Membrane-Based Pervaporation Methods
Pervaporation is another technique that can be used in the dehydration of ethanol-water azeotropic mixture to fuel grade ethanol. In the pervaporation process, the membrane acts as a selective barrier between the two phases, the liquid phase feed and the vapor phase. It allows the desired component(s) of the liquid feed to transfer through it by vaporization. Separation of components is based on a difference in transport rate of individual components through the membrane. This process is used by a number of industries for several different processes, including purification and analysis, due to its simplicity and in-line nature. Typically, the upstream side of the membrane is at ambient pressure and the downstream side is under vacuum to allow the evaporation of the selective component after permeation through the membrane. The driving force for the separation is the difference in the partial pressure of the components on the two sides and not the volatility difference of the components in the feed.
The driving force for transport of different components is provided by a chemical potential difference between the liquid feed/retentate and vapor permeates at each side of the membrane. The retentate is the remainder of the feed leaving the membrane feed chamber, which is not permeated through the membrane. Separation of components in water-ethanol mixture is based on the differences in transport rate of individual components through the membrane. This transport mechanism can be explained using the solution-diffusion model based on the rate/ degree of dissolution of a component into the membrane and its velocity of transport (expressed in terms of diffusivity) through the membrane, which will be different for each component and membrane type leading to separation. A schematic diagram of a membrane pervaporation system is shown in Figure 15.3.
Membrane-based pervaporation is an emerging technology for the bioethanol industry and has the potential to reduce energy usage and operating costs [35-40]. In pervaporation, a fraction of the liquid feed is selectively evaporated, significantly reducing the amount of energy required relative to technologies in which the entire stream is evaporated. The ideal pervaporation membrane would achieve high permeability, high water selectivity, be easy to fabricate, and assemble into module forms. A number of research groups around the world have developed polymeric [41-52], inorganic [53-56], and composite membranes [57-59,48,60] for ethanol-water separations.
Polymeric membranes are attractive because they are relatively easy and economical to fabricate. However, polymeric membranes typically display a permeability-selectivity tradeoff. This is because permeability normally varies inversely with selectivity. Thus, membranes with desirable permeabilities often do not meet selectivity criteria. In addition, the performance of most polymeric water-selective
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Pervaporation Figure 15.3 Schematic diagram of a membrane pervaporation system. (Reprinted with permission from reference [34]; copyright 2007 Elsevier). |
membranes is a strong function of the water concentration. High water concentrations cause membrane swelling, resulting in higher permeabilities and lower selectivities. At low water concentrations, the benchmark dehydration membrane material, poly(vinyl alcohol) (PVA), becomes glassy and exhibits a very low water permeability. In addition, at low water concentrations, the driving force for transport across the membrane can be quite low, resulting in extremely low observed water fluxes. For example, the partial vapor pressure of water at 70°C in equilibrium with liquid ethanol containing 0.5 wt% water is only 7.5 Torr (1k Pa). Under these conditions, pervapo — ration experiments are challenging to perform. Also, most literature studies on ethanol dehydration by pervaporation focus on water concentrations at or above the standard azeotropic composition. Several research groups have studied the PVA membranes for dehydration of wet ethanol. Pervaporation data reported in the literature for selected PVA-based membranes evaluated at 10 wt% water and at similar temperatures are shown in Table 15.5.