Polymeric catalytic membrane reactors modelling

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. 

Sousa J M and Mendes A M (2003), Modelling a dense polymeric catalytic membrane reactor with plug-flow pattern , Catal Today, 82,241-254. 

Because of their intrinsic characteristics, mostly dense rubbery polymers have been considered for the preparation of polymeric catalytic membranes. The mass-transport mechanism considered has been the well-known sorption-diffusion model.ii Modelling the kinetics of the reaction(s) occurring at the occluded catalyst level is a much more complex task. The reaction may be carried out under special operating conditions, for example in a batch reactor where the catalyst is dispersed in a support - or directly inside the catalytic membrane, -" a reaction-rate model is assumed and the related parameters are determined by fitting the global model to the experimental data. In other cases, the kinetic models determined by other authors are used. - In some theoretical studies, a hypothetical reaction-rate model and the respective model parameters are assumed. 

In Part I the various aspects related to polymeric, dense metallic and composite membranes for membrane reactors are extensively considered. The volume starts with Chapter 1, in which the authors (Vital and Sousa) give an overview of the polymeric membranes used in membrane reactors. After introducing some basic concepts of polymer science and polymer membranes, two different types of polymeric membrane reactors (inert and catalytic) are discussed. Various examples of the main reactor types (extractors, forced-flow or contactors) are also given. Finally, the modelling aspects of membrane reactors with dense polymeric catalytic membranes are also presented in detail. It is followed by Chapter 2 (Basile,Tong and Millet), which... 

Yawalkar et al. (2001) has developed a model for a three-phase reactor based on the use of a dense polymeric composite membrane containing discrete cubic zeolite particles (Fig. 4.5) for the epoxidation reaction of alkene. Catalytic particles of the same size are assumed vdth a cubic shape and uniformly dispersed across the polymer membrane cross-section. Effects of various parameters, such as peroxide and alkene concentration in liquid phase, sorption coefficient of the membrane for peroxide and alkene, membrane-catalyst distribution coefficient for peroxide and alkene and catalyst loading, have been studied. The results have been discussed in terms of a peroxide effidency defined as the ratio of flux of peroxide through the membrane utilized for alkene oxidation to the total flux of organic peroxide through the membrane. The paper aimed to show that, by using an organophilic dense membrane and the catalysts confined in the polymeric matrix, the oxidant concentration (in that reaction peroxides) can be controlled on the active site with an improvement of the peroxide efficiency and selectivity to desired products. 

The modelling of CMRs is extensively reported in the literature. However, most of such works concern metallic or ceramic membranes. Only a few papers consider the modelling of membrane reactors with polymeric membranes, more specifically catalytic polymeric membranes. 

The following discussion will focus exclusively on membrane reactors with polymeric catalytic dense membranes. Cases of membrane reactors with a polymeric non-catalytic membrane are not presented and discussed, as the membrane only performs some separation task and the models to be considered are independent of the type of membrane, considering that an adequate transport equation is considered. The cases of porous polymeric membranes with catalyst supported in the porous walls, typical in the biomembrane reactors, photocatalysis, amongst others, are also not considered here. 

In spite of the growing research effort, with the exception of fuel cells, there are only a few examples of industrial applications of non-biocatalytic polymeric membrane reactors, such as the Remedia Catalytic Filter System for the destruction of dioxins and furans from industrial combustion sources or pervaporation-assisted esterification processes. More research is required in order to find long-lasting high-performance and cheap polymeric materials and catalysts that can effectively compete with the traditional processes. On pursuing this quest, mathematical modelling and simulation are fundamental tools for the better understanding of membranes behaviour and optimization. 

Sousa J M, Cruz P and Mendes A M (2001), Modeling a catalytic polymeric non-porous membrane reactor , J Membr Sci, 181,241-252. 

Sousa et al [5.76, 5.77] modeled a CMR utilizing a dense catalytic polymeric membrane for an equilibrium limited elementary gas phase reaction of the type ttaA +abB acC +adD. The model considers well-stirred retentate and permeate sides, isothermal operation, Fickian transport across the membrane with constant diffusivities, and a linear sorption equilibrium between the bulk and membrane phases. The conversion enhancement over the thermodynamic equilibrium value corresponding to equimolar feed conditions is studied for three different cases An > 0, An = 0, and An < 0, where An = (ac + ad) -(aa + ab). Souza et al [5.76, 5.77] conclude that the conversion can be significantly enhanced, when the diffusion coefficients of the products are higher than those of the reactants and/or the sorption coefficients are lower, the degree of enhancement affected strongly by An and the Thiele modulus. They report that performance of a dense polymeric membrane CMR depends on both the sorption and diffusion coefficients but in a different way, so the study of such a reactor should not be based on overall component permeabilities. 

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