5-25: Effects of mixing and power consumption on the enzymatic hydrolysis of lignocelluloses

To realize a successful scale-up of lignocellulose ethanol production, a high ethanol concentration after the fermentation is highly important. Clearly, a high content of water insoluble solids (WIS) is needed in the enzymatic hydrolysis (EH). Increasing the WIS content is, however, not without problems since this will result in high viscosities, with problems of poor mixing and/or high power consumptions as a result. In order to adequately design and cost-optimise a large-scale process it is therefore important to understand the impact of mixing on the EH both in terms of hydrolysis yield and needed energy input.

In the current work, the effects of mixing on the EH of steam pretreated spruce were studied in stirred bioreactors, operated at fixed impeller speeds. Different enzyme loadings (10 and 20 FPU g^-1 glucan), of a commercially available enzyme, were tested. Experiments were performed at an initial WIS content of 10 %. It was shown that impeller speed greatly impacts the enzymatic hydrolysis (e.g. two times higher conversion after 96 hours at 500 RPM compared to 75 RPM), an effect that remained throughout the 96 hour hydrolysis. Estimations of power input needed to operate the impeller were made and related to the degree of conversion, taking into account the changes in rheology throughout the process. A high impeller speed requires a power input which will not be feasible in large scale applications. The possibilities of achieving the same conversions by altering the enzyme load, impeller speed (i.e. energy input) and residence time are discussed.