NO151786B - PROCEDURE FOR THE PREPARATION OF UREA GRANULES - Google Patents

Description

Urea granules are widely used for fertilizing purposes, either as such or as a component in a mixture of different fertilisers. The size of the urea granules depends on the application and is, for example, 0.5-4 mm for bulk mixtures and 5-10 mm for forest fertilisers.

Urea granules are produced by granulating a urea melt which is produced in urea synthesis units. In order to keep capital and operating costs down, it is usually found desirable that the granulation of the entire production in a urea synthesis

unit is carried out in a single granulator unit. This is possible by the urea-prille process, which involves spraying a substantially anhydrous urea melt at the top of a tower and cooling the resulting droplets as they fall countercurrently with an upward-flowing cooling gas, so that they solidify. Ureapril's in-

holds internal voids that are formed due to the contraction that takes place during the sudden solidification of the material, and which causes internal stresses in the pearl grains. As a result, ureaprils are mechanically weak; they have low crushing strength, low impact resistance and a tendency to form flying dust due to abrasion, which properties, among other objections, make the prill material unsuitable for pneumatic transport. Fly ash is an extremely fine, hydroscopic powder that pollutes the air at the workplace and is therefore harmful to the personnel who must process the material. Furthermore, this dust gives rise to problems when the plastic bags in which the prill material is packed are to be sealed or closed.

In an article in "Nitrogen" 95, pp. 31-36 (1975), two methods are described for the production of urea granules which have greater hardness and strength4 and, if desired, a larger diameter than ureapril's. According to both of these methods, a substantially anhydrous urea melt is sprayed onto urea seed, in the one case in a cylinder granulator and in the other case in a drum granulator of special construction. The urea granules obtained by these methods have better physical properties than ureaprils. A disadvantage of these methods, however, is that both cylinder granulators and drum granulators with practically acceptable dimensions only have a limited capacity, so that one often has to use more than one granulator to convert the production from a urea synthesis unit into granules.

It is an aim of the present invention to provide a method for the production of urea granules which have a desired size between 1.5 and 25 mm or even larger dimensions, a good sphericity and an even closed surface, high crushing strength, high impact resistance and a small tendency to form flying dust during abrasion, so that, firstly, they are suitable for pneumatic transport and secondly, they are free-flowing even after long-term storage, moreover, they have a desirable chemical composition: low values for the content of water, biuret , free NH^ and C02 (low buffer capacity), is very well suited for technical applications and forms a good basis for the production of slow-acting urea (such as sulphur-coated urea).

The invention relates to a method for production

of urea granules, where an aqueous urea solution with a urea concentration of at least 70% by weight is injected onto fluidized urea seeds at a temperature at which the water evaporates from the solution sprayed onto the seeds, and urea crystallizes on the seeds while forming granules with a desired size, characterized in that formaldehyde or a water-soluble addition or condensation product of formaldehyde and urea is added as a crystallization retarder for urea to the aqueous urea solution in an amount of 0.1 - 2% by weight, calculated as formaldehyde on the weight of the urea solution, and the resulting solution is sprayed onto the fluidized urea germs in the form of small droplets with an average droplet diameter of 20 - 120 µm, and the resulting granules are optionally subjected to subsequent drying with air at 100 - 150°C, so that the temperature of the granules is kept between 70 and 90°C, followed by cooling the granules to 30°C or lower. Preferred embodiments are stated in claims 2 - 6, and reference is made to these.

During the fluidized bed granulation, the granules that are formed will be continuously exposed to strong collision and friction with other particles, which is suitable for causing abrasion of the top layer of the granules during the formation of fine dust.

It has been found that this dust formation can be reduced by spraying the urea solution in the form of very fine droplets with an average size of 20-120 µm, which - after being deposited on the granules - dry so quickly that the outermost layer is constantly kept "dry"; this dry, i.e. water-free outer layer is far more resistant to abrasion than a "wet" layer, i.e. water-containing outer layer.

It has further been found that the formation of dust can essentially be avoided when it is added to

the urea solution to be sprayed, a crystallization retarder for urea which delays the crystallization of the urea deposited on the granules, so that the outer layer, although free of water, contains a relatively large proportion of the liquid phase for some time and thereby remains plastic, which has shown able to significantly improve resistance to dust formation. This addition is all the more important the lower the urea concentration in the urea solution that is sprayed, and this concentration is therefore preferably below 95% by weight and especially below 90% by weight.

The use of this preferred embodiment of the method according to the invention results in urea granules with an exceptionally high resistance to dust formation.

Among the crystallization retarders for urea that are used are water-soluble addition and/or condensation products of formaldehyde and urea. The production of water-soluble addition products of formaldehyde and urea is known, for example from the U.S. patent 3,067,177, and the preparation of water-soluble condensation products of formaldehyde and urea is described in U.S. Pat. patent 3 112 343. It is also possible to use addition products of formaldehyde and urea which are first produced in an alkaline medium and then condensed in an acidic medium to form thin to syrup-like liquids, such as the liquid adhesives used in the cardboard industry. The crystallization retarders are added, as mentioned, in an amount of 0.1-2.0%, preferably 0.1-1.0%, more preferably 0.5-1.0%, calculated as formaldehyde on the weight of the urea solution.

The aqueous urea solution that is sprayed onto the fluidized urea seeds has a urea concentration of 70-99.9% by weight, preferably 85-96% by weight. The use of a solution with a urea concentration of 85-96% by weight offers various advantages.

Firstly, in the urea synthesis unit, it is no longer necessary to use the rather expensive concentration of the solution to an essentially anhydrous product, as is the case in the prilling process and in granulation using cylinder or drum granulators. Secondly, such a solution can have a low content of biurea, as this content increases markedly, particularly when the solution is evaporated to a mainly anhydrous product. Despite the fact that the solution to be granulated by the method according to the invention does not

is anhydrous, but often contains 4 - 15% water, it has been found that a high specific granulation capacity (the weight amount of urea that can be granulated per unit of fluid bed surface area) of 2-4 tonnes per hour per m 2 fluidized bed surface area can be achieved.

The urea solution is sprayed using a gas, such as air. Preferably, the solution is sprayed inside the fluidized bed of urea seeds, as spraying onto the fluidized bed entails a risk of the sprayed droplets being carried away with the fluidization air from the fluidized bed. The pressure of the air used for spraying is preferably 147-392 kPa (1.5-4 ata). This pressure has a very large effect on the size of the sprayed droplets.

The higher the pressure used, the smaller the drops will be.

The mean droplet diameter can be calculated on the basis of empirical formulas, as according to W. R. Marshall, "Atomization and Spray Drying", A. I. Ch. Monograph, Vol. 50 pp. 74-75 can be developed depending on the size of the air diffusers used. In the case of spreaders for granulation, the following formula of Nukiyama and Tanasawa can be used:

where:

D = the mean droplet diameter in µm

V = the relative velocity of the air in relation to the liquid i

m/sec.

s = the surface tension in, dyne/cm

v = viscosity of the liquid in poise

d = density of the liquid in g/crn"^

Q-^/O^ = the ratio between the liquid volume and the gas volume

The density can be calculated according to M. Frejaques, "Les bases théorique de la synthese industrielle de l'urée", Chimie et Industrie 6_0 (1948), pp 22-35.

The surface tension is calculated according to the formula: 1 /4

S' = k.d. (S. Glasstone, Textbook of Physical Chemistry, 2nd Edition, (1956), p. 494). The surface tension at the boiling point is

(J.H. Perry, Chemical Engineers Handbook, 4th Edition, (1963) pp. 3-221 and 223). The surface tension under the experimental conditions is

The viscosity of the urea solution is determined by

using the International Critical Tables. The temperature T is expressed in °Kelvin in all cases.

The air flow rate after adiabatic expansion at a temperature T from a supercritical initial pressure P is given by Q2 = k.P \/T. From this, the constant k can be calculated, since the spray devices used in the examples, at room temperature (20°C) and an initial pressure of 216 kPa (2.2 ata), give a flow rate of 30 m<3>/hour. It follows that k = 0.008 m<3>/hour kPa.°K (0.80 m<3>/hour atm.°K). From this air flow rate, the air speed is calculated, at a nozzle diameter (head diameter) of 5.8 mm: Air speed = Q2 (im 3 /hour/3600, surface area (im<2>). As the liquid speed is negligible in relation to the air speed, V can is set equal to the air velocity. The flow rate of urea in each spray device is known. The calculation of the average droplet size will be explained in more detail in Example I.

The size of the urea seeds that are supplied to the fluidized bed where the granulation takes place is generally between 0.2 and 4 mm and can be increased within this range when larger urea granules are to be produced.

The temperature of the fluidized bed of urea seeds is generally between 70 and 110°C, preferably between 80 and 100°C. Within these limits, the temperature can be the lower the higher the urea concentration in the solution that is sprayed on the seeds. The temperature of the fluidized bed can be regulated by suitable selection of the temperature of the fluidizing air and the temperature of the urea solution.

The urea solution is sprayed over the urea seeds in the form of very fine droplets with an average diameter of 20-120 µm. Under the influence of the temperature that prevails in the fluidized bed, the water evaporates from the solution, and urea crystallizes on the surface of the urea seeds. Due to the small droplet sizes, the drops will normally only be able to cover part of the surface of the individual urea seed. This prevents the formation of granules with an onion-like structure, where the germ is successively coated with essentially overlapping layers. The granules obtained according to the invention consequently do not exhibit the stresses that accompany an onion-like structure. It is considered that the very good mechanical properties that the urea granules produced according to the invention exhibit are due to the absence of these stresses. A further advantage of the small droplet sizes when spraying the urea solution is that the water can evaporate completely within a short time.

To remove surface moisture, the resulting granules can, if desired, be subjected to subsequent drying for approx. 5-10 minutes with air of 100-150°C, so that the temperature of the granules is kept between 70 and 90°C, followed by cooling the granules to a temperature of approx. 30°C or lower. The cooling can be carried out in any suitable cooling device, e.g. in a fluidized bed cooler.

The product obtained by the method according to the invention contains only small amounts of free NH^, CO^, water and biuret, and it has such mechanical properties that it is suitable for pneumatic transport, and remains free-flowing even after longer storage. A particular advantage achieved by the present method is that the formation of the biure during the granulation can be prevented almost completely by spraying a urea solution whose crystallization point is below 100°C. By spraying a urea solution with a urea content of, for example, 75-85% by weight and a biuret content of less than 0.1%, urea granules with a biuret content of less than 0.1% can thus be obtained. Such urea granules are particularly desirable for certain plants, such as tobacco and tomato plants.

The urea granules obtained according to the present method are very well suited for coating with, for example, sulfur for the production of slow-acting granules, as the required amount of the coating material is minimized due to the granules' high sphericity and their closed surface.

The method according to the invention can be carried out in

any type of fluid bed granulator. An example of suitable apparatus is schematically shown in the drawing, which shows a granulator 1 divided into a number of chambers 2, 3, 4, 5 for the granulation, as well as a chamber 6 for the subsequent drying of the urea granules. The latter chamber does not necessarily have to be used, as a subsequent drying will only be used if the granules still contain surface water, which may be the case if the relatively diluted urea solution is used. The granulator 1 has a grate 7,

which supports the fluidized bed and allows the fluidization air to pass through; this is heated by means of one or more heating devices that are not shown in the drawing, and the air is supplied through line 8. The space below the grate 7 can be divided into chambers in the same way as the space above the grate, in which case the fluidizing air is supplied to each chamber. Furthermore, the granulator 1 is provided at the bottom with pneumatic spray devices 9, 10, 11,

12, which protrudes above the grate 7. It is also possible to use two or more spray devices in each chamber. Through these spray devices, the urea solution supplied from line 13, to which a crystallization retarder has been added, is sprayed inside the granulation chambers 2, 3, 4, 5 by means of the spray air supplied through line 14. The fluidized bed consists of urea seed, which is supplied by using a screw conveyor 15. For the subsequent drying of the granules in the chamber 6, the granulator 1 is provided with a line 16 for supplying drying air.

Air and possibly trapped dust particles flow out of the granulator 1 through lines 17 and 18, which are connected to a cyclone 19 where very small granules, with a size of approx. 100-500 µm, are separated and fed to a screw conveyor 15

through line 20. The air from the cyclone 19 is led through line 21 to a device 22, in which the air is washed with a diluted urea solution to remove fine dust and any remaining very small granules. To achieve a high washing effect, water can be sprayed into the air through a diffuser 23. The thus purified air escapes through line 24, and the diluted urea solution that is formed is withdrawn through line 25.

The granulator 1 further comprises an outlet 26 arranged at the bottom for urea granules, which opens onto a vibrating chute 27, which leads the granules to a screening device 28, in which they are separated into a number of fractions, namely into a sub-grain fraction, a fraction with the desired sizes and an overgrain fraction. The fraction with the desired sizes is passed through a cooler 29 to a storage location, where a further separation into fractions for different purposes can be carried out. If desired, the cooler can be arranged upstream of the screening device. The fraction of excess that is separated in screening device 28 is transported after cooling to a crusher 30, where this fraction is crushed to the same sizes as, or smaller sizes than, the grain sizes of the sub-grain fraction. The sub-grain fraction that is separated in the screening device 28 is led through line 32 to line 31, through which it is led to the screw conveyor 15 together with the fraction from the crusher 30.

The method according to the invention can be carried out both continuously and in batches. A urea solution is supplied through line 13 and sprayed with the help of air supplied through line 14 via spray devices 9, 10, 11, 12 inside the fluidized bed of urea seed in chambers 2, 3, 4, 5 of granulator 1.

The amount of urea granules that is removed from the fluidized bed via the chamber 6, into which no urea solution is injected, and the outlet 26, is replaced with urea seeds supplied by the screw conveyor 15.

The size of the product granules depends on several factors, such as the number of urea seeds in the fluidized bed, the size of these seeds, the amount of urea solution that is injected per unit of time, and the residence time of the seeds in the fluidized bed. For example, larger product granules will be obtained if the number of seeds in the fluid bed is reduced and the residence time is increased. When a predetermined particle size distribution in the product is to be maintained, it is necessary that the content of the fluidized bed is kept as constant as possible with regard to both the particle size distribution and the number of germs. This can be achieved by ensuring that the amount by weight of urea seed, with the correct particle size distribution, which is to be added to the fluidized bed, is at all times consistent with the amount by weight of the product granules that are removed from the fluidized bed.

If for any reason deviations occur in

the desired product sizes during the granulation process, such deviations will be automatically corrected in the above-described embodiment of the process: If the product becomes too coarse,

a larger overgrain fraction will be separated in the section direction 2o, the supply to the crusher 30 will increase, and a larger number of seeds will be supplied through line 31 and the screw conveyor 15 to the fluidized bed in the granulator 1, thereby reducing the average diameter of the granules. The operation of the crusher 30 should be regulated in an appropriate manner: if the crushed product is too fine, too much dust will be supplied to the fluidized bed, where it is either carried down by the fluidizing gas or causes agglomeration; if the crushed product is too coarse, an insufficient number of seeds will be supplied to the fluidized bed.

Due to the division of the fluidized bed in the chamber, a fractionation of the growing granules can be achieved. In each chamber, the granules that have the largest dimensions will mainly be found in the lower part of the vortex layer and will easily be carried to the next chamber.

The following examples will further illustrate the invention.

Example I

In an apparatus similar to that shown in the drawing, but with a granulation chamber in which two spray devices were arranged, 10 granulation trials were carried out in batches.

The spray devices used had an air nozzle diameter of 5.8 mm, a liquid pipe diameter of 3 mm and an air capacity of 500 l/minute at an air pressure of 216 kPa (1.2 atm. overpressure). In all experiments, the vortex layer was approx. 30 cm high. In each experiment, 35 kg of urea germ was used as starting material. Other data as well as the results are given in Table A. In this table "Formurea 80" is given as one of the crystallization retarders. "Formurea 80" is a commercially available product, the manufacture of which is described in US patent 3,067,177; it is an aqueous solution consisting of approx. 20 parts by weight water, 23 parts by weight urea and 5.7 parts by weight formaldehyde, where approx. 55% of the formaldehyde is essentially bound as trimethylolurea, while the rest of the formaldehyde exists in an unbound state.

In order to show the effect of the pressure of the spray air, the concentration of the urea solution used and other factors on the average droplet diameter, and the effect of the average droplet diameter on the physical properties of the product granules, the average droplet diameter was calculated in all experiments. The data required for this calculation, and the calculated values for the average droplet diameter, are given in tables B and C.

The TVA abrasion test is described in Special Report S-444 (1970) of Applied Research Branch, Division of Chemical Development, Tennessee Valley Authority, Alabama.

Example II

In the same apparatus as used in example I

two granulation trials were carried out using the continuous process, where 30-35 kg of urea seed with a grain size of 0.5-2.0 mm was fluidized in a granulator using approx. 700 m of fluidizing air of 120°C per hour. As soon as the urea germs had reached a temperature of approx. 120°C, an 80% aqueous urea solution to which 0.5% by weight "Formurea 80" had been added was injected through two spray devices at a rate of 120 kg/hour using air of 120°C and 245 kPa (2, 5 ata). The mean droplet diameter of the urea solution was

about. 80 µm. In the granulator, the temperature settled at 90-100°C due to the heat of crystallization developed by urea and the evaporation of water from the urea solution.

The average size of the product granules is a function of the size of the supplied urea germs and of the amount of sprayed urea solution. In experiment 11 granules of 2.5-4 mm were obtained and in experiment 12 granules of 4-6 mm.

The size distribution of the product granules in the fluidized bed was as follows:

With continuous withdrawal of approx. 170 kg of product granules per hour, the amount of granules in the fluidized bed was kept constant. The product granules were separated into three fractions by sieving:

The overgrain granules were crushed in a crusher and returned to the granulator together with the undergrain fraction.

In this way, the size distribution of the granules in the granulator was kept constant.

The product granules of the desired size were subjected to subsequent drying for 5-10 minutes with air of 100-150°C to a product temperature between 70 and 90°C. The product granules were then cooled to approx. 30°C.

During the granulation, 2.8% by weight of fine flying dust was formed, calculated on the amount of sprayed urea solution.

The properties of the resulting urea granules were as follows:

Claims (6)

1. Process for producing urea granules, where an aqueous urea solution with a urea concentration of at least 70 percent by weight is injected onto fluidized urea seeds at a temperature at which the water evaporates from the solution sprayed onto the seeds, and the urea crystallizes on the seeds while forming granules with a desired size, characterized in that formaldehyde or a water-soluble addition or condensation product of formaldehyde and urea is added as a crystallization retarder for urea to the aqueous urea solution in an amount of 0.1 - 2 percent by weight, calculated as formaldehyde on the weight of the urea solution, and the resulting solution is sprayed on the fluidized urea seeds in the form of small droplets with an average droplet diameter of 20 - 120 µm, and the resulting granules are optionally subjected to a subsequent drying with air of 100 - 150°C, so that the temperature of the granules is kept between 70 and 90 °C, followed by cooling the granules to 30°C or lower. 2. Method according to claim 1, characterized in that the urea solution contains 85 - 96 weight percent urea. 3. Method according to claim 1 or 2, characterized in that the urea solution is sprayed in the form of small droplets with an average droplet diameter of 30 - 100 um. 4. Method according to claim 3, characterized in that the drops have an average diameter of 30 - 60 ym. 5. Method according to one of the preceding claims, characterized in that the crystallization retarder is used in an aqueous solution consisting of approx. 20 parts by weight water, 23 parts by weight urea and 57 parts by weight formaldehyde, where approx. 55% of the formaldehyde is essentially bound as trimethylolurea and the rest of the formaldehyde exists in an unbound state. 6. Method according to one of the preceding claims, characterized in that the urea solution that is sprayed has a crystallization point below 100°C, whereby practically no biuret is formed during the granulation.