The effects of mixing on the enzymatic hydrolysis of lignocellulosic biomass
Abstract: Biorefining of lignocellulosic biomass into biofuels and chemicals can help replace fossil resources and decrease anthropogenic greenhouse gas emissions. This thesis is focused on the effects of mixing on the enzymatic hydrolysis of pretreated biomass. Two different types of biomass were studied: softwood (Norway spruce and Scots pine), and the energy grass giant reed. Before enzymatic hydrolysis, the biomass was pretreated by either steam or sulfite pretreatment. The first part of the work concerns the connection between particle morphology and rheology of pretreated biomass, how such properties change during the course of enzymatic hydrolysis, and how the changes are influenced by reactor mixing. The second part examines the effects of mixing in stirred tank reactors on the enzymatic hydrolysis of different pretreated materials, and also attempts to explain the mechanisms behind the observed phenomena.The particle size reduction during enzymatic hydrolysis of steam pretreated spruce was primarily driven by reactor agitation. In the case of steam pretreated giant reed the particle size was mainly reduced by enzymatic hydrolysis. The rapid reduction in particle size of giant reed coincided with a rapid liquefaction. For steam pretreated softwood, the viscosity in fact increased at the beginning of enzymatic hydrolysis, followed by a gradual decrease during the remainder of the hydrolysis. This interesting phenomenon was in part linked to the type of pretreatment used on the softwood biomass. In contrast to steam pretreated softwood, the viscosity of sulfite pretreated spruce decreased rapidly during enzymatic hydrolysis. Efficient viscosity reduction in sulfite pretreated spruce was also achieved with very low doses of pure endoglucanase enzymes (0.1 mg protein per g glucan) without significant glucose release.The effect of mixing on the enzymatic hydrolysis was in part determined by the viscosity of the pretreated biomass. For steam pretreated spruce at low solid loading, decreasing the agitation rate had little effect on the the enzymatic hydrolysis. However, if the viscosity was increased by the addition of a thickening agent, the effect of agitation was much larger. For a substrate that underwent rapid initial viscosity reduction, such as steam pretreated giant reed, the enzymatic hydrolysis was almost independent of agitation rate. Another important factor determining the effect of mixing on the enzymatic hydrolysis was the level of product inhibition. If the glucose and cellobiose concentrations were high, as during high solid hydrolysis of steam pretreated spruce, low agitation rate had a large negative effect on the enzymatic hydrolysis. However, if the product concentration was kept low, as during SSF, the effect of agitation was much weaker. Overall, the results indicate that the decrease in hydrolysis rate occurred due to increased local product inhibition, caused by mass transfer limitations in the stagnant zones, formed in the reactor volume when under low intensity mixing. The rate of enzymatic hydrolysis appeared to be determined by flow regime, i.e. Reynolds number, rather than specific mixing power input. This implies that the negative effects of low agitation rate will be less of a problem in larger reactors.
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