Nonselective membrane reactors

A packed-bed nonpermselective membrane reactor (PBNMR) is presented by Diakov et al. [31], who increased the operational stability in the partial oxidation of methanol by feeding oxygen directly and methanol through a macroporous stainless steel membrane to the PB. Al-Juaied et al. [32] used an inert membrane to distribute either oxygen or ethylene in the selective ethylene oxidation. By accounting for the proper kinetics of the reaction, the selectivity and yield of ethylene oxide could be enhanced over the fixed-bed reactor operation. 

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A schematic of a reactor made from a nonselective membrane for preventing the slip of an excess reactant is shown in Figure 24.2g. The principle of this reactor was outlined before. In the particular design shown, one of the reactants (5) is continuously recirculated on one side of the membrane so that complete conversion of A can be achieved on the opposite side without any slip. We refer to such a catalytic nonselective membrane reactor without packing as a CNMR-E. When packed, it is referred to as a CNMR-P. Another nonselective... 

Catalytic nonselective membrane reactor with no packing (CNMR)... 

New results on the improvement of membrane reactors developed and applied to prevent nonselective oxidation by oxygen are still reported.550 551 For example, yttria-doped Bi2C>3 gives C2 selectivity about 30% higher at the same C2 yield than that found in the co-feed fixed-bed reactor.550 The best values are 17% yield with 70% selectivity. 

Catalytic Nonselective Hollow Membrane Reactor for Multiphase Reactions (CNHMR-MR)... 

Due to the availability of the mentioned overviews it is not the goal of this chapter to consider the whole field of membrane reactors. Rather, the discussion below will be focused on presenting simplified and more detailed mathematical models capable of describing the performance of membrane reactors. Although there are several studies available for analyzing the combination of reaction and membrane separation (e.g. Salomon et al., 2000 Struis and Stucki, 2001 Wielandet al., 2002 Patil etal., 2005 Rohde etal., 2005) there is a need to analyze in more detail specific features of membrane reactors. The focus of this chapter will be the development and application of simplified and also more detailed mathematical models for packed-bed membrane reactors in which certain reactants are dosed over the reactor wall using nonselective membranes. This type of membrane reactor is sometimes also-called a distributor (Dalmon, 1997 Julbe et al, 2001). Despite this restricted focus of the work, most of the concepts considered should be applicable also in the analysis of other types of membrane reactors. 

Figure 13.2 Types of membrane reactors, (a) IMR-P, (b) IMMR-P, (c) hollow membrane tube reactor with catalyst in shell (another version of IMR-P), (d) fluidized-bed Inert selective membrane reactor (IMR-F), (e) CMR-E, (f) CMR-P, (g) catalytic nonreactive membrane reactor (CNMR-E), (h) catalytic nonselective hollow membrane reactor (CNHMR-E) for multiphase reactions G = gas, L = liquid, and (i) immobilized-enzyme membrane reactor (lEMR). (Adapted from Shao, X., Xu, S., and Govind, R AlChE Symp. Ser., 268, 1, 1989.)...

Catalytic nonselective hollow membrane reactor for multiphase reactions (CNHMR-MR) 424... 

In the following we attempt to describe the acetylcholinesterase/choline acetyltransferase enzyme system inside the neural synaptic cleft in a simple fashion see Figure 4.49. The complete neurocycle of the acetylcholine as a neurotransmitter is simulated in our model as a simple two-enzymes/two-compartments model. Each compartment is described as a constant-flow, constant-volume, isothermal, continuous stirred tank reactor (CSTR). The two compartments (I) and (II) are separated by a nonselective permeable membrane as shown in Figure 4.50. 

When the catalyst is immobilized within the pores of an inert membrane (Figure 25.13b), the catalytic and separation functions are engineered in a very compact fashion. In classical reactors, the reaction conversion is often limited by the diffusion of reactants into the pores of the catalyst or catalyst carrier pellets. If the catalyst is inside the pores of the membrane, the combination of the open pore path and transmembrane pressure provides easier access for the reactants to the catalyst. Two contactor configurations—forced-flow mode or opposing reactant mode—can be used with these catalytic membranes, which do not necessarily need to be permselective. It is estimated that a membrane catalyst could be 10 times more active than in the form of pellets, provided that the membrane thickness and porous texture, as well as the quantity and location of the catalyst in the membrane, are adapted to the kinetics of the reaction. For biphasic applications (gas/catalyst), the porous texture of the membrane must favor gas-wall (catalyst) interactions to ensure a maximum contact of the reactant with the catalyst surface. In the case of catalytic consecutive-parallel reaction systems, such as the selective oxidation of hydrocarbons, the gas-gas molecular interactions must be limited because they are nonselective and lead to a total oxidation of reactants and products. For these reasons, small-pore mesoporous or microporous... 

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