Reactant distributor

Membranes in catalysis can be used to improve selectivity and conversion of a chemical reaction, improve stability and lifetime of the catalyst, and improve the safety of operation. The most well-known example is in situ removal of products of an equilibrium-limited reaction. However, many more ways of application of a membrane can be thought of [1-3], such as using the membrane as a reactant distributor to control the reactant concentration levels in the reactor, or performing catalysis inside the membrane and having control over reactant feed and product removal. 

Novel unit operations currently being developed are membrane reactors where both reaction and separation occur simultaneously. Through selective product removal a shift of the conversion beyond thermodynamic equilibrium is possible. The membrane itself can serve in different capacities including (i) a permselective diffusion barrier, (ii) a non-reactive reactant distributor and (iii) as both a catalyst and permselective membrane [44]. 

Several profound theoretical and experimental studies performed on the laboratory scale have been reported which focus on the use of various configurations of membrane reactors as a reactant distributor in order to improve selectivity-conversion performances. In particular, several industrially relevant partial oxidations have been investigated, including the oxidative coupling of methane [56], the oxidative dehydrogenations of propane [57], butane [58], methanol [59, 60], the epoxidation of ethylene [61], and the oxidation of butane to maleic anhydride [62]. 

Zeolite-Membrane Reactors as Reactant Distributors for Selectivity Enhancement.301... 

Figure 1.11 Principles of membrane reactors to enhance the reaction process (a,b) membrane as a product extractor (c,d) membrane as a reactant distributor (e,f) membrane as an active contactor.

The membrane serves as a reactant distributor (Figure l.ll(c,d)). A reactant is added to the reaction zone in a controlled manner through the membrane. As a consequence, the side reactions are limited, leading to increased selectivity and yield. In addition, it is... 

As mentioned in Chapter 1, the catalyst in porous MRs may just be placed on the membrane (illustrated in Figure 1.12(a)). The reaction takes place in the catalyst phase and the membrane only serves either as a product extractor or as a reactant distributor but does not participate directly in chemical reactions. It is not always easy to obtain a true inert membrane since the porous membrane materials such as alumina, silica, titania, zirconia, zeolite or the components used to modify membrane permeation properties (e.g., pore-filling materials) can make a contribution to reactions. In order to reduce non-selective catalytic activity, the membrane used in selective oxidation reactions often has to be modified significantly by using controlled sintering to reduce surface area, or by doping with alkaline compounds to decrease surface acidity [19]. 

The majority of zeolite MR applications reported in the literature to date fall into the category of PBMRs. The reactor consists of a zeolite membrane with a conventional catalyst present in the form of a packed bed of pellets. The reaction takes place in the catalyst bed while the zeolite membrane serves mainly as a product separator (for H2 or H2O separation) [27] or a reactant distributor (for O2 distribution) [28]. Figure 3.5 illustrates a FAU-type zeolite PBMR combined with a packed bed reactor for dehydrogenation of cyclohexane [29]. Half of the catalyst is packed in the area upstream of the permeation portion to enhance the conversion, otherwise cyclohexane will preferentially permeate at the front end of the zeolite membrane, resulting in a decrease in conversion. 

In FBMRs, the membranes are inserted inside the fluidized catalyst bed, serving as a product extractor or a reactant distributor. Figure 7.1 shows a typical FBMR structure for selective removal of a product (hydrogen) [4,5]. Pd-membrane tubes are placed vertically in the FBMR.The reactant gas is fed through the gas distribution plate at the bottom of the reactor to fluidize the fine particulate catalysts. Entrained solids are separated from the reaction product gas stream by internal cyclone separator and then returned to the reactor catalyst bed. 

Zeolite membranes, due to their microporous nature and hence the low diffusivity coefficients, could work under mass transfer limited regime due to the high thickness of the zeolite layer. When the membrane acts as a reactant distributor the mass transfer should help to obtain the desired reactant profile in the reactor. In a catalytic zeohte membrane reactor, when the intrinsic reaction rate is higher than the diffusion rate, the mass transfer becomes a limiting factor for the conversion. 

Principle of membrane reactor to promote reactions (a) selective permeation of by-product of an equilibrium-limited reaction (b) selective permeation of an intermediate product (c) dosing a reactant through the membrane and (d) supplying a well-defined reaction interface. In (a) and (b) the membrane functions as a product extractor, and as a reactant distributor in (c). (In the figure, A and B stand for two different reactants, C for a by-product, P for the desired product and nB for n mole of the reactant B.)... 

In order to improve the catalytic activity and selectivity of the membrane reactor, a catalyst has to be used. A simple way is to place the catalyst pellets on/next to the membrane, as shown in Fig. 7.9a. The membrane mainly functions as either a product extractor or a reactant distributor, although it also plays some role in the reaction.The reaction selectivity is mainly determined by the catalyst. This incorporation mode is most popular in practical use and is easily operated. Since the catalyst is physically separated from the membrane, only the separation function of the membrane needs to be controlled. The high selectivity of the dense ceramic membranes leads to highly attractive results (pure Hj extraction in dehydrogenation reactions and direct use of air in partial oxidation reaction). But the permeability of the membrane has to be improved as high as possible. 

Dense ceramic membranes allow oxygen or hydrogen permeation in a dissociated or ionized form other than the conventional molecular diffusion, and thus exhibit extremely high selectivity (up to 100%). They can be incorporated into membrane reactors for a variety of oxidation and dehydrogenation reactions where the membrane functions as either a product extractor or a reactant distributor. Three configurations, that is, disc/flat-sheet, tubular and hollow fibre membranes have been applied in membrane reactors. They exhibit respective advantages/disadvantages in terms of the ease and cost of fabrication, the effective membrane area/volume ratio, and... 

Abstract This chapter discusses the research and development of porous ceramic membranes and their application as membrane reactors (MRs) for both gas and liquid phase reaction and separation. The most commonly used preparation techniques for the synthesis of porous ceramic membranes are introduced first followed by a discussion of the various techniques used to characterise the membrane microstructure, pore network, permeation and separation behaviour. To further understand the structure-property relationships involved, an overview of the relevant gas transport mechanisms is presented here. Studies involving porous ceramic MRs are then reviewed. Of importance here is that while the general mesoporous natnre of these membranes does not allow excellent separation, they are still more than capable of enhancing reaction conversion and selectivity by acting as either a product separator or reactant distributor. The chapter closes by presenting the future research directions and considerations of porous ceramic MRs. 

Porous ceramic membranes have also found application in MRs as reactant distributors, particularly for catalytic oxidative dehydrogenation reactions where the intermediate or final products are more reactive towards oxygen than the original hydrocarbons, which limits reaction selectivity. As a result, total rather than partial oxidation of the original hydrocarbon is often achieved. Systems which can control the oxygen concentration along the reactor length, such as membranes by means of an MR, can increase the reaction selectivity towards the intermediate or final products (Coronas eta/., 1995). 

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