17-19: In-situ rheological characterization of different softwood by torque measurements during high solid enzymatic hydrolysis

Understanding the complex rheology of concentrated biomass slurries will be vital in order to produce concentrated sugar syrups from lignocellulosic biomass. In the production of second generation bioethanol for example, it is crucial to achieve high ethanol titers after the fermentation step, something which requires concentrated sugar syrups after the hydrolysis. Pretreated lignocellulosic biomass exhibits very high, non-Newtonian, viscosities which need to be considered in process design and optimization. We have previously performed an extensive rheological characterization of steam-pretreated spruce and linked the rheological data to process conditions, e.g. mixing power. The results show that viscosity, and in particular its change during enzymatic hydrolysis, has a strong influence on both power requirements and obtained sugar yields. It is not an easy task to measure viscosities of these dense fiber-slurries at high concentrations due to both particle settling and slip problems. It would be a significant advantage to be able to measure, or estimate, viscosities without sampling during enzymatic hydrolysis.

In this study, changes in viscosity have been studied in situ for different softwood materials during high solid enzymatic saccharification. Firstly, viscosities of model substrates (e.g. glycerol, xantan gum) were measured in a standard rheometer. These viscosities were then compared with torque measurements in a hydrolysis reactor to establish a correlation between torque and viscosity. Secondly, a set of hydrolysis experiments were performed in the reactor and evaluated both with respect to hydrolysis yield, viscosity changes and mixing-power requirements. A discussion on how these parameters influence the process will be presented.