WO2005099884A1

WO2005099884A1 - Process for optimizing a fluid bed granulator

Info

Publication number: WO2005099884A1
Application number: PCT/NL2005/000204
Authority: WO
Inventors: Willem Frederik Roos
Original assignee: DSM IP Assets BV
Current assignee: DSM IP Assets BV
Priority date: 2004-04-15
Filing date: 2005-03-18
Publication date: 2005-10-27
Anticipated expiration: 2006-10-15
Also published as: NL1025954C2; AR050498A1

Abstract

The invention relates to a process for optimizing a fluid bed granulator for the production of urea granulate comprising sprayers for the spraying of urea melt, whereby mist sprayers for the spraying of urea melt are replaced with film sprayers. The invention also relates to a process for optimizing a fluid bed granulator whereby the granulator comprises a distribution plate through which fluidization air is supplied and above which the fluid bed is located, with the distribution plate comprising openings in which the sprayers are mounted and with sprayers being added during the optimization.

Description

PROCESS FOR OPTIMIZING A FLUID BED GRANULATOR The invention relates to a process for optimizing a fluid bed granulator for the production of urea granulate comprising sprayers for the spraying of urea melt. Fluid bed granulation processes are known from the Encyclopedia of Chemical Technology, third edition, volume 23, pages 566-572. It describes the fluid bed granulation processes of NSM and Mitsui

Toatsu/Toyo Engineering. In these processes the urea melt is sprayed in the fluidized bed of urea particles, as a result of which the particles grow to form urea granulate of the desired size. The aforementioned processes employ mist sprayers that spray the urea melt in the form of fine droplets. Application of such sprayers has the drawback that much and fine dust develops during granulation. This has a negative effect on the run-time of the granulator. Granulation additives are often added to prevent such dust formation. This is described in for example US-4, 219,589. According to that patent publication a granulation additive, preferably formaldehyde, is added to the urea melt before it is sprayed so as to prevent dust formation during granulation. A drawback of adding a granulation additive is that such additives are costly, which has an adverse effect on the cost price of granulated urea. The object of the invention is to eliminate these drawbacks. The invention is characterized in that the mist sprayers for spraying urea melt are replaced with film sprayers. It is thereby surprisingly achieved that the so-called run time of the granulator is up to 5 times longer. Another advantage is that the amount of granulation additive that needs to be present in the urea melt in order for granulation to be effected without dust formation may be significantly smaller than customary according to the state of the art. As a consequence, urea granulate can be produced more cheaply. A further advantage of replacing mist sprayers with film sprayers is that by utilizing film sprayers granulate is obtained that possesses very good roundness. As a result, large (4-8 mm) and small (1.7-2.8 mm) urea granules can be produced more easily. An other advantage is that by replacing mist sprayers with film sprayers urea granulate with a greater uniformity is obtained. The fluid bed granulator for production of urea granulate comprises sprayers for the spraying of urea melt. A sprayer normally comprises a supply line for supplying urea melt and concentric thereto a channel for supplying a gas stream. The form in which the urea melt leaves the sprayer is influenced by inter alia the shape of the sprayer head and the velocity of the urea melt and the gas stream when leaving the sprayer. According to the invention, a mist sprayer is replaced with a film sprayer. A mist sprayer sprays the urea melt in the form of very fine droplets, so that a mist of urea melt is formed. These fine droplets deposit on urea particles from the fluid bed and ensure that the granules grow in size. In the case of a film sprayer, the urea melt leaves the sprayer in the form of a thin film. A gas stream exits from the film sprayer beneath the urea melt and draws urea particles from the fluid bed. Subsequently, the entrained urea particles contact the urea melt film leaving the sprayer and are coated therewith, as a result of which the particles grow in size. By adding film sprayers^' during the optimization of the fluid bed granulator it is possible not only to optimize the fluid bed granulator but also to increase the granulator throughput. The fluid bed granulator may comprise a distribution plate through which the fluidization air is supplied. The fluid bed of urea particles is located above the distribution plate. The sprayers are mostly located in the granulator by mounting them in openings in the distribution plate. When mist sprayers are replaced with film sprayers they may be placed in the same openings. Film sprayers may be added for example by providing additional openings in the distribution plate. To that end, the distribution plate may be replaced with another distribution plate. However, it is also possible to increase the number of openings in the existing distribution plate. The number of film sprayers may also be increased in another manner. By replacing the distribution plate, the design of such plate may be so adapted that more fluidization air can be passed through it per unit time for improved cooling of the urea granules. The distribution plate in the granulator may also be expanded so that more fluidization air can be supplied and improved cooling of the granulate is obtained. The distribution plate preferably has a thickness in excess of 2 mm. The distribution plate is subjected to heavy loading during granulation. Such minimum thickness is desired in order to reduce as much as possible the risk of mechanical damage. In replacing the mist sprayers with film sprayers, the film sprayers are preferably so positioned that the height of the outflow opening of the urea melt from the sprayers is 10-100 mm above the distribution plate. More preferably, such outflow opening is located at 30-70 mm above the distribution plate. The height of the outflow opening above the distribution plate exhibits an optimum. When the outflow opening is lower or higher above the distribution plate, more secondary air will be needed during granulation. For an equal number of sprayers this means that the secondary air exits from the sprayer at a higher velocity. The supply of more secondary air with, in addition, a higher velocity requires more energy. The urea melt supplied to the granulator preferably has a urea concentration greater than or equal to 97 wt.%. The urea concentration in the urea melt may be achieved by for example concentrating the urea melt by evaporation. The higher the urea concentration in the melt, the less dust develops during granulation and the better the properties of the urea granulate obtained. Preferably the urea concentration in the urea melt.is greater than 98 wt.%. The throughput of the fluid-bed granulator may also be increased for example as a result of the fluidization air in the granulator containing very finely atomized water that is added to the fluidization air.

The fluidization air preferably contains 0.0001-10 wt.% of water relative to the amount of sprayed urea melt. The addition of finely atomized water may take place at various locations and also in various manners in the granulator. The finely atomized water may for example be added to the fluidization air beneath the distribution plate. This is possible by placing sprayers in the underside of the granulator but also by atomizing water in the fluidization air supply lines. The water may also be added to the fluidization air at the elevation of the distribution plate; for example by atomization from sprayers in the distribution plate. Most preferably, the water is added to the fluidization air by atomization of water in one or more fluidization air supply lines. This is effected by atomization of the water from one or more sprayers in the supply line. This is preferably a single sprayer that is positioned at the centre of the inlet line. The sprayers are preferably positioned some metres ahead of the outflow opening of the supply line for fluidization air in the granulator. If the water is sprayed in one or more fluidization air supply lines, the atomized water may be distributed highly homogeneously in the granulator using as few sprayers as possible. For the spraying of water sprayers are used that are capable of finely atomizing the water. Preferably the water is so atomized that the maximum droplet size of the atomized water is less than 50 μm; more preferably less than 40 μm and most preferably less than 20 μm. The smaller the water droplets, the faster the water evaporates during granulation and the more effective cooling will be. Effective cooling during granulation results in increased throughput of the granulator. By adding finely atomized water to the fluidization air it is possible to produce 10-50 wt.% more urea granulate in a granulator of equal size. As sprayers for the spraying of the water use may be made of any useful sprayer provided that the maximum droplet size of the atomized water is less than 50 μm. Examples of such sprayers are two-phase sprayers and sonic sprayers. Additionally, the water may be atomized by what is known as flashing water that is above the boiling point. The water is normally sprayed at a temperature of between 0 and 150 °C, preferably between 15 and 50°C and at a pressure of between 0.2 and 5.0 MPa. Film sprayers are described in for example US-4.619.843. The urea melt is introduced from the bottom up in the fluidized bed of nuclei with the aid of film sprayers that are provided with a central channel through which the urea melt is supplied and a channel concentric thereto through which a gas stream is supplied at a linear upward velocity greater than that of the fluidization gas, which gas stream creates a rarefied zone in the bed above the film sprayer, with the urea melt, on exiting from the central channel, entering the rarefied zone. The gas stream, prior to impinging on the film, sucks up and entrains urea particles from the bed as a result of which it decelerates so that both the film and the gas stream are deflected on impingement and the entrained urea particles penetrate through the film, in which process they are moistened with a minor amount of urea melt that subsequently has the opportunity in the rarefied zone to solidify to the point where the particles, on exiting from the rarefied zone, are dry enough to prevent agglomeration. The film that leaves the sprayer may in principle have any of various configurations, but a closed, conical film is preferred. A closed, conical film (hereafter referred to as "film") may in principle be obtained in various ways. For example, the urea melt may be formed into a film with the aid of a tapered insert at the end of the outflow channel. Preferably, the film is obtained by imparting rotation to the urea melt. Of course, besides the rotational speed imparted to the urea melt, the hydrostatic pressure on the urea melt is important here. In general, the urea melt is supplied at a hydrostatic pressure of 0.15 to 0.60 MPa, in particular 0.20 to 0.40 MPa. Use is preferably made here of a sprayer that is provided with a rotation chamber. It has been found that it is advantageous for the film to have a slightly corrugated surface in that a uniform distribution of the urea melt over the passing urea particles is obtained. This is influenced by inter alia providing the outflow section of the sprayer with a smooth surface. Furthermore, it should be ensured that the urea melt in the film does not have excessive internal turbulence. It has been found that the dimensionless Weber number (We_δ) is characteristic for obtaining a sufficiently smooth film. This number is expressed as:

p_LU δ We_δ = σ

where p_L density of urea melt in kg/m³, U|_ potential velocity of urea melt in m/sec, σ_L surface tension of the urea melt in N/m, and δ film thickness on exiting from the central channel in meters.

The Weber number should be less than 2500, in particular less than 2000.

It has been found that to that end the velocity of the urea melt should in general be maximum 30 m/sec and preferably 10-25 m/sec. The gas stream picks up urea particles and thus slows down before it hits the film. This is preferably accomplished by positioning the outlet of the gas channel below the urea melt channel in the fluidized bed. In this way, the gas stream is given the opportunity of entraining urea over some distance and imparting to them a certain velocity before they hit the film. This so-called free distance may vary between wide limits, for example 0.5-5.0 cm. It is preferred to apply a free distance of 1-4 cm. It is preferred to use air as the gas stream, which should be supplied at a velocity of at least 50 m/sec, in particular 50-400 m/sec, generally at a pre- pressure of 0.11 to 0.74 MPa. The temperature of this gas stream may vary. In general a gas stream is used with a temperature approximately equal to that of the urea melt. Where film sprayers are used, the required amount of this gas stream is very small. In general, a gas : urea melt mass ratio of between 0.1 and 0.8, in particular between 0.2 and 0.6 is used. On leaving the gas channel, the gas stream sucks up urea particles from the bed and entrains them in it. As a result, the velocity of the gas stream decreases while on the other hand the urea particles are given a certain velocity, for example 0.5-5 m/sec. On hitting the film, the velocity of the gas stream has decreased such that the impulse of the gas and the impulse of the film are approximately equal. Impulse here refers to the product of mass flow rate and velocity. If in a collision the impulse of the film is far greater than that of the gas stream, the gas stream is strongly deflected outward, so disturbing the rarefied zone. If on the other hand the impulse of the gas stream in a collision is far greater than that of the film, the film is deflected inward such that a significant number of urea particles will not come into contact with the urea melt and so will not be moistened. In a collision, both the film and the gas stream are deflected somewhat, with hardly any mixing of the gas and the melt taking place. The extent to which the film is deflected inward and the extent to which the gas stream is deflected outward are determined by the aforementioned impulses and, to a lesser degree, by the impact angle. The angle is determined by the apex of the film and the angle, if any, at which the gas stream converges. It has been found that the gas stream, on leaving the gas channel, converges somewhat in the bed of its own accord, so that in many cases it will not be necessary to apply a converging gas channel. The powerful gas stream may optionally be caused to converge at the outflow opening at an angle of 5-25°, in particular 5-10°. In general, the impact angle will be 50 to 85°, in particular 60-70°. In a collision between the film and the gas, the urea particles entrained in the gas stream, on account of their mass, fly virtually straight ahead, so through the film. As a result, such particles are moistened by a thin layer of urea melt that solidifies completely or almost completely in the rarefied zone. The absorbed amount of urea melt is dependent on inter alia the film thickness and the granule diameter. The film thickness in a collision is generally 50-250 urn. The granule diameter may vary between wide limits depending on the nature of the material, the size of the urea particles introduced in the bed and the number of times such a particle has already been moistened. Consequently, the gas stream serves to convey particles and also to create the rarefied zone above the sprayer. This zone should have adequate height, for example approximately 30 cm, in order to allow the urea melt on the particles to adequately solidify, on the other hand, in view of dust emissions, the bed surface should be prevented from being locally ruptured. Determining factors for these conditions are the mass and velocity of the gas and the bed height, which is for example 40-100 cm. It has been found that the width of the gas stream on leaving the gas channel is important for satisfactory granulation. In the case of a very wide gas zone a number of particles are found to be entrained on the outside of the gas stream, which particles are not moistened by the film. In general, the width of this gas zone is chosen between 0.25 and 4 times the average diameter of the urea particles. As urea particles in the fluidized. bed use may in principle be made of any of various granules, for example prills separately prepared from a portion of the urea melt to be sprayed or from a melt obtained by melting the oversize fraction obtained on screening the granulate. Preferably, the urea particles are granules obtained in screening and/or crushing the granulate obtained from the bed. The average diameter of such urea particles may vary, depending in part on the desired grain size of the product. The quantity of urea particles that are introduced may also vary. The bed of urea particles is kept in fluidized condition by an ascending gas, in particular air. This fluidization gas should have a minimum superficial velocity in order to ensure that the bed is completely kept in fluidized condition. On the other hand, this velocity should be as low as possible in view of energy costs and to prevent dust emissions. In general, a fluidization gas is used with a superficial velocity of 1.5 to 3.5 m/sec, in particular 1.8-3.0 m/sec. The temperature of the fluidization gas may vary, depending in part on the desired bed temperature, which is customarily adjusted by suitable selection of the temperatures of the urea melt, the atomizing gas, the urea particles supplied and the fluidization gas. The urea melt may contain a granulation additive. Examples of granulation additives are formaldehyde, methylol urea, formurea, hexamethylene tetramine, sodium CMC and synthetic water-soluble polymers, such as for example SMA, polyacrylic acid and PVA. Formaldehyde is preferably used as a granulation additive to reduce dust formation during granulation, to increase the mechanical strength of the urea granules and to reduce adhesion between the urea granules during storage (caking behaviour). Formaldehyde may be added as a gaseous or liquid stream essentially containing formaldehyde, formalin, paraformaldehyde, paraformaldehyde solution or as urea formaldehyde precondensate. Formaldehyde is usually added as urea formaldehyde precondensate. Formaldehyde precondensate contains for example 60 wt% of formaldehyde. To the urea may be added 0.01-1 wt.% of formaldehyde relative to urea. The urea melt preferably contains 0.1-0.4 wt.% of formaldehyde relative to urea. The invention is elucidated with reference to the following example, without being limited thereto. ^' :

Comparative experiment A . A granulator with a plate area of 7,4 m² with 60 mist sprayers receives 600 tons of urea melt per day. The urea melt had a urea concentration of 96% and contained 0.5 wt.% of formaldehyde. The run time of the granulator was 20 days. There was obtained a granulate with a NSP (non-spherical parts) content of 14 %.

Example I The granulator of comparative experiment A was optimized in accordance with the invention by replacing the mist sprayers with film sprayers and expanding the plate area to 9.0 m². 625 tons of urea melt per day are supplied with 128 film sprayers. The urea melt had a urea concentration of 98.5% and contained 0.25 wt.% of formaldehyde. The run time of the granulator was more than 100 days. There was obtained a granulate with a NSP (non-spherical parts) content of 10 %.

Claims

1. Process for optimizing a fluid bed granulator for the production of urea granulate comprising sprayers for the spraying of urea melt, characterized in that mist sprayers for the spraying of urea melt are replaced with film sprayers.

2. Process according to claim 1 characterized in that film sprayers are also added.

3. Process according to claim 1 or 2 wherein the granulator comprises a distribution plate through which fluidization air is supplied and above which the fluid bed is located and with the distribution plate comprising openings wherein the sprayers are mounted, characterized in that openings and sprayers are added to the distribution plate during the optimization.

4. Process according to claim 3, characterized in that the distribution plate is replaced with a distribution plate in which more openings are present.

5. Process according to any one of claims 1-4, characterized in that the height of the outflow opening of the urea melt from the sprayers is 10-100 mm above the distribution plate.

6. Process according to any one of claims 1-5, characterized in that the urea melt supplied to the granulator has a urea concentration greater than or equal to 97 wt.%.

7. Process according to any one of claims 1-6, wherein the granulator comprises a distribution plate through which fluidization air is supplied, characterized in that the fluidization air contains very finely atomized water.

8. Process according to claim 7, characterized in that the fluidization air contains 0.0001-10 wt.% of water relative to the sprayed amount of urea melt.

9. Process according to any one of claims 7-8, characterized in that the finely atomized water is added to the fluidization air beneath the distribution plate.

10. Process according to any one of claims 7-9, characterized in that the maximum droplet size of the atomized water is less than 50 μm.

11. Process according to any one of claims 1-10, characterized in that the amount of formaldehyde in the urea melt is 0.1-0.4 wt.%.