WO2007017446A1

WO2007017446A1 - Self-cleaning filter

Info

Publication number: WO2007017446A1
Application number: PCT/EP2006/064980
Authority: WO
Inventors: Valerio Vernocchi
Original assignee: Valerio Vernocchi
Priority date: 2005-08-05
Filing date: 2006-08-02
Publication date: 2007-02-15
Also published as: EP1909941A1; ITMI20051553A1; WO2007017446B1

Abstract

A filter module for fluids, comprising a pleated filtering membrane (6) the surface of which separates a pre-filtration volume, upstream of the filtering membrane (6), from a post-filtration volume, downstream of the filtering membrane (6), said pleated filtering membrane (6) presenting, on its surface, pleats bounded by contiguous crests (7), said filtering membrane comprising, on the pre-filtration volume side, a conveyor (10) for causing the filtrate to flow substantially between the pleats of the pleated filtering membrane (6).

Description

SELF-CLEANING FILTER

The present invention relates to a self-cleaning filter in accordance with the introduction to the main claim. Liquids and fluids in general, the word "fluids" including both liquids and gases, are filtered by modules of various types, of which those using filtering baffles and membranes are the most effective and less bulky. Depending on the diameter of the particulate to be retained, filtration is divided into microfiltration, ultrafiltration, nanofiltration and reverse osmosis.

For simplicity of description, all filtration processes including reverse osmosis are indicated in the text of the present description by the generic term "filtration". Instead, the generic term "filtering membrane" or simply "membrane" indicates both the filtering baffles and the membranes used in filtration and reverse osmosis processes.

The filtrate capacity of a filter module depends on the membrane surface, other parameters being equal. For this reason many filter modules use pleated membranes to present a large surface of reduced volume. The filtrate capacity is also proportional to the transmembrane pressure. Precisely for this reason a filter module with an indeformable permeable support (patent application MI2004A000070) has been recently proposed to enable high transmembrane pressures to be applied, with increased production. In the present state of the art, the problem of the soiling and subsequent cleaning of filter modules arises in filtration. Pleated membranes comprise many interstices in which impurities easily accumulate. Increased transmembrane pressure produces a faster impurities accumulation, to completely clog the filter module within a short time. Various methods and devices are currently used to wash the membrane and regenerate the filter. The filtration process is normally suspended during washing. In some cases the membrane is washed by high velocity liquid jets. In other cases the membrane is hand washed, after extracting it from the filter. Sometimes, the normal flow direction is reversed to remove the clogging. Filter modules other than the membrane type also exist, which perform their filtering action by a series of capillaries through which the liquid to be filtered passes. The high fluid velocity within the capillaries maintains them clean by a mechanical action. These types of filter modules have however considerable drawbacks compared with membrane filter modules. Firstly the transmembrane pressures are necessarily low. If then one of the capillaries breaks, the impurities pass to the filtrate side and the entire filter module has to be replaced.

Hence the soiling and the consequent clogging of the filter module constitutes a serious problem in membrane filter modules. An object of the present invention is therefore to provide a membrane filter module by which the said drawbacks are overcome.

A particular object is to provide a membrane filter module of reduced soiling, which hence only rarely requires washing with interruption of the filtration process. Another object is to provide a robust filter module of low cost. Said objects are attained by the filter module the inventive characteristics of which are defined by the claims. The invention will be more apparent from the ensuing detailed description, provided by way of non-limiting example, of three embodiments shown in the accompanying drawings in which:

Figure 1 shows a longitudinal section through a first preferred embodiment of the filter module of the invention;

Figure 2 shows an exploded perspective view of that part within the filter module casing in the first embodiment;

Figure 3 shows a second embodiment of the filter module without the casing; Figure 4 shows a third embodiment of the filter module with an indeformable permeable support for the filtering membrane;

Figure 5 shows a detail of the end of the indeformable permeable support in the third embodiment;

Figure 6 shows a fourth embodiment with an indeformable permeable support which internally presents a cylindrical cavity.

From Figure 1 it can be seen that the membrane filter module, in its first embodiment, comprises an outer casing 1 with an inlet aperture 2 and an outlet aperture 3, and within which a filtrate collection tube 4 is disposed, surrounded by a pleated filtering membrane 6. Within the portion surrounded by the filtering membrane 6, the collection tube 4 presents a lateral surface permeable to the filtered liquid, by virtue of the presence of a plurality of small holes represented in the drawing of Figure 1 by dotting.

The impermeable lower end of the collection tube 4 emerges from the outer casing 1 for filtrate outflow. The pleated filtering membrane 6 forms a laterally closed cylindrical surface, with a substantially star-shaped cross-section. The pleating of the membranes 6 is formed by pleats disposed longitudinally, each bounded by contiguous crests 7. The filtering membrane 6 divides the filter module into a pre-filtration volume in which the fluid to be filtered is present before passing through the membrane 6 (upstream of the membrane 6), and a post-filtration volume in which the filtered fluid is present after passing through the membrane 6 (downstream of the membrane 6). The upper end of the cylindrical surface formed by the filtering membrane 6 is sealed by a first cap 8, the lower end being sealed by a second cap 9. A conveyor 10, which in this embodiment is a cylindrical jacket of circular cross-section, presents an active surface 19, in this case the inner surface, tangential to the crests 7 formed by the vertices of the pleats on the filtering membrane 6. These crests 7, and the valleys formed by the pleating, can present a sharp or rounded edge. The outer surface of the jacket 10 is in contact with the inner surface of the casing 1. From the drawing of Figure 1 it can be seen that said jacket has a thickness S. Said jacket has a height H less than the height K of the filtering membrane 6 and is disposed substantially about the intermediate central part of the filtering membrane 6, so that the ends of the cylindrical surface formed by the filtering membrane 6 emerge from the jacket 10 by a length L. The thickness S of the jacket 10 enables the fluid to be filtered to pass about the caps 8 and 9 disposed on the ends of the cylinder formed by the filtering membrane 6. A first and a second spacer 11 and 12 are disposed on the ends of the filtering membrane 6, below the caps 8 and 9, to maintain the jacket 10 in a substantially central position with respect to the surface of the filtering membrane 6, by virtue of the presence of arms 13 and 14 (visible in Figure 2). In the operation of the self-cleaning filter the fluid to be filtered enters the filter module casing 1 through the inlet aperture 2. It then, compelled by the conveyor 10, enters between the pleats of the filtering membrane 6, to move within their interior in a substantially longitudinal direction. The unfiltered fluid leaves the pleats of the filtering membrane 6 in proximity to their upper end and emerges from the casing 1 through the outlet aperture 3. During its path in contact with the filtering membrane 6, a part of the fluid to be filtered passes through this membrane, to hence undergo filtration. The filtered fluid then enters the collection tube 4 through the plurality of holes present in the surface of said collection tube 4 and flows out of the casing 1 through the open lower end of the collection tube 4, which is free of holes in its lateral walls. The fluid leaving through the outlet aperture 3 is then again fed several times into the casing 1 through the inlet aperture 2 by means of a forced circulation system using a pump. The fluid path within the casing 1 is shown by arrows.

As the fluid to be filtered during normal operation of the filter module is compelled by the active surface of the conveyor 10 to transit longitudinally between the pleats of the filtering membrane 6, a sufficiently fast flow is created between the pleats to prevent impurities depositing on the upstream surface of the filtering membrane 6.

The jacket 10 can be advantageously formed by an inflatable air chamber of suitable shape and dimensions.

The filtering membrane 6 may be internally supported by an indeformable permeable support such as that described in patent application MI2004A000070 to enable a higher transmembrane pressure to be applied and increase filtrate production. The first and second cap 8 and 9 are formed by positioning on the ends of the cylindrical surface formed by the filtering membrane 6 two rigid discs of slightly larger diameter than the end diameter of said cylindrical surface. The end diameter means the diameter of a cylinder of circular directrix which completely contains the filtering membrane 6 and which is substantially tangential to the crests 7. The two ends of the pleated filtering membrane together with the rigid discs are then immersed in a molten plastic material, which on solidifying seals them. The function of the rigid discs is to create, together with the solidified plastic material, a barrier impenetrable to the fluid. In the described embodiment, the function of the rigid discs is performed by the spacers 11 and 12. The function of the conveyor 10 is essentially to block radial transit of the fluid to be filtered and to force it to move substantially longitudinally within the pleats in contact with the filtering membrane 6. It must cover the pleated filtering membrane 6 while remaining tangential to the pleating crests 7, in order to create a plurality of flow tubes, each of which is defined by a pleat of the membrane 6 lying between its contiguous crests and the active surface 19 of the conveyor 10. It is not strictly necessary for the conveyor 10 to touch the crests of the pleated membrane 6, but this evidently improves the efficiency of the filter module by preventing part of the fluid from transiting outside the pleating and hence not in contact with the filtering membrane. The conveyor 10 must leave free two portions in proximity to the ends of the membrane 6, so that the fluid is able to pass about the caps and firstly enter between the pleats by moving with radial centripetal motion, and then leave the pleats with radial centrifugal motion. The conveyor 10 presents a thickness S which enables the fluid to be filtered to move longitudinally in proximity to the ends of the filtering membrane 6, before the fluid enters between the pleats with radial movement, to then pass about the caps 8 and 9. The conveyor 10 can be advantageously formed from an inflatable air chamber, to obtain in this manner a greater lightness and lesser problems of contact between the conveyor 10 and the crests 7 of the pleated membrane 6. An inflatable conveyor 10 can also be more easily assembled, as it can be mounted in its deflated state by drawing it over the end, even after the first and second cap 8 and 9 have been formed, to be then inflated.

The conveyor 10 can also be formed in one piece with the casing 1 and consist of a reduction in the inner diameter thereof. Depending on the dimensions of the pleating and the fluid viscosity, it may be advantageous for the conveyor to present an undulated cylindrical surface which follows the pleating of the filtering membrane 6 and partially penetrates between the pleats of the filtering membrane without completely obstructing the flow tubes formed by the pleats and the active surface, but reducing its passage cross-section and hence increasing the flow velocity therein.

The casing 1 does not necessarily have to be of closed cylindrical shape with the pleated membrane being closed about a tube and hence presenting a closed cylindrical envelope. Figure 3 (in which corresponding elements carry the same numbers as those of Figures 1 and 2 increased by 100) shows a second embodiment in which the pleated filtering membrane 106 is disposed in a plane and the conveyor 110 is disposed in proximity to the crests of the pleating within the pre-filtration volume. The same figure shows the first and second cap 108 and 109 disposed on the end edges of the filtering membrane 106. The fluid flow is indicated by the arrows. The filtrate is collected below and is indicated by the downward facing arrow. This second embodiment is not conceptually different from the first and is obtainable therefrom by making the diameter of each element of closed cylindrical form tend to infinity. Figure 4 (in which corresponding elements carry the same numbers as those of Figures 1 and 2 increased by 200) shows a third embodiment in which on the ends of the filtering membrane 206 a first cap 208 and a second cap 209 (not visible in the figure) are present, they being of star shape with arms which follow the pleats of the filtering membrane 206 on the post-filtration volume side, so that from the pre-filtration volume side the fluid is able to enter the pleats by moving substantially in a straight line, as shown by the arrows, without having to pass around the caps. Although this embodiment comprising star-shaped caps is more complicated, it has the advantage that the conveyor 210, tangential to the crests, can extend for the entire length of the filtering membrane, as the fluid to be filtered is able to penetrate into the flow tubes formed between the membrane 206 and conveyor 210 directly from the ends. In this embodiment the two spacers serve no purpose and the conveyor thickness is not important. In this case the conveyor and casing can be the same element, without any diameter reduction being required at the central lateral part of the membrane 206. To support the transmembrane pressure the caps 208 should be made of a rigid material with a plastic cover to obtain the necessary seal. This results in greater production difficulty for this third embodiment.

If on the filtrate side an indeformable permeable support 216 for the membrane 206 (Figure 5) is present (as in the said MI2004A000070) of prismatic form with a substantially star-shaped cross-section, the indeformable permeable support advantageously presents at its two ends a rim 215 which extends beyond the thickness of the filtering membrane 206 resting on the indeformable permeable support 216. In this case the cap 208 can be made by immersing in molten plastic material the ends of the indeformable permeable support 216 surrounded laterally by the filtering membrane 206 and then allowing the plastic material to solidify. The rim 215 of the indeformable permeable support 216 ensures a good seal for the cap 208 when made in this manner. A fourth embodiment can also be provided (Figure 6) comprising a prismatic indeformable permeable support 316 presenting an inner cylindrical cavity of substantially star-shaped cross-section and a pleated filtering membrane 306 disposed adhering to the lateral surface of said internal cavity. In this case the conveyor 310 comprises a cylinder closed at its ends to be inserted into the cylindrical internal cavity of said indeformable permeable support 316, to create a plurality of flow tubes. The fluid motion is shown in Figure 4 by the arrows. It can thus be seen that the fluid to be filtered enters from the ends into the flow tubes formed by the pleats of the filtering membrane and the lateral surface of the conveyor 310, moving substantially in a straight line. Part of it passes through the membrane 306, being filtered and leaving laterally from the indeformable permeable support 316. Again in this case the conveyor can present an undulated active surface to be partially inserted between the pleats and reduce the cross-section of the flow tubes but without obstructing them, in order to increase the fluid velocity and hence its capacity to remove impurities.

Claims

1. A filter module for fluids, comprising a pleated filtering membrane (6) the surface of which separates a pre-filtration volume, upstream of the filtering membrane (6), from a post-filtration volume, downstream of the filtering membrane (6), said pleated filtering membrane (6) presenting pleats bounded by contiguous crests (7) on its surface, characterised by comprising, on the pre-filtration volume side, a conveyor (10) for causing the fluid to be filtered to flow substantially between the pleats of the pleated filtering membrane (6).

2. A filter module as claimed in claim 1 , characterised in that said conveyor (10) comprises an active surface (19) disposed at the crests (7) of the filtering membrane (6).

3. A filter module as claimed in claim 2, characterised in that said active surface (19) of the conveyor (10) is tangential to the crests (7) of the pleated filtering membrane (6), to form, together with the pleats of the filtering membrane (6), flow tubes through which the fluid to be filtered is conveyed.

4. A filter module as claimed in claim 1 , characterised by comprising a casing (1 ) within which the pleated filtering membrane (6) is disposed.

5. A filter module as claimed in claim 1 , characterised in that said filtering membrane (6) is closed in such a manner as to form, bounded by a lower end and an upper end, a laterally closed cylindrical surface of substantially star-shaped cross-section with pleats disposed in the longitudinal direction.

6. A filter module as claimed in claims 2 and 5, characterised in that said conveyor active surface (19) is defined by the inner surface of a cylindrical jacket (10) which surrounds the cylindrical surface of star- shaped cross-section formed by the pleated filtering membrane (6).

7. A filter module as claimed in claims 2 and 4, characterised in that said active surface (19) forms part of the inner surface of the casing (1 ).

8. A filter module as claimed in claim 6, characterised in that said conveyor is a cylindrical jacket (10) of circular-ring cross-section, presenting two ends (17 and 18) and an inner active surface (19) which surrounds the pleated filtering membrane (6) forming a cylindrical surface of star-shaped cross-section.

9. A filter module as claimed in claim 6 or 8, characterised in that said jacket (10) is disposed about the central part of the cylindrical surface of the filtering membrane (6), the ends of the filtering membrane (6) emerging by a predefined length (L) from said jacket (10), to form fluid passage apertures.

10. A filter module as claimed in the preceding claim, characterised in that two spacers (11 , 12) are disposed on the ends of the filtering membrane (6) to maintain the position of the jacket (10) substantially central on the lateral surface of the filtering membrane (6).

11. A filter module as claimed in claim 5, characterised by comprising two caps (8, 9) each disposed on one of the ends of the filtering membrane (6), said caps (8, 9) preventing fluid passage from the pre- filtration volume to the post-filtration volume and vice versa, through the ends of the cylinder formed by the filtering membrane (6).

12. A filter module as claimed in claims 4 and 6, characterised in that the outer surface of said jacket (10) is in contact with the inner wall of the casing (1 ).

13. A filter module as claimed in claim 6, characterised in that said jacket (10) comprises an inflatable air chamber.

14. A filter module as claimed in claim 1 , characterised by comprising two caps (208), each disposed on an end edge of the filtering membrane (206), at least one of said caps (208) presenting arms which follow the pleats of the filtering membrane (206) such that from the pre-filtration volume side the fluid is able to enter the flow tubes by moving substantially in a straight line.

15. A filter module as claimed in the preceding claim, characterised in that the active surface (219) extends at said caps (208) as far as the end edge of the filtering membrane (206).

16. A filter module as claimed in claim 2, characterised in that said active surface is longitudinally undulated such as to partially penetrate between the pleats of the filtering membrane.

17. A filter module as claimed in claim 1 , characterised in that said pleated filtering membrane (206) is disposed adhering to the lateral surface of a prismatic indeformable permeable support (216) substantially of star-shaped cross-section.

18. A filter module as claimed in the preceding claim, characterised in that said indeformable permeable support (216) presents at its ends a rim

(215) which extends beyond the thickness of the filtering membrane (206) when resting on the indeformable permeable support (216).

19. A filter module as claimed in claim 1 , characterised by comprising a prismatic indeformable permeable support (316) presenting a cylindrical internal cavity of substantially star-shaped cross-section, said pleated filtering membrane (306) being disposed adhering to the lateral surface of said internal cavity.

20. A filter module as claimed in claim 19, characterised in that said conveyor (310) comprises at its ends a closed cylinder to be inserted into the cylindrical internal cavity of said indeformable permeable support (316).