Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Chemical and Biochemical Engineering

Supervisor(s)

Dr. Lars Rehmann

Abstract

Extrusion pretreatment of lignocellulosic biomasses is a physical continuous pretreatment method for subsequent fermentative ethanol production. Twin-screw elements (conveying, kneading and reverse) have different effects on enzymatic digestibility according to their geometries and functions. The first study evaluated the effects of individual functional screw elements on enzymatic digestibility and found that extrusion enhanced enzymatic digestibility under all tested condition. Reverse and kneading screw elements however resulted in lignin re-distribution over the cellulose fibres, blocked pores (shown via SEM) and resulted in less digestible biomass, compared to conveying screw elements, only. Lignin removal via NaOH reversed this effect and the highest digestibility was found for biomass extruded in the presence of reverse screw elements (the most severe conditions tested in this study).

Based on these finding a process was developed for separating xylose from lignocellulosic biomass (steam-exploded corncobs) in a co-rotating twin-screw extruder, through water addition and solid-liquid separation. Eight screw configuration profiles were evaluated to define the best performance on xylose recovery. Subsequently, operating conditions (barrel temperature, screw speed and water flow rate) were examined with respect to xylose recovery and specific mechanical energy consumption. It was found that liquid/solid separation highly depended on the position and spacing of reverse screw elements. In addition, more xylose was recovered as the screw speed decreased and water flow rate and barrel temperature increased. Furthermore, operating conditions influenced the specific mechanical energy consumption of the motor drive.

The extruder was then used to produces differently treaded biomass samples, and a response surface method (RSM)-based model using IV-optimal design was used to study the combined effects of various enzymatic hydrolysis process variables (enzyme loading, surfactant addition, and hydrolysis time) with two differently extruded corncobs (7% xylose removal, 80% xylose removal) on glucose conversion. The results showed that the extrusion process led to an increase in cellulose crystallinity, while structural changes could also be observed via SEM. A quadratic polynomial model was developed for predicting the glucose conversion and the fitted model provided an adequate approximation of the true response as verified by the analysis of variance (ANOVA).


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