WO2016008484A1

WO2016008484A1 - A vertical flash tube dryer, and method for cooling product deposits in such a flash dryer

Info

Publication number: WO2016008484A1
Application number: PCT/DK2014/050225
Authority: WO
Inventors: Thomas Filholm; Søren FJORDGAARD; Claus LEMB
Original assignee: Gea Process Engineering A/S
Priority date: 2014-07-16
Filing date: 2014-07-16
Publication date: 2016-01-21
Also published as: CN106537073B; CN106537073A

Abstract

The vertical flash tube dryer (1) defines an axial direction (a) and comprises a drying chamber (2) having a lower section (3) and an upper section (4). A feed inlet (5) for product to be dried and an inlet (6) for axial introduction of a drying gas are provided in the lower section. A cooling arrangement (8) for cooling a wall of the drying chamber is provided, and the cooling arrangement (8) consists of a gas cooling arrangement including an inlet (81) for cooling gas located in the wall of the lower section (3) at a position between the inlet (6) for drying gas and the feed inlet (5) as seen in the axial direction.

Description

A vertical flash tube dryer, and method for cooling product deposits in such a flash dryer

Field of the invention

The present invention relates to a substantially vertical flash tube dryer defining an axial direction, comprising a drying chamber having a lower section and an upper section, in which the lower section includes a substantially cylindrical wall defining a radial direction perpendicular to the axial direction, a feed inlet for product to be dried and an inlet for axial introduction of a drying gas provided in the lower section, and in which the upper section includes a wall and an outlet for dried product and exhaust drying gas, and a cooling arrangement for cooling at least a part of at least one wall of the drying chamber. The invention furthermore relates to a method for cooling at least one wall of such a flash dryer.

Background of the invention

Flash drying is defined as the drying of particles that are suspended and conveyed in a hot air stream. The particles have their origin in a viscous feed including pastes, filter cakes and highly viscous liquids, such as agrochemicals, ceramics, dye-stuffs/pigment, inorganic and organic chemicals, and waste products, e.g. sludge, sediments etc. Flash drying is typically used as an alternative to or upstream of other drying units such as fluid bed dryers. In a flash dryer the product to be dried is fed to a drying chamber, through which the product is passed once and some of its liquid contents is evaporated and discharged. The flash dryer may either be a so-called vortex disintegrator dryer, one example of such a dryer being the SWIRL FLUIDIZER™ (GEA Niro), or a vertical flash tube dryer. In a vortex disintegrator dryer, the feed is introduced into a drying chamber which is provided with a vertical rotary disintegrator at the lower part thereof. Above the bottom, the drying chamber has an air inlet for introducing hot air in a tangential air flow, thus creating the controllable swirling air flow together with the rotary disintegrator. The impeller thus disintegrates the feed, and the liquid contents of the feed are evaporated while being transported upwards in a swirling movement in the drying chamber.

However, in the type of flash dryer according to the present invention, i.e. a vertical flash tube dryer, the feed is dispersed solely by the introduction of the drying gas, due to the high air velocity prevailing in the flash dryer.

Regardless of the kind of flash dryer, the short residence time allows for high temperatures of the inlet drying air due to the cooling effect provided by the fast evaporation of the moisture from the particle surfaces. The spent drying air containing the dried particles flows to a combined exhaust air cleaning and product recovery system through a product/air outlet placed in the top of the drying chamber.

Typical products that are suited for flash drying are chemicals such as polymers, e.g. s-PVC, PAN, and ABS/MBS, or dyes, pigments etc. Other products are food and feed products, e.g. algea products.

The flash dryer typically operates at an inlet temperature of the drying gas of often up to 250°C depending on the heat sensitivity of the product. If a wanted low residual moisture is not obtained easily in the simple flash dryer special designs can be considered, such as ring dryers, or further drying can take place. This could be a second flash dryer or more often a fluid bed where longer residence time at lower and safer temperatures is possible and unwanted over-drying can be avoided.

As the inlet temperature of the drying gas is high, there is a risk of overheating the product in the lower part of the flash dryer, and consequently, many plants incorporate means for cooling of the wall in the lower part. In the upper parts, evaporation of the water contents of the product cools the mixture of product and drying gas, thus reducing the temperature.

In the prior art, the means for cooling typically include a cooling jacket enclosing the wall, for instance by forming the wall of a double- walled configuration, in which cooling water circulates. Water has excellent heat transfer properties and provides for reliable cooling.

Another problem in flash dryers, other than the risk of overheating of the product, is constituted by deposits formed by product adhering to the inside of the wall. This problem is particularly pronounced when the product is sticky due to thermoplasticity or to hygroscopicity, or for instance due to a high content of fat, or if the maximum processing temperature lies relatively close to the melting point of the product, in which it may happen that partial melting occurs.

Deposits in themselves are not desirable, but only turn into a substantive problem when burnt. Not only does the burning render the cleaning more difficult, but burnt deposits may be entrained by the drying gas and mix with the finish product, thus deteriorating the quality. This is known in the field as the formation of black specks, which is particularly undesirable if miscolouring a white or light-coloured product.

Summary of the invention

It is an object of the invention to provide a vertical flash tube dryer in which the risk of burnt deposits is reduced.

In a first aspect, this and further objects are achieved by the provision of a vertical flash tube dryer of the kind mentioned in the introduction, which is furthermore characterized in that the cooling arrangement consists of a gas cooling arrangement and includes an inlet for cooling gas located in the wall of the lower section substantially at the level of the feed inlet. In this manner, the relevant parts of the flash dryer are cooled in an efficient manner by a cooling gas, which after cooling the wall in the lower section follows the exhaust drying gas and dried product through the outlet. It turns out that the consequences of introducing a gas of a lower temperature locally, along the wall of the lower section, do not affect the drying process negatively to any significant degree. The problems of overheated product and burnt deposits encountered in the prior art are overcome. Without wishing to be bound by theory, it is believed that the fact that the deposits formed on the wall of the lower section are submerged in the flow of cooling gas reduces the temperature of the deposit to such an extent that burning is prevented. The provision of an external cooling jacket for cooling water is made redundant, thus reducing the overall costs and complexity for manufacture and operation.

In one preferred embodiment, the cooling gas inlet is located at a position between the inlet for drying gas and the feed inlet as seen in the axial direction. This provides for a particularly effective cooling, as the wall of the lower section is cooled from the point of entry of the feed and upwards.

Alternatively, the cooling gas inlet may be located within a distance corresponding to a diameter of the flash tube lower section above the feed inlet.

In one embodiment, the cooling gas inlet is formed as a substantially circumferential gap in the wall of the lower section.

In principle, the gap could be formed as a slit, slit portions or nozzles, in the wall of the lower section. However, in a preferred development of the above embodiment, the substantially circumferential gap is provided by forming the wall of the lower section by a first wall section having a first diameter and a second wall section having a second diameter, the second diameter being larger than the first diameter such that the gap is provided with a pre-defined extension in the radial direction. This provides for a mechanically simple configuration which provides a reliable formation of a layer of cooling gas along the wall. In addition, the first and second wall section may move slightly relative to each other to accommodate changes of dimensions of the parts of the flash dryer due to temperature changes.

The pre-defined extension of the gap in the radial direction may lie in the interval 5 to 50 mm, preferably 10 to 30 mm. The exact dimensions may be chosen according to other dimensions of the flash dryer.

In a further development pertaining to the wall of the lower section, an embodiment is preferred wherein the first wall section has such an extension relative to the extension of the second wall section in the axial direction that an overlap in the axial direction is formed between the first and second wall sections, the overlap preferably lying in the range of 50 to 500 mm. In this manner, the formation of the axial flow of cooling gas is facilitated.

In another embodiment, which is advantageous as regards the introduction of the cooling gas, the cooling arrangement comprises a disperser for cooling gas connected to the cooling gas inlet.

In a preferred development of this embodiment, a lower part of the cooling gas disperser is connected to the first wall section and an upper part of the cooling gas disperser is connected to the second wall portion. This configuration is advantageous in that the change of dimensions due to temperature changes is accommodated, as the wall sections are able to move slightly relative to each other, and to the air disperser.

Additionally, in an even more preferred embodiment, the cooling gas disperser is provided with a plurality of vanes positioned in connection with said gap. In addition to providing a reliable means of achieving the flow of cooling gas along the wall of the lower section, the vanes function as spacer elements such that the gap is sufficiently wide throughout the circumference, as the vanes keep the distance between the first and second wall sections, even in such cases in which manufacturing tolerances and/or temperature related dimensional changes act to reduce the extension of the gap at specific points along the circumference.

Each vane may have a pre-defined extension in the radial direction which corresponds to or is slightly lower than the pre-defined extension of the gap. The vanes may be connected to one or both wall sections, for instance only to the first wall section.

In a preferred embodiment, the vanes extend in the radial direction and parallel to the axial direction. This configuration is advantageous in directing the flow of the cooling gas in the axial direction only.

In the particular embodiment mentioned above, the first wall section has such an extension relative to the extension of the second wall section in the axial direction that an overlap in the axial direction is formed between the first and second wall sections, the overlap preferably lying in the range of 50 to 500 mm. In a further development of this embodiment it is furthermore preferred that the extension of the vanes in the axial direction corresponds to the dimension of the overlap between the first and second wall sections. By this configuration, the vanes act as spacer elements throughout the overlap and furthermore have a sufficient length in the axial direction to ensure proper axial flow of the cooling gas along the wall of the lower section.

In another preferred embodiment, the wall of the upper section has a diameter varying along the axial direction, the diameter of the upper section preferably increasing for a certain length in the axial direction starting at the transition from the lower section. As the drying gas introduced into the lower section of the flash dryer, thus having a relatively smaller diameter, a venturi effect arises, increasing the velocity of the drying gas locally. This enhances the effect of disintegrating and entraining the feed, which at the point of introduction may still be relatively compact and wet.

The dimensions of the flash dryer are chosen in accordance with desired specifications. In one embodiment, the diameter of the lower section lies in the range 0.2 to 3 m, preferably 0.5 to 2 m.

In order to ensure that the feed is introduced into the drying gas, the inlet or inlets protrudes into the lower section, preferably by a distance of 10 to 500 mm, in one embodiment.

The ratio between the distance that the feed inlet protrudes into the lower section and the diameter of the lower section is chosen according to specifications, but will advantageously lie in the range 0.02 to 0.45.

In a further development of the embodiment in which the predefined extension of the gap in the radial direction lies in the interval 5 to 50 mm, preferably 10 to 30 mm, the ratio between the pre-defined extension of the gap in the radial direction and the diameter of the lower section lies in the range 0.01 to 0.05.

The invention is applicable to most sizes of vertical flash tube dryers, with appropriate dimensioning of the parts. The total height of the drying chamber may for instance lie in the interval 5 to 25 m and the height of the lower section in the interval 0.5 to 5 m.

In a second aspect, a method for cooling product deposits in a flash dryer as described above is provided, the method comprising the steps of supplying a feed of product via the feed inlet, supplying a drying gas at a pre-defined temperature and a pre-defined flow rate, contacting the product with the drying gas to provide an upward flow of drying gas and product to be dried in the axial direction, supplying a cooling gas via the inlet of the cooling arrangement to provide an upward flow of cooling gas in the axial direction along the wall of at least the lower section of the drying chamber, and allowing the flow of cooling gas to reduce the temperature of deposits at the wall of at least the lower section.

Further details and advantages will appear from the detailed description and appended claims.

Brief description of the drawings

In the following, the invention will be described in further detail by means of the following description of preferred embodiments and with reference to the drawings, in which

Fig. 1 shows a schematic overview of a plant incorporating a flash dryer in an embodiment of the invention;

Figs 2 and 3 show a schematic side and front view, respectively, of details of an embodiment of a flash dryer according to the invention;

Fig. 4 is a schematic partial sectional view, on a larger scale, of details of an embodiment of a flash dryer corresponding to the flash dryer shown in Figs 2 and 3;

Fig. 5 is a schematic partial perspective view of details of the flash dryer shown in Fig. 4;

Fig. 6 is a schematic view seen from the above of the details of the flash dryer shown in Figs 4 and 5;

Fig. 7 is an enlarged schematic partial perspective view corresponding to Fig. 5, of another embodiment of the flash dryer according to the invention;

Figs 8 to 1 1 show graphic illustrations of a computer-simulated example of the temperature distribution in a flash dryer operated with the cooling arrangement according to the invention (Figs 8 and 10) and without operation of the cooling arrangement (Figs 9 and 1 1 ); and

Figs 12 and 13 are schematic partial sectional side views illustrating the operating principles of cooling of a prior art flash dryer (Fig. 12) and operated with the cooling arrangement according to the invention (Fig. 13), respectively. Detailed description of the invention and of preferred embodiments

In Fig. 1 , a vertical flash tube dryer generally designated 1 is shown in a position in a dryer plant including a number of well-known operational units. Feed constituted by product to be dried is introduced at a feed inlet 5 at the lower end of the flash dryer 1 and drying gas is introduced via drying gas inlet 6, which in turn is connected to heating means, a supply conduit 62 and an air box 61 in a manner known as such. At the upper end of the flash dryer 1 , an outlet 7 is provided for dried product and exhaust drying gas. Further operational units include cyclone and/or filter units, further drying units and product recovery units, not described in detail.

Referring now to Figs 2 and 3, it is shown how the vertical flash tube dryer 1 defines an axial direction a, and comprises a drying chamber 2 having a lower section 3 and an upper section 4. The lower section 3 includes a substantially cylindrical wall defining a radial direction perpendicular to the axial direction a and is provided with the feed inlet 5 for product to be dried and the inlet 6 for axial introduction of a drying gas. The upper section 4 includes a wall and the outlet 7. A cooling arrangement to be described in further detail below is provided for cooling at least a part of at least one wall of the drying chamber 2, here the wall of the lower section 3. There may be more than one inlet 5, e.g. two inlets 5 placed opposite each other at the same horizontal level.

The cooling arrangement consists of a gas cooling arrangement generally designated 8, thus being the only cooling arrangement of the flash dryer. As shown in detail in the schematic view of Fig. 4, the cooling arrangement 8 includes an inlet 81 for cooling gas located in the lower section 3. In the presently preferred embodiment, the cooling gas inlet 81 is located at a position between the inlet 6 for drying gas and the feed inlet 5 as seen in the axial direction. Alternatively, the cooling gas inlet may be located within a distance corresponding to a diameter of the flash tube lower section above the feed inlet.

In the embodiment shown, the cooling gas inlet is formed as a substantially circumferential gap 81 in the wall 31 , 32 of the lower section 3. The substantially circumferential gap 81 is provided by forming the wall of the lower section 3 by a first wall section 31 having a first diameter and a second wall section 32 having a second diameter, the second diameter being larger than the first diameter such that the gap 81 is provided with a pre-defined extension in the radial direction. The gap thus has an extension represented by the distance between line g denoting the outline of the second wall section 32 and the outline of the first wall section 31 . The pre-defined extension of the gap 81 in the radial direction typically lies in the interval 5 to 50 mm, preferably 10 to 30 mm. In the embodiment shown, the extension of the gap 81 is approximately 20 mm. The parts of the flash dryer 1 including the wall sections 31 , 32 are typically made of metal such as steel of suitable dimensions and properties.

As shown most clearly in the schematic view of Fig. 4, the first wall section 31 has such an extension relative to the extension of the second wall section 32 in the axial direction a that an overlap in the axial direction is formed between the first and second wall sections 31 , 32. The overlap is as shown the difference between the upper edge of the first wall section 31 , represented by line h31 , and the lower edge of the second wall section 32, represented by line h32. The overlap will typically lie in the range of 50 to 500 mm, here 200 mm.

In the embodiment shown, the cooling arrangement 8 comprises a disperser 82 for cooling gas connected to the cooling gas inlet 81 . The cooling gas disperser 82 is connected to supply means for cooling gas, not shown.

A lower part 82a of the cooling gas disperser 82 is connected to the first wall section 31 and an upper part 82b of the cooling gas disperser 82 is connected to the second wall portion 32. The connection may take place in any suitable manner, for instance by welding.

The cooling gas disperser 82 is provided with a plurality of vanes 83 positioned in connection with said gap 81.

Each vane 83 has a pre-defined extension in the radial direction represented by line w. The extension is here slightly lower than the predefined extension of the gap 81 , but the vanes 83 may also substantially span the entire distance between the first wall section 31 and the second wall section 32.

In the embodiment shown, the vanes 83 are connected only to the first wall section 31 , also typically by welding. The vanes 83 generally extend in the radial direction and parallel to the axial direction. The extension of the vanes 83 in the axial direction here corresponds to the dimension of the overlap between the first and second wall sections 31 , 32. The vanes 83 have a substantially larger dimension in the axial direction than in the radial direction.

The invention is applicable to most sizes of vertical flash tube dryers, with appropriate dimensioning of the parts. The total height of the drying chamber may for instance lie in the interval 5 to 25 m and the height of the lower section in the interval 0.5 to 5 m. Here, the total height is approximately 15 m and the height of the lower section 3 m. The diameter of the lower section 3 typically lies in the range 0.2 to 3 m, preferably 0.5 to 2 m. Here, the diameter is approximately 1 m.

Returning to Figs 2 and 3, it is shown how the wall of the upper section 4 has a diameter varying along the axial direction, the diameter of the upper section 4 increasing for a certain length in the axial direction starting at the transition from the lower section 3. Towards the upper end, the diameter of the upper section 4 decreases again in the direction of the outlet 7.

In the embodiment shown, the inlet 5 protrudes into the lower section 3, preferably by a distance of 10 to 500 mm. Here, the distance is approximately 350 mm. The ratio between the distance that the inlet 5 protrudes into the lower section 3 and the diameter of the lower section 3 lies in the range 0.02 to 0.45. In the specific embodiment, the ratio is hence approximately 0.035. As shown in Fig. 7, the inlet 5 is provided with a hat section 51 which mainly prevents that product collects and deposits on top of the inlet, but may also have some importance on reducing pressure loss from the flow of drying and cooling gas.

It follows that the ratio between the pre-defined extension of the gap 81 in the radial direction and the diameter of the lower section 3 lies in the range 0.01 to 0.05. In the specific embodiment, the ratio is approximately 0.02.

In the following, operation of the vertical flash tube dryer 1 will be described in some detail. Reference is additionally made to Fig. 13 illustrating the operational principle.

The method for cooling product deposits in a flash dryer of the above-described kind comprises the steps of

supplying a feed of product via the feed inlet 5,

supplying a drying gas at a pre-defined temperature and a predefined flow rate,

contacting the product with the drying gas to provide an upward flow F of drying gas and product to be dried in the axial direction,

supplying a cooling gas via the inlet 81 of the cooling arrangement 8 to provide an upward flow of cooling gas CG in the axial direction along the wall of at least the lower section 3 of the drying chamber 2, and allowing the flow of cooling gas CG to reduce the temperature of deposits at the wall 31 , 32 of at least the lower section 3.

The deposits are represented by deposit 100a on the wall, here the second wall section 32, in Fig. 13. In Fig. 12, representing a prior art cooling arrangement in the form of a cooling jacket in a double-walled configuration, in which cooling water CW circulates, a corresponding deposit 100b is illustrated.

The values of the temperature, flow rate and flow velocity of the cooling gas are chosen in accordance with the dimensions of the flash dryer and the corresponding parameters of the drying gas. Stated here are typical intervals, and a simulation with specific values will be described further on. The cooling gas is supplied at a flow velocity of 30 to 90 m/s. The flow rate of cooling gas compared to the accumulated rate of drying gas and cooling gas is in an amount of 2 to 20%, preferably 4 to 15%. The cooling gas is supplied at flow velocity substantially corresponding to the pre-defined flow velocity of the drying gas. The cooling gas is supplied at a temperature of ambient air to 1 10°C, preferably 50 to 90°C. The pre-defined temperature of the drying gas lies in the range 150 to 250°C. The choice of cooling gas temperature depends on the drying gas temperature and product properties, but may often be either ambient air or as indicated above.

Example

To illustrate operation of the vertical flash tube dryer, computer simulations of a configuration as in the above embodiment of the flash dryer 1 were carried out. Figs 8 to 1 1 show graphic illustrations of the temperature distributions when the flash dryer was operated with the cooling arrangement according to the invention (Figs 8 and 10), and without operation of the cooling arrangement (Figs 9 and 1 1 ), respectively.

The temperature of the drying gas at the inlet 6 was set to 210°C and the temperature of the feed at the inlet 5 to 50°C. The temperature of the cooling gas was set to 80°C.

The flow rate measured in kg/h of the drying gas was 127,000 kg/h and of the cooling gas 15,000 kg/h and the flow rate of the cooling gas compared to the accumulated rate of drying gas and cooling gas was 10.6%.

The velocity of the drying gas was 70 m/s. The simulation was carried out with a velocity of the cooling gas of 30, 50, 70 and 90 m/s. A velocity of 30 m/s resulted in cooling of only a smaller portion of the wall of the lower section. A velocity of 90 m/s resulted in a relatively large impact on the flow of drying air in the upper section of the flash dryer. Velocities of 50 and 70 m/s resulted in satisfactory cooling and moderate impact on the drying air.

The flow of cooling gas CG was able to reduce the measured temperature of deposits 100a at the wall of the lower section, and burning of deposits 100a was reduced or even eliminated as compared to corresponding deposits 100b when not operating the cooling arrangement 8. Even though the flash dryer in the embodiments shown is described as a vertical flash tube dryer having substantially axial flow of the drying gas only, it is conceivable that the principles underlying the invention could function in a swirl flash dryer operating with a swirling flow of drying gas and with an impeller in the lower section to disintegrate the feed.

The invention should not be regarded as being limited to the embodiment shown and described in the above but various modifications and combinations of features may be carried out without departing from the scope of the following claims.

Claims

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

1 . A substantially vertical flash tube dryer (1 ) defining an axial direction (a), comprising a drying chamber (2) having a lower section (3) and an upper section (4), in which the lower section includes a substantially cylindrical wall defining a radial direction perpendicular to the axial direction (a), a feed inlet (5) for product to be dried and an inlet (6) for axial introduction of a drying gas provided in the lower section, and in which the upper section (4) includes a wall and an outlet (7) for dried product and exhaust drying gas, and a cooling arrangement (8) for cooling at least a part of at least one wall of the drying chamber, c h a r a c t e r i z e d in that the cooling arrangement (8) consists of a gas cooling arrangement and includes an inlet (81 ) for cooling gas located in the wall of the lower section (3) substantially at the level of the feed inlet (5).

2. A flash dryer according to claim 1 , wherein said cooling gas inlet (81 ) is located at a position between the inlet (6) for drying gas and the feed inlet (5) as seen in the axial direction.

3. A flash dryer according to claim 1 , wherein said cooling gas inlet is located within a distance corresponding to a diameter of the flash tube lower section (3) above the feed inlet (5).

4. A flash dryer according to any one of the preceding claims, wherein said cooling gas inlet is formed as a substantially circumferential gap (81 ) in the wall (31 , 32) of the lower section (3).

5. A flash dryer according to claim 4, wherein the substantially circumferential gap (81 ) is provided by forming the wall of the lower section (3) by a first wall section (31 ) having a first diameter and a second wall section (32) having a second diameter, the second diameter being larger than the first diameter such that the gap (81 ) is provided with a pre-defined extension (g) in the radial direction.

6. A flash dryer according to claim 5, wherein the pre-defined extension (g) of the gap (81 ) in the radial direction lies in the interval 5 to 50 mm, preferably 10 to 30 mm.

7. A flash dryer according to claim 5 or 6, wherein the first wall section (31 ) has such an extension (h31 ) relative to the extension (h32) of the second wall section (32) in the axial direction (a) that an overlap in the axial direction is formed between the first and second wall sections (31 , 32), the overlap preferably lying in the range of 50 to 500 mm.

8. A flash dryer according to any one of the preceding claims, wherein the cooling arrangement (8) comprises a disperser (82) for cooling gas connected to the cooling gas inlet (81 ).

9. A flash dryer according to any one of claims 5 to 7 and 8, wherein a lower part (82a) of the cooling gas disperser (82) is connected to the first wall section (31 ) and an upper part (82b) of the cooling gas disperser (82) is connected to the second wall portion (32).

10. A flash dryer according to claim 9, wherein the cooling gas disperser (82) is provided with a plurality of vanes (83) positioned in connection with said gap (81 ).

1 1 . A flash dryer according to claim 10, wherein each vane (83) has a pre-defined extension (w) in the radial direction which corresponds to or is slightly lower than the pre-defined extension (g) of the gap (81 ).

12. A flash dryer according to claim 10 or 1 1 , wherein the vanes (83) are connected only to the first wall section (31 ).

13. A flash dryer according to any one of claims 10 to 12, wherein the vanes (83) extend in the radial direction and parallel to the axial direction.

14. A flash dryer according to any one of claims 10 to 13 when dependent on claim 7, wherein the extension of the vanes (83) in the axial direction corresponds to the dimension of the overlap between the first and second wall sections (31 , 32).

15. A flash dryer according to any one of the preceding claims, in which the wall of the upper section (4) has a diameter varying along the axial direction, the diameter of the upper section (4), preferably increasing for a certain length in the axial direction starting at the transition from the lower section (3).

16. A flash dryer according to any one of the preceding claims, wherein the diameter of the lower section (3) lies in the range 0.2 to 3 m, preferably 0.5 to 2 m.

17. A flash dryer according to any one of the preceding claims, wherein the feed inlet (5) protrudes into the lower section (3), preferably by a distance of 10 to 500 mm.

18. A flash dryer according to claim 16 and 17, wherein the ratio between the distance that the feed inlet (5) protrudes into the lower section (3) and the diameter of the lower section (3) lies in the range 0.02 to 0.45.

19. A flash dryer according to claim 6 and 17, wherein the ratio between the pre-defined extension (g) of the gap (81 ) in the radial direction lies and the diameter of the lower section (3) lies in the range 0.01 to 0.05.

20. A flash dryer according to any one of the preceding claims, wherein the total height of the drying chamber (2) lies in the interval 5 to 25 m and the height of the lower section (3) in the interval 0.5 to 5 m.

21 . A method for cooling product deposits in a flash dryer according to any one of claims 1 to 20, comprising the steps of

supplying a feed of product via the feed inlet (5),

supplying a drying gas at a pre-defined temperature and a predefined flow rate,

contacting the product with the drying gas to provide an upward flow (F) of drying gas and product to be dried in the axial direction,

supplying a cooling gas via the inlet (81 ) of the cooling arrangement (8) to provide an upward flow of cooling gas (CG) in the axial direction along the wall of at least the lower section (3) of the drying chamber (2), and

allowing the flow of cooling gas (CG) to reduce the temperature of deposits at the wall (31 , 32) of at least the lower section (3).

22. The method of claim 21 , wherein the cooling gas is supplied at a flow velocity of 30 to 90 m/s.

23. The method of claim 21 or 22, wherein the flow rate of cooling gas compared to the accumulated rate of drying gas and cooling gas is in an amount of 2 to 20%, preferably 4 to 15%.

24. The method of claim 22 or 23, wherein the cooling gas is supplied at flow velocity substantially corresponding to the pre-defined flow velocity of the drying gas.

25. The method of any one of claims 21 to 24, wherein the cooling gas is supplied at a temperature of ambient air to 1 10°C, preferably 50 to 90°C.

26. The method of claim 25, wherein the pre-defined temperature of the drying gas lies in the range 150 to 250°C.

27. The method of any one of claims 21 to 26, wherein the flow of cooling gas (CG) takes place in the axial direction only.

28. The method of any one of claims 21 to 27, wherein the step of allowing the flow of cooling gas (CG) to reduce the temperature of deposits at the wall (31 , 32) of at least the lower section (3) prevents burning of deposits.