The assessment of the environmental impacts generated during the fuel ethanol production process is a key criterion to select different conversion technologies during early stages of the design of such process. If different feedstocks are evalu­ated, the analysis of the environmental performance for the proposed technological configurations is even more important. In a previous work (Cardona et al., 2005a), the environmental performance of several process alternatives for fuel ethanol production from two types of feedstock was determined. The types of feedstock corresponded to lignocellulosic materials (herbaceous biomass, wood chips, sug­arcane bagasse, and waste paper) and starchy materials (corn, wheat, and cassava). Using the commercial process simulator Aspen Plus, different configurations were analyzed considering two options for ethanol dehydration step: adsorption using molecular sieves and azeotropic distillation.

The results obtained for the environmental performance of these processes (in terms of PEI per mass of product) are shown in Figure 10.5 for the three most rep­resentative process alternatives: (1) ethanol production from corn by dry-milling technology using molecular sieves for the dehydration step, (2) ethanol produc­tion from lignocellulosic biomass (wood chips) using azeotropic distillation for the dehydration step, and (3) ethanol production from lignocellulosic biomass (wood chips) using molecular sieves for the dehydration step. It is evident that the produc­tion of ethanol from starch has a lower impact on the environment than the bio­mass ethanol process. Because processes using biomass involve a pretreatment step where inorganic acids are used, which tends to increase the PEI. In comparing two

FIGURE 10.5 Potential environmental impact (PEI) per mass of product streams for different ethanol production flowsheets according to the eight impact categories addressed by the WAR algorithm: HTPI = human toxicity potential by ingestion, HTPE = human toxicity potential by either inhalation or dermal exposure, TTP = terrestrial toxicity poten­tial, ATP = aquatic toxicity potential, GWP = global warming potential, ODP = ozone depletion potential, PCOP = photochemical oxidation or smog formation potential, AP = acidification or acid-rain potential. Dehydration technologies: AD = azeotropic distilla­tion, MS = molecular sieves.

kinds of separation technologies for the same feedstock (wood biomass), the utili­zation of molecular sieves for recovery of the product has slightly lower PEI than the process involving azeotropic distillation due to the release of relatively small amounts of the entrainer (in this case, the toxic benzene) into the output streams. In this sense, the adsorption with molecular sieves is a cleaner separation technology and it is currently being used in the bioethanol industry.