Study of Propane Dehydrogenation to Propylene in an Integrated

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Study of Propane Dehydrogenation to Propylene in an Integrated Fluidized Bed Reactor Using Pt-Sn/Al-SAPO-34 Novel Catalyst Zeeshan Nawaz, Yue Chu, Wei Yang, Xiaoping Tang, Yao Wang, and Fei Wei* Beijing Key Laboratory of Green Chemical Reaction Engineering & Technology (FLOTU), Department of Chemical Engineering, Tsinghua UniVersity, Beijing 100084, China

Direct propane dehydrogenation is the most economical route to propylene, but very complex due to endothermic reaction requirements, equilibrium limitations, stereochemistry, and engineering constraints. The state of the art idea of bimodal particle (gas-solid-solid) fluidization was applied, in order to overcome alkane dehydrogenation reaction barriers in a fluidized bed technology. In this study, the propane dehydrogenation reaction was studied in an integrated fluidized bed reactor, using Pt-Sn/Al-SAPO-34 novel catalyst at 590 °C. The results of fixed bed microreactor and integrated bimodal particle fluidized bed reactors were compared and parametrically characterized. The results showed that the propylene selectivity is over 95%, with conversion between 31 and 24%. This significant enhancement is by using novel bimodal particle fluidization system, owing to uniform heat transfer throughout the reactor and transfer coke from principal catalyst to secondary catalyst, which increases principal catalyst’s stability. Experimental investigation reveals that the novel Pt-Sn/Al-SAPO-34 catalyst and proposed intensified design of fluidized bed reactor is a promising opportunity for direct propane dehydrogenation to propylene, with both economic and operational benefit. Introduction Light olefins production from alkane dehydrogenation has been in practice since 1930.1 At first chromia-alumina and then platinum based catalysts got preference in alkane dehydrogenation. Time and time again, catalyst design breakthroughs have made major contributions to dehydrogenation technology. Propane and butane are cheep and easily available raw materials as they are produced through a number of petrochemical processes, while the propylene market demand is rapidly increasing.2-4 The petrochemical industry is trying to shift toward direct propane dehydrogenation technology as it does not require a large investment and has room for easy integration with existing production facilities. The first butane dehydrogenation plant was designed by UOP (Universal Oil Products, USA) and ICI, England, in 1940.5 Soon after, other companies, Phillips Petroleum, Houdry, Shell, Gulf, and Dow, also built similar dehydrogenation technologies. Phillips Petroleum built a multitubular dehydrogenation reactor in 1943, with an oxidehydrogenation approach.1 Houdry designed dehydrogenation process at less than atmospheric pressure for higher conversions, for the production of butenes using chromia-alumina catalyst (Catadiene).6 Later this process was commercialized by the Petro-Tex Chemical Corporation, with the name Oxo-D.6 A varity of light olefins were produced by thermal and catalytic cracking in bulk quantity as byproduct, forces to shutdown direct olefins production in the 1970s. In the late 1980s, the application of chromia-alumina catalysts was extended by Houdry for dehydrogenation of propane and isobutene; they renamed the process Catofin, and about ten units were commercialized.7 Out of these, two processes were particularly designed for propane dehydroegenation to propylene, of about 250 000 MTA propylene capacities. The Catofin technology used an adiabatic fixed bed reactor at 570-630 °C and 0.5 bar, reported at 40-65% conversion.1,7 The Catofin technology is currently owned by Sud-Chemie and license by ABB Lummus. Phillips Petroleum’s, * To whom correspondence should be addressed. E-mail: wf-dce@ tsinghua.edu.cn.

STAR Krupp-Uhde (steam active reforming) process based on a fixed-bed fired-tube reactor, operating at a positive super atmospheric pressure and isothermal temperature conditions.8 During the 1990s, a fluidized bed iso-butane dehydrogenation unit for about 450 000 MTA iso-butylene was commercialized by Snamprogetti in Saudi Arabia, based on Yarsintez (Russian) technology.9-11 UOP commercialized a typical radial flow adiabatic fixed-bed (or slowly moving bed) reactor design using modified Pt-alumina catalysts for alkane dehydrogenation, but the performance is unsatisfactory due to number of reasons.1 BASF and State Oil also built pilot plants for propane dehydrogenation named Linde and Sintef, respectively. Recently, Mitsubishi Chemical, Japan, also claimed a novel process for oxidative dehydrogenation of alkane by using a fixed bed reactor, but no information was available through open resources.12 On the other hand, Pt-Sn-based catalysts supported on amorphous (Al2O3, SiO2, etc.) and zeolite (ZSM-5, SAPO-34, etc.) supports were discussed in many studies, and promising results have been reported.13-18 It is believed that the support has a very important role in stabilizing the activity and performance. A number of drawbacks were also observed due to the supports; those affect the catalyst performance in a distinct manner. Al2O3 supported catalyst has very short lifetime (quickly deactivated).19,20 The dehydrogenation performance of Pt-Snbased catalysts depends largely on Pt, Sn, and support interaction, and deactivation occurs due to aggregation/sintering of Pt particles.21 Given the rapid development of the Chinese petrochemical industry in last two decades, the total production capacity of zeolites is more than 12 000 t/y. Therefore, the PtSn/ZSM-5 zeolite catalyst has been developed and a number of attempts have been made to improve Pt-Sn/ZSM-5 performance by incorporating more metallic promoter, like Na, Zn, La, Ca, Ce, etc., and/or by increasing the Si/Al ratio.22-26 However, the performance of ZSM-5 zeolite supported bimetallic catalysts is still objectionable due to the effect of frequent regenerations and to it taking part in cracking to some extent.27 A highly selective catalyst, Pt-Sn/SAPO-34 gives a new technological trend in light olefin production via the direct

10.1021/ie902043w  2010 American Chemical Society Published on Web 04/07/2010

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Table 1. XRF Analysis of Pt-Sn/Al-SAPO-34 Catalyst SAPO-34 supported

Pt content (wt %)

Sn content (wt %)

Al content (wt %)

Pt-Sn/Al

0.47

0.86

17.6

dehydrogenation route.13,14 The advantages of the novel Pt-Sn/ SAPO-34 catalyst is its better stereochemistry control over propane and butane dehydrogenation to propylene.13,14 The SAPO-34 supported catalyst is inherently resistant toward hydrothermal treatment, having higher selectivity for light olefins due to shape selectivity.2,4 No study to date has focused on the Pt-Sn/Al-SAPO-34 novel catalyst and an intensified bimodal particle fluidized bed reactor. Bimodal particle fluidized bed reactor technology is proposed and applied first time for alkane dehydrogenation to propylene. The study is conceived to explore reaction engineering pedagogy of propane dehydrogenation using fluidized bed technology. Experimental Section Catalyst Preparation. The SAPO-34 support was prepared by mixing Al2O3:P2O5:SiO2:TEA:H2O in the molar ratio of 1:1:0.5:2:100.2,4 The catalyst was palletized with a binder Al (20 wt %). The bimetallic Pt-Sn-based samples of specific metallic content were prepared by a sequential impregnation method with calcined SAPO-34 (BET surface area 441 m2/g).13,14,28,29 The support after palletizing was first impregnated with an aqueous solution of SnCl2 · 2H2O at 80 °C, to dope 1 wt % Sn in the catalyst. After impregnation, the samples were dried at 110 °C for 3 h and calcined at 500 °C for 4 h. Later the Sn doped SAPO-34 was coimpregnated again with an aqueous solution of H2PtCl6 at 75 °C to give a 0.5 wt % Pt in the final catalyst. The final composition was confirmed by X-ray fluorescence (XRF) analysis using a Shimadzu XRF 1700 fluorimeter. The results are shown in Table 1. The catalysts were dechlorinated at 500 °C for 4 h with N2 mixed dilute steam and then reduced under flowing H2 (8 mL/min) at 500 °C, for 8 h. Feed. The 99.5% pure propane provided by Zhong Ke Hui Jie (HJAT), Beijing, China, was used as feed. The reaction

Figure 2. Performance comparison of novel catalyst in fixed bed with proposed GSS-fluidized bed reactor. Table 2. Deactivation Rate and Amount of Coke Formed on Principal Catalyst (Pt-Sn/Al-SAPO-34) reactor fixed bed fluidized Bed

coke (wt %)a

deactivation (%)b

0.41 0.24

54 45

a O2-pulse coke analysis. b Deactivation ) [(X0 - Xf)/X0 × 100], where, X0 is the initial conversion at 5 min and Xf is the final propane conversion.

Figure 1. Hot-model bimodal particle fluidized bed reactor (FBR) apparatus.

mixture composed of H2 and C3H8 was charged into the reactor. The mixture was composed of H2/C3H8 molar ratios 0.5, 0.15 and 0.25. Experimental Setup. The performance of the prepared PtSn/Al-SAPO-34 catalyst sample was first confirmed at the fixedbed microreactor. Measured amounts of catalyst samples were loaded into the reactor in order to obtain the desired WHSV (i.e., 3, 5.6, and 9) and operated at 590 °C. The operating fluidization velocity is between 0.1 and 0.3 m/s. The technology and design features of GSS-FBR can be found elsewhere. The optimum operating parameters were identical with our previously studied system.30 The product distribution was analyzed by an online gas chromatography system having an Al2O3

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Figure 3. Influence of WHSV on catalysts performance in a novel bimodal particle (gas-solid-solid) fluidized bed reactor.

capillary column equipped with a flame ionization detector (FID). The systematic design of the microrector setup can be find elsewhere.24,31 All the values were calculated in weight percentages and using following relationships. conversion of propane (%) ) [propane in feedstock (wt %) propane in product (wt %)] × 100/propane in feedstock(wt %) propylene selectivity (%) ) [propylene in products (wt %) propylene in products (wt %)] × 100/ [propane in feedstock (wt %) - propane in feedstock (wt %)] A pilot hot model fluidized bed reactor was designed as shown in Figure 1, including a feeding system, reaction-regeneration system, and a product sampling system. The fluidized bed reactor is a 180 mm long, 20 mm i.d, stainless steel tube placed coaxially in a furnace coil. The operation specification of this apparatus are as follows: feed rate of gas 2-10 L/min, catalyst reserve 10-50 g, reaction (in accordance with WHSV) temperature 580-600 °C, H2/C2H8 molar ratio 1-3, WHSV 2.9-9, regeneration temperature with steam 500-650 °C. Results and Discussion The experimental results of propane dehydrogenation using novel catalyst Pt-Sn/Al-SAPO-34 were compared for fixed bed microreactor and pilot scale fluidized bed reactor. The results are shown in Figure 2. The influence of fluidization mode in an integrated bimodal particle fluidized bed was investigated for propane dehydrogenation to propylene. It is observed that the propylene selectivity in a fluidized bed reactor was improved a lot after 1 h operation, when the reactor reaches steady state conditions. The steady and uniform conversion and yield is also achieved. Actually, in fixed bed reactor coke deposition is high as compared to fluidized bed reactor (see Table 2). In the two particles cofluidized system, it was observed that the coke

deposited on SAPO-34 (fine catalyst particles) is higher than the metal incorporated SAPO-34 (principal catalyst). Therefore, it is easy for principal dehydrogenation catalyst (Pt-Sn/AlSAPO-34) to sustain its activity for longer duration. Moreover, in the continuous processing the small catalysts (those serve as heat carrier) were continuously regenerated, and the process efficiency was improved. Above 96% propylene selectivity was obtained at 8 h timeon-stream. Sustainable conversion with lower deactivation rate (see Table 2) was also observed. The lower propylene yield initially was due to lower conversion and selectivity, which increased gradually with time. Therefore we can say that the impressive results were obtained using this integrated fluidized bed reactor. Moreover, in fluidized the bed we tried to maintain our previously explored optimum operating values.30 It was interesting to find that the reaction stability and activity of Pt(0.5 wt %)-Sn(1 wt %)/Al(20 wt %)-SAPO-34 catalysts is superior, but also superior in coke management. The deactivation trends in micro and pilot scale fluidized-bed reactors and the amount of coke were measured by O2-pulse analysis. The results are shown in Table 2. It is generally believed that the coke on the platinum is responsible for the deactivation of the catalyst. Nevertheless, deactivation and/or activity loss of the bimetallic catalysts is due to coke deposition and Pt sintering.14,30 But it is also known now that the presence of promoters facilitate intermediate species adsorption and coke transfer from active Pt sites to support, and ultimately enhance catalyst performance.30 Therefore, in coke analysis of bimodal particle fluidized bed catalyst, it was noted that a large amount of coke is deposited over nonmetallic SAPO-34, that is in a continuous recirculation through the regenerator, in a continuous setup. It is an effective way to protect catalyst activity for longer times with stable activity, and so-called coke management. Since the hydrodynamic behavior of the fluidized bed is quite different from the fixed bed and the bimodal particle fluidization system is much more complex in operation, variation in weight

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Figure 4. Influence of H2/C3H8 molar ratio on catalyst performance in a fluidized bed reactor.

Figure 5. OPE with respect to yield and selectivity.

hourly space velocity (WHSV) has been investigated. The relationship between time on stream (TOS), selectivity, and reaction rates at different WHSV’s are shown in Figure 3.

Varying WHSV from 5.6 h-1 leads to significant decrease in conversion. Moreover, propylene selectivity somehow suffers at the cost of changing fluidization properties, as we adjust

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Table 3. Influence of Hydrothermal Treatment and Catalyst Performance in Continuous Operationa cycle I Al-SAPO-34 supported

a

conversion

cycle II selectivity

conversion

cycle III selectivity

conversion

selectivity

TOS

1h

8h

1h

8h

1h

8h

1h

8h

1h

8h

1h

8h

Pt-Sn-based

29.1

26.2

85.4

96.8

28.3

25.5

85.9

96.9

27.2

24.6

87.9

97.4

Reaction conditions: T ) 590 °C. WHSV ) 5.6 h-1. H2/C3H8 molar ratio ) 0.25.

WHSV as a function of catalyst weight. At higher WHSV above 5.6 h-1, selectivity shows almost identical trends. Therefore, well-mixed fluidization is particularly important for integrated operation. It is known that the presence of hydrogen prevents the catalyst from coke formation and maintains catalytic activity without affecting the secondary propylene formation reaction.14,30 PtSn supported on Al-SAPO-34 exhibited good stability and a relatively lower rate for hydrogen transfer reactions during the propane dehydrogenation. While the optimum H2/C3H8 molar ratio of fixed bed may be affected due to equilibrium in fluidized bed reactor. In order to remove ambiguity, different H2/C3H8 molar ratios were tested in a fluidized bed reactor. The results are shown in Figure 4. It is observed that the operation with in the H2/C3H8 molar ratio 0.15-0.25 is acceptable; while below this range, both selectivity and conversion drop. The overall picture of selective propane dehydrogenation to propylene over Pt(0.5 wt %)-Sn(1 wt %)/SAPO-34 at 590 °C is shown in the OPE plot in Figure 5. The data was plotted with respect to yields and selectivity. The best propane conversion range to have high propylene yield and selectively is observed to be between 24 and 28% conversion. In the designated operating range, the propylene yield is above 25% and selectivity is as high as 96%. While at higher conversions, both propylene yield and selectivity dropped sharply, with the increase in ethane formation. It is further noted that the higher conversion favors both cracking and a hydride transfer reaction with the decrease in dehydrogenation rate. Moreover, the deactivation of the catalyst may also lead to cracking. The performance of the Pt-Sn/Al-SAPO-34 is evaluated in a continuous mode of reaction-regeneration for three cycles. The results are shown in Table 3. The catalysts were regenerated with nitrogen mixed steam for 4 h at 600 °C. After regeneration, the Pt was redispersed using C2Cl2H4 solution, injected with nitrogen at 500 °C. The detailed chlorination method can be found elsewhere.28,29 After the regeneration and redispersion of Pt, the catalyst was reduced in hydrogen environment, and reused for next reaction cycle at identical conditions. The results clearly demonstrate hydrothermal stability of the catalyst. Therefore, the robustness of the proposed design of the bimodal particle (gas-solid-solid) fluidized bed reactor and Pt-Sn/AlSAPO-34 is successfully proved. Conclusion In this study we experimentally investigate the performance of Pt-Sn/Al-SAPO-34 novel catalyst in an integrated bimodal particle fluidized bed reactor, and compared these results with fixed bed microreactor. Uniform heat transfer with desired catalyst contact was achieved in the gas-solid-solid fluidized bed design. The stable activity was obtained in an intensified gas-solid-solid fluidized bed reactor, as maximum coke was transferred from principal catalyst (Pt-Sn/Al-SAPO-34) to secondary heat carrier (SAPO-34) catalyst. The operational optimization is explored experimentally over a range of operating parameters for superior catalytic performance. WHSV 5.6 h-1 is found to be optimum. High propylene selectively (above 95%) and yield (above

24%) from propane dehydrogenation is obtained in H2/C3H8 molar ratio 0.15-0.25. The detailed OPE of fluidized bed reactor suggested that the best conversion range is 24-28%. It is further confirmed from continuous reaction-regeneration cycles that the catalyst is hydrothermally stable with recoverable activity, and sophisticated in operation using proposed gas-solid-solid fluidized bed reactor (GSS-FBR) technology. Acknowledgment This research was supported by the Higher Education Commission, Islamabad, Pakistan (2007PKC013), and Natural Scientific Foundation of China (Nos. 20606020, 20736004, and 20736007). Literature Cited (1) Bhasin, M. M.; McCain, J. H.; Vora, B. V.; Imai, T.; Pujado., P. R. Dehydrogenation and oxydehydrogenation of paraffins to olefins. Appl. Catal. A. Gen. 2001, 221, 397. (2) Nawaz, Z.; Tang, X. P.; Zhu, J.; Wei, F.; Naveed, S. Catalytic cracking of 1-hexene to propylene using integrated SAPO-34 catalysts topologies. Chin. J. Catal. 2009, 30, 1049. (3) Zhou, H. Q.; Wang, Y.; Wei, F.; Wang, D. Z.; Wang, Z. Kinetics of the reactions of the light alkenes over SAPO-34. Appl. Catal. A. Gen. 2008, 348, 135. (4) Nawaz, Z.; Tang, X. P.; Wei, F. Hexene catalytic cracking over 30% SAPO-34 catalyst for propylene maximization: Influence of Reaction Conditions and Reaction Pathway Exploration. Braz. J. Chem. Eng 2009, 26, 705. (5) Hornaday, G. F.; Ferrell, F. M.; Mills, G. A. Manufacture of monoand diolefins from paraffins by catalytic dehydrogenation. In AdVances in Petroleum Chemistry and Refining; InterScience: Pans, 1961; Vol. 4. (6) Waddams, A. L. Chemicals from Petroleum, 4th ed.; Gulf Publishing Company: Houston, 1980. (7) Craig, R. G.; Spence, D. C. Catalytic dehydrogenation of liquefied petroleum gas by the Houdry Catofin and Catadiene processes. Handbook of Petroleum Refining Processes; Meyers, R. A., Ed.; McGraw-Hill: New York, 1986. (8) Dunn, R. O.; Brinkmeyer, F. M.; Schuette, G. F. The Phillips STAR process for the dehydrogenation of C3, C4, and C5 paraffins. Proceedings of the NPRA Annual Meeting, New Orleans, LA, 1992, pp 22-24. (9) Sanfilippo, D.; Buonomo, F.; Fusco, G.; Miracca, I. Paraffins Activation through Fluidized Bed Dehydrogenation: the Answer to Light Olefins Demand Increase, Elsevier, Amsterdam. Stud. Surf. Sci. Catal. 1998, 119, 919. (10) Iezzi, R.; Bartolini, A. Process for dehydrogenating light paraffins in a fluidized bed reactor. U.S. Patent 5,633,421, May 27, 1997; assigned to Snamprogetti. (11) Luckenbach, E. C.; Zenz, F. A.; Papa, G.; Bertolini, A. Fluidized bed reactor and process for performing reactions therein. U.S. Patent 5,656,243, August 12, 1997; assigned to Snamprogetti. (12) Setoyama, T. Recent topics on catalyst research at Mitsubishi Chemical. 17th Saudi Arabia-Japan Joint Symposium, Dhahran, Saudi Arabia, November 11-12, 2007. (13) Nawaz, Z.; Wei, F. Pt-Sn-Based SAPO-34 Supported Novel Catalyst for n-Butane Dehydrogenation. Ind. Eng. Chem. Res. 2009, 48, 7442. (14) Nawaz, Z.; Tang, X. P.; Zhang, Q.; Wang, D. Z.; Wei, F. SAPO34 supported Pt-Sn-based novel catalyst for propane dehydrogenation to propylene. Catal. Commun. 2009, 10, 1925. (15) Liersk, H.; Volter, J. State of tin in Pt-Sn/Al2O3 reforming catalysts investigated by TPR and chemisorption. J. Catal. 1984, 90, 96. (16) Barias, O. A.; Holmen, A.; Blekkan, E. A. Propane Dehydrogenation over Supported Pt and Pt-Sn Catalysts: Catalyst Preparation, Characterization, and Activity Measurements. J. Catal. 1996, 158, 1.

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ReceiVed for reView December 23, 2009 ReVised manuscript receiVed February 24, 2010 Accepted March 6, 2010 IE902043W