Optimize Aspect Ratio in Industrial Fermenters

The ratio of a fermenter’s liquid height to diameter can impact its heat-transfer surface area, blend time, and required agitator power. This article describes the factors that go into determining the optimum aspect ratio for a given set of conditions.

Industrial fermenters are used to make all manner of products using live organisms as chemical reactors. Engineers and designers are tasked with determining the optimum aspect ratio of a fermenter in the design phase of a process. Aspect ratio — also known as Z/T — is the ratio of liquid height (Z) to the tank diameter (T).

This article discusses how to determine the optimum aspect ratio of agitated gas-liquid fermenters. In these fermenters, air is the gas most often added, but syngas, methane, or other gases can be used, depending on the application. Although some fermenters do not require agitation (e.g., bubble column or airlift approaches), this article focuses on mechanically agitated fermenters because they are the most versatile. Agitated fermenters can achieve higher mass-transfer rates and handle higher viscosities than unagitated systems.

The optimum aspect ratio depends on every facet of the design requirements, including the desired mass-transfer rate, dissolved gas uniformity, cost of electric power, cost of the agitator, cost of the vessel, cost of real estate, heat-transfer requirements, the mass-transfer correlation used, backpressure imposed on the tank, site restrictions, time horizon of the plant economic calculations, etc.

This article describes the main impacts of the aspect ratio on fermenter design and performance, and works through an example application to show how to determine the ideal aspect ratio.