Forced-flow membrane reactors

Kobayashi, M., Togawa, J., Kanno, T., et al (2003). Dramatic Innovation of Propene Epoxidation Efficiency Derived from a Forced Flow Membrane Reactor, J. Chem. Technol. BiotechnoL, 78, pp. 303-307. 

Forced-flow membrane reactors Only applied to PCMRs. In this type of reactor a porous membrane is used. The flow is mainly convective, taking place through the membrane pores, where the catalytic active species are located. The effects of the permselectivity properties of the polymer are negUgible and the membrane works mainly as a catalyst support. 

In forced-flow membrane reactors functionalized porous membranes are used instead of dense membranes. The advantages of this type of membrane reactor lie in the small resistance to mass transfer of the porous membranes, in comparison with their dense counterparts. However, this is also their... 

Figure 8.6 Schematic difference between an enzyme-immobilized column reactor and a forced-flow membrane bioreactor (Nakajima et al., 1988]...

In the case of dense membranes, where only hydrogen can permeate (permselectivity for H2 is infinite), the permeation rate is generally much lower than the reaction rate (especially when a fixed bed is added to the membrane). Experimental conditions and/or a reactor design which diminishes this gap will have positive effects on the yield. An increase of the sweep gas flow rate (increase of the driving force for H2 permeation) leads to an increase in conversion and, if low reactant flow rates are used (to limit the H2 production), conversions of up to 100% can be predicted [55]. These models of dense membrane reactors explain why large membrane surfaces are needed and why research is directed towards decreasing the thickness of Pd membranes (subsection 9.3.2.2.A.a). 

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... 

FIGURE 10.22 (See color insert following page 588.) Applications of catalytic membrane reactors as (a) contactors using opposing reactant mode, (b) interfacial contactors for triphasic reactions, and (c) efficient gas-soUd contactor using forced flow mode. 

A series of original synthesis strategies has been also reported recently such as flow-through reactors for the homogeneous synthesis of zeolite membranes [77], centrifugal force field [114] or electrophoresis [115] for the preparation of A-type membranes, and pulse laser deposition (PLD) for the secondary growth of oriented MCM-22 membranes [116]. 

On the other hand, in nature a continuous uptake of substrate and release of product without loss of biocatalysts is not achieved by carrier fixation but by means of cellular membranes. Efficient immobilized enzyme reactor systems for technical applications can therefore be established replacing cellular membranes by ultrafiltration or reverse osmosis synthetic membranes, and the activated transport through the cellular wall by a forced flow across the membrane.7... 

Contact modalities and concentration profiles in catalytic membrane reactors for three-phase systems.The concentration of reactants is represented on the y-axis and the spatial coordinate along the membrane cross-section is represented on the x-axis. Below the scheme of each case the sequence of the mass transfer (MT) resistances and of the reaction event (R) are reported. (a)Traditional slurry reactor (b) supported thin porous catalytic layer with the liquid impregnating the porosity and the gas phase in contact with the catalytic layer (c) supported thin porous catalytic layer with the liquid impregnating the porosity and the liquid phase in contact with the catalytic layer (d) supported dense membrane which is perm-selective to the gas-phase reactant (e) dense catalytic membrane perm-selective to both reactants in the gas and liquid phases (f) forced flow of the liquid phase enriched with the gas-phase reactant through the thin catalytic membrane layer. 

Figure 4.3f shows a way of contact of the reactants on the catalytic membrane based on the forced flow of both reactants through the catalytic layer directly from one membrane side. Tliis configuration with respect to traditional reactors can offer an important improvement in the contact of reactants with the catalytic sites. Mass transfer from the gas phase to the liquid phase will occur in the same way as traditional reactors the liquid phase needs to be previously saturated with the gas reactant. The features of this last feeding configuration have been reported by Reif and Dittmeyer (2003) for both the catalytic nitrite reduction and the dechlorination of chloroform. 

Preparatory work for the steps in the scaling up of the membrane reactors has been presented in the previous sections. Now, to maintain the similarity of the membrane reactors between the laboratory and pilot plant, dimensional analysis with a number of dimensionless numbers is introduced in the scaling-up process. Traditionally, the scaling-up of hydrodynamic systems is performed with the aid of dimensionless parameters, which must be kept equal at all scales to be hydrodynamically similar. Dimensional analysis allows one to reduce the number of variables that have to be taken into accoimt for mass transfer determination. For mass transfer under forced convection, there are at least three dimensionless groups the Sherwood number, Sh, which contains the mass transfer coefficient the Reynolds number. Re, which contains the flow velocity and defines the flow condition (laminar/turbulent) and the Schmidt number, Sc, which characterizes the diffusive and viscous properties of the respective fluid and describes the relative extension of the fluid-dynamic and concentration boundary layer. The dependence of Sh on Re, Sc, the characteristic length, Dq/L, and D /L can be described in the form of the power series as shown in Eqn (14.38), in which Dc/a is the gap between cathode and anode Dw/C is gap between reactor wall and cathode, and L is the length of the electrode (Pak. Chung, Ju, 2001) ... 

Fritsch, Randjelovic, and Keil (2004) studied a forced-flow catalytic membrane reactor for dimerization of isobutene to isooctene. They examined several catalysts, such as sihca-supported Nafionl (Nafionl SAC-13), Nafionl NR50, AmberlystTM... 

Fritsch, D., Randjelovic, 1., Keil, F. (2004). Application of a forced-flow catalytic membrane reactor for the dimerisation of isobutene. Catalysis Today, 98, 295—308. 

The partial oxidation of propene (PP) to propylene oxide over a Ag-Re catalyst immobilized in a porous ceramic tube membrane reactor was studied by a continuous forced flow system under a differential reactor condition. The steady state rate equations for the production of propylene oxide (PO) and carbon dioxide were separately proposed by two different reaction pathways as follows. 

Figure 12.25 Membrane reactors for FT synthesis from the literature (a) distributed feeding of reactants A and B, (b) in situ water removal by selective membrane (F, feed S, sweep), (cl) plug-through contactor membrane (PCM) with wide transport pores, (c2) forced-through flow membrane contactor, product and heat removal by circulated liquid product, (d) zeolite encapsulated FT catalyst, P, modified product [123].

Increasing the inert gas flow on the retenate side increased the driving force for the steam reforming and water-gas shift reactions due to the thermodynamic equilibrium of methane steam reforming and water-gas shift. Consequently, conversion increased. While increasing pressure decreases (equilibrium) conversion in conventional methane steam reformers, conversion could be increased at elevated pressure in a membrane reactor under certain conditions. In a similar way, the water-gas shift reaction is pushed in a favourable dhection in a membrane reactor, while the reaction is pressure independent in a conventional reactor (see Section 3.10.1). 

Abstract The objective of this chapter is to give an overview of the use of polymeric membranes in membrane reactors. Since the stndy of polymeric membrane reactors is a multidisciplinary activity, the chapter begins with some basic concepts of polymer science and polymer membranes. In the following, the different types of polymeric membrane reactors, classified into two main groups - polymeric inert membrane reactors (PIMRs) and polymeric catalytic membrane reactors (PCMRs), are presented and discussed. For each of these group , examples of the main reactor types are given extractors, forced-flow or contactors. Finally, there is a discussion of the modelhng aspects of membrane reactors with dense polymeric catalytic membranes reported in the literature. 

Key words catalytic/inert polymeric membranes, polymeric membranes preparation, membrane reactors, extractor-type, distributor/contactor-type, forced-flow-type, polymeric inert membrane reactors (PIMRs), polymeric catalytic membrane reactors (PCMRs), modelling. 

The dimerization of isobutene carried out in a forced-flow polymeric catalytic membrane reactor was reported by D. Fritsch and co-workers. The authors prepared composite porous membranes consisting of a catalytic layer made of solid add catalysts, such as siUca supported Naflon , Nafion NR50, Amberlyst 15 and silica supported tungstophosphoric add dispersed in polymeric binders such as Teflon AF, Hyflon AD, polytrim-ethylsilylpropyne, or polydimethylsiloxane (PDMS), cast on microporous support membranes made of polyacrylonitrile (PAN) or Torlon . The membranes were assembled in the membrane reactor into which isobutene was fed in the retentate side with a build-up pressure of 4 bar. The liquid product was collected on the permeate side. 

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