Ethanol Recovery and Dehydration

The recovery of ethanol produced by different technological configurations and from diverse types of feedstocks is accomplished in a very similar way. The ethanol content in the culture broth resulting from fermentation processes oscil­lates between 2.5 and 10 % (by weight). The utilization of fuel ethanol as a gasoline oxygenate requires a high-purity ethanol, so it is necessary to concentrate the ethyl alcohol up to 99.5 % obtaining the anhydrous ethanol, which is the suitable form used for ethanol-gasoline blends. The first step of the ethanol recovery scheme is the concentration of ethanol contained in culture broths. This process is carried out in a distillation (concentration) column achieving ethanol content about 50 %. This product is removed from the column by a side stream. The overhead vapors contain mostly CO2 (about 84 %), a significant amount of ethanol (12 %), and a small amount of water. The following step is the rectifi­cation of this concentrated stream in order to obtain a product with 90-92 % ethanol composition, which is near to the azeotropic mixture of ethanol and water (95.6 %). To achieve 99.5 % or more of ethanol purity from streams containing 90-92 % ethanol, it is necessary to employ nonconventional separation operations like pressure-swing distillation, azeotropic distillation, extractive distillation, adsorption, and pervaporation. All these operations have found industrial appli­cation in the fuel ethanol industry (Cardona et al. 2010b).

The adsorption is one of the most important unit operations currently employed in the biofuel industry for ethanol dehydration. In this operation, the ethanol-water mixture passes through an apparatus usually cylindrical that contains a bed with an adsorbent material. Due to the difference in the affinity of molecules of water and ethanol with respect to the adsorbent, the former remains entrapped in the bed while the ethyl alcohol passes through this same bed increasing its concentration in the stream leaving the apparatus. Adsorption of water employing the so-called molecular sieves to dehydrate ethanol has been the technology that has acquired more development in the last years in the fuel ethanol industry. In fact, this technology has been replacing the azeotropic distillation (Cardona et al. 2009).

The adsorption operation requires that, once the adsorbent bed is saturated with the water that needs to be removed, its desorption should be accomplished to make possible the reutilization of the adsorbent material (regeneration cycle). For regeneration of the sieves, hot gas is needed. This rapidly deteriorates them espe­cially if the bed is fed with a liquid stream during the previous water adsorption cycle. To counter this deterioration, the pressure-swing adsorption (PSA) technol­ogy was developed. This technology involves the use of two adsorption beds. While one bed produces vapors of anhydrous ethanol superheated under pressure, the other one is regenerated under vacuum conditions by recirculating a small portion of superheated ethanol vapors through the saturated sieves. The system feeding is carried out using the overhead vapors from the rectification column. The ethanolic vapors obtained in the regeneration cycle and that can contain 28 % water are recirculated to the rectification column (Montoya et al. 2005; Wooley et al. 1999b). In this way, the molecular sieves life can be prolonged for several years that, in turn implies very low costs related to the replacement of adsorbent material, and therefore reduced operating costs (Guan and Hu 2003; Madson and Monceaux 1995). The process flowsheet for ethanol recovery and dehydration in the case of the adsorption with molecular sieves is depicted in Fig. 16.2.

Fig. 16.2 Process flowsheet of product concentration, dehydration by molecular sieves and effluent treatment steps for ethanol production from lignocellulosic biomass; 1 SSCF bioreactor, 2 scrubber for recovery of ethanol vapors, 3 preheater, 4 concentration column, 5 rectification column, 6 9 heat exchangers, 7 molecular sieves, 8 regenerate tank, 10 product cooler, 11 centrifuge, 12 anaerobic digester, 13 activated sludge tank, 14 clarifier, 15 boiler/burner, 16 turbogenerator

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