Comparisons of Common Dehydration Methods
There are several methods available for dehydration of wet ethanol to fuel grade nearly anhydrous ethanol. The choice of the method depends on the energy requirements and the capital cost of the technology. Dilute ethanol-water mixture is usually first concentrated by fractional distillation to approximately 90% ethanol and then is dehydrated by one of the several processes discussed earlier to produce fuel grade ethanol. The energy requirements of ethanol purification by fractional distillation remain essentially constant for feeds containing more than 15-20 wt% ethanol and less than 92-94 wt% ethanol. As ethanol concentration in the feed decreases, the reflux ratio required must increase dramatically, and this results in increased energy requirements. The fractional distillation requires a disproportionate raise in energy at product concentrations above 92-94 wt% ethanol due to the shape of the vapor-liquid equilibrium curve for the ethanol-water system. Table 15.6 shows a number of common dehydration methods and also shows the amount of energy necessary to accomplish the
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Table 15.6 Energy consumption of various processes for dehydration of wet ethanol to fuel grade ethanol [2].
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water removal from aqueous ethanol as a fraction of energy in a kilogram of anhydrous ethanol.
The combustion energy of anhydrous ethanol is about 29.5 MJ/ kg, and the amount of energy required for drying ethanol as a percentage of energy in ethanol is shown in the last column. Extractive distillation with ethylene glycol requires the highest amount of energy of 18.84 MJ/kg, and extractive distillations with salts require less energy compared to other distillation methods. Extractive distillations with calcium chloride require the least amount of energy of 5.02 MJ/kg.
The non-distillation processes for the production of anhydrous ethanol includes adsorption and membrane pervaporation. In comparison to all other methods, adsorption on molecular sieves requires only a distinctly small amount of energy of 0.528 MJ/kg, which is only 0.89% of the energy in processing equal weight of ethanol. This includes the energy required to regenerate the molecular sieve after adsorption of water from ethanol vapor containing 7.4% water, heat required to vaporize the feed, the energy needed to heat the regenerating air from ambient to an inlet temperature of 95°C and heat losses from the overall system. The heat of adsorption is retained in the bed if adsorption is stopped when the concentration wave begins to leave the adsorption column. Hence, it is desirable to adsorb up flow and regenerate by passing gas down flow, thus making use of some of the heat of adsorption stored at the upper part of the bed to drive off adsorbed water from the bottom of the bed. In 2008, Kaminski et al. reported a comparison of vapor permeation, pervaporation, azeotropic distillation and adsorption on molecular sieve processes on the basis of cost of production of anhydrous ethanol [68]. For small installations (100 dm3/day) they showed the cost of ethanol dehydration by azeotropic distillation is twice as high in the case of adsorption, and 1.5 times higher than that in pervaporation.
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