Optimization of Water Consumption in Second Generation Bioethanol Plants

In this work we address the water consumption optimization of second generation bioethanol production plants from lignocellulosic switchgrass when using thermo-chemical, thermo-biochemical, or biochemical routes considering corn-based ethanol as a reference. To optimize the water consumption a three stage method is used. First, energy consumption is optimized in the production processes, which reduces the cooling needs of the processes and thus, the water losses by evaporation and drift in the cooling tower. Next, a number of technologies are considered to partially substitute the use of water as cooling agent. Finally, the optimal water networks for each of the ethanol production processes are designed by determining water consumption, reuse, and recycle and the required treatment using a superstructure optimization approach. The resulting water consumption ratios range from 1.5 to 3 gal/gal, which are in the range or even below the amount of water needed for gasoline production and with low or no water discharge depending on the process. Further reduction can be obtained by using air cooling and if the water released from the crop can be properly recovered and treated. Under these conditions the water consumption ratios range from 0.4 to 1.7 gal/gal and with no or low water discharge.