RECIRCULATION OF PROCESS WATER IN THE PRODUCTION OF ETHANOL FROM SOFTWOOD

In the production of ethanol from lignocellulosics, large quantities o f water are needed in various process steps. In industrial processes, recycling of process streams is necessary to minimise fresh water requ irements and decrease the amount of wastewater produced. This leads to increases in various substances, both volatile and non-volatile which inhibit fermentation. In the present Study ethanol was produced from softwood in a bench-scale process development unit. The stillage strea m from the distillation step was fractionated by evaporation and vario us evaporation fractions characterised and their inhibitory effect on fermentation investigated. irt was found that the volatile fractions h ave no negative effect on the fermentation while the non-volatile frac tion inhibited fermentation and resulted in a decreased yield. Simulat ion of a six-effect evaporation unit based on the experimental data sh ows that the intermediate evaporation condensate fractions are most su ited for disposal. The chemical and biological oxygen demands are only about 10% of the value in the stillage stream. Evaporation has been s hown to be an effective way to drastically diminish the build-up of in hibitory compounds when process streams are recirculated, but the ener gy demand is high. Various process configurations to reduce the energy demand are suggested. (C) 1997 Elsevier Science Ltd.