In a second generation bioethanol process, reducing overall production cost by recycling enzymes has gained increasing attention. Possible points of where to introduce recycling in a high-solid bioethanol process from biomass are discussed on a conceptual basis (figure 1) and investigations on factors which could limit the successful introduction of recycling concepts are presented:
1) Recycling after initial hydrolysis, provided feedback inhibition is avoided by continuously removing sugars. Recycling at this step requires methods to reuse the enzymes without reintroducing sugars.
2) Recycling after fermentation holds the advantage of retaining enzyme activities which have not endured the conditions of distillation. Separating solids and liquids for desorption of enzymes on solids is an option.
3) Recycling after distillation by reintroducing slurry to freshly pretreated biomass was tested with two cellulase preparations with and without presence of surfactant PEG 6000, using two different temperatures in the hydrolysis (40°C and 50°C).
Best result for recyclability was 65% and 41% of the original enzyme load of each of the two cellulase preparations, respectively. One cellulase preparation displayed improved recyclability with conditions of low temperature, short hydrolysis time and addition of PEG 6000. Individual enzyme components adsorbed differently to substrate and the effect of temperature on enzyme activities and enzyme precipitation differed between cellulase preparations. Borderline issues with enzyme thermostability and precipitation at normal process conditions suggested recycling might not be relevant yet.
Figure 1: Overall process flow in industrial bioethanol process and suggested points of recycling