Insights
November / December 2018

Aeration Equipment in Aerobic Fermentation Processes

Travis McGarrah

A wide range of pharmaceutical products is produced using two complex fermentation processes. Anaerobic fermentation takes place in the absence of oxygen, and aerobic fermentation requires oxygen, supplied via blower and compressor systems, to yield microorganisms and produce the desired product. This article examines the role of aeration equipment in aerobic fermentation, considerations when designing aeration systems, and compressor features that contribute to reliable plant production.

Travis McGarrah

Compressors in Aerobic Fermentation

To begin, we will identify the differences between a blower, single-stage compressor, and typical two-stage screw compressor, and discuss their uses in fermentation applications.

A blower is a machine that supplies high volumes of air at pressures typically under 20 psi(g).* A compressor produces air at higher pressures and typically at lower volume. Single-stage screw compressors are designed to operate between 25 and 60 psi(g); two-stage compressors are typically designed for pressures above 100 psi(g). Large-scale production of pharmaceuticals is carried out in deep fermentation tanks that require pressures between 25 and 45 psi(g).

It is not uncommon for the air supply in fermenters to be normal plant or instrument air that has been compressed to approximately 100 psi(g) for use in other areas of the plant. The actual pressure required to aerate fermentation tanks is usually much lower, how- ever—roughly equal to the pressure of the liquid column height in the tank. It may seem convenient to use instrument air that is already available, but it can lead to unintended electricity costs. For example, a tank that is 60 feet deep and filled with liquid that has a density similar to water will only require air at ~25 psi(g) to overcome the tank liquid pressure.

*Pounds per square inch (gauge): Air pressure measured relative to ambient atmospheric pressure

Figure 1: Aerobic fermentation system with a common header and multiple compressors

Table A: Energy costs for 200-cfm airflow at varying pressures
Pressure,
psi(g)
Flow, cfm Power,
kilowatts
Energy cost
per year*
25 200 26 $20,820 45 200 37$29,471
100 200 53 $42,792 *Assuming 8,000 hours per year at$0.10 per kilowatt-hour

We recommend having aeration equipment that is dedicated to the fermentation process and sized appropriately for process needs. Energy savings from supplying only the required air pressure can be quite significant in continuous applications.

Why is this important? The electrical cost for operating compressor equipment continuously can be quite high. Compressing air to 100 psi and then throttling it down to 25 psi is a large and unnecessary expenditure of energy. In general, we recommend having aeration equipment that is dedicated to the fermentation process and sized appropriately for process needs. Table A shows the energy required to produce air at these pressures vs. instrument air that was compressed to 100 psi for a flow of 200 cubic feet per minute (cfm).

Features for Reliable Operation

The compressor equipment should have several features to meet fermentation process requirements.

Equipment reliability: A backup compressor for each header system should always be in place, since unreliable equipment can lead to multiple machines down and a system unable to meet oxygen demands. Although nonfunctioning equipment is most often the result of extreme conditions or unexpected circumstances, the cost of unusable end product(s) may be greater than the compressor equipment investment.

Energy efficiency: As we have seen, single-stage equipment designed for the lower operating pressures of fermentation tanks can greatly reduce operating costs. VFD equipment further improves this efficiency to accommodate variable oxygen demands at various stages of fermentation.

Controllability: A supervisory control and data acquisition (SCADA) or distributed control (DCS) system in the plant should be able to operate compressor equipment seamlessly, varying the speed as the system requires. Compressors that utilize integrated controls can perform this function while they monitor critical parameters, making sure the equipment is protected while being controlled remotely.

Conclusion

In summary, aeration is a critical element for producing pharmaceuticals via aerobic fermentation processes. Consideration should be given to the system layout, control scheme, and compressor equipment to ensure that fermentation tanks always have a reliable source of air to meet the oxygen demand. Following these guidelines will ultimately lead to smooth, reliable plant production at the lowest possible operational cost.