WO2014001134A1

WO2014001134A1 - Pleated filter

Info

Publication number: WO2014001134A1
Application number: PCT/EP2013/062559
Authority: WO
Inventors: Stefan Vandendijk; Marie Nguyen
Original assignee: Bekaert Advanced Filtration Sa
Priority date: 2012-06-28
Filing date: 2013-06-18
Publication date: 2014-01-03

Abstract

The invention discloses an elongated filter pack for use in a filter candle for the filtration of non-newtonian fluid and/or molten polymers. The elongated filter pack comprises a pleated filter medium wherein the pleat density of the filter medium on the inner circumferential circle in the elongated filter pack is higher than 70% over a length of at least 20 mm of the elongated filter pack. The pleat density on the inner circumferential circle of the pleated filter medium in the elongated filter pack is less than75% over at least 50% of the length of the elongated filter pack. The pleat density on the inner circumferential circle of the elongated filter pack is defined as the thickness of the filter pack multiplied by the number of pleats, wherein one pleat is comprising two legs with a fold in between the two legs of the pleated filter medium,and multiplied with two, divided by the circumference of the circle that is enclosing the cross section perpendicular to its longitudinal axis of the pleated filter medium in the elongated filter pack and multiplied by 100, the value of the pleat density is obtained expressed as a percentage.

Description

Pleated filter

Description

Technical Field

[1 ] The invention relates to the field of filters, in particular to the field of filter candles (also called candle filters or tubular filters) and more specifically to filters for use in filtration at high pressure of fluids with non-newtonian behaviour, such as molten polymer filtration. The invention also relates to elongated filter packs (or elongated mesh packs) for such filtration applications. In the field of melt spinning of synthetic filaments the molten polymer is pressed through a filter candle where hydraulic pressures can be in the range of 150 and 250 bar or even higher. The invention also relates to filtration methods employing elongated filter packs, filter candles or filter apparatuses as described in the invention.

Background Art

[2] Various apparatuses for polymer filtration are described in the prior art.

Such apparatuses utilize a metal filtration medium in order to obtain the desired characteristics for polymer filtration. Typical properties desired for such filtration relate to strength, temperature, chemical resistance and medium migration. Various structures of metal filtration medium can be utilized in apparatuses designed for polymer filtration, and numerous factors can be considered when evaluating the various medium types. There are various types of filter elements utilizing different types of filter medium including flat filters, candle filters, basket filters and disc filters.

[3] WO94/1 1082 discloses a filter including a cylindrical filter element having a longitudinal axis, first and second end surfaces, and a plurality of longitudinal pleats. Each of the pleats has a pair of legs with first and second surfaces. The pleats are in a laid-over state in which the first surface of each leg is in intimate contact with the first surface of an adjoining leg and the second surface of each leg is in intimate contact with the second surface of an adjoining leg. [4] Filters with pleated filter medium have the benefit that they have a higher effective surface area for a same space occupancy, therefore, for a same space, pleated filters are more efficient.

Filter candles of cylindrical shape with pleated filtration medium are known for polymer filtration, e.g. from US5279733, WO2003/020396,

WO2008/000468 and WO2009/109435. However, such prior art systems are not suited for use in filtration systems foreseen with a screen changer as such filter candles cannot withstand the forces during screen change.

[5] Filter candles with pleated filter medium that have a conical shape are known from US2009/017183 for filtration of fluids at low pressure. The filter candle is preferably having a taper between 2° and 10°, in particular approximately 5°. The filter candle is made tapered in order to facilitate the replacement of the filter candle in the filter apparatus in which it is used. The pleated filter medium and the filter candle in which it is used is suited for low pressure liquid filtration, it is not suited for molten polymer filtration and will not withstand forces at changing screens if used for molten polymer filtration.

[6] It is a problem that prior art pleated filter medium for filter candles for non- newtonian fluids, when using the filter candle in molten polymer filtration systems equipped with a screen changer, are not resisting the forces that are exerted on the pleated filter medium at the moment of screen changing. With screen changer is meant a mechanism that allows to automatically move a filter candle from a filtration position to a

backwashing position. As a consequence, when using a screen changer in high pressure filtration, the benefits of pleated filter medium cannot be exploited.

Disclosure of Invention

[7] The primary objective of the invention is to provide an elongated filter pack for a filter candle for filtering non-newtonian fluids, e.g. at high pressure, and to provide such a filter candle, that offer the benefits of a pleated filter medium while allowing the use of a screen changer for the filter. [8] In order to achieve the objective, in a first aspect of the invention an elongated filter pack is provided for use in a filter candle for the filtration of non-newtonian fluid and/or of molten polymers (the majority of molten polymers demonstrate a non-newtonian behaviour). An elongated filter pack is also known under the name of an elongated mesh pack. The elongated filter pack comprises a pleated filter medium. The pleat density of the pleated filter medium on the inner circumferential circle of filter medium in the elongated filter pack is higher than 70%, preferably higher than 75%, more preferably higher than 80%, even more preferably higher than 85%, even more preferably higher than 90%, and even more preferably higher than 95%, and still even more preferably 100%, over a length of at least 20 mm, preferably over a length of at least 50 mm, of the length of the elongated filter pack, as measured along the axis of the elongated filter pack. This zone is preferably located at one of the ends of the elongated filter pack. A pleat density of over 75% is as such not advisable for filtration performance, but solves the problem of lack of mechanical resistance of elongated filter packs with pleated filter medium when using screen changers.

The pleat density of the filter medium on the inner circumferential circle of the filter medium in the elongated filter pack is less than 75%, preferably less than 70%, more preferably less than 60% and preferably higher than 40%, over at least 50% (and preferably over at least 75%) of the length of the elongated filter pack.

[9] The combination of the length of the elongated filter pack with pleat

density higher than 70% preferably higher than 75%, more preferably higher than 80%, even more preferably higher than 85%, and even more preferably higher than 90%, even more preferably higher than 95%; and the majority of the length of the elongated filter pack having a pleat density less than 75%, preferably less than 70%, more preferably less than 60% and preferably higher than 40%, ensures a good filtration performance combined with resistance of the elongated filter pack to screen changer operation. Consequently, such elongated filter packs according to the invention can advantageously be used in filter systems equipped with a screen changer, while providing good filtration performance.

[10] The pleat density on the inner circumferential circle of the elongated filter pack is calculated as the thickness of the filter pack (which includes supporting meshes if present) multiplied by the number of pleats (one pleat is comprising two legs with a fold in between the two legs) of the pleated filter medium and multiplied with two, divided by the circumference of the circle that is enclosing the cross section perpendicular to its longitudinal axis of the pleated filter medium in the elongated filter pack. Multiplied by 100, the value of the pleat density is obtained expressed as a percentage. The value for the thickness of the filter medium that is considered in the calculation of the pleat density is the thickness in the zone for which the pleat density is calculated, and more specifically the thickness at the circumference of the inner circle. Such thickness values for a filter medium can vary, even within a mesh pack (e.g. along the length of the elongated filter pack), e.g. where the filter medium is compressed to a lower thickness.

[1 1 ] In a preferred embodiment the pleat density can be 100% in some parts of the elongated filter pack. Such embodiments provides an even better resistance of the elongated filter pack against the forces occurring at screen changing.

[12] In a preferred embodiment the pleat density is 100% over a length of at least 20 mm, preferably over a length of at least 50 mm, of the length of the elongated filter pack, as measured along the axis of the elongated filter pack. This is possible by compressing the pleated filter medium, reducing its thickness as compared to in other zones of the elongated filter pack.

[13] A pleat density can be calculated for each position along the length of the longitudinal axis of the elongated filter pack. In state of the art elongated filter packs with pleated filter medium for the filtration of non-newtonian fluid and/or of molten polymers, the pleat density is constant over the length of the elongated filter pack and is normally between 40% and 70%.

[14] The elongated filter pack can comprise at one or both of its ends an end fitting that is fixed to the filter medium. The fixation can be obtained via a crimp connection or via welding, e.g. after local densifying of the pleated filter medium. A crimp connection can be established via crimping the end fitting over the end of the filter medium. It needs to be mentioned that at such ends where local densifying is applied, such ends can no longer be considered as locations with pleated filter medium, but as densified ends for connection purposes.

[15] Examples of non-newtonian fluids that can be filtered with the elongated filter pack are molten polymers, e.g. at temperatures over 100°C and e.g. at pressures over 100 bar.

[16] In a preferred embodiment the elongated filter pack is conical in shape over the length of at least 20 mm, preferably of at least 50 mm, where the pleated filter medium in the elongated filter pack has on its inner circumferential circle a pleat density higher than 70%, preferably higher than 75%, more preferably higher than 80%, even more preferably higher than 85%, and even more preferably higher than 90%, even more preferably higher than 95%. Preferably, the taper of that conical part of the elongated filter pack is at least 5°. And preferred is a taper of less than 20°. A preferred range for the taper is between 5° and 10°. The preferred range offers the benefit that elongated filter packs can be made in an efficient way.

With the elongated filter pack is conical in shape is meant that the diameter of the circle that encloses the cross section perpendicular to the longitudinal axis of the elongated filter pack is different for different positions along the length of the elongated filter pack over which it is conical in shape.

[17] In a more preferred embodiment, the elongated filter pack is conical over its full length, wherein a taper of at least 5°, and preferably between 7° and 15° has shown to be particularly beneficial as it allows to obtain an elongated filter pack that is very suitable for use in a filter element with a screen changer device.

[18] In a preferred embodiment an elongated filter pack is formed by pleating a rectangular shaped filter medium and forming it to shape the elongated filter pack with a conical part or that is conical over its full length. The rectangular shape can be found back when disassembling the elongated filter pack and straightening the pleats. It is a benefit of elongated filter packs of this embodiment that the filter medium material can be used in a very efficient way.

[19] In a specific example of the invention, the filter medium of the elongated filter pack comprises a sintered metal fibre web and/or one or more wire meshes (e.g. woven wire meshes). The filter medium can comprise one or more sintered metal fibre webs. The sintered metal fibre web is preferably a multi-layered web (preferably sintered together to form one metal fibre web), wherein the layers have different porosity and/or metal fibres with different equivalent diameters. The sintered metal (preferably stainless steel) fibre web can be positioned between wire meshes to obtain the filter medium (one or more meshes at the flow in side and/or at the flow out side of the filter medium). Preferably, at the flow in side of the filter medium, a finer mesh or finer meshes is/are used than at the flow out side.

Preferably, the sintered metal fibre web is not sintered to the wire mesh or meshes, nor bonded in any other way to the sintered metal fibre web. The filter medium is pleated in order to obtain the pleated filter medium and is bent to shape to form the elongated filter pack.

[20] Preferably, metal fibres in the average equivalent diameter range of 1 to

40 μιτι are used, more preferably in the average equivalent diameter range of 12 to 18 μιτι. With equivalent diameter is meant the diameter of a perfectly round fibre that is having the same cross sectional surface as a fibre that is having a cross section that is not necessarily perfectly round, but is e.g. hexagonal in cross section. Fibre web porosity between 60 and 90%, e.g. between 65 and 80% is preferred for best filtration results.

[21 ] Alternatively or in addition, the filter medium can comprise glass fibre web layers.

[22] An alternative example of a filter medium can comprise or can be a stack of wire meshes (e.g. woven wire meshes). The wire meshes are preferably put on top of each other without bonding them together. The stack of wire meshes is pleated in order to obtain the pleated filter medium, and is bent to shape to form the elongated filter pack. [23] In a preferred embodiment, the elongated filter pack has a smallest outer diameter of at least 35 mm. And preferably the largest outer diameter of the elongated filter pack is smaller than 200 mm.

[24] In a preferred embodiment, the elongated filter pack has a smallest inner diameter of at least 20 mm.

[25] In preferred embodiments, the pleat height is between 5 and 9 mm. The pleat height is defined as the difference between the outer and the inner radius of the elongated filter pack with the pleated filter medium.

[26] In a preferred embodiment, and especially beneficial for elongated filter packs with an internal diameter up to 100 mm, the number of pleats of the elongated filter pack is in between the smallest inner diameter expressed in mm of the elongated filter pack minus and plus 6%. For instance if the smallest inner diameter of the elongated filter pack is 50 mm, the preferred range for the number of pleats is between 47 and 50.

[27] Preferred length of elongated filter packs (measured along the central axis of the elongated filter pack) according to the invention is between 50 and 200 mm.

[28] According to a second aspect of the invention a filter candle is provided comprising a hollow core tube with fluid permeable wall (e.g. a tube comprising welded wires, or a tube that has been perforated or a tube formed with perforated plate metal) and an elongated filter pack as in the first aspect of the invention, wherein the elongated filter pack is supported by the hollow core tube.

[29] In a preferred embodiment the filter candle comprises an outer guard to protect the elongated filter pack. The outer guard can be a tube

comprising welded wires.

[30] In a preferred embodiment the elongated filter pack is provided slideable in the filter candle for replacement in the filter candle by sliding the elongated filter pack from the hollow core tube and sliding another elongated filter pack over the hollow core tube.

[31 ] The sliding replacement can be arranged in any way, e.g. as known in the art. The sliding replacement preferably includes a way to seal between the hollow core tube and the elongated filter pack in order to prevent short circuit of fluid flow that could result in unfiltered fluid. A particularly suited way is as is described in WO2008/000468 where a filter candle is described that comprises:

- an elongated filter pack for filtering a fluid, the elongated filter pack defines a mesh pack axial opening. For example, the mesh pack comprises a filter medium for filtration of a fluid and has an end fitting securely fixed to the filter medium. The end fitting can provide a first end of the mesh pack and has an end inner surface defining an end fitting opening through the end fitting, which end fitting opening is co-axial with the axial opening of the mesh pack.

- a hollow core tube having a fluid permeable wall, the core tube is positioned within the axial opening of the mesh pack and is removably slidable through the mesh pack axial opening. The core tube can have a first outer end having an outer end surface. The core tube can have an end flange fixed to the outer end surface, which end flange is suitable for engaging the end fitting on the mesh pack.

- a ring-shaped seal provided between and making contact with the outer end surface of the core tube and the end inner surface of the end fitting and the end flange. The seal can encircle the outer end surface of the outer end of the core tube. The seal can be a porous ring-shaped structure comprising a multitude of pores, which pores have an average pore size being equal or less than the filter rating of the filter medium.

A third aspect of the invention relates to a filtration apparatus for the filtration of non-newtonian fluids, comprising an elongated filter pack as in the first aspect of the invention and/or a filter candle as in the second aspect of the invention and comprising a screen changer, with screen changer is meant a mechanism that allows to automatically move a filter candle from a filtration position to a backwashing position. The screen changer can be of any type, e.g. of the piston type or of the type operating by means of rotation. [33] A fourth aspect of the invention is a method for the filtration, e.g. at a pressure of at least 100 bar, of non-newtonian fluids and/or of molten polymers, e.g. for filtration of molten polymers, wherein an elongated filter pack is used as in the first aspect of the invention and/or where a filter candle is used as in the second aspect of the invention or where a filtration apparatus is used as in the third aspect of the invention.

[34] As an example, the temperature of the non-newtonian fluid - e.g. molten polymer - that is filtered is higher than 80°C, e.g. up to 380°C. The pressure drop of the molten polymer over the filter is e.g. higher than 100 bar and e.g. below 250 bar, e.g. below 200 bar. As an example the pressure of the molten polymer at the inlet of the filter is 200 bar and is 50 bar at the outlet of the filter, meaning a pressure drop over the filter of 150 bar.

[35] In a preferred method, a filter candle is used wherein the flow of the non- newtonian fluid and/or of the molten polymer is from the outside to the inside of the elongated filter pack.

[36] In a further preferred method of filtration a filter candle is automatically moved from a filtration position to a backwashing position. Such operation is also known as a screen changing operation.

[37] Several technologies exist for performing the screen changing operation. A first technique is piston screen changing in which filter candles are pushed in another position. Filter candles can be in operating position during which they are used as filter. Filter candles can also be pushed in a position where they are cleaned by means of backwashing.

An alternative technique is screen changing by rotation. At screen changing, the screen changer assembly comprising a number of filter elements is rotated, wherein filter elements in filtration position are moved by a rotation of elements of the filtration apparatus into non-filtration position and filter elements that were not in filtration position are put into filtration position. The filter elements (and filter candles) that are turned out of filtration position can be cleaned by means of backwashing. Brief Description of Figures in the Drawings

[38] Figure 1 shows in a schematic way a cross section of a mesh pack

according to the invention to illustrate the way of calculation of the pleat density.

Figure 2 shows an example of a filter candle and of a mesh pack according to the invention.

Mode(s) for Carrying Out the Invention

[39] Figure 1 shows in a schematic way a cross section 100 perpendicular to the axis of a mesh pack according to the invention, in order to illustrate the way of calculation of the pleat density. The internal diameter of the cross section of the mesh pack 100 is D. The mesh pack comprises a pleated filter medium 1 10, e.g. a sintered fibre web between two woven wire meshes. The thickness of the filter medium 1 10 is indicated with d. The value for the thickness of the filter medium that is considered in the thickness in the zone for which the pleat density is calculated and at the internal diameter of the cross section. Such thickness values for a filter medium can vary, even within a mesh pack, e.g. where the filter medium is compressed to a lower thickness. The pleat height, which is the difference between the outer and the inner radius of the pleated filter medium in the elongated filter pack, is indicated with h. The mesh pack has n pleats, each pleat consisting of two legs, this means that the example shown in figure 1 has 1 1 pleats. The pleat density (in percentage) can be calculated according to the formula: 200*n*d/(n*D). A value of 100% is possible, e.g. where a thicker filter medium is compressed during pleating to lower thickness to achieve a pleat density of 100%. The thickness value in the formula is taken at the internal diameter of the cross section.

[40] Figure 2 shows an example of a filter candle 200 and a mesh pack 215 according to the invention. The filter candle 200 had a hollow core tube with fluid permeable wall 225 which is supporting the elongated filter pack 215, which comprises a pleated filtration medium. Di is the smallest outer diameter of the hollow core tube 225, equal to 57 mm; while D₂ is the biggest outer diameter of the hollow core tube 225, equal to 1 15 mm. The length L of the filter candle 200 is 250 mm. The hollow core tube and the filter candle have a conical shape, with a taper a of 20°.

In an example, the pleated filter medium consists out of a sintered stainless steel fibre medium of 1200 g/m² (consisting out of 600 g/m² of 30 μιτι stainless steel fibres and 600 g/m² of 40 μιτι stainless steel fibres) with a porosity of 85%, between on the one side a 60 mesh (meaning 60 openings in the mesh per linear inch) woven from 160 μιτι diameter stainless steel wire and between on the other side a 37 mesh (meaning 37 openings in the mesh per linear inch) woven from 200 μιτι diameter wire. The total thickness of such filter medium is 1 .8 mm. The filter medium is pleated from a rectangle and formed into the elongated filter pack with 50 pleats and a pleat height of 5 mm. The pleat density at the lowest diameter of the filter candle is 100% (the filter medium is compressed to a lower thickness to achieve 100% pleat density at this location) and the pleat density at the biggest diameter of the filter candle is 50%. At a distance - measured along the axis of the filter candle - of 20 mm from the lowest diameter cross section of the filter, the pleat density is 93%. At a distance of 50 mm from the lowest diameter cross section of the filter, the pleat density is 83%. The pleat densities at different lengths from the smallest diameter are:

- at length 30 mm: pleat density 89.6%

- at length 60 mm: pleat density 80.9%

- at length 90 mm: pleat density 73.6%

- at length 120 mm: pleat density 67.6%

- at length 150 mm: pleat density 62.5%

- at length 180 mm: pleat density 58.1 %

- at length 210 mm: pleat density 54.3%

- at length 240 mm: pleat density 50.9%.

The filter candle can be provided with an outer guard to protect the elongated filter pack (not shown in figure 2).

Claims

1 . An elongated filter pack for use in a filter candle for the filtration of non- newtonian fluid and/or molten polymers,

wherein said elongated filter pack comprises a pleated filter medium, wherein the pleat density of the filter medium on the inner circumferential circle in the elongated filter pack is higher than 70% over a length of at least 20 mm of the elongated filter pack,

wherein the pleat density on the inner circumferential circle of the pleated filter medium in the elongated filter pack is less than 75% over at least 50% of the length of the elongated filter pack;

with pleat density on the inner circumferential circle of the elongated filter pack is meant the thickness of the filter pack multiplied by the number of pleats, wherein one pleat is comprising two legs with a fold in between the two legs of the pleated filter medium, and multiplied with two, divided by the circumference of the circle that is enclosing the cross section perpendicular to its longitudinal axis of the pleated filter medium in the elongated filter pack and multiplied by 100, the value of the pleat density is obtained expressed as a percentage.

2. The elongated filter pack as in claim 1 , wherein said length of at least 20 mm of the elongated filter pack where the pleat density on the inner circumferential circle of the filter medium in the elongated filter pack is higher than 70%, comprises a zone with pleat density 100%.

3. The elongated filter pack as in any of the preceding claims, wherein the

elongated filter pack is conical in shape over the length of at least 20 mm where the mesh pack has on its inner circumferential circle a pleat density higher than 70%.

4. The elongated filter pack as in claim 3, wherein in said length of the elongated filter pack of at least 20 mm where said elongated filter pack is conical in shape, the taper of said conical shape is at least 5°.

5. The elongated filter pack as in claims 3 or 4, wherein the elongated filter pack is conical over its full length.

6. The elongated filter pack as in claims 3 to 5, wherein the pleated filter medium is formed by pleating a rectangular shaped filtration medium and forming it to shape the elongated filter pack.

7. The elongated filter pack as in any of the preceding claims, wherein the filter medium comprises a sintered metal fibre web and/or one or more wire meshes.

8. Filter candle comprising,

- a hollow core tube with fluid permeable wall,

- an elongated filter pack as in any of the preceding claims,

and wherein the elongated filter pack is supported by said hollow core tube.

9. Filter candle as in claim 8, wherein the filter candle comprises an outer guard to protect the elongated filter pack.

10. Filter candle as in claims 8 to 9, wherein the elongated filter pack is provided slideable in the filter candle for replacement by sliding the elongated filter pack from the hollow core tube and by sliding another elongated filter pack over the hollow core tube.

1 1 . Filtration apparatus, comprising an elongated filter pack as in claims 1 to 7 or a filter candle as in claims 8 - 10; wherein the filtration apparatus comprises a screen changer, with screen changer is meant a mechanism that allows to automatically move a filter candle from a filtration position to a backwashing position.

12. Method for the filtration at a pressure of at least 100 bar of non-newtonian

fluids and/or of molten polymer, wherein an elongated filter pack is used as in claims 1 to 7 and/or where a filter candle is used as in claims 7 to 10 or where a filtration apparatus is used as in claim 1 1 .

13. Method as in claim 12, wherein a filter candle is automatically moved from a filtration position to a backwashing position.