving
4
,i
rate
4
" N 0
0 04
C
2
*
of N^
4
1 /
2
P
4
NO
, l.o /
H
N 0 2
(o) 4
.I " N Q
,l,o/ N 0
C
s
into
2
H
4
2
63%) can be c o n s i d e r e d as pure p h y s i c a l p r o c e s s .
acid
I t c a n be c o n c l u d e d
that
N^O^ i s t h e a c t i v e s p e c i e s d u r i n g t h e a b s o r p t i o n p r o c e s s . The s o l u b i l i t y o f N^O^ i n c o n c e n t r a t e d n i t r i c
a c i d s o l u t i o n s was c a l c u l a t e d from t h e t o t a l
p r e s s u r e d a t a o f t h e system NgO^HNOg-HgO, and i t can be c o n c l u d e d law
that
vapour Henry's
i s valid.
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H., J. Chem. Eng. of Japan,
1978, 11, 25.
23. Counce, R.M., Master T h e s i s , U n i v e r s i t y o f Tennessee, 24. L o n g s t a f f , J.V.L. 25. Karavaev, 1974,
Chem. USSR (Engl.
Transl.),
1001.
26. Atroshchenko, 1958,
and S i n g e r , K., J. Chem. Soa. , 1954, 2610.
M.M. and V i s l o g u z o v a , V.G., J. Appl.
47,
K n o x v i l l e , 1978.
31,
V . l . and Kaut, V.M., J. Appl.
Chem. USSR (Engl.
Transl.),
340.
27. W e i n r e i c h , G.H., PhD T h e s i s , U n i v e r s i t y o f T o u l o u s e , F r a n c e , 1955. 28. Klemenc, A. and Rupp, J . , Z. Anorg. 29. Karavaev, 1967,
M.M. and Yarkovaya,
Allg.
Chem., 1930, 194, 51.
V.A., J. Appl.
Chem. USSR (Engl.
Transl.),
40, 2340.
30. Karavaev,
M.M. and Bessmertnaya,
A . I . , The Soviet
Chemical
Industry,
1969,
7, 30. 31. A u d i n o i s , R., J. Chim. Phys.,
1965, 62, 439.
32. A u d i n o i s , R., J. Chim. Phys. Physicochim. 33. A u d i n o i s , R., CR.
Acad. Sei. Paris
Biol.,
1969, 66, 489.
Sec. C. , 1968, 266,
117.
34. T e c h n i c o n A u t o - A n a l y z e r I I , I n d u s t r i a l Method No. 230-72A/Tentative 1974. 35. Dekker, W.A., PhD T h e s i s , D e l f t U n i v e r s i t y o f Technology, 36. Danckwerts,
D e l f t , 1958.
P.V., G a s - L i q u i d r e a c t i o n s , M c G r a w - H i l l , London, 1970.
37. H i s a t s u n e , I.e., J. Phys. Chem., 1961, 65, 2249. 38. B o d e n s t e i n , M. and Bogs, F., Z. Physik. 39. F o r s y t h e , W.R. and Giauque,
Chem., 1922, 100, 68.
W.F., J. Am. Chem. Soa., 1942, 64, 48.
5. THE OXIDATION AND ABSORPTION OF NO BY NITRIC ACID
5.1 INTRODUCTION
The
oxidation
o f NO t o N 0
by c o n c e n t r a t e d n i t r i c
2
a c i d s o l u t i o n s may be o f
importance f o r the p r o d u c t i o n o f c o n c e n t r a t e d n i t r i c concentrated n i t r i c nitrogen
o x i d e s from t h e t a i l
attractive properties.
gas o f n i t r i c
Firstly,
e a s i l y o x i d i z e NO t o N0 .
2
p h y s i c a l l y very well
acid plants,
i t i s a very strong
Secondly, N 0
2
dissolves
a c i d . In a d d i t i o n
a c i d may be used as a s c r u b b i n g l i q u i d
4
f o r t h e removal o f
since
i t has two
o x i d i z i n g agent and i t can
which i s i n e q u i l i b r i u m
into concentrated n i t r i c
with
In t h i s C h a p t e r the mechanism and the k i n e t i c s o f t h i s o x i d a t i o n nitric
acid are investigated
to obtain
a b s o r b e r s . Furthermore some p r e l i m i n a r y o f NO i n t o 40% n i t r i c
acid solutions
gathered i n the concentrated region
N0 , 2
acid. by 63%-78%
data f o r the design o f i n d u s t r i a l experiments c o n c e r n i n g the
absorption
a r e c a r r i e d out t o check i f t h e r e s u l t s apply a l s o t o t h e d i l u t e d system.
5.2 PROPOSED MECHANISM
The
r e a c t i o n o f NO w i t h n i t r i c
a c i d i s presented with the f o l l o w i n g
overall
reaction:
NO + 2HN0
This
3
reaction
acid solutions
t 3N0
2
+ H 0
i s the r e v e r s e d
investigate
t h i s phenomenon
stream c o n t a i n i n g
that walls
r e a c t i o n of a c i d formation. Concentrated
o f 55%-80% have a c o n s i d e r a b l e
T h e r e f o r e , t h i s r e a c t i o n may
gas
(1)
2
take p l a c e
nitric
a c i d vapour p r e s s u r e .
i n t h e gas phase [ 1 ] . In o r d e r t o
e x p e r i m e n t s were c a r r i e d out by p a s s i n g a
1% NO o v e r a 65% n i t r i c
acid solution.
c l o s e t o t h e g a s - l i q u i d i n t e r f a c e . Furthermore, l a r g e N0
2
nitrogen
I t was o b s e r v e d
t h e produced water vapour condensed i n t h e gas phase on t h e g l a s s
brown c o l o u r e d
nitric
reactor
amounts o f t h e
were found i n t h e gas phase. Tereshchenko e t a l [2]
67
i n v e s t i g a t e d t h e o x i d a t i o n o f NO i n a n i t r o g e n gas stream s o l u t i o n s o f 60% t o 80% i n a b u b b l i n g apparatus
by n i t r i c
acid
and found t h a t t h e o x i d a t i o n
r a t e was c o n t r o l l e d by gas phase d i f f u s i o n o f NO from t h e gas b u l k t o t h e g a s liquid
i n t e r f a c e . These o b s e r v a t i o n s can be e x p l a i n e d by r e g a r d i n g t h e gas
phase r e a c t i o n between NO and n i t r i c
a c i d vapour as i n f i n i t e l y
f a s t . The
r e a c t i o n may t a k e p l a c e i n a s m a l l r e a c t i o n zone o r i n an a s y m p t o t i c r e a c t i o n plane very c l o s e to the g a s - l i q u i d which p r o c e e d if
1 Absorption-oxydation
The and
[11,12],
model
r e a c t i o n i s a c t u a l l y much more c o m p l i c a t e d than e q u a t i o n
(1) s u g g e s t s ,
i t may p r o c e e d v i a a mechanism composed o f t h e f o l l o w i n g s t e p s :
k
l
NO + HNOg
HN0
+ N0
(2)
2
= 0.2 - 9 m /kmol.sec a t 298°K
HN0
2
produced
HN0
g
2
+ HN0
= 6 x 10
3
a c i d vapour
[4,5,6].
2
+
3
[4,5,6].
reacts very r a p i d l y with n i t r i c
k
68 k
2
3
k
The
c a s e on a
(see F i g . 1 ) . R e a c t i o n s
i n a r e a c t i o n zone may be t r e a t e d as though they are i n s t a n t a n e o u s
t h e r e a c t i o n zone i s n o t t o o l a r g e
Fig.
interface
2N0
2
- 9 x 10
+ H 0
(3)
2
3
3
m /kmol.sec a t 300°K
[4,5,9],
The
r e a c t i o n rate constant k
constant the
o f the
assumed t h a t
the n i t r o u s
J
2C
HN0 HNO
phase r e a c t i o n i s
/Z„ D
.
HNO,
J, 1
i n which J„„„ i s the HNUg a r e a i f the
D
reaction the
i s not
N0
the
+ NO
2
NO
+ N0
From the
be
reactions
+ H0
X
2HN0
X
N 0
2
2
literature
react with n i t r i c
2
g
From the
+ HN0
that
2
2
be
It
an a u t o c a t a l y t i c
catalytic reaction
2
HN0
2
+ HN0
and
N 0 2
D
.
by
the
and
(4) HN0„
, 1
O
e v a p o r a t e d per surface
condition (2)
unit
retained
reaction. Using
the
f o r instantaneous
i s too
slow. Near
water vapour are p r e s e n t , i n the
reaction
and
zone:
^0^
reactions
produced by
(5) and these
to, r e s p e c t i v e l y , equation
(6)
are
reactions (3)
and
(7)
2
concluded that
the
gas
phase o x i d a t i o n
o f NO
by
nitric
under t h e s e c i r c u m s t a n c e s i t seems
reaction. the
l i q u i d phase o x i d a t i o n
o f NO
by d i l u t e d
which o n l y n i t r o u s a c i d p r o d u c e s , i s a l s o an
[14,15,16,30]. P r e s e n t l y ,
NOg
t
%
(6)
gas
2
for
3
mechanism t o c o n f i r m
2N0
i t i s formed.
(5)
c o n c e r n i n g k i n e t i c s and phase r e a c t i o n . The
that
It i s
2
(2) . The
s h o u l d be n o t e d t h a t (< 25%)
O
./
reaction
important
a c i d vapour a c c o r d i n g
above i t can
acid solutions
HNO,
[3,7,8,10,13] i t i s known t h a t
2N0
3
are
a c i d vapour i s a very complex r e a c t i o n , and t o be
2C
shown t h a t
which i m p l i e s
following
as
condition
i n the neighbourhood o f the
i t can
much f a s t e r than r e a c t i o n
N 0
N0,o
i n s t e a d o f becoming d e p l e t e d
fulfilled,
the
a c i d which would be
i n t e r f a c e , however, l a r g e amounts o f N0
therefore
C
NO
amount o f n i t r i c
r e a c t i o n r a t e c o n s t a n t k^
found
rate
[11,12]:
O
o f NO
i t was
instantaneously
a c i d i n t o a gas,
/Ik.
concentration
i t s bulk value C
reaction
phase c o u l d be n e g l e c t e d .
J°mO,
,/
3
gas
acid reacts
t r a n s i e n t e v a p o r a t i o n of n i t r i c
i n s t a n t a n e o u s gas
times h i g h e r than the
p r i m a r y r e a c t i o n . With i n f r a r e d a n a l y s i s
amount o f n i t r o u s a c i d vapour i n the
therefore For
i s about 10
2
however, too the
little
conditions
produced i s i n e q u i l i b r i u m w i t h
for
nitric
auto-
i s known instantaneous
NgO^.
(8)
4
69
T h i s e q u i l i b r i u m i s e s t a b l i s h e d very t h a t N0„ and A
rapidly
At the g a s - l i q u i d i n t e r f a c e o n l y N 0 2
nitric
4
5.3
EXPERIMENTAL
The
experiments we
(Chapter 2 and
5.4
c a r r i e d out
C h a p t e r 4 ) . The
for nitrogen
o x i d a t i o n was
b u l k and
The
1.
i n the equipment which was
previously and
and
o u t - g o i n g gas
liquid
a mass b a l a n c e around the
found t h a t
the d e v i a t i o n was
described were N0-
l e s s than
5%.
RESULTS
to t h e proposed model t h e o x i d a t i o n
phase d i f f u s i o n from the gas
r a t e o f NO
i s c o n t r o l l e d by
gas
b u l k t o t h e r e a c t i o n zone o r r e a c t i o n p l a n e .
Gas
phase mass t r a n s f e r i n t h e wetted w a l l column t a k e s p l a c e by d i f f u s i o n only concentration C
i n the r a d i a l d i r e c t i o n and change o f NO
i n the
C„„ NO,o gas
-a
—
4 I n=l
a
exp
t h e r e f o r e the
phase can
fractional
be w r i t t e n
as:
The
it
(
)
(9)
Gz„„ NO
n
phase d i f f u s i o n c o e f f i c i e n t
r e l a t i o n o f Chapman-Enskog (I>
given
gas
molecular
2 1
oo
=
N0
of NO
= 1.98
e x p e r i m e n t a l r e s u l t s f o r 78%,
i n F i g s . 2,
oxidation
3 and
r a t e f o r 78%
c o n t r o l l e d and
4. and
in nitrogen -5 2
x 10 63%
and
m /s 57%
at 20 C and nitric
From t h e s e f i g u r e s i t can 63%
nitric
was c a l c u l a t e d u s i n g o
change o f NO
concentration
o f NO.
concentration
change o f NO
s u g g e s t s , t h a t the
From T a b l e s 1 and
should
be
2 i t can
1.0132 bar)
acid solutions
be c o n c l u d e d t h a t
a c i d i s c o m p l e t e l y gas
agrees w i t h the proposed model. A c c o r d i n g
f r a c t i o n a l concentration
70
the
(see Chapter 4 ) .
in-going
I t was
the
oxidation
According
The
from
to the g a s - l i q u i d i n t e r f a c e .
dissolves physically into
o x i d e s c o n t e n t and
established.
MATHEMATICAL MODEL AND
NO
gas
a c i d , i n which i t i s h i g h l y s o l u b l e
proposed model i s p r e s e n t e d i n F i g .
analysed
i t i s assumed
N„0 d i f f u s e i n c o n t i n u o u s e q u i l i b r i u m w i t h each o t h e r ¿1 4
r e a c t i o n zone o r r e a c t i o n p l a n e t o the
concentrated
[17]. T h e r e f o r e ,
[24].
are the
phase d i f f u s i o n
t o e q u a t i o n (9)
independent o f the be
the
the
inlet
c o n c l u d e d t h a t the f r a c t i o n a l
tends t o i n c r e a s e w i t h i n c r e a s i n g P„_ . This NO, o r e a c t i o n i s very complicated.
71
Fig.
3
Fractional
concentration
number for 63% nitric
72
change of NO as a function acid
(0: 20°C; A : 30°C; —
of the equation
Graetz(9)).
'Or
Fig.
4
Fractional
concentration
number for
67% nitric
change of NO as a function acid
(0: 20°C;
equation
of the
Graetz-
(9)).
73
Exp..
h
h xlC >
6
m
T sec
T °C
p
o
c
bar
3. m /sec
kmol N
m
P
P NO, o
bar
N0
P N 2
bar
2°4 bar
P N
2°3 bar
2 V
\
°4 xlO kmol 6
°N0 C
N0,o
N
2° / m. s
3
4
2
1.1
5 44
0 346
1 26
20
1 093
6 5
0 0419
0 0284
0 00825
0 000268
5 .25
0.327
1.2
5 44
0 346
1 26
20
1 093
6 5
0 0866
0 0388
0 01541
0 000845
11. 50
0.365
1.3
5 44
0 346
1 26
20
1 093
3 8
0 1274
0 0509
0 02648
0 00177
18. 92
0.396
1.4
5 44
0 346
1 26
20
1 093
3. 8
0 1683
0 0599
0 0367
0 00252
24. 71
0.363
1.5
5 44
0 346
1 26
20
1 093
3 8
0 2099
0 0689
0 0486
0 00395
31. 73
0.396
1.6
5 44
0 346
1 26
20
1 093
3 8
0 0433
0 0248
0 00628
0 000258
5 28
0.349
2.1
7 38
0 346
1 03
20
1 115
2 2
0 0410
0 0224
0 00515
0 000265
5 64
0.420
2.2
7 38
0 346
1 03
20
1 115
2 2
0 0782
0 0304
0 00947
0 000750
11 34
0.458
2.3
7 38
0 346
1 03
20
1 115
2 2
0 1075
0 0373
0 0143
0 00125
15 73
0.452
2.4
7 38
0 346
1 03
20
1 115
2 2
0 1596
0 0479
0 0235
0 00245
23 08
0.466
0 193
0 0560
0 0321
0 00362
27 66
0.486
0 0264
0 00749
0 000578
9 93
0.508
2.5
7 38
0 346
1 03
20
1 115
2 2
3.1
11 05
0 346
0 79
20
1 148
10 6
0 0626
3.2
11 05
0 346
0 79
20
1 148
10 6
0 0989
0 0340
0 01184
0 00117
15 28
0.509
3.3
11 05
0 346
0 79
20
1 148
20 7
0 1247
0 0390
0 01553
0 00180
17 85
0.538
3.4
11 05
0 346
0 79
20
1 148
20 7
0 1553
0 0454
0 0211
0 00256
23 95
0.527
37 0
0 1764
0 0487
0 0243
0 00314
28 88
0.531
8 35
0.270
3.5
11 05
0 346
0 79
20
1 148
5.1
3 45
0 346
1 71
20
1 087
2 5
0 0811
0 0469
0 0223
0 000708
5.2
7 38
0 346
1 71
20
1 087
2 5
0 1362
0 0636
0 0413
0 00148
14 47
0.247
9.1
7 38
0 137
0 43
20
1 069
3 3
0 1631
0 0435
0 0194
0 00317
36 41
0.649
7 38
0 137
0 43
20
1 069
3 3
0 0788
0 0288
0 00851
0 00102
17 60
0.651
9.2
Table
1
Experimental
r e s u l t s of N0-oxidation
by 78% HNO
E X P
-
h
*l xlO 3 m
' m
T
T
sec
P
°C
P
c bar
xlO kmol
N0,o bar
P
P
N0, bar
P
NO
J
N n
bar
N
bar
W
/SeC
Ul
_^N0_ 6
xlO kmol
NO, o
W
m3
20 1
n
m2
0
0 0104
0 000635
0 0498
0 0253
0 00224
0 0150
0 00231
0 000182
0 0232
0 00549
0 000523
5 87
0 677
0 0752
0 0285
0 00832
0 00102
9 98
0 689
0 1124
0 0363
0 0135
0 00197
14 07
0 701
5 06
0 1410
0 0387
0 0153
0 00269
17 9
0 714
1 93
0 0259
0 0233
0 00555
0 000194
2 23
0 467
3 17
0 137
0 75
20
1 060
1 11
0 0593
0 0318
20 2
3 17
0 137
0 75
20
1 060
22 1
7 69
0 137
0 42
20
1 087
1 11
0 1208
5 06
0 0263
22 2
7 69
0 137
0 42
20
1 087
22 3
7 69
5 06
0 0483
0 137
0 42
20
1 087
5 06
22 4
7 69
0 137
0 42
20
1 087
5 06
22 5
7 69
0 137
0 42
20
1 087
24 1
7 69
0 346
1 05
20
1 103
5 25
0 489
12 43
0 541
2 69
0 669
24 2
7 69
0 346
1 05
20
1 103
1 93
0 1085
0 0479
0 0235
0 00169
8 19
0 471
26 1
4 15
0 346
1 58
20
1 091
2 30
0 0309
0 0311
0 00992
0 000244
1 53
0 369
26 2
4 15
0 346
1 58
20
1 091
2 30
0 1277
0 0648
0 0429
0 00247
7 16
0 433
Table
2
Experimental
results
of the NO-oxidation
by 63% HNO
In some experiments w i t h 63% n i t r i c oxidation
investigated. it
a c i d the i n f l u e n c e
r a t e o f NO f o r e q u i m o l a r i n l e t
o f N0
2
on t h e
q u a n t i t i e s o f NO and N 0
2
was
The e x p e r i m e n t a l r e s u l t s a r e g i v e n i n T a b l e 3 and from t h i s
can be c o n c l u d e d t h a t
the i n f l u e n c e o f N0
change o f NO i n t h e gas phase i s o f minor The
oxidation
r a t e o f NO by 57% n i t r i c
on t h e f r a c t i o n a l
2
importance. acid solutions
phase d i f f u s i o n c o n t r o l l e d , and t h e o x i d a t i o n liquid is the
phase (see
i s not c o m p l e t e l y gas
a l s o takes place
i n the
F i g . 4 ) . Under t h e s e c i r c u m s t a n c e s t h e gas phase r e a c t i o n r a t e
very low n i t r i c 2
a c i d vapour p r e s s u r e . At more d i l u t e d n i t r i c
was found i n t h e gas phase, and under t h e s e c o n d i t i o n s
takes place
only
i n the l i q u i d
phase. I t s h o u l d be noted t h a t
cannot e x i s t i n d i l u t e d n i t r i c f i n a l product The
liquid
2N0
+
acid
phase o x i d a t i o n
HN0
(< 40%),
overall
(< 40%)
N0
2
and/or N 0
and i n t h i s case n i t r o u s
2
4
acid i s
+
3
H 0 2
o f NO by d i l u t e d n i t r i c
a c i d c a n be p r e s e n t e d
reaction:
•* 3HN0
(10)
2
Some e x p e r i m e n t a l r e s u l t s w i t h 40% n i t r i c
a c i d a r e g i v e n i n T a b l e 4.
I f t h e t h e o r y o f mass t r a n s f e r w i t h a r a p i d pseudo f i r s t liquid
acid
the r e a c t i o n
[14,15,16].
with the following
the
partially
t o o slow t o be c o n s i d e r e d as i n s t a n t a n e o u s , a f a c t which may be caused by
no N 0
the
table
concentration
phase may be a p p l i e d
the absorption
order r e a c t i o n i n
rate per unit surface
a r e a can
be w r i t t e n a s :
1 provided that The
kx »
absorption
straight
line
regression
rate
(J„„) NO
p l o t t e d as a f u n c t i o n o f P . should give a NO, 1
through t h e o r i g i n w i t h a s l o p e
o f J J Q ^ ^ D ^ (see F i g . 5 ) . With H
a n a l y s i s t h e s l o p e was found t o be:
HJJQ ^/~ki>£
The
1.
oxidation
=
5
2
2.81 + 0.15 x 1 0 ~ kmol/m .bar.sec a t 20°C
o f NO i n t o 5-25% n i t r i c
acid solutions
(12)
seems t o be a u t o c a t a l y t i c
[14,15,16,25,26,27,28,29,30]. A b e l e t a l [14,25,26,27,28] proposed the f o l l o w i n g reaction
HN0
76
3
scheme:
+ HN0
2
"* N 0 2
4
+ H 0 2
(13)
model
measured Exp.
N„0„ 2 4,o
N0„ 2,o
NO,o
p
P
P
p
C
N0
P N 2
2°4
°N0,o bar
bar
bar
bar
C
C
N0
V Q,i C
Q,o" Q,i
bar
N0
°N0,o
C
X
10
C
V Q,i Q,o
J n
2°4
Q,i
6
X
10
6
2
2 kmol/ra
kmol/m .s
1.90
0.490
0.610
1.76
0.67
7.03
0.490
0.610
7.68
0.62
4.76
0.490
0.610
4 . 85
9.78
0.490
0.610
0.53
40.1
0.0199
0.0171
0.00296
0.0293
0.00871
0.482
40.2
0.0704
0.O4O3
0.0164
0.0574
0.0334
0.546
0.0213
0.523
0.0455
0.544
0.69
41.1
0.0484
0.0285
0.00819
0.0458
41.2
0.0941
0.0486
0.0239
0.0671
Table
3
Influenae
of N0 on the oxidation g
of NO by 63% HNO y
.s
(T - 20°C, T = 1.05 sea,
10 bar).
10.62
N 0 2
+ 2N0 + 2H 0
4
2
*
4HN0
In t h i s r e a c t i o n mechanism is
established
overall
reaction
(13) i s r a t h e r
very r a p i d l y . The i n i t i a l
reaction
and f i r s t
slow, w h i l e e q u i l i b r i u m (14)
formation r a t e o f n i t r o u s a c i d o f the
(10) was found t o be f i r s t
acid concentration concentration
(14)
2
order with respect
order with respect
to the n i t r i c
to the nitrous acid
[15,16]. Furthermore t h e o v e r a l l r e a c t i o n r a t e was found t o be
independent o f t h e p a r t i a l p r e s s u r e o f NO [15,16]. From t h e above i t can be concluded that within not
valid
t h e measured c o n d i t i o n s
f o r the oxidation
t h i s a u t o c a t a l y t i c behaviour i s
o f NO by 40% n i t r i c
acid solutions.
g x 1 0 £
5
h'
T
p C
HN0 2, o
X
l
2
°
3
N0
3
N0 2 kmol/m . s e c .
3, m /s
m
sec.
1.21
0 136
0.284
2.65
0 0402
1.40
1.21
0 136
0.284
2.65
0 1038
2.72 0.97
bar
kmol/m
1.21
0 346
0.722
1.92
0 0302
1.21
0 346
0.722
2.62
0 0578
1.91
1.21
0 346
0.722
2.62
0 0870
2.58
1.21
0 346
0,722
2.62
0 109
3.14
1.21
0 346
0.722
2.62
0 129
3.59
0.722
2.62
0 0274
0.88
0 346
1.21
Table
4
Experimental (T = 20°C; P
78
results
of the absorption
-1.16 bar).
rate
of NO into
40% HN0
Fig.
5
The absorption
rate
of NO into
40% nitric
acid
(0: x =0.722
sec; A ;
x = 0.284 sec).
UOp/IlpO^ diffusion
from the reaction
A c c o r d i n g t o t h e proposed model N 0
plane
2
and N^O^, which a r e i n c o n t i n u o u s e q u i -
l i b r i u m w i t h each o t h e r , d i f f u s e from t h e r e a c t i o n p l a n e t o t h e gas b u l k and t o the g a s - l i q u i d physically
i n t e r f a c e . At t h e g a s - l i q u i d
i n the concentrated n i t r i c
i n t e r f a c e only N„0 d i s s o l v e s & 4
acid. For a quantitative description o f
these d i f f u s i o n processes the c o n c e n t r a t i o n o f N0
2
and N^O^ on t h e r e a c t i o n
p l a n e s h o u l d be known. I t s h o u l d be noted t h a t t h e d i s t a n c e from t h e r e a c t i o n p l a n e t o t h e g a s - l i q u i d i n t e r f a c e (6 ) v a r i e d i n o u r experiments from 0 -4 3 x 10
m (moving boundary).
c o n c e n t r a t i o n decrease o f N0 liquid N 0 2
4
2
From t h e s e r e s u l t s and NgO
i t can be d e r i v e d t h a t t h e
from t h e r e a c t i o n p l a n e t o t h e g a s -
i n t e r f a c e i s s m a l l (< 5 % ) , and t h e r e f o r e t h e c o n c e n t r a t i o n o f NOg and
on t h e r e a c t i o n p l a n e was assumed t o be e q u a l t o t h a t a t t h e i n t e r f a c e .
The c o n c e n t r a t i o n o f N 0 mass b a l a n c e around situation - 3D, NO Q = N0
[18,19]. 3C, NO
and N 0 2
Q
D
8r 2
2
Q
+ 2N 0 2
4
on t h e r e a c t i o n p l a n e i s c a l c u l a t e d
from a
t h e r e a c t i o n p l a n e by assuming a q u a s i - s t a t i o n a r y
3r
- 2D
(15)
4
i n which t h e s o l u b i l i t y m i s d e f i n e d as:
79
C N
m
=
= 2
For
H N
2°4,£,i
~
0
2 4 2T~
16
< )
4,g,i
small values of the contact
b o t h be c o n s i d e r e d
time t h e gas phase and t h e l i q u i d phase may
t o be i n f i n i t e l y
deep, and t h e r e f o r e t h e l o c a l mass f l u x can
be c a l c u l a t e d from t h e p e n e t r a t i o n
theory.
The l o c a l mass f l u x o f NO from t h e gas b u l k
N
NO " " NO T I = m,o^k D
c
R-6 For
zero
initial
and N^O^
N0
2
from t h e r e a c t i o n p l a n e
=
Q
If there
- D
i n t h e gas phase t h e l o c a l mass f l u x o f N 0
i s N0„ p r e s e n t
2
t o gas b u l k becomes: / D
-—• 3r ^
Q
(17)
f
concentration
3C
N
t o the r e a c t i o n plane i s :
I
= C Q, i
V—
(18) K
TTt
i n t h e i n - g o i n g gas stream, t h e l o c a l mass f l u x N
'
can
0,
4
be w r i t t e n a s :
N
Q
"
(
C
Q , i -
C
Q , o
)
V
19
i ?
The l o c a l mass f l u x o f N 0 2
< >
4
i n t h e l i q u i d phase can be d e s c r i b e d by:
3C
V.
=
• V,« "
=
L a ,
V,,,
" V m . . * ^ * * (20)
A f t e r s u b s t i t u t i o n i n t o t h e mass b a l a n c e
3C
the f o l l o w i n g equation
V — — = (C - C )¥ — t 2 ( 1 C„ „ NO.o' TTt Q,i Q V TTt 2°4,g,i N
D n
(21)
are only v a l i d
plane
than t h e c o n c e n t r a t i o n o f N 0
D
N
X
These e q u a t i o n s i s higher
- C „ ) x 2°4,£,o
2°4 I * > 4
coefficient
i s obtained:
i f the concentration of N0 2
2
on t h e r e a c t i o n
i n t h e gas phase. The d i f f u s i o n
was c a l c u l a t e d as was d e s c r i b e d i n C h a p t e r 4. With t h e known 2 e q u i l i b r i u m constant K„ = P /P„„ , t h e c o n c e n t r a t i o n s o f N0„ and N„0„ on "2 N2O4 NO2 2 2 4 the r e a c t i o n p l a n e can be c a l c u l a t e d from e q u a t i o n (21) by means o f an i t e r a t i o n Q
80
p r o c e d u r e . Note t h a t t h e s e value.
concentrations
a r e independent o f the c o n t a c t
I f the gas phase may n o t be c o n s i d e r e d
t o be i n f i n i t e l y
c o n c e n t r a t i o n o f N0„ and N „ 0 . on t h e r e a c t i o n p l a n e o v e r t h e e f f e c t i v e 2 2 4 h e i g h t h' was c a l c u l a t e d from an o v e r a l l mass b a l a n c e : 2 o o l a i T 3
T7t
with:
- C_
Q,o
22
Zl '
*
equation (21).
d i f f u s i o n of N0
— G„
2
o f NOg and NgO^ on t h e r e a c t i o n p l a n e
(22) d e v i a t e under o u r e x p e r i m e n t a l
film
2 °° i a i r ) ( 1 - 4E - r exp ()) + '° n=l a Gz„ . n Q
)Y W
2 4,g,i
equation
be d e s c r i b e d
C Q
-C W
The
Q
_
time
deep, an average
according
to equation
r a t e s and the t h e o r e t i c a l zero N O concentrations ^ concentration
(24).
(24)
the t h e o r e t i c a l l y
predicted
In F i g . 8 and F i g . 9 t h e measured
predicted absorption
rates are p l o t t e d f o r i n i t i a l
i n t h e gas phase. The i n f l u e n c e o f i n i t i a l
i n t h e gas phase on t h e N 0
3. From t h e above i t can be c o n c l u d e d
2
4
absorption
absorption
r a t e i s given
t h a t t h e measured N 0 2
4
N0 2 o
i n Table
absorption rate i s
r a t h e r w e l l p r e d i c t e d by t h e model. 81
Fig.
6
The diffusion for
82
78% nitric
of iVOg and N 0
4
acid
from
the reaction
(0: 20°C; A : Z0°C;
plane
equation
to the gas bulk (22)).
10
0-5h
O U o u o IU
0
G
Fig.
7
The diffusion for
63% nitric
of N0
2
acid
z
Q
05
0-10
, red
and Nfl^ from the reaction (0: 20°C; A: 30° C;
plane equation
to the gas
bulk
(23)).
83
84
85
5.5 DISCUSSION
In the proposed mechanism a few assumptions were made which w i l l be d i s c u s s e d i n more d e t a i l . a) A c c o r d i n g t o t h e p r o p o s e d model water vapour i s produced on t h e r e a c t i o n plane very c l o s e to the g a s - l i q u i d
interface.
l e a v i n g t h e wetted w a l l column was
a n a l y s e d f o r i t s water vapour c o n t e n t . I t
was
found t h a t
In some e x p e r i m e n t s t h e gas phase
the amount o f water vapour i n t h e gas phase c o u l d be n e g l e c t e d .
T h i s i m p l i e s t h a t a l l t h e water vapour produced condenses on the n i t r i c liquid
film. Nitric
a c i d d i f f u s e s from t h e l i q u i d
film
i n t o t h i s t h i n water
l a y e r and water d i f f u s e s from t h e i n t e r f a c e i n t h e n i t r i c c o n c e n t r a t i o n g r a d i e n t s may
acid
acid film.
These
have some i n f l u e n c e on t h e s o l u b i l i t y o f NgO^.
can be shown t h a t t h e average t h i c k n e s s o f t h i s l a y e r i s v e r y s m a l l _7 (6 < 3 x 10 m). The c o n c e n t r a t i o n g r a d i e n t o f water i n t h i s l a y e r layer
It
was
r o u g h l y c a l c u l a t e d under s t a t i o n a r y c o n d i t i o n s w i t h : DH 0 2
AC H
2°
(25) H
layer
2°
i n which J
i s t h e c o n d e n s a t i o n r a t e o f water vapour. From t h i s v a l u e i t was -2 3 c a l c u l a t e d t h a t AC < 10 kmol/m under o u r e x p e r i m e n t a l c o n d i t i o n s . T h i s H2O 2
i m p l i e s t h a t t h e a c i d s t r e n g t h i n t h e t h i n l a y e r may equal to the s t r e n g t h i n the n i t r i c
acid l i q u i d
a l s o be assumed t o be
film.
b) In the c a l c u l a t i o n s t h e i n f l u e n c e o f a temperature change n e a r t h e i n t e r f a c e as a r e s u l t
o f heat o f r e a c t i o n , heat o f c o n d e n s a t i o n o f t h e water
vapour, heat o f m i x i n g o f the condensed water and n i t r i c e v a p o r a t i o n o f the n i t r i c was n e g l e c t e d
NO + 2HN0g
3N0
2HN0
->
2HN0
H0
N
Table 86
2
2°4
5
(g)
acid
f N
Heat effects
N
1
=
38.6 X 10
3
(g)
AH
2
=
78.6
AH
3
AH AH
(A)
2
->
AH
H0
2°4
2°4
near the
interface
Reference
298 1
2
*
2
H0
(g)
2
3N0
(£)
into n i t r i c
(see T a b l e 5 ) . A H
3
a c i d , heat o f
a c i d and heat o f s o l u t i o n o f NgO^
3
J
[23]
X io
3
J
[23]
= -44.2
x io
3
J
[23]
= -85.9
x io
3
4
J
[23]
= -25.3
X 19
3
S
J
t h i s work
The
temperature
change near
t h e i n t e r f a c e was c a l c u l a t e d from
a heat b a l a n c e by
assuming t h a t t h e heat o f m i x i n g may be n e g l e c t e d and t h a t a l l heat o n l y c o n t r i b u t e t o a temperature AH
< 1
*
A H
2
+
A H
3
+
^
A H
change i n t h e l i q u i d phase near t h e i n t e r f a c e .
4
) C
N0,o
yf%0,
P
+
effects
V
(26)
pc P
v
i n which y r e p r e s e n t s t h e f r a c t i o n o f t h e NO^ produced
which i s c o n v e r t e d t o
N^O^. W i t h i n t h e e x p e r i m e n t a l c o n d i t i o n s t h e temperature change near t h e i n t e r o o face variei f a c e v a r i e d from - 0.6 C t o 0 C. T h i s was found t o be s m a l l enough t o be neglected.
5.6 CONCLUSIONS
The be
o x i d a t i o n o f NO by c o n c e n t r a t e d n i t r i c
acid
(63-78%) can be c o n s i d e r e d t o
an i n s t a n t a n e o u s gas phase r e a c t i o n i n a r e a c t i o n zone o r on a r e a c t i o n
plane very c l o s e to the g a s - l i q u i d
i n t e r f a c e . P r e s e n t l y too l i t t l e
c o n c e r n i n g t h e mechanism and k i n e t i c s t o prove criteria
i s known
t h i s hypothesis using the
f o r instantaneous r e a c t i o n s .
I t was found
t h a t Danckwerts' s o l u t i o n s f o r i n s t a n t a n e o u s
irreversible
r e a c t i o n s i n t h e l i q u i d phase can a l s o be a p p l i e d t o gas phase r e a c t i o n s . The NOg and ^ 0 ^ produced, which a r e i n c o n t i n u o u s
e q u i l i b r i u m w i t h each o t h e r
d i f f u s e from t h e r e a c t i o n zone o r r e a c t i o n p l a n e t o t h e gas b u l k and t o t h e gas-liquid
i n t e r f a c e . At t h e i n t e r f a c e o n l y NgO^ d i s s o l v e s p h y s i c a l l y
concentrated n i t r i c
a c i d . The mathematical
d i f f u s i o n p r o c e s s e s was found The
model p r e s e n t e d
into the
to d e s c r i b e these
t o be i n good agreement w i t h t h e experiments.
a b s o r p t i o n o f NO by 40% n i t r i c
phase and under t h e s e c i r c u m s t a n c e s
a c i d s o l u t i o n s takes place i n the l i q u i d
n i t r o u s acid i s the f i n a l
p r o d u c t . The
a b s o r p t i o n r a t e can be d e s c r i b e d by the t h e o r y o f mass t r a n s f e r w i t h a r a p i d pseudo f i r s t
order r e a c t i o n i n the l i q u i d
phase.
REFERENCES
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R.J., C a l v e r t , J.G. and Shaw, J.H., Paper
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Chemistry
American
Chemical
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6. AN ABSORPTION MODEL FOR THE DESIGN OF A DILUTED NITRIC ACID ABSORBER AND METHODS TO DECREASE THE NO CONTENT IN TAIL GASES x
6.1
INTRODUCTION
A l t h o u g h many i n v e s t i g a t i o n s can be
found
absorption of n i t r o g e n oxides into n i t r i c still
not w e l l u n d e r s t o o d
a c i d . T h i s i s m a i n l y due and NgO^ liquid
i n absorbers
i n the l i t e r a t u r e c o n c e r n i n g
f o r the p r o d u c t i o n o f d i l u t e d
a l l p l a y an important
r o l e i n the a b s o r p t i o n p r o c e s s nitric
t o NOg.
T h i s o x i d a t i o n i s an u n u s u a l
temperature In
coefficient
t h i s Chapter
as was
i n both
shown by B o d e n s t e i n
an a b s o r p t i o n model, based
the p r o d u c t i o n o f d i l u t e d n i t r i c
acid.
ABSORPTION MODEL FOR
The
overall reaction
3N0
2
THE
negative
on g e n e r a l c h e m i c a l
absorbers
In a d d i t i o n , v a r i o u s methods o f gases o f n i t r i c
PRODUCTION OF
acid
DILUTED NITRIC ACID
f o r the a c i d f o r m a t i o n i n the a b s o r p t i o n column can
(N 0 ) 2
4
+
H0
%
2
a given composition
2HN0
3
+
be
NO
the maximum a c i d c o n c e n t r a t i o n t h a t can be
v e r y p o o r l y s o l u b l e i n aqueous s o l u t i o n s ,
+
0
2
•*
2N0
(1)
o f the n i t r o u s gases ( C h a p t e r 4 ) . The
where i t r e a c t s w i t h m o l e c u l a r
2N0
reaction
with:
T h i s e q u i l i b r i u m determines at
can
be b r i e f l y d i s c u s s e d .
6.2
presented
the
[1].
d e c r e a s i n g the c o n c e n t r a t i o n s o f n i t r o g e n o x i d e s i n t a i l plants w i l l
NO^
i n the gas phase o x i d i z i n g
r e a c t i o n w i t h an apparent
e n g i n e e r i n g c o n s i d e r a t i o n s , i s d e r i v e d f o r the d e s i g n o f i n d u s t r i a l for
NgO^,
a c i d as w e l l as n i t r o u s a c i d
be formed i n b o t h phases. Oxygen i s n o r m a l l y p r e s e n t NO
nitric
t o t h e f a c t t h a t v a r i o u s n i t r o g e n o x i d e s NO,
and the gas phase. Furthermore
the
a c i d s o l u t i o n s , the mechanism i s
2
and
NO
obtained
produced
i s t r a n s f e r r e d t o t h e gas
is phase,
oxygen.
(2) 89
The
r e a c t i o n r a t e o f t h i s o x i d a t i o n can be e x p r e s s e d
-dt-
The
k
•
P
by [ 1 - 5 ] :
(
N0 \
r e a c t i o n r a t e constant
k increases with decreasing
3
)
temperature. The r e v e r s e
r e a c t i o n may be n e g l e c t e d under t h e c o n d i t i o n s p r e v a i l i n g
i n the absorption
column. E s p e c i a l l y a t t h e t o p o f t h e a b s o r p t i o n column where t h e p a r t i a l pressure
o f NO i s low, t h e r e o x i d a t i o n r a t e o f NO i s s m a l l . As a f i r s t
a p p r o x i m a t i o n t h e o x i d a t i o n o f NO can be c o n s i d e r e d step i n the absorption The
t o be t h e r a t e
determining
process.
NO produced has a c o n s i d e r a b l e
i n f l u e n c e on t h e a b s o r p t i o n
rate of
NgO^ i n t o water and d i l u t e d a c i d . T h i s e f f e c t may be due t o t h e f o r m a t i o n o f N
HNOg and 2 ^ 3
N0
2
NO
The
i
n
t
+
NO
+
N0
nitric
n
e
*=
+
as
P
H 0 2
2
h a s e :
%
2HN0
X
N 0
a c i d formation
2
2
3
(g)
(4)
(g)
(5)
i n t h e gas phase seems t o be o f minor
under t h e c o n d i t i o n s p r e v a i l i n g i n t h e a b s o r p t i o n column constants
of reactions
concentrations to
the high
of N 0 2
and HN0
3
solubilities
A first
[ 6 , 7 ] . The e q u i l i b r i u m
(4) and (5) were g i v e n i n C h a p t e r 4 ( T a b l e 1 ) . The 2
a r e s m a l l under e q u i l i b r i u m c o n d i t i o n s but due
and t h e r a p i d e s t a b l i s h m e n t
t r a n s f e r o f NgO^ and HN0 neglected.
2
o f these
from t h e gas phase t o t h e l i q u i d phase can n o t be
an a b s o r p t i o n model which i s s c h e m a t i c a l l y p r e s e n t e d
i n F i g . 1. In t h e from t h e gas phase
t h e l i q u i d phase. In t h i s work a model i s s e t up i n which t h e NgOg t r a n s f e r
and
t h e HNOg t r a n s f e r a r e b o t h taken i n t o account. The model i s based on:
a) D i f f u s i o n o f N 0
and NgO^ from t h e gas b u l k
2
k =23 N0
= N 0
2
2
b) T r a n s f e r o f liquid
N 2
0
4
RT
4
N
0 2
to the g a s - l i q u i d interface
2k g,NO
J
90
was
[ 8 ] . More r e c e n t l y H o f t i j z e r and Kwanten [7] proposed
m a t h e m a t i c a l d e s c r i p t i o n they n e g l e c t t h e t r a n s f e r o f N „ 0
The
e q u i l i b r i a , the
attempt t o d e s c r i b e such a complex a b s o r p t i o n p r o c e s s
done by Andrew and Hanson
to
importance
4'
N
2°3
_ (P N0
a
n
d
N
2
H N 0
- P ) + N0 .> 2>
2
f
r
o
m
t
n
e
e> 2°4 — (P - P ) RT 2 4, 2°4,i N
g a s - l i q u i d i n t e r f a c e to the
phase.
r e a c t s w i t h water t o produce n i t r i c
a c i d and n i t r o u s a c i d , as was
(6)
3HN0
-HN0 >2NO»
2
3
H 0 2
LIQUID - BULK
Fig.
1 Absorption
discussed
model according
i n Chapter 4. A c c o r d i n g
to Hoftijzer
and Kwanten [?].
to Corriveau
[9] t h e N^O^ r e a c t s r a p i d l y w i t h
water i n the l i q u i d phase t o produce n i t r o u s a c i d .
N
2HN0„
«2°
2°3
(7)
T h i s r e a c t i o n may be c o n s i d e r e d
t o be a r a p i d pseudo f i r s t
order
r e a c t i o n . The
n i t r o u s a c i d formed i n t h e gas phase d i s s o l v e s p h y s i c a l l y i n t o t h e s o l u t i o n . The a b s o r p t i o n
J
N0 °2
=
2
J
N
r a t e can then be w r i t t e n as:
N„0. = "2"4
2 P
+
N"2"4,i
4 P
"2"4
HNO„
+
/«HNO 2, i
2
Vo„
/ V o 2 3,i
2 3
A (8)
In a b s o r p t i o n
columns f o r the p r o d u c t i o n
o f d i l u t e d a c i d the l i q u i d
phase may
be assumed t o be n e a r l y s a t u r a t e d w i t h NO and under t h e s e c o n d i t i o n s t h e reverse
H
r e a c t i o n becomes i m p o r t a n t . The a b s o r p t i o n
=
2
v
"
4
2
\ '
p
» o
2
A
v « i f o
(
1
•
(
^
r a t e i s then r e p r e s e n t e d
)
2
/
3
3. i H
HN0 ^P 2
P
4
P
P
• N0, i • N 0 ^ . • H 0 , 2
k„(l-(-^)
by:
)
1/6 )
2
kD„
8. 1/3 (1 - ( ~ ) ) K
(9)
91
Values o f the e q u i l i b r i u m constants K
p
, Kp , K
p
and Kp
were g i v e n i n Chapter
4 ( T a b l e 1 ) . The v a l u e o f p\ i s d e f i n e d a s :
P
P
NO,i
N 0
2,i
Note t h a t e q u a t i o n N0 /N 0 2
2
(10)
3
(9) may o n l y be a p p l i e d t o d i l u t e d a c i d . The a b s o r p t i o n o f
i n t o c o n c e n t r a t e d a c i d s h o u l d be c o n s i d e r e d t o be p u r e l y p h y s i c a l .
4
c) T r a n s f e r o f NO from t h e g a s - l i q u i d
J
In
N0
=
J
3 N0
= 2
~wT^
( P
N0,i
P
" N0
i n t e r f a c e t o the gas-bulk.
}
t h e gas b u l k t h e r e o x i d a t i o n o f NO w i t h oxygen t a k e s p l a c e [ 1 - 5 ] .
It may be assumed t h a t t h e gas phase i s s a t u r a t e d w i t h water. The water vapour p r e s s u r e as a f u n c t i o n o f t h e a c i d s t r e n g t h can be taken HN0,-H 0 measured by Vandoni and Laudy
from
t h e b i n a r y system
[10] . The v a l u e s o f H
f u n c t i o n o f t h e a c i d s t r e n g t h were g i v e n i n Chapter be found
•ƒ kD„ as a
4. L i t t l e
i n f o r m a t i o n can
i n the l i t e r a t u r e concerning the values of H _ and H %ƒ kD„. HN0 N 0 V and Neusser [12] determined t h e HN0 vapour p r e s s u r e above n i t r o u s a c i d H
m i n
3
n
2
d
H
H
2
n
Abel
3
2
s o l u t i o n s . V a l u e s o f H_„ „
were c a l c u l a t e d from
t
Hi\L)
Theobald The
t h e e q u i l i b r i u m measurements o f
2
[13] c o n c e r n i n g t h e heterogeneous system n i t r i c
a c i d / n i t r o u s gases.
partial
p r e s s u r e s o f HN0_ i n t h e gas phase were c a l c u l a t e d from P , P„„ , £ NO NO2 P _ and t h e e q u i l i b r i u m c o n s t a n t K_ . The v a l u e s o f H „ . as a f u n c t i o n o f t h e n 0 P4 HNO2 a c i d s t r e n g t h thus found a r e about t w i c e t h e v a l u e measured by A b e l and Neusser tI
TI
2
[12]
(see F i g . 2 ) . T h i s discrepancy r e q u i r e s f u r t h e r i n v e s t i g a t i o n . Values of
H„ „ U kD. were c a l c u l a t e d from 2°3 N
V
t h e a b s o r p t i o n measurements o f H o f m e i s t e r and
l
Kohlhaas
[14]. The r e s u l t s a r e g i v e n i n T a b l e 1. C o r r i v e a u [9] used a l a b o r a t o r y
a b s o r b e r c o n t a i n i n g f i v e wetted spheres t o i n v e s t i g a t e t h e a b s o r p t i o n r a t e o f N„0„ i n t o water. From T a b l e 1 i t can be seen t h a t t h e v a l u e o f H „ _ \/ kD„ *s 3 "2O3* " r e p o r t e d by C o r r i v e a u [9] i s much lower than t h a t o f H o f m e i s t e r and Kohlhaas [14].
No i n f o r m a t i o n was found
i n the l i t e r a t u r e concerning the i n f l u e n c e of
the n i t r i c
a c i d s t r e n g t h on t h e v a l u e s o f H \l kD.. As a f i r s t a p p r o x i m a t i o n N 0 N 2 O 3 » * t h i s i n f l u e n c e may be c a l c u l a t e d from t h e d e c r e a s e o f H „ _ w i t h i n c r e a s i n g N 0 i o n i c s t r e n g t h . I t i s c l e a r t h a t more work i s needed t o o b t a i n r e l i a b l e d a t a 2
3
2
92
3
liquid
method o f
2 kmol/m Hofmeister, Corriveau
Table
1
Kohlhaas
[14]
[9]
5 x 1.58
Comparison water
of
literature
x
10~
water
laminar j e t
10~
water
wetted
data
concerning
(see a l s o H o f t i j z e r
a) h i g h p a r t i a l b) low
the
absorption
spheres
of N^O^
into
at 25.0°C
From the proposed model i t can be c o n c l u d e d i n c r e a s e d by
measurement
.s.bar
t h a t the a b s o r p t i o n r a t e w i l l
be
and Kwanten [ 7 ] ) :
p r e s s u r e s o f the n i t r o g e n o x i d e s ;
temperatures
i n both
phases;
c) h i g h degree o f o x i d a t i o n o f the n i t r o g e n o x i d e s ; d) l a r g e g a s - l i q u i d
interfacial
area. 93
F o r a s i m p l i f i e d mathematical nitric
6.3
model o f the a b s o r p t i o n column i n the
a c i d p r o d u c t i o n the r e a d e r i s r e f e r r e d t o the l i t e r a t u r e
METHODS TO
DECREASE THE
NO
diluted
[16,17],
CONTENT IN TAIL GASES OF NITRIC ACID PLANTS x
Tail
gases o f n i t r i c
o x i d e s and
tail
a c i d p l a n t s c o n t a i n between 100
and
3000 ppm
e f f e c t on the ecosystem an e f f e c t i v e removal o f N0^ the e m i s s i o n l e v e l
i s 1.5
kg NO
i s n e c e s s a r y . At
( c a l c u l a t e d as N0 ) £t o
X
p l a n t s a l e v e l o f 400
ppm
will
c o u n t r y t o c o u n t r y . F o r new the l o c a l
n i t r o g e n oxides content
present
per ton a c i d f o r
p l a n t s i n the U n i t e d S t a t e s . T h i s i s e q u i v a l e n t t o about 200
depending on
of nitrogen
gases o f some v e r y o l d p l a n t s even more. Because o f i t s harmful
ppm.
For
new existing
be r e q u i r e d . In Europe t h e l i m i t v a r i e s
p l a n t s a l i m i t o f 400-500 ppm
may
from
be assumed,
s i t u a t i o n . P r e s e n t l y s e v e r a l methods t o d e c r e a s e i n these t a i l
the
gases a r e known i n the l i t e r a t u r e [15,18].
T a b l e 2 g i v e s a r e v i e w o f the most important
methods. Extended a b s o r p t i o n (water s c r u b b i n g )
•— Wet
.H 0
process
2
2
scrubbing
I—HN0, s c r u b b i n g
[19,20,21]
[22]
[23-37]
NO abatement
—
Dry
process
Adsorption
[41-43]
. N o n - s e l e c t i v e r e d u c t i o n [44-49]
S e l e c t i v e r e d u c t i o n [15,44,45,46, 50,51]
Table
2
Methods plants
94
to decrease
the NO^ content
in tail
gases of nitric
acid
6.3.1
Wet P r o c e s s e s
6.3.1.1 Extended a b s o r p t i o n
Increasing tail
[19,20,21]
of the absorption
gases o f n i t r i c
oxides i n these t a i l
volume d e c r e a s e t h e n i t r o g e n
oxides content i n
a c i d p l a n t s . The degree o f o x i d a t i o n o f t h e n i t r o g e n gases i s about 0.5. In t h e l i q u i d phase m a i n l y HN0 i s 2
produced.
NO
+
N0
o
+
¿
The
2HN0 (£)
•>
2
HN0
(12)
^
HNOg may be decomposed
3HN0
The
HO
2
+
3
2N0
NO produced i s v e r y
partially:
+
H 0
(13)
2
poorly
s o l u b l e i n aqueous s o l u t i o n s , hence i t i s
t r a n s f e r r e d t o t h e gas phase where i t r e a c t s w i t h oxygen. At the t o p o f t h e a b s o r b e r t h e r e o x i d a t i o n r a t e o f NO w i l l be very o f NO i s s m a l l . T h i s required
for a high
implies that
a relatively
slow as t h e p a r t i a l
large absorption
degree o f o x i d a t i o n . The extended a b s o r p t i o n
r a t h e r o f t e n a p p l i e d i n new p l a n t s . By working a t a p r e s s u r e a b s o r b e r and by c o o l i n g t h e a b s o r p t i o n tail
method i s now
o f 12 b a r i n t h e
system w i t h water t h e N 0
x
content i n the
gas may be reduced t o 200 ppm. Even i n e x i s t i n g p l a n t s extended
can be a p p l i e d , p r o v i d e d (Fig.
that the pressure
r e s u l t i n g weak a c i d becomes t h e f e e d
absorption
i n t h e main a b s o r b e r i s n o t t o o low
3 ) . The extended a b s o r b e r i s p o s i t i o n e d down stream r e l a t i v e l y
e x i s t i n g a b s o r b e r . Condensate i s c o o l e d
pressure
volume i s
and e n t e r s
t o an
t h e extended a b s o r b e r . The
f o r t h e main a b s o r b e r ( F i g . 3 ) . I f t h e
degree o f t h e o x i d a t i o n o f NO i s low such a p r o c e s s i s not e c o n o m i c a l due t o the
large absorption
6.3.1.2 H O
A 2
The
tail
volume
scrubbing
p r o c e s s [22]
gas o f t h e a c i d a b s o r b e r
following overall reactions
NO
2N0
required.
+
+
N0
2
3H 0
+
2H 0 2
2
•* 2HN0
(A) i s s c r u b b e d w i t h H 0 2
2
(see F i g . 4 ) . The
occur:
2HN0
+
3
2H 0
+
HgO
(14)
(15)
95
o NH 2
3
1 HpO
A
2 .r
0°/. H N O ,
Fig.
3
Simplified
flow
extended
absorption.
A: converter;
sheet for the production
B: cooler/condenser;
1: feed to converter; ppm N0 ; x
nitric
x
to
C: absorber;
6: water;
nitric
acid
D: extended
2: 10% NO; 3: NO oxidized
5: 200-400 ppm N0 ;
acid
of diluted
with
absorber.
to NO^; 4: 2000-600
7: weak nitric
acid;
8: 60%
bleacher.
HoO u 2
2
H 02
N0
2
60 % HNO3
Fig.
4
Simplified A: acid
flow absorber;
1: feed to acid
B; U^O^ absorber;
4: weak nitric
acid with
H0;
to acid
0
96
sheet of the N^O^ scrubbing
0
7: water
process
[22],
scrubber. 2: 2000-4000 ppm N0 ; x
unreaated
absorber;
3: 200-400 ppm N0 ; x
H^O^; 5: recycling 8: 60% nitric
acid
E^O^; 6: to
fresh
bleacher.
The
tail
gas treatment
t a k e s p l a c e at ambient temperatures
and
at a p r e s s u r e
e q u a l t o t h a t i n the a c i d a b s o r b e r . The weak a c i d l e a v i n g the s c r u b b e r c o n t a i n i n g some u n r e a c t e d H O ¿1 r e a c t i o n equations
(14) and
e n t e r s the top o f the a c i d a b s o r b e r .
experiments
H„0
solutions.
was
(15)
i t can be seen
t h a t t h i s p r o c e s s overcomes the
i n the extended
a b s o r p t i o n method.
were c a r r i e d out t o i n v e s t i g a t e the a b s o r p t i o n r a t e o f NO I t was
found t h a t H O
into
decomposed r a t h e r r a p i d l y as soon as i t
a c t i v a t e d by r e a c t i o n . The m o l e c u l a r oxygen produced
liquid
From the
2
c h e m i c a l and p h y s i c a l l i m i t a t i o n s which e x i s t Own
and
diffused
from
the
phase t o the gas phase. In the gas phase the oxygen i s r a t h e r i n -
e f f e c t i v e . The
loss of H 0 2
by d e c o m p o s i t i o n
2
i s a s e r i o u s disadvantage
of
this
process. 6.3.1.3 N i t r i c
acid scrubbing
a) D i l u t e d n i t r i c
[23-40]
a c i d scrubbing
[23-29,40]
The Humphreys/Glasgow and Bolme p r o c e s s uses a 30% n i t r i c scrubbing l i q u i d stream
[23,24].
from t h e a c i d a b s o r b e r
which N0„ and NO
NO
+
2N0
The
A simplified
N0
+
2
HNOg
H0
+
2
+
H0
2HN0
*
2
N0^
enters scrubber
at ambient temperature
scrubber
(A) i n
(16)
3HN0
(17)
2
c o n t a i n s 150-250 ppm o
NO
i s r e g e n e r a t e d by h e a t i n g t o 70 C and
c o o l e d and r e c y c l e d t o the a c i d a b s o r b e r . The can a l s o be
absorbed
are known i n l i t e r a t u r e b) C o n c e n t r a t e d S0LN0X n i t r i c
and
. The
nitric
CW
bar).
acid leaving
the
s t r i p p i n g w i t h a i r o r steam. N0^
produced
is
main advantage o f t h i s p r o c e s s i s
recovered. S e v e r a l v a r i a n t s of t h i s
process
[25-29].
nitric
acid
scrubbing
[30-37]
a c i d p r o c e s s o f Ugine Kuhlmann produces weak a c i d o f 60-63%
and c o n c e n t r a t e d n i t r i c
a c i d o f 80%
[30,31,32], An
SOLNOX-process i s the d i s s o l u t i o n o f NgO^ dissolution
gas
2
Under t h e s e c o n d i t i o n s the n i t r o u s a c i d i s decomposed. The
t h a t NO
as
according to
p r e s s u r e i n the s c r u b b e r i s about the same as i n the a c i d a b s o r b e r
A f t e r s c r u b b i n g the gas
The
acid solution
i s g i v e n i n F i g . 5. The
c o n t a i n i n g 2600 ppm
can be absorbed
+
flow sheet
important
i s a l s o the method f o r c l e a n i n g the t a i l
sheet
i s presented
first
e n t e r s a precondenser
s t e p i n the
into concentrated n i t r i c
i n F i g . 6. The combustion gas
acid;
gas. A s i m p l i f i e d
l e a v i n g the c o n v e r t e r
(A). In the precondenser
the gas
this
flow (1)
i s cooled with 97
3
1% Fig.
5
Simplified
flow
sheet
of
the
Humphreys/Glasgow
and
Bolme process
[23,
24] . A: n i t r i c 1:
tail
tail
acid
gas
gas
recovered
scrubber;
of acid
(200 ppm NO^
acid
from
NO^);
4: 30%
column.
2: 30% n i t r i c n i t r i c acid
absober;
acid
solution;
containing
6: stream
or air
c o o l e d weak a c i d . The
the l i q u i d
c o o l e d gas
circuit
a r e both c o o l e d t o 0°C w i t h c o l d b r i n e . In t h i s way
a c o n c e n t r a t i o n o f about 62-63%. The
d i s s o l v e d i n t o a 80% n i t r i c p r e s s u r e o f about 8 b a r . The
% o f NgO^
98
acid gas
a c i d . The
acid solution
l e a v i n g the t o p o f the a b s o r b e r lowered
c o l d , dry 2
o f -10
t o 200
concentrated n i t r i c
ppm
the
leaving
(C) i n which N 0 / N 0 o
s o l u t i o n at a temperature
T h i s can be e a s i l y
water o r d i l u t e d n i t r i c
nitric
nitric
o x i d i z e d gas then e n t e r s a p h y s i c a l a b s o r b e r
NOg.
5:
and a p o r t i o n o f the c o o l e d weak
the c o - c u r r e n t condenser has
ppm
acid;
regeneration
pass t o the c o - c u r r e n t condenser (B) where
the gas phase. The
about 600
treated
nitrous for
water vapour i s removed from
fully
3:
liquid.
the precondenser
the gas and
absorber;
to main acid
of scrubbing
circulating
B: regeneration
2
4
and
is
t o 0 C and
a
(5) c o n t a i n s
by s c r u b b i n g i t w i t h
a c i d c o n t a i n i n g 10-30
e n t e r s r e a c t o r (D). In the r e a c t o r t h e d i s s o l v e d NgO^ i s c o n v e r t e d o o a c i d w i t h water and a i r at 60 -80 C and a p r e s s u r e o f 8 b a r .
wt to
N
2°4
+
H
+
2°
i0
2
"*
Under t h e s e c o n d i t i o n s p a r t o f the
reactor
a l s o Chapter 1).
Fig.
6
column; D:
i s produced
[38,39]. The
partially
SOLNOX p r o c e s s can
B: co-current
from converter
condenser; to physical
80% nitric
strong
a c i d i n the
lower
r e c y c l e d t o the p h y s i c a l a b s o r b e r .
be
found i n the
condenser;
literature
[33-37]
[30], C: physical
acid;
80% nitric
acid;
6: 60% nitric
8: gas containing
11: unbleached
13: bleached
to precondenser;
3: weak nitric absorber;
acid containing
bleacher;
(18)
3
absorption
reactor.
1: gas stream
nitric
0
flow sheet of the SOLNOX process
A: precondenser;
stream
N
i s b l e a c h e d and
(see
current
H
no NO
P r o c e s s e s s i m i l a r t o the
Simplified
2
4: cooled acid;
weak acid; 7: bleached
NO^ to physical
10-30% by weight 60% nitric
2: gas stream
acid
N^Q^S
absorber;
10: 60% nitric
to reactor;
to co5: NO^ and
gas
cooled
9: 80% acid
12: air to
to reactor;
acid.
99
6.3.2 Dry p r o c e s s e s
6.3.2.1 A d s o r p t i o n
NO
x
[41-43]
can be removed and r e c o v e r e d from n i t r i c
a d s o r p t i o n on m o l e c u l a r capacity f o r N0
2
a t ambient temperatures,
nitric
2
gas streams by f i x e d - b e d
s i e v e s . M o l e c u l a r s i e v e s show a h i g h a d s o r p t i o n
v e r y low. In t h e p r e s e n c e o x i d a t i o n o f NO t o N 0
acid t a i l
but t h e a d s o r p t i o n c a p a c i t y f o r NO i s
o f oxygen t h e m o l e c u l a r s i e v e s can c a t a l y z e t h e
which i s adsorbed
on t h e m o l e c u l a r
sieves. T a i l
a c i d p l a n t s c o n t a i n water vapour and water vapour w i l l
T h i s decreases
gases o f
f i r s t be adsorbed.
t h e a d s o r p t i o n c a p a c i t y f o r NOg. An e m i s s i o n l e v e l o f 50 ppm N 0
can be o b t a i n e d . The adsorbed
N0
2
i s periodically
desorbed
and r e c y c l e d t o t h e
a c i d a d s o r b e r . The d e s o r p t i o n p r o c e s s t a k e s p l a c e a t a temperature o 150-250 C. A s i m p l i f i e d flow sheet i s p r e s e n t e d i n F i g . 7.
1
o f about
3
4
2
Fig.
7
Adsorption acid
process
for the removal
of NO^ from
tail
gases
of n i t r i c
plants.
A: fixed
bed adsorption
column;
B: regeneration
of the
adsorption
column. I: tail gas
gas from n i t r i c
containing
acid
absorber
50 ppm N0^; 3: recovered
4: gas for regeneration
Non-selective reduction of n i t r o g e n oxides
100
( C H , CO, Hg, naphta, 4
N0^ to n i t r i c
of the adsorption
6.3.2.2 N o n - s e l e c t i v e r e d u c t i o n p r o c e s s e s
a r e d u c i n g agent
(2000 ppm NO^); 2: treated acid
tail
absorber;
column.
[44-49]
i s c h a r a c t e r i z e d by t h e r e a c t i o n o f
etc.) with N0
x
and oxygen i n t h e
x
p r e s e n c e o f a c a t a l y s t . Noble m e t a l c a t a l y s t s are used based on P t , Pd and d e p o s i t e d on a s u i t a b l e i n e r t if
carrier.
a l l t h e oxygen i s removed. T a i l
In t h i s p r o c e s s N 0
gases o f n i t r i c
can o n l y be
i s needed f o r the r e d u c t i o n o f NO^.
r i s e o f the t a i l
gas more energy
e x p a n s i o n t u r b i n e . Van den B l e e k and Van den Berg e x p l a i n i n g why be b r i e f l y
3%
content. T h i s implies
x
heat e v o l v e d by r e a c t i o n can be r e c o v e r e d i n a waste heat b o i l e r . due t o the temperature
reduced
a c i d p l a n t s c o n t a i n about
oxygen which i s an orde o f magnitude h i g h e r than the N 0 t h a t a l a r g e amount o f r e d u c i n g agent
x
Rh,
The
Furthermore,
can be r e c o v e r e d at the
[46] put forward a h y p o t h e s i s
t h e s e p r o c e s s e s are n o n - s e l e c t i v e r e l a t i v e t o oxygen. T h i s w i l l
reviewed. The
r e a c t i o n s o c c u r r e d can be p r e s e n t e d as
follows:
cat N0
2
NO in
+
Red
-*
NO
+
Red
cat •*
N N0
which Red
its
+
Red 0
(19)
+
Red 0
(20)
2
2
and Red 0 r e p r e s e n t a r e d u c i n g agent
(CH^, H , 2
o x i d a t i o n p r o d u c t , r e s p e c t i v e l y . A c c o r d i n g t o Van
Berg
[46] , N 0
2
main r e a s o n why
NO
The NO
+
0
2
produced
i s easily NO
reduced t o NO
i s not reduced
cat •+
N0
selectively
naphtha)
den B l e e k and Van
and den
(20) i s very slow.
i s due t o the
The
reaction
(21)
2
by r e a c t i o n
oxygen. In t h i s way
while reaction
CO,
(19) w i l l be e a s i l y
and q u i c k l y o x i d i z e d w i t h
the r e d u c t i o n c y c l e has t o s t a r t
a g a i n consuming
another
q u a n t i t y o f r e d u c i n g agent. Only when a l l o f the oxygen i s consumed r e a c t i o n (20) becomes i m p o r t a n t .
6.3.2.3 S e l e c t i v e r e d u c t i o n p r o c e s s e s
[15,44,45,46,50,51]
A s e l e c t i v e r e d u c t i o n of n i t r o g e n oxides i n t a i l p o s s i b l e w i t h NH^
gases o f n i t r i c
acid plants i s
as a r e d u c i n g agent. T h i s saves a l a r g e amount o f r e d u c i n g
agent. The r e d u c t i o n can be c a r r i e d out i n a f i x e d bed at a temperature o f o o 200 -500 C u s i n g a c a t a l y s t based on P t , Pd, Rh o r metal o x i d e s such as V„0_, z 5 Fe 0 , Cr 0 and CuO [51]. In t h i s way the NO c o n t e n t i n t h e s e t a i l gases can Z
o
Z
o
X
be e a s i l y d e c r e a s e d t o 200 ppm. result Van is
The
temperature
rise
i n the t a i l
gas
as a
o f the heat e v o l v e d from the r e d u c t i o n r e a c t i o n s i s s m a l l (about 2 0 ° C ) .
den B l e e k and Van
den Berg
[46] p o s t u l a t e d why
s e l e c t i v e r e l a t i v e t o oxygen. The
a r e d u c t i o n of N0
x
with
NH^
f o l l o w i n g r e a c t i o n s are considered i n
t h e i r hypothesis: 101
N0
2
+
NH
3
+
NH
3
cat -
2/3
"
"NH N0
2/3
"
4
cat NO
+
cat N 0 -> ^ 2
"NH N0
+
^0
(21)
c a t NgO 4
+
+
H
R
2
(
°
2
2
)
2 On
the c a t a l y s t
s u r f a c e they assume t h e f o r m a t i o n o f n i t r a t e o r n i t r a t e - l i k e
complexes as i n t e r m e d i a t e p r o d u c t s . These n i t r a t e o r n i t r a t e - l i k e decompose t o N
and N O . In t h i s way no NO i s produced,
complexes
and which o f c o u r s e
i m p l i e s t h a t t h e r e o x i d a t i o n o f NO a c c o r d i n g t o r e a c t i o n
(20) can n o t t a k e
place.
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44. K l i m i s c h , R.L. and L a r s o n , J.G., Oxides,
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F r e i t a g , W. and P a c k b i e r , M.W.,
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104
A p p e n d i x 1. THE A D D I T I V I T Y OF RESISTANCES FOR MASS TRANSFER IN A WETTED WALL COLUMN
1. INTRODUCTION AND GENERAL THEORY
In c h e m i c a l e n g i n e e r i n g d e s i g n t h e a d d i t i v i t y o f i n d i v i d u a l mass t r a n s f e r resistances derived
for gas-liquid
from
1 K
k
og
originally
1 mk.
(1)
I
g
It s h o u l d be n o t e d steady
systems i s o f t e n a p p l i e d , which was
the two-film theory:
t h a t t h e use o f t h i s r u l e
s t a t e t r a n s f e r a t a l l times
i s based
on t h e assumption
i n both phases and e q u a t i o n
of
(1) w i l l
hold
t r u e i f t h e f o l l o w i n g two c o n d i t i o n s a r e met:
1. The d i s t r i b u t i o n c o e f f i c i e n t m must be a c o n s t a n t o r known as a f u n c t i o n o f t h e t r a n s f e r r e d component
i n the l i q u i d
phase.
2. No o t h e r r e s i s t a n c e may be p r e s e n t o t h e r than t h o s e e x p r e s s e d by k
1/mk^
The
and V g -
gas phase mass t r a n s f e r c o e f f i c i e n t
and t h e l i q u i d phase mass t r a n s f e r
c o e f f i c i e n t may vary w i t h t h e c o n t a c t time o f renewable s u r f a c e s o r may over a f i n i t e
surface. Equation
the mass t r a n s f e r
K
og,local
In p r a c t i c e
(1) may then be a p p l i e d
k
g,local
(2)
mk„ , I,local s i n g l e phase mass t r a n s f e r
I f , f o r i n s t a n c e , t h e s i n g l e mass t r a n s f e r c o e f f i c i e n t s
the c o n t a c t time o f renewable s u r f a c e s we get f o r t h e time average transfer
values of
coefficients.
i t i s customary t o d e f i n e average
coefficients.
f o r the l o c a l
vary
vary with
mass
coefficients: T
£ K
k
o &,local T
d
t
(3)
105
and T ƒ"* k
=
, g,local
dt (4)
-
g
The
t r u e average o v e r a l l mass t r a n s f e r c o e f f i c i e n t
(K
&
with:
og,true
) can be
calculated
T — ƒ K , og,local o
—
dt
i
—
~
r + — mk„
>
o k , _ g,local
~
« (5)
, t,local
og,true T In
T
c h e m i c a l e n g i n e e r i n g d e s i g n the average o v e r a l l mass t r a n s f e r c o e f f i c i e n t i s ,
however, o f t e n d e r i v e d from the a d d i t i v i t y o f i n d i v i d u a l
average mass t r a n s f e r
resistances: K
In
1 I T + — — k mk„ g £
og, addition
—
general equation
equation
1
r • o
k
t
g.local — T
1
r — • i mk„ , ,
dt +
o
(5) i s not e q u a l t o e q u a t i o n
(5) i s e q u a l t o e q u a t i o n
c o n d i t i o n s are
1
1
dt
I,local
T
(6). King
[1] p o i n t e d out t h a t
(6) o n l y i f the f o l l o w i n g a d d i t i o n a l
fulfilled:
3. The mass t r a n s f e r c o e f f i c i e n t s o f the gas phase and the phase must not
4. The
local
interact.
v a l u e o f mk„/k A
R -
C
=
£
C„
(9) ,o
h=0
r < R - 6
h > 0
C
t
r = 0
h>0
The
c
r
h>0
The double
penetration
An
asymptotic
if
the gas
transfer
phase may gas
the p e n e t r a t i o n
=
and
calculated
= D
m C . = C . . g,i fc.,i
model (asymptotic
solution)
a l s o be
equations
considered
phase and
i n the
can
t o be
be
infinitely
i
with
o f i n d i v i d u a l phase r e s i s t a n c e s can
are be
(
+
2m^
Hoornstra
1
5
)
D^/TTT
[5] d e s c r i b e d t h e p h y s i c a l a b s o r p t i o n
of c h l o r i n e
model.
solution
(7-14) have been a p p r o x i m a t e d by [3,4]
between the gas
and
i n the
analytical
concentration
and
the
then s o l v e d by
finite
d i f f e r e n c e s according
a Gaus-Seidel
iteration
i n t e r f a c e f o r h = 0 where no
liquid.
liquid
Therefore
the
s o l u t i o n o f the p e n e t r a t i o n
theory
profiles
first
[ 4 ] . The
zero the
gas
concentration
solubility
i n the
liquid
i n the
gas
s t e p by means o f
fractional
change o f the t r a n s f e r r e d component i n the gas
for i n i t i a l
to
procedure.
equilibrium exists
concentration
phase were approximated i n t h e
o f the Graetz-number and 108
mass
described
i
A problem a r i s e s at the
calculated
deep. The
then be
with:
Crank-Nicolson
the
(14)
then the t r u e average o v e r a l l mass t r a n s f e r c o e f f i c i e n t
Berg and
phase and
(13)
found at l a r g e Graetz-numbers
l i q u i d phase can
i n t o benzene by means o f t h i s double p e n e t r a t i o n
Equations
6
-tt- -
r = R - 6. f
2 ^ D /TTT
Numerical
3C
Ç
D
theory.
og,true
den
_ 6
R
f o u r c o n d i t i o n s f o r the a d d i t i v i t y
fulfilled
Van
(12)
Je, o
are:
s o l u t i o n o f these
i n the
(11)
= C „
3C h>0
K
0
ic
interface conditions
All
=
g
—
(10) g.o
3C /3r
- + c o
r
= C
g
phase
was
phase as a f u n c t i o n
(see F i g . 2 ) . From t h i s
f i g u r e i t can
liquid jfiim]
gas
5 f
/
Fig,
Fig.
1
2
Flow model and coordinate
The fractional
system.
concentration
gas phase as a function
of
change of a transferred TT/GS
for D./D x,
(
Numerical
solution,
—
component in the
= 0.0001. g Double penetration
model). 109
be
seen t h a t t h e a s y m p t o t i c
s o l u t i o n (double p e n e t r a t i o n
model) i s o n l y
valid
at Graetz-number l a r g e r than 100. The t r u e average o v e r a l l mass t r a n s f e r c o e f f i c i e n t was c a l c u l a t e d from t h e n u m e r i c a l s o l u t i o n by e s t a b l i s h i n g a simple mass b a l a n c e around t h e wetted w a l l column. Based on t h e l o g a r i t h m i c mean d r i v i n g f o r c e between t h e i n l e t
C (h)) g
1> (C g g,o
K
A
V
and o u t l e t t h e mass b a l a n c e can be w r i t t e n as:
g,o
m
/
og
v g
h) (16)
'l,o B,o
In
C (h)
L e A comparison o f t h e t r u e average o v e r a l l mass t r a n s f e r c o e f f i c i e n t w i t h t h e average o v e r a l l mass t r a n s f e r c o e f f i c i e n t i n d i v i d u a l phase r e s i s t a n c e s
c a l c u l a t e d from t h e a d d i t i v i t y o f
i s o n l y p o s s i b l e i f a l l mass t r a n s f e r c o e f f i c i e n t s
are based on t h e same d r i v i n g f o r c e . F o r t h e l i q u i d phase i t was found under c e r t a i n c o n d i t i o n s theory.
t h e mass t r a n s f e r can be d e s c r i b e d
that
by t h e p e n e t r a t i o n
The average Sherwood number based on t h e l o g a r i t h m i c mean d r i v i n g f o r c e
between t h e i n l e t
and t h e o u t l e t can be w r i t t e n
2
In
Fo„
CO
O Z
e
,2 , V n «- n=l
X
p
(
"
as [ 6 ] :
(17)
2 Fo ) n t
11
i n which
(18)
The
eigenvalues 0
m , t h e c o e f f i c i e n t s A and t h e f u n c t i o n s F a r e g i v e n i n n n n
T a b l e 1. In C h a p t e r 2 i t was found t h a t t h e average gas phase Sherwood-number can be described
by the s o l u t i o n o f t h e G r a e t z - p r o b l e m . 2
Sh
= - — g
The
V
values The
In I n=l
^ a
2
n
of a are given n
(
a
IT
\
(19)
exp
Gz i n T a b l e 1 o f C h a p t e r 2.
d e v i a t i o n o f t h e t r u e average o v e r a l l mass t r a n s f e r c o e f f i c i e n t
compared t o t h e average o v e r a l l mass t r a n s f e r c o e f f i c i e n t a d d i t i v i t y o f i n d i v i d u a l phase r e s i s t a n c e w i t h e q u a t i o n s
obtained
c a l c u l a t e d f o r mk„/k a 1 at which t h e d e v i a t i o n i s pronounced. y» g
110
from t h e
( 1 9 ) , (17) and (6) was
For
initial
plotted in
z e r o gas c o n c e n t r a t i o n
the c a l c u l a t i o n o f t h e t r u e
about 5 °/oo t h i s d e v i a t i o n c a l c u l a t i o n of the l i q u i d logarithmic be
average o v e r a l l mass t r a n s f e r
i s that
k„
This
=
c o e f f i c i e n t , based on t h e
the t h i c k n e s s o f the l i q u i d
to define
an average l i q u i d
2V —
(20)
been based on an a r i t h m e t i c a l
from t h e p e n e t r a t i o n
driving
average o v e r a l l mass t r a n s f e r
(19) and e q u a t i o n
(20).
t h e o r y and i t has
force. c o e f f i c i e n t following
o f t h e a d d i t i v i t y o f t h e i n d i v i d u a l phase r e s i s t a n c e s equation
f i l m must
phase mass
follows:
e q u a t i o n can be d i r e c t l y d e r i v e d
The
coefficient i s
The d i s a d v a n t a g e i n t h e
phase mass t r a n s f e r
mean d r i v i n g f o r c e ,
c o e f f i c i e n t , as
(see F i g . 3 ) . Because t h e i n a c c u r a n c y
i s not r e l e v a n t .
known. T h e r e f o r e i t i s u s e f u l
transfer
i n t h e l i q u i d phase t h e d e v i a t i o n i s
o f t h e TT/Gz-number
as a f u n c t i o n
from the a p p l i c a t i o n
was c a l c u l a t e d
I t s h o u l d be n o t e d t h a t
from
i n t h i s case the
s i n g l e phase mass t r a n s f e r c o e f f i c i e n t s have n o t been based on t h e same d r i v i n g f o r c e . The l i q u i d driving
phase mass t r a n s f e r
f o r c e , w h i l e t h e gas phase mass t r a n s f e r
logarithmic-mean d r i v i n g The
c o e f f i c i e n t i s based on an a r i t h m e t i c a l
true
force.
average o v e r a l l mass t r a n s f e r
n u m e r i c a l s o l u t i o n , was t h e r e f o r e a logarithmic-mean d r i v i n g force
coefficient, calculated
defined as:
(C In
g,o
of t h i s true
individual
phase r e s i s t a n c e s
2 1 and f o r z e r o i n l e t
was p l o t t e d
measured c o n d i t i o n s ,
there
a r e added, l e a d i n g
from t h e a d d i t i v i t y o f
as a f u n c t i o n
gas c o n c e n t r a t i o n
ir/Gz-numbers. T h i s
unequal d r i v i n g f o r c e s
_
c o e f f i c i e n t obtained
From t h i s f i g u r e i t can be seen t h a t creases at increasing
m
average o v e r a l l mass t r a n s f e r c o e f f i c i e n t w i t h t h e
average o v e r a l l mass t r a n s f e r
mk./k
M >
- ^ ) m
g
deviation
with the
based on a c o m b i n a t i o n o f an a r i t h m e t i c a l and
*g «g,o .î«»,.ï >- " V ° - V g og
The
c o e f f i c i e n t i s based on a
o f t h e ir/Gz-number f o r
i n the l i q u i d
phase ( F i g . 4 ) .
i s a positive deviation,
deviation
which i n -
i s caused by t h e f a c t
that
to a systematic e r r o r . Within the
however, t h i s d e v i a t i o n
i s s m a l l enough t o be n e g l e c t e d .
Ill
1-05
1 00
0 95
0 0 5
0-1
TI Gz
Fig.
3
The deviation
of the additivity
mass transfer 0.0001; Fo
in a wetted wall
of individual
phase resistances
column as a function
for
of tt/Gz (D^/V^ =
= 0.01).
i
(Average
overall
driving
force.)
mass transfer
coefficient
based on a
logarithmic-mean
105
100
o
0
95
_L_ 0 0 5
0-1
Tt Gz
Fig.
4
The deviation
of the additivity
mass transfer
in a wetted wall
of individual
phase resistances
column as a function
for
of TT/GZ (D„/D J6 g
0.0001). (Average
overall
arithmetical 112
mass transfer
coefficient
and a logarithmic-mean
driving
based on a combination force.)
of an
F
n
n
1
2.26313
1.33823
0.393429
2
6.29782
-0.54556
-0.118857
3
10.30802
0.35893
0.067046
4
14.31325
-0.27211
-0.045787
5
18.31657
0.22113
0.034377
6
22.31892
-0.18732
-0.027320
7
26.32070
0.16313
0.022551
8
30.32213
-0.14488
-0.019128
9
34.32432
0.13060
0.016559
10
38.32519
-0.11908
0.014565
Table
1
Eigenvalues
3. CONCLUSIONS
The
s i n g l e phase mass t r a n s f e r c o e f f i c i e n t s
i n a p r e v i o u s l y developed
wetted
w a l l column do not v a r y w i t h t h e same power o f t h e c o n t a c t time o f renewable s u r f a c e s . Under t h e s e c o n d i t i o n s t h e a d d i t i v i t y o f i n d i v i d u a l phase r e s i s t a n c e s f o r mass t r a n s f e r does not h o l d and d e v i a t i o n s may o c c u r . In o r d e r t o study t h i s d e v i a t i o n , a t r u e average c o e f f i c i e n t was c a l c u l a t e d from
a numerical
o v e r a l l mass t r a n s f e r
s o l u t i o n , which was compared w i t h
the v a l u e o b t a i n e d from t h e a d d i t i v i t y o f i n d i v i d u a l phase r e s i s t a n c e s . be c o n c l u d e d
I t can
t h a t t h e d e v i a t i o n i s s m a l l enough t o be n e g l e c t e d .
REFERENCES
1. K i n g , J.C., A. I. Ch. E. Journal,
1964, 10, 671.
2. S z e h e l y , J . , Chem. Eng. Sai. , 1965, 20, 141. 3. C r o f t , D.R. and L i l l e y ,
D.G., Heat T r a n s f e r C a l c u l a t i o n s U s i n g
Finite
D i f f e r e n c e E q u a t i o n s , A p p l i e d S c i e n c e P u b l i s h e r s L t d . , London, 1977. 4. Crank, J . , The Mathematics o f D i f f u s i o n , C l a r e n d o n
P r e s s , Oxford, 1975.
5. Van den Berg, H. and H o o r n s t r a , R. , Chem. Eng. J., 1977, 12!, 6. Brauer,
191.
H., S t o f f a u s t a u s c h , S a u e r l a n d e r A.G., Aarau, 1971.
113
NOMENCLATURE
roots o f the equation thermal
J (a ) = 0 o n
2, m /sec
diffusivity
area of i n t e r f a c e coefficient concent r a t i o n
kmol/m 3 kg/m
specific
Joule/kg.
heat
d
diameter
D
diffusion
F
K
2, ra /sec
coefficient
funct ion n
F o u r i e r number
Fo
g
gravitational
Gz
Graetz
number
Graetz
number c o r r e c t e d f o r t h e i n a c t i v e f i l m
Gz
red
film
acceleration
length or co-ordinate
m/sec
height
o f l e n g t h i n flow
d i r e c t ion
m
h'
effective
Ah
inactive
H
Henry's law c o n s t a n t
3 kmol/m .bar
heat o f r e a c t i o n
Joule/kmol
heat o f s o l u t i o n
Joule/kmol 3 k-ion/m 2 kmol/m .sec
AH
film film
length
m
length
m
r
AH s
ionic
strength
I
absorption
r a t e per u n i t of s u r f a c e
area
J
Bessel
f u n c t i o n of the f i r s t
k i n d and z e r o
Bessel
f u n c t i o n of the f i r s t
k i n d and f i r s t
order
J„
reaction rate gas
og,addit ion
constant
sec
phase mass t r a n s f e r c o e f f i c i e n t
liquid _og
order
m/sec
phase mass t r a n s f e r c o e f f i c i e n t
overall
mass t r a n s f e r c o e f f i c i e n t
overall
gas phase mass t r a n s f e r c o e f f i c i e n t
from t h e a d d i t i v i t y o f i n d i v i d u a l
m/sec
based on gas s i d e
average mass t r a n s f e r
resistances K
og,true
K
solubility 114
m/sec
true o v e r a l l equilibrium
m/sec
derived
gas phase mass t r a n s f e r c o e f f i c i e n t constant
(= C. ,/C .) A,l g , l
m/sec bar
eigenvalues m(T)
q u a n t i t y o f gas absorbed p e r u n i t o f s u r f a c e a f t e r contact
n
time
area
I
kmol/m 2 kg/m
number
N
local
P
pressure N0„
mass
kmol/m .sec bar
2N 0 2
distance
Sh
flux
4
in radial direction
m
r a d i u s o f wetted w a l l column
m
gas
o Joule/kmol. K
law
constant
Sherwood number
t
time
T
temperature
sec
surface
°K
v e l o c i t y o f the l i q u i d
mass flow
film
r a t e o f the gas
co-ordinate
m/sec kg/sec
of length across
flow d i r e c t i o n
m
v a l e n c i e s o f ions
GREEK SYMBOLS
the
layer
f r a c t i o n o f NO^ c o n v e r t e d
t o ^^0^
thickness
o f the l a m i n a r
thickness
o f f i c t i t i o u s water l a y e r
distance at
instantaneous
reactions
viscosity
2, m /sec 3
density
kg/m
contact
time
flow
liquid
film
from r e a c t i o n p l a n e t o g a s - l i q u i d i n t e r f a c e
kinematic
gas
liquid
sec
rate
flow
rate
3.
m /sec 3,
m /sec
SUBSCRIPTS
c
wetted w a l l column
f
liquid
g
gas phase
film
i
gas-liquid interface
&
liquid
local
local
phase values 115
o
inlet
Q
N0
r
reaction
s
liquid
2
+ 2N 0 2
4
plane
surface
SUPERSCRIPTS
b u l k average
116
or mixing
up v a l u e
; —
•
— - -
-
!
S T E L L I N G E N
1. B i j g a s - v l o e i s t o f c o n t a c t b e s t a a t gasfase in
de
t u s s e n een
v l u c h t i g e component u i t de v l o e i s t o f f a s e en een
sprake
zijn
van een
o n e i n d i g s n e l mag
(Dit
reactant
i n d i e n de r e a c t i e a l s
worden beschouwd.
Proefschrift)
dig
gepaard gaande met
s n e l l e r e a c t i e i n de v l o e i s t o f f a s e t u s s e n een o p g e l o s t
tant
i n de v l o e i s t o f f a s e z i j n een
(Dit
i n p r i n c i p e tevens
een
onein-
gas en een
reac-
g e l d i g voor systemen waar-
v l u c h t i g e component u i t de v l o e i s t o f f a s e o n e i n d i g s n e l r e a g e e r t i n
de g a s f a s e met
3. De
r e a c t i e v l a k i n de g a s f a s e
algemene r e l a t i e s b e t r e f f e n d e g a s a b s o r p t i e
bij
i n de
gasfase.
E r kan
2. De
de m o g e l i j k h e i d dat r e a c t i e o p t r e e d t
een
d a a r i n aanwezige r e a c t a n t .
Proefschrift)
s t e r k t e van
verdund s a l p e t e r z u u r a l s w a s v l o e i s t o f voor de v e r w i j d e r i n g
van n i t r e u z e n u i t a f g a s s e n t i e g r a a d van NO
wordt voor een
en de o p l o s b a a r h e i d
van
groot
deel bepaald
door de
oxida-
salpeterigzuur i n salpeterzuur.
"V.
4. Het
ondergronds v e r g a s s e n
van k o o l i s o n g u n s t i g
vanuit reactorkundig
oog-
punt .
5. Het
succes
van
i n n o v a t i e - g e r i c h t e onderzoekprogramma's aan
en h o g e s c h o l e n z a l s t e r k afhangen van om
6.
de n o d i g e i n f o r m a t i e en gegevens t e
De k w a l i t e i t
van
schikbare
verstrekken.
researchwerkzaamheden i s
minimum n i v e a u o n a f h a n k e l i j k van
financiële m i d d e l e n .
(New Scientist,
de b e r e i d h e i d van het b e d r i j f s l e v e n
i n groepsverband uitgevoerde
boven een b e p a a l d
1979, 84_ (1176),
91)
universiteiten
de g r o o t t e van
de
be-
7. De samenleving zou een b e t e r i n z i c h t hebben i n de wetenschap en t e c h n i e k i n d i e n met name j o u r n a l i s t e n en T . V . - p r e s e n t a t o r e n h e i d en bekwaamheid op d i t t e r r e i n
8. De s t e l l i n g n a m e heersen
een g r o t e r e
deskundig-
bezaten.
d a t i n h e t " v r i j e " Westen v r i j h e i d van m e n i n g s u i t i n g zou
b e r u s t op een v o o r o o r d e e l .
9. De r e g e r i n g i s met name door de Wet op de I n v e s t e r i n g s r e k e n i n g verantwoordelijk
(WIR) mede-
voor de h u i d i g e z u i v e l o v e r s c h o t t e n i n ons l a n d .
10. " M a c r o b i o t i c i " kunnen i n bepaalde o p z i c h t e n beschouwd worden a l s " l u x e wilde
11.
beesten".
Hoogbouw i s l a a g - b i j - d e - g r o n d s .
Delft,
12 maart 1980
J.B.
Lefers
