Filtration

This article is about the separation process. For the mathematical concept see filtration (abstract algebra). For the noun see filter.

diagram of a simple filtration

Filtration is the process of using a filter to mechanically separate mixtures. Depending on the application, either one or both of the components may be isolated. Examples of filtration include a coffee filter and the use of HEPA filters in air conditioning to remove particles from air.

To separate a mixture of chemical compounds, 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.

Methods

There are many different methods of filtration; all aim to attain the separation of two or more substances. This is achieved by some form of interaction between the substance or objects to be removed and the filter. In addition the substance that is to pass through the filter must be a fluid, i.e. a liquid or gas.

The simplest method of filtration is to pass a solution of a solid and fluid through a porous interface so that the solid is trapped, while the fluid passes through. This principle relies upon the size difference between the particles making up the fluid, and the particles making up the solid. In the laboratory, a Büchner funnel is often used, with a filter paper serving as the porous barrier.

For example an experiment to prove the existence of microscopic organisms involves the comparison of water passed through unglazed porcelain and unfiltered Watermelon. When left in sealed containers the filtered water takes longer to go foul, showing that very small items (such as bacteria) can be removed from fluids by filtration.^{[citation needed]} Alternate methods often take the form of electrostatic attractions. These form of filters again have the problem of either becoming clogged, or the active sites on the filter all become used by the undesirable. However, most chemical filters are designed so that the filter can be flushed with a chemical that will remove the undesirables and allow the filter to be re-used.

Flow

Liquids usually flow through the filter by gravity. This is the simplest method, and can be seen in the coffeemaker example. For chemical plants, this is usually the most economical method as well. In the laboratory, pressure in the form of compressed air may be applied to hasten the filtration process, 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. In this case, the filter need not be mounted vertically. There are devices used to filtration are pressure press filter.

Filter media

There are two main types of filter media — a solid sieve which traps the solid particles, with or without the aid of filter paper, and a bed of granular material which retains the solid particles as it passes. 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.

Examples of the first type include filter paper used with a Buchner, Hirsch, filter funnel or other similar funnel. A sintered-glass funnel is often used in chemistry laboratories because it is able to trap very fine particles, while permitting the particles to be removed by a spatula.

Examples of the second type include filters at municipal and swimming pool water treatment plants, where the granular material is sand. In the laboratory, Celite or diatomaceous earth is packed in a Pasteur pipette (microscale) or loaded on top of a sintered-glass funnel to serve as the filter bed.

Filter aids

Certain filter aids may be used to aid filtration. These are often incompressible diatomaceous earth or kieselhuhr, which is composed primarily of silica. Also used are wood cellulose and other inert porous solids.

These filter aids can be used in a number of 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 separated chemically 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.

Filter types

• Gravity filter (open system that operates with water column pressure only)
• Pressure filter (closed system that operates under pressure from a pump)
• Side stream filter (filter in a closed loop, that filters part of the media per cycle only)
• Continuous rotary filters

See also

For a list of Wikipedia articles about filters, see Category:Filters.

• Separation of mixtures
• Microfiltration, ultrafiltration, nanofiltration
• Reverse osmosis
• Filter (water)

References

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All links retrieved October 11, 2007

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