Continuous ethanol production by Zymomonas mobilis in a fluidized bed reactor. Part II: Process development for the fermentation of hydrolysed B-starch without sterilization

Abstract

A continuous fluidized bed reactor operation system has been developed for ethanol production by Zymomonas mobilis using hydrolysed B-starch without sterilization. The operation system consists of two phases. In the first phase macroporous glass carriers in a totally mixed fluidized bed reactor were filled up totally with a monoculture of Z. mobilis by fast computer-controlled colonization, so that in the subsequent production phase no contaminants, especially lactic-acid bacteria, could penetrate into the carrier beads. In the production phase the high concentration of immobilized Z. mobilis cells in the fluidized bed reactor permits unsterile fermentation of hydrolysed B-starch to ethanol at short residence times. This results in wash-out conditions for contaminants from the substrate. Long-term experimental studies (more than 120 days) of unsterile fermentation of hydrolysed B-starch in the laboratory fluidized bed reactor (2.2 l) demonstrated stable operation up to residence times of 5 h. A semi-technical fluidized bed reactor plant (cascade of two fluidized bed reactors, each 55 l) was operated stably at a mean residence time of 4.25 h. Glucose conversion of 99% of the unsterile hydrolysed B-starch was achieved at 120 g glucose/l−1 in the substrate, resulting in an ethanol concentration of 50 g·l−1 and an ethanol space-time yield of 13 g·l−1·h−1. This is a factor of three compared to ethanol fermentation of hydrolysed B-starch with Z. mobilis in a continuous stirred tank reactor, which can only be operated stably under sterile conditions.