Energy considerations for a SSF-based softwood ethanol plant

Ethanol can be produced from softwood by steam pretreatment followed by simultaneous saccharification and fermentation (SSF). However, the final ethanol concentration in the SSF step is usually rather low (around 4 wt%) and as a result the energy demand in the downstream processing will be high. In an effort to reduce the energy consumption various alternatives for the downstream processing part of the process were evaluated from a technical-economic standpoint. With experimental data as a basis, the whole process was modelled using the commercial flowsheeting program Aspen Plus. The results were used in the subsequent economic evaluation, which was performed using Icarus process evaluator. A base case configuration, consisting of three thermally coupled distillation columns and multiple-effect evaporation was established. For a feed containing 3.5% ethanol (w/w) to the distillation step, the overall process demand for steam was estimated to be 19.0 MJ/L ethanol and the ethanol production cost 4.14 SEK/L (0.591 USD/L). Different alternatives were considered, such as integration of a stripper with the evaporation step, increasing the number of evaporation effects and the application of mechanical vapour recompression to the evaporation step. Replacement of evaporation with anaerobic digestion was also considered. Among these alternatives, evaporation using mechanical vapour recompression and the anaerobic digester alternative both resulted in significantly lower energy demand than the base case, 10.2 and 9.8 MJ/L, respectively, and productions costs of 3.82 (0.546 USD/L) and 3.84 SEK/L (0.549 USD/L).