Ethanol-rejecting membranes

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. 

The low ethanol rejection and the instability of the hollow-fiber NS-100 membranes preclude the use of this membrane for practical ethanol enrichment. Nevertheless, for the purpose of demonstrating the concept of CCRO using hollow-fiber membranes, CCRO experiments were conducted at the reduced ethanol concentration of 10 vol%. The permeate fluxes of NS-100 modules were measured at 250 psi in the absence and presence of recirculation with a 10-volX ethanol solution. The results were varied recirculation brought about flux increases ranging from 5% to about 20%. The limited flux increase may again be explained in terms of the formation of a polyamine gel during NS-100 membrane fabrication. Nevertheless, the flux increase shows that the hollow-fiber geometry is a viable one for CCRO operation. 

Estimated Process Performance with a Hypothetical Membrane. The CCRO performance of a hypothetical membrane was calculated as an estimate of the improvement in process performance that may be realized when better membranes are developed. Assume that a membrane could be developed to be totally ethanol-resistant and to also have better selectivity than does 3N8. Specifically, its transport parameters are assumed to be the same as those of 3N8, but its ethanol permeability coefficient, u, is only one-tenth as high. Because of its better selectivity, the hypothetical membrane would exhibit higher ethanol rejection and lower fluxes than does 3N8 at any concentration. 

Figure 2. Rejection performance of five different RO membranes (6). A, methanol B, aniline C, formaldehyde D, methyl acetate E, acetic acid F, urea G, ethanol H, acetone 1, hydroquinone J, isopropyl alcohol Kt glycerol L, sodium chloride M, ethyl ether N, phenol. Conditions pressure, 40.8 atm temperature, 24 °C feed, 0.30 gal/min.

In any process, if one component is enriched at the membrane surface, then mass balance dictates that a second component is depleted at the surface. By convention, concentration polarization effects are described by considering the concentration gradient of the minor component. In Figure 4.3(a), concentration polarization in reverse osmosis is represented by the concentration gradient of salt, the minor component rejected by the membrane. In Figure 4.3(b), which illustrates dehydration of aqueous ethanol solutions by pervaporation, concentration polarization is represented by the concentration gradient of water, the minor component that preferentially permeates the membrane. 

In the case of reverse osmosis, the enrichment factors (E and Ea) are less than 1.0, typically about 0.01, because the membrane rejects salt and permeates water. For other processes, such as dehydration of aqueous ethanol by pervaporation, the enrichment factor for water will be greater than 1.0 because the membrane selectively permeates the water. 

To improve process economics, an integrated process shown conceptually in Fig. 27 has been proposed. A per-vaporation subsystem is equipped with a membrane selectively permeable to water and ammonia, but rejects ethanol and ethyl lactate. The retentate stream carrying these reactants may be returned to the reactor to help drive the reaction toward completion. 

The extrapolation is to what is called pervaporation, where the feed mixture is a liquid, but the permeate vaporizes during permeation, induced by the relatively low pressure maintained on the permeate side of the membrane. Accordingly, the reject or retentate remains a liquid, but the permeate is a vapor. Thus, there are features of gas permeation as well as hquid permeation. The process is eminently apphcable to the separation of organics and to the separation of organics and water (e.g., ethanol and water). In the latter case, either water vapor may be the permeate, as in dehydration, or the organic vapor may be the permeate. The obvious, potential application is to the separation of azeotropic mixtures and close-boiling mixtures—as an alternative or adjunct to distillation or liquid-liquid extraction methods. 

All of the membranes tested degraded to some extent upon exposure to concentrated ethanol solutions. Degradation was manifested by lower rejections and higher fluxes when membranes exposed to high ethanol concentrations were retested at low feed concentrat ion. 

Membranes have also been integrated at a second stage in edible-oil processing to recover the extraction solvents. Koseoglu reported the use of RO and UF in the separation of cotton-seed oil from isopropanol, ethanol and hexane. These membranes, originally developed for aqueous applications, showed acceptable performances in isopropanol and ethanol, but were often damaged or hardly permeable, as in the case of hexane [28]. Schmidt tested a PDMS membrane in the separation of corn-seed oil from hexane, with high permeability and rejection values of 90%... 

Shukla and Cheryan [18] studied the effect of conditioning on the performance of UF polymeric membranes in solutions with various concentrations of ethanol in water. These authors analyzed membranes made of materials such as PS, polyether-sulfone, PAN, PVDF and methyl cellulose acetate. Methods of conditioning consisted of gradual change in the levels of ethanol in aqueous solution at 0%-70%, with the increase of 10%-10% (method 1), a direct change of 0%-70% ethanol (method 2), exposure to direct solution of 70% ethanol (method 3), and reduction from 100% to 70% ethanol (method 4). After each conditioning method, the membranes were tested for flow and rejection of zein (protein present in corn, soluble in ethanol. 

Van der Bruggen, B., Geens, J., and Vandecasteele, C. (2002) Fluxes and rejections for nanofiltration with solvent stable polymeric membranes in water, ethanol and n-hexane. Chemical Engineering Science 57, 2511-2518. 

Various water soluble solvents such as acids and alcohols have been used to treat TFC membrane surfaces [20]. Mixtures of alcohol (ethanol and iso-propanol) and acid (hydrofluoric and hydrochloric add) in water have been used to improve flux and rejection, a result of partial hydrolysis and skin modification initiated by the alcohol and acid. The presence of hydrogen bonding is believed to encourage interaction between the acid... 

The RO and NF membrane processes are discussed in detail in Chapter 1. RO membranes are weU-suited to rejecting dissolved ions and most organics (some organics such as ethanol and acetone have very low rejections of 45-55%). The rate of water transport through a membrane depends on membrane properties (polymeric, chemical, morphological), water temperature, and the difierence in applied pressure across the membrane, less the difference in osmotic pressure between the concentrated and dilute solutions. Osmotic pressure is proportional to the solution concentration and temperature, and depends on the type of ionic species present. For solutions of predominandy sodium chloride at 25°C, a mle of thumb is that the osmotic pressure is 0.7 bar per 1000 mg/1 concentration (see Table 6.11 for osmotic pressures of various solutions). 

Several types of zeolite membranes such as A-type, Y-type, silicalite, ZSM-5, etc. have been developed, and have been applied mainly to gas and pervapo-ration separations. Kumakiri et al. [43] prepared A-type zeolite membranes by hydrothermal synthesis with seed growth, and applied these to the reverse osmosis separation of water/ethanol mixtures. The zeolite A membrane showed a rejection of 40% and a permeate flux of 0.06 kg m h for 10 wt% ethanol at a pressure difference of 1.5 MPa, while a permeate flux of 0.8 kgm h and a separation factor of 80 were obtained in PV. 

This chapter has studied the control of a column-pervaporation process for producing high-purity ethanol to overcome the azeotropic limitation encountered in distillation. A conventional control structure is developed that provides effective dismrbance rejection for both production rate and feed composition changes. A simple pervaporation model is developed in Aspen Custom Modeler that captures the important dynamic features of the process. The model uses pervaporation characteristic performance curves to determine diffusivities. Component fluxes depend upon composition driving forces between the retentate and permeate sides of the membrane. The dynamics of the pervaporation cells are assumed to be dominated by composition and energy capacitance of the liquid retentate. 

In aortas implant of biological origin, it is necessary to inactivate and/or remove living cells to avoid rejection postoperative. Sawada et al., [126] used modified CO2-SFE with ethanol for 20 minutes at 150 bar and 37 C to remove nucleus and cell membranes fixim porcine aorta, thereby there was obtained a dry, free tissue cells without the need for surfactants and aldehydes in the process. 

Kim et al. made a more detailed study of the effect of solvent in the SPPO coating solution on the performance of SPPO TFC membranes. In their study solute rejection of NaCl and MgS04 was used as a measure for the membrane selectivity. Four solvents including methanol, 2-methoxy ethanol, 2-ethoxy ethanol and 2-butoxy ethanol were used to make 0.5 wt. % SPPO... 

Kim et al. studied further the effect of solvent evaporation temperature on the reverse osmosis performance. Butoxy ethanol solution containing 0.5 wt.% SPPO was used as coating solution. Figure 9 illustrates the effect of evaporation temperature on the solute rejection and product permeation flux of the SPPO TFC membranes. The figure shows that the salt rejection increased while the product permeation flux decreased when the... 

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