A study of continuous ethanol production using a highly flocculent yeast in the gas lift tower fermenter

Abstract

A highly flocculent strain of the yeast Saccharomyces uvarum was used to convert glucose to ethanol and CO 2 continuously at a temperature of 33 °C and a pH of 4.5. The yeast was grown in a single-stage gas lift tower fermenter, the design of which allowed the reactor to run anaerobically for long periods with a biomass holdup exceeding 100 g l −1 (dry weight) and permitted virtually 100% cell retention at hydraulic dilution rates greater than 2.5 h −1. Unstructured parabolic and linear equations were proposed to describe the variation in yeast growth and ethanol production with ethanol concentration. In combination with the fermenter model, these equations enabled prediction of all steady state parameters for ethanol concentrations of 0 – 73 g l −1. Such parameters were in close agreement with the experimental data obtained. Yeast growth could not be correlated with ethanol production in the traditional manner, although non-competitive ethanol inhibition was observed and the dependence of product formation on glucose concentration was in agreement with Michaeli—Menten kinetics. Evaporative ethanol losses were accurately calculated to be less than 0.6% of the total ethanol produced and were therefore neglected. The maximum theoretical ethanol concentration and productivity attainable were 75 g l −1 and 50 g l −1 h −1 respectively with a mean ethanol yield on glucose of 83% of the theoretical value.