Urea and melamine production

Abstract

The present disclosure relates to the production of urea using in series: an MP carbamate condenser for condensing at least urea synthesis section off-gas; a gas / liquid separation producing a gas stream and a liquid stream; and a second carbamate condenser receiving off-gas from melamine production and the liquid stream.

Description

Technical Field

[0001] This invention relates to an integrated process for the production of urea and melamine. Background Technology

[0002] Melamine plants generate waste gas containing significant amounts of CO2 and NH3. It is desirable to supply these gases to a urea plant, where they will be used as reactants. Various possibilities for coupling and integrating urea and melamine plants have been proposed in the art.

[0003] The "Urea" chapter of Ullmann's Encyclopedia of Industrial Chemistry, 2010 (hereinafter referred to as Ullmann's Urea, 2010), describes an example urea production plant. Some well-known urea plant types include conventional plants (without high-pressure stripping) and stripping plants using designs such as Stamicarbon CO2 stripping, Toyo CO2 stripping, and Snamprogetti hot stripping.

[0004] As used in this article, HP indicates high pressure, MP indicates medium pressure, and LP indicates low pressure.

[0005] The article “Urea-melamine plant integration,” in *Nitrogen + Syngas 321*, January-February 2013, pp. 44-54, describes a method for urea-melamine plant integration. One method involves condensing the melamine plant exhaust gas by mixing it with a lean carbamate solution received from the urea plant. Another method used in the case of a Sinanpugeti-type urea plant is described as condensing the exhaust gas at 20 bar (g) inside the melamine plant and delivering the resulting carbamate solution to the high-pressure urea section. For this type of urea plant, another method is described as condensing the exhaust gas in existing or parallel condensers, as appropriate, in the low-pressure, medium-pressure, or high-pressure sections of the urea plant.

[0006] The Ullman Encyclopedia of Industrial Chemistry, in its chapter on melamine (2003), describes various melamine production processes.

[0007] US2016 / 0194293 describes an integrated process for urea and melamine production, wherein exhaust gas from a melamine plant is condensed at medium pressure (20-30 bar) in a condenser that also receives urethane from a low-pressure recovery section of the urea plant. In one embodiment, exhaust gas from the melamine plant is condensed in a first medium-pressure (MP) urethane condenser that also receives urethane solution from a low-pressure (LP) recovery section. The effluent from the condenser is supplied to a second MP urethane condenser, which also receives a gas stream from a separator that receives a portion of the urea synthesis solution directly from the reactor; another portion of the urea synthesis solution is supplied to a high-pressure CO2 stripper. The urea plant uses a high-pressure (HP) scrubber. Gas from the reactor is supplied to the scrubber, where it is washed with urethane solution, and non-condensable gas is discharged as "inert gas." A portion of the CO2 feed is supplied to the LP urethane condenser.

[0008] Another reference for the coupling of urea plants and melamine plants is US2007 / 0282102.

[0009] High-pressure scrubbers (HPs) in urea plants are complex and expensive pieces of equipment, posing certain safety risks due to operation under high pressure and inert gas concentrations. It should be noted that the inert gases typically contain O2, used to prevent corrosion through passivation in the synthesis section, and are usually supplied as passivation air; they also contain N2, as well as H2 from the CO2 feed. Ullman Urea, page 25, discusses the necessity of avoiding the formation of explosive H2 / O2 mixtures at the venting points of non-condensable gases in urea plants. It is generally desirable to reduce the number of HP units. Background reference US2019 / 0015811 discusses the advantages of using HP scrubbers for distribution.

[0010] US2016 / 0318883 illustrates a process scheme for a urea plant with a CO2 HP stripping tower in Figure 10, where the stripped urea solution undergoes adiabatic flash evaporation. The resulting gas, along with exhaust gas from the reactor and exhaust gas from the melamine plant, is condensed. Summary of the Invention

[0011] The present invention aims to provide better plants and processes for the integrated production of urea and melamine, for example, with a relatively low steam consumption (energy consumption) rate.

[0012] The present invention generally relates to a urea production process and plant, the urea production process and plant using in series the following: an MP first urethane condenser for condensing waste gas from at least the urea synthesis section to produce a medium-pressure (MP) condenser effluent; gas / liquid separation of the MP condenser effluent to produce a gas stream and a liquid stream; and a second urethane condenser for receiving waste gas and the liquid stream from melamine production.

[0013] One aspect of the present invention relates to an integrated production process for urea and melamine, comprising: producing urea in a stripping-type high-pressure urea synthesis section, thereby generating a urea solution also containing carbamates, said urea solution preferably being a stripped urea solution, and a separate urea synthesis section waste gas stream containing inert gases, NH3, and CO2; producing melamine, thereby providing a melamine waste gas stream; preferably subjecting said urea solution (preferably a stripped urea solution) to MP treatment, wherein at least a portion of the carbamates contained in said urea solution dissociates into CO2 and NH3, thereby generating an MP urea solution and a first MP gas stream; and subjecting said urea synthesis section waste gas stream and preferably the first MP gas stream to MP treatment. At least a portion of the P gas stream is condensed in a first MP urethane condenser to produce MP condenser effluent and / or the gas from the exhaust gas outlet of the urea synthesis section, and preferably the gas from the MP treatment, is condensed in the first MP urethane condenser; the MP condenser effluent is subjected to gas / liquid separation to produce a second MP gas stream containing the inert gas and a first MP urethane solution; the melamine exhaust gas stream is condensed in a second MP urethane condenser in the presence of at least a portion of the first MP urethane solution to produce a second MP urethane solution; and the second MP urethane solution is supplied directly or indirectly to the urea synthesis section.

[0014] This invention also relates to an integrated urea and melamine production plant, comprising: a stripping-type high-pressure urea synthesis section having an outlet for a urea solution further comprising carbamates, and a separate outlet for a urea synthesis section waste gas stream comprising inert gas, NH3, and CO2; a melamine production section having an outlet for a melamine waste gas stream; preferably an MP treatment unit for treating the urea solution at medium pressure, wherein at least some of the carbamates contained in the urea solution dissociate into CO2 and NH3, the MP treatment unit having an outlet for the MP urea solution and an outlet for a first MP gas stream; and a first MP carbamate condenser. The first MP carbamate condenser is used to condense at least a portion of the urea synthesis section waste gas stream and preferably the first MP gas stream to produce MP condenser effluent; a gas / liquid separator is used to subject the MP condenser effluent to gas / liquid separation to produce a second MP gas stream and a first MP carbamate solution; a second MP carbamate condenser is used to condense the melamine waste gas stream in the presence of at least a portion of the first MP carbamate solution to produce a second MP carbamate solution; and a liquid flow line is used to supply the second MP carbamate solution directly or indirectly to the urea synthesis section.

[0015] The present invention also relates to a method for modifying an existing urea plant, the existing urea plant comprising: a stripping-type high-pressure urea synthesis section, the stripping-type high-pressure urea synthesis section having an outlet for a urea solution further comprising carbamates, and a separate outlet for a urea synthesis section waste gas stream comprising inert gases, NH3, and CO2; wherein the method involves adding the following, if not already present in the existing urea plant: a melamine production section, the melamine production section having an outlet for a melamine waste gas stream; preferably an MP treatment unit, the MP treatment unit for treating the urea solution at medium pressure, wherein at least some of the carbamates contained in the urea solution dissociate into CO2 and NH3, the MP treatment unit having an outlet for the MP urea solution and an outlet for a first MP gas stream. The method comprises: a first MP carbamate condenser for condensing at least a portion of the urea synthesis section waste gas stream and preferably the first MP gas stream to produce an MP condenser effluent; wherein the method includes adding: a gas / liquid separator for gas / liquid separation of the MP condenser effluent to produce a second MP gas stream and a first MP carbamate solution; a second MP carbamate condenser for condensing the melamine waste gas stream in the presence of at least a portion of the first MP carbamate solution to produce a second MP carbamate solution; and a liquid flow line for directly or indirectly supplying the second MP carbamate solution to the urea synthesis section. Attached Figure Description

[0016] Figure 1 An example process scheme according to the present invention is illustrated schematically.

[0017] Any embodiments shown in the accompanying drawings are merely examples and do not limit the invention. Detailed Implementation

[0018] This disclosure is broadly based on the insightful observation that, through upstream gas / liquid separation, the condensation of melamine waste gas is advantageously carried out with a relatively low water content in the resulting second urethane solution. Therefore, the condensation of the melamine waste gas can be performed using the amount of water already contained in the first MP urethane solution; the minimum water volume in the first MP urethane condenser is determined by the need to sufficiently condense the NH3 and CO2 from the urea synthesis section waste gas into urethane. The inert gases (e.g., passivating air) in the urea synthesis section waste gas do not negatively affect the melamine waste gas condensation temperature and pressure, nor do they negatively affect the water content of the urethane recovery stream. The presence of non-condensable inert gases in the second MP urethane condenser will cause a decrease in the condensation temperature of the second MP urethane condenser at a given operating pressure.

[0019] The present invention relates to coupling a melamine plant and a urea plant by supplying waste gas (containing CO2 and NH3) from a melamine plant to a urea plant, specifically indirectly to a high-pressure synthesis section.

[0020] The integrated urea and melamine production process of this invention comprises urea production and melamine production. As used herein, "integrated" means that melamine waste gas (i.e., waste gas from a melamine plant) is supplied to a high-pressure urea synthesis section after condensation. Urea production involves producing urea in a stripping-type high-pressure urea synthesis section, thereby generating a urea solution comprising urea and water, and also containing carbamate, and generating a separate urea synthesis section waste gas stream comprising inert gases, NH3, and CO2. For the purposes of the process of this invention, the inert gases are non-condensable and contain, for example, N2 and O2. As an alternative to the term 'inert' or otherwise, the waste gas may be specified to contain gaseous components other than NH3, CO2, and H2O.

[0021] Based on the total inert gases, such as the urea synthesis section exhaust gas stream obtained from the urea synthesis section containing, for example, at least 2.0 vol.% of inert gases, or for example, at least 4.0 vol.%, for example, from 2 vol.% to 12 vol.% of inert gases, and / or wherein the inert gases are components other than NH3, CO2 and H2O; and / or the exhaust gas stream contains a total of at least 2.0 vol.% of N2, O2, H2 and CH4, for example, at least 4.0 vol.%, for example, in the range of 2.0-10 vol.%; or 4.0-10%, or in the range of 4.0-8.0 vol.%.

[0022] Urea production involves the reaction of NH3 with CO2 to form carbamate, and the carbamate is dehydrated to yield urea and water. The high-pressure urea synthesis section includes a reaction zone operating at above 100 bar, preferably above 120 bar, and a high-pressure stripping tower operating at pressures above 80 bar, for example, above 120 bar. The stripping tower is, for example, a CO2 stripping tower or a hot stripping tower. The synthesis section also includes a condensation section for condensing the gas from the stripping tower, operating at pressures above 80 bar or above 120 bar. The exhaust gas from the urea synthesis section typically originates from the reactor and / or carbamate condenser contained within the synthesis section.

[0023] The process involves melamine production, thereby providing a melamine waste gas stream. The pressure of the melamine waste gas stream is at least 10 bar and / or less than 130 bar, or less than 80 bar, or less than 60 bar, or less than 40 bar, for example 10-130 bar, or 10-80 bar, or 10-60 bar, or 10-40 bar. The melamine waste gas contains NH3 and CO2. Melamine production uses urea as a feedstock. The melamine synthesis pressure is typically higher than 70 bar, and the gaseous components in the melamine synthesis effluent are typically depressurized to less than 80 bar, more preferably less than 60 bar or less than 40 bar, and typically at least 10 bar or at least 20 bar.

[0024] The process preferably involves subjecting a urea solution from the synthesis section, preferably a stripped urea solution from an HP stripper, to a medium-pressure treatment, i.e., treatment at a medium pressure, for example, at least 10 bar and / or less than 80 bar. During this treatment, at least some of the carbamates contained in the urea solution dissociate into CO2 and NH3, thereby obtaining a first MP gas stream and an MP urea solution.

[0025] The most common MP treatment typically involves reducing the pressure of a urea solution from HP to MP using a pressure-reducing unit (e.g., a valve), thereby releasing gas from the solution, and performing gas / liquid separation of the first MP gas stream from the urea solution. Pressure reduction causes the carbamates contained in the urea solution to spontaneously dissociate into NH3 and CO2, at least a portion of which is released from the solution as gas, i.e., flash evaporation, thus forming the first MP gas stream. Therefore, MP treatment preferably involves flash evaporation, optionally combined with heating.

[0026] For example, the process can involve adiabatic flash evaporation. Adiabatic flash evaporation involves pressure reduction and gas / liquid separation.

[0027] MP treatment may also involve, for example, depressurization to MP, heating under MP, and gas / liquid separation under MP. The treatment may also involve, for example, depressurization to MP, gas / liquid separation under MP (e.g., flash evaporation), wherein the flash evaporation is, for example, adiabatic, heating of the urea solution under MP, and gas / liquid separation under MP, preferably these three steps being performed in this order. Heating under MP can be performed using heat exchange with a heating fluid (e.g., steam, steam condensate) or process-to-process heat exchange with fluids in the synthesis section; for example, MP heating is performed within a tube bundle of a pool condenser or pool reactor contained in the urea synthesis section. Optionally, the gas / liquid separated MP urea solution is contacted with gas from an MP (adiabatic) flash evaporator to distill the urea solution under MP, wherein the distilled urea solution expands to LP and is supplied to an LP dissociator in the LP recovery section, and wherein gas from the MP distillation unit is supplied together with urea synthesis section exhaust gas, and preferably also with gas from the MP gas / liquid separation.

[0028] Optionally, the treated urea solution is further subjected to countercurrent contact with CO2 under medium pressure to remove NH3 from the treated urea solution.

[0029] The process involves subjecting part or all of the urea synthesis section waste gas stream, preferably part or all of the first MP gas stream, to condensation at medium pressure in a first MP urethane condenser to produce MP condenser effluent. Specifically, the effluent is a fluid containing both gas and liquid at medium pressure.

[0030] Furthermore, the process involves subjecting the MP condenser effluent to gas / liquid separation at medium pressure to produce a second MP gas stream and a first MP carbamate solution. For example, the separation is carried out in a gas / liquid separation unit having both gas and liquid outlets. However, in embodiments where the first MP carbamate condenser has separate outlets for gas and liquid, the separation can also be performed within the first MP carbamate condenser. The gas / liquid separation unit (separator) and the first MP carbamate condenser can also be integrated into a single container or device, for example, with gas / liquid separation performed at the outlet side.

[0031] Furthermore, the process involves subjecting a melamine waste gas stream to condensation at medium pressure in a second MP urethane condenser in the presence of at least a portion (preferably at least 90 wt.%) of a first MP urethane solution to produce a second MP urethane solution. The second MP urethane condenser operates at pressures in the MP range (e.g., 10 to 80 bar), or suitably at 10-130 bar. The pressure of the second MP urethane condenser may be the same as or different from that of the first MP urethane condenser, preferably no more than 1 bar lower. The process also involves supplying the second MP urethane solution directly or indirectly to a high-pressure urea synthesis section. The urethane can then be converted to urea in the urea synthesis section.

[0032] Melamine production is typically carried out in non-catalytic high-pressure melamine plants (melamine synthesis pressure above 70 bar), specifically in non-catalytic high-pressure melamine reactors. Melamine synthesis is based on the pyrolysis of molten urea, and the melamine-forming reaction produces CO2 and NH3 as byproducts.

[0033] The urea feedstock can come from the same urea plant that receives melamine waste gas, or from different urea plants, or from a combination thereof; in all these embodiments, the production of urea and melamine is integrated through the processing of melamine waste gas.

[0034] Melamine synthesis is carried out at pressures above 70 bar, but typically at pressures below 130 bar. EP2385043A1 describes an example suitable melamine production process operating at 80 bar. US2004 / 0162429A1 describes another example suitable melamine production process with a washing section for using urea melt to form anhydrous waste gas from the feedstock.

[0035] In some embodiments, the melamine synthesis pressure is more than 10 bar lower than the urea synthesis pressure, for example, more than 20 bar lower, and at least 10 bar or at least 20 bar lower than the pressure of the HP reactor, HP stripping tower, and HP urethane condenser. In some embodiments, the melamine synthesis pressure is between 70 bar and 90 bar, and the urea synthesis pressure is at least 120 bar, for example, 120-200 bar.

[0036] The steps of providing the melamine waste gas stream, for example, involve preferably washing the feed waste gas stream from the melamine synthesis reactor with urea. A raw waste gas stream can be obtained by subjecting the melamine synthesis mixture or melamine synthesis effluent to gas / liquid separation to separate molten melamine from the gas phase, producing a raw waste gas stream containing a small amount of melamine. The process typically involves washing the raw waste gas stream to recover melamine, thereby producing purified waste gas. The washing involves, for example, water washing or urea washing, i.e., washing the raw waste gas stream with water or urea. In principle, any method for separating melamine from the waste gas can be used; anhydrous separation is preferred.

[0037] For example, the process also involves reducing the pressure of the raw waste gas stream and / or reducing the pressure of the purified waste gas. The purified waste gas is supplied as a gaseous phase to the second MP urethane condenser in the process of the present invention.

[0038] Relative to the total gas stream, the melamine waste gas stream (specifically after washing) contains, for example, less than 15 wt.% water, or less than 10 wt.% water, or less than 2 wt.% water, where water indicates H2O; for example, in embodiments using urea washing; or preferably less than 15 vol.% H2O, or less than 10 vol.% or less than 2 vol.% H2O. For example, urea washing can produce an anhydrous waste gas containing less than 0.5 wt.% water. In embodiments where the water content of the melamine waste gas is relatively low, the aqueous fraction of the first MP urethane solution can be fully utilized to condense the melamine waste gas. The melamine waste gas contains, for example, at least 95 vol.% or at least 98 vol.% NH3 and CO2.

[0039] Melamine exhaust gas contains, for example, less than 0.10 vol.% of inert gases, such as negligible amounts of inert gases.

[0040] Melamine exhaust gas is typically provided at a pressure ranging from 10 bar to 130 bar, for example, from 10 bar to 80 bar, preferably from 15 bar to 45 bar, and more preferably from 20 bar to 30 bar. In some embodiments, the exhaust gas is provided at a pressure more than 10 bar lower than the urea synthesis pressure, for example, more than 20 bar lower, and for example, at a pressure at least 10 bar or at least 20 bar lower than the pressure of the HP synthesis section, i.e., the pressure of each of the HP reactor, HP stripping tower, and HP carbamate condenser.

[0041] In the process of this invention, the aqueous fraction of the first MP carbamate solution is used to condense the melamine waste gas to form a second carbamate solution. Therefore, the second carbamate solution contains the aqueous fraction of the first MP carbamate solution. Thus, at least a portion of the first MP carbamate solution is present in the condensation zone, specifically in the second MP carbamate condenser, which receives the melamine waste gas.

[0042] The resulting second carbamate solution has a certain minimum water content to prevent carbamate crystallization at a given temperature and therefore condensation pressure, and at a given N / C ratio of the carbamate solution. The minimum water content decreases with increasing temperature and condensation pressure. Since the carbamate solution generated from the condensation of melamine waste gas is supplied to the urea synthesis section, specifically the urea synthesis zone or urea synthesis reactor, a low water content is desirable, as the presence of water in the urea synthesis zone or reactor is detrimental to urea production.

[0043] In this invention, it is highly advantageous that the condensation of melamine waste gas is carried out through upstream gas / liquid separation while the water content of the resulting second carbamate solution is relatively low.

[0044] Therefore, the condensation of melamine exhaust gas can be carried out using the amount of water already contained in the first MP carbamate solution; the minimum amount of water in the first MP carbamate condenser is given by the requirement to fully condense NH3 and CO2 from the exhaust gas from the urea synthesis section into carbamate.

[0045] In this invention, in a first aspect, the exhaust gas from the urea synthesis section is advantageously reduced to medium pressure and subjected to condensation at medium pressure. This eliminates the need for a high-pressure scrubber, thereby advantageously simplifying the HP urea synthesis section, particularly for CO2 stripping type urea plants.

[0046] One aspect of the invention relates to the use of two MP urethane condensers in series, wherein a first MP urethane condenser is used to condense the exhaust gas from the urea synthesis section, and a downstream second MP urethane condenser is used to condense the melamine exhaust gas. Melamine exhaust gas condensation is improved by including a gas / liquid separation unit in the fluid flow line from the first MP urethane condenser to the second MP urethane condenser. Thus, the condensation temperature in the second MP urethane condenser can be higher than that in the first MP urethane condenser, for example, in embodiments where the absolute total pressure in the second MP urethane condenser is lower than or substantially the same as that in the first MP urethane condenser. In any case, removing inert, non-condensable gases from the effluent of the first MP urethane condenser located upstream of the second MP urethane condenser allows the water content of the second urethane solution at the liquid outlet of the second MP urethane condenser to be lower than the water content achieved without such gas / liquid separation. Because the urethane solution is recycled to the HP urea synthesis section, the urea conversion rate in the urea synthesis section is improved. Optionally, the first carbamate solution is pumped to the second MP carbamate condenser.

[0047] Preferably, the condensation temperature in the second MP urethane condenser is at least 5°C higher than the condensation temperature in the first MP urethane condenser, for example at least 10°C higher, such as 10°C to 20°C higher.

[0048] For example, the water content of the first urethane solution from the first MP urethane condenser is in the range of 22 wt.% to 26 wt.%.

[0049] In percentage terms, the water content in the second MP urethane condenser is preferably 1-5 wt.% lower than that in the first MP urethane condenser.

[0050] Preferably, based on the liquid outlet composition, the N / C ratio in the first MP urethane condenser is in the range of 2.0 to 2.5, more preferably in the range of 2.1 to 2.4, for example, about 2.3. Using these N / C values, advantageous and complete condensation can be achieved in the second MP gas stream with a relatively low amount of NH3.

[0051] The first MP urethane condenser may comprise one or more stages. Optionally, the urea synthesis section is subjected to condensation upstream of the first MP urethane condenser, wherein the gas condensed upstream is supplied to the first MP urethane condenser. Optionally, the first MP gas stream is subjected to condensation upstream of the first MP urethane condenser, wherein the gas condensed upstream is supplied to the first MP urethane condenser. Preferably, the plant includes a condenser that receives gas from the gas outlet of the HP urea synthesis section and gas from the gas outlet of the MP treatment unit; and the process involves a condensation step in a condensation zone that receives gas from the gas outlet of the HP urea synthesis section and gas from the gas outlet of the MP treatment unit, i.e., receiving at least a portion, for example, all of the urea synthesis section exhaust gas and at least a portion, for example, all of the first MP gas stream.

[0052] Preferably, the urethane condensation in the first MP urethane condenser is carried out in heat exchange contact with the urea solution to be heated, specifically with the urea solution directly or indirectly from the LP dissociator, to produce a heated urea solution. This heating, preferably carried out with moisture removal, is used, for example, to concentrate the urea solution from about 72 wt.% urea to about 75-80 wt.% urea (urea includes biuret). Preferably, the urea solution is at sub-atmospheric pressure, for example, 0.3-1.0 bar. The plant, for example, includes a shell-and-tube heat exchanger in which the first MP gas stream and the urea synthesis waste gas stream are condensed in the shell, i.e., the gas from the MP treatment and the gas from the urea synthesis section are condensed in the shell, and the urea solution is heated in the tubes; more preferably, it has vertical tubes. The shell, as used in a shell-and-tube heat exchanger, refers to the shell-side space of the heat exchanger. Optionally, the urea solution undergoes gas / liquid separation to remove water vapor downstream of the tubes; this gas / liquid separation of the urea solution is performed separately and is distinct from the gas / liquid separation of the condensate effluent (i.e., the effluent from the shell). In an example embodiment, the shell has an outlet for an effluent containing both liquid and gas, and the effluent from the shell undergoes gas / liquid separation in a separate unit. In embodiments where the shell surrounds a vertical tube bundle, the gas inlet into the shell is preferably located at the bottom of the shell, and the outlet for the two-phase effluent is located at the top, or vice versa.

[0053] In other embodiments, the first MP urethane condenser is, for example, an absorber, which does not involve indirect heat exchange with the cooling liquid; for example, if the amount of exhaust gas from the urea synthesis section is small.

[0054] Optionally, the first MP urethane condenser has a first outlet for liquid and a separate second outlet for gas, and gas / liquid separation takes place inside the condenser; for example, the outlets are vertically spaced apart, having a bottom liquid outlet and a top gas outlet.

[0055] Typically, the process involves gas / liquid separation of a urethane solution formed in a first MP urethane condenser from a non-condensable gas in the first MP condenser, and selectively supplying the urethane solution to a second urethane condenser. Preferably, the separated gas is expanded to LP and supplied partially or entirely to a unit operating at LP, such as a separate absorber or scrubber. The gas / liquid separation can be performed within the first MP urethane condenser or in a separate unit.

[0056] The second MP urethane condenser may comprise one or more stages. Preferably, the urethane condensation in the second MP urethane condenser is carried out in heat exchange contact with a urea solution to be heated, for example with a urea solution directly or indirectly from the LP dissociator, more preferably with a heated urea solution from the tube side of the first MP urethane condenser. Preferably, the urea solution is at sub-atmospheric pressure, for example, 0.3-1.0 bar. The plant, for example, includes a shell-and-tube heat exchanger in which melamine exhaust gas is condensed in the shell and the urea solution is heated in the tubes, preferably wherein the heated urea solution is further heated in the tubes. The pressure in the tubes is, for example, 0.2-0.5 bar. Optionally, the urea solution is further heated downstream of the heating in the second MP urethane condenser, for example, with steam. This further heating can be carried out in a single shell-and-tube heat exchanger having a shell (shell-side space) divided into two compartments, the first compartment serving as the second MP urethane condenser and the second compartment containing steam. For example, the first chamber has an inlet for gas at the bottom and an outlet for the two-phase fluid effluent at the top, or vice versa, specifically a vertical tube bundle. The urea solution flow optionally undergoes gas / liquid separation to remove water vapor downstream of the tubes. In a preferred embodiment, a vertical tube bundle is used, wherein an inlet for the urea solution is located at the upper end of the tube bundle, the liquid flows downwards in the tubes, and the gas flows upwards during operation. For example, a urea melt containing at least 95 wt.% urea is obtained from the tubes.

[0057] In other embodiments, the second MP urethane condenser is, for example, an absorber, which does not involve indirect heat exchange with the cooling liquid; for example, if the amount of melamine exhaust gas is relatively small.

[0058] Preferably, both the first and second MP urethane condensers are in heat exchange contact with an evaporation stage for heating the urea solution via their walls, for example, as shell-and-tube heat exchangers, where condensation occurs in the shell and heating of the urea solution occurs in the tubes. Preferably, the first MP urethane condenser is in heat exchange contact with the first evaporation stage, and the second MP urethane condenser is in heat exchange contact with the second evaporation stage, and the urea solution is preferably supplied from the first evaporation stage to the second evaporation stage via gas / liquid separation. Thus, the lower inert content in the second MP urethane condenser facilitates further concentration of the urea solution in the second evaporation stage. Increasing the urea concentration through water evaporation helps to provide a urea melt suitable, for example, for melamine production or urea refining.

[0059] Optionally, the second MP urethane condenser has a first outlet for liquid and a separate second outlet for gas. Optionally, the second MP urethane condenser has an outlet for an effluent containing both gas and liquid, said outlet being connected to a gas / liquid separation unit having a liquid outlet in liquid flow communication with the reaction zone of the urea synthesis section. For example, gas from the second MP urethane condenser is fed to, for example, an absorber operating at MP. The gas from the second MP urethane condenser (if obtained as a separate stream) contains, for example, at least 90 vol.% or at least 98 vol.% of total NH3 and CO2.

[0060] If necessary, an aqueous stream, such as process condensate or purified process condensate, is supplied to the second MP urethane condenser in addition to the first MP urethane solution. Preferably, no additional water is supplied to the second MP urethane condenser besides the first urethane stream. Optionally, the process involves concentrating a urea solution from the urea synthesis section by water evaporation to form a concentrated urea solution or melt having a concentration of at least 80 wt.% urea (including biuret), and using a portion of the concentrated urea solution or melt for purposes other than melamine production, such as for the production of solid or liquid fertilizers. The water vapor generated by evaporation can be condensed and is typically supplied, in part, directly or indirectly, to the second MP urethane condenser, for example, through the first MP urethane condenser.

[0061] For example, a second MP gas stream from a first MP urethane condenser is supplied to an MP scrubber, which preferably also receives urethane streams from the LP recovery section of the urea plant, specifically from the LP urethane condenser of the urea plant. The MP scrubber has a liquid outlet preferably connected to the inlet of the first MP urethane condenser. The MP scrubber also has a gas outlet connected to an absorber (e.g., an LP absorber). Therefore, inert gases from the urea synthesis section are preferably supplied to the absorber and can be discharged from the absorber. The liquid from the absorber is typically supplied directly or indirectly to a wastewater treatment section. For example, the absorber uses a scrubbing liquid, such as purified process condensate.

[0062] The second MP carbamate solution is supplied directly or indirectly to the HP urea synthesis section via a liquid flow line. The carbamate solution is typically pumped to the HP urea synthesis section after gas / liquid separation to remove any uncondensed vapors. For example, vapors are also supplied to the MP scrubber. Alternatively, the carbamate solution may be supplied directly to the reactor or the HP carbamate condenser.

[0063] The urea synthesis section is of the stripping type and includes an HP stripping tower, a reaction zone, and a condensation zone. For example, the reaction zone and condensation zone may be provided as zones within a single vessel, as in the case of a horizontal pool reactor. Pool reactors can also be combined with vertical urea reactors. For example, the reaction zone and condensation zone may also be provided as separate units, such as a urea reactor (typically a vertical urea reactor) and an HP carbamate condenser. The HP synthesis section has a liquid outlet for the stripped urea solution.

[0064] Urea reactors are typically vertical urea reactors with one or more inlets at the bottom and an outlet for (e.g., using downcomers) to draw urea solution from the top of the reactor; and are typically provided with trays. The reactor also typically has an inlet for urethane solution from an HP urethane condenser.

[0065] HP urethane condensers are typically shell-and-tube heat exchangers. Suitable HP urethane condensers are, for example, horizontal urethane condensers with U-shaped tube bundles containing a cooling fluid, and condensation taking place in the shell. Alternatively, they can be vertical urethane condensers with U-shaped tube bundles and cooling fluid in the tubes, or vertical shell-and-tube heat exchangers where condensation takes place in the tubes, for example, where the gas to be condensed is supplied in the form of bubbles in the tubes.

[0066] In another embodiment, the HP urethane condenser is a shell-and-tube heat exchanger in which condensation takes place in tubes, such as a kettle boiler or a falling film condenser.

[0067] Optionally, the reactor and condenser are combined in a single vessel, such as a pool reactor. A pool reactor is a horizontal vessel with a U-shaped tube bundle extending horizontally above a portion of the vessel, and condensation takes place in the shell-side space at the tube bundle, with a reaction portion located in a portion of the shell-side space between the tube bundle and the shell (wall) of the vessel. The shell-side space of the pool reactor has an outlet for liquids connected to the reactor and an inlet for gases from a stripping tower.

[0068] Typically, the cooling fluid for HP urethane condensers is, for example, boiler feedwater or a urea solution to be heated, or the cooling fluid may be used in combination, for example, in two separate tube bundles.

[0069] HP stripping towers are heat exchangers configured for a falling film of a urea solution in countercurrent contact with a gas stream. They have a top inlet for the urea solution, a bottom outlet for the stripped urea solution, and a top outlet for the gas stream, the outlet being connected to an HP urethane condenser for at least a portion of the gas stream. Stripping towers are typically shell-and-tube heat exchangers, with a heating fluid in the shell and a urea solution in the tubes. HP stripping towers are preferably CO2 HP stripping towers, having a bottom inlet for CO2 as the stripping gas. HP stripping towers can also be hot stripping towers.

[0070] Depending on the type of synthesis section used, the HP synthesis section may include additional equipment components, such as injectors or urethane separators.

[0071] In a preferred embodiment, the stripper is a CO2 HP stripper that uses at least part or all of the CO2 feedstock as the stripping gas, and the synthesis zone is operated at an N / C ratio of 3.0 to 3.5, for example 3.0-3.2, and / or at a synthesis pressure of, for example, 120-160 bar, for example (about) 140 bar; preferably these N / C ratios and pressure combinations.

[0072] When the N / C ratio in the synthesis section is higher than 3.5, it becomes difficult or special measures are required to condense the urea synthesis waste gas into a carbamate solution at MP (especially 10-30 bar).

[0073] Preferably, in this embodiment, the plant does not include a dedicated NH3 recirculation flow line with CO2 HP stripping. Therefore, the first MP urethane condenser preferably operates with NH3 (and CO2) substantially completely condensed. Supplying a first MP gas stream from MP processing, preferably MP processing with a reduced urea solution temperature, and more preferably from MP adiabatic flash evaporation, to the first MP urethane condenser may be advantageous in this regard.

[0074] Preferably, the urea solution is subjected to high-pressure CO2 stripping prior to MP treatment; therefore, the stripped urea solution from the CO2 HP stripper is preferably subjected to MP treatment when the urea solution temperature at the outlet is lower than that at the inlet, more preferably to adiabatic flash evaporation. In such embodiments, the N / C ratio of the first gas stream is sufficiently low to provide optimal urethane condensation when combined with the exhaust gas from the urea synthesis section under MP. Additionally, MP flash evaporation can help reduce steam consumption in the HP stripper.

[0075] In one embodiment, urea production uses HP hot stripping, and the plant or process is, for example, of the Sinanpuji type. In an example embodiment, the urea solution from the hot HP stripping tower is subjected to MP dissociation (as MP treatment) in an MP decomposer, which is, for example, a shell-and-tube heat exchanger, with the urea solution in the tubes. A first MP gas stream from the MP decomposer is condensed together with the urea synthesis section exhaust gas in a first MP carbamate condenser. The effluent from the condenser is subjected to gas / liquid separation in an ammonia-carbamate separation column; the resulting second MP gas stream is supplied, for example, to an ammonia condenser, and the resulting first MP carbamate solution is supplied to a second MP carbamate condenser that also receives melamine exhaust gas. The second MP carbamate solution from the second MP carbamate condenser is supplied to the synthesis section. The effluent from the ammonia condenser is subjected to gas-liquid separation, and the resulting gas stream containing inert gases is typically discharged after further purification, for example, in an ammonia scrubber. Uncondensed gas from the second MP urethane condenser may be supplied, for example, to a unit in the LP recovery section or MP section of the plant, or directly or indirectly to an ammonia scrubber, or discharged.

[0076] In another embodiment, the urea synthesis section includes a CO2 stripping tower, and is of the ACES21 type, for example, supplied by Toyo Corporation. In an example embodiment of the process, the reactor operates at a pressure of at least 150 bar and an N / C ratio greater than 3.5; and HP stripping uses CO2 as the stripping gas. The stripped urea solution is supplied to the MP treatment in an MP decomposer, which is a shell-and-tube heat exchanger heated by steam. The resulting first MP gas stream is supplied directly or indirectly to an MP absorber, which is a heat exchanger for cooling and also receives urethane solution from the LP recovery section and provides gas / liquid separation. The first urethane solution from the MP absorber is supplied to a second MP urethane condenser, which also receives melamine waste gas. The effluent from the second MP urethane condenser is supplied to a gas / liquid separator; the liquid is supplied to the HP synthesis section via an HP urethane pump, specifically to the HP urethane condenser. HP urethane condensers are, for example, condensers in which condensation takes place in a shell and has a cooling fluid in a vertical U-shaped tube bundle.

[0077] In another embodiment, the process involves HP CO2 stripping and subjecting the stripped urea solution to MP treatment by heating it under MP, the MP heating being carried out by flowing the liquid under MP through the tube bundle of an HP urethane condensation unit, for example by supplying the liquid under MP to the tube bundle of a pool reactor, the HP urethane condensation unit condensing the gas from the stripping tower in a shell under HP. The effluent from the tube bundle is subjected to gas / liquid separation to produce a first MP gas stream. Furthermore, the process involves condensing the gas stream and urea synthesis section exhaust gas in a first MP urethane condenser; subjecting the resulting effluent to gas / liquid separation; and supplying the resulting first MP urethane solution to a second MP urethane condenser that also receives melamine exhaust gas. The pool reactor is a horizontal vessel comprising a condensation zone and a reaction zone, wherein a U-shaped tube bundle is positioned in the condensation zone. For example, the reaction zone is provided between the bend of the U-shaped tube bundle and the shell.

[0078] In one embodiment, the urea synthesis section includes a CO2 HP stripper, and the process involves supplying a first portion of the urea synthesis solution from the HP reaction zone to an MP dissociator, bypassing the HP stripper, and supplying a second portion of the urea synthesis solution from the reactor to the HP stripper. The first portion of the urea solution is expanded to MP pressure and subjected to MP treatment, such as adiabatic flash evaporation, heating, and / or countercurrent contact with an MP CO2 gas stream to generate a first MP gas stream and an MP urea solution. The gas stream and waste gas from the urea synthesis section are condensed together in a first MP urethane condenser to form an MP urethane solution, which is supplied via an MP gas / liquid separator to a second MP urethane condenser that also receives melamine waste gas. The gas from the MP gas / liquid separator contains an inert gas. The urea synthesis section typically does not include an HP scrubber. The urea solution and MP urea solution from the HP stripper are supplied to one or more LP recovery sections. The MP urea solution can be subjected to countercurrent contact with the MP CO2 stream, and the resulting MP gas stream can be supplied to the first and / or second MP urethane condenser.

[0079] Typically, the MP urea solution from the MP process is expanded to LP and subjected to LP carbamate decomposition (usually by heating) to produce an LP urea solution and an LP gas stream. The LP urea solution is typically supplied directly or indirectly to an evaporation section, which typically includes one or more vacuum evaporation stages for producing a urea melt having, for example, at least 95 wt.% urea (including biuret) or at least 98 wt.% urea (including biuret). For example, the urea melt is supplied partially or entirely to a melamine production section. In some embodiments, the urea melt is not supplied to the melamine production section, and the melamine production section uses urea from another urea plant. In some embodiments, at least a portion of the urea melt is subjected to refining, i.e., solidification, to form a solid urea product, such as granulation or pelleting.

[0080] The vapor from the evaporation section is typically condensed, and the resulting condensate is usually supplied to a wastewater treatment section that includes, for example, a desorber and a hydrolysis unit, which produces purified process condensate and a diluted carbamate solution.

[0081] In a preferred embodiment, 5 wt.% to 40 wt.% of the total urea produced in the high-pressure urea synthesis section is supplied to the melamine synthesis section or reactor, while the remaining urea is transported to units other than the melamine plant, such as a refining section or a unit for producing liquid fertilizer. Preferably, the 5 wt.% to 40 wt.% portion of the total urea produced from the melamine production feedstock is used. Suitably, the remaining urea portion is concentrated by water evaporation.

[0082] Preferably, the amount of melamine waste gas flow corresponds to the amount of 5 wt.% to 40 wt.% of the total urea produced in the high-pressure urea synthesis section that is converted into melamine. A relatively large amount of melamine waste gas relative to the urea production in the synthesis section may be particularly unfavorable for CO2 stripping-type urea synthesis sections, because the amount of carbamate recycled becomes relatively large compared to the amount of feed CO2.

[0083] Water vapor from the evaporation section is condensed to form process condensate, which is typically purified in the wastewater treatment section using methods such as desorption and hydrolysis.

[0084] The LP gas stream is supplied to the LP urethane condenser to produce an LP urethane solution. The LP urethane solution is typically supplied directly or indirectly to the first MP urethane condenser, for example, via an MP scrubber or MP absorber.

[0085] LP urethane condensers typically receive an aqueous stream to provide water and prevent urethane precipitation. This aqueous stream originates, for example, from a wastewater treatment section. The amount of water in the LP urethane condenser can be adjusted as needed for the water requirements of the first and second MP urethane condensers.

[0086] Figure 1 An example urea production process according to a non-limiting embodiment of the present invention is illustrated schematically. It should be noted that only some, not all, of the units and connections shown in the drawings are referenced in the independent claims; other units and connections are not essential and are preferably used. The urea plant (100) includes an HP synthesis section (101) comprising a reactor (1), an HP stripping tower (2), and an HP carbamate condenser (3).

[0087] The urea reactor (1) is typically a vertical urea reactor having one or more inlets at the bottom and an outlet for a urea synthesis solution (4), the outlet being optionally used to draw the urea synthesis solution from the top of the reactor using a downcomer for the urea synthesis solution. For example, the reactor is provided with trays.

[0088] The HP stripper (2) is shown as a CO2 HP stripper that receives part or all of the CO2 feed. The stripper has an inlet for the urea synthesis solution (4), an outlet for the gas stream (5) located at the top relative to the HP carbamate condenser (3), and an outlet at the bottom portion for the stripped urea solution (8). In operation, the urea synthesis solution stream (4), or a portion thereof, is stripped in the HP stripper (2) to produce the stripped urea solution (8). Optionally, a portion of the urea synthesis solution stream (4) may be provided directly to the unit (9) for MP processing, bypassing the HP stripper (2).

[0089] The HP urethane condenser (3) is, for example, a horizontal urethane condenser with a U-shaped tube bundle containing a cooling fluid, and condensation takes place in a shell. The HP urethane stream (6) from the HP urethane condenser (3) is supplied to the urea reactor (1).

[0090] The HP synthesis section (101) also has an inlet (not shown) for NH3 feed, for example leading to the HP urethane condenser (3).

[0091] The HP synthesis section (101) has a separate outlet for the urea synthesis section exhaust gas stream (7). For example, the outlet is provided at the urea reactor (1) (as shown) or at the HP carbamate condenser (3). The urea synthesis section exhaust gas stream (7) contains an inert gas, such as one or more of the following: O2 for passivation, H2, CH4 and possibly other hydrocarbons contained in the CO2 feed, and argon and N2 from the CO2 and / or NH3 feed, and any air contained in the urea synthesis section. Passivation refers to corrosion protection in the urea synthesis section. In embodiments, the urea synthesis section is operated with, for example, at least 0.2 vol.% or at least 0.5 vol.% O2 relative to the CO2 feed; wherein O2 is introduced as air into any associated N2 in the embodiments of the urea synthesis section. Passivation oxygen or air is added, for example, to the CO2 feed.

[0092] It should be noted that these gaseous components are typically non-condensable and are discharged in urea plants.

[0093] The stripped urea solution (8) is expanded from HP to MP and supplied to a unit (9) for MP treatment, where at least gas / liquid separation is performed. The unit (9) for MP treatment is, for example, an adiabatic flash evaporation unit, or, for example, a dissociator comprising a heating unit (such as a heat exchanger) and gas / liquid separation. The unit (9) for MP treatment has an outlet connected to a first MP carbamate condenser (11) for a first MP gas stream (10), and an outlet for the MP urea solution (20). The MP urea solution is supplied to an LP recovery section (not shown).

[0094] The first MP urethane condenser (11) has an outlet for the MP condenser effluent (12) containing urethane solution and gas.

[0095] The first MP urethane condenser (11) is typically provided as part of a heat exchanger and has a heat exchange wall that is exposed on a first side to the condensation process medium of the first MP urethane condenser and on a second side to a cooling fluid (such as a urea solution).

[0096] The first MP urethane condenser (11) is preferably provided as a shell-and-tube heat exchanger, with cooling fluid in the tubes and condensation taking place in the shell. Preferably, in operation, at least a portion of the urea synthesis section waste gas stream (7) and the first MP gas stream (10) are condensed in the shell space of the first MP urethane condenser (11) to produce MP condenser effluent (12), while the urea solution (21) is heated in the tubes of the first MP urethane condenser (11) to produce heated urea solution (22).

[0097] The first MP urethane condenser (11) also receives the exhaust gas stream (7) from the synthesis section, which includes inert components, NH3 and CO2.

[0098] Based on the liquid outlet, the N / C ratio in the first MP urethane condenser (11) is preferably in the range of 2.0 to 2.5, more preferably in the range of 2.1 to 2.4, for example, about 2.3. The preferred N / C ratio of up to 2.4 avoids the loss of gaseous NH3 through the second MP gas stream (14) and avoids the need for separate NH3 condensation and liquid NH3 recycling to the HP synthesis section.

[0099] Advantageously, the combined condensation of the synthesis section exhaust gas stream (7) and the first MP gas stream (10) can provide NH3 to CO2 molar ratios within these ranges. Specifically, the synthesis section exhaust gas stream (7) may have a relatively high NH3 to CO2 molar ratio, such as 3.0-4.0, and the first MP gas stream (10) may have a relatively low NH3 to CO2 molar ratio, such as less than 2.0, for example in the range of 1.0-2.0. Specifically, in the case of adiabatic flash evaporation of the stripped urea solution (8) in the MP treatment unit (9), the NH3 to CO2 molar ratio of the first MP gas stream (10) can be 1.0-1.5. Due to the upstream CO2 stripping, the stripped urea solution (8) from the CO2 HP stripping tower typically has a relatively low N / C ratio.

[0100] The MP condenser effluent (12) undergoes gas / liquid separation (13) at medium pressure, for example, in a gas / liquid separation zone, to produce a second MP gas stream (14) and a first MP carbamate solution (15) (the first MP carbamate liquid stream). Typically, the inert components of the urea synthesis feed stream end up in the second MP gas stream (14). The gas / liquid separation (or separation zone) is, for example, a dedicated unit (as shown), or, for example, integrated into a single unit with the first MP carbamate condenser. For example, the first MP carbamate condenser has two separate outlets, for example, a top outlet for gas and a bottom outlet for liquid.

[0101] The relatively low N / C ratio in the first MP urethane condenser (11) enables more complete condensation in the condenser (11) and a relatively low NH3 content in the second MP gas stream (14). Thus, in an embodiment, the second MP gas stream (14) is supplied directly or indirectly to a scrubber or absorber (not shown), such as an LP absorber, which, for example, has a liquid outlet connected to a wastewater treatment section (WWT section). The increase in WWT load is advantageously relatively small, which contributes to better energy efficiency of the plant.

[0102] Therefore, the first MP carbamate solution (15) is essentially degassed and supplied part or all of it to the second MP carbamate condenser (17) with a very low content of gaseous inert components, which also receives MP melamine waste gas stream (16) from the melamine production section (102).

[0103] The MP melamine waste gas stream (16) is at least partially, preferably substantially entirely, condensed and / or absorbed in the second MP urethane condenser (17) to produce a second MP urethane solution (18), which is supplied directly or indirectly to the HP synthesis section (101), preferably to the HP urethane condenser (3). Preferably, the second MP urethane condenser (17) is provided, for example, as a shell-and-tube heat exchanger, wherein condensation takes place in the shell and the tubes serve as a second evaporation stage (25). Preferably, in operation, at least a portion of the melamine waste gas stream (16) undergoes condensation in the shell-side space of the second MP urethane condenser (17) to produce the second MP urethane solution (18), wherein a heated urea solution (22) is heated in the tubes of the second MP urethane condenser (17) to produce an additional heated urea solution (23).

[0104] Due to the lower portion of the inert gas pressure and the corresponding higher portion of the vapor pressure of NH3 and CO2 in the second MP urethane condenser (17), the very low inert gas content in the first MP urethane solution (15) (i.e., as obtained from gas / liquid separation) allows the second MP urethane condenser (17) to be operated at a relatively higher temperature than any fixed absolute (or total) operating pressure used for the second MP urethane condenser (17), compared to processes in which gas / liquid separation is not used. The relatively higher condensation temperature allows for a lower water content in the second MP urethane solution (18) without the risk of urethane crystallization or precipitation, thereby reducing the water content of the reaction mixture in the reactor and advantageously increasing the urea yield, i.e., increasing the urea conversion rate in the synthesis section. The water content of the first MP urethane solution (15) is generally sufficient for operating the second MP urethane condenser (17) and is generally set to the minimum required for safe operation of the first MP urethane condenser (11). The water content of the first MP carbamate solution (15) can be adjusted, for example, by the amount of aqueous liquid supplied to the LP carbamate condenser. Suitable sources of the aqueous liquid are diluted carbamate solution from the wastewater treatment section (associated with the evaporation section) and purified process condensate from the wastewater treatment section.

[0105] The first MP urethane condenser (11) also receives a urethane solution stream (19) preferably directly or indirectly from the LP urethane condenser (not shown). This urethane solution stream (19) contains H2O to prevent urethane crystallization in the first and second MP urethane condensers.

[0106] The first MP urethane condenser (11) is in heat exchange contact with the urea solution (21) to be heated in the first evaporation stage (24) via its walls. The urea solution (21) is derived, for example, directly or indirectly from the LP recovery section (not shown). The heated urea solution (22) is supplied to the second evaporation stage (25) after gas / liquid separation to remove water vapor. The second evaporation stage is in heat exchange contact with the second MP urethane condenser (17) via its walls, where the urea solution is heated to provide an additional heated urea solution (23). This urea solution can then be further heated, for example by using steam, to produce urea melt.

[0107] The present invention also provides an integrated urea and melamine production plant, preferably suitable for implementing the process of the present invention. The plant includes a gas / liquid separator or separation zone for subjecting the effluent from the MP condenser to gas / liquid separation; this separator or separation zone may be integrated, for example, downstream of a first MP urethane condenser, or provided, for example, as a separate unit. The gas / liquid separator or separation zone has a liquid outlet in fluid communication with the inlet liquid flow of a second MP urethane condenser. The gas / liquid separator or separation zone also has a separate gas outlet in fluid connection with the inlet fluid of, preferably, an absorber or scrubber, which is preferably operated at LP.

[0108] Preferably, the melamine production section is a non-catalytic high-pressure type.

[0109] Preferably, the melamine production section includes a melamine synthesis reactor and a scrubbing section for washing the raw waste gas stream from the melamine synthesis reactor with urea.

[0110] Preferably, the high-pressure urea synthesis section is of the CO2 stripping type, which includes a high-pressure CO2 stripping tower having an outlet for urea solution connected to the MP processing unit.

[0111] Preferably, a first MP urethane condenser is in heat exchange contact with a first evaporation stage, a second MP urethane condenser is in heat exchange contact with a second evaporation stage, and the plant includes liquid flow lines for supplying urea solution from the first evaporation stage (preferably via gas / liquid separation) to the second evaporation stage. The first and second evaporation stages are also included in the plant; the first evaporation stage is directly or indirectly connected to receive urea solution specifically derived from the MP processing unit, typically via an LP recovery section.

[0112] A method for retrofitting an existing urea plant is also provided. The existing urea plant includes an HP urea synthesis section of the stripping type, having an outlet for a urea solution (also containing carbamates) and a separate outlet for the urea synthesis section waste gas stream (containing inert gases, NH3, and CO2). The method involves adding a melamine production section (if not already present) to the existing urea plant, having an outlet for the melamine waste gas stream. Thus, in one embodiment, the existing plant is already used for the integrated production of melamine and urea; in other embodiments, the plant is retrofitted to such a purpose.

[0113] The method involves adding the following (if not already present) to an existing urea plant: an MP treatment unit for treating a urea solution under medium pressure, wherein at least a portion of the carbamate contained in the urea solution dissociates into CO2 and NH3, the MP treatment unit having an outlet for the MP urea solution and an outlet for a first MP gas stream; and a first MP carbamate condenser for condensing at least a portion of the urea synthesis section waste gas stream and the first MP gas stream to produce an MP condenser effluent. The method further includes adding: a gas / liquid separator for gas / liquid separation of the MP condenser effluent to produce a second MP gas stream and the first MP carbamate solution; a second MP carbamate condenser for condensing the melamine waste gas stream in the presence of at least a portion of the first MP carbamate solution to produce a second MP carbamate solution; and a liquid flow line for directly or indirectly supplying the second MP carbamate solution to the urea synthesis section. The modified plant is preferably a plant according to the invention and is preferably adapted to the urea production process of the invention.

[0114] As used herein, the term 'carbamate' when used in the urea production field refers to ammonium carbamate.

[0115] As used herein, for the process flow in a urea plant (i.e., not for steam lines), high pressure (HP) is above 100 bar, for example, 120 bar to 300 bar, or 140 bar to 200 bar. Medium pressure (MP) is, for example, 10 bar to 80 bar (including intermediate pressures of 30 bar to 70 bar), specifically 15 bar to 30 bar, and low pressure (LP) is, for example, 0 bar to 10 bar, specifically 1 bar to 8 bar or 2 bar to 5 bar. All pressures are absolute pressures (bara).

[0116] The terms 'typical' and 'in particular' are used to indicate features that may be used in some embodiments but are not mandatory. Preferred features are also not mandatory.

[0117] For all instances of the phrase “at least a portion” of the stream, preferably at least 50 wt.% or at least 90% of the stream, optionally the entire stream.

[0118] The N / C ratio of a gas stream indicates the molar ratio of NH3 to CO2. The N / C ratio used in this paper for the urea synthesis section reflects the composition of the so-called initial mixture before urea production, as used in urea plants, consisting only of NH3, CO2, and H2O, and is expressed in molar ratio. The N / C ratio of a carbamate solution indicates the molar ratio of corresponding amounts of NH3 to CO2, i.e., the amounts of NH3 and CO2 based on the total amount of free and carbamate, where 1 mol of carbamate corresponds to 2 mol of NH3 and 1 mol of CO2. The N / C ratio of a carbamate condenser refers to the N / C ratio of the carbamate solution at the relevant outlet.

[0119] As used herein, the term 'melamine exhaust gas' refers to an exhaust gas stream originating from the melamine production area, and specifically refers to a gas stream primarily containing NH3, CO2, and possibly H2O and small amounts of inert gases.

[0120] The term 'first' as used herein for a unit or step allows for the existence of additional instances upstream of such a unit or step.

[0121] All process-related priorities and details discussed also apply to the plant, and vice versa. All plant-related priorities and details described also apply to the methods used to transform the plant.

[0122] The embodiments of the present invention will now be further illustrated by the following examples, which do not limit the invention or the claims.

[0123] Example 1

[0124] In a design example, according to Figure 1 The integrated urea-melamine plant with a capacity of 3700 MTPD (metric tons / day) urea was designed to operate according to the process design parameters given in Table 1. 1200 MTPD of urea was used for melamine production, and the remaining urea was obtained as a solid urea product after refining.

[0125] like Figure 1 As shown, the first MP urethane condenser receives urea synthesis waste gas and gases from the MP treatment unit, and selectively supplies liquid from the first MP urethane condenser to a second MP urethane condenser that receives melamine waste gas. Non-condensable gases from the first MP urethane condenser are degraded to the LP unit and processed separately.

[0126] The example urea plant includes an HP CO2 stripping tower, and the MP process is adiabatic flash evaporation.

[0127] Compared to a reference process using a single MP urethane condenser (receiving urea synthesis section exhaust gas, melamine exhaust gas, gas from the MP treatment, and urethane solution from the LP urethane condenser), approximately 20-25 kg of steam per tonne of urea can be saved by using two MP urethane condensers in series and removing inert gases between them. Steam consumption refers to the steam consumption of the HP stripper, and specifically the 20-25 bar steam used in the HP stripper. The lower water recirculation in urea synthesis allows for less heating operation of the HP stripper, thus reducing steam consumption.

[0128] In Table 1, the percentage of inert gases refers to the vol.% (based on outlet temperature) of inert gases in the gas phase from the condenser. Besides inert gases, the gas phase contains NH3, CO2, and H2O.

[0129] Both MP urethane condensers operated at the same (total) pressure of 24 bara, but the partial vapor pressures of NH3 and CO2 in the second urethane condenser were higher than those in the first urethane condenser. This was because the inert gas levels in the second urethane condenser were lower, resulting in a higher condensation temperature. The higher temperature, in turn, allowed the second urethane condenser to operate with a lower liquid phase wt.% without the risk of urethane crystallization. Consequently, the amount of water (kg water / tonne urea production) recycled from the second urethane condenser to the urea synthesis section was lower than that in the comparative embodiment using a single MP urethane condenser, which received melamine waste gas, urea synthesis section waste gas, and gases from the MP treatment.

[0130] Both MP urethane condensers are provided as shell-side compartments of the corresponding shell-and-tube heat exchangers, with the gas to be condensed in the shell and the heated urea solution in the tubes.

[0131] Table 1

[0132]

[0133]

Claims

1. An integrated urea and melamine production process, comprising: - Urea is produced in a stripping-type high-pressure (HP) urea synthesis section (101), thereby producing a stripped urea solution (8) that also contains carbamate, and a separate urea synthesis section exhaust gas stream (7) containing inert gases, NH3 and CO2. -Produces melamine, thereby providing melamine waste gas flow (16). - subject the stripped urea solution (8) to medium pressure (MP) treatment (9), wherein at least some of the carbamates contained in the stripped urea solution (8) dissociate into CO2 and NH3, thereby generating MP urea solution (20) and a first MP gas stream (10). - At least a portion of the urea synthesis section exhaust gas flow (7) and the first MP gas flow (10) are condensed in a first medium-pressure (MP) urethane condenser (11) to produce MP condenser effluent (12). - The MP condenser effluent (12) is subjected to gas / liquid separation (13) to produce a second MP gas stream (14) containing the inert gas and a first MP carbamate solution (15). - The melamine waste gas stream (16) is condensed in a second MP urethane condenser (17) in the presence of at least a portion of the first MP urethane solution (15) to produce a second MP urethane solution (18); and - The second MP carbamate solution (18) is supplied directly or indirectly to the urea synthesis section (101).

2. The process according to claim 1, wherein the melamine production is carried out in a non-catalytic high-pressure melamine reactor.

3. The process according to claim 1 or 2, wherein the condensation temperature in the second MP urethane condenser (17) is at least 5°C higher than the condensation temperature in the first MP urethane condenser (11).

4. The process according to claim 1, wherein the melamine waste gas stream (16) contains less than 15 wt.% water.

5. The process according to claim 4, wherein the melamine waste gas stream (16) contains less than 2 wt.% water.

6. The process according to claim 1, wherein the production of melamine includes washing the raw waste gas stream from the melamine synthesis reactor with urea.

7. The process according to claim 1, wherein the HP urea synthesis section (101) is of the CO2 stripping type, and the urea synthesis solution (4) from the reaction zone is subjected to high-pressure CO2 stripping before the MP treatment (9).

8. The process according to claim 7, wherein the MP treatment (9) includes depressurizing the urea solution from HP to MP in a depressurization unit, thereby releasing gas from the solution, and performing gas / liquid separation on the first MP gas stream from the urea solution.

9. The process of claim 7, wherein the MP treatment (9) comprises depressurizing the urea solution from HP to MP in a valve, thereby releasing gas from the solution, and performing gas / liquid separation on the first MP gas stream from the urea solution.

10. The process according to claim 8 or 9, wherein the MP treatment (9) is adiabatic flash evaporation.

11. The process according to claim 1, wherein both the first MP urethane condenser and the second MP urethane condenser (11, 17) are in heat exchange contact with the first evaporation stage (24) and the second evaporation stage (25) to heat the urea solution.

12. The process according to claim 11, wherein the first MP urethane condenser (11) is in heat exchange contact with the first evaporation stage (24), the second MP urethane condenser (17) is in heat exchange contact with the second evaporation stage (25), and heated urea solution (22) is supplied from the first evaporation stage (24) to the second evaporation stage (25).

13. The process according to claim 12, wherein the heated urea solution (22) is supplied from the first evaporation stage (24) to the second evaporation stage (25) via gas / liquid separation.

14. The process according to claim 12 or 13, wherein the first MP urethane condenser is provided as a shell-and-tube heat exchanger, wherein at least a portion of the urea synthesis section waste gas stream (7) and the first MP gas stream (10) are condensed in the shell space of the first MP urethane condenser (11) to produce the MP condenser effluent (12), wherein a urea solution (21) is heated in the tubes of the first MP urethane condenser (11) to produce the heated urea solution (22), wherein the second MP urethane condenser (17) is provided as a shell-and-tube heat exchanger, wherein at least a portion of the melamine waste gas stream (16) is condensed in the shell-side space of the second MP urethane condenser (17) to produce the second MP urethane solution (18), and wherein the heated urea solution (22) is heated in the tubes of the second MP urethane condenser (17) to produce a further heated urea solution (23).

15. An integrated urea and melamine production plant, comprising: - A stripping-type high-pressure (HP) urea synthesis section (101), the stripping-type HP urea synthesis section having an outlet for a stripped urea solution (8) that also contains carbamate, and a separate outlet for a urea synthesis section exhaust gas stream (7) containing inert gases, NH3 and CO2. - Melamine production section (102), the melamine production section having an outlet for melamine waste gas flow (16); -MP processing unit (9), the MP processing unit is used to process the urea solution at medium pressure, wherein at least some of the carbamates contained in the stripped urea solution (8) dissociate into CO2 and NH3, the MP processing unit having an outlet for MP urea solution (20) and an outlet for a first MP gas stream (10); - First MP carbamate condenser (11), the first MP carbamate condenser is used to condense at least a portion of the urea synthesis section waste gas flow (7) and the first MP gas flow (10) to produce MP condenser effluent (12). - Gas / liquid separation zone (13), which is used to subject the MP condenser effluent (12) to gas / liquid separation to produce a second MP gas stream (14) and a separate first MP carbamate solution (15). - A second MP carbamate condenser (17), the second MP carbamate condenser being used to condense the melamine waste gas stream (16) in the presence of at least a portion of the first MP carbamate solution (15) to produce a second MP carbamate solution (18); and - A liquid flow line for supplying the second MP carbamate solution (18) directly or indirectly to the urea synthesis section (101).

16. The integrated urea and melamine production plant according to claim 15, wherein the melamine production section (102) is of the non-catalytic high-pressure type and includes a melamine synthesis reactor and a washing section for washing the raw waste gas stream from the melamine synthesis reactor with urea, and wherein the HP urea synthesis section (101) is of the CO2 stripping type and includes a high-pressure CO2 stripping tower (2) having an outlet for urea solution (8) connected to the MP treatment unit (9).

17. The integrated urea and melamine production plant according to claim 15 or 16, wherein the first MP urethane condenser (11) is in heat exchange contact with the first evaporation stage (24), the second MP urethane condenser (17) is in heat exchange contact with the second evaporation stage (25), and the plant includes a liquid flow line for supplying heated urea solution (22) from the first evaporation stage (24) to the second evaporation stage (25).

18. The integrated urea and melamine production plant according to claim 17, wherein the liquid flow line is used to supply heated urea solution (22) from the first evaporation stage (24) to the second evaporation stage (25) via gas / liquid separation.

19. A method for retrofitting an existing urea plant, the existing urea plant comprising: - A stripping-type high-pressure (HP) urea synthesis section (101), the stripping-type HP urea synthesis section having an outlet for a stripped urea solution (8) that also contains carbamate, and a separate outlet for a urea synthesis section exhaust gas stream (7) containing inert gases, NH3 and CO2. The method includes: If the following devices are not installed in the existing urea plant, then add the following devices to the existing urea plant: - Melamine production section (102), the melamine production section having an outlet for melamine waste gas flow (16); -MP processing unit (9), the MP processing unit is used to process the urea solution at medium pressure, wherein at least some of the carbamates contained in the urea solution dissociate into CO2 and NH3, the MP processing unit has an outlet for MP urea solution (20) and an outlet for first MP gas stream (10); - First MP carbamate condenser (11), the first MP carbamate condenser is used to condense at least a portion of the urea synthesis section waste gas flow (7) and the first MP gas flow (10) to produce MP condenser effluent (12). - Gas / liquid separation zone (13), the gas / liquid separation zone is used to subject the MP condenser effluent (12) to gas / liquid separation to produce a second MP gas stream (14) and a first MP carbamate solution (15). And the method described therein includes adding: - Second MP carbamate condenser (17), the second MP carbamate condenser is used to condense the melamine waste gas stream (16) in the presence of at least a portion of the first MP carbamate solution (15) to produce a second MP carbamate solution (18). as well as - A liquid flow line for supplying the second MP carbamate solution (18) directly or indirectly to the urea synthesis section (101).

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