ETHANOL PRODUCTION FROM LIGNOCELLULOSIC BIOMASS USING HIGH CELL DENSITY FERMENTATION

Abstract

The development and implementation of fermentation strategies for utilizing the lignocellulosic biomass to produce bioethanol is the essential need since long time back. This dissertation focuses on the development of different fermentation strategies using synthetic xylose and glucose then implements those successful strategies on real hydrolysates i.e., XRF and GRF. First, the optimum xylose and glucose concentrations were found out in batch fermentations using Pichia stipitis and Zymomonas mobilis respectively. After this, the batch fermentations were conducted with XRF and GRF. Thereafter, mass transfer coefficient (KLa) was optimized for the Pichia stipitis using 0.5 L bioreactor and optimized KLa was validated on 1 L and 5 L bioreactor. Then high cell (inoculum) density fermentations were carried out with those optimum concentrations at both flask and bioreactor level to further enhance the ethanol yield and productivity. These experiments resulted in 1.56 folds and 2.48 folds increment in ethanol productivity respectively from xylose (60 g/L) and glucose (60 g/L) in comparison of their batch productivities of 1.08 g/L/h and 2.43 g/L/h respectively. Further these high cell density experiment concepts were applied in fed-batch (constant volume) fermentation with 20-90% retention of cell culture. The fed-batch (constant volume) fermentation of glucose with high cell density through 50% retention of culture was resulted in an ethanol yield and productivity of 0.50 g/g and 5.85 g/L/h after 20 fed-batch cycles. Similarly, the fed-batch (constant volume) fermentation of xylose with high cell density through 50% retention of culture was resulted in an ethanol yield and productivity of 0.35 g/g and 1.33 g/L/h after 12 fed-batch cycles. These fed-batch strategies shown no inhibition during performed fed-batch cycles thus these may be used to run bioreactor continuously with limitless feedings. Further these strategies were applied on XRF hydrolysate and resulted in an overall ethanol yield and productivity of 0.29 g/g and 0.78 g/L/h after 5 fed-batch cycles. There were no inhibition phenomena observed in these 5 fed-batch cycles. Thus, unlimited feeding may be possible with real hydrolysate too.