WO2020192851A1

WO2020192851A1 - Filter bag and spray drying system

Info

Publication number: WO2020192851A1
Application number: PCT/DK2019/050102
Authority: WO
Inventors: Stig Moller ANDERSEN; Martin SKANDERBY; Tobias Johansson; Sikker ROSENDAL; Todd SIWIK; Madaline DREWISKE; Göran Tobias LINDBERG; Lisa Sofia Maria KORSGREN
Original assignee: Gea Process Engineering A/S
Priority date: 2019-03-25
Filing date: 2019-03-25
Publication date: 2020-10-01

Abstract

The invention relates to a filter bag for solid-gas separation, the filter bag comprising a filter material, the filter material having a first end portion, a second end portion and a tubular portion extending therebetween, the filter bag having a substantially elongate shape, wherein the filter material of the filter bag comprises casein and/or at least one other component selected from the group consisting of: biopolymers, biopolymer derivatives and bioplastics. The invention also relates to a spray drying system and a method for solid-gas separation.

Description

Title of Invention

Filter Bag and Spray Drying System

Field of Invention

The present invention relates to a filter bag and to a spray drying system.

Background of the Invention

Filter bags for the separation of solids, such as powder particles, from gases are well known in the art. An example is provided in W02008/119346, the contents of which are herein incorporated by reference.

Filter bags, such as the ones disclosed in W02008/119346, can be uti lised in a number of ways, including in spray drying systems. In an example of a conventional spray drying system which is particularly suited to dairy pro cessing to form milk powders and infant powder formulas, three stages are used.

In a first stage of the process, a spray dryer is used to produce a powder product from a liquid feed. Flowever, the spray dryer is not able to capture all of the powder at the outlet, and so as a second step, unrecovered powder is passed to one or more cyclones. The cyclones enable additional powder to be recovered. Flowever, again, not all of the particles are captured in the cyclone step, and the gas leaving the cyclone(s) contains too many particles to simply be vented as exhaust gas. Thus, as a third step, a filtration process is used to capture remaining particles and thereby generate gas which is sufficiently free from particles to be suitable for dispensation as an exhaust gas.

The filtration step typically involves passing powder-containing gas through a filter bag housing containing a plurality of filter bags. The particles are captured by the filter bags and the gases passing through the filter can be passed to an exhaust outlet.

Conventional filter bags used in processes such as these are typically made from plastic materials such as polyester, full name polyethylene tereph- thalate. These materials have been found to provide acceptable performance. When considering performance of the filter bags, the following points, amongst others, need to be taken into consideration:

Hygiene: This is extremely important in plants which are producing prod ucts for human consumption. As part of maintaining hygiene levels that are sufficient to enable the powder to be fit for human consumption, cleaning-in- place procedures are often used. This involves washing the system in multiple stages, including with warm water, under alkaline conditions and/or under acidic conditions. It is important that the filter bag allows for hygienic pro cessing and can withstand rigorous cleaning conditions.

Safety: In Europe, Commission Regulation (EU) No 10/201 1 relates to plastic materials and articles intended to come into contact with food. There is also an FDA equivalent in the US and other requirements elsewhere. These are relevant, for example in dairy processing systems. The safety requirements for the market(s) of interest need to be satisfied, particularly in relation to the relevant migration tests, amongst others.

Lifetime: The filter bags need to be able to withstand the degree of load ing typically used on a commercial scale plant. The lifetime is important since plant downtime is required to replace filter bags. A filter bag housing for a typical plant may contain in excess of 100 filter bags and so the man hours associated with replacing filter bags is significant. The longer the lifetime of the bags, the less downtime is required which is desirable from an economic per spective.

Permeability: The filter bags need to capture particles as desired and thus the pore sizes need to be sufficient to prevent passage of particles, but with acceptable gas permeability.

Other factors such as hygroscopicity, temperature resistance and cost are also taken into consideration.

Conventional filter bags, such as those made from polyethylene tereph- thalate, are effective at satisfying the aforementioned requirements. However, conventional filter bags have a tendency to shed plastic fibres during the filtra tion process. Thus, the powder captured in the filtration step contains some plastic fibres from the filter bag. The loss of fibres from the filter bag typically only occurs to a very small extent. For example, for every ton of powder captured using the filter bags, there may be less than 10 g of plastic fibres from the filter bag contained within the powder. More specifically, there may be in the region of 0.1 - 0.5 g of plastic fibres from the filter bag for every ton of powder captured. Nevertheless, where the powder is intended for human consumption, the presence of plastic fibres in the powder remaining after filtration may not be tolerated and the pow der is therefore usually discarded.

Filter bags are commonly used in large scale industrial plants. Thus, the filtration stage will be carried out in a bag house often containing more than 50 filter bags, or in some cases more than 100 filter bags, with each filter bag usually being over 5 m in length. As a result, as much as 120 kg per hour of product can be captured by the bag filters. Thus, very large quantities of pow der are being discarded because of contamination, even at a low level, with plastic fibres.

The problem with this is two-fold. Firstly, it means that large quantities of product are wasted. Even for relatively inexpensive products, this is signifi cant waste. The cost is magnified for more expensive end products such as infant formulas. Secondly, the powder material waste product will have to be either disposed of or transformed into inexpensive products with less demand and thereby loses much of its value.

Whilst this is tolerated within conventional systems, it would be advan tageous to provide apparatus in which the losses of product are reduced. The present invention has been made from a consideration of this.

Summary of the Invention

Thus, according to a first aspect of the present invention, there is pro vided a filter bag for solid-gas separation, the filter bag comprising a filter ma terial, the filter material having a first end portion, a second end portion and a tubular portion extending therebetween, the filter bag having a substantially elongate shape, the filter bag being characterised in that filter material com prises casein and/or at least one other component selected from the group consisting of: biopolymers, biopolymer derivatives and bioplastics.

It has surprisingly been found that the materials conventionally used to make the filter material of the filter bag, for example polyethylene terephthalate, can be replaced, at least partially, with casein and/or at least one other compo nent selected from the group consisting of: biopolymers, biopolymer derivatives and bioplastics. These filter materials are surprisingly able to fulfil the require ments on the filter bag. It was previously understood that these materials, for example casein, would be unsuitable in view of the demanding conditions that a filter bag needs to withstand, especially in a commercial plant.

When a filter bag according to the present invention is used in place of a conventional filter bag, the result is decreased contamination of the filtered solid material with conventional materials such as plastic, since there is less or no plastic in the filter bag.

When casein is present, like plastic fibres, casein fibres are shed during filtration, but casein fibres are a natural material that are suitable for human consumption and as such are deemed to be a more acceptable contaminant of the captured solids. Thus, the presence of casein in the filter material reduces the extent of undesired contamination. With a lower degree of plastic contam ination, there is greater scope to use the captured powder rather than needing to discard it. This results in a higher product yield and reduced waste disposal costs.

The same applies when other biopolymers, biopolymer derivatives and bioplastics are used, either in addition to casein or instead of casein. Fibres of the biopolymer, for example, are shed during filtration, but these are deemed to be a more acceptable contaminant of the captured solids.

Further, by replacing conventional materials, namely plastic, with a nat urally occurring, more environmentally friendly material, the negative environ mental impact of systems utilising such a filter bag is reduced.

These advantages can be obtained by incorporating filter bags accord ing the present invention in newly built plants, and also by retrofitting the filter bags in place of conventional filter bags in existing plants. When the filter bags of the present invention are used in new plants, the plants can be designed without any cyclones since they are unnecessary if solids are recovered after filtration. Thus, new plants incorporating the filter bags are more cost effective.

The term“biopolymer” refers to polymers that occur in nature i.e. are naturally occurring. However, it does not mean that the polymers need to have been created in a natural process. Thus, it also encompasses polymers that are naturally occurring but which have been made via a synthetic route. Alter natively, the biopolymers may be created in a naturally occurring process and then isolated by further treatment(s).

The term “biopolymer derivatives” refers to components that are de rived/obtained from biopolymers. An example is polydextrose, which is a syn thetic polymer of glucose. It is formed from dextrose which can be obtained by hydrolysis of starch. Starch is a biopolymer and polydextrose is thus derived from this.

The term“bioplastic” refers to plastic materials that are derived from bi ological substances rather than from petroleum.

Casein is an example of a biopolymer. Other biopolymers and biopoly mer derivatives include cellulose, hemicellulose, lignin, beta-glucans, pectin, gum arabic, mucilage, polydextrose polyols, psyllium, starch, wheat dextrin, zein, soy protein, lentil protein, peanut protein, whey protein, gelatine, pullulan, guar gum, collagen, chitosan, carrageenan or amylose. These biopoly mers/derivatives may be present in the filter material in addition to casein or as an alternative to casein. The presence of other biopolymers can be advanta geous as the biopolymer can be used to adjust the properties of the filter bag, whilst still giving rise to the advantages discussed above. The same applies to biopolymer derivatives.

Bioplastics may be present in the filter material in addition to casein or as an alternative. The bioplastic may be polylactic acid. The presence of bio plastics can be advantageous as the bioplastic can also be used to adjust the properties of the filter bag, whilst still giving rise to the advantages discussed above.

The biopolymer and derivatives may be edible. The term“edible” is well known in the art. Edible species are those which are suitable or safe to be eaten by humans. Edible species need not necessarily be digestible by hu mans, but must be non-toxic to humans and therefore safe for consumption.

The filter material may comprise at least 50 wt% biopolymer, or at least 75 wt% biopolymer, or derivatives thereof, based on the weight of the filter ma terial. Likewise, the filter material may comprise at least 50 wt% bioplastic, or at least 75 wt% bioplastic, based on the weight of the filter material.

The filter material may comprise at least 50 wt% casein, or at least 75 wt% casein, based on the weight of the filter material.

It is generally the case that the more conventional material e.g. polyeth ylene terephthalate, is replaced with biopolymer, for example casein, the smaller the extent of undesired contamination by fibres from the filter bag. However, the less conventional material there is in the filter bag, the less influ ence the conventional material has on the properties of the bag. It was previ ously believed that materials such as biopolymers did not have appropriate characteristics for use in a filter bag. However, it has now surprisingly been found that biopolymers, for example casein, or biopolymer derivatives or bio plastics can be present, even as the major component of the filter material, and the filter bag can still have the required properties. In such circumstances, the contamination by plastics may be reduced to such an extent that it is deemed to be negligible, even for powders intended for human consumption. Thus, the resulting captured powder has greater scope to be used rather than discarded.

The filter material may further comprise one or more additives selected from the group consisting of: citric acid.

It has been found that the properties of the filter bag can be improved by the presence of additives that are natural products, or natural product precur sors, and suitable for human consumption. It will be understood that natural materials are materials that occur in nature. However, the materials need not necessarily have been produced by natural processes. Thus, the term natural materials encompass materials that are naturally occurring but have been gen erated by a synthetic route.

Any suitable additive can be used. For example, citric acid can be used as an additive. It has been found that citric acid combines with the casein to provide mechanically stronger fibres compared with equivalent fibres of pure casein. Thus, the lifetime of the corresponding filter bag is increased. Citric acid also has antimicrobial properties and therefore increases the level of hy giene of the filter bag. Citric acid may also be used with other, biopolymers, biopolymer derivatives and bioplastics.

For additive(s) that are natural products or natural product precursors, that are suitable for human consumption, if they also shed during the filtration process, the result is a safe and acceptable contamination and so the filtered solids/powder can still be used, even when it is intended for human consump tion.

The filter material may consist essentially of, or consist of, one or more edible biopolymers and optionally citric acid.

Thus, in these embodiments, the filter material contains no plastic. Therefore, the only fibres that would be shed from the filter bag and end up in the powder when the bag is in use would be casein fibres and/or other edible biopolymers, plus any additional citric acid. The result is that the corresponding powder is free from plastic contaminants and contains only powder and fibres that are safe for human consumption. These filter bags are particularly suitable for processes where even very low levels of plastic contamination are deemed to be unacceptable. For example, in the production of infant formula, safe lev els of plastic contamination are still rejected by many customers, since con sumers do not generally approve of even very low level plastic contamination. For embodiments where the filter material contains no plastic material, the pow der will only be contaminated with safe, acceptable materials from the filter bag.

The filter material may consist essentially of, or consist of, casein.

Thus, the filter material contains no plastic material. The only fibres that are shed from the filter bag and end up in the powder are casein fibres. This can be particularly beneficial when the filter bag is to be used in processes for drying milk powder or infant formula. The powder is likely to contain fibres from the filter bag, but the fibres will be purely casein. The result is that the only contaminant in the powder, e.g. milk powder, is casein, which is a component of milk. Citric acid may optionally be present. The filter bag may comprise only the filter material i.e. no other compo nents are present. Thus, the filter bag itself may consist essentially of, or con sist of, biopolymers, biopolymer derivatives or bioplastics or may consist es sentially of, or consist of casein.

In embodiments where the filter bag has components other than the filter material, the other components will preferably be inert i.e. arranged so that they do not come into contact with the solid material being separated, or formed so that they do not in any way contaminate the solid material. Thus, the captured solids are only contaminated with fibres from the filter material, and the filter material is formulated so as to only give acceptable contamination.

Alternatively, other components of the filter bag may be formulated to only give acceptable contamination.

The filter material may be in the form of woven multifilament or needle punched fibres.

These forms of filter material have acceptable porosity, gas permeability and strength properties and are cost effective.

The filter material may have an air permeability in the range of from 100 to 300 dm³/dm²,min as measured according to EN ISO 9237. This gives ac ceptable performance, particularly in terms of the pressure drop that can be expected from the filter bag in use.

The filter bag is particularly suitable for separation of nanoparticles from gas.

The filter bag may be adapted for the separation of milk powder or infant formula powder from a gas. The gas may be any suitable gas, such as air or other process gas.

The filter bag may be adapted for connection with a support structure.

Any suitable adaptations, such as one or more connection means, for example flaps or connectors, may be used. Filters bags that are adapted for connection with a support structure are advantageous as the filter bag does not in itself need to have the required properties to maintain the desired shape in use. So, for example, the filter bags may be made from a flexible material which alone would not be able to maintain the desired shape when the filter bag is in use, but which can maintain the desired shape when used with an appropriate support structure.

In a second aspect of the present invention, there is provided a bag filter comprising at least one filter bag according to the first aspect of the present invention. Whereas the filter bag comprises the filter material which effects gas-solid separation, the bag filter additionally includes a support structure and optionally other ancillary items such as connectors, which facilitate use of the bag filter in filtration units.

In a third aspect of the present invention, there is provided a spray drying system comprising a spray dryer in fluid communication with a filter unit, the filter unit comprising at least one filter bag according to any embodiment of the first aspect of the invention, and/or at least one bag filter according to any em bodiment of the second aspect of the invention.

The spray drying system incorporating one or more filter bags according to the first aspect of the present invention benefits from the advantages of the first aspect of the invention. Likewise, the spray drying system incorporating one or more bag filters according to the second aspect of the present invention benefits from the advantages of the first and second aspects of the invention.

As a result, the spray drying system has a significantly higher yield than conventional systems because the reduced contamination of the product of fil tration means that the powder can be retained as product rather than dis carded.

The fluid communication between the spray dryer and the filter unit may be direct, or may be via intermediate components such as a cyclone.

Optionally, the spray drying system does not comprise one or more cy clones.

As previously noted, in conventional systems, cyclones are often used after the spray drying step to increase yield of product. However, as the filter bag of the present invention reduces undesired contamination and thereby al lows the product after filtration to be retained rather than discarded, the cyclone step can be omitted. Particles which are not recovered in the spray drying step can be passed directly to a filtration step, and since the powder recovered has reduced plastic contamination, it can be retained for use as product. As a re sult, the spray drying system has a significantly higher yield than conventional systems. These embodiments of the spray drying system are also cheaper than conventional systems using a cyclone as there is no need to pay for in stallation and maintenance of one or more cyclones. The corresponding foot print of the spray drying system is also smaller, again reducing costs compared with conventional systems.

In a fourth aspect of the present invention, there is provided a method for solid-gas separation, the method comprising the steps of:

i) providing a liquid containing particles;

ii) passing the liquid containing particles to a spray dryer;

iii) recovering a gas stream from the spray dryer, wherein the gas stream contains gas and particles;

iv) passing the gas stream to a filtration unit, wherein the filtration unit comprises one or more filter bags according to any embodiment of the first aspect of the invention and/or wherein the filtration unit comprises one or more bag filters according to any embodiment of the second aspect of the invention; and

v) recovering particles from the filtration unit.

The method benefits from the advantages discussed above.

In the method, the liquid containing particles may comprise a dairy prod uct. Dairy product includes, but is not limited to, dried milk powder, whole milk, skimmed milk and infant formula. The result is a method of providing a dried dairy product benefitting from the aforementioned advantages.

In a further aspect of the invention, there is provided use of a filter bag for solid-gas separation, wherein the filter bag comprises a filter material and the filter material comprises casein. The use may apply to all embodiments outlined in connection with the first aspect of the invention.

It will be understood that embodiments outlined in connection with the first aspect of the invention may be implemented in all other aspects of the invention. Brief Description of Drawings

Embodiments of the present invention will now be described, with refer ence to the following non-limiting examples and figures, in which:

Fig. 1 shows a schematic view of a filter bag according to the present invention;

Fig. 2 shows a schematic side view of a filter unit including two of the bag filters of the present invention;

Fig. 3 shows a perspective view of a support structure for use with the filter bag;

Fig. 4 shows a schematic representation of the conventional three stage process described in the Background of the Invention;

Fig. 5 shows a schematic representation of a spray drying system ac cording to the invention. Description of Embodiments

Fig. 1 illustrates schematically a filter bag 2 according to the present invention. The filter bag 2 is flexible, but in the extended configuration shown it is substantially cylindrical, with a diameter of 100-250 cm and a length of 0.7- 7 m, for example 7 m.

In the embodiment shown, the shape of the filter bag 2 is defined by the shape of the filter material 4 combined with a support structure (not shown). The filter material has a first end portion 4a, a second end portion 4c and a tubular portion 4b. The filter material is made from needle pressed casein fi bres and has a thickness in the range of approximately 1-3 mm, as measured according to EN ISO 5084. The filter material contains casein. In the shown embodiment, the filter bag is adapted for connection with a support structure (not shown) via a collar (also not shown).

At the top end 6 of the filter bag 2 as shown, the bag is open. In some embodiments, the end portion of the filter material forming the top end 6 of the filter bag may be combined with a rim component to add rigidity to the opening and/or to assist with affixing the filter bag 2 in position for use, either in combi nation with a support structure or otherwise. At the lower end 8 of the filter bag, the bottom end is closed by the filter material. The closed end may be formed integrally with the tubular portion of the filter material, or connection means such as a sewn seam may be used for the attachment of separate pieces of filter material.

In other embodiments, the filter bag may have at its lower end an end closure of an impermeable, either flexible material or a rigid and strong mate rial, such as steel, stainless steel or another metal, polymers or ceramics. Thus, an end portion of the filter material may combine with another component to define the lower end of the filter bag.

Filter bags according to the present invention may have alternative ge ometrical shapes, for example, the cross section may be circular, rectangular, triangular, square, oval, or any other suitable shape.

The proportions of the filter bag may be selected according to the appli cation. For example, the filter bags may have a cross section of from 1 m² to 6.25 m² per bag. Likewise, the filter bags may have a length of from 1 m to 12 m, for example at least 3 m, or at least 5 m, or at least 6 m.

The filter bag of the present invention may be made by any suitable means. For example, rather than needle punching the filter material, it may be woven. The manufacturing process may involve heat setting, and/or singeing and/or calendering.

Commercially available fibres may be used to make the filter material, or alternatively, casein fibres may be formed from milk according to processes known to the person skilled in the art. A method of making edible casein fibres is disclosed in US patent application no. 3,865,959. Casein can broadly be categorised into two groups: rennet casein and acid casein. Both types are suitable for use in the present invention.

The air permeability of the filter bag may be selected according to the application, although generally an air permeability in the range of from 100 to 300 dm³/dm²,min as measured according to EN ISO 9237 is acceptable.

Likewise, the strength required for the filter bag may depend on the ap plication. However, filter materials that have a tensile strength of around 1400 - 1500 N/5cm, as measured according EN ISO 13934-1 , will be suitable for many applications.

Fig. 2 illustrates schematically a filter unit 10 employed for separating product particles from a process gas coming from e.g. a spray drying appa ratus, 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, dye stuffs, chemical products etc. Preferably, the products are dairy or food prod ucts. The process gas can be e.g. heated air or drying gas or special gas com- positions inert to the products treated in the apparatus.

In the embodiment of Fig. 2, the filter unit is shown as a separate ex ternal 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 pro cessing unit producing the particle loaded gas, such as a spray drying appa- ratus or a fluid bed apparatus. In the following description, the terms“filter”, “bag filter”,“filter bags” etc. denote elements forming part of either such a sep arate unit or an integrated unit.

A filter unit housing is composed of a vertically arranged cylindrical upper section 12 connected with a downward tapered lower section 14. An inlet (not shown) for process gas with product to be filtered off is arranged in the lower part of the cylindrical section 12a and an outlet (not shown) for filtered clean gas is arranged in the upper part of cylindrical section 12b. At the bottom of the lower section 14, an outlet port 16 is arranged for extraction of retained product.

A horizontal suspension plate 18 is arranged in the upper part of the cylindrical section 12 and divides the housing in an upper outlet side with a clean-gas chamber 20 and a lower inlet side 22. The plate 18 has a number of holes, in which filter bags 2 are suspended approximately vertically with up- wards-open ends that deliver filtered gas to the clean-gas chamber 20.

The filter bags 2 are inserted into apertures (not shown) in the suspen sion plate 18 and the first end portion 4a of each filter is connected with the suspension plate 18 in a manner to be described in further detail below. The number of filter bags in the filter unit depends on the desired filter capacity. The smallest filter has a single filter element. Plants for treating, handling or produc ing pharmaceuticals can use smaller filter units having e.g. from 2 to 25 filter bags, and plants for foodstuffs, dairies and chemicals can comprise many hun dreds of filter bags. For all of the above-mentioned applications, the filter bags may be located either in a separate filter unit or integrated in the plant.

As will be described in further detail below, each filter bag may be pre sent as a bag filter 24, which will comprise a filter bag 2 and a support structure 26. Where present, 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 26 is shown in Fig. 3. This support structure 26 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.

The support structure can be made in several sections mounted in ex tension of each other. The top end of the support structure is suspended in the hole in the suspension plate 18 by means of an upper collar of a larger diameter than the hole and being placed on the upper side of plate 18.

Further details of support structures can be found in W02008/1 19345, the contents of which are herein incorporated by reference.

During operation of the filter unit 10, process gas carrying product en ters 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 24 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 24. 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 ex tracted through the outlet port 16. During filtration, a flow of filtrated gas streams vertically up into clean-gas chamber 20. 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.

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 com plete interior of the filter unit is washed with a cleaning liquid. Such cleaning of the filter unit carried out in between periods of operation involves cleaning noz zles (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 com munication 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 may be drained off by means at the bottom of each bag filter, e.g. by a gravity forced valve or by a controlled valve.

Fig. 4 shows a schematic representation of the conventional spray dry ing system described in the background of invention section. It uses a spray dryer 500, cyclone 502 and filter unit 504. The filter unit 504 contains a plurality of filter bags made from polyethylene terephthalate.

Product to be treated is introduced into the spray dryer via inlet 506. The powder is dried within the spray dryer and dried powder particles are re covered in the lower part of the spray dryer via powder outlet 508. From the outlet, the powder product may pass to a product container 510, and/or may be processed further.

Not all of the powder particles can be captured in this way and a mixed stream containing gas, for example air or other inert process gas, and powder particles passes from the spray dryer 500 to cyclone 502 via a first connection conduit 512.

In the cyclone 502, further powder particles are separated from the gas. Captured powder is recovered via cyclone outlet 514 and passes to prod uct container 510 for storage and/or further treatment. The remaining powder and gas passes through second connection conduit 516 to the filter unit 504.

In the filter unit, remaining particles are separated from the gas using polyethylene terephthalate filter bags (not shown). The particle free gas is then released in exhaust gas outlet 518 and the powder is released in filtered pow der outlet 520. In these conventional systems where the filter bags are made from polyethylene terephthalate, the powder recovered in filtered powder outlet 520 is either disposed of or transformed into inexpensive products.

Fig. 5 shows an equivalent system according to the present invention. The filter unit 600 comprises a plurality of filter bags of the type shown in Fig. 1 . The need for any cyclones is obviated. Thus, the spray dryer 500 can be connected directly to the filter via third connection conduit 602. The use of filter bags according to the present invention (not shown) in the filter unit 600 allows powder to be recovered and passed to the product container 510 via powder recovery line 604, rather than discarded as in conventional systems.

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

Whilst the use of an edible biopolymer, for example casein, has been described in connection with filter bags, it will be understood that the principle may also be used in other applications. For example, the filter need not nec essarily have the form of a filter bag, and may instead be formed so that it can be used at the inlet to the spray dryer, for example, instead of HEPA filters.

Filters comprising an edible biopolymer, for example casein, may also be used in alternative post-processing of powders and in powder conveyer sys tems. Likewise, in these circumstances, the filter need not be shaped as a filter bag, but instead may have the form of a plate, a sheet, a cone etc. Edible biopolymers, for example casein, may also be used in packaging for powder materials.

The same applies to biopolymer derivatives and bioplastics.

List of Reference Numerals

2 filter bag

4 filter material

4a first end portion of filter material

4b tubular portion of filter material

4c second end portion of filter material

6 top end of the filter bag

8 lower end of the filter bag

10 filter unit

12 vertically arranged cylindrical upper section of filter unit 12a lower part of the cylindrical section

12b upper part of the cylindrical section

14 downward tapered lower section of filter unit

16 outlet port

18 horizontal suspension plate

20 clean-gas chamber

22 lower inlet side

24 bag filter

26 support structure

121 , 122 threads or rods of support structure

127 rings or annular rods of thread

500 spray dryer

502 cyclone

504 filter unit (prior art)

506 spray dryer inlet

508 spray dryer powder outlet

510 product container

512 first connection conduit

514 cyclone outlet

516 second connection conduit

518 exhaust gas outlet filtered powder outlet filter unit (invention) third connection outlet powder recovery line

Claims

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

1. A filter bag for solid-gas separation, the filter bag comprising a filter material, the filter material having a first end portion, a second end portion and a tubular portion extending therebetween, the filter bag having a substantially elongate shape,

the filter bag being c h a r a c t e r i z e d in that filter material comprises casein and/or at least one other component selected from the group consisting of: biopolymers, biopolymer derivatives and bioplastics.

2. A filter bag according to Claim 1 , wherein the biopolymers and biopol- ymer derivatives are selected from the group consisting of: cellulose, hemicel- lulose, lignin, beta-glucans, pectin, gum arabic, mucilage, polydextrose polyols, psyllium, starch, wheat dextrin, zein, soy protein, lentil protein, peanut protein, whey protein, gelatine, pullulan, guar gum, collagen, chitosan, carrageenan or amylose.

3. A filter bag according to any of Claims 1 or 2, wherein the biopoly- mer(s) and/or derivatives are edible.

4. A filter bag according to any preceding claim, wherein the at least one bioplastic is selected from the group consisting of: polylactic acid.

5. A filter bag according to any preceding claim, wherein the filter mate- rial comprises at least 50 wt% casein, or at least 75 wt% casein, based on the weight of the filter material.

6. A filter bag according to any preceding claim, wherein the filter mate rial further comprises one or more additives selected from the group consisting of: citric acid.

7. A filter bag according to any preceding claim, wherein the filter mate rial consists essentially of, or consists of, one or more edible biopolymers and optionally citric acid.

8. A filter bag according Claim 7, wherein the filter material consists essentially of, or consists of, casein and optionally citric acid.

9. A filter bag according to any preceding claim, wherein the filter mate rial is in the form of woven multifilament or needle punched fibres.

10. A filter bag according to any preceding claim, wherein the filter ma terial has an air permeability in the range of from 100 to 300 dm³/dm²,min as measured according to EN ISO 9237.

1 1 . A filter bag according to any preceding claim, wherein the filter bag is adapted for the separation of milk powder or infant formula powder from a gas.

12. A filter bag according to any preceding claim, wherein the filter bag is adapted for connection with a support structure.

13. A bag filter comprising:

at least one filter bag according to any one of Claims 1 to 12 and;

a support structure.

14. A spray drying system comprising a spray dryer in fluid communi cation with a filter unit, the filter unit comprising at least one filter bag according to any one of Claims 1 to 12, and/or at least one bag filter according to Claim 1 1 .

15. A spray drying system according to Claim 14, which does not com prise one or more cyclones.

16. A method for solid-gas separation, the method comprising the steps of:

i) providing a liquid containing particles;

ii) passing the liquid containing particles to a spray dryer;

iv) passing the gas stream to a filtration unit, wherein the filtration unit comprises one or more filter bags according to any of Claims 1 to 12 and/or wherein the filtration unit comprises one or more bag filters accord ing to Claim 13; and

v) recovering particles from the filtration unit.

17. A method according to Claim 16, wherein the liquid containing particles comprises a dairy product.

18. Use of a filter bag for solid-gas separation, wherein the filter bag comprises a filter material and the filter material comprises casein and/or at least one other component selected from the group consisting of: biopolymers, biopolymer derivatives and bioplastics.