Influence of Impeller Design On Power Input and Performance During High Solid Enzymatic Hydrolysis of Pretreated Lignocellulose

Reaching high ethanol titers after the fermentation step is crucial in cellulosic ethanol production in order to make the process cost competitive and commercially available. To achieve this, a high amount of water insoluble solids (WIS), i.e. lignocellulose fibers, is needed in the hydrolysis step. However, a high WIS content in the hydrolysis step is likely to give a high viscosity. The power requirements needed to operate the impeller at high viscosities can be quite substantial, and high WIS processes therefore may need to be operated with a poor mass- and/or heat transfer. We have previously shown that stirrer speed (or more accurately – mixing-power) strongly affects the hydrolysis of certain lignocellulosic materials, in particular pretreated spruce. An almost 100 % yield increase was achieved by increasing the stirrer speed from 75 to 500 rpm. In contrast, the power input seems to have only a very marginal impact on the hydrolysis performance when using for example the energy crop Arundo Donax (Giant Reed). Another difference between these particular materials is their relative change in rheology due to enzymatic hydrolysis. For example, Arundo Donax quickly (within 2-4 hours) losses its network strength, leaving no significant difference in needed power input between different WIS contents (within the range of 10-20 % WIS). Spruce on the other hand retains its strength, i.e. high viscosity, during a much longer time which results in significantly higher power inputs during a longer time. For the same stirrer rate, approximately, four times as much energy is needed for mixing of spruce compared to Arundo Donax at 20 % WIS.

In the present study, enzymatic hydrolysis at high solid loadings (<20% WIS) was investigated using different impeller types. In addition to sugar yields, impeller torques (and hence power input) were monitored throughout the hydrolysis to compare the suitability of the impeller type for this process. Furthermore, it was possible to control the power input at a fixed set-point value throughout the hydrolysis period by adjusting the stirrer speed. This enables a proper comparison between the different impellers based on an energy input basis. The factors affecting high solid hydrolysis will be discussed.