Bioreactor and Extraction Heat Exchangers

Designing TCU, sterilizer and extraction exchangers

A bioreactor temperature control unit is a small closed water loop with a pump, a heater and a cooler. The heater is a steam-to-water exchanger, often a U-tube or a plate, that adds heat during start-up and trims during the run; the cooler removes the metabolic heat of a dense culture, which on a 2,000 liter mammalian reactor is a few kilowatts and on a 20,000 liter microbial fermenter can be several hundred. The exchangers are sized on the peak cooling load at the warmest chilled water with a 30 percent margin, and the heater on the start-up time from ambient. Turndown is the design problem: the same cooler that removes 300 kW at peak must remove 3 kW at the end of the run without cycling the jacket temperature, which is done with a small approach, a well-sized control valve and a loop volume that damps the swings.

Continuous media sterilization heats media to 121 C, holds it for a validated time in a sloped hold tube and cools it to inoculation temperature, and it is the process that decides whether the batch is contaminated. The heater is double tubesheet because the media is made on WFI and the utility is plant steam; the cooler is double tubesheet for the same reason on tower water; regeneration is media-against-media through a third double tubesheet unit where the plant wants the steam savings. Media with dissolved sugars and salts fouls the heater over a campaign, and the run length is set by area margin the way a dairy UHT heater's is.

Ethanol chilling to -40 F is a viscosity and gasket problem. Ethanol at -40 F is about 4 cP, glycol at -45 F is about 60 cP, and the coefficient on the glycol side collapses; the exchanger is sized on the glycol side, the glycol flow is generous, and a low-temperature fluid replaces propylene glycol where the chiller can supply it. The plate gaskets are chosen for ethanol at that temperature, and the frame is insulated and vapor-sealed so the plates do not ice from room air. The holdup of a plate pack is small, which matters when a cold solvent is also a flammable one.

Solvent recovery is a condenser sized on the evaporator's vapor rate at the chilled water or glycol available. Ethanol at 173 F atmospheric boiling condenses on tower water; under vacuum at 100 to 120 F it needs chilled water, and the condensate is sub-cooled before it returns to the tank so the vent does not carry ethanol. Hydrocarbon recovery condenses propane and butane at pressure on chilled glycol, and the exchanger, the skid and the electricals are specified for a classified area.