DRASTIC ENHANCEMENT OF PROPENE YIELD FROM 1-HEXENE ...jestec.taylors.edu.my/Vol 4 Issue 4 December...

Journal of Engineering Science and Technology Vol. 4, No. 4 (2009) 409 - 418 © School of Engineering, Taylor’s University College

409

DRASTIC ENHANCEMENT OF PROPENE YIELD FROM 1-HEXENE CATALYTIC CRACKING USING A SHAPE

INTENSIFIED MESO-SAPO-34 CATALYST

ZEESHAN NAWAZ*, JIE ZHU, FEI WEI

Beijing Key Laboratory of Green Chemical Reaction Engineering

and Technology (FLOTU), Department of chemical Engineering,

Tsinghua University, Beijing, 100084, China.

* Corresponding author: [email protected]

Abstract

A shape intensified Meso-SAPO-34 catalyst was designed and used to improve

the yield and selectivity of propene from 1-hexene cracking. The propene was

produced with an optimal selectivity of 73.9 wt.% with high feed conversion

98.2 wt.% at 14 per hour WHSV. Robust exponential control of the

stereochemistry was observed over the Meso-SAPO-34 shape selective

catalyst’s cracking. The influence of the operating parameters on 1-hexene

catalytic cracking, such as reaction temperature, time-on-stream effect on

product distribution and conversion variations were systematically studied. The

yield of propene and conversion rapidly increased with the reaction

temperature, until 575oC. Shape intensification and topological integration of

SAPO-34 increases the diffusion opportunities for feed, and this phenomenon

was found to be responsible for drastic increase in 1-hexene conversion and

propene yield. One other reason for this increase is the suppression of surface

reactions (isomerization and hydride transfer) owing to better diffusion

opportunities. About 55 wt.% propene yield and higher total olefins content was

achieved over Meso-SAPO-34.

Keywords: Cracking, Propene, Shape intensification, Diffusion, Meso-SAPO-34.

1. Introduction

Propene is the principal raw material for production of important

petrochemicals, such as polypropylene, acrylonitrile, propylene oxide, cumene,

phenol, oxo-alcohols, alkylates blends, isopropylic alcohol, acrylic acid,

isopropyl alcohol, polygas chemicals [1, 2]. The continuous increase in demand

410 Zeeshan Nawaz et al.

Journal of Engineering Science and Technology December 2009, Vol. 4(4)

Abbreviations

BET Brunauer-Emmett-Teller (m2/g)

CTO Catalyst to oil ratio

DCC Deep catalytic cracking

FCC Fluid catalytic cracking

FID Flame ionization detector

GC Gas chromatography

IUPAC International union of pure and applied chemistry

MSCC Millisecond catalytic cracking

PIONA Paraffin’s, i-paraffins, olefins, naphthenes, and aromatics

SAPO Silicoaluminophosphate

TOS Time-on-stream

WHSV Weight hourly space velocity (h-1)

XRD X-ray diffraction

of propene has lead to the development of new catalysts and on purpose

propene producing processes.

Fluid catalytic cracking (FCC) products were containing higher olefins while it

shows selectivity towards alkane and the processes were governed by hydrogen-

transfer reactions. Moreover olefins yield from FCC is still limited due to its

complexity and process severity. It is also known that increasing process

severities, leads to maximize propene production in FCC units up to 7% [3].

The on-purpose propene technologies are currently being developed to obtain

higher yields of propene from FCC units by upgrading low value refinery streams

like C4–C6 olefins and their overall reaction kinetics is recently proposed by

Huaqun et al. [4]. Recent developments in reactor’s integration, like millisecond

catalytic cracking technology (MSCC), the down-flow reactor technology

(Downer), deep catalytic cracking (DCC), two-stage riser FCC and the new

catalytic cracking technologies for producing light olefins are on the verge of

communication but no drastic improvement has been reported [5]. Catalyst

intensification on the nano-scale was also currently being investigated, e.g.

SAPO-34 in the fixed-bed catalytic cracking process ‘‘Propylur’’, developed by

Lurgi in 1996. In this process Lurgi claims to convert approximately 60 wt.% of

C4 and C5 olefins to propene and ethene using steam cracker [6].

The catalytic enhancement of SAPO-34 zeolite catalysts is still not well

understood and not available through open resources; but it does claim to

demonstrate better performance in hexene catalytic cracking [5]. The effect of

topology on SAPO-34 catalyst performance has been studied here to ascertain any

possible propene production enhancements. In previous studies SAPO-34 was

observed to be more effective for butene cracking and hexane [5, 7, 8].

In the work presented here, 1-hexene cracking over shape intensified

catalyst Meso-SAPO-34 was studied and found that the shape selectivity not

only increased the reaction rate but also improved the propene yield and feed

conversion. Both SAPO-34 and Meso-SAPO-34 results were obtained under

similar operating values and compared in order to explain superiority. The

overall 1-hexene process was parametrically characterized to explored

operational optimization.

Drastic Enhancement of Propene Yield from 1-Hexene Catalytic Cracking 411


2. Experimental

2.1. Preparation of Meso-SAPO-34 catalyst and characteristics

Conventional, solid, acidic, zeolite-based catalysts are widely used in the

petrochemical industry for hydrocarbon conversion. In significant features of

SAPO-34 catalyst, it has high catalytic activity for cracking reactions and low

activity for hydrogen-transfer reactions. Characteristics of SAPO-34 including

packing density (0.84 g/cm3), the internal pore size (0.34 Ao) and average

diameter of the particles (~5µm) [9]. The shape intensified catalyst; Meso-SAPO-

34 was designed by Zhu Jie et al. of Beijing Key Lab of Green Reaction

Engineering & Technology (FLOTU), Tsinghua University, Beijing, China [10].

The kaolin (a source of aluminum and silicon), phosphorus, template and de-

ionized water are mixed together and stirred to obtain uniform crystallization

solution. The Al2O3: SiO2: P2O5: H2O molar ratio is 0 ~ 1.5:0 ~ 1.2:0.8 ~ 1.5:2 ~

4:10 ~ 500. The silicon aluminum phosphate catalyst of slit shape i.e. Meso-

SAPO-34 is available shortly after crystallization. The shape intensified Meso-

SAPO-34 catalyst was characterized through X-ray diffraction (XRD). The Meso-

SAPO-34 catalyst possesses an internal structure similar to cube shape SAPO-34,

but with different quoin and topology. Although in many experimental studies

SAPO-34 cracking ability has proven to be uniform among available cracking

catalysts, the slit shaped topology of Meso-SAPO-34 offers a more robust design

for exponential enhancement of activity by increasing pore diffusivity.

The structure of Meso-SAPO-34 is shown in Fig. 1 and its XRD pattern is

shown in Fig. 2 [8]. The internal pore size, acidity, and XRD pattern of Meso-

SAPO-34 is almost similar to SAPO-34 [8], except the outer shaping quo’s (see

Fig. 1). Therefore characterization of Meso-SAPO-34 is useless but their cracking

performance can only be justified on the experimental grounds. The single point

surface area at P/Po is 0.2012: 540.3451, BET surface area 522.3917 and

Langmuir surface area was 689.102.

Fig. 1. SEM Image of the

Outer Shaping Quo’s of

Meso-SAPO-34 Catalyst.

0 10 20 30 40 50

Intensity (a. u.)

2 Theta (angle)

Fig. 2. XRD of Meso-SAPO-34.



2.2. Feed stock and products

All alkenes have the general formula CnH2n, so propene is CH3-CH=CH2 and

molar mass of 42.08 g/mol [11]. Although the official IUPAC nomenclatures for

saturated and unsaturated hydrocarbons are alkanes and alkenes, respectively by

IUPAC, these compounds are also known as paraffin’s and olefins. Thus many

acronyms and industrial concepts are also based on these common names e.g.

“PIONA” (paraffin’s, i-paraffins, olefins, naphthenes, and aromatics) analysis and

“olefinicity” (the relative amount of olefins within a certain product group) [12].

Here we refer to individual species according to IUPAC nomenclature; e.g.

ethane, propene and use non-IUPAC nomenclature (olefins and paraffin’s) when

addressing groups of hydrocarbons. Analytical grade 98 % pure 1-hexene

(IUPAC name: hex-1-ene) of AlfaAesar (A. Johanson Matthey Company, UK)

was used as a model FCC feed in current experimental investigations in a micro-

reactor. All the values were calculated and presented in weight percentages.

2.3. Feed stock and products

The experimental setup was designed to analyze the robustness and exponential

control of the Meso-SAPO-34 catalyst’s on a model FCC component 1-hexene

cracking in a micro-reactor. After the cracking reaction the product gas was

analyzed using on-line Gas Chromatography equipped with FID (Techcomp

Holdings Ltd., Model GC-7890II). The used FID has a sensitivity of Mt = 1×10-

11 g/s and was used at 200oC. The measured amount of Meso-SAPO-34 catalyst

was mixed with 0.2 g of inert material. The rector pressure was maintained at

0.02 MPa and the feed loading rate was adjusted at 10 ml/min in all experiments

in order to obtain the desired WHSV i.e 14 h-1.

3. Results and Discussions

The activity of intensified topological SAPO-34 catalyst (Meso-SAPO-34) has been

experimentally studied and found suitable for propene enhancement. As

isomerization rate is much faster than cracking [13], but Slit-SAPO-34 better

diffusion ability reduces the possibilities of surface reactions. Extensive

experimentation has been carried out between the temperature range of 450-600oC

at varying TOS and fixed WHSV, 14 h-1.

Initially, the selectivity of propene increased, but then decreased at 500oC with

a sudden increase in conversion. After 500oC, both selectivity and yield gradually

increased with the increasing conversion of propane on TOS = 1 min. (see Fig. 3).

At higher TOS (5 min.) the catalyst quickly deactivates above 575oC with a

maximum 66.5% propene yield and 73.9% selectivity was obtained (see Fig. 4).

The present conversion begins to decline after TOS = 4 min. and temperature

575oC with a sudden decrease in yield and selectivity (see Fig. 5).

1-hexene was first cracked over Lewis acid site and as the L/B ratio increased with

the increase in temperature, conversion also increased [14]. It has long been reported

that at higher temperatures and lower pressures, the adsorption of hydrocarbons

decreases and monomolecular mechanism will enhanced; in fact dominating the

dimeric cracking mechanism [12]. The cracking of olefins at FCC reaction conditions

favors the formation of small olefins such as propene experimentally explained by



450 475 500 525 550 575 600

40

50

60

70

80

90

100

% Conversion of 1-Hexene (X)

% Yield of Propene (Y)

% Selectivity of Propene (Z)X / Y / Z wt%

Temperature oC

TOS = 1min

Ea = 18 kJ/mol

WHSV = 14 h-1

Fig. 3. Effect of Temperature on 1-hexene Cracking

at TOS = 1 min. in a Micro-reactor.

450 475 500 525 550 575 600

20

30

40

50

60

70

80

90

TOS = 5min

Ea = 20.5 kJ/mol

WHSV = 14 h-1



% Selectivity of Propene (Z)

X / Y / Z wt%

Temperature oC

Fig. 4. Effect of Temperature on 1-hexene Cracking

at TOS = 5 min. in a Micro-reactor.

1 2 3 4 5

60

70

80

90

100

CTO5.16.48.412.525.1



% Selectivity of Propene (Z)

X / Y / Z wt%

TOS, min

Temperature= 575oC

WHSV = 14 h-1

Fig. 5. At Constant Temperature 575oC, TOS Influence

on % Conversion, % Yields and % Selectivity of Propene.



Xaioping et al. [5]. It has also been demonstrated that the medium pore size zeolite

catalyst, SAPO-34 has better cracking ability owing to its shape selectivity as reported

by Tabak et al. [15, 8].

In current experimentation, however we observed that for the same catalyst

pore size, changes in quo’s and topology can have a drastic effect in enhancing

the desired product. This enhancement can be attributed to better diffusion

abilities provide by Meso-SAPO-34. There is still controversy regarding the

nature of acid sites action for sustained hydride transfer and it may prolong

alkylation activities [16]. The decrease of the hydrogen transfer coefficient with

the reaction temperature indicates that higher temperatures favor

monomolecular cracking and enhancement of light olefin formation [17].

Comparison of Meso-SAPO-34 and SAPO-34 catalytic performance for

1-hexene cracking under identical conditions has been shown in Fig. 6, and

similar results were shown in literature [8]. Significant difference in conversion

can be seen while slightly higher propylene selectivity at higher conversion

over Meso-SAPO-34 was observed. Detailed experimental results obtained at

different temperatures and TOS were show in Table 1, except propene because

it has been presented through graphs.

1 2 3 4 5

0

20

40

60

80

100

Conversion, %

TOS, min

Meso-SAPO-34 at 550oC

SAPO-34 at 550oC

WHSV = 14 h-1

Fig. 6. 1-Hexene Conversion over SAPO-34 and Meso-SAPO-34 at 550oC.

The product distribution obtained at optimum cracking temperature of 575oC

is shown in Fig. 7. Under fixed feed flow rate of 10 ml/min, increasing the TOS

from 1-5 min. decreases the CTO to 25.1, 12.5, 8.4, 6.4 and 5.1 respectively. The

% yield of propene and % selectivity increases with the increase in TOS up to 4

min and then becomes stagnant. The C3/C2 ratio also increases with time and

temperature up to 575oC. In all the current experiments, at WHSV = 14 h-1, no

methane and C7+ carbons were detected. Buchanan et al. [18] explained all

possible cracking schemes after extensive experimentation; the dominant cracking

reaction is C-type β-scission (see Scheme 1) while using Slit-SAPO-34 as a

catalyst [18, 8].



Table 1. Percentage Yield of Total Olefins and Ethene with

Percentage Conversion of Feed (in Temperatures Range

of 450-600oC and TOS = 1 - 5 min., at WHSV = 14 h-1).

Temperature, oC

TOS,

min.

% conversion

of 1-hexene

% yield of

total olefins

% yield of

ethene

1 63.949 95.229 4.237 2 47.187 96.905 1.862 3 42.246 96.712 1.492 4 38.989 96.945 1.276

450

5 36.861 97.359 1.142 1 95.836 89.087 8.180 2 94.813 90.389 8.014 3 91.341 93.133 6.647 4 87.456 94.815 5.230

500

5 82.874 95.776 4.204 1 97.400 92.771 12.636 2 96.447 94.824 10.897 3 95.162 95.973 8.556 4 93.175 96.859 6.869

550

5 88.310 97.416 5.404 1 98.207 93.305 13.689 2 97.848 94.913 12.376 3 96.231 96.516 9.915 4 94.139 97.281 8.060

575

5 89.965 97.862 6.335

1 2 3 4 5

0

2

4

6

8

10

12

14

% Selectivity

TOS, min

Ethane

Ethene

Propane

Iso-Butane

n-Butane

trans-2-butene

1-butene

Iso-Butene

cis-2-butene

Pentane

Pentene

Hexane

Hexene

25.1 12.5 8.4 6.4 5.1 CTO

Fig. 7. At Temperature 575oC and WHSV 14 h-1,

TOS and CTO Influence on Product Distribution.

Scheme 1. Carbenium Ion Formation in 1-hexene Cracking,

C-type β-Scission Mode [8].



At the same reaction conditions the balance between mono-molecular and

bimolecular mechanisms will depend on the characteristics of the catalyst. The

medium pore size of Meso-SAPO-34 will favors the monomolecular mechanism.

Since the bimolecular reaction intermediates cannot be formed during the course

of cracking. The conversion of 1-hexene and yield of propene shows a rapid

increase with the increase in reaction temperature and decreases with reference to

the catalyst-to-oil ratio. However, at lower catalyst-to-oil ratios the hydrogen-

transfer reactions were negligibly low. The optimum operating conditions are

therefore moderate residence time for high yields of propene combined with (i)

lower yields of dry gas and (ii) a lower apparent hydrogen-transfer coefficient.

One of the interesting findings of our current experimentations is the initially high

yield and selectivity of propene, which decreased rapidly up to 80% by increasing

TOS at all temperatures (Fig. 8). Therefore, optimum TOS and catalyst

deactivation integration provides a significant guidance towards severity of the

operating parameters with the Meso-SAPO-34 catalyst.

480 500 520 540 560 580 600

0

1

2

3

4

5

6

7

8

Yield of Propene (%)

Temperature oC

TOS=1min

TOS=2min

TOS=3min

TOS=4min

TOS=5min

Fig. 8. Influence of Temperature on % Yield of Propane

during 1-hexene cracking over Meso-SAPO-34.

Meso-SAPO-34 catalyst exhibits high activity and can reduce the production of

by-products such as methane and coke. It should be noted that the ethene and butane

are the second prominent products which promotes a significant impact in overall %

yield of total olefins. Meso-SAPO-34 catalyst has a significant advantage as the

higher desired product yield and gives less coke formation than SAPO-34.

4. Conclusion

The influence of primary operating parameters such as reaction temperature, catalyst-

to-oil ratio and residence time on product distribution and conversion was

systematically studied. Because of very high conversion rates, even at high WHSV,

therefore lower CTO is needed during cracking. The Meso-SAPO-34 catalyst

promotes C-type β-scission and significantly retards HT-surface reactions.

Increasing catalyst-to-oil ratio can enhance 1-hexene cracking rate and

improves the yield of propene even at higher temperatures to some extent. While

increase in catalyst-to-oil ratio will leads towards secondary reactions and

decrease the desired product yield and selectivity. The percentage selectivity of



propene and conversion showed a rapid increase with increasing reaction

temperature up to 575oC. But further increase in propene selectivity was at the

cost of lower feed conversion along with the increase of by-products were

observed and found unsuitable. This drastic increase in propene yield was

attributed to an enhanced opportunity for diffusion owing to the change in

existing SAPO-34 catalyst intensification.

The results suggest that at the optimum operating parameters investigated were

575oC and TOS = 1 min. at WHSV = 14 h-1, the conversion of 1-hexene achieved

was approximately 98.2 wt.%. The optimum propene selectivity was observed to be

73.9 wt.% at 575oC and at TOS = 5 min, propene yield reached a maximum

66.5 wt.%. At optimum conditions, the yield of total olefins was 97.9 wt.%.

This new generation Meso-SAPO-34 catalyst with improved quo’s and

topology provides a significant increase in propene yield and may emerge as a

significant milestone in commercial catalyst design and development for the

enhancement of propene via petroleum based route.

Acknowledgments

The authors are gratefully acknowledged the resources and technical support

provided by Bejing Key Laboratory for Green Chemical Reaction Engineering &

Technology (FLOTU), Department of Chemical Engineering, Tsinghua

University, Beijing, China and to Higher Education Commission, Islamabad,

Pakistan for their financial support.

References

1. Ladwig, P.K.; Asplin, J.E.; Stuntz, G.F.; Wachter, W.A. and Henry, B.E. (2000).

Process for selectively producing light olefins in a fluid catalytic cracking

process. US Patent 6,069,287 (to Exxon Research and Engineering Corporation).

2. Wei, F.; Tang, X.P.; Zhu, H. and Nawaz, Z. (2008). Development of

propylene production enhancement technology. Petrochemical Technology,

37(10), 979-986.

3. Upson, L.L. (2000). FCC process with high temperature cracking zone. US

Patent No. 6,113,776.

4. Zhu, H.; Wang, Y.; Wei, F.; Wang, D.Z. and Wang, Z. (2008). Kinetics of

the reactions of the light alkenes over SAPO-34. Applied Catalysis A:

General, 348(1), 135–141.

5. Xioping, T. (2008). Study on the Downer Fluid Catalytic Cracking and Light

Olefin Conversion for Maximizing Propylene. PhD Thesis, Department of

Chemical Engineering, Tsinghua University, Beijing, China.

6. Nawaz, Z.; Tang, X.P.; Zhu, J.; Naveed, S. and Wei, F. (2009). Catalytic

Cracking of 1-Hexene to Propylene Using SAPO-34 Catalysts with Different

Bulk Topologies, Chinese Journal of Catalysis, 30(11), 1049-1057.



7. Nawaz, Z.; Tang, X.P. and Wei, F. (2009). Hexene Catalytic Cracking over

SAPO-34 Catalyst for Propylene Maximization: Influence of Reaction

Conditions and Reaction Pathway Exploration, Brazilian Journal of

Chemical Engineering, 26(4), 705-712.

8. Nawaz, Z.; Zhu, J. and Fei, W. (2009). Structural Topology Integration of

SAPO-34 Catalyst at nano-level enhances its catalytic activity for FCC model

feed to propylene. Presented at 6th International Bhurban Conference on

Applied Sciences & Technology (IBCAST), Islamabad, Pakistan.

9. Min, P.; Zhi-hong, L.; Yan-jun, G.; Dong, W. and Yuhan, S. (2003). Ab-

initio study on the isomerization of 1-hexene to 2-hexene over the surface of

aluminosilicate molecular sieves. Journal of Material Science Letters,

22(14), 955– 957.

10. Zhu, J.; Cui, Y.; Wang, Y. and Wei, F. (2009). Direct synthesis of

hierarchical zeolite from a natural layered material. Chemical

Communications, 22, 3282-3284.

11. Budavari, S. (1996). Propylene. The Merck Index, Twelfth Edition, New

Jersey: Merck & Co., 1348-1349.

12. Hollander, M.A.; Wissink, M.; Makkee, M. and Moulijn, J.A. (2002).

Gasoline conversion: reactivity towards cracking with equilibrated FCC and

ZSM-5 catalysts. Applied Catalysis A: General, 223(1), 85–102.

13. Choung, S.J. and Butt, J.B. (1990). The selectivity changes in 1-hexene

isomerization and its relation to acid properties of Pd/SAPO-11, SAPO-11,

AZSM-5, and H-Mordenite catalysts. Korean Journal of Chemical

Engineering, 7(3), 175-181.

14. Jung, J.S.; Kim, T.J. and Seo, G. (2004). Catalytic cracking of n-Octane over

zeolites with different pore structures and acidities. Korean Journal of

Chemical Engineering, 21(4), 777-781.

15. Tabak, S.A.; Krambeck, F.J. and Garwood, W.E. (1986). Conversion of

propylene and butylene over ZSM-5 catalyst, AIChE Journal. 32(9), 1526–1531.

16. Feller, A.; Zuazo, I.; Guzman, A.; Barth, J. O. and Lercher, J. A. (2003).

Common mechanistic aspects of liquid and solid acid catalyzed alkylation of

isobutane with n-butene. Journal of Catalysis. 216(1-2), 313-323.

17. Chen, J.W. and Cao, H.C. (2005). Catalytic cracking technology and

engineering. China Petrochemical Press, Beijing, Chapter 2, 154–159.

18. Buchanan, J.S.; Santiesteban, J.G. and Haag, W.O. (1996). Mechanistic

considerations in acid-catalyzed cracking of olefins. Journal of Catalysis,

158(1), 279–287.

Date post:	24-Aug-2020
Category:	Documents
Upload:	others
View:	0 times
Download:	0 times

DRASTIC ENHANCEMENT OF PROPENE YIELD FROM 1-HEXENE ...jestec.taylors.edu.my/Vol 4 Issue 4 December...

Documents