High Solids Loading in Ethanol Production - Effects on Heat Integration Opportunities and Economics

Production of lignocellulosic ethanol still suffers the drawback of high production cost. Methods for making the process more cost efficient are therefore of high importance for commercialising large-scale production. One way of improving the process economics is to ensure efficient heat integration within the process, thereby reducing process energy costs. Another opportunity to improve production costs is to design the process for high solids loading in the bioreactors. By operating the process with higher solids concentrations (so-called high-gravity), water flows through the process will be reduced, which would enable smaller equipment and lower energy demand for downstream separation and purification of the ethanol product. However, problems with pumping and mixing are also likely to occur due to higher viscosities. Furthermore, the yield will be lower due to increased concentration of inhibitors. In addition, the opportunities for heat integration within the process will be affected. In this paper, the effect on heat integration opportunities of the high gravity concept is investigated for ethanol production in a stand-alone process in which fractionation of the softwood raw material is assumed to be achieved with steam explosion. Estimations of how the process economics will be affected are also provided.