Overview of industrial filtration technology and its applications

Abstract

Filtration is a process where solid particles present in a suspension are separated from liquid or gas employing a porous medium. This article includes some advances in sintered metal filters, different types of filter media and their applications in various industries. Recent applications of ultra filtration and some advances in bag and belt filtration systems have also been discussed. This article also focuses on advantages of self cleaning filters over manual and mechanical cleaning. Thus, advances in filtration technology include the development of continuous processes to replace old batch process technology. Different self cleaning filters reduce product loss, required minimal operator intervention and improves flow consistency. Using programmable logic controllers in humidity ventilation and air conditioning system which often use sensors can cut installation and labor costs. On the global scale, we are surrounded by different filters. Thus by using latest filters and filter media it is possible to apply it in various food, starch and sugar industries, which reduces the time as well as give better quality products.

Full text

Indian Journal of Science and Technology Vol. 3 No. 10 (Oct 2010) ISSN: 0974- 6846
Popular article “Industrial filtration technology” Rakesh et al.
©Indian Society for Education and Environment (iSee) http://www.indjst.org Indian J.Sci.Technol.
1121
Overview of industrial filtration technology and its applications
Rakesh Patel, Devarshi Shah, Bhupendra G. Prajapti and Manisha Patel
Dept. of Pharmaceutics & Pharmaceutical Technology, S. K. Patel College of Pharmaceutical Education & Research,
Ganpat University, Ganpat Vidyanagar, Kherva, Mehsana-Gozaria Highway, Pin-382711, Mehsana, Gujarat, India
raka_77us@yahoo.com
Abstract
Filtration is a process where solid particles present in a suspension are separated from liquid or gas employing a
porous medium. This article includes some advances in sintered metal filters, different types of filter media and their
applications in various industries. Recent applications of ultra filtration and some advances in bag and belt filtration
systems have also been discussed. This article also focuses on advantages of self cleaning filters over manual and
mechanical cleaning. Thus, advances in filtration technology include the development of continuous processes to
replace old batch process technology. Different self cleaning filters reduce product loss, required minimal operator
intervention and improves flow consistency. Using programmable logic controllers in humidity ventilation and air
conditioning system which often use sensors can cut installation and labor costs. On the global scale, we are
surrounded by different filters. Thus by using latest filters and filter media it is possible to apply it in various food, starch
and sugar industries, which reduces the time as well as give better quality products.
Keywords: filter media, ultra filtration, bag filtration, belt filtration, HVAC system.
Introduction
Filtration is a process whereby solid particles present
in a suspension are separated from the liquid or gas
employing a porous medium, which retains the solids but
allows the fluid to pass through. When the proportion of
solids in a liquid is less, the term clarification is used. It is
a common operation which is widely employed in
production of sterile products, bulk drugs, and in liquid
oral formulation. The suspension to be filtered is known
as slurry. The porous medium used to retain the solids is
known as filter medium. The accumulated solids on the
filter are referred as filter cake & the clear liquid passing
through the filter is filtrate. The pores of the filter medium
are smaller than the size of particles to be separated.
Filter medium like filter paper or muslin cloth is placed on
a support. When feed is passed over the filter medium,
the fluid flows through the filter medium by virtue of a
pressure differential across the filter. Gravity is acting on
the liquid column. Therefore, solids are trapped on the
surface of the filter medium. After a particular point of
time, the resistance offered by the filter cake is high that
stops the filtration (Sambhamurthy, 2005).
Types of filtration
Based on the mechanism, three types of the filtration
are known.
Surface filtration:
It is a screening action by which pores
or holes of the medium prevent the passage of solids.
The mechanisms, straining and impingement are
responsible for surface filtration. For this purpose, plates
with holes or woven sieves are used. Example is
cellulose membrane filter (Matteson, 1987).
Depth filtration:
This filtration mechanism retains
particulate matter not only on the surface but also at the
inside of the filter. This is aided by the mechanism
entanglement. It is extensively used for clarification.
Examples are ceramic filters and sintered filters
(Stephan, 2003).
Advances in sintered metal filters
(Mottcorp, 1999):
• Filtration technology utilizing sintered metal media
provides excellent performance for separation of
particulate matter. Sintered metal filter media are widely
used in the chemical process, petrochemical and power
generation industries.
• Advances in filtration technology include the
development of continuous processes to replace old
batch process technology. Liquid/solids filtration using
conventional leaf filters is messy and hazardous to
clean and require extended re-circulation time to obtain
clean product. Traditional gas/solids separation
systems such as cyclones, Electro static precipitators
and disposable filters are being replaced by sintered
fiber metal filtration systems.
• Sintered metal filters should be operated within the
design parameters to prevent premature blinding of the
media due to fluctuations in process operations. Use of
flow control assures the filter will not be impacted with a
high flow excursion. Filter efficiency increases as the
filter cake forms. The cake becomes the filter media
and the porous media acts as a septum to retain the
filter cake. Filter cakes can be effectively washed in-situ
and backwashed from the filter housing. A gas assisted
pneumatic hydropulse backwash has proven to be the
most effective cleaning method for sintered porous
metal filters.
• Sintered metal filters can be fully automated to
eliminate operator exposure and lower labour costs
while providing reliable, efficient operation.
Case study of depth filtration (Carey, 2008):
Several forces have driven changes in filtration
technology during the last couple of decades, including
environmental concerns, the health and safety of winery

Indian Journal of Science and Technology Vol. 3 No. 10 (Oct 2010) ISSN: 0974- 6846
Popular article “Industrial filtration technology” Rakesh et al.
©Indian Society for Education and Environment (iSee) http://www.indjst.org Indian J.Sci.Technol.
1122
workers, and wine quality. The major active component in
traditional depth filtration is diatomaceous earth, which
has several major problems. First, it is difficult to dispose
of because it does not decompose. Second, it can cause
symptoms similar to coal miners' "black lung" disease
when inhaled over long periods of time. In the United
States this problem can be overcome by using cross flow
filtration. The main benefit of cross flow filtration is that it
uses a membrane with an absolute pore size to clarify
wine without the need for media to act as the sieve for
removal of particles from wine.
Example of cross flow filtration
• Nanofiltration is a recent membrane filtration process
used most often with low total dissolved solids water,
with the purpose of softening and removal of
disinfection by-product precursors such as natural
organic matter and synthetic organic matter.
• It is a cross-flow filtration technology which ranges
somewhere between ultra filtration and reverse
osmosis. The nominal pore size of the membrane is
typically below 1 nanometer, thus Nanofiltration.
Nanofilter membranes are typically rated by molecular
weight cut-off rather than nominal pore size. The
transmembrane pressure required is considerably lower
than the one used for RO, reducing the operating cost
significantly. However, NF membranes are still subject
to scaling and fouling and often modifiers such as anti-
scalants are required for use (Hillie, 2007).
Ultra filtration
: Ultra filtration is a pressure-driven
membrane transport process that has been applied, on
both the laboratory and industrial scale. Ultra filtration is a
separation technique of choice because labile streams of
biopolymers (proteins, nucleic acids & carbohydrates)
can be processed economically, even on a large scale,
without the use of high temperatures, solvents, etc.
Shear denaturation can be minimized by the use of low-
shear (e.g., positive displacement) pumps (Goldsmith
et
al.,
1974).
Following types of ultra filtration membranes are used
prominently (Smolders, 1980):
• Asymmetric skinned membranes made from synthetic
polymers by the "phase-inversion" methods.
• Inorganic membranes, utilizing inorganic porous
supports and inorganic colloids, such as ZrC*2 or
alumina with appropriate binders.
• Melt-spun, "thermal inversion" membranes.
• "Composite" and "dynamic" membranes with selective
layers formed in situ.
Recent & developing application of ultra filtration
• Ultra filtration is becoming a powerful separation tool
for the rapidly growing biotechnology industry.
Examples are cell harvesting, depyrogenation of
injectable drugs, and enzyme purification.
• Ultra filtration offers some important advantages over
centrifugation for harvesting of bacteria; the asymmetric
character of ultra filtration membranes renders them
less prone to clogging by cells and debris than micro
porous filters.
• Plasma product processing is another promising
application of ultra filtration. When human plasma is
fractionated by the Cohn process or some new
methods, a need arises for concentration of the
important protein fractions (albumin & globulins) or for
removal of alcohol and salt from these fractions. This
can be conveniently accomplished by
ultrafiltration.
• Production of a new generation of miniaturized
computer microchips has created very stiff acceptance
criteria for minimum tolerable particle size in the so-
called ultra pure water used in rinsing operations. Thus,
increased use of ultra filtration for production of ultra
pure water in the computer and electronics industry.
Cake filtration:
By this filtration mechanism, the cake
accumulated on the surface of the filter is itself used as a
filter. A filter consists of a coarse woven cloth through
which a concentrated suspension of rigid particles is
passed so that they bridge the holes and form a bed.
Example is cake made from diatomite. This cake can
remove sub micrometer colloidal particles with high
efficiency.
Theory of filtration
Depending on dispersing medium filtration is divided in
two parts: 1) gas filtration and 2) liquid filtration.
Gas filtration theory
It mainly includes filtration of aerosols and lyosols.
Membrane filters and nucleopore filters are based on
these below mechanisms.
Mechanism of gas filtration (Wilson & Cavanagh, 1969)
Diffusion deposition:
The trajectories of individual small
particles do not coincide with the streamlines of the fluid
because of Brownian motion. With decreasing particle
size the intensity of Brownian motion increases and, as a
consequence, so does the intensity of diffusion
deposition.
Direct interception:
This mechanism involves the finite
size of particles. A particle is intercepted as it approaches
the collecting surface to a distance equal to its radius. A
special case of this mechanism is the so-called sieve
effect, or sieve mechanism.
Inertial deposition:
The presence of a body in the flowing
fluid results in a curvature of the streamlines in the
neighbourhood of the body. Because of their inertia, the
individual particles do not follow the curved streamlines
but are projected against the body and may deposit there.
It is obvious that the intensity of this mechanism
increases with increasing particle size and velocity of
flow.
Gravitational deposition:
Individual particles have a
certain sedimentation velocity due to gravity. As a
consequence, the particles deviate from the streamlines
of the fluid and, owing to this deviation; the particles may
touch a fiber.

Indian Journal of Science and Technology Vol. 3 No. 10 (Oct 2010) ISSN: 0974- 6846
Popular article “Industrial filtration technology” Rakesh et al.
©Indian Society for Education and Environment (iSee) http://www.indjst.org Indian J.Sci.Technol.
1123
Electrostatic deposition:
Both the
particles and the fibers in the filter may
carry electric charges. Deposition of
particles on the fibers may take place
because of the forces acting between
charges or induced forces.
Liquid filtration theory (Melia & Weber,
1972)
The term solid-liquid filtration
covers all processes in which a liquid
containing suspended solid is freed of
some or the entire solid when the sus-
pension is drawn through a porous
medium.
Kozeny-Carman equation:
()
Llr P
dtAdv
+
Δ
=
μ
.
.1 (1)
Where, A = filter area; V = total
volume of filtrate delivered; t =
filtration time; ΔP = pressure drop
across cake and medium; r = specific
cake resistance; μ = filtrate viscosity; l
= cake thickness;
L = thickness of cake equivalent to medium resistance.
Limitations of Kozeny-Carman equation (Chowdiah et al.,
1981)
This equation does not take into account of the fact
that depth of the granular bed is lesser than the actual
path traversed by the fluid. The actual path is not straight
throughout the bed, but it is sinuous or tortuous.
Poiseulle’s law:
This Law considered that filtration is
similar to the streamline flow of a liquid under pressure
through capillaries.
)(.
.1
CM RR P
dtdv A
+
Δ
=
μ
(2)
Cake resistance:
A
W
RM
α
=(3)
Specific cake resistance
α=α׳∆Ps (4)
The filter resistance is much less than the cake
resistance.
Rc
<<
Rm
)'(.
.1 WAP
P
dtdvAsΔ
Δ
=
αμ
(5)
Where,
V=Filtrate volume; A= Filter area; t=Time; ΔP=Pressure
driving force; μ=Broth viscosity
W=Mass of filter cake; R=Resistance; α=Specific cake
resistance; S= Compressibility factor.
Filter media (Subramanyam et al.,
2005)
The filter medium acts as a
mechanical support for the filter cake
and it is responsible for the collection
of solids. Minimum cake thickness of
discharge for different types of filter is
presented in Table 1.
Materials used as filter media
(Rushton, 2008)
Different types of materials used as
filter media are presented in Table 2.
Woven materials such as felts or
cloths
: woven material is made of
wool, cotton, silk & synthetic fibers
etc. are used. Synthetic fibers have
greater chemical resistance than
wool or cotton. The choice of fiber
also depends on the physical state &
chemical constitution of the slurry. It
includes mainly of two types.
Monofilament woven cloth (Fig.1):
The yarns of a monofilament fabric
are not only impermeable but also fairly smooth and
cylindrical.
Orifice analogy and drag theory approaches
have been the most successful in predicting the
resistance of these materials to fluid flow.
Multifilament woven cloth:
The chief difficulty encountered
when dealing with multifilament media is the highly
complex geometry of the fibers and yarns that make up
the cloth. Even in a fabric of apparently simple weave
and construction, such as a plain-weave, continuous-
filament cloth, some of the flow takes place in the highly
tortuous channels present in the yarns (Wardsworth,
2007).
Perforated sheet metal:
stainless steel plates have pores
which act as channels as in case of Meta filter.
Bed of granular solid built up on a supporting medium:
examples of granular solids are gravel, sand, asbestos,
paper, pulp & kieselguhr.
Prefabricated porous solid unit:
sintered glass, sintered
metal, earthenware and porous plastics are material used
for fabrication.
Membrane filter media:
it includes surface & depth type of
cartridges.
Criteria for choice of filter medium (Purchas, 2000)
There are three criteria for choice of filter medium.
1. Size of particle retained by the medium.
2. The permeability of the clean medium.
3. The solid holding capacity of the medium and the
resistance to fluid flow of the used medium.
Measurement of pore size & particle retention (Lach &
Wright, 2004):
In some cases, the desirable component in the slurry
is the liquid, which may be required in clarified form e.g.,
beverage filtration; here the choice of deep-bed elements
of precoated candles of large solids-holding capacity may
Table 1. Minimum cake thickness for
discharge (Andrew et al., 2002).
Filter type Minimum design
thickness
Belt 3.0-5.0
Roll discharge 1.0
Standard scraper 6.5
Coil 3.0-5.0
String discharge 6.5
Horizontal belt 3.0-5.0
Horizontal table 19.0
Fig.
1
. Plain &
t
will weave monofilamen
t
.

Indian Journal of Science and Technology Vol. 3 No. 10 (Oct 2010) ISSN: 0974- 6846
Popular article “Industrial filtration technology” Rakesh et al.
©Indian Society for Education and Environment (iSee) http://www.indjst.org Indian J.Sci.Technol.
1124
be indicated. While, where the solids are valuable, a
sieve like mechanism is favored, so that information
about the pore size of the medium may be of more direct
use in media selection. The pore structure of the medium
will determine the feasibility of a separation.
The pore size of a medium particularly for filters of the
edge, simple wire or monofilament type is of use in
deciding the upper limit of aperture size required by a
particular process. In filters composed of random fibers,
sintered or porous elements, staple or natural fiber cloths,
the mean pore size will have less significance and use in
predicting media behaviour. In certain cases, the
geometry of septum allows direct measurement of
aperture or pore size. In random situation, where complex
weave pattern produce a distribution of pore sizes, such
as a bubble point test or a permeability test are used.
Filter aids:
It forms a surface deposit which screens out
the solids and also prevents the plugging of the
supporting filter medium. They are also used as filter
media in recoat filtration. Some of the properties of filter
aid material are presented in Table 3. The ideal
characteristics of filter aid materials are chemically inert
to the liquid being filtered & free from impurities, low
specific gravity (so that filter aids remain suspended in
liquid), porous rather than dense (so that previous cake
can be formed) and recoverable (Hunt, 2001).
Filtration equipments:
Different forms of equipment are
employed for filtration. The factors which should be
considered, while selecting the equipment and operating
conditions are given below.
Equipment selection is done on the basis of Shirato
(1978): Material related:
Properties of the fluid: viscosity;
Nature of the solids: particle size, shape, size distribution
etc; Concentration of solids in suspension; Quantity of
material to be handled.
Equipment & process related:
Flow rate; the limit to size
of particles passing through the filter should be known; It
should be sterilized by heat, radiation or gas; It should be
economical.
Belt filter:
The belt filter is an industrial machine, used for
solid/liquid separation processes, particularly the
dewatering of sludge’s in the chemical industry, mining
and water treatment. The process of filtration is primarily
obtained by passing a pair of filtering cloths and belts
through a system of rollers (Fig. 2).
Operation:
The feed sludge to be dewatered is introduced
from a hopper between two filter cloths (supported by
perforated belts) which pass through a convoluted
arrangement of rollers. As the belts are fed through the
rollers, water is squeezed out of the sludge. When the
belts pass through the final pair of rollers in the process,
the filter cloths are separated and the filter cake is
scraped off into a suitable container.
Improvements in belt filters
(
US Patent, 1996)
1. The effectiveness of the operation can be increased by
creating a pressure difference across the filter cloth.
The filter cloth is directed though a zone where either
pressure or vacuum pushes water from the filter cloths
and ultimately to drain.
2. The sludge can be combined with a filter aid or
flocculants which help the filtration process and reduce
blinding of the filter cloth.
Table 2. Type of filter media, characteristics and their application.
Type of filter media Characteristics
A
pplication References
Metal fiber media
(non-woven metal fiber)
Excellent durability, corrosion &
abrasion resistance
Polymer & gas industry
Wardsworth
(2007)
Multilayer sintered mesh It can be reused Gas industry Wardsworth
(2007)
Stainless steel (plain, twill & Dutch
type)
Water proof inside & plastic woven
cloth outside
Oil, chemical, food, pharmaceutical
& aviation industry Hunt (2001)
Anthracite filter media It has high efficiency Water purification Hunt (2001)
Filter media treated by graphite Made up of fiberglass Used in cement & steel industry.
Used as filter cloth for air filter
Wardsworth
(2007)
A
ctivated carbon fabric
(non-woven type)
Little air current resistance, strong
strength
Used in air conditioner as auto air
filter or carbon air filter
Wardsworth
(2007)
Biocell High biological activity Sanitary sewage& industrial waste
processing Hunt (2001)
Aramide filter fabric Easiness of cake peeling, high
stability, anti-distortion
Used in ore dressing, chemical &
brewing industry, equipped in filter
presses, vacuum filters etc.
Hunt (2001)
Autoroll filter media It has metal structure, saves energy
& work stably Used in air filtrate Hunt (2001)
Laminating PTFE membrane Felt type of filter Used in cement company &
incineration fields Hunt (2001)
Air filter Pocket type of filter
A
ir conditioner & electronic
industry, food industry, applied to
the pre-filtration of coarse
efficiency
Hunt (2001)

Indian Journal of Science and Technology Vol. 3 No. 10 (Oct 2010) ISSN: 0974- 6846
Popular article “Industrial filtration technology” Rakesh et al.
©Indian Society for Education and Environment (iSee) http://www.indjst.org Indian J.Sci.Technol.
1125
3. Filter cloths can be cleaned throughout the operation of
the process by means of water sprays positioned on the
return section of the belt.
Bag filter (Parksanfilters, 2010)
• Filter Bags are made of felt material, which has the
advantage of providing three dimensional filter media
and offers both, a surface and depth filtration effect.
• Filter Bags are available in different types of filter
media in different ratings. Polypropylene filter bag
offers a broad range of chemical compatibility and is
suitable for many applications.
Polyester filter bags are
suitable for high temperature
with compatibility for acids and
petroleum based fluids.
• Bag filter is mainly used for the
preparation of adhesives, fruit
juices, petroleum products and
high viscosity fluids (Fig. 3).
Advances in bag filtration system
• Innovations in both the filter housing and the bag
filtration media now make it possible for Eaton bag
filters to be used in applications that previously
required more costly types of filtration equipment.
• Eaton filtration's new patent pending HAYFLOW(TM)
filter element. This element combines the best of both
bag and cartridge filters into one single filtration
element for enhanced performance.
Self cleaning filters (Parksanfilters, 2010)
Various methods for cleaning the filters are used.
1. Manual cleaning: Manual filters are cleaned manually
and this is usually done by jet of water. Sometimes the
filters have to be soaked in chlorine or detergent.
2. Mechanical cleaning: In this the cleaning is done by
brush or knife.
Automation minimizes disposable waste and labor
costs.
Self-cleaning filter systems
• Reduces product loss.
• Required minimal operator intervention.
• Improves flow consistency.
A wide variety of self-cleaning filters:
1. MCF magnetically coupled industrial filters
2. DCF Self-cleaning Filters
3. Tubular backwashing filters including
clearamine and reactogard filters (Fig. 4).
Mainly 3 types of flushing are there.
Direct flushing:
this method causes the water to
flow at high velocity at a tangent to the screen and
removes the filter cake from filter element.
Back flushing:
it is used for flushing sand and
gravel filters in these filters the direction of filtration
is from top to bottom. During flushing, water flows
in the opposite direction. The water pressure which
operates from bottom to top causes suspension of the
bed inside filter. Light particles are washed out with
water. The particles which are heavy stay in the vessel.
At the end of flushing the sand settles & recreates the
filter bed.
Forced back flushing:
in these filters there is a suction
device which scans the screen and causes a local forced
back flushing.
Application of filtration
In heating, ventilation & air conditioning (HVAC) (Dorman,
1964):
Poor IAQ (indoor air quality) significantly
influences the occurrence of
communicable respiratory illnesses and
allergy, asthma and sick-building
symptoms. Some of the airborne
triggers for these illnesses include
microorganisms, respirable particles
such as dust and smoke; volatile
organic compounds and allergens.
• Ideally, these triggers are eliminated
or reduced significantly by the air filters in a building's
HVAC system. Advances in air filtration have led to the
development of systems that provide superior IAQ
while reducing energy costs and helping commercial
and institutional buildings achieve green-building
milestones.
• Filtration efficiency is defined by how well a filter cleans
indoor air by removing airborne particles. Low-
efficiency filters — those that are 25% efficient in
removing particles 3 to 10 µm in size — typically are
used to keep lint and dust from clogging the heating
and cooling coils of HVAC systems. Medium and high-
efficiency filters — those that are up to 95% efficient in
removing particles measuring 3-10 µm in size—typically
are used to remove mold, pollen, soot, and other small
particles.
Advances of HVAC (Dorman, 1964)
Using PLCs (programmable logic controllers) in HVAC
is the trend nowadays. Companies are adopting wireless
Fig. 4. Tubular backwashing filters (Tekleen, 2000).
Fig. 2. Belt filter (Wikipedia, 2002).
Fig. 3. Bag filter (Advantec, 2006).

Indian Journal of Science and Technology Vol. 3 No. 10 (Oct 2010) ISSN: 0974- 6846
Popular article “Industrial filtration technology” Rakesh et al.
©Indian Society for Education and Environment (iSee) http://www.indjst.org Indian J.Sci.Technol.
1126
technology after they found out that networking HVAC
controllers, which often use sensors, can eventually cut
installation and labor costs. A lot of engineers are also
focused on further improving this technology through the
use of mesh wireless setup, which will work for both the
wireless sensor and wireless controller networks.
Advances in filtration technology are making new
products possible in food & beverage (Higgins, 2003):
Micro filtration has served the food industry in a variety
of areas for years, but refinements in membrane
technology and a better understanding of the impact
membranes have on the molecules that pass through are
opening up a new world of possibilities. For e.g., bacteria
and spoilage organisms in milk are easily removed by
micro filters with pore sizes ranging from 0.1 to 20
microns. Canada's dairy land dairy, now a unit of Saputo,
promotes this benefit with pure and fresh micro filtered
milk. Ultra filtration units with pores ranging from 0.01-0.2
microns have been shown to affect the appearance and
sensory properties of fluid milk because of the protein
molecules that can be retained and then added back.
Molecules that manage to work their way through the
membrane exhibit different organoleptic properties, with a
richer mouth feel attributed to the squeezed proteins. A
significant body of research on the sensory, nutritional
and bacteria-removal effects of membrane filtration has
been compiled in the last decade by David M. Barbano
and other food scientists at the northeast dairy foods
research center at Cornell University.
Application of filtration in cane & beet sugar industries
• The sugar industry in developed countries has been
under pressure for some time due to high-energy and
labor costs, and environmental challenges. Many
technologies are being constantly explored to improve
sugar yields and quality with reduced energy
consumption.
• Membrane filtration technology offers economic and
technical advantages, when used either as a
standalone process or in combination with other more
established technologies such as ion exchange and
chromatographic separators.
• Ultra filtration/ Micro filtration process in cane sugar
production acts as a pretreatment prior to other
separation technologies by removing impurities from
the raw juice, including starch, dextran, gums, waxes,
proteins and polysaccharides.
Application of filtration in starch & sugar industry
In a very short duration, cross flow membrane filtration
has become a mainstream unit operation in the starch
and sweetener industry. Membrane filtration processes,
namely reverse osmosis, nanofiltration, and microfiltration
by their versatility have gained acceptance. Microfiltration
of saccharification tank liquor removes unliquified starch,
polysaccharides, proteinaceous matter and other
impurities. The process has been successfully applied to
sweeteners derived from various starch sources–corns,
wheat, tapioca, potatoes or cassava. The process
eliminates use of diatomaceous earth (kieselgur) in rotary
vacuum filters, while at the same time producing a
superior quality product. Microfiltration is used for
clarification of maltodextrins, depyrogenation of dextrose,
final filtration of dextrose and fructose syrups. Reverse
osmosis is used for concentration of dilute sugar streams
and in some cases as a pre-concentration step prior to an
evaporator.
Filter evaluations & testing
Integrity testing:
Integrity testing sterilizing filters is
fundamental requirement of critical process filtration
applications in the pharmaceutical industry. Two
classifications of integrity testing are destructive and
nondestructive. There are three types of non-destructive
testing which are available to show that the system has
no leaks and is correctly assembled. These are referred
to as the bubble point test, the fixed pressure test, and
the gas diffusion test.
Bubble point test:
The bubble point is a direct measure of
the largest pore in the filter. The membrane or cartridge is
first wetted and the inlet side of the housing drained free
of liquid. The outlet, still containing liquid, is connected
via tubing to a vessel containing the wetting liquid. Air
pressure is then applied at the inlet until a continuous
stream of bubbles appears in the vessel. The pressure at
which this occurs is the bubble point. The size of the
largest pore may then be related to the pressure via
Darcy's law. The bubble point will vary with pore size,
wetting liquid, filter media, and temperature (Choa
et al.,
2009).
Fixed pressure test:
The basis for this test is the ability of
the filter of cartridge to hold a certain level of gas
pressure applied at the inlet. The filter is wetted and the
inlet side drained. A fixed pressure, about 70% of the
bubble point, is then applied to the inlet side. The
pressure level is then observed. And any changes over a
period of time are indicative f small leaks in the inlet side
of the system. An instrument that is capable of both the
bubble point and the fixed pressure tests.
Diffusion test:
This test is usually recommended for
multicartridge or other systems with high filtration areas.
When pressure is applied to a wetted membrane filter, air
dissolves in the liquid, diffuses through the film, and is
released on the low pressure side in the form of bubbles
in an inverted burette
.
Pore size measurement of filter
(Chu, 2004): There are
two main methods of measuring the pore sizes in a filter:
challenge testing and porometry.
Challenge testing
: It is a simplistic and easy to
understand process of challenging a filter with a liquid
suspension or dust cloud of particles. Measuring the
relative concentration of particles before and after the
filter determines the filter efficiency while measuring the

Indian Journal of Science and Technology Vol. 3 No. 10 (Oct 2010) ISSN: 0974- 6846
Popular article “Industrial filtration technology” Rakesh et al.
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1127
largest particles passing determines the cut point of the
filter.
Disadvantage:
Challenge testing has been perceived as
being limited and inaccurate in that it could not measure
pore size distribution, only the cut point or maximum pore
size. The inaccuracy stems from the fact that test dusts
had been used in the past. The test dusts were wide in
particle size distribution and irregular in shape, which led
to large variations from lab to lab.
Porometry:
It is a process which relies on the
interpretation of gas flow through a ‘wetted’ filter. Wetting
the pores effectively ‘blind’ the filter making it impervious
to the passage of gas. However, as the gas pressure is
raised, it overcomes the surface tension of the liquid and
blows through the pores starting with the largest and
finishing with the smallest. The gas pressure/flow rate
curve can then be interpolated to determine the pore size
distribution in the filter. It has had an advantage over the
challenge test method in that it can measure a pore size
distribution, not just the maximum pore size. One of the
disadvantages is in the theory used to convert flow rate
through pores to pore sizes. In the critical application in
the pharmaceutical industry, uncertainty in pore size
measurement of filters can be fatal.
Advances in porometer (lboro, 2002):
The latest
instrument from Benelux scientific1 incorporates state of
the art flow meters and pressure transducers enabling the
pore size range to be extended down to 14 nm and up to
300 µm.
Conclusion
There are few areas in our lives that are not touched
by filtration. We are surrounded by filters in the home,
from tea bags and coffee makers to dish washers, food
products in our cupboards from mustard and flour to
sugar and cereal, all of which involve some form of
filtration. Thus, a self cleaning filter reduces product
waste and minimizes time. Sintered metal filters which is
discussed here is continuous type of process which
replaces old batch process. Many patent pending filters
are also available which combines the best use of any
two filters like HAYFLOW system which combines both
types like bag and cartridge types. In this way, by using
innovative filter equipments we can reduce labor cost and
can get better quality products, maximize the yield in
lesser time.
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