r. shanthini 25 nov 2011 1 sterilization (continued) cp504 – lecture 17 - learn about thermal...

R. Shanthini 25 Nov 2011

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Sterilization (continued)

CP504 – Lecture 17

- Learn about thermal sterilization of liquid medium

- Learn about air sterilization

- Learn to do design calculations


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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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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 Injection Type

Holding section(where most of the

sterilization takes place)

SteamRawmedium Expansion

valvevacuum

Flashcooler

Sterilemedium

- Direct steam injection for heating (relatively a rapid process)

- Flash cooling (rapid process)



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- 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 anti-corrosive agents

- Foaming may occur during both heating and cooling

Continuous Injection Type


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Continuous Heat Exchanger Type

Raw medium

Holding section

Sterilemedium

Cooling water

Steam

Indirect steam heating in

plate-and-frame (or shell-and-tube) heat exchanger



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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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Most of the sterilization in the continuous sterilization process may occur in the holding section.

Therefore

holdln

no

nt=

λhold

L uav

Since the holding section is a long pipe,

=length of the pipe

average fluid velocity in the pipe


= kd λhold = kd0 exp(-Ed/RT) λhold


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uav

The ratio of average velocity to maximum velocity

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.


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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.

lnn0

nt= = 37.2 = kd dt

0

t

= ln144x1014

1

n0 = (5 x 1012 per m3) x (2 m3/h) x (24 h/day) x (60 days)

= 14400 x 1012

nt = 1

Solution to Example 10.4:

The above integral should give 37.2.


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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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Since plug flow is assumed,

velocity through the pipe =


Therefore, L = (245 m/h) x (0.099 h) = 24.26 m

2 m3/h

π (0.102/2)2 m2= 245 m/h

What happens if the flow is not plug flow?


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b) If axial dispersion is considered, then we use the following table:


kd L uav

nt

n0

uav L

DPeclet number =

Axial dispersion coefficient


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uav L

DPeclet number = = (245 m/h) x ( L m)

(20 m2/h)

kd L uav

X-coordinate = = (375.3 per h) x ( L m) (245 m/h)

nt n0

Y-coordinate = is obtained from the table


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uav L

DPeclet number = = (245 m/h) x ( 25 m)

(20 m2/h)

kd L uav

X-coordinate = = (375.3 per h) x ( 25 m) (245 m/h)

nt n0

Y-coordinate = =

If L = 25 m is tried, we get

= 306

= 38.3

> 6.9 x 10-17


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uav L

DPeclet number = = (245 m/h) x ( 27.5 m)

(20 m2/h)

kd L uav

X-coordinate = = (375.3 per h) x ( 27.5 m) (245 m/h)

nt n0

Y-coordinate = =

If L = 27.5 m is tried, we get

= 337

= 42.1

> 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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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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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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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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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.

r. shanthini 25 nov 2011 1 sterilization (continued) cp504 – lecture 17 - learn about thermal...

Documents

sterilization time

batch sterilization

pipecontinuous sterilization

air sterilization

frame heat exchanger

steam injector

live steam

high temperatures