Conroy, NS; (2017) Establishment of an ultra scale-down model of a lignocellulosic ethanol process. Doctoral thesis , UCL (University College London).

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Abstract

Lignocellulosic bioethanol processes have significant potential to reduce global greenhouse gas emissions. Such processes involve several steps, all of which require experimental optimisation. A significant aid to this research would be a validated Ultra Scale-Down (USD) platform that could be used to perform rapid and wide ranging screening and optimisation experiments on key operations in the process; namely enzymatic saccharification and microbial fermentation. Such experimental systems are widely used in the pharmaceutical industry but their applicability to lignocellulosic ethanol processes has been limited by the engineering complexity presented by biomass substrates, through, for example, the high viscosity and high levels of suspended solids involved. As a basis for development of the USD methods, initial studies focused on the engineering characterisation of the current standard laboratory-scale, 4L, Stirred Tank Reactors (STRs) used for lignocellulosic bioprocess development. These yielded quantitative values of key engineering parameters that needed to be matched in the small scale models such as solid-liquid suspension, liquid mixing and oxygen mass transfer. Along with practical considerations related to the volumes of sample required for analysis, orbitally shaken, conical bottomed tubes (30 mL) were identified as a potential USD bioreactor geometry. The suitability of the USD tubes as a model system for enzyme hydrolysis and fermentation processes was evaluated using Distillers’ Dried Grains with Solubles (DDGS). Detailed studies on enzyme hydrolysis of DDGS (steam pre-treated, solids loadings from 5 – 25 % (w/w)) in the USD tubes closely match those obtained from STRs, in terms of the rate, composition and concentrations of sugars released. This represents an eighty fold scale reduction. The utility of the USD approach was further illustrated by investigating factors that may be limiting hydrolysis yields at high solids loadings. Washing the residual solids periodically during hydrolysis allowed 100% of the available sugar to be hydrolysed using commercially available enzymes. The biomass hydrolysate is next used for fermentation by the thermophilic organism Geobacillus thermoglucosidasius. Here the presence of the suspended solid complicates the use of existing engineering correlations used as a basis for scale transfer in conventional STRs. Consequently, specific correlations were developed for prediction of kLa as a function of gassed power input and superficial air velocity at the USD and laboratory scales, accounting for the effect of suspended solids. This led to successful demonstration of comparability of fermentation performance between the USD tubes and STRs in terms of product titres, conversion yields and substrate utilisation rates. This again represented an 80-fold reduction in scale of operation and material requirements. The final studies aimed at validating the results from the USD saccharification and fermentation steps by using the USD data to predict operational expenditure, process step timings and feedstock conversion yields for a planned DDGS to ethanol production facility. These predictions are based on a validated process model from ReBio and can be validated against existing large scale data. Several model outputs were examined for scenarios using USD platform and demonstration scale data, for example the volume of ethanol produced from 100,000 tons of DDGS and the predicted rate of return on investment. There was good agreement between the outputs of the two models. Likewise a breakdown of expenditure by plant area showed a close agreement between the two models. The work in this thesis has therefore established the shaken USD tubes as a small scale alternative to STRs for early stage bioprocess development of lignocellulosic bioethanol processes. The USD tubes have been used successfully within ReBio in order to carry out strain screening experiments under conditions that closely reflect target process operating conditions.

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