Membrane reactors classification

In this section, after a classification of the different types of membrane reactors, selected examples including some of the most recent developments in asymmetric synthesis, highlight the potential of this approach (cf. Sections 2.9 and 3.3.1). 

According to lUPAC a membrane reactor is a device for simultaneously carrying out a reaction and membrane-based separation in the same physical enclosure . This type of reactor has been studied intensively by many groups, resulting in more than 1400 publications since 1994 [5]. The membrane may act in several ways. A rough classification is illustrated in Figure 1. Examples of each type are listed in Table 1. 

There is a multitude of different configurations that have been proposed in the literature in order to combine the membrane separation module and the reactor into a single unit (Figure 1.4b). Sanchez and Tsotsis [1.24] have classified these configurations for catalytic membrane reactors into six basic types, as indicated in Table 1.1 and Figure 1.5. This classification and acronyms are also applicable to other types of membrane reactors, and will be used throughout this book. 

Table 1.1, Classification of Membrane Reactors (Adapted from Sanchez and Tsotsis, [1.24]).

As previously noted, a broader classification of membrane reactors can be made relevant to the role the membrane plays with respect to the removal/addition of various species [1.25, 1.49, 1.67]. Membrane reactors could be classified as reactive membrane extractors when the membrane s function is to remove one or more products. Such action could result in increasing the equilibrium yield, like in the catalytic dehydrogenation re-... 

Classification of membrane reactors with catalysts is based on the location of the catalyst. The notation introduced by Tsotsis et al. has been widely adopted, and will be used here. Table 1 gives a list of the acronyms used in this paper. 

The many attractive featuies of membrane reactors described in the previous section underscore the potential of these reactors in organic chemical technology and synthesis. A broad classification of these reactors is given in Table 24.1 and sketches of a few specific ones are given in Figure 24.2. 

As a main scope, the present chapter will give an overview on the general classification of the membranes, paying particular attention to the palladium-based membranes and their applications, pointing out the most important benefits and the drawbacks due to their use. Finally, the application of palladium-based membranes in the area of the membrane reactors will be illustrated and such reaction processes in the issue of hydrogen production will be discussed. 

Membrane reactors were classically grouped according to the hydrodynam-ics/configuration of the system in CSTR and PFR types [106]. However, this proved vmable to comprise some commonly used types in UF, such as flat membranes or dead-end operated modules and multiphase bioreactors. A classification based on the contact mechanisms that bring together substrate and biocatalyst was thus proposed [110]. Thus, membrane reactors could be divided into direct contact, diffusion contact, and interfacial contact reactors. 

Table 7.3 A general classification of membrane reactor (MR) types...

These proposed classifications can be further extended in two ways considering the reactor design (how it is clearly shown in the matrix of Figure 19.10) and considering the possibility of using inert membranes (inert pervaporafion membrane reactors (I-PVMRs)) or catalytic membranes (catalytic pervaporation membrane reactors (C-PVMRs)), as shown in Figure 19.11. 

Figure 13.1 Broad classification of membrane reactors. E = empty, F = fluidized bed, P = packed bed.

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. 

Table 27.1. Classification of inorganic membranes most often employed in membrane reactors.

While the role of the membrane is straightforward in membrane reactors using dense membranes (simply to selectively permeate a reactant or product), the use of porous membranes has created a large variety of configurations, depending on the characteristics of the reaction. Therefore, for this kind of membrane, a classification based on the reactor structure rather than on the membrane material is more appropriate, since the same membrane can be used in different ways to improve the performance of different reactions. The following four operating modes, illustrated in Fig. 27.3, will be considered ... 

A possible classification of polymeric membrane reactors is based on the role of the membrane in the catalytic process ... 

Membrane reactors can also be classified according to the transport function of the membrane. A possible classification may be that depicted in Fig. 1.3 with three different reactor types for PCMRs and only two for PIMRs ... 

Another classification of zeolite membrane reactors is based on the aim of the application, in the same way as that generally adopted for inorganic membrane reactors ... 

The selection of a module shape depends on a number of factors, including cost, heat management, manufacturability, maintainability, operability, efficiency and membrane replacement. Membrane modules and thus membrane reactors can be combined into number of stages. The options for membrane system layout are virtually endless, as stages can be combined in various ways incorporating both compressors and recycle streams. Furthermore, membrane reactors inevitably contain catalysts and there are several ways in which the catalyst can be incorporated (Tsotsis et al., 1993). The classification of membrane reactors that incorporate catalysts is mainly based on the location of the catalyst with respect to the membrane as shown in Table 9.2. 

In this chapter, we first give an overview of carbon membrane materials (Section 10.2) and the classification of carbon membranes (Section 10.3). Then, unsupported carbon membranes, based on planar membranes and asymmetric hollow fiber membranes are discussed (Section 10.4). In Section 10.5, the supported CMSMs are reviewed in detail in terms of precursors, supports, fabrications and problems. In Section 10.6, carbon-based membrane reactors are discussed in detail, based on the topics of dehydrogenation reactions, hydration reactions, hydrogen production reactions, H2O2 synthesis, bio-diesel synthesis, and new carbon membranes for carbon membrane reactors (CMRs). In the end, the new concept of using carbon membranes in microscale devices (microcarbon-based membrane reactor) is outlined (Section 10.7). 

Because there are many different ways to combine a catalyst with a membrane, there are numerous possible classifications of the CMRs. However, one of the most useful classifications is based on the role of the membrane in the catalytic process we have a catalytically active membrane if the membrane has itself catalytic properties (the membrane is functionalized with a catalyst inside or on the surface, or the material used to prepare the membrane is intrinsically catalytic) otherwise if the only function of the membrane is a separation process (retention of the catalyst in reactor and/or removal of products and/or dosing of reagents) we have a catalytically passive membrane. The process carried out with the second type of membrane is also known as membrane-assisted catalysis (a complete description of the different CMRs configurations will be presented in a specific chapter). 

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