Urea production with MP carbamate condensation heat recovery

Abstract

The disclosure pertains to a stripping-type urea production process wherein urea synthesis solution is treated at MP to give MP gas that is condensed in an MP carbamate condensation chamber with heat recovery, in a preferred embodiment the heat recovery is to the HP stripper.

Description

P138160PC00Title: UREA PRODUCTION WITH MP CARBAMATE CONDENSATION HEAT RECOVERYField

[0001] The invention pertains to the production of urea from CO₂ and NH₃.Introduction

[0002] Ullmann’s Encyclopedia of Industrial Chemistry, chapter Urea, 2010, provides a general discussion of plants and processes for producing urea, including the stripping type process. Herein, the heat supplied is effectively used twice (N=2). In particular, heat imported into the HP (high pressure) stripper is recovered in the HP carbamate condenser and used to raise LP (low pressure) steam, which is used, e.g., in heaters.

[0003] The HP stripper is a vertical shell-and-tube heat exchanger with a falling film of urea solution in the tubes and steam in the shell. The HP carbamate condenser is a shell-and-tube heat exchanger with a condensation compartment operated at HP and a compartment receiving boiler feed water that is used for raising LP steam (typically 4.5 bar steam).

[0004] EP4331717A1 describes a urea production process, wherein the HP stripper receives steam at a pressure of at least 15 bar (absolute), called high-pressure steam in said publication. The publication mentions that high-pressure steam is conventionally produced by boilers, which burn fossil fuels, such as natural gas or oil, and thus produce CO2, and that high-pressure steam can also be obtained from a steam turbine as steam extraction, if the plant comprises a steam turbine, for example, to drive a CO2 compressor. However, this requires producing steam at an even higher pressure, thereby increasing CO2 emissions from the boilers, according to the document. In Fig. 1 of said document, the HP stripper receives steam from a steam network. In one part of the steam network, steam is received from a compressor that compresses the LP steam raised in the HP carbamate condenser. A second part of the steam network is also illustrated.

[0005] It is desired to improve said process, particularly in terms of energy efficiency.Summary

[0006] The invention aims to provide an energy-efficient urea production process, in particular (but not exclusively) for processes wherein a part or all of the urea product (e.g. at least 10% of the produced urea) is a liquid aqueous urea product with a molar ratio of water to urea of at least 1.5 or at least 2.

[0007] The invention also pertains, in embodiments, to a urea production process that advantageously uses electricity, e.g., from renewable sources and / or from sources other than the combustion of fossil fuels. As a result, the CO₂ emissions of the urea production process can be relatively low.

[0008] The invention pertains in a first aspect to a process for the production of urea from ammonia and carbon dioxide in a urea plant (urea production plant), wherein the urea plant comprises a high-pressure (HP) synthesis section comprising an HP reaction zone, an HP stripper which is a first heat exchanger that is a shell- and-tube heat exchanger configured to operate with a falling-film of urea solution in the tube bundle and having an inlet for steam in the shell, and an HP carbamate condenser. The plant further comprises an MP treatment section (MP recovery section) comprising an MP treatment unit and an MP carbamate condensation chamber. The process comprises: condensing gas from said HP stripper in said HP carbamate condenser to provide an HP carbamate-containing liquid stream; treating carbamate-containing urea solution from the HP synthesis section by carbamate decomposition in said MP treatment unit to give a treated MP urea solution and an MP gas stream, condensing said MP gas stream in said MP carbamate condensation chamber to give an MP carbamate solution, and raising steam (“first steam”) in a steam boiler, wherein the MP carbamate condensation chamber and said steam boiler are provided as two heat-exchanging compartments of a second heat exchanger. The process preferably comprises: compressing a part or all of said first steam (or: compressing said first steam) in one or more steam compressors, preferably in one or more stages, and supplying the compressed steam to said inlet for steam of said shell of said HP stripper.

[0009] The invention also provides a urea production plant comprising a high-pressure (HP) synthesis section. The HP synthesis section comprises an HP reaction zone, an HP stripper and an HP carbamate condenser, wherein the HP stripper is a first heat exchanger that is a shell-and-tube heat exchangerconfigured to operate with a falling film of urea solution in the tube bundle and has an inlet for steam in the shell. The plant further comprises: an MP treatment unit that is connected to receive carbamate-containing urea solution from the HP synthesis section and configured to treat said solution by carbamate decomposition to give a treated MP urea solution and an MP gas stream, an MP carbamate condensation chamber that is connected by a gas flow line to receive a part or all of said MP gas stream and is configured to subject said gas to carbamate condensation to give an MP carbamate solution, and a steam boiler configured to raise steam from boiler feed water (i.e, raise “first steam”), wherein the MP carbamate condensation chamber and said steam boiler are provided as two heat¬ exchanging compartments of a second heat exchanger. Preferably, the plant comprises one or more steam compressors connected to receive first steam from the steam boiler, and a steam connection to supply compressed steam from the one or more steam compressors to said inlet for steam of said shell of said HP stripper. The plant is preferably suitable for the process; the process is preferably carried out in the inventive plant.

[0010] Also provided is a method of modifying an existing urea production plant.Brief description of the drawings

[0011] Figure 1 schematically illustrates a process scheme of an example urea production plant and process according to the invention.Figure 2 schematically illustrates an example heat exchanger that can be used in an inventive urea production plant and process.Figure 3 schematically shows an example steam compression section used in an example process and plant of the invention.Figure 4 schematically shows an example embodiment with a steam stripper.

[0012] Any embodiments illustrated in the figures are examples only and do not limit the invention.Detailed description

[0013] In the inventive process, the urea solution from the synthesis section, still containing carbamate, is subjected to heating at MP to decompose carbamate to form MP gas and a treated, i.e. purified, MP urea solution. This heating isconducted using process-process heat exchange with the condensing gas mixture in the HP carbamate condenser. Furthermore, the MP gas is condensed in heat exchange with a steam boiler. Thereby, the heat of a part of the steam supplied to the shell of the HP stripper is used three times for evaporation, namely for the HP stripping of HP urea solution, for the MP carbamate decomposition from MP urea solution, and for raising (first) steam from boiler feed water in the steam boiler.

[0014] Ullmann’s Encyclopedia of Industrial Chemistry, chapter Urea, 2010, Fig. 14 shows the conceptual heat balance for a typical stripping plant, with N=2 heat integration. Heat supplied to the HP stripper is recovered in the HP carbamate condenser and used for heating in LP heaters (decomposers and water evaporators). In practice, a part of the steam from the HP carbamate condenser can also be used (as live steam) for injection into process condensate inside a desorber of a process condensate treatment section and in a vacuum ejector of an evaporation section.

[0015] Fig. 15 of said chapter shows a conceptual diagram of N=3 heat integration of a urea plant; however, therein, the heat is used in three heaters for the urea solution.

[0016] In Fig. 25 of said chapter, a process scheme of a so-called Snamprogetti self-stripping type urea production process is shown, wherein urea solution from the HP stripper is heated in an MP decomposer (usually with steam or hot steam condensate) and the resulting MP gas is condensed in heat exchange with a preheater for urea solution. Hence, in both figures, an MP carbamate condensation chamber is a part of a process-process heat exchanger.

[0017] US 2015 / 0119603A1 illustrates a process scheme wherein heat is used three times, namely for stripping of urea solution, carbamate decomposition in MP urea solution by heating, and heating of urea solution under vacuum conditions in an evaporator that forms a compartment of a vertical shell- and-tube heat exchanger used as an MP condenser / evaporator (condensation in the shell, evaporation in the tubes).

[0018] In a first embodiment, the steam from the steam boiler can be used in one or more steam-consuming units of the urea plant or be exported, or a combination thereof, with preferably compression of a part or all of the steam to ahigher pressure using a steam compressor, wherein the steam compressor is preferably an electric steam compressor.

[0019] The present invention provides for improved energy efficiency over EP4331717A1, in particular in terms of a lower duty of the HP stripper and corresponding lower steam consumption of the HP stripper, which is combined with treating urea solution from the synthesis section, e.g. stripped urea solution, further in an MP treatment unit to effect carbamate decomposition, in particular adiabatically or by heating by heat integration. It is observed that the MP treatment judiciously cooperates with the HP stripper, especially a HP stripper that is operated with a lower duty and corresponding lower stripping efficiency alpha. Generally, the invention provides for a process with an advantageously low steam consumption of the HP stripper; and the MP treatment contributes to said advantageous relatively low steam consumption of the HP stripper. In the preferred embodiment with the supply of compressed steam from the second heat exchanger to the shell of the HP stripper, the steam consumption of the HP stripper, in terms of ‘external steam’, is reduced, possibly to nil, e.g. steam extracted from the CO₂ compressor steam turbine (if used in the plant) or from raised in a utility boiler.

[0020] In the invention, the HP stripper is a first heat exchanger, and a second heat exchanger provides the MP carbamate condensation chamber and the steam boiler (as two heat-exchanging compartments of the second heat exchanger).

[0021] In a preferred embodiment, the MP treatment unit comprises an MP heater, and the plant comprises a third heat exchanger comprising the HP carbamate condenser and MP heater as the two heat-exchanging compartments. In such an inventive plant, an N=3 heat integration concept is used.

[0022] In a further embodiment, the MP treatment unit comprises an MP flash vessel for expanding the urea solution from HP to MP, forming a biphasic (gas / liquid) mixture via carbamate decomposition (decomposition of carbamate contained in the urea solution from the synthesis section), and a gas-liquid separator. Preferably, an adiabatic flash vessel is used. In an embodiment with the MP heater, the MP treatment unit comprises an adiabatic flash vessel and the MP heater in series. A background reference is the paper “Experiences with Launch melt flash design”, Stamicarbon Symposium 2022, wherein it is said that the adiabatic flashing is combined with heat integration between the MP carbamatecondensation of the gas from the flashing and a pre-evaporation section of the plant. This heat integration reduces steam consumption and permits reducing the stripping efficiency from 78% in a standard design to 68% (i.e. permits reducing the steam supply and the steam pressure to the HP stripper shell) and enables achieving 80 wt.% urea solution from the pre-evaporator.

[0023] Especially in embodiments wherein the HP stripper is a CO2 stripper, such a preferred MP flash is useful. Especially in embodiments wherein the HP stripper is a CO2 stripper, the preferably used third heat exchanger comprising the MP heater and the HP carbamate condenser heat exchanger is useful.

[0024] It may be observed that the (one or more) steam compressors in the inventive plant and process do not need to supply the heat of vaporisation of the steam.

[0025] Due to the excellent energy efficiency, the HP stripper can be operated with a relatively low steam supply in the inventive plant. For example, the HP stripper is operated with a stripping efficiency in the range of 0.60 — 0.73, or in the range of 0.62 - 0.70, preferably 0.62 - 0.68.

[0026] The HP stripper preferably is operated with steam on the shell side, the steam having a pressure of 12 to 16 bar.

[0027] The relatively low stripping efficiency is also beneficial when making a product with low biuret content. A background reference in this respect isWO 2023 / 158314A1. However, the product from evaporation during heat exchange with the condensing MP gas stream is a urea melt containing 94 wt.% urea, which is used for urea finishing.

[0028] Compared to e.g. US 2015 / 0119603A1, the third stage heat integration (recovery of heat released by the condensing MP gas) is achieved in a different way, namely through a steam boiler and steam compressor, thereby in particular departing from process-process heat integration (heat integration between two process streams) that is used in the art for heat integration of the MP carbamate condensation chamber. The steam compressor compresses the steam to the pressure necessary to ensure the steam condenses at a sufficiently high temperature to effect carbamate decomposition in the urea solution in the HP stripper tube bundle, e.g., to a pressure of at least 12 bar, typically in the range 12 - 20 bar. /

[0029] The invention is especially useful in plants and processes in which a part or all of the urea in the treated MP urea solution is used to make a liquid urea product. In such processes, a part or all of the MP urea solution does not, after heating in the LP section, need to be heated further at a pressure lower than the operating pressure of the LP dissociator. Particularly, in such processes, a part (or all) of the urea solution originating from the MP treatment section (MP treatment unit) is not subjected to a step of heating at sub-atmospheric pressure. For example, if only a small first part of the urea solution is to be subjected to pre¬ evaporation (because a second part is used for making liquid urea product), the amount of the heat of condensation available from the MP gas, is larger than the amount of heat that can be taken up in that pre-evaporator, resulting in noncondensed MP gas.

[0030] The invention is also especially useful in plants and processes wherein the urea solution at the inlet of the MP treatment section (treatment unit) has a relatively high carbamate content, resulting in a relatively large amount of gas to be condensed in the MPCC. Again, even if a pre-evaporator is used for a part or all of the urea solution, the amount of gas to be condensed at MP may be too large for heat recovery in the pre-evaporator (PEV) only.

[0031] The invention is also especially useful in plants and processes wherein a liquid urea product is obtained (as the sole urea product or in combination with other urea products, e.g. in combination with a more concentrated liquid urea product and / or a solid urea product), wherein the liquid urea product is e.g. liquid stream comprising urea and water in a molar ratio of water to urea of at least 1.5. For a mixture consisting of water and urea (including biuret), this corresponds to max. 70 wt.% urea. In particular, for achieving urea concentrations above 70 wt.% starting from the LP urea solution from an LP recovery section (e.g. at 4.0 bar), often evaporation or pre-evaporation is necessary (i.e. heating the urea solution under vacuum); conversely, no pre-evaporation is necessary for urea products having a lower urea-to-water ratio. Preferably, the liquid urea product has a water-to-urea molar ratio of at least 1.5, or at least 2, or at least 3, or at least 4, or at least 5, or at least 6. A molar ratio of water to urea of at least 6 corresponds to a water-urea mixture with up to 36 wt.% urea. Preferably, this liquid urea product contains at least 10% or at least 20% of the urea production from the synthesis section. Preferably, the liquid urea product, having such a water-to-urea molarratio, originates from the synthesis section, and the amount of the liquid urea product is at least 10 mol.% of the treated MP urea solution, on the basis of moles urea, or at least 20 mol.% or at least 50 mol.%. In such embodiments, the need for pre-evaporation, if any, is small relative to the amount of urea solution treated in the MP treatment unit and, hence, relative to the amount of MP gas from the MP treatment unit.

[0032] In embodiments of such a plant, there is no or limited need for water evaporation from the urea solution by heating downstream of the LP decomposer. For example, the plant may produce a high-purity aqueous urea solution, e.g. for use as DEF, and / or UAN.

[0033] DEF stands for Diesel Exhaust Fluid and is, in a typical embodiment, a 32.5 wt.% aqueous urea solution; other urea concentrations are also possible for DEF, e.g. up to 50 wt.% urea. DEF and other types of high-purity aqueous urea solutions are used for NOx abatement (reduction) in flue gas from (carbon and hydrocarbon) combustion, e.g. from engines, for vehicles, ships, and stationary burners. For instance, for marine applications, a urea content of 40% wt. ± 1% can be used.

[0034] UAN is urea ammonium nitrate and is an aqueous solution of urea and ammonium nitrate that is used as a liquid fertiliser.

[0035] For instance, in the paper “Experiences with Launch melt flash design”, Stamicarbon Symposium 2022, the energy consumption of the technology (plant design) presented in that paper is compared to a standard plant wherein an evaporator with steam heating is used to obtain a concentrated 80 wt.% urea solution. The high level of heat integration described in the concept of that paper reduces flexibility if a less concentrated urea solution is desired as a product.

[0036] Correspondingly, the invention provides for a new scope of application of N=3 heat integration in urea plants, namely, especially for plants where a liquid urea product is produced as the sole product or in combination with a solid urea product.

[0037] Generally, the invention is useful in plants wherein no pre -evaporator is used or wherein the urea solution flow rate (mol / hr) in the pre-evaporator PEV (mol water / hr to be evaporated) is relatively small compared to the amount of gas (CO₂ and NH₃) to be condensed at MP and hence also relatively small compared tothe flow rate of carbamate formed at MP, i.e. from the MP carbamate condensation chamber (mol / hr carbamate). It is observed that the maximum urea concentration (lowest water concentration) in a pre-evaporator is specified by the condensation temperature of the MP gas and the typical temperature (10°C) difference over the heat exchanging wall between the PEV and MP carbamate condensation chamber as heat exchanging compartments, in embodiments wherein a pre-evaporator is used.

[0038] In the invention, the (first) steam is raised in said steam boiler, in particular at a pressure in the range of 0.5 to 3.5 bar absolute, for example, in the range of 1.0 to 2.5 bar absolute (also referred to as LLP steam). The heat exchange with the condensing MP gas stream allows raising steam (by boiling of water) at such a pressure (and corresponding temperature). This is a different pressure level than the LP steam typically raised in urea plants in the art (3 - 5 bar).

[0039] The invention provides, in an aspect, the insight to use the heat released in the MP carbamate condensation chamber for heat integration with the steam side of the HP stripper by virtue of the steam compressor. For example, the steam at 0.5 - 3.5 bara from the steam boiler is compressed to a pressure of 12 to 16 bar.

[0040] In a further aspect of the invention, the steam from the steam boiler is used, preferably without compression, e.g. in a steam stripper. In a preferred embodiment, a first part of the steam boiler is compressed and supplied to the shell of the HP stripper, and a second part of the steam from the steam boiler is used in another unit, preferably without compression, e.g. in a steam stripper. Hence, the two uses of the steam from the steam boiler (compression to the shell of the HP stripper, and use in a steam stripper) can be applied independently, and also in combination with each other.

[0041] Preferably, the third heat exchanger comprises a third compartment that is used for raising a second steam, wherein the second steam has a higher pressure than the (first) steam raised in the steam boiler that is comprised in the second heat exchanger, e.g. at least 0.50 bar higher, e.g. 0.5 — 5 higher, and e.g. at a pressure in the range of 3.0 to 6.0 bar. For example, the pool condenser contains a second tube bundle used for raising the second steam; for example, a pool condenser or pool reactor with two U-shaped tube bundles is used.

[0042] In particular, the HP carbamate condenser may be a shell- and-tube heat exchanger with two tube bundles, with the shell used for condensation of gas from the stripper, the first tube bundle used as the MP heater, and the second tube bundle used as a second steam boiler used for raising second steam at a pressure of e.g. 3.0 - 5.0 bar absolute. For instance, a horizontal pool condenser with two U-shaped tube bundles as described in US 2015 / 0119603A1 can be used. It may be observed that the vessel of such a pool condenser may also provide the HP reaction zone; this is known as a pool reactor in the art (see e.g. US 5,767,313 for a pool reactor with a single tube bundle).

[0043] The MP treated urea solution is typically expanded to an LP decomposer that comprises a heater that is a heat exchanger using the second steam (e.g. 4.5 bar steam) for heating, with gas supplied to an LP carbamate condenser LPCC and with the LP treated urea solution supplied e.g. to an atmospheric flash vessel. The LP carbamate condenser is typically a heat exchanger, typically for heat exchange with cooling water.

[0044] The LP carbamate solution formed in the LPCC is usually supplied to the MP carbamate condensation chamber. The LPCC also receives an aqueous liquid stream containing water, which is used as a solvent for the carbamate formed in the LPCC.

[0045] The disclosure hence pertains to a stripping-type urea production process and plant wherein urea synthesis solution is treated at MP to give MP gas that is condensed in an MP carbamate condensation chamber with heat recovery, in particular, heat recovery to the HP stripper shell side. In particular, the MP condensation is carried out in heat exchange with a steam boiler. The resulting steam is compressed and is supplied as heating fluid to the HP stripper shell. The MP carbamate condensation yields an MP carbamate stream that is supplied to the HP synthesis section, in particular to the HP carbamate condenser.

[0046] The one or more steam compressors are, for example, electrical compressors. This provides the important advantage that a portion of the energy for the urea production process can be supplied as electricity, e.g., from wind energy or solar energy, or from other electricity-generating processes that do not use combustion of hydrocarbons and / or coal. Thereby, the electricity source can have low or zero CO2 emissions. It is highly advantageous to use such energysources to drive the urea production process at least in part, and to introduce energy from electricity into the process via a steam compressor.

[0047] Preferably, first steam is compressed in at least two stages using at least a first and a second steam compressor in series. In other words, highly preferably, a multi-stage compressor is used, with two or more stages in series.

[0048] In a preferred embodiment, a condensate stream is added to the steam at an interstage section between two compressor stages, for de-superheating of the steam. The added condensate stream (liquid water stream) evaporates in the interstage section, thereby increasing the amount of HP steam produced.Furthermore, the de-superheating may help to avoid excessively high temperatures at the downstream compressor stage. For example, after the de-superheating, the steam at the inlet of the downstream compressor is superheated by about 5°C (i.e., has a temperature about 5°C above the saturation temperature).

[0049] Optionally, a part of the second steam (raised in the heat exchanger of the HP carbamate condenser) is also compressed, in particular by a second steam compressor. The compressed steam is supplied to the shell (shell side compartment) of the HP stripper.

[0050] Advantageously, in this embodiment wherein a part of the second steam is compressed and the resulting second steam is (also) supplied to the shell of the HP stripper, simulations show that it is possible to run the inventive urea production process without a need to import high pressure steam from battery limit and without a need for an auxiliary steam boiler (e.g. without a need for a steam boiler using fuel), for example in embodiments wherein all urea is used for making DEF. Hence, simulations show that it is advantageously possible that all the heat of vaporisation of the steam used in the HP stripper is provided by heat integration in the inventive urea production process.

[0051] Preferably, in this embodiment, multi-stage compression is used to compress the first steam, and the plant comprises a compressor used to compress the second steam and the steam from a first-stage compressor of the first steam.

[0052] Typically, the treated urea solution from the MP heater is further treated in at least an LP dissociator (heater) to remove further carbamate by decomposition. Urea solution from the LP dissociator may be further flashed to atmospheric pressure.

[0053] In a preferred embodiment, a part or all of the urea solution from the MP heater, for example from the LP dissociator or from the atmospheric flash, is used for making DEF and is supplied to a steam stripper (DEF steam stripper) that is used for reducing the NH3 level of the urea solution, preferably to a desired low level that is suitable for making DEF. In a preferred embodiment of the steam stripper, steam is injected into the urea solution (i.e., live stream). Hence, the steam stripper differs from the HP stripper in that the steam mixes with the urea solution in the steam stripper. In the present invention, preferably, the first steam from the second heat exchanger (MP carbamate condensation chamber / steam boiler) is used, in part or entirely, in the steam stripper.

[0054] In an embodiment, a part or all of the urea solution originating from the MP treatment unit, preferably originating from the MP heater, for example, urea solution from the LP dissociator or from the atmospheric flash, is supplied to a steam stripper. In the steam stripper, steam is injected into the urea solution (i.e., live stream). Preferably, a first part of the first steam from the steam boiler (that is comprised in the second heat exchanger) is compressed and supplied to the HP stripper, and a second part is injected into the urea solution in the steam stripper. Preferably, the second part of the steam is not compressed. In this embodiment, the plant comprises a liquid flow line from the MP treatment unit to the steam stripper, for supply of urea solution from the MP treatment unit to the steam stripper, preferably through the LP dissociator and preferably also the atmospheric flash unit. In a preferred embodiment, the plant comprises, in series for urea solution, the MP treatment unit, the LP dissociator, the atmospheric flash unit, and the steam stripper.

[0055] The steam stripper is based on direct injection of steam, preferably in counter-current contact with the urea solution, in a vessel; the vessel is preferably kept at a pressure of less than 10 bar, or less than 5 bar, less than 3.0 bar, less than 2.0 bar, or less than 1.5 bar (absolute), or at 0.10-1.1 bar (absolute), e.g. at less than 1.0 bar absolute, for example at 0.010 to 0.50 bar, or at 0.4 to 0.5 bar (absolute). The preferred low operating (absolute) pressure of the purification section advantageously enables purification by ammonia evaporation at lower temperatures, thereby reducing biuret formation. The steam stripper preferably operates at a lower pressure than the steam boiler. This enables the flow of steam from the steam boiler to the steam stripper.

[0056] The steam stripper is preferably a vessel with a urea solution inlet at an upper side, a steam inlet at a lower side, a gas outlet at the top, and a liquid outlet for steam -stripped urea solution at the bottom. The gas stream from the gas outlet contains the NH₃ stripped from the urea solution, and water vapor. The urea solution is e.g. heated before entering the steam stripper.

[0057] It was found that advantageously by using a first steam, that is raised in the steam boiler in the second heat exchanger by water boiling at a pressure of 1.0 - 2.0 bar (i.e. a part or all of the first steam), in particular by using the resulting saturated steam having the corresponding temperature, for the steam stripping by direct injection of the first steam, a sufficient purification of the urea solution can be obtained in the steam stripper, in particular to achieve a desirable low NH₃ level to make DEF, without excessive water content, and with optimum, i.e. neither too high nor too low, grade of heat (of the live steam), and hence with the efficient use of heat by process-process heat integration between the MP carbamate condensation and the steam stripping through the steam boiler. This use of first steam for the steam stripping is preferably combined with compressing a part of the first steam and supplying the resulting compressed steam to the shell of the HP stripper, but may also be used independently thereof.

[0058] Also disclosed is an embodiment of the process wherein the first steam (from the steam boiler) is A) in part or entirely compressed and supplied to the shell of the HP stripper and / or B) is used for the steam stripping of urea solution originating from the MP treatment unit, and an embodiment of the plant that is configured for A and / or B; with preferences and details for A and B as described hereinabove and hereinbelow.

[0059] By comparison, US 2018 / 0071653A1, Figures 3 and 5, shows DEF production using a steam stripper using direct injection of “LP” steam and steam condensate. The stripper is a vessel configured for counter-current contacting of urea solution and steam.

[0060] US 2019 / 0185422A1 mentions the use of steam stripping by direct injection of steam for producing DEF, and mentions that the pressure of the injected steam is 1 to 30 bar (absolute), preferably 2 to 15, typically 2 to 6 bar (absolute).

[0061] Optionally, a part of the treated MP urea solution is, after further purification at LP by an LP carbamate decomposer, used to produce UAN by mixing with a liquid stream containing ammonium nitrate. The liquid stream may be an aqueous stream or could be AN melt. The concentration of AN in the liquid stream, and correspondingly the water content of the liquid stream, can be adjusted to achieve a desired water concentration of the UAN liquid product. For instance, an initial AN aqueous solution may be concentrated to form a concentrated AN solution, which is then mixed with the treated urea solution.

[0062] The invention provides a process for producing urea from ammonia and carbon dioxide in a urea production plant. The urea plant comprises a high-pressure (HP) synthesis section. In the process, NH₃ and CO₂ are reacted in the HP synthesis section to form carbamate (in the HP carbamate condenser), and the resulting carbamate is further reacted to water and urea (in the reaction zone).

[0063] The HP synthesis section comprises an HP reaction zone, an HP stripper, and an HP carbamate condenser (HPCC). The synthesis section has an inlet for CO2 feed, e.g. to the HP stripper, an inlet for NH₃ feed, e.g. to the HP carbamate condenser, and an outlet for urea synthesis solution, e.g. an outlet for stripped urea solution from the HP stripper. The synthesis section furthermore has an outlet for an off-gas stream.

[0064] The HP stripper is a first heat exchanger, and is a shell-and-tube heat exchanger that is configured to operate with a falling film of urea solution in the vertical tube bundle and with steam as heating fluid in the shell, i.e. in the shell side space. The HP stripper has an inlet for steam in the shell. The HP stripper preferably is a CO2 stripper and preferably has an inlet for CO2 used as strip gas, the strip gas is supplied to the tubes. The HP stripper has an upper chamber with an inlet for the urea solution and a gas outlet connected to an inlet of the HP carbamate condenser. The upper chamber receives the upper tube ends of the tube bundle, such that in operation, urea solution flows from the upper chamber into the tube bundle. Typically, each tube is provided with a liquid divider or ferrule in the upper chamber that provides for the formation of a falling film of urea solution in the tubes. The stripper further typically contains a bottom chamber where stripped urea solution from the bottom end of the tubes is collected and has a liquid outlet. The bottom chamber may also comprise means for introducing the CO2 strip gas.The HP stripper shell typically has an outlet for steam condensate, arranged at a lower elevation than the inlet for steam. The stripper shell may comprise baffles.

[0065] In principle, two or more HP strippers in parallel or in series can be used. The HP strippers may receive a part or all of the urea synthesis solution from the reaction zone. In an example embodiment, a part of the urea synthesis solution from the reaction zone bypasses the HP stripper and is supplied to an MP dissociator or to the MP treatment unit.

[0066] The HP reaction zone may be provided by one or more units or parts of units in series or in parallel. For instance, a vertical urea reactor can be used. The vertical urea reactor has an inlet at a bottom part that is connected to an outlet of the HP carbamate condenser to receive the carbamate stream, and has an outlet for withdrawing urea synthesis solution from a top part, e.g. through a downcomer. The reaction zone may also be provided by a part of a vessel, e.g. in case of a pool reactor that provides both the HP carbamate condenser and a reaction zone. A pool reactor and a vertical urea reactor may also be combined.

[0067] The HP carbamate condenser (HPCC) is a heat exchanger, typically a shell-and-tube heat exchanger, where gas from the stripper is condensed to form a carbamate solution. The HP carbamate condenser also receives an aqueous carbamate solution from a downstream section of the plant, in particular from the MP carbamate condensation chamber. Preferably, the HP carbamate condenser is a submerged shell-and-tube heat exchanger operated with condensation of gas from the stripper in a compartment with liquid as a continuous phase.

[0068] For example, the HP carbamate condenser is a shell-and-tube heat exchanger with condensation in the shell side compartment and with cooling liquid in a tube bundle or with at least two tube bundles for different cooling liquids. The tube bundles are e.g. U-shaped tube bundles or straight tube bundles. In an example embodiment, a horizontal shell-and-tube heat exchanger is used as the HPCC with a U-shaped tube bundle (or two of such tube bundles) with horizontal legs of the tubes and with the condensation in the shell side space. In the operation of such an HPCC, the tube bundle is submerged in the liquid phase in the shell. Preferably, the HPCC comprises a sparger in the shell side for introducing the gas from the stripper in a horizontally distributed manner. Preferably, the strip gas sparger extends horizontally below the U-shaped tube bundle. Optionally, the HPCC comprises a second sparger for NH₃ feed (optionally with MP carbamatesolution), for introducing this feed in a horizontally distributed manner in the shell side space.

[0069] The shell side space may comprise a compartment between the bend of the U-shaped tube bundle and the shell to provide for a residence time for the formed condensate to allow the urea formation liquid to occur at least partially, e.g. in case of a pool reactor.

[0070] In an embodiment, two or more HPCC units are used in series and / or in parallel.

[0071] The urea synthesis solution obtained from the reaction zone comprises urea, water, and carbamate. The carbamate is, for a part, decomposed into CO₂ and NH₃ in the HP stripper to give a stripped urea solution still comprising carbamate.

[0072] The plant further comprises an MP treatment section comprising an MP treatment unit and an MP carbamate condensation chamber (MPCC). The MP treatment unit and MP carbamate condensation chamber together provide an MP recovery section.

[0073] The MP treatment unit may be provided by one or more pieces of equipment in series or in parallel, and may also be provided by parts of equipment, for instance by chambers or compartments of heat exchangers.

[0074] The MP carbamate condensation chamber (MPCC), as used herein, refers to a compartment of a heat exchanger that has a further heat exchanging compartment: the heat exchanger may be called an MP carbamate condenser.

[0075] The MP carbamate condensation chamber is connected by a gas flow line to receive a part or all of said MP gas stream from the MP treatment unit and is configured to subject said gas to condensation to form an MP carbamate solution. The gas flow line may comprise one or more pipes, tubes, conduits, and units for transporting the gas exclusively in the gaseous phase.

[0076] In prior’ art processes, often an MP carbamate condenser is used, which unit is a heat exchanger with a condensation chamber and a cooling liquid chamber, e.g. a shell-and-tube heat exchanger with condensation in the shell (the shell being the condensation chamber) and with cooling liquid in the tubes.

[0077] The inventive plant may comprise one or more MP carbamate condensation chambers in series or in parallel.

[0078] The MP treatment section receives urea solution from the synthesis section, e.g. stripped urea solution from the HP stripper, and typically comprises an expansion valve for that urea solution; this valve delimits the HP section from the MP section. The urea solution received by the MP treatment section contains carbamate, and expanding the solution from HP to MP (reducing its pressure) causes carbamate decomposition and gas formation, resulting in biphasic flow.

[0079] In an embodiment, the MP treatment unit comprises a flash unit, preferably a flash vessel, e.g. an adiabatic flash vessel, to separate the MP gas (comprising NH₃ and CO₂) from the MP urea solution, with the gas stream supplied to the MPCC. The flash vessel typically has an inlet for urea solution, a gas outlet at a higher elevation than the inlet, and a separate liquid outlet at a lower elevation than the inlet.

[0080] The flashed urea solution has a higher wt.% urea than the urea solution received by the expansion valve. In such an embodiment, the MP treatment unit may be provided by the HP-MP expansion valve and the MP flash vessel. The gas from the flash is typically at least in part, directly or indirectly, supplied to the MP carbamate condensation chamber, such that a part or all of the NH₃ and / or CO₂ released by the flashing is condensed into carbamate in said chamber.

[0081] In a further embodiment, the MP treatment unit comprises an MP heater, typically a heat exchanger, for effecting carbamate decomposition by heating, thereby forming gas. The MP treatment unit in this embodiment can be referred to as an MP decomposer. The heater may be arranged downstream of an MP flash vessel. The MP heater, for example, uses steam as heating fluid.

[0082] Typically, downstream of the MP heater, an MP gas-liquid separation vessel is provided for the separation of gas and liquid, to separate the MP gas formed in the heater from the urea solution. The MP flash gas is supplied to the MP carbamate condensation chamber.

[0083] Preferably, the MP heater is provided as a compartment of a heat exchanger, which heat exchanger also provides, in a further compartment, the HP carbamate condenser (more specifically, an HP carbamate condensation chamber or compartment). For instance, the plant comprises a shell-and-tube heat exchanger having a shell-side compartment used as the HPCC and a tube bundle used as the MP heater; the heat exchanger may comprise a further tube bundleused to raise steam (typically LP steam). Preferably, in such a case, the tube bundle is a horizontal tube bundle, e.g. a U-shaped tube bundle with horizontal legs, e.g. the heat exchanger is operated as a pool condenser or a pool reactor with MP urea solution in a tube bundle.

[0084] In this embodiment, HP CO2 stripping is conveniently used, and preferably the NH₃ feed is supplied at least in part to the HPCC. The relatively high amount of gas to be condensed in the HPCC in the case of HP CO2 stripping allows for achieving significant carbamate decomposition in the MP urea solution in the MP heater by heat exchange.

[0085] For example, the MP heater is provided by a tube bundle of a pool condenser, as shown e.g. in US 2015 / 0119603A1.

[0086] Generally, it is preferred to use the MP flash and MP heating in series, with the flash upstream of the heating. Preferably, an MP gas / liquid separation vessel is provided downstream of the MP heater. The urea solution from the separation vessel is, for instance, contacted with the gas from the flash to decrease the ammonia content of the urea solution, with the gas from the contacting unit also supplied to the MP carbamate condensation chamber, preferably for combined condensation with the gas from the MP gas / liquid separation vessel.

[0087] The process comprises condensing gas from the HP stripper, i.e. a part or all of the gas from said HP stripper, in said HP carbamate condenser to provide an HP carbamate-containing liquid stream, and supplying that liquid stream to the reaction zone. Optionally, two or more HP carbamate condensers are used in series or in parallel. Typically, all of the gas from the HP stripper is condensed in the one or more HP carbamate condensers, subject to some off-gas from the synthesis section comprising inerts and minor amounts of condensable gas. Typically, all HP carbamate-containing liquid is supplied to a reaction zone for the carbamate dehydration reaction (urea formation reaction), which produces urea and water. The transport of the HP carbamate-containing liquid stream can occur between two separate vessels, e.g., from a pool condenser to a vertical urea reactor, or within a single vessel, e.g., in a pool reactor.

[0088] The process comprises treating carbamate-containing urea solution from the HP synthesis section by carbamate decomposition in the MP treatmentunit, e.g. in the MP flash and / or MP heater, to give a treated MP urea solution and, separately, an MP gas stream containing NH₃ and CO₂.

[0089] A part or all of the carbamate-containing urea solution from the HP synthesis section, e.g, of the stripped urea solution, is treated in the MP treatment unit, e.g. in the MP flash followed by the MP heater.

[0090] The process comprises condensing said MP gas stream at least in part in said MP carbamate condensation chamber, at MP, to give an MP carbamate solution. The MP carbamate condensation chamber typically also receives an aqueous stream, e.g. LP carbamate solution from an LP carbamate condenser.

[0091] Two or more MP carbamate condensers and / or MP carbamate condensation chambers are optionally used in series or in parallel. A part or all of the gas stream from the MP treatment unit, i.e. at least some of the gaseous NH₃ and CO₂ released by the MP treatment unit, preferably all, is subjected to condensation in the MP carbamate condensation chamber for carbamate condensation.

[0092] The MP carbamate solution from the MP carbamate condensation chamber is typically recycled to the HP synthesis section, e.g. to the HPCC, typically using a carbamate pump. Typically, all the MP carbamate solution is recycled to the HP synthesis section, Non-condensed gas is typically separated from the MP carbamate solution and processed separately, e.g., by being supplied to an absorber.

[0093] The treated MP urea solution from the MP treatment unit(s) is typically supplied to an LP recovery section, comprising an LP dissociator and, typically, an LP carbamate condenser. The LP dissociator is typically a heater, usually a heat exchanger, where the urea solution is heated at LP to decompose a part or all of the remaining carbamate yielding an LP urea solution and an LP gas stream. The LP dissociator is e.g. a heat exchanger, e.g. using steam as heating fluid, e.g. steam raised in the HPCC. In an example embodiment, the LP dissociator uses as the heating fluid a part of the second steam raised in a second tube bundle of the HPCC.

[0094] The LP gas stream is supplied to the LP carbamate condenser. The LP carbamate condenser (chamber) usually also receives an aqueous stream, which provides the water provides the solvent for the carbamate. The resulting LPcarbamate solution is preferably supplied to the MP carbamate condensation chamber.

[0095] One LP dissociator can be used, or two or more LP dissociators in series and / or in parallel.

[0096] Preferably, a part or all of the LP urea solution is flashed to atmospheric pressure to remove NH3 and carbamate from the urea solution. The resulting gas can be condensed in an atmospheric condenser.

[0097] The resulting urea solution (from the LP dissociator or from the atmospheric flash) is, for example, further subjected to sub-atmospheric flash, e.g. to a pressure in the range 0.3 to 0.9 bar. The resulting gas is e.g. supplied to a suitable condenser with the aid of a (steam-driven) ejector.

[0098] Preferably, in the invention, for a part or all of the urea solution, no heating is used downstream of the LP dissociator, namely for the urea solution that is used for making liquid urea product, e.g., DEF and UAN. Preferably, at least 10% of the produced urea is not subjected to heating downstream of the LP dissociator. Preferably, 0 to 90 % of the treated MP urea solution, on the basis of the weight of urea, is subjected to heating downstream of the LP dissociator, or 0 - 50 %. Preferably, the process comprises subjecting 0 to 90 wt.% (on the basis of urea) of the treated MP urea solution to both LP dissociation in the LP dissociator and heating downstream of the LP dissociation, or 0 to 50 wt.% or 0 to 30 wt.%. Thereby, the need for heating in the downstream LP section is small, relative to the amount of urea solution treated in the MP treatment unit and corresponding MP carbamate condensation.

[0099] In the invention, the MP carbamate condensation chamber is provided as a compartment of a heat exchanger that also comprises a second compartment that provides the steam boiler. Hence, the plant comprises a heat exchanger comprising an MP carbamate condensation compartment and a steam boiler compartment; these compartments being separated by a heat exchanging wall. The heat exchanger is, for instance, a shell-and-tube heat exchanger, with the tube bundle providing one compartment and the shell (shell-side space) providing another compartment. For instance, the tube bundle is used for raising steam, and the shell-side space is used for carbamate condensation.

[0100] The steam boiler compartment has an inlet for boiler feed water (BFW) and an outlet for steam. The steam boiler raises saturated steam. Hence, the plant comprises a supply line for boiler feed water to the steam boiler compartment.

[0101] The process comprises raising steam in the steam boiler, compressing a part or all of said steam in one or more steam compressors, and supplying a part or all of the compressed steam to the inlet for steam of the shell (shell side space) of the HP stripper.

[0102] By way of comparison, US 2015 / 0119603A1 uses a heat exchanger with MP carbamate condensation in the shell and heating of the urea solution in the tube bundle, namely for falling-film evaporation. The heating of the urea solution in the tubes results in water evaporation; the document mentions that this water evaporation is done under vacuum conditions in that heat exchanger. The processing of the water vapour is not shown in that document. However, since the vapour comes from a urea solution pre-evaporator, it is not suitable for entry into the plant's steam network, as it would contaminate the steam network with traces of urea.

[0103] In the invention, the boiler feed water of the (preferably used) steam boiler comprises at least 99 wt.% H₂O.

[0104] Preferably, in the plant, the boiler feed water is supplied by a steam network of the plant. Preferably, the steam condensate from the HP stripper shell is supplied to said steam network.

[0105] The steam network of the plant comprises the fluid connections and vessels for steam, distinct (and separate) from the urea-containing and carbamate-containing process streams.

[0106] In operation, the temperature of the MP carbamate condensation chamber is the condensation temperature of the condensing mixture, at the pressure of the chamber, and the temperature of the steam in the steam boiler is e.g. 5°C or 10°C lower than said condensation temperature, to permit efficient heat exchange. The pressure of the steam can be set accordingly to raise saturated steam at the available temperature. A higher MP pressure can be used to obtain higher-pressure steam, but it reduces the degree of carbamate decomposition in the MP treatment unit.

[0107] In an embodiment, steam is raised in the steam boiler at a pressure in the range of 1.0 to 2.5 bar absolute.

[0108] A preferred embodiment of the method involves compressing a part or all of, preferably at least 50%, the steam from the steam boiler, with a stream compressor, e.g. with a single-stage or multi-stage steam compressor. As said, it is very advantageous to use one or more electrical steam compressors to benefit from electricity, e.g., from renewable sources.

[0109] The compression is to the pressure of the shell side of the HP stripper; said pressure in turn being set by the desired water condensation temperature in the shell side space of the HP stripper. This temperature is in turn determined by the desired temperature for heating the urea solution in the stripper tubes to effect carbamate decomposition.

[0110] Optionally, a part (portion) of the steam from the steam boiler is compressed to an intermediate pressure level only and is used, at that intermediate pressure level, elsewhere in the plant, e.g. as LP steam having a pressure in the range 3 - 6 bar.

[0111] A preferred embodiment of the method involves supplying at least a part, preferably all, of the compressed steam to the inlet for steam of the shell of said HP stripper, using an appropriate steam connection.

[0112] The invention is also suitable for modifying an existing urea plant, in particular an existing urea plant that has an HP stripper, an MP treatment unit, and an MP carbamate condenser (i.e., a heat exchanger comprising an MP carbamate condensation chamber and a cooling fluid compartment) that operates with cooling water (in the cooling fluid compartment). The plant can be modified by connecting the existing MP carbamate condenser to operate with boiler feed water as cooling fluid, thereby providing a steam boiler compartment, and preferably adding one or more steam compressor (s), and connecting the steam compressor(s) to compress steam from the steam boiler (i.e. the cooling fluid compartment) and to supply the compressed steam to the shell side space of the HP stripper. The invention also provides a method of modifying an existing urea plant, the existing urea plant comprising a synthesis section with a HP stripper, and further comprising the MP carbamate condenser (i.e., a heat exchanger comprising an MP carbamate condensation chamber and a cooling fluid compartment); the methodcomprising modifying the MP carbamate condenser to use boiler feed water in the cooling fluid compartment and preferably adding a steam compressor configured and connected to compress steam from that cooling fluid compartment of the MP carbamate condenser (in the modified plant functioning as steam boiler) and supply the compressed steam to the shell side space of the HP stripper. It is also possible to add a steam stripper, configured to receive urea solution originating from the MP treatment and a part or all, preferably a part, of the steam from the cooling fluid compartment of the MP carbamate condenser. In a preferred embodiment, the plant is modified to connect an outlet for steam of the modified cooling fluid compartment of the MP carbamate condenser (in the modified plant functioning a steam boiler) to the shell side space of the HP stripper and to the steam stripper, to supply a part of the steam from said compartment to the HP stripper and a part to the steam stripper.

[0113] All preferences for the inventive plant also apply to the modified plant.

[0114] Figure 1 schematically illustrates a plant and a process according to the invention. The urea production plant (100) comprises a high-pressure (HP) synthesis section (101). This HP section comprises an HP reaction zone (1), an HP stripper (2) and an HP carbamate condenser (3), which are connected to each other to form a loop. Urea solution (22) from the reaction zone (shown as exemplary vertical urea reactor) is supplied to the stripper, gas (12) from the stripper (2) is supplied to the HP carbamate condenser (HPCC) (3). Carbamate solution (13) (liquid stream comprising carbamate) is supplied from the HPCC to the reactor (1). Furthermore, CO2 feed (23) is supplied to the stripper (2) (in particular, to the bottom of the tube bundle (5)) and ammonia feed (24) is supplied to the HP carbamate condenser (3).

[0115] The HP stripper (2) is a first heat exchanger (4), in particular a vertical shell-and-tube heat exchanger configured to operate with a falling-film of urea solution (22) in the vertical tube bundle (5) (only two stripper tubes of the tube bundle are shown for simplicity). The HP stripper has a shell side space (6a) and an inlet (21a) for steam in the shell (6), to receive steam in the shell side space. The shell (6) also comprises an outlet (21b) for fluid from the shell side space.

[0116] The plant comprises an MP treatment section (102) comprising an MP treatment unit (7) and an MP carbamate condensation chamber (8). The MP treatment unit (7), e.g. flash vessel, is connected to receive carbamate-containingurea solution (14) from the HP synthesis section (as shown, stripped urea solution) and is configured to treat said solution by carbamate decomposition to give a treated MP urea solution (15) and an MP gas stream (16). The treatment involves pressure reduction, optionally with heating. The treatment unit may be provided by two or more units in series, e.g. a flash vessel followed by a heater.

[0117] The MP carbamate condensation chamber (8) is connected to receive the MP gas stream (16) and to subject that gas to condensation at MP to give an MP carbamate solution (17). The MP carbamate solution is supplied to the HP carbamate condenser (3) using a pump (pump not shown).

[0118] The plant further comprises a (first) steam boiler (9) configured to raise steam (20) from boiler feed water (19). As shown, the MP carbamate condensation chamber (8) and said steam boiler (9) are provided as two heat-exchanging compartments of a second heat exchanger (11). The compartments are separated by a heat-exchanging wall (18). A shell-and-tube heat exchanger is conveniently used, in particular with a tube bundle providing the steam boiler (9).

[0119] The plant also comprises one or more steam compressors (10) connected to receive the steam (20) from the steam boiler (9). The plant comprises a steam connection to supply compressed steam (21) from the one or more steam compressors (10) to said inlet for steam (21a) of said shell (6) of said HP stripper (2), i.e. to supply compressed steam (21) into the shell side space of the shell-and-tube heat exchanger used as the HP stripper. The shell side space may receive additional steam from other sources.

[0120] Figure 2 schematically shows an example embodiment of the preferably used third heat exchanger (25). As shown, the third heat exchanger (25) is a shell-and-tube heat exchanger with a shell-side space (26) and a first tube bundle (27) and a second tube bundle (28), each being a U-shaped tube bundle with horizontal legs. The shell side space (26) of the third heat exchanger provides the HP carbamate condenser (3), the first tube bundle (27) provides the MP heater (7) that is used as (a part of) the MP treatment unit (preferably downstream of MP flash). Hence, the first tube bundle (27) receives carbamate-containing urea solution (14), at MP, originating from the synthesis section, and provides a biphasic stream at the end of the tube bundle, comprising the treated MP urea solution (15) and the MP gas stream (16). A gas / liquid separator is provided downstream of the first tube bundle to separate the gas stream from the treated urea solution. Theprocess involves, as a further preference, raising (second) steam (30) from boiler feed water (29) in the second tube bundle (28). The second tube bundle is hence used as a second steam boiler. The second steam has a higher pressure than the (first) steam raised in the (first) steam boiler (9). The arrangement of the first and second tube bundles above each other, as illustrated, is merely an example. In practice, e.g. concentric U-shaped tube bundles could be used in the third heat exchanger.

[0121] The second steam can be used in various ways, e.g. in units consuming LP steam, such as in an LP dissociator, and / or compressed to the pressure of the HP stripper shell side space using one or more steam compressors. In an example embodiment, a part of the second steam is compressed and another part of the second steam is used by units consuming LP steam (e.g. 4.0 - 6.0 bar steam).

[0122] Figure 3 schematically shows an example steam compression section used in an example process and plant of the invention. A first steam (20) from the (first) steam boiler (9) is compressed in a first stage compressor (10a). A part or all of the first steam from the steam boiler is supplied to the first stage compressor. The intermediate steam is combined with the steam (30) from the second tube bundle (28) of the third heat exchanger (25). The combined steam is further compressed in a second stage compressor (10b) to give the compressed steam (21) that is supplied to the shell side space of the HP stripper (2).

[0123] Figure 4 schematically shows an example embodiment of the plant and process according to the invention. The treated MP urea solution (15) is supplied, indirectly, to a steam stripper (401) which also receives first steam (20) from the steam boiler (9) that is comprised in the second heat exchanger (11). As illustrated, a part (20a) of the first steam (20) is supplied to the steam stripper, and a further part (20b) is supplied to another unit, e.g. the compressor. The steam stripper (401) is configured for the direct injection of the first steam (20), and is configured for counter-current contact between the urea solution (15) and the injected steam. The steam stripper has an outlet (402) for urea solution, that is preferably suitable as DEF product, and a top outlet (403) for a gas stream. In the illustrated example embodiment, the treated MP urea solution (15) from the MP treatment unit (7) is supplied to an LP dissociator (404) and though an atmospheric flash unit (405) (each having a gas outlet, not shown, and a separate liquid outlet), to give the urea solution (15a) received by the steam stripper (401). The connection for the MP gasstream (16) from the MP treatment unit (7) to the MP carbamate condenser (8) is omitted.

[0124] The MP heater is typically operated, and the MP urea solution is typically provided, at a pressure generally in the range 10 - 60 bar and preferably in the range 15-40 bar.

[0125] All pressures are absolute pressures.

[0126] Carbamate indicates ammonium carbamate, as that, term is used in the art.

[0127] HP is high pressure, e.g. of at least 100 bar. MP is medium pressure, e.g. 10 to 60 bar. LP is low pressure of 1 to 10 bar. These pressures refer to process streams and not to the pressure of steam.

[0128] The process of the invention is preferably carried out in the inventive plant. The inventive plant is preferably suitable for the inventive process. All preferences discussed in connection with the process also apply to the plant, and vice versa.

[0129] The term ‘typically’ indicates a feature that is used in many embodiments but is not strictly mandatory.

[0130] As used herein, the stripping efficiency alpha = (2 * wt.% urea / 60) / ((2*wt.% urea / 60)+(wt.% NHa / 17)), measured at the liquid outlet of the stripper, wherein wt.% NHs includes all ammonia species, including ammonium carbamate, and wt.% urea includes biuret. The term ‘stripping efficacy’ could also be used. A lower stripping efficiency in practice permits a lower steam consumption in the HP stripper..> 1

[0131] The invention will now be further illustrated by the following nonliming example.

[0132] A urea production plant was simulated, having the features of Fig. 1, 2 and 3, with the first tube bundle of the third heat exchanger providing the MP heater.

[0133] The HP stripper (stripping efficiency 64%) required 592 kg HP steam / ton urea. The LP steam produced in the second tube bundle (4.4 bar) was 544 kg / ton urea, of which 257 kg / ton urea was used by downstream steam consumingunits and of which 287 kg / ton urea was supplied to the 2ndstage steam compressor. The LLP steam (1.86 bar) produced in the MP carbamate condenser was 398 kg / ton urea, of which 124 kg / ton urea was used by steam consuming units, including the DEF steam stripper. The remaining 273 kg LLP steam / ton urea was supplied to the 1ststage compressor and thereafter combined with the 287 kg / ton urea LP steam, such that in total 560 kg steam / ton urea was supplied to the second compressor stage. The condensate water injection for interstage cooling was 32 kg / ton urea, giving 592 kg / ton urea of HP steam available for the HP stripper.

Claims

Claims1. A process for the production of urea from ammonia and carbon dioxide in a urea plant (100), wherein the urea plant comprises a high-pressure (HP) synthesis section (101) comprising an HP reaction zone (1), an HP stripper (2) and an HP carbamate condenser (3), wherein the HP stripper (2) is a first heat exchanger (4) b that is a first shell- and-tube heat exchanger configured to operate with a falling film of urea solution in the tube bundle (5) and that has an inlet for steam in the shell (6),wherein the urea plant further comprises:an MP treatment section (102) comprising an MP treatment unit (7) and an MP carbamate condensation chamber (8),a steam boiler (9), andone or more steam compressors (10);wherein the process comprises:treating carbamate-containing urea solution (14) from the HP synthesis 5 section by carbamate decomposition in said MP treatment unit (7) to give a treated MP urea solution (15) and an MP gas stream (16), and condensing said MP gas stream (16) at least in part in said MP carbamate condensation chamber to give an MP carbamate solution (17),raising steam (20) from boiler feed water (19) in the steam boiler (9), 0 compressing a part or all of said steam (20) in the one or more steam compressors (10), andsupplying at least a part of the compressed steam (21) to said inlet for steam of said shell (6) of said HP stripper (2),wherein said MP carbamate condensation chamber (8) and said steam boiler (9) are provided as two heat-exchanging compartments of a second heat exchanger (11).

2. The process according to claim 1, wherein said MP treatment unit comprises an MP heater (7), and wherein said HP carbamate condenser (3) and said MP heater (7) are provided as two heat-exchanging compartments of a third heat 0 exchanger (25).

3. The process according to claim 2, wherein the third heat exchanger (25) is a shell- and-tube heat exchanger with shell side space (26) and a first tubebundle (27) and a second tube bundle (28), wherein the shell side space (26) provides the HP carbamate condenser (3), the first tube bundle (27) provides the MP heater (7), and wherein the process comprises raising steam in the second tube bundle.

4. The process according to any of the preceding claims, wherein said MP treatment unit comprises a flash unit.

5. The process according to claim 4, having the features of claim 2, wherein MP urea solution is supplied from said flash unit to said MP heater.

6. The process according to any of the preceding claims, wherein the steam is raised in said steam boiler at a pressure in the range of 0.5 to 3.5 bar absolute.

7. The process according to any of the preceding claims, wherein at least one of said steam compressors is an electrical steam compressor.

8. The process according to any of the preceding claims, wherein the treated MP urea solution (15) is used in part or entirely for preparing a liquid urea product comprising urea and water and having a water-to-urea molar ratio of at least 1.5, preferably at least 2.

9. The process according to any of the preceding claims, wherein the boiler feed water received by the steam boiler (9) comprises at least 99 wt.% water.

10. The process according to any of the preceding claims, further comprising: condensing gas (12) from said HP stripper (2) in said HP carbamate condenser (3) to provide an HP carbamate-containing liquid stream (13);supplying the HP carbamate-containing liquid stream (13) to the reaction zone (1);allowing the urea formation reaction to take place in the reaction zone to yield a urea synthesis solution (22) which comprises urea, water, and carbamate;supplying the urea synthesis solution (22) to the tube bundle (5) of the HP stripper (2).

11. The plant according to claim 10, wherein the HP stripper is a CO2 stripper, and wherein a COa feed (23) is supplied to the HP stripper and ammonia feed (24) is supplied to the HP carbamate condenser (3).

12. The process according to any of the preceding claims, wherein the MP carbamate solution (17) from the MP carbamate condensation chamber (8) is recycled to the HP synthesis section, e.g. to the HP carbamate condenser, preferably with a carbamate pump.

13. The process according to any of the preceding claims, wherein a part or all of the urea solution (15a) originating from the MP treatment unit (7), preferably from the MP heater, is supplied to a steam stripper (401), and wherein a part of the steam (20) from the steam boiler (9) is injected into the urea solution inside the steam stripper (401).

14. A process for the production of urea from ammonia and carbon dioxide in a urea plant (100), wherein the urea plant comprises a high-pressure (HP) synthesis section (101) comprising an HP reaction zone (1), an HP stripper (2) and an HP carbamate condenser (3), wherein the HP stripper (2) is a first heat exchanger (4) that is a first shell-and-tube heat exchanger configured to operate with a falling film of urea solution in the tube bundle (5) and that has an inlet for steam in the shell (6),wherein the urea plant further comprises:an MP treatment section (102) comprising an MP treatment unit (7) and an MP carbamate condensation chamber (8),a steam boiler (9), andone or more steam compressors (10);wherein the process comprises:treating carbamate-containing urea solution (14) from the HP synthesis section by carbamate decomposition in said MP treatment unit (7) to give a treated MP urea solution (15) and an MP gas stream (16), and condensing said MP gas stream (16) at least in part in said MP carbamate condensation chamber to give an MP carbamate solution (17), andraising first steam (20) from boiler feed water (19) in the steam boiler (9),wherein said MP carbamate condensation chamber (8) and said steam boiler (9) are provided as two heat-exchanging compartments of a second heat exchanger (11); wherein a part or all of the urea solution (15a) originating from the MP treatment unit (7), preferably from the MP heater, is supplied to a steam stripper (401), and wherein a part or all of the first steam (20) from the steam boiler (9) is injected into the urea solution inside the steam stripper (401); and preferably having the features recited in any of claims 2-12.

15. A urea production plant (100) comprising a high-pressure (HP) synthesis section (101) comprising:an HP reaction zone (1).an HP stripper (2) andan HP carbamate condenser (3), wherein the HP stripper (2) is a first heat exchanger (4) that is a shell-and-tube heat exchanger configured to operate with a falling-film of urea solution in the tube bundle (5) and has an inlet for steam in the shell (6),the plant further comprising:an MP treatment unit (7) that is connected to receive carbamate-containing urea solution (14) from the HP synthesis section and configured to treat said solution by carbamate decomposition to give a treated MP urea solution (15) and an MP gas stream (16),an MP carbamate condensation chamber (8) that is connected by a gas flow line to receive at least a part of said MP gas stream (16) and is configured to subject said gas to carbamate condensation to give an MP carbamate solution (17), a steam boiler (9) configured to raise steam (20) from boiler feed water (19); one or more steam compressors (10) connected to receive steam (20) from the steam boiler,a steam connection to supply compressed steam (21) from the one or more steam compressors (10) to said inlet for steam of said shell (6) of said HP stripper (2),wherein said MP carbamate condensation chamber (8) and said steam boiler (9) are provided as two heat-exchanging compartments of a second heat exchanger (11).

16. The plant according to claim 15, wherein the plant comprises a third heat exchanger (25) that is a shell-and-tube heat exchanger with a shell side space (26),a first tube bundle (27) and a second tube bundle (28), wherein the shell side space (26) provides the HP carbamate condenser (3), the first tube bundle (27) provides an MP heater that provides at least a part of the MP treatment unit (7), and the second tube bundle is connected for raising second steam.

17. The plant according to claim 15 or 16, wherein the plant comprises a second steam connection to supply the second steam through one or more steam compressors to the shell (6) of said HP stripper (2).

18. The plant according to any of claims 15-17, wherein the plant comprises a flow line for transport of the MP carbamate solution (17) from the MP carbamate condensation chamber (8) to the HP synthesis section, e.g. to the HP carbamate condenser, preferably with a carbamate pump.

19. The plant according to any of claims 15-18, wherein the plant comprises: a steam stripper (401) wherein a part or all of the urea solution (15a) originating from the MP treatment unit (7), preferably from the MP heater, is supplied to the steam stripper (401), and wherein a part or all of the first steam (20) from the steam boiler (9) is injected into the urea solution inside the steam stripper (401).

20. A urea production plant (100) comprising a high-pressure (HP) synthesis section (101) comprising:an HP reaction zone (1),an HP stripper (2) andan HP carbamate condenser (3), wherein the HP stripper (2) is a first heat exchanger (4) that is a shell-and-tube heat exchanger configured to operate with a falling-film of urea solution in the tube bundle (5) and has an inlet for steam in the shell (6),the plant further comprising:an MP treatment unit (7) that is connected to receive carbamate-containing urea solution (14) from the HP synthesis section and configured to treat said solution by carbamate decomposition to give a treated MP urea solution (15) and an MP gas stream (16),an MP carbamate condensation chamber (8) that is connected by a gas flow line to receive at least a part of said MP gas stream (16) and is configured to subject said gas to carbamate condensation to give an MP carbamate solution (17), a steam boiler (9) configured to raise steam (20) from boiler feed water (19); a steam stripper (401) wherein a part or all of the urea solution (15a) originating from the MP treatment unit (7), preferably from the MP heater, is supplied to the steam stripper (401), and wherein a part or all of the first steam (20) from the steam boiler (9) is injected into the urea solution inside the steam stripper (401); and preferably having the features recited in any of claims 16-18.

21. A method of modifying an existing urea plant, the existing urea plant comprising a synthesis section with a HP stripper (2), and further comprising a MP carbamate condenser comprising a cooling fluid compartment; the method comprising:modifying the MP carbamate condenser to use boiler feed water (19) in the cooling fluid compartment; andadding a steam compressor to compress steam from said cooling fluid compartment of the MP carbamate condenser and to supply the compressed steam to the shell side space of the HP stripper.

Images