Question

A fermenter containing 40 m 3 of medium (25˚C) is going to be sterilized by the direct
injection of saturated steam. The typical bacterial count of the medium is about 5 X 10 12 m -3 ,
which needs to be reduced to such an extent that the chance for a contaminant surviving the
sterilization is 1 in 1,000. The stream (345 kPa, absolute pressure) will be injected with a
flow rate of 5,000 kg/hr, which will be stopped when the medium temperature reaches 122
˚C. During the holding time, the heat loss through the vessel is assumed to be negligible.
After a proper holding time, the fermenter will be cooled by passing 100 m 3 / hr of 20˚C
water through the cooling coil in the fermenter until the medium reaches 30 ˚C. The coil has a
heat – transfer area of 40 m 2 and for this operation the average overall heat – transfer
coefficient (U) for cooling is 2,500 kJ/hr m 2 K. The heat resistant bacterial spores in the
medium can be characterized by an Arrhenius coefficient (K d0 ) of 5.7 X 10 39 hr -1 and
activation energy (E d ) of 2.834 X 10 5 kJ / kmol (Deindoerfer and Humphrey, 1959). The heat
capacity and density of the medium are 4.187 kJ/kg K and 1,000 kg/m 3 , respectively.
Estimate the required holding time.

Answer 1

Answer:

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