The Oxygen Mass Transfer, Carbon Dioxide Inhibition, Heat Removal, and the Energy and Cost Efficiencies of High Pressure Fermentation

Abstract

This study focuses on the enhancement of oxygen transfer in aerobic fermentation processes through the use of the reactor headspace pressure as a process variable. Our investigation was performed using an example 5 m³ fermentor, and by applying the mass transfer (sorption) characteristics obtained from measurements with Pseudomonas putida CA-3 in a mineral medium. By applying reactor pressures up to 11 bar (10 bar overpressure) the oxygen transfer capacities of stirred tank reactors can theoretically be increased to values above 1 kmol m^–3 h^–1, although the problem of heat removal limits that value to levels of 0.7 kmol m^–3 h^–1. However, this is still sufficient for the oxygenation of most aerobic fermentation processes, even at very high cell densities. Possible inhibition by the carbon dioxide partial pressure at elevated total pressures is discussed. It is shown that increasing reactor pressure enhances the energy efficiency, defined as the ratio of the oxygen transfer capacity to the total power consumption of the reactor system. The cost efficiency of the oxygen mass transfer, defined as the ratio of the oxygen transfer capacity to the overall cost of the system (overall purchase cost divided by a project lifetime of ten years, plus total energy cost) also increases with reactor pressure up to 5.6 bar.