This document discusses cell separation methods used in downstream processing. It describes filtration and centrifugation as the two main methods. For filtration, it explains the theory and types of filters used like plate and frame, pressure leaf, and rotary vacuum filters. For centrifugation, it outlines stock's law and describes basket, tubular bowl, solid bowl (decanter), multi-chamber, and disc bowl centrifuges. Cross flow filtration is also introduced as an alternative to overcome issues with other filtration methods. In under 3 sentences, this document provides an overview of cell separation techniques used in downstream processing and discusses filtration and centrifugation methods in detail.

1. CELL SEPARATION
BY:DHARATI JOSHI
M.Sc. Biotechnology
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2. Contents:
Introduction of Down stream processes
Introduction of cell separation
Methods of cell separation:
1.Filtration
2.Centrifugation
Theory of Filtration
Types of Filter
Types of centrifuge
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3. Downstream process:
It is a process by which we can separate &
purify our desired product. It is collective term which
includes many processes which are helpful to isolate and
purify our desired product. In downstream processing all
processes are done step wise by which finally we can get
product. One of the basic step of these downstream
processes is cell separation process.
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5. WHAT IS CELL SEPARATION
According to fermentation, cell separation is a process in
which we can separate cells which may be useful or
waste during downstream process.
If product is extracellular at that time cells which
produce this product these cells are consider as a waste
material which should separate to isolate a pure
product.
 If product is intracellular at that time we must separate
cells from broth to get our product. At that time cells are
consider as useful thing. To get product from these cells
further cell disruption processes are require.
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6. There are several methods by which cell
separation is done such as:
1.Filtration
2.Centrifugation
3.Precipitation
4.Foam separation etc.
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7. Filtration:
Conventional filtration is often the starting point for
separating cells from the fermentation broth.
It is one of the most common process used at all scales
of operation.
In filtration suspended particles are separated from a
liquid or gas using porous medium which retains the
particles but allow the liquid or gas to pass through.
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8. Theory of filtration:
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9. In the filtration apparatus a support is present which is
covered with a porous filter cloth.
As filtration proceed, particles or cells which have more
size than pore size of filter cloth are deposited on the
filter cloth and small particles & liquid is pass through
the filter cloth which is called as filtrate.
The particles which are deposited on the filter cloth is
called as filter cake.
 As the thickness of filter cake is increases the
resistance to flow will gradually increase.
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10. When pressure is applied to the surface of filter cloth is
constant at that time the flow of filtrate will be decrease with
time goes due to the accumulation of filter cake.
If you want to maintain constant flow of filtrate through filter
cloth at that time you should increase the pressure as filter
cake is increase.
If particles of filter cake are smooth & compressible at that
time the flow rate of filtrate is decrease due to blockage of
pores of filter cloth.
 If particles are compressible at that time you should not
apply increase pressure to maintain the constant flow rate.
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11. So, compressible particle will cause big challenge.
 Flow through a uniform & constant depth porous bed
can be represented by the Darcy equation:
Rate of flow = dV/dt = KA P/µL ….(1)
Where, µ = Liquid viscosity,
L = Depth of filter bed,
P = Pressure difference across the filterbed
A = Area of the filter cloth exposed to liquid
K = Constant
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13. If fermentation broth contain gelatinous material such
as bacteria at that time we use filter aid along with filter
bed to prevent the blockage of filter bed.
Kieselguhr or diatomaceous earth is widely used as filter
aid.
When it is mix with cell suspension, it improves porosity
of filter cake & improves the flow rate of filtration.
 The quantity of filter aid which is used in filtration are
different according to scale of operation.
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14. Demerits of kieselguhr:
1. It is costly.
2. It is also absorb some amount of filtrate.
How kieselguhr is use?:
1. A thin layer of kieselguhr is applied to the filter to form
a precoat prior to perform broth filtration.
2. The appropriate quantity of filter aid is mixed with the
harvested broth. Filtration is started to build up a
satisfactory filter bed. The initial raffinate is returned
to the remaining broth prior to starting the true
filtration.
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15. 3.When vacuum drum filter are to be used which are
fitted with advancing knife blades, a thick precoat filter is
initially built up on the drum.
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17. Plate & frame filters
It is pressure filter. It contain plates & frames which are
arranged alternatively.
The hollow frame is separated from the plate by the filter
cloth. The press is closed either by use of a hand screw
or hydraulically.
The slurry is introduced through a port in each frame &
the filtrate passes through the cloth & plate then
discharged through outlets.
The solids are retain in the frame & filtration is stopped
when frames are completely filled.
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Pressure leaf filters
These filters have numbers of leaves consisting of metal
framework which is covered with a fine wire mesh or
filter cloth & precoated with a layer of cellulose fibers.
The slurry is fed into the filter which is operated under
pressure with a vacuum pump.
The filters are totally enclosed so it is possible to sterilize
with steam.
Some types of pressure leaf filters are 1.Vertical metal
leaf filter 2.Horizontal metal leaf filter 3.Stacked disc
filter

20. 1.Vertical metal leaf filter:
This filter consist many porous metal leaves which are
mounted on a horizontal hollow shaft. The solids from the
slurry builds up on the surface of the leaves & filtrate is
removed from the plates through hollow shaft. Solids are
removed by blowing air in opposite direction through
hollow pipe.
2.Horizontal metal leaf filter:
In this metal leaves are arranged on a vertical hollow
shaft within pressure vessel. Only upper surface of leaves
are porous.
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21. Filtration is continued until cake fills the space between
the disc shaped leaves. At the end of process the cake can
be discharge by releasing the pressure & spinning the
shaft with a drive motor.
3.Stacked disc filter:
One kind of filter of this type of filter is Metafilter. It is
very robust device because it do not contain filter cloth. It
contain many precision made rings which are stacked on
fluted rod. The stack of ring is placed in vessel. The
filtrate passes between the discs & is removed through
grooves of fluted rods, while solids are deposited on the
filter coating.
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25. Rotary Vacuum filters:
It is a type of continuous filter. It consists of a rotating,
hollow, segmented drum covered with a metal filter
which is partially immersed in a trough containing broth
to be filtered.
The slurry is fed on to the outside of the revolving drum
& vacuum pressure is applied internally so that the
filtrate is drawn through filter, into the drum & finally to
a collecting vessel.
To discharge the cake internal pressure is apply in
opposite direction.
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26. Many rotary vacuum drum filters are available which are
differ in the mechanism of the cake discharge from the drum:
1.String discharge
2.Scarper discharge
3.Scarper discharge with precoating of the drum
String discharge:
In this filter cake is remove from drum by string discharge.
Long lengths of string 1.5cm apart are threaded over drum &
round two rollers. The cake is lifted free from upper part of
the drum when the vacuum pressure is released & carried to
small rollers where it falls free.
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27. Scraper discharge:
The filter cake which builds up on the drum is removed
by an accurately positioned knife blade. Because knife is
close to drum, there may be gradual wearing of the filter
cloth on the drum.
Scraper discharge with precoating of the drum:
In this, drum is precoated with filter aid 2 to 10 cm
thick. The cake builds up on the drum during operation
is cut away by the knife blade.
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28. 1.String discharge 2.Scarper discharge
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30. Cross flow filtration (Tangential filtration):
In previous all filtration process, the flow of broth was
perpendicular to the filter cloth. So, blockage of the
membrane occur which led to lower rates of filtration.
To overcome this problem cross flow filtration is used. In this
case the flow of medium is tangential to the membrane & no
filter cake builds up on the membrane.
Advantage:
Efficient separation.
Separation is independent of cell & media densities in
contrast to centrifugation.
No addition of filter aid.
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Centrifugation
When filtration is not satisfactory for removal of
microorganism at that time centrifugation is used.
Mostly centrifuges are all operated in semi-continuous
or continuous operation.
According to stock’s law, the rate of sedimentation of a
spherical particle is proportional to the square of the
diameter of the particle, thus the rate of sedimentation
of a particle in a centrifuge is:
Vc=dω2
r(ρP-ρL)/18µ

33. Where Vc = rate of sedimentation in the centrifugation
ω = angular velocity of rotor
r = radial position of the particle
d = particle diameter
ρ = density (p for particle, L for liquid)
µ = viscosity
Some factors influence the rate of sedimentation such as
diameter of cells, viscosity of the liquid. Ideally the cells
should have a large diameter, there should be a large
density difference between cell & liquid. The angular
velocity & diameter of centrifuge are also major factors.
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34. The types of centrifuges:
1.Basket centrifuge(perforated bowl basket centrifuge):
The centrifuge is used with a perforated bowl which is
lined with filter bag.
A continuous feed is used, & when the basket is filled with
filter cake it is possible to wash the cake before removing
it.
These centrifuges are normally operated up to 4000 rpm.
The basket centrifuge may be considered to be a
centrifugal filter.
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36. 2.Tubular bowl centrifuge:
The tubular bowl centrifuge is used primarily for
continuous separation of liquids from liquids or very fine
particles from liquids. Although in some cases it is
employed as a batch type centrifuge.
It is used when higher centrifugal fields are required for
separation. The rotating bowl of tubular centrifuge
consists of a long hollow tube.
The feed is enter from the nozzle. During operation
solids sediment on the wall of the bowl while the liquids
separates into heavy phase in zone(G) & the light phase
in the central zone (H). 1/18/20
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38. 3.The solid bowl centrifuge (Decanter centrifuge):
In this slurry is fed through the spindle of an archimedean
screw within the horizontal rotating solids bowl.
The solids are settling on the walls of the bowl are scraped to
the conical end of the bowl.
The liquid phase is discharge from the opposite end of the
bowl.
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40. 4.Multichamber centrifuge:
In this centrifuge, a series of
concentric chambers are
mounted within rotor chamber.
The broth enters via the
central spindle & then takes a
circuitous route through the
chambers.
Solids collect on the outer
faces of each chamber. It has
greater solid capacity than
tubular bowls.
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41. 5.The Disc bowl centrifuge:
It contains discs in the rotor or bowl. A central inlet pipe
is surrounded by a stack of conical discs.
The broth flows outwards from the central feed pipe , then
upwards & inwards between the discs at an angle of 45°
to the axis of rotation.
During sedimentation solids then slides to the edge of
bowl & eventually accumulate on the inner wall of the
bowl. The sediment make sludge which flows rather than
a particulate sediment.
This sludge is removed by nozzle or intermittent opening
of bowl. 1/18/20
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43. REFERENCES:
Principles of fermentation technology by
Whittekar & Stanbury
Visual encyclopedia of Chemical Engineering-
University of Michigan
Fermentation Microbiology & biotechnology By
A.R.Allman
Filtration video By Sajid Majeed
Bioprocess Engineering by Nelson
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