Oxygen and carbon dioxide mass transfer and the aerobic, autotrophic cultivation of moderate and extreme thermophiles: A case study related to the microbial desulfurization of coal

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Abstract
Mass transfers of O2, CO2, and water vapor are among the key processes in the aerobic, autotrophic cultivation of moderate and extreme thermophiles. The dynamics and kinetics of these processes are, in addition to the obvious microbial kinetics, of crucial importance for the industrial desulfurization of high-pyritic coal by such thermophiles. To evaluate the role of the temperature on the gas mass transfer, kLa measurements have been used to supplement the existing published data. Oxygen mass transfer from gas (air) to liquid (5 mM H2SO4 in water) phase as a function of the temperature has been studied in a laboratory-scale fermentor. At 15, 30, 45, and 70°C, (kLa)o values (for oxygen) were determined under three different energy input conditions by the dynamic gassing in/out method. The (kLa)o was shown to increase under these conditions with increasing temperature, and straight lines were obtained when the logarithm of (kLa)o was plotted against the temperature. By multiplying the equilibrium concentration of O2 in water with (kLa)o maximal, O2 transfer capacities were calculated. It appeared that in finite of a decreased solubility of O2 at elevated temperature in mechanically mixed fermentors the calculated transfer capacities showed only minor changes for the range between 15 and 70°C. However, in an air-mixed fermentor the transfer capacity of O2 decreased slowly but steadily. Carbon dioxide mass transfer was predicted by calculations on the basis of the data for oxygen transfer. The maximal CO2 transfer capacity, calculated as the product of the equilibrium CO2 concentration times (kLa)c, decreased slowly as the temperature increased over the range 15–70°C under all three energy input conditions. Subsequent process design calculations showed that for aerobic, autotrophic cultures, CO2 limitation is more likely to occur than O2 limitation.