Integrated pervaporation-reaction

The basic concepts of integrated pervaporation-reaction processes... 

Peters, T. A., van der Tuin, J., Houssin, C. et al. 2005c. Preparation of zeolite-coated pervaporation membranes for the integration of reaction and separation. 104 288-295. 

Catalytic reactions can be combined in membrane-assisted integrated catalytic processes with practically all the membrane unit operations available today. Many examples of integration of membrane contactors, pervaporation, gas separation, nanofiltration, microfiltration, and ultrafiltration operations together with catalytic reactions, have been proposed in the literature. 

The qualifiers continuous and discrete as applied to pervaporation refer to different aspects of the process. In fact, analytical pervaporation is a continuous technique because, while the sample is in the separation module, mass transfer between the phases is continuous until equilibrium is reached. Continuous also refers to the way the sample is inserted into the dynamic manifold for transfer to the pervaporator. When the samples to be treated are liquids or slurries, the overall manifold to be used is one such as that of Fig. 4.18 (dashed lines included). The sample can be continuously aspirated and mixed with the reagent(s) if required (continuous sample insertion). Discrete sample insertion is done by injecting a liquid sample, either via an injection valve in the manifold (and followed by transfer to the pervaporator) or by using a syringe furnished with a hypodermic needle [directly into the lower (donor) chamber of the separation module when no dynamic manifold is connected to the lower chamber]. In any case, the sample reaches the lower chamber and the volatile analyte (or its reaction product) evaporates, diffuses across the membrane and is accepted in the upper chamber by a dynamic or static fluid that drives it continuously or intermittently, respectively, to the detector — except when separation and detection are integrated. 

Fig. 4.21. Different ways of improving pervaporation efficiency. (A) By halting the flow in the acceptor chamber without disrupting the overall dynamic system. (B) By on-line retention of transferred volatile species and elution in the opposite direction. (C) By use of a packed flow-cell to integrate reaction and detection. E eluent, lEC ion-exchange column, PR preconcentration, EL elution. (For other abbreviations, see previous figures.) (Reproduced with permission of Wiley Sons.)...

Keurentjes et al. [98] studied the esterification of tartaric acid with ethanol using pervaporation. The equilibrium composition could be shifted significantly towards the final product diethyltartrate by integration of pervaporation with hydrophilic poly(vinyl alcohol)-based composite membranes in the process. Based on the kinetic parameters, an optimum membrane surface area could be calculated that results in a minimal reaction time for the esterification reaction. Where the membrane surface area to volume ratio is too low, the water removal is rather slow, whereas at high surface area to volume ratios significant amounts of ethanol are removed as well. 

Although low molecular weight esterifications (and etherifications) can benefit substantially from an integration with pervaporation, to our knowledge no large-scale applications have emerged so far. Probably closer to final application is the development by Akzo Nobel on the application of ceramic pervaporation membranes in polycondensation reactions [99, 100]. In the production of alkyd coating... 

Abstract Pervaporation is a peculiar membrane separation process which is currently being considered for integration with a variety of reactions in promising new applications. Indeed, pervaporation membrane reactors have some specific uses in sustainable chemistry, which is an area currently growing in importance. The fundamentals of this type of membrane reactor are presented in this chapter, along with the advantages and limitations of different processes. A number of applications are reviewed with particular attention given to potential future developments. 

Pervaporation, as a non-integrated process, is typically utilized for dehydration and for the recovery or removal of organics from aqueous solutions and sometimes also for the separation of organic mixtures (Neel, 1995). Also many hybrid processes have been developed where PV is coupled with other processes, such as different membrane processes (e.g., reverse osmosis, or organophilic pervaporation coupled with hydrophilic pervaporation), distillation, reactive distillation and, of course, reaction. With these aspects in mind, PV appears particularly suitable to keep the concentration of a by-product low, or to continuously recover a product while it is formed. Note that these are the main objectives typically pursued in membrane reactors. 

The experimental results obtained with catalytic pervaporation membranes for the well-explored case of esterification reactions are generally not as good as those observed in integrated reaction-pervaporation processes with non-catalytic membranes. For the reasons mentioned above, although pervaporation catalytic membranes are potentially interesting, they require additional research in order to better analyse these phenomena and optimize the immobilization techniques of the catalyst in the membranes. 

For scheme 2(ii) high integration effectiveness is reached if the volume, V of each reactor is small in order to get again Da 1. In this case, despite the small conversion given by each reactor, higher overall conversion of the system can be obtained by using N reaction-pervaporation blocks in series. If the total volume of the reactors, y and the total membrane area, A are almost evenly distributed in the N blocks, then when N regardless of whether the reactors are PFRs or CSTRs, scheme 2(ii) equals the performance that can be obtained with scheme 3(ii) with the same total reactor volume and the same total membrane area. 

There are two major difficulties in the integration of a biological reaction with pervaporation the difficulty of operating at a relatively high temperature to avoid the thermal death of the microorganisms or the deactivation of enzymes, and the (bio-)fouling effect of the fermentation broth on the... 

In 1995, Ni et al. studied the PVMR involving the reaction between valeric acid and ethanol to give ethyl valerate. A hydrophilic modified aromatic polyimide membrane and p-toluene sulfonic acid as catalyst were used. The integration in a membrane reactor of reaction and pervaporation allowed for obtaining a conversion rate of 95.2%. 

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