In order for commercial scale lignocellulosic bioethanol to be economically feasible , high solid operation throughout the whole process is regarded as a necessity. This will not only reduce the need for high amounts of water and waste water treatment , but also increase ethanol titers after the fermentation step , which will lower energy demands in the following distillation step. However , since concentrated biomass slurries are associated with very high non-Newtonian viscosities , achieving sufficient mixing during high consistency enzymatic hydrolysis will be a challenging task. Poor mixing can lead to mass- and/or heat transfer problems as well as inhomogeneous enzyme distribution , all with possible yield reduction as an effect. Moreover , we have previously shown that stirrer speed (or more accurately – mixing-power) strongly affects the hydrolysis yield of certain lignocellulosic materials , in particular spruce , where an almost 100 % yield increase was achieved by increasing the stirrer speed from 75 to 500 rpm. On the contrary , other materials such as for example the energy crop Arundo Donax (Giant Reed) seems rather unaffected by mixing power input during enzymatic hydrolysis. This presentation summarizes what we have learned during the last years about rheology and mixing of high consistency biomass during enzymatic hydrolysis. The fact that different materials respond completely different to mixing power input during hydrolysis will be discussed and seems to be related to the viscosities of the materials. Furthermore , a discussion about liquefaction of biomass versus hydrolysis yield and the effect of particle size distribution on hydrolysis rates will be presented.
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