Rheology and effects of mixing on enzymatic hydrolysis and simultaneous saccarification and fermentation

The increased awareness of climate changes during the last years has put a lot of interest in the development of renewable fuels. Bioethanol production from lignocellulosic material is considered to have a large potential, especially the simultaneous saccarification and fermentation process. However no full scale plant is operating at the moment. Scale-up problems from laboratory or pilot plant scales are often related to mixing problems and will certainly be so in the SSF as well since the used material consist of a highly concentrated fiber suspension. In this work a set of enzymatic hydrolysis and SSF experiments were conducted at different stirrer speeds in order to investigate the effects of mixing. The effects that arose from these experiments were further examined in new experiments. In addition the rheology of pretreated spruce was investigated at different contents of water insoluble solids (WIS) and different degrees of hydrolysis. This study proves that the rate of hydrolysis is favored by high stirrer speeds, probably due to an increase in turbulence. The ethanol productivity in the SSF was, on the other hand initially lowered by an increased stirrer speed. However after 96 hours no significant influence of stirrer speed was observed. Several possible effects were examined to explain the lower initial productivity, but the only investigated factor that did show an influence was the presence of oxygen which seemed to slow down the glucose uptake rate together with the fermentation rate. The rheology measurements indicated a Bingham plastic flow behavior for the pretreated spruce at WIS contents above 6 %. It was concluded that higher WIS content resulted in an increased viscosity and further it was shown that the degree of hydrolysis affected the material viscosity. The viscosity could however not be correlated only to the fiber concentration, which indicates that also shape and composition of the fibers is essential. Introduction The effect of fossil fuel on the climate has been paid more and more attention to in the recent years, especially since it has been commonly accepted that observed climate changes, can be derived from human actions. This effect originates in the emission of carbon dioxide and in order to reduce the emissions of CO2, a change to renewable fuels is a good option. Bioethanol can be produced from lignocellulosic materials and simultaneous saccarification and fermentation (SSF) has shown to be a promising process proposal [1,2]. This is, however, an advanced process subjected to lots of research and today no full scale plant is operating [3]. In order to get an economically feasible process it has been shown that as high ethanol concentration as possible should be obtained in the SSF step [4]. This can be obtained by increasing the substrate load, i.e. the content of water insoluble solids (WIS). However, some problems arise due to an increased WIS content, for example higher inhibitor concentrations as well as an increased viscosity, although successful SSF experiments have been reported at 10 % WIS [5]. Scale-up problems from laboratory or pilot plant scales are often related to mixing problems and will certainly be so in the bioethanol case as well since the slurry consist of a highly concentrated fiber suspension. This study focuses on the effects of mixing on enzymatic hydrolysis and SSF, but also on