Design of Membrane Reactors

The design equations of a membrane reactor consist of the mass, heat, and momentum balances in combination with the membrane permeation and the reaction kinetics conducted on the feed side and permeate side, respectively. Table 9.1 summarizes the design equations to describe the phenomena occurring in membrane reactors. Among these equations. 

Inorganic Membrane Reactors Fundamentals and Applications, First Edition. Xiaoyao Tan and Kang Li. 2015 John Wiley Sons, Ltd. Published 2015 by John Wiley Sons, Ltd. 

Mass balance MR 2 X number of species Conversion, selectivity, 

Heat balance configuration MR in the reaction system 2, for both the feed and yield Temperature profiles 

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It should be evident, therefore, that the design of membrane reactors is largely concerned with finding membranes that have sufficient area, catalytic activity, mechanical strength, and freedom from cracks, holes, and clogging. 

Some new work on assessing the limits of membrane reactors, and on comparing them to other reactor configurations, appeared at the ISCRE-15 conference. McGregor et a/.have extended attainable region theory to include separation processes. This work aims to synthesize the structures of optimal reactor-separator networks, which has implications for the design of membrane reactors. The question of what conversion is achievable in a membrane reactor has been revisited,with identification of operating conditions at which maximum conversion occurs. 

The economic analysis reported in this chapter with the case studies already described will offer an idea of the methods that can be applied to the project and design of membrane reactors and bioreactors. New investments, as well as the introduction of bioreactors in existing plants, have been taken into account with the aim of offering a general view of their potentialities also from an economic point of view. 

In summary, the design of membrane reactors for electrochemical desulfurization of transportation fuels should take into consideration the issues shown in Figure 14.10. 

In addition to the reaction kinetics and the membrane permeation parameters, values of the mass and heat transfer coefficients are necessary for the design of membrane reactors. These can be calculated using the local physical properties of the reaction system (Table 9.2). 

Figure 11.4 Schematic diagram of membrane reactor. The reaction is designed so that reagent X may pass through the membrane and is therefore available for reaction. The substrate and product cannot pass through and the phases remain separate...

Furthermore, it can be shown that, in the limiting cases of first-order kinetics [Equation (11.35) also holds for this case] and zero-order kinetics, the equal and optimal sizes are exactly the same. As shown, the optimal holding times can be calculated very simply by means of Equation (11.40) and the sum of these can thus be used as a good approximation for the total holding time of equal-sized CSTRs. This makes Equation (11.31) an even more valuable tool for design equations. The restrictions are imposed by the assumption that the biocatalytic activity is constant in the reactors. Especially in the case of soluble enzymes, for which ordinary Michaelis-Menten kinetics in particular apply, special measures have to be taken. Continuous supply of relatively stable enzyme to the first tank in the series is a possibility, though in general expensive. A more attractive alternative is the application of a series of membrane reactors. 

Under certain conditions, scale-up of membrane reactors is straightforward. Provided that (i) the reactor contents are well mixed so that the reactor is operated as a CSTR, and that (ii) the membrane is configured for filtration in the tangential mode, the pertinent design criterion, besides constant residence time T in the reactor, is constant fluidity F of the substrate/product solution through the membrane at all reactor scales. Fluidity is defined by Eq. (19.36) (V = ultrafiltered volume, AP = transmembrane pressure, t = filtration time, and A = membrane area). 

Onstot, W.J., Minet, R.G. and Tsotsis, T.T. (2001) Design aspects of membrane reactors for dry reforming of methane for the production of hydrogen. Industrial e[ Engineering Chemistry Research, 40, 242-251. 

Finally, possible causes for deactivation of catalytic membranes are described and severad aspects of regenerating catalytic membrane reactors are discussed. A variety of membrane reactor configurations are mentioned and some unique membrane reactor designs such as double spiral-plate or spiral-tube reactor, fuel cell unit, crossflow dualcompartment reactor, hollow-fiber reactor and fluidized-bed membrane reactor are reviewed. 

Yoshida, H. et al., Preliminary design of fusion reactor fuel clean-up system hy the palladium alloy membrane method, Nucl. Technol./ Fusion, 3, 471, 1983. 

Takeuchi et al. 7 reported a membrane reactor as a reaction system that provides higher productivity and lower separation cost in chemical reaction processes. In this paper, packed bed catalytic membrane reactor with palladium membrane for SMR reaction has been discussed. The numerical model consists of a full set of partial differential equations derived from conservation of mass, momentum, heat, and chemical species, respectively, with chemical kinetics and appropriate boundary conditions for the problem. The solution of this system was obtained by computational fluid dynamics (CFD). To perform CFD calculations, a commercial solver FLUENT has been used, and the selective permeation through the membrane has been modeled by user-defined functions. The CFD simulation results exhibited the flow distribution in the reactor by inserting a membrane protection tube, in addition to the temperature and concentration distribution in the axial and radial directions in the reactor, as reported in the membrane reactor numerical simulation. On the basis of the simulation results, effects of the flow distribution, concentration polarization, and mass transfer in the packed bed have been evaluated to design a membrane reactor system. 

By appropriately designing the membrane reactor, the possibility of decreasing the reactor volume to a given, required capacity with respect to that of a conventional unit or conversely increasing the capacity given the reactor volume is equally important. In addition, the energy balance can be improved considerably using membrane reactors, as reported by several authors. 

M. Tashimo, et. al., Advanced Design of Fast Reactor - Membrane Reformer , Proceedings of OECD/NEA Second Information Exchange Meeting on Nuclear Production of Hydrogen, Argonne USA, 2-3, October 2003, p.267 (2003). 

The authors would like to thank Dr. Srin Babu, Dr. Naresh Nayyar, Steven Lee, James Stein in CRD and Dr. Barbara Potts, Christine Albizati, Jason Ewanicki in ARD for experimental support. We also thank Dr. Andreas Liese at the Institute of Biotechnology, Julich, Germany for helpful suggestions in designing a membrane reactor. Finally, we thank Dr. Kim Albizati and Dr. Van Martin for fruitful discussions in preparing this manuscript. 

O. Wolfrath, L. Kiwi-Minsker, A. Renken, Filamenteous catalytic beds for the design of membrane micro-reactor propane dehydrogenation as a case study, in M. Matlosz, W. Ehrfeld, J.P. Baselt (Eds.), Proceedings of the 5th International Conference on Microreaction Engineering (IMRET 5), Springer, Berlin, 2001, p. 191. 

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