Water dehydration, from composite membranes

The phenomenon of PV was described for the first time by Kober in 1917 (Kober, 1919). The real breakthrough of this membrane process started in the 1980s with the development of a series of industrial appUcations and the use of poly(vinyl alcohol)-poly(acryl nitrile) (PVA-PAN) composite membranes for the dehydration of alcohol/water azeotropic mixtures (Thsel and Bruschke, 1985). The other main PV commerdal application was developed by Membrane Technology Research (MTR) and is based on the removal of VOCs from contaminated water (Cox and Baker, 1998). 

Although hydrophobic pervaporation membranes can be used to recover alcohol from fermentation broth, hydrophilic pervaporation membranes allow for the dehydration of water/alcohol mixmres (Figure 11.7). Hydrophilic pervaporation membranes can be applied to separate water from highly concentrated alcohol (>85%) (Abels et al., 2013). Two types of membranes have been explored in recent studies (1) zeoUte-based membranes and (2) polymeric composite membranes with polyvinylalcohol or polyimide as the active layer. 

The first application of pervaporation was the removal of water from an azeotropic mixture of water and ethanol. By definition, the evaporative separation term /3evap for an azeotropic mixture is 1 because, at the azeotropic concentration, the vapor and the liquid phases have the same composition. Thus, the 200- to 500-fold separation achieved by pervaporation membranes in ethanol dehydration is due entirely to the selectivity of the membrane, which is much more permeable to water than to ethanol. This ability to achieve a large separation where distillation fails is why pervaporation is also being considered for the separation of aromatic/aliphatic mixtures in oil refinery applications. The evaporation separation term in these closely boiling mixtures is again close to 1, but a substantial separation is achieved due to the greater permeability of the membrane to the aromatic components. 

Over the last 50 years, pervaporation (PV) has been developed as a technology with particular use in dehydration of solvents, alcohol/water separation, VOC removal from water, and separation of organic mixtures. Chapter 9 reviews the improvement of PV performance using membrane modification with a discussion on different strategies for surface modification of synthetic polymeric membranes and composite polymer/inorganic membranes. 

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