Ethanol-permselective membrane

Except poly(dimethylsiloxane), all of the conventional polymers such as cellulose acetate, poly(phenylene oxide), nylon, polyethylene, and nafion are known to permeate water preferentially against ethanol111,112). However, in order to separate ethanol from dilute aqueous solution, it is favorable to use a membrane which permeates ethanol preferentially (ethanol-permselective membrane). 

Pervaporation is a method of separating liquid mixtures, especially ethanol-water mixtures, withtheuseofapolymermembrane. Poly(TMSP)membranehasprovedhighlypermselective for ethanol in the ethanol-water pervaporation. Thus, the separation factor, a(EtOH/ H2O), reaches 17 at 10 wt% ethanol in the feed. This value is similar to that for poly(dimethylsiloxane), a well-known ethanol-permselective membrane. In contrast, other substituted poly acetylenes are nonpermselective or rather water-permselective. On the basis of its excellent adsorptivity for organic compounds, poly(TMSP) has been applied to a regenerate column packing which removes trace organic solvents from water ". ... 

Ethanol permselective membrane system has been used for the extraction of ethanol from fermentation broth. However, both membrane distillation and polymeric silicon rubber membranes showed low separation factors of ethanol and were invalid in this case. M. Nomura et al. [26] investigated the continuous extraction of ethanol from ethanol fermentation broth through a silicalite-1 membrane. From 4.73wt.% ethanol concentration of broth, the permeate ethanol concentration was 81.0wt.%. In our group, we have also investigated the potentiality of silicalite-1 membrane for alcohol extraction from aqueous solution [27]. 

A trimethylsilyl or trifluoromethyl group containing poly(phenylacetylenes) for oxygen and ethanol permselective membranes had been prepared by WCle and a Rh complex [Rh(cyclooctadiens) (triphenyl-phosphine) 2] PFe (RhP). ... 

A water-permselective membrane is useful in accelerating chemical reactions in which water has to be removed. For example, the reaction, R-OH + R -COOH R-OOC-R + H20, is accelerated by the removal of H20 from the reaction bath by pervaporation. Figure 6.40 shows that pervaporation to remove water substantially increases the conversion in esterification of oleic acid with ethanol.231 This technique has been used in the esterification of oleic acid with methanol and ethanol.232 The concept is also applicable in accelerating chemical reactions near the equilibrium state. 

Vapor permeation and pervaporation are membrane separation processes that employ dense, non-porous membranes for the selective separation of dilute solutes from a vapor or liquid bulk, respectively, into a solute-enriched vapor phase. The separation concept of vapor permeation and pervaporation is based on the molecular interaction between the feed components and the dense membrane, unlike some pressure-driven membrane processes such as microfiltration, whose general separation mechanism is primarily based on size-exclusion. Hence, the membrane serves as a selective transport barrier during the permeation of solutes from the feed (upstream) phase to the downstream phase and, in this way, possesses an additional selectivity (permselectivity) compared to evaporative techniques, such as distillation (see Chapter 3.1). This is an advantage when, for example, a feed stream consists of an azeotrope that, by definition, caimot be further separated by distillation. Introducing a permselective membrane barrier through which separation is controlled by solute-membrane interactions rather than those dominating the vapor-liquid equilibrium, such an evaporative separation problem can be overcome without the need for external aids such as entrainers. The most common example for such an application is the dehydration of ethanol. 

Matsuda, H., Yanagishita, H., Kitamoto, D. et al. 2001. Preparation of silicate pervaporation membrane with ethanol permselectivity by qa 2-step hydrothermal synthesis. Sep. Sci. Technol. 36(15) 3305-3310. 

Pervaporation. Pervaporation differs from the other membrane processes described so far in that the phase-state on one side of the membrane is different from that on the other side. The term pervaporation is a combination of the words permselective and evaporation. The feed to the membrane module is a mixture (e.g. ethanol-water mixture) at a pressure high enough to maintain it in the liquid phase. The liquid mixture is contacted with a dense membrane. The other side of the membrane is maintained at a pressure at or below the dew point of the permeate, thus maintaining it in the vapor phase. The permeate side is often held under vacuum conditions. Pervaporation is potentially useful when separating mixtures that form azeotropes (e.g. ethanol-water mixture). One of the ways to change the vapor-liquid equilibrium to overcome azeotropic behavior is to place a membrane between the vapor and liquid phases. Temperatures are restricted to below 100°C, and as with other liquid membrane processes, feed pretreatment and membrane cleaning are necessary. 

A process referred to as vapor-arbitrated pervaporation addresses these issues by manipulating the transmembrane activity gradients of water and ethanol in a pervaporation system. Using a permeate side sweep stream that contains water vapor at a partial pressure corresponding to the activity of water on the feed side, permeation of water is halted while ethanol continues to diffuse through the membrane into the sweep stream and is removed. In this way, the native permselectivity of the membrane system can be altered in a controlled fashion to extract one or more volatile components from a solution. 

Similarly, Sano et al. [1994] added colloidal silica to a stirred solution of tetrapropylammonium bromide and sodium hydroxide to synthesize a hydrogel on a stainless steel or alumina support with a mean pore diameter of 0.5 to 2 pm. The composite membrane is then dried and heat treated at 500 C for 20 hours to remove the organic amine occluded in the zeolite framework. The silicalite membranes thus obtained are claimed to be free of cracks and pores between grains, thus making the membranes suitable for more demanding applications such as separation of ethanol/water mixtures where the compound molecules are both small. The step of calcination is critical for synthesizing membranes with a high permselectivity. 

Real membranes are not perfectly permselective the rate of ethanol permeation across the skin layer of the membrane determines the actual osmotic pressure gradient. The lower the membrane selectivity, the less likely it is that the flow of permeate would be stopped due to high osmotic pressures. However, using a membrane with poor ethanol rejection necessarily compromises the efficiency of separation. 

Uragami, T., Kato, S., and Miyata, T. 1997. Structure of N-alkyl chitosan membranes on water-permselectivity for aqueous ethanol solutions, J. Membr. Sci., 124 203-211. 

Extended immersion of the PIM-PI membrane in alcohol obviously affects the properties of permeation. After immersion in alcohol, the residual solvents can be removed and polymer chain relaxation may be achieved in the swollen state. It was observed that both ethanol and methanol treatment have similar effects on gas permeability. PIM-PI membranes treated with methanol or ethanol for several days show higher gas permeabilities than the films cast directly from chloroform, even though these films are subjected to removal of the chloroform in a vacuum at room temperature for a few days until a constant weight of the film is achieved (Table 5.3). These effects were also observed in PIM-1, though PIM-1 has the higher permeability coefficient of the films. These phenomena indicate local interactions between low molecular weight alcohol and some binding sites on the PIM structure. It should be noted that an increase of permeability is always accompanied by a decrease in permselectivity. 

Nagase et al. have studied the grafting of polyacetylene and its derivative onto poly(dimethylsiloxane) (PDMS) for membrane applications [118-121]. Poly(l-trimethyl-silyl-l-propyne) (PTMSP) is a polymer known to have excellent gas permeability but suffers from relatively low selectivity and a decrease in gas permeability over time. Graft copolymers of poly(l-phenyl-1-propyne) (PPP) onto PDMS were found to have improved gas permeability and selectivity, and the performance was related non-linearly to the PDMS content. A minimum oxygen permeability coefficient was found for a copolymer with 55 mol% PDMS [118]. The same membrane series was also foimd to be permselective for a range or organic liquids, including an ethanol/water mixture, and was used in pervaporation applications... 

Their applications and improved long-term stability for gas separation and pervaporation were further investigated. A maximum in the separation factor and rate was obtained for the PTMSP copolymer with 12 mol% PDMS. The membrane was able to convert a 7 wt% ethanol mixture to over 70 wt% in ethanol [120]. Good oxygen permeability and stability of over a month was also achieved for the PTMSP copolymers with over 60% PDMS [121]. In a separate study, PPP was foimd to have good permselectivity to water while PTMSP is alcohol permselective. These copolymers as well as a series of poly(phenyl acetylene) graft PTMSP copolymers were further studied for pervaporation applications [122]. 

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