Fluidized bed granulation

Abstract

The present disclosure relates to a fluidized bed granulation process, for example for urea melts. Embodiments of the method include providing a fluidized bed of particulate matter in a granulation compartment of a fluidized bed granulator; forming a film of a granulation liquid in the form of a hollow conical frustum protruding into the granulation compartment by supplying the granulation liquid through a first channel of a granulation nozzle; a second gas flow is supplied into the granulation compartment via a second gas channel of the granulation nozzle, where the second gas channel is arranged as a ring around the first channel, where an outlet of the second gas channel is directly exposed to the fluidized bed in the granulation compartment, wherein particulate matter from the fluidized bed is entrained in the second gas stream.

Description

Technical Field

[0001] The present invention relates to the granulation of, for example, urea in a fluidized bed granulator. Background Art

[0002] Fluidized bed granulation of urea is an important technology for the large-scale production of solid urea granules suitable for use as fertilizer. The capacity of modern urea fluidized bed granulation units (single lines) typically exceeds 1000 metric tons of urea product per day (mt / d), such as more than 2000mt / d or even 3000mt / d, or even higher (metric tons = 1000kg). Therefore, high-capacity granulation nozzles are desired, which have the ability to spray urea melt far above 100kg / h in a single nozzle. In addition, the entire granulator is expected to achieve a high conversion rate of the sprayed urea melt to urea granules of suitable size (i.e., low urea dust formation and a low proportion of undersized and oversized granules) so as to desirably have a relatively low dry cycle. In general, a fluidized bed granulator comprises a granulation compartment in which a fluidized bed of granules is maintained by fluidizing air supplied via a perforated bottom plate. A plurality of granulation liquid nozzles arranged at the bottom of the granulation compartment supply granulation liquid upwards into the granulation compartment. Typically a plurality of granulation compartments connected in series are used. Typically, seed granules are provided at one side of the granulator and granules are removed at the other side of the granulator. This type of fluidized bed granulator is used, for example, for the production of urea granules. The present invention relates to granulation in a fluidized bed granulator of this type.

[0003] Ullmann's Encyclopedia of Industrial Chemistry, Chapter Urea, 2010 describes various fluidized bed granulation techniques for fertilizing bed granulation.

[0004] In some of these known methods, a liquid material, for example in the form of a solution, melt or suspension, is sprayed with the aid of a gas into droplets which solidify on a fluidized core to form particles of the desired size. For the granulation process to proceed smoothly, the surface of the growing core must solidify quickly enough to prevent agglomeration of the individual particles. Therefore, it is necessary to ensure that the sprayed liquid material crystallizes quickly and that the water present evaporates quickly.

[0005] W. Roos, Urea Granulation–Part 1, Fertilizer Focus May June 2013 describes the Stamicarbon granulation technology using film spraying. High velocity hot air (secondary air) and urea melt are supplied through a nozzle. The secondary air stream is supplied through an annulus around the urea melt nozzle. In operation, the granules in the fluidized bed are drawn into the liquid film of urea melt discharged from the nozzle by the secondary air and are "wetted" by the urea melt. The wetted granules flow upward into a portion of the fluidized bed.

[0006] EP 0141437 describes a granulation process in which the liquid material is made to emerge from a central channel in the form of an almost closed conical film, the thrust of which exceeds that of the powerful airflow. With the help of the powerful airflow, the film is atomized into very fine droplets. In the examples, it was observed that upon impact, the film atomized almost immediately in the airflow. The accompanying drawings show that the outlet of the concentric channel for the powerful airflow (second gas) is isolated from the fluidized bed by a cover.

[0007] EP 0141436 describes a granulation process in which the liquid material is passed from a central channel into a fluidized bed as an almost closed conical film. The cores from the bed pass through the film with the help of a strong gas stream and, subsequently, the liquid material absorbed by the cores solidifies during the transport of the cores thus moistened through the region above the nozzles.

[0008] Important expectations for a granulation process include low dust formation and high throughput, as well as a high proportion of particles of the desired size in the granules removed from the granulator.

[0009] It would also be desirable to provide a fluidized bed granulation process and nozzles for use in such a process, wherein the throughput per nozzle is relatively high (eg kg urea melt per nozzle per hour). Summary of the invention

[0010] The first aspect of the present invention relates to a fluidized bed granulation method, which includes growing particles in a fluidized bed granulator by solidifying a granulating liquid on the particles. The method includes: providing a fluidized bed of particles in a granulation compartment of a fluidized bed granulator; forming a granulation liquid film in the form of a hollow conical frustum (i.e., provided as a hollow conical frustum protruding into the granulation compartment) by supplying granulating liquid through a first channel of a granulation nozzle; supplying a second gas flow into the granulation compartment via a second gas channel of the granulation nozzle, wherein the second gas channel is arranged as an annulus surrounding the first channel, wherein the outlet of the second gas channel is directly exposed to the fluidized bed in the granulation compartment, wherein particles from the fluidized bed are entrained in the second gas flow; and supplying the entrained particles to the membrane by directing the second gas flow to the membrane, depositing the granulating liquid on the surface of the particles, and solidification of the deposited liquid causes the growth of the particles. In an embodiment, the mass ratio of the second gas stream to the granulation liquid stream is at least 0.40, and preferably in the range of 0.40 to 1.0.

[0011] The present invention also provides a granulation nozzle for a fluidized bed granulation method, comprising a first channel for a granulation liquid and a second gas channel, wherein the second gas channel is arranged as a ring around the first channel, wherein the outlet of the first channel comprises a diffuser, which is inclined and bent outwardly to provide a convex surface for the granulation liquid to leave the first channel; in particular, a surface protruding in a vertical section and exposed to the granulation liquid ejected from the first channel during operation is provided. A fluidized bed granulator comprising the nozzle, a urea device comprising the fluidized bed granulator, and a granulation method using the nozzle are also provided. The method of the first aspect is optionally carried out with the granulation nozzle of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 An example granulator ( Figure 1 A: cross section, Figure 1 B; details; Figure 1 C: Detailed view of a preferred embodiment).

[0013] Figure 2 A cross-section of an example granulation nozzle that may be used in the present invention is schematically shown.

[0014] Figure 3 Schematically illustrated is an impact height, as used in an embodiment of the method of the present invention.

[0015] Figure 4 An example granulation nozzle with a diffuser, as provided by aspects of the present invention, is schematically illustrated.

[0016] Figure 5 Shows Figure 4 Top view (AA) of the nozzle.

[0017] Figure 6 The conceptual operation of a nozzle, in particular a nozzle with a hollow conical membrane, is shown schematically.

[0018] Any embodiments shown in the accompanying drawings are examples only and do not limit the present invention. DETAILED DESCRIPTION

[0019] The present invention relates in one aspect to a fluidized bed granulation process. The process involves growing granules in a fluidized bed granulator by solidifying a granulating liquid on the granules.

[0020] The fluidized bed granulation process of the present invention comprises growing granules in a fluidized bed granulator. Figure 1 A schematically shows an example of a fluidized bed granulator. The granulator (1) comprises a granulation compartment (2) configured to contain a fluidized bed of granules. The granulator (1) also comprises a nozzle (3). Figure 1 B shows in more detail that the nozzle (3) comprises a first channel (4) for the granulating liquid (a) and a second gas channel (5) for the second gas (b), which is arranged as a ring (annular object) around the first channel. The channel has an outlet at the top that is directly exposed to the fluidized bed of particulate matter. The outlet of the first channel is, for example, vertically above the outlet of the second gas channel. The granulator also includes a perforated fluidizing plate (6) for the fluidizing gas (c). The nozzle (3) preferably comprises an integral part (3a). Part (3a) has a channel for the granulating liquid (i.e., the first channel (4)), and has a conical shoulder (7) and provides a convex inner wall (5a) of the annular channel (5) for the second gas. Another part (12) provides a concave inner wall (5b) of the annular channel (5) and is, for example, cylindrical.

[0021] In operation, granulation liquid (a) is ejected from the first passage (4) and applied to the granules (13) which are kept in a fluidized state by the fluidizing gas (c).

[0022] In the granulator and therein can be Figure 1 In the typical embodiment of the granulator for carrying out the method shown in C, the reference numerals not discussed are the same as those Figure 1 Same as in A. Figure 1 The features of C are preferred for all embodiments of the invention.

[0023] The fluidizing gas is introduced into the granulation compartments (2); in particular, for each of them, via perforations in a perforated fluidizing plate (6) provided at the bottom of the granulation compartment (2), typically from one or more fluidizing gas chambers (14) located below the perforated fluidizing plate. In a preferred embodiment, the granulator is provided with a plurality of granulation compartments (2, 2a, 2b) connected in series, for example at least 3, for example 3 to 5 granulation compartments connected in series, each granulation compartment comprising a plurality of nozzles (3) (operating with a specific ratio of second gas to granulation liquid and / or having a curved diffuser), and these compartments are separated from each other by vertical partitions (15) which divide the fluidized bed into compartments and which provide a passage (21) for the particles at the bottom end (i.e. the space between the bottom of the partition and the fluidizing plate) so that preferably larger particles preferentially move between adjacent compartments and effectively move from an upstream compartment to an adjacent downstream compartment by the action of particle inlets and outlets at opposite sides of the granulator. The dividing plate is for example mounted to a side wall of the granulator.The use of a plurality of granulation compartments arranged in series, separated by such a divider and each comprising a nozzle provides for a better controlled residence time of the granules in the granulator.

[0024] The particles are removed by a particle outlet (16) arranged at the downstream side of a series of granulation compartments, and are preferably supplied to size classification. Gas is discharged by a gas outlet (17) at the upper part of the granulator. Granulation liquid (e.g., urea melt) is supplied to the nozzle by a supply manifold (18). A second gas (e.g., air) is also supplied to the nozzle using a second gas supply manifold (19). The dry cycle is optionally supplied as seed particles by a particle inlet (20), which is arranged at the upstream side of a series of granulation compartments. In certain embodiments, the dry cycle is melted into a urea melt separately and solidified in a separate unit (e.g., granulator) to obtain seed particles introduced via a particle inlet (20). In certain embodiments, the dry cycle is directly introduced into the particle inlet (20).

[0025] exist Figure 1 In a vertical section of the granulator in the length direction of the granulator C, the particle inlet and the particle outlet are arranged on opposite sides of the granulator. In operation, as a series of granulation compartments maintain a gradient of average particle size, the average particle size in the downstream compartment is larger; downstream means the compartment closer to the particle outlet. The granulation compartments are arranged in series along the length direction of the granulator. Optionally, as shown in the figure, the downstream compartment is a cooling compartment that receives fluidizing air but has no nozzle. Typically, the fluidized bed of particles is maintained in all granulation compartments and any cooling compartments. The growth of particles is based on the layer-by-layer accumulation of solidified granulation liquid to achieve a significant volume increase of particles of desired size and individual particles.

[0026] In a preferred embodiment, the fluidized bed granulator comprises an upstream granulation compartment and a downstream granulation compartment in series, as discussed, wherein the upstream compartment comprises a particle inlet, and the downstream compartment comprises a particle outlet or is adjacent to a downstream cooling compartment comprising a particle outlet. Optionally, one or more intermediate granulation compartments are provided between the upstream granulation compartment and the downstream granulation compartment.

[0027] The present method can be used for the granulation of various liquid materials. The granulation liquid is, for example, in the form of a solution, a melt or a suspension. The granulation liquid should contain or consist essentially of a component or one or more components that are solid at ambient temperature (e.g., 20°C) so as to allow the liquid to solidify. The granulation liquid, for example, contains at least 80% by weight of one or more compounds that are solid at 20°C in total. The granulation liquid contains, for example, less than 10% by weight or less than 5% by weight of water. The method can be used, for example, for the granulation of aqueous liquid materials, where rapid evaporation of water should occur in addition to solidification. The present method is used, for example, for the granulation of materials such as ammonium salts, such as ammonium nitrate, ammonium sulfate or ammonium phosphate and mixtures thereof; single fertilizers, such as calcium ammonium nitrate, magnesium ammonium nitrate; composite NP and NPK fertilizers; urea and urea-containing compounds; sulfur; and organic substances, such as bisphenol and caprolactam, etc. In a preferred embodiment, the method is used for the granulation of urea and urea-containing compounds.

[0028] In a particularly preferred embodiment, the granulation liquid is a urea melt, which contains, for example, at least 80% by weight of urea. The urea melt contains, for example, at most 3% by weight of water. The urea melt contains, for example, at least 90% by weight of urea and biuret, at most 3% by weight of water, and, for example, at most 7% by weight of other components, such as, for example, formaldehyde, or an ammonium salt or sulfur, or a combination thereof. In some embodiments, the granulation liquid contains, for example, at least 97.5% by weight of urea, which includes biuret.

[0029] Typically, the temperature of the granulating liquid in the nozzle is 5°C to 15°C higher than the solidification temperature of the granulating liquid.

[0030] The bed of particles is kept fluidized in the granulation compartment by an upwardly flowing fluidizing gas, in particular an air stream. In embodiments where the average particle size of the final product is from 2 to 4 mm, a fluidizing gas having a superficial velocity of from 1.5 to 2.5 m / s, in particular from 1.7 to 2.2 m / s, is preferably used. The height of the bed can be selected within wide limits (e.g., from 50 to 150 cm). The fluidizing gas is typically also used to cool the granulation liquid on the particles to solidify them. The temperature of the fluidizing gas is, for example, below 100° C., for example below 80° C.

[0031] Typically, a perforated fluidizing plate is used to uniformly supply fluidizing gas into the granulation compartment from the bottom of the granulation compartment.The granulation nozzles are typically mounted via openings in the fluidizing plate which are larger than the openings for supplying the fluidizing gas.

[0032] A fluidized bed of granules is maintained in the granulation compartment. These granules may also be referred to as core or seed granules. Conventionally, seed granules are typically supplied and product granules are removed on opposite sides of the granulator. In addition, the granules are kept in motion in the fluidized bed. Typically, a granulator comprises two or more granulation compartments connected in series. Specifically, the fluidized bed is typically divided by vertical baffles, and a relatively heavy stream of granules flows from one compartment to the next via openings at the lower side of the baffles.

[0033] The composition of the seed is not particularly limited and may have the same or different composition as the granulation liquid. For example, crushed oversized and undersized particles can be recycled to the granulator and used as seed, or pellets from the granulator or pellets from the granulation tower can be used as seed. Other types of seeds are also possible.

[0034] The seeds for example have an average diameter of at least 0.75 mm, for example in the range of 1 to 3 mm, for example 1.5 to 2.5 mm. Suitably the average seed diameter is the weighted median diameter.

[0035] The amount of seed introduced may vary. For example, an amount of seed is introduced such that the weight ratio of seed to granulation liquid is between 1:1 and 1:3; the same example range applies to the volume ratio.

[0036] The method involves growing the granules in a fluidized bed by solidifying the granulating liquid on the granules. The method of seed growth to product granules of the desired size typically involves multiple passages of the granules through a (hollow) conical film of granulating liquid above a nozzle. Thus, the growth method is based on a layer-by-layer accumulation of solidified granulating liquid.

[0037] The product particles are typically (substantially) spherical and have a diameter of, for example, at least 2.0 mm, for example in the range of 2.0 to 8.0 mm.

[0038] The method involves forming a granulation liquid film (i.e., a conical film) in the form of a hollow conical frustum protruding into a granulation compartment by supplying granulation liquid through a first channel of a granulation nozzle. In the method, the granulation liquid is ejected from the first channel as a granulation liquid film having the shape of a hollow conical frustum.

[0039] like Figure 2A schematically shows that the hollow conical frustum (f) of the granulating liquid film (a) has a smaller radius r1 at the nozzle and a larger radius r2 at a height h above the nozzle (3) in the upward direction, and the method includes contacting the film with the second gas flow (b) at a height h (impact height). The conical frustum has a vertex angle (θ). The nozzle (3) (particularly part (3a)) has an inclined shoulder (7) that guides the second gas (b) from the second gas channel (5) to the hollow conical film (f). For example and as shown in the figure, the lower end (8) of the shoulder (7) is directly adjacent to the second gas channel (5). This inclined shoulder is also preferred for the fluidized bed granulation method of the present invention, regardless of the use of a diffuser. Figure 2 B shows a horizontal cross section of the granulated liquid film (a) through BB, which shows that the film is in the shape of a hollow conical frustum (f). The example film thickness at the impact height is calculated to be about 90 μm.

[0040] Thus, the nozzle is suitably a hollow cone nozzle.

[0041] A granulation nozzle is provided at the bottom of the granulation compartment, and the nozzle is arranged with an outlet in an upward direction. The granulation liquid leaves the nozzle in an upward direction.

[0042] Preferably, the tapered membrane is formed by causing a rotational movement of the liquid material, in particular in the first channel; a rotation in a horizontal section or plane, ie a rotation about a vertical axis.

[0043] For example, a nozzle provided with a swirl chamber (also referred to as a vortex chamber) is used. The swirl chamber (vortex chamber) is, for example, a cylindrical chamber upstream of the granulation liquid outlet, which has a tangential inlet opening, for example, is arranged in the cylindrical wall of the chamber for granulation liquid in the form of a slit; the chamber is connected (particularly directly connected) to the outlet for granulation liquid at the top. The outlet channel typically has a smaller diameter than the swirl chamber. In this arrangement, the liquid is pressed through one or more channels of the tangential introduction swirl chamber under hydrostatic pressure. The material (that is, granulation liquid) moves along the wall of the outlet channel (first channel) in the form of a liquid film, and has a horizontal velocity component generated by rotation and a vertical velocity component depending on the production volume (that is, in the outlet channel). Therefore, the method preferably includes supplying via the first channel when the granulation liquid is rotated in a horizontal section as a rotating film.

[0044] Example Suitable rotation chambers are shown and described in EP 0141437A and EP 0141436A.

[0045] Hollow cone nozzles with swirl chambers are also called tangential flow hollow cone nozzles.

[0046] In a further embodiment, the nozzle is an axial hollow cone nozzle.Such nozzles typically include a swirl insert with spiral grooves to give the liquid a swirl.

[0047] In general, the liquid material is preferably supplied at a hydrostatic pressure of at least 3 bar absolute, or at least 4 bar absolute, for example up to 7 bar absolute, or up to 5 bar absolute.

[0048] Preferably, the liquid velocity for the granulation liquid is at least 15 m / s, more preferably at least 17 m / s, and for example up to 50 m / s or up to 25 m / s.As used herein, liquid velocity typically includes a rotational component.

[0049] The apex angle of the conical frustum of the membrane is, for example, in the range of 50 to 160°, preferably in the range of 70 to 110°, in particular in the range of 80 to 100°. The apex angle θ is schematically shown as Figure 2 shown.

[0050] Preferably, the film has a thickness of at least 300 μm, or at least 400 μm, more preferably at least 450 μm at the outlet of the first channel, and the thickness is for example less than 600 μm. Suitably, the granulation nozzle is of a type that allows the film to have a thickness of at least 400 μm, for example in the range of 400 μm to 600 μm. The film thickness at the impact height is for example in the range of 80 to 120 μm.

[0051] Preferably, the first channel has a diameter of at least 3.0 mm or at least 4.0 mm at the outlet. A relatively large diameter facilitates a larger production volume.

[0052] In addition, the method comprises supplying the second gas into the granulation compartment via the second gas channel of the granulation nozzle. The second gas channel is arranged to be an annulus around the first channel and is typically concentric with the first channel. The second gas channel is preferably adjacent to the granulation nozzle so that the nozzle includes a wall that contacts the granulation liquid on one side and contacts the second gas on the other side when in operation.

[0053] The second gas is supplied substantially upwards into the granulation compartment and may be, for example, converging at a small angle of, for example, 5 to 25° and preferably 5 to 15°. In some embodiments, the outlet openings for the second gas are made to converge at a small angle of, for example, 5 to 25° and preferably 5 to 15°.

[0054] Importantly, the outlet of the second gas channel entering the granulation compartment is directly exposed to the fluidized bed in the granulation compartment. Therefore, the outlet is in unobstructed contact with the fluidized bed. In this way, the particles from the fluidized bed are entrained by the high-speed second gas and brought to the conical film of the granulation liquid. In particular, the outlet of the second gas channel is not isolated from the fluidized bed.

[0055] The second gas has a relatively high velocity at the outlet of the second gas channel, for example at least 200 or at least 250 m / s, for example 200 to 350 m / s. The velocity refers to the velocity of the second gas in the second gas channel at the outlet of the channel.

[0056] High velocity can help to effectively entrain and supply the particles to the conical membrane. The gas feed pressure is, for example, 1.1 to 1.5 bar absolute. The second gas is preferably air. For example, a gas stream having a temperature equal to or at most 20° C. higher than the temperature of the granulation liquid material is used.

[0057] The second gas contacts the conical film at a height h, for example, in the range of 5 mm to 15 mm above the nozzle outlet for the conical film, more preferably in the range of 7.0 mm to 12 mm. A larger height may contribute to a larger side surface area of ​​the conical film, thereby providing more efficient granulation. This height h may be referred to as the impact height.

[0058] The second gas stream is unrestricted and typically flows generally upwards. The second gas stream flows upwards, for example, as a converging annulus (ring).

[0059] For example, the second gas stream is directed by a conical or inclined shoulder of the granulating liquid nozzle, such as Figure 2 As shown in . The shoulder of the granulation liquid nozzle converges in an upward direction to provide a conical surface (conical frustum) with a smaller radius at the top. For example, the angle between the shoulder of the granulation liquid nozzle and the vertical direction is 10 ° to 40 °, in particular 20 ° to 30 °. The lower end of the nozzle shoulder is preferably directly adjacent to the second gas channel, for example, the vertical spacing between the lower end of the nozzle shoulder and the second gas channel is less than 1 mm and the horizontal spacing is less than 1 mm. The second gas leaving the annular second gas channel is thus directly guided by the shoulder.

[0060] In the embodiment where the granulating liquid nozzle has a conical or inclined shoulder converging in the upward direction, the impact height (h) of the second gas with the granulating liquid film can be considered to be the height of the geometric intersection of the conical frustum of the granulating liquid (diverging in the upward direction) and the conical frustum defined by the projection of the shoulder in the upward direction. Figure 3 Schematically shown in the figure, there is a conical shoulder (7) and a second gas channel (5), as well as a protrusion (9) where the shoulder intersects (10) with the conical membrane (f), thereby defining a height (h) between the nozzle (3) (particularly the outlet of the first channel (4)) and the intersection surface (10). Figure 3 All other reference numerals in Figure 2 The same as in.

[0061] Preferably, the mass ratio of the second gas stream (preferably air as the second gas) to the granulation liquid stream is at least 0.40, or at least 0.45, or at least 0.50, and / or for example up to 1.0 or up to 0.70, for example 0.40 to 1.0 or for example 0.45 to 0.70. A relatively large amount of the second gas can advantageously contribute to low dust formation and can allow operation with high liquid throughput, particularly in combination with the large side surface area of ​​the tapered film. A mass ratio that is too high (for example, higher than 1.0) may increase the risk of interfering with the granulation method. The relatively high mass ratio of the second gas to the granulation liquid can advantageously contribute to sufficiently small dry circulation percentage, for example, less than 70% by weight of dry circulation relative to the appropriate size particulate matter. In this article, the particulate matter of appropriate size is for example a particle diameter of 2 to 4mm. Without wishing to be bound by theory, the combination of relatively high gas to liquid mass ratio and relatively high gas velocity can also provide atomized film, thereby obtaining the benefit of additional coating by droplets.

[0062] The measured volumetric gas flow rate can be converted to the weight density of the second gas using the ambient pressure (1 atm, 101.325 kPa absolute pressure) and the density at 20°C, i.e., 1.18 kg / m if air is used as the second gas. 3 (Dry air is 1.188kg / m 3 , 60% relative humidity is 1.182kg / m 3 ).

[0063] Therefore, preferably, for air as the second gas, the second gas flow is 0.34 to 0.85 Sm per kg of granulation liquid. 3 The second gas, Sm 3 It is expressed in m at the reference conditions of 20°C and 101.325 kPa absolute pressure. 3 Preferably, the second gas flow is at least 0.34 or at least 0.38 or at least 0.42 Sm per kg of granulation liquid. 3 Second gas (mass ratio of at least 0.40, at least 0.45 or at least 0.50, respectively), and / or for example at most 0.84 or at most 0.59 Sm per kg of granulation liquid 3 Second gas (mass ratio up to 1.0 or up to 0.70); for example 0.34 to 0.85 Sm per kg of granulation liquid 3 Second gas (mass ratio 0.40 to 1.0 or 0.38 to 0.59 Sm per kg granulation liquid 3 Second gas (mass ratio 0.45 to 0.70). The measured volumetric gas flow rate and the measured temperature can be converted to Sm 3 Gas flow rate in the second gas. The advantages of the gas volume ratio per kg of liquid are the same as the mass ratio.

[0064] In the example, if the volume flow of the granulation liquid is measured, the density of the granulation liquid is optionally set to 1220 kg / m 3 , wherein the granulation liquid is a urea melt, which contains, for example, at least 80% by weight of urea. The liquid density at the granulation liquid temperature measured at or near the granulator nozzle inlet can be used for the conversion.

[0065] In an example embodiment, the method involves dry recycling of part of the granules removed from the granulator. Thus, in an example embodiment, the method comprises removing granules from the granulator, classifying the granules by size into granules of a desired size, undersized granules and oversized granules, and recycling the undersized granules and / or the crushed oversized granules as seed to the granulator. Preferably, the proportion of the recycled granules is at least 5% by weight or at least 10% by weight, and / or at most 70% by weight or at most 60% by weight relative to the unrecycled granules of the desired size.

[0066] In the present invention, the opening of the annular channel for the second gas is directly exposed to the granulation compartment and thus directly exposed to the fluidized bed of particulate matter. Therefore, before the second gas flow hits the granulation liquid film, at least some of the particulate matter from the fluidized bed in the granulation compartment is entrained by the second gas flow. In particular, the opening of the second gas channel of the nozzle is not isolated from the fluidized bed. The second gas channel is preferably arranged next to the granulation liquid channel. The nozzle preferably comprises an integral part having a channel for the granulation liquid and a conical shoulder and providing an inner wall of the annular channel for the second gas.

[0067] Without wishing to be bound by theory, particulate matter at the lower portion of the fluidized bed may be drawn into the high velocity secondary gas stream, moving upward and generally radially inward with the gas stream until the secondary gas stream strikes the conical membrane.

[0068] In a preferred embodiment, the second gas channel has an outlet (outlet opening) entering the fluidized bed at a vertical level that is at least 5 mm or at least 10 mm lower than the outlet (outlet opening) of the first channel, preferably this vertical distance is 10 to 20 mm. In this way, it is advantageous that the high-speed second gas can better suck and entrain particles from the fluidized bed before contacting the granulating liquid film. The conical shoulder of the granulating liquid nozzle guides the second gas flow upwards through this vertical distance. The vertical distance between the outlets is such as Figure 2 As shown by the arrow (h2).

[0069] Preferably, the second gas is in direct contact with the fluidized bed of particles as the second gas travels through the vertical distance. Thus, the vertical distance provides (or contributes to) a free path distance for the second gas.

[0070] The method comprises supplying the entrained particles to the membrane by directing a second gas stream to the membrane, depositing a granulation liquid on the surface of the particles and solidification of the deposited liquid causing the growth of the particles. In particular, a second gas stream with entrained particles is directed to the membrane. This causes the granulation liquid to be deposited on the surface of the particles from the membrane. The method also relates to solidification of the deposited liquid, causing the growth of the particles. The process generally involves contacting the membrane with a second gas containing the entrained particles. This contact causes the deposition of the granulation liquid on the surface of the particles. The method also relates to solidification of the deposited liquid, causing the growth of the particles.

[0071] The contact angle between the conical liquid film and the second gas flow is preferably in the range of 40° to 80°, more preferably 60° to 80°.

[0072] By the strong upward airflow of the second gas around the nozzle, a zone is formed in the fluidized bed above the nozzle. Solidification of the deposited liquid can occur in this zone. The particulate matter on the top of this zone is typically fully dry, that is, the applied granulation liquid is completely or almost completely solidified. Thereby avoiding the reunion of the particulate matter. The relatively high ratio of the second gas to the granulation liquid can contribute to the sufficient size of the zone for solidification.

[0073] In operation, each nozzle has its own zone which is used to dry the particles located above it.

[0074] Suitably, the granulation liquid and the second gas are A ratio in the range of 1.5 to 10, for example 3 to 8 (referred to as dynamic pressure ratio in Example 1) is used.

[0075] Where ρ is the density in kg / m 3 , and V is the linear outlet velocity in m / s. The density for the urea melt is optionally set to 1220 kg / m 3 .

[0076] The film preferably has a Weber number of at least 1000 and / or at most 5000, more preferably 2600-5000. The Weber number We here is:

[0077] We=ρU 2 δσ -1

[0078] Where ρ is the density of the liquid in kg / m 3; U is the velocity of the liquid (including the rotational component) in m / s; σ is the surface tension of the liquid in N / m, and δ is the film thickness when discharged from the first channel for granulation liquid in m. The surface tension for the urea melt can be taken as 0.07 N / m. The density for the urea melt is optionally set to 1220 kg / m 3 .

[0079] Liquid films having a Weber number within this range advantageously provide sufficient internal turbulence in the film for a good granulation process.

[0080] The fluidized bed granulation process is advantageously carried out with a throughput of at least 200 kg / hr of granulation liquid per nozzle, or at least 300 kg / hr, or at least 400 kg / hr, or at least 450 kg / hr (e.g., for urea melt). The fluidized bed granulation process is advantageously carried out with a throughput of at least 0.15 m3 of granulation liquid per nozzle. 3 / hr of granulation liquid, or at least 0.30m 3 / hr, or at least 0.40m 3 / hr (for example, for urea melt).

[0081] For example, a higher throughput per nozzle can be used to reduce the number of nozzles in a granulator, for example, by increasing the distance between granulators, and / or increasing the capacity of the granulator.

[0082] Preferably, the conical membrane of the granulating liquid has a diameter of at least 200 mm. 2 , more preferably at least 300 mm 2 or at least 400mm 2 Or even at least 450mm 2 The lateral surface area of ​​the conical frustum is preferably at least 0.15 m2, in particular the throughput of granulating liquid per nozzle is at least 300 kg / hr, or at least 400 kg / hr, or at least 450 kg / hr, and / or the lateral surface area of ​​the conical frustum is preferably at least 0.15 m2 3 / hr, or at least 0.30m 3 / hr or at least 0.40m 3 / hr; using, for example, urea melt as the granulation liquid. Here, the conical frustum has a smaller radius r at the nozzle 1 , and has a larger radius r at a height h above the nozzle in the upward direction 2 , and the method involves exposing the membrane to a second gas stream at a height h (impact height h; e.g. Figure 2 Specifically, r 1is equal to the radius of the first channel at the outlet for the granulation liquid, and the frustum is conceptually composed of a cone horizontally intercepted by the nozzle outlet face and the impact height h. The apex angle θ of the cone is defined by the apex angle of the conical film of the granulation liquid. Therefore, the conical frustum is defined by the diameter of the outlet opening of the first channel, the angle of the conical film and the impact height h, wherein the frustum diverges in the direction from the outlet opening of the nozzle to the impact height.

[0083] In a preferred embodiment, the tapered membrane has a width of at least 400 mm. 2 The lateral surface area of ​​the conical frustum is at least 0.5 m, the throughput of the granulating liquid per nozzle is at least 400 kg / hr, and / or the granulating liquid per nozzle is at least 0.30 m 3 / hr, wherein the granulation liquid is, for example, urea melt.

[0084] A relatively large lateral area, especially in combination with a relatively high ratio of the second gas to the granulation liquid, ensures that the membrane has sufficient area for the particles to be wetted (or covered) by the membrane. Too small a lateral area may result in stunted particle growth, especially at relatively high granulation liquid throughputs, e.g. for larger film thicknesses at the nozzle outlet.

[0085] A larger lateral surface area of ​​the tapered membrane can be obtained by one or more of a larger first channel outlet diameter, a wider cone apex angle θ, a larger impact height; in particular, by a combination of these characteristics.

[0086] A granulation nozzle which is particularly suitable for the method of the invention is also provided. The invention also relates to such a granulation nozzle

[0087] refer to Figure 4 In the example nozzle observed in a vertical section, the granulation nozzle (3) comprises a first channel (4) for the granulation liquid and a second gas channel (5), wherein the second gas channel is arranged as a ring around the first channel. The outlet of the first channel comprises a diffuser (11), which is inclined and curved outwards, especially in the vertical section. The curvature of the diffuser can increase the top angle θ of the tapered membrane by the Coanda effect. This in turn increases the lateral surface area of ​​the tapered membrane, which ensures that the membrane has enough area for the particles to be wetted by the membrane as described above.

[0088] The diffuser in particular provides or comprises a convex surface, i.e. a surface of the channel which protrudes along the trajectory of the (emitted) granulating liquid in a vertical section and which is exposed to the granulating liquid during operation. Typically, at least a portion of the diffuser is in contact with the granulating liquid film during operation, i.e. the diffuser portion which provides this surface. The diffuser is thus a curved, concave diffuser for the granulating liquid film. The vertical section mentioned is a section through the nozzle and more specifically through the central axis of the first channel. The first channel is typically circular in a top view or in a horizontal section through it, and the diffuser is typically curved and concave in a top view or in a horizontal section. Alternatively or additionally, the direction of the first channel defines the length direction (in Figure 4 and vertical in operation) and the radial plane is perpendicular to the length direction, i.e. Figure 4 and running level.

[0089] In an embodiment (not shown), the outlet of the first channel includes a vertical straight portion, an arcuate convex portion serving as a diffuser, and an outwardly inclined straight portion in the liquid flow direction (ie, upward direction); in particular, the vertical cross-section of the diffuser is convex.

[0090] The radius of curvature of the diffuser portion is, for example, 50 to 150% of the radius of the first channel. For a curved surface, the radius of curvature is the radius of the circle of the most conforming normal section or a combination thereof. The surface at the upper end of the diffuser (the downstream end for liquid flow) is, for example, inclined at 20° to 60° from the vertical. Figure 4 Shows Figure 1 B Modification of the nozzle with the addition of a diffuser (11).

[0091] Preferably, the nozzle has an inclined (or tapered) shoulder from the outlet of the second gas channel (5) upwards towards the first channel. More preferably, the inclined shoulder converges in an upward direction. The inclined shoulder can be configured to direct the gas from the second gas channel (5) towards the tapered film of granulation liquid from the first channel.

[0092] Preferably, in operation of the nozzle with diffuser, a hollow conical film of granulating liquid is ejected from the first channel by imparting a rotational motion (rotation in a horizontal plane) to the liquid material.

[0093] For example, the nozzle comprises a rotation chamber (also called a vortex chamber) upstream of the first passage for the granulation liquid; with all the preferences and details of the vortex chamber as discussed above. For example, the nozzle with the diffuser is a tangential flow hollow cone nozzle. In another embodiment, the nozzle with the diffuser is an axial hollow cone nozzle. For example, the nozzle comprises a vortex insert with spiral grooves to induce rotation of the granulation liquid.

[0094] Figure 5 A top view of the nozzle is shown ( Figure 4 The gray part indicates Figure 4 The metal part in the horizontal section BB, white represents the annular opening in said section. The diffuser (11) is hatched. The first channel (4) is concave in the horizontal section BB, and the diffuser (11) is similarly concave in the horizontal section through it.

[0095] Figure 6 The conceptual operation of a nozzle, in particular a nozzle with a hollow conical membrane, is shown schematically. Figure 2 The same as in. Without wishing to be bound by theory, the second gas (B) flows from the second gas channel (5) through the inclined shoulder (7) and entrains the particles (20). In other words, the particles from the lower part of the fluidized bed are entrained in the second gas flow with a high velocity. The second gas flow with the entrained particles hits the hollow conical film (F) of the granulation liquid ejected from the channel (4), and the entrained particles are therefore covered (i.e., wetted) by a layer of granulation liquid (21), which then solidifies in the continuation of the gas flow. It is worth noting that the film may no longer be a perfect film at the impact area and may have been deformed when encountering the second gas. At the impact area, the film may be broken to a certain extent due to the action of the second gas. The arrow (FD) indicates the direction of liquid flow in the vertical section. The background reference for granulation using the ejected conical film of granulation liquid is EP 0141436.

[0096] The present invention also relates to a fluidized bed granulator including a granulation compartment. The granulation compartment includes at least one granulation nozzle as described herein, i.e., with a curved diffuser as discussed. The compartment also includes a perforated fluidizing plate located at the lower portion of the granulation compartment, for example, as the bottom of the compartment. The granulation nozzle with a diffuser extends upward from the bottom portion of the granulator (i.e., the nozzle extends vertically upward) through the fluidizing plate, wherein the outlet of the first channel protrudes upward into the granulation compartment. Preferably, the granulator includes two or more granulation compartments in series, and each granulation compartment preferably includes at least 10 such granulation nozzles. Preferably, the granulation compartments are separated from each other by the vertical partition plate discussed, with a channel for particles at the bottom of the vertical partition plate; preferably, the seed particle inlet is located on one side of the granulator and the particle outlet is preferably located on the opposite side of the granulator, as discussed, wherein these sides are relative in a horizontal plane, and the particle outlet defines one side of the granulator, wherein the downstream granulator compartment of a series of granulator compartments is located on this side. Preferably, in operation, a gradient of mean particle size is maintained over the granulation compartments in series, with larger particles being located in downstream granulation compartments closer to the particle outlet.

[0097] The granulator typically has an outlet for exhaust gas, an inlet for seed particles and an outlet for formed particles at the top or upper part, and a supply manifold for granulation liquid from the inlet for granulation liquid of the granulator to a plurality of first channels of a plurality of nozzles. In addition, the granulator typically also includes a fluidizing air compartment arranged below the granulation compartment for receiving fluidizing gas (air), wherein the fluidizing plate is located between the bottom compartment and the granulation compartment; so that in operation the fluidizing gas flows from the lower compartment through the perforations of the fluidizing plate into the granulation compartment, in particular in an upward direction. The vertical direction can also be referred to as the longitudinal direction, which is defined by the direction of the first channel; and the horizontal plane can then be referred to as a radial plane perpendicular to the longitudinal direction. When in operation or installation, the nozzle extends vertically upward relative to gravity, which ensures that the fluidized bed of particles is correctly formed in operation.

[0098] In an embodiment, the process of the present invention is preferably carried out in the fluidized bed granulator, wherein a curved diffuser is optional.

[0099] The invention also relates to a urea plant comprising a urea melt plant and a fluidized bed granulator as described above. The outlet for the urea melt of the urea melt plant is connected to the inlet for the granulation liquid of the fluidized bed granulator. The urea melt plant comprises, for example, a synthesis section for forming urea, typically a recovery section, and an evaporation section with an outlet for the urea melt. The urea plant is, for example, of the stripping type. The synthesis section is a high-pressure synthesis section and comprises a reactor and, for a stripping type urea plant, also a stripper and a condenser, wherein the reactor and the condenser are optionally combined in a single container. The synthesis section has a urea recovery section for NH 3 Feed and CO 2 The recovery section may comprise one or more dissociators and typically comprises a condenser for condensing the gases from the dissociators and has a recycle line for the carbamate from the condenser to the synthesis section.

[0100] The invention also provides a fluidized bed granulation method with a nozzle with a diffuser as described and / or in a fluidized bed granulator comprising such a nozzle with a diffuser. The method comprises providing a fluidized bed of granules in a granulation compartment of the fluidized bed granulator. The method also comprises forming a film of granulation liquid in the form of a hollow conical frustum protruding into the granulation compartment by supplying granulation liquid via a first channel of the granulation nozzle. Thus, in this method, the granulation liquid is ejected from the first channel and a film of granulation liquid is formed or provided, which film has the shape of a hollow conical frustum.

[0101] The method also includes that the second gas channel of granulation nozzle is supplied with the second gas flow (preferably air) in the granulation compartment, wherein the second gas channel is arranged to the annulus around the first channel. The outlet of the second gas channel is directly exposed to the fluidized bed in the granulation compartment, and the particulate matter from the fluidized bed is entrained in the second gas flow. The method also includes that (at least some) particulate matter entrained is supplied to the film by directing the second gas flow to the film. Therefore, the second gas flow with the particulate matter entrained is directed to the film. This makes the granulation liquid deposited on the surface of the particulate matter and the solidification of the deposited liquid causes the growth of the particulate matter.

[0102] The fluidized bed granulation process with nozzles with diffusers is advantageously carried out with a throughput of at least 300 kg / hr of granulation liquid per nozzle, or at least 400 kg / hr, or at least 450 kg / hr (e.g. for urea melt). The fluidized bed granulation process is advantageously carried out with at least 0.30 m3 of granulation liquid per nozzle (each individual nozzle). 3 / hr of granulation liquid, or at least 0.40m 3 / hr (for example, for urea melt). Note that each nozzle is 0.30m 3 The granulation liquid at a rate of 83 ml per second is supplied through a membrane with a diameter of about 5 mm and a thickness of about 500 μm at the liquid outlet.

[0103] All preferences for the granulation method according to the present invention are also applicable to a fluidized bed granulation method performed with a nozzle with a diffuser as described and / or in a fluidized bed granulator comprising such a nozzle with a diffuser. The granulation method described herein (e.g., with a specific ratio of the second gas to the granulation liquid) is preferably performed using a nozzle with a diffuser as described and / or in a fluidized bed granulator comprising such a nozzle with a diffuser; the same preferences are present for the nozzle and the fluidized bed granulator. The granulation liquid is preferably a urea melt comprising at least 70% by weight or at least 90% by weight urea and preferably less than 5% by weight water. All preferences for the nozzle configuration discussed in conjunction with the fluidized bed granulation method (e.g., a conical or inclined shoulder) are also applicable to a nozzle with a diffuser. The second gas-liquid ratio is optional, but is preferred for the granulation liquid with a diffuser. The diffuser is optional, but is preferred for the granulation method with the second gas-liquid ratio.

[0104] At least the terms "typically", "usually" and "particularly" indicate features that are frequently used but are not mandatory for all embodiments.

[0105] Examples

[0106] Aspects of the invention are illustrated in the following examples, which neither limit the invention nor the claims.

[0107] Example 1

[0108] Two nozzles according to the present invention were simulated using CFD. The method settings are shown in Table 1. The inner diameter of the second gas channel of Nozzle 2 is larger than that of Nozzle 01, and the diameter of the front head of the nozzle is also larger. Nozzle 2 has a curved diffuser, as described in the present disclosure and as shown in Table 1. Figure 4 Schematically shown. Air is used as the second gas.

[0109] Table 1

[0110]

[0111] Example 2

[0112] Nozzles 1 and 2 of Example 1 were tested in a pilot-scale unit for urea granulation. The results are shown in Table 2 below. The dry recycle consists of fines (undersized particles) and crushed oversized particles. The dry recycle is given as a percentage of the appropriate size product. The dry recycle is reintroduced into the granulator as particles through a separate inlet rather than through a nozzle. For nozzle 1 with a load of 200 kg / h, the flow at the granulator particle outlet consists of 200 kg / h of particles of appropriate size and 104 kg / h of dry recycle. For nozzle 1, the dry recycle is 99% fine material and 1% coarse material. The particle mixture at the granulator outlet is size graded by screening with two sieves, and the oversized particles are crushed. The granulator is provided with a gas outlet; the gas exhaust contains some urea dust, so the output is lower than the melt load.

[0113] Dry cycles for commercial plants are typically in the range of 30 to 60%.

[0114] For nozzle 2, the dust level (including dust from the oversize pellet mill) was 3.8% at 500 kg / hr, within the range of 2.8 to 4% observed for commercial pelletizing units. Air was used as the second gas.

[0115] Table 2

[0116]

Claims

1. A fluidized bed granulation method comprising growing granules in a fluidized bed granulator by solidifying a granulating liquid on the granules, the method comprising: a. providing a fluidized bed of the granules in the granulation compartment of the fluidized bed granulator; b. forming a film of the granulating liquid in the form of a hollow conical frustum protruding into the granulating compartment by supplying the granulating liquid through a first channel of a granulating nozzle; c. supplying a second gas flow into the granulation compartment via a second gas channel of the granulation nozzle, wherein the second gas channel is arranged as an annulus around the first channel, wherein an outlet of the second gas channel is directly exposed to the fluidized bed in the granulation compartment, wherein granules from the fluidized bed are entrained in the second gas flow; d. supplying the entrained particles to the membrane by directing the second gas stream toward the membrane, causing the granulation liquid to be deposited on the surface of the particles and solidification of the deposited liquid causing growth of the particles, The mass ratio of the second gas flow to the granulation liquid flow is in the range of 0.40-1.0, and / or the second gas flow is 0.34 to 0.85 Sm per kg of granulation liquid. 3 Gas (20°C, 101.325 kPa absolute pressure).

2. The fluidized bed granulation method according to claim 1, wherein the second gas flow has a velocity of at least 200 m / s, preferably at least 250 m / s at the outlet of the second gas channel.

3. The fluidized bed granulation method according to claim 1 or 2, wherein the outlet of the second gas channel is arranged to be at least 10 mm lower than the outlet of the first channel.

4. Fluidized bed granulation process according to any one of the preceding claims, wherein the membrane has a thickness of at least 400 μm, preferably at least 450 μm, at the outlet of the first channel.

5. A fluidized bed granulation process according to any one of the preceding claims, wherein the conical frustum has a height h of at least 400 mm between the outlet of the first channel and the second gas stream contacting the membrane. 2 wherein the height h is preferably in the range of 7 to 12 mm from the granulation liquid outlet.

6. The fluidized bed granulation process according to claim 5, wherein the first channel has a diameter of at least 4.0 mm at the outlet, and the granulation liquid has a velocity of at least 15 m / s at the outlet.

7. A fluidized bed granulation process according to any one of the preceding claims, wherein the second gas flow is 0.34 to 0.85 Sm per kg of granulation liquid. 3 Gas (20°C, 101.325 kPa absolute pressure).

8. The fluidized bed granulation process according to any one of the preceding claims, wherein the second gas is air and the mass ratio of the second gas flow to the granulation liquid flow is in the range of 0.40-1.

0.

9. A fluidized bed granulation process according to any one of the preceding claims, wherein the conical film is formed by imparting a rotational motion to the granulation liquid in a horizontal plane in the first channel.

10. A fluidized bed granulation method according to any one of the preceding claims, which is carried out in a granulator comprising a plurality of granulation compartments connected in series, wherein the fluidizing gas is introduced into each of the granulation compartments (2) via perforations in a perforated fluidizing plate (6) arranged at the bottom of the granulation compartment (2), each of the plurality of granulation compartments (2) comprises a plurality of nozzles (3), the plurality of nozzles operating with a specific ratio of a second gas to a granulation liquid, and wherein the granulation compartments are separated from each other by vertical partitions (15), the vertical partitions providing a passage for the particles from one granulation compartment to an adjacent granulation compartment at the bottom portion.

11. Fluidized bed granulation process according to any of the preceding claims, wherein the granulation liquid is a urea melt comprising, for example, at least 80% by weight of urea.

12. A fluidized bed granulator (1) comprising a granulation compartment (2) and at least one nozzle, the granulation compartment comprising a perforated fluidizing plate (6) located at the lower part of the compartment, wherein the nozzle comprises a first channel (4) for a granulation liquid and a second gas channel (5), wherein the second gas channel is arranged as a ring around the first channel, wherein the outlet of the first channel comprises a diffuser (11), which is inclined and curved outwardly to provide a convex surface for the granulation liquid to leave the first channel in a vertical section, and wherein the nozzle extends through the fluidizing plate, wherein the outlet of the first channel protrudes upward into the granulation compartment.

13. A fluidized bed granulator according to claim 12, wherein the nozzle has an inclined shoulder from the outlet of the second gas channel (5) upwards towards the first channel.

14. A fluidized bed granulator according to claim 12 or 13, wherein the granulator comprises a plurality of granulation compartments connected in series, wherein the fluidizing gas is introduced into each of the granulation compartments (2) via perforations in a perforated fluidizing plate (6) arranged at the bottom of the granulation compartment (2), each of the plurality of granulation compartments (2) comprises a plurality of nozzles (3), the plurality of nozzles operating with a specific ratio of a second gas to a granulation liquid, and wherein the granulation compartments are separated from each other by vertical partitions (15), the vertical partitions providing a passage for the particles at the bottom portion from one granulation compartment to an adjacent granulation compartment.

15. A urea plant comprising a urea melt plant and the fluidized bed granulator according to claim 12, 13 or 14.

16. A fluidized bed granulation method, the fluidized bed granulation method being carried out in a fluidized bed granulator according to any one of claims 12 to 14, the method comprising: a. providing a fluidized bed of granules in the granulation compartment of the fluidized bed granulator; b. forming a film of the granulation liquid in the form of a hollow conical frustum protruding into the granulation compartment by supplying the granulation liquid through the first channel of the granulation nozzle; the granulation nozzle having the diffuser (11) which is inclined and curved outwardly to provide a convex surface for the granulation liquid to leave the first channel in a vertical section; c. supplying a second gas flow into the granulation compartment via the second gas channel of the granulation nozzle, wherein the second gas channel is arranged as an annulus around the first channel, wherein the outlet of the second gas channel is directly exposed to the fluidized bed in the granulation compartment, wherein the particles from the fluidized bed are entrained in the second gas flow; d. supplying entrained particles to the membrane by directing the second gas stream containing entrained particles toward the membrane, causing the granulation liquid to be deposited on the surface of the particles and solidification of the deposited liquid causing growth of the particles, Furthermore, it preferably has the features according to any one of claims 1 to 11.

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