Process and facilities for urea synthesis

Abstract

A process for the synthesis of urea from ammonia and carbon dioxide in which: Urea synthesis is carried out by a rejection process in a synthesis loop comprising at least one reactor (1), a stripper (2) and a condenser (3); the reactor effluent is treated in the stripper to remove unreacted ammonia and carbon dioxide; the urea solution (14) from the stripper is sent to a low-pressure recovery section (4); the stripper vapors are divided into a first portion (151) which is directed to the reactor and a second portion (152) which is sent to the condenser; the condenser (3) is a shell-and-tube condenser in which the condensation of the stripper vapors is carried out on the tube side (30); a The carbamate-containing effluent (20) from the condenser is returned to the reactor.

Description

Process and facilities for urea synthesis Description Field of application The invention relates to a process and a plant for the synthesis of urea. The invention particularly relates to an improvement in the CO2 removal process for the synthesis of urea. Prior knowledge An overview of industrial processes for urea synthesis can be found in Meissen, “Urea”, Ullmann Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag, 2010. The name of the CO2 stripping process comes from the use of gaseous CO2 as a stripping aid in the processing of the urea-containing effluent solution leaving the reactor. This stripping step is typically carried out in a steam-heated shell and tube stripper in which the reactor effluent flows in a film-forming regime from the side of the tubes and gaseous CO2 is fed in countercurrent from the bottom of the tubes. The heat generated by the hot steam dissociates the unconverted ammonium carbamate present in the solution, and the gaseous CO2 lowers the partial pressure of ammonia, aiding in the decomposition of the carbamate. The stripping step thus results in a urea solution collected at the bottom of the apparatus, with a reduced content of unconverted ammonium carbamate, and a gas stream above the stripper head consisting mainly of ammonia and carbon dioxide. The urea solution is further processed in the recovery section to remove unconverted carbamate. The overhead gas stream is typically condensed in a high-pressure condenser with the aid of the recovered carbamate solution and the resulting condensate is returned to the reactor. The condenser cooling medium is usually evaporated water to produce steam. Accordingly, a CO2 removal plant typically consists of a high-pressure synthesis section consisting of a reactor, a stripper, a condenser, and a scrubber. These form a so-called high-pressure loop in which the reactor effluent goes to the stripper, the overhead gas from the stripper goes to the condenser, and the condensate from the condenser is recycled to the reactor. The problem encountered in CO2 removal plants is ensuring proper circulation between these items in this high pressure synthesis loop. To do this, it may be necessary to install these items at different heights, for example, placing the scrubber above the condenser to ensure condensate flow from the scrubber to the condenser and placing the condenser above the reactor to ensure condensate flow from the condenser to the reactor. However, this installation is expensive. EP 2 297 094 shows an evolution of the CO2 removal process in which the condenser is a horizontal kettle device providing additional synthesis space. The condenser is a shell and tube device in which condensation of the stripper gas is carried out on the shell side and cooling water is fed to the tube side. This design may reduce the equipment height thanks to the horizontal condenser layout and the distribution of fresh CO2 between the reactor and stripper. However, this solution has some disadvantages. The first drawback is that the condensation is done on the shell side, which means that the entire condenser pressure vessel must withstand the synthesis pressure (well above 100 bar) and the aggressive environment of the urea process fluids. Therefore, the condenser of this design is very expensive. The second drawback is that diverting some of the fresh CO2 directly to the reactor reduces the ammonia stripper efficiency. WO 2019 / 083367 discloses a high-pressure carbamate condenser for urea plants. Summary of the invention The invention aims to improve the CO2 removal urea process and related plants. In particular, the invention aims to reduce the cost of equipment and to provide a low-altitude synthesis loop that is simpler and less expensive to install compared to current solutions. Accordingly, another aim of the invention is to reduce the capital cost of the synthesis section of a CO2 removal plant. These objectives are achieved by a process and plant in accordance with the claims. In the invention, the CO2 gas fed to the stripper represents the full amount of CO2 entering the synthesis loop, i.e. there is no direct CO2 feed to the reactor. A related advantage is the optimized stripper efficiency. The stripper overhead gas (also called stripper vapors), i.e. the gas containing ammonia and carbon dioxide leaving the stripper, is divided into a first portion sent to the reactor and a second portion sent to the condenser. This division of the stripper vapors can be carried out in accordance with EP 1 036 787. A related advantage is that the accumulation of inert gases in the reactor is minimized. The condenser is a shell-and-tube kettle condenser with one tube side and one shell side, and the second gas portion above the stripper head is sent to the tube side for condensation. A related advantage is that the high-pressure and aggressive process fluids are confined to the tube side of the condenser. The shell side, and especially the large pressure vessel, does not have to be designed to withstand the high pressure and corrosive attack of the urea solution containing the carbamate. This reduces the capital cost of the condenser. The carbamate-containing condensate stream leaving the condenser is then recycled to the reactor. A synthesis section according to the invention requires a reduced height for installation. The height can be further reduced in preferred embodiments, for example by providing an ejector to feed the condensate stream to the reactor, and / or by adopting a modified vertical reactor design having a reduced height and increased size (e.g. diameter). Accordingly, the capital cost for the construction and installation of the synthesis section equipment is competitive. Preferred Examples of the Invention The carbamate-containing condensate stream, which exits the condenser tube side, may be recycled to the reactor either by itself or through a carbamate separator. The carbamate separator may be provided to separate the condenser effluent, which is usually a two-phase mixture, into a carbamate-containing liquid and a vapor phase. The liquid is then sent to the reactor. The vapor phase may consist primarily of non-compressible gases and can be discharged. The carbamate-containing liquid, preferably after phase separation in the carbamate separator mentioned above, may be fed to the reactor via an ejector. More preferably, the ejector drive stream is a stream of fresh ammonia. In an alternative embodiment, the carbamate-containing liquid may flow to the reactor by gravity. The ejector embodiment is preferred because it does not require a condenser to be installed above the reactor, thus reducing the height of the equipment. The fresh ammonia entering the ejector may represent the majority of the fresh ammonia input to the synthesis loop, preferably at least 80% of the fresh ammonia input. The remaining portion of the fresh ammonia may enter the loop by mixing with a portion of the stripper overhead gas sent to the condenser. Depending on the model, the stripper overhead gas directed to the condenser may be mixed with one or more of the following: a stream of inert gas exiting the reactor; a stream of fresh ammonia; a carbamate-containing recycle solution coming from the recovery section. Mixing may occur before entering the condenser tube side. In one example, the urea synthesis section (high pressure loop) does not include a high pressure scrubber. Accordingly, a stream of inert gas discharged from the reactor is mixed with a second portion of stripper vapors before the stripper vapors enter the condenser tube side. In a preferred embodiment, a portion of the stripper overhead gas directed to the condenser is mixed with fresh ammonia, reactor exhaust gas, and carbamate recovery solution from the recovery section. The fresh ammonia fed to the condenser may be a small portion of the ammonia fed to the synthesis loop, e.g., 20% or about 20%. The remaining portion may be fed directly to the reactor, e.g., via an ejector, if provided. The reactor overhead gas must be removed to prevent the accumulation of inert gas in the reactor itself. The reactor overhead gas, which can be discharged from the top of the reactor, contains some ammonia and carbon dioxide. Mixing this reactor vent gas with the stripper gas directed to the condenser has the advantage that the reactants (ammonia and carbon dioxide) in the vent gas can be recovered by condensation. Another advantage of this example is the potential improvement in overall process efficiency by transferring waste heat to the fresh ammonia fed to the synthesis loop to preheat the fresh ammonia. All of the heat provided to this stream is ultimately recovered in the condenser as low-pressure steam usable in the recovery section. This gives you the opportunity to reduce the MP stripper steam consumption by about half the heat transferred to the ammonia in the preheater. In a different example, the reactor vent gas may be contacted with the carbamate recovery solution in a high-pressure scrubber, thus producing a solution that is sent to the condenser tube. Preferably the shell side of the condenser is passed through by a cooling fluid, preferably boiling water, at a pressure greater than 6 bar, preferably 2 to 6 bar. The condenser tubes are preferably a bundle of U-tubes. The condensation carried out on the tube side (i.e., process side) of the condenser is preferably complete condensation. This term indicates that the gas phase is almost completely condensed to the liquid state, apart from the incompressible fraction. In a preferred embodiment, the urea reactor is a vertical reactor with a height of less than 20 meters, preferably in the range of 12 to 18 meters, and more preferably 12 to 16 meters. This height is significantly less than the typical height of urea reactors, which is between 20 and 35 meters. According to an aspect of the invention, this reduced height is compensated for by increasing the height to diameter ratio (h / D). The resulting structure, which supports the high pressure synthesis section items, may have a maximum height of no more than 40 meters, for example 30 to 38 meters, compared to the typical 45 to 60 meters of the prior art. This is a significant advantage in terms of installation cost. A preferred example includes the following: the condensate stream leaving the condenser is fed to the urea synthesis reactor through an ejector; the overhead gas is removed from the reactor to prevent the accumulation of inert gas in the reactor itself; the reactor vent gas is mixed with the stripper gas directed to the condenser to recover the ammonia and carbon dioxide contained in the vent gas; the stripper is a vertical shell and tube device operating at the same pressure as the reactor; the condenser is a high-pressure equipment operating at the same pressure as the reactor and stripper; the reactor, stripper and condenser are connected to form a high-pressure synthesis loop. The following detailed description relates to preferred embodiments, which are illustrated by way of a non-limiting example. Brief description of the problem Figure 1 is a diagram of a urea synthesis process and plant in accordance with an example of the invention. Detailed description Figure 1 shows the following main points: Urea reactor 1 High Pressure Stripper 2 High pressure condenser 3 Low pressure recovery section 4 Carbamate separator 5 Ejector 6 Control valve 7. Urea is formed in reactor 1 under high pressure, for example 140 bar. Urea reactor 1 is a vertical device divided internally by plates. The urea-containing solution is collected by gas-carrying tube 10 and passes through line 11 to stripper 2. Stripper 2 is a vertical shell and tube apparatus operating at essentially the same pressure as reactor 1. The reactor effluent 11 is fed into the stripper 2 tube. Gaseous CO2 is fed down the tube via a line 12. The effluent solution thus flows down the tubes in a falling film regime in countercurrent to the gaseous CO2. The shell surrounding the tubes is heated by steam entering line 13. In stripper 2, some of the unreacted carbamate in the reactor effluent is decomposed into gaseous ammonia and carbon dioxide. The dissolved effluent from stripper 2 is sent to recovery section 4 via line 14. Stripper overhead gas, consisting primarily of ammonia and carbon dioxide, exits the top of stripper 2 via line 15. This line 15 is divided into a first line 151 which goes to the reactor 1 and a second line 152 which goes to the condenser 3. The flow rate of line 152 is controlled by valve 7. Accordingly, the first portion of the stripper overhead gas is returned to reactor 1 and the remaining second portion is sent to condenser 3 for condensation. Preferably, valve 7 is adjusted so that the flow in line 151 directed to reactor 1 is 20% to 40% of the total flow leaving the stripper in line 15. Condenser 3 is a horizontal kettle apparatus with a package of 30 U-tubes. Condenser 3 is also a high-pressure equipment that operates at a pressure approximately equal to that of reactor 1 and stripper 2. The second part (line 152) of the gas is sent above the stripper head to condenser tube 3, as shown in the figure. More specifically, the second portion of the stripper overhead gas line 152 is mixed with the reactor off-gas 16, a stream of fresh ammonia 17, and a stream of recycled solution 18 from recovery section 4. It is optional to mix the stripper vapors 152 with any of streams 16 (reactor vent gas), 17 (fresh ammonia), and 18 (carbamate recycle), or a combination thereof. Figure 1 shows a preferred embodiment in which the stripper vapors 152 are mixed with all of streams 16, 17, and 18. The resulting mixed stream 19, which includes a portion of the stripper vapors from line 152, is condensed on the tube side 30 of condenser 3. The shell side 31 is passed through by a cooling fluid, e.g. boiling water (not shown). The condenser effluent is sent to the carbamate separator 5 via line 20. The condenser effluent in line 20 is generally a two-phase mixture. This mixture is separated into a liquid fraction and a gaseous fraction in the carbamate separator 5. The liquid fraction leaves the separator 5 via line 21 and is returned to the reactor 1 via ejector 6 and its outlet line 23. The fresh ammonia stream 22 drives the ejector 6. The gaseous fraction separated in the separator 5 is mainly composed of non-compressible gas and can be discharged via line 25. The recovery section 4 operates at low pressure, for example 2 to 6 bar. The recovery section 4 is known and does not need to be explained in detail. Essentially, this section comprises at least one low-pressure carbamate decomposer and a low-pressure condenser; gaseous ammonia and carbon dioxide are removed from the urea solution in the decomposer and said gas is condensed into a carbamate-containing recovery solution in the low-pressure condenser. Accordingly, the recovery section 4 produces a solution containing carbamate 18 and a urea solution 24, which consists essentially of urea and water. As mentioned above, the carbamate solution 18 is preferably returned to the condenser 3 together with the stripper vapors portion 152. The carbamate solution 18 may assist in condensing said stripper vapors in the condenser. It can be noted that the reactor 1, the stripper 2 and the condenser 3 are connected to form a high pressure synthesis loop. The loop may also include a scrubber in some embodiments. The items in the synthesis loop operate at the same or nearly the same pressure, which is preferably in the range of 120 to 180 bar. Figure 1 shows a preferred embodiment in which the high pressure loop does not include a scrubber. Accordingly, the vent gas 16 leaving the reactor 1 goes directly to the condenser 3 after mixing with the vapor portion 152. It can also be noted that fresh CO2 only enters the said loop through line 12 connected to stripper 2. In other words, the only fresh input is CO2 which enters loop 2 as the removal medium. Fresh ammonia enters the loop from line 22 connected to ejector 6 and possibly from line 17 where it is mixed with stream 152 before condenser 3. If ammonia enters from both locations, it is preferred that the majority of the ammonia enters through the ejector, i.e. from line 22. In one embodiment, the gas discharged from reactor 1 via line 16 may be sent to a high-pressure scrubber and scrubbed with a carbamate solution, for example a portion of solution 18. The liquid effluent from the scrubber may be sent to condenser tube 3.

Claims

Process and plant for urea synthesis Claims 1. A CO2-removal urea plant for the synthesis of urea from ammonia and carbon dioxide, comprising: a synthesis loop comprising at least one reactor (1), a stripper (2) and a condenser (3); a line (11) arranged to feed a urea-containing reactor effluent from the reactor to the stripper, and a line (12) arranged to feed fresh CO2 as a removal medium to said stripper; a line (14) arranged to send a urea-containing solution effluent from the stripper to a low-pressure recovery section (4) for further processing, said line (12) arranged to feed fresh CO2 to the stripper which is the sole CO2 feed to the synthesis section; A line (151) arranged to feed a first portion of the stripper overhead vapors to the reactor, and a line (152) arranged to feed a second portion of said stripper vapors to the condenser, the condenser being a shell and tube kettle condenser with a tube side (30) and a shell side, and said line (152) connects the second portion of the stripper vapors to the condenser tube side, such that the second portion of the stripper vapors is fed to the condenser tube side for condensation, a line (20) arranged to flowFeed the carbamate-containing condensate leaving the condenser to the reactor; a plant further comprising a structure for supporting the reactor (1), the stripper (2) and the condenser (3), wherein the maximum height of said structure is not more than 40 meters and preferably in the range of 30 to 38 meters.

2. A plant according to claim 1, further comprising a carbamate separator (5) arranged to separate the condensate discharged from the condenser tube side into a carbamate-containing liquid, which is recycled to the reactor, and a vapor or gas phase.

3. A plant according to claim 2, further comprising an ejector (6) arranged to feed carbamate-containing liquid from the carbamate separator to the urea reactor, and a line arranged to feed fresh ammonia as a drive stream to the ejector.

4. A plant according to any one of claims 1 to 3 further comprising one or more lines for mixing the second portion of stripper vapors, before it enters the condenser tube, with a stream of fresh ammonia and / or with a carbamate-containing recycle solution coming from the recovery section.

5. A plant according to any one of claims 1 to 4, wherein the synthesis loop does not include a high pressure scrubber and a line is arranged to mix a stream of inert gas (16) discharged from the reactor with a second portion of stripper vapors (152) before said stripper vapors enter the condenser tube side.

6. A plant according to any one of claims 1 to 5, wherein the urea reactor is a vertical reactor with a height of less than 20 meters, preferably in the range of 12 to 18 meters.

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