Urea and melamine production

Abstract

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

Description

Technical Field

[0001] The present invention relates to an integrated process for producing urea and melamine. Background Art

[0002] Melamine plants produce large amounts of CO 2 and NH 3 It is desirable to supply these gases to a urea plant, where they are used as reactants. Various possibilities for coupling and integrating a urea plant and a melamine plant have been proposed in the art.

[0003] Ullmann's Encyclopedia of Industrial Chemistry, chapter "Urea", 2010 (hereinafter referred to as Ullmann's Urea, 2010) describes example urea production plants. Some well-known types of urea plants include conventional plants (without high pressure stripping) and plants using, for example, Stamicarbon CO 2 Stripping Design, Toyo CO 2 Stripping plants designed by Snamprogetti and thermal stripping.

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

[0005] The article "Urea-melamine plant integration", Nitrogen + Syngas 321, January-February 2013, pages 44-54, describes urea-melamine integration methods. One method is to condense the melamine plant off-gases by mixing them with a lean carbamate solution received from the urea plant. Another method used in the case of a Sinampujiti type urea plant is described as condensing the off-gases at 20 bar(g) inside the melamine plant and delivering the resulting carbamate solution to the urea high pressure section. For this type of urea plant, another method is described as condensing the off-gases in the low pressure, medium pressure or high pressure section of the urea plant, as appropriate, in an existing condenser or in parallel condensers.

[0006] Ullmann's Encyclopedia of Industrial Chemistry, chapter "Melamine", 2003 describes various melamine production processes.

[0007] US2016 / 0194293 describes an integrated process for the production of urea and melamine, wherein the offgases from the melamine plant are condensed at medium pressure (20-30 bar) in a condenser which also receives carbamate from the low pressure recovery section of the urea plant. In one embodiment, the offgases from the melamine plant are condensed in a first medium pressure (MP) carbamate condenser which also receives carbamate solution from the low pressure (LP) recovery section. The effluent of the condenser is supplied to a second MP carbamate condenser which also receives a gas stream from a dissociator which 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 CO condenser. 2 Stripper. Urea plants use high pressure (HP) scrubbers. The gas from the reactor is fed to the scrubber where it is scrubbed with a carbamate solution and the non-condensable gases are discharged as "inert gases". Part of the CO 2 The feed is supplied to the LP carbamate condenser.

[0008] Another reference for coupling a urea plant with a melamine plant is US 2007 / 0282102.

[0009] The high-pressure scrubber in a urea plant is a complex and expensive piece of equipment that has certain safety risks due to the high pressure and inert gas concentration it operates under. It should be noted that inert gases usually contain O 2 , used to prevent corrosion by passivation in the synthesis section, usually supplied as passivation air, also contains N 2 , and from CO 2 Feed H 2 Ullmann Urea, page 25 discusses avoiding the formation of explosive H in urea plants where non-condensable gases are discharged. 2 / O 2 Necessity of mixture. It is generally desirable to reduce the amount of HP equipment. US2019 / 0015811 Background reference discusses the advantages of using HP scrubbers for distribution.

[0010] US2016 / 0318883 shows in FIG. 10 a 2 Process scheme of a urea plant with an HP stripper, in which the stripped urea solution is subjected to an adiabatic flash evaporation. The resulting gas is subjected to condensation together with the offgas from the reactor and the offgas from the melamine plant. Summary of the invention

[0011] The present invention aims to provide a better plant and process 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, which uses in series: an MP first carbamate condenser, which is used to condense the off-gases of at least the urea synthesis section to produce a medium pressure (MP) condenser effluent; a gas / liquid separation of the MP condenser effluent to produce a gas stream and a liquid stream; and a second carbamate condenser, which is used to receive the off-gases from melamine production and the liquid stream.

[0013] One aspect of the present invention relates to an integrated process for the production of urea and melamine, comprising: producing urea in a high-pressure urea synthesis section of the stripping type, thereby producing a urea solution also comprising carbamate, preferably a stripped urea solution, and a urea solution comprising an inert gas, NH 3 and CO 2 a separate urea synthesis section off-gas stream of urea synthesis section; producing melamine, thereby providing a melamine off-gas stream; preferably subjecting the urea solution, preferably the urea solution being a stripped urea solution, to an MP treatment, wherein at least a portion of the carbamate contained in the urea solution is dissociated into CO 2 and NH 3 , thereby producing an MP urea solution and a first MP gas stream; subjecting the urea synthesis section offgas stream and preferably at least a portion of the first MP gas stream to condensation in a first MP carbamate condenser to produce an MP condenser effluent and / or subjecting gases from the offgas outlet of the urea synthesis section and preferably gases from the MP treatment to condensation in the first MP carbamate condenser; subjecting the MP condenser effluent to gas / liquid separation to produce a second MP gas stream comprising the inert gas and a first MP carbamate solution; subjecting the melamine offgas stream to condensation in a second MP carbamate condenser in the presence of at least a portion of the first MP carbamate solution to produce a second MP carbamate solution; and supplying the second MP carbamate solution directly or indirectly to the urea synthesis section.

[0014] The invention also relates to an integrated urea and melamine production plant comprising: a high-pressure urea synthesis section of the stripping type having an outlet for a urea solution further comprising carbamate, and an outlet for a urea solution comprising an inert gas, NH 3 and CO 2a separate outlet for a urea synthesis section offgas stream; a melamine production section having an outlet for a melamine offgas 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 are dissociated into CO 2 and NH 3 , the MP treatment unit having an outlet for an MP urea solution and an outlet for a first MP gas stream; a first MP carbamate condenser, the first MP carbamate condenser being used to subject the urea synthesis section offgas stream and preferably at least a portion of the first MP gas stream to condensation to produce an MP condenser effluent; a gas / liquid separator, the gas / liquid separator being 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, the second MP carbamate condenser being used to subject the melamine offgas stream to condensation 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, the liquid flow line being used to supply the second MP carbamate solution directly or indirectly to the urea synthesis section.

[0015] The invention also relates to a method for converting an existing urea plant comprising: a high-pressure urea synthesis section of the stripping type having an outlet for a urea solution further comprising a carbamate, and an outlet for a urea solution comprising an inert gas, NH 3 and CO 2 a separate outlet for a urea synthesis section off-gas stream of the urea synthesis section; wherein the method involves: adding the following, if not already present in the existing urea plant: a melamine production section having an outlet for a melamine off-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 are dissociated into CO 2 and NH 3, the MP treatment unit having an outlet for an MP urea solution and an outlet for a first MP gas stream; a first MP carbamate condenser, the first MP carbamate condenser being used to subject the urea synthesis section offgas stream and preferably at least a portion of the first MP gas stream to condensation to produce an MP condenser effluent; wherein the method comprises adding: a gas / liquid separator, the gas / liquid separator being 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, the second MP carbamate condenser being used to subject the melamine offgas stream to condensation 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, the liquid flow line being used to supply the second MP carbamate solution directly or indirectly to the urea synthesis section. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0017] Any embodiments shown in the accompanying drawings are examples only and do not limit the present invention. DETAILED DESCRIPTION

[0018] The present disclosure is broadly based on the sensible insight that, by upstream gas / liquid separation, the condensation of the melamine off-gases is advantageously carried out with a relatively low water content of the resulting second carbamate solution. Thus, the condensation of the melamine off-gases can be carried out with the amount of water already contained in the first MP carbamate solution; the minimum amount of water in the first MP carbamate condenser is determined by the amount of NH4+ from the urea synthesis section off-gases that needs to be removed. 3 and CO 2 The inert gas in the urea synthesis section off-gas (e.g., passivation air) does not negatively affect the melamine off-gas condensation temperature and pressure, and does not negatively affect the water content of the carbamate recovery stream. The presence of non-condensable inert gas in the second MP carbamate condenser will cause the condensation temperature of the second MP carbamate condenser to decrease at a given operating pressure.

[0019] The present invention relates to a method for treating melamine plant waste gas (which contains CO 2 and NH 3 ) is supplied to the urea plant, specifically indirectly to the high pressure synthesis section to couple the melamine plant with the urea plant.

[0020] The integrated urea and melamine production process of the present invention comprises urea production and melamine production. Integration as used herein means that melamine waste gas (i.e., waste gas from a melamine plant) is supplied to a high-pressure urea synthesis section after condensation. The production of urea involves producing urea in a high-pressure urea synthesis section of the stripping type, thereby producing a urea solution comprising urea and water, and also comprising carbamate, and producing a urea solution comprising inert gas, NH 3 and CO 2 For the purposes of the process of the present invention, inert gases are non-condensable and comprise, for example, N 2 and O 2 As an alternative to or in addition to the term 'inert', it may be specified that the exhaust gas contains 3 , CO 2 and H 2 Gas components other than O.

[0021] A urea synthesis section offgas stream, such as obtained from a urea synthesis section, comprises, for example, at least 2.0 vol.% inert gases, or for example at least 4.0 vol.%, such as from 2 vol.% to 12 vol.% inert gases, based on the total inert gases, and / or wherein the inert gases are excluding NH 3 , CO 2 and H 2 O; and / or the exhaust gas stream contains at least 2.0 vol.% N 2 , O 2 H 2 and CH 4 , such as at least 4.0 vol.%, such as in the range of 2.0-10 vol.%; or 4.0-10%, or 4.0-8.0 vol.%.

[0022] Urea production involves NH 3 With CO 2 The high pressure urea synthesis section comprises a reaction zone operated at a pressure of more than 100 bar, preferably more than 120 bar, and a high pressure stripping tower operated at a pressure of more than 80 bar, for example more than 120 bar. The stripping tower is, for example, a CO 2 The synthesis section also comprises a condensation section for condensing the gases coming from the stripper, operating at a pressure above 80 bar or above 120 bar. The urea synthesis section offgas stream typically comes from the reactor and / or the carbamate condenser contained in the synthesis section.

[0023] The process relates to 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, such as 10-130 bar, or 10-80 bar, or 10-60 bar, or 10-40 bar. The melamine waste gas comprises NH 3 and CO 2 Melamine production uses urea as a raw material. The melamine synthesis pressure is usually higher than 70 bar, and usually at least the gaseous components in the melamine synthesis effluent are depressurized to a pressure of less than 80 bar, more preferably less than 60 bar or less than 40 bar, and usually 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 the HP stripper, to a medium pressure treatment, i.e. a treatment at medium pressure, for example a pressure of at least 10 bar and / or less than 80 bar. In this treatment, at least some of the carbamates contained in the urea solution are dissociated into CO 2 and NH 3 , thereby obtaining a first MP gas flow and a MP urea solution.

[0025] The most commonly used MP process generally involves depressurizing the urea solution from HP to MP using a depressurization unit (e.g., a valve), thereby releasing gas from the solution, and performing a gas / liquid separation of the first MP gas stream from the urea solution. The depressurization causes the carbamate contained in the urea solution to spontaneously dissociate into NH 3 and CO 2 , at least a portion of which is released from the solution as a gas, i.e. flashed, so that a first MP gas stream is formed. Thus, the MP treatment preferably involves flashing, optionally in combination with heating.

[0026] For example, the process may involve an adiabatic flash. An adiabatic flash involves pressure reduction and gas / liquid separation.

[0027] The MP treatment may also involve, for example, decompression to MP, heating at MP and gas / liquid separation at MP. The treatment may also involve, for example, decompression to MP, gas / liquid separation (e.g., flashing) at MP, wherein the flashing is, for example, adiabatic, heating the urea solution at MP, and gas / liquid separation at MP, preferably these three steps are performed in this order. Heating at MP may be performed using heat exchange with a heating fluid (e.g., steam, steam condensate) or process-process heat exchange with a fluid in the synthesis section, for example, the MP heating is performed in a bundle of a pool condenser or pool reactor contained in the urea synthesis section. Optionally, the MP urea solution after gas / liquid separation is contacted with gas from an MP (adiabatic) flasher to rectify the urea solution at MP, wherein the rectified urea solution is expanded to LP and supplied to the LP dissociator of the LP recovery section, and wherein the gas from the MP rectification unit is supplied together with the urea synthesis section off-gas, and preferably also supplied together with the gas from the MP gas / liquid separation.

[0028] Optionally, the treated urea solution is further subjected to CO 2 Countercurrent contact to remove NH from the treated urea solution 3 .

[0029] The process involves subjecting part or all of the urea synthesis section off-gas stream, preferably as well as part or all of the first MP gas stream, to condensation in a first MP carbamate condenser at medium pressure to produce an MP condenser effluent. The effluent is specifically a fluid comprising gas and liquid at medium pressure.

[0030] In addition, the process involves subjecting the MP condenser effluent to a gas / liquid separation at medium pressure to produce a second MP gas stream and a first MP carbamate solution. For example, the separation is performed in a gas / liquid separation unit having a gas outlet and a liquid outlet. However, in embodiments in which the first MP carbamate condenser has separate outlets for gas and liquid, the separation may also be performed in the first MP carbamate condenser. It is also possible to integrate the gas / liquid separation unit (separator) and the first MP carbamate condenser in a single container or apparatus, for example, performing a gas / liquid separation at the outlet side.

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

[0032] The steps of producing melamine are usually carried out in non-catalytic high-pressure type melamine plants (melamine synthesis pressure above 70 bar), i.e. in non-catalytic high-pressure melamine reactors. Melamine synthesis is based on the pyrolysis of molten urea and the melamine formation reaction forms CO 2 and NH 3 As a by-product.

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

[0034] Melamine synthesis is carried out at a pressure above 70 bar, usually at a pressure of less than 130 bar. An example suitable melamine production process operating at 80 bar is described in EP2385043A1. US2004 / 0162429A1 describes another example suitable melamine production process with a scrubbing section for forming a water-free waste gas from the raw waste gas using a urea melt.

[0035] In some embodiments, the melamine synthesis pressure is more than 10 bar lower than the urea synthesis pressure, such as more than 20 bar lower, and for example at least 10 bar lower or at least 20 bar lower than the pressure of the HP reactor, HP stripper and HP carbamate 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, such as 120-200 bar.

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

[0037] For example, the process also involves depressurizing the raw off-gas stream and / or depressurizing the purified off-gas.The purified off-gas is supplied in gas phase to the second MP carbamate condenser in the process of the present invention.

[0038] The melamine waste gas stream (particularly after washing) contains, for example, less than 15 wt.% of water, or less than 10 wt.% of water, or less than 2 wt.% of water, relative to the total gas stream, the water indicating H 2 O; for example, in embodiments using urea scrubbing; or preferably less than 15 vol.% H 2 O, or less than 10 vol.% or less than 2 vol.% H 2 O. For example, urea scrubbing can produce an anhydrous off-gas containing less than 0.5 wt.% water. In embodiments where the water content of the melamine off-gas is relatively low, the water fraction of the first MP urethane solution can be fully utilized to condense the melamine off-gas. The melamine off-gas contains, for example, at least 95 vol.% or at least 98 vol.% NH 3 and CO 2 .

[0039] Melamine waste gas contains, for example, less than 0.10 vol. % of inert gases, for example negligible inert gases.

[0040] The melamine off-gas is typically provided at a pressure in the range of 10 to 130 bar, for example 10 to 80 bar, preferably 15 to 45 bar, more preferably 20 to 30 bar. In some embodiments, the off-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 least 10 bar lower or at least 20 bar lower than the pressure of the HP synthesis section, i.e. the pressure of each of the HP reactor, the HP stripper and the HP carbamate condenser.

[0041] In the process of the present invention, the water fraction of the first MP carbamate solution is used to condense the melamine off-gases to form a second carbamate solution. Thus, the second carbamate solution contains the water 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 off-gases.

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

[0043] In the present invention, it is very advantageous if, by upstream gas / liquid separation, the condensation of the melamine offgases is carried out with a relatively low water content of the resulting second urethane solution.

[0044] Thus, the condensation of the melamine offgases can be carried out with the amount of water already contained in the first MP carbamate solution; the minimum amount of water in the first MP carbamate condenser is determined by the need to convert the NH ions from the urea synthesis section offgases into urea. 3 and CO 2 Fully condensed to give the carbamate.

[0045] In the present invention, in a first aspect, the urea synthesis section off-gas is advantageously decompressed to medium pressure and subjected to condensation at medium pressure. Thus, there is no need to use a high-pressure scrubber, thereby advantageously simplifying the HP urea synthesis section, in particular for CO 2 Urea plant of stripping type.

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

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

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

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

[0050] Preferably, the N / C ratio in the first MP carbamate condenser is in the range of 2.0 to 2.5, more preferably in the range of 2.1-2.4, such as about 2.3, based on the liquid outlet composition. With these N / C values, it is possible to obtain a relatively low NH 3 The amount is advantageously sufficiently completely condensed.

[0051] The first MP carbamate condenser may comprise one or more stages. Furthermore, the urea synthesis section is optionally subjected to condensation upstream of the first MP carbamate condenser, wherein the gas from this upstream condensation is supplied to the first MP carbamate condenser. Optionally, the first MP gas stream is subjected to condensation upstream of the first MP carbamate condenser, wherein the gas from this upstream condensation is supplied to the first MP carbamate condenser. Preferably, the plant comprises a condenser which receives gas from the gas outlet of the HP urea synthesis section and receives gas from the gas outlet of the MP treatment unit; and the process involves a condensation step carried out in a condensation zone which receives gas from the gas outlet of the HP urea synthesis section and receives gas from the gas outlet of the MP treatment unit, i.e. receives at least a portion, e.g. all, of the urea synthesis section off-gas and at least a portion, e.g. all of the first MP gas stream.

[0052] Preferably, the carbamate condensation in the first MP carbamate condenser is carried out in heat exchange contact with a urea solution to be heated, in particular with a urea solution directly or indirectly coming from a LP dissociator, to produce a heated urea solution. This heating, preferably carried out with water removal, is used, for example, to concentrate the urea solution from about 72 wt.% urea to about 75-80 wt.% urea (urea including biuret). Preferably, the urea solution is at subatmospheric pressure, for example 0.3-1.0 bar. The plant, for example, comprises a shell and tube heat exchanger, in which the first MP gas stream and the urea synthesis offgas stream are condensed in the shell, i.e. the condensation of the gases from the MP treatment and the gases from the urea synthesis section in the shell, and the urea solution is heated in the tubes; more preferably with vertical tubes. The shell as used for a shell and tube heat exchanger refers to the shell side space of the heat exchanger. Optionally, the urea solution is subjected to a gas / liquid separation to remove water vapor downstream of the tubes; this gas / liquid separation of the urea solution is performed separately and distinct from the gas / liquid separation of the condensed 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 is subjected to a gas / liquid separation in a separate unit. In embodiments in which 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 carbamate condenser is, for example, an absorber, ie does not involve indirect heat exchange with a cooling liquid; for example, if the amount of urea synthesis section off-gas is small.

[0054] Optionally, the first MP carbamate condenser has a first outlet for liquid and a separate second outlet for gas, and the gas / liquid separation is performed internally in the condenser; for example, the outlets are vertically spaced apart with a bottom liquid outlet and a top gas outlet.

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

[0056] The second MP carbamate condenser may comprise one or more stages. Preferably, the carbamate condensation in the second MP carbamate 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 a LP dissociator, more preferably with a heated urea solution from (the tube side of) the first MP carbamate condenser. Preferably, the urea solution is at a subatmospheric pressure, for example 0.3-1.0 bar. The plant, for example, comprises a shell and tube heat exchanger, in which the melamine waste gas is condensed in the shell and the urea solution is heated in the tube, preferably wherein the heated urea solution is further heated in the tube. The pressure of the tube is, for example, 0.2-0.5 bar. Optionally, the urea solution is further heated downstream of the heating in the second MP carbamate 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 being used as the second MP carbamate condenser, and the second compartment having steam. For example, the first shell compartment has an inlet for the gas at the bottom and an outlet for the two-phase fluid effluent at the top, or vice versa, in particular with a vertical tube bundle. The urea solution stream is optionally subjected to a gas / liquid separation to remove water vapor downstream of the tubes. In a preferred embodiment, a vertical tube bundle is used, with an inlet for the urea solution at the upper end of the tube bundle, the liquid flowing downwards in the tubes and the gas flowing upwards in operation. For example, a urea melt containing at least 95 wt.% of urea is obtained from the tubes.

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

[0058] Preferably, both the first MP carbamate condenser and the second MP carbamate condenser are in heat exchange contact with an evaporation stage for heating the urea solution through a wall, for example as a shell and tube heat exchanger, wherein condensation takes place in the shell and heating of the urea solution takes place in the tubes. Preferably, the first MP carbamate condenser is in heat exchange contact with the first evaporation stage, the second MP carbamate 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 by gas / liquid separation. Thus, a lower inert content in the second MP carbamate condenser contributes to further concentrating the urea solution in the second evaporation stage. Increasing the urea concentration by water evaporation contributes to providing a urea melt suitable, for example, for melamine production or urea finishing.

[0059] Optionally, the second MP carbamate condenser has a first outlet for liquid and a separate second outlet for gas. Optionally, the second MP carbamate condenser has an outlet for an effluent containing both gas and liquid, which is connected to a gas / liquid separation unit, wherein the gas / liquid separation unit has a liquid outlet, which is in liquid flow communication with the reaction zone of the urea synthesis section. For example, the gas from the second MP carbamate condenser is sent to an absorber operating, for example, under MP. The gas from the second MP carbamate condenser (if obtained as a separate stream) contains, for example, at least 90 vol.% or at least 98 vol.% of total NH 3 and CO 2 .

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

[0061] For example, the second MP gas stream from the first MP carbamate condenser is supplied to the MP scrubber, which preferably also receives the carbamate stream from the LP recovery section of the urea plant, specifically from the LP carbamate condenser of the urea plant. The MP scrubber has a liquid outlet preferably connected to the inlet of the first MP carbamate condenser. The MP scrubber has a gas outlet connected to an absorber (e.g., LP absorber). Therefore, the inert gas from the urea synthesis section is preferably supplied to the absorber and can be discharged from the absorber. The liquid from the absorber is usually supplied directly or indirectly to the wastewater treatment section. For example, the absorber uses a washing liquid, such as a 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 usually pumped to the HP urea synthesis section after gas / liquid separation to remove any uncondensed vapors. For example, the vapors are also supplied to the MP scrubber. For example, the carbamate solution is directly supplied to the reactor or the HP carbamate condenser.

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

[0064] The urea reactor is usually a vertical urea reactor with one or more inlets at the bottom and an outlet for extracting urea solution from the upper part of the reactor (for example using a downcomer); and is usually provided with trays. The reactor usually has an inlet for carbamate solution coming from a HP carbamate condenser.

[0065] HP carbamate condensers are usually shell and tube heat exchangers. Suitable HP carbamate condensers are, for example, horizontal carbamate condensers with a U-shaped tube bundle with a cooling fluid in the tubes and the condensation taking place in the shell. For example, vertical carbamate condensers with a U-shaped tube bundle with a cooling fluid in the tubes are also possible, or vertical shell and tube heat exchangers with the condensation taking place in the tubes, for example, in which the gas to be condensed is provided in the form of bubbles in the tubes.

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

[0067] Optionally, the reactor and the condenser are combined in a single vessel, for example as a pool reactor. A pool reactor is a horizontal vessel whose U-shaped tube bundle extends horizontally over a portion of the vessel, and the condensation takes place in the shell side space at the tube bundle, and there is a reaction part 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 liquid connected to the reactor and an inlet for gas from the stripping column.

[0068] Typically, the cooling fluid for the HP carbamate condenser is, for example, boiler feed water or the urea solution to be heated, or the cooling fluids are combined, for example in two separate tube bundles.

[0069] The HP stripper is a heat exchanger configured for falling film of urea solution in countercurrent contact with a gas stream, having an inlet for urea solution at the top, an outlet for stripped urea solution at the bottom and an outlet for the gas stream at the top connected to the HP carbamate condenser for at least a portion of the gas stream. The stripper is typically a shell and tube heat exchanger with a heating fluid in the shell and urea solution in the tubes. The HP stripper is preferably a CO 2 HP stripper with CO at the bottom for use as stripping gas 2 The HP stripper can also be a hot stripper.

[0070] Depending on the type of synthesis section used, the HP synthesis section may comprise further plant components, such as ejectors or carbamate separators.

[0071] In a preferred embodiment, the stripping column is a CO 2 HP stripper, using at least some or all of the CO 2 The feedstock acts as stripping gas and the synthesis zone is operated with an N / C ratio of 3.0 to 3.5, e.g. 3.0-3.2, and / or with a synthesis pressure of e.g. 120-160 bar, e.g. (about) 140 bar; preferably these N / C ratios and pressure combinations.

[0072] At N / C ratios above 3.5 in the synthesis section, condensation of the urea synthesis offgas to carbamate solution at MP (especially 10-30 bar) becomes difficult or requires special measures.

[0073] Preferably, in this embodiment, the plant does not contain any 2 Special NH for HP stripping 3 Recycle flow line. Therefore, the first MP carbamate condenser is preferably in NH 3 (and CO 2) is operated with substantially complete condensation. It may be advantageous to supply a first MP gas stream from an MP treatment, preferably an MP treatment with reduced temperature of the urea solution, more preferably from an MP adiabatic flash, to the first MP carbamate condenser.

[0074] Preferably, the urea solution is subjected to high pressure CO prior to MP treatment. 2 stripping; therefore, preferably the 2 The stripped urea solution of the HP stripper is preferably subjected to MP treatment, more preferably adiabatic flashing, with the urea solution temperature at the outlet lower than the urea solution temperature at the inlet. In such embodiments, the N / C ratio of the first gas stream is low enough to provide optimal carbamate condensation in combination with the urea synthesis section off-gas under MP. Additionally, MP flashing can help reduce the steam consumption of the HP stripper.

[0075] In an embodiment, urea production uses HP thermal stripping, and the plant or process is, for example, of the Sinapuji stripping type. In an exemplary embodiment, the urea solution from the hot HP stripper is subjected to MP dissociation (as MP treatment) in an MP decomposer, which is, for example, a shell and tube heat exchanger, for example, with urea solution in the tubes. The first MP gas stream from the MP decomposer is condensed in a first MP carbamate condenser together with the urea synthesis section off-gases. The effluent of the condenser is subjected to gas / liquid separation in an ammonia-carbamate separation column; the resulting second MP gas stream is, for example, supplied to an ammonia condenser, and the resulting first MP carbamate solution is supplied to a second MP carbamate condenser which also receives melamine off-gases. 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 usually finally discharged after further purification, for example in an ammonia scrubber. The uncondensed gases from the second MP carbamate condenser are supplied, for example, to a unit of the LP recovery section or the MP section of the plant, for example directly or indirectly to an ammonia scrubber, or discharged.

[0076] In a further embodiment, the urea synthesis section comprises CO 2 The stripping column is of the ACES21 type supplied by Toyo, for example. In an exemplary embodiment of the process, the reactor is operated at a pressure of at least 150 bar and the N / C ratio is higher than 3.5; and the HP stripping uses CO 2as stripping gas. The stripped urea solution is supplied to the MP treatment in the MP decomposer, which is a shell and tube heat exchanger heated with steam. The resulting first MP gas stream is supplied directly or indirectly to the MP absorber, which is a heat exchanger for cooling, which also receives the carbamate solution from the LP recovery section and provides gas / liquid separation. The first carbamate solution from the MP absorber is supplied to a second MP carbamate condenser, which also receives melamine waste gas. The effluent from the second MP carbamate condenser is supplied to the gas / liquid separation; the liquid is supplied to the HP synthesis section, specifically to the HP carbamate condenser, via an HP carbamate pump. The HP carbamate condenser is a condenser that performs condensation, for example, in a shell and has a cooling fluid in a vertical U-tube bundle.

[0077] In another embodiment, the process involves HP CO 2 Stripping, and subjecting the stripped urea solution to MP treatment by heating under MP, the heating under MP being carried out by flowing the liquid under MP through the tube bundle of the HP carbamate condensation unit, for example, by supplying the liquid under MP to the tube bundle of the pool reactor, which condenses the gas from the stripper in the shell under HP. The effluent from the tube bundle is subjected to gas / liquid separation to produce a first MP gas stream. In addition, the process involves condensing the gas stream and the urea synthesis section off-gas in a first MP carbamate condenser; subjecting the resulting effluent to gas / liquid separation, and supplying the resulting first MP carbamate solution to a second MP carbamate condenser which also receives melamine off-gas. The pool reactor is a horizontal vessel comprising a condensation zone and a reaction zone, wherein the 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 comprises CO 2 The process involves supplying a first portion of the urea synthesis solution of the HP reaction zone to the MP demultiplexer, 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 flashing, heating and / or mixing with MP CO 2The gas streams are contacted countercurrently; to produce a first MP gas stream and an MP urea solution. The gas stream is condensed together with the off-gas from the urea synthesis section in a first MP carbamate condenser to form an MP carbamate solution, which is supplied to a second MP carbamate condenser also receiving melamine off-gases via an MP gas / liquid separation. The gas from the MP gas / liquid separation contains inert gases. The urea synthesis section generally does not contain an HP scrubber. The urea solution from the HP stripper and the MP urea solution are supplied to one or more LP recovery sections. The MP urea solution may be subjected to a reaction with the MP CO 2 The MP gas streams are contacted countercurrently and the resulting MP gas stream can be supplied to the first and / or second MP carbamate condenser.

[0079] The MP urea solution from the MP treatment is typically expanded to LP and subjected to LP carbamate decomposition (typically by heating) to produce a LP urea solution and a LP gas stream. The LP urea solution is typically supplied directly or indirectly to an evaporation section, which typically comprises 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 partially or completely supplied to the 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 finishing, i.e., solidification to form a solid urea product, such as granulation or pelletization.

[0080] The vapors from the evaporation section are typically condensed and the resulting condensate is typically supplied to a wastewater treatment section comprising, for example, a desorber and a hydrolysis unit, which produces a 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 zone or reactor, while the remaining urea is conveyed to units other than the melamine plant, such as the finishing section, or a unit for producing liquid fertilizers. Preferably, the melamine production feeds said 5 wt.% to 40 wt.% portion of the total urea produced separately. Suitably, the remaining urea portion is subjected to concentration by water evaporation.

[0082] Preferably, the amount of the melamine offgas stream corresponds to an amount of 5 wt.% to 40 wt.% of the total urea produced in the high-pressure urea synthesis section being converted into melamine. Larger amounts of melamine offgas relative to the urea production of the synthesis section may be particularly detrimental to CO 2 The stripping type urea synthesis section is 2Compared with the amount of , the amount of carbamate recycling becomes relatively large.

[0083] The water vapor from the evaporation section is condensed to form a process condensate which is typically purified in a wastewater treatment section using, for example, desorption and hydrolysis.

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

[0085] The LP carbamate condenser typically receives an aqueous stream to provide water to prevent carbamate precipitation. The aqueous stream comes from, for example, a wastewater treatment section. The amount of water in the LP carbamate condenser can be set as appropriate for the water required in the first and second MP carbamate condensers.

[0086] Figure 1 An exemplary urea production process scheme according to a non-limiting embodiment of the present invention is schematically shown. It should be noted that only some, but not all, of the units and connections shown in the drawings are cited in the independent claims; other units and connections are not necessary and are preferably used. A urea plant (100) comprises an HP synthesis section (101) comprising a reactor (1), an HP stripper (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 urea synthesis solution (4) for withdrawing the urea synthesis solution from the upper part of the reactor, optionally using a downcomer for the urea synthesis solution. For example, the reactor is provided with trays.

[0088] The HP stripper (2) is shown receiving some or all of the CO 2 Feed CO 2 HP stripper. The stripper has an inlet for 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 for the stripped urea solution (8) located in the bottom part. In operation, the urea synthesis solution stream (4) or a part thereof is subjected to stripping in the HP stripper (2) to produce a stripped urea solution (8). Optionally, a part of the urea synthesis solution stream (4) can be provided directly to the unit for MP treatment (9), bypassing the HP stripper (2).

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

[0090] The HP synthesis section (101) also has a 3 The inlet for the feed (not shown) leads, for example, to the HP carbamate condenser (3).

[0091] The HP synthesis section (101) has a separate outlet for the urea synthesis section off-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 off-gas stream (7) comprises an inert gas, such as one or more of the following: O for passivation 2 , from CO 2 Feed H 2 , CH 4 and possible inclusion in CO 2 Other hydrocarbons in the feed and from CO 2 and / or NH 3 Feed argon and N 2 , and as contained in any air used for passivation. Passivation refers to corrosion protection in the urea synthesis section. In an embodiment, the urea synthesis section is operated with: for example, relative to CO 2 Feed, at least 0.2 vol.% or at least 0.5 vol.% O 2 ; , where O 2 Any associated N in the embodiment introduced as air into the urea synthesis section 2 Passivating oxygen or air, for example, is added to the CO 2 Feeding.

[0092] It should be noted that these gas components are usually non-condensable in a urea plant and are vented.

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

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

[0095] The first MP carbamate condenser (11) is usually provided as part of a heat exchanger and has heat exchange walls which are exposed on a first side to the condensing process medium of the first MP carbamate condenser and on a second side to a cooling fluid such as urea solution.

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

[0097] The first MP carbamate condenser (11) also receives the synthesis section offgas stream (7), which includes inert components, NH 3 and CO 2 .

[0098] Based on the liquid outlet, the N / C ratio in the first MP carbamate condenser (11) is preferably in the range of 2.0 to 2.5, more preferably in the range of 2.1-2.4, such as about 2.3. The preferred N / C ratio of maximum 2.4 avoids the gaseous NH 3 The loss of the second MP gas stream (14) and the avoidance of a separate NH 3 Condensation and liquid NH 3 The need for recycle to the HP synthesis section.

[0099] Very advantageously, the combined condensation of the synthesis section offgas stream (7) and the first MP gas stream (10) can provide NH 3 With CO 2 Specifically, the synthesis section offgas stream (7) may have a relatively high NH 3 With CO 2 molar ratio, such as 3.0-4.0, and the first MP gas stream (10) may have a relatively low NH 3 With CO 2 The molar ratio is, for example, less than 2.0, for example in the range of 1.0-2.0. In particular, in the case of adiabatic flash evaporation of the stripped urea solution (8) in the MP treatment unit (9), the NH 3 With CO2 The molar ratio can be 1.0-1.5. 2 Stripping, from CO 2 The stripped urea solution (8) of the HP stripper generally has a relatively low N / C ratio.

[0100] The MP condenser effluent (12) is subjected to a 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) (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 is, for example, integrated in a single unit with the first MP carbamate condenser. For example, the first MP carbamate condenser has two independent 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 carbamate condenser (11) enables more complete condensation in said condenser (11) and a relatively low NH 3 Thus, in embodiments in which 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 the load of the WWT is advantageously relatively small, which helps to achieve better energy efficiency of the plant.

[0102] Thus, the first MP carbamate solution (15) is substantially degassed and supplied partly or completely with a very low content of gaseous inert components to the second MP carbamate condenser (17), which also receives the MP melamine offgas stream (16) from the melamine production section (102).

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

[0104] Due to the lower partial pressure of the inert gas in the second MP carbamate condenser (17) and the corresponding NH 3 and CO 2 The very low inert gas content in the first MP carbamate solution (15) (i.e. as obtained from the gas / liquid separation) permits the operation of the second MP carbamate condenser (17) at a relatively high temperature for any fixed absolute (or total) operating pressure of the second MP carbamate condenser (17) compared to processes in which no gas / liquid separation is used. The relatively high condensation temperature permits a lower water content of the second MP carbamate solution (18) without the risk of carbamate 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 in the synthesis section. The water content of the first MP carbamate solution (15) is generally sufficient for operating the second MP carbamate condenser (17) and is generally set to the minimum value required for safe operation of the first MP carbamate 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 aqueous liquid are dilute carbamate solutions from a wastewater treatment section (associated with an evaporation section) and purified process condensate from a wastewater treatment section.

[0105] The first MP carbamate condenser (11) also receives a carbamate solution stream (19) which preferably originates directly or indirectly from a LP carbamate condenser (not shown). This carbamate solution stream (19) contains H 2 O, for preventing carbamate crystallization in the first and second MP carbamate condensers.

[0106] The first MP carbamate condenser (11) is in heat exchange contact through the wall with a urea solution (21) to be heated in a first evaporation stage (24). The urea solution (21) originates, for example, directly or indirectly from a LP recovery section (not shown). The heated urea solution (22) is supplied, after gas / liquid separation to remove water vapor, to a second evaporation stage (25) which is in heat exchange contact through the wall with a second MP carbamate condenser (17), where the urea solution is heated to provide a further heated urea solution (23). This urea solution can in turn be further heated, for example by heat exchange with steam, to produce a urea melt.

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

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

[0109] Preferably the melamine production section comprises a melamine synthesis reactor and a scrubbing section for scrubbing the raw offgas stream from the melamine synthesis reactor with urea.

[0110] Preferably the high pressure urea synthesis section belongs to the CO 2 Stripping type, which contains a high pressure CO with an outlet for urea solution connected to the MP treatment unit 2 Stripping tower.

[0111] Preferably, a first MP carbamate condenser is in heat exchange contact with the first evaporation stage, a second MP carbamate condenser is in heat exchange contact with the second evaporation stage, and the plant comprises a liquid flow line for supplying urea solution from the first evaporation stage (preferably via gas / liquid separation) to the second evaporation stage. The first evaporation stage and the second evaporation stage are also comprised in the plant; the first evaporation stage is directly or indirectly connected to receive urea solution originating in particular from the MP treatment unit, typically via a LP recovery section.

[0112] A method for converting an existing urea plant is also provided. The existing urea plant comprises a HP urea synthesis section of the stripping type having an outlet for a urea solution (also comprising carbamate) and an outlet for a urea synthesis section offgas stream (comprising inert gases, NH 3 and CO 2 ). The method involves adding a melamine production section in an existing urea plant (if not already present) having an outlet for the melamine off-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 converted to such a plant.

[0113] The method involves adding to an existing urea plant the following (if not already present): an MP treatment unit for treating a urea solution at medium pressure, wherein at least a portion of the carbamate contained in the urea solution is dissociated into CO 2 and NH 3 , the MP treatment unit having an outlet for the MP urea solution and an outlet for the first MP gas stream; and a first MP carbamate condenser for subjecting the urea synthesis section off-gas stream and at least a portion of the first MP gas stream to condensation to produce an MP condenser effluent. The method comprises adding: a gas / liquid separator for subjecting 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 for subjecting the melamine off-gas stream to condensation 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 supplying the second MP carbamate solution directly or indirectly to the urea synthesis section. The modified plant is preferably a plant according to the invention and is preferably suitable for the urea production process of the invention.

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

[0115] As used herein, for the process streams of a urea plant (i.e. not for the steam line), high pressure (HP) is above 100 bar, for example 120 bar to 300 bar, for example 140 bar to 200 bar. Medium pressure (MP) is for example 10 bar to 80 bar (including an intermediate pressure 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 a stream, preferably at least 50 wt. % or at least 90 % of the stream, optionally the entire stream, is used.

[0118] The N / C ratio of the gas stream indicates NH 3 With CO 2 The N / C ratio as used herein for the urea synthesis section reflects the composition of the so-called initial mixture before urea production, as used in the field of urea plants, consisting only of NH 3 , CO 2 and H 2 O composition and is a molar ratio. The N / C ratio of the carbamate solution indicates the corresponding amount of NH 3 With CO 2 The molar ratio is based on the total amount of free and carbamate NH 3 , CO 2 The amount of NH 3 and 1 mol CO 2 The N / C ratio of the carbamate condenser refers to the N / C ratio of the carbamate solution at the relevant outlet.

[0119] The term 'melamine off-gas' as used herein indicates an off-gas stream from the melamine production section and refers to an off-gas stream containing mainly NH 3 , CO 2 and possible H 2 O and a small amount of inert gas.

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

[0121] All priorities and details discussed in relation to the process also apply to the plant, and vice versa. All priorities and details described in relation to the plant also apply to the method of modifying the plant.

[0122] Embodiments of the 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 1An integrated urea-melamine plant with a capacity of 3700 MTPD (metric tons per day) of urea is designed to operate as per the process design parameters given in Table 1. 1200 MTPD urea is used for melamine production and the remaining urea is obtained as solid urea product after finishing.

[0125] like Figure 1 As shown, a first MP carbamate condenser receives urea synthesis off-gas and gases from MP process and liquid from the first MP carbamate condenser is optionally supplied to a second MP carbamate condenser receiving melamine off-gas. Non-condensable gases from the first MP carbamate condenser are dropped to LP and processed separately.

[0126] The example urea plant contains HP CO 2 Stripper, and MP treatment is adiabatic flash.

[0127] By using two MP carbamate condensers in series and removing inert gas between them, about 20-25 kg steam / ton urea can be saved compared to a reference process using a single MP carbamate condenser (receiving urea synthesis section off-gas, melamine off-gas, gas from MP treatment and carbamate solution from LP carbamate condenser). The steam consumption refers to the steam consumption of the HP stripper and specifically to the 20-25 bar steam used in the HP stripper. The lower water recycle of the urea synthesis permits operating the HP stripper with less heating and thereby reduces the steam consumption.

[0128] In Table 1, the inert gas percentage refers to the vol.% of inert gas in the gas phase from the condenser (based on the outlet temperature). In addition to the inert gas, the gas phase contains NH 3 , CO 2 and H 2 O.

[0129] Both MP carbamate condensers were operated at the same (total) pressure of 24 bara, but the NH 3 and CO 2 The partial vapor pressure of is higher than in the first carbamate condenser because the inert gas level in the second carbamate condenser is lower, resulting in a higher condensation temperature in the second carbamate condenser. The higher temperature in turn permits the second carbamate condenser to operate with a lower liquid phase wt.% without the risk of carbamate crystallization. Therefore, the amount of water (kg water / ton of urea produced) recycled from the second carbamate condenser to the urea synthesis section is lower than in the comparative example in which a single MP carbamate condenser is used, which receives melamine off-gas, urea synthesis section off-gas and gases from the MP process.

[0130] Both MP carbamate condensers are provided as shell-side compartments of a respective shell-and-tube heat exchanger, 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: - producing urea in a high-pressure (HP) urea synthesis section (101) of the stripping type, thereby producing a stripped urea solution (8) which also contains carbamate, and a urea solution (9) containing inert gases, NH 3 and CO 2 a separate urea synthesis section exhaust gas stream (7); - producing melamine, thereby providing a melamine waste gas stream (16); - subjecting the stripped urea solution (8) to a medium pressure (MP) treatment (9), wherein at least some of the carbamates contained in the stripped urea solution (8) are dissociated into CO 2 and NH 3 , thereby producing an MP urea solution (20) and a first MP gas stream (10); - subjecting the urea synthesis section offgas stream (7) and at least a portion of the first MP gas stream (10) to condensation in a first medium pressure (MP) carbamate condenser (11) to produce an MP condenser effluent (12); - subjecting the MP condenser effluent (12) to a gas / liquid separation (13) to produce a second MP gas stream (14) comprising the inert gas and a first MP carbamate solution (15); - subjecting the melamine offgas stream (16) to condensation in a second MP carbamate condenser (17) in the presence of at least a portion of the first MP carbamate solution (15) to produce a second MP carbamate 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 carbamate condenser (17) is at least 5°C higher than the condensation temperature in the first MP carbamate condenser (11).

4. The process according to any one of the preceding claims, wherein the melamine offgas stream (16) contains less than 15 wt.% water.

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

6. Process according to any one of the preceding claims, wherein the production of melamine involves scrubbing a raw off-gas stream from a melamine synthesis reactor with urea.

7. The process according to any one of the preceding claims, wherein the HP urea synthesis section (101) belongs to a CO 2 stripping type, and subjecting the urea synthesis solution (4) from the reaction zone to high pressure CO before said MP treatment (9) 2 Steam stripping.

8. The process according to claim 7, wherein the MP treatment (9) involves decompressing the urea solution from HP to MP in a decompression unit, e.g. in a valve, thereby releasing gas from the solution, and performing a gas / liquid separation of the first MP gas stream from the urea solution, Preferably wherein the MP treatment (9) is an adiabatic flash evaporation.

9. Process according to any of the preceding claims, wherein the first MP carbamate condenser and the second MP carbamate condenser (11, 17) are both in heat exchange contact with an evaporation stage (24, 25) for heating the urea solution.

10. The process according to claim 9, wherein the first MP carbamate condenser (11) is in heat exchange contact with a first evaporation stage (24), the second MP carbamate condenser (17) is in heat exchange contact with a second evaporation stage (25), and the heated urea solution (22) is supplied from the first evaporation stage (24) to the second evaporation stage (25), preferably by gas / liquid separation.

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

12. An integrated urea and melamine production plant comprising: a high-pressure (HP) urea synthesis section (101) of the stripping type having an outlet for a stripped urea solution (8) also comprising carbamate, and for a urea solution comprising inert gases, NH 3 and CO 2 A separate outlet for the urea synthesis section exhaust gas stream (7); a melamine production section (102) having an outlet for a melamine waste gas stream (16); - an MP treatment unit (9) for treating the urea solution at medium pressure, wherein At least some of the carbamate contained in the stripped urea solution (8) is dissociated into CO 2 and NH 3 , the MP treatment unit having an outlet for an MP urea solution (20) and an outlet for a first MP gas stream (10); a first MP carbamate condenser (11) for subjecting at least a portion of the urea synthesis section offgas stream (7) and the first MP gas stream (10) to condensation, to produce an MP condenser effluent (12); a gas / liquid separation zone (13) for subjecting 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) for subjecting the melamine offgas stream (16) to condensation 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).

13. An integrated urea and melamine production plant according to claim 12, wherein the melamine production section (102) is of the non-catalytic high pressure type and comprises a melamine synthesis reactor and a washing section for washing the raw off-gas stream from the melamine synthesis reactor with urea, and wherein the HP urea synthesis section (101) is of the CO 2 Stripping type, including high pressure CO 2 Stripping tower (2), the high pressure CO 2 The stripping column has an outlet for urea solution (8) which is connected to the MP treatment unit (9).

14. An integrated urea and melamine production plant according to claim 12 or 13, wherein the first MP carbamate condenser (11) is in heat exchange contact with a first evaporation stage (24), the second MP carbamate condenser (17) is in heat exchange contact with a second evaporation stage (25), and the plant comprises a liquid flow line for supplying heated urea solution (22) from the first evaporation stage (24) to the second evaporation stage (25), preferably by gas / liquid separation.

15. A method for renovating an existing urea plant, the existing urea plant comprising: a high-pressure (HP) urea synthesis section (101) of the stripping type having an outlet for a stripped urea solution (8) also comprising carbamate, and for a urea solution comprising inert gases, NH 3 and CO 2 A separate outlet for the urea synthesis section exhaust gas stream (7); The method involves: Add the following, if not already present in the existing urea plant: a melamine production section (102) having an outlet for a melamine waste gas stream (16); - an MP treatment unit (9) for treating the urea solution at medium pressure, wherein At least some of the carbamate contained in the urea solution dissociates into CO 2 and NH 3 , the MP treatment unit having an outlet for an MP urea solution (20) and an outlet for a first MP gas stream (10); a first MP carbamate condenser (11) for subjecting the urea synthesis section offgas stream (7) and at least a portion of the first MP gas stream (10) to condensation to produce an MP condenser effluent (12); a gas / liquid separation zone (13) for subjecting 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 wherein the method comprises adding: a second MP carbamate condenser (17) for subjecting the melamine offgas stream (16) to condensation 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).

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