Urea melamine device with melamine tail gas condenser

Abstract

The present disclosure relates to melamine production co-production with urea production using a melamine off-gas condenser operating at a pressure of at least 25 bar. In an embodiment, a carbamate solution from the recovery section of urea production is supplied to the melamine off-gas condenser. In an embodiment, a carbamate solution from melamine mother liquor treatment is supplied to the LP carbamate condenser of the recovery section.

Description

Technical Field

[0001] The present invention relates to the co-production or integrated production of urea and melamine.

[0002] Introduction

[0003] Various types of urea production methods are described in Ullmann’s Encyclopaedia, chapter Urea, 2010.

[0004] The article “Urea-melamine plant integration”, Nitrogen+Syngas 321, January - February 2013, pages 44 to 54 describes a method of urea-melamine integration. In the method scheme shown, an aqueous stream from the melamine plant is supplied to the wastewater treatment section of the urea plant.

[0005] The wastewater treatment section of the urea plant is generally based on hydrolysis and desorption and is energy-intensive.

[0006] US2004 / 0162429A1 describes a high-pressure non-catalytic type of melamine production method using urea washing of tail gas. The mother liquor is subjected to deammoniation treatment, especially to obtain an aqueous ammonia solution containing CO2. Makeup water is supplied to the quench tower, in which the melamine stream is quenched.

[0007] An exemplary melamine plant is described in EP 2385043A1. This plant uses high-pressure non-catalytic melamine synthesis. In Figure 3 of this document, a washing section is used in which an anhydrous tail gas stream is brought into contact with an aqueous washing stream. The tail gas saturated with water is sent as it is to the urea plant, or is sent to the urea plant after being treated (such as by condensation in aqueous solution absorption). The melamine mother liquor is treated in an ammonia recovery section to obtain an aqueous solution of carbonate (i.e., carbamate), which can be sent to the urea plant.

[0008] Another exemplary melamine plant is described in EP 3597641A1. In Figure 3 of this document, a steam 27 containing CO2 from the deammoniation treatment is combined with a stream 33 containing H2O, NH3 and CO2 obtained from the decomposition step (such as hydrolysis) of the organic components in the deammoniated mother liquor; the combined stream can be sent to the urea plant.

[0009] Generally, the tail gas containing CO2 and NH3 from the melamine plant needs to be directly or indirectly supplied to the urea synthesis section, where carbamate is formed by the reaction of CO2 and NH3 and the carbamate is dehydrated to form urea. In this way, the production of melamine and urea can be co-produced. Typically, the tail gas from the melamine plant is condensed into an aqueous carbamate solution, which is then supplied to the synthesis section, especially if the melamine synthesis pressure is lower than the urea synthesis pressure. However, the presence of water in the reaction mixture in the synthesis section, especially in the reaction zone, is detrimental to the urea yield. The minimum water content of the carbamate solution increases with the decrease in the degree of condensation. Summary of the Invention

[0010] In an embodiment, the present invention aims to provide the co-production of melamine and urea with high energy efficiency (relatively low steam consumption) and / or relatively low water supply to the urea synthesis section, thereby providing a higher urea conversion rate.

[0011] In a first aspect, the present invention relates to a method for the co-production of urea and melamine, the method comprising: producing urea in a urea synthesis section of a urea production section to obtain a first urea stream; subjecting the first urea stream to purification in at least a low-pressure (LP) recovery section; producing melamine in a high-pressure non-catalytic melamine synthesis section to obtain molten melamine and anhydrous tail gas, wherein the melamine synthesis section is included in a melamine production section. The method preferably relates to: washing the anhydrous tail gas by contacting it with urea in a washing unit and condensing the washed tail gas in a tail gas condenser at a pressure of at least 25 bar to form a tail gas condensate, the tail gas condenser preferably being included in the melamine production section; or more generally, the method preferably relates to: condensing the anhydrous tail gas in a tail gas condenser at a pressure of at least 25 bar to form a tail gas condensate.

[0012] In a preferred embodiment, the method comprises, in a melamine processing section, dissolving the molten melamine in an aqueous solution and crystallizing the melamine with the formation of a crystalline melamine stream and a mother liquor stream, and treating part or all of the mother liquor to obtain a recovered mother liquor and preferably an aqueous carbamate solution.

[0013] The method preferably relates to: supplying part or all of the aqueous carbamate solution (i.e., from mother liquor treatment) to a carbamate condenser included in the low-pressure recovery section, where the gas from the LP dissociator is condensed into carbamate to form a carbamate-rich solution.

[0014] Preferably, the method involves supplying the carbamate solution (e.g., at least part of the carbamate-rich solution) from the LP carbamate condenser directly or indirectly to the off-gas condenser; and preferably supplying the off-gas condensate to the urea synthesis section or the urea reaction zone.

[0015] In some embodiments, the method includes washing the off-gas with urea, condensing the anhydrous washed off-gas, and supplying the carbamate solution from the LP carbamate condenser included in the LP recovery section to the off-gas condenser. Preferably, it further includes supplying the aqueous carbamate solution from the mother liquor treatment to the LP carbamate condenser. The LP carbamate condenser receives the gas stream from the purification step in the LP recovery section.

[0016] In a preferred embodiment, the method involves treating the melamine mother liquor and supplying the carbamate solution from the mother liquor treatment to the low-pressure carbamate condenser in the low-pressure recovery section of the urea production section (preferably directly). In this embodiment, there is no particular limitation on the processing of the melamine off-gas.

[0017] In a preferred embodiment, the method involves supplying the carbamate solution from the low-pressure carbamate condenser in the low-pressure recovery section of the urea production section to the melamine off-gas condenser that receives the anhydrous off-gas from the melamine synthesis section to produce a carbamate solution, which is supplied to the urea reaction zone, preferably to the urea reaction zone in the urea synthesis section of the urea production section. In this embodiment, there is no particular limitation on the way of treating the melamine mother liquor.

[0018] In another preferred embodiment, the following features are combined: supplying the carbamate solution formed by the mother liquor treatment to the LP carbamate condenser, and supplying the carbamate solution from the LP carbamate condenser to the melamine off-gas condenser.

[0019] In one aspect, the present invention provides a method for the co-production of urea and melamine, the method comprising: producing urea in a urea synthesis section of a urea production section to obtain a first urea stream; subjecting the first urea stream to purification in at least a LP recovery section including a low pressure (LP) carbamate condenser; producing melamine in a high pressure non-catalytic melamine synthesis section to obtain molten melamine and anhydrous tail gas, wherein the melamine synthesis section is included in a melamine production section; in a melamine processing section, dissolving the molten melamine in an aqueous solution and crystallizing the melamine with the formation of a crystallized melamine stream and a mother liquor stream, and treating part or all of the mother liquor to obtain a recovered mother liquor and an aqueous carbamate solution; and supplying the aqueous carbamate solution obtained from the mother liquor treatment to the LP carbamate condenser.

[0020] In one aspect, the present invention provides a method for the co-production of urea and melamine, the method comprising: producing urea in a urea synthesis section of a urea production section to obtain a first urea stream; subjecting the first urea stream to purification in at least a LP recovery section including a low pressure (LP) carbamate condenser; producing melamine in a high pressure non-catalytic melamine synthesis section to obtain molten melamine and anhydrous tail gas, the melamine synthesis section being included in a melamine production section; preferably washing the anhydrous tail gas by contacting it with urea in a washing unit; condensing the tail gas (preferably the washed tail gas) in a tail gas condenser at a pressure of at least 25 bar to form a tail gas condensate, wherein the tail gas condenser is preferably included in the melamine production section; and supplying the carbamate solution from the LP carbamate condenser directly or indirectly (preferably directly) to the tail gas condenser; wherein the method preferably further comprises dissolving the molten melamine in an aqueous solution and crystallizing the melamine with the formation of a crystallized melamine stream and a mother liquor stream, and treating part or all of the mother liquor to obtain a recovered mother liquor and an aqueous carbamate solution.

[0021] The present invention also relates to a combined production apparatus for urea and melamine, the apparatus comprising a urea production section including a urea synthesis section for producing urea to obtain a first urea stream; and a low-pressure recovery section including a unit for purifying the first urea stream, such as a dissociator. The apparatus further includes a melamine production section, the melamine production section including: a high-pressure non-catalytic melamine synthesis section for producing melamine to obtain molten melamine and anhydrous tail gas; preferably, a washing unit for washing the anhydrous tail gas by contacting with urea; and an apparatus, preferably the melamine production section includes a tail gas condenser for condensing the tail gas at a pressure of at least 25 bar to form a tail gas condensate. The apparatus preferably includes a melamine processing section including a unit for dissolving the molten melamine in an aqueous solution, and a crystallizer for crystallizing the melamine to form a crystallized melamine stream and a mother liquor stream; and a processing unit for processing part or all of the mother liquor to obtain a recovered mother liquor and preferably an aqueous carbamate solution. The apparatus preferably further includes a liquid flow line for supplying part or all of the aqueous carbamate solution from the processing unit (for processing part or all of the mother liquor) to a carbamate condenser included in the low-pressure recovery section, the carbamate condenser having an outlet for a rich carbamate solution.

[0022] The apparatus preferably includes: a liquid flow line for directly or indirectly supplying a carbamate solution (preferably at least part of the rich carbamate solution) to the tail gas condenser, and preferably, a liquid flow line for supplying the tail gas condensate to the urea synthesis section.

[0023] The apparatus preferably includes: a first liquid flow line for supplying a carbamate solution from a LP carbamate condenser included in the LP recovery section to the tail gas condenser, and / or a second liquid flow line for supplying an aqueous carbamate solution from a preferably used mother liquor treatment to the LP carbamate condenser. The LP carbamate condenser includes an inlet connected to receive a gas stream from a purification step in the LP recovery section.

[0024] The present invention also relates to a method for retrofitting an existing apparatus for combined production of urea and melamine.

[0025] Accordingly, the present invention relates to an apparatus and a method for combined production of urea and melamine. Preferably, a lean carbamate solution is supplied from the melamine production section to a recovery section of the urea production section, and / or a carbamate solution (e.g., a rich carbamate solution) is supplied from the urea production section to a tail gas condenser of the melamine production section. Description of the Drawings

[0026] Figure 1 Schematically shows an exemplary method embodiment according to the present invention.

[0027] Any embodiment shown in the drawings is merely exemplary and does not limit the present invention. Detailed Description

[0028] In the method and apparatus of the present invention, melamine is produced. Preferably, in the melamine processing section, molten melamine is dissolved in an aqueous solution, and melamine crystallization is carried out with the formation of a crystalline melamine stream and a mother liquor stream, and part or all of the mother liquor is treated to obtain a recycled mother liquor and an aqueous carbamate solution.

[0029] In a preferred embodiment, the carbamate solution from the mother liquor treatment is supplied from the melamine production section to the condenser of the LP recovery section.

[0030] As shown in Example 1, this has the advantage of reducing the energy-consuming wastewater treatment in the urea production section.

[0031] In a preferred embodiment, the carbamate solution (e.g., a carbamate-rich solution) is supplied from the LP recovery section (e.g., from the LP carbamate condenser) to the tail gas condenser, where the tail gas from the melamine plant is condensed at a higher pressure than the operating pressure of the LP recovery section. The resulting carbamate solution is preferably supplied to the urea reaction zone, such as the urea synthesis section. Thus, the amount of water supplied to the urea reaction zone (e.g., the urea synthesis section) is effectively reduced, which is advantageous. In a preferred embodiment, water is used for condensing the carbamate three times: in the melamine production section, in the LP recovery section, and in the tail gas condenser.

[0032] The present invention relates to a method and apparatus for the co-production of urea and melamine. Herein, co-production means that the tail gas from the melamine production section is directly or indirectly supplied to the urea synthesis section, especially after condensation.

[0033] In an embodiment, the method is also integrated, i.e., the urea melt from the urea production section is supplied to the melamine production section, preferably at least 5%, or at least 10%, or at least 20%, for example 10 wt.% to 80 wt.%, 20 wt.% to 70 wt.% or 30 wt.% to 70 wt.% of the urea formed in the synthesis section is supplied to the melamine production section. In other embodiments, the melamine production section is fed with urea from other sources. Preferably, the amount of urea melt consumed in the melamine production section is at least 10% or at least 20% and / or less than 70% of the amount of urea produced in the urea production section. The remaining portion of the urea production is supplied to the finishing section, such as a granulator or a prilling tower.

[0034] The method includes producing urea in a urea synthesis section to obtain a first urea stream; the synthesis is carried out by reacting NH3 with CO2 at high pressure (above 100 bar absolute pressure). The urea synthesis section of the apparatus and method can be, for example, of a type that does not employ a high-pressure (HP) stripper, or of a type that employs a high-pressure stripper (such as a CO2 or a thermal high-pressure stripper). The synthesis section employing an HP stripper is preferred; in this embodiment, the synthesis section further includes an HP carbamate condensation section. In a preferred embodiment, the urea synthesis section includes an HP CO2 stripper.

[0035] The synthesis section includes a reaction zone. The first urea stream contains urea, water, and ammonium carbamate.

[0036] The urea synthesis section includes a urea synthesis zone, which is provided, for example, by a urea reactor or by a combined vessel comprising an HP condensation section and a reaction section. An example of a combined vessel is a so-called pool reactor. A urea reactor is typically a vertical reactor with trays, having one or more inlets at the bottom and an outlet for withdrawing the liquid stream containing urea from the top of the reactor. The synthesis section preferably has a separate outlet for the gas stream containing inert gases.

[0037] The HP stripper (if used) is a unit for counter-current contacting of a urea solution with a gas stream under heating, and is preferably a shell-and-tube heat exchanger with a falling film of urea solution in the tubes and steam in the shell, wherein the top of the stripper has a liquid inlet for the urea solution to be stripped, a gas outlet connected to the HP carbamate condenser at the top and a liquid outlet for the urea solution at the bottom, and in an embodiment employing an HP CO2 stripper, further has an inlet for the CO2 feed used as the stripping gas at the bottom, and all outlets are connected to the tube side.

[0038] In an embodiment, the urea synthesis section operates at a pressure at least 10 bar or at least 20 bar higher than the tail gas condenser.

[0039] The first urea stream is a urea solution that also contains water, carbamate, and NH3.

[0040] The method involves subjecting the first urea stream to purification in at least a low-pressure (LP) recovery section and optionally treating the urea solution in an MP treatment section between the urea synthesis section and the LP recovery section. Thus, in an embodiment, the first urea stream is subjected to purification in the MP treatment section and subsequently in the LP recovery section. Accordingly, the apparatus includes an LP recovery section. The LP recovery section includes an LP dissociator and an LP carbamate condenser. The LP dissociator receives the urea solution and has a gas outlet and a separate liquid outlet; the gas outlet is connected to the LP carbamate condenser. The LP carbamate condenser also receives an aqueous carbamate solution, for example, directly or indirectly from a reflux condenser associated with the WWT of the urea plant. Thus, even in embodiments where the LP carbamate condenser does not receive a carbamate solution from the melamine production section, the carbamate solution at the outlet of the LP carbamate condenser can be described as a carbamate-rich solution.

[0041] Purification refers to the removal of components other than urea from the stream. Purification at low pressure preferably involves decomposing ammonium carbamate in the LP dissociator to obtain a gas stream containing CO2 and NH3, which is partially or completely condensed in the LP carbamate condenser. The LP dissociator is typically a heat exchanger, more specifically a shell-and-tube heat exchanger, for example, having the urea solution to be heated in the tubes and a heating fluid (typically steam) in the shell. The LP dissociator has a liquid outlet for the urea solution to be purified and a gas outlet for the steam containing NH3 and CO2, and both outlets are connected to the LP carbamate condenser (which can also be an absorber), which is included in the LP recovery section.

[0042] The MP treatment section (if optionally used in the plant or process) is provided to receive the urea solution from the HP synthesis section and has a liquid outlet for the urea solution to the LP recovery section. The treatment involves purifying the urea solution by removing NH3 and CO2 (optionally present as carbamate) from the urea solution at medium pressure. The treatment can involve, for example, flashing (e.g., adiabatic flashing), or heating, or a series combination of both (flashing and heating). The treatment generally involves gas / liquid separation, having a liquid outlet for the urea solution connected to the LP recovery section and a gas outlet for the MP gas stream connected to the MP carbamate condenser (which can also be an absorber). The MP carbamate condenser is preferably a heat exchanger configured for indirect heat exchange between the urea solution to be heated and the MP gas to be condensed. The MP treatment section with the MP carbamate condenser can be referred to as the MP recovery section. The carbamate solution from the MP carbamate condenser is supplied directly or indirectly to the urea synthesis section.

[0043] In an exemplary embodiment, the MP treatment includes supplying the urea solution at MP through a tube bundle provided in the HP carbamate condenser and condensing the gas from the HP stripper in the shell side space, as described, for example, in US 2015 / 0119603A1.

[0044] The MP carbamate condenser typically receives part or all of the carbamate solution from the LP carbamate condenser. Preferably, the gas stream containing inert substances from the urea synthesis section is also condensed in the MP carbamate condenser.

[0045] In an embodiment, the preferably used MP carbamate condenser (e.g., as an absorber) also functions as a melamine off-gas condenser. In a further embodiment, the MP carbamate condenser and the melamine off-gas condenser in the preferably used urea production section are two separate and independent condensers.

[0046] The method involves producing melamine in a high-pressure non-catalytic melamine synthesis section, obtaining molten melamine and anhydrous off-gas; and thus the plant includes a melamine synthesis section. The melamine synthesis is based on the pyrolysis of urea feed to produce melamine. The melamine synthesis section is included in the melamine production section. The melamine synthesis pressure is higher than 70 bar absolute pressure and, for example, up to 140 bar or up to 120 bar. The present invention is particularly advantageous if the melamine synthesis pressure is at least 10 bar lower than the pressure in the urea synthesis reaction zone, because at such a pressure difference, the off-gas is preferably condensed into a carbamate solution, which can be pumped to the urea synthesis section.

[0047] The method typically purifies the tail gas in an anhydrous manner and preferably involves washing (purifying) the anhydrous tail gas from melamine synthesis by contacting it with urea (melt) in a (tail gas) scrubbing unit to recover melamine from the tail gas. The urea melt used for washing also serves as a reactant in melamine synthesis. The thus-recovered melamine is recycled to melamine synthesis. The urea used for washing is supplied in the form of a urea melt. The washed tail gas contains, for example, less than 2 wt.% water or less than 1 wt.% water, and a tail gas having less than 2 wt.% water or less than 1 wt.% water can be referred to as an anhydrous tail gas. The washed tail gas contains, for example, 45 wt.% to 55 wt.% NH3 and 45 wt.% to 55 wt.% CO2 and less than 2 wt.% water.

[0048] As an alternative anhydrous washing or purification of the tail gas, sublimation cooled by a gaseous fluid can optionally be used, as described, for example, in WO03080584A1.

[0049] Furthermore, in an optional embodiment, the anhydrous melamine tail gas can be condensed as it is without washing or purification, as described in US20210261498A1, paragraph

[0059] .

[0050] The method involves subjecting the washed tail gas or anhydrous tail gas to condensation in a tail gas condenser at a pressure of at least 25 bar to form a tail gas condensate, preferably at a pressure of at least 50 bar, or at least 70 bar, or at least 80 bar, or at least 100 bar, and for example less than 140 bar. Accordingly, the apparatus includes a tail gas condenser.

[0051] Preferably, the tail gas is condensed at a pressure substantially the same as the melamine synthesis pressure (e.g., 0 bar to 5 bar lower than the melamine synthesis pressure).

[0052] Condensing the tail gas at a higher pressure is desirable because this allows reducing the water content in the formed carbamate solution without the risk of carbamate precipitation. The relatively high condensation pressure enables the condensation temperature to reach, for example, higher than 120 °C, or even higher than 140 °C, and for example less than 160 °C.

[0053] The tail gas condenser is preferably included in the melamine production section. The tail gas condenser is, for example, a heat exchanger that generates steam on the cooling fluid side. The steam is used, for example, for the stripper of the melamine plant, for example to remove condensates, or for the hydrolysis unit to purify an aqueous stream, or for heating the mother liquor to be purified. The steam can also be used for steam tracing, for example for tracing the urea melt pipeline. The relatively high condensation pressure (e.g., at least 80 bar) is beneficial for generating useful steam.

[0054] The tail gas condenser can also be an absorber, especially an absorber that does not perform heat exchange with the cooling fluid, for example, if the melamine production capacity is relatively small compared to the urea production capacity.

[0055] The condensation unit includes, for example, a shell-and-tube heat exchanger.

[0056] In an optional embodiment, the melamine tail gas condenser is as described, for example, in WO 2023 / 280684A1 and is, for example, a partial condensation unit. For example, additional NH3 is supplied to the condensation section to promote urea formation in the melamine tail gas condenser. In some additional embodiments, the tail gas condensation does not involve significant urea formation.

[0057] Therefore, the device includes a melamine tail gas condenser. In an embodiment, the melamine tail gas condenser is included in the melamine production section. For example, the melamine tail gas condenser is a heat exchanger for generating steam used in the melamine production section.

[0058] The method involves dissolving molten melamine (typically after one or more purification steps) and crystallizing melamine from the solution with the formation of a crystalline melamine stream and a mother liquor stream, which is an aqueous liquid stream containing water, ammonia, melamine, and typically some urea and oxy-amino-triazine (OAT). Therefore, the device includes a melamine processing section that includes a unit for dissolving melamine, such as a quench tower and a crystallizer.

[0059] The method includes, for example, contacting the liquid phase from the melamine synthesis unit with gaseous NH3 to remove dissolved CO2 and completing the pyrolysis reaction of urea to melamine in a post-reactor. A background reference for an example of such a step is US2004 / 0162429A1. Therefore, the device can include a melamine post-reactor.

[0060] The method can include, for example, dissolving a liquid melamine product (e.g., from the said post-reactor) in an ammonia aqueous solution in a unit for dissolving molten melamine, such as in a quench tower. A background reference for an example of such a step is US2004 / 0162429A1.

[0061] The method preferably involves treating part or all of the mother liquor in a treatment unit to obtain a recycled mother liquor and an aqueous carbamate solution. Therefore, the device includes a treatment unit, especially a mother liquor treatment unit. Here, the aqueous carbamate solution refers to an aqueous solution of NH3 and CO2, for example, as carbonate species. The aqueous carbamate solution is provided by the mother liquor treatment unit at a pressure of, for example, at least 10 bar or at least 15 bar, such as 10 bar to 30 bar.

[0062] Treatment of the mother liquor stream to be processed (part or all of the mother liquor) includes, for example, heating the mother liquor or the aqueous fraction of the mother liquor, which causes hydrolysis of the organic components (such as unreacted urea and melamine) into CO2 and NH3, which are obtained as the aqueous carbamate solution after condensation. The heating step is used, for example, for deammoniation of the mother liquor or for decomposition of the organic components. In an embodiment, two or more heating steps are used for gas / liquid separation, which produces a gas stream containing CO2 and NH3, and these gas streams are recovered as the aqueous carbamate solution. For example, the first heating step is used for deammoniation and the second heating step is used for decomposition, and the CO2 and NH3 obtained from either or both heating steps are provided as the aqueous carbamate solution after appropriate absorption and / or condensation. For example, the heating step is carried out in a heating unit having a gas outlet and a liquid outlet, and the gaseous stream from the gas outlet is cooled (e.g., relative to the ambient temperature), preferably using cooling water or using an air cooler, with the formation of an aqueous carbamate solution.

[0063] For example, the method involves subjecting part or all of the mother liquor to a deammoniation treatment to form a gaseous stream containing NH3, a carbamate solution, and an aqueous stream containing the deammoniated mother liquor. In some embodiments, part of the mother liquor is directly recycled to the quench tower. The deammoniation treatment is carried out, for example, in a deammoniation tower as described in US2004 / 0162429 A1 (also published as US7125992A), and uses, for example, distillation as described in US 3161638A. An exemplary deammoniation section is also described in paragraph

[0017] of EP 2385043A1.

[0064] The carbamate solution produced by the mother liquor treatment (preferably the deammoniation treatment) is preferably supplied to the LP carbamate condenser.

[0065] Thus, compared with the reference method of transporting the lean carbamate solution from the mother liquor treatment to the WWT unit in the urea production section, it is advantageous that the lean carbamate solution from the mother liquor treatment is processed in an energy-saving manner in the urea production section. In this embodiment, the manner of processing the tail gas from the melamine synthesis is not particularly limited, and, for example, anhydrous condensation of the tail gas can be optionally used as described in US6111138A. However, condensation of the tail gas to form a carbamate solution that also contains water is preferred.

[0066] The aqueous stream containing the deammoniated mother liquor is further processed, for example, in a section for eliminating oxyamino triazine (OAT), for example, by decomposition into NH3 and CO2, as described, for example, in EP 2385043A1 and EP 3597641A1.

[0067] A gaseous stream containing NH3 from the deamination treatment is, for example, suitably supplied to a quench column after being absorbed into an aqueous stream for reconstructing the mother liquor. Thus, the deamination treatment is preferably also used for the purpose of ammonia recovery. At least some of the ammonia contained in the mother liquor is removed and recovered in the deamination treatment. Generally, the treatment of the mother liquor can recover the mother liquor by reconstructing the mother liquor from an ammonia stream and an aqueous stream.

[0068] In an embodiment, thus the treatment of the mother liquor includes deaminating the mother liquor to form a gaseous stream containing NH3, an aqueous solution of carbamate, and an aqueous stream containing the deaminated mother liquor. Preferably, at least part of the aqueous solution of carbamate is supplied to a urea plant, particularly an LP carbamate condenser.

[0069] An example of the aqueous solution of carbamate produced in this way is stream 27 in Figures 2 and 3 of EP2385043A1.

[0070] In an embodiment, thus the treatment involves deaminating the mother liquor to at least form a gaseous stream containing NH3 and an aqueous stream containing the deaminated mother liquor, and subjecting at least part of the deaminated mother liquor to decomposition to recover organic compounds as NH3 and CO2, thereby obtaining a first liquid stream and a second liquid stream. The first aqueous stream is, for example, substantially pure water, and the second liquid stream is a carbamate solution. The second liquid stream is obtained, for example, by condensation of a gas stream from the decomposition step. Examples of such treatment are discussed in EP3597641A1, paragraphs

[0026] and

[0073] and are shown in the drawings of the said document as unit EO; wherein the second liquid stream is shown in the drawings 2 and 3 of the said document as flow line 33. The condensation to produce the second liquid stream may involve absorption of the gas stream from the decomposition into an aqueous stream.

[0071] In the present invention, the second liquid stream (i.e., the carbamate solution) is preferably supplied in part or in whole to a urea plant, more preferably to an LP carbamate condenser.

[0072] In an embodiment, thus the treatment involves deaminating the mother liquor to at least form a gaseous stream containing NH3 and an aqueous stream containing the deaminated mother liquor, and subjecting at least part of the deaminated mother liquor to decomposition (e.g., hydrolysis) to form a gas stream containing NH3 and CO2, which gas stream is condensed to at least form part of the carbamate solution. The condensation may involve absorption in an aqueous stream. The gas stream may be contacted with a liquid before condensation. The decomposition involves, for example, an organic compound (such as oxyamino triazine (OAT)) being decomposed into NH3 and CO2.

[0073] Suitably, make-up water is supplied to the melamine processing section to balance the water losses from the melamine unit, in particular the water contained in the tail gas condensate supplied directly or indirectly to the urea synthesis section.

[0074] The method preferably involves supplying part or all of the carbamate aqueous solution (i.e., the lean carbamate solution) to the LP carbamate condenser (LPCC) included in the LP recovery section, and the apparatus preferably includes a corresponding liquid flow line for the carbamate aqueous solution from the mother liquor treatment unit to the LP carbamate condenser. The gas stream containing CO2 and NH3 from the LP dissociator is condensed in the LP carbamate condenser, thereby forming a carbamate solution, such as a rich carbamate solution. The water fraction of the carbamate solution from the deamination treatment is preferably used as a solvent for the carbamate in the LP carbamate condenser. Preferably, at least 50 wt.% of the carbamate solution is supplied to the LPCC. The LP recovery section may include two or more LP carbamate condensers in series or in parallel, and the carbamate stream or a portion thereof may be supplied to one or more of the LP carbamate condensers. If a portion of the carbamate solution is supplied to the LPCC, the other portion is supplied to, for example, the wastewater treatment section of the urea unit, or is supplied to, for example, a recovery section operating at an absolute pressure of, for example, 3 bar to 20 bar, and is recycled from this recovery section to the urea synthesis section.

[0075] The water content of the carbamate solution supplied from the melamine production section to the LP carbamate condenser of the urea production section is, for example, at least 30 wt.%, at least 40 wt.% or at least 45 wt.%, for example up to 80 wt.%; and is thus a lean carbamate solution. The carbamate solution received by the LP carbamate condenser contains, for example, at least 10 wt.% NH3 and at least 5 wt.% CO2 (including the amount as carbamate). The carbamate solution received by the LP carbamate condenser contains, for example, 10 wt.% to 40 wt.% carbamate, for example 15 wt.% to 35 wt.%.

[0076] The composition of the carbamate solution supplied to the LP carbamate condenser is, for example: 30 wt.% to 60 wt.% water, 20 wt.% to 40 wt.% NH3 and 10 wt.% to 20 wt.% CO2 (including the amount as carbamate).

[0077] The amount of the carbamate aqueous solution, relative to the urea produced in the synthesis section, is, for example, less than 0.2 tons of carbamate solution / ton of urea produced, for example in the range of 30 kg to 100 kg of carbamate solution / 1000 kg of urea produced, and is thus, for example, in the range of 0.3 wt.% to 1.0 wt.%.

[0078] The amount of the aqueous carbamate solution is 1.0 wt.% to 4 wt.%, more preferably 1.5 wt.% to 3.0 wt.%, relative to the urea feed of the melamine unit.

[0079] In addition, the LPCC typically also receives the aqueous carbamate solution from the WWT of the urea unit, particularly from the reflux condenser of the WWT of the urea unit, and typically also from the atmospheric condenser.

[0080] The LPCC receiving the carbamate solution preferably operates at a pressure of at least 4.5 bar, more preferably 4.5 bar to 5.5 bar, to support condensation, wherein the N / C ratio of the formed carbamate solution is 3.0 to 3.5.

[0081] If the amount of the carbamate solution supplied to the LPCC is small relative to the amount of CO2 and NH3 released in the LP dissociator connected to the LPCC, the N / C ratio of the total amount of the formed carbamate solution will be low. The uncondensed gas from the LPCC can be sent to the atmospheric absorber or the atmospheric condenser, as known in the art.

[0082] If appropriate, an MP heating step can be used as part of the MP treatment section, for example, using steam as the heating fluid in a heat exchanger to reduce the N / C ratio of the urea solution received in the LP recovery section. In addition, the urea solution supplied to the LP dissociator can be contacted with a gaseous stream containing CO2, for example, a gaseous stream with an N / C ratio less than 2.0, preferably less than 1.6 (such as a gas stream obtained from flashing, for example, adiabatic flashing, at medium pressure). Any uncondensed NH3 from the LP recovery section is, for example, absorbed in the (atmospheric) absorber or is further condensed, for example, in the atmospheric condenser (optionally together with other vapors).

[0083] Advantageously, the carbamate solution bypasses the (typical) wastewater treatment section (WWT) of the urea production section and is supplied to the LP carbamate condenser, which improves the energy efficiency of the plant, particularly compared to a plant where the carbamate solution is supplied to the WWT section of the urea unit and the LP carbamate condenser receives the carbamate solution obtained by condensing the vapors from the WWT (particularly from the typical desorber and hydrolyzer of the WWT).

[0084] In particular, if the carbamate solution from the melamine production section at least partially originates from the deammoniation unit upstream of the hydrolysis-based decomposition unit of the melamine production section, the load on the decomposition unit is advantageously reduced.

[0085] The carbamate solution from the LP carbamate condenser or, preferably, supplied to the off-gas condenser (e.g., the carbamate-rich solution), for example, has a water content of less than 40 wt.%, such as 20 wt.% to 40 wt.%, such as 20 wt.% to 35 wt.%, or 25% to 40%, or 30 wt.% to 40%, and, for example, provides a pressure of 4.0 bar to 8.0 bar, more preferably 4.5 bar to 5.5 bar. This water content preferably also applies to the carbamate-rich solution at the inlet of the melamine off-gas condenser. A water content of at least 20 wt.% or at least 25 wt.% provides the advantage that sufficient water is supplied to the off-gas condenser to prevent the carbamate solution from crystallizing in and from the off-gas condenser, and a relatively small amount of carbamate is transported from the urea plant to the melamine plant, for example, compared to supplying the carbamate solution from the MP carbamate condenser.

[0086] The amount of make-up water supplied to the LP carbamate condenser (e.g., sourced from WWT) can be reduced by an amount corresponding to the amount of water effectively provided by the carbamate-lean solution.

[0087] The method preferably involves directly or indirectly supplying at least part or all of the carbamate-rich solution (e.g., the carbamate-rich solution) from the LP carbamate condenser to the off-gas condenser included in the melamine production section, where the (carbamate-rich) solution is brought into contact with the off-gas to be condensed. Accordingly, the apparatus preferably includes a corresponding liquid flow line.

[0088] In an optional embodiment, part or all of the (carbamate-rich) solution is supplied from the LP carbamate condensation zone to the off-gas condenser via an optional MP recovery section, in particular via the MP carbamate condenser included therein. Depending on the amount of water required in the off-gas condenser, for example, part of the carbamate solution from the MP carbamate condenser is supplied to the off-gas condenser. The (carbamate-rich) solution can be pumped as appropriate. The off-gas condenser can also receive other water streams included, such as streams from the melamine production section units. In some embodiments, at least 80% of the water source of the carbamate solution at the outlet of the off-gas condenser is from the (carbamate-rich) solution from the LP carbamate condenser. In some embodiments, bypassing the off-gas condenser, a first part of the (carbamate-rich) solution is supplied to the off-gas condenser, and a second part of the (carbamate-rich) solution is supplied to the urea synthesis section.

[0089] In some embodiments, the carbamate solution of the first part (such as obtained from the liquid outlet of the LP carbamate condenser) is directly or indirectly supplied to the off-gas condenser, and the carbamate solution of the second part bypasses the off-gas condenser and is directly or indirectly supplied to the urea synthesis section. The first part is, for example, a carbamate solution of 10 wt.% to 50 wt.% (such as obtained from the liquid outlet of the LP carbamate condenser), for example 10 wt.% to 25 wt.%.

[0090] Due to the higher pressure (typically at least 10 bar or at least 30 bar or at least 60 bar higher) in the off-gas condenser than in the LP carbamate condenser, even if the water content (wt.%) in the (rich) carbamate solution is not significantly higher than the minimum water content allowed in the LP carbamate condenser, the water fraction in the (rich) carbamate solution from the LP carbamate condenser can act as a solvent for the carbamate formed by the condensation of the off-gas from melamine synthesis. Thus, in a preferred embodiment, the carbamate solution at the (liquid) inlet of the off-gas condenser has a high enough water content to prevent the crystallization of carbamate in the melamine off-gas condenser and in the resulting carbamate solution in the liquid flow line. Preferably, at least 90 mol% of the water received in total by the melamine off-gas condenser is supplied by the (rich) carbamate solution and is from the LP carbamate condenser.

[0091] In alternative embodiments, only one or more aqueous streams other than the rich carbamate solution are supplied to the off-gas condenser to provide an absorbent for the off-gas to be condensed. In further embodiments, at least part or all of the rich carbamate solution is directly or indirectly supplied to the off-gas condenser included in the melamine production section, while other aqueous streams are supplied to the (LP) carbamate condenser.

[0092] The rich carbamate solution is initially formed in the LP carbamate condenser included in the low-pressure recovery section. In embodiments, part or all of the rich carbamate solution is indirectly supplied to the off-gas condenser, for example, optionally through an absorber and / or a purifier, and then supplied to the off-gas condenser. The purifier and / or absorber are used to absorb CO2 and NH3 from the gas stream into the liquid. These gas streams are, for example, the uncondensed steam from the condenser. This may result in an increase in the N / C ratio of the rich carbamate solution.

[0093] The method preferably involves supplying the tail gas condensate (i.e., carbamate solution) from the tail gas condenser to the urea reaction zone or the urea synthesis section; in an embodiment, it is supplied to the urea reaction zone included in the urea synthesis section. For example, the tail gas condensate is directly or indirectly supplied to the urea reaction zone, such as a urea reactor or a high-pressure carbamate condenser in the synthesis section. Thus, the supplied carbamate is converted to urea in the urea synthesis section. The water fraction of the tail gas condensate (carbamate solution) is unfavorable to the urea yield, and a relatively low water fraction is advantageous. However, considering also the risk of cold spots in, for example, relatively long transfer pipelines, a certain minimum water content is beneficial for the reliable transfer of the carbamate solution, especially for transferring the carbamate solution from the tail gas condenser included in the melamine production section to the synthesis section of the urea production section. The tail gas condensate is preferably supplied (e.g., pumped) to the urea synthesis section, where the urea synthesis operates at a pressure preferably at least 10 bar or at least 20 bar higher than that of the tail gas condenser. For a preferred embodiment where the tail gas condenser operates at a pressure higher than 40 bar or higher than 50 bar, the tail gas condensate contains, for example, 5 wt.% to 20 wt.% water, such as 10 wt.% to 15 wt.% water. A person skilled in the art can determine the pressure required to achieve these water levels. The tail gas condensate contains, for example, 40 wt.% to 50 wt.% NH3 and 40 wt.% to 50 wt.% CO2.

[0094] In some further exemplary embodiments, the tail gas condensate is supplied to a separate reaction zone, such as a pre-reactor, or the reactor section of the tail gas condenser, or a dedicated synthesis section of the associated urea plant. The manner of supplying the tail gas condensate to the urea reaction zone is not particularly limited. However, supplying the carbamate solution to the synthesis section of the urea production zone offers the advantage of simple design (e.g., using a single urea reactor).

[0095] At least a part of the urea solution from the LP recovery section is supplied to an evaporation section that includes one or more evaporators where water is evaporated to form urea melt. In some embodiments, a part of the urea melt is supplied to the melamine unit and another part is supplied to the finishing section. Preferably, a part of all the urea melt is supplied to the melamine production section. The vapor stream from the evaporation section is condensed in the condensation section. The process condensate from the condensation section is supplied to the wastewater treatment section WWT (also called the process condensate treatment section) of the urea production section for purification, where the urea, NH3, and CO2 (also in the form of carbamate) contained therein are removed by hydrolysis and desorption to provide clean process condensate and a gas stream containing H2O, CO2, and NH3, which is typically supplied to a reflux condenser and condensed therein. This purification step is usually energy-intensive.

[0096] Exemplary apparatuses and methods of the present invention are schematically shown in Figure 1 . The apparatus (100) includes a urea production section (101) and a melamine production section (102). The urea production section (101) includes a urea synthesis section (1) and an LP recovery section (3).

[0097] The melamine production section includes a melamine synthesis section (4), a washing unit (7), a tail gas condenser (9), a melamine processing section (11), and a treatment unit (14) (mother liquor treatment section).

[0098] In the urea synthesis section (101), urea is produced by the following method: CO2 reacts with NH3 to obtain a first urea stream (2), which also contains water and carbamate, in particular a urea solution. The first urea stream is purified in at least the LP recovery section (3), preferably in series MP and LP recovery sections, to decompose and remove at least carbamate from the urea stream in the LP dissociator (20) to obtain a purified urea solution (19) and a gas stream (21), and the gas stream is condensed in the LP carbamate condenser (18). The LP carbamate condenser (18) typically also receives an aqueous carbamate solution (22), for example from a reflux condenser. In the melamine production section (102), melamine is produced by pyrolyzing urea in a high-pressure non-catalytic melamine synthesis section (4) to obtain molten melamine (5) and an anhydrous tail gas (6). The anhydrous tail gas is typically purified (e.g., washed (or cleaned)) by contacting it with urea (in particular urea melt) in a washing unit (7). The washed tail gas (8) is condensed in a tail gas condenser (9) at a pressure of at least 25 bar, preferably at least 60 bar or at least 70 bar, to form a tail gas condensate (10), which is typically a high-pressure (<70 bar) carbamate solution. The molten melamine (5) is dissolved in an aqueous solution in the melamine processing section (11), and melamine is crystallized from the solution with the formation of a crystalline melamine stream (12) and a mother liquor stream (13). Part or all of the mother liquor is treated (14) to obtain a recycled mother liquor (15) and an aqueous carbamate solution (16). Preferably, part or all of the carbamate solution (16) is supplied (e.g., pumped) to the LP carbamate condenser (18) included in the LP recovery section (3), thereby forming a carbamate-rich solution (17) by condensing the gas stream from the LP dissociator of the LP recovery section. The first part of the (rich) carbamate solution (17) from the LP carbamate condenser (18) is directly supplied to the tail gas condenser (9), and the tail gas condensate (10) (carbamate solution) from the tail gas condenser is preferably supplied to the urea synthesis section. The second part of the (rich) carbamate solution (17a) is supplied to the urea synthesis section (1), preferably through an MP recovery section (not shown), in particular through the MP carbamate condenser of the MP recovery section.

[0099] The present invention also provides a method for retrofitting an existing plant for the co-production of urea and melamine. Refer to Figure 1(This does not limit the present invention), the existing plant includes a urea production section (101) (including a urea synthesis section (1)) and a low-pressure recovery section (3) for purifying a first urea stream, which section includes an LP carbamate condenser (18) with an outlet for a carbamate-rich solution (17). In the existing plant, the carbamate-rich solution (17) is supplied to the urea synthesis section (1). The LP carbamate condenser (18) receives an aqueous carbamate solution in the existing plant, and the carbamate-rich solution (17) is rich in carbamate relative to the aqueous carbamate solution. The low-pressure recovery section (3) includes a unit (dissociator) for purifying the first urea stream, which unit has a gas outlet connected to an inlet of the LP carbamate condenser (18). The existing plant also includes a melamine production section (102), or this section is added. The melamine production section (102) includes a melamine synthesis section (4), a tail gas scrubbing unit (7), a tail gas condenser (9) for condensing the tail gas (8) (the tail gas condenser is configured to operate at a pressure of at least 25 bar to form a tail gas condensate (10)), and a melamine processing section (11) (the melamine processing section includes a unit for dissolving molten melamine in an aqueous solution), and a crystallizer for crystallizing melamine with the formation of a crystallized melamine stream (12) and a mother liquor stream (13). The melamine production section (102) further includes a treatment unit (14) for treating part or all of the mother liquor (13) to obtain a recovered mother liquor (15) and an aqueous carbamate solution (16). The existing plant includes a liquid flow line from an outlet of the tail gas condenser (9) for the tail gas condensate (10) to an inlet of the urea synthesis section. More details and preferred embodiments of the melamine production section (102) are as specified in the apparatus and method of the present invention.

[0100] A method for retrofitting an existing plant includes providing a plant having a liquid flow line from an outlet of the treatment unit (14) for the carbamate solution (16) to the carbamate condenser (18) included in the low-pressure recovery section (3); and / or providing a plant having a liquid flow line from an outlet of the LP carbamate condenser (18) for the carbamate-rich solution (17) directly or indirectly to the tail gas condenser (9). In other methods for retrofitting an existing urea plant, a melamine production section is added and co-production is carried out using liquid flow lines as specified. However, the apparatus of the present invention is also very suitable as a new plant.

[0101] As used herein, the term 'carbamate', when used in the field of urea production, refers to ammonium carbamate. In an aqueous carbamate stream, this component may be present as a carbonate substance; thus, an aqueous carbamate solution may also be referred to as a carbonate solution. The amounts of NH3 and CO2 in the water stream include the amounts present as carbonate substances.

[0102] As used herein, for the process streams in the urea production section (i.e., not steam pipelines; nor the melamine production section), 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 intermediate pressures of 30 bar to 70 bar), particularly 15 bar to 30 bar, and low pressure (LP) is for example 0 bar to 10 bar, particularly 1 bar to 8 bar or 2 bar to 5 bar. All pressures are in bar absolute (bar).

[0103] The terms 'typically' and 'particularly' are used to indicate features that may be used in some embodiments but are not mandatory. Preferred features are also not mandatory.

[0104] The N / C ratio of a gas stream indicates the molar ratio of NH3 to CO2. As used herein, the N / C ratio for the urea synthesis section reflects the composition of the so-called initial mixture consisting only of NH3, CO2, and H2O before urea production, as used in the field of urea plants, and is the molar ratio of NH3 to CO2. The N / C ratio of a carbamate solution indicates the molar ratio of the corresponding amounts of NH3 to CO2. The N / C ratio of a carbamate condenser refers to the N / C ratio of the carbamate solution at the liquid outlet. The H / C ratio of the urea synthesis section reflects the composition of the so-called initial mixture consisting only of NH3, CO2, and H2O before urea production, as used in the field of urea plants, and is the molar ratio of H2O to CO2.

[0105] As used herein, the term'melamine off-gas' refers to the off-gas from the melamine production section and refers to a gas stream mainly containing NH3, CO2, and possibly H2O.

[0106] As used herein, the term 'first' for a unit or step allows for additional instances upstream of such unit or step.

[0107] As used herein, a liquid flow pipeline indicates a flow pipeline for transporting a liquid stream, the fluid being in a liquid state throughout the liquid flow pipeline, and does not include a flow pipeline for transporting a gaseous stream. A liquid flow pipeline may also be referred to as a liquid flow connection and may pass through a unit, particularly from the liquid inlet to the liquid outlet of the unit.

[0108] All priorities and details related to the method discussed also apply to the apparatus, and vice versa. All priorities and details related to the apparatus described also apply to the method of retrofitting the apparatus. The urea method of the present invention is preferably carried out in the apparatus of the present invention. The apparatus of the present invention is preferably suitable for the method of the present invention.

[0109] Embodiments of the present invention will now be further illustrated by the following examples, which do not limit the present invention or the claims.

[0110] Example 1

[0111] A 3790 MTPD urea plant of the HP CO2 stripping type is co-produced with a melamine production section. For example, 30 wt.% of the urea is supplied to a non-catalytic high-pressure melamine production section, which has a melamine synthesis pressure of 110 bar and urea scrubbing of the tail gas. The co-production plant is schematically shown in Figure 1 . Note that other melamine synthesis pressures are also feasible, such as at least 80 bar, and the melamine tail gas condenser preferably operates at substantially the same pressure as the melamine synthesis section.

[0112] The carbamate solution (A) (at about 50 °C) from the ammonia stripping unit of the mother liquor from the melamine production section is supplied to an LP carbamate condenser operating at 5 bar and about 65 °C. In addition, a part B1 of the resulting rich carbamate solution (B) from the LP carbamate condenser is directly supplied to a melamine tail gas condenser operating at 110 bar and about 150 °C, which also receives melamine synthesis (C) in the form of substantially anhydrous tail gas. Those skilled in the art understand that these features can also be used independently.

[0113] The remaining part B2 of the rich carbamate solution is supplied to an MP condenser in the MP recovery section operating at 25 bar. The carbamate solution (D) formed in the melamine tail gas condenser is supplied to the urea synthesis section. The composition of the streams is as given in Table 1.

[0114] Advantageously, 5.5 ton / hr of water contained in stream A in the example is supplied from the melamine plant to the LP carbamate condenser without energy-consuming treatment in the wastewater treatment section of the urea production plant.

[0115] It can be seen that, as a further advantage, compared with the reference example in which the carbamate-rich solution is entirely supplied to the urea synthesis section, optionally via an MP recovery section operating at 25 bar, in the example, 7.6 ton / hr of the carbamate-rich solution is supplied to the tail gas condenser and thus effectively used twice. The LP carbamate condenser receives make-up water where appropriate.

[0116] Compared with the reference plant in which all the carbamate-rich solution is supplied to the MP recovery section and the carbamate-lean solution is directly supplied to the tail gas condenser, the water recovery in the urea synthesis section is reduced by 3.4 ton / hr (from 50.0 ton / hr to 46.5 ton / hr, a relative reduction of 7%).

[0117] The H / C ratio in the urea synthesis section of the inventive plant is 0.62, while that of the reference plant is 0.67, in which the carbamate solution from the LP recovery section is directly supplied to the synthesis section, and in which the melamine tail gas condenser uses a separate aqueous solution and the resulting condensate is separately recycled to the urea synthesis section of the comparative plant.

[0118] Compared with the said reference plant, the H / C ratio of 0.62 in the inventive plant results in an additional 0.5 wt.% increase in the urea conversion in the reactor and, for the same stripping efficiency, a reduction in the steam requirement of the high-pressure stripper by 20 kg to 25 kg of steam / ton of urea.

[0119] Table 1

[0120]

[0121] (*) NH3 and CO2 are also present as carbamate in the liquid stream; (**) reference Figure 1 。

Claims

1. A method for co-production of urea and melamine, the method comprising: - Producing urea in a urea synthesis section (1) of a urea production section (101) to obtain a first urea stream (2); - Subjecting the first urea stream to purification in at least a low pressure (LP) recovery section (3); - Producing melamine in a high-pressure non-catalytic melamine synthesis section (4) to obtain molten melamine (5) and anhydrous tail gas (6), the melamine synthesis section being included in a melamine production section (102); - In a washing unit (7), washing the anhydrous tail gas by contacting it with urea, and condensing the washed tail gas (8) in a tail gas condenser (9) of the melamine production section at a pressure of at least 25 bar to form a tail gas condensate (10); - In a melamine processing section (11), dissolving the molten melamine in an aqueous solution, and crystallizing the melamine with the formation of a crystallized melamine stream (12) and a mother liquor stream (13), and treating part or all of the mother liquor (14) to obtain a recovered mother liquor (15) and an aqueous carbamate solution (16).

2. The method according to claim 1, the method comprising: - Supplying at least a part of a carbamate solution (17) from a carbamate condenser (18) included in the low pressure recovery section (3) directly or indirectly to the tail gas condenser (9).

3. The method according to claim 2, wherein the carbamate solution (17) from the carbamate condenser (18) included in the low pressure recovery section (3) is directly supplied to the tail gas condenser (9).

4. The method according to any one of the preceding claims, the method further comprising: - Supplying part or all of the aqueous carbamate solution (16) to a carbamate condenser (18) included in the low pressure recovery section (3) to form a carbamate-rich solution (17).

5. The method according to claim 4, the method further comprising: - Supplying at least part of the carbamate-rich solution (17) directly or indirectly to the tail gas condenser (9); and - Preferably, supplying the tail gas condensate (10) to a urea reaction zone and / or the urea synthesis section.

6. The method according to any one of the preceding claims, wherein the treatment (14) of the mother liquor comprises: - Deammoniating the mother liquor to form a gaseous stream containing NH3, at least part of the carbamate solution (16), and an aqueous stream containing deammoniated mother liquor.

7. The method according to any one of the preceding claims, wherein the treatment (14) of the mother liquor comprises: - Deammoniating the mother liquor to at least form a gaseous stream containing NH3 and an aqueous stream containing deammoniated mother liquor, and subjecting at least part of the deammoniated mother liquor to decomposition to form a gas stream, which is condensed to form at least part of the carbamate solution (16).

8. The method according to any one of the preceding claims, wherein the purification in the LP recovery section involves dissociating the ammonium carbamate contained in the first urea stream (2) in an LP dissociator to obtain a gas stream (21) containing CO2 and NH3, which gas stream is condensed in an LP carbamate condenser (18).

9. The method according to any one of the preceding claims, wherein the urea synthesis section (1) comprises a high-pressure CO2 stripper tower.

10. The method according to claim 9, wherein the purification of the first urea stream involves flashing the urea stream at medium pressure (MP), optionally accompanied by further heating at medium pressure, the flashing and optional heating yielding a first urea solution supplied to the low-pressure recovery section and an MP gas stream supplied to an MP carbamate condenser.

11. The method according to claim 10, wherein the carbamate solution (17), preferably the carbamate-rich solution, is partially supplied to the MP carbamate condenser.

12. The method according to any one of the preceding claims, wherein the off-gas condenser (9) operates at a pressure of at least 80 bar.

13. The method according to any one of the preceding claims, wherein the urea synthesis section operates at a pressure at least 10 bar or at least 20 bar higher than that of the off-gas condenser.

14. The method according to any one of the preceding claims, wherein at least 10 wt.% of the urea formed in the synthesis section is supplied to the melamine production section.

15. An apparatus (100) for the co-production of urea and melamine, the apparatus comprising: a urea production section (101), which comprises - a urea synthesis section (1) for producing urea to obtain a first urea stream (2); - a low-pressure recovery section (3) for purifying the first urea stream; and a melamine production section (102), which comprises: - a high-pressure non-catalytic melamine synthesis section (4) for producing melamine to obtain molten melamine (5) and anhydrous off-gas (6); - a washing unit (7) for washing the anhydrous off-gas by contact with urea; - an off-gas condenser (9) for condensing the off-gas (8) at a pressure of at least 25 bar to form an off-gas condensate (10); - a melamine processing section (11), which comprises a unit for dissolving the molten melamine in an aqueous solution and a crystallizer for crystallizing the melamine with the formation of a crystallized melamine stream (12) and a mother liquor stream (13); and - a treatment unit (14) for treating part or all of the mother liquor to obtain a recovered mother liquor (15) and an aqueous ammonium carbamate solution (16), wherein the apparatus comprises: a carbamate condenser (18) included in the low-pressure recovery section (3), the carbamate condenser having an outlet for the carbamate solution (17). and preferably, a liquid flow line for supplying part or all of the aqueous carbamate solution (16) to the carbamate condenser (18).

16. The device according to claim 15, wherein the device comprises: A liquid flow line for supplying at least part of the carbamate solution (17) directly or indirectly to the tail gas condenser (9), and preferably, a liquid flow line for supplying the tail gas condensate (10) to the urea reaction zone and / or the urea synthesis section.

17. The apparatus according to claim 15 or 16, wherein the processing unit comprises: A tower for deaminating the mother liquor, the tower having a gas outlet for a gaseous stream containing NH3 and a liquid outlet for an aqueous stream containing the deaminated mother liquor; and a decomposition unit for subjecting at least part of the deaminated mother liquor to decomposition, the decomposition unit having a gas outlet for a gas stream connected to a condensation unit for condensing the gas stream to form at least part of the carbamate solution (16).

18. The apparatus according to any one of claims 15 to 17, wherein the urea synthesis section (1) includes a high-pressure CO2 stripping tower.

19. The apparatus according to claim 16, wherein the liquid flow line is configured to supply at least part of the carbamate solution (17) directly from the LP carbamate condenser (18) to the tail gas condenser (9).

20. A method for retrofitting an existing apparatus for the co-production of urea and melamine, the existing apparatus comprising: A urea production section (101), which includes - a urea synthesis section (1) for producing urea to obtain a first urea stream (2); - a low-pressure recovery section (3) for purifying the first urea stream, the low-pressure recovery section including a carbamate condenser (18) having an outlet for a rich carbamate solution (17); - a melamine production section (102), which includes: - a high-pressure non-catalytic melamine synthesis section (4) for producing melamine to obtain molten melamine (5) and anhydrous tail gas (6); - a washing unit (7) for washing the anhydrous tail gas by contacting it with urea - a tail gas condenser (9) for condensing the tail gas (8) at a pressure of at least 25 bar to form a tail gas condensate (10); - a melamine processing section (11), which includes a unit for dissolving the molten melamine in an aqueous solution, and a crystallizer for crystallizing the melamine with the formation of a crystallized melamine stream (12) and a mother liquor stream (13), and - a processing unit (14) for treating part or all of the mother liquor (13) to obtain a recovered mother liquor (15) and an aqueous carbamate solution (16), - and a liquid flow line from the outlet of the tail gas condenser (9) for the tail gas condensate (10) to the inlet of the urea synthesis section; The method includes providing for the apparatus: - a flow line from the outlet of the processing unit (14) for the aqueous carbamate solution (16) to the carbamate condenser (18) included in the low-pressure recovery section (3); and / or - Provide a liquid flow line from the outlet for the amino formate-rich solution (17) directly or indirectly to the off-gas condenser (9).

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