SEPARATION-SEPARATION INTEGRATION FOR BIOETHANOL PRODUCTION

The development of technologies for separation-separation integration has been linked to the development of the different unit operations involved during down­stream processes and to new approaches for process intensification. The examples of separation-separation integration in the case of ethanol production mostly cor­respond to integration of the conjugated type, i. e., when integrated processes are carried out in different equipments closing the flowsheet by fluxes or refluxes.

Integration possibilities are particularly important for ethanol dehydration. In Chapter 8, the features and advantages of the integrated process of extractive distillation were emphasized. In the specific case of fuel ethanol production, the utilization of salts as extractive agents (saline extractive distillation) has dem­onstrated certain energetic advantages compared to other dehydration schemes according to some reports (Barba, et al. 1985; Llano-Restrepo and Aguilar-Arias, 2003). Cited results indicate that energy costs of saline distillation were lower than is the case of azeotropic distillation (using benzene, pentane, or diethyl ether), extractive distillation (using ethylene glycol or gasoline), or solvent extrac­tion, being almost the same as the costs of pervaporation. Pinto et al. (2000) employed Aspen Plus for the simulation and optimization of the saline extractive distillation for several substances (NaCl, KCl, KI, and CaCl2). This configura­tion was compared to the simulated scheme of conventional extractive distil­lation with ethylene glycol and with data for azeotropic distillation. Obtained results showed considerably lower energy consumption for the process with salts. However, for this latter case, the recovery of salts was not simulated. Thus, if evaporation and recrystallization of salts is contemplated, energy requirements could significantly increase, taking into account the energetic expenditures. In this way, the utilization of commercial simulators shows the viability for pre­dicting the behavior of a given process configuration provided the appropriate thermodynamic models of studied systems has been completed.

Gros et al. (1998) describe the process synthesis for ethanol dehydration using near critical propane. To this end, these authors combined thermodynamic mod­els for the description of ethanol recovery under supercritical conditions based on Group Contribution Associating Equation of State (GCA-EOS) with robust methods of simulation and optimization (integrating the SQP with MINLP). Considering the energy consumption as the objective function, the developed software analyzed the main units required by the configuration: high-pressure multistage extractors, distillation columns, and multiphase flash separators. Obtained results showed that configurations involving vapor recompression and feed preconcentration are competitive alternatives in comparison to azeotropic distillation (Cardona and Sanchez, 2007).

The utilization of pervaporation for the production of absolute (anhydrous) ethanol through its coupling with the previous distillation step has been reported (Cardona and Sanchez, 2007). The modeling and optimization of the process using MINLP tools showed 12% savings in the production costs with a 32% increase in membrane area and a reduction in both reflux ratio and ethanol concentration in the distillate of the column (Lelkes et al., 2000; Szitkai et al., 2002). Through pilot plant studies, the integration of distillation process with the pervaporation has been attained resulting in good indexes in terms of energy savings. These sav­ings are due to the low operation costs of pervaporation and to the high yield of dehydrated ethanol, typical of pervaporation processes (Tsuyomoto et al., 1997). The comparison between azeotropic distillation using benzene and the pervapo- ration system using multiple membrane modules showed that, at same ethanol production rate and quality (99.8 %), operation costs, including the membrane replacement every two to four years, for the pervaporation system are approxi­mately one-third to one-quarter those of azeotropic distillation.

complexed cellulose systems. Biotechnology and Bioengineering 8 (7):797-824.

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