Coproduct urea melamine production with hp co2 stripping

Abstract

The present disclosure relates to an integrated plant process for the production of urea and melamine, said process having a urea synthesis section with HP CO2 stripping and a part of the CO2 feed is supplied to a recovery section.

Description

Technical Field

[0001] This invention relates to the co-production of urea and melamine.

[0002] introduce

[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, pp. 44-54, describes various methods for urea-melamine plant integration.

[0005] Currently, there is still a need for co-production of relatively large melamine production units with improved urea production methods, featuring a CO2 stripping-type HP urea synthesis section. This is particularly important compared to urea production via an HP urea synthesis section, where a relatively large amount of tail gas from the melamine unit is recovered and fed back into the HP urea synthesis section. High energy efficiency and low equipment costs are particularly required. This co-production unit and method includes a urea production section with a urea synthesis section, a recovery section, and a wastewater treatment (WWT) section. WWT includes, for example, hydrolyzers and desorbers, and is energy-intensive (steam consumption per kg of treated water is relatively high). The WWT section typically also includes a reflux condenser. Summary of the Invention

[0006] In a first aspect, the present invention relates to a co-production method for urea and melamine, the method comprising: producing urea in a high-pressure urea synthesis section including a high-pressure (HP) CO2 stripping tower to produce a stripped urea solution, wherein the HP CO2 stripping tower uses a first portion of a gaseous CO2 feed stream as a stripping agent; producing melamine in a melamine production section, thereby also releasing melamine tail gas; the melamine tail gas optionally being supplied directly or indirectly as a condensate to the high-pressure urea synthesis section; the stripped urea solution being supplied directly or indirectly via a liquid flow connection to a low-pressure (LP) dissociator to produce a purified urea solution and a low-pressure gas stream; and subjecting the low-pressure gas stream to condensation in a low-pressure (LP) carbamate condensation zone together with a second portion of the gaseous CO2 feed stream to form a carbamate solution.

[0007] The present invention also relates to a co-production apparatus for producing urea and melamine, the apparatus comprising: a high-pressure urea synthesis section for producing urea, the high-pressure urea synthesis section including a high-pressure (HP) CO2 stripping tower having an outlet for stripped urea solution and an inlet for a first portion of the gaseous CO2 feed stream serving as a stripping agent; a melamine production section having an outlet for melamine and an outlet for melamine tail gas; and a fluid flow connection for the melamine tail gas optionally being used directly or indirectly as a condensate. The apparatus includes a ground supply to the high-pressure urea synthesis section; a liquid flow connection for directly or indirectly supplying the stripped urea solution to a low-pressure (LP) dissociator, which has an outlet for purified urea solution and an outlet for low-pressure gas stream; and a low-pressure (LP) carbamate condensation zone configured to condense the low-pressure gas stream together with a second portion of the gaseous CO2 feed stream to form a carbamate solution, the apparatus including a gas flow connection from the CO2 supply unit to the LP carbamate condensation zone.

[0008] Therefore, the present invention relates to a method and apparatus for the co-production of urea and melamine, the method having a urea synthesis section using HP CO2 stripping and a portion of the CO2 feed being supplied to an LP recovery section. Attached Figure Description

[0009] Figure 1 An example method scheme according to the present invention is illustrated schematically.

[0010] Any embodiments shown in one or more of the accompanying drawings are merely exemplary and do not limit the invention. Detailed Implementation

[0011] The method and apparatus of the present invention can combine a relatively large melamine production method with a urea production method employing a CO2 stripping-type HP urea synthesis section; that is, the amount of melamine tail gas recovered to the urea synthesis section is relatively high, both in terms of the urea production rate and the fresh NH3 and CO2 feeds, and particularly does not interfere with downstream sections (such as the LP recovery section) and / or significantly increase the load on the wastewater treatment section. Furthermore, the method and apparatus of the present invention do not require the use of expensive equipment (such as dedicated NH3 condensers) and allow the use of relatively small condensers in the LP section, and in a preferred embodiment, also allow for the use of relatively small condensers in the WWT.

[0012] What is particularly ingenious is that, when the amount of melamine tail gas recovered to the synthesis section is relatively large, the apparatus and method of the present invention can alleviate the relative lack of CO2 feedstock available for stripping in the HP urea synthesis section.

[0013] One advantage of this invention is that the steam consumption of the HP stripping tower may be relatively low.

[0014] Ullmann's Encyclopaedia, section Urea, 2010, mentions that in (Stamicarbon) CO2 stripping urea methods, integration with melamine units is unnecessary because the ammonia and carbon dioxide concentrations in the stripped urea solution are low, requiring only a low-pressure recovery stage. On the other hand, this invention allows for a relatively high N / C ratio in the stripped urea solution.

[0015] Various aspects of the invention will now be further described, with reference to [reference needed]. Figure 1 The figure illustrates an exemplary method and apparatus according to the present invention, and does not limit the invention or the claims.

[0016] This invention provides a co-production method and apparatus for producing urea and melamine. The method and apparatus are co-production because tail gas from melamine production is supplied to urea production. Optionally, the method can also be integrated, as molten urea from urea production is supplied to the melamine production section.

[0017] The unit generally includes a urea production section, which includes a high-pressure urea synthesis section (1); a recovery section; and typically WWT and evaporation sections.

[0018] The method comprises producing urea in a high-pressure urea synthesis section (1) including a high-pressure (HP) CO2 stripping tower (2), producing a stripped urea solution (3). The HP CO2 stripping tower (2) uses a gaseous CO2 feed stream, particularly as the first portion (4) of the high-pressure gaseous CO2 stream, as the stripping agent. CO2 stripping is used to generate a low NH3 partial vapor pressure in the stripping tower tubes to promote the decomposition of carbamates in the urea solution, and further heating is employed. The gaseous stream used for CO2 stripping contains at least 90 vol.% CO2, typically at least 95 vol.% CO2, with the balance being inert gases, typically including some air for passivation. The stripping tower is typically a shell-and-tube heat exchanger configured to allow the tubes of the urea solution to be stripped to fall film to come into countercurrent contact with the gaseous stripping agent. A stripping tower typically has a liquid inlet for urea solution at the top, a liquid outlet for stripped urea solution at the bottom, an inlet for CO2 gas used as the stripping gas at the bottom, and an outlet for the gas feed stream at the top; all outlets are on the tube side. Heating fluids (such as steam) are located on the shell side.

[0019] The synthesis section further includes reaction zones and condensation zones. These zones can be provided as independent units (such as urea reactors and high-pressure carbamate condensers) or combined in a single vessel, such as a pool reactor. Optionally, two or more condensation zones can be used, for example, in parallel or in series. Optionally, two or more reaction zones can be used, for example, in parallel or in series.

[0020] A urea reactor is typically a vertical urea reactor, with one or more inlets at the bottom and one or more outlets at the top for discharging the urea synthesis solution. The urea synthesis solution contains urea, water, carbamate, and NH3. An HP stripping tower is used to strip the urea synthesis solution to separate and remove the carbamate fraction.

[0021] HP carbamate condensers are typically shell-and-tube heat exchangers, preferably but not limited to those where the gas to be condensed is located in the shell-side space and the coolant is located in the tube bundle. The HP carbamate condenser has an outlet from the HP carbamate feed to the urea reactor; the gas outlet of the stripping tower is connected to the HP carbamate condenser. In embodiments where the gas from the stripping tower is condensed in the shell-side space of the HP carbamate condenser, some urea may already be formed in the HP carbamate condenser.

[0022] In embodiments employing a pool reactor, a horizontal vessel is provided comprising a condensation zone and a reaction zone; it has an inlet for gas from the stripper to the condensation zone and an outlet for urea synthesis solution from the reaction zone to the stripper. The condensation zone of the pool reactor is provided by a U-shaped tube bundle section within the vessel for cooling liquids and is in fluid communication (gas and liquid) with the reaction zone provided between the tube bundle bend and the vessel wall. Typically, the pool reactor includes separate gas and liquid outlets provided at the reaction zone. Advantageously, the pool reactor includes a distributor (gas distributor) for gas from the HP stripper, which extends through the condensation and reaction zones. Optionally, the pool reactor is combined with a downstream urea reactor (e.g., a vertical urea reactor) arranged between the reaction zone of the pool reactor and the HP stripper. Optionally, the pool reactor or HP carbamate condenser includes two tube bundles for two different coolants.

[0023] In a preferred embodiment, the reaction zone operates at a pressure of 130 to 150 bar. In another embodiment, the reaction zone, condensation zone, and HP stripping tower all operate at pressures in the range of 130 to 150 bar.

[0024] In a preferred embodiment, the reaction zone operates at an N / C ratio of 3.0 to 3.2. The relatively low N / C ratio in the reaction zone facilitates the recovery of carbamates from downstream processes, particularly ensuring that only carbamate solution is recovered from the recovery process.

[0025] The method involves supplying the stripped urea solution (3) directly or indirectly via a liquid flow connection to a low-pressure (LP) dissociator (8). Treatment in the LP dissociator, particularly heating, produces a purified urea solution (9) and a low-pressure gas stream (10). The LP dissociator includes, for example, a shell-and-tube heat exchanger for decomposing carbamates from the stripped urea solution. The LP gas stream comprises CO2 and NH3.

[0026] In an exemplary embodiment, the purified urea solution expands, for example, to atmospheric pressure, and preferably undergoes gas / liquid separation flash evaporation. Preferably, the resulting gas is condensed in a condenser (e.g., an atmospheric pressure condenser), and the liquid (i.e., the urea solution) is supplied to a heating unit, preferably a pre-evaporator operating at, for example, sub-atmospheric pressure of 0.2 bar to 0.5 bar; and the resulting heated urea solution undergoes gas-liquid separation. The urea solution from the pre-evaporator is supplied, for example, via a urea solution storage tank to, for example, a vacuum evaporation section, and the resulting urea melt is, for example, partially supplied to a melamine production section, and, for example, partially supplied to a urea refining section to solidify into a solid urea product.

[0027] The relatively low free N / C ratio of the urea solution at the atmospheric pressure flash evaporation inlet is beneficial to the operation of the atmospheric pressure condenser and also to the condensation of the gas obtained from the downstream urea solution in the atmospheric pressure flash evaporation.

[0028] The method involves condensing a low-pressure gas stream in a low-pressure (LP) carbamate condensation zone (11) to form a carbamate solution (12). The LP carbamate condensation zone is provided, for example, as a single condensation unit, or as two or more condensation units connected in series and / or in parallel, such as using first and second LP carbamate condensation units connected in series.

[0029] For example, in a possible embodiment, the first gas stream is condensed in a first condenser (which also receives a water stream), and the resulting carbamate solution is supplied to a second condenser that receives the second gas stream to obtain a second carbamate solution.

[0030] For all configurations of the LP carbamate condensation zone, the carbamate solution is recovered to the HP urea synthesis section, optionally via, for example, an MP carbamate condenser. Therefore, the apparatus includes an LP recovery section comprising an LP dissociator and an LP carbamate condensation zone. Optionally, a portion of the condensate from the first LP carbamate condensation unit (e.g., within the range of 10% to 60% of the total CO2 supplied to the condenser) is supplied to the condenser, and the gases (including NH3 and CO2) from the first LP carbamate condensation unit undergo further condensation in the second LP carbamate condensation unit. The first and second carbamate condensation units can be operated with different coolants. The temperature of the carbamate outlet (e.g., shell side) of the first carbamate condensation unit is, for example, in the range of 80°C to 100°C; and for the second carbamate condensation unit, it is, for example, below 75°C, in the range of 50°C to 75°C; for example, the carbamate outlet temperature of the second unit is at least 10°C lower than that of the first unit. The second LP carbamate condensation unit preferably uses cooling water and has a preferred minimum carbamate outlet temperature of 50°C.

[0031] The LP carbamate condensation zone (especially the first carbamate condensation unit) preferably also receives an aqueous liquid stream, such as condensate or clarified carbamate solution from the urea plant wastewater treatment section (WWT), or condensate from an atmospheric pressure condenser.

[0032] Preferably, the condensation in the LP carbamate condensation zone (11) undergoes at least partial indirect heat exchange with the urea solution to be heated, particularly with the urea solution in the pre-evaporator. Preferably, the condensation in the first LP carbamate condensation unit undergoes indirect heat exchange with the urea solution to be heated.

[0033] The heated urea solution is, for example, at a pressure of 0.5 bar or lower (e.g., 0.2 bar to 0.5 bar). Therefore, the first LP carbamate condenser unit is preferably provided on the shell side of a shell-and-tube heat exchanger containing the urea solution to be heated in tubes, with the tube side providing a pre-evaporator for the urea unit. Preferably, this first LP carbamate condenser unit receives CO2 feed as part of a gaseous stream.

[0034] Preferably, the amount of CO2 and NH3 condensed into carbamate in the first LP carbamate condensing unit that indirectly exchanges heat with the urea solution to be heated is at least 80% or at least 100% of the amount of CO2 and NH3 condensed into carbamate in the downstream second carbamate condensing unit (which uses, for example, cooling water).

[0035] In the method of the present invention, a second portion (13) of the gaseous CO2 feed stream is also condensed in the LP carbamate condensation zone (11), for example, in the preferred first LP carbamate condensation unit. Specifically, the second portion of the gaseous CO2 feed stream is introduced into the carbamate condenser as a gaseous stream with an N / C ratio of less than 2.0, preferably less than 1.8. This advantageously reduces the N / C ratio of the formed carbamate solution (12), allowing for a high degree of condensation of the gas supplied to the LP carbamate condensation zone as an LP carbamate solution. This avoids the transport of NH3 to downstream processes, thereby reducing the load on the WWT.

[0036] The lower N / C ratio in the LP carbamate condensation zone also contributes to a more favorable higher condensation temperature, allowing less water to be recycled to the urea synthesis section and improving urea conversion. Furthermore, through preferred thermal integration, particularly via a pre-evaporator, the condensation heat of the CO2 feed stream is advantageously at least partially recovered and used to heat the urea solution.

[0037] For example, based on the composition of the final liquid stream, the N / C ratio of the carbamate solution obtained from the LP carbamate condensation zone is 2.0 to 3.2.

[0038] For example, based on the outlet pressure, the LP carbamate condenser operates at a pressure of at least 3.0 bar, at least 4.0 bar, at least 4.5 bar, at least 5.0 bar, or at least 5.5 bar, typically up to 10 bar. Preferably, the LP carbamate condenser receives a second portion (13) of the gaseous CO2 feed stream as a gaseous stream at said pressure. The LP carbamate condenser is preferably connected to an LP disintegrator operating at the same pressure; and receives gas from the LP disintegrator and a second portion (13) of the gaseous CO2 feed stream as a gaseous stream, and in an exemplary embodiment, the gaseous streams are merged to form a merged gaseous stream, which is supplied to the LP carbamate condenser.

[0039] Preferably, based on CO2, the second portion (13) of the gaseous CO2 feed stream is, for example, at least 2 mol.%, or at least 5 mol.%, or at least 10 mol.%, and / or, for example, less than 25 mol.%, or less than 20 mol.%, or less than 15 mol.%, optionally in the range of 2 mol.% to 20 mol.%, or 2 mol.% to 10 mol.%, or 10 mol.% to 20 mol.%. In embodiments with contact units (e.g., LP stripper), these values ​​may specifically refer to the gas inlet of said unit. Preferably, the amount of CO2 in the gas stream at the gas outlet of the contact unit (e.g., LP stripper) is also within these ranges, and preferably the gas at this outlet is directly supplied to the gas inlet of the LP condensation zone. In this way, the heat of condensation of CO2 contained in the gas at the gas outlet of the contact unit is preferably recovered through thermal integration with a preferred pre-evaporator.

[0040] For example, 10% to 50% of the CO2 supplied to the LP countercurrent contact unit (specifically, the stripper) is absorbed by the liquid in the unit, and the remainder, for example, at least 50% of the CO2 received at the gas inlet, leaves the unit (LP stripper) through the gas outlet and is supplied to the LP condensation zone.

[0041] Typically, the gaseous CO2 feed stream is obtained from the boundary zone and is preferably supplied at least partially in uncondensed gaseous form to the inlet of the LP urethane condensation zone. For example, in the boundary zone, the maximum pressure available for the gaseous CO2 feed stream is 10 bar.

[0042] In another possible embodiment, a portion of the CO2 feed is supplied to another unit of the urea production plant, such as a vertical urea reactor.

[0043] In a preferred embodiment, particularly at LP, preferably at a pressure of at least 3.0 bar, at least 4.0 bar, at least 4.5 bar, at least 5.0 bar, or at least 5.5 bar, and typically up to 10 bar, the purified urea solution (9) is countercurrently contacted (15) with a second portion (13) of the gaseous CO2 feed stream in the contact unit. Preferably, this contact is carried out at a pressure substantially the same as that of low-pressure dissociation (e.g., at most 0.10 bar lower than dissociation). Therefore, flash evaporation between dissociation and countercurrent contact is not preferred.

[0044] Countercurrent contact is preferably carried out when the temperature at the urea solution inlet is at least 120°C (e.g., 120°C to 150°C).

[0045] This advantageously reduces the N / C ratio of the purified urea solution and advantageously removes at least some NH3 from the urea solution. This advantageously reduces the loss of NH3 to downstream processes and can reduce the load on the WWT.

[0046] The contact step generates a liquid stream (17) (urea solution) and a gaseous stream (16), and the gaseous stream is supplied to the LP carbamate condensation zone (11). In this embodiment, the gaseous stream is supplied from the contact unit (15) to the LP carbamate condensation zone (11) via a gas flow line (16).

[0047] This contact step can be particularly advantageous in embodiments where the purified urea solution expands to a lower pressure (e.g., atmospheric pressure) after countercurrent contact, preferably accompanied by gas / liquid separation (e.g., atmospheric flash evaporation). Preferably, the resulting gaseous stream is condensed in one or more units (such as condensers or absorbers), wherein the resulting condensate is supplied directly or indirectly via a liquid flow connection to the wastewater treatment section. The wastewater treatment (WWT) section is typically contained within the urea production section, including, for example, hydrolyzers and desorbers, and is energy-intensive (with relatively high steam consumption per kg of treated water). The WWT section also typically includes a reflux condenser. The contact step advantageously reduces the load on the WWT and improves energy efficiency.

[0048] Countercurrent contact is preferably thermally adiabatic.

[0049] In a preferred embodiment, a second portion (13) of the gaseous CO2 feed stream is at least partially used for low-pressure stripping of the purified urea solution (9), wherein the stripping is preferably adiabatic to achieve the countercurrent contact.

[0050] Preferably, the method includes medium-pressure treatment of the stripped urea solution upstream of the LP dissociator (i.e., between the HP stripper and the LP dissociator). In embodiments, the MP treatment includes or consists of adiabatic flash evaporation. In embodiments, the MP treatment includes heating the urea solution at medium pressure and subsequently performing gas / liquid separation; preferably, the preceding flash evaporation is performed at medium pressure. Flash evaporation involves the expansion of the urea solution and gas / liquid separation, for example in a flash evaporation unit having a gas outlet and a separate liquid outlet. MP heating is carried out, for example, in a heat exchanger using steam as the heating fluid; or, for example, by supplying the MP urea solution, after expanding to MP and undergoing gas / liquid separation, to a tube bundle of an HP carbamate condenser and condensing the gas from the HP stripper within the shell to heat the MP urea solution. Overall, using the MP treatment is beneficial for reducing the total carbamate load in the LP section, and thereby can improve urea conversion.

[0051] The MP process generates an MP gaseous feed stream containing CO2 and NH3, which is supplied to the MP carbamate condenser (or absorber). It typically also receives carbamate solution from the LP carbamate condenser to form an MP carbamate solution, which is then recycled to the HP urea synthesis section. The MP carbamate condenser can be provided by one or more condensation units connected in series and / or in parallel. A gaseous feed stream containing inert components from the urea synthesis section can also be condensed in the MP carbamate condenser. Advantageously, a scrubber may not be necessary in the HP urea synthesis section.

[0052] This method involves producing melamine (6) in a melamine production section (5), thereby also releasing melamine tail gas (7). There are no particular limitations on the type of melamine production section. Both high-pressure (>70 bar absolute pressure) and low-pressure (<70 bar absolute pressure) melamine production methods can be used, with high-pressure methods preferred, particularly high-pressure non-catalytic liquid-phase melamine synthesis. Various suitable melamine production methods are described in Ullmann's Encyclopedia of Industrial Chemistry, Volume 21, Chapter Melamine and Guanamines, 2003. Further examples of suitable melamine production methods are described in US20040162429A1, EP2385043A1, and EP3597641. However, other types of melamine production methods may also be used. Melamine tail gas includes NH3 and CO2, and possibly water. In some embodiments, the melamine production section comprises two parallel melamine production lines.

[0053] At least a portion, preferably all, of the melamine tail gas (7) is supplied directly or indirectly to the urea reaction zone, preferably to the urea production process, particularly preferably to the high-pressure urea synthesis section (1), optionally as a condensate. The tail gas is introduced into the urea synthesis section, for example, in gaseous form, or as a condensate (carbamate solution). The tail gas is introduced into the urea synthesis section, for example, in gaseous form, and condensed in a carbamate condenser included in said section. The tail gas enters, for example, a melamine tail gas condenser, and the resulting condensate is supplied to the urea synthesis section.

[0054] In yet another embodiment, melamine tail gas is supplied to a dedicated urea reaction zone. For example, the melamine tail gas is condensed in a melamine tail gas condenser, and the resulting condensate is supplied to a dedicated urea reactor. Alternatively, for example, the melamine tail gas is condensed in a melamine tail gas condenser (e.g., a pool reactor or pool condenser) that also includes a reaction zone. The urea reactor may be a dedicated or complementary urea unit, as described, for example, in US20210060519.

[0055] In yet another embodiment, the exhaust gas is condensed in a melamine exhaust gas condenser operating at, for example, a pressure of at least 80 bar, and the resulting carbamate solution is supplied to a dedicated urea reaction zone (i.e., a separate and independent reaction zone from the urea synthesis section). This zone can be a separate container (e.g., a urea pre-reactor) or part of a melamine exhaust gas condenser (e.g., in the case of a pool condenser), such that a portion of the carbamate is converted to urea. The resulting urea-containing solution is then supplied to the urea synthesis section, wherein the urea-containing solution typically contains carbamate.

[0056] In the case of condensation, a dedicated tail gas condenser (part of or between a melamine plant, urea plant) can be used, or combined condensation can be used, for example, in the MP recovery section of a urea plant. Combinations are also possible. Indirect recovery specifically refers to recycling via a condensation section (e.g., the condensation section of a urea production plant, such as the condensation section of an MP recovery plant). Some exemplary configurations in which tail gas is supplied to a urea plant are described in US20160318883A1.

[0057] There are no particular restrictions on how exhaust gas is recovered and fed into the HP urea synthesis section.

[0058] In embodiments where the tail gas is supplied as condensate, the tail gas is condensed into a carbamate solution, for example, between 20 bar and 110 bar, and is supplied, for example, to an HP carbamate condenser. In embodiments where the melamine synthesis section operates at a pressure higher than that of the HP carbamate condenser, the melamine tail gas is introduced into the urea synthesis section, for example, in gaseous form.

[0059] The method and apparatus of the present invention are particularly advantageous when the amount of melamine tail gas is relatively large relative to urea production.

[0060] Preferably, the amount of melamine tail gas supplied to the HP urea synthesis section corresponds to at least 5%, at least 10%, or at least 20%, or at least 30% and / or up to 50% (preferably 10 wt.% to 45 wt.%) of the total urea produced by the high-pressure urea synthesis section, when the amount of urea converted into melamine is at a mass flow rate.

[0061] Preferably, the amount of CO2 supplied to the melamine tail gas stream in the HP urea synthesis section is 5%, or at least 10%, or at least 15%, and / or up to 25% of the amount of CO2 converted into urea in the HP urea synthesis section; all amounts are in kilograms per hour. The remaining CO2 is supplied as gaseous CO2 feed.

[0062] For example, the urea production rate in the synthesis section is 100 tons / hr, and the amount of melamine tail gas corresponds to a 15-ton / hr urea melt feed in the melamine production section. In some embodiments, at least 10 wt.%, or at least 20 wt.%, or at least 30 wt.%, and / or up to 50 wt.% of the urea produced in the urea synthesis section is supplied to the melamine synthesis. In some embodiments, the melamine synthesis also receives urea from another urea unit. In some embodiments, the tail gas volume comes from two or more melamine units.

[0063] When the amount of melamine tail gas recovered is relatively large compared to the urea production rate, a problem arises: the efficiency of the HP stripper (stripping efficiency α) decreases, while the steam consumption and required steam pressure of the HP stripper increase. This is because the amount of fresh CO2 feed (which can also be used for stripping) in the synthesis section is smaller compared to the amount of urea synthesis solution to be stripped. Consequently, the N / C ratio of the stripped urea solution (calculated based on free NH3, CO2, and carbamate, with urea negligible) increases to, for example, 2.3 or higher, in the range of 2.4 to 3.0. The method of the present invention advantageously mitigates this effect by supplying a second portion of fresh CO2 feed to the LP carbamate condensation zone. Therefore, the method involves reducing the N / C ratio of the LP carbamate condensation section.

[0064] It is noteworthy in this invention that, advantageously, a sufficiently low N / C ratio in the stripped urea solution and a sufficiently low N / C ratio in the LP carbamate condensation section can be achieved without operating the HP stripping tower at very high energy consumption.

[0065] Advantageously, a preferred embodiment of the method does not involve recovering the condensed NH3 stream from the recovery section (i.e., a section containing a unit for processing urea solution from the HPCO2 stripping tower to recover NH3 and CO2) to the HP urea synthesis section as a separate stream from the carbamate solution recovery stream. A preferred embodiment of the apparatus does not include an NH3 condenser having a fluid flow connection to the HP synthesis section, independent of and supplemented to the liquid flow connection from the LP carbamate condensation zone to the carbamate solution in the HP urea synthesis section. By avoiding the separate recovery of condensed NH3, a dedicated NH3 condenser is eliminated, thereby reducing equipment costs. Furthermore, NH3 condensation (a complex and sensitive operation) is avoided, and pure NH3 treatment is also avoided.

[0066] Preferably, a first portion of the purified urea solution is used to produce melamine, and a second portion of the purified urea solution is used to prepare one or more urea products, such as solid urea products. Preferably, the first and second portions are subjected to evaporation in a (vacuum) evaporation section included in the urea production section to form urea melt. Preferably, the first portion of the urea melt is supplied to the melamine production section, and preferably the second portion of the urea melt is supplied to, for example, a refining section, such as a granulation tower or granulator. Preferably, the first portion used for melamine production is at least 10 wt.%, or at least 20 wt.%, or at least 30 wt.%, and / or up to 50 wt.% of the total urea yield. Optionally, a third portion of the purified urea solution is used to prepare liquid urea products.

[0067] The present invention also provides a co-production apparatus for producing urea and melamine. All the preferences and details discussed in relation to the method also apply to the apparatus. Furthermore, the method of the present invention is preferably carried out in an apparatus according to the invention. The apparatus includes a urea production section, which includes a high-pressure urea synthesis section (1) and a recovery section. The high-pressure urea synthesis section (1) for producing urea includes a high-pressure (HP) CO2 stripping tower (2). The HP stripping tower has an outlet for a stripped urea solution (3) and an inlet for a first portion (4) of the gaseous CO2 feed stream, which serves as a stripping agent. The HP urea synthesis section also includes a reaction zone and a condensation zone, as described above. The apparatus includes a melamine production section (5) which has an outlet for a melamine (6) product and a separate outlet for melamine tail gas (7) (as a gas or as a condensate). The preferred embodiment of the melamine production section (5) discussed for this method also applies to this apparatus, particularly the use of high-pressure and low-pressure melamine synthesis sections. The melamine production section may include, for example, a high-pressure (>70 bar) melamine synthesis reactor, a melamine melt processing section (e.g., including a quenching unit), a crystallization unit for crystallizing melamine crystals, a mother liquor treatment section, and a tail gas scrubbing section, for example, using water or urea for scrubbing.

[0068] The apparatus includes a fluid flow connection, such as a gas flow connection or a liquid flow connection, or a combination thereof, for the direct or indirect supply of the melamine tail gas (7) (optionally as a condensate) to the high-pressure urea synthesis section (1). The apparatus optionally includes a condenser for the tail gas. The tail gas condenser can be a dedicated condensation unit or a unit for combining the melamine tail gas with other gaseous streams for condensation. The condenser is included, for example, in the urea production section, such as in the recovery section. The fluid flow connection may include, for example, a gas flow connection, or a combination of a gas flow connection, a condensation unit, and a liquid flow connection.

[0069] The apparatus includes a liquid flow connection for supplying the stripped urea solution (3) directly or indirectly as a liquid to a low-pressure (LP) dissociator (8). This dissociator (e.g., a heat exchanger) has an outlet for a purified urea solution (9) and an outlet for a low-pressure gas stream (10). The apparatus includes a low-pressure (LP) carbamate condensation zone (11), configured to condense the low-pressure gas stream together with a second portion of the gaseous CO2 feed stream to form a carbamate solution (12). The apparatus includes a gas flow line for the low-pressure gas stream (10) from the low-pressure (LP) dissociator (8) to the LP carbamate condensation zone (11).

[0070] The apparatus includes a gas flow connection (13), such as a gas flow line, directly or indirectly connecting the CO2 supply unit (14) to the LP carbamate condensation zone (11); this gas flow connection may pass through, for example, a gas / liquid contact unit, from the gas inlet to the gas outlet of this unit. The CO2 supply unit is typically a CO2 supply line at the boundary zone. In an exemplary embodiment of the apparatus, the CO2 originates from a syngas unit, i.e., a unit that produces hydrogen for an upstream co-production ammonia unit, which feeds NH3 into a urea unit, and also includes a CO2 generation unit connected to the LP carbamate condensation section. For example, the apparatus includes a gas flow connection for transferring CO2 from the CO2 removal unit (particularly a desorber) of the syngas unit to the LP carbamate condensation zone (11). However, other sources of CO2 are also possible, such as from a waste incinerator, or generally from a flue gas feed stream.

[0071] In some embodiments, the gas flow connection (13) extends directly or indirectly from the CO2 removal unit to the LP carbamate condensation zone (11). For example, the CO2 removal unit is a unit comprising an absorption unit and a desorption unit, used to remove CO2 from the gaseous stream using a solvent, and to release the CO2 stream from the desorption unit into the gas flow connection.

[0072] In some embodiments, the gas flow connection (13) is from the hydrogen removal unit to the LP carbamate condensation zone (11), the unit being configured to remove hydrogen from the CO2 gas stream by catalytic combustion. Using this unit (also referred to as a hydrogen converter), air is added to the CO2 stream, for example.

[0073] In some embodiments, the gas flow connection (13) is from the CO2 compressor, particularly the intermediate stage of the compressor, to the LP carbamate condensation zone (11).

[0074] Therefore, in some embodiments, the CO2 supply unit (14) is a compressor, hydrogen converter, or CO2 removal unit of the syngas apparatus. Thus, the CO2 supply unit (14) may have a CO2 inlet. In some embodiments, the gas flow connection (13) extends from the CO2 removal unit of the syngas apparatus through the compressor and hydrogen converter to the LP carbamate condensation zone (11).

[0075] Preferably, the apparatus includes a contact unit (15), preferably an LP stripper, for contacting the purified urea solution (9) with the second portion (13) of the gaseous CO2 feed stream. The contact unit has a first outlet for the liquid stream and a second outlet for the gaseous stream, wherein the second outlet is connected to an LP carbamate condensation zone (11). The unit also has a gas inlet for the second portion (13) of the gaseous CO2 feed stream and a liquid inlet for the purified urea solution (9). The apparatus is preferably configured for countercurrent contact, wherein the gas inlet of the second portion (13) of the gaseous CO2 feed stream is located at the bottom and the gas outlet at the top, the liquid inlet of the purified urea solution (9) is located at the top, and the liquid outlet is located at the bottom, and the apparatus is, for example, a packed bed or structured bed, thereby providing an LP stripper, particularly configured for adiabatic stripping at low pressure.

[0076] In a preferred embodiment, the LP carbamate condensation zone (11) is at least partially provided as a first compartment for indirect heat exchange with a second compartment of the urea solution to be heated. Preferably, the device includes a liquid flow line for the urea solution from the LP dissociator (8) to the second compartment. Preferably, the first and second compartments are provided as shell-and-tube heat exchangers. Preferably, the second compartment is provided by a tube bundle (i.e., tubes), and the first compartment is provided by a shell-side space. Preferably, the first compartment operates as a pre-evaporator. Preferably, the device includes a gas / liquid separator located downstream of the tube outlet to separate water vapor from the urea solution.

[0077] As used herein, a gas flow connection refers to a flow connection for a continuous gaseous fluid. A gas flow connection extends from a gas inlet through a unit (such as a gas / liquid contact unit) to a gas outlet of the unit. As used herein, a liquid flow connection refers to a flow connection for a continuous liquid fluid. The unit provides a liquid flow connection from a liquid inlet to a liquid outlet. For example, a decomposer and condenser connected via a gas flow connection from a decomposer to a condenser do not provide a liquid flow connection from the decomposer's liquid inlet to the condenser's liquid outlet. However, the condenser provides a liquid flow connection from the unit's liquid inlet to its liquid outlet.

[0078] As used herein, the term 'carbamate' when used in the urea production field refers to ammonium carbamate. In aqueous carbamate streams, this component may exist as a carbonate substance. The amounts of NH3 and CO2 in the water stream include the amounts present as carbonate substances.

[0079] As used herein, for the method flow of a urea plant (i.e., not for steam lines or melamine plants), the high pressure (HP) is above 100 bar, for example, 120 bar to 300 bar, or 140 bar to 200 bar. The intermediate 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 the low pressure (LP) is, for example, 0 bar to 10 bar, particularly at least 1 bar, preferably at least 2 bar or at least 4 bar; and typically at most 10 bar, or preferably at most 8 bar. For example, LP is in the range of 1 bar to 8 bar, or 2 bar to 10 bar, or 3 bar to 10 bar, or 4 bar to 10 bar, or 2 bar to 5 bar. All pressures are absolute values ​​in bars.

[0080] The terms “typical,” “suitable,” and “particularly,” and their derivatives, are used to indicate features that may be used in some embodiments but are not mandatory. Preferred features are also not mandatory.

[0081] As used in this article, the term "melamine tail gas" refers to the tail gas from the melamine production process, and specifically refers to a gaseous stream that primarily contains NH3, CO2, and possibly H2O.

[0082] As used herein, the term "first" for a unit or step allows for the existence of additional instances upstream of such a unit or step.

[0083] As used in this paper, the stripping efficiency α = (2 * wt.% urea / 60) / ((2 * wt.% urea / 60) + (wt.% NH3 / 17)) is measured at the liquid outlet of the stripping tower, where wt.% NH3 represents all types of ammonia, including ammonium carbamate.

[0084] The N / C ratio represents the molar ratio of NH3 to CO2 in the gas stream, based on NH3 and CO2; for carbamate solutions, it is based on NH3, CO2, and carbamate; and for the synthesis stage, it is based on the theoretical initial reaction mixture consisting of H2O, NH3, and CO2. The N / C ratio for carbamate condensers refers to the N / C ratio at the liquid outlet. The free N / C ratio for urea solutions is calculated based on NH3, CO2, and carbamate (excluding urea).

[0085] As used herein, indirect heat exchange refers to heat exchange through a heat exchange wall, which in the context of this invention involves heat exchange through a heat exchange wall that is in contact with a urea solution to be heated on a first side and with a gas containing CO2 and NH3 (which condenses to form a carbamate solution) on a second side of the wall.

[0086] The invention will now be further illustrated by one or more examples, which do not limit the invention or the claims.

[0087] Example 1

[0088] Consider and simulate a urea plant in which 30 wt.% of molten urea is supplied to the melamine production section and a corresponding amount of melamine tail gas is recovered to the urea synthesis section. Tons are metric tons, and values ​​are approximations.

[0089] The overall design of the device is as follows Figure 1 As shown, the urea solution undergoes MP adiabatic flash evaporation between the HP stripper and the LP dissociator.

[0090] Case 1: The HP CO2 stripper has a CO2 feed rate of 86 tons / hour for HP stripping; the LP section (especially the LP stripper) has a CO2 feed rate of 10 tons / hr. The LP carbamate condenser section comprises two condensation units connected in series. The first LP carbamate condenser receives gas from the LP dissociator and the LP CO2 feed stream; and provides the shell side of a pre-evaporator for urea solution, where the urea solution to be heated is in tubes. The second LP carbamate condenser receives gas and liquid from the first LP carbamate condenser and operates using cooling water or circulating cooling water.

[0091] Under optimal operating conditions, the first LP condenser unit operates at a liquid outlet temperature of 93°C, and the second LP condenser unit operates at a liquid outlet temperature of 66°C at approximately 5.4 bar. The N / C ratio of the urea solution at the HP stripper outlet is 2.8 (free N / C, i.e., based on NH3, CO2, and carbamate, excluding urea). The N / C ratio of the carbamate solution at the liquid outlet of the second LP condenser unit is 2.77. This carbamate solution is supplied to the MP carbamate condenser, which has a liquid outlet N / C ratio of 2.35, and is recovered from this condenser to the HP urea synthesis section. The MP carbamate condenser also receives gases from the MP flash evaporation.

[0092] Case 2, Less Optimal: The CO2 feed rate for the LP section is reduced to 3 tons / hour; the CO2 feed rate for the HP section is 93 tons / hour. Under optimal operating conditions, the first LP condenser unit operates at a liquid outlet temperature of 91°C, and the second LP condenser unit operates at a liquid outlet temperature of 57°C at approximately 5.4 bar. The N / C ratio of the urea solution at the HP stripper outlet is 2.7 (free N / C). The N / C ratio of the carbamate solution at the liquid outlet of the second LP condenser unit is 3.2. This carbamate solution is supplied to the MP carbamate condenser, which has a liquid outlet N / C ratio of 2.55, and is recovered from this condenser to the HP urea synthesis section.

[0093] In Case 2, the surface area of ​​the LP carbamate condenser must be larger, and the WWT load is also increased. Furthermore, the reflux condenser temperature drops from 68°C in Case 1 to 50°C in Case 2; a larger reflux condenser is required in the WWT. Due to the increased CO2 feed flow, the reduction in total steam consumption of the HP stripper is only minor. Therefore, the cogeneration unit and method in Case 1 perform better than in Case 2. The higher N / C ratio in MP and LP results in a higher WWT load in Case 2, leading to a higher absorber load for uncondensed gases from the corresponding condensers, while the aqueous solution from the absorbers will be treated in the WWT section.

[0094] Case 3 (Comparison): The CO2 feed rate in the LP section is reduced to 0 tons / hour; the CO2 feed rate in the HP section is 96 tons / hour. The goal is to achieve 30% melamine integration while maintaining MP flash evaporation. Simulation results show that in Case 3, the recovery section requires a separate NH3 condenser and separate recovery of condensed NH3; otherwise, a significant amount of ammonia will be lost into the atmosphere in Case 3.

[0095] Example 2

[0096] Case 1A: Case 1A is the same as Case 1, with the following additional details. In the first LP carbamate condenser, a total steam feed of 78 ton / h is supplied (of which 65 ton / h of this steam comes from LP distillation / dissociation and 13 ton / h from LP CO2 stripping). Approximately 57 ton / h of aqueous carbamate is added to the first carbamate condenser, supplied from approximately 33 ton / h from the reflux condenser and 24 ton / h from the atmospheric condenser. The average water concentration of the added aqueous carbamate is approximately 50 wt.%. From a single outlet at the first condenser outlet to the inlet of the second condenser, the feed consists of approximately 105 ton / h of liquid (N / C approximately 2.4) at approximately 95°C and approximately 30 ton / h of steam at 95°C. At the outlet of the second condenser, the total liquid is approximately 132 ton / h (N / C approximately 2.8) at approximately 69°C; and the uncondensed gas is approximately 3 ton / h at 69°C. This steam is supplied to the atmospheric condenser.

Claims

1. A method for the co-production of urea and melamine, the method comprising: - Urea is produced in a high-pressure urea synthesis section (1) including a high-pressure CO2 stripping tower (2) to produce a stripped urea solution (3), wherein the high-pressure CO2 stripping tower uses the first part (4) of the gaseous CO2 feed stream as the stripping agent. - Melamine (6) is produced in the melamine production section (5), thereby releasing melamine tail gas (7); - The melamine tail gas (7) may optionally be supplied directly or indirectly as condensate to the high-pressure urea synthesis section (1); - The stripped urea solution (3) is supplied directly or indirectly via a liquid flow connection to the low-pressure dissociator (8) to produce a purified urea solution (9) and a low-pressure gas stream (10); -The low-pressure gas stream is condensed together with the second part (13) of the gaseous CO2 feed stream in the low-pressure carbamate condensation zone (11) to form a carbamate solution (12).

2. The method according to claim 1, wherein the purified urea solution (9) is countercurrently contacted with the second portion (13) of the gaseous CO2 feed stream in the contact unit (15) to generate a liquid stream and a gaseous stream (16), and the gaseous stream (16) is supplied to the low-pressure carbamate condensation zone (11).

3. The method according to claim 2, wherein the second portion (13) of the gaseous CO2 feed stream is used for low-pressure stripping of the purified urea solution (9).

4. The method according to any one of the preceding claims further includes condensing the melamine tail gas (7) into a first carbamate stream, the first carbamate stream being supplied to the high-pressure urea synthesis section.

5. The method according to any one of claims 1-3, further comprising supplying the melamine tail gas (7) as a gaseous stream to the high-pressure urea synthesis section (1) and condensing it into the first carbamate stream in a carbamate condenser included in the high-pressure urea synthesis section (1).

6. The method according to claim 1, wherein the condensation in the low-pressure carbamate condensation zone (11) is at least partially subject to indirect heat exchange with the urea solution to be heated.

7. The method according to claim 1, wherein the method comprises subjecting the stripped urea solution to medium-pressure treatment upstream of the low-pressure dissociator.

8. The method according to claim 7, wherein the medium-pressure treatment comprises or consists of adiabatic flash evaporation.

9. The method according to claim 7, wherein the medium-pressure treatment includes heating the stripped urea solution.

10. The method according to claim 1, wherein the second portion (13) of the gaseous CO2 feed stream is 2 mol% to 20 mol% of the total CO2 feed stream.

11. The method of claim 1, wherein the amount of CO2 in the melamine tail gas stream supplied to the high-pressure urea synthesis section, in kg / hr, is at least 5% of the amount of CO2 converted into urea in the high-pressure urea synthesis section, in kg / hr.

12. The method according to claim 1, wherein the purified urea solution optionally expands to atmospheric pressure and is accompanied by the formation of a gaseous stream (16) after countercurrent contact with the second portion (13) of the gaseous CO2 feed stream, wherein the gaseous stream (16) condenses at atmospheric pressure.

13. The method according to claim 1, wherein the low-pressure carbamate condensation zone (11) operates at a pressure of at least 3.0 bar, and wherein the low-pressure carbamate condensation zone receives the second portion (13) of the gaseous CO2 feed stream at a pressure of at least 3.0 bar.

14. The method of claim 1, wherein the low pressure is at least 2 bar.

15. The method according to claim 1, wherein, based on CO2, the second portion (13) of the gaseous CO2 feed stream is at least 5 mol.% and less than 25 mol.% of the total CO2 feed stream.

16. A co-production unit for producing urea and melamine, the unit comprising: - A high-pressure urea synthesis section (1) for producing urea, comprising a high-pressure CO2 stripping tower (2) having an outlet for stripped urea solution (3) and an inlet for a first part (4) of the gaseous CO2 feed stream used as a stripping agent. - Melamine production section (5), which has an outlet for melamine (6) and an outlet for melamine tail gas (7); - A fluid flow connection for supplying the melamine tail gas (7) as a condensate directly or indirectly to the high-pressure urea synthesis section (1); - A liquid flow connection for directly or indirectly supplying the stripped urea solution (3) to a low-pressure dissociator (8), the low-pressure dissociator having an outlet for purified urea solution (9) and an outlet for low-pressure gas stream (10); - A low-pressure carbamate condensation zone (11) configured to condense the low-pressure gas stream together with a second portion (13) of the gaseous CO2 feed stream to form a carbamate solution (12). The device includes a gas flow connection from the CO2 supply unit (14) to the low-pressure carbamate condensation zone (11) and a gas flow line for the low-pressure gas stream (10) from the low-pressure dissociator (8) to the low-pressure carbamate condensation zone (11).

17. The cogeneration unit according to claim 16, wherein the gas flow connection is from the CO2 removal unit to the low-pressure carbamate condensation zone (11).

18. The cogeneration unit according to claim 16, wherein the gas flow connection is from the CO2 compressor to the low-pressure carbamate condensation zone (11).

19. The cogeneration unit according to claim 18, wherein the gas flow connection is from the intermediate stage of the CO2 compressor to the low-pressure carbamate condensation zone (11).

20. The co-production apparatus according to claim 16, the co-production apparatus comprising a contact unit (15) for contacting the purified urea solution (9) with the second portion (13) of the gaseous CO2 feed stream, the contact unit having a first outlet for the liquid stream and a second outlet for the gaseous stream, wherein the second outlet is connected to the low-pressure carbamate condensation zone (11).

21. The cogeneration unit according to claim 16, wherein the low-pressure carbamate condensation zone (11) is at least partially provided as a first compartment for indirect heat exchange with a second compartment for a urea solution to be heated.

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