Separate Hydrolysis and Fermentation of Pretreated Spruce

Abstract: Bioethanol from lignocellulose is expected to be the most likely fuel alternative in the near future. SEKAB E-Technology in Örnsköldsvik, Sweden develops the technology of the 2nd generation ethanol production; to produce ethanol from lignocellulosic raw material. The objective of this master’s thesis was to achieve a better knowledge of the potential and limitations of separate hydrolysis and fermentation (SHF) as a process concept for the 2nd generation ethanol production. The effects of enzyme concentration, temperature and pH on the glucose concentration in the enzymatic hydrolysis were investigated for pretreated spruce at 10% DM using a multiple factor design. Enzyme concentration and temperature showed significant effects on the glucose concentration, while pH had no significant effect on the concentration in the tested interval of pH 4.5-5.5. To obtain the maximum glucose concentration (46.4 g/l) for a residence time of 48 h, the optimal settings within the studied parameter window are a temperature of 45.7⁰C and enzyme concentration of 15 FPU/g substrate. However, a higher enzyme concentration would probably further increase the glucose concentration. If enzymatic hydrolysis should be performed for very short residence times, e.g. 6 h, the temperature should be 48.1⁰C to obtain maximum glucose concentration. The efficiency of the enzymes was inhibited when additional glucose was supplied to the slurry prior to enzymatic hydrolysis. It could be concluded that end product inhibition by glucose occurs and results in a distinct decrease in glucose conversion. No clear conclusions could be drawn according to different techniques for slurry and enzymes, i.e. batch and fed-batch, in the enzymatic hydrolysis process. Investigations of the fermentability of the hydrolysate revealed that the fermentation step in SHF is problematic. Inhibition of the yeast decrease the fermentation efficiency and it is therefore difficult to achieve the 4% ethanol limit. Residence time for enzymatic hydrolysis (48 h) and fermentation (24 h) need to be prolonged to achieve a sufficient SHF process. However, short processing times are a key parameter to an economically viable industrial process and to prolong the residence times should therefore not be seen as a desirable alternative. SHF as a process alternative in an industrial bioethanol plant has both potential and limitations. The main advantage is the possibility to separately optimize the process steps, especially to be able to run the enzymatic hydrolysis at an optimal temperature. Although, it is important to include all the process steps in the optimization work. The fermentation difficulties together with the end product inhibition are two limitations of the SHF process that have to be improved before SHF is a preferable alternative in a large scale bioethanol plant.