Technical grade urea granulate

Abstract

Disclosed is a method of producing technical grade urea in the form of granulate. This requires the ability to produce urea granules without normally essential granulation additives such as formaldehyde. The invention provides a method of operating a fluidized bed urea granulation unit by temporarily dispensing with such additives, in such a way that the production of technical grade urea can be realized.

Description

[0001] Title: TECHNICAL GRADE UREA GRANULATE

[0002] Field of the Invention

[0003] The invention is in the field of producing technical grade urea. Particularly, the invention relates to an improved urea granulation process.

[0004] Background of the Invention

[0005] Urea is generally produced from ammonia and carbon dioxide. It can be prepared by introducing an ammonia excess together with carbon dioxide at a pressure between 12 and 40 MPa and at a temperature between 150°C and 250°C into a urea synthesis section. Typical urea production plants further comprise a recovery section and a finishing section. In the recovery section non-converted ammonia and carbon dioxide are recovered and recirculated to the synthesis section. The recovery section is generally followed by an evaporation section. Therein the urea concentration is further increased by the evaporation of water, resulting in a highly concentrated solution that is generally referred to as a urea melt. In the finishing section, typically, the urea melt is brought into a desired solid, particulate form, generally involving techniques such as prilling, granulation, or pelletizing.

[0006] Technical grade urea refers to a relatively pure form of urea. Particularly, this concerns specifications minimizing the presence of impurities such as biuret (which is formed from urea upon heating) and formaldehyde (which is a well-known additive, notably employed in urea granulation).

[0007] An interesting technical grade urea product is a solution for NOx abatement such as used in selective reduction, which may be a non-catalytic thermal process or a selective catalytic reduction (SCR) process. An example of a solution for SCR is diesel exhaust fluid (DEF), which term is used in this description to generally refer to urea solutions for NOx abatement. Particularly in such SCR process, the specifications for the applied urea solution are highly stringent, since the catalyst is typically very sensitive to any contaminants (poisons), such as metals and any organic compounds which can plug the pores of the catalyst, either by forming a film on the surface or by being converted into carbon particles.

[0008] For automotive applications, DEF is a 32.5 wt.% urea solution in demineralized water with a composition that has maximum 0.3 wt.% biuret and maximum 0.2 wt.% of alkalinity as ammonia. Other commercial grades include marine grade DEF ISO standard 18611-1, which has 40 wt.% of urea. DEF is marketed under the (commercial) trade names Ad-Blue®, Airl®, Aria 32 and AUS-32 and is injected in the tail gas of combustion engines to capture NOXto prevent it from escaping to the atmosphere. The purpose of the DEF is to convert the NOXinto harmless nitrogen (N2) and water. By being injected into a high temperature gas stream, the urea from DEF decomposes into ammonia and carbon dioxide. The ammonia acts as a reductor for the NOX. Reduction of NOXfrom combustion engines is widely applied as NOXis one of the main sources for environmental pollution indicated for global warming such as the Global Warming Potential (GWP), Tropospheric Ozone Formation Potential (TOFP) and Ozone Depletion Potential (ODP).

[0009] The production of Diesel Exhaust Fluid (DEF) is generally achieved by dissolving solid urea product in demineralized water. The solid urea product, for example produced via one of the aforementioned finishing technologies and the demineralized water are combined and the solution is mixed until the urea is fully dissolved. The finishing technologies, however, come with drawbacks that are not easily solved. Prilling has the advantage that this does not normally introduce additional impurities. However, urea prills are not suitable for shipping over longer distances as the strength and the caking tendency are not sufficient to allow such shipping. Granulation is typically used to produce bulk amounts of solid urea which can be shipped over longer distances.

[0010] Generally, urea granules are produced in a granulation plant, typically in a fluidized bed urea granulation unit. To this end a highly concentrated urea aqueous solution, typically known as a urea melt, having a concentration of above 95 wt.%, is fed as a granulation liquid to the granulation unit. Such urea melt is generally produced in a urea production plant in which an aqueous urea solution is produced that, in a final step preceding the granulation, is subjected to evaporation in an evaporation section.

[0011] A fluidized bed granulation unit employs fluidization air which, as a result of urea solidification will become laden with urea dust. A background reference on fluidized bed granulation is EP 141436. Herein, generally, a process is disclosed for preparing granules, from a variety of materials, by making solid nuclei grow by means of a liquid feed to a fluidized bed. The disclosure teaches a wide list of materials that can be granulated. Specifically for urea, as described and as shown in the examples, it is taught that a high formation of dust occurs, which can be diminished by adding formaldehyde to the liquid feed. In EP 141436 it is stated that the disclosed process leads to such a small amount of dust that the addition of formaldehyde to the liquid feed would be substantially superfluous. Essentially, however, this finding is based on granulation in a stand-alone granulation device, not associated with a urea production plant, and not involving any recycle of urea dust. Moreover, the apparatus used is a circular, small (46 cm diameter) granulator. This is incomparable with commercial-size granulators used in connection with commercial urea production plants.

[0012] Notably, since urea dust still represents urea product, it will generally be collected and recirculated. To this end, downstream of a urea fluidized bed granulation unit generally a dust scrubbing section is provided in which used, urea dust-containing fluidization air is subjected to dust scrubbing. This is accomplished by means of an aqueous dust scrubbing liquid, which itself may be dilute urea solution. The resulting urea containing scrubbing liquid is then subjected to evaporation, either in the evaporation section of the urea production plant or in a dedicated evaporation section, to form a highly concentrated urea solution, typically a urea melt, which is suitable to be recirculated to the granulation unit, to be subjected to granulation.

[0013] Other than with small-scale granulation, not linked to urea production, as disclosed in EP 141436, industrial granulation of urea is invariably done with formaldehyde additive or alternative crushing strength improving additives. This is in line with the general experience in the art that such an additive is indispensable in order to produce granules on an industrial scale, typically preferably in a granulator wherein particles are made to grow over multiple compartments in series. This concerns granules that meet requirements for crushing strength, and reduction of dust formation to have an acceptable operation time before cleaning the granulator.

[0014] In order to keep the formation of urea dust manageable, one or more granulation aids can be added, for which typically formaldehyde is used. Producing granules without formaldehyde will typically lead to a very high dust formation and hence to a load on the dust scrubber and a high recycle of captured urea dust. This high recycle will typically lead to overloading the evaporation section producing the urea melt from which the granules are produced. This will then lead to lower melt concentration and again higher dust formation. Also the increased dust formation will lead to fouling, or even blocking of the granulation unit. Typically excess dust will accumulate on the walls and ducts of the granulator, thereby building up in a thick layer which eventually leads to blocking. Typically problems start to occur when large blocks of urea break off from the walls and fall on the fluidization plate. Thereby they can block some nozzles and the granulator does not work properly anymore. This results in a need to shut down the process and clean the granulator, which is most undesirable.

[0015] In view hereof, urea granules invariably are produced with formaldehyde as a granulation aid. Granulation therefore cannot normally be used to produce technical grade urea due to the presence of formaldehyde.

[0016] Alternatively, DEF is produced directly from an aqueous urea solution obtained in a urea plant upstream of evaporation and finishing (EP 1856038). It will be understood, however, that shipping of such a solution is undesirable, as this would require shipping much larger volumes than in the event of solid urea, the majority of which is water. Another alternative is pelletizing. This also has drawbacks, inter alia being more capital intensive than prilling or granulation, due to the much lower capacity that pelletizers have. Yet another alternative is to produce solid urea by flash crystallization (WO2016 / 030412). This results in a powder that then needs to be pelletized, thus adding a capital intensive further step. Moreover, whilst flash crystallization results in a suitable solid technical grade urea product, it presents quite a different urea finishing process, requiring additional equipment. It would be desired to produce solid technical grade urea in accordance with a standard urea finishing process, notably in a urea granulation plant, preferably without adding extra finishing equipment.

[0017] Summary of the Invention

[0018] In order to better address the foregoing desire, the invention presents, in one aspect, a method of operating a fluidized bed urea granulation unit, wherein said unit has an inlet for a concentrated urea aqueous solution to be subjected to granulation and an inlet for clean fluidization air, an outlet for urea granules and an outlet for used fluidization air comprising urea dust, wherein the outlet for used fluidization air is in fluid communication with a dust scrubbing section for recovering urea dust from said air, wherein said dust scrubbing section has an outlet for urea-containing scrubbing liquid which is in fluid communication with an inlet of an evaporation section, said evaporation section having an outlet for concentrated aqueous urea solution which is in fluid communication with an inlet for urea melt of the granulation unit, the method comprising temporarily preventing the urea-containing scrubbing liquid from being sent to the evaporation section.

[0019] In another aspect, the invention provides a process for the production of solidified technical grade urea, comprising providing a highly concentrated urea aqueous solution and subjecting said solution to a fluid bed granulation process, thereby obtaining urea granules and fluidization air comprising urea dust, wherein said air is subjected to recovery of urea dust so as to provide recovered urea dust; wherein the urea solution subjected to granulation has a purity adapted for the production of technical grade urea, and wherein at least part of the recovered urea dust is not recirculated to the fluid bed granulation process without first being collected separately from such recirculation.

[0020] In yet another aspect, the invention provides a method of modifying a pre-existing fluidized bed urea granulation unit, wherein said unit has an inlet for a urea melt to be subjected to granulation and an inlet for clean fluidization air, an outlet for urea granules and an outlet for used fluidization air comprising urea dust, wherein the outlet for used fluidization air is in fluid communication with a dust scrubbing section for recovering urea dust from said air, wherein said dust scrubbing section has an outlet for urea-containing scrubbing liquid which is in fluid communication with an inlet of an evaporation section, said evaporation section having an outlet for urea melt which is in fluid communication with an inlet for urea melt of the granulation unit, the method comprising adding a connection to a storage section having an inlet for urea-containing scrubbing liquid in fluid communication with the outlet for urea-containing scrubbing liquid of the dust scrubbing section, and an outlet for urea- containing scrubbing liquid in fluid communication with an inlet of the evaporation section, said inlets and outlets for urea-containing scrubbing liquid being provided with a closable aperture.

[0021] In a still further aspect, the invention provides a process for the production of urea granules, comprising providing a highly concentrated urea aqueous solution and subjecting said solution to a fluid bed granulation process, thereby obtaining urea granules and fluidization air comprising urea dust, wherein said air is subjected to scrubbing resulting in a used aqueous scrubbing liquid comprising recovered urea dust, said used scrubbing liquid being recirculated, with evaporation, to the granulation process; wherein the urea solution subjected to granulation periodically contains a crushing strength improving additive such as formaldehyde and periodically does not contain such additive, and wherein during granulation of the urea solution not containing formaldehyde, the recirculation of the used scrubbing liquid urea to the granulation process is at least temporarily prevented.

[0022] In yet another aspect, the invention presents two alternatives for the production of DEF. One comprises producing urea granules by a process as described in the preceding paragraph, and dissolving said granules in demineralized water to the desired concentration in a range of from 30 wt.% to 40 wt.%. The other comprises sending pure utilized urea-containing scrubbing liquid, obtained by virtue of the above-identified method of operating a fluidized bed urea granulation unit, and subjecting said liquid to evaporation to provide an aqueous urea solution having a concentration in a range of 30 wt.% to 40 wt.% , preferably 32.5 wt.% or 40 wt.%. Detailed description of the Invention

[0023] The invention is based on the judicious insight that urea granules can be produced without formaldehyde, in the event that a provision is made for handling the resulting overload of urea dust.

[0024] Accordingly, in the aforementioned method of operating a fluidized bed urea granulation unit, a technical measure is to temporarily prevent urea-containing used scrubbing liquid from being sent to the evaporation section. Generally, such technical measure will comprise sending the used scrubbing liquid to a destination, within or outside of the involved urea granulation plant, different from recirculation to granulation via the evaporation section.

[0025] Thus, in one embodiment, the method of the invention can be performed by sending said used scrubbing liquid to a storage or processing facility outside of the urea plant. Within the urea granulation plant, this destination will preferably be a storage facility, typically a storage tank. From a storage tank, outside or, preferably within the urea plant, the stored liquid can still be recirculated at a later point in time, when production had changed back from technical grade urea granules to regular urea granules. The stored liquid can then be recirculated at will to evaporation and granulation._This way, advantageously, any resulting higher load of the evaporation section can be managed better, notably better divided over time, than in the event of an immediate recirculation of all of the additional urea dust resulting from granulation without formaldehyde.

[0026] The temporary diversion of used scrubbing liquid to a destination other than recirculation, presents an advantageous combination of effects.

[0027] The advantages particularly relate to switching the granulation from normal operation, which comprises adding a crushing strength additive such as formaldehyde to the granulation liquid, i.e., the highly concentrated urea aqueous solution to be granulated, to an operation without. After terminating the feeding of said additive to the granulation liquid, it will generally require (depending on process parameters) some 6 to 8 hours before the granulation unit is free from contamination with said additive. By preventing the urea-containing scrubbing liquid from being sent to the evaporation section, and thus effectively preventing recycling this to the granulation unit, the granulation unit will be free from the additive in a much shorter period of time, typically about one third of the time otherwise needed, such as about 2 hours. The absence of remaining additive can be easily determined by analyzing the purity of the granules produced at a given point in time. This presents a clear advantage of the method of operating a granulation unit according to the invention. Particularly, the shorter time interval that is required before switching production from standard, fertilizer-grade urea granules to TGU, suitable for the production of DEF. As a result, the amount of produced granulate that it is still off-spec as regards TGU, is reduced. Moreover, the considerably shorter time period involved with switching production, results in a more efficient operation time of the plant.

[0028] After the granulation unit has thus been cleaned from additive, it is preferred not to recommence sending the scrubbing liquid to the evaporator at that point. Dust scrubbers are relatively large, require a relatively large amount of water, and the scrubbing system generally involves a further reservoir, known as a dissolving tank. In these and other places in the process in which urea dust is subjected to wet recycling, dust will be present and mixing will take place. As a result, any contamination from previously received dust may still be taken up by used scrubbing liquid for a further period of time after the granulation unit per se is no longer contaminated. As a result another 3-5 hours, typically about 4 hours, will be required until also the used scrubbing liquid is free from contamination with previously added crushing strength additive, such as formaldehyde.

[0029] At this point, one option is to recommence sending used scrubbing liquid to the evaporator. It will be understood that the granulation without additives, particularly without a crushing strength additive such as formaldehyde, will result in extra dust formation. Dust scrubbing will therefore result in an increased aqueous ammonium carbamate streams to be recycled as well as, associated therewith, an increased load on the plant’s wastewater treatment system. In view hereof, preferably, the recirculation of the used dust scrubbing liquid by sending it to the evaporator, is therefore further postponed, preferably for the entire duration of the production of TGU.

[0030] Another option, advantageously in conjunction with said further postponement, is to separately collect the no longer contaminated used dust scrubbing liquid, and process it as a liquid product. Such processing will generally comprise evaporation, in a separate evaporator directed at producing a pure urea aqueous solution of desired concentration. This will preferably be a solution having a concentration required for DEF, i.e. 32.5 wt.% for automotive application and 40 wt.% for marine application.

[0031] In this respect, the invention also pertains to a process for the production of an aqueous solution of urea suitable for use as Diesel Exhaust Fluid (DEF). This processes initially comprises operating a fluidized bed urea granulation unit as described hereinbefore. The process involves allowing a period of time until a urea-containing pure scrubbing liquid is obtained that is free from containing a crushing strength improving additive such as formaldehyde. In this period of time, the used scrubbing liquid is not suitable for the production of DEF. Once it is determined that the used scrubbing liquid has the desired purity, the process comprises subjecting the pure scrubbing liquid to evaporation to provide an aqueous urea solution having a concentration in a range of 30 to 40 wt.% , preferably 32.5 wt.% or 40 wt.%.

[0032] Incidentally, it will be understood that the urea granules produced with the method of the invention, as such are particularly suitable for the production of DEF. In this respect, the invention also pertains to a process for the production of an aqueous solution of urea suitable for use as Diesel Exhaust Fluid, comprising preparing urea granules according to the process described herein, and dissolving the granules in demineralized water to the desired concentration in a range of 30 to 40 wt.% , preferably 32.5 wt.% or 40 wt.%.

[0033] The invention employs fluidized bed granulation. Therein a highly concentrated urea aqueous solution, preferably a urea melt is sprayed on granules that grow in size as the process continues. The term “urea melt” is known in the art, and is applied to urea having less than 10% of water, such as less than 5% of water and preferably up to 1.5 % of water. The concentration of urea takes usually place at high temperatures and sub- atmospheric pressures. Usually concentration of the urea solution to the desired moisture content in the anhydrous urea melt takes place in an evaporation section comprising one or a sequence of one or more evaporators in series.

[0034] The highly concentrated urea aqueous solution leaving the evaporation section is usually conveyed by a pump to a urea finishing section, in this case a urea granulation finishing section, For urea granulation, the desired urea concentration in the highly concentrated urea aqueous solution is preferably in between 96 and 99 % by weight, more preferably 98.0-99.0 wt.%. The urea melt sent to the finishing section comprises urea, including biuret, water and small amounts of ammonia. The ammonia concentration in the urea melt sent to said urea finishing section amounts in between 100 and 900 ppm by weight.

[0035] In an interesting embodiment, the concentrated urea solution (urea melt) employed as a feed liquid to granulation, has the unusually high concentration of between 99.1 wt.% and 99.9 wt.,%, preferably 99.6 wt.% to 99.8 wt.%, such as most preferably 99.7 wt.%. This is based on the unexpected finding that such a higher concentration of urea melt correlates with a lower dust production. Accordingly, the choice, according to this aspect of the invention, to present a urea granulation liquid at such high urea concentrations has the advantage of allowing producing granules without granulation additives such as, particularly, without formaldehyde.

[0036] It will be understood that achieving such a high urea concentration requires a greater extent of evaporation than usual. This can be achieved by extending the duration of evaporation prior to feeding the urea melt to a granulation section. Preferably, this is achieved by adding a further evaporator, or further evaporation section. The latter can be done in series. In that event, in operation a choice can be made to employ the additional evaporator or additional evaporation section in the event of producing granulate without formaldehyde, and not operating the additional evaporator or additional evaporation section in the event of switching to regular operation of the granulation section without formaldehyde.

[0037] Preferably, an additional evaporator or additional evaporation section is positioned in parallel, thus enabling an easy switch between granulation processes with and without additives such as formaldehyde. Still more preferably, such parallel additional evaporator or additional evaporation section is in fluid communication with a parallel granulation section. In that event, the resulting plant can be in full operation with parallel production of both regular urea granulate with formaldehyde and technical grade urea granulate without formaldehyde. Of course, also in this embodiment, the manufacturer will have a choice of the extent to which either of the parallel granulation sections is operated.

[0038] The invention unexpectedly enables producing technical grade urea granules. This can be accomplished by the judicious operation of an existing urea granulation plant. It can also be accomplished by adapting a urea granulation plant, or building a dedicated technical grade urea granulation plant. A “granulation plant” generally is a urea production plant, comprising a urea synthesis section and one or more recovery sections enabling a total recycle of unconverted reactants, and a urea granulation finishing section, In the present invention, the urea granulation finishing section is a fluidized-bed granulation section. A urea granulation plant can also be a separate granulation plant, which is fed with urea sourced from elsewhere. However, since urea granulation requires employing the aforementioned highly concentrated urea aqueous solution as a granulation liquid, it is less convenient to obtain urea from elsewhere, than to use a hot urea melt obtained from a connected urea synthesis section.

[0039] A fluidized bed granulator typically comprises an inlet for seed particles, an outlet for granule products, a perforated bottom plate for distributing fluidizing air, and a gas outlet. A preferred granulator further comprises a plurality of compartments configured in series between said inlet for seed particles and outlet for granule products. Each compartment contains at least one or a plurality of nozzles for feeding a concentrated urea aqueous solution as a granulation liquid. Different types of nozzles can be employed, such as atomization nozzles (generating droplets) and film spray nozzles (also known as film-forming nozzles), the latter serving to feed the melt to the fluidized bed in the form of a film. The granulator is configured to move the particles from the inlet to the outlet whereby the particle size increases from the inlet to the outlet.

[0040] The present invention specifically addresses a problem occurring in connection with industrial granulators. A typical granulator as used in industry has an oblong shape. This leads to a relatively high ratio of surface to volume, which means that dust formation leads more easily to dust building up on the walls of the granulator, over time. This is further enhanced as a result of the residence time in an industrial granulator, which typically is longer than in lab -scale granulators. Lab scale granulators, such as the granulator described in EP 141436, are typically only used for a couple of hours, for granulating a particular type of material. The dust is not recycled but only removed by a typical vacuum cleaner type exhaust. Before starting a new run, the lab granulator would simply be rinsed out. An industrial scale granulator is further distinguished from a typical lab-scale granulator, by having multiple compartments in series. In normal operation, a urea granulation plant will be fed with a urea granulation liquid comprising formaldehyde. If the granulation plant is operated for a short time without formaldehyde and without any recycle of urea dust, as discussed above, the final product will not contain any formaldehyde and can be used as technical grade urea.

[0041] After a limited period of time, i.e., preferably for the entire duration of the production of TGU before switching back to producing fertilizer-grade urea, the formaldehyde supply and the recycle of the urea to the evaporation section will be restarted. It will be understood that, just as in the event of the production of regular, fertilizer grade urea granules, the granulation unit will be subject to periodical cleaning, generally every 5 to 15 days of operation. Surprisingly, it was found that whilst operating periodically without formaldehyde, the amount of dust was increased, but this did not negatively affect the interval between cleaning sessions. This is counter-intuitive, as the skilled person would expect that in the event of more dust, the interval between necessary cleanings would be shorter. It was also found that employing a higher urea concentration has a positive effect on cleaning intervals when operating without crushing strength additive. Without wishing to be bound by theory, the inventors believe that the higher concentration of urea melt in the granulator and the correspondingly higher operating temperature, causes the urea dust to be less ‘sticky’ and thus easier to be removed

[0042] Operating without formaldehyde for, e.g., 8 hours can yield in a 2000 MTPD plant in a batch of 650 MT TGU (technical grade urea). In this way by changing the operation procedure and without any extra investments, TGU can be produced in existing granulation plants.

[0043] It will be understood that, with temporarily halting the recycle of urea dust scrubbing liquid, said liquid will need to be collected separately. This can be done by scrubbing the dust in a conventional way and redirecting the scrubbing liquid (often called wet recycle) to a new or an existing storage tank, such as a urea storage tank generally being present from which the evaporation section is fed. This tank usually comprises several compartments. The scrubbing liquid is preferably fed to a different compartment than the compartment from which the evaporation section is fed. In general there are two tanks in a urea production plant: a scrubber tank / dissolving vessel in the granulation section and a urea solution tank for urea production. The storage of used dust scrubbing liquid should take place in the granulation plant in the dissolving vessel or an alternative tank. Typically the wet recycle is send from the dissolving vessel to a compartment of the urea solution tank. This recycle is disrupted in the operation mode of the present invention. In an alternative embodiment, urea dust can be sent to a scrubber of a second parallel urea plant, and recycled to the evaporation section of such plant.

[0044] In another aspect, the invention also provides a method of modifying an existing urea granulation plant, by adding a connection to a storage section such as a tank. This is reflected in the invention method of modifying a pre-existing fluidized bed urea granulation unit as defined above. In an embodiment, the modification comprises adding a connection, such as providing a pipe, typically provided with a valve, to a storage section, typically a tank, that is already present. Alternatively, the modification comprises adding a storage facility, typically adding a storage tank.

[0045] In accordance with the invention, technical grade urea granules can also be produced in a plant not otherwise used for making urea granulate with formaldehyde.

[0046] In the present invention, when performing urea granulation without formaldehyde, it is preferred to employ film-forming granulation nozzles, e.g., such as disclosed in WO 2024 / 04923 or EP141436.

[0047] In accordance with the invention, the urea solution subjected to granulation has a purity adapted for the production of technical grade urea. Reference is made to ISO-2241-1-2019 which lists the relevant specifications, such as an alkalinity reflected by a maximum of 0.2 % (mass fraction) of NH3, a maximum of 0.3 % (mass fraction) of biuret, and a maximum of 5 mg / kg of aldehydes. Specifically, the urea solution having a purity adapted for the production of technical grade urea does not contain a crushing strength improving additive such as formaldehyde.

[0048] In the process of the invention, the urea can be synthesized by any suitable method. A frequently used process for the preparation of urea according to a stripping process is the carbon dioxide stripping process as for example described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. In this process, the synthesis section is followed by one or more recovery sections. The synthesis section comprises a reactor, a stripper, a condenser and a scrubber in which the operating pressure is in between 12 and 18 MPa and preferably in between 13 and 16 MPa. In the synthesis section the urea solution leaving the urea reactor is fed to a stripper in which a large amount of non-converted ammonia and carbon dioxide is separated from the aqueous urea solution. Such a stripper can be a shell and tube heat exchanger in which the urea solution is fed to the top part at the tube side and a carbon dioxide feed to the synthesis is added to the bottom part of the stripper. At the shell side, steam is added to heat the solution. The urea solution leaves the heat exchanger at the bottom part, while the vapor phase leaves the stripper at the top part. The vapor leaving said stripper contains ammonia, carbon dioxide and a small amount of water. Said vapor is condensed in a falling film type heat exchanger or a submerged type of condenser that can be a horizontal type or a vertical type. A horizontal type submerged heat exchanger is described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. The heat released by the exothermic carbamate condensation reaction in said condenser is usually used to produce steam that is used in a downstream urea processing section, generally indicated as being an evaporation section, for heating and concentrating the urea solution. Since a certain liquid residence time is created in a submerged type condenser, a part of the urea reaction takes already place in said condenser. The formed solution, containing condensed ammonia, carbon dioxide, water and urea together with the non-condensed ammonia, carbon dioxide and inert vapor is sent to the reactor. In the reactor the above mentioned reaction from carbamate to urea approaches the equilibrium. The ammonia to carbon dioxide molar ratio in the urea solution leaving the reactor is generally in between 2.5 and 4 mol / mol. It is also possible that the condenser and the reactor are combined in one piece of equipment. An example of this piece of equipment as described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. The formed urea solution leaving the urea reactor is supplied to the stripper and the inert vapor containing non-condensed ammonia and carbon dioxide is sent to a scrubbing section operating at a similar pressure as the reactor. In that scrubbing section the ammonia and carbon dioxide is scrubbed from the inert vapor. The formed carbamate solution from the downstream recovery system is used as absorbent in that scrubbing section. The urea solution leaving the stripper in this synthesis section requires a urea concentration of at least 45 % by weight and preferably at least 50 % by weight to be treated in one single recovery system downstream the stripper. The recovery section comprises a heater, a liquid / gas separator and a condenser. The pressure in this recovery section is between 200 to 600 kPa. In the heater of the recovery section the bulk of ammonia and carbon dioxide is separated from the urea and water phase by heating the urea solution. Usually steam is used as heating agent. The urea and water phase, contains a small amount of dissolved ammonia and carbon dioxide that leaves the recovery section and is sent to a downstream urea processing section where the urea solution is concentrated by evaporating the water from said solution.

[0049] The invention is not limited to any particular urea production process. Other processes and plants include those that are based on technology such as total recycle plants, the HEC process developed by Urea Casale, the ACES process developed by Toyo Engineering Corporation and the process developed by Snamprogetti. All of these processes, and others, may be used in the method of the invention.

[0050] Preferably, the urea solution is produced in such a way as to have a low biuret content. Reference is made to processes as disclosed in WO 2023 / 158314 or W 2019 / 093891. This is preferably accomplished by injection of NH3 in the produced urea melt, as disclosed in WO 2017 / 192031.

[0051] In sum, a method is disclosed of producing technical grade urea in the form of granulate. This requires the ability to produce urea granules without normally essential granulation additives such as formaldehyde. The invention provides a method of operating a fluidized bed urea granulation unit by temporarily dispensing with such additives, in such a way that the production of technical grade urea can be realized.

[0052] Example

[0053] In a urea production plant a 98.5 wt.% urea melt is produced, in accordance with the process described in WO 2017 / 192031. The plant is provided with a fluidized bed granulation section using film spraying nozzles. The process was tested in 2 test runs, in which the addition of formaldehyde was turned off. With the further normal operation of the granulation process continuing, fines as well as crushed oversize particles are recycled. I.e., as of stopping the formaldehyde feed, amounts of formaldehyde will initially still be present in the granulator as result of said recycle originating from prior operation with formaldehyde. The formaldehyde level in the final product dropped below the 15 ppm level after 150 mins of the start of the run. This resulted in technical grade urea meeting the required specifications. The first test run lasted 24 hours, the second run 17 hours and the biuret concentration lowered by about 0.05 wt.% to an average level of 0.88 wt.% . The amount of dust generated was more than twice the dust generation during normal operation.

Claims

Claims1. A method of operating a fluidized bed urea granulation unit, wherein said unit has an inlet for a concentrated urea aqueous solution to be subjected to granulation and an inlet for clean fluidization air, an outlet for urea granules and an outlet for used fluidization air comprising urea dust, wherein the outlet for used fluidization air is in fluid communication with a dust scrubbing section for recovering urea dust from said air, wherein said dust scrubbing section has an outlet for urea-containing scrubbing liquid which is in fluid communication with an inlet of an evaporation section, said evaporation section having an outlet for concentrated aqueous urea solution which is in fluid communication with an inlet for urea melt of the granulation unit, the method comprising temporarily preventing the urea-containing scrubbing liquid from being sent to the evaporation section.

2. A method according to claim 1, wherein the granulation unit comprises a granulator wherein particles are made to grow over multiple compartments in series.

3. A process for the production of solidified technical grade urea, comprising providing a highly concentrated urea aqueous solution and subjecting said solution to a fluid bed granulation process in an industrialscale granulation unit, thereby obtaining urea granules and fluidization air comprising urea dust, wherein said air is subjected to recovery of urea dust so as to provide recovered urea dust; wherein the urea solution subjected to granulation has a purity adapted for the production of technical grade urea, and wherein the granulation is conducted by operating the granulation unit in accordance with the method of claim 1 or 2.

4. A process according to claim 3, wherein the highly concentrated urea aqueous solution is obtained from an evaporation section of a urea production plant.

5. A process according to claim 3 or 4, wherein the recovery of urea dust comprises liquid scrubbing.

6. A process according to any one of the claims 3 to 5, wherein the granulation unit comprises film-forming granulation nozzles.

7. A process for the production of urea granules, comprising providing a highly concentrated urea aqueous solution and subjecting said solution to a fluid bed granulation process, thereby obtaining urea granules and fluidization air comprising urea dust, wherein said air is subjected to scrubbing resulting in a used aqueous scrubbing liquid comprising recovered urea dust, said used scrubbing liquid being recirculated, with evaporation, to the granulation process; wherein the urea solution subjected to granulation periodically contains a crushing strength improving additive such as formaldehyde and periodically does not contain such additive, and wherein during granulation of the urea solution not containing formaldehyde, the recirculation of the used scrubbing liquid to the granulation process is at least temporarily prevented.

8. A process according to any one of the claims 3-7, wherein the highly concentrated urea aqueous solution has a urea concentration of between 99.1 wt.% and 99.9 wt.,%, preferably 99.6 wt.% to 99.8 wt.%.

9. A process according to claim 7 or 8, wherein the highly concentrated urea aqueous solution is obtained from an evaporation section of a urea production plant.

10. A process according to claim 9, wherein the fluid bed granulation process is conducted in an industrial-scale granulator.

11. A process according to claim 10, wherein the granulator comprises film-forming granulation nozzles.

12. A method of modifying a pre-existing fluidized bed urea granulation unit, wherein said unit has an inlet for a urea melt to be subjected to granulation and an inlet for clean fluidization air, an outlet for urea granules and an outlet for used fluidization air comprising urea dust, wherein the outlet for used fluidization air is in fluid communication with adust scrubbing section for recovering urea dust from said air, wherein said dust scrubbing section has an outlet for urea-containing scrubbing liquid which is in fluid communication with an inlet of an evaporation section, said evaporation section having an outlet for urea melt which is in fluid communication with an inlet for urea melt of the granulation unit, the method comprising adding a connection to a storage section having an inlet for urea-containing scrubbing liquid in fluid communication with the outlet for urea-containing scrubbing liquid of the dust scrubbing section, and an outlet for urea-containing scrubbing liquid in fluid communication with an inlet of the evaporation section, said inlets and outlets for urea-containing scrubbing liquid being provided with a closable aperture.

13. A method according to claim 12, wherein the step of adding a connection to a storage section comprises adding a storage tank.

14. A process for the production of an aqueous solution of urea suitable for use as Diesel Exhaust Fluid, comprising operating fluidized bed urea granulation unit in accordance with the method of claim 1, allowing a period of time until a urea-containing pure scrubbing liquid is obtained that is free from containing a crushing strength improving additive such as formaldehyde, and subjecting the pure scrubbing liquid to evaporation to provide an aqueous urea solution having a concentration in a range of 30 to 40 wt.% , preferably 32.5 wt.% or 40 wt.%.

15. A process for the production of an aqueous solution of urea suitable for use as Diesel Exhaust Fluid, comprising preparing urea granules according to any one of the claims 3 to 11, and dissolving the granules in demineralized water to the desired concentration in a range of 30 to 40 wt.% , preferably 32.5 wt.% or 40 wt.%.