CA2245334A1 - Filtration apparatus - Google Patents

Abstract

Filter apparatus (2) comprises end plates (13),(14) between which are arranged a plurality of parallel filter membrane plates (15). Feed ports (21), return ports (23), filtrate ports (22) and backflush ports (24), and seals (41),(41),(43),(44) between adjacent plates define a turbulent feed flow path across one side of the membrane to set up vibrations in the membranes to reduce clogging.

Description

FILTRATION APPARATUS
The present invention relates to filtration apparatus for separating solid particles from liquids.
BACKGROUND ART
Filtration systems utilizing one or more filter membranes of metallic or non-metallic porous material, such as wire mesh, have been used for many years to separate and remove suspended solid particles from their liquid medium. Each filter membrane is typically comprised of a number of pores of predetermined diameter through which the liquid to be filtered (or feed liquid) is directed. Solid particles of the same or slightly larger size than the size of the pores in the filter membrane are trapped within or on the surface of the filter membrane but the filtrate liquid is able to pass through the membrane pores. When some or all of the solid particles are of small diameter, effective separation of the solid particles from the liquid will not occur unless the pores of the filter membrane are of sufficiently small size to trap the solid particles.
A disadvantage of conventional filtration systems is that filter membranes having pores of small size (say~5~m or less) tend to blind, or plug up or become clogged during the filtration process. The result is that a filter cake or particulate may build up on the surface of the blinded filter membrane which interrupts the further filtration process and specifically the flow of the feed liquid through the pores of that filter membrane. In these circumstances the filter membranes have to be cleaned before further filtration can take place. Typically, the filter membranes can be cleaned by back flushing. In this process, air or steam is passed through the filter membrane in a direction opposite to the normal direction of the filtrate flow to dislodge the filter cake from the filter membrane. The cake can then be removed from the filter system as sludge through a sludge outlet. The time taken to complete this cleaning of the filter membranes presents a disadvantage because it interrupts the active filtration process.

SUMMARY OF THE INVENTION
According to one aspect of the present invention there is disclosed filtration apparatus for liquids comprising a filter apparatus including at least one filter element comprising a flat filter membrane, feed means and return means communicating with a cross flow side of said membrane such that feed liquid flows across the cross flow side of the membrane from the feed means to the return means and filtrate is drawn through the membrane to a filtrate side of the membrane, and filtrate removal means communicating with the filtrate side of the membrane.
Preferably, the filter membranes are formed of non-woven metal fibres, preferably sintered together, which are selected from the group consisting of stainless steel and Incanol.
Preferably, the membranes of said module have a pore size in the range of 0.5 to 10~m, I S more preferably to S Vim.
BRIEF DESCRIPTION OF THE DRAWINGS
A number of embodiments of the present invention will now be described with reference to the drawings in which:
Fig. 1 is a schematic diagram of the filtration system of a first embodiment;
Fig. 2 is a schematic diagram of a second embodiment;
Fig. 3 is a schematic view of a third embodiment;
Fig. 4 is an exploded perspective view of a single preferred filter module used in the above embodiments, depicting the flow path of the feed liquid;
Fig. 5 further depicts the filter module of Fig. 4 to illustrate the movement of filtrate during the filtering operation;
Fig. 6 is a vertical cross-section depiction of the module of Fig. 4 illustrating operation of the filter;

Fig. 7 depicts a back flush operation of the module of Fig. 4;
Fig. 8 is an exploded perspective view of an alternative filter module;
Fig. 9 is a vertical cross-sectional view of one of the sandwich plates of Fig. 8;
Fig. 10 is a partial transverse vertical cross section of the plate of Fig. 9;
Fig. 11 is an end elevation of end plates for use in a further embodiment;
Fig. 12a is an elevation of the cross flow side of a membrane plate used in the embodiment of Fig. 1 l;
Fig. 12b is an elevation of the filtrate side of the membrane plate of Fig.
12a;
Fig. 13 is a cross-sectional detail depicting a seal formed on the side of membrane plate depicted in Fig. 12b; and Fig. 14A to 14C schematically illustrate the arrangements of membranes and seals in a generalised embodiment.
BEST MODE OF CARRYING OUT THE INVENTION
Fig. 1 depicts a filtration system 1 incorporating a single filter module 2, a liquid reservoir 3 to hold the feed liquid, feed pump 4 and a programmable logic controller (PLC) 5. During the filtering operation the liquid is drawn from liquid reservoir 3 along a feed supply pipe 6 through a non-return valve 12 to the filter module 2 by feed pump 4.
Leading from the filter module 2 is a filtrate outlet pipe 7 that can be opened by using a controllable valve. Linked to the filtrate outlet pipe 7 is a flow meter 8 which provides a measure of output flow rate to the PLC S. Return pipe 9 conveys unfiltered liquid exiting the filter module 2 back to the liquid reservoir 3. Also leading into the filter module 2 is back flush inlet pipe 10 through which air or steam can be introduced to the filter module 2 for cleaning of the filter module 2 during back flush operation. Branching from the supply pipe 6 proximate its entry into filter module 2 is sludge outlet pipe 11 also having a valve controllable by the PLC 5. Non return valve 12 prevents return of sludge through supply pipe 6 to liquid reservoir 3 during backflushing. The PLC 5 is used to control the flow of liquid and filtrate into and out of the filter module 2 during the filtering operation and also to control the introduction of air or steam into the filter module 2 and the flow of sludge out of filter module 2 during back flushing.
Fig. 2 illustrates a filtration system 40 which includes a reservoir 3 and pump 4 connected in the manner shown in Fig. 1 to an array of six parallel connected filter modules corresponding to module 2 of Fig. 1. The sludge outputs of each are supplied to a further filter module 2a, as in Fig. 1, which includes only sludge and filtrate outputs.
Fig. 3 illustrates a system 50 which includes a parallel combination of six of the filter modules 2 of Fig. 1. A further filtering system module 2b, like Fig. 1, accepts the sludge outputs of modules 2, and each module includes its own pump 4.
As best illustrated in Fig. 4, the module 2 consists of two facing end plates 13 and 14, preferably of stainless steel. Between the end plates 13 and 14 are four filter membrane plates 15, individually designated 15a, 15b, 15c and 15d. The membrane plates 15a to 15d each include a sintered mat of non-woven metal fibres formed preferably of grade stainless steel or Incanol, defining pores (not illustrated) of predetermined size.
Each membrane plate 15a to 15d can be provided with a woven backing, for example a woven wire mesh, for added strength. Because high grade metal fibres are used in the construction of the membrane plates 15a to 15d, the filtering system 1 is of particular value in the food industry and can be effectively operated in a temperature range of 0° to 460°C. It is preferable for membrane plates 15a to 15d to be each comprised of separate layers of metal fibres where the pore sizes vary between the different layers in each plate.
The pore size of the membrane is generally in the range 0.5~m-10~m.
As can be seen generally from the exploded view of Fig. 4, the end plates 13, 14 and membrane plates 15a to 15d are generally rectangular and abut together in a parallel configuration. Each of the membrane plates have a number of ports extending therethrough, with seals surrounding the ports defining flow paths for the feed liquid (Fig.
4), filtrate (Fig. S) and back flushing (Fig. 7).
The end plates and membrane plates are held together by rods (not shown) which pass through small aligned apertures 20 about the periphery of the end plates (see Fig. 11 ), outwardly of the edges of the membrane plates, These small apertures are omitted from the other figures for the sake of clarity.
Each membrane plate has a peripheral seal 37 around its edge which seals against the adjacent membrane or end plate to form a sealed exterior of the filter unit.
In addition, each port in the membrane plates has a surrounding raised seal protruding from either the filtrate or cross flow side of the membrane to contact and seal against the adjacent plate and thus isolate the port from the fluid flow path on that side of the membrane.
With reference to Fig. 4, the membrane plates are paired together so that the cross flow sides of membrane plates 15a to 15d face each other, as do the cross flow sides of membrane plates 15c and 15d. The feed ports 21 at the bottom of the membranes align with the feed inlet port 17 in end plate 13 and return ports 23 at the top of the membranes align with return outlet 19 in end plate 13. Each of these ports have their surrounding seals 41,43 on the filtrate side of the membranes so that the fluid flow path between the feed ports and return ports lies on the cross flow side of the membranes. Feed liquid entering feed inlet 17 will pass through the feed port 21 in membrane 15a, from where some feed liquid will be diverted to flow through the small spacing between the cross flow sides of membranes 15a and 15b and the remainder will flow through ports 21 of membranes 15b and 15c and then flow over the cross flow sides of membranes 15c and 1 Sd.
Some of the liquid from the feed slurry will be filtered through the exposed surface of the filter membranes, as discussed below with reference to Figs. 5 and 6. The remainder of the slurry, with a higher solids content due to removal of the filtrate, will form a collective return flow, generally designated at y, through return ports 23 in the membranes and exit the filter unit via return outlet 19 in end plate 13, from where it is recycled to the liquid reservoir 3 as shown in Fig. 1.
As illustrated in Fig. 6, cross flow of feed liquid in the space between membrane plates 15a and 15b will give rise to a region of high pressure PEA relative to regions of lower pressure PZA and P2B resident in the adjacent spaces between the end plate 13 and membrane plate 15a and the space between membrane plates 15b and 15c respectively.
Similarly, cross flow of liquid in the space between membrane plates 15c and 15d will give rise to another region of high pressure P1B relative to regions of lower pressure P2B
and PZC resident in the spaces between membrane plates 15b and 15c and between membrane plates 15d and end plate 14 respectively. Because of these regions of differential pressure, a portion of the general liquid cross flow in region P1A will be attracted to the lower pressure regions of PZA and P2B and will move through the pores of the adjacent membrane plates 15a and 15b. Similarly, a portion of the general liquid cross flow in the region P1B will be attached to the lower pressure regions PZB and PZC, and will move through adjacent membrane plates 15c and 15d. Filtrate which has passed through the membrane plates 15a to 15d will form part of a collective cross flow of filtrate, in the regions P2A, Pzs ~d Pzc in an opposite direction to that of the cross flow of the general liquid body travelling in regions P, A and P i B. This movement of filtrate generally designated at x is best illustrated in Fig. 5.
Fig. 5 shows the flow path of the filtrate which passes through the membranes.
The feed port 21 and return port 23 of each membranes are isolated from the filtrate side of the filter membranes by seals 41, 43 as discussed above and thus the filtrate cannot mix with the slurry. The filtrate travels across the filtrate side of the membrane to filtrate ports 22, which have their raised seals 42 on the cross flow side of the membranes and thus communicate only with the filtrate side of the membranes. The filtrate forms a collective flow, generally designated x, through the filtrate ports 22 to the filtrate outlet 26 in end plate 14.
Figs. 11 to 13 illustrate a filter module with a different port arrangement from that illustrated in Figs. 4 and 5. In particular, end plate 13 has a pair of feed inlets 17 near the bottom of the end plate, and a pair of return outlets 19 near its top. Each membrane plate (Figs. 12A and 12B) has corresponding pairs of feed ports 21 and return ports 23, and also has small filtrate ports 29 in each corner. The seals 37,38 on the membrane plates 15 are hot moulded into and upon the surface of each plate. The seals 37,38 are raised from the surface of the membrane plate by 2.4mm, this being the spacing between adjacent membrane plates.
Seals 37 on the cross flow side, shown in cross hatching in Fig. 12A, surround the periphery of the plate and also surround and isolate the small filtrate side, shown in cross hatching in Fig. 12B, surrounds the periphery and also isolates the feed ports 21 and return ports 23.
The principle of operation is the same as that of Figs. 4 and 5, the feed entering the space between adjacent membrane plates and flowing across the cross flow sides of the membranes from feed ports 21 to return ports 23, while filtrate passes through the membrane and is removed through filtrate ports 29.
Referring to Figs. 14A to 14C, a membrane configuration 60 is shown which includes a combination of three filter membranes 61, 62 and 63 each constructed in accordance with a preferred embodiment. Each of the membranes 61-63 includes a filtrate side 64 and a cross flow side 65 that are defined by seals associated with each of the feed ports and the filtrate ports. For the purposes of clarity, a seal which surrounds the periphery of each of the filtrate sides 64 and the cross flow sides 65 has been omitted from these drawings.

As illustrated, each membrane 61-63 includes a lower feed port 66 and a upper return port 67. Similarly, a lower filtrate port 68 and a upper filtrate port 69 are also provided. On the filtrate side 64 of each membrane the feed and return 66 and 67 are each surrounded by annular seals 70 and 71 respectively. Filtrate ports 68 and 69 are not sealed on the filtrate side thus permitting filtrate liquid to pass from out of the filtrate side and into either one of the apertures 68 or 69.
As seen in Fig. 14C, on the cross flow side 65 each of the apertures 68 and 69 are surrounded by respective seals 72 and 73. As seen in Fig. 14B the seals 72 and 73 act to isolate the filtration apertures 68 and 69 from the cross flow path between adjacent membranes 61 and 62 however the absence of corresponding seals on the filtrate side 64 permits the flow of filtrate as described above and as seen between the membranes 62 and 63.
Using the configuration illustrated in Figs. 14A to 14C any stack of filter membranes including two or more such membranes will permit the generation of cross flow and filtration flow. Further, those skilled in the art, based on the above description will appreciate that the arrangements of Figs. 14A to 14C have been exaggerated for explanatory purposes. In practice the individual membranes have thickness of approximately 0.3 to 1.Omm and the separation between the membranes can be between about 1.0 to 6mm. The separation between adjacent membranes is one thickness of each of the seals as they protrude from the cross flow side 65 and filtrate side 64 respectively.
Because of the cross flow movement of liquid on the feed side of the filter membranes, the tendency of membrane plates 15a to 15d to blind during the filtering process will be minimised. Consequently, filters with pores of small size (S~m or less) can still be effectively used with less interruption of the filtering operation required for cleaning.
The configuration of membrane plates and end pates in the present invention has the further advantage that it encourages fully turbulent flow of feed liquid between the cross flow sides of adjacent membranes. This results in a high frequency vibration being set up in each of the membrane plates 15a to 15d, which also assists in minimising blinding of the membrane plate pores.
The distinction between laminar and turbulent flow is well understood in the art. During passage through a pipe the flow of a liquid is said to be laminar or turbulent depending on the liquid velocity, pipe size and liquid viscosity. For any given liquid and pipe size these factors can be expressed in terms of a dimensionless number called the Reynolds number R where:-R=VD
v V = average velocity ft/sec D = average internal dia. ft v = Kinematic viscosity of the fluid ft2/sec (for pure fresh water @
60°F, v =1.1216 x 10-S ft2/sec) For flow within a smooth-bored pipe values of R less than approximately 2000 correspond too laminar flow, ie. particles of the liquid follow separate non-interesting paths with little or no eddying or turbulence. Where R is above 4000 turbulent flow is considered to exist. Values of R between 2000 and 4000 are in the critical zone where the flow is generally considered to be turbulent for the purpose of frictionless or pressure drop calculations.
High velocity flow can cause the layer of turbulent flow to increase to the point where no laminar flow will exist, at which point high frequency vibrations will form in the pipe.
An example of this occurs with hydraulic oil applications at high velocities.
However, for most applications, fully turbulent flow will not usually occur in a round tube, but instead laminar flow occurs near the centre of the pipe and turbulent flow near the walls. This central laminar flow region absorbs vibrations from the turbulent flow and thus prevents excessive vibration of the pipe.

5 The present inventor has discovered, however, that the problem of vibration due to turbulent flow can be turned to an advantage in the instant application, and that fully turbulent flow can be encouraged by a filter module construction in accordance with the invention, in which cross flow of the feed occurs between closely spaced membrane plates. The inventor has found that this arrangement allows fully turbulent flow between 10 adjacent membranes, thus causing vibration of the membranes to reduce blinding, without excessive pumping power requirements.
The roughness of the membrane surface also encourages turbulence, as rougher surfaces cause thicker and greater turbulent flow at any given velocity.
With the reduced tendency of the pores of membrane plates 15a to 15d to blind in the filter system 1, cleaning is not required as often as in other filter systems.
Nevertheless, a back flushing system is necessary. When cleaning of membrane plates 15a to 15d is required to remove filter cake or particulate built up on membrane plate surfaces, the filtration process is discontinued and air or steam is introduced into port 28 through inlet pipe 10.
Fig. 7 illustrates the back flushing operation. Back flushing ports 24 in the membranes communicate with a back flushing inlet 28 in end plate 14 and are isolated from fluid flow on the cross flow side of the membranes by raised seals 44. Air or steam introduced to back flushing inlet 28 will pass to the spaces between the filtrate sides of the membranes and be forced through the membranes from the filtrate side to the cross flow side, i.e. opposite to the normal flow of filtrate during filtering operation.
This reverse flow of air or steam removes filter cake or particulates blinding the filter membrane, to form a sludge which is removed from the filter module through feed inlet 17 and sludge outlet pipe 11. Typically the filter module 2 can be effectively cleaned by back flushing within 3 to 90 seconds.
To further enhance speedy cleaning of filter membranes 15a to 15d, a modified form of the filter module 2 can be used. An example of the modified form of the filter module 2 is illustrated in Figs. 8 to 10. In this arrangement the filter membrane plates 15a and 15b are paired, as are the filter membrane plates 15c and 15d. Sandwich plates 31 to 33 are positioned between end plate 13 and membrane plate 15a, between membrane plates 15b and 15c and between membrane plate 15d and end plate 14, so that there is a sandwich plate adjacent the filtrate side of each membrane. Preferably, the sandwich plates 31, 32 and 33 are made of cast aluminium or stainless steel.
As illustrated in Figs. 9 and 10 each sandwich plate 31, 32 and 33 carries a central pipe 34 down its length with transverse branch arms 35. Each branch 35 has a number of small exit holes 36 through which air or steam introduced into central pipe 34 is directed onto the filtrate side of each membrane, in addition to the back flushing operation. The air or steam expelled from holes 36 will help clean sediment cake from the adjacent membrane plate.
As seen from Figs. 9 and 10, the central pipe 34 can be slidably mounted relative to the sandwich plate, so that the pipe 34 and branch arm 35 can be moved up and down while the air or steam is being directed onto the membranes by exit holes 36.
Specifically, the present invention has a wide variety of uses and is suitable for use in a number of different industries, including food and beverage processing, fuel oil and waste water industries. It is to be understood, however, that the present invention is not limited to these industries.

The foregoing describes only a number of embodiments to the present invention and modifications, obvious to those skilled in the art can be made thereto without departing from the scope of the present invention.

Claims (22)

1. Filter apparatus including at least one filter element comprising a flat filter membrane, feed means and return means communicating with a cross flow side of said membrane such that feed liquid flows across the cross flow side of the membrane from the feed means to the return means and filtrate is drawn through the membrane to a filtrate side of the membrane, and filtrate removal means communicating with the filtrate side of the membrane.

2. Filter apparatus according to claim 1 having a plurality of said membranes in parallel configuration.

3. Filter apparatus according to claim 2 wherein said membranes are disposed in pairs having their cross flow sides facing each other, and said feed means and return means cause said flow of feed liquid through a space bounded by the cross flow sides of said pair of membranes.

4. Filter apparatus according to claim 3 wherein the space between the cross flow surfaces is less than 6mm.

5. Filter apparatus according to claim 4 wherein said space is between about 1.0 and 6mm.

6. Filter apparatus according to any preceding claim wherein said feed means causes said cross flow of feed liquid is turbulent and induces vibrations in said filter membranes.

7. Filter apparatus according to claim 6 wherein said vibrations act to dislodge particulate material entrapped on or within said membrane.

8. Filter apparatus according to claim 2 wherein said membranes are retained in stacked configuration between a pair of end plates.

9. Filter apparatus according to claim 8 wherein membrane plates each including a said membrane are retained in stacked configuration between said end plates.

10. Filter apparatus according to claim 9 wherein said feed means includes aligned feed ports in each of the membrane plates.

11. Filter apparatus according to claim 10 wherein said feed ports seal to feed ports on the filtrate side of adjacent membrane plates.

12. Filter apparatus according to claim 10 wherein said membrane plates further comprise aligned return ports co-operating to form said return means and aligned filtrate ports co-operating to form said filtrate removal means.

13. Filter apparatus according to claim 12 wherein said return ports seal to return ports on the filtrate side of adjacent membrane plates and said filtrate ports seal to filtrate ports on the cross flow side of adjacent membrane plates.

14. Filter apparatus according to claim 11 or 13 wherein said ports are sealed to each other by means of a raised seal surrounding said port contacting the adjacent membrane plate.

15. Filter apparatus according to claim 2 wherein said membranes are formed of non-woven metal fibres.

16. Filter apparatus according to claim 15 wherein said fibres are sintered.

17. Filter apparatus according to claim 15 or 16 wherein said fibres are made from metals selected from stainless steel and Incanol.

18. Filter apparatus according to any of claims 15 to 17 wherein said membranes have a pore size in the range 0.5µm to 10µm.

19. Filter apparatus according to claim 18 wherein said pore size is in the range of 1µm to 5µm.

20. Filter apparatus according to any of claims 15 to 19 wherein said membranes are retained between a pair of end plates.

21. Filter apparatus substantially as herein described with reference to Figs.
4 to 7, 8 to 10, 11 to 13 or 14A to 14C.

22. A filter system substantially as herein described with reference Figs. 1, 2 or 3.