Filters are apparatuses for cleaning liquids and gases from suspended particles by filtration. The main component of a filter is a filter membrane which lets liquids and gases pass and removes suspended particles which settle on the surface or in the pores of the membrane.
The simplest filter (Figure 1) is a vessel separated by a filter membrane into two sections, dirty and clean. A pressure difference, giving rise to fluid flow through the filter membrane, is produced between them.
Filter membranes are made of cotton, wool, synthetic, glass, and metal fibers and cloths, of ceramic, cermet, synthetic, and other porous materials shaped as plates, pipes, and elements of other shapes, and of granular bulk materials of the same nature.
Depending on their mode of operation, filters are divided into batch-type filters and continuous filters. In turn, filters of both groups vary in the method of producing pressure difference (pressure or vacuum), the geometry of filter membranes (plane, tubular, etc.) and the type and the material of the filter membrane.
In batch-type filters, filtration proceeds simultaneously over the entire surface of the filter membrane, covering it with a layer of captured particles and clogging up the pores. Then, on achieving a certain limiting pressure difference the filter is switched off, regenerated, and put into operation once again.
Continuous filters are divided into sections, where the same operations proceed uninterruptedly and independently, but with some phase shift permitting the degeneration of each section one by one without switching off the filter as a whole.
Although the basic principle of operation is the same, filters are commonly classified into liquid filters for suspension separation and gas filters for aerosol separation and cleaning of dustladen gases.
Liquid filters can be both batch-type and continuous. Filtration of suspensions gives rise to a sediment which, on the one hand, favors a finer cleaning and, on the other hand, increases pressure drop and reduces filter capacity. The latter circumstance has a severe effect on separation of fine-dispersed particles or easily deformed flocculi. In this case the filter capacity can be raised making an addition in a suspension of a subsidiary substance in the form of relatively coarse solid particles. The substance (a "filter aid") must possess a low bulk density and be chemically inert in relation to the liquid filtered and the sediment. The most commonly used is diatomite and, in food industry, cellulose. The quantity of the subsidiary substance needed is generally not large, and it may be recovered by washing.
Filtration can be performed under both hydrostatic pressure, pressure that is produced by pumping the suspension, or by a vacuum on the filtrate side.
The hydrostatic pressure is low and, as a rule, does not exceed a few meters of water gauge, and is used in a filter with easily filtering materials or at low filtration rates. The main advantage of these filters—known as Nutsche filters—is simple design and low cost. They have a perforated bottom with various filtering materials, from paper to sand and gravel, stacked on it. The suspension is fed from the top while the cleaned liquid (filtrate) is discharged into the lower chamber. The sediment is removed by either liquid back-flushing or mechanically, the filtration membrane is often merely replaced by a new one.
In order to enhance filter capacity, the pressure drop may be increased across the filtration membrane. If this is achieved by increasing the pressure on the suspension side (on the other side of filtration membrane the pressure approaches atmospheric in this case), the filters are considered to operate under pressure.
Pressurized filters, which run under periodic duty, include chamber and frame filter presses. In a frame filter press, the suspension is fed at a pressure of up to 7 MPa into spaces between alternating plates and frames (Figure 2). The ribbed-surface plates serve as a support for the filtration membrane and afford filtrate drainage. Solid particles are collected in the frame plane as a compact mass which is removed when the plates are drawn apart.
The operation of a chamber filter press is similar to that of a frame filter press, but the working cavities are made by recessions in the plates. In a Karver filter press operating on this principle, the sediment before ejection is pressed at 42 MPa pressure, dried, solidified, and afterwards automatically ejected when the plates are drawn apart. A variety of filter press designs are used in industry.
Commonly encountered systems are the plate and pipe filters in which the filtering membrane is made of solid porous materials such as cermet cloth fastened on a strong screen membrane with a skeleton or on a grooved plate. An example is the pipe filter. This is also a batch-type filter: the pipe pores plugged with sediment are washed by water back-flushing and air-blown.
Continuous filters can also operate under pressure, but in this case removal of sediment on the pressure side of filtration membrane presents a problem which is solved much more readily in vacuum filters with atmospheric pressure on the pressure side. The drum-type vacuum filter, which has gained wide acceptance, contains, as the main component, a drum whose internal space is separated into three sections. This makes it possible to run in one rotation of the drum through the filtration stages, washing the sediment with water, drying with air, and cake removal.
There are also disk, band, rotary, and other types of vacuum filters of batch type in which the sediment is either washed with water or removed mechanically.
The pressure difference can also be exerted by the centrifugal effect. The filters based on this principle are hollow rotating drums, the internal surfaces of which are covered with a filtration material. The suspension fed is centrifuged on the surface of the filtering material, the liquid flows through it, and is discharged out of the drum either by grooves or through perforations, while the sediment is left on the filtration membrane and later is removed mechanically. The subsidiary filtering substances described above (filter aids) may be used in Centrifugal filters.
Gas filters aimed at gas cleaning are, as a rule, continuous. They are designed for large volumetric flow rates of gas when it contains dust up to tens of grams per cubic meter. For the gases to conform to environmental requirements (see Air Pollution), it is necessary to achieve no less than a 99% dust collection. Multielement cloth filters in housings fit best of all for the purpose. Filtration elements may take the form of a plane broad cloth resting on a support grid, but more often they have the form of cylindrical bags stretched on a grid or grid-free. Their diameter varies within wide ranges, but does not exceed 600 mm. The bags from 125 to 300 mm in diameter and from 2.5 to 3.5 m long (high) are commonly used. In grid-free bag filters, dustladen gas is supplied inside the bag (fixed from either above or below) and inflating it. The collected dust settles on the inside of the bag. In the supported filters, filtration may proceed inwardly, with sedimentation on the outward side of the bag. The number of elements in one housing may exceed 10,000.
Filters are regenerated by mechanically shaking off the dust layer and/or back blowing by the cleaned gas. The latter technique is more efficient. The dust shaken off is collected in a hopper from which it is removed mechanically or pneumatically.
Sectional design of filters makes it possible to regenerate it section by section without switching it off. A schematic diagram of this multisectional bag filter with shaking off and back blowing is presented in Figure 3.
Cleaning smaller volumes of industrial gases contaminated with reactive condensed substances is carried out in fibrous or grain filters, as well as, dry or wet filters. In the latter case, filters may operate under self-cleaning regime, which is also the case when mist droplets are captured.
Felt made of special ultra thin fibers, in which filtration membranes are supported by a frame structure, is applied for fine and superfine gas cleaning. The filtration membrane is placed in the frame structure as a tape between pi-shaped frames. Corrugated separators are mounted between the adjacent layers. These filters permit the cleaning of gases with the 99.99% efficiency and higher; however, as a rule, they require precleaning. Combined filters (Figure 4) are designed to account for this.
Special cases are air filters aimed at cleaning dust from atmospheric air supplied to a room or technological apparatus, where ingress of dust is not allowable. A characteristic property of these filters is a higher rate of filtration, up to 3 m/s, and, hence, the risk of carrying away the particles already settled.
Air is commonly purified either by periodically replaced coarse fiber filtration membranes made of wire, or coarse synthetic fibers up to 250 μm in diameter, or by screens wetted in oil and mounted in the form of cassettes, or by oil self-cleaning filters with filtration membranes as a plane woven cloth. A filter of this type is shown in Figure 5. An endless tape moves in a vertical plane, passes through the filtration zone, collects dust on its surface, and then passes through an oil bath, where the cloth is washed of dust and oiled once again. The dust settles on the bath bottom as slime and is removed as the oil is recovered.
For fine air cleaning, filters of the same type are used as those for a fine and superfine gas cleaning described above.
Perry, R. H. and Chilton, C. H. Eds. (1973) Chemical Engineer's Handbook, 5th ed., McGraw-Hill, New York.