High solid handling
In the aqueous-phase biomass hydrolysis route, biomass saccharification and fermentation steps are usually carried out in dilute solutions. Handling of large solvent volumes demands energy and any advancement in high solid loading is an energy saving. Development of new technologies for high solid handling is also a high priority, challenging area.
Distillation is another challenging area in large-scale operations. The current three column distillation technique requires a substantial amount of energy for distilling a dilute ethanol solution, or "beer," which contains only 10-12% ethanol to produce a ~90% ethanol azeotrope. The energy for the distillation process comes from burning lignin and dried fermentation wastes. Alternatively, lignin can be used for conversion to value-added products or burned to generate electricity, adding profits to integrated cellulosic ethanol plants. Further improvements in column/tray designs or combining distillations with other techniques like pervaporation may cut the energy cost associated with distillation. Substantial progress has been made in membrane-based pervaporation technologies for separation of ethanol out of the beer. However, more R&D is required in scaling up distillation-pervaporation integrated configurations.