CP504 – Lecture 17
Sterilization (continued)
- Learn about thermal sterilization of liquid medium
- Learn about air sterilization
- Learn to do design calculations
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25 Nov 2011
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Continuous Sterilization:
- Simplifies production planning
- Therefore gives maximum plant utilization and minimum
delays
- Provides reproducible conditions
- Can be operated at high temperature (1400C)
- Therefore sterilization time can be reduced (2 to 3 min)
- Requires less steam and less cooling water
- Suitable when the capacity of operation is high
- High initial capital investment (use of aseptic transfer system
for the sterile broth to be transported to a sterile vessel)
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Continuous Sterilization:
It is the only option if the medium is to be exposed to high
temperatures for a short time (HTST process) to avoid
denaturation of proteins or to avoid destruction of some of
the enzymes, etc., since it is not possible in commercial
scale operations in batch sterilization to quickly heat large
volumes of the medium in short time and cool it also in short
times.
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Continuous Sterilization:
Continuous Injection Type
Steam
Raw
medium
Expansion
valve
Holding section
(where most of the
sterilization takes place)
vacuum
Flash
cooler
- Direct steam injection for heating
(relatively a rapid process)
- Flash cooling (rapid process)
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25 Nov 2011
Sterile
medium
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Continuous Sterilization:
Continuous Injection Type
- Capital investment is low
- Easy to clean and maintain the system
- Heating and cooling periods are shorter
- Steam efficiency is very high as live steam is directly injected
into the medium
- Direct contact of the steam with the medium makes it
necessary that the steam should be clean and free of any anticorrosive agents
- Foaming may occur during both heating and cooling
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Continuous Sterilization:
Sterile
medium
Continuous Heat Exchanger Type
Steam
Holding section
Cooling water
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25 Nov 2011
Raw medium
Indirect steam
heating in
plate-and-frame (or
shell-and-tube)
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heat exchanger
Continuous Sterilization:
Continuous Heat Exchanger Type
plate-and-frame heat exchanger has larger heat transfer area
than shell-and-tube heat exchanger, and therefore more
effective
plate-and-frame heat exchanger is favourable with high
viscous system
plate-and-frame heat exchanger is limited to lower pressures
(less than 20 atm) due to its weak structural strength
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Continuous Sterilization:
Most of the sterilization in the continuous sterilization process
may occur in the holding section.
Therefore
hold =
no
ln
nt
= kd λhold = kd0 exp(-Ed/RT) λhold
Since the holding section is a long pipe,
λhold
=
L
uav
length of the pipe
average fluid velocity
in the pipe
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25 Nov 2011
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Continuous Sterilization:
The ratio of average velocity to maximum velocity
uav
umax
=
0.5 for laminar flow of Newtonian fluids
through a smooth round pipe
=
0.75 for turbulent flow
=
0.87 for turbulent flow with the Reynolds
Number of 1000,000
Therefore, using the average velocity to calculate
the length of the pipe required for sterilization may
leave some portion of the medium understerilized,
which may cause contamination problem
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Example 10.4: Estimating the time required for continuous sterilization
A continuous sterilizer with a steam injector and a flash cooler
will be employed to sterilize medium continuously with the flow
rate of 2 m3/h. The time for heating and cooling is negligible with
this type of sterilizer. The typical bacterial count of the medium is
about 5 x 1012 per m3, which needs to be reduced to such an
extent that only one organism can survive during two months of
continuous operation. The sterilizer will be constructed with the
pipe with an inner diameter of 0.102 m. Steam at 600 kPa
(gauge pressure) is available to bring the sterilizer to an
operating temperature of 125oC. For the heat resistant bacterial
spores: kdo = 5.7 x 1039 per h; Ed = 2.834 x 105 kJ / kmol. For the
medium: c = 4.187 kJ/kg.K; ρ = 1000 kg/m3; μ = 4 kg/m.h.
a) What length should the pipe be in the sterilizer if you assume
ideal plug flow?
b) What length should the pipe be in the sterilizer if the effect of
axial dispersion is considered? Assume an axial dispersion
coefficient
of 20 m2/h.
R.
Shanthini
10
25 Nov 2011
Solution to Example 10.4:
Problem statement: The typical bacterial count of the medium
is about 5 x 1012 per m3, which needs to be reduced to such
an extent that only one organism can survive during two
months of continuous operation.
n0 = (5 x 1012 per m3) x (2 m3/h) x (24 h/day) x (60 days)
= 14400 x 1012
nt = 1

=
t
n0
ln
nt
=
144x1014
= 37.2 =
ln
1

kd dt
0
The above integral should give 37.2.
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25 Nov 2011
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Solution to Example 10.4:
a) Since the temperature at the holding section in constant,
hold
= kd λhold = 37.2
For the given data,
kd = kd0 exp(-Ed/RT)
= (5.7 x 1039 per h) exp[- 2.834x105 / 8.314x(273.15+125)]
= 375.3 per h
Therefore, λhold = 37.2 / kd = 37.2 / 375.3 per h = 0.099 h
Length of the pipe, L = velocity through the pipe x λhold
= velocity through the pipe x 0.099 h
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25 Nov 2011
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Solution to Example 10.4:
Since plug flow is assumed,
velocity through the pipe =
2 m3/h
π (0.102/2)2 m2
= 245 m/h
Therefore, L = (245 m/h) x (0.099 h) = 24.26 m
What happens if the flow is not plug flow?
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25 Nov 2011
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Solution to Example 10.4:
b) If axial dispersion is considered, then we use the following
table:
uav L
Peclet number =
D
nt
n0
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25 Nov 2011
Axial dispersion
coefficient
kd L
uav
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Solution to Example 10.4:
=
(245 m/h) x ( L m)
(20 m2/h)
X-coordinate =
kd L
uav
=
(375.3 per h) x ( L m)
(245 m/h)
Y-coordinate =
nt
n0
is obtained from the table
uav L
Peclet number =
D
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25 Nov 2011
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Solution to Example 10.4:
If L = 25 m is tried, we get
=
(245 m/h) x ( 25 m)
(20 m2/h)
X-coordinate =
kd L
uav
=
(375.3 per h) x ( 25 m)
= 38.3
(245 m/h)
Y-coordinate =
nt
n0
=
uav L
Peclet number =
D
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25 Nov 2011
= 306
> 6.9 x 10-17
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Solution to Example 10.4:
If L = 27.5 m is tried, we get
=
(245 m/h) x ( 27.5 m) = 337
(20 m2/h)
X-coordinate =
kd L
uav
=
(375.3 per h) x ( 27.5 m)
= 42.1
(245 m/h)
Y-coordinate =
nt
n0
=
uav L
Peclet number =
D
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25 Nov 2011
> 6.9 x 10-17
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Filtration:
Sterilize solutions that may be damaged or denatured by
high temperatures or chemical agents.
The pore size for filtering bacteria, yeasts, and fungi is in
the range of 0.22-0.45 μm
The pore size for filtering viruses and some large proteins
is in the range of 0.01 μm
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Methods for gas (air) sterilization:
- Aerobic fermentation require huge volumes of air (a
50,000 L fermenter requires 7x106 to 7x107 L per day
of air) which must be sterilized.
- Adiabatic compression of process air can increase air
temperature (150oC to 220oC). A temperature typically
of 220oC for 30 s is required to kill spores.
- An air-filtration step is almost always used to ensure
sterility of process air.
- Depth filters use glass wool, and rely on inertial
impaction, interception, diffusion and electrostatic
attraction (explained later).
- Surface filters using membrane cartridges use the
sieving effect (explained later).
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Filtration mechanisms operating in depth filters
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Filtration mechanisms operating in depth filters
Inertial impaction (or impingement) occurs when a particle
travelling in the air stream and passing around a fibre,
deviates from the air stream (due to particle inertia) and
collides with the fibre.
Impaction is the dominant collection mechanism for
particles larger than 0.2 μm.
It is important in removing bacteria.
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Filtration mechanisms operating in depth filters
Interception occurs when a large particle, because of its
size, collides with a fibre in the filter that the air stream is
passing through.
Interception is the dominant collection mechanism for
particles greater than 0.2 μm.
It is important in removing bacteria.
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25 Nov 2011
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Filtration mechanisms operating in depth filters
Diffusion occurs when the random (Brownian) motion of a
particle causes that particle to contact a fibre.
Diffusion is dominant for particles less than 0.2 μm.
It may be important for virus removal, but bacteria are
sufficiently large that diffusion is relatively unimportant.
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Filtration mechanisms operating in depth filters
Electrostatic attraction plays a very minor role in mechanical
filtration. After fibre contact is made, smaller particles are
retained on the fibres by a weak electrostatic force.
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Filtration mechanisms operating in surface filters
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Concerns in air sterilization:
Pressure drop is critical in a filter.
Energy input for compressed air for a commercial-scale
process is significant.
Air treatment can account for 25% of total production
costs.
Design engineer has to balance the assurance sterility
against the pressure drop.
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180M3 Fermenter Plant
Air Compressor
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Air Filters in Fermentation Plant
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Air sterilization note from Lee was handed over.
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