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ABSTRACT
This study presents a 2D-axisymmetric computational fluid dynamic - CFD - to investigate the performance of Pd-based membrane reactor - MR - during isobutane dehydrogenation reaction for hydrogen and isobutene production. The proposed CFD model provides the local information of velocity, pressure and component concentration for the driving force analysis. The validation of model results was carried out by experimental data and a good agreement between model results and experimental data was achieved. In MR model, a commercial Cr2O3/Al2O3 catalyst in reaction zone was considered. The effects of some important operating parameters such as reaction temperature and reaction pressure on the performance of Pd-based MR were studied in terms of isobutane conversion and hydrogen recovery. The CFD results showed that the Pd-based MR during isobutane dehydrogenation reaction presents higher performance in terms of isobutane conversion and hydrogen recovery with respect to fixed-bed reactor - FBR - , in all the studied cases - 34% isobutane conversion enhancement at 800 K - .
1. INTRODUCTION
The demand for olefins and olefinic products continues to increase and commercially has resulted in a number of catalytic dehydrogenation processes[1]. Current processes for propene and butenes production from the corresponding alkanes employ mainly fixed bed operation, but because of the necessarily high temperatures involved in these reversible endothermic reactions, catalyst coking rapidly occurs, so that frequent catalyst regeneration is required [2]. To overcome this problem a number of alternative procedures have been suggested. An alternative is the use of a membrane reactor - MR - . In recent years, the possibility of overcoming the equilibrium constraint in reversible reactions such as dehydrogenations has attracted wide attention [2]. Separation of the hydrogen product permits a higher conversion or better selectivity to be achieved. Alternatively, operation may be made under less severe conditions with a consequent reduction in the extent of coking and less need to regenerate the catalyst.
Porous membranes such as glass and alumina have been used to study the catalytic dehydrogenation of hydrocarbons and conversions as high as two to seven times the equilibrium level has been observed for some reactions [2, 3]. However, these membranes exhibit poor selectivity, due largely to the nature of the flow process in their pore structure. It has been demonstrated both experimentally and theoretically that the MRs, inside which dense palladium or palladium alloy membranes permeable only to hydrogen are equipped to remove the hydrogen produced [4, 5], can provide high performance for dehydrogenation even in a single stage[6, 7]. Isobutane dehydrogenation using catalytic MR has been studied previously by a number of researchers. Ionnides and Gavalas [8] used a dense silica membrane reactor and obtained increases in selectivity and yield over a fixed bed reactor - FBR - at higher space times. Matsuda et al. [9] employed a palladium MR and tested both a chromia/alumina and a platinum alumina catalyst. The isobutene yield for both was greater than for a conventional reactor. Bernald et al. [10] used a membrane reactor with a very thin palladium film, while Casanave et al. used both microporous and zeolite membrane reactors for this reaction [11]. Further zeolite MR results by Ciavarella et al. [12] discussed the effect of operating conditions.
Moreover, concerning to high cost of experimental works, numerical models [13, 14] could be useful to avoid high experimental costs and to develop a better understanding of the effects of various parameters for the design and study of Pd-based MRs in the isobutane dehydrogenation and also specific features and constraints like the necessity of obtaining a high purity hydrogen stream. To this purpose, computational fluid dynamic - CFD - tool is a feasible method to simulate detailed gas flow characteristics at any point of a membrane system. Indeed, the CFD approach can be used for virtual prototyping of chemical reactors and separators and since it is based on control volume methodology, the local variations of the fluid, thermal and mass transport properties can be visualized in comparison to simple models and can be used to design the MRs [15]. In the present work, as a first approach, a dense Pd-based MR performance was modeled using the CFD method to investigate effects of the most important operating parameters in comparison with the FBR system during the isobutane dehydrogenation reaction. Then, A set of simulation results is provided illustrating some significant points about the dense Pd-based MRs performance in terms of isobutane conversion and hydrogen recovery for isobutane dehydrogenation.
2. Development of CFD model
Fig. 1 shows a simple scheme of Pd-based MR performance for hydrogen production during isobutane dehydrogenation reaction. Fig. 1. Schematic of fixed-bed catalytic MR for isobutane dehydrogenation.
Moreover, the main assumptions of this CFD model are summarized as follows:
- 1 - Isothermal conditions;
- 2 - plug flow in both the feed and permeate sides;
- 3 - No axial or radial diffusion;
- 4 - Permeation through the membrane is proportional to the difference in partial pressures between the feed and permeate sides;
- 5 - Dehydrogenation reactions take place only on the catalysts packed in the feed side.
2.1. Governing equations
Briefly, the CFD model is mathematically expressed by the governing equations consisting of continuity equation - Eq. - 2 - - , momentum balance - Eq. - 3 - - and species transport reaction equation - Eq. - 4 - - :
where mi is the mass fraction of species i, Rj the reaction rate with a corresponding stoichiometric coefficient vij , WKH YRLG IUDFWLRQ RI WKH SDFNHG EHG DQG 6H2 the source/sink term of hydrogen component. For the reaction zone - Fig.
1 - , which is packed with catalyst pellets, the model includes the reaction terms - Rj denotes the rate of a reaction j - , the sink term - SH2 - that accounts for H2 UHPRYDO E\ WKH PHPEUDQH' DQG WKH IULFWLRQ WHUP - .X - WR DFFRXQW IRU SUHVVXUH ORVVHV DORQJ WKH SDFNHG EHG. 7KHUH LV QR FDWDO\VW LQ WKH SHUPHDWH ]RQHV - 1 - DQG' therefore, there is no reaction there - Rj =0 - . The source/sink term that accounts for mass flow of H2 across the dense Pd-based membrane is given by Eq. - 6 - ,
where H2-permeate is the hydrogen molar flow rate that permeates through the Pd-based membrane, H2-retentate the hydrogen molar flow rate in retentate side. It should be noted that the Eq. - 14 - is related to the Pd-based MRs simulation.
2.2. Numerical method
Numerical simulations were performed using the commercial CFD package COMSOL Multiphysics 5.3 the finite element method was used to solve the governing equations in the two-dimensional CFD model for present work.
