GB2236693A - Filtration - Google Patents

Abstract

The invention relates to cross-flow filters using self-supporting filter plates or membranes (7). Each self-supporting membrane has two major faces, each being adapted to receive sealing means (7A) around its periphery. Two or more membranes are abutted with their major faces opposed and their peripheries in sealing engagement, the major faces being spaced apart by the sealing means to provide flow channels for a liquid to be filtered and for the permeate. The self-supporting membranes enable support plates conventionally used to be eliminated and more compact design achieved. <IMAGE>

Description

FILTRATION This invention relates to filtration and, more particularly, to a means of cross-flow filtration utilising self-supporting filter plates, hereafter referred to as self-supporting membranes.

A known type of cross-flow filter comprises a thin permeable polymeric membrane which is supported on a support plate. A series of support plates are stacked together with a membrane supported on each side of each support plate so that flow channels for the liquid to be filtered are provided across the membranes' surfaces. Inlet and outlet passages are provided for the liquid and these communicate with the flow channels. Liquid flowing through a membrane, i.e.

permeate, is collected separately from liquid passing across the membrane face, i.e. retentate, and the latter may be continuously recirculated around the system.

Ceramic membrane micro- and ultra-filters for use in cross-flow mode have also been available for a number of years. The normal arrangement has been to form the ceramic membrane units into tubes or multichannels (see U.K. Patent Application GB 2176715A) and incorporate these into a circulatory system such that the liquid to be filtered passes from a pump via a pipe to the ceramic membrane tube, through the membrane tube and via a further pipe to a heat exchanger and thence through further pipework back to the circulatory pump.

The present invention aims to provide an improved system that, for example, eliminates much of this pipework, especially that between the ceramic membrane filter and the heat exchanger and to make thereby a cheaper and more compact filter which will better fit into modern plant design.

Accordingly, in a first aspect the present invention provides a membrane for a cross-flow filter, the membrane being of porous material formed into a self-supporting shape having two major faces, each major face being adapted to receive sealing means around its periphery, whereby two or more membranes may be abutted with their peripheries in sealing engagement, the major faces of adjacent membranes being spaced apart by the peripheral sealing means, whereby the spaces between the major faces of adjacent membranes provides flow channels for a fluid.

The major faces of the membranes may be formed with corrugations as this can greatly increase the surface area that the liquid to be filtered must pass across. Alternatively, each membrane may have one flat major face to assist permeate flow. In this embodiment pairs of membranes are arranged with their corrugated faces opposed to each other and each with its flat face opposed to the flat face of the membrane of a next adjacent pair. The space between the opposed flat faces provides the permeate flow channel and the gap may be very narrow to reduce permeate volume.

If desired, both major faces ct each membrane may be flat which could be advantageous in pumping viscous liquids.

The pores of the membranes may need to be sealed, e.g. by impregnation, around the edges and adjacent the ports to prevent retentate or permeate migrating through the porous structure to leak at the sides or into ports where it is not wanted. Thus, the filtration area of the membrane is sealed off and leakfree.

Alternatively, a ceramic glaze could be applied to those portions of the membrane.

A plurality of the self-supporting membranes can be installed as a stack in a suitable frame, e.g. a standard heat exchanger frame, using standard plate heat exchanger ends, connections and gaskets. For example, a typical assembly so formed could contain up to 300, or even more, self-supporting membranes.

In a preferred embodiment the membranes each have two inlet ports and two exit ports, one inlet and one exit being closed and one inlet port open to receive the liquid to be circulated and filtered and one exit port being open to exit the liquid to be filtered. When a series of membranes is stacked to abut together, the first inlet port between a first pair of membranes, i.e. membrane one and membrane two, is open to the channel between the membranes and the first exit port is open. The second inlet port and the second exit port are closed by suitable sealing means between the membranes. At the second pair of membranes, constituted by membrane two and membrane three, the first inlet port is sealed off from the channel between the membranes so that unfiltered liquid cannot pass along that channel. The second inlet port is open.The first exit port of this pair of membranes is sealed off and the second exit port is open. This alternating arrangement is repeated along the stack, so that at the third pair of membranes, i.e. membrane three and membrane four, the first inlet port is open, the second inlet port is closed, the first exit port is open and the second exit port is closed. Thus, when liquid to be filtered is pumped around the unit, unfiltered liquid can pass between alternate pairs of membranes through the first exit port of that pair and be recirculated. Liquid (permeate) which passes through a membrane into the channels between the second, fourth and so on pairs, passes out through the second exit port and is collected.

Accordingly, the invention also provides a cross-flow filtration unit comprising a plurality of self-supporting porous membranes, each having two major faces, each major face having sealing means around its periphery, the membranes being fitted into a suitable frame so that the peripheries of the major faces of adjacent membranes sealingly abut with the faces spaced apart to provide flow channels between the membranes, the flow channels between alternate pairs of major faces communicating with ports through which liquid (retentate) pumped into the unit can be recirculated and the flow channels between the other pairs of major faces being closed to the retentate flow path and only receiving permeate that has passed through a membrane and communicating with an exit port through which permeate can be collected.

In a particular embodiment of the present invention there is provided a filter which can be arranged so that banks of membranes may be added in series, each bank having the same or different pore size to its adjacent bank.

In another embodiment of the present invention there is provided a filter which can be combined with a plate heat exchanger in the same frame to provide a method of controlling the filtration temperature.

It is also possible to provide a method of automatically cleaning the filter unit.

The membrane may be of orgar.ic or inorganic material provided that it is self-supporting and of the required porosity. It could be of polymeric material or of metal but it is preferably of a ceramic material.

Thus the membrane may be, for example, of silica, alumina, zircon, silicon carbide, an aluminosilicate or a partially stabilised zirconia.

Alternatively, it could be a composite material, e.g. a sheet of porous polymeric material bonded to a supporting layer of porous ceramic material.

For microfiltration, the required pore size would be in a range from O.S to 10 microns, while smaller pore sizes would be appropriate for ultrafiltration, e.g. from 1000 to 40 Angstroms and even smaller for reverse osmosis.

In another embodiment of the invention, the pore size may increase from one major face through the thickness of the structure to the other major face, the smallest pores being of the desired size for the required filtration effect. For example, the smallest pores could be provided in a porous sheet of polymer material that is bonded to a porous ceramic supporting layer of coarser pore size.

It is also possible readily to cater for the efficient filtration of liquid of varying ranges of viscosity by means of the present invention, by widening or narrowing the gap, i.e. flow channels, between adjacent membranes by incorporating a sealing gasket of greater or lesser thickness as required.

The present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is an elevation of a self-supporting corrugated ceramic membrane plate of the invention; Figure 2 is a diagrammatic illustration in cross-section showing how corrugated membranes in a stack fit together in one embodiment; Figure 3 is a cross-sectIon showing the stacking of membranes in an alternative embodiment.

Figure 4 is a diagrammatic illustration of a simple multi-plate cross-flow filter showing the liquid flow pattern; Figure 5 is a similar illustration showing two banks of plates installed so that they operate in series; Figure 6 is a similar illustration showing the addition of standard heat exchanger plates; and Figure 7 is a similar illustration showing a unit in cleaning configuration.

Referring firstly to Figure 1, the basic component of the filter is the filter plate or membrane 7 which is of porous, self-supporting ceramic material.

The liquid to be filtered is caused tc enter via inlet 4 from whence it fills the space between plate 7 and the next adjacent plate, leaving the filter through exit 3 from which it will be returned by external pumping to inlet 4. Port 5 is the inlet permeate channel if the permeate is to be circulated and also serves as the permeate drain means. It is isolated from plate 7 but connected to alternate plates 8 (see Figure 4). Port 6 is the outlet permeate channel also isolated from plate 7 and connected to alternate plates 8.

As can be seen, ports 5 and ; are of smaller cross-sectional area than ports 3 and . Although not essential, this has the advantages of increasing the plate area available for filtration or using larger ports for the retentate circulation. Iso, savings may be made on pumping costs when the permeate is circulated.

Since the retentate side of the filter is maintained at a higher pressure than the permeate side, liquid will pass through plate 7 into the space between plate 7 and plate 8. It will leave plate 8 via outlet 6 and will similarly leave from all other alternate outlet plates.

A sealing gasket 7A is shown extending around the periphery of the membrane. Gasket 7A is provided with vent holes 5A adjacent to port 5 and 6A adjacent to port 6, both ports being closed tc liquid passage from plate 7. These vent to the atmosphere and would give an immediate visual indication oi leaking liquid should any such leakage problem arise. Likewise, alternate plates have similar vents (not shown) at ports 3 and 4 which are closed to liquid from plates 8.

Figure 2 is a cross-section through a number of corrugated membranes showing how the corrugations fit together to allow a high surf ace area to be contained in a small compact filter. The liquid to be filtered (retentate) 1 and the filtered liquid (permeate) 2 are shown.

In Figure 3, each membrane 3A has one flat major face and one corrugated major face. The membranes are arranged so that the corrugated faces of adjacent pairs face each other with the retentate flow channel 1A between them and so that the flat faces of adjacent pairs also face each other with the permeate flow channel 2A between them.

Figure 4 shows a plurality of ceramic membrane plates. (It will be appreciated that, for convenience, the membranes are shown spaced apart whereas in practice they will be urged into sealing peripheral engagement with adjacent membranes.) The liquid to be filtered enters through the inlet retentate port 4, flows upwards through the spaces immediately to the right of plates 7 and exits through port 3 to be returned to port 4 by external pumping. The permeate passes through all of the plates into the spaces immediately to the right of plates 8, collects in the header and exits through port 6. Port 5 is used for draining the filter.

Referring now to Figure 5, which shows two banks of plates operating in series, outlet 3 is now closed and a special plate 8A is inserted into the filter in place of one of the plates 8. This plate has its bottom port 9 blanked off. Outlet 10 is opened.

Thus the retentate flows into inlet 4 as before up through the spaces to the right of the first two plates 7 along the top header and down through the spaces immediately to the right of the second two plates 7 and exits from the filter through port D. The permeate collects in the same spaces as befcre and exits as before through port 6.

Figure 6 shows the system flow arrangement when the filter is combined with a heat exchanger. In this configuration the retentate is fed into the filter at port 12 into the header from which it passes up the space immediately to the right of plate 7 from which permeate may pass to the permeate space adjacent to plate 8. The retentate passes into the header and then down stainless steel (or other heat exchange medium) plate 16a where its temperature is adjusted. It passes out of the filter at port 15.

The filter and heat exchanger are separated by a standard connector plate 14.

Permeate is collected in the permeate space adjacent to plate 8 and is exited from the filter at port 16.

Plate 13 is a stainless steel heat exchanger plate, the purpose of which is to contain, in conjunction with plate 18, the temperature controlling fluid and to maintain continuity of the retentate and temperature controlling fluid channels.

Plate 13A is also a stainless steel plate, the purpose of which is to contain, in conjunction with plate 8, the permeate and to maintain continuity of the retentate and permeate channels.

Temperature controlling fluid is introduced at port 17, passes down stainless steel (or other heat exchange medium) plate 18 and exits at port 10.

Referring to Figure 7, the filter is cleaned by passing cleansing liquids at high velocity through both sides of the filter The cleansing liquid is first passed into port 4 along the header and up all plates 7, into the header and exits at port 3.

Secondly, the cleansing liquid is passed into port 5 along the header and up all plates 8, into the header and exits at port 6.

The filtration membranes and units according to the invention have several advantages over those hitherto.

1) The system can be designed for maximum cross-flow and turbulence and permits a variety of membrane designs to be used to maximise these characteristics.

2) The system is designed in such a way that it can incorporate standard heat exchanger plates in the same frame.

3) It can be designed for minimum pressure drop and therefore, minimum pumping cost.

4) It permits easy maintenance and replacement of filter plates where necessary.

5) It is possible to design the filter with a variety of inter-plate gaps to accommodate different viscosity liquids.

6) It can accommodate two filters on one frame.

7) It is particularly amenable to allowing the permeate also to be pumped around the system in order to equalise trans-membrane pressure between retentate and permeate to give improved filtration efficiency.

8) The elimination of metal containment structure reduces energy consumption when heating/cooling cycles are required.

9) Low permeate volume in the filtration unit.

In addition to micro- and ultrafiltration, the membranes and filtration units of the present invention may also have applicability in techniques such as reverse osmosis, dialysis, diafiltration, electrodialysis, transmembrane distillation, pervaporation, immobilised cell culture and hot or cold gas separations. Thus, it will be appreciated that by filtration is meant to include separation techniques such as liquid-liquid, gas-gas and gas-liquid separations.

Claims (15)

1. A membrane for a cross-flow filter, the membrane being of porous material formed into a selfsupporting shape having two major faces, each major face being adapted to receive sealing means around its periphery, whereby two or more membranes may be abutted with their peripheries in sealing engagement, the major faces of adjacent membranes being spaced apart by the peripheral sealing means, whereby the spaces between the major faces of adjacent membranes provide flow channels for a fluid.

2. A membrane according to Claim 1, which comprises a porous material in which the pore size increases from one major face through the thickness of the membrane to the other major face, the smaller pores being of the desired size for the required filtration effect.

3. A membrane according to Claim 1 or 2, in which the filtration pore size is from 0.1 to 10 microns.

4. A membrane according to Claim 1 or 2, in which the filtration pore size is from 40 to 1000 Angstroms.

5. A membrane according to Claim 1 or 2, in which the filtration pore size is les than 40 Angstroms.

6. A membrane according to any one of Claims 2 to 5, in which the smallest pores are provided in a porous sheet of polymer material that is bonded to a porous ceramic supporting layer of coarser pore structure.

7. A membrane according to any of the preceding claims, in which at least one of the two major faces of the membrane is formed with corrugations.

8. A cross-flow filtration unit comprising a plurality of self-supporting porous membranes each having two major faces, each major face having sealing means around its periphery, the membranes being fitted into a frame so that the peripheries of the major faces of adjacent membranes sealingly abut with the faces spaced apart to provide flow channels between the membranes, the flow channels between alternate pairs of major faces communicating with ports through which the liquid (retentate) pumped into the unit can be recirculated and the flow channels between the other pairs of major faces being closed to the retentate flow path to receive only permeate that has passed through a membrane and communicating with exit ports through which the permeate can be collected.

9. A unit according to Claim 8, in which the sealing means is provided with vent holes to the atmosphere adjacent the ports that are closed to liquid flow from an adjacent membrane.

10. A unit according to Claim 8 or 9, in which the width of the flow channels between membranes can be changed by use of sealing means of appropriate thickness.

11. A unit according to Claim 8, 9 or 10, which contains up to 300 membranes.

12. A unit according to any one of Claims 8 to 11, which comprises banks of membranes in series.

13. A unit according to Claim 12, in which each bank has membranes of different pore size to the adjacent banks.

14. A unit according to any one of Claims 8 to 13, which includes a plate heat exchanger in the frame to control the filtration temperature.

15. A cross-flow filtration unit, substantially as hereinbefore described with reference to and as shown in the accompanying drawings.