Liquid purification or separation – Processes – Separating
Patent
1993-03-30
1994-11-29
Spear, Frank
Liquid purification or separation
Processes
Separating
210269, 210409, B01D 2414
Patent
active
053687523
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of washing filters and also to liquid filters which comprise a granular filter bed.
In filtering liquids which contain dispersed particles, emulsified and dissolved substances, the bed becomes blocked and clogged after a while and must be cleansed of the material caught in the bed, i.e. the bed must be washed. This is effected by introducing a stream of liquid into the bed, from beneath. The bed granules are fluidized in the liquid flow, porosity of the bed increases and the material captured in the bed are carried away with the liquid flow. Another method comprises the steps of removing granular material from the bed, cleansing the granules externally of the bed and then replacing the granules.
The particles suspended in a liquid to be filtered may extend from 1 nm up to several nm, and the filtering mechanism is adapted in accordance therewith. Particles which are larger than 0.1 times the grain diameter of the bed are separated or extracted by mechanical screening in the outer layers of the filter. Smaller particles are attracted to the upper surface of the bed grains or granules with which they collide, and are held thereto These particles penetrate deeply into the bed, whereas the coarser particles rapidly seal the outer bed layers and necessitate washing of the filter bed, although the bed has not been clogged throughout the whole of its depth. In order to extract small particles from the contaminated liquid to be filtered, it is necessary to destabilize the dispersion; precipitation in contact with bed granules is referred to as contact filtration and the particle extraction effect increases with filtering time. Bed washing at short intervals is undesirable. Because it is impossible to avoid mechanical screening on the filter surfaces and clogging or blocking of the filter, the majority of filters are directed to surface filtration and small particles are floculated prior to being introduced to the bed, therewith enabling the particles to be sieved mechanically.
Known filter constructions which endeavour to improve bed utility are multimedia filters, continuously washed filters with movable beds which are washed outside the filters, and upstream filters. In the case of multimedia filters, the problem is distributed to surface screening in each medium, although the problem is nevertheless not solved satisfactorily. A better solution for effective downstream filtration is not known in practical use, however. Other solutions that have been proposed have not been brought into practical use, often because of small details and the unknown filtering mechanisms. Movable bed filters have a poor effect in contact filtration processes; the material caught in the bed is torn from the bed granules, which are in constant movement. The use of upstream filters in contact filtration processes is often encumbered with problems impossible to solve.
The increase resistance in the bed is a decisive factor when cleaning filters, and contact filters in particular. This resistance also describes the complete filtering mechanism. I have found the following relationship by taking measurements: .multidot.1.sub.2 +k.sub.3 .multidot.1.sub.3 +k.sub.4 .multidot.1.sub.4 filter.
The inventive filter and the inventive filter-washing method and regeneration of the bed are adapted to this equation. The rapidly blocked surface layers are washed, or flushed, separately at least once between two sequential washing operations. When washing the outer layers, the deeper bed layers are not touched and the bed in these locations therewith retains precipitated and collected material, sludge. Washing or flushing of the bed is carried out in a manner to prevent layering of the bed material. This reduces other factors in the equation. The inventive filter is also provided with bottom drainage means through which filtrate is discharged from the bed, means for introducing bed-flushing air and filtrate for bottom flushing, and also with pumps, pipes and control means, etc. for carrying out the inventive meth
REFERENCES:
patent: 1767729 (1929-04-01), Baynard et al.
patent: 3731803 (1973-05-01), Hirs
patent: 4627923 (1986-12-01), Ross
Spear Frank
Walker W. L.
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