Optimize Aerobic Fermenter Operation

Fermenter optimization involves determining a profile of agitator and compressor speeds that satisfy the required oxygen transfer rates. Optimization minimizes power consumption, which will save you money.

A previous CEP article (1) presented a procedure to minimize the total power consumption (agitator plus compressor) of an aerobic fermenter. It focused on minimizing the power needed at the peak oxygen-transfer rate (OTR), i.e., the rate at which oxygen moves from air bubbles to the fluid, which is defined as:

where k_La is the overall mass-transfer coefficient and F is the driving force for mass transfer.

That article, however, did not address how to minimize total energy consumption under conditions of varying OTR. This article expands on Ref. 1, and provides a simplified method for minimizing total energy consumption at any OTR.

The procedure

This procedure for achieving substantial energy savings involves simply adjusting the agitation and compressor speed (which varies the air flowrate). It overcomes the complications of detailed pressure drop calculations and varying compressor efficiency inherent in other approaches with two simplifying assumptions: compressor efficiency is constant, and pressure drop is proportional to the square of the air flowrate. These are reasonable assumptions for low-pressure systems (less than 3 bar).

Optimization starts with optimizing the total power consumption at peak OTR and at the maximum allowable pressure drop. (At actual operating conditions, the air flowrate may vary at different OTRs, and thus, the pressure drop will also vary.) Then the optimization is repeated at varying pressure drops for each change in OTR. This creates an idealized operating profile. During actual operation, the air flowrate is altered as a function of time according to the idealized profile and fine-tuned by varying the agitator speed.

The following steps aim to produce a system design that operates with minimum power consumption.

Step 1. Based on peak OTR, calculate the minimum air flowrate required at 100% utilization (i.e., when all of the oxygen in the feed gas is transferred to the fluid to achieve the required OTR):

where Q_min is the minimum air flowrate, V is the volume of the liquid, and C_air is the concentration of oxygen in air. At normal conditions, 1 nm³ of air has 9,366 mmol of oxygen, assuming the air is not enriched (21% oxygen).

Step 2. For the first design calculation, choose an initial air flowrate that is greater than the theoretical minimum calculated in Step 1. Try 20% above Q_min.

Step 3. Perform a mass balance and determine the exit gas flowrate and composition, including the amount of CO₂ respired.

Step 4. Calculate the driving force for mass transfer. For pilot-scale equipment (typically less than 500 L), the dissolved oxygen concentration does not vary significantly, so:

where C_sat is the dissolved oxygen concentration at saturation and C is the dissolved oxygen concentration.

In full-scale equipment, there may be...