Porous membranes contactor function

Similarly to partially overlapping channels, microchannels with mesh contactors (Figure 7.2h) are used to create the partial contact of fluids. The advantage of these contactors is that both modes of operation, cocurrent and countercurrent, can be apphed. Besides, the flow is stabilized because of the solid support between two fluids. The solid contactors are porous membrane [9, 10] and metal sheets with sieve-like structure [11]. Similarly to parallel flow, the mass transfer in both cases is only by diffusion and the flow is under laminar flow regime dominated by capillary forces. The membrane contactor has the advantage of being flexible with respect to the ratio of two fluids. In addition to flow velocities, the mass transfer is a function of membrane porosity and thickness. In another type of microextractor, two microchaimels are separated by a sieve-like wall architecture to achieve the separation of two continuous phases. However, the hydrodynamics in both types of contactors is more complex because of interfadal support and bursting of fluid... 

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... 

A carbon membrane reactor constitutes one of the most promising applications of carbon membranes. The performance of a carbon membrane for gas separation and for the dehydrogenation of cyclohexane to benzene was examined by Itoh and Ha-raya [29], They concluded that the performance of their caibon membrane reactor for dehydrogenation was fairly good compared with that of a normal reactor, i.e. functioning at equilibrium [29], On the other hand, Lapkin [30] used a macro-porous phenolic resin carbon membrane as a contactor for the hydration of propene in a catalytic reactor. He found that the use of this porous contactor-type reactor for his high-pressure catalytic reaction is practical. 

In porous MRs, the membrane may function as an extractor, a distributor, or an active contactor, as listed in Table 2.6. The extractor mode corresponds to the earlier applications of MRs and has been applied to increase the conversion of a number of equilibrium-limited reactions, such as alkane dehydrogenation, by selectively extracting the hydrogen produced. Other H2-producing reactions - such as water gas shift (WGS), steam reforming of methane, and the decomposition of H2S and HI -have also been investigated successfully with the MR extractor mode. The H2 perm-selectivity of the membrane and its permeability are two important factors controlling the efficiency of the processes [17]. 

First, membrane contactors can first be used in order to achieve intensified gas-liquid absorption. In that case, no separation function is expected from the membrane and the energy requirement of the process will remain essentially unchanged compared to a packed tower. The main interest of using porous, non-selective membranes in that case is due to the possibility to decrease the size (and weight) of the installations and consequently the capital costs (CAPEX) of the capture process." ... 

Volkov et describe the use of porous polypropylene hollow fibre membranes loaded with palladium particles on the external surface, as dis-tributer/contactor membrane reactors for the catalytic removal of dissolved oxygen (DO) from water. Hydrophobic porous catalytic membrane serves three key functions ... 

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