Reverse osmosis ethanol/water separation

Ethanol/water separation distillation. 387, 394, 424 reverse osmosis, 642 Ethylene... 

Cadotte ( ) presents a comprehensive review of the development of the composite membrane with emphasis on the pros and cons of the four preparation methods mentioned above and on the polymer chemistry Involved. Cadotte points out that while each of the four methods continues to receive some attention, the Interfaclal polymerization method appears to be the most versatile. This method can be used to produce skin layers from polyamines, polylmlnes, polyurethanes, polyesters and other polymers. Elsewhere In this volume, Lee and co-workers (45) discuss the advantages and problems associated with using these composite membranes for ethanol-water separations via counter-current reverse osmosis. Also, Cabasso (44) discusses double-layer composite membranes. 

Ethanol-Water Separation by Countercurrent Reverse Osmosis... 

Ethanol produced by fermentation is conventionally dehydrated by distillation, an inefficient process that consumes energy equivalent to a large fraction of the energy content of the product ethanol.(X) Reverse osmosis (RO) has been considered before for ethanol-water separation because of its inherent energy efficiency. However, a difficulty encountered in using RO is the high osmotic pressures associated with concentrated ethanol solutions. For example, the osmotic pressure of a 15-volX ethanol solution is about 960 psi, and that of a 50-volX solution is about 3700 psi.(2,) Because most... 

Pervaporation is a relatively new process with elements in common with reverse osmosis and gas separation. In pervaporation, a liquid mixture contacts one side of a membrane, and the permeate is removed as a vapor from the other. Currendy, the only industrial application of pervaporation is the dehydration of organic solvents, in particular, the dehydration of 90—95% ethanol solutions, a difficult separation problem because an ethanol—water azeotrope forms at 95% ethanol. However, pervaporation processes are also being developed for the removal of dissolved organics from water and the separation of organic solvent mixtures. These applications are likely to become commercial after the year 2000. 

The most significant application of reverse osmosis has been in the field of desalination to produce drinking water. Other important apphcations include the treatment of industrial waste water, concentration of fruit juices, and concentration of weak solutions such as aqueous ethanol [3-6]. The rest of the chapter will focus almost entirely on semi-permeable membranes used for reverse osmosis based applications. We chose this focus in view of the importance of reverse osmosis as a rather efficient separation technique for separating a wide range of solutions, especially very dilute solutions—which are usually notoriously difficult to handle using conventional techniques such as distillation. 

In hot-climate viticulture it is a common practice to lower the high ethanol content of wines made from overripe fruit by partial dealcoholisation. This objective can be achieved by vacuum distillation, where the spinning cone column technique allows even more viscous liquids to be processed. Alternatively, a water-ethanol fraction can be separated from wine by reverse osmosis, followed by distillation of the water-ethanol permeate to yield high-grade ethanol and pure water. The latter will be added back to the treated wine. 

Because of the low energy requirements of separations by reverse osmosis, much attention has been devoted to other separations of aqueous solutions, at least on a laboratory scale, for instance, or ethanol/water. Membranes have been found that are moderately effective, but the main obstacle to the process is the very... 

The wet cellulose acetate membranes prepared for reverse osmosis purposes can be used for gas separation when they are dried. The water in the cellulose acetate membrane cannot be evaporated in air, however, because the asymmetric structure of the membrane will collapse. Instead, the multistage solvent exchange and the evaporation method is applied. In this method, the water in the membrane is first replaced by a water-miscible solvent such as ethanol. Then, the first solvent is replaced by a second volatile solvent such as hexane. The second solvent is subsequently air-evaporated to obtain a dry membrane. 

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. 

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