EP2419201A1

EP2419201A1 - Tubular filter with stellated inner cross-section

Info

Publication number: EP2419201A1
Application number: EP10715320A
Authority: EP
Inventors: Andrew Clark; Roger White
Original assignee: FAIREY FILTRATION SYSTEMS Ltd
Current assignee: MANTEC TECHNICAL CERAMICS Ltd
Priority date: 2009-04-18
Filing date: 2010-04-16
Publication date: 2012-02-22
Also published as: GB201006421D0; GB2469582A; WO2010119258A1; GB0906751D0; GB2469582B

Abstract

A tubular filter (1) is described, which comprises a porous material (2) defining a passage (3). The porous material has an inner surface (6) having finer pores than an outer surface (7). The cross section of the inner surface (6) is castellated or stellated, and comprises an odd number of projections.

Description

TUBULAR FILTER WITH STELLATED INNER CROSS-SECTION

The present invention relates to filters and more particularly to porous tube filters in which a flow of fluid including particulate matter is presented to the filter whereby through cross flow particulate matter is removed from the fluid.

It is known from a number of documents and prior arrangements to create tubular cross flow filters and filtration systems. In such filtration systems and filters a liquid which contains particulate matter, whether in suspension or dissolved, is arranged to flow across the filter having a porous medium. The porous medium has a pore size small enough to prevent passage of the particulate matter but allows the fluid to flow across the medium generally outwards but possibly in an inwards direction. The fluid which passes through the porous medium is typically referred to as the permeate whilst the fluid which is retained within the filter is called the retentate. It will be understood a problem with such filtration systems relates to clogging or blinding of particularly the inner surface by embedded and deposited particulate matter from the fluid to be filtered. Ideally the retentate and the fluid should create sufficient surface turbulence to minimise such clogging.

In order to achieve surface turbulence a number of techniques have been utilised. For example as described in UK patent number 2223690B an inner surface or layer is defined for the filter which typically incorporates finer pores than the outer layer. The inner layer is stellated and typically has a spiral configuration to maximise turbulence in the flow through the filter. Such configuration and structures as indicated create turbulence within the flow to avoid or inhibit clogging upon the usually finer pores of the inner surface. Nevertheless, such an approach is not idealised in that typically in order to achieve supposedly desirable filtration processes, that is to say even flow through the porous material generally a symmetrical and even number of stellations is taught. By such an approach previously it was considered an even distribution upon the porous material would occur and therefore greater benefits with regard to filtration efficiency and use of the available filter porous material surface area.

In accordance with aspects of the present invention there is provided a filter comprising a porous material extending between an inner surface having finer pores than an outer surface, the porous material defining a passage with the inner surface castellated or stellated with an odd number of castellations or stellations projecting towards a centre of the passage.

Typically, the castellations or stellations have a principal axis substantially normal to the outer surface. Generally, the castellations or stellations are evenly distributed about the inner surface. Generally, the castellations or stellations are symmetrical in predetermined cross sections of the filter. Generally, the castellations or stellations are symmetrical about a principal plane.

Generally, the inner surface defines a spiral along the length of the passage. Generally, the spiral is of a consistent spiral pitch length. Alternatively, the spiral has a variable pitch length along the passage. Possibly, the spiral has up to 500 turns per metre. Further alternatively, the inner surface defines a straight passage along the filter from one end to the other end.

Generally, the inner surface is formed from a synthetic polymer in a porous form. Possibly, the porous material itself comprises a metal, particulate stone, ceramic or synthetic polymer compositions. The ceramic compositions may be based on aluminium and/or zirconium oxides or other suitable materials. The polymer compositions may be polyamides, such as nylons, cellulosic polysulphones, polyethylenes and polytetra fluroethyline (PTFE).

Possibly, the inner surface is configured to provide filtration in a micro- or ultra- filtration range. Possibly, a trough portion between castellations or stellations is flat. Alternatively, a trough portion between the castellations varies in terms of thickness between the inner surface and the outer surface thereabout.

Also in accordance with aspects of the present invention there is a provided a filtration system comprising one or more filters as described above associated with a source of fluid flow whereby the filter separates the fluid flow between a retentate and a permeate either side of the porous material.

Further in accordance with aspects of the present invention there is provided a method of removing liquid from a liquid medium containing suspended or dissolved solids whereby the liquid medium is caused to flow through a filtration system as described above such that the liquid medium is separated into the permeate and the retentate with solids remaining substantially in the retentate.

An embodiment of aspects of the present invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a schematic illustration of a filter in accordance with aspects of the present invention;

Figure 2 is a schematic illustration of alternative passage cross sections in accordance with aspects of the present invention; and,

Figure 3 provides schematic illustrations of a filter in accordance with aspects of the present invention in terms of flow cross-sections.

As indicated above filters of a tube or passage nature are known. Figure 1 provides a schematic side cross section of a filter 1 comprising a porous medium 2 which defines a passage 3 through which a fluid flow 4 passes in the direction of the arrowheads. In accordance with filtering operation a permeate flow 5 passes through the porous medium or media 2 leaving a retentate 4a continuing along the passage 3. In such circumstances the fluid 4 passes along the passage 3 in the direction of the arrowheads and the permeate 5 is rendered substantially free of solids on the other side of the medium 2. It will be understood that generally an inner surface 6 will have a finer pore size in comparison with an outer surface 7 as this is generally the surface upon which separation of the solids from the flow 4 is achieved. Such narrow pore sizes towards the surface 6 in comparison with larger pore sizes towards the outer surface 7 aids a flow pressure gradient. It will be understood that it is important that the flow 4 within the passage 3 is turbulent such that the fine pore surface 6 does not become overly clogged or as it is referred to in the technology "surface blinded" with deposits of solids rendering the filter less than totally functional. It will be understood that clogging of the surface 6 will lead to that surface becoming totally or partially impermeable and therefore little or no permeate liquid will pass through the medium 2.

Generally filters in accordance with aspects of the present invention will be of a tube construction. However, what is most important is creating turbulence in the filter 1 and in such circumstances it is known to provide multi filter or tube constructions. In such arrangements within a cross section several tubes or channels or passages will be created along which the flow to be filtered passes whereby the permeable porous medium or material removes solids from the flow appropriately.

Filters can be formed from a base ceramic and porous material or alternatively the inner surface of the filter may be provided with a porous surface film. The film will clearly be permeable to allow separation of solids from the retentate flow into the permeate flow. In any event it is the porous medium which separates out the solids and it is important that the inner layer is inhibited from premature clogging. This inhibition to clogging is achieved by creating turbulence. A traditional filter tube simply comprises a generally round centre passage along which the flow passes under pressure such that there is flow through the porous material in order to separate the solids from the flow or retentate to leave the permeate on the other side towards the outer surface of the filter. Such circular or oval or other smooth surface constructions themselves do not generally create a high turbulence relatively within the filter. Typically turbulence with such traditional circular or smooth surfaced filters has been achieved through the pumping process of pumping the fluid at a high flow rate through the filter.

More recently as described in UK patent number 2223690B it has been known to provide castellated or stellated surfaces. The stellated or castellated cross section increases the effective surface area of the inner flow channel or passage and decreases the cross section of that flow channel or passage. In such circumstances there is a relative reduction in fluid mass flow rate compared to the velocity of the liquid at the filter surface, that is to say adjacent to the inner surface. It will be understood that effectively by such stellations or castellations there is greater contact with the fluid flow and therefore turbulence created. The stellated or castellated surface increases the tendency for turbulent flow to take place so that lower flow rates can be used relatively or comparatively within a filter without clogging.

In order to achieve effective filtering it will be understood that there will generally be a recommended surface velocity at the filter surface, that is to say the surface or inner surface where there is separation of solids from the flow in order to create the permeate. Generally such flow rates will be in the range of 4 to 6 metres per second. Thus, with regard to a filter tube having a 4mm internal diameter such considerations lead to a flow rate of 0.27m³ per hour. Furthermore, such a tube has a surface area of about 0.012m³ per metre length for a circular passage. However, with regard to passages which have a stellated or castellated surface increases in engaged surface area by the fluid can be in the order of about 25% above cylindrical or circular cross section passages having the same diameter. Such greater surface engagement and therefore turbulence can lead to a reduction in the necessary flow rate for adequate turbulence to inhibit clogging and surface blinding by in the order of 50%. The stellations or castellations induce turbulence which may provide even greater reductions in necessary flow rate generated by a pump for adequate filter life in comparison with clogging rates. In such circumstances stellated or castellated surfaces generally are preferable on a like for like basis with their reduced necessity for high flow rates for turbulence and therefore greater acceptability with less pumping requirements.

A further feature with regard to increasing relative turbulence is to provide within the passage a spiral along its length. It is known to provide spirals which have up to 500 turns per metre. Combining a spiral shape passage along the length of the filter with stellations or castellations in cross section further improves the filter efficiency and provides a high rate of permeate flow across the porous medium for a lower rate of pumped fluid flow through the filter than required by prior known filters having a cylindrical or circular cross sectional passage. Essentially turbulence is required as indicated to avoid clogging.

For illustration purposes typically filters in accordance with aspects of the present invention will be up to two metres in length but can be longer. Normally filters are provided in 600mm or 1200mm standard lengths. Furthermore, the channel or passage dimension of the filter may be in the range of 3mm to 10mm in diameter and outer dimensions in the range 20mm to 30mm. The porous filter area provided by the stellated inner surface of tubes in accordance with prior stellated arrangements is greater than are available with a conventional tube having a circular inner surface. The number, as well as possibly, distribution of channels or passages will be determined in order to provide a desired surface area. Typically, there will be up to 20 channels or passages in a filter but the number will be sufficient to provide required surface area as indicated whilst being practical to form and manufacture with adequate strength for purpose.

Prior teaching in such circumstances was to provide stellations with regard to the inner surface in order to increase surface area available and therefore decrease the necessary flow rate to achieve adequate turbulence to avoid clogging. Unfortunately increased surface area also increases resistance to flow due to surface friction between the fluid flow and the increased surface contacted. Such resistance is translated into necessity for greater pump torque power which can inhibit and diminish pump reliability. What is required is a balance between achieving turbulence to avoid clogging with acceptable pump operation for reliability and convenience.

The approach with regard to stellations as indicated relates to increasing the available surface area for added turbulence. In such circumstances prior stellated and castellated inner surfaces for filters have been symmetrical in order to maximise the surface area gained provided by the stellations or castellations. It will be understood that by such symmetry a known surface area distribution is provided for even surface usage about the periphery of the filter. Aspects of the present invention provide asymmetric passages in terms of stellations or castellations which may be regularly or uniformly distributed but typically will not be completely symmetrical in all planes. It will also be understood that providing asymmetric passage cross- sections may allow in combination with spiralling and variation in asymmetry along the length of the filter improved turbulence or control of that turbulence within that filter. In some filters it may be advantageous to alter the turbulence at different positions along the length of the passage in the filter.

Figure 2 provides illustrations with regard to alterations in the filter passage in accordance with aspects of the present invention. Essentially, filter passages in accordance with aspects of the present invention have an asymmetry in that an odd number of stellations or castellations are provided rather than an even symmetrical distribution of such castellations or stellations in a symmetrical cross section. Although illustrated in figures 2a to 2e with five stellations or castellations it will be understood that a greater number of odd points or flats such as seven, nine or more could be provided as well as three stellations or castellations if required. By providing an odd number of castellations or stellations an increased surface area in comparison with a simple smooth round or oval passage cross section is achieved but with an asymmetry for enhanced turbulence.

Figure 2a illustrates a first alternative with regard to a passage 13 in a porous medium 12. The passage 13 has a five point star cross section with five stellations penetrating inwards towards the centre of the passage 13. In such circumstances there is an odd number of stellations which enhances turbulence whilst increasing surface area engagement with the flow and therefore necessary flow rate for filtering effect. The greater turbulence as described will reduce surface clogging and therefore overall filter efficiency.

Figure 2b illustrates a further alternative with regard to aspects of the present invention in which again five stellations are presented inwards of a passage 23 in a porous medium 22. However, between the stellations land portions 24 are provided to again alter the cross section and flow characteristics with greater turbulence in operation.

Figure 2c again illustrates five stellations with lands 34 in a passage 33. The passage 33 is presented within a porous medium 32. In comparison with prior arrangements it will be noted that the depth of penetration for the stellations towards the centre of the passage 33 varies at different peripheral locations. The variations are exaggerated for clarity in comparison with a portable practical embodiment. This variation may be consistent along the length of the filter medium 32 and therefore the filter in accordance with aspects of the present invention. Alternatively the variation may alter along the length to again alter turbulence flow rates and therefore create greater turbulence and so improve filter efficiency.

Figure 2d provides a further alternative in that rather than a pointed, or nearly pointed, stellations or castellations more rounded or smooth structures are provided in a passage 43. The passage 43 is again within a porous medium 42. Between the three castellations or stellations as illustrated in figure 2d there are curved and essentially circular profile lands 44 provided or alternatively dependent upon requirements and as illustrated by dotted line 44a a flat projection land created between castellations. In any event, an odd number of castellations or stellations is provided which will create an appropriate turbulence response.

Figure 2e provides a side cross section of a further alteration in accordance with aspects of the present invention. As indicated above stellations or castellations can be projected in an odd number inwardly of a passage in accordance with aspects of the present invention. These stellations or castellations may be consistently presented in terms of penetration depth along the length of the filter or as illustrated in figure 2e provide variation in an undulating inner surface for a passage 54 in a medium 52 in accordance with aspects of the present invention. In such circumstances in addition to the odd number of stellations or castellations variations along the length of the filter can also be utilised.

By aspects of the present invention castellation and stellation with regard to a passage is provided but with an odd number of such castellations or stellations for asymmetric flow passage cross sections and therefore greater turbulence. In addition the depth of penetration by the individual castellations or stellations can vary and the castellation size and cross sectional configurations can be consistent for all castellations or stellations in a passage or different configurations and sizes provided at different circumferential periphery positions.

As indicated above it is also known to provide spiralling with respect to passages utilised in filters. Such spiralling will extend along the length of the filter. In such circumstances with prior arrangements spiralling of up to 500 turns a metre has been achieved. Spiralling in accordance with aspects of the present invention may extend to variations in the respective stellations or castellations in terms of the number of the twists of the spiral along the length and therefore greatly increase turbulence in the flow to avoid blockage and clogging. It will also be understood that rather than there being a consistent repeat cycle for all the spirals in accordance with aspects of the present invention the pitch or repeat length for the spirals may vary with position along the length of the filter. Thus, for example the first 30cms of filter length may have two hundred spirals, the next 30cms one hundred spirals and the next 30cms fifty spirals in progressive repeated progress along the filter length in order to again create further turbulence control in accordance with aspects of the present invention. However, it will also be understood that aspects of the present invention also provide a straight passage which extends between ends of the filter or along parts of the filter.

It is by achieving a symmetry with regard to the passage that improved performance is achieved. Whereas prior systems had symmetrical castellations and stellations by providing an uneven number, that is to say an asymmetric and greater than one number of castellations or stellations, an improved flow performance is achieved. The flow profile increases the differing intersecting flow velocities by the offsets of the parabolic flow profiles away from the main tubular flow centreline. The utilisation of an odd number in an asymmetric distribution leads to the creation of greater turbulence at lower cross flow volumetric flow rates and average velocity resulting in a reduction in energy, that is to say pumping requirements.

Figure 3 provides an illustration with regard to a five sided star cross section. Figure 3a illustrates the cross section from one end. Figure 3b illustrates respective flow planes D1 , D2 across the cross section as depicted in figure 3a. As can be seen the parabolic flow velocities 60, 61 respectively for D1 and D2 have an offset 65a, 65b either side of a centreline for the passage 63. This offset is asymmetric about the centreline of the passage 63 in comparison with even numbered castellation or stellation passages as described previously. By providing an asymmetric offset about the centreline for the passage 33 through an odd number of castellations or stellations turbulence is introduced as a lower average cross sectional flow velocity for the fluid through the passage 63. Such turbulence will reduce clogging for an equivalent cross flow velocity and therefore allows equivalent performance. By such reduced flow velocities lower energy input is required by the pump as less volumetric flow is necessary. This will reduce pumping energy requirements.

Modifications and alterations to aspects of the present invention will be understood by persons skilled in the technology. Thus as illustrated an odd number of castellations or stellations are provided, these castellations may be of varying depth about the circumferential periphery of the passage and spirals may be created along the length of the filter which may incorporate variations in the stellations and castellations. Additionally, combinations of castellations and stellations may be created where a castellation will develop into a pointed stellation and vice versa along the length of a spiral and the filter.

Claims

1. A filter comprising a porous material defining a passage, the porous material including an inner surface having finer pores than an outer surface, wherein the inner surface is castellated or stellated, and comprises an odd number of castellations or stellations.

2. A filter in accordance with claim 1 , wherein each castellation or stellation defines a respective flow plane.

3. A filter in accordance with claim 2, wherein a plane perpendicular to a centreline of each flow plane is offset from a centreline of the passage.

4. A filter in accordance with any preceding claim, wherein the castellations or stellations are substantially evenly distributed about the inner surface.

5. A filter in accordance with any preceding claim, wherein each castellation or stellation defines a projection towards a centre of the passage, and comprises a depth of penetration.

6. A filter in accordance with claim 5, wherein at least one projection comprises a different depth of penetration to at least one other projection.

7. A filter in accordance with claim 5 or 6, wherein the depth of penetration of at least one projection varies along the length of the passage.

8. A filter in accordance with any preceding claim, wherein each castellation or stellation is substantially symmetrical in cross-section.

9. A filter in accordance with any preceding claim, wherein the castellations of stellations define a spiral along the length of the passage.

10. A filter in accordance with claim 9, wherein the spiral is of a consistent spiral pitch length.

11. A filter in accordance with claim 9, wherein the spiral has a variable pitch length along the passage.

12. A filter in accordance with any preceding claim, wherein the inner surface is formed from a synthetic polymer.

13. A filter in accordance with any preceding claim, wherein the porous material comprises a metal, particulate stone, ceramic or synthetic polymer composition.

14. A filter in accordance with claim 12 or 13, wherein the polymer comprises at least one of polyamides, nylons, cellulosic polysulphones, polyethylenes and polytetra fluroethyline (PTFE).

15. A filter in accordance with claim 13, wherein the ceramic comprises at least one of aluminium oxide and zirconium oxide.

16. A filter in accordance with any preceding claim, wherein the inner surface comprises at least one land portion between adjacent castellations or stellations.

17. A filter in accordance with any preceding claim, wherein the filter is a cross-flow filter tube.

18. A filtration system comprising one or more filters as described in any one of claims 1 to 17.