Membrane liquid-phase reactions

Typically liquid-phase reactions do not require high temperatures, and as such organic membranes may be suitable for the membrane reactor applications. Justification of using inorganic membranes for these applications comes from such factors as better chemical stability and beuer control and containment of the catalysts. 

Gas-phase reactions are usually performed with a series of fixed bed adiabatic reactors with re-heating between each catalytic bed. Liquid-phase reactions are usually performed in a slurry reactor with a circulating inert gas. The new trends in dehydrogenation reactors and conditions are membrane reactors [12], wall reactors [13], reactions performed in supercritical water [14], and oxidative dehydrogenation [11],... 

In addition to the liquid phase membrane reactor models previously discussed, there have been a number of other interesting modeling investigations involving liquid reactants and products. Garayhi et al [5,69] have reported a general isothermal model for a CMR where a liquid phase reaction takes place. The model does not consider the separation of species by the membrane. 

NaA/polyelectrolyte multilayer-pervaporation membrane showing a greater stability under acidic conditions in comparison with a pure zeolite A membrane and maintaining a high selectivity for water over alcohols. For the same purpose, Kita et al. [181-183] proposed a zeolite T membrane, prepared by ex situ crystallization, for the per-vaporation-aided or vapor-permeation-aided esterification of acetic acid with ethanol. This membrane has a higher acid resistance and can be directly immersed in the liquid-phase reaction mixture. The conversions achieved exceed the equilibrium limit and reached to almost 100% after a stabilization period of 8 h. 

Kiatkittipong et al. (2002) investigated a PV membrane reactor for the synthesis of ethyl icri-butyl ether (ETBE) from a liquid phase reaction between EtOH and TEA. Supported p-zeolite and PVA membrane were used as catalyst and as membrane in the reactor, respectively. The permeation studies of water-EtOH binary system revealed that the membrane worked effectively for water removal for the mixtures containing water lower than 62 mol%. The permeation studies of quaternary mixtures (water-EtOH-TBA-ETBE) were performed at three temperature levels of 323, 333, and 343 K. It was found that the manbrane was preferentially permeable to water. 

Passive mass transfer In the case of passive mass transfer, the membrane liquid phase consists of an organic solvent or a mixture of organic solvents. The transfer of the analyte across both membrane/solu-tion interfaces is governed by its partition coefficient. Figure 2A illustrates schematically the passive transport of an organic acid across a liquid membrane involving suitable protolytic reactions in both the feed and receiver solutions. If the volume of the receiver solution is smaller than the volume of the feed solution the analyte of interest can be concentrated as well. The mechanism of passive liquid membrane separation is analogous to that involved in separation based on solid SP and gas-diffusion membranes. However, unlike these membranes, liquid membranes... 

Schematic diagram of a PMR with photocatalytic membrane for liquid phase reaction operating in a cross-flow mode. F, flowmeter VP, vacuum pump Rpl and Rp2, permeate reservoirs (adapted from Zhang et a ., 2006a). 

Most of the research pertained on membrane reactors is carried out on gas phase reactions i using ceramic, metal or zeolite membranes. However, the concept can be used as well for liquid phase reactions. A verj specific class of reactions are the condensation or polycondensation reaction in which water is one of the products. Water is easily be removed by pervaporation (see section Vl.4.3 ) and therefore pervaporation can be applied if the reaction temperature is not too high. As example we will use here an esterification reaction [89,90]. This reaction may be carried out in a batch reactor coupled with a pervaporation unit in which water is removed constantly (see figure VI - 76). 

Contactor-type polymeric membrane reactors have been also applied to liquid-phase reactions other than hydrogenation or oxidation. The hydration of a-pinene has been carried out successfully over polymeric membranes consisting of mixed matrixes of PDMS embedded USY or beta zeolites or sulfonated activated carbon. The membranes were assembled in a flat contactor-type reactor configuration, separating the aqueous and organic phases. Sulfonated PVA membranes were also reported to be effective in the acid catalysed methanolysis of soybean oil carried out in a flat contactor-type membrane reactor configuration. ... 

Abstract This chapter discusses the research and development of porous ceramic membranes and their application as membrane reactors (MRs) for both gas and liquid phase reaction and separation. The most commonly used preparation techniques for the synthesis of porous ceramic membranes are introduced first followed by a discussion of the various techniques used to characterise the membrane microstructure, pore network, permeation and separation behaviour. To further understand the structure-property relationships involved, an overview of the relevant gas transport mechanisms is presented here. Studies involving porous ceramic MRs are then reviewed. Of importance here is that while the general mesoporous natnre of these membranes does not allow excellent separation, they are still more than capable of enhancing reaction conversion and selectivity by acting as either a product separator or reactant distributor. The chapter closes by presenting the future research directions and considerations of porous ceramic MRs. 

Intelligent engineering can drastically improve process selectivity (see Sharma, 1988, 1990) as illustrated in Chapter 4 of this book. A combination of reaction with an appropriate separation operation is the first option if the reaction is limited by chemical equilibrium. In such combinations one product is removed from the reaction zone continuously, allowing for a higher conversion of raw materials. Extractive reactions involve the addition of a second liquid phase, in which the product is better soluble than the reactants, to the reaction zone. Thus, the product is withdrawn from the reactive phase shifting the reaction mixture to product(s). The same principle can be realized if an additive is introduced into the reaction zone that causes precipitation of the desired product. A combination of reaction with distillation in a single column allows the removal of volatile products from the reaction zone that is then realized in the (fractional) distillation zone. Finally, reaction can be combined with filtration. A typical example of the latter system is the application of catalytic membranes. In all these cases, withdrawal of the product shifts the equilibrium mixture to the product. 

As for the former problem, the researchers of GA found that the mixed acid solution produced by the Bunsen reaction separates spontaneously into two liquid phases in the presence of excess amount of iodine [17]. The heavier phase is mainly composed of HI, I2, and H20, and is called "Hix" solution. The main components of the lighter phase are H2S04 and H20. The phenomenon (liquid-liquid (LL)-phase separation) offered an easy way of separating the hydriodic acid and the sulfuric acid. As for the HI processing, some ideas have been proposed by GA [17], RWTH Aachen [18], and JAEA. JAEA studied the utilization of membrane technologies for concentrating the Hix solution to facilitate the HI separation and also for enhancing the one-pass conversion of HI decomposition [19,20]. 

The unique ability of crown ethers to form stable complexes with various cations has been used to advantage in such diverse processes as isotope separations (Jepson and De Witt, 1976), the transport of ions through artificial and natural membranes (Tosteson, 1968) and the construction of ion-selective electrodes (Ryba and Petranek, 1973). On account of their lipophilic exterior, crown ether complexes are often soluble even in apolar solvents. This property has been successfully exploited in liquid-liquid and solid-liquid phase-transfer reactions. Extensive reviews deal with the synthetic aspects of the use of crown ethers as phase-transfer catalysts (Gokel and Dupont Durst, 1976 Liotta, 1978 Weber and Gokel, 1977 Starks and Liotta, 1978). Several studies have been devoted to the identification of the factors affecting the formation and stability of crown-ether complexes, and many aspects of this subject have been discussed in reviews (Christensen et al., 1971, 1974 Pedersen and Frensdorf, 1972 Izatt et al., 1973 Kappenstein, 1974). 

Onium salts, such as tetraethylammonium bromide (TEAB) and tetra-n-butylammonium bromide (TBAB), were also tested as PTCs immobilized on clay. In particular, Montmorillonite KIO modified with TBAB efficiently catalyzed the substitution reaction of a-tosyloxyketones with azide to a-azidoketones, in a biphasic CHCI3/water system (Figure 6.13). ° The transformation is a PTC reaction, where the reagents get transferred from the hquid to the solid phase. The authors dubbed the PTC-modified catalyst system surfactant pillared clay that formed a thin membrane-hke film at the interface of the chloroform in water emulsion, that is, a third liquid phase with a high affinity for the clay. The advantages over traditional nucleophilic substitution conditions were that the product obtained was very pure under these conditions and could be easily recovered without the need for dangerous distillation steps. 

When lipases are used for enzymatic conversions, the enzyme is mainly active at a phase boundary, which can effectively be provided by a membrane. Additionally, for conversions requiring two phases (e.g. fat splitting [84—86] and esterifications [87]), the membrane also keeps the two liquid phases (an oil and an aqueous phase, respectively) separated. This is schematically depicted in Fig. 13.11. The equilibrium reactions involved are... 

The two main mechanisms of solute separation by liquid membranes involve chemical reactions. In the case illustrated in Fig. 15.2a, the solute first dissolves in the liquid membrane, then diffuses toward phase 3 due to the buildup of a concentration gradient, and finally transfers to phase 3 at the... 

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