WO2008119346A1

WO2008119346A1 - A filter bag

Info

Publication number: WO2008119346A1
Application number: PCT/DK2007/050039
Authority: WO
Inventors: Ole HØJGAARD; Martin Novak; Michelle Milling Madsen
Original assignee: Niro A/S
Priority date: 2007-03-30
Filing date: 2007-03-30
Publication date: 2008-10-09

Abstract

The filter bag (50) has a substantially elongated shape and extends in a general longitudinal direction and comprises a tube part (51) of filter material formed by a substantially rectangular width of material. The tube part may be made of Teflon ® (PTFE) and is formed by rolling a width of material to form an elongated shape and two opposed edge portions (51b, 51c) of said width of material are connected to each other in a longitudinally extending glue joint (55). The predefined area may have been subjected to a pre-treatment, such as a plasma- polymerisation process or a pressurized carbon dioxide process, in order to secure a sufficient adhesion between the overlapping edge portions (51b, 51c). In case the filter bag furthermore comprises a top part (52) and a bottom part, for instance made of stainless steel, the tube part (51) may be connected to the top part (52) and the bottom part by means of gluing.

Description

A filter bag

Field of the invention

The present invention relates to a filter bag for filtering gas, i.e. for separating particulate matter from a gas. In particular, the invention relates to sanitary filter bags suitable for cleaning-in-place processes

(CIP) in filter units or filters integrated in process equipment, for the pharmaceutical industry and other industries. More particularly, the invention relates to a filter bag for filtering a process gas from a process creating and/or treating a powdery product. The powdery product may be an agglomerated or granulated product. Processes may be such as spray drying, spray cooling, fluid bed processing, agglomeration, granulation or coating.

Background of the invention

The filter bag may form part of a bag filter in a filter unit or of a filter integrated in process equipment such as e.g. spray dryers and fluid bed processing equipment. Such bag filters are well known in the prior art and are adapted to separate particulate matter from a gas. One example of use of the bag filter integrated in the process equipment is shown in US patent No. 6,463,675 (Assignee: Niro A/S). Bag filters forming part of a filter unit are disclosed in, i.a., US patents Nos

6,676,720, 6,149,716 and 6,332,902 (Assignee of all patents: Niro A/S).

The filter bags may be of any material suitable to the purpose, and may for instance be made of felt or woven of e.g. polyester or Teflon® (PTFE). A felt or polyester material may be permeable to the cleaning fluid, while Teflon® is not. Alternatively, a filter bag may be manufactured from a permeable material and subsequently coated with an impermeable material, e.g. a polyester material coated with Teflon®. In general, the material used in such filter bags is flexible. Within the technical field of filtering gas in process equipment, such as e.g. spray dryers and fluid bed processing equipment, the filter bag traditionally includes a tube part of filter material of an elongated shape, e.g. a cylindrical shape. The tube part may e.g. be formed by a width of material rolled to form a cylinder and two opposed edge portions are connected to each other in a longitudinally extending joint. Furthermore, the tube part may be connected to a top part and a bottom part.

Traditionally, such a filter bag is, on its inside, supported by a support structure in the form of a basket to prevent the flexible bag to collapse due to the process gas flow from the outside to the inside of the bag. Such a support structure traditionally includes a number of threads or rods extending in the longitudinal direction of the filter bag and spaced from each other, seen in the circumferential direction, in suitable intervals. The longitudinally extending rods are held by a number of rings or annular rods that are spaced from each other, seen in the longitudinal direction, in suitable intervals. Most often, the filter bag is positioned with the longitudinal direction extending substantially vertically in the mounted position, e.g. when the top of the filter bag is mounted in the filter unit or in the process equipment. Hence, in such a position the longitudinally extending rods are substantially vertical and the annular rods are substantially horizontal.

In between periods of operation the filter unit is cleaned, e.g. by a cleaning-in-place process (CIP) involving cleaning nozzles located at least in the clean gas chamber at the upper outlet side of the bag filters. The cleaning nozzles are supplied with a cleaning fluid that possibly includes a cleaning agent. It is also possible to supply the cleaning nozzles with gas pulses in combination with the supply of cleaning fluid. During cleaning the cleaning fluid is washed down into the bag filters together with any entrained particles or powder from the clean gas side. Similarly, the bag filters are cleaned on the outside.

A great demand on the quality of the CIP exists. In the chemical industry, e.g. for dyes, it is essential to avoid cross-contamination when shifting from one colour to another. In the dairy and food industry, the quality of the cleaning is very important for bacteriological reasons. Furthermore, the cleaning quality is essential in the pharmaceutical industry, again for bacteriological and health reasons and to meet requirements of authority regulations.

In the pharmaceutical industry, the active components may not escape from the process equipment and therefore, impermeable filter bags may be prescribed for this area of application. For this purpose, a filter bag made of a woven Teflon® support layer with an outer layer of Teflon® membrane or film is used. During operation, process gas is blown through the filter bag.

Furthermore, the filter bag is subjected to pressure pulses ejected from a suitable supply at intervals to shake off product adhering to the outside of the filter bag. Consequently, the filter bag is exposed to a number of stress loads. In turn, this entails that in particular the tube part of filter material is subjected to wear and tear, both of the tube part material itself, of the joint connecting the edge portions to each other, and of the joints between the tube part and the top and bottom parts.

In order to ensure a firm connection that does not crack or rupture, the joint is traditionally provided as a seam joint carried out by means of sewing. However, in sanitary filter bags to be used in process equipment requiring a very high degree of cleaning quality, e.g. as in the above-mentioned pharmaceutical industry, the joints have proven to be a source of possible contamination. This is partly due to the overlap between the edge portions at the joint, which does not abut against the opposite edge portion and hence has a tendency to become frayed when the process gas is blown into the filter bag. In one prior art filter bag, a supplemental cover element sewn into the joint on the outer side of the tube part strengthens the joint and reduces to some extent the edge portion from being frayed or otherwise damaged. Another risk of contamination lies in the fact that residual product penetrates into the stitching holes in the seams and that this residue may not be fully removed by the CIP process.

Summary of the invention With this background it is an object of the invention to provide a filter bag, in which the sanitary conditions are improved and the inconveniences with fraying or other damage of the joint are reduced.

This and further objects are achieved by a filter bag for filtering gas, the filter bag having a substantially elongated shape and extending in a general longitudinal direction, said filter bag comprising a tube part of filter material formed by a substantially rectangular width of material, said width of material being rolled to form an elongated shape and two opposed edge portions of said width of material being connected to each other in a longitudinally extending joint, said filter bag being characterized in that at least a major part of said longitudinally extending joint is a glue joint.

By forming the longitudinally extending joint at least partly as a glue joint it has turned out that this connection provides for the required strength while at the same time the sanitary conditions are improved to a significant degree. This is believed to be due to the fact that the glued connection results in a reduction of the number and size of cavities such as stitch holes or frayed edges, which may serve for concealing residual material that may not be removed by normal CIP processes. The term "glue joint" and similarly the term "gluing" are used to denote any joint or any process involving only adhesive mutual connection between the surfaces of two elements. The gluing may cover processes involving application of a glue of any suitable kind on one of or both surfaces, but also processes involving welding, melting etc., i.e. any process not involving mechanical connection as such.

Preferably, the joint is formed by overlapping edge portions of said width of material, at least one strip of glue being applied to a predefined area on at least one of the mutually facing sides of the overlapping portions. This is an easy and well-recognized manner of applying glue.

In order to secure a sufficient adhesion between in particular the outer overlapping portion and the overlapped opposite edge portion, said predefined area may have a larger extension in the circumferential direction than the overlap between the overlapping edge portions. Depending on the filter material used in the tube part, it may be necessary to prime the surfaces to be connected to each other. Hence, in a preferred embodiment, at least said area has been subjected to a pre-treatment.

Preferably, the pre-treatment comprises a plasma- polymerisation process or a pressurized carbon dioxide process, but other processes e.g. involving flame, corona or plasma processes may be used.

The part of the longitudinally extending joint to be provided solely by said glue joint depends on a number of factors, such as the length of the filter bag, the load pattern on the tube part etc. Preferably, however, the glue joint extends along the entire longitudinally extending joint.

Particularly for use within the pharmaceutical industry, the tube part may be made of Teflon® (PTFE).

In the cases, in which the filter bag comprises a top part and a bottom part, it is advantageous if the tube part is connected to the top part and the bottom part by means of gluing. This provides for an all- glued filter bag, without any cavities to form a risk of contamination. Preferably, the top part and the bottom part are made of stainless steel.

In another aspect of the invention, a bag filter comprising a support structure and a filter bag is provided.

Yet another aspect of the invention provides for a method of manufacturing a filter bag, comprising the steps of providing a substantially rectangular width of filter material, rolling said width of material to form an elongated shape, connecting two opposed edge portions of said width of material to each other by means of gluing in a longitudinally extending joint to provide a tube part.

Brief description of the drawings

Fig. 1 shows a schematic side view of a filter unit including two bag filters;

Fig. 2 is a perspective view, on a larger scale, of a bag filter in- eluding a filter bag and a support structure;

Fig. 3 is a perspective view, on a still larger scale, of a support structure for use with the filter bag;

Fig. 4 is a partial perspective view corresponding to Fig. 3 of another support structure; Fig. 5 is a partial perspective view, on a larger scale, corresponding to the filter bag in its mounted position shown in Fig. 2 and illustrating a support structure interacting with a filter bag;

Fig. 6 is a perspective view of the top portion of a filter bag in one embodiment of the invention;

Fig. 7 is a perspective view of the bottom portion of a filter bag in one embodiment of the invention;

Fig. 8 is a perspective view, on a larger scale, of a detail of a prior art filter bag; Fig. 9 is a sectional view, on a still larger scale, of the detail shown in Fig. 8;

Fig. 10 is a view corresponding to Fig. 8 showing a detail of the filter bag in an embodiment of the invention; and

Fig. 11 is a view corresponding to Fig. 9 of the detail shown in Fig. 10.

Detailed description of the invention and of preferred embodiments

Fig. 1 illustrates a filter unit 1 employed for separating product particles from a process gas coming from e.g. a spray drying apparatus, a fluid bed apparatus, a drying apparatus, an agglomeration apparatus or the like using air or a gas in the treatment of particulate or dust-like products, or from other industrial processes, such as flue gas cleaning. The products can be e.g. foodstuffs, dairies, pharmaceuticals, dyestuffs, chemical products etc. The process gas can be e.g. heated air or drying gas or special gas compositions inert to the products treated in the apparatus. In the embodiment of Fig. 1, the filter unit is shown as a separate external unit connected to a gas outlet for particle loaded processing gas in a plant (not shown). Alternatively, the filter unit can be integrated into a processing unit producing the particle loaded gas, such as a spray drying apparatus or a fluid bed apparatus. In the following description, the terms "filter", "bag filter", "filter bags" etc. denote equal or analogous elements forming part of either such a separate unit or an integrated unit.

A filter unit housing is composed of a vertically arranged cylin- drical upper section 2 connected with a downward tapered lower section 3. An inlet (not shown) for process gas with product to be filtered off is arranged in the lower part of the cylindrical section 2 and an outlet (not shown) for filtered clean gas is arranged in the upper part of cylindrical section 2. At the bottom of the lower section 3, an outlet port 4 is arranged for extraction of retained product.

A horizontal suspension plate 5 is arranged in the upper part of the cylindrical section 2 and divides the housing in an upper outlet side with a clean-gas chamber 6 and a lower inlet side 7. The plate 5 has a number of holes, in which elongated tubular bag filters 10 are suspended approximately vertically with upwards-open ends that deliver filtered gas to the clean-gas chamber 6. Fig. 2 shows that each bag filter 10 extends between a first end 11 and a second end 12 in a generally longitudinal direction indicated by axis x. The bag filters 10 are inserted into apertures (not shown) in the suspension plate 5 and the first end 11 of each filter is connected with the suspension plate 5 in a manner to be described in further detail below. The number of bag filters in the filter unit depends on the desired filter capacity. The smallest filter has a single filter element. Plants for treating, handling or producing pharmaceuticals can use smaller filter units having e.g. from 2 to 25 bag filters, and plants for foodstuffs, dairies and chemicals can comprise many hundreds of bag filters. For all of the above-mentioned applications, the bag filters may be located either in a separate filter unit or integrated in the plant. As will be described in further detail below, each bag filter 10 comprises a filter bag and a support structure. The support structure is located on the inside of the filter bag and supports the filter bag, particularly in the radial direction.

One example of a support structure 120 is shown in Fig. 3. This support structure 120 has the form of a thread basket made of threads or rods 121, 122 extending in the longitudinal direction of the bag filter and being attached to rings or annular rods 127 of thread. Such thread baskets are well known in the art and comprise at least three longitudinal rods and at least two annular rods, but typically more than five rods and typically at least four annular rods per meter of length of the bag filter. Traditionally, the filter bag has at its lower end an end closure of either a permeable filter material or an impermeable, either flexible material or a rigid and strong material, such as steel, stainless steel or another metal, polymers or ceramics. The support structure can be made in several sections mounted in extension of each other. The top end of the support structure is suspended in the hole in the suspension plate 5 by means of an upper collar of a larger diameter than the hole and being placed on the upper side of plate 5. Fig. 4 shows another example of a support structure. This support structure is described in further detail in Applicant's co-pending application filed on the same day as the present application. As in the support structure shown in Fig. 3, the support structure 20 shown in Fig. 4 comprises a plurality of elements extending in the longitudinal direction x from a first end to a second end, and a plurality of substantially annular elements. However, instead of being formed as solid rods, the longitudinally extending elements of this embodiment include six tubular pipe elements 21-26. As in the prior art support structure, each annular element 27 is located in a transverse plane perpendicular to the longitudinal direction x, and the annular elements 27 are located at a distance from each other in the longitudinal direction. Each annular element 27 is connected to the tubular pipe elements 21-26, however, each annular element 27 includes a number of parts 28, each part having two ends and extending between adjacent tubular pipe elements. For instance, the indicated part 28 extends between tubular pipe elements 21 and 22, each end of part 28 being connected to a respective tubular pipe element 21 and 22, respectively. The general shape of the support structure is cylindrical. It is noted that the term "cylindrical" describes the general shape of the support structure. Hence, in the embodiment shown, the outer shape is defined by the six longitudinally extending tubular pipe elements 21-26. Other general shapes are conceivable as well, for instance shapes having an oval, polygonal, or a rectangular, e.g. square, cross-section. In the shown embodiment, however, each part 28 of the annular element 27 is concave, seen in the longitudinal direction, and the general shape of the support structure 20 is thus cylindrical with longitudinally extending concavities. The parts 28 are shown as solid elements having any suitable cross-section. In order to prevent material filtered through the filter bag material from collecting on top of the parts 28, the upper side of each part 28 is preferably made with a rounded or otherwise sloping shape. Alternatively, the substantially annular elements 27 may include tubular pipe elements as well.

During operation of the filter unit 1 process gas carrying product enters the filter unit through the inlet of the filter unit and flows into the area around the bag filters. The gas is filtrated through the tubular walls of the bag filters 10 and flows out through the outlet of the filter unit. As the gas passes the filter walls product carried by the process gas is retained by the bag filters 10. The retained material is partially left on the bag filters and partially drops down and accumulates in the lower section. The accumulated product can then be extracted through the outlet port 4. During filtration a flow of filtrated gas streams vertically up into clean-gas chamber 6. As the filtration proceeds some of the filtered off particles or dust accumulate on the outside of the bag filters, and has to be cleaned away in order to avoid building up of dust cakes. Cleaning is effected during continuous operation of the filter unit by using high pressure reverse pulse gas cleaning at intervals.

As shown in Fig. 5, the tube part 51 of the filter bag 50 is being pressed against the support structure 20 during operation due to the process gas flow. It is noted that the tensile load on the filter material of the tube part 51 is notably lower with the support structure 20 than in the support structure 120. This is partly due to the fact that the tubular pipe elements 21-26 have larger diameters than their prior art counterparts, partly to the fact that the annular element parts 28 are formed with a concave shape, seen in the longitudinal direction. As indicated by area 51a in Fig. 5, the material of the tube part 51 has a slightly sloping path on each side of the only slightly protruding part 28. In a ring-shaped annular element the slope of the path would be considerably larger, thus increasing the wear and tear on the filter material in the tube part 51. When a filtration procedure is completed and the filter unit needs cleaning, for sanitary reasons or because it is to be used for filtration of another product, a cleaning-in-place (CIP) process is carried out during which the complete interior of the filter unit is washed with a cleaning liquid. Such throughout cleaning of the filter unit carried out in between periods of operation involves cleaning nozzles (not shown) located at least in the clean-gas chamber at the upper outlet side of the bag filters. The cleaning nozzles are supplied with a cleaning liquid or cleaning gas that possibly includes a cleaning agent. It is also possible to supply the cleaning nozzles with gas pulses in combination with the supply of cleaning liquid. In the CIP process, the cleaning liquid or cleaning gas may be introduced into the interior of the filter bag through the above-mentioned communication port at the second end of the support structure and/or from other locations. During cleaning the cleaning liquid is washed down into the bag filters together with any entrained particles or powder from the clean gas side. The liquid flows through the bag filters and particles or powders accumulate at the bottom of the bag filters, which is drained off by means at the bottom of each bag filter, e.g. by a gravity forced valve or by a controlled valve.

In the following, the general design of the filter bag will be described with particular reference to Figs 5 to 7 and 10 to 11.

The filter bag 50 has a generally elongated shape, which in the embodiment shown is substantially cylindrical, and extends in a general longitudinal direction, i.e. in the direction of axis x in the mounted position shown in Fig. 2. The cross-section of the filter bag 50 may be of any shape suitable to the purpose, for instance oval, polygonal, rectangular, e.g. square. The filter bag has a tube part 51 of any suitable filter material such as felt or woven of e.g. polyester or Teflon® (PTFE). In the embodiment shown and described, the filter bag is manufactured from Teflon®. This may be a woven Teflon layer provided with a Teflon membrane on the outside. However, other suitable polymer materials may be used. The filter material is cut to the appropriate size as a substantially rectangular width of material, and the width of material is rolled to form an elongated shape, i.e. in the embodiment shown a cylindrical shape. In the rolled-up position, two opposed edge portions of the width of material are connected to each other in a longitudinally extending joint 55 to form the tube part 51. Subsequently, the tube part 51 is connected to a top part 52 designed as a ring-shaped stiffener and a bottom part 53 shaped as a bowl-shaped stiffener. Alternatively, the connection to the top part 52 and the bottom part 53 may be carried out when shaping the cylindrical shape of the tube part 51.

The top part 52 has an upper end portion 52a having a larger diameter than the lower portion of the top part 52. A number of flaps 52b, here three, are connected to the upper end portion 52a. In the embodiment shown, the diameter of the upper end portion 52a is slightly larger than the aperture in the suspension plate 5 (cf. Fig. 1) intended to receive the bag filter. In the mounted position, the upper end portion 52a and flaps 52b are clamped between a top element of the support structure 20 and the aperture in the suspension plate by means of the suitable fastening device, thus holding the entire filter bag 50 securely in place. The top part 52 is formed by any suitable material, e.g. stainless steel. At the second or bottom end 12, the bag filter 10 has a bottom part 53 formed as substantially bowl-shaped stiffener, cf. also Fig. 2, likewise formed by any material suitable to the purpose, e.g. stainless steel. The tube part 51 is connected to the bottom part 53 at a flange portion 53a. The bottom part 53 has a central aperture 53b adapted to receive a valve to be connected with the support structure or the otherwise connected to the interior of the bag.

Figs 8 and 9 show the tube part 151 of a prior art filter bag. The longitudinally extending joint 155 in this filter bag is provided by rolling a width of filter material to an elongated shape, i.e. in the embodiment shown a cylindrical shape, overlapping two opposed edge portions 151b and 151c and connecting the edge portions 151b and 151c to each other. The connection is carried out by sewing to provide three longitudinally extending seams, here represented by a single stitch 156 in the middle seam. Prior to the sewing operation, a cover element 157 has been positioned on top of the outwards facing edge portion 151c, the cover element 157 being sewn into the seams during the sewing operation.

In contradistinction to this mechanical connection, the joint 55 of the filter bag tube part 51 according to the invention is carried out, at least partly, by means of gluing. In the embodiment shown, the entire longitudinally extending joint 55 is a glue joint 58, however, the glued joint may extend only over a limited part of the longitudinal extension of the tube part 51. As shown most clearly in Figs 10 and 11, the joint 55 is formed by overlapping edge portions 51b, 51c of the width of material, and then applying at least one strip of glue 59a, 59b to a predefined area on at least one of the mutually facing sides of the overlapping portions 51b, 51c. In the embodiment shown a strip of glue 59a, 59b is applied to each overlapping portion 51b, 51c to form the glue joint 58.

In the embodiment shown, the area, in which the glue is ap- plied, has a larger extension d in the circumferential direction than the overlap between the overlapping edge portions 51b, 51c. This entails that the glue covers the portion adjacent the end edge of the respective edge portion 51b, 51c. In turn, this entails that each overlapping edge portion 51b and 51c is attached very securely to the opposite edge portion 51c and 51b, respectively. The secure attachment is most crucial on the outwards facing side of the filter bag tube part 51, as this is the side facing the process gas to be filtered and the product.

In order to improve the adhesion properties of the surfaces to be glued together, at least the predefined area is subjected to a pre- treatment. The pre-treatment may be any process resulting in better adhesion properties of the surface, e.g. corona treatment, application of a primer, and in particular a plasma-polymerisation process or a pressurized carbon dioxide process.

In the embodiment shown, the tube part 51 is connected to the top part 52 by means of gluing as well in a top edge portion 51d of the width of material forming the filter bag tube part 51. Likewise, the bottom part 53 is connected to the tube part 51 by means of gluing a bottom edge portion 51e to the flange portion 53a.

In the filter bag according to the invention, a number of glue joints are thus carried out, partly between filter material and filter material, partly between filter material and top and bottom parts. In the embodiment shown and described, the filter bag is an all-glued filter bag involving gluing by Teflon® to Teflon®, and by Teflon® to stainless steel.

The invention is not limited to the embodiments shown and described in the above, but various modifications and combinations may be carried out without departing from the scope of the appended claims.

Claims

P A T E N T C L A I M S

1. A filter bag (50) for filtering gas, the filter bag having a substantially elongated shape and extending in a general longitudinal direction, said filter bag comprising: a tube part (51) of filter material formed by a substantially rectangular width of material, said width of material being rolled to form an elongated shape and two opposed edge portions of said width of material being connected to each other in a longitudinally extending joint (55), c h a r a c t e r i z e d in that at least a major part of said lon- gitudinally extending joint (55) is a glue joint (58).

2. A filter bag according to claim 1, wherein the joint (55) is formed by overlapping edge portions (51b, 51c) of said width of material, at least one strip of glue (59a, 59b) being applied to a predefined area on at least one of the mutually facing sides of the overlapping portions (51b, 51c).

3. A filter bag according to claim 2, wherein said predefined area has a larger extension (d) in the circumferential direction than the overlap between the overlapping edge portions (51b, 51c).

4. A filter bag according to claim 2 or 3, wherein at least said area has been subjected to a pre-treatment.

5. A filter bag according to claim 4, wherein the pre-treatment comprises a plasma-polymerisation process or a pressurized carbon dioxide process.

6. A filter bag according to any one of the preceding claims, wherein said glue joint extends along the entire longitudinally extending joint (55).

7. A filter bag according to any one of the preceding claims, wherein the tube part (51) is made of Teflon® (PTFE).

8. A filter bag according to any one of the preceding claims, fur- thermore comprising a top part (52) and a bottom part (53), wherein the tube part (51) is connected to the top part (52) and the bottom part (53) by means of gluing.

9. A filter bag according to claim 8, wherein said top part (52) and said bottom part (53) are made of stainless steel.

10. A bag filter comprising a support structure and a filter bag according to any one of the preceding claims.

11. A method of manufacturing a filter bag according to any one of claims 1-9, comprising the steps of: providing a substantially rectangular width of filter material, rolling said width of material to form an elongated shape, connecting two opposed edge portions of said width of material to each other by means of gluing in a longitudinally extending joint to provide a tube part.

12. The method of claim 11, whereby said gluing is carried out by applying at least one strip of glue to a predefined area on at least one of the mutually facing sides of the overlapping edge portions.

13. The method of claim 12, whereby said area is subjected to a pre-treatment prior to the application of said at least one strip of glue.

14. The method of claim 13, whereby said pre-treatment comprises a plasma-polymerisation process or a pressurized carbon dioxide process.

15. The method of any one of claims 11-14, whereby said glue joint is provided along the entire longitudinally extending joint.

16. The method of any one of claims 11-15 for the manufacture of a filter bag having in addition to said tube part, a top part and a bottom part, whereby said tube part is connected to said top part and said bottom part by means of gluing.