Filter element

Abstract

Filter element, such as a candle filter or a pressure leaf filter. The filter element comprises a stack of a support screen and at least one outer fine screen. The outer fine screen is a stainless steel sheet with machined filter openings, e.g., made by laser cutting. The filter openings may be slots, tapering in cross section.

Description

[0001] 41212-Zo / ml

[0002] FILTER ELEMENT

[0003] TECHNICAL FIELD

[0004] The present disclosure relates to a filter element, in particular for cake filtration, such as a pressure leaf filter or a candle filter .

[0005] BACKGROUND

[0006] Filter leaves for pressure leaf filters typically comprise a perimetral tube frame holding a stack of screens including one or more central support screens and outer fine wire screens at opposite sides of the central support screen. The support screen typically comprises one or more inner coarse meshes with an open area that is considerably larger than the open area of the outer fine screens . Therefore it does not contribute to the filtration, but has better mechanical strength and rigidity and spaces the two outer fine screens apart so as to provide a drainage space in communication with the interior of the perimetral tube . This perimetral tube has an outlet, usually centrally at a lower side of the frame, opening into a manifold or discharge channel . Most common filter leaves have five layers : the support screen comprising a central support weave screen providing mechanical strength and stability and a drain weave mesh screen at either side of the central support weave . The outer fine screens are at both outer sides of the filter leaf . Typical examples of such filter leaves are disclosed in US 2, 468, 296 and WO 94 / 14518.

[0007] There are two main types of pressure leaf filters : horizontal and vertical . Examples of vertical pressure leaf filters are disclosed in US 2, 874, 848 and WO 2019 / 241248.

[0008] Pressure leaf filters are for example used in beermaking, winemaking, sugar refining, starch industry,processing of edible oil, liquid sulphur filtration, oil processing, bauxite refining, etc .

[0009] Pressure leaf filters are used for filtering fluids containing solid particles, such as dispersions or slurries . The slurry is forced into the pressure vessel through the outer fine wire weave screens . The solids tend to form a cake on the outer surface of the outer fine screens, while the liquid permeate passes the screens into the inner space of the leaves formed by the support screen. The liquid permeate is then discharged via the perimetral tube to the discharge channel .

[0010] Candle filters are tubular, e . g. , cylindrical having a tubular support screen and a filtering cloth or mesh forming an outer fine screen. The tubular support screen serves to collect and discharge liquid permeate and to support the outer fine screen .

[0011] The filtrate on the outer fine screens forms a filter cake . For some applications, filter aids, such as diatomaceous earth, perlite, or activated carbon are used to form a filter cake precoating the pressure leaf filter before filtering the slurry. These filter aid cakes can achieve finer filtration. The filter cake grows during the filtration process . Flow resistance of the filter cake increases with increasing layer thickness . If the filter cakes grow too thick, the filter cakes on adj acent filter elements can grow together . Thicker filter cakes also result in increased pressure build-up and, consequently, in higher energy consumption during the filtering process . To avoid this, the filter cake has to be removed when it becomes too thick, e . g. by backflushing or vibration.

[0012] To remove the filter cake it must first be dried for 10 - 20 minutes . In a next step, the filter is vibrated for about 5 minutes to shake off the dried cake . These steps interrupt the filtration process and substantially increase overall process costs . If the filter cake is too sticky it can be necessary toopen the pressure vessel for manual cleaning of the filter leaves . During manual cleaning wires can be damaged or broken.

[0013] Even if the filter cakes are removed regularly, the filter openings gradually become clogged with filter cake residues . Some filter aids, such as diatomaceous earth, are acidic . Clogging residues of diatomaceous earth can oxidise the wires .

[0014] Another problem with wire mesh screens is that it is hard to connect the filter edges to the peripheral frame without leaks . If filter cake material, in particular filter aid material, leaks through the fine screen mesh, the batch must be reprocessed and the downstream parts of the system need to be cleaned .

[0015] It was also found that the prior art cake filtration filters generate a substantial pressure drop and leave part of the filtrate in the cake .

[0016] It has been proposed to make fine screens for pressure leaf filters by electroforming. However, this electroforming typically makes use of metals that are less suitable for use in food industry. These pressure leaf filters are not resistant to the disinfectants commonly used in practice, and have a short technical lifespan.

[0017] It is an obj ect of the present invention to provide filter leaf for a pressure leaf filter with a lower pressure drop, that allow easier and more effective filtration at a lower cost of ownership, leaving less filtrate in the filter cake .

[0018] SUMMARY

[0019] The obj ect of the invention is achieved with a filter leaf comprising a stack of at least one support screen and an outer fine screen at one or both outer sides of the stack, wherein the outer fine screen is a stainless steel sheet with machined filter openings . It was found that such screens can be cleaned much more efficiently and easily. It was also found thatsuch outer screens can be connected to the peripheral frame with substantially less risk of leakage . Surprisingly, it was also found that these screens show 5 - 10% more production capacity per cm2 filter surface, compared to the usual mesh filters . The filter leaf also has a substantially improved mechanical strength and rigidity. The better mechanical strength makes it also possible to use less support layers . For example, instead of the commonly used 5-layer structure, a 3-layer structure can be used.

[0020] In a specific embodiment, the filter openings are laser cut, e . g. , by pulse laser cutting. This results in openings with plane inner walls, allowing a smooth flow of permeate liquid and efficient cleaning.

[0021] Alternatively, the filter openings can be made by photo-chemical machining, using a photoresist and etchants to corrosively machine the filter openings .

[0022] Flow through the filter openings can be further smoothed by electro-polishing the screen.

[0023] The filter openings can have any suitable shape or size, e . g. , dependent on the type of material to filtered and process conditions such as pressure . The filter openings can for example be slot shaped. This allows combining a relatively large open area with a relatively high mechanical strength. Slotshaped openings are less susceptible to clogging, so the operational batch times are longer . Slot shaped filter openings maintain a higher flux per hour . The slot shaped openings can be parallel and aligned or staggered relative to the slot shaped openings of a next row.

[0024] The filter openings can have any suitable width, dependent on the particle size distribution of the solids in the slurry to filtered. For openings that have a width that varies in flow direction ( for example, openings that taper in crosssection of the filter leaf ) , the width refers to the smallest width. The width will typically be in the range of about 20 -about 200 micron, but can also be larger or smaller, if so desired. Smaller openings, e . g. , having a width of 20 - 50 micrometer are particularly suitable if no filter aid is used. Filter openings with a larger width, e . g. , up to about 200 micrometer, e . g. , about 100 - 150 micron, are particularly suitable for use without a filter aid.

[0025] The filter elements can have any suitable total open area, dependent on the type of material to filtered and process conditions such as pressure . The open areas of the outer fine screens are less than the open area of the support screen and can for example have a total open area of at least 15 % of the total surface .

[0026] The outer fine screens can have any suitable thickness, e . g. , dependent on the type of material to filtered and process conditions such as pressure . In practice, the thickness will be typically below 1 mm.

[0027] If the outer fine screen is used with a pressure leaf filter, the outer fine screen may be provided with unperforated side edge sections, without filter openings . These side edge sections can be connected to the peripheral frame in a leak tight manner . These sections can for example have a width of about 5 - 20 mm and may extend along at least a part of the contour of the filter leaf .

[0028] Good results are achieved if the filter openings of the outer fine screen are tapered in longitudinal cross section, e . g. , widening in a flow-through direction, i . e . , the flow direction of the liquid passing the filter during normal operation. It was found that this further reduces the risk of clogging. A process of laser cutting filter openings with such a tapered configuration is disclosed in WO 03 / 026832, herewith incorporated by reference .

[0029] The outer screens are made of a corrosion resistant metal, in particular steel, e . g. , stainless steel grade 316 or super duplex steel .Optionally, the filter element is a pressure leaf, the stack comprising at least one of said outer fine screen at either side of the support screen. In that case, the stack will typically be substantially flat, the stack being framed by a peripheral tube defining a discharge channel connected to a space between the two outer fine screens .

[0030] Alternatively, the filter element can be a candle filter element, the support screen and the at least one outer fine screen being coaxial tubes, the outer fine screen surrounding the support screen.

[0031] The invention also pertains to a process of filtering edible liquids, such as edible oils or beer, using a filter element comprising at least one fine screen of a stainless steel, with machined filter openings, in particular filter openings made by laser cutting, e . g. , by pulsed laser cutting. This filtering process can for example be carried out as cake filtration. However, it was found that for filtering edible liquids, in particular edible oils, no cake is required, particularly if the filter openings have a width of about 20 -50 micron. The filter element can for example be a tubular filter or a filter sheet .

[0032] BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The above-described aspects will hereafter be more explained with further details and benefits with reference to the drawings showing a number of embodiments by way of example .

[0034] DETAILED DESCRIPTION OF EMBODIMENTS

[0035] Figure 1 : shows a schematic drawing of an embodiment of a pressure leaf filter;

[0036] Figure 2 : shows a detail of a pressure leaf in cross section;Figure 3A: shows an exemplary embodiment of a pressure leaf with the filter stack layers have been partly broken away in stages;

[0037] Figure 3B : shows in detail a corner section of the pressure leaf of Figure 3A.

[0038] Figure 4 : shows a detail of an outer screen of the pressure leaf of Figure 3 in front view;

[0039] Figure 5 : shows a detail of the outer screen of Figure 4 in cross section;

[0040] Figure 6 : shows an exemplary embodiment of a candle filter;

[0041] Figure 7 : shows a cross section of a filter element of the candle filter of Figure 6.

[0042] Figure 1 schematically shows a vertical pressure leaf filter 1 for separating a liquid slurry or suspension comprising solid particles into a cake of filtered solids and a liquid filtrate . The pressure leaf filter 1 comprises a vertical cylindrical pressure vessel 2 with a dome-shaped top end 3, a conical lower end 4 narrowing in downward direction and a substantially cylindrical middle part 5.

[0043] In Figure 1, the cylindrical middle part 5 is shown in cross section to show the internals . A series of parallel plate shaped vertical pressure leaves 6 is arranged within the cylindrical middle part 5.

[0044] Figure 2 shows one of the filter leaves 6 in cross section. Each one of the filter leaves 6 comprises a tubular peripheral frame 7 holding a filter stack 8. At the top ends of the filter leaves 6, the tubular peripheral frame 7 is connected to a supply channel 9 (see Figure 1 ) for the supply of back flow liquid via an inlet 10 for flushing the filter leaves 6. The lower ends of the tubular peripheral frames 7 are connected to a collecting channel 11 for collecting and discharging clean filtrate liquid via an outlet 12.The filter stack 8 of each filter leaf 6 includes two outer screens 13, and a support screen or drain weave 14 between the two outer screens 13. Optionally one or more wire mesh layers can be present between the support screen 14 and the outer screens 13. The support screen 14 serves to space the outer screens 13 from each other to form a hollow interior 15 for collecting liquid filtrate flowing downward to the collecting channel 11.

[0045] Back to Figure 1, the vessel 2 of the pressure leaf filter 1 has an inlet 16 for a slurry or suspension to be filtered. The slurry is transferred into the vessel 2 and pressurized to force the slurry to pass the outer screens 13 of the filter stack 8. Solids in the slurry form a cake of growing thickness on the outer surfaces of the outer screens 13. The cake itself also serves as a filter . Liquid filtrate passes the outer screens 13 flowing in the interior space 15 between the two outer screens 13 and downward into the collecting channel 11 for discharge via the outlet 12.

[0046] The filter leaves 6 can be cleaned to blow a back flush gas or liquid via the inlet 10 and the supply channel 9. Other cleaning mechanisms can also be used.

[0047] Figures 3A and 3B shows an alternative filter leaf 6' in perspective view, with the respective layers of the filter stack stepwise broken away. The filter leaf 6' has additional support weaves 17 between the support screen and the outer screens 13. These additional support weaves 17 have a finer weave than the central support screen 14 and serve to increase mechanical rigidity and strength of the filter stack 8. However, with the laser cut outer screens 13 of the present invention, such additional supports may become redundant for most cases . The outer screens 13 of the filter stack 8 are made of a corrosion resistant metal, in particular steel, having arrays of filter openings 18 which are too small to be visible in Figures 3A and 3B, but are shown on a large scale in Figure 4. Thesefilter openings 18 are slot shaped and made by laser cutting photochemical machining, or similar machining processes . In the shown embodiment of Figure 4, the filter openings 18 are slot shaped, e . g. , having a length of about 1 to about 1, 3 mm and a width of about 80 to about 130 micron.

[0048] Other patterns or sizes of filter openings 18 can also be used, depending on the type of use and the slurries to be filtered .

[0049] Edge sections 19 of the outer screens 13 are free of filter openings 18. The tubular peripheral frame 7 is mounted to these opening-free edge sections 19 of the outer screens 13.

[0050] Figure 5 shows a filter opening 18 in a longitudinal cross section along line V-V in Figure 4, i . e . a cross section though a plane perpendicular to the upstream and downstream surfaces of the filter leaf 6 spanning the shortest width of the filter openings 18. The filter openings 18 widen in flow direction A of the liquid filtrate . Consequently, the filter opening is narrower at upstream side 20 than at the downstream side 21. This helps to avoid clogging of the filter openings 18. In the shown embodiment, the filter opening 18 has a trumpet shape . Other tapering configurations can also be used.

[0051] Figure 6 schematically shows an exemplary embodiment of a candle filter 30, with a vessel 31. In the drawing, the left side of the vessels' wall is broken away to show the internals . The vessel 31 is cylindrical with a conical bottom end 32 and a convex top end 33. A substantially horizontal support plate 34 separates the internal space of the candle filter 30 into a lower filtration section 35 and an upper filtrate discharge section 36.

[0052] Candle filter elements 37 hang down from openings in the support plate 34. The candle filter elements 37 are tubular having open top ends 38, closed bottoms 39 and a tubular wall 40 with filter openings .The conical bottom end 32 of the candle filter 30 is provided with an inlet 41. The filtrate discharge section 36 is provided with an outlet 42. A fluid to be filtered enters the filtration section 35 via the inlet 41 and is forced through the filter openings of the candle filter elements 37 and then flows upward through the open top ends 38 of the candle filter elements 37 into the filtrate discharge section 36 and subsequently through the outlet 42.

[0053] Figure 7 shows a cross section of a candle filter element 37. The candle filter element 37 has a central tubular support screen 43 and a tubular outer fine screen 44 coaxially surrounding the support screen. The support screen has radial ribs 45 spacing the outer fine screen 44 to define a drain channel 46. The outer fine screen 44 has first filter openings 47. Although the drawing shows these filter openings 47 as relatively large, the filter openings 47 are laser cut openings approximately 80 - 130 micrometer wide, as discussed in relation to Figure 4 .

[0054] The central support screen 43 has second filter openings 48. The first filter openings 47 of the outer fine screen are smaller than the second filter openings 48 of the support screen 43, but the total open area of the first filter openings 47 is less than the total open area of the second filter openings 48.

[0055] The outer fine screen 44 is precoated with a layer 49 of a filter aid, such as such as diatomaceous earth, perlite, or activated carbon.

Claims

CLAIMS1. Filter element, such as a candle filter or a pressure leaf filter, the filter element comprising a stack of a support screen and at least one outer fine screen, wherein the at least one outer fine screen is a stainless steel sheet with machined filter openings .

2. Filter leaf according to claim 1, wherein the filter openings are laser cut, e . g. , by pulsed laser cutting.

3. Filter leaf according to claim 1 or 2, wherein the filter openings are machined by photo-chemical machining.

4. Filter leaf according to any one of the preceding claims, wherein the filter openings are slot shaped.

5. Filter leaf according to any one of the preceding claims, wherein the filter openings of the at least one outer fine screen make up a total open area of 35 % of the total surface .

6. Filter leaf according to any on of the preceding claims, wherein at least one outer screen is electropolished.

7. Filter leaf according to any one of the preceding claims, wherein at least one of the outer screens is provided with a peripheral edge without filter openings .

8. Filter leaf according any one of the preceding claims, wherein at least a part of the filter openings widens in a flow-through direction.

9. Filter element according to any one of the preceding claims, wherein the support screen and the at least one outer fine screen are coaxial tubes, the outer fine screen surrounding the support screen.

10. Filter element according to any one of the preceding claims 1-8, wherein the stack comprises at least one of said outer fine screens at either side of the support screen.

11. Filter element according to claim 10, wherein the stack is substantially flat, the stack being framed by a peripheral tube defining a discharge channel connected to a space between the two outer fine screens .

12. Process of filtering edible liquids, such as edible oils or beer, using a filter element comprising at least one fine screen of a stainless steel, with machined filter openings, in particular filter openings made by laser cutting, e . g. , by pulsed laser cutting.

13. Process according to claim 12, wherein filtering is carried out as cake filtration.

14. Process according to claim 12 or 13 using a tubular filter or a filter sheet as said filter element .

15. Process according to any one of claims 12 - 14, wherein the machined filter openings have a width of 20 - 50 micrometer .

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