Bioreactor Design and Operation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 936

Heat exchanging fluid

x x+dx

Q

Q

Feed

Effluent

C X0

C Xe

Heat exchanging fluid

FIGURE 18.11 Sterilization in a well-mixed reactor.

Integrating Eqn (18.40) , we obtain the probability of a contaminant cell surviving the heat
treatment

pð s Þ¼ C X e k d s

C X 0 ¼

(18.41)

where is the space time of the fluid medium through the whole length of the sterilization

section L, that is

s

s ¼ A cr L
Q

(18.42)

Therefore sterilization in a PFR is quite similar to that of the batch process. The probability of
the entire population in one reactor-full process fluid (of volume V) not vanishing from the
heat treatment is

e k d s
1þ C X 0 V 1
2

C X 0 V

P 1 ð s Þ¼1P 0 ð s Þ¼1½1pð s Þ C X 0 V ¼ 1ð1 e k d s Þ C X 0 V z

(18.43)

e k d s

which corresponds to the dimensionless sterilization time of

t S ¼ k d s lnðC X0 VÞ (18.44)

Eqn (18.44) is nearly identical to Eqn (18.25) . Thus, sterilization in a PFR is just as sterilization
in a batch reactor. However, the heating up and cooling down requirements are not as strin-
gent as batch sterilization.
Continuous sterilization in a PFR set-up, particularly a high-temperature short-
exposure time process, can significantly reduce damage to medium ingredients while
achieving high levels of cell destruction. Other advantages include improved steam
economy and more reliable scale-up. The amount of steam needed for continuous tube
flow sterilization is 20 e 25% that used in batch processes. The time required is also
significantly reduced because heating and cooling are virtually instantaneous. Therefore,
for sterilizing process fluids, it is advantageous to design the sterilization in a PFR
setting.