Patent Description:
In the field of powder drying, high demands to the sanitary conditions of the system are present in general, and cleaning requirements for the drying and powder handling equipment are normally prescribed, typically by use of automated cleaning-in-place systems (CIP systems).

Cleaning of the individual operational units of the powder drying system typically takes place by means of a dedicated cleaning arrangement provided in connection with or in the operational unit itself. This applies also to the filter unit of the powder drying system. An example of a prior art arrangement is seen in Applicant's <CIT> where the filter unit is provided with a number of filter elements in the form of bag filters and filter cages. A further improvement of this arrangement is shown and described in Applicant's published <CIT>.

While these cleaning arrangements have proven to function well, there is an ongoing aspiration to improve the overall operating conditions of in particular the filter unit even further.

This has proven particularly important as the bag filters of the filter units of the powder drying system are made longer, and as a consequence, the cleanability of the bag filters is reduced as it is difficult to provide a burst of air that sufficiently cleans a long bag. Furthermore, longer bag filters create new problems when maintenance of the bag filters is required. Long bag filters are heavy which introduces health and safety issues for the workers who handle these bags. Long bag filter cages also generally require a joint as most installation sites do not have enough clearance above the filter unit. This joint increases the risk of incorrect assembly which can lead to cage separation and equipment damage. Additionally, long bag filters also lead to reduced utilization of the filter area and an increased pressure drop, just as the length in itself gives rise to problems with vibrations.

Attention has turned also to the issue of sufficiently distributing the gas to be filtered in the filtering chamber of the filter unit. Various different inlet arrangements exist in the prior art, for example <NUM>° scrolled inlet, <NUM>° scrolled inlet, single tangential inlet and double tangential inlet. An example of a single tangential inlet is seen in <CIT>. The scrolled inlets have the advantage over the tangential inlets of requiring a smaller plant footprint, since additional space around the filter unit is not required. However, the geometry of the scrolled inlet introduces problems with bag vibration mainly due to flow instabilities, causing unnecessary bag filter wear. The tangential inlets mitigate some of these problems with vibrations but do not completely solve the problem and requires a large amount of plant footprint increasing the space requirement of the powder drying system. All of these inlet types also require a long straight section before the inlet into the filter unit to reduce flow instabilities to an acceptable level. These straight sections generally require high flow speeds to ensure that particles or dust carried by the flow are not separated from the flow and fall to the bottom of the duct sections.

However, such high speed of the gas to be filtered at the entry into the filtering chamber is not desirable from an operational point of view. These challenges are particularly pronounced in fields such as food and dairy production, in which the demands to the sanitary conditions are strict, and hence, the interior walls of all vessels, ducting and other conduits need to be smooth. As a consequence, it is not desirable to provide such sanitary systems with vanes and other flow restrictors that could otherwise diffuse the flow in the inlet ducting.

Also <CIT> discloses a powder drying system and method according to the prior art.

With this background, it is therefore an object of the present invention to provide a powder drying system, by which it is possible to mitigate some of the drawbacks of the prior art.

The invention is a powder drying system according to claim <NUM> and a method of operating a powder drying system according to claim <NUM>.

In a first aspect, these and further objects are obtained by a powder drying system of the kind mentioned in the introduction, which is furthermore characterized in that the inlet of the filtering chamber is positioned at the top portion of the filter unit, and that said inlet includes at least one inlet duct section adjacent the filtering chamber and arranged substantially centrally in the top portion of the filter unit.

Surprisingly, a substantially central inlet at the top portion of the filter unit decreases the required footprint for the inlet. The previously used footprint for a prior art inlet can instead be used for additional bag filters. This allows more space to be allocated to bag filters, increasing the number of bag filters included in the filter unit. This is an advantage since a better utilization of the total filter area is achieved by having a larger number of shorter bag filters in comparison with a smaller number of longer bag filters. This is due to the fact that the pressure drop in short bag filters is substantially lower than in long bag filters. The possibility to accommodate more bag filters in the same volume thus more than compensates for the reduced bag filter height in terms of capacity. The substantially centrally located inlet also has the advantage of allowing a slower inlet flow speed leading to fewer problems with bag filter cage vibrations. The term "substantially central" should be interpreted as encompassing also such positions of the inlet in which the center of the inlet is located somewhat off-center relative to the exact geometrical center of the filter unit.

The inventive concept is based on the recognition that the inlet types of powder drying systems of the prior art suffer from the same problem. Either the inlets of the prior art require a lot of plant footprint in order to ensure an acceptable flow behavior or an ill-behaved flow is let in the filter unit causing bag filter vibrations and wear. Furthermore, a large portion of the plant foot print is reserved to reduce flow instabilities of the inlet and thus not exploited as filtering space for bag filters. The improved utilization of the available space entails that it is either possible to increase the capacity of a filter unit with unchanged length of the bag filters, or reduce the length of the bag filters. In case it is chosen to reduce the length of the bag filters, the inventive concept thus alleviates the disadvantages of the prior art arrangements in that it is possible to obtain a reduced weight of each bag filter and without the need for joints, in addition to reducing the problems associated with pressure drop and vibrations.

According to the invention, said at least one inlet duct section is arranged substantially in parallel with the central vertical axis such that the gas entering the filter unit is allowed to flow substantially downwards. This configuration increases the advantages of the invention and contributes in particular to the reduction of detrimental flow conditions in the filtering chamber.

According to the invention, said at least one inlet duct section is substantially coaxial with the central vertical axis such that the gas is allowed to enter the filtering chamber substantially along the central vertical axis. This increases the advantages even further.

In a second aspect, a method of operating a filter unit of such a powder system is devised.

Further presently preferred embodiments and further advantages will be apparent from the following detailed description and the appended dependent claims.

The invention will be described in more detail below by means of non-limiting examples of presently preferred embodiments and with reference to the schematic drawings, in which:.

<FIG> shows a schematic view of the main components of a powder drying system comprising a powder processing unit which in the embodiment shown is in the form of a spray drying system <NUM>. In a manner known per se, the spray drying system <NUM> comprises a spray dryer with a drying chamber <NUM> and a process air/gas supply device <NUM>, typically including an air/gas disperser. It is noted that the term "gas" will be used alongside with the term "air" as "air/gas" and is to be interpreted as encompassing any gas that is suitable as process gas in such a spray drying system. The drying chamber <NUM> also incorporates atomizing means, such as nozzles and/or an atomizer wheel. The term "powder drying system" is intended to encompass such systems in which a powdery or particulate material is formed and/or processed. The material may either be provided as a feed of powdery or particulate material, or as a liquid feed to be dried. The powder drying system is also intended to cover cooling of the particulate material. In addition, or alternatively, to the spray dryer described, such a system could include one or more fluid beds, pneumatic dryers etc. The powder drying system thus incorporates a unit for forming powder in any suitable manner. Here, the powder forming unit is a powder processing unit in the form of a spray dryer with a drying chamber.

At the lower end of the drying chamber <NUM>, an outlet <NUM> for dried material is provided. In the shown spray drying system <NUM>, an after-treatment unit in the form of vibrating or static fluid bed <NUM> is provided. At one end, the vibrating or static fluid bed <NUM> receives dried material from the outlet <NUM> of the drying chamber <NUM> for further treatment of the material, which is then to be collected at an outlet at the other end of the vibrating or static fluid bed. Further upstream or downstream equipment may be present as well, but is not relevant to the present invention.

Furthermore, the powder drying system comprises in addition to the spray drying system <NUM> a filter unit <NUM>, to which spent process air/gas with particles entrained in the process air/gas is conducted. The filter unit <NUM> has a configuration which will be described in further detail below. In <FIG> is shown an inlet <NUM> for spent process air/gas from one or more of the upstream operational units, a plurality of bag filters <NUM> and a clean air outlet <NUM>. The filter unit <NUM> may form part of a series of powder recovery units including further filter units and cyclones or bag filters, or any combination thereof. Furthermore, a cleaning arrangement <NUM> is shown in <FIG>.

A number of conveying lines connect the operational units with each other in a manner known per se and will not be described in detail.

The general configuration of the filter unit <NUM> will now be described in more detail with particular reference to <FIG>.

The filter unit <NUM> defines a central vertical axis <NUM> and includes a filtering chamber <NUM>, a top portion <NUM> and an exhaust chamber <NUM>. The top portion <NUM> here has the form of a cylinder with closed top surface. The top portion may also have other geometrical shapes, such as square, rectangular, hexagonal or other polygonal shape. Additionally, the filter unit <NUM> has a bottom portion <NUM>, which is here substantially frusto-conical, but which may in principle have any suitable configuration. The bottom portion <NUM> delimits the lower section of the filtering chamber <NUM>. A central portion <NUM> is provided between the top portion <NUM> and the bottom portion <NUM> of the filter unit. The plurality of bag filters <NUM> is in the embodiment shown located in the central portion <NUM>, which is here provided as a substantially cylindrical portion of the filter unit <NUM> adjacent the top portion <NUM>, i.e. in the upper section of the filtering chamber <NUM>.

The filter unit <NUM> is arranged such that the gas entering from the inlet <NUM> flows into the filtering chamber <NUM>, through the bag filters <NUM>, out through the top opening of the bag filters <NUM> and into the exhaust chamber <NUM>. As is apparent, the inlet <NUM> and the outlet <NUM> are connected to the top portion <NUM> of the filter unit <NUM> and arranged such that the gas enters the filter unit <NUM> along the central vertical axis <NUM> in the specific embodiment shown. The powder may be collected at the bottom portion <NUM> of the filter unit <NUM>.

It is important that the inlet is arranged such that the gas flow, when it enters the filter unit, has as few flow instabilities as possible. Flow instabilities can be defined as the gas flow having a transient character as opposed to a steady state flow. According to the invention said inlet <NUM> and outlet <NUM> are connected to the top portion of the filter unit <NUM> and arranged such that the gas enters the filter unit <NUM> along a central vertical axis. This has the advantage of reducing the susceptibility of bag filters to flow instabilities by having a gas flow in parallel with the length axis of the bag filters <NUM> thus reducing the transient perpendicular force on the bag filters <NUM>.

The top portion <NUM> of the filter unit <NUM> is located at the uppermost part of filter unit <NUM>, for example as a substantially cylindrical portion with a closed top, along the central vertical axis <NUM>.

According to the invention, the at least one inlet duct section of the inlet <NUM>, adjacent the filtering chamber <NUM> comprises an angled duct <NUM>, a transitional duct <NUM>, and an internal duct <NUM>. The angled duct <NUM> connects the duct having gas carrying dust from the previous component of the spray drying system with the transitional duct <NUM>. According to the invention, the angled duct <NUM> has the form of pipe segments forming substantially a <NUM>-degree bend, and the transitional duct <NUM> may have a shape changing from hexagonal to circular. The shape of the internal duct is chosen to improve the explosion pressure resistance of the top portion <NUM>. The transitional duct <NUM> is attached to the top portion <NUM> of the filter unit <NUM> and the internal duct <NUM>. The internal duct <NUM> connects the transitional duct <NUM> with the filtering chamber <NUM> of the filter unit <NUM>. Thus, the inlet <NUM> allows gas from the previous or upstream component of the spray drying system to enter the filtering chamber <NUM> of the filter unit <NUM>.

In the embodiment shown, the plurality of bag filters <NUM> is located in the space between the periphery of the filter unit <NUM> defined by the central portion <NUM> and the periphery of the extension of the internal duct <NUM> which here forms the inlet duct section adjacent the filtering chamber <NUM>. This has the advantage that the stream of gas to be filtered flows into the filtering chamber <NUM> at a location where no bag filters <NUM> are present. This is particularly clear from <FIG> and <FIG>.

Referring now in particular also to <FIG>, the outlet <NUM> comprises a transitional duct <NUM> and a straight portion <NUM> as shown in <FIG>. In one embodiment of the invention the edges 421a between the exhaust chamber <NUM> and the transitional duct <NUM>, and the edges 421b between the transitional duct <NUM> and the straight portion 421b are rounded. This has the advantage of allowing a shorter transitional duct <NUM> which reduces the plant footprint required by the filter unit <NUM>. It also reduces the risk of remaining deposits and/or cleaning agent after cleaning-in-place.

In the filter unit <NUM>, a number of elongated tubular filter elements or bag filters <NUM> are suspended substantially vertically in a support structure. The bag filters of the filter elements have a filter surface which at least includes the bag filter wall, which is typically cylindrical. The bag filter walls are typically made of filter wall material that can be a substantially soft material, such as a felt, a polymer mesh or weave, supported by a basket in the interior of the bag, or the filter wall material can be a self-supporting substantially rigid porous material, such as metal fibers or ceramic fibers. The bottom of each bag filter <NUM> may either be provided by the same filter material as the bag filter wall, or provided as a solid bottom, possibly also provided with draining means.

Turning now to <FIG>, the configuration of the bag filters <NUM> into a plurality of bag filter segments is shown.

The bag filters <NUM> of the filter unit <NUM> can be arranged in bag filter segments <NUM>, as shown in the embodiment of <FIG>. According to the invention, the filter unit <NUM> can comprise a plurality of bag filter segments <NUM>. In this embodiment the filter unit <NUM> comprises six bag filter segments <NUM> and the number of sides of the internal duct <NUM> correspond to the number of bag filter segments. The bag filter segments <NUM> allow increased stiffness of the supporting structure and also contribute to facilitating the handling of the components of the filter unit <NUM> during installation and maintenance, including release and removal of the individual filter bag segments <NUM>. The number of bag filter segments may also be for instance two, three, four or any other suitable number. The distribution of bag filters in each segment may be equal or vary.

The dimensions and number of filter elements including bag filters <NUM> in the filter unit <NUM> depends on the desired filter capacity. The smallest filter has a single filter element. Plants for treating, handling or producing pharmaceuticals typically use smaller filter units having for instance from <NUM> to <NUM> filter elements, and plants for foodstuffs, dairies and chemicals typically use very large filter units with many hundreds of filter elements in a single filter unit. The bag filters can be arranged in a square grid as shown in <FIG>, alternatively the bag filters could be arranged in triangular grid, a hexagonal grid, or any other type of polygonal grid. The diameter of each bag filter is shown as D and the distance between each bag filter is shown as S. As a general rule the capacity of the system increases with increasing filtering area. The filtering area being defined as the surface area of the bag filters. The individual filter element according to the invention has a length in the range from <NUM> to <NUM>, for instance the length is typically at least <NUM> meters long, but lengths over <NUM> meters long or even over <NUM> meters long are conceivable as well.

The diameter D of each bag filter is typically in the range of <NUM> to <NUM>, preferably in the range <NUM> to <NUM>, and even more preferably in the range <NUM> to <NUM>, in the embodiment shown about <NUM>.

The distance between neighboring bag filters, S, is typically no more than <NUM>, preferably no more than <NUM> and even more preferably no more than <NUM>.

The number of bag filters <NUM> may be at least <NUM> bag filters <NUM>, preferably at least <NUM> bag filters <NUM>, more preferably at least <NUM> bag filters, and more preferably at least <NUM> bag filters.

During operation of the filter unit <NUM> process gas carrying product enters the unit through inlet <NUM> and flows into the area around the filter elements in the form of individual bag filters <NUM>. The gas continues through the bag filter walls of the filter elements <NUM> and flows up to an upper outlet side for clean filtered gas and eventually exits through the outlet <NUM>. As the gas passes the filter walls product carried by the process gas is retained by the filter elements <NUM>. The retained material is partially left on the filter elements and partially drops down and accumulates in the lower section of the filter unit <NUM>. The accumulated product can then be extracted through an outlet port. The filter unit may be a separate external unit connected to a gas outlet for particle loaded processing gas in a plant, or be integrated into a processing unit producing the particle loaded gas, such as a spray drying apparatus or a fluid bed apparatus. As the filtration proceeds some of the filtered off particles or dust accumulate on the outside of the filter element <NUM>, and has to be removed in order to avoid building up of dust cakes. Cleaning is effected during continuous operation of the filter unit by using high pressure reverse flow gas cleaning.

A cleaning arrangement, here generally designated <NUM> in <FIG>, includes a filter cleaning nozzle <NUM> positioned at a distance A' above the filter element <NUM>. The nozzle <NUM> ejects a burst of cleaning gas down into the filter element <NUM> at intervals adapted to the current filtration process.

The jet-like burst of reverse flowing cleaning gas produces a very quick pressure increase inside the filter element so that the filter wall accelerates outwards. The pulse of cleaning gas has a very short duration, such as from <NUM> to <NUM>, typically about <NUM>, and the filter wall is therefore immediately inflated to the maximum diameter by the gas pressure difference across the filter. Especially for non-rigid filter materials the result of the cleaning action is consequently of mainly mechanical nature, because the particles or dust on the filter element are shaken or kicked loose by the movement of the filter material.

A pressure vessel <NUM> contains pressurized primary cleaning gas. In this prior art cleaning arrangement, the cleaning gas is provided at a pressure in the range of <NUM> to <NUM> barg, typically from <NUM> to <NUM> barg. A gas supply device <NUM>, such as a compressor, delivers compressed air or another gas at a set pressure. The nozzle <NUM> is positioned at the end of a nozzle lance <NUM> and a closure device <NUM> is positioned in connection with the pressure vessel <NUM>. The setting of the pressure depends on the length of the filter element <NUM> and the size of the nozzle <NUM>. One and the same nozzle size can be used for several different lengths of filter elements by suitably varying the setting of said pressure so that a higher pressure is used for longer elements and vice versa. This setting of the pressure can be done at the commissioning of the filter. The gas supply device can also be of a type allowing adjustment of the gas pressure during operation in order to accommodate for variations in the filtration conditions, possibly dynamically controlled by the pressure drop over the filter or by clogging of the filter.

The dimensions of the nozzle <NUM> include a predefined nozzle inlet diameter i, a nozzle throat diameter t, and a nozzle exit diameter e shown in <FIG>. A membrane valve <NUM>' is provided, including membrane valve slots <NUM>' and membrane valve openings <NUM>'.

Further details regarding the nozzle <NUM> of the specific embodiment include that the nozzle has a continuous curve from the throat 431t to the exit 431e.

The nozzle <NUM> comprises a membrane valve <NUM>, which has a valve opening <NUM> shown in <FIG> of at least <NUM>.

The nozzle <NUM> may have a nozzle exit diameter e of up to <NUM>, preferably up to <NUM>.

The nozzle <NUM> has a nozzle throat diameter t of up to <NUM>, preferably up to <NUM>.

The length of the nozzle <NUM> is shown in <FIG> as L. The ratio of the throat diameter 431t to the length of the nozzle <NUM> may be <NUM> to <NUM>, preferably <NUM> to <NUM>, and even more preferably <NUM> to <NUM>.

Although not described in detail, the distance between the nozzle exit 431e and a top of said plurality of bag filters <NUM> may lie between -<NUM> and +<NUM>. The nozzle can be moved with a rotating and/or robotic nozzle system so that a nozzle can be used to pulse different bag filters.

For example, the inlet can be of a different shape than hexagonal, for example round, oval, triangular, square, pentagonal, octagonal, or any other polygonal.

The invention offers several advantages over different prior art solutions. Due to allowing a larger part of the plant footprint to be allocated to bag filters and thus enabling a more compact system, the wear on the individual bag filters is reduced. The possibility to reduce the length of the bag filters while maintaining or even increasing the capacity entails that the weight of the bag filters is reduced, just as there is no need for a joint. The pressure drop is reduced, and the problems of vibrations also reduced.

Claim 1:
A powder drying system comprising
at least one powder processing unit (<NUM>), and
at least one filter unit (<NUM>) defining a central vertical axis (<NUM>) and including
a filtering chamber (<NUM>) accommodating a plurality of bag filters (<NUM>), each having a bag filter wall, a top opening and a length in the range of from <NUM> to <NUM>,
a top portion (<NUM>), and
an exhaust chamber (<NUM>) at or near the top portion (<NUM>),
in which the filtering chamber (<NUM>) is provided with an inlet (<NUM>) configured to allow entry of processed powder carrying gas to be filtered by passing through the bag filter wall of the bag filters (<NUM>), and the exhaust chamber (<NUM>) has an outlet (<NUM>) configured to allow exhaust of filtered gas flowing upwards from the bag filters through the top opening thereof,
wherein the inlet (<NUM>) of the filtering chamber (<NUM>) is positioned at the top portion (<NUM>) of the filter unit (<NUM>), and said inlet (<NUM>) includes at least one inlet duct section adjacent the filtering chamber (<NUM>) and arranged substantially centrally in the top portion (<NUM>) of the filter unit (<NUM>),
which inlet duct section is arranged substantially coaxial with the central vertical axis (<NUM>) such that the gas entering the filter unit (<NUM>) is allowed to flow substantially downwards along the central vertical axis,
wherein said at least one inlet duct section comprises an internal duct (<NUM>) and a transitional duct (<NUM>),
which internal duct (<NUM>) is adjacent the filtering chamber (<NUM>) and extends substantially in parallel with the central vertical axis (<NUM>),
which transitional duct (<NUM>) connects the internal duct (<NUM>) to an angled duct (<NUM>),
which angled duct (<NUM>) has the form of pipe segments forming a substantially <NUM>-degree bend to the internal duct (<NUM>),
wherein
the distance between the angled duct (<NUM>) and the end of the internal duct (<NUM>) adjacent the filtering chamber (<NUM>) exceeds the length of a bag filter (<NUM>).