GB2192812A - Filtration device - Google Patents

Abstract

A filtration device comprises a housing (4,5) having a first flow passage communicating with the interior of the housing (4,5) at an upper end thereof and a second flow passage (2) communicating with the interior of the housing (4,5) at a lower end thereof. A plurality of flexible filter elements (13) each comprising a semi-permeable membrane envelope enclosing a permeable medium are supported at their upper ends only by support means (12) on the upper part of the housing (4,5). Each filter element (13) has an opening through the membrane whereby permeate can be removed from the permeable medium, the opening being in the region of the support means (12) and communicating with a third flow passage (11) opening from the interior of the housing (4,5). <IMAGE>

Description

SPECIFICATION Filtration device This invention relates to a filtration device, and particularly to a filtration device comprising semipermeable membranes.

It is known that semi-permeable membranes may be used to separate liquids into two fractions by microfiltration, ultrafiltration or reverse osmosis. In each of these processes the fluid to be treated is forced to flow over the membrane, whereupon filtrate permeates through the membrane and can be collected from the opposite side thereof. The flow of feed fluid has a sweeping action which reduces the build up of separated components on the membrane surface or in the boundary layer adjacent to it. Membranes for microfiltration may generally contain pores in the range of 0.02 micron to 10 microns, membranes for use in ultrafiltration may generally contain pores in the range of 0.001 micron to 0.02 micron, while membranes for use in reverse osmosis have pores even smaller than those for ultrafiltration.

The techniques described in the foregoing paragraph may conveniently be referred to as cross-flow filtration. Many systems for effecting cross-flow filtration are known, and perhaps the commonest of these are those known as the plate and frame systems. In such systems the filter comprises a set of spaced parallel filter plates with feed spaces interspersed between the plates. Each filter plate comprises at least one membrane surface forming one boundary of a filtrate'cham- ber within the thickness of the plate. The feed is passed across the surfaces of the membranes at high velocity and liquid permeating through the membranes into the filtrate chambers is drawn off from those chambers.

Examples of such systems are shown in British patents 1390671, 1406485, 1555389 and 2080144. In these filters the plates are fixed in a more or less rigid array, with small feed spaces between adjacent plates. There is a danger of the feed spaces becoming blocked by large particles jamming between adjacent plates, this tendency being increased by the tendency of the plates to deflect and warp in response, for example, to temperature changes. Known forms of plate and frame cross-flow filters are also costly to manufacture, and replacement of an individual mem brane within a filter is difficult and labour intensive.

According to.the present invention a filtration device comprises a housing, a first flow passage communicating with the interior of thehousing adjacent to a firstend thereof, a second flow passage communicating with the interior of the housing adjacent to a second end thereof, remote from the first end, a plu rality of flexible filter elements each comprising a semi-permeable membrane envelope enclosing a permeable medium, support means at one end region only of each filter element,the support means being secured in the housing adjacent to the first end thereof, and each filter element having an opening through the membrane whereby permeate can be removed from the permeable medium, the opening being in the region of the support means and communicating with a third flow passage opening from the interior of the housing.

This differs from known cross-flow filtration devices in that each flexible filter element is only supported at one end region thereof, and the elements, while being held within the housing, are thus allowed a degree of free movement as liquid flows past them. This free movement is advantageous in that it allows large particles which would otherwise block channels between adjacent elements to pass through those channels, and in that it allows the individual elements to contract or expand freely upon changes in temperature which may otherwise cause the elements to warp. Furthermore, there is a constant tendency to straighten out deflections in individual elements due to the flow pattern of liquid past the elements during operation, so that substantially uniform flow channels between adjacent elements are maintained.Filtration devices of the invention will primarily be designed for process applications handling large volumes of liquid with high efficiency, rather than as smaller devices destined for analytical and medical work.

Preferably the first end of the housing is an upper end thereof, the elements thus being supported at their upper ends and hanging freely down within the housing. Freedom of movement for the elements is thus maximised.

Flow of feed liquid through the filter may be either downwardly from the first to the second flow passage or upwardly from the second to the first flow passage. If the housing is disposed so that the elements do not hang freely downwards then it is preferred that the flow of feed liquid is from the first to the second flow passage.

Preferably each filter element is in the form of a flat envelope, adjacent elements are spaced apart at their upper regions, and the spacing means form part of the support means.

Conveniently each filter element has its up per region sandwiched in fluid-tight manner between two spacing members, and the stack of spacing members is secured in the housing adjacent to the first end thereof.

Using a construction of this nature a filter may rapidly be dismantled and one or more of the envelopes replaced as required. The spac ing members may be in the form of hollow press studs further to facilitate the assembly and dismantling of the stack of elements. The hollow centres of the studs will communicate with those of adjacent studs and form the third flow passage for permeate to flow from the filter.

The flat envelopes may be of any convenient shape, but it is presently preferred to have the envelopes of rectangular shape and to mount them in a substantially parallelopipedal housing.

In order that the invention may be better understood specific embodiments of filtration devices in accordance therewith will now be described in more detail, by way of example only, with reference to the accompanying drawings in which: Figure 1 diagrammatically shows a first embodiment of a#filter#according to the invention; Figure 2 shows detail AA from Figure 1; Figure 3 shows a membrane envelope from the filter to Figure 1; Figure 4 shows diagrammatically a second embodiment of filter according to the invention; Figure 5 is a cross-section on line V-V of Figure 4; Figure 6 is an enlarged cross-section on line VI-VI of Figure 5; and Figure 7 is a further enlarged view of part of Figure 6.

Figure 1 shows a filter embodying the invention and comprising ahousing having a first wall 4 and a second, parallel wall 5. Walls 4 and 5 are rectangular and are interconnected by walls 6 and 7. The four walls are sealed by end caps 8, 9 and together they define a chamber 10. End cap 8 has an inlet 1 for the feed to be concentrated. Inlet 1 communicates with chamber 10 through an opening in wall 6. End cap 9 has an outlet 2 for the concentrated feed.

Located within the chamber are a plurality of flexible filter elements 13, each comprising an envelope 14 of semi permeable membrane material supported by and enclosing a permeable substrate. As shown, the permeable substrate is a fibre mesh fabric 15, the fibres being of any suitable material, preferably a synthetic plastics material. However, it is preferred that the permeable substrate be a porous mat of non-woven fibres that are inert to the feed stock or permeate, the non-woven mat being chosen to give the element sufficient strength to prevent it tearing away from its mounting. The membranes that form the envelope are preferably supported membranes, in order to increase their strength during handling.

Each element 13 is embedded at its upper end in an cured resin compound, parts 12 of which serve to space adjacent elements apart.

The cured resin has a downwardly angled surface 12a adjacent to the inlet 1. A filtrate channel 11 is drilled through the assembly of resin and envelopes 14, the channel terminating at a filtrate outlet 3 in the end cap 8.

In use, feed liquid supplied to the inlet 1 flows downwardly through the chamber 10, as shown by the arrows, to the outlet 2. The downwardly angled surface 12a assists in preventing any stagnation in liquid in the upper part of the filter. An excess pressure differential is maintained between the inlet and the outlet 2, and accordingly filtrate flows through the membranes 14 into the interior of the envelopes, according to the pore size of the membranes. That filtrate flows through the permeable substrate 15 and is discharged from the aperture 1 1a of each element into the channel 11 and so flows to the filtrate outlet 3.

As already stated, each filter element 13 is flexible, so requiring both the membrane and the substrate material to be flexible. This flexibility allows any deflection in any element to be straightened out by the action of the liquid flow during operation, so maintaining substantially uniform flow channels between adjacent elements. The elements are suspended freely in the liquid and are thus allowed a degree of free movement which allows large particles to pass through the channels and which also allows the elements to contract or expand freely upon temperature changes which could otherwise cause them to warp.

Figures 4 to 7 show a further embodiment of filter comprising a parallelopipedal housing 20 having a feed inlet 21 at the upper part thereof and a feed outlet 22 at the lower part thereof. A plurality of filter elements 22 are suspended adjacent to their upper ends from the upper part of the chamber as will be described in more detail. Each filter element comprises an envelope formed by two layers 23, 24 (Figure 7) of flexible semi-permeable membrane material, the layers being supported by and enclosing a flexible porous substrate 25 of non-woven fibres.

Each filter element is suspended by way of an assembly comprising two spacing members 26, 27, each of which is conveniently moulded from a suitable plastics material. As will be seen from Figure 7 a hole is punched in the filter element and the members 26 and 27 are inserted from opposite sides of the hole until castellated inner ends 28 and 29 of the two members abut one against the other.

The end surfaces are joined by ultrasonic welding or any other suitable means so that the members are secured in position on the filter element, desirably with very slight compression of the element being caused between the facing flanges 30 and 31 of the two members 26, 27. The castellated ends of the two members lie in the region of the porous non-woven fabric 25 and thus allow filtrate to drain out of this fabric body into the hollow bores 32, 33 of the members 26, 27.

It will be seen that the free end of the member 27 has a frustoconical recess 34 coaxial with the bore 33, and that the free end of the member 36 has a mating, frusto conical projection 35 coaxial with the bore 32.

The projection 35 of one such member may be mated into the recess 34 of an adjacent member as shown in Figure 6, with the matching tapers forming a fluid-tight seal. The stack of spacing members, and thus the stack of filter elements, may be self-supporting, or may be held in assembled form by a bolt 36 and nut 37, the shank of the bolt passing through the bores 32, 33 of the aligned spacing members, and the bolt also securing the stack to one wall of the housing 20. An outlet 38 for filtrate may be secured to the housing in the region of the bolt 36.

It will be appreciated that operation of the filter is similar to that already described, with feed liquid supplied to the inlet 21 and flowing downwardly through the chamber to the outlet 22. Alternatively, the flow of feed liquid may be upwardly. In either case an excess pressure differential is maintained between the inlet and the outlet so that filtrate flows through the membranes 23, 24 into the porous substrates 25 and thence through the bores 32, 33 in the spaces to the filtrate outlet 38.

It will be understood that numerous changes may be made to the apparatus shown in the drawings. The elements have-been described as of rectangular shape enclosed in a parallopipedal housing. However, this is not essential and elements of polygonal, circular, elliptical or irregular shape may be provided, accommodated in a housing of appropriate shape. Use of spacers both to effect spacing between adjacent envelopes and also to mount the envelopes in the housing is convenient, but it will be clear that other mounting means may be used. Similarly, aligned passages in the envelopes and spacing means in order to allow filtrate to flow from the apparatus is a convenient way of defining the filtrate passage, but other arrangements are also possible.

Claims (7)

1. A filtration device comprising a housing, a first flow passage communicating with the interior of the housing adjacent to a first end thereof, a second flow passage communicating with the interior of the housing adjacent to a second end thereof, remote from the first end, a plurality of flexible filter elements each comprising a semi-permeable membrane envelope enclosing a permeable medium, support means at one end region only of each filter element, the support means being secured in the housing adjacent to the first end thereof, and each filter element having an opening through the membrane whereby permeate can be removed from the permeable medium, the opening being in the region of the support means and communicating with a third flow passage opening from the interior of the housing.

2. A filtration device according to claim 1 in which the first end of the housing is an upper end thereof, the elements thus being supported at their upper ends and hanging freely down within the housing.

3. A filtration device according to claim 2 in which each filter element is in the form of a flat envelope, adjacent elements are spaced apart at their upper regions, and the spacing means form part of the support means.

4. A filtration device according to claim 3 in which each filter element has its upper region sandwiched in fluid-tight manner between two spacing members, and the stack of spacing members is secured in the housing adjacent to the upper end thereof.

5. A filtration device according to claim 4 in which the spacing members are in the form of hollow press studs, the hollow centres of the studs communicating with those of adjacent studs and forming at least part of the third flow passage.

6. A filtration device according to any one of the preceding claims in which the flat envelopes are of rectangular shape and the housing is parallelopipedal.

7. A filtration device substantially as herein described with reference to the accompanying drawings.