GB2279890A

GB2279890A - Filter element

Info

Publication number: GB2279890A
Application number: GB9314414A
Authority: GB
Inventors: Clive Richard Smith; Robert Jeffrey Coleman
Original assignee: BTR PLC
Current assignee: BTR PLC
Priority date: 1993-07-13
Filing date: 1993-07-13
Publication date: 1995-01-18
Anticipated expiration: 2013-07-13
Also published as: GB9314414D0; GB2279890B; DE4424719A1

Description

2279890 ELTER This invention relates to a filter to separate solid

particles from a fluid medium in which the particles are entrained. The filter is particiAarly useful for separating dust particles entrained in gas, e.g. atmospheric air, which particles are produced in industrial manufacturing processes but is not so limited in its application.

Substantially stff, self-supporting porous filter elements are well known. They may be made by sintering particulate powder ot for example, plastics material, ceramic material or metal, to bond the particles together to borm a porous body of the desired shape and porosity for the intended end use.

It is also known that cleaning such filter elements, for example by the well-known "reverse pulse" method, is not always entirely successfal. Some of the solid particles that have been filtered out of the fluid stream and trapped on the outer surface of the filter element may in fact be tapped inside the pores of the filter element where they are difficult to remove. Moreover, the filter elements are often of corrugated or other non-planar shape so that there may be regions where filtered particles collect and are sheltered to an extent from thereverse-pulse.

In order to overcome these problems it has been proposed to coat the surface of the filter element which is to receive the fluid stream to be filtered,, i.e. its inlet surface, with a layer of a powder of finer grain size than the surface pore size whereby the pore size is effeCtiVCIY reduced. For example, polytetrafluorethylene powder has been used to cover the 1.

pores in a filter element made of sintered polyethylene powder. The polytetrafluorethylene (PTFE) is conveniently applied to the polyethylene filter element by spraying or brushing onto the inlet surfaces of the element a PTFE/resin miKture whereby the PTFE particles are bonded in or over the pores of the element. By this means, the solid particles filtered out of the fluid stream cannot so readily become lodged in the pores of the filter element. Moreover they do not adhere well to the PTFE and both of these factors aid cleaning. Other fine gram materials have been proposed instead of PTFE, including, for example, glass beads.

It is also known that for certain applications, where the dust to be filtered out is potentially explosive, it is necessary that the filter element be anti-static. This has been achieved by applying fine carbon particles to the filter element surface. The carbon particles are substantially of submicron size and are applied in a mixture with a suitable resin. Thus when it is desired to improve the cleaning of the anti-static filter elements by the application of the PTFE coating as described above, it will be appreciated that two separate applications must be made to the filter element surface with attendant increased manufactming time and expense.

"Me present invention aims to provide a filter which has both antistatic properties and good cleaning properties and that can be made more conveniently and cheaply than heretofore.

Accordingly, in one aspect the invention provides a filter for separating solid particles from a fluid medium in which they are entrained,, the filter comprising a porous element whose inlet smface through which - the fluid is to pass has a porous coating of carbon particles of finer size than the inlet surface pore size.

The coating of carbon particles preferably does not completely flu the inlet surface pores but the particles are of sufficient size so that a number of particles can bridge across each pore opedng to provide a diin sIdn or coating layer which only partially fills the outermost pores.

In another aspect the invention provides a method of making a filter for separating solid particles from a fluid medium in which they are entrained, which comprise providing a porous filter element and applying to its inlet surface through which the fluid is to pass a mixture of carbon particles in a binder, the carbon particles being of finer size than the inlet surface pore size, whereby the carbon particles are bonded to the inlet surface to provide a porous coating.

Pteferably, a substantial proportion., e.g. at least 80%, of the carbon particles has a diameter of from 1 to 10 microns, especially 2 to 9 and more especially from 3 to 5 microns. Thus, for example, a porous surface of pore size from 30 to 70 microns can be modified to have an effective pore size of from 3 to 8 microns.

The coating of carbon particles may be applied by sprayming and/or brushing to any desired thickness and the desired wnount can conveniently be determined by measuring the pressure drop across the element before the coating is applied and after the applied coating has dried for a particular through flow rate. For example, a pressure drop of 2 to 3 mbar at a filtration velocity of 1,55 metres per minute has been found to correspond to a preferred coating thickness.

It will be appreciated that it is not necessary that the pore size of the surffice coatiag of the filter be. smaller than the diameters of the particles which are to be caught. When filtration commences, the filter pores become bridged by the particles which wedge themselves across pore entrances, i.e. in a similar manner to the formation of the above, preferred coating layer. This occurs even though, for example, the pores may be 5 microns in diameter and the particles only 1 micron. Once this initial particle deposition has occurred on the suffice of the filter, further and more efficient filtution takes place as the formed "dust cake" of particles on the filter surface becomes, effectively, its own filter medium. Subsequent filtration efficiency is, therefore, dependent on the porosity and permeability of this dust cake and the original filter medium acts in ellect just as a support structure.

However, as noted above, it is important to allow as little as possible of the dust particles to migrate into the filter body where they are difficult to remove by reverse pulsing and the invention is particularly effective in this respect.

The coating of carbon particles applied according to the invention may not necessarily be a layer of continuous touching carbon particles but the carbon particles may be held in a lattice of solidified strands of the binder. This can produce a more open structure of good air permeability while remaining impervious to particles of dust.

The invention conveniently provides a filter element that is both antistatic and can readily be cleaned by reverse pulsing or by other means, e. g. mechanical rapping or shaking. The inlet surface of the filter element is sufficiently fine-pored by virtue of its carbon coating to prevent even very fine sub-micron solid particles, particularly fine dust particles, from penetrating into the filter element. The filter deposits on the filter surface are, therefore, easily removable and normal cleaning can be carried out without having to close down the continuous filtering operation. By virtue of its anti-static properties the filter can be safely used even with potentially explosive dusts and it is unnecessaiy for special anti-static grades to be stocked in 'addition to non-anti-static grades of filter. Moreover, unlike PTFE, the carbon coating has no adverse properties which would prevent disposed elements being incinerated or recycled.

In the method of the invention, it may be found advantageous to include a dist liquid in the carbon particle/binder mixture to aid spreading of the mixture evenly over the porous surface of the filter element. Water is a preferred dispersant liquid but other dispersants, e. g. benzene, toluene and methyl isobutyl ketone may be used, if desired, depending on the inature of the binder and the filter material.

The binder is preferably a resin and should be capable of drying to form an open matrix structure with the carbon particles, which structure is permeable to air but capable of supporting a cake of deposited solid particles, e.g- a dust cake. Suitable resin binders include epoxy resins.

A typical coating inixture may, for example, be forrnuiated as follows:

1 litre of carbon particles of about 5 microns size 1 litre of vinyl acrylic resin 14 litres of water These proportions may be varied widely, e.g. from 50% to 200% of the above quantities.

The mixture may conveniently be blended with an electric mixer and is then kept agitated until used.

The invention is applicable to a wide variety of different types of filter elements. It may be applied to flexible filter bags but is preferably applied to self-supporting filter elements. Thus the coating may be applied to any of the above-mentioned filter elements formed by sintering particulate powder of plastics material, ceramic material or metal. It may. equally, be applied to other types of self-supporting filter elements, for example, incorporating fkMc materials which have been rigidised by thermal and/or resin treatment Examples of filter elements of this type are disclosed in our GB 2220589A Filters of the invention have wide applicability. They may be used to remove dust from gas streams in a wide variety of industries and manufacturing processes, including those producing., handling and processing powdered and granulated materials or using dust generating machines. They may also be used, for example in boiler flues, incinemtors and motor vehicle exhausts. They may also fmd applicability in the fields of rainin and air conditioning. They are not limited to use with gaseous streams and may equally find applicability in removing solid from liquid streams, e.g. oil or water. They can operate over a very wide range of temperatures, e.g. from -4011 to above 200"C and a range of conditions from acid to alkaline.

The filters of the invention not only have good anti-static properties but they are easy to clean as the accumulated deposits do not adhere well to the carbon particles/fesin coating. The coating layer is cheaper than the previously proposed PTFE layer and, as indicated above, only a single coating layer is required to achieve the objectives previously requiring two separate coatings.

The invention wiU now be described by way of example only with refe:rence to the accompanying drawings in which:

Figure I is an elevation of a filter with its cover removed; Figure 2 is an enlarged plan view of a type of filter element for use in the filter of Figure. 1; and Figure 3 is an enlarged view of region ITI of Figure 2.

The filter 10 of Figure I has a housing 11 containing a number of suspended filter elements 12, an inlet 13 for the particulate-ocmtaining gas to be fiftered and an outlet 14 for the filtered, gas. Above the filter el=ents, is a header 15 into which the filtered gas, after passing through a filter element, is collected on its passage to the outlet.

Each filter element 16 is of generally rectangular corrugated plan shape as shown in Figure 2. It has a hollow core 17 defined by two pm-allel porous corrugatf,-d walls 18 and two porous end walls 19. It is made by sintering particles of polyethylene in an appropriately shaped cored mould. A mixture of high and low molecular weight polyethylene and different grain sizes may be used to optimise the properties of the fmished element.

8_ The exterior surfaces of the waUs of the filter element have been coated with a carbon parficle/resin/water mixture according to the invention and the coating allowed to dry. An enlarged detail of the porous surface resulting from this treatment is shown in Figure 3. Particles 20, 21, 22 of polyethylene are sintered together to form a structure with large pores 23. Cmbon particles 24 have formed a surface layer over, and have partially filled, pores 23, thereby reducing the effective surface pore size by a factor of from 5 to 10.

It will be appreciated that the mvention is applicable to a wide range of types of filter elements and filtering processes. Tbus, for example, when applied to filtration of liquids, the filter elements maybe in the form of known plate or dnnn types.

ExamDle A conventional sintered polyethylene filter was provided with a carbon particle surface coating according to the invention. The filter had a total surface area of about 40m2. Air was passed through the coated filter at a flow rate of 3600m3/hour and -%rith an inlet dust loading of Sogjnm3. The outlet emission was less than lmgl=3, showing an efficiency of 99.998%.

Pulse cleaning of the filter maintained a worldng pressure drop of 175 Yn tn. w9Z (wa Merasguareu&n)f of the resistivity of the carbon coating indicated a value lower than 100 mcgohms, which is more than adequate to discharge any build-up of static charge.

It can be seen, therefore, that the invention provides a filter of filtration efficiency at least as good as for conventional filters while having the advantages of ease of manufacture, good anti static properhes and which is readily diq)osable eT recyclable when its useful life is over.

a 0

Claims

CLAIMS:

7 1. A filter for separating solid particles from a fluid medium in which they are entrained, the filter comprising a porous element whose inlet surface through which the fluid is to pass has a porous coating of carbon particles of finer size than the inlet surface pore size.

2. A filter according to Claim 1, in which the coating of carbon particles bridges across each inlet surface pore opening of the porous element to provide a thin coating layer which only partially fills the outermost pores.

3. A filter according to Claim 1 or 2, in which at least 80% of the carbon particles have a diameter of from 1 to 10 microns.

4. A filter according to Claim 3, in which the carbon particles have a diameter of from 2 to 9 microns.

5. A filter according to Claim 4, in which the carbon particles have a diameter of from 3 to 5 microns.

6. A filter according to any one of the preceding claims, in which the inlet surface of the porous element has a pore size of 30 to 70 microns and the carbon coating reduces the effective pore size to 3 to 8 microns.

A filter according to any one of the preceding claims, in which the coating results in a pressure drop of 2 to 3 mbar across the element at a filtration velocity of 1.55 metres per minute.

8. A filter according to any one of the preceding claims, in which the coating of carbon particles is in the form of a lattice of solidified strands of a binder in which the carbon particles are held.

9. A filter according to any one of the preceding claims, in which the porous element is formed of a sintered particulate powder of plastics material, ceramic material or metal.

10. A filter according to any one of Claims 1 to 8, in which the porous element includes fabric material which has been rigidised by thermal and/or resin treatment.

11. A filter according to any one of claims I to 8, in which the porous element is a flexible filter bag.

12. A filter for separating solid particles from a fluid medium in which they are entrained substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

13. A filter assembly comprising a housing with an inlet for particulatecontaining gas to be filtered and an outlet for the filtered gas, the housing containing one or more filters according to any one of the preceding claims.

14. A method of making a filter for separating solid particles from a fluid medium in which they are entrained, which comprises providing a porous filter element and applying to its inlet surface through which the fluid is to pass a mixture of carbon particles in a binder, the carbon particles being of finer size than the inlet surface pore size, whereby carbon particles are bonded to the inlet surface to provide a porous coating.

15. A method according to Claim 14, in which at least 80% of the carbon particles have a diameter of from I to 10 microns.

0 a 16. A method according to Claim 15. in which the carbon particles have a diameter of from 2 to 9 microns.

17. A method according to Claim 16, in which the carbon particles have a diameter of from 3 to 5 microns.

18. A method according to any one of Claims 14 to 17, in which the coating of carbon particles is applied by spraying and/or brushing to a desired thickness.

19. A method according to any one of Claims 14 to 18, in which a dispersant liquid is included in the carbon particlelbinder mixture.

20. A method according to Claim 19, in which the dispersant is water.

21. A method according to Claim 19, in which the dispersant is benzene, toluene or methyl isobutyl ketone.

22. A method according to any one of Claims 14 to 21, in which the binder is a resin which dries to form an air-permeable matrix structure with the carbon particles.

23. A method according to Claim 22, in which the resin is an epoxy resin.

24. A method according to any one of Claims 14 to 23, in which the coating mixture has the formulation:

1 litre of carbon particles of about 5 microns size; 1 litre vinyl acrylic resin; and 14 litres water.

25. A method according to any one of Claims 14 to 24, in which the coating mixture is blended in an electric mixture and then kept agitated until applied to the porous filter element.

0 26. A method of making a filter for separating solid particles from a fluid medium in which they are entrained substantially as hereinbefore described with reference to the Example. 27. A filter made by the method of any one of Claims 14 to 26.

Amendments to the claims have been filed as follows 16. A method according to Claim 1 s, in which the carbon particles have a diameter of from 2 to 9 microns.

17. A method according to Claim 16, in which the carbon particles have a diameter of from 3 to 5 microns.

18. A method according to any one of Claims 14 to 17, in which the coating of carbon particles is applied by Spraying andlor brushing to a desired thickness.

19. A method according to any one of Claims 14 to 18, in which a dispersant liquid is included in the carbon particlelbinder mixture.

20. A method according to Claim 19, in which the dispersant is water.

21. A method according to Claim 19, in which the dispersant is benzene, toluene or methyl isobutyl ketone.

22. A method according to any one of Claims 14 to 21, in which the binder is a resin which dries to form an air-permeable matrix structure with the carbon particles.

23. A method according to Claim 22, in which the resin is an epoxy resin.

24. A method according to any one of Claims 14 to 23, in which the coating mixture has the formulation:

1 litre of carbon particles of about 5 microns size; 1 litre vinyl acrylic resin; and 14 litres water.

25. A method according to any one of Claims 14 to 24, in which the coating mixture is blended in an electric mLxer and then kept agitated until applied to the porous filter element.