Pervaporation membrane reactor applications

T. S. Chung, J. J. Shieh, J. Qin, W. H. Lin, and R. Wang, Polymeric membranes for reverse osmosis, ultrafiltration, microfdtration, gas separation, pervaporation, and reactor applications. In Advanced Functional Molecules and Polymers, H. S. Nalwa (ed.). Chapter 7, Gordon Breach, pp. 219-264 (2001). 

Pervaporation membrane reactors (PVMR) are an emerging area of membrane-based reactive separations. An excellent review paper of the broader area of pervaporation-based, hybrid processes has been published recently [3.1]. The brief discussion here is an extract of the more comprehensive discussions presented in that paper, as well as in an earlier paper by Zhu et al [3.2]. Mostly non-biological applications are discussed in this chapter. Some pervaporation membrane bioreactor (PVMBR) applications are also discussed additional information on the topic can be found in a recent publication [3.3], and a number of other examples are also discussed in Chapter 4. 

In the middle of the last century, the original form of zeolite membranes were synthesized by dispersing the zeolite crystals in polymer membrane matrixes, which were used for gas separation and pervaporative alcohol/water separations. In the last few decades, the researches of polycrystalline zeolite membranes that supported on ceramic, glass, or metal substrates have grown into an attractive and abundant field. Their applications for gas separation, pervaporation, membrane reactors, sensors, low-k films, corrosion protection coatings, zeolite modified electrodes, fuel cells, heat pumps et al. have been wildly explored. In the following text, the applications of supported polycrystalline zeolite membranes for energy and fuels will be presented. 

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. 

A novel type of membrane reactor, emerging presently, is the pervaporation reactor. Conventional pervaporation processes only involve separation and most pervaporation set-ups are used in combination with distillation to break azeotropes or to remove trace impurities from product streams, but using membranes also products can be removed selectively from the reaction zone. Next to the polymer membranes, microporous silica membranes are currently under investigation, because they are more resistant to chemicals like Methyl Tertair Butyl Ether (MTBE) [23-24], Another application is the use of pervaporation with microporous silica membranes to remove water from polycondensation reactions [25], A general representation of such a reaction is ... 

The efforts and advances during the last 15 years in zeolite membrane and coating research have made it possible to synthesize many zeolitic and related-type materials on a wide variety of supports of different composition, geometry, and structure and also to predict their transport properties. Additionally, the widely exploited adsorption and catalytic properties of zeolites have undoubtedly opened up their scope of application beyond traditional separation and pervaporation processes. As a matter-of-fact, zeolite membranes have already been used in the field of membrane reactors (chemical specialties and commodities) and microchemical systems (microreactors, microseparators, and microsensors). 

Several plenary lectures were given at the conferences, including a call from Tsotsis for renewed emphasis on using membrane reactors to reduce or eliminate the separation task. An update to his 1994 review paper was given at the ISCRE-15 conference by van Swaaij, who concluded that the outlook for membrane reactors was perhaps more optimistic than a few years ago. Dalmon presented a timely survey of membrane catalysis for liquid applications, and several recent publications have focused on this, including pervaporation. Dalmon also emphasized the area of fine chemicals production, where the use of membranes for three-phase contacting could see a revival of interest. An example of this was presented at the ISCRE-15 conference. 

Hence, their application field is not only restricted to their use in gas separation, pervaporation, and membrane reactors, but are also applicable in microscale devices (microreactors, microseparators, microvalves, microneedles, etc.) and for the preparation of functional materials (adsorbents for pollutant removal, controlled release systems, bactericidal, anticorrosive or antirefiective coatings, chemical sensors, and so on). 

This chapter gives an overview of the synthesis procedures and applications of zeolite membranes (gas separation, pervaporation, and zeolite-membrane reactors), as well as new emerging applications in the micro- and nanotechnology field. It is important to note that, up to now, pervaporation is the only large-scale industrial application and gas separation is still not implemented at industrial level for zeolite membranes. Related areas such as new zeolite and zeolite-related materials for membranes, alternative supports, and scale-up issues are also discussed. 

Gas separation (GS) is a relatively young technology and accounted for about US 230 million/year in 1998, but is growing fast with a rate higher than 15% a year. The development of membrane reactors is opening a number of new gas applications. For the electrically driven membrane processes the sales in 1998 were around US 180 million. For pervaporation (PV) in 1996 the market was about US 26 million, with a growth rate of 20%. 

Zeolite membranes show high thermal stability and chemical resistance compared with those of polymeric membranes. They are able to separate mixtures continuously on the basis of differences in the molecular size and shape [18], and/or on the basis of different adsorption properties [19], since their separation ability depends on the interplay of the mixture adsorption equilibrium and the mixture. Different types of zeolites have been studied (e.g. MFI, LTA, MOR, FAU) for the membrane separation. They are used still at laboratory level, also as catalytic membranes in membrane reactors (e.g. CO clean-up, water gas shift, methane reforming, etc.) [20,21]. The first commercial application is that of LTA zeolite membranes for solvent dehydration by pervaporation [22], Some other pervaporation plants have been installed since 2001, but no industrial applications use zeolite membranes in the GS field [23]. The reason for this limited application in industry might be due to economical feasibility (development of higher flux membranes should reduce both costs of membranes and modules) and poor reproducibility. 

Waldburger R M and Widmer F (1996), Membrane reactors in chemical production processes and the application to the pervaporation-assisted esterification ,... 

Membrane Processes in Separation and Purification published in 1993, contains chapters on pervaporation, facilitated transport membrane processes, membrane gas absorption processes, hollow fiber contactors, membrane reactors, and the preparation and application of inorganic membranes. In addition to an introductory chapter by the editors. Polymeric... 

Membrane reactors for energy applications and basic chemical production Edited by Angelo Basile, Luisa Di Paolo, Faisal Hai and Vincenzo Piemonte 11 Pervaporation, vapour permeation and membrane distillation Principles and applications... 

CNT membranes open a new door for membranes used for separation purposes. These membranes can be used for water separation, gas separation, pervaporation, fuel cells, membrane reactors, and medical science. However, the application of CNT membranes needs a lot of research for its development. 

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. 

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