Membrane Reactor Configuration

Because Pd-alloy membranes operate at high temperatures in the range of WGS reaction and on the lower end of methane reforming reaction, they can be used in a membrane reactor configuration for the simultaneous separation of hydrogen. As discussed earlier,... 

Figure 7.1. Typical membrane reactor configurations (a) reactor with plate-shaped membranes, (b) tubular-shaped membrane in double pipe configuration and (c) multichannel monolith.

It is important to attain as high an area as possible for a membrane reactor. Configurations with multilayer planar membranes, coiled membranes, or as multiple tubes also can be used for similar processes with potentially very high surface areas, as sketched in Figure 12-6. 

Partial oxidation of methane in the membrane reactor configuration shown in Figure 1 will not lead to higher yields of desired products than a plug flow reactor unless the diffusivity of the intermediate product, formaldehyde, is approximately four times that of methane. Presently available membranes that can withstand partial oxidation temperatures do not satisfy this criterion. 

Membrane reactors have, for a long time, been the focus of intensive research, and a variety of membrane reactor configurations have either been developed or suggested. The state of the art with regard to this broad field has been the subject of several excellent reviews [1-10], while comprehensive summaries have recently been provided by Sanchez Marcano and Tsotsis [11] and Dixon [12]. Modern developments were reported on a regular basis during the International Congresses on Catalysis in Membrane Reactors - ICCMR [13]. 

A multistage membrane reactor configuration appears to be attractive for the enhancement of yields. 

Zaspalis and Burggraaf [47] have summarized typical membrane reactor configurations, different membrane/ catalyst combinations, and a large number of membrane reactor studies. Their article clearly shows that inorganic membranes prepared by the sol-gel method, with their dual ability in catalysis and separation, have many unique advantages over other product forms. At the same time, it is important to realize that the parameters which affect a membrane s characteristics and the advantages which the sol-gel process offers are similar to what has been presented thus far. 

Figure 8.7 Schematic diagrams showing two electrocatalytic membrane reactor configurations (a) eicctrochemicai oxygen pumping and (b) solid oxide fuel cell operation.

In a generalized case, both the tube and the shell sides are packed with catalyst beds (e.g., in a reaction coupling situation explained in Chapter 8) and the membrane layer is catalytic either inherently or through impregnation on the pore surfaces. Two commonly occurring membrane reactor configurations will be treated as special cases of this generalized model. 

The direction of the feed stream relative to the orientation of the membrane surface and the general membrane reactor configuration also affect the catalytic and reaction aspects of the membrane reactor. 

Membranes that arc catalytically active or impregnated with catalyst do not suffer from any potential catalyst loss or attrition as much as other membrane reactor configurations. This and the above advantage have the implication that the former requires a lower catalyst concentration per unit volume than the latter. It should be mentioned that the catalyst concentration per unit volume can be further increased by selecting a high "packing density" (surface area per unit volume) membrane element such as a honeycomb monolith or hollow fiber shape. 

Enhancement of reaction conversion by employing a permselective membrane often has the implication that, for a given conversion, it is possible to run the reaction at a lower temperature in the membrane reactor than in a conventional reactor. Catalyst deactivation due to coke formation generally becomes more severe as the reaction temperature increases. Therefore, the use of a membrane reactor to replace a conventional one should, in principle, reduce the propensity for coke formation because for the same conversion the membrane reactor configuration may be operated at a lower temperature than a conventional reactor. This is particularly true for such reactions as dehydrogenation. 

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. 

The different types of membrane reactor configurations can also be classified according to the relative placement of the two most important elements of this technology the membrane and the catalyst. Three main configurations can be considered (Figure 25.13) the catalyst is physically separated from the membrane the catalyst is dispersed in the membrane or the membrane is inherently catalytic. The first configuration is often called the inert membrane reactor (IMR), in contrast to the two other ones, which are catalytic membrane reactors (CMRs).5o... 

As shown in Figure 7.3, two membrane reactor configurations can be proposed one with free catalyst and the other one with catalyst fixed on the membrane. In the first case, the membrane insures rejection of catalyst and keeps it in a restricted part of the system where reaction takes place. In the second case, the catalyst is fixed on the surface or in the pores of the membrane and reaction takes place at crossing. 

Before proceeding further it would be appropriate for our readers to familiarize themselves with the few additional acronyms that will be used in this chapter and which are listed in Table 11.1. They are used to describe some of the most common membrane reactor configurations that have been studied in the technical literature. By far the most commonly referred to reactor is the PBMR, in which the reaction function is provided by a packed bed of catalysts in contact with the membrane. The membrane is not itself catalytic at least not intentionally so. Some of the commonly utilized inorganic and metal membranes, on the other hand, are intrinsically catal) ically active. The PBMR clcissification, therefore, should be assigned with caution. When the packed bed... 

Four basic catalytic membrane reactor configurations, when the membrane and reactor are in the same physical unit, can be distinguished [32] ... 

The packed bed ceramic membrane reactor configuration (PBMR) has been chosen as the reactor set-up (see Section 14.2.2). In the PBMR configuration three possible sub-configurations can be envisioned for a specific sweep gas in combination with a hydrogen or oxygen selective membrane for the dehydrogenation of ethylbenzene. These sub-configurations are shown in Fig. 14.10. 

The objective of this study was to determine the conversion and selectivity of the methane partial oxidation reaction when using high feed rates and low methane/oxygen feed ratios in the membrane reactor configuration. 

Figure 5.21. Effect of pressure and pore size on the asphaltene conversion and maltene weight fraction yield for membrane reactor configuration A at 350 The flat surface is for the conventional reactor. From [5.70], with permission from the American Chemical Society.

Figure 1 Schematic of three membrane reactor configurations...

The first section of this chapter gives a brief survey of major membrane concepts and different membrane reactor configurations. Membrane materials are discussed in the second section. The third section will present the recent development of OITM reactors for selective oxidation of light alkanes. 

Figure 7.3 Schematic of ambient pressure membrane reactor configuration for promoting the spontaneous conversion of natural gas into synthesis gas.

The state-of-the-art regarding the various membrane-reactor configurations has been reviewed in several comprehensive summaries (e.g. Sanchez Marcano and Tsotsis, 2002 Dixon, 2003 Seidel-Morgenstern, 2005). Modern... 

Caravella A, Di Maio FP, Di Renzo A (2010) Computational study of staged membrane reactor configurations for methane steam reforming. I. Optimization of stage lengths. AIChE J 56(l) 248-258... 

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