Industrial fermenters are used to make all manner of products using live organisms as chemical reactors. For this article, we will concentrate on gas-liquid fermenters. Though some processes are successfully carried out without agitation (the bubble column or airlift approaches, for example), this article will focus on mechanically agitated fermenters, as they are the most versatile. They can handle higher mass transfer rates and higher viscosities than unagitated systems.
A common question asked about such fermenters is, “What is the optimum aspect ratio?” (Aspect ratio is also known as Z/T, where Z is the liquid height and T is the tank diameter). The answer is, “It depends”. The optimum depends on every aspect of the design requirements, including, but not limited to: the mass transfer rate desired, dissolved gas uniformity, cost of electric power, cost of the agitator, cost of the vessel, cost of real estate, heat transfer requirements, mass transfer correlation used, back pressure imposed on the tank, site restrictions, time horizon of the plant economic calculations and probably a few others. The only way to find the optimum is to do detailed calculations over an array of Z/T ratios, and then interpolate if needed. What this article seeks to do is identify the main effects of Z/T ratio, and then illustrate the concept with an example fermenter problem.