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Stirrer speed adaption is beneficial for liquefaction of cellulose

corresponding

THOMAS HAHN1*, MATTHIAS NOTHACKER1, ALINE KÖVILEIN1, 3, ALEXANDER BECK2, SUSANNE ZIBEK1
*Corresponding author
1. Fraunhofer Institute for Interfacial Engineering and Biotechnology, Department Molecular Biotechnology, Stuttgart, Germany
2. Institute of Interfacial Engineering and Plasma Technology IGVP, University of Stuttgart, Germany
3. Karlsruhe Institute of Technology, Institute of Process Engineering in Life Sciences, Section II: Technical Biology, Karlsruhe, Germany

Abstract

Enzymatic hydrolysis of cellulose is a key step in the valorisation of lignocellulosic material subsequent to pretreatment. The optimization of hydrolytic enzymes is in the focus of intensive research of different institutes and companies whereby not much attention is being paid to the process engineering parameters such as the stirrer velocity. 

We therefore focused on a stirrer speed adaption strategy during enzymatic cellulose hydrolysis in larger lab-scale. Mixing time measurements were performed with carboxymethyl cellulose solutions in a 10 L-tank reactor equipped with a segmented helical stirrer to determine agitator speed levels adapted to the cellulose suspensions’ viscosities at the certain hydrolysis time. Comparing the optimized hydrolysis with adaptation of stirrer speed to the hydrolysis performed at a constant stirrer velocity, a 30% higher glucose yield and 25% lower energy input was observed. Overall, the enzymatic hydrolysis of cellulose applying a step-wise adaption of the stirrer velocity thus resulted in a faster liquefaction, while at the same time the overall power requirement was reduced.


INTRODUCTION

Lignocellulose from second generation resources contains two major fractions: a lignin fraction that can provide a natural source of aromatic compounds, and a carbohydrate fraction consisting of cellulose and hemicellulose, which can both be utilised for microbial fermentation, for example. One of the most promising techniques to separate the different fractions is the organosolv process, which is characterized by high separation efficiencies (1). This technique applies organic solvents and potentially an acidic catalyst along with temperatures above 100 °C to prepare the fractions for successive processing. The residual solid after pretreatment mainly consists of cellulose, which requires a subsequent (enzymatic) hydrolysis step to obtain monosaccharides (e.g. glucose) as appropriate substrate for fermentation. A major challenge concerning the use of enzymes for saccharification of cellulose is the adsorption of these enzymes to residual lignin as well as unproductive binding to cellulose (2, 3). Since the enzymes provide the main cost factor of this process step (4), several researchers already highlighted the relevance of enzyme production, optimi ...




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