WO2017092758A1

WO2017092758A1 - Powder drying system with improved filter unit cleaning arrangement and method for cleaning the system

Info

Publication number: WO2017092758A1
Application number: PCT/DK2015/050380
Authority: WO
Inventors: Thomas Filholm
Original assignee: Gea Process Engineering A/S
Priority date: 2015-12-03
Filing date: 2015-12-03
Publication date: 2017-06-08

Abstract

A powder drying system comprising at least one powder processing unit (1), at filter unit (4) including a plurality of bag filters (7), and a cleaning arrangement (10) including at least one gas nozzle (11) associated to a respective bag filter (7) for cleaning thereof, said nozzle (11) having a nozzle inlet (11i), a nozzle throat (11t) and a nozzle exit (11e), the dimensions of the nozzle including a predefined nozzle inlet diameter (i), a nozzle throat diameter (t), and a nozzle exit diameter (e). The nozzle (11) is provided with a nozzle exit diameter (e) which is larger than the nozzle throat diameter (t) by a ratio in the interval 1.05:1 to 2:1, preferably 1.1:1 to 1.5:1, and even more preferably 1.2:1 to 1.4:1, and most preferred 1.2:1 to 1.3:1.

Description

Powder drying system with improved filter unit cleaning arrangement and method for cleaning the system Field of invention

The present invention relates to a powder drying system comprising at least one powder processing unit, a filter unit including a plurality of bag filters, and a cleaning arrangement including at least one gas nozzle associated to a respective bag filter for cleaning thereof, said nozzle having a nozzle inlet, a nozzle throat and a nozzle exit, the dimensions of the nozzle including a predefined nozzle inlet diameter, a nozzle throat diameter, and a nozzle exit diameter. The invention furthermore relates to a method for cleaning such a system. Background of the invention

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. During operation, the filter elements, or bag filters, of the filter unit separate the processing air or gas and powder exiting the powder processing unit. In this separation process, some of the powder will attach to the bag filters. In order to remove the powder from the filter, the filter is shaken. This can be done be either mechanically shaking the entire filter, or by blowing a burst of air through the filter, thus shaking the bag filter walls. Because sanitary conditions are desired, bursts of air are mostly used.

Bag filters may have different geometrical appearances, for example, the cross section may be circular, rectangular, triangular, square, oval etc.

An example of a prior art arrangement is seen in Applicant's European patent No. 1 251 933 B1 where the filter unit is provided with a number of filter elements in the form of bag filters. The filter unit is provided with a cleaning arrangement including a nozzle associated to each bag filter for shaking the bag filter. When the spray drying system including the filter unit has run for a period of time, a shaking process begins by a burst of air or other gas being blown into the bag filter from the nozzle. By pressurizing the gas to high pressure the gas accumulates an amount of kinetic energy. This kinetic energy is transferred to the bag filter, resulting in a bag filter wall movement. The more kinetic energy is transferred from the air to the bag filter, the more the bag filter is shaken.

While the cleaning arrangement of the prior art filter unit has proven to function very well over a long time, bag filters are increasingly made longer in order to obtain more capacity within the same plant foot-print. One way of accommodating the increased length of the bag filters is to increase the pressure of the cleaning gas. However, as supplying pressurized gas is relatively costly, in particular if the required pressure exceeds that of a standard compressor.

Also, it is desired to increase the time interval between CIP procedures thus increasing the operational time of the drying plant.

Summary of the invention

With this background, it is therefore an object of the present invention to provide a powder drying system, by which it is possible to perform efficient and cost-effective cleaning in a larger range of filter units. In a first aspect of the invention, these and further objects are obtained by a powder drying system of the kind mentioned in the introduction, which is furthermore characterized in that said nozzle is provided with a nozzle exit diameter which is larger than the nozzle throat diameter (t) by a ratio in the interval 1 .05: 1 to 2: 1 , preferably 1 .1 : 1 to 1 .5: 1 , and even more preferably 1 .2: 1 to 1 .4: 1 , and most preferred 1 .2: 1 to 1 .3: 1 . Without wishing to be bound by theory, the inventive concept is based on the recognition that the nozzles of the prior art cleaning arrangements suffered from a relatively large loss in energy. Consequently, not all, or even only a small portion, of the kinetic energy provided by a larger pressure was utilized and converted to shaking of the bag filter. In turn, it is believed that this was at least partly due to choking at the nozzle throat where a shock developed. Surprisingly, it was found that providing a much smaller ratio between the nozzle exit diameter and the nozzle throat diameter than in the prior art nozzle made it possible to obtain a good cleaning effect of the bag filters, even with a relatively narrower jet. The narrow jet turned out to entrain a sufficient amount of cleaning gas and an excellent shaking effect and cleaning was attained.

The interval of the ratio between the nozzle exit diameter and the nozzle throat diameter makes it possible to obtain a nozzle design in which the exit pressure substantially equals ambient, i.e. substantially atmospheric, pressure.

In general, the following definitions are given: a nozzle is designed to have a pressure increase up to a maximum pressure at the nozzle throat. In such supersonic nozzle, after the throat, the gas expands, lowering the pressure and accelerating the gas. Just past the throat, the pressure of the gas is higher than ambient pressure, and needs to be lowered between the throat and the nozzle exit via expansion. If the pressure of the jet leaving the nozzle exit is still above ambient pressure then a nozzle is said to be under- expanded; if the jet is below ambient pressure then it is over-expanded.

Slight over-expansion causes a slight reduction in efficiency, but otherwise does little harm. However, if the exit pressure is less than approximately 40% that of ambient then 'flow separation' occurs. This can cause jet instabilities according to literature.

Because pressure cannot travel upstream through the supersonic flow, the exit pressure can be significantly below the ambient pressure into which it exhausts, but if it is too far below ambient, then the flow will cease to be supersonic, or the flow will separate within the expansion portion of the nozzle, forming an unstable jet that may flow turbulently around within the nozzle.

The nozzles seen in the prior art are highly over-expanded. This results in a so-called normal shock, which carries with it a high efficiency loss.

By redesigning the nozzle of the cleaning arrangement of the inventive powder drying system, with a goal of improving the efficiency of the nozzle, a surprising effect was found in that limiting the ratio between the nozzle throat and the nozzle exit resulted in a highly increased efficiency. This was achieved because the gas in the nozzle no longer over-expanded.

In an embodiment of the invention, the inner surface of the nozzle between the throat and the exit has a substantially continuous cross section. By a continuous cross section, it is meant that any sudden geometrical changes in the inner nozzle diameter should be avoided, and the surface should be substantially smooth. As such, the inner surface cross section could have a linear increase in diameter from the throat to the exit, or it could have a slightly curved cross section, as long as there are no sharp corners of sudden changes in the inner nozzle diameter. This has the advantage that it limits both over-expansion of the gas, as well as decreasing the risk of flow disturbances in the nozzle. Both over-expansion and flow disturbances will decrease the efficiency of the nozzle. A further advantage of having a substantially continuous inner surface cross section is that the nozzle will be easily cleaned and less contamination can be retained in the nozzle. These qualities are desired in powder drying systems, which are to be used especially for the dairy, pharmaceutical and foodstuff industries. However, other industries such as chemical, biotechnological, etc. could also benefit from such a filter unit. In particular, a linear configuration provides the outlet section of the nozzle with a diverging conical configuration which has advantages also in relation to manufacture of the nozzle.

In another embodiment of the invention, the nozzle has a membrane valve, which has a valve opening of at least 4 mm. The membrane valve is located upstream of the nozzle inlet, and serves to limit the inlet mass flow and inlet pressure. By having a larger valve opening in the membrane valve, a larger mass flow is allowed for the same supply pressure.

In another embodiment, the nozzle has a nozzle exit diameter of up to 20 mm, preferably up to 17 mm. By reducing the exit diameter, the gas will over expand less, resulting in a more efficient nozzle. Further the jet stream exiting the nozzle will be more concentrated, resulting in a longer reach of the pressurized gas.

In a further embodiment, the nozzle has a nozzle throat diameter of up to 20 mm, preferably up to 14 mm. By having a small throat diameter, a lower supply pressure can be used.

In another embodiment of the invention, the powder drying system has a plurality of bag filters, which are at least 3 meters long, preferably at least 5 meters long and even more preferably at least 6 meters long. In some case the bag filters may be up to 12 meters long. By increasing the nozzle efficiency, and by concentrating the jet stream, the bag filter length can be increased. By increasing the length of the bag filter, more dry powder product can be filtered in the filter unit. This will improved the overall yield and efficiency of the powder drying system.

In a further embodiment, the bag filters each have a diameter of at least 10 cm, preferably at least 15 cm and even more preferably at least 20 cm. By increasing the diameter of the bag filters, a higher nozzle inlet pressure or nozzle orifice may be used. Although the optimal nozzle exit pressure is the ambient atmospheric pressure, a slight under expansion can be preferred, in order to avoid a loss in efficiency. As such, the larger the bag filter diameter is, the more tolerant they are to a higher supply pressure, as there is a longer distance to the bag filter wall, wherein the increased jet velocity can diffuse.

In another aspect of the invention, the nozzle exit has a distance to the top of bag filters. If the exit of the nozzle is flush with the top of the bag filter, it is defined as having a distance of 0 mm. If the nozzle exit is inside the bag filter opening, it is defined as having a negative distance. If the nozzle exit is outside the bag filter opening, it is defined as having a positive distance. In this embodiment of the invention, the distance between the nozzle exit and the top of the bag filters is between -20 mm and +700 mm. The advantages of having the distance in this interval is an increase in efficiency of the bag filter cleaning by the nozzle, without damaging or clogging the bag filters. If the nozzle has a negative distance, which is numerically greater, than 20 mm, the ambient air inside the bag filter, surrounding the nozzle, will be pulled into the stream. If the nozzle is too far into the bag filter, this will result in the bag filter walls and outside ambient air being pulled towards the jet stream exiting the nozzle. This can result in wear of the bag filter, or dried powder product being forced further into the filter, resulting in a clogging effect of the filter. If the distance is too great in the positive direction, the jet stream exiting the nozzle will diffuse too much before reaching the bag filter, thus decreasing the efficiency of the bag filter cleaning process.

In another embodiment of the invention, the filter unit comprises at least 50 bag filters, preferably at least 100 bag filters, and even more preferably at least 200 bag filters. In large filter units there may even be 350, 400, 450, or 500 filter bags. The more bag filters/filter area there are present, the more dried powder product can be filtered in the filter unit, leading to a higher capacity, and a more efficient powder drying system.

In a second aspect of the invention, a method for cleaning a plurality of bag filters of powder drying system, each bag filter having a predefined length, comprises the steps of

selecting a set supply pressure,

selecting a time period between cleaning, and

applying a mass flow rate in accordance with the set pressure and time period between cleaning.

In an embodiment of the inventive method, a supply pressure to the nozzle is up to 6 barg. It is advantageous in a powder drying system of the type according to the invention to limit the supply pressure of the filter bag cleaning nozzle to a maximum of 6 barg, as the auxiliary gas in other parts of the powder drying system according to the invention, also operate at 6 barg of pressure. As such, there is no need for further compression of the process gas for the nozzle, thus resulting in a more energy efficient powder drying system and less costly system since it doesn't require compressed air boosting equipment.

By using a cleaning arrangement nozzle of the type according to the first aspect of the invention, it is possible to reduce the supply pressure, or maintain the same supply pressure, but increase the velocity and mass flow rate, thus resulting in a more efficient shake of the bag filter. By a more efficient shake, the bag filters will be cleaned more, thus the pressure drop over the filter unit will decrease, which can further result in an increase in the time period between cleaning processes. This increase in the time period will both result in a higher running time of the filter unit, as well as decrease the wear on the bag filters, which will result in a less frequent replacement of the bag filters, which will further improve the running time of the filter unit, and the powder drying system as a whole.

Brief description of the drawings

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. 1 shows a schematic view of the main components of a powder drying system in an embodiment of the invention;

Fig. 2 shows a schematic partial cross-sectional view of a filter unit including a cleaning arrangement of a prior art powder drying system;

Figs 3a and 3b show a membrane valve of a cleaning arrangement of a prior art powder drying system;

Figs 3c and 3d are views of a nozzle forming part of a cleaning arrangement of a prior art powder drying system;

Figs 4a and 4b are illustrations of the pressure distribution and temperature distribution, respectively, of a simulation carried out on the nozzle of Figs 3c to 3d;

Figs 5a and 5b are views corresponding to Figs 3a and 3b of a membrane valve of a cleaning arrangement of a powder drying system in an embodiment of the invention;

Figs 5c and 5d are views corresponding to Figs 3c and 3d of a nozzle of a cleaning arrangement of a powder drying system in an embodiment of the invention;

Figs 6a and 6b are illustrations corresponding to Figs 4a and 4b of simulations carried out on the nozzle in the embodiment of Figs 5c to 5d;

Fig. 7 is an illustration showing comparative results of the mass flow rate over time of the prior art nozzle of Figs 3c to 3d and the nozzle in the embodiment of Figs 5c to 5d at various pressure conditions;

Figs 8a to 8c are illustrations of the pressure over time of the prior art nozzle of Figs 3c to 3d and the nozzle in the embodiment of Figs 5c to 5d at various positions in a bag filter; and

Fig. 9 is a graph showing the efficiency of the nozzle in dependence on the ratio between nozzle exit diameter and nozzle throat diameter.

Detailed description of embodiments of the invention

Fig. 1 shows a schematic view of the main components of a powder drying system in the form of a spray drying system 1 . In a manner known per se, the spray drying system 1 comprises a spray dryer with a drying chamber 2 and a process air/gas supply device 3, 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 2 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 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, flash dryers etc. The powder drying system thus incorporates a powder processing unit, here a spray dryer with a drying chamber. At the lower end of the drying chamber 2, an outlet 5 for dried material is provided. In the shown spray drying system 1 , an after-treatment unit in the form of vibrating or static fluid bed 6 is provided. At one end, the vibrating or static fluid bed 6 receives dried material from the outlet 5 of the drying chamber 2 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 spray drying system 1 comprises a filter unit 4, to which spent process air/gas with particles entrained in the process air/gas is conducted. The filter unit 4 has a configuration which will be described in further detail below. The filter unit 4 may form part of a series of powder recovery units including further filter units and cyclones or bag filters, or any combination thereof.

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 4 will now be described with reference to the prior art filter unit shown in Fig. 2, which corresponds to the embodiment of above-mentioned EP 1 251 933 B1 to which reference is hereby explicitly made. Elements are denoted by reference numerals corresponding to those that will be used for elements having the same or analogous function of embodiments of the invention to which a mark " ' " has been added.

In the filter unit 4', a number of elongated tubular filter elements or bag filters 7' are suspended substantially vertically in a support structure. The filter elements 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 fibres or ceramic fibres.

The number of filter elements 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 typically use smaller filter units having for instance from 2 to 25 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 individual filter element typically has a length in the range from 1 to 8 m and a diameter in the range of 8 to 30 cm, for instance about 20 cm.

During operation of the filter unit 4' process gas carrying product enters the unit through inlet and flows into the area around the filter elements 7'. The gas continues through the walls of the filter elements 7' and flows up to an upper outlet side for clean filtered gas and eventually exits through the outlet. As the gas passes the filter walls product carried by the process gas is retained by the filter elements 7'. The retained material is partially left on the filter elements and partially drops down and accumulates in the lower section of the filter unit 4'. 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 7', 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 10', includes a filter cleaning nozzle 1 1 ' positioned at a distance A above the filter element 7'. The nozzle 1 1 ' ejects a burst of cleaning gas down into the filter element 7' 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 0.10 s to 0.50 s, typically about 0.2 s, and the filter wall is therefore immediately pressed back to the filtration position 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 12' contains pressurized primary cleaning gas. In this prior art cleaning arrangement, the cleaning gas is provided at a pressure in the range of 3 to 10 barg, typically from 4 to 6 barg. A gas supply device 13', such as a compressor, delivers compressed air or another gas at a set pressure. The setting of the pressure depends on the length of the filter element 7' and the size of the nozzle 1 1 '. 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 configuration of a commercially available nozzle 1 1 ' is shown in Figs 3a to 3d.

The nozzle 1 1 ' has a nozzle inlet 1 1 i', a nozzle throat 1 1 t' and a nozzle exit 1 1 e', and the dimensions of the nozzle include a predefined nozzle inlet diameter i, a nozzle throat diameter t, and a nozzle exit diameter e. A membrane valve 16' is provided, including membrane valve slots 17' and membrane valve openings 18'.

A cleaning arrangement of a powder drying system includes a nozzle 1 1 and other details in a presently preferred embodiment which are shown in Figs 5a to 5d.

Further details regarding the nozzle 1 1 of the specific embodiment is that the nozzle has a continuous curve from the throat 1 1 t to the exit 1 1 e.

As in the prior art nozzle, the nozzle 1 1 comprises a membrane valve 16, which has a valve opening 18 of at least 4 mm.

The nozzle 1 1 may have a nozzle exit diameter e of up to 20 mm, preferably up to 17 mm. The nozzle 1 1 has a nozzle throat diameter t of up to 20 mm, preferably up to 14 mm.

The length of the nozzle 1 1 is not crucial but may be chosen according to the design of other parts of the cleaning arrangement of the spray drying system.

The length of the bag filters may also vary. For instance, the length is typically at least 3 meters long, but lengths over 5 meters long or even over 6 meters long are conceivable as well.

The diameter of each bag filter is typically at least 10 cm, preferably at least 15 cm and even more preferably at least 20 cm. Although not described in detail, the distance between the nozzle exit 1 1 e and a top of said plurality of bag filters 7 may lie between -20 mm and +700 mm.

The number of bag filters 7 may be at least 50 bag filters 7, preferably at least 100 bag filters 7, and even more preferably at least 200 bag filters.

The nozzle 1 1 may form part of the nozzle system used for supplying a CIP liquid in an overall CIP phase of the whole filter unit. The nozzle can be moved with a rotating and/or robotic nozzle system so that a nozzle can be used to pulse different filter bags.

Example 1 (prior art nozzle)

The nozzle throat diameter of the prior art nozzle was 13.5 mm and the nozzle exit diameter e 29.5.

A simulation of the flow conditions in the prior art nozzle was carried out and the result is shown in Figs 4a and 4b. The flow in the nozzle is visualized using contour plots. The black line indicates Mach 1 (speed of sound). The flow enters the nozzle at high pressure and low velocity. At the throat the velocity exceeds Mach 1 and the flow is choked. Hence, the flow rate can only be increased by increasing the upstream pressure. Downstream of the throat the pressure is very low (~10 000 Pa) and the velocity high (~700 m/s). There is a so-called normal shock just before the exit. Example 2 (nozzle of the presently preferred embodiment)

A nozzle 1 1 as shown in Figs 5a to 5d was designed with a nozzle throat diameter t was 13.5 mm and a nozzle exit diameter e 17.0 thus rendering a ratio of 1 .26. The new nozzle is designed for 6 bar absolute pressure at the inlet and atmospheric pressure at the outlet. This requires a marginally larger exit area than throat area.

A simulation of the flow conditions in the nozzle 1 1 of the presently preferred embodiment was carried out and the result is shown in Figs 6a and 6b. The black line indicates Machl , and it can be seen that the supersonic region has increased substantially. An example of the improved performance of the new nozzle type is seen in Fig. 7. Fig. 7 shows a comparison of mass flow rates of air through a bag filter of a prior art nozzle and the new nozzle.

The graph shows the mass flow of air in the bag filter during a burst of air, meant to shake the bag filter in order to dislodge dried product from the bag filter. The mass flow through the bag filter correlates to the kinetic energy of the air, which is to be absorbed in the bag filter in the form of the bag shaking.

As seen from the graph, the new nozzle has an increase in mass flow when compared to the prior art nozzle. Even when only supplying 5 bar pressure to the nozzle, it will have a higher mass flow rate than the prior art nozzle with a 6 bar supply pressure.

Comparative examples

Based on the above examples 1 and 2, a number of comparative examples were carried out by means of simulation.

The results are presented in Figs 7 and 8a to 8c.

The flow field for the prior art and new nozzle is shown in Fig. 4a, 4b, 6a and 6b. The nozzle pressure is 5 barg (6 bar absolute pressure). The flow in the current nozzle separates and a shock is present in the diverging part. The new design shows a well attached flow and no shocks. Due to the higher exit velocity the momentum from the nozzle increased approximately 25% from the current to the new nozzle.

The prior art nozzle has a low pressure zone in the diverging part. The pressure is about 20% of the ambient pressure. Due to the significant pressure difference a normal shock is present.

The new nozzle features a smooth pressure gradient. This should lead to a better utilization of the compressed air.

As seen in Figs 8a to 8c the air pressure in the bag filter varies in dependence of the location in the bag filter. As seen here, the pressure is higher for the new nozzle compared to the prior art nozzle.

As furthermore seen from Figs 8a to 8c the pressure in each section is proportionally increased by using the new nozzle compared to the prior art nozzle. This is advantageous, as the bag filter will wear in a similar manner as with the prior art nozzle, which enables use of the same bag filter.

Finally, Fig. 9 shows a graph indicating the efficiency of a nozzle according to the present invention in dependence on ratios between the nozzle exit diameter and the nozzle throat diameter. The nozzle efficiency is measured as the mass flow into the bag in kg/s and as it clearly appears, the preferred embodiment described in the above and having a ratio of 1 .26 provides for an excellent efficiency. The efficiency outside the most preferred interval of 1 .2: 1 to 1 .3: 1 is lower but still highly acceptable in the more preferred interval of 1 .2: 1 to 1 .4: 1 and the preferred interval of 1 .1 : 1 to 1 .5: 1 , whereas the efficiency for ratios above 2: 1 is low.

The invention should not be regarded as being limited to the embodiments described in the above but several modifications and combinations are possible.

Claims

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

1. A powder drying system comprising

at least one powder processing unit (1),

a filter unit (4) including a plurality of bag filters (7), and

a cleaning arrangement (10) including at least one gas nozzle (11) associated to a respective bag filter (7) for cleaning thereof, said nozzle (11) having a nozzle inlet (11 i), a nozzle throat (111) and a nozzle exit (11e), the dimensions of the nozzle including a predefined nozzle inlet diameter (i), a nozzle throat diameter (t), and a nozzle exit diameter (e),

characterized in that

said nozzle (11) is provided with a nozzle exit diameter (e) which is larger than the nozzle throat diameter (t) by a ratio in the interval 1.05:1 to 2:1, preferably 1.1:1 to 1.5:1, and even more preferably 1.2:1 to 1.4:1, and most preferred 1.2:1 to 1.3:1.

2. A powder drying system according to claim 1 , wherein said nozzle

(11) has a continuous curve from the throat (111) to the exit (11 e).

3. A powder drying system according to claim 2, wherein the continuous curve is substantially linear.

4. A powder drying system according to any of the preceding claims, wherein said nozzle (11) comprises a membrane valve (16), which has a valve opening (18) of at least 4 mm.

5. A powder drying system according to any of the preceding claims, wherein said nozzle (11) has a nozzle exit diameter (e) of up to 20 mm, preferably up to 17 mm.

6. A powder drying system according to any of the preceding claims, wherein said nozzle (11) has a nozzle throat diameter (t) of up to 20 mm, preferably up to 14 mm.

7. A powder drying system according to any of the preceding claims, wherein said plurality of bag filters (7) are at least 3 meters long, preferably at least 5 meters long and even more preferably at least 6 meters long.

8. A powder drying system according to any of the preceding claims, wherein said plurality of bag filters (7) has a diameter of at least 10 cm, preferably at least 15 cm and even more preferably at least 20 cm.

9. A powder drying system according to any of the preceding claims, wherein a distance between the nozzle exit (1 1 e) and a top of said plurality of bag filters (7) is between -20 mm and +700 mm.

10. A powder drying system according to any of the preceding claims, wherein said plurality of bag filters (7) comprises at least 50 bag filters (7), preferably at least 100 bag filters (7), and even more preferably at least 200 bag filters (7).

1 1 . A powder drying system, wherein the powder process unit comprises a spray dryer (1 ) with a drying chamber (2).

12. A method for cleaning a plurality of bag filters (7) of a powder drying system as defined in any one of claims 1 to 1 1 , each bag filter (7) having a predefined length, comprising the steps of

selecting a set supply pressure,

selecting a time period between cleaning, and

13. The method of claim 12, the set supply pressure to the nozzle (1 1 ) is up to 6 barg.