Stripping type urea plant for DEF production.

Abstract

A stripping-type urea plant and urea production process is provided that is adapted to make DEF or a precursor thereof in a DEF production unit downstream of a treatment section that receives a portion of the urea synthesis stream from the reaction zone, in particular bypassing the HP stripper.

Description

[Technical field]

[0001] The present invention relates to the production of urea, in particular the production of high purity urea solutions suitable for use, for example, as diesel exhaust fluid (DEF) in stripping-type urea plants. [Background technology]

[0002] Urea is often produced in fertilizer grade quality. Higher purity urea products are used to convert NO from combustion exhaust gases, for example for the production of diesel exhaust fluid (DEF). x DEF is required for use in reducing NOx emissions from exhaust gases. The standard for DEF is ISO 22241-1:2006 (edition of October 15, 2006). DEF should have a low biuret content. x Urea refers to an aqueous solution of urea for use in the selective catalytic reduction of fuel (SCR solution).

[0003] A urea product having a low biuret content can be prepared, for example, by crystallization from a mother liquor. The urea crystals can be dissolved to produce DEF.

[0004] Ullmann's Encyclopedia of Industrial Chemistry, chapter Urea, 2010, describes various urea production processes, including the preparation of urea with low biuret content using crystallization and stripping processes such as the Stamicarbon CO2 stripping process.

[0005] Many existing urea plants are of the stripping type having a high pressure stripper, e.g., a CO2 stripper or a thermal stripper, in the synthesis section.

[0006] US 2013 / 207035 A1 describes a process in which a first urea-containing aqueous stream is taken and diluted directly from or after a recovery section of a urea production process, the recovery section being located downstream of the synthesis section.

[0007] US2017 / 204054 A1 describes a process for making DEF using flash crystallization.

[0008] US2018 / 0071653 A1 strips the urea reaction solution and dilutes a portion of the aqueous urea stream with water to produce NO x The present invention describes a method comprising obtaining a solution suitable for use in a unit for the reduction of

[0009] EP 3862345 A1 to Casale describes a process for purifying an aqueous urea-containing stream, which includes removing biuret from the urea-containing stream by reverse osmosis to produce an SCR solution.

[0010] The present invention is directed to the production of urea products suitable for use as DEF or suitable as DEF precursors in stripping-type urea plants, in particular the production of urea products suitable for dilution by adding water to form DEF. DEF precursors are typically converted to DEF outside of the urea plant by adding sufficiently pure water. It remains difficult to produce DEF or DEF precursor urea products with sufficiently high purity in stripping-type urea plants.

[0011] DEF is NO x DEF is used in vehicles with diesel engines to reduce emissions. The composition of DEF for vehicles is standardized in ISO 22241-1:2006. DEF for vehicles has about 32.5 wt.% urea (i.e., essentially a eutectic composition) and has a very low concentration of impurities. The product urea solution contains, for example, NO xIt can also be used in industrial plants, ships and trains to reduce NOx. x For reduction, a urea solution of about 40 wt.% is used in accordance with ISO 186111-1:2014. x For reduction, typically a 50 wt.% urea solution is used. The term "DEF" is used in this disclosure and for the present invention, particularly for NO x It is used to refer to a urea solution that is suitable, adapted and / or specified for use in abatement, e.g. a urea solution according to any of the specifications, more specifically a urea solution according to ISO 22241-1:2006.

[0012] When making DEF, the concentration of urea is important to allow for accurate dosing of the liquid to the SCR catalyst. A low concentration of organic impurities in the DEF is important to avoid blockages and coke formation on the catalyst surface. A very low concentration of inorganic impurities in the DEF, especially heavy metals, is important because these impurities contribute to the poisoning of the SCR catalyst. The metals accumulate on the catalyst, thereby reducing its lifespan.

[0013] In the referenced specifications for DEF, the maximum limits for metal impurities are close to the typical achievable concentrations in the urea stream obtained from a typical urea stripping plant. Particularly during upset or start-up conditions, the amount of metals in the urea solution from the urea stripping plant can frequently exceed the DEF specifications. For urea plants specifically designed for DEF production only and not solid urea production (e.g., plants without a urea solidification section), this can lead to significant amounts of urea material that are outside the product specifications and cannot be handled in the plant (e.g., inside the battery limits).

[0014] US 2004 / 0116743 A1 describes a urea plant having a high pressure (HP) stripper which receives a first portion of a urea synthesis solution, a second portion being fed to a medium pressure (MP) dissociator and a medium pressure (MP) stripper and then to a low pressure (LP) recovery section which also receives the stripped urea solution from the HP stripper.

[0015] Jan Mennen, Nitrogen 2005, "The MEGA urea plant concept is now reality at SKW Piesteritz, Germany" illustrates a urea production process in which a first portion of the urea solution from a urea reactor is fed to a HP stripper using CO2 strip gas, a second portion is fed to a MP dissociator, and the urea solutions from the HP stripper and MP dissociator are combined and fed to a low pressure (LP) rectifier.

[0016] EP2086928 B1 describes a process in which a first portion of the urea solution from a urea reactor is fed to an HP stripper, a second portion is fed to an MP dissociator, and the urea solutions from the HP stripper and from the MP dissociator are combined and sent to a LP recovery section. [Prior art documents] [Patent documents]

[0017] [Patent Document 1] US2013 / 207035 A1 [Patent Document 2] US2017 / 204054 A1 [Patent Document 3] US2018 / 0071653 A1 [Patent Document 4] EP3862345 A1 [Patent Document 5] US2004 / 0116743 A1 [Patent Document 6] EP2086928B1 [Non-patent literature]

[0018] [Non-Patent Document 1] Ullmann's Encyclopedia of Industrial Chemistry,chapter Urea,2010 [Non-Patent Document 2] Jan Mennen,Nitrogen 2005,“The MEGA urea plant concept is now reality at SKW Piesteritz,Germany” Summary of the Invention [Problem to be solved by the invention]

[0019] The present invention is directed to the production of urea products suitable for use as DEF or as DEF precursors in stripping-type urea plants, in particular the production of urea products suitable for dilution by adding water to form DEF. DEF precursors are typically converted to DEF outside of the urea plant by adding sufficiently pure water. It remains difficult to produce DEF or DEF precursor urea products with sufficiently high purity in stripping-type urea plants. [Means for solving the problem]

[0020] The present invention relates in a first aspect to a urea production plant for producing a first urea-containing product and a second urea-containing product, the second urea-containing product being diesel exhaust fluid (DEF) or a DEF precursor, the plant comprising a high pressure (HP) synthesis section comprising a reaction zone, a high pressure (HP) stripper, a condensation zone and a flow splitter adapted to split a urea synthesis stream from the reaction zone into a first stream and a second stream, and a flow line from the first stream to the HP stripper, the plant comprising a flow line from the HP stripper to a urea synthesis section comprising a flow splitter adapted to split a stripped urea solution from the HP stripper into the first urea-containing product. the urea processing section for purifying the stripped urea solution, the urea processing section including a recovery section comprising one or more recovery section crackers for processing the stripped urea solution, an expansion device for expanding the second stream to obtain an expanded second stream, a processing section comprising one or more processing section crackers for purifying the expanded second stream into a purified urea solution, the one or more processing section crackers being arranged in parallel with the one or more recovery section crackers, and preferably a DEF production unit for producing DEF and / or DEF precursors from the purified urea solution.

[0021] The invention also provides in one aspect a process wherein a urea synthesis stream is split into a first and a second portion, the first portion is stripped in a high pressure (HP) stripper, recovered in a recovery section comprising one or more recovery section crackers and processed into a first urea containing product, preferably a urea fertilizer product, and the second stream is sent to a processing section comprising one or more processing section crackers without passing through the HP stripper, the one or more processing section crackers operating separately and in parallel with the one or more recovery section crackers, and the processing section produces a stream comprising urea suitable for use as diesel exhaust fluid or for preparation of diesel exhaust fluid by dilution with water, preferably wherein no urea solution is fed from the HP stripper to the processing section, and preferably wherein no urea solution is fed from the recovery section to the processing section.

[0022] The invention also provides in a further aspect a method for modifying an existing urea plant of stripping type comprising a high pressure (HP) synthesis section comprising a reaction zone, a condensation zone and a high pressure HP stripper, and a recovery section, the recovery section comprising a recovery section low pressure (LP) cracker, the method comprising adding to the existing plant a flow splitter for splitting a urea synthesis stream from the reaction zone into a first and a second portion, the first portion being fed to the HP stripper, and a processing section for processing said second portion, the processing section comprising a processing section LP cracker in parallel with the recovery section LP cracker. [Brief description of the drawings]

[0023] [Figure 1] 1 illustrates generally an exemplary urea plant and process according to the present invention. [Diagram 2] 1 illustrates generally an exemplary urea plant and process according to the present invention. [Diagram 3] 1 illustrates generally an exemplary urea plant and process according to the present invention.

[0024] Any embodiments illustrated in the figures are merely examples and are not intended to be limitations of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The present invention is in one aspect broadly based on the insight of producing DEF in a stripping type urea plant by treating a portion of the urea synthesis solution in a processing section operating at medium and / or low pressure without passing through a high pressure (HP) stripper, and processing the treated urea solution into a DEF product essentially without the addition of urea from the stripped urea solution, which advantageously allows for the preparation of a high purity urea solution in the processing section, in order to achieve a particularly low metal content.

[0026] The present invention is directed in certain aspects to a urea plant and the processes carried out in such a plant. The urea plant comprises a high pressure (HP) synthesis section comprising a reaction zone, a high pressure (HP) stripper, and a condensation zone. The urea plant may be, for example, of the CO2 stripping type or the thermal stripping type.

[0027] The HP synthesis section has an inlet for NH3 feed and an inlet for CO2 feed. In this process, urea is formed by the reaction of NH3 with CO2.

[0028] The reaction zone is provided, for example, as a vertical urea reactor, although other configurations are possible. A vertical urea reactor has, for example, one or more inlets at the bottom and one or more outlets at the top, for example, one or more inlets at the bottom and an outlet with a downcomer. The urea reactor may be equipped with horizontal trays to improve fluid flow in the reactor. The reactor may have one outlet for the urea synthesis stream, which includes both gas and liquid, or separate outlets, for example, for gas and liquid.

[0029] The reaction zone and the condensation zone may be combined in a single unit, for example in a single vessel or in a so-called pool reactor, as illustrated for example in Ullmann's Encyclopedia, chapter Urea, 2010, Fig. 19. A pool reactor is a horizontal vessel comprising a reaction zone and a condensation zone.

[0030] One or more reactors can be used in series or in parallel. One or more condensers can be used, for example, in series or in parallel.

[0031] The reaction zone, condensation zone, and stripper are operated at pressures in the high pressure range. The reactor, condenser, and stripper can be operated at substantially the same pressure or at different pressures. For example, in a self-stripping plant, the reactor is typically operated at 155-160 bar and the stripper at 145-150 bar. In a CO2 stripping type plant, the reaction zone, condensation zone, and stripper are, for example, all operated at the same pressure, for example, 138-142 bar.

[0032] The reactor outlet temperature is, for example, greater than 180°C, for example, in the range of 180 to 190°C.

[0033] The urea solution at the reactor outlet has an N / C ratio of, for example, 3.0 to 3.8, for example, 3.0 to 3.1 in a plant equipped with a high pressure (HP) CO stripper, and an N / C ratio of, for example, 3.3 to 3.6 in a plant equipped with a high pressure (HP) thermal stripper.

[0034] The synthesis section may comprise further high pressure (HP) units, such as HP scrubbers, or prereactors, or eductors.

[0035] The HP Stripper receives some, but not all, of the urea solution contained in the urea synthesis stream from the reactor. In particular, the HP Stripper receives the first stream from the flow splitter. The urea solution received by the HP Stripper also includes carbamates.

[0036] The stripper uses, for example, CO2 as a stripping gas or uses thermal stripping (self-stripping). The stripper is typically a shell-and-tube heat exchanger that uses the urea solution as a falling film in the tube and a heating fluid, typically steam, in the shell-side space. The gas outlet of the stripper is connected to the condensation zone. All or part of the gas from the stripper is fed to the condensation zone. In some embodiments, part of the gas from the stripper is fed to the reactor to improve the heat balance in the reactor. The stripper has an inlet for the urea synthesis solution and an outlet for the stripped urea solution. For example, a CO2 stripping type stripper has an inlet for the urea synthesis solution at the top, a gas outlet at the top, an inlet for CO2 used as a stripping gas at the bottom, and an outlet for the stripped urea solution at the bottom, all connected to the stripper tube, as well as an inlet and an outlet for the heating fluid on the shell side.

[0037] The present invention is particularly suitable, but not limited to, embodiments in which stripping in the HP synthesis section is carried out with steam at a temperature of at least 195° C., e.g., from 195 to 225° C., and / or with saturated steam at, e.g., from 14 to 24 absolute bar.

[0038] The stripper tube is made, for example, of a corrosion-resistant material, such as a stainless steel alloy, in particular a ferritic-austenitic duplex stainless steel. Exemplary duplex steel alloys are described in WO95 / 00674 A1, WO2017 / 013180 A1, and WO2017 / 014632 A1. The parts of the reactor in contact with the reaction mixture are preferably made of duplex stainless steel, more preferably also of a duplex steel alloy as described. This contributes to a low metal content of the urea synthesis solution.

[0039] In a particular embodiment, the stripper is a CO2 stripper with a stripper tube made of duplex ferritic-austenitic stainless steel.

[0040] Duplex stainless steels suitable for stripper tubes include steels with the composition 29Cr-6.5Ni-2Mo-N, available for example as Safurex® steel and also designated by UNS S32906, or steels with the composition 27Cr-7.6Ni-1Mo-2.3WN, available for example as DP28W™ steel and also designated by ASME Code 2496-1 and by UNS S32808. Duplex stainless steels for stripper tubes have for example the following composition (in mass%): C: max 0.05, Si: max 0.8, Mn: 0.3-4.0, Cr: 28-35, Ni: 3-10, Mo: 1.0-4.0, N: 0.2-0.6, Cu: max 1.0, W: max 2.0, S: max 0.01, Ce: 0-0.2, with the remainder Fe and (unavoidable) impurities. Preferably, the ferrite content is 30-70% by volume, more preferably 30-55%. More preferably, the steel contains the following (in weight %): C max 0.02, Si max 0.5, Cr 29-33, Mo 1.0-2.0, N 0.36-0.55, Mn 0.3-1.0, the balance Fe and (unavoidable) impurities.

[0041] Also advantageous is a duplex stainless steel having the following composition weight percent (wt.%): C max 0.030, Si max 0.8, Mn max 2.0, Cr 29.0-31.0, Ni 5.0-9.0, Mo less than 4.0, W less than 4.0, N 0.25-0.45, Cu max 2.0, S max 0.02, P max 0.03, residual Fe and unavoidably occurring impurities, Mo+W content is more than 3.0 to less than 5.0 (wt.%), preferably more than 3.0 to less than 4.0 wt.%, more preferably having the steel composition described in WO2017 / 014632 A1, which is incorporated herein by reference.

[0042] The condensation zone, used as a high pressure (HP) carbamate condenser, is provided, for example, as a vertical or horizontal HP carbamate condenser, typically as a horizontal or vertical shell-and-tube heat exchanger for heat exchange against a cooling fluid. The gas condensed from the stripper is received in the tubes in the shell side space of the shell-and-tube heat exchanger. The condensation zone receives all or a portion of the gas stream from the HP stripper, typically at least 60 vol.%, for example at least 90 vol.%. Optionally, a portion of the gas stream from the HP stripper, for example 5-40 vol.%, is fed to the reaction zone.

[0043] The HP carbamate condenser has an outlet for a fluid stream containing a liquid containing ammonium carbamate in the liquid. The outlet is connected to the inlet of the reactor for liquid flow connection. Typically, carbamate recycle solution from one or more low pressure (LP) and / or medium pressure (MP) carbamate condensers included in the plant is also fed to the part of the condenser that receives the gas from the stripper. In an embodiment where the gas is condensed in the vertical tubes of the condenser, the carbamate solution is fed, for example, to the top (falling film) or to the bottom of the tube. In another embodiment, the gas to be condensed is fed to the tubes of a U-shaped horizontal tube bundle (kettle type). In an embodiment where the gas is condensed in the shell side space, the condenser typically comprises a tube bundle for the cooling fluid, the tube bundle being, for example, vertical or horizontal. The HP carbamate condenser comprises, for example, a straight or U-shaped tube bundle. The condenser may comprise two or more tube bundles. The cooling fluid is typically the liquid at the cooling fluid inlet of the condenser, e.g., water used to raise the steam and / or process stream to be heated, such as a urea solution that also contains a carbamate. Two or more cooling fluids can be used, e.g., with dedicated tube bundles. An exemplary horizontal submerged condenser is the pool condenser, illustrated in Ullmann's Encyclopedia, 2010, chapter Urea, FIG. 18.

[0044] A liquid containing ammonium carbamate is fed to the reaction zone, optionally together with any uncondensed gases, which are optionally separated from the condensate in a gas / liquid separator and fed to another unit, such as a scrubber or a medium pressure (MP) section.

[0045] The CO2 feed and NH3 feed are fed, completely or partially, directly or indirectly, to the synthesis section, e.g., in some embodiments, the CO2 and NH3 feeds are both fed to the reactor (e.g., in combination with thermal stripping). In some embodiments with HP CO2 stripping, some or all of the CO2 is fed to a stripper and some or all of the NH3 feed is fed, for example, to a HP carbamate condenser.

[0046] The synthesis section comprises a flow splitter. The flow splitter, or divider, is adapted to split the urea synthesis stream from the reaction zone into a first stream and a second stream that are fed to the HP stripper. Both the first stream and the second stream contain urea. In particular, the urea synthesis solution from the reaction zone is split into a first stream and a second stream.

[0047] In some embodiments of the process, up to 50 wt.%, or such as up to 35 wt.%, e.g. 10-30 wt.%, of the urea from the reaction zone is fed as a second stream to the treatment section. The flow splitter is suitable for example to provide an adjustable fraction of the urea synthesis solution to the HP stripper and an adjustable fraction of the urea synthesis solution to the treatment section, e.g. a fraction ranging from no urea solution to the total urea solution to the HP stripper, or to simultaneously provide part of the urea synthesis solution to the HP stripper and part to the treatment section.

[0048] The first and second streams are, independently, optionally in some cases, liquid streams. In some embodiments, the process involves gas / liquid separation of the reaction mixture in the reaction zone upstream of the flow splitter, for example, already in the reactor or in a gas / liquid separator between the reaction zone and the flow splitter. In such embodiments, the flow splitter receives only the urea synthesis solution.

[0049] The urea synthesis solution comprises urea, water, ammonium carbamate, and unconverted ammonia. The N / C ratio of the urea synthesis solution received by the flow splitter and by the expansion device is preferably the same as at the outlet of the reaction zone.

[0050] Preferably, the plant comprises a control means for adjusting the mass ratio of the first and second streams. In this way, flexible fractions of the urea synthesis solution can be fed to the processing section to the HP stripper, respectively. For example, when the demand for fertilizer urea product is low, relatively more urea can be fed to the processing section to make DEF. During some periods, the entire urea solution can be fed to the processing section to the HP stripper, respectively.

[0051] The plant includes a urea processing section for processing the stripped urea solution from the HP stripper into a first urea-containing product. The urea processing section includes a recovery section, an evaporation section, e.g., downstream of the recovery section, and a finishing section, e.g., downstream of the evaporation section.

[0052] The recovery section includes one or more recovery section crackers for processing the urea solution from the HP stripper, and the plant typically includes one or more condensers connected to the gas outlets of the one or more recovery section crackers.

[0053] The term "recovery section" is used broadly herein to refer to the recovery of urea from the stripped urea solution, i.e., the processing of the stripped urea solution to at least partially remove non-urea components such as carbamates and ammonia. The recovery section may comprise, for example, a medium pressure (MP) cracker and a downstream low pressure (LP) cracker in series, or, for example, a LP cracker that receives the stripped urea solution directly from a HP stripper.

[0054] The cracker is, for example, a heat exchanger using a heating fluid to crack the carbamates and remove NH3 and CO2. The cracker has an outlet for gas. The cracker is, for example, a shell-and-tube heat exchanger for indirect heat exchange with a heating fluid. The heating fluid is, for example, steam or a process stream, for example, a condensed process gas stream.

[0055] A gas stream comprising CO2 and NH3 from the recovery section cracker, preferably from the MP or LP recovery section cracker, is fed to a carbamate condenser, for example. The carbamate solution formed in the carbamate condenser is typically recycled to the reaction zone, optionally possibly through a HP carbamate condenser. Thus, the recovery section is typically a "total recycle" design.

[0056] The urea solution from the cracker may be further processed, for example, in an atmospheric flash, a sub-atmospheric flash, and / or in a pre-evaporator.

[0057] The recovery section may, for example, in particular have an HP CO2 stripper and, optionally, in some embodiments, an LP decomposer having an MP treatment unit, such as an MP adiabatic flash unit, between the outlet for the stripped urea solution of the HP stripper and the inlet of the LP decomposer.

[0058] The recovery section may, for example, comprise a downstream low pressure (LP) cracker, eg, a medium pressure (MP) cracker with a HP thermal stripper.

[0059] As the term "recovery section" is used in this disclosure, the recovery section may include units operating at different pressures, and may, for example, include both an MP recovery section and an LP recovery section.

[0060] The stripped urea solution after passing through the recovery section, e.g., after passing through the LP cracker, comprises, e.g., at least 75 wt.% urea, e.g., 75-85 wt.% urea, and water, as well as biuret and metal impurities. The urea solution can be used, e.g., to make urea ammonium nitrate (UAN) liquid fertilizer. In some embodiments, the plant includes a UAN generation unit downstream of the recovery section, and the urea solution from the recovery section is mixed with an ammonium nitrate (AN) solution.

[0061] The first urea-containing product of the process is, for example, the urea solution from the LP cracker.

[0062] The urea solution is preferably converted to a urea melt containing at least 90 wt.% urea in the evaporation section for water removal. For example, at least 90 wt.%, or at least 95 wt.%, or for example, all of the urea in the stripped urea solution is converted to such a urea melt. The urea melt may contain, for example, at least 95 wt.% urea or at least 99 wt.% urea. The evaporation section may, for example, comprise one or more vacuum evaporators.

[0063] The urea melt is subjected to urea finishing, for example by granulation or prilling, to form a solid urea product. Other uses of the urea melt are also possible, such as for melamine production. The first urea-containing product of the process is, for example, a solid urea product or a urea melt.

[0064] The water vapor removed in the evaporation section is, for example, condensed and the condensate is treated, for example, in a waste water treatment (WWT) section, which comprises, for example, a hydrolyzer and a desorber. The cleaned process condensate from the WWT is, for example, used to make DEF, in particular by dilution of a urea solution, and / or is fed to a low pressure (LP) carbamate condenser, for example to avoid crystallization of carbamates.

[0065] Very sophisticatedly, the plant optionally comprises a liquid flow connection for the urea solution from the treatment section to the evaporation section, preferably a liquid flow line for the urea solution from the treatment section to the evaporation section. For example, the plant comprises a liquid flow line from the treatment section LP cracker to the evaporation section, whereby the outlet of the treatment section for the urea solution is connected for liquid flow with the inlet of the evaporation section. The treatment section is preferably connected with the inlet of the DEF generation unit by a flow line for the urea solution in parallel with the flow line to the evaporation section.

[0066] The liquid flow connection can be provided, for example, as a flow line for the urea solution from the treatment section to the recovery section, and the evaporation section receives the urea solution from the recovery section. Preferably, the flow line is connected at the inlet of the flow line to the MP cracker, the intermediate pressure (MP) contacting unit (such as an intermediate pressure (MP) adiabatic stripper), the LP cracker, or the residual ammonia removal unit of the treatment section. For example, the plant comprises a flow line for the urea solution from the MP cracker or the MP contacting unit of the treatment section to the inlet of the LP cracker of the recovery section. For example, the plant comprises a flow line for the urea solution from the residual ammonia removal unit to the inlet of the evaporation section.

[0067] In a preferred embodiment, the plant comprises a flow line for the urea solution from the treatment section to the inlet of the evaporation section. The liquid flow connection, including the preferred embodiment of the flow line, offers the advantage that the urea solution from the treatment section can be converted to urea melt when the demand for DEF is low and / or the demand for (solid) urea fertilizer is high. This can also be used during plant upset, ensuring the production of fertilizer grade urea, although the DEF quality may not be guaranteed. The liquid flow connection therefore offers an improved flexibility of the plant.

[0068] Preferably, the plant comprises a liquid flow connection for the urea solution from the processing section LP cracker to the inlet of the evaporation section, e.g. with a preferred steam stripper, and a liquid flow connection for the urea solution from the processing section LP cracker to a preferred residual ammonia removal unit, which provides improved flexibility without increased loading of the recovery section.

[0069] Optionally in some cases, the process involves feeding the urea solution from the treatment section to a preferably used evaporation section, or to a recovery section, for example, or to a UAN production section, for example.

[0070] In yet a further embodiment, the plant comprises a flow line for the urea solution from the treatment section to the UAN production unit, which also provides improved flexibility.

[0071] In one embodiment, the plant is of the CO2 stripping type and the HP stripper uses CO2 as stripping gas. Preferably, the reaction zone is operated at a pressure of preferably 140-160 bar with an N / C ratio of 3.0-3.2 (reaction zone urea solution outlet). Preferably, the HP reactor, HP condenser and HP stripper are operated at essentially the same pressure. Preferably, gravity flow is used for fluid transport from the condenser to the reactor and from the reactor to the stripper. Preferably, the stripper tube is made of stainless steel, for example a duplex austenitic-ferritic stainless steel, such as the steels discussed herein above.

[0072] Advantageously, since only a portion of the urea formed in the reaction zone is processed in the HP stripper, a relatively larger portion of the CO2 feed is available for stripping, which can be used to reduce energy consumption in the HP stripper and / or achieve high stripping efficiency. When only a portion of the CO2 feed is used for stripping, another portion of the CO2 feed can be fed directly to the reaction zone or can be used to adjust (reduce) the N / C ratio in one or more carbamate condensers (N / C ratio measured at the carbamate solution outlet).

[0073] The stripped urea solution is, for example, fed directly to an LP decomposer, which is, for example, operated at 2-6 bar, for example at about 4 bar.

[0074] In some embodiments, the HP stripper is a thermal stripper, and the stripper comprises a stripper tube, e.g., a bimetallic stripper tube. Also for thermal stripping, the advantage of lower metal content for DEF production can be achieved with the process and plant of the present invention. For example, the inner tube of a bimetallic stripper tube, e.g., made of Zr or Ti, may be prone to corrosion leading to loss of metal into the stripped urea solution. Furthermore, if a stainless steel stripper tube is used, there may be loss of metal into the urea solution, e.g., by passive corrosion.

[0075] In an embodiment with thermal HP stripping, the recovery section typically includes a medium pressure (MP) cracker and a downstream low pressure (LP) cracker. The HP synthesis section typically includes a high pressure (HP) ejector for transporting liquid containing carbamates from the HP condenser to the vertical urea reactor. The HP ejector typically uses feed NH3 as the motive fluid. The reactor is operated, for example, with an N / C ratio of 3.2 to 3.6 at the reactor outlet.

[0076] The plant comprises an expansion device for expanding the second stream to give an expanded second stream, e.g., to reduce the pressure of the second stream from high pressure to intermediate pressure. The expansion device is, for example, an expansion valve.

[0077] The processing section includes one or more processing section crackers for purifying the expanded second stream, which comprises a urea solution, into a purified urea solution. The processing section is for processing the unstripped urea solution. The processing section receives a portion of the urea synthesis stream, i.e., the second stream, from the reaction zone. In particular, the portion of the urea synthesis stream, i.e., the second stream, bypasses the HP stripper.

[0078] The processing section may, for example, comprise a medium pressure (MP) cracker, a low pressure (LP) cracker, or, for example, both MP and LP crackers in series.

[0079] The processing section cracker and the recovery section cracker are separate units. In particular, the processing section cracker is separate from and additional to the recovery section cracker. The processing section cracker and the recovery section cracker are arranged in parallel. Thereby, mixing of the urea solutions from the first and second streams can be avoided.

[0080] The one or more crackers have an inlet for the urea solution, an outlet for the urea solution, and an outlet for a gas stream containing NH3 and CO2. During operation, at least some of the carbamates contained in the urea solution are cracked in the crackers. The crackers are, for example, heat exchangers that use a heating fluid, for example, steam, or, for example, a condensed process stream, or, for example, a steam condensate. The crackers are typically shell-and-tube heat exchangers.

[0081] The gas outlet of the cracker is typically connected to a carbamate condenser. The carbamate condenser is typically a heat exchanger that uses a cooling fluid, for example water or a urea solution that is heated. The carbamate condenser has an outlet for the condensate and typically an outlet for the uncondensed gas. The carbamate solution from one or more carbamate condensers is recycled directly or indirectly to the reaction zone. The carbamate condenser is optionally in common with the recovery section. In particular, the gas stream from the treatment section cracker is preferably combined with the gas stream from the recovery section cracker, and the combined gas stream is subjected to condensation in a condenser, and optionally in some cases one or both of these gas streams are already partially condensed before being combined.

[0082] The processing section may comprise additional urea solution processing units operating at medium and / or low pressures, such as an adiabatic flash vessel. In an exemplary embodiment, the processing section comprises an adiabatic flash vessel and a downstream LP decomposer, optionally without an MP heat exchanger decomposer. The flash vessel has an inlet for the urea solution, an outlet for the urea solution, and an outlet for the flash vapor. The flash vessel is located downstream of an expansion device, such as an expansion valve, or the flash vessel comprises an expansion device. The flash vessel is operated at medium pressure and is used for adiabatic flashing of the urea solution from high to medium pressure.

[0083] The flash steam at medium pressure is fed to, for example, an MP carbamate condenser, which also receives, for example, fresh CO2.

[0084] Preferably, the stripped urea solution (stripped in the HP stripper) is not fed to the treatment section. For example, the urea solution at the outlet of the treatment section contains less than 5 wt.% urea from the stripped urea solution. This contributes to the low metal content and low biuret content of the urea solution obtained from the treatment section.

[0085] Optionally in some cases, steam stripping, especially with injection of steam into the urea solution, can be used to remove traces of NH3 from the urea solution in the treatment section.

[0086] In embodiments in which the processing section comprises a processing section LP decomposer, the processing section LP decomposer is separate from the LP decomposer included in the recovery section, and thus is disposed in parallel with the recovery section LP decomposer.

[0087] In particular, the processing section LP cracker is a dedicated LP cracker, which provides an important difference from prior art processes in which the urea solution from the HP stripper and from the intermediate pressure (MP) processing section is expanded and combined, and the resulting combined urea solution is fed to the LP cracker.

[0088] In one embodiment, the processing section comprises an intermediate pressure (MP) cracker and a low pressure (LP) cracker in series, optionally with an intermediate pressure (MP) contacting unit between the MP cracker and the LP cracker. Gases from the processing section MP cracker are typically condensed in an intermediate pressure (MP) carbamate condenser. This MP carbamate condenser may also optionally receive gases from the recovery section intermediate pressure (MP) cracker.

[0089] A portion of the CO2 feed can be fed to the MP carbamate condenser to adjust the N / C ratio therein. Optionally, in some cases, the portion of the CO2 feed is first contacted with the urea solution in the treatment section, for example in an MP contact unit arranged in the flow line for the urea solution between the MP decomposer and the LP decomposer, to adjust the N / C ratio of the urea solution. The MP contact unit has an outlet for a gas stream connected to the inlet of the MP carbamate condenser. The MP contact unit is used, for example, for MP stripping of the urea solution, more preferably for MP adiabatic stripping, and is preferably a medium pressure (MP) adiabatic stripper. The MP stripping involves a countercurrent flow of a gas stream in direct contact with the urea solution. The MP adiabatic stripping can contribute to sufficient condensation in the low pressure (LP) carbamate condenser connected to the low pressure (LP) decomposer that receives the urea solution from the MP adiabatic stripping.

[0090] The treatment section optionally may comprise a residual ammonia removal unit, typically downstream of the LP cracker. The residual ammonia removal unit may be, for example, a steam stripper, or a stripper using, for example, an inert gas such as N2, or a unit operating at a lower pressure than the LP cracker, for example, a unit operating at sub-atmospheric pressure, i.e., less than 1.0 bar absolute. The residual ammonia removal unit is typically operated at LP (1-10 bar) or less than 1.0 bar pressure.

[0091] The residual ammonia removal unit is, for example, a steam stripper for treating the urea solution from the processing section cracker, typically a low pressure (LP) urea solution from a low pressure (LP) processing section cracker. Any steam stripper is, for example, operated at a pressure less than 1.0 absolute bar. The steam stripper is, for example, a vessel having an inlet for steam and configured to inject steam into the urea solution. The steam stripper can be used to control and reduce the amount of NH3 in the DEF precursor, and other units and methods can also be used to control and reduce the amount of NH3 in the DEF precursor. The urea solution at the liquid outlet of the steam stripper meets, for example, DEF specifications, for example, NH3 is max. 0.2 wt.%, and for example, alkalinity as NH3 is max. 0.2 wt.% in the urea solution at the liquid outlet of the steam stripper. Other purification devices can also be used, and in some embodiments, the urea solution obtained from the processing section cracker already meets the DEF specifications.

[0092] The plant preferably further comprises a DEF production unit downstream of the treatment section, the DEF production being a unit and preferred part of the plant, which is preferably also used in the urea production process.

[0093] The DEF production unit is, for example, a dilution unit configured to add water to the purified urea solution obtained from the treatment section in order to obtain the desired urea concentration. In particular, high purity water, for example clean process condensate, such as from the wastewater treatment section of a urea plant, is added. In principle, dilution water can additionally or alternatively be added upstream in the process, but this is generally less preferred in view of the purification steps.

[0094] In some embodiments, the DEF generation unit includes a dry flushing unit, for example, as described in US2017 / 204054A1.

[0095] The dry flushing unit comprises a liquid distributor, in particular a spray nozzle. The dry flushing is preferably carried out at a pressure of less than 15 kPa, preferably in the range of 1-10 kPa. Preferably, the urea solution subjected to the dry flushing has a urea concentration in the range of 85 wt.% to 95 wt.%, more preferably 87 wt.% to 92 wt.%. The DEF production unit may comprise an evaporator for increasing the urea concentration of the urea solution upstream of the dry flushing unit. Typically, in an embodiment in which the DEF production unit comprises a dry flushing unit, the treatment section does not comprise a residual ammonia removal unit, e.g. a steam stripper. The dry flushing preferably results in a free-flowing urea powder. Preferably, the DEF production unit comprises a packaging unit, e.g. a bagging unit, for packaging the powder. Preferably, the free-flowing urea powder is packaged with a moisture content of less than 0.2 wt.%, preferably in a water-impermeable packaging. Preferably, the plant comprises a conveying line for conveying the powder from the dry flushing unit to the packaging unit. The powder is easily transported, easily packaged, and then easily removed from its packaging and dissolved to produce DEF.

[0096] By feeding the second stream from the reaction zone to the treatment section without HP stripping, the urea solution can have a lower metal content. In particular, the processing of the second stream involves less corrosive process conditions than HP stripping. The intermediate reaction product ammonium carbamate contained in the urea synthesis solution is extremely corrosive at high temperatures, especially in the high pressure stripper. In particular, in a stripping plant, the reactor can operate at, for example, about 185°C, while the peak temperature at the top of the HP stripper is, for example, 210-220°C. Furthermore, the HP stripper is typically a shell-and-tube heat exchanger with the urea solution in the stripper tubes and steam on the shell side, and in the HP stripper, the number of tubes is high, so the amount of metal surface in contact with the ammonium carbamate containing urea solution is high.

[0097] The lower metal content of the urea synthesis solution as a source of the second urea-containing product is advantageous because these systems contain (expensive) catalysts for selective catalytic reduction of NO x It is important for use as a DEF and reductant in reduction systems. The presence of metal species in urea solutions shortens the life of selective catalytic reduction (SCR) catalysts due to metal accumulation. Reducing the metal content (i.e., metal concentration) will result in longer life of the SCR catalyst.

[0098] Biuret is an undesirable component in urea used in selective catalytic reduction (SCR) systems because exposure of the biuret component to the catalyst causes isocyanuric acid formation which shortens the life of the SCR catalyst. Thus, a lower biuret content in the supplied urea solids used in the DEF solution extends the life of the SCR catalyst.

[0099] Because the precursor solution for the production of DEF in the present invention is not exposed to the typical high temperature conditions in an HP stripper, and therefore the process avoids biuret formation and corrosion resulting from these conditions, the DEF produced in the DEF production unit will have significantly lower biuret and metals content compared to processes in which the DEF production unit receives urea solution from a recovery section.

[0100] The first urea-containing product obtained from the urea treatment section and also in the urea production process of the present invention is a final product, such as, for example, a solid urea product, or an intermediate product, such as, for example, a urea melt that can be used for melamine production, or a urea solution that can be used for UAN production. The first urea-containing product is preferably fertilizer grade urea.

[0101] The second urea-containing product, typically obtained downstream of the treatment section in a DEF production unit, is preferably diesel exhaust fluid (DEF) or a DEF precursor, DEF precursor denotes a urea product that can be converted to DEF by adding (pure) water.

[0102] DEF is NO x Catalytic selective reduction of NO x A wide range of urea solutions suitable for reduction are shown.

[0103] The high purity urea solution obtained from the treatment section is used to make DEF or DEF precursors, for example, by diluting the urea solution with water to a desired target urea concentration, for example, up to about 32.5 wt.% urea for DEF according to ISO 22241-1:2006, up to about 40 wt.% urea solution according to ISO 186111-1:2014, or for example, to prepare a NO2 solution in an industrial plant. x For reduction, dilution to approximately 50 wt.% urea solution.

[0104] The purified urea solution from the treatment section and the DEF product and / or DEF precursor product, for example from a DEF production unit, has low impurities. The same applies to the urea-containing stream suitable for use as DEF or for its preparation prepared by the process of the present invention. Preferably, the impurities are in accordance with the use envisaged as DEF or as dilution to DEF by adding water, in particular DEF, according to ISO 22241-1:2006 (i.e. 32.5 wt.% urea) and / or ISO 186111-1:2014. Generally, biuret is max. 0.3 wt.%. Generally, NH3 is max. 0.2 wt.%, for example alkalinity as NH3 is max. 0.2 wt.%. Preferably, carbonate as CO2 is max. 0.2 wt.%. Additionally, aldehydes are typically at most 5 ppm (by weight) and / or insolubles are at most 20 ppm (by weight). PO4, Ca, Fe, Al, Mg, Na, and K are typically at most 0.5 ppm each. Cu, Zn, Cr, and Ni are typically at most 0.2 ppm each (all ppm by weight). The same applies to urea-containing streams suitable for use as or preparation of DEF prepared by the process of the present invention.

[0105] For DEF precursor solutions, the impurities are preferably such that the impurity levels are obtained after dilution by adding water to the urea content of the DEF specification (e.g., by dilution by adding water to 32.5 wt.% urea). The purity level values ​​above are given at 32.5 wt.% urea. In general, it is desirable and advantageous for the purity to exceed these limits, e.g., for the DEF product or DEF precursor to have a higher purity.

[0106] The purified urea solution from the treatment section, as well as the DEF product and / or DEF precursor product, e.g., from the DEF production unit, preferably has an alkalinity as NH3 of max 0.2 wt.%, biuret max 0.3 wt.%, PO4, Ca, Fe, Al, Mg, Na, and K max 0.5 wt.ppm each, and Cu, Zn, Cr, and Ni max 0.2 wt.%, and a urea content of at least 32.5 wt.%. The same preferred levels apply to urea-containing streams suitable for use as or for the preparation of diesel exhaust fluid prepared with the process of the invention.

[0107] The purified urea solution from the treatment section, as well as the DEF product and / or DEF precursor product, e.g., from the DEF production unit, preferably has an alkalinity as NH3 of max 0.2 wt.%, biuret max 0.3 wt.%, PO4, Ca, Fe, Al, Mg, Na, and K max 0.5 wt.ppm each, and Cu, Zn, Cr, and Ni max 0.2 wt.ppm each, calculated on 32.5 wt.% urea by adding water as needed, and an actual urea content of at least 32.5 wt.%. The same preferred levels apply to streams containing urea suitable for use as or for the preparation of DEF prepared by the process of the present invention.

[0108] Advantageously, the second stream has a lower metal content relative to urea (wt.% relative to urea) than the stripped urea solution. The relatively lower temperatures preferably in the treatment section than in the HP stripper contribute to maintaining this relatively lower metal content, which is advantageous for making DEF or DEF precursors.

[0109] In some embodiments, the processing section operates entirely at temperatures below 170°C, or below 165°C, or even below 160°C.

[0110] Preferably, the treatment section results in a urea solution having at least 32.5 wt.% urea, calculated on 32.5 wt.% urea, i.e. for urea solutions with higher urea content, by adding water as necessary to reach 32.5 wt.% urea, with max 0.3 wt.% biuret and / or max 0.2 wt.% NH.

[0111] The present invention in one aspect provides a urea production process in which a urea synthesis stream is split into a first portion and a second portion. The urea production process is preferably carried out in a plant according to the present invention, and all preferences relating to the plant apply equally to the process carried out in a plant according to the present invention. The first portion is stripped in a HP stripper, recovered in a recovery section and processed into a first urea-containing product, as a preferred feature, with the preferences and details described in relation to the plant. The recovery section comprises one or more recovery section crackers, with the details discussed in relation to the plant.

[0112] In this process, the second stream is sent to a processing section without passing through a HP stripper. The processing section comprises one or more processing section crackers, which operate separately and in parallel with the one or more recovery section crackers. The processing section produces a urea-containing stream suitable for use as DEF or for preparation of DEF by dilution with water. The urea-containing stream is preferably the same as the purified urea solution discussed in connection with the plant.

[0113] The urea-containing stream is fed, for example, to a DEF production unit, where it is diluted, for example, by adding clean water, or is subjected, for example, to a dry flashing. The dry flashing product is a DEF precursor that can be converted to DEF by adding clean water. The urea-containing stream at the inlet of the dry flashing unit typically already has a sufficiently high purity that it can be converted to DEF by adding water. The details of the dry flashing are the same as those discussed in connection with the plant. The added clean water is, for example, a clean process condensate from the wastewater treatment section of the urea plant.

[0114] The processing section and the processing therein have, as preferred features, the same preferences and details as described in relation to the plant.

[0115] Preferably, the urea containing stream obtained from the treatment section comprises at least 32.5 wt.% urea, preferably 32.5 wt.% urea with max. 0.2 wt.% alkalinity as NH3, max. 0.3 wt.% biuret, max. 0.5 wt.ppm each of PO4, Ca, Fe, Al, Mg, Na and K, and max. 0.2 wt.ppm each of Cu, Zn, Cr and Ni. These impurity limits are calculated based on 32.5 wt.% urea, i.e., based on adding water to the urea solution as needed to obtain 32.5 wt.% urea.

[0116] Preferably, the recovery section comprises a recovery section LP decomposer and the treatment section comprises a separate treatment section LP decomposer. Preferably, these LP decomposers are arranged in parallel. Thus, the process particularly involves subjecting the urea solution in the treatment section to decomposition in a dedicated LP decomposer.

[0117] Preferably, in the process, neither the stripped urea solution from the HP stripper nor the urea solution derived from the stripped urea solution from the HP stripper is combined with the second stream or with a urea solution derived from said second stream. In other words, preferably, no urea solution is fed from the HP stripper or from the recovery section to the treatment section, whereby the urea solution in the treatment section is not contaminated with any impurities in the stripped urea solution from the HP stripper.

[0118] Thus, preferably the plant does not include a flow line for such transport of the urea solution. Preferably the plant does not comprise a flow line for the stripped urea solution from the HP stripper, or for the urea solution derived from the stripped urea solution from the HP stripper, to a flow line or unit in the plant receiving the second stream, or a flow line or unit receiving the urea solution derived from said second stream.

[0119] Preferably, less than 1.0 wt.%, preferably 0 wt.%, of the urea in the DEF product, DEF precursor product, and urea stream from the treatment section is derived from the HP stripper.

[0120] The invention in one aspect also provides a method for modifying an existing urea plant of stripping type, comprising a synthesis section comprising a reaction zone, a condensation zone, and a HP stripper, and a recovery section. The existing plant also comprises, for example, an evaporation section and a wastewater treatment section. The method comprises adding a flow splitter and a treatment section to the existing plant. The flow splitter is arranged to split the urea synthesis stream from the reaction zone into a first part and a second part, the first part being fed to the HP stripper. The treatment section is adapted to treat said second part. The recovery section comprises a recovery section LP cracker, and the added treatment section preferably comprises a separate treatment section LP cracker arranged in parallel to the recovery section LP cracker in the modified plant. Preferably, the method also comprises adding a steam stripper included in the treatment section downstream of the treatment section LP cracker. The steam stripper is configured for injecting steam into the purified urea solution in the treatment section. The modified plant is preferably a urea production plant according to the invention. The method also preferably involves adding a DEF production unit downstream of the treatment section, such as a unit for diluting the urea solution from the treatment section or a unit for subjecting the urea solution to dry flashing. Preferably, the method involves adding a flow line for clean process condensate from the wastewater treatment section to the DEF production unit. All details and preferences of the urea production plant of the present invention also apply to the modified plant of the present method.

[0121] 1 shows a schematic of an exemplary urea plant and process according to the invention, comprising a HP synthesis section comprising a reactor (R), an HP stripper (S), a carbamate condenser (C), and a flow splitter (FS). The urea synthesis solution (1) from the reactor (R) is split in the flow splitter (FS) in a first stream (1a) comprising urea that is fed to the HP stripper (S) and a second stream (1b) also comprising urea.

[0122] The gas stream (6) from the HP stripper (S) is fed to a carbamate condenser (C) and the liquid containing carbamates (7) from the condenser is fed to the reactor (R). NH3 feed and CO2 feed are also fed to the synthesis section. Inert gases are removed from the synthesis section (not shown).

[0123] The stripped urea solution (2) is fed to a stripping section (RS) where the urea content of the solution (2) is increased by removal of NH3, CO2 and ammonium carbamate to obtain a urea stream (3). The stripping section (RS) comprises at least one stripping section decomposer (RSD), which is, for example, a heat exchanger for heating the urea solution. The stripping section (RS) is included in a urea treatment section (UPS), which may further comprise, for example, an evaporation section and a finishing section downstream of the stripping section.

[0124] The second stream (1b) is expanded in an expansion device, e.g., a valve (V1), and fed as expanded stream (1c) to a processing section (TS), where it is processed to a high-purity urea solution (4). The high-purity urea solution (4) is used to make DEF and / or DEF precursors (5) in a DEF production unit (DEF). In practice, the plant comprises elements such as further units, flow lines, and additional and control valves. The processing section (TS) comprises at least one processing section cracker (TSD) arranged in parallel with a recovery section cracker (RSD).

[0125] FIG. 2 illustrates in schematic form an exemplary urea plant and process according to the invention with a CO2 HP stripper (S). Unless otherwise stated, references are the same as in FIG. 1. The reactor (R) comprises a downcomer connected to a liquid outlet (1) and a separate gas outlet (not shown) at the top. The stripped urea solution (2) is fed directly to a recovery section LP pressure cracker (RSLPD) contained in a LP recovery section (LPRS). Gas (8) from the LP cracker (RSLPD) is condensed in a LP carbamate condenser (LPCC) and the resulting carbamate solution (9) is fed to a HP carbamate condenser (C) which also receives an NH3 feed. The treatment section (TS) comprises a MP cracker (MPD) and a LP cracker (TSLPD) arranged in series to treat the expanded second urea stream (1c). The gas (11) from the LP cracker (TSLPD) is condensed, for example in a dedicated LP carbamate condenser (not shown) or is fed to the LP carbamate condenser (LPCC) of the LP recovery section (LPRS), as illustrated. The treatment section LP cracker (TSLPD) and the recovery section LP cracker (RSLPD) are separate units arranged in parallel. The urea treatment section may further comprise an evaporation section (EVAP) for concentrating the LP urea solution (3) from the recovery section LP cracker (RSLPD) by water evaporation to form a urea melt (12). The urea melt (12) is fed to a finishing section (FIN), for example a prilling tower or a granulator, to form a solid urea product (13).

[0126] The treatment section (TS) further comprises a residual ammonia removal unit (RAR) from which the high purity urea solution (4) is fed to a DEF generation unit (DEF), which is a dilution unit having an inlet (10) for clean water.

[0127] Advantageously, the precursor solution (4) for the production of DEF is not exposed to the typical high temperature conditions in the HP stripper (S), thus avoiding biuret formation and metal incorporation due to these conditions and the corrosion that occurs in the HP stripper (S).

[0128] In reality, the plant comprises elements such as further units, flow lines, and additional and control valves.

[0129] FIG. 3 illustrates in a schematic manner an exemplary urea plant and process according to the invention. Unless otherwise stated, references are the same as in FIG. 2. The residual ammonia removal unit (RAR) is implemented as a steam stripper (StS). The steam stripper (StS) is a vessel for injection of steam into the urea solution to be stripped. The steam stripper (StS) has a supply line (not shown) for steam and a gas outlet (not shown) connected to a condenser (C). The plant further comprises a flow line (14) for the LP urea solution from the treatment section LP cracker (TSLPD) to the evaporation section (EVAP), which can be used in case of low demand for DEF and / or high demand for solid urea product or in case of upset conditions.

[0130] Other configurations of the preferred liquid flow connections from the treatment section (TS) to the evaporation section (EVAP) are also possible, for example through a recovery section LP decomposer (RSLPD) (not shown) or by feeding the urea solution (4) to the evaporation section (EVAP) (not shown).

[0131] In summary, embodiments of the present invention provide a stripping type urea plant and urea production process adapted to make DEF or its precursors in a DEF production unit downstream of a treatment section, which receives a portion of the urea synthesis stream from a reaction zone, in particular bypassing the HP stripper.

[0132] As used herein, HP (high pressure) denotes a pressure of at least 100 bara, e.g., 120-250 bara, or e.g., 110-160 bara, MP (medium pressure) denotes a pressure of, e.g., 15-80 bara, 20-80 bara, and LP (low pressure) denotes a pressure of, e.g., 1-10 bara, e.g., 4-10 bara, these pressure ranges being for process streams, particularly solutions, and not necessarily the same for steam and heating fluids. The abbreviation "bara" means absolute bar. Pressures are absolute pressures unless otherwise indicated.

[0133] As used herein, the term "typically" denotes a feature that is frequently used, but not required.

[0134] As used herein, "carbamate" refers to ammonium carbamate.

[0135] The terms "typically," "generally," and "particularly" are used to indicate features that may be used in some embodiments, but are not required in all embodiments. Preferred features are not required.

[0136] The N / C ratio used herein for the urea solution at the reaction zone outlet reflects the composition of the so-called initial mixture before urea production, consisting only of NH3, CO2 and H2O, as the term is used in the art of urea plants, and is a molar ratio. The N / C ratio for the gas stream indicates the molar ratio of NH3 to CO2. The N / C ratio for the carbamate condenser indicates the molar ratio of NH3 to CO2 at the outlet for the carbamate solution.

Claims

1. A urea production plant for producing a first urea-containing product and a second urea-containing product, the second urea-containing product being diesel exhaust fluid (DEF) or a DEF precursor, the plant comprising: a reaction zone (R), a high pressure stripper (S) configured to operate at a pressure of at least 100 bara (10,000 kPa), a condensation zone (C) and a high pressure (HP) synthesis section configured to operate at a pressure of at least 100 bara (10,000 kPa), the high pressure stripper (S) being configured to operate at a pressure of at least 100 bara (10,000 kPa), the condensation zone (C) and a flow splitter (FS) adapted to split a urea synthesis stream (1) from the reaction zone (R) into a first stream (1a) and a second stream (1b), and a flow line for the first stream (1a) to the HP stripper (S), the plant comprising: - a urea processing section (UPS) for processing the stripped urea solution (2) from the HP stripper (S) into the first urea-containing product (3), the urea processing section (UPS) comprising a recovery section (RS) equipped with one or more recovery section crackers (RCD) for processing the stripped urea solution (2); - an expansion device (V1) for expanding said second stream (1b) to give an expanded second stream (1c), - a treatment section (TS) comprising one or more treatment section crackers (TCD) for purifying said expanded second stream (1c) into a purified urea solution (4), said one or more treatment section crackers (TCD) being arranged in parallel with said one or more recovery section crackers (RCD); - a DEF production unit (DEF) for producing DEF and / or DEF precursors (5) from said purified urea solution (4).

2. 2. The urea production plant of claim 1, wherein the treatment section comprises a treatment section low pressure (LP) cracker (TSLPD) configured to operate at a pressure of 1 to 10 bara (100 to 1000 kPa) and the recovery section (RS) comprises a recovery section low pressure (LP) cracker (RSLPD) in parallel with the treatment section LP cracker (TSLPD) configured to operate at a pressure of 1 to 10 bara (100 to 1000 kPa).

3. 3. Urea production plant according to claim 1 or 2, wherein the treatment section (TS) comprises a steam stripper (StS) for stripping the urea solution by injection of steam.

4. 2. The urea production plant according to claim 1, wherein the DEF production unit (DEF) comprises an inlet (10) for adding clean water to the purified urea solution (4).

5. 2. The urea production plant according to claim 1, wherein the DEF production unit (DEF) comprises a dry flashing unit.

6. 2. A urea production plant according to claim 1, wherein the plant does not comprise a flow line for the stripped urea solution from the HP stripper (S) or for the urea solution obtained from the stripped urea solution (2) from the HP stripper (S) to a flow line or unit in the plant receiving the second stream (1b) or receiving a urea solution derived from the second stream (1b).

7. At least one of the one or more recovery section crackers (RCD) is the recovery section LP cracker (RSLPD), the recovery section (RS) optionally optionally comprises a medium pressure (MP) cracker between the outlet for the stripped urea solution (2) of the HP stripper (S) and the recovery section LP cracker (RSLPD) configured to operate at a pressure of 15 to 80 bara (1500 to 8000 kPa), and the plant comprises 2. The urea production plant according to claim 1, further comprising a low pressure (LP) carbamate condenser (LPCC) configured to operate at a pressure of 1-10 bara (100-1000 kPa) for condensing gases (8) from a recovery section LP cracker (RSLPD), said LP carbamate condenser (LPCC) being optionally also used in some cases to condense gases (11) from a processing section LP cracker (TSLPD) comprised in said processing section (TS).

8. The HP stripper (S) is a CO ２ 2. The urea production plant of claim 1, which uses at least a portion of the feed.

9. 9. A urea production plant according to claim 8, wherein the stripper (S) is a shell-and-tube heat exchanger with a stripper tube made of duplex stainless steel.

10. 2. Urea production plant according to claim 1, wherein the urea processing section (UPS) comprises an evaporation section (EVAP) for converting a urea solution (2) into a urea melt (12) downstream of the recovery section (RS) and a liquid flow connection (14) for the urea solution from the treatment section (TS) to the evaporation section (EVAP).

11. 1. A process for producing urea, comprising: a urea synthesis stream (1) split into a first portion (1a) and a second portion (1b), said first portion (1a) being stripped in a high pressure (HP) stripper (S) configured to operate at a pressure of at least 100 bara (10,000 kPa), recovered in a recovery section (RS) comprising one or more recovery section crackers (RCD) and processed into a first urea-containing product (3), said second stream (1b) being sent to a treatment section (TS) comprising one or more treatment section crackers (TCD) without passing through said HP stripper, said one or more treatment section crackers (TCD) operating separately from and in parallel with said one or more recovery section crackers (RCD), said treatment section (TS) producing a stream (4) comprising urea suitable for use as diesel exhaust fluid (DEF) or for preparing DEF by dilution with water.

12. 12. The urea production process according to claim 11, wherein no urea solution is fed from the HP stripper (S) to the treatment section (TS).

13. 13. Urea production process according to claim 11 or 12, wherein no urea solution is fed from the recovery section (RS) to the treatment section (TS).

14. The urea-containing stream (4) at 32.5 wt. % urea contains up to 0.2 wt. % NH ３ Alkalinity as PO, Biuret max 0.3 wt.%, PO ４ 12. The process of claim 11, having a maximum of 0.5 ppm by weight each of Ca, Fe, Al, Mg, Na, and K, and a maximum of 0.2 ppm by weight each of Cu, Zn, Cr, and Ni.

15. 15. The process of any one of claims 11, 12 and 14, wherein the stripping section (RS) comprises a stripping section low pressure decomposer (RSLPD) configured to operate at a pressure of 1 to 10 bara (100 to 1000 kPa) and the treatment section (TS) comprises a treatment section low pressure decomposer (TSLPD) configured to operate at a pressure of 1 to 10 bara (100 to 1000 kPa) arranged in parallel with the stripping section low pressure decomposer (RSLPD) configured to operate at a pressure of 1 to 10 bara (100 to 1000 kPa).

16. A urea production plant for producing a first urea-containing product and a second urea-containing product, the second urea-containing product being diesel exhaust fluid (DEF) or a DEF precursor, the plant comprising: a reaction zone (R), a high pressure stripper (S) configured to operate at a pressure of at least 100 bara (10,000 kPa), a condensation zone (C) and a high pressure (HP) synthesis section configured to operate at a pressure of at least 100 bara (10,000 kPa), the high pressure stripper (S) configured to operate at a pressure of at least 100 bara (10,000 kPa), the high pressure stripper (S) configured to operate at a pressure of at least 100 bara (10,000 kPa), the condensation zone (C) and a flow splitter (FS) adapted to split a urea synthesis stream (1) from the reaction zone (R) into a first stream (1a) and a second stream (1b), and a flow line for the first stream (1a) to the HP stripper (S), the plant comprising: - a urea processing section (UPS) for processing the stripped urea solution (2) from the HP stripper (S) into the first urea-containing product (3), the urea processing section (UPS) comprising a recovery section (RS) equipped with one or more recovery section crackers (RCD) for processing the stripped urea solution (2); - an expansion device (V1) for expanding said second stream (1b) to give an expanded second stream (1c), - a treatment section (TS) comprising one or more treatment section crackers (TCD) for purifying said expanded second stream (1c) into a purified urea solution (4), said one or more treatment section crackers (TCD) being arranged in parallel with said one or more recovery section crackers (RCD); - a DEF production unit (DEF) for producing DEF and / or DEF precursors (5) from said purified urea solution (4), The process of claim 11 , wherein the process is carried out in

17. 1. A method of retrofitting an existing urea plant of the stripping type comprising a high pressure (HP) synthesis section comprising a reaction zone, a condensation zone, and a HP stripper configured to operate at a pressure of at least 100 bara (10,000 kPa), and a stripping section, the stripping section comprising a low pressure cracker configured to operate at a pressure of 1 to 10 bara (100 to 1000 kPa); The method further comprises: - a flow splitter (FS) for splitting a urea synthesis stream from said reaction zone into a first portion (1 a) and a second portion (1 b), said first portion (1 a) being fed to said HP stripper; - a processing section (TS) for processing said second portion (1b), The process of claim 1, wherein the processing section (TS) comprises a processing section low pressure (LP) cracker (TSLPD) configured to operate at a pressure of 1 to 10 bara (100 to 1000 kPa) in parallel with the recovery section low pressure (LP) cracker (RSLPD) configured to operate at a pressure of 1 to 10 bara (100 to 1000 kPa).

18. 18. The method of claim 17, wherein the added processing section (TS) comprises a steam stripper (StS).

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