Adsorption Method
Adsorption processes employ solid adsorbent material that selectively adsorb water molecule leaving ethanol on the basis of the difference in molecular size between water and ethanol. These adsorbent materials are known as molecular sieves as well because the separation depends on the size of molecules. A molecular sieve is a material containing tiny pores of a precise and uniform size. A number of materials with these characteristics are known, which include synthetic zeolites, clays, porous glasses, microporous charcoals, active carbons, as well as a variety of plant-derived adsorbents, including cornmeal, straw, and sawdust. The zeolite-type molecular adsorbents can be made to be both size and sorption-selective for water, thereby achieving a high selectivity for water; therefore, hydrophilic molecular sieves remain an industry standard [3].
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Table 15.1 Typical concentrations of common impurities in fermentation-produced crude ethanol [4].
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Impurities in fermentation-produced crude ethanol are mostly higher alcohols, methanol, esters, and aldehydes, but the concentration of these impurities are small. Typical concentrations of common impurities in fermentation-produced crude ethanol are shown in Table 15.1. As zeolite is highly selective to water, these small amounts of impurities are not removed by molecular adsorbents.