Scale-up of high-solid enzymatic hydrolysis of steam-pretreated softwood: the effects of reactor flow conditions

The importance of flow conditions during scale-up of high-solid enzymatic hydrolysis of steam-pretreated spruce was demonstrated by comparing hydrolysis rates between laboratory (2 L) and demonstration (4 m3) scale. A positive effect of increased agitation speed on the rate of enzymatic hydrolysis was found regardless of scale. Importantly, the hydrolysis rate was higher at the larger scale when compared at similar specific power inputs. Changes in the rheological properties of the pretreated material during the hydrolysis were followed by off-line measurements of apparent viscosity. This information was used to estimate the flow conditions in the reactors, i.e., average Reynolds numbers, which together with measured mixing power consumptions enabled a more detailed comparison between the scales. The hydrolysis yields correlated better with average Reynolds numbers than specific power input over the different scales. This indicates that mass transport limitations, caused by insufficient bulk flow, likely play a decisive role in determining the rate of enzymatic hydrolysis.