Bioreactor Design and Operation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 960

(a) the probability of a successful sterilization and
(b) what fraction of the vitamin remains active?
18.8. In cultivation of baker's yeast in a stirred and aerated tank, lethal agents are added to
the fermentation medium to kill the organisms immediately. Increase in DO
concentration upon addition of lethal agents is followed with the aid of a DO analyzer
and a recorder. Using the data in Table P18.8 , determine the oxygen transfer
coefficient, k L a, for the reactor. Saturation DO concentration is C ¼

9 mg/L.

TABLE P18.8

Time, min 1 2 2.5 3 4 5

DO, mg/L 1 3 4 5 6.5 7.2

18.9. A stirred-tank reactor is to be scaled down from 10 to 0.1 m 3 . The dimensions of the
large tank are: D R ¼ 100 rpm.

(a) Determine the dimensions of the small tank (D R , D i , H) by using geometric
similarity.
(b) What would be the required rotational speed of the impeller in the small tank if the
following criteria were used?
(1) Constant tip speed
(2) Constant impeller Re number
18.10. An autoclave malfunctions, and the temperature reaches only 119.5 C. The
sterilization time at the maximum temperature was 20 min. The jar contains 10 L of
complex medium that has 10 5 spores/L. At 121 C, k d ¼

2m;D i ¼ 0.5 m; u¼

1.0/min and E ad ¼ 380 kJ/mol.

What is the probability that the medium was sterile?
18.11. A10m 3 chemostat is operated with a dilution rate of 0.1 h 1 . A continuous sterilizer
with steam injection and flash cooling is to be designed to deliver sterilized medium to
the fermentor. Medium in the holding section of the sterilizer is maintained at 125 C.
The concentration of contaminants in the raw medium is 5 10 12 m 3 ; an acceptable

contamination risk is 0.01 on a weekly basis. The activation energy and pre-
exponential factor of the cell thermal death rate are estimated at 300 kJ/mol and
2.3 10 36 s 1 . The sterilizer tube inner diameter is 0.15 m. The liquid medium density

is 1000 kg/m 3 , and viscosity is 0.9 10 3 Pa $ s.

(a) Determine the length of the holding section and space time required;
(b) Determine the length of the holding section and space time if the inner tube
diameter is 1 m instead.
18.12. A12-m 3 fermentor is available to carry out the fermentation. The medium stream of
1.5 m 3 /h is to be sterilized continuously. Medium in the holding section of the
sterilizer is maintained at 121 C. The concentration of contaminants in the raw
medium is 2.5 10 12 m 3 ; an acceptable contamination risk is 0.1 on a monthly basis.

The activation energy and pre-exponential factor of the cell thermal death rate are
estimated at 300 kJ/mol and 2.3 10 36 s 1 . The sterilizer tube inner diameter is

1.5 m. The liquid medium density is 1200 kg/m 3 ,andviscosityis1.5 10 3 Pa $ s.

(a) Determine the length of the holding section and space time required;
(b) Determine the dimensionless time required.