A process for producing urea with low biuret

Abstract

A process for producing urea with low biuret

Description

A process for producing low-biuret urea Description Field of invention The invention relates to the field of urea or urea-based products. The invention relates in particular to the removal of biuret from aqueous urea solutions. Prior knowledge Urea is synthesized industrially by the reaction of ammonia and carbon dioxide. An overview of the relevant processes can be found in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag. Urea is typically produced by the reaction of ammonia and carbon dioxide in a urea synthesis section at the appropriate urea synthesis pressure to form a reaction effluent containing urea. This effluent is essentially an aqueous urea solution containing unreacted ammonia and carbon dioxide, mostly in the form of ammonium carbamate. In a known example, the synthesis section comprises a reactor, a stripper and a condenser forming a high pressure loop. The reactor effluent is heated in the stripper, possibly with the aid of a gaseous stripping agent, to remove a gaseous stream containing ammonia and carbon dioxide. This gaseous stream exiting the stripper is condensed in a condenser, possibly with the aid of a solution recovered from the recovery section. The resulting condensate is recycled to the reactor. The well-known CO2 stripping process of Steamicarbon uses CO2 as a stripping agent. Another stripping process uses gaseous ammonia as a stripping agent. The reaction effluent is typically processed in a recovery section, comprising one or more recovery steps at a recovery pressure lower than the synthesis pressure, to remove unreacted ammonia and carbon dioxide from the reaction effluent and to obtain an aqueous urea solution consisting essentially of urea and water. For example, the recovery step includes heating the solution to obtain the carbamate cleavage and condensing the resulting vapors to a recovery solution containing the carbamate. This solution may be recycled to the synthesis section, for example to the synthesis loop condenser. The aqueous solution leaving the recovery section typically contains 60% to 90% urea by weight. This solution may be processed to remove water and obtain a highly concentrated solution or a urea melt to feed the granulation section or the granulation section where solid urea is produced. It is known that a granulation section requires an incoming urea melt containing at least 96% urea by weight; a prilling section requires a urea melt of at least 99.7% concentration. Another economically attractive application of urea is the production of an aqueous urea solution for use in the selective catalytic reduction of NOx from exhaust gases (SCR solution). The urea content in the SCR solution may vary; a solution for automotive use, called diesel exhaust fluid (DEF), typically contains 30 to 35% by weight, preferably 31.8% to 33.2% and more preferably 32.5% urea. For this purpose, the urea solution from the recovery section may be diluted with water until it reaches the target urea concentration, as disclosed, for example, in EP 1 856 038. The solution from the recovery section is mostly urea and water, but it also contains some impurities. One of the most problematic impurities is biuret. Biuret formation occurs at virtually every stage of urea production and is enhanced by time spent at elevated temperatures. Biuret has the formula H2N-CO-NH-CO-NH2 and is formed when urea is heated above its melting point according to the reaction: 2 Urea → Biuret + NH3 The quality requirements for the final product in terms of maximum acceptable biuret content are stringent and difficult to achieve. A typical target for solid urea is 0.9% by weight or less, calculated as kilograms of biuret per kilogram of solid product. This target is usually required for the use of solid urea as a soil fertilizer; a foliar grade fertilizer may have a significantly lower limit of acceptable biuret. As explained above, the starting material for the production of solid urea is an aqueous solution containing 60 to 80% by weight of urea, which is treated to remove water, and the resulting highly concentrated melt is granulated or pelletized. It is difficult to maintain such a low biuret content in the final product. Since biuret content increases significantly with water removal, the urea producer may have to reduce the concentration of the urea melt sent to the granulation or pelletizing section to achieve maximum biuret in the solid urea. However, the pelletizing or pelletizing process is severely affected by any excess water content in the urea melt feed. Similar requirements exist in terms of maximum biuret in the production of SCR solutions. For example, the maximum acceptable biuret in DEF is usually 0.3% by weight, as prescribed for example by the DIN V70070 standard. Considering a urea concentration of 30 to 35% in DEF, this means that the dissolved solid urea for the production of DEF must not exceed 0.9% biuret. If DEF is produced directly by diluting a 70% solution, the initial solution must not exceed 0.6% biuret (all percentages by weight). Biuret control is also complicated by production fluctuations. For example, when a urea plant is operating at part load, the residence time of the urea melt at high temperature may be longer, resulting in more biuret formation. A well-known process for obtaining low-biuret solid urea from the aqueous solution discharged from the recovery section is concentration by crystallization. In this process, crystals of very pure urea are obtained and subsequently melted to produce a urea melt. However, crystallization is expensive. It requires centrifugation to separate the crystals from the solution; careful handling of the crystals, for example with pneumatic means; and a melter to melt the crystals. All of the above require materials that are expensive and difficult to work with. There is a need to provide a process for obtaining low-biuret urea solution that is cost-effective, easy to implement and manage, efficient in partial loads, and compatible with the evaporation-based concentration sector. Summary of the invention The object of the invention is to overcome the above-described disadvantages of the prior art. The object of the invention is to provide a cost-effective and practical process for removing biuret from an aqueous urea solution. In particular, the object of the invention is to provide a process for removing biuret that can be used in the urea production process, including evaporation concentration. It is also another object to provide a process for making urea with a low biuret content to meet today's stringent quality requirements. It is another object to provide a process for removing biuret that is also effective in partial loads of a urea plant. In one application, the object of the invention is to produce solid urea with less than 0.9% by weight of biuret, preferably less than 0.7% by weight, by a process comprising concentration by evaporation and granulation or perlification. Referring to another preferred application, one object of the invention is a process for producing an SCR solution with a low biuret content, although the initial solid product to be dissolved or the diluted urea solution contains a high level of biuret. In particular, one object is to produce an SCR solution meeting the quality requirements of DIN 70070, which contains no more than 0.3% by weight of biuret. The above objects are achieved by a process according to the claims. The dependent claims disclose preferred embodiments. This invention is based on the innovative idea of ​​removing biuret from an aqueous solution containing urea using reverse osmosis. Reverse osmosis (RO) is a well-known process that involves passing an aqueous stream through a semipermeable membrane and separating the permeate from the non-permeate. In the present invention, the reverse osmosis process separates biuret from an aqueous solution of water and urea through a semipermeable membrane. The applicant has experimentally tested that biuret molecule can be effectively separated from urea solution in a membrane-based RO process. The preferred membrane for carrying out the invention is a thin film composite (TFC) membrane. Preferably, the inventive process is carried out with a membrane having a nominal retention factor equal to or greater than 99.0% on NaCl. The invention preferably applies to the aqueous urea solution discharged from the recycling section of a urea plant, which is essentially composed of urea and water. One aspect of the invention is a process that includes the following: The reaction of ammonia and carbon dioxide under urea formation conditions and urea synthesis pressure in the urea synthesis section to form a reaction effluent containing urea; Processing the reaction effluent containing urea in a recovery section, comprising one or more recovery steps with a recovery pressure lower than the urea synthesis pressure, to remove unreacted ammonia and carbon dioxide from the reaction effluent and obtain an aqueous urea solution; Purification of aqueous urea solution to remove biuret by a reverse osmosis process. One aspect of the invention is that biuret can be removed from the aqueous urea solution by reverse osmosis before the urea solution is sent to the concentration section for the production of solid urea. Accordingly, one aspect of the invention is also a process for the production of solid urea, which includes the steps of taking an aqueous urea solution from the recycling section of a urea synthesis plant, optionally after holding the solution in a tank, removing biuret by reverse osmosis; subsequently concentrating the resulting low biuret solution to remove water; and processing the resulting concentrated or molten solution to obtain a solid urea product, for example by granulation or perlification. In another interesting application, an aqueous urea solution for SCR (SCR solution), preferably containing 30 to 35 wt% urea, is purified from biuret by reverse osmosis. The aqueous solution may be obtained by dissolving solid urea in water or simply by diluting a more concentrated solution (e.g., from the recovery section) with water. Another aspect of the invention is a urea production plant according to the claims. The process of the invention does not separate urea from water. Accordingly, a solution with a reduced biuret content obtained by the process of the invention may have the same or nearly the same water to urea ratio (kg / kg) as the input solution. Removal of biuret alone without affecting the water to urea ratio can be achieved by an appropriate pressure difference across the membrane. Said pressure difference represents the pressure difference between the permeate and non-permeate sides of the membrane and is commonly referred to as the delta pressure. The delta pressure across the membrane is greater than the first osmotic pressure 1 and less than the second osmotic pressure 2, where: the first osmotic pressure 1 is the osmotic pressure that can be calculated for the aqueous urea solution assuming that biuret is soluble and the urea / water mixture is the solvent; the second osmotic pressure 2 is the osmotic pressure that can be calculated for the aqueous urea solution assuming that urea is soluble and water is the solvent. By choosing a delta pressure in this range, a significant amount of biuret can be removed and a permeate with a water to urea ratio approximately similar to that of the inlet solution can be obtained. Preferred examples In this description and in the claims, all percentages are by weight unless otherwise specified. The reverse osmosis process of the present invention is preferably carried out with an inlet urea-containing stream at a temperature of 60°C to 90°C, and preferably 70°C to 80. Particularly preferably, the inlet stream temperature is 70°C to 75°C. The RO process may be carried out in a single RO stage or, preferably, in multiple RO stages in a cascade. Each stage preferably operates within the temperature range mentioned above. The term cascade indicates that at least one of the permeate, the non-permeate, or both of at least one stage is further processed in one or more subsequent stages. In multi-step embodiments, various preferred process embodiments disclosed in this description may be applied to at least one step or, preferably, to all steps. The pressure difference across the RO stage or each RO stage in the case of multiple stages is preferably 30 bar to 70 bar, preferably 35 bar to 50 bar and more preferably 40 bar or about 40 bar. The permeability of a stage may be, for example, about 10 liters per hour per square meter. A reverse osmosis stage produces a permeate and a non-permeate. The permeate is a purified solution that contains less biuret than the inlet solution; the non-permeate contains the biuret removed from the inlet solution and therefore has a relatively high biuret content, usually greater than 1% by weight. In a multi-stage example, the input solution may be processed in the first RO stage, yielding the first permeate and the first non-permeate. The first permeate may be processed through the first set of one or more subsequent RO stages, with the permeate of the nth stage being sent to the (n+1)th stage for further biuret removal. The permeate of the last stage represents the purified solution produced by the entire RO process. The first retentate may be processed through a second set of one or more RO stages. The retentate from the last RO stage of said second set ultimately forms a biuret-rich stream. The non-permeate stream(s) from said first set of RO stages together with the non-permeate stream(s) taken from said second set of RO stages may be recycled together with the inlet solution to the inlet of the first RO stage. The urea inlet aqueous solution, after leaving the recovery section, may be stored in a urea solution tank. According to this example, the urea aqueous solution that is subjected to reverse osmosis to remove biuret is taken from said tank. Preferably, the feed solution contains at least 25% urea. At this concentration, the calculated osmotic pressure for the binary mixture in which water is the solvent and urea is the solute is significantly higher than 100 bar. Preferably, urea and water together constitute at least 90% by weight of the solution, and preferably at least 95% by weight of the solution. When the feed solution is a solution obtained from the recycling section of a urea plant, it preferably contains 60% to 90% by weight of urea. The balance is mainly water and includes biuret and possibly other impurities. In some embodiments, a flash evaporation or pre-evaporation step may be performed at subatmospheric pressure of the aqueous urea solution prior to storage of the solution in the tank. The term subatmospheric pressure refers to a pressure of less than 1 bar absolute, and preferably less than 0.5 bar. This initial flash evaporation or pre-evaporation step is useful for maintaining a low concentration of carbonates in the solution stored in the tank. Preferably, the carbonates are kept below 0.2% by weight, and preferably below 0.1%. A low carbonate content in the solution may be useful to maintain the osmotic pressure of the concentrate below a desired level, e.g. below 70 bar or preferably below 40 bar. In this regard, it should be noted that a semipermeable membrane is usually very selective to salts. For example, in a multi-stage RO process, salts in the feed solution may be almost completely removed in the first stage. For this reason, a high salt content (e.g. carbonates) in the feed solution may lead to an undesirable increase in the osmotic pressure. The purified low-biuret solution obtained after the reverse osmosis process may have a biuret content of half the input concentration. The purified solution obtained after the reverse osmosis process may be subjected to an evaporation step to remove water. Particularly preferably, said evaporation step yields a highly concentrated or molten urea solution suitable for granulation or perlification. The solid product from the purified low-biuret solution obtained after the reverse osmosis process may not contain more than 0.7% biuret by weight. A biuret-rich stream (non-permeate) produced in the RO process may be used as a raw material to obtain a biuret-based secondary product, e.g. feed grade biuret. This biuret-rich stream may also be recycled to a urea plant, e.g. added to the recovery section condenser to assist in the condensation of ammonia and CO2-containing vapors. In this case, the rate of the non-permeate stream recycled to the recovery section is preferably no greater than 10% of the rate of flow of the aqueous solution entering the reverse osmosis treatment process. In various embodiments of the invention, the aqueous urea solution undergoing RO purification may be considered as a binary mixture in which biuret is a solute and the water-urea mixture is a solvent. That is, the water-urea mixture can be considered the biuret solvent. Also, if the input solution contains significant amounts of carbonate and / or ammonia, this approach can still be used by considering biuret and carbonates as solutes and the water, urea, and ammonia mixture as solvents. Osmotic pressure can be calculated using the following formula: Where: Π is the osmotic pressure (Pa); R is the universal gas constant (J K-1mol-1); T is the absolute temperature (K); vsolvent is the molar volume of the solvent (m3mol-1); asolvent is the activity (dimensionless) of the solvent. For a dilute solution, the solvent activity can be approximated by the mole fraction of the solvent. It should be noted that the carbonate rejection coefficient is likely to be dissolved in the input solution, due to the dissociation of carbonates being significantly greater than the biuret rejection coefficient. The term carbonates refers to salts of carbonic acid. The invention is applicable to all known processes and plants for the synthesis of urea. A preferred application is for a stripping process, preferably a CO2 stripping process. The invention, in its various embodiments, allows a low-biuret solid urea or low-biuret urea solution to be produced without the expense and complexity of a crystallization section. The invention is now further described with reference to preferred embodiments and accompanying figures. Description of the problem Figure 1 is a diagram of a process for producing urea in an example of the invention. Figure 2 is a diagram of a multi-stage reverse osmosis section that can be used to practice the invention. Description with details Referring to Figure 1, a urea synthesis plant UP produces an aqueous urea solution 1 from urea. Said solution 1 is taken from the recovery section of the UP plant. The UP plant in more detail may include a high pressure synthesis section, for example a CO2 stripping synthesis section, and a low pressure recovery section from which solution 1 is obtained. Said solution 1 is stored in the urea solution tank T. Solution 2 taken from tank 2 is sent to the RO reverse osmosis section including a membrane package which performs the reverse osmosis process to remove biuret from said solution 2. Low-biuret urea solution 3 is obtained from the RO section. This low-biuret solution 3 is sent to the evaporation section EV where water is removed to obtain a highly concentrated solution 4. This highly concentrated solution 4 is processed in a fining section FIN to obtain solid urea U in the form of beads or granules. Biuret-rich solution 5 is also produced in the RO section. Solution 5 contains the biuret removed from feed solution 4 and typically contains more than 1% biuret. Solution 5 is recycled to the UP plant. The preferred use of solution 5 in the plant is to send solution 5 to the ammonia and CO2 vapor condenser. In another interesting application, water may be added to stream 3 to produce a urea solution for use in SCR for NOx removal. Figure 2 shows an example of an RO section. An inlet solution F (e.g. solution 2 of Figure 1) is sent to the first reverse osmosis stage RO-1 together with internal recycle streams 20, 21. The RO-1 stage therefore receives a mixed stream 22 and produces a first permeate 1P and a first non-permeate 1R. The first 1P permeate is processed in a series of RO-1.1 and RO-1.2 stages where the permeate is progressively purified. In particular, the 2P permeate of RO-1.1 stage is further purified in RO-1.2 stage to produce a P permeate which is the first output of the process (e.g. stream 3 of Figure 1). The first 1R retentate is processed in a series of stages RO-2.1 to RO-2.3. The retentate of each stage forms the input to the next stage. The retentate R of the last stage RO-2.3 is another output of the process, for example stream 5 in Figure 1. Stream P has the lowest biuret content while stream R has the highest. The permeate streams of stages RO-2.1 to RO-2.3 and the non-permeate streams of stages RO-1.2 and RO-1.3 are streams with medium biuret content. As shown in Figure 2, they can be recycled to the inlet of the first stage RO-1 via lines 20, 21. For example, in a preferred embodiment, the streams of Figure 2 have the following flow rates (m3 / h) and biuret mass fraction wB. Flow m3 / h WB F950.50 222900.77 P11940.50 P21070.34 P880.25 R74.0 20901.05 The present invention achieves the above-mentioned objectives, namely to provide a cost-effective process for removing biuret from urea solutions and producing low-biuret urea.

Claims

Claims 1. A process for purifying an aqueous urea-containing stream comprising the step of removing biuret from the urea-containing stream by reverse osmosis, wherein the urea-containing stream is an aqueous urea solution obtained from the recovery section of a urea plant, preferably containing 60 to 90% by weight of urea.

2. A process according to claim 1 wherein reverse osmosis is carried out with a thin film composite membrane.

3. A process according to any one of claims 1 to 2, wherein reverse osmosis is carried out with a urea-containing stream at a temperature of 60°C to 90°C, preferably 70°C to 80°C.

4. A process according to any one of the preceding claims wherein the reverse osmosis is carried out with one or more reverse osmosis stages in a cascade.

5. A process according to claim 4 wherein the pressure difference across or at each stage of the reverse osmosis process is 30 bar to 70 bar, preferably 35 bar to 50 bar, and more preferably 40 bar or about 40 bar.

6. A process according to any preceding claim, wherein the process produces a purified solution with a lower biuret content than the input solution and having a water to urea ratio that is the same or nearly the same as the input solution.

7. A process according to any one of the preceding claims, wherein the aqueous urea solution contains at least 25% by weight of urea.

8. A process comprising: reacting ammonia and carbon dioxide under urea-forming conditions and urea synthesis pressure in a urea synthesis section to form a reaction effluent containing urea; processing the reaction effluent containing urea in a recovery section, comprising one or more recovery steps with a recovery pressure lower than the urea synthesis pressure, to remove unreacted ammonia and carbon dioxide from the reaction effluent and obtain an aqueous urea solution; purifying the aqueous urea solution to remove biuret by a process according to any one of the preceding claims.

9. A process according to claim 8, wherein all or some of the aqueous urea solution discharged from the recovery section is stored in a urea solution tank, and the aqueous urea solution subjected to said reverse osmosis is withdrawn from said tank.

10. A process according to claim 9, comprising a step of rapid evaporation or pre-evaporation at subatmospheric pressure of the aqueous urea solution before said solution is stored in said tank.

11. A process according to claim 9 or 10 comprising an evaporation step to remove water from the pure urea solution obtained by the reverse osmosis process.

12. A process according to any one of claims 8 to 11, wherein the osmosis process produces a permeate which is a purified solution containing low-biuret urea and a non-permeate which contains biuret removed from the inlet solution, and wherein at least a portion of said non-permeate is recycled to the recovery section.

13. A process according to claim 12, wherein the impervious part is used in the recovery section in the condensation step of vapors containing CO2 and ammonia as a means of improving condensation.

14. A process according to claim 12 or 13, wherein the flow rate of the recycled non-permeate to the recycling section is not greater than 10% of the flow rate of the aqueous solution under reverse osmosis treatment process conditions.

15. A process according to any one of claims 8 to 14 further comprising producing biuret or feed grade biuret, wherein at least a portion of said impermeable material is used to produce said biuret or feed grade biuret.

16. A process according to any one of claims 8 to 15, wherein urea is synthesized by a stripping process, preferably a CO2 stripping process.

17. A plant for the synthesis of urea comprising: a urea synthesis section adapted to produce urea by reacting ammonia and carbon dioxide at a urea synthesis pressure; a recovery section arranged to process a reaction effluent containing urea produced in the urea synthesis section, the recovery section operating at one or more recovery pressures lower than said urea synthesis pressure to remove unreacted ammonia and carbon dioxide from the reaction effluent and obtain an aqueous urea solution; a purification section arranged to remove biuret from the aqueous urea solution obtained in the recovery section, said purification section comprising one or more reverse osmosis stages, wherein biuret is removed from the solution using a reverse osmosis process.

18. A plant according to claim 17, wherein the one or more reverse osmosis stages are upstream of a concentration section and a finishing section for producing solid urea, such that the biuret removal by reverse osmosis is carried out before the solution is concentrated by removing water.

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