WO1995015805A1

WO1995015805A1 - A septum for a filter, and a filter fitted therewith

Info

Publication number: WO1995015805A1
Application number: PCT/GB1994/002685
Authority: WO
Inventors: Howard Wallis Green
Original assignee: Flexibulk Limited
Priority date: 1993-12-08
Filing date: 1994-12-06
Publication date: 1995-06-15
Also published as: GB9325174D0

Abstract

This invention relates to a septum for a filter, and in particular to a septum for a plate and frame filter. There is disclosed a septum (24) for a filter (10) comprising a sheet with a plurality of apertures, the apertures being of a size to permit the passage of filtrate characterised in that the sheet is of metal. Also disclosed is a filter (10) which includes such septum; and in particular a filter which includes a number of filter sub-assemblies, each sub-assembly comprising a septum postioned between two frames (14), the septa being folded to either side of a supporting plate (12), the septa having an area greater than double that of a frame.

Description

A SEPTUM FOR A FILTER, AND A FILTER FITTED THEREWITH

FIELD OF THE INVENTION

This invention relates to a septum for a filter and to a filter fitted therewith, and in particular to a septum for a plate and frame filter-

In this specification, "filter" as a verb refers to the action of separating solid impurities from a liquid host material; "filter" as a noun refers to the machine which performs this action. "Filtrate" is liquid which has been filtered, whereas "feedstock" is the unfiltered liquid and solids to be filtered.

BACKGROUND TO THE INVENTION

The separation of solid impurities from liquids is a widely used process in industry. The size of the solids removed is diverse, from pieces many centimetres across which may be removed by sieving, down to particles milli-microns across which may be removed using reverse osmosis/dialysis techniques.

Between these extremes are particles in the size range 0.1 micron to 100 microns, and it is to the filtration of such "intermediate" particle sizes that this invention relates. The filtration (or clarification) of liquids such as beer and lager, and the removal of solids during the refining of sugar typically fall into this range.

DESCRIPTION OF THE PRIOR ART

For the range of particle sizes mentioned in the preceding paragraph, filtration is usually accomplished by forcing the feedstock (the liquid containing the impurity), under pressure through a filtering medium. The filtering medium may be a fine mesh cloth or screen, having apertures of a size which prevent the passage of the solids but allow the passage of the filtered liquid or filtrate therethrough. However, as the quantity of solids extracted from the feedstock and captured by the filtering medium builds up, the passage of further filtrate through the filter is impaired, and thus the filter performance drops. With solids which retain their rigid particulate form this performance drop is relatively slow, since the spaces between the captured solid particles allow the continued passage of filtrate. But, with some feedstocks, the captured solids combine on the surface of the filtering medium into a slimy or gelatinous mass which is substantially or totally impervious to the filtrate, so that the filtering medium can become blocked relatively quickly. To assist in the filtration of solids, particularly those forming such a gelatinous mass, it is known to use "filteraids" . These "filteraid" materials, typically comprising rigid particles less than 100 microns in size, are porous, or substantially so, to the filtrate but not to the solid impurity; when added to the feedstock, the selected filteraid material is laid down upon the surface of the cloth or screen i.e. with the filtered solids as filtering proceeds. However, since the selected filteraid particles are porous to the filtrate, they can then provide pathways in the gelatinous mass by means of which filtrate can more readily continue to pass to and flow through the cloth or screen during filtering, despite the build up of solids thereupon.

In addition to its use with solids which form into a gelatinous mass, a filteraid may also be used for solids which retain their rigid particulate form but which comprise very small particles e.g. 0.1 microns in size. With such impurities, a cloth or screen filtering medium, even with the smallest grade of aperture, could allow a high proportion of the particles to pass therethrough.

For applications requiring a filteraid it is usual that a layer, or precoat, of filteraid will be built up on the surface of the cloth or screen prior to filtration commencing; upon the commencement of filtration, it will be the filteraid precoat, and the further filteraid (if any) deposited from the feedstock, which wholly or mainly acts to capture the suspended solids, with the cloth or screen providing a support for the filteraid. The term "septum" is used in the filtering art to describe a cloth or screen adapted to support a filteraid precoat, the septum itself providing little or no direct filtering action.

Many types of industrial filter are available for filtering solids in the given "intermediate" size range, one such is a plate and frame filter.

A plate and frame filter comprises many plates and many frames, the plates and frames being arranged alternately, so that each plate is sandwiched between two frames, and, with the exception of the frames adjacent the ends of the filter, each frame is sandwiched between two plates. In use, the plates and frames are pressed together by an hydraulic ram to prevent leakage of feedstock between their facing walls.

The plates each include a pair of spaced perforated partitions, through each of which the filtrate can pass. A septum, comprising a sheet of cloth or cellulose having suitably sized apertures therethrough, is draped over the plate, i.e. so that it is carried by the plate, and extends downwardly to each side of the plate whereby to lie against the plate so as to cover all of the perforations in both of the respective partitions. The partitions are selected to be of a strength and rigidity to prevent or inhibit "inward" deformation of the septum, i.e. towards the central plane of the respective plate, as the feedstock is introduced, and during subsequent feedstock flow, i.e. the plate is a septum carrier and a support means for the septum.

The frames are all connected to the feedstock input of the filter. The feedstock is introduced under pressure into the hollow interior of each frame, and is then forced through the filteraid precoat, whereupon the solids are captured; the filtrate then passes through the septum and partition, into the gap between the partitions of the plate, and then into the outlet of the filter.

As the process continues, the solids, along with the filteraid (if any) which has been added to the feedstock, is deposited as a "cake" on the septa. When the performance of the filter drops to a certain level, i.e. the filter frames are full of cake, usually so that the pressure required to continue feedstock pumping has exceeded a predetermined level, the supply of feedstock is halted. The hydraulic pressure clamping the frames and plates together is removed, whereupon the plates and frames may be separated, allowing the cake to drop off, or be shaken off, each septum, into a collection hopper. Following removal of all of the cake, the filter can be reassembled and reused. One disadvantage of the known septa for plate and frame filters is that, following separation of the plates and frames for cleaning, should some of the cake be retained thereupon i.e. it does not drop off spontaneously or upon shaking of the plate and septum, then it is necessary to clean the septum, and it is known that such cleaning can damage the septum. For example, it is difficult to pressure wash a cellulose septum, and a cloth septum may have its working life shortened by this process, even if carried out by careful operators. Thus, the removal of any retained cake, without causing irreparable damage to the septum, is a difficult and time consuming task.

Another disadvantage is that the septa are required to withstand the hydraulic clamping load of the filter, and this load is applied and removed repeatedly as the filter is used, cleaned, and then reused.

It is known for both cloth and cellulose septa to be in service for only a matter of weeks before the damage caused both by the removal of any retained cake, and the repeated application of the clamping load, mean that the septa must be replaced.

The replacement of all of the septa of a filter is also a time consuming task, during which the filter must be taken out of service. It is known to take two men three-four hours to replace all of the septa of a plate and frame filter having one hundred plates, and this time period is unacceptably long for some users of plate and frame filters.

STATEMENT OF THE INVENTION

There is now proposed a septum for a filter which will overcome or reduce the above disadvantages, i.e. a septum from which any retained cake can more easily be removed, e.g. by pressure washing, and which will withstand a larger number of clamping cycles, so reducing the expense both in downtime of the filter and in the purchase of new septa.

Thus there is provided a septum for a filter comprising a sheet with a plurality of apertures, the apertures being of a size to permit the passage of filtrate, wherein the sheet is of metal.

Preferably, the septum has sealing means affixed thereto. The sealing means is usefully compressible and resilient and is positioned to engage the plate or frame when the septum is fitted to the plate, so that upon clamping of the plates and frames, the sealing means occupies the space between the facing walls of the adjacent plate and frame and provides the required sealing contact therebetween. Usefully, the sealing means is a strip of rubberised material permanently affixed, as by chemical bonding, to the sheet. Preferably, the sheet is foldable, whereby it may be positioned over the plate to provide the septum covering both partitions of the plate.

Desirably, the sheet is a mesh of metal wires, preferably stainless steel wires, though other metals can be used if required for particular filtrates or feedstocks.

There is also provided a filter having plates fitted with a septum as above disclosed.

The sealing means will be chosen to withstand the repeated clamping loads of the filter (e.g. two hundred bar), and whilst clamped will provide a seal against feedstock leakage between the facing wall of the frame and septum; the feedstock will be under a typical pressure of up to ten bar.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described, by way of example, with reference to the accompanying drawings, in which:-

Fig.l is a schematic side view of a plate and frame filter; Fig.2 is a perspective view of a frame of the filter of Fig.1;

Fig.3 is a perspective view of a plate of the filter of Fig.l, with a septum according to the invention fitted thereto;

Fig.4 is a plan view of the septum of Fig.3; and

Fig.5 is a partial view along the line V-V of Fig.4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in Fig.l, a plate and frame filter 10 comprises many plates 12 and many frames 14 arranged alternately.

The filter has ground engaging end members 16,18, which are rigidly connected together by tie bars (not shown) . The plates 12 and frames 14 are supported upon the tie bars, and may be slid therealong (towards end member 18) during cleaning of the filter. Passing through end member 18 is a piston 20 of an hydraulic ram (not shown), which is connected to platen 22. Prior to operation of the filter, the ram clamps the plates and frames together, i.e. the plates and frames are sandwiched between the ram and the end member 16. Typically, clamping loads of up to two hundred bar may be utilised.

Positioned over each plate 12 is a septum 24. The septum is slightly larger than twice the area over which each partition of the plate is perforated, so that it may completely cover the perforations on both partitions of a given plate. A part of each septum 24 is clamped between the facing walls of an adjacent plate 12 and frame 14.

The filter 10 has an inlet 26 for feedstock, and an outlet 28 for filtrate.

As shown in Fig.2, each frame 14 comprises a box section, with a facing wall 30 defining an opening 32. Adjacent each corner of the frame is a conduit 40,42. The lowermost

(in the orientation as drawn and typically in the orientation of use) conduits (marked 40) are open laterally to the frame through a portway 44 (only one of which is seen in Fig.2). The uppermost conduits (marked 42) are not open to the frame i.e. the conduits 42 are continuous and do not have a portway into the frame. Thus, feedstock under supply pressure enters along conduits 40, and passes through respective portways 44 into the frame 14, and then flows towards the septum and plate.

In other embodiments of frame, there is a baffle plate adjacent each portway 44, to prevent turbulence from inrushing feedstock dislodging any previously deposited cake. Also, there may be strengthening ribs connecting opposite corners of the frame.

As seen in Fig.3, each plate 12 has a similar cross section to the frames 14, and also has conduits 46,48 adjacent each corner. The conduits 46 are not open to the plate i.e. there is no entry for feedstock directly into the plate, but the conduits 48 are open to the plate between the partitions 50 via portways (not shown), which portways allow filtrate to pass from the gap between the partitions into the conduits 48.

When the plates 12 and frames 14 are fitted to form a filter 10 and clamped together, the conduits 46 will line up with the conduits 40, and the conduits 48 will line up with the conduits 42. Thus feedstock can enter through the inlet 26, and can pass along the conduits 40,46, entering each frame 14 through its respective portways 44; filtrate can pass from between the partitions 50 of each plate into the conduits 48, and can pass through conduits 42,48 to the filter outlet 28.

The plate 12 has a pair of partitions 50 (only one of which can be seen in this figure); each partition 50 has perforations 52. The area of each partition which is perforated is similar in size and shape to the opening 32 of the adjacent frame. In an alternative embodiment, the perforations in the partitions are provided by gaps in a mesh. The partitions are covered by the septum 24 which is positioned over the plate 12. In this embodiment, the septum is slightly larger than twice the perforated area of each partition, so that a single septum covers both partitions 50, one to each side of the plate 12. In an alternative, though less preferred embodiment, the septa for each partition of a plate are separate.

The septum 24 comprises a metallic sheet having a stainless steel mesh 56 extending thereacross. The mesh 56 can provide the filtering medium, though it will more usually provide a support for a filteraid precoat. Secured to the sheet is sealing means or sealant 58 (see also Figs.4,5). The sealant in this embodiment is neoprene, chemically bonded to the sheet, though in other embodiments it may be a rubber compound. The choice of sealant material will be determined largely by the chemicals in the filtrate and the feedstock, i.e. the sealant, and its bonding agent (if used) must not be liable to significant chemical attack.

The sealant is affixed to the sheet so that in its position of use it is located between the facing wall 30 of a frame and the corresponding facing wall part of a plate.

In this embodiment, the sealing means are provided on only one side of the sheet, i.e. the side which contacts the frame. In use, this side will be under the greater pressure (the pressure to the other or plate side of the septum typically being close to atmospheric pressure) ; at such a greater pressure, the clamping load will be sufficient to seal the metal septum and the plate to prevent or inhibit leakage therebetween.

In another embodiment, sealing means are provided on both sides of the sheet, to seal against both the facing wall of the frame and the facing wall of the plate.

In the embodiment shown, the sheet is solid at its outer periphery (Fig.5) i.e. the mesh 56 does not extend to the edges of the sheet, but is surrounded by a solid border 62 to which the sealant 58 is secured. In the alternative embodiments in which the mesh does extend to the edges of the sheet, the sealing means must cooperate with the mesh to occupy all of the apertures which exist in the peripheral regions of the sheet. Thus, in addition to the mesh having apertures which allow the filtrate to pass through in the direction towards and away from the top of the paper in the orientation of Fig.5, if the mesh extended to the edges of the sheet, it would also have apertures which could allow the passage of filtrate from left to right, i.e. beneath the sealant 58, and so these apertures must themselves also be sealed. In these embodiments, the chemical bonding agent for the sealant, such as epoxy resin, is selected so that in its initial liquid state it can flow into the mesh to occupy all of these apertures; when it hardens these apertures will be closed off.

Alternatively, the material of the sealant can be made integral with the mesh, for example by being manufactured as a liquid which passes through the required apertures of the mesh before solidifying into a sealant on both sides of the sheet, and forming a permanent barrier thereacross.

The septum 24 has two fold lines 60, enabling it to be folded over the plate 12. In other embodiments, the septum may be sufficiently flexible to permit it to be positioned over the plate without fold lines; in yet another embodiment the septum is thinned around its centre line to allow it to be bent over the plate.

Though we prefer the sealing means to be permanently affixed to the sheet, we do not discount the temporary engagement of the sealing means therewith. Such a termporary engagement might be provided by a filter in which the frame or plates carry a sealing strip, which sealing strip only engages the septum when this is positioned over the plate and the plates and frames are clamped together. The sealing strip in this embodiment might be adhered to the plate or frame, or might be located in a recess in the facing wall part 30 of the frame (or the corresponding wall part of the plate).

Claims

1. A septum (24) for a filter (10) comprising a sheet with a plurality of apertures, the apertures being of a size to permit the passage of filtrate, characterised in that the sheet is of metal.

2. A septum as claimed in claim 1 characterised in that the septum has two parallel fold lines (60) intermediate two spaced non-adjacent septum sides whereby the septum is adapted to be bent around one edge of a support plate (12).

3. A septum as claimed in claim 1 characterised in that the septum has sealing means (58) affixed thereto, the sealing means enclosing said apertures.

4. A septum as claimed in claim 1 characterised in that the septum has two sets of apertures, each set being surrounded by separate sealing means (58), each set and sealing means being to a respective side of a septum fold line (60) whereby the septum is adapted to be folded with said apertures aligned and said separate sealing means facing outwardly one away from the other.

5. A septum as claimed in claim 1 characterised in that the sheet comprises a wire mesh (56), and in that the wire is of stainless steel.

6. A filter (10) which includes a septum (24) characterised in that the septum is as claimed in claim 1.

7. A filter (10) which includes a number of filter sub- assemblies, each sub-assembly comprising a septum (24) positioned between two frames (14) characterised in that the septa is as claimed in claim 1, in that the septa is folded to either side of a supporting plate (12) and in that the septa has an area greater than double that of a frame.

8. A filter according to claim 7 characterised in that the frames (14) and plates (12) each enclose a respective opening, the plate opening being between two septum parts each with apertures, in that the frames and plates have respective first through conduits (40,46) and second through conduits (42,48), said conduits being aligned to provide first and second through passageways, in that the plates each have a first portway connecting a first through passagway (46) with the plate opening and in that the frames each have a second portway (44) connecting the plate openings (32,) with a second through passageway.

9. A filter according to claim 8 characterised in that the frames (14) and plates (12) are slidably mounted on tie bars, and in that an hydraulic ram (20) is positioned to press the frames and plates into sealing engagement.

10. A filter according to claim 7 characterised in that each plate (12) has a pair of spaced partitions (50) adapted to inhibit substantial deformation of the portions of the septum lying adjacent said partitions.