Dissolved carbon dioxide measurement and its correlation with operating parameters in fermentation processes

A silicone tubing probe was used to measure dissolved carbon dioxide in two industrial fermentation processes. The two processes differed markedly in their broth viscosities: one had high viscosity (2000-3000 cP) and the other had low viscosity (< 100 cP). The goal was to study changes in dissolved carbon dioxide concentration in fermentation broth when operating parameters, such as total pressure, air flow rate and, agitation rate, were modified. It was shown that in high-viscosity fermentation broths the dissolved carbon dioxide concentration can reach as high as 190% of that calculated by assuming an equilibrium between the air coming out of the fermentor and the fermentation broth. For low-viscosity broth, the highest dissolved carbon dioxide concentration was found to be about 133% of the equilibrium value. On the basis of the material balance over the entire fermentor, the ratio of actual dissolved carbon dioxide partial pressure and carbon dioxide partial pressure in air leaving the fermentor was correlated to the operating parameters of pressure, air flow, and carbon dioxide transfer coefficient. Since carbon dioxide transfer coefficients are not readily available, we recommend a method of using oxygen transfer coefficients to develop a correlation that can be used on-line to predict changes in dissolved carbon dioxide concentration when operating parameters are changed or when process improvements and scale-up are undertaken.