Filtration is commonly the mechanical or physical operation
which is used for the separation of solids from fluids (liquids or gases) by
interposing a medium through which only the fluid can pass. The fluid that
passes through is called the filtrate. Oversize solids in the fluid are retained, but
the separation is not complete; solids will be contaminated with some fluid and
filtrate will contain fine particles (depending on the pore size and filter
thickness). Filtration is also used to describe some biological processes,
especially in water treatment and sewage treatment in which undesirable
constituents are removed by absorption into a biological film grown on or in
the filter medium as in slow sand filtration.
Applications
Filtration is used
to separate particles and fluid in a suspension, where the fluid can be a
liquid, a gas or a supercritical fluid. Depending on the application, either
one or both of the components may be isolated.
Filtration, as a
physical operation is very important in chemistry for the separation of
materials of different chemical composition. A solvent is chosen which
dissolves one component, while not dissolving the other. By dissolving the
mixture in the chosen solvent, one component will go into the solution and pass
through the filter, while the other will be retained. This is one of the most
important techniques used by chemists to purify compounds.
Filtration is also
important and widely used as one of the unit operations of chemical
engineering. It may be simultaneously combined with other unit operations to
process the feed stream, as in the biofilter, which is a combined filter and
biological digestion device.
Filtration differs
from sieving, where separation occurs at a single perforated layer (a sieve).
In sieving, particles that are too big to pass through the holes of the sieve
are retained (see particle size distribution). In filtration, a multilayer
lattice retains those particles that are unable to follow the tortuous channels
of the filter. Oversize particles may form a cake layer on top of the filter
and may also block the filter lattice, preventing the fluid phase from crossing
the filter (blinding). Commercially, the term filter is applied to membranes
where the separation lattice is so thin that the surface becomes the main zone
of particle separation, even though these products might be described as
sieves.
Filtration differs
from adsorption, where it is not the physical size of particles that causes
separation but the effects of surface charge. Some adsorption devices
containing activated charcoal and ion exchange resin are commercially called
filters, although filtration is not their principal function.
Filtration differs
from removal of magnetic contaminants from fluids with magnets (typically
lubrication oil, coolants and fuel oils), because there is no filter medium.
Commercial devices called "magnetic filters" are sold, but the name
reflects their use, not their mode of operation.
The remainder of this article focuses primarily on liquid
filtration.
Methods
There are many different methods of filtration; all aim to
attain the separation of substances. Separation is achieved by some form of
interaction between the substance or objects to be removed and the filter. The
substance that is to pass through the filter must be a fluid, i.e. a liquid or
gas. Methods of filtration vary depending on the location of the targeted
material, i.e. whether it is dissolved in the fluid phase or suspended as a
solid.
Filter media
Two main types of filter media are employed in any chemical
laboratory— surface filter, a solid sieve which traps the solid particles, with
or without the aid of filter paper (e.g. Büchner funnel, Belt filter, Rotary
vacuum-drum filter, Cross-flow filters, Screen filter), and a depth filter, a
bed of granular material which retains the solid particles as it passes (e.g.
sand filter). The first type allows the solid particles, i.e. the residue, to
be collected intact; the second type does not permit this. However, the second
type is less prone to clogging due to the greater surface area where the
particles can be trapped. Also, when the solid particles are very fine, it is
often cheaper and easier to discard the contaminated granules than to clean the
solid sieve.
Filter media can be cleaned by rinsing with solvents or
detergents. Alternatively, in engineering applications, such as swimming pool
water treatment plants, they may be cleaned by backwashing. Self-cleaning
screen filters utilize point-of-suction backwashing to clean the screen without
interrupting system flow.
Achieving flow through the filter
Fluids flow through a filter due to a difference in pressure
— fluid flows from the high pressure side to the low pressure side of the
filter, leaving some material behind. The simplest method to achieve this is by
gravity and can be seen in the coffeemaker example. In the laboratory, pressure
in the form of compressed air on the feed side (or vacuum on the filtrate side)
may be applied to make the filtration process faster, though this may lead to
clogging or the passage of fine particles. Alternatively, the liquid may flow
through the filter by the force exerted by a pump, a method commonly used in
industry when a reduced filtration time is important. In this case, the filter
need not be mounted vertically.
Filter aid
Certain filter aids may be used to aid filtration. These are
often incompressible diatomaceous earth, or kieselguhr, which is composed
primarily of silica. Also used are wood cellulose and other inert porous solids
such as the cheaper and safer perlite.
These filter aids can be used in two different ways. They
can be used as a precoat before the slurry is filtered. This will prevent
gelatinous-type solids from plugging the filter medium and also give a clearer
filtrate. They can also be added to the slurry before filtration. This
increases the porosity of the cake and reduces resistance of the cake during
filtration. In a rotary filter, the filter aid may be applied as a precoat;
subsequently, thin slices of this layer are sliced off with the cake.
The use of filter aids is usually limited to cases where the
cake is discarded or where the precipitate can be chemically separated from the
filter.
Alternatives
Filtration is a more efficient method for the separation of
mixtures than decantation, but is much more time consuming. If very small
amounts of solution are involved, most of the solution may be soaked up by the
filter medium.
An alternative to filtration is centrifugation — instead of
filtering the mixture of solid and liquid particles, the mixture is centrifuged
to force the (usually) denser solid to the bottom, where it often forms a firm
cake. The liquid above can then be decanted. This method is especially useful
for separating solids which do not filter well, such as gelatinous or fine
particles. These solids can clog or pass through the filter, respectively.
Examples
Filter flask (suction flask, with sintered glass filter
containing sample). Note the almost colourless filtrate in the receiver flask.
Examples of filtration include
The coffee filter
to keep the coffee separate from the grounds.
HEPA filters in
air conditioning to remove particles from air.
Belt filters to
extract precious metals in mining.
Horizontal plate
filter, also known as Sparkler filter.
Furnaces use
filtration to prevent the furnace elements from fouling with particulates.
Pneumatic
conveying systems often employ filtration to stop or slow the flow of material
that is transported, through the use of a baghouse.
In the laboratory,
a Büchner funnel is often used, with a filter paper serving as the porous
barrier.
An experiment to prove the existence of microscopic
organisms involves the comparison of water passed through unglazed porcelain
and unfiltered water. When left in sealed containers the filtered water takes
longer to go foul, demonstrating that very small items (such as bacteria) can
be removed from fluids by filtration.
In the kidney, renal filtration is the filtration of blood
in the glomerulus, followed by selective reabsorbtion of many substances
essential for the body to maintain homeostasis.
SUBSCRIBERS - (
LINKS) :FOLLOW / REF / 2 /
findleverage.blogspot.com
Krkz77@yahoo.com
+234-81-83195664
No comments:
Post a Comment