3-37: The influence of process parameters on prehydrolysis prior to high solids batch simultaneous saccharification and fermentation of spruce to ethanol

Bio-ethanol can be produced from cellulose-rich materials, such as softwood, using enzymatic hydrolysis and fermentation. Simultaneous saccharification and fermentation (SSF) of steam pretreated SO₂-impregnated spruce has been shown to result in almost complete conversion of the fermentable sugars in the raw material to ethanol at moderate dry matter contents up to around 10% WIS (water insoluble solids), i.e. 15-20% total solids.

To reduce production cost, the energy demand in the distillation step needs to be reduced. To do so, the ethanol concentration after fermentation needs to be higher than that achieved using 10% WIS. More substrate has thus to be added to the process. Increasing the solids loading in the SSF step, however, results in significant reduction of the overall ethanol yield due to mixing difficulties, and increased concentrations of compounds inhibitory to the enzymes and yeast used in the process.

Experiments performed with prehydrolysis at 48 °C prior to SSF at 32 °C resulted in  a final ethanol concentration of 48 g/L, and an overall ethanol yield of 72% over the combined prehydrolysis and SSF steps using 13.7% WIS (24.4% total solids) of unwashed steam-pretreated spruce. Previously, the addition of a prehydrolysis step prior to SSF of steam-pretreated spruce was not able to improve ethanol concentration or yield at moderate WIS contents (up to 10% WIS). To understand the mechanism of prehydrolysis prior to high solids SSF, a systematic study on the influence of the process parameters viscosity, initial sugar concentration, inhibitor concentration and initial WIS content was performed.