Catalysts
Developing Engineered Catalysts for Fluid Catalytic Cracking Units a report by
N e g a h d a r H o s s e i n p o u r , A b b a s A l i K h o d a d a d i , M o h a m m a d S a l e h A g h a k h a n i and Y a d o l l a h M o r t a z a v i Catalysis and Nanostructured Materials Research Laboratory, School of Chemical Engineering, University of Tehran
A study of the synergistic effects of the principal components of fluid
the process. In the regenerator, air is introduced to burn off the coke.
catalytic cracking (FCC) catalysts, i.e. Y-zeolite and amorphous
The exothermic heat from the burn-off is absorbed by the regenerated
silica-alumina (ASA), on the cracking of large molecules is of great
catalyst, providing the heat for endothermic cracking reactions in the
importance for developing engineered catalysts appropriate for heavy
riser reactor.
feed processing. Here, the synergetic effects of Y-zeolite and ASA as principal components of FCC catalysts on tri-isopropylbenzene (TiPB)
Demand for higher amounts of diesel fuels and high-octane gasoline is
cracking and formation of coke are studied.
growing worldwide. On the other hand, there are vast resources of
Fluid Catalytic Cracking Process Description
feedstocks tends to produce more coke than lighter feeds because of
heavy feeds available. In addition to this, the cracking of heavy The FCC of hydrocarbons is the most widely used process for the
the wide variety of unsaturated species that they contain and/or are
production of diesel fuels and high-octane gasoline from
formed. These challenges necessitate the development of engineered
atmospheric and vacuum gas oils. FCC units consist of a riser reactor
FCC catalysts that are tailor-made for the cracking of heavy
and a regenerator. Feed in contact with hot catalyst particles is
hydrocarbon molecules.
vaporised, cracked and passed through the riser, resulting in the production of lower-molecular-weight products and the formation
Unsaturated hydrocarbons are adsorbed on high-density acid sites of
of coke on the catalyst. In the regenerator of the FCC plants, the
the catalyst and become increasingly hydrogen deficient, ultimately
coke is burned off and the regenerated catalyst is transferred back
forming coke via cyclisation. These molecules readily form ion radicals
into the riser reactor.
with the acid sites, polymerise with other unsaturated hydrocarbons and then dehydrogenate to aggregates of coke.2,3
During cracking in the riser, the catalyst surface is covered with layers of coke deposit as a result of side reactions. The accessible surface
Since the acid site density and pore structure of the FCC catalysts have
area of the catalyst therefore decreases. Active acid sites become
a great influence on the catalyst activity for cracking of
coated with coke deposits and the catalyst pores are blocked with
difficult-to-process heavy feeds, some provisions should be considered
coke build-up. This leads to catalyst deactivation with time on stream.1
when making the catalyst. The tailor-made catalyst should provide a
The concept of the movement of the catalyst from the reactor to the
exposure of heavy feeds to its high-density acid sites.
good accessibility for heavy hydrocarbons, while preventing the direct regenerator is based on the need for continuous operation of Engineered Fluid Catalytic Cracking Catalyst Design Negahdar Hosseinpour is a PhD candidate at the School of Chemical Engineering at the University of Tehran. His research interests are in the area of catalyst and photocatalyst design, and band and reaction engineering, especially in fluidised bed reactors for hydrogen production and the cracking of heavy hydrocarbons. He received his BSc in chemical engineering from Persian Gulf University in Bushehr and his MSc in chemical engineering from University of Tehran. Abbas Ali Khodadadi is Head of the Nanoscience and Nanotechnology Research Centre and a faculty member of the School of Chemical Engineering at the University of Tehran. His research interests include catalysis, reaction engineering and nanostructured materials as applied to nanoparticles, carbon nanotubes and plasmacatalytic conversion of natural gas to liquids and chemicals. He received his PhD in catalysis and reaction engineering from the University of Waterloo in Canada. Mohammad Saleh Aghakhani is a Research Assistant in the Catalysis and Reaction Engineering Laboratory at the University of Tehran. His research interests are in the area of nanostructured catalytic materials, especially for the cracking of vacuum distillates. He received his MSc in chemical engineering from the University of Tehran.
Typical FCC catalysts consist of crystalline Y-zeolite, active matrices, low-activity fillers and proprietary additives. One of the most common active matrices of the FCC catalysts is ASA with nanometre-sized particles. The cracking activity of ASA is enhanced with alumina content; however, its coking tendency also increases.3 An acidic Y-zeolite with highly stable crystalline framework of 7.4Å openings is the most active and selective component of the FCC catalysts, with a high coke formation propensity.1,2 In the case of cracking heavy feeds, many reactant molecules are either too large to penetrate the micropores of Y-zeolite or their diffusion is strongly limited.4 The most important function of active matrices in the FCC catalyst is the pre-cracking of heavy portions of the feeds and decreasing their
Yadollah Mortazavi is Chair of the Nanotechnology Department at the College of Engineering at the University of Tehran. His research interests include catalysis and reaction engineering, especially in C1 chemistry, environmental catalysis, chemical gas sensors and nanostructured materials. Professor Mortazavi received his BSc in chemical engineering from he University of Shiraz in Shiraz, and his MSc and PhD in chemical engineering from the University of Waterloo in Canada.
zeolite pores and be further cracked on the stronger acid sites.2 Thus,
E:
[email protected]
while processing heavy feeds, the most selective component of the
direct contact with the surface of Y-zeolite crystals that have high-density acid sites with high coking tendency. The pre-cracked products with lower molecular weights are able to diffuse into the
catalyst, i.e. Y-zeolite, and the active matrices synergistically result in
24
© TOUCH BRIEFINGS 2010
Developing Engineered Catalysts for Fluid Catalytic Cracking Units Figure 1: Schematic Representation of Y-zeolite, SA-Y, Core-shell Composite and SA.Y Catalysts
Figure 3: Field-emission Scanning Electron Micrographs of the Core-shell Composite, Y-zeolite and Amorphous Silica-alumina
SA-Y 50wt% SA + 50wt% Y-zeolite
Y-zeolite
CSC
SA. Y 50wt% SA + 50wt% Y-zeolite
Core-shell composite (CSC) 50wt% SA + 50wt% Y-zeolite
Y-zeolite
Figure 2: X-ray Diffraction Patterns of the Synthesised Y-zeolite, Core-shell Composite and Amorphous Silica-alumina 1,400
1,200
Intensity (a.u.)
1,000
800
ASA
600
400
200
0 5
10
15
20
25
30
35
40
45
50
2 theta (º) Y-zeolite
CSC
ASA
ASA = amorphous silica-alumina; CSC = core-shell composite
the formation of higher amounts of high-value products and lower
CSC stands for a core-shell composite of Y-zeolite and ASA where
amounts of coke. In this work, crystalline Y-zeolite and ASA were
Y-zeolite crystals were covered with an equal amount of ASA
synthesised by hydrothermal and coprecipitation methods,
particles. SA-Y is designated for a series configuration of ASA and
respectively, and ammonium was exchanged. For studying the
Y-zeolite samples in which a bed of ASA was placed upstream of the
synergistic effects of Y-zeolite and ASA on the cracking of heavy
same amount of Y-zeolite in the reactor. For the SA.Y sample, equal
feeds, TiPB with a kinetic diameter of 9.4Å, (higher than the
amounts of ASA and Y-zeolite were physically mixed. The SA.Y
apertures of Y-zeolite), was used as the probe molecule.2,3
sample resembled the common configuration of Y-zeolite and ASA in FCC catalysts.
The TiPB cracking activity of three different configurations of Y-zeolite and ASA, i.e. core-shell composite (CSC), SA-Y and SA.Y, was investigated in
X-ray diffraction patterns of the synthesised Y- zeolite, CSC, and
a fixed bed reactor. The schematic representation of the catalysts is
ASA are given in Figure 2. The X-ray diffraction pattern of the Y-
depicted in Figure 1.
zeolite sample shows only the Y-zeolite structure. No peak
HYDROCARBON WORLD – VOLUME 5 ISSUE 2
25
Developing Engineered Catalysts for Fluid Catalytic Cracking Units Figure 4: Tri-isopropylbenzene Conversion as a Function of the Catalysts Time on Stream
0.5µm. In the CSC sample, Y-zeolite crystals are well covered with small particles of ASA, protecting Y-zeolite from direct exposure to TiPB molecules during the cracking experiments.
100
95
Tri-isopropylbenzene Cracking Results
90
shown in Figure 4. All the samples show a high activity at lower times
85
even on week surface acid sites. The activity of SA-Y and CSC is higher
80
pre-cracking effects of ASA with mesopores that provide the products
TiPB conversion (%)
The conversion of TiPB on different catalysts versus time on stream is on stream because the dealkylation of TiPB may readily proceed, than that of the SA.Y sample at all times. This can be attributed to with possible diffusion into the zeolite micropores.2 At three minutes of 75
time on stream (see Figure 4), all of the catalyst samples showed a high TiPB conversion and their cracking activity decreased in the order of
70 0
5
10
15
20
25
30
35
40
SA-Y
SA.Y
ASA
SA-Y ≥CSC >SA.Y >Y-zeolite >ASA. The TiPB cracking activity of SA-Y and CSC are almost the same at three minutes. Compared to the CSC
Time on stream (min) CSC
Y-zeolite
sample, a higher deactivation rate is observed for the SA-Y catalyst. The activity of ASA is the slowest and decreases sharply with time on
ASA = amorphous silica-alumina; CSC = core-shell composite
stream. Since the ASA activity decreases sharply, the activity of SA-Y Figure 5: Distribution of Products at Three Minutes on a Stream of TiPB Cracking on the Catalysts
decreases with a higher pace compared to that of the CSC sample. The activity SA-Y sample resembles the ideal protection of Y-zeolite from direct contact with TiPB molecules.
80
Molar yields for various products at three minutes of time on cracking
70
are shown in Figure 5. For all catalysts, cumene (isopropylbenzene) and di-isopropylbenzene (DiPB) are the main products of TiPB cracking. The
60
yield of benzene plus cumene as deep cracking products on SA-Y and CSC samples is 45.4% and 32.1% higher, respectively, than that on
Molar yeild (%)
50
the SA.Y catalyst. The higher yield of deep cracking products for SA-Y and CSC may be explained by the pre-cracking of TiPB to DiPB mainly
40
on ASA and further cracking of mostly 1,4-DiPB to cumene and then to 30
benzene on the zeolite strong acid sites. After the cracking experiment
20
were estimated by the temperature programmed oxidation (TPO)
had proceeded for 39 minutes, the coke content of the catalyst beds technique. The coke content of the SA-Y, CSC and ASA samples, 10
measured by CO and CO2 evolution profiles appeared in the TPO spectra, were 6.5, 2.2 and 44% lower, respectively, than that of the
0 Propylene
Benzene SA-Y
Cumene CSC
SA.Y
1,3-DiPB Y-zeoline
1,4-DiPB
SA.Y catalyst. However, the coke content of Y-zeolite sample, which was 2.10wt%, was 27% higher than that of the SA.Y catalyst.
ASA
Furthermore, compared with the other catalysts, slightly heavier coke
ASA = amorphous silica-alumina; CSC = core-shell composite
was formed on ASA, indicated by about a 20ºC higher peak temperature in TPO profiles.
characterising other zeolites is observed in the spectrum. No peak in the X-ray diffraction pattern of the silica-alumina is observed,
Conclusion
indicating formation of ASA. The X-ray diffraction pattern of CSC
Protecting the Y-zeolite from direct exposure to heavy feeds of FCC
samples
units enhanced the catalyst cracking activity and selectivity to high-value
shows
all
the
Y-zeolite
peaks
with
lower
intensities/background ratio. Since 50wt% of the CSC sample is
products and decreased the catalyst’s tendency to form coke. n
amorphous, this is a main source for the production of background in the X-ray diffraction pattern. The field-emission scanning electron
Acknowledgments
microscopy pictures of ASA, CSC and Y-zeolite samples are
The National Iranian Oil Refining & Products Distribution Company is
presented in Figure 3. Surface morphology of the ASA sample shows
acknowledged for partially funding of this research work. The authors
irregular agglomerates with varied sizes. The Y-zeolite sample has
would also like to thank Mr A Bazyari and Mr S Najafi for their help
irregular polyhedra crystals with an average particle size of about
during building the experimental set-ups and testing the catalysts.
1. 2.
26
Rase HF, Handbook of Commercial Catalysts: Heterogeneous Catalysts, CRC Press LLC, New York, 2000: 365–71. Hosseinpour N, Mortazavi Y, Bazyari A, et al., Synergetic effects of Y-zeolite and amorphous silica-alumina as main FCC catalyst components on tri-isopropylbenzene cracking
3.
and coke formation, Fuel Proc Techy, 2009;90:171–9. Hosseinpour N, Khodadadi AA, Mortazavi Y, et al., Nano-ceria–zirconia promoter effects on enhanced coke combustion and oxidation of CO formed in regeneration of silica–alumina coked during cracking of tri-isopropylbenzene,
4.
Applied Catalysis A: General, 2009;353:271–81. Bazyari A, Khodadadi AA, Hosseinpour N, et al., Effects of steaming-made changes in physicochemical properties of Y-zeolite on cracking of bulky 1,3,5-tri-isopropylbenzene and coke formation, Fuel Processing Technology, 2009;90:1226–33.
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