NL2019964A - Plant and process for the production of solid urea in granules - Google Patents

Abstract

The present invention concerns a plant and a process for the production of solid urea in granules. The plant (1 0,1 00,200,300,400,500) for the production of solid urea in granules comprises at least one first tank (11,110,210,310, 410,510) for the formation of granules of urea and a dripping unit (12, 120,220,320,420) provided with at least one feeding hole (12a,222,422) for feeding molten urea in the form of drops into the first tank (11, 110,210,310,410,51 0) and is characterized in that the first tank (11,110,210,310,410,510) is filled with at least one cooling liquid (14, 140,240,340,440,540) immiscible with molten urea.

Description

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The Netherlands © 2019964 (21) Application number: 2019964 © Application submitted: 23/11/2017

APPLICATION FOR APPLICATION (51) Int. CL:

B01J 2/06 (2018.01)

© Priority: 11/25/2016 IT 102016000119699 © Applicant (s): EUROTECNICA MELAMINE, LUXEMBOURG, ZWEIGNIEDERLASSUNG IN ITTIGEN in ITTIGEN, Switzerland, CH. © Application registered: 08/06/2018 © Inventor (s): © Request published: Giacomo Cavuoti in RIMINI (IT). 08/06/2018 © Authorized representative: ir. H.V. Mertens et al. In Rijswijk.

54) PLANT AND PROCESS FOR THE PRODUCTION OF SOLID UREA IN GRANULES © The present invention concerns a plant and a process for the production of solid urea in granules. The plant (10,100,200,300,400,500) for the production of solid urea in a granules comprising at least one first tank (11,110,210,310, 410,510) for the formation of granules of urea and a dripping unit (12,120,220,320,420) provided with at least one feeding hole (12a, 222,422) for feeding molten urea in the form of drops into the first tank (11,110,210,310,410,510) and is characterized in that the first tank (11,110,210,310,410,510) is filled with at least one cooling liquid (14,140,240,340,440,540) immiscible with molten urea.

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NL A 2019964

This publication corresponds to the documents originally submitted.

P33407NL00 / MKO

PLANT AND PROCESS FOR THE PRODUCTION OF SOLID UREA IN GRANULES

The present invention concerns a plant and a process for the production of solid urea in granules.

In particular the present invention refers to a plant for the production of solid urea in granules, capable of operating with a lower energy consumption and having reduced dimensions.

The urea is obtained from a reaction of ammonia and carbon dioxide, from which a salt is obtained which melts at 135 ° C and which, in solution, has a clear colorless appearance. Once produced, the molten urea undergoes a finishing phase in which it is transformed into a format suitable for subsequent use as an end product, for example fertilizer, animal feed or another type of end product.

This finishing phase generally entails treating the molten urea by means of appropriate plants for the production of urea granules, generally in the form of prills.

In the present description and in the following claims, by prill we mean a substantial spherical ball or solidified urea.

A first known urea granule production plant consisting of a cylindrical tower, generally made of reinforced concrete, with typical dimensions up to a height of 50 meters and a diameter of 20 meters, which is called prilling tower.

At the top of the prilling towers a perforated factory distribution basket is provided which distributes a shower of liquid urea as it rotates.

As the urea droplets fall, they pass from the liquid state to the solid state by cooling, otherwise forming the urea prills.

The urea prills thus formed are collected at the bottom of the tower and transported on a conveyor belt to the storage warehouses.

The Applicant has observed that said plant, in addition to having imposing dimensions, only allows prills with very small diameters to be obtained, generally equal to 1-2.5 mm. In particular for use as fertilizer in agriculture, prills with much larger diameter, for example equal to or greater than 8 mm, also known as mega-prills, are highly desirable in order to obtain a gradual release of nitrogen, uniformly distributed overtime.

Alternative to the prilling towers, the finishing phase can be performed by means of a plant for the production of solid urea in granules known as fluidized bed granulator which comprises a tank with air blown in from below at a temperature of approx. 40-60 ° C.

Again, liquid urea is sprayed from below which, once solidified into small particles, is kept suspended by the air blown in. Continuing to spray liquid urea from below, the particles swell

-2while they are moved by the air, as the liquid urea adheres to their surface and, solidifying, causes them to increase in diameter.

The Applicant has found that said plant, like the prilling towers, has imposing dimensions. Furthermore, in order to obtain prills with large diameters, the solidified particles are kept in suspension for a long time. This results in an enormous consumption of energy.

CA666178 or GB937447, in which the molten urea is dripped in a bath of a liquid having a boiling temperature at the working pressure (about atmospheric pressure) some tenths or degrees lower than the starting temperature of the molten urea droplets. In said plants of cooling of the prill, the molten urea drops are generated through nozzles or feeding hole or small diameter (about 1 mm) and dropped above the free surface of the cooling bath from a variable height depending on the case from 12 cm to about 2 cm (5-1 inches). By adopting a low-boiling liquid maintained at a temperature well below the starting temperature of the drops, the nozzles must be kept sufficiently distant from the liquid to prevent the early solidification of the melt, but close enough to prevent crushing or the drop in the collision with the free surface of the cooling bath. The formation of droplets in the gaseous intersection entails an upper limit to their size determined by the balance between surface tension at the nozzle mouth and drop weight less floating force. The drop, as it is generated in this gaseous layer, has a very small floating force due to the low density of the gas, and therefore cannot grow beyond a certain size (4-5 mm as stated in the above cited documents). The cooling of the prills is therefore, in all cases, for the first part of the bath, obtained by evaporation of the bath liquid and formation of a vapor film around the drop that prevents it from contact with the liquid.

The cooling mechanism for evaporation is the predominant one, taking in account the great difference between the melting temperature of the urea drop (which tends to remain constant even if the cooling occurs by concentric layers of the prill) and the bath temperature (equal to or less than the boiling point or the cooling liquid).

Further, taking into account the low vaporization heat of the low-boiling hydrocarbons or chlorinated hydrocarbons provided in those documents (50-100 kcal / kg compared to 500 kcal / kg or water), large amounts of vapors are produced, which must be re-condensed externally, cooled and pumped back into the prilling tank. A similar system is also described in GB 1535842.

The Applicant has found that also such systems do not allow the obtaining of prills with large diameters and require the recycling or high amounts of solvents. This results in an enormous consumption of energy.

Last but not least, with all the known plants described above, substantially spherical urea granules are obtained, since it is not possible to intervene in any way on the shape of the end product.

- 3Depending on the applications, lenticular or toroidal shapes could be interesting, for example due to the different surface-volume ratio offered by these shapes.

The Applicant has therefore highlighted the need to provide a plant for the production of solid urea in granules which has reduced dimensions and is able to operate with lower energy consumption than the known plants.

The Applicant has also recognized the need to provide a plant for the production of solid urea in granules, which is able to produce, in addition to the granule shapes and sizes so far obtainable, urea granules with larger dimensions and different shapes.

The problem underlying the present invention is therefore that of producing a plant for the production of solid urea in granules which has compact dimensions and is characterized by reduced energy consumption.

In the context of this problem, one object of the present invention is to conceive a plant for the production of solid urea in granules which is able to obtain urea granules with both diameters larger and smaller than 8 mm.

A further object of the present invention is to conceive a plant for the production of solid urea in granules which further allows granules with controlled dimensions and both a spherical and non-spherical shape to be obtained.

According to a first aspect, the invention therefore concerns a plant for the production of solid urea in granules including at least one first tank for the formation of urea granules and a dripping unit provided with at least one hole for feeding molten urea in the form of drops into the first tank, characterized in that the first tank is filled with at least one cooling liquid immiscible with molten urea, at least one molten urea feeding hole or the dripping unit is immersed in at least one cooling liquid.

In the present description and in the subsequent claims, the expression cooling liquid means a liquid kept at a temperature below the melting temperature of the urea and in particular at a temperature that facilitates the solidification of the molten urea.

In particular, the layer of the least one cooling liquid in which the least one molten urea feeding hole or the dripping unit is immersed, is maintained at a temperature comprised between 110 and 125 ° C, preferably of from 110 to 120 ° C , namely at a temperature slightly lower than that of solidification or urea which is equal to 132 ° C.

This allows the geometric stabilization of the drop after the release from the formation nozzle or feeding hole and before the drop / prill solidification begins.

In fact, the Applicant has observed that by filling the solidification tank with a liquid immiscible with molten urea kept at a temperature below that of the urea melting temperature, it is possible to obtain solidification of the drops of urea, greatly reducing the dimensions of the plant.

Furthermore, the energy necessary to maintain the cooling liquid at a sufficiently low

-4temperature to obtain the solidification of the drops of urea, is a minimum percentage with respect to the consumption of the known plants.

The Applicant has in fact ascertained that, depending on the immiscible liquid used, it is possible to obtain solidification also when the drop travels through the liquid for less than half a meter, ie one hundred times shorter than the distance or that required by the prilling towers.

Furthermore, the Applicant has found that carrying out the solidification in a cooling liquid offering several further advantages.

For example, by appropriately choosing the density of the cooling liquid it is possible to produce prills with diameters or up to 10 - 14 mm.

In addition, the large floating force due to the high density of liquid surrounding the droplet allows to form even larger drops, up to 15 mm or more, which, by solidifying, can produce the so-called mega-prills or mega-granules (namely with a size or more than 8-9 mm), used as fertilizer in very important agronomic fields such as rice cultivation.

Performing solidification in a liquid with an appropriate density also allows urea granule shapes to be obtained, different from the spherical shape, for example lenticular or toroidal.

In particular, as cooling liquid, any organic or inorganic compound can be used with a density below the density of the molten urea, in which the urea is insoluble, and the residue of which left on the solidified urea granules, is compatible in terms of quantity and quality with use of the product.

According to a second aspect, the invention concerns a process for the production of solid urea in granules which is characterized by the fact that it comprises the phases consisting of dripping molten urea into a cooling liquid immiscible with molten urea.

Advantageously, the process for the production of solid urea in granules according to the invention achieves the technical effects described above in relation to the plant for the production of solid urea in granules according to the invention.

The present invention, at least one of the above-mentioned aspects, can have at least one of the following preferred characteristics; the latter are in particular combined with one as required in order to meet the specific application needs.

As said above, at least one molten urea feeding hole or the dripping unit is immersed in at least one cooling liquid.

This advantageously prevents freezing from the molten urea drops, which could occur if they were fed into air at ambient temperature.

Immersing the feeding hole in the liquid ensures that the solidification process takes place gradually and uniformly, therefore extending right through up to the center of the drop.

Preferably, at least one cooling liquid has a density below the density of the molten urea, i.e.

lower than approximately 1300 kg / m3, and the dripping unit is positioned on the free surface

- 5of the cooling liquid.

Said configuration advantageously allows the solidification of the urea drops in downflow conditions.

In the present description and in the following claims, by the expression downflow configuration we mean a configuration in which the urea drops are fed from above and solidify as they fall downwards.

The molten urea feeding hole or the dripping unit is immersed in the cooling liquid and, in the downflow configuration, the liquid layer in which the feeding hole is dipped is preferably kept at a temperature ranging from 110 to 120 ° C.

In fact, the lower density of the cooling liquid with respect to the density of the molten urea, a descent by gravity once delivered into the liquid.

More preferably, at least one cooling liquid with a density lower than the density of the molten urea is chosen from the group consisting of:

- Edible and vegetable oils;

- Mineral oils;

- Light liquid paraffin;

- Molten waxes;

- Silicone oils with a density lower than the density of the molten urea.

By way of example, the edible vegetable oils can be seed oils such as sunflower seed, soybean oil, palm oil, castor oil, olive oil, rapeseed oil and so on.

Analogously, mineral oils can be, for example, Vaseline oil.

With reference to molten waxes, they preferably have a melting temperature lower than 80 ° C. Said cooling liquids have a boiling temperature far above the maximum temperature of molten drops of urea (132-140 ° C) and therefore cooling is carried out without evaporation, with liquid-liquid direct contact between the molten urea drop and the bath cooling liquid, with a convective thermal exchange due to the relative drop speed or urea droplet / prill compared to the quiescent oil bath. Cooling is carried out in situ, suitably arranged heat exchange surfaces immersed in the tank, without need or extracting an oil stream, cooling it in an external exchanger and pumping it back into the tank. Cooling is then carried out in the liquid phase using non-toxic substances (some substances are also used in the food industry) and without production and condensation or vapors or potentially dangerous, toxic or flammable substances (such as toluene, cyclohexane, chlorobenzene, carbon tetrachloride, hexane).

Advantageously, if vegetable oils or waxes are used as cooling liquid, a coating of the formed granules is automatically obtained which prevents caking, i.e. fusion between individual granules as a result of heat and humidity.

Preferably, at least one polymer resin is added to at least one cooling liquid.

-6Advantageously, said resin adheres to the surface of the granule generated, forming a coating, thus making it unnecessary to purposely coat the granules in order to slow down the rapid drainage from rainwater and regular the release of urea over time, especially in association with the use of mega granules (granules with diameters equal to or greater than 89 mm).

Preferably, heavy powders and / or micropowders are added to at least one cooling liquid.

By way of example, silica, glass microspheres, metal powders, fossil flour and so on can be used as powders and / or micropowders.

Advantageously, it is thus possible to obtain a liquid with an increased density to increase the diameter of the prill that can be obtained.

Alternatively or additionally, means for generation of a current or at least one cooling liquid from the bottom to the top are provided.

In this way, the characteristic of floating or the drops is increased, prolonging the falling times and thus also obtaining solidification or prills with large dimensions of approximately 10-14 mm.

More preferably, on the bottom of at least one first tank and inclined wall is provided, which extends between a portion positioned at a higher level and a portion positioned at a lower level, a transport unit being arranged at the surface portion at a higher level for conveying urea granules flowing out of at least one first tank.

Alternatively or additionally, a unit for generation or a current or cooling liquid is preferably provided at the bottom or at least one first tank.

More preferably, the unit for generation or a current or cooling liquid is designed to generate a current directed towards the conveying unit.

Advantageously, the inclined wall positioned on the bottom and / or the current of cooling liquid conveyors the prills deposited on the bottom towards the transport unit, facilitating the outflow from the tank.

Preferably, the transport unit is partially immersed in at least one first granule formation tank and is preferably made like a perforated conveyor belt with scrapers or perforated bucket elevator or screw elevator.

More preferably, at least one blower for evaporation or cooling liquid is associated with the transport unit.

Preferably, the cold current or cooling liquid which flows on the bottom is kept separate from the cooling fluid above by means of a multiple of flow separating partitions.

In this manner it is advantageously possible to separate the bottom part of the tank from the upper part, so that the current created on the bottom does not interfere with the drops still not completely solidified during the descent.

Preferably, at least one first tank, various or cooling plates is arranged.

- 7 In this way it is possible to control the temperature of the liquid in the upper part of the tank, maintaining it at the ideal temperature for a gradual solidification of the drops. Preferably, at least one first tank is filled in addition with a second cooling liquid immiscible with molten urea and having a density higher than the density of the molten urea, a connection opening being provided at the bottom or at least one first tank to a second tank, the second tank being filled with the second cooling liquid.

In this way, the prills generated in downflow remain suspended at the interface between the two liquids with different densities and are driven into the second chamber where they surface and can be easily collected.

Preferably, at least one cooling liquid has a density higher than the density of the molten urea and the dripping unit is positioned at the bottom or at least one first tank.

Said configuration advantageously allows the solidification of the urea drops in upflow conditions.

In the present description and in the following claims, the expression configuration in upflow indicates a configuration in which the urea drops are fed from below and solidify during their ascent towards the top.

In fact, the higher density of the cooling liquid with respect to the density of the molten urea, an ascent of the latter towards the free surface of the liquid with a higher density, once delivered into said liquid.

More preferably, at least one cooling liquid with higher density than the density of the molten urea is chosen from the group consisting of:

- Molten sulphur;

- Oils with the addition of heavy powders;

- perchlorethylene;

- Perfluoropolyethers, also known by the trade name Fomblin®;

- Silicone oils with a density higher than the density of the molten urea.

Preferably, at the free surface or at least one cooling liquid, a transport unit is provided designed to move the urea granules that have risen to the surface from at least one first tank or, alternatively or additionally, means for the generation of a surface laminar flow of a cooling liquid.

Preferably, the dripping unit in an upflow configuration is provided with means for the generation of a central pressure impulse.

Advantageously, via the generation of a central pressure impulse during delivery of the drop, the urea delivered takes on a toroidal bubble shape and, solidifying, generates correspondingly shaped granules.

Preferably, at least one cooling liquid has a density higher than the density of the molten urea and the dripping unit is positioned at, preferably above, the free surface of the cooling liquid,

- 8means for generation of a surface laminar flow or the cooling liquid being provided. Advantageously said configuration allows granules to be obtained with lenticular shape since the urea drops delivered from the dripping unit to remain on the surface while they solidify, losing their classic spherical bubble shape.

Preferably, the dripping unit comprises at least one manifold or at least one dripping tube provided with a variety of feeding holes for feeding molten urea in the form of drops into the first tank.

Preferably, centrally to at least one feeding hole for feeding molten urea in the form of drops, a central needle is provided arranged for insertion or an additive in each urea drop during formation of the drop.

Advantageously, it is thus possible to create granules that contain an additive, for example an anti-urease substance, zinc, sulphur or other nutrients which are released once the urea granule has been consumed.

Further characteristics and advantages of the present invention will become clearer from the following detailed description of some preferred alternative, with reference to the accompanying drawings.

The different characteristics in the individual configurations can be combined with one another as required according to the preceding description, to attain the advantages specifically resulting from a particular combination.

In said drawings, figure 1 is a schematic representation in lateral elevation or a first preferred embodiment or a plant for the production of solid urea in granules according to the present invention in the downflow configuration;

- figure 2 is a schematic representation in lateral elevation or a second preferred embodiment or a plant for the production of solid urea in granules according to the present invention in the downflow configuration;

- figure 3 is a schematic representation of a variation of the plant or figure 2;

- figure 4 is a schematic representation in lateral elevation or a third preferred embodiment or a plant for the production of solid urea in granules according to the present invention in the downflow configuration;

- figure 5 is a schematic representation in lateral elevation or a fourth preferred embodiment or a plant for the production of solid urea in granules according to the present invention in the downflow configuration;

- figure 6 is a schematic representation in lateral elevation or a fifth preferred embodiment or a plant for the production of solid urea in granules according to the present invention in the upflow configuration;

- figure 7 is a schematic representation in lateral elevation or a sixth preferred embodiment of

- 9a plant for the production of solid urea in granules according to the present invention in the mixed configuration;

- figure 8 is a schematic representation in a section of a mega-scale available with the plants of figures 1 to 7.

In the following description, for illustration of the figures, identical reference numbers are used to indicate construction elements with the same function. Furthermore, for clarity of illustration, some numerical references may not be repeated in all the figures.

With reference to figure 1, a first edition of a plant for the production of solid urea in granules is shown, indicated overall by the number 10.

The plant 10 according to the first embodiment comprises a first tank 11 filled with a cooling liquid 14, immiscible with urea, and a dripper 12 from which drops 13 or molten urea are fed into the liquid through at least one feeding hole 12a.

The feeding hole 12a or the dripper 12 is immersed in the cooling liquid 14 to avoid freezing of the drops or molten urea 13, if they are theoretically fed into air at ambient temperature.

The cooling liquid 14 is preferably chosen from the fluids suitable for the food chain, in order not to alter the properties of the potentially edible urea granules. Examples of suitable fluids are vegetable and edible oils, such as seed oil, soybean oil, olive oil, palm oil, and so on, mineral oils such as, for example, Vaseline oil and silicone oils.

Different prill diameters can be obtained depending on the density of the cooling liquid 14 into which the drops of molten urea 13 are fed and the diameter of the feeding hole 12a or the dripper 12.

For this purpose, heavy micropowders are preferably added to the cooling liquid 14 to increase the density and thus obtain larger prill diameters.

In particular, in the embodiment illustrated in Figure 1, a cooling liquid 14 is used having a density lower than the density of the molten urea so as to obtain the configuration in downflow.

In order to maintain the cooling liquid at a temperature suitable for solidification of the urea, for example at a temperature ranging between 40-80 ° C, a cooling system for cooling said cooling liquid 14 is provided.

In the embodiment of figure 1, the cooling system comprises a second tank 15 containing a cooling fluid 16, for example cold water, in which the first tank 11 is immersed.

With reference to figure 2, a second embodiment of a plant for the production of solid urea in granules is shown, indicated overall by 100.

The plant 100 according to the second embodiment comprises a tank 110 filled with a cooling liquid 140, immiscible with urea, and a dripping unit 120 including at least a dripping manifold 121 immersed in the cooling liquid 140 on the free surface 141 or said liquid.

A reservoir 101 or molten urea is connected via a pump 102 to the dripping unit 120 to feed

- 10the latter with molten urea.

The manifold 121 or dripping unit 120 is provided with a variety of holes (not illustrated) for feeding molten urea in the form of drops into the cooling liquid 140.

The cooling liquid 140 has a density lower than the density of the molten urea so as to obtain the downflow configuration.

The drops of molten urea delivered from the dripping unit 120 solidify as they fall downwards. On the bottom of the tank 110 filled with cooling liquid 140, an inclined wall 111 is provided on which, the prills 130, formed from solidification of the drops, are deposited, rolling by gravity and entrainment due to a current or cold fluid towards a transport unit 170, said current being purposely generated on the bottom of the tank.

The transport unit 170 comprises a perforated conveyor belt with scrapers 171, partially immersed in the tank 110 and in the cooling liquid 140, which defines a conveying path for the prills 130, from the tank 110 to a collection tank 180.

In the embodiment illustrated in figure 2, the conveyor scraper 171 comprises a first vertical branch 171a which transports the prills 130 outside the tank 110, and a second horizontal branch 171b along which the residual liquid left around the prills 130 is drained from the holes provided on the belt. At said second horizontal branch 171b, a variety of hot air blowers 172 are also arranged for a more rapid evaporation and downward entrainment of the residual liquid and consistent drying of the prills 130.

At the end of the horizontal branch 171b, the prills 130 drop by gravity into the collection tank 180.

Also the collection reservoir 180 comprises a blower 181 which completes the drying of the prills 130 collected.

Lastly, a cooling system 150 is provided, illustrated in figure 2 only schematically, which, in general terms, comprises a withdrawing duct 151 for drawing the cooling liquid 140 from the tank 110, a cooling unit 152 outside the tank 110, a circulation pump 154 and a duct 153 for re-introduction of the liquid 140 into the tank 110.

The re-introduction duct 153 is arranged at the height of the inclined wall 111 in order to generate a cold current at the tank inlet which facilitates the movement of the prills towards the transport unit 170.

Alternatively to the included wall 111 and the transport unit 170 provided in the plant 100 or figure 2, a transport unit 170 'or the type illustrated in figure 3 can be provided.

Said transport unit 170 'comprises a variety of scrapers 171' which, by scraping the bottom of the tank 110 ', convey the prills deposited there towards an inclined outlet wall 112' or the tank 110 '.

With reference to figure 4, a third edition of a plant for the production of solid urea in granules is shown, indicated overall by 200.

- 11 The plant 200 according to the third embodiment comprises a tank 210 filled with a cooling liquid 240, immiscible with urea, and a dripping unit 220 including a multiple of dripping ducts 221, in the figure only the head duct is shown, immersed in the cooling liquid 240 at the free surface 241 or said liquid.

The molten urea is fed in a controlled manner to the dripping unit 220 via a feeding unit 201 (not illustrated in detail).

Each duct 221 or the dripping unit 220 is provided with a variety of feeding holes 222 for feeding molten urea in the form of drops into the cooling liquid 240.

The cooling liquid 240 has a density lower than the density of the molten urea so as to obtain the downflow configuration.

The drops of molten urea delivered from the dripping unit 220 solidify as they fall downwards, forming prills 230.

On the bottom of the tank 210 filled with cooling liquid 240 and inclined wall 211 is provided on which, the prills 230 formed from solidification of the drops deposit, rolling by gravity and entrainment due to a current or cold fluid towards a transport unit 270, said current being purposely generated on the bottom of the tank.

The transport unit 270 comprises a bucket elevator 271, partially immersed in the tank 210 and in the cooling liquid 240 which defines a conveying path for the prills, from the tank 210 to a belt conveyor 280 which transports the prills towards a collection tank (not illustrated).

In the embodiment illustrated in figure 4, the bucket elevator 271 comprises a first vertical branch 271a which transports the prills outside the tank 210, and a second horizontal branch 271b, at the end of which the prills drop by gravity onto the belt conveyor 280.

271a a first blower 273 is provided for a first drying of the prills as they are vertically transported.

Furthermore also along the second horizontal branch 271b a variety of second blowers 272 are arranged which continue drying of the prills as they are transported horizontally.

The buckets 271 of the elevator are perforated in order to allow dripping during the transport and are associated with a vibration unit (not illustrated) which accelerates the dripping phase. 280 a blower 281 is also provided which continues drying of the prills collected before reaching the tank.

Lastly, a cooling system 250 is provided in Figure 4 only schematically, which, in general terms, comprises a withdrawal duct 251 for drawing the cooling liquid 240 from the tank 210, a liquid cooling unit 252 outside the tank 210 and a re- introduction duct 253 for reintroducing the cooled liquid 240 into the tank 210.

The re-introduction duct 253 is arranged at the height of the inclined wall 211 in order to generate a cold current at the tank inlet which facilitates the movement of the prills 230 towards the transport unit 270.

- 12In order to maintain a separation between the cold current that flows on the bottom of the tank 210 and the cooling fluid 240 above, a multiple of flow separating partitions 212 are provided inside the tank 210.

In this way it is possible to maintain a temperature difference between the bottom part and the upper part of the tank 210.

The cooling unit 250 illustrated in figure 4 further comprises a variety of cooling plates 256 arranged inside the tank 210, above the flow separating partitions 212, and immersed in the cooling liquid 240 which control the temperature of the liquid 240 present in the upper part of the tank 210.

With reference to figure 5, a fourth edition of a plant for the production of solid urea in granules is shown, indicated overall by 300.

The plant 300 according to the fourth embodiment comprises a tank 310 filled with a cooling liquid 340, immiscible with urea, and a dripping unit 320 positioned below the free surface 341 of said liquid. The cooling liquid 340 has a density lower than the density of the molten urea so as to obtain the downflow configuration.

On the bottom of the tank 310 filled with cooling liquid 340 a first conical wall 311 is provided converging towards a lower level positioned centrally to the tank 310.

The prills 330 formed from the solidification of the drops roll by gravity towards said lower level, reaching a transport unit 370 positioned centrally to the tank 310.

The transport unit 370 has the form of a vertical screw lift and is partially immersed in the cooling liquid 340.

At the top of the transport unit 370 a second conical wall 375 is provided which, from the outlet 371 of the transport unit 370 positioned centrally, develops radially towards a lower level, facilitating rolling of the prills 330 towards the periphery or said surface 375.

At said periphery, an outlet 374 is provided for collection of the prills 330.

With reference to figure 6, a fifth edition of a plant for the production of solid urea in granules is shown, indicated overall by 400.

The plant 400 according to the fifth edition comprises a tank 410 filled with a cooling liquid 440, immiscible with urea, and a dripping unit 420 including a various or dripping ducts 421, illustrated in the figure only, the head duct is illustrated, immersed in the cooling liquid 440 at the bottom of the tank 410.

The molten urea is fed in a controlled manner to the dripping unit 420 through a feeding unit 401 (not illustrated in detail).

Each duct 421 or the dripping unit 420 is provided with a variety of feeding holes 422 for feeding molten urea in the form of drops into the cooling liquid 440.

The cooling liquid 440 has a density higher than the density of the molten urea so as to obtain the upflow configuration.

- 13The drops of molten urea delivered from the dripping unit 420 solidify as they ascend towards the free surface 441 or the cooling liquid 440, forming prills 430.

At the free surface 441 of the liquid, a transport unit 470 is provided in the form of a conveyor belt with scrapers 471, the scrapers scraping away from the free surface 441 of the liquid, the prills 430 that have surfaced and pushes them towards a lateral side opening 472 through which the prills 430 drop into a collecting tank 480.

Alternative to the conveyor belt with scrapers 471, the prills 430 can be conveyed towards the outlet via a cold laminar flow or cooling liquid 440 which flows over the surface.

In the embodiment illustrated in figure 6, a cooling system 450 is provided only schematically shown which, in general terms, comprises a variety of extraction elements 453 for extracting the vapor of the cooling liquid, a condenser 452 outside the tank 410 and a duct 451 for reintroducing into the tank 410 condensed cooling liquid vapors 440, therefore in liquid phase. With reference to figure 7, a sixth edition of a plant for the production of solid urea in granules is shown, indicated overall by 500.

The plant 500 according to the sixth embodiment comprises a first tank 510 filled partly with a first cooling liquid 540, immiscible with urea, having a density lower than the density of the molten urea, and partly filled with a second cooling liquid 545, immiscible with urea, having a density higher than the density of the molten urea.

The two cooling liquids are arranged in layers, with the liquid having a lower density 540 positioned in the upper part.

The first tank 510 is connected in the lower part to a second tank 550 via a connection opening 515 positioned on the bottom of both the tanks. The second tank 550 is filled only with the second cooling liquid 545.

A system 560 for generation of a current of the second cooling liquid 545 is also provided with a withdrawal duct 561 for drawing the second cooling liquid 545 from the second tank 550, a pump 562 and a duct 563 for re-introduction into the first tank 510. Both the ducts 563,561 are positioned at the same level, at the bottom of the respective tank 510,550.

A dripping unit is also provided (not illustrated) immersed in the first cooling liquid 540 at the free surface 541 or said liquid 540.

The drops of molten urea delivered from the dripping unit solidify as they drop downwards, forming prills 530 which remain suspended at the boundary interface between the two phases consisting of the two cooling liquids 540,545.

Furthermore, the current generated by the respective system 560 drags the prills 530 in suspension into the second tank 550 where, due to the density lower than the second cooling liquid 545, they come to the surface.

According to a further embodiment (not illustrated), a tank filled with a cooling liquid, immiscible with urea, and a dripping unit positioned on the free surface of the cooling liquid

- 14can be provided.

The cooling liquid used has a density higher than the density of the molten urea so as to obtain the upflow configuration and is made to flow on the surface as a laminar flow or cold liquid.

In this way, the drops of urea delivered from the dripping unit on the surface, taking on a lenticular shape, and solidify due to the cold laminar flow of liquid.

From the description, the characteristics of the plant for the production of solid urea in granules and the relative process for the production of solid urea in granules subject of the present invention are clear, and likewise the relative advantages.

From the outlined described above, further variations are possible, without departing from the teaching of the invention.

For example, a central needle can be provided at each urea feeding hole or the dripping unit for the insertion, in each drop or urea, or an additive in the drop formation phase. In this way a prill 20 or the type illustrated in figure 8 is created in which, within the sphere of solid urea 22, a centrally positioned additive 21 is present. The surface of the sphere 22 is further coated in a protective film 23 created by the drying of the cooling fluid used in the production process.

Furthermore, the dripping unit used in the plant according to the embodiment of figure 6 can be provided with means for the generation of a central pressure impulse, around which the urea delivered is distributed, generating a toroidal drop.

Lastly, it is clear that a plant for the production of solid urea in granules and a process for the production of solid urea in granules conceived as above are subject to numerous modifications and variations, all falling within the invention; furthermore, all the details can be replaced by technically equivalent elements. In practice both the materials used and the dimensions can be modified according to technical requirements.

Claims (15)

CONCLUSIONS An installation (10, 100, 200, 300, 400, 500) for the production of solid urea in pellets comprising at least one first tank (11, 110, 210, 310, 410, 510) for the formation of urea pellets and a drip unit (12, 120, 220, 320, 420) provided with at least one feed opening (12a, 222, 422) for supplying molten urea in the form of drops into the first tank (11, 110, 210, 310, 410, 510), characterized in that the first tank (11, 110, 210, 310, 410, 510) is filled with at least one cooling liquid (14, 140, 240, 340, 440, 540) which is immiscible with molten urea, wherein at least one molten urea feed hole (12a, 222, 422) of the drop unit (12, 120, 220, 320, 420) is immersed in at least one cooling fluid (14, 140, 240, 340, 440, 540). The plant (10, 100, 200, 300, 500) for the production of solid urea in pellets according to claim 1, wherein at least one cooling fluid (14, 140, 240, 340, 540) has a density lower than the density of the molten urea and the drop unit (12, 120, 220, 320, 420) is positioned on the free surface (141, 241, 341) of the cooling fluid (14, 140, 240, 340, 440, 540). The plant (10, 100, 200, 300, 500) for the production of solid urea in pellets according to claim 1 or 2, wherein the layer of the at least one cooling liquid in which the at least one molten urea feed hole (12a, 222, 422) of the dripping liquid unit (12, 120, 220, 320, 420) is immersed, is maintained at a temperature between 110 and 125 ° C, preferably between 110 and 120 ° C. The plant (100, 200, 300) for the production of solid urea in granules according to claim 2, wherein an inclined wall (111,211, 311) is arranged on the bottom of at least one first tank (110, 210, 310) extending between a portion positioned at a higher level and a portion positioned at a lower level, a transport unit (170, 270, 370) being provided on the portion of the surface positioned at a lower level for transporting of urea granules which flow from at least one first tank (111,211, 111). An installation (500) for the production of solid urea in granules according to any of claims 1 to 3, wherein at least one first tank (510) is additionally filled with a second cooling liquid (545) which is immiscible with molten urea and has a density higher than the density of the molten urea, there is a connection opening (515) for connection to a second tank (550) provided on the bottom of at least one first tank (510), the second tank (550) ) is filled with the second coolant (545). The plant (100, 200, 500) for the production of solid urea in pellets according to any of claims 2 to 5, wherein a generation unit (150, 250, 560) for generating a stream of coolant (140) , 240, 545) is provided on the bottom of at least one first tank (110, 210) and / or a generation unit is provided for generating a - 16 flow of coolant (140, 240, 340, 540) from the bottom to the top of at least one first tank (110, 210, 310, 510). The plant (200) for the production of solid urea in pellets according to claim 6, wherein the cold flow of cooling fluid (240) flowing over the bottom is kept separate from the cooling fluid above (240) by means of a plurality of flow partition walls (212). The plant (200) for the production of solid urea in granules according to any of the preceding claims, wherein a plurality of cooling plates (256) are arranged in at least one first tank (210). The plant (400) for the production of solid urea in pellets according to claim 1, wherein at least one cooling fluid (440) has a density that is higher than the density of the molten urea and the drop unit (420) is placed on the bottom of at least one first tank (410). The plant (400) for the production of solid urea in granules according to claim 9, wherein on the free surface (441) at least one cooling fluid (440) is provided with a transport unit (470) designed to feed granules of urea have risen to the surface of at least one first tank (441) or, alternatively or additionally, generation means for generating a laminar flow on the surface of the cooling fluid (440). Process for the preparation of solid urea in granules, characterized in that it comprises the step of dripping molten urea in a cooling liquid (14, 140, 240, 340, 440, 540) which is immiscible with molten urea. The method for preparing solid urea in granules according to claim 11, wherein the cooling liquid (14, 140, 240, 340, 540) has a density lower than the density of the molten urea and the dripping phase takes place from top to bottom. The method for preparing solid urea in granules according to claim 12, wherein the cooling liquid (14, 140, 240, 340, 540) is selected from the group consisting of: - edible and vegetable oils; - mineral oils; Light liquid paraffins; - molten washing; - silicone oil with a density lower than the density of the molten urea, - heavy powders and / or heavy micropowders and / or at least one polymer resin which is preferably added to the cooling liquid (14, 140, 240, 340, 540). The method for preparing solid urea in pellets according to any of claims 11 to 13, comprising the step of generating a stream of at least one cooling fluid (14, 140, 240, 340, 540) from the bottom to the top. 1715. The method for the preparation of solid urea in granules according to claim 11, characterized in that the cooling liquid (440) has a higher density than the density of the molten urea and the dripping step takes place from the bottom to the top. The method for producing solid urea in granules according to claim 15, wherein the cooling liquid (440) is selected from the group consisting of: - molten sulfur; Perchlorethylene; Perfluoropolyethers; silicone oils with a density higher than the density of the molten Urea - heavy powders and / or heavy micropowders and / or at least one polymer resin which is preferably added to the cooling liquid (440). 1/6