PROCESS FOR THE PREPARATION OF GRANULES AND GRANULES OBTAINED BY THIS PROCESS

Abstract: Process for the preparation of granules from a liquid composition by spraying the liquid composition in a granulation zone (1) onto or over solid particles, present in the granulation zone, that are being kept in motion, as a result of which layered growth of these particles takes place, withdrawing a particle stream (2) from the granulation zone, dividing this stream into a stream of particles of the desired size (5), a stream of oversize particles (7) and a stream of undersize particles (6), the stream of undersize particles (6) being returned at least in part to the granulation zone and the stream of oversize particles being stored in a buffer vessel (8) and subsequently being returned to the granulation zone (1) at constant flow rate after size reduction in a size reduction device (11) to a size desirable for use as nuclei.

Description: The invention relates to a process for the preparation of granules from a liquid composition by spraying the liquid composition in a granulation zone onto or over solid particles, present in the granulation zone, that are being kept in motion, as a result of which layered growth of these particles takes place, withdrawing a particle stream from the granulation zone, dividing this stream into a stream of particles of the desired size, a stream of oversize particles and a stream of undersize particles, the stream of undersize particles being returned at least in part to the granulation zone and the stream of oversize particles being stored in a buffer vessel and subsequently . being returned at least in part, after size reduction, to the granula­ tion zone. Various granulation processes are known in which layered growth of the particles in the granulation zone takes place. TVA bulletin Y 158 describes TVA drum granulation process, in which granulation is effected via a so-called falling curtain process, in which the particles are moved upwards by blades and subsequently fall down as a curtain. The liquid composition is sprayed by sprayers that are directed at the particle curtain, as a result of which the liquid is deposited on the particles. European patent 26918 describes a so-called spouted bed gra­ nulation in which the liquid composition is centrally passed through a mass of particles, from the bottom upwards in a gas stream and a quan­ tity of the particles of this mass is entrained by the gas stream, the liquid composition being deposited on the particles, which sub­ sequently, when the velocity of the gas stream decreases, drop back on the surface of the mass of particles. In both of the above-mentioned processes it has been found necessary to take measures to counteract instability of the granula­ tion process, which is manifested in a strongly fluctuating granule size distribution of the product stream leaving the granulation zone. This variation in granule size distribution is highly undesirable as it has an adverse effect on the production capacity and also because extra facilities are necessary to keep the granule temperature, often an important quality factor, constant. Another type of granulation process with layered particle growth is the process for granulation in a fluid bed. Such a process is described in US patent 4,219,589. In this process the liquid composition is sprayed in the form of fine droplets that solidify on nuclei in a fluidized bed. In this way of spraying it appears that new nuclei are readily formed, and these are entrained either with the product, or with the air discharged from the fluidiza­ tion zone, and must be separated therefrom. Perhaps droplets not trapped by particles in the bed are responsible for this. It has been found that such granulation processes suffer less often from instability. An important disadvantage of these processes is that the requisite very fine distribution of the liquid composition entails a high energy consumption and a high dust emission. European patent 141436 discloses a fluid bed granulation pro­ cess in which the liquid composition leaves a liquid distribution device in the form of a virtually closed, conical film, and in which nuclei from the bed are passed through the film by means of a powerful gas stream, so that they are moistened. This method of liquid supply requires a lower energy consumption and results in less dust being emitted. Another consequence, however, is that stable process opera­ tion is difficult. This probably has to do with the fact that few new nuclei are formed during distribution of the liquid composition. It has been found that there is no process instability if during distribution of the liquid composition in the bed a substantial amount of new nuclei is formed. It is known to improve the stability of granulation processes in which there is layered particle growth by installation of buffer vessels in the recycle streams to the granulation zone. A process in which such a measure is applied is known from the above-mentioned European patent 26918. In it, the process stability is imporved by returning both the stream of oversize partciels and the stream of undersize particles via a buffer vessel in metered amounts to the gra­ nulation zone, the amounts depending on the measured variation in the average particle size of the granules discharged from the granulation zone. A disadvantage of this process is that a buffer vessel must be included both in the stream of oversize particles and in the stream of undersize particles. Since the caking tendency of particles increases with decreasing size and with increasing temperature, the process further has the disadvantage that cooling generally is necessary before storage. Another disadvantage is that the control is based on the measured variation in the size distribution of the gra­ nule stream discharged from the granulation zone, necessitating use of a measurement and control system, which requires additional equipment. The invention aims to provide a simplified process that does not have the above-mentioned disadvantages while stable process opera­ tion still is realized. According to the invention the process is characterized in that a particle stream is returned at a constant flow rate from the buffer vessel to the granulation zone after size reduction in a size reduction device to a size desirable for use as nuclei. A constant flow rate can, for instance, be achieved in a simple manner by use of a buffer vessel from which a variable mass flow is discharged to the size reduction device by means of a feed screw. The stream of undersize particles can be returned direct to the granulation zone. An advantage of this process is that only one buffer vessel needs to be installed. This advantage is even augmented because the stream of undersize particles, which in the process according to the invention can be returned direct to the granulation zone, is much larger than the stream of oversize particles, so that a relatively small buffer vessel will suffice. Another advantage of the process according to the invention is that, since there is no buffer vessel in the stream of undersize particles, cooling of the recycle stream to the granulation zone can often be done without, on account of which less heat neads to be supplied to reach the desired granulation temperature. In principle, all kinds of liquid material, in the form of a solution, a melt as well as a suspension, can be applied. Examples of materials that can be granulated are ammonium salts, such as ammonium nitrate, ammonium sulphate or ammonium phosphate, as well as mixtures thereof, single fertilizers, such as calcium ammonium nitrate, magne­ sium ammonium nitrate, compound NP and NPK fertilizers, urea and urea­ containing compositions, sulphur, organic substances such as bisphenol and caprolactam, and the like. The process can suitable be used for various granulation pro­ cesses in which layered growth of the particles in the granulation zone takes place, for instance fluid bed granulation, spouted bed granulation, disk granulation or drum granulation, more in particular falling curtain granulation. The flow rate of the stream of particles of reduced size that is returned to the granulation zone, is of consequence for the size distribution of the granule stream from the granulation zone, and con­ sequently for the feed to the buffer vessel. Practice has shown that a small buffer capacity will suffice. In an advantageous embodiment of the process according to the invention there is a possibility of sending, besides the stream of oversize particles, if necessary, a stream of particles having a suitable size to the buffer vessel and/or of separating a portion of the particles in the buffer vessel and, if desired, removing them from the process. These measures permit of further reduction of the buffer vessel volume. The particles that are separated can be sent to a melting device or a dissolving vessel, and the melt or solution obtained can, optionally mixed with the liquid composition to be gra­ nulated, subsequently be passed to the granulator. By preference, the flow rate of the particle stream that is discharged from the buffer vessel and returned to the granulation zone after size reduction, is set such that the latter flow rate is vir­ tually equal to the flow rate of the stream of oversize particles. The result of the process according to the invention is improved if a stream of dust particles is separated from the particle stream returned to the granulation zone after size reduction. This may take place, for instance, in a wind sifter. Dust particles are here understood to mean particles having a size smaller than 1 mm, more in particular smaller than 0.75 mm. The utmost limit of the particle stream that is separated, is considered to be a limitation to par­ ticles having a size smaller than 0.2 mm. The advantage of this measure is that the risk of caking of particles, resulting in lower roundness and strength of the particles, is reduced, as is dust emission. In addition, it is desirable to remove not only the dust par­ ticles but also the oversize particles from the recycle flow. This will prevent the presence of fragments in the end product. In par­ ticular the particles having a size equal to or larger than that of the particles having the desired size are removed. Both the oversize particles and the dust particles are transported to a melting device or dissolving vessel and the melt or solution obtained there optionally is subsequently mixed with the liquid composition to be granulated. For good process operation it is important that the spread in particle size in the particle stream leaving the size reduction device is minimal. In practice, therefore, preferably a pressure crusher, in particular a roll crusher, is used. If there are large differences in the size of the particles in the buffer vessel, there is advantage in using a double-roll crusher. The process according to the invention will be elucidated with reference to the figure. From granulation device 1, through 17 a liquid composition is distributed over an amount of particles that are being kept in motion. To this end, the particles can be brought into a fluidized state and the feed can be supplied by means of a number of distribution devices, regularly divided over the bed and preferably projecting into the bed from the bottom upwards, such as sprayers. Other granulation devices, such as those operating with a spouted bed of solid particles or with a falling curtain of solid particles, may, however, also be used. Through 2, the crude product is passed over a fine screen 3 and sub­ sequently over a coarse screen 4, after which the product of the desired granule size is discharged through 5. The stream of undersize particles from the fine screen is returned direct to the granulation zone through 6. The stream of oversize particles from the coarse screen is sent to buffer vessel 8 through 7. From buffer vessel 8, a particle stream is passed through 10 to crusher 11 at a constant flow rate, use being made of a feed screw 9. From the particles stream sent from the crusher through line 12 to the granulation device, in a screening section 13 the oversize and undersize particles are separated and subsequently sent through 14 to a melting or dissolving device 15, following which the liquid composition, mixed with the liquid composition supplied through 17, is passed through 16, to the granulation zone. The residual particle stream is returned through 18 to the granulation zone. Through 19 particles of the desired size from product stream 5 can be passed to buffer vessel 8. Through 20 par­ ticles can be discharged from the buffer vessel. The process will be elucidated further with reference to the following examples. Example I Urea is granulated in a fluid bed granulator. In a cylindri­ cal fluid bed with a diameter of 45 cm, granules are kept at a tem­ perature of 110◊C. At the bottom the fluid bed is bounded by a porous plate consisting to 6 % of holes with a diameter of 1.8 mm. Through the holes, cold air flows into the fluid bed at a superficial velocity of 2 m/sec. At the top of the bed, 70 cm above the bottom plate, there is an overflow. In the centre of the bottom place there is a liquid distribution device, which is of the type described in European patent 141436. 200 kg/h urea melt of 140◊C with 0.5 wt.% water is supplied, and the liquid distribution device is operated with 90 kg/h air of 140◊C. The urea granules (2) leaving the bed are cooled to 40◊C and screened on flat deck screens (3,4). Almost 200 kg/h product having a granule size between 2 and 4 mm is obtained, 30 kg/h granules larger than 4 mm and 145 kg/h granules smaller than 2 mm. The dosing rate from the buffer vessel (8) to the size reduction device (11) is 30 kg/h. Size reduction device (11) consists in a double roll crusher set at an average granule size of 1.4 mm. The recycle stream (12) is sub­ sequently screened on flat deck screens (13), granules larger than 2 mm and smaller than 0.75 mm being separated and sent to smelting vessel (15). This is a stream of 7 kg/h. Very stable process operation is achieved. The stream of granules of the desired size is virtually constant. The stream of smaller granules varies between 135 and 155 kg/h, and the stream of larger granules between 26 and 34 kg/h. Example II Ammonium nitrate is granulated in a spouted bed granulator (1). The spouted bed is accomodated in a cylindrical vessel with a diameter of 45 cm, which at the bottom has been provided with a coni­ cal section that converges downwards at an angle of 30 to the ver­ tical and ends in the air feed tube. At a rate of 40 m/sec, 400 kg/h air at a temperature of 35◊C flows into the apparatus. The granules in the spouted bed have a tem­ perature of 100◊C. The overflow is in the cylindrical part of the spouted bed, 30 cm above the transition from the conical part to the cylindrical part. In the air stream 100 kg/h ammonium nitrate melt of 180◊C, with 0.5 wt.% water and 1.5 wt.% Mg(NO₃)₂, is sprayed. The ammonium nitrate granules (2) leaving the bed are passed to a screening device consisting in flat deck screens (3,4) without being cooled. Almost 100 kg/h product with a granule size between 2 and 4 mm is obtained, 10 kg/h granules larger than 4 mm (7) and 75 kg/h granules smaller than 2 mm (6). The crusher (11) is fed at a rate of 10 kg/h. The size reduction device (9) is set at an average granule size of 1.4 mm. The recycle stream (12) is subsequently screened, the granu­ les larger than 2 mm and smaller than 0.75 mm being separated and sent to a melting vessel. This is a stream of 20 kg/h. The good product is cooled to 40◊C in a fluid bed cooler and is then stored. Very stable process operation is achieved. The stream of granules of the desired size is virtually constant. The stream of smaller granules fluctuates between 71 and 79 kg/h, and the stream of larger granules fluctuates between 9 and 11 kg/h.

Claims: 1. Process for the preparation of granules from a liquid composition by spraying the liquid composition in a granulation zone onto or over solid particles, present in the granulation zone, that are being kept in motion, as a result of which layered growth of these particles takes place, withdrawing a particle stream from the gra­ nulation zone, dividing this stream into a stream of particles of the desired size, a stream of oversize particles and a stream of undersize particles, the stream of undersize particles being returned at least in part to the granulation zone and the stream of oversize particles being stored in a buffer vessel and sub­ sequently being returned at least in part, after size reduction, to the granulation zone, this process being characterized in that a particle stream is returned at constant flow rate from the buffer vessel to the granulation zone after size reduction in a size reduction device to a size desirable for use as nuclei. 2. Process according to claim 1, characterized in that, besides the stream of oversize particles, a portion of the stream of particles of the desired size is returned to the granulation zone via the buffer vessel and the size reduction device. 3. Process according to claim 1, characterized in that a portion of the particles in the buffer vessel is sent to a melting or dissolving device and the liquid composition formed here is sub­ sequently fed to the granulation zone. 4. Process according to any one of claims 1-3, characterized in that a stream of particles having a size smaller than 1 mm is separated from the particle stream being returned to the granulation zone after size reduction, which stream of particles is returned to the granulation zone after dissolving or melting. 5. Process according to claim 4, characterized in that a stream of particles having a size smaller than 0.75 mm is separated from the particle stream being returned to the granulation zone after size reduction, which stream of particles is returned to the granula­ tion zone after dissolving or melting. 6. Process according to any one of claims 1-5, characterized in that a stream of particles consisting mainly of particles having a size equal to or larger than the desired size is separated from the particle stream being returned to the granulation zone after size reduction, which stream of particles is returned to the granula­ tion zone after dissolving or melting. 7. Process according to any one of claims 1-6, characterized in that granulation takes place in a fluid bed. 8. Process according to any one of claims 1-6, characterized in that granulation takes place in a spouted bed.

Inventors: MONHEMIUS, LEO, MUTSERS, STANISLAUS MARTINUS PETRUS
Applicants: STAMICARBON B.V.
Application year: 1986
Main classification: B01J2
IPC codes: B01J2