Urea prilling method

Abstract

A method for granulating urea melt, comprising using calcium lignosulfonate or carboxymethyl starch as additives to obtain formaldehyde-free urea products.

Description

Technical Field

[0001] This invention relates to a method for granulating urea. Background Technology

[0002] Urea is industrially produced by reacting ammonia and carbon dioxide under suitable urea formation conditions, typically at high pressure and high temperature.

[0003] Urea is synthesized at a synthesis pressure above 100 bar, yielding a reaction effluent containing urea, water, and unconverted reagent, primarily in the form of ammonium carbamate. Due to the equilibrium reached in the reaction environment, the amount of unconverted substance in the reaction effluent is significant, and it is typically treated to recover it.

[0004] In widely used stripping methods, the reaction effluent is heated in a high-pressure stripping column, possibly in the presence of a stripping agent, to decompose ammonium carbamate and extract gaseous ammonia and carbon dioxide. These are condensed in a high-pressure condenser and recycled to the synthesis reactor. The stripping agent is typically gaseous carbon dioxide or gaseous ammonia.

[0005] The high-pressure stripper and high-pressure condenser can operate at substantially the same pressure as the synthesis reactor, thus forming a high-pressure synthesis section or loop. The urea-containing effluent from the stripper is then treated at lower pressure in one or more recovery sections to further recover unconverted reagents, yielding a purified aqueous solution of urea. The purified solution is primarily composed of urea and water, and may contain, for example, about 65 to 70% by weight of urea, with the balance being water and unavoidable impurities.

[0006] Many applications require urea in solid form. The production of solid urea is also known as refining or product molding.

[0007] An overview of urea synthesis and subsequent product molding technologies can be found in relevant literature, such as the Ullmann's Encyclopedia of Industrial Chemistry.

[0008] Granulation is one of the two most commonly used techniques for urea formation; the other is granulation. Solid particles obtained by granulation are called spheres, while particles obtained by granulation are called granules.

[0009] For use in the granulation method, the aqueous solution is first converted into urea melt by removing water, for example, in a suitable evaporation section; the urea melt thus obtained is distributed in the form of droplets in the granulation equipment, wherein the urea droplets are solidified as they fall in the presence of a countercurrent cooling airflow.

[0010] Granulation equipment typically takes the form of a granulation tower. The urea melt can contain more than 98% or 99% urea. A purity of 99.5% or even higher may be required. This contrasts with pelleting, where a higher water content in the urea source, such as 3% or 4%, is generally acceptable.

[0011] Droplets can be generated using nozzles or a suitable granulation barrel mounted at the top of the granulation tower. The granulation barrel is a rotating drum with perforated side surfaces for supplying molten urea, wherein rapid rotation of the drum generates droplets. A granulation method in a tower with a rotating barrel is described, for example, in WO2004 / 101131. In this granulation method, the droplets solidify as they fall downwards through the tower without further addition of urea. The solid particles collect at the bottom of the granulation tower.

[0012] An important aspect of product forming technology is the mechanical properties of the resulting product, particularly its crushing strength. Granulation methods are generally considered inferior to pelletizing methods in terms of crushing strength; however, granulation remains widely used, and many granulation towers are in operation, thus there is interest in improving the quality of granulated products.

[0013] It is known that the mechanical properties of granules can be improved through suitable additives. For this purpose, formaldehyde is a known and widely used additive, which can be added as is or in the form of a formaldehyde-containing solution. However, the addition of formaldehyde raises health and environmental sustainability concerns. Any formaldehyde added prior to the granulation process will inevitably contaminate the urea product. Formaldehyde is particularly undesirable for certain applications, such as the production of feed-grade urea.

[0014] The term feed-grade urea refers to urea suitable for direct use as a feed ingredient for animals such as ruminants like cattle. Feed-grade urea is typically produced using granulation technology because the usual size of the pellets is suitable for this application.

[0015] Another area where formaldehyde is not required is, for example, the production of DEF (diesel exhaust fluid) grade urea. This term refers to urea used for the selective catalytic removal of NOx from flue gas. Urea used for this application must meet stringent quality requirements, such as conforming to DIN 70070 standards.

[0016] Therefore, efforts are being made to find a suitable additive to replace formaldehyde in the granulation process for the production of formaldehyde-free urea. Any such additive must be safe and economically acceptable. Furthermore, the additive should be effective at low concentrations to maintain the desired properties of the urea product, such as the nutritional value of cattle feed-grade urea. Summary of the Invention

[0017] The present invention aims to solve the above-mentioned problems. In particular, the present invention aims to find a safe and economically acceptable additive for urea granulation methods that can improve the mechanical properties of urea pellets, thereby replacing formaldehyde.

[0018] This invention is based on the discovery that calcium lignosulfonate and carboxymethyl starch can be used for the aforementioned purposes. These additives are completely safe, do not cause health problems, and are similar in cost to common formaldehyde-containing solutions such as LIF80.

[0019] Therefore, this objective was achieved by a granulation method for urea melt according to the following scheme.

[0020] This invention provides a method for granulating urea melt, wherein:

[0021] The granulation method includes distributing urea melt in the form of droplets in a granulation tower, and solidifying the urea droplets as they fall down the tower countercurrently with the upward-flowing air.

[0022] The urea melt contains at least one of calcium lignin sulfonate and carboxymethyl starch as a granulation additive, and

[0023] The urea melt contains no formaldehyde.

[0024] The urea melt is produced in a urea synthesis unit, wherein urea is synthesized from ammonia and carbon dioxide under synthesis pressure to obtain a reaction effluent containing urea, water and unconverted ammonium carbamate.

[0025] The reaction effluent is treated at low pressure in one or more recovery stages to recover unconverted reagents and obtain a purified aqueous solution of urea.

[0026] The aqueous solution is treated in an evaporation section to remove water and obtain the urea melt, wherein:

[0027] The granulation additive is added to the purified aqueous solution before evaporation or to the urea melt.

[0028] The applicant has experimentally discovered that the crushing strength of urea balls provided by the above-mentioned additives is comparable to that obtained by adding formaldehyde. This result can be achieved with a total additive content of no more than 1.0%, meaning that the additives of the present invention are effective at low concentrations and can be used without significantly altering the urea product. The total amount may include only one or two of the additives mentioned above. These additives can be used together in mixture form.

[0029] In particular, feed-grade urea products with desired properties can be obtained, including acceptable nitrogen content. In most cases, the minimum required nitrogen content in feed-grade urea is 46% (by weight), and formaldehyde-free urea produced using the method of this invention meets this requirement.

[0030] Regarding cost, under current market conditions, the additive of this invention may incur an additional cost of approximately €4.5 to €5.5 per metric tonne of urea. Conventionally adding 0.5% UF80 would incur a cost of approximately €4.6 per metric tonne. Therefore, the additive of this invention is also economically feasible.

[0031] Preferred Implementation

[0032] The total amount of additives in the urea melt is preferably 0.1% to 1.0% by weight, based on a dry basis of urea. More preferably, this amount is 0.3% to 0.8% by weight, based on a dry basis of urea.

[0033] The additives of this invention can be used alone or in combination.

[0034] In an embodiment using only one additive, the total amount of the additive is the amount of the selected additive, namely carboxymethyl starch or calcium lignin sulfonate.

[0035] In an embodiment using two additives, the total amount of the additives is the amount of carboxymethyl starch plus the amount of calcium lignosulfonate.

[0036] In embodiments using both additives, the weights of carboxymethyl starch and calcium lignin sulfonate are preferably the same, i.e., in a 1:1 ratio.

[0037] In implementations using two additives, they can be added individually or as a mixture.

[0038] When the granulation additive includes calcium lignosulfonate, a particularly preferred amount of calcium lignosulfonate in the urea melt is 0.7% by weight or about 0.7% by weight (on a dry basis of urea). For example, this amount can be from 0.65% to 0.75%.

[0039] When the granulation additive includes carboxymethyl starch, a particularly preferred amount of carboxymethyl starch in the urea melt is 0.5% by weight or about 0.5% by weight (on a dry basis of urea). For example, this amount can be from 0.45% to 0.55%.

[0040] The term urea melt refers to a highly concentrated solution, typically obtained by evaporating water from an aqueous solution of urea. Urea melt may contain more than 98% by weight of urea, preferably more than 99% by weight. More preferably, urea melt contains more than 99.5% urea, for example, 99.6% or 99.7% urea. This is because water contained in the urea source is detrimental to the granulation process and may affect the strength of the spheres obtained by this method.

[0041] In the granulation method, urea melt can be converted into droplets using one or more nozzles or one or more rotating granulation barrels. The nozzles or granulation barrels can be installed at the top of the granulation tower.

[0042] The urea balls obtained by the method of the present invention can have an average diameter of not more than 2.0 mm, preferably not more than 1.0 mm.

[0043] In a preferred application of the invention, feed-grade urea pellets are produced. Particularly preferred feed-grade pellets have an average diameter of 0.5 mm or about 0.5 mm.

[0044] Urea melt can be produced in urea synthesis equipment according to one of the known methods of industrial urea synthesis. These methods include, for example, the known CO2 stripping method introduced by Stamicarbon and the ammonia stripping or self-stripping method introduced by Snamprogetti.

[0045] In practice, urea is synthesized from ammonia and carbon dioxide under synthetic pressure to obtain a reaction effluent containing urea, water, and unconverted ammonium carbamate; the reaction effluent is treated at a lower pressure in one or more recovery stages to recover the unconverted reagents and obtain a purified aqueous solution of urea; the aqueous solution is then treated with an evaporation step in a suitable evaporation stage to remove water and obtain urea melt.

[0046] The granulation additives of this invention can be added before or after the evaporation step. That is, each additive (calcium lignosulfonate or carboxymethyl starch), or a mixture thereof, can be added to the purified aqueous solution of urea before evaporation, or to the urea melt obtained after evaporation. The additives or mixtures can be added directly to the urea solution stream or melt stream, or can be added to a suitable tank.

[0047] As described above, a preferred application of the present invention is a method for producing solid urea, comprising:

[0048] Ammonia and carbon dioxide react under synthesis pressure to obtain a reaction effluent containing urea, water, and unconverted ammonium carbamate; the reaction effluent is treated at a lower pressure to recover the unconverted reagent and obtain a purified aqueous solution of urea; the aqueous solution of urea is evaporated to remove water and obtain urea melt, the method further comprising:

[0049] At least one of calcium lignosulfonate and carboxymethyl starch is added as a granulation additive to purified aqueous solution or urea melt.

[0050] In this case, no formaldehyde was added to the purified aqueous solution or urea melt, so that the urea melt does not contain any added formaldehyde.

[0051] A granulation method is used to convert molten urea containing additives into a solid product. The granulation method includes distributing molten urea in the form of droplets in a granulation tower, wherein the droplets fall down the tower and solidify in the presence of countercurrent upward flowing cooling air. Attached Figure Description

[0052] Figure 1 An embodiment of the invention is shown. The following markings and process flow are illustrated.

[0053] Label 1 represents the synthesis section, in which ammonia (NH3) and carbon dioxide (CO2) react under high temperature and pressure to produce urea. Synthesis section 1 may include at least a reactor, a high-pressure stripper, and a high-pressure condenser.

[0054] Stream 2 is an aqueous solution containing urea, water, and unconverted ammonium carbamate. This solution can be extracted from the stripper of Synthesis Section 1.

[0055] Section 3 is the recovery section, in which unconverted reagents in solution 2 are recovered and recycled back to section 1. The unconverted reagents are typically recovered through one or more of the following steps: heating the urea-containing solution to decompose ammonium carbamate into gaseous ammonia and CO2, and condensing the reagent into a carbamate-containing solution, which can be pumped back to the reactor or condenser of section 1. Recovery section 3 can operate at one or more pressure levels.

[0056] Stream 4 is a purified solution obtained from recovery section 3. This purified solution 4 contains urea, water, and unavoidable impurities.

[0057] Mark 5 is the evaporation section from which water is removed from solution 4.

[0058] Flow 6 is urea melt.

[0059] Mark 7 is the urea melt tank.

[0060] Mark 8 is the additive container.

[0061] Mark 9 indicates the additive metering device.

[0062] Stream 10 indicates the additives (calcium lignosulfonate and / or carboxymethyl starch) added from tank 7 to the urea melt.

[0063] Flow 11 is a urea melt containing additives.

[0064] Mark 12 is the granulation tower.

[0065] Mark 13 is the granulation barrel installed in the granulation tower 12.

[0066] Stream 14 is the solid product (urea pellets) collected at the bottom of granulation tower 12.

[0067] In an alternative implementation, additive 10 can be added directly to the melt tank 7 or it can be added to the urea solution 4 before evaporation.

[0068] Additive 10 may be a mixture of calcium lignosulfonate and carboxymethyl starch. In some embodiments, calcium lignosulfonate and carboxymethyl starch may be added separately to urea solution 4 and / or urea melt 6. Separate tanks and metering devices may be provided for each additive. When added separately, calcium lignosulfonate and carboxymethyl starch may be added at the same or different locations. The total amount of both additives, on a dry basis of urea, is preferably in the range of 0.1% to 1.0% by weight.

Claims

1. A method for granulating urea melt, wherein: The granulation method includes distributing urea melt in the form of droplets in a granulation tower, and solidifying the urea droplets as they fall down the tower countercurrently with the upward-flowing air. The urea melt contains at least one of calcium lignin sulfonate and carboxymethyl starch as a granulation additive, and The urea melt contains no formaldehyde. The urea melt is produced in a urea synthesis unit, wherein urea is synthesized from ammonia and carbon dioxide under synthesis pressure to obtain a reaction effluent containing urea, water and unconverted ammonium carbamate. The reaction effluent is treated at low pressure in one or more recovery stages to recover unconverted reagents and obtain a purified aqueous solution of urea. The aqueous solution is treated in an evaporation section to remove water and obtain the urea melt, wherein: The granulation additive is added to the purified aqueous solution before evaporation or to the urea melt; The urea balls mentioned therein are feed grade.

2. The method according to claim 1, wherein, The total amount of the granulating additive in the urea melt is from 0.1% to 1.0% by weight, based on the dry basis of urea.

3. The method according to claim 1, wherein, The total amount of the granulating additive in the urea melt is 0.3% to 0.7% by weight, based on the dry basis of urea.

4. The method according to claim 1, wherein, The amount of calcium lignosulfonate in the urea melt is 0.7 wt% on a dry basis.

5. The method according to claim 1, wherein, The amount of carboxymethyl starch in the urea melt is 0.5% by weight, based on the dry basis of urea.

6. The method according to claim 1, wherein, The urea melt contains calcium lignosulfonate and carboxymethyl starch as granulation additives in a weight ratio of 1:

1.

7. The method of claim 1, wherein the urea melt contains more than 98% by weight of urea.

8. The method of claim 1, wherein the urea melt contains more than 99% by weight of urea.

9. The method of claim 1, wherein the urea melt is converted into droplets by means of one or more nozzles or by means of one or more rotating granulation tanks.

10. The method of claim 1, wherein the average diameter of the urea balls thus obtained is not greater than 2.0 mm.

11. The method of claim 1, wherein the average diameter of the urea balls thus obtained is not greater than 1.0 mm.

12. A method for producing solid urea, comprising: Ammonia and carbon dioxide react under synthesis pressure to produce a reaction effluent containing urea, water, and unconverted ammonium carbamate. The reaction effluent was treated at a lower pressure to recover the unconverted reagent and obtain a purified aqueous solution of urea. The aqueous solution of urea is evaporated to remove water and obtain the urea melt. The method further includes: At least one of calcium lignosulfonate and carboxymethyl starch is added as a granulating additive to the purified urea aqueous solution or the urea melt. In this case, neither the purified aqueous solution nor the urea melt contains formaldehyde, thus ensuring that the urea melt does not contain added formaldehyde. A granulation method is used to convert urea melt containing the granulation additive into a solid product, wherein the granulation method includes distributing the urea melt in the form of droplets in a granulation tower, wherein the droplets fall down the tower and solidify in the presence of countercurrent upward flowing cooling air. The urea mentioned above is feed grade.

13. The method of claim 12, wherein the granulating additive is added to the aqueous solution or the urea melt in an amount of 0.1% to 1.0% by weight on a dry basis of urea.

14. The method of claim 12, wherein the granulating additive is added to the aqueous solution or the urea melt in an amount of 0.3% to 0.7% by weight on a dry basis of urea.

15. The method of claim 12, wherein both calcium lignosulfonate and carboxymethyl starch are used as granulation additives, and they are added individually or as a mixture to the purified aqueous solution before evaporation or to the urea melt.

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