Sterilization of air: Methods

Sterilization of air: Methods
Most industrial fermentations are operated under
vigorous aeration and the air supplied to the
fermenter must be sterilized.
To prevent contamination of either the fermentation by
Microorganisms or the environment by aerosols
generated within the fermenter, both air input and
air exhaust ports have air filters attached.
Suspended solids may be separated from a fluid
during filtration by the following mechanisms:
(j) Inertial impaction.
(ii) Diffusion.
(iii) Electrostatic attraction.
(iv) Interception.
Advantages of continuous sterilization over
batch Sterilization
(i) Superior maintenance of medium quality.
(ii) Ease of scale-up - discussed later.
(iii) Easier automatic control.
(iv) The reduction of surge capacity for steam.
(v) The reduction of sterilization cycle time.
(vi) Under certain circumstances, the reduction of
fermenter corrosion.
Advantages of batch sterilization over
continuous sterilization
Lower capital equipment costs.
(ii) Lower risk of contamination - continuous
processes require the aseptic transfer of the
sterile broth to the sterile vessel.
(iii) Easier manual control.
(iv) Easier to use with media containing a high
proportion of solid matter.
Filters and design of depth filters
• Filtration is one of the most common processes
used at all scales of operation
to separate suspended particles
from a liquid or gas,
using a porous medium called filter
which retains the particles
but allows the liquid or gas when pass through filter.
The theory of depth filters:
Aiba et al. (1973) have given detailed quantitative analysis of these
mechanisms but this account will be limited to a description of the overall
efficiency of operation of fibrous filters.
It is assumed that if a particle touches a fibre it remains attached to it, and
that there is a uniform concentration of particles at any given depth in the
filter, then each layer of a unit thickness of the filter should reduce the
population entering it by the same proportion; which may be expressed
mathematically as:
dN/dx = -KN
• Where,
• N is the concentration of particles in the air at a depth, x, in the filter and
• K is a constant.
On integrating equation (1) over the length of the
filter it become:
N/No = e-kx
No is the number of particles entering the filter and
N is the number of particles leaving the filter.
On taking natural logarithms, equation (2) becomes:
In (N / No) = -Kx.
Equation (3) is termed the log penetration
• The efficiency of the filter is given by the ratio of the
number of particles removed to the original number
present, thus:
E = (No - N)/No
where E is the efficiency of the filter.
(No - N)/No = 1 - (N / No).
Substituting N/No = e-Kx ,Therefore
(No - N)/No 1 e-Kx
E = 1 - e-Kx
The log penetration relationship [equation (3)] has been
used by Humphrey and Gaden (1955) in filter design, by
using the concept X9O ' the depth of filter required to
remove 90% of the total number of particles entering
the filter; thus:
If No were 10 and x were X 9O ' then N would be 1:
In (1/10) = -KX90
or 2.303 10glO(1/IO) = - KX9O
2.303( -1) = -KX90
X 90 = 2.303/K.
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