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catalytic cracking (FCC) catalysts, i.e. Y-zeolite and amorphous silica-alumina (ASA), on the ... of coke are studied. Fluid Catalytic Cracking Process Description .... Rase HF, Handbook of Commercial Catalysts: Heterogeneous. Catalysts, CRC ...
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

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© 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

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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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