Reactant distribution, membrane reactors

It is evident, that in any membrane reactor operation mode there are important parameters which determine the performance of the process (Shah, Remmen and Chiang 1970). These are (1) the total and partial pressures on both sides of the membrane, (2) the total and partial pressure differences across the membrane, (3) the diffusion mechanism through the support and the membrane layer (membrane structure), (4) the thickness of the membrane, (5) the reactant configuration (i.e. whether the reactants are supplied from the same or from opposite sides of the membrane, in counter or co-current flow) and (6) the catalyst distribution. 

The core to the concept of an inorganic membrane reactor is to steer a membrane-selective species of a reaction mixture in or out of the membrane, which also serves as part of the reactor boundaries, to improve either the reaction conversion or product distribution. This is accomplished primarily by equilibrium displacement due to the permselective removal of a product or by avoiding or reducing undesirable side reactions (between certain chemical species including intermediate products) via controlled addition of some key reactant to the reaction zone through the membrane. 

FIGURE 10.21 (See color insert following page 588.) Traditional applications of inorganic membrane reactors for (a) conversion enhancement by product removal, (b) permeation of products and reaction coupling, and (c) selectivity enhancement by reactant distribution. 

It has been considered traditional applications of zeolite-membrane reactors those based on reactor concepts already demonstrated using mesoporous or dense membranes. These include conversion enhancement by equilibrium displacement or by the removal of inhibitors, and selectivity enhancement by reactant distribution. For such cases, the zeolite membrane is usually catalytically inert and is coupled with a conventional fixed bed of catalyst placed on one of the membrane sides. 

As discussed in Chapter 2, in a number of membrane reactor applications the membrane is non-permselective, and it simply acts as a contactor device (when it is catalytic), or simply as a means to distribute one of the reactants in a more uniform manner (when it is inert). In modeling such reactors one must take into consideration, in addition to Knudsen diffusion, the presence of molecular diffusion and convective transport. The Dusty Gas Model... 

The distributed-reactant membrane reactor has also been studied for several oxidative dehydrogenation reactions ethane to ethylene, propane to propylene and butane to butene. The results for these reactions have shown more promise, with higher yields for the membrane reactor when compared with a fixed bed, over certain ranges of the operating parameters. 

Zaspalis conducted a theoretical study of an asymmetric membrane with a thin, small-pore toplayer on a large-pore support, in both flat and tubular geometries for the simple isothermal reaction A - B. The best conversion was in the case where all of the catalyst was near the outer surface of the toplayer, on the reactant side. In fact, for this reaction, reactant loss meant that the membrane reactor did worse than the fixed bed reactor. He also claimed that the optimal distribution is a delta function, for both geometries, for all possible kinetics, when diffusion of a reactant occurs from one side only. For segregated reactants, for the reaction A + B - C as illustrated in Figure 22, the optimal location of the catalyst was at the toplayer/support interface, assuming that diffusion was the only transport mechanism through the support. 

In this chapter a specific type of membrane reactor, the so-called distributor was analyzed theoretically. In contrast to conventional tubular fixed-bed reactors (FBR), where all reactants are introduced together at the reactor inlet (cofeed mode), packed-bed membrane reactors (PBMR) allow dosing of one or several reactants via membranes over the reactor wall along the axial coordinate (distributed-feed mode). 

Catalytic membrane reactors belong to the class of structured reactors. The catalytic membrane performs several functions (e.g., separation, reactant distribution, catalytic and/or interface roles) in order to enhance the process productivity (in terms of conversion, selectivity or yield of desired products) and/or the process safety. Due to their unique property... 

As can be observed, the main difference between conventional three-phase reactors and catalytic membrane reactors hes in the relative positions of the mass transfer resistances with respect to the catalytic phase. In a conventional porous catalyst the catalytic sites in the pores have only one way or path of access. The gaseous reactant will encounter the first two mass transfer resistances at the gas-liquid interface, where the solvation equilibrium of the species from one phase to the other wiU take place. The dissolved species will diffuse towards the surface of the catalytic pellet for quite a long path in the hquid phase and will meet an additional mass transfer resistance at the hquid-sohd catalyst interface. It then needs to diffuse and react in the porous structure of the catalyst as well as the other reactant already present in the liquid phase. In the case of a traditional three-phase reactor (Fig. 4.3a), the concentration of at least one of the reacting species is hmited by its solubility and diffusion in the other fluid phase with a long diffusion path and in some cases unknown interfadal area (e.g., bubbles with variable size depending on the type of the gas feeding and distribution device in slurry reactors, not uniform phase contact and distribution in trickle-bed reactors). 

The thickness of the catalytic layer in a membrane reactor can be very low (e.g., in porous catalytic membranes usually 1-10 pm) compared to the pellet size of a traditional reactor (from 100 pm to few mm) and, as a consequence, depending on the specific reaction rate, the Thiele modulus can be low enough to achieve an intrinsic effectiveness of about 1, which corresponds to full and efficient catalyst utilization in the reactive process. Moreover, the distributed reactant feeds on the two sides of the catalytic layer improves the mass transfer of the reactants from the surface of the catalytic layer to the catalytic sites in the catalytic layer internal structure. 

Figure 12.25 Membrane reactors for FT synthesis from the literature (a) distributed feeding of reactants A and B, (b) in situ water removal by selective membrane (F, feed S, sweep), (cl) plug-through contactor membrane (PCM) with wide transport pores, (c2) forced-through flow membrane contactor, product and heat removal by circulated liquid product, (d) zeolite encapsulated FT catalyst, P, modified product [123].

Important advantages of processes in membrane reactors, as compared to conventional reactors, include the shift of the reaction equUibrium to the product side, enhanced conversion, prevention of side reactions, distributive feeding of reactants to allow for more control on reaction selectivity and integration of the separation step in one unit. Packed bed membrane reactors have been very popular as a research topic (see, e.g., Andres et al., 2011 ... 

Controlled addition of one of the reactants in a bimolecular reaction using an IMR-P There are several instances of industrial organic reactions that are bimolecnlar and exothermic. An important example is the production of chloromethanes. The temperature rise can be controlled by axially distributed addition of chlorine at several discrete points into a packed bed, fluidized bed, or empty tube reactor through which methane is passed (Doraiswamy et al., 1975). The membrane reactor... 

This chapter reviews the possibilities that the application of a membrane in a catalytic reactor can improve the selectivity of a catalytic oxidation process to achieve a more compact system or to otherwise increase competitiveness. Classification differentiates between those reactors using dense membranes and those using porous membranes. Dense membranes provide high selectivity towards oxygen or hydrogen and the selective separation of one of these compounds under the reaction conditions is the key element in membrane reactors using such membranes. Porous membranes may have many different operation strategies and the contribution to the reaction can be based on a variety of approaches reactant distribution, controlled contact of reactants or improved flow. Difficulties for the application of membrane reactors in industrial operation are also discussed. 

Diakov, V. and Varma, A. (2002). Reactant Distribution by Inert Membrane Enhances Packed-Bed Reactor Stability, Chem. Eng. Sci, 57, pp. 1099-1105. 

Rihko-Struckmann L K,Munder B,Chalakov L and Sundmacher K (2010), Solid electrolyte membrane reactors , in Seidel-Morgenstem A, Membrane Reactors, Distributing Reactants to Improve Selectivity and Yield, Weinheim, Wiley-VCH Verlag, 193-233. 

---