2.5.2. Gas exchange and mass transfer
One of the most critical factors in the operation of a fermentor is the provision of adequate gas exchange. Oxygen is the most important gaseous substrate for microbial metabolism, and carbon dioxide is the most important gaseous metabolic product.
When oxygen is required as a microbial substrate, it is frequently a limiting factor in fermentation. Because of its low solubility, only 0.3 mM O2, equivalent to 9 mg/l, dissolves in one liter of water at 20 degrees Celsius in an air/water mixture. This amount of oxygen will be depleted in a few seconds by an active and concentrated microbial population unless oxygen is supplied continuously. In contrast, during the same period the amount of other nutrients used is negligible compared to the bulk of concentrations. Therefor most aerobic microbial processes are oxygen limited. This is the reason why the concept of gas-liquid mass transfer in bioprocesses is centered on oxygen transfer even if other gasses such as carbon dioxide, hydrogen, methane and ammonia can also be involved.
Due to the influence of the culture nutrients, the maximal oxygen content is actually lower than it would be in pure water.
The solubility of gasses follows Henry's law in the gas pressure range over which fermentors are operated. This means that if the oxygen concentration in the gas phase increases, the O2 proportion of the nutrient solution increases. Consequently the highest O2 partial pressure are attained during aeration with pure oxygen. Compared to the value in air (9 mg O2/l), 43 mg O2/l dissolves in water when pure oxygen is considered.
As temperature rises, the O2 solubility decreases. For example the solubility at 33
degrees Celcius is