(374ac) Product Inhibition Kinetics and Mass Transport Limitations in Reactor Design for Improved Enzymatic Hydrolysis of Lignocelluloses

The better understanding of the hydrolysis kinetics and the mass transfer phenomena during the reaction is believed to be the key for the design of improved reactors for the enzymatic lignocellulose hydrolysis and for the feasible hydrolysis process that is moreover one of the several main factor that delay the rapid deployment of a second generation (cellulosic) bioethanol production at industrial scale. One of major requirements in the cost-efficient lignocellulosic biomass processing to biofuels and biochemicals is to employ reactor or reactors systems that will ensure the maximal conversion of the cellulose with the minimum use of the cellulolytic enzymes. Thus, the efficient bioconversion of the substrate designated as kg glucose/kg cellulose will be essential in a process that inherently is limited by the low ethanol yield i.e. ~ 250-300 L/ton dry biomass [1-2]. The effective use of the biocatalyst, i.e. the yield of glucose obtained per unit amount of enzyme(s), referred to as kg glucose/kg cellulase (i.e. ?'specific cellulase yield''), may also be regarded as one of the parameters of a large importance for the design and operation of bioreactor systems for the lignocellulose conversion to fuels and chemicals. The requirement for high specific cellulase yield is likely to persist as long as the cellulolytic enzymes remain to be one of the most expensive single cost contributors in the overall biomass processing to bioethanol [1-4]. The cellulase/β-glucosidase reuse should in principle improve obtained kg glucose/kg cellulase. However, as cellulases are: (a) heavily inhibited by the intermediate and final hydrolysis product, cellobiose and glucose, respectively, (b) susceptible to the different forms of the deactivation and non-specific adsorption, (c) strongly adsorbed onto the cellulosic material, and as (d) strong mass transfer resistances are found within the pretreated lignocellulose slurries in the hydrolysis reaction and enzyme microenvironment, the effective recovery of cellulolytic enzymes remains a major challenge. In order to achieve the complete and fast cellulose hydrolysis to glucose, the hydrolysis reactor system must thus provide the favourable conditions for the enzyme-substrate contact and in general for the mass transfer of reactants and products, in the reaction microenvironment free of cellobiose and glucose inhibition [5-6]. In contrast, rather high biomass concentration slurries (e.g. > 20-25 % (w/w)) are required to obtain the sufficient levels of glucose in the hydrolyzate (e.g. > 100 g/L) for the reduction of the hydrolysis reactor and downstream steps operating costs and especially for the achievement of feasible ethanol recovery from the fermentation mixture (which requires > 5 % (w/w) of ethanol in the mixture). The hydrolysis reactor operation at these conditions is, however, a rather difficult task as the high pretreated lignocellulose loadings (> 5-10 % (w/w)) are connected to high apparent viscosities and poor mass transfer, result of which is a particularly slow hydrolysis rate and high energy expenditure. Based on experimental and modelling studies, the major features of the likely mechanisms of cellulase product inhibition by cellobiose and glucose are initially highlighted, together with the role of the inhibition in the hydrolysis kinetics and the quantitative effects that the inhibition exhibits on the reaction rate. The second part of the presentation deals with the rheological characterization of the pretreated lignocelluloses slurries (hydrothermally treated wheat straw), the possible mass transport phenomena occurring inside the cellulosic particles during the hydrolysis that can harm the reaction rate, and their relation to the design of mixing equipment. Finally, new strategies that focus on reactor designs encompassing glucose removal during enzymatic cellulose hydrolysis, enzyme reuse and the advanced lignocellulose slurry mixing technologies are discussed in the light of product inhibition kinetics and enzyme and sugar mass transport limitations, and are supported by results from several experimental set-ups with different reactor configurations and kinetic modelling studies.

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