Operating at High Solid Content

Operating at high solid content in the enzymatic hydrolysis process is crucial for large-scale development of bioproduct and biofuel production processes. The aim of utilizing high solid content is to reach high sugar concentrations and subse­quently high concentrations of fermentation products, such as ethanol (Jprgensen et al. 2007a; Hodge et al. 2009). Furthermore, maintaining high substrate con­centrations throughout the conversion process is important for the energy balance and economic viability of biofuels production. Obtaining high concentration of fermentation product reduces global production cost since downstream processing and water consumption can be lowered. In case of ethanol production, distillation increases significantly the energy demand of the process, especially when ethanol concentrations are below 4 % (Ohgren et al. 2006).

In general, higher substrate loadings results in higher concentration of sugars. However, it has been shown that enzyme performance gradually decreases as substrate concentration increased. This can be attributed to enzyme inhibition by end products or toxics, presence of high concentrations of lignin and mass transfer limitations (Jprgensen et al. 2007a; Kristensen et al. 2009). In addition, some recent studies have reported a decrease in the adsorption capacity of cellulase enzymes to cellulose at high substrate loadings due to the effect of hydrolysis products (Kristensen et al. 2009; Wang et al. 2011). To overcome these barriers, different process configurations and strategies have been suggested to increase solids concentration in bioconversion processes.

Operating at high initial solids content (above 10-15 % (w/w)) involves tech­nical barriers. Viscosity of the pretreated materials is usually very high, which implies mass transfer limitations and mixing difficulties. Operating fed-batch processes by adding fresh substrate when viscosity decreases has been shown as an effective strategy to increase substrate concentrations in fermentations processes (Ballesteros et al. 2002; Varga et al. 2004). Another possibility is carrying out a prehydrolysis prior to initiate the simultaneous saccharification and fermentation (SSF) process. Using this strategy, the enzymes act at optimum temperature and reduce viscosity, which can result in higher substrates loadings (Rosgaard et al. 2007; Manzanares et al. 2011). A recent advance for operating at high consistency is the development of novel bioreactors with improved mixing capacity and low energy consumption (Jprgensen et al. 2007b; Zhang et al. 2009).

Another problem when operating at high substrate concentration is product inhibition. Cellobiose, glucose, and hemicellulose-derived sugars have been shown to inhibit the enzymes action (Xiao et al. 2004). In SSF processes, sugars released by the action of the enzymes are converted directly to ethanol by the fermenting microorganism, which reduces end-product inhibition (Ballesteros et al. 1994; Olsson et al. 2006). Constant removal of glucose during the process has been also proposed (Andric et al. 2010).

Degradation compounds originated from carbohydrates and lignin during the pretreatment affect the enzymatic hydrolysis (Tengborg et al. 2001; Ganna-Aparicio et al. 2006) and the fermenting microorganisms (Palmqvist and Hahn-Hagerdal 2000a; Oliva et al. 2003; Oliva et al. 2004). At high substrate loadings, the concentration of these compounds increases, therefore their influence in the bioconversion process can become more significant. Washing the pretreated material has been typically employed to eliminate toxic compounds and increase enzymatic hydrolysis and fermentation yields. To avoid washing and use the whole slurries, detoxification strategies such as laccase treatments have been studied to reduce the concentration of phenolic compounds and increase substrate concentrations in fermentation (Moreno et al. 2012).

Different articles reported the utilization of high substrate concentrations for ethanol production. Using wet oxidized and steam exploded corn stover, a sub­strate consistency of 15 % and 10-30 % dry matter (DM) in fermentation experiments, respectively, was studied (Varga et al. 2004; Lu et al. 2010). Using corn stover pretreated by combination of stream explosion and alkaline hydrogen peroxide, it was reached a solids loading of 30 % (Yang et al. 2010). With hydrothermal pretreated and steam pretreated wheat straw, it was possible to carry out hydrolysis and SSF at high substrate concentrations up to 20-30 % (Jprgensen 2009; Ballesteros et al. 2011), and with steam pretreated spruce it could be reached a consistency of 14 % (Hoyer et al. 2010).

6.4 Conclusion

Efficient utilization of lignocellulosic materials in a biorefinery depends on the advances in pretreatment technologies, enzyme saccharification, and fermentation of sugars to fuels and chemicals. Optimization of pretreatment and enzymatic hydrolysis processes is crucial to make bioconversion processes from lignocellu — losic biomass viable and cost-effective. The aim of the pretreatment is increasing the digestibility of carbohydrates while minimizing degradation of sugars and generation of toxic compounds. The pretreatment has to be adapted to the different raw materials and should be validated at large scale. The cost and efficiency of enzyme products still represents a major bottleneck to improve the economy of industrial biorefineries. To reduce costs of enzymatic hydrolysis processes, it is required the optimization of enzymatic mixtures in order to increase sugars pro­duction yields, reduce pretreatment severity, and decrease enzyme dosages. Complexity of lignocellulosic substrates involves that enzyme cocktails should be adapted for each raw material and type of pretreatment. In addition, operating at high solid content should be considered as a key issue for biofuels production. Finally, the integration of all the process steps has a remarkable importance to increase overall process efficiency and promote large-scale development. The type of biomass and pretreatment determines the process configuration requirements for hydrolysis and fermentation as each step has a large impact on all subsequent stages.


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