Liquid purification or separation – Processes – Including controlling process in response to a sensed condition
Patent
1987-04-01
1990-02-27
Jones, W. Gary
Liquid purification or separation
Processes
Including controlling process in response to a sensed condition
210748, 210780, 209269, 209363, B01D 3300, B01D 3520
Patent
active
049043943
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to filters and in the more important example to microporous filters capable of separating micron sized particles such as microorganisms and ultrafiltration membranes capable of retaining macromolecules such as proteins. The invention also relates to methods of filtering.
Filters are presently constructed of polymers (such as cellulose derivatives, teflon and nylon), ceramics, metals (for example stainless steel) and metal oxides (for example aluminium). It has been proposed (see U.S. Pat. No. 4,279,751) to produce a hydraulic fluid filter element which comprises a body of porous piezoelectric material. An alternating voltage is supplied to metallized surfaces of the body to effect vibration, thereby reducing clogging of the filter and promoting the breakdown of contaminants in the hydraulic fluid.
SUMMARY OF THE INVENTION
In this invention, vibration of the filter body in a direction transverse to the flow of filtrate may be controlled so as selectively to block or pass a size of particle or macromolecule to which the filter body is permeable in the stationary state. For example, a porous pizeoelectric filter body may be permeable, in the stationary state, to two species of macromolecule. For each species, a factor can be identified which is related to the size of the particle or macromolecule and the diffusion coefficient. This factor can be thought of as a measure of transport time across the upper surface of the filter body. If the filter body is vibrated in a direction lying in the surface plane, at a rate which exceeds the factor for a particular species, it will be recognised that entry into the pores of the filter body is impeded. The other species of macromolecule in the example would have a higher factor and at the same frequency of vibration this other species would still be passed by the filter body. In other words, by vibrating the filter body at a particular frequency, selectivity will be exhibited in favour of species having a higher factor, these being generally smaller species.
It is believed that to maximise this selectivity effect, the amplitude of vibration should be relatively low and, in the preferred form, about twice the pore dimension. It is believed that the wider range of 0.5 to 10 times the pore dimension will also exhibit useful effects.
Preferably, the surface of the filter body opposing the sample to be filtered is planar and relatively smooth so that shear is maximised and the production of pressure waves causing the fluid and particular species to move with the membrane is minimised.
In a filter according to this invention comprising a body of porous piezoelectric material, the vibratory motion of the filter body will be determined not only by the applied voltage and the physical properties of the body but also by process parameters such as the viscosity and density of both the suspension and the medium and flow characteristics including the flow rate through the filter and, in the case of a cross-flow arrangement, across the filter. It is a further object of this invention to provide an improved filter means capable of providing information concerning the filtration process.
Accordingly, the present invention consists in yet a further aspect in filter or analysis means comprising a body of porous piezoelectric material which may be exposed to a solution or suspension in liquid medium so at least the medium permeates the pores of the body, electrode means associated with the body, means for applying a periodic voltage to the electrode means to cause vibration of the body and means for monitoring the vibratory movement of the body.
Information concerning the filtration process obtained in the above manner or by use of other techniques such as light scattering analysis of the filtrate, can be used in feedback to control the filtration process. Thus, for example, where it is desired to exhibit selectivity in favour of a particular species, the regime of amplitude and frequency of vibration applied to the filter body can be
REFERENCES:
patent: 3517811 (1970-06-01), Newfarmer
patent: 4253962 (1981-03-01), Thompson
patent: 4279751 (1981-07-01), Fishgal
patent: 4352570 (1982-10-01), Firth
Chem. Abstracts vol. 69, 1968 p. 6413 abstract No. 68529j and SU, A, 217765 (5/7/68).
Chem. Abstracts vol. 96, No. 2, 1/11/82 Benzinger et al "Development of non-fouling piezoelectric U.F. membrames" p. 316 no. 11455k.
Patent Abst of Japan vol. 100 No. 12 (3/19/84) abstract no. 870174a.
Patent Abst of Japan vol. 7 No. 281 (12/15/83), JP, A, 58159811 (9/22/83).
Blake-Coleman Barrie C.
Clarke David J.
Jones W. Gary
Lee Coreen Y.
Public Health Laboratory Service Board
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