Co-produced urea melamine production with HP CO2 stripping

Abstract

The present disclosure relates to a co-production plant process for the production of urea and melamine, the process having a urea synthesis section with HP CO2 stripping and a portion of the CO2 feed being supplied to a recovery section.

Description

Field of the Invention

[0001] The present invention relates to the co-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 - 54 describes various methods of urea-melamine integration.

[0005] There is still a need for an improved co-production of a relatively large melamine production plant with a urea production method having a HP urea synthesis section of the CO2 stripping type, in particular with respect to urea production in the HP urea synthesis section, to recycle a relatively large amount of off-gas from the melamine plant to this HP urea synthesis section. There is in particular a need for high energy efficiency and low equipment costs. The co-production plant and method comprise a urea production section having a urea synthesis section, a recovery section and a wastewater treatment (WWT) section. The WWT includes, for example, a hydrolyser and a desorber and is energy-consuming (relatively high steam consumption per kg of treated water). 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 producing urea and melamine, the method comprising: producing urea in a high-pressure (HP) urea synthesis section comprising a HP CO2 stripper, producing a stripped urea solution, wherein the HP CO2 stripper uses a first portion of a gaseous CO2 feed stream as a stripping agent; producing melamine in a melamine production section, thereby also releasing a melamine off-gas; optionally supplying the melamine off-gas directly or indirectly as a condensate to the high-pressure urea synthesis section; supplying the stripped urea solution directly or indirectly via a liquid flow connection to a low-pressure (LP) dissociator, producing a purified urea solution and a low-pressure gas stream; subjecting the low-pressure gas stream to condensation together with a second portion of the gaseous CO2 feed stream in a low-pressure (LP) carbamate condensation zone to form a carbamate solution.

[0007] The present invention also relates to a combined production unit for producing urea and melamine, the unit comprising: a high-pressure urea synthesis section for producing urea, the high-pressure urea synthesis section including a high-pressure (HP) CO2 stripper having an outlet for the 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; a fluid flow connection for the melamine tail gas to be optionally supplied directly or indirectly to the high-pressure urea synthesis section as condensate; a liquid flow connection for the stripped urea solution to be supplied directly or indirectly to a low-pressure (LP) dissociator having an outlet for the purified urea solution and an outlet for a low-pressure gas stream; a low-pressure (LP) carbamate condensation zone configured to subject the low-pressure gas stream together with a second portion of the gaseous CO2 feed stream to condensation to form a carbamate solution, the unit including a gas flow connection from a CO2 supply unit to the LP carbamate condensation zone.

[0008] Accordingly, the present invention relates to a combined production method and unit for producing urea and melamine, the method having a urea synthesis section with HP CO2 stripping and a portion of the CO2 feed being supplied to an LP recovery section. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 Schematically shows an exemplary method scenario according to the present invention.

[0010] Any embodiment shown in one or more of the drawings is merely exemplary and does not limit the present invention. DETAILED DESCRIPTION

[0011] The method and unit of the present invention are capable of combining a relatively large melamine production method with a urea production method employing an HP urea synthesis section of the CO2 stripping type; that is, the amount of melamine tail gas recycled to the urea synthesis section is relatively high in terms of the amount recycled to the urea synthesis section and compared to the urea production rate and compared to the fresh NH3 feed and CO2 feed, and in particular does not interfere with downstream sections (such as the LP recovery section) and / or does not significantly increase the load on the wastewater treatment section. Further, the method and unit of the present invention do not require the use of expensive equipment (such as a dedicated NH3 condenser), and allow the use of a relatively small-sized condenser in the LP section, and in a preferred embodiment, also allow a relatively small-sized condenser for the WWT.

[0012] Very cleverly, in the case where the amount of melamine tail gas recycled to the synthesis section is relatively large, the unit and method of the present invention can alleviate the relative lack of CO2 feed available for stripping in the HP urea synthesis section.

[0013] One advantage of the present invention is that the steam consumption of the HP stripper may be relatively low.

[0014] Ullmann’s Encyclopaedia, chapter Urea, 2010 mentions that in the (Stamicarbon) CO2 stripping type urea process, i.e., without integration with a melamine plant, only a low-pressure recovery stage is required due to the low ammonia and carbon dioxide concentrations in the stripped urea solution. On the other hand, the present invention allows a relatively high N / C ratio of the stripped urea solution.

[0015] The various aspects of the present invention will now be further described. For convenience, reference is made to Figure 1 , which shows an exemplary method and apparatus according to the present invention and does not limit the present invention or the claims.

[0016] The present invention provides a combined production method and apparatus for urea and melamine. The method and apparatus are combined because the tail gas from melamine production is supplied to urea production. Optionally, the method may also be integrated because the urea melt from urea production is supplied to the melamine production section.

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

[0018] The method includes producing urea in a high-pressure urea synthesis section (1) including a high-pressure (HP) CO2 stripper (2), generating a stripped urea solution (3). The HP CO2 stripper (2) uses a gaseous CO2 feed stream, particularly the first part (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 stripper tubes to promote the decomposition of carbamate in the urea solution, and heating is further used. The gaseous stream for CO2 stripping contains at least 90 vol.% CO2, typically at least 95 vol.% CO2, with the balance being inert gases, typically containing some air for passivation. The stripper is typically a shell-and-tube heat exchanger configured for countercurrent contact of the tubes for the falling film of the urea solution to be stripped with the gaseous stripping agent. The stripper typically has a liquid inlet for the urea solution at the top, a liquid outlet for the stripped urea solution at the bottom, an inlet for the CO2 gas used as the stripping gas at the bottom, and an outlet for the gas stream at the top, all outlets being on the tube side. The heating fluid (such as steam) is located on the shell side.

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

[0020] The urea reactor is typically a vertical urea reactor having one or more inlets at the bottom and one or more outlets for discharging the urea synthesis solution at the upper part of the reactor. The urea synthesis solution contains urea, water, carbamate, and NH3. The HP stripper is used to strip the urea synthesis solution to separate and remove a portion of the carbamate.

[0021] The HP carbamate condenser is typically a shell-and-tube heat exchanger, preferably but not limited to having the gas to be condensed in the shell-side space and the coolant in the tube bundle. The HP carbamate condenser has an outlet for the HP carbamate stream to the urea reactor; the gas outlet of the stripper is connected to the HP carbamate condenser. In embodiments where the gas from the stripper 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 that contains a condensation zone and a reaction zone; having an inlet for the gas from the stripper to the condensation zone and an outlet for the 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 portion in the vessel for cooling the liquid and is in fluid communication (gas and liquid) with the reaction zone provided between the tube bundle elbows 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 the gas from the HP stripper that extends through the condensation zone and the reaction zone. Optionally, the pool reactor is combined with a downstream urea reactor (such as a vertical urea reactor) that is arranged between the reaction zone of the pool reactor and the HP stripper. Optionally, the pool reactor or the 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 bar to 150 bar. In a further embodiment, the reaction zone, the condensation zone, and the HP stripper all operate at a pressure within the range of 130 bar to 150 bar.

[0024] In a preferred embodiment, the reaction zone operates at an N / C ratio of 3.0 to 3.2. A relatively low N / C ratio in the reaction zone is beneficial for the recovery of carbamate from the downstream section, especially for enabling the recovery of only the carbamate solution from the recovery section.

[0025] The method comprises that the stripped urea solution (3) is supplied directly or indirectly via a liquid flow connection to a low pressure (LP) dissociator (8). The treatment carried out in the LP dissociator, in particular heating, produces a purified urea solution (9) and a low pressure gas stream (10). The LP dissociator comprises, for example, a shell and tube heat exchanger for decomposing carbamate 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 flashing. Preferably, the resulting gas is condensed in a condenser (e.g., an atmospheric condenser), and the liquid (i.e., the urea solution) is supplied to a heating unit, preferably a pre-evaporator operating at a sub-atmospheric pressure, for example, from 0.2 bar to 0.5 bar; and gas / liquid separation is carried out on the resulting heated urea solution. 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 a part of the resulting urea melt is supplied, for example, to a melamine production section, and a part of it is supplied, for example, to a urea finishing section to solidify into a solid urea product.

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

[0028] The method involves condensing the 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, with a single condensation unit or with two or more condensation units in series and / or parallel, for example, using a first and a second LP carbamate condensation unit 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 receiving a second gas stream to obtain a second carbamate solution.

[0030] For all configurations of the LP carbamate condensation zone, the carbamate solution is recycled to the HP urea synthesis section, optionally via, for example, an MP carbamate condenser. Thus, the plant includes an LP recovery section, which includes an LP dissociator and an LP carbamate condensation zone. Optionally, the condensation in the first LP carbamate condensation unit is partially (e.g., in the range of 10% to 60% of the total CO2 in the total supply condenser) supplied to the condenser, and the gas (including NH3 and CO2) from the first LP carbamate condensation unit is subjected to further condensation in the second LP carbamate condensation unit. The first carbamate condensation unit and the second carbamate condensation unit can be operated with different coolant liquids. The temperature at 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, e.g., 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 clarified process condensate or clarified carbamate solution from the urea plant wastewater treatment section (WWT), or condensate from, for example, an atmospheric condenser.

[0032] Preferably, the condensation in the LP carbamate condensation zone (11) is at least partially in indirect heat exchange with the urea solution to be heated, especially with the urea solution in the pre-evaporator. Preferably, the condensation in the first LP carbamate condensation unit is in 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). Thus, the first LP carbamate condensation unit is preferably provided on the shell side of a shell-and-tube heat exchanger having the urea solution to be heated in the tubes, and the tube side provides the pre-evaporator of the urea plant. Preferably, this first LP carbamate condensation unit receives a part of the CO2 feed as a gaseous stream.

[0034] Preferably, the amount of CO2 and NH3 condensed to carbamate in the first LP carbamate condensation unit in indirect heat exchange with the urea solution to be heated is at least 80% or at least 100% of the amount of CO2 and NH3 condensed to carbamate in the downstream second carbamate condensation 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 subjected to condensation in the LP carbamate condensation zone (11), for example, in the preferably used first LP carbamate condensation unit. In particular, the second portion of the gaseous CO2 feed stream is introduced into the carbamate condenser as a gaseous stream at an N / C ratio of less than 2.0, preferably less than 1.8. Thereby, the N / C ratio of the formed carbamate solution (12) is advantageously reduced, allowing a high degree of condensation to be achieved for the gas that is supplied to the LP carbamate condensation zone as the LP carbamate solution. Thereby, the transport of NH3 to the downstream section is avoided, thus reducing the load on the WWT.

[0036] The lower N / C ratio in the LP carbamate condensation zone also contributes to generating a favorable higher condensation temperature, allowing less water to be recycled to the urea synthesis section and increasing the urea conversion rate. In addition, through preferred heat integration, in particular through a pre-evaporator, the condensation heat of the CO2 feed stream portion 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 condensation zone 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, usually up to 10 bar. Preferably, the LP carbamate condensation zone receives the second portion (13) of the gaseous CO2 feed stream as a gaseous stream at the said pressure. The LP carbamate condensation zone is preferably connected to an LP dissociator operating at the same pressure; and receives the gas from the LP dissociator and the second portion (13) of the gaseous CO2 feed stream as a gaseous stream, and in an exemplary embodiment, the said gaseous streams are combined to form a combined gaseous stream, which is supplied to the LP carbamate condensation zone.

[0039] Preferably, based on CO2, the second part (13) of the gaseous CO2 feed stream is, for example, at least 2 mol.%, or at least 5 mol.%, or at least 10 mol.% of the total CO2 feed stream, and / or for example less than 25 mol.%, or less than 20 mol.%, or less than 15 mol.%, optionally in the range from 2 mol.% to 20 mol.%, or from 2 mol.% to 10 mol.%, or from 10 mol.% to 20 mol.%. In embodiments having a contacting unit (e.g., an LP stripper), these values may particularly refer to the gas inlet of said unit. Preferably, the amount of CO2 in the gas stream at the gas outlet of the contacting unit (e.g., an 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 the CO2 contained in the gas at the gas outlet of the contacting unit is preferably recovered by heat integration with a preferred pre-evaporator.

[0040] For example, 10% to 50% of the CO2 supplied to the LP counter-current contacting unit (particularly, a stripper) is absorbed by the liquid in said unit, and the remaining part, 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 partly in an uncondensed gaseous form to the inlet of the LP carbamate condensation zone. For example, in the boundary zone, the maximum pressure available for the gaseous CO2 feed stream is 10 bar.

[0042] In a further possible embodiment, a further part 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 in counter-current contact (15) with the second part (13) of the gaseous CO2 feed stream in a contacting unit. Preferably, this contact is carried out at a pressure substantially the same as that of the low-pressure dissociation (e.g., at most 0.10 bar lower than the dissociation). Therefore, flashing between dissociation and counter-current contact is not preferred.

[0044] The counter-current contact is preferably carried out at a temperature at the urea solution inlet of at least 120 °C (e.g., 120 °C to 150 °C).

[0045] Thereby, the N / C ratio of the purified urea solution is advantageously reduced, and at least some NH3 is advantageously removed from the urea solution. This advantageously reduces the loss of NH3 to the downstream section and can reduce the load on the WWT.

[0046] The contacting step produces 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 contacting unit (15) to the LP carbamate condensation zone (11) through a gas flow line (16).

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

[0048] The countercurrent contacting is preferably adiabatic.

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

[0050] Preferably, the method includes a medium-pressure treatment of the stripped urea solution upstream of the LP dissociator (i.e., between the HP stripper and the LP dissociator). In an embodiment, the MP treatment includes or consists of an adiabatic flash. In an embodiment, the MP treatment includes heating the urea solution at medium pressure and then performing gas / liquid separation; preferably, the previous flash is performed at medium pressure. The flash involves the expansion of the urea solution and gas / liquid separation, for example, in a flash unit having a gas outlet and a separate liquid outlet. The MP heating is performed, 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 performing gas / liquid separation to the tube bundle of the HP carbamate condenser and condensing the gas from the HP stripper in 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 can thereby increase the urea conversion rate.

[0051] MP processing produces an MP gas stream containing CO2 and NH3 and is supplied to the MP carbamate condensation section (which can also be an absorber), and usually also receives a carbamate solution from the LP carbamate condensation zone 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 in series and / or in parallel. The gas stream containing inert components from the urea synthesis section can also be condensed in the MP carbamate condensation section. Advantageously, in the HP urea synthesis section, it may not be necessary to provide a scrubber.

[0052] The method includes producing melamine (6) in a melamine production section (5), thereby also releasing a melamine off-gas (7). The type of melamine production section is not particularly limited. Both high-pressure (>70 bar absolute pressure) melamine processes and low-pressure processes (<70 bar absolute pressure) can be used, with high-pressure melamine processes being preferred, especially 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 can also be used. The melamine off-gas includes NH3 and CO2 and possibly water. In some embodiments, the melamine production section includes two parallel melamine production lines.

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

[0054] In yet another embodiment, the melamine off-gas is supplied to a dedicated urea reaction zone. For example, the melamine off-gas is condensed in a melamine off-gas condenser, and the resulting condensate is supplied to a dedicated urea reactor. Or, for example, the melamine off-gas is condensed in a melamine off-gas condenser (such as a pool reactor or a pool condenser) that also includes a reaction zone. The urea reactor can be part of a dedicated or integrated urea plant, as described, for example, in US20210060519.

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

[0056] In the case of condensation, a dedicated off-gas condenser (part of the melamine plant, the urea plant, or between them) can be used, or combined condensation can be used, such as in the MP recovery section of the urea plant. Combinations are also possible. Indirect recovery particularly refers to recycling through a condensation section (e.g., the condensation section of a urea production plant, such as the condensation section of the MP recovery section). Some exemplary configurations for supplying the off-gas to the urea plant are described in US20160318883A1.

[0057] The manner of recycling the off-gas to the HP urea synthesis section is not particularly limited.

[0058] In an embodiment where the off-gas is supplied as a condensate, the off-gas is condensed into a carbamate solution, for example, between 20 bar and 110 bar, and is, for example, supplied to a HP carbamate condenser. In an embodiment where the melamine synthesis section operates at a pressure higher than that of the HP carbamate condenser, the melamine off-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 in cases where the amount of melamine off-gas is relatively large relative to urea production.

[0060] Preferably, the amount of the melamine off-gas stream supplied to the HP urea synthesis section corresponds to the amount of urea converted to melamine being at least 5%, at least 10%, or at least 20%, or at least 30% and / or up to 50% (preferably 10 wt.% to 45%) of the total urea produced by the high-pressure urea synthesis section; all amounts are preferably in terms of mass flow rate.

[0061] Preferably, the amount of CO2 in the melamine off-gas stream supplied to 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 to urea in the HP urea synthesis section; all amounts are in kg / h. The remaining part of the CO2 is provided as a gaseous CO2 feed.

[0062] For example, the urea production rate in the synthesis section is 100 ton / hr, and the amount of melamine tail gas corresponds to a urea melt feed of 15 ton / hr 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 an additional urea plant. In some embodiments, the tail gas amount is from two or more melamine plants.

[0063] When the amount of recycled melamine tail gas 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 the required steam pressure of the HP stripper increase, because the fresh CO2 feed rate (which can also be used for stripping) in the synthesis section is small compared to the amount of urea synthesis solution to be stripped. As a result, the N / C ratio (calculated based on free NH3, CO2, and carbamate, neglecting urea) of the stripped urea solution increases to, for example, 2.3 or higher, for example, in the range of 2.4 to 3.0. The method of the present invention advantageously mitigates this effect by supplying the fresh CO2 feed of the second part to the LP carbamate condensation zone. Therefore, the method involves reducing the N / C ratio in the LP carbamate condensation section.

[0064] It is noteworthy in the present invention that it is advantageous to achieve a sufficiently low N / C ratio of the stripped urea solution and a sufficiently low N / C ratio in the LP carbamate condensation section without operating the HP stripper at a very high energy consumption.

[0065] Advantageously, the preferred embodiment of the method does not involve recycling the condensed NH3 stream from the recovery section (i.e., the section containing the unit for treating the urea solution from the HP CO2 stripper to recover NH3 and CO2) to the HP urea synthesis section as an additional stream of the carbamate solution recycle stream. The preferred embodiment of the apparatus does not include an NH3 condenser having a fluid flow connection to the HP synthesis section that is independent of and additional to the liquid flow connection of the carbamate solution from the LP carbamate condensation zone to the HP urea synthesis section. By avoiding the separate recycling of the condensed NH3, a dedicated NH3 condenser is not required, thus reducing the equipment cost. In addition, the NH3 condensation (which is a complex and sensitive operation) is avoided, and the handling of pure NH3 is also avoided.

[0066] Preferably, a first portion of the purified urea solution is used for the production of melamine, and a second portion of the purified urea solution is used for the preparation of one or more urea products, such as solid urea products. Preferably, the first and second portions are subjected to evaporation in an (evacuated) evaporation section included in the urea production section to form a urea melt. Preferably, a first portion of the urea melt is supplied to the melamine production section, and preferably a second portion of the urea melt is supplied to a finishing section, such as a prilling tower or a granulator, for example. Preferably, the first portion used for the production of melamine 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 production. Optionally, a third portion of the purified urea solution is used to prepare a liquid urea product.

[0067] The present invention also provides a combined production apparatus for producing urea and melamine. All the priorities and details related to the method discussed also apply to the apparatus. In addition, the method of the present invention is preferably carried out in an apparatus according to the present 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 stripper (2). The HP stripper has an outlet for the stripped urea solution (3) and an inlet for a first portion (4) of the gaseous CO2 feed stream used 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 the melamine (6) product and a separate outlet for the melamine tail gas (7) (either as a gas or as a condensate). The preferred embodiments of the melamine production section (5) discussed for the method also apply to the present apparatus, and in particular, high-pressure and low-pressure melamine synthesis sections can be used. 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 the crystallization of melamine crystals, a mother liquor treatment section, and a tail gas scrubbing section using, for example, water scrubbing or urea 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 melamine tail gas (7) (optionally as a condensate) to be directly or indirectly supplied to the high-pressure urea synthesis section (1). The apparatus optionally includes a condenser for the tail gas. The condenser for the tail gas can be a dedicated condensation unit or a unit for condensing the melamine tail gas in combination with other gaseous streams. The condenser is included, for example, in the urea production section, such as 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 device includes a liquid flow connection for the stripped urea solution (3) to be supplied directly or indirectly as a liquid to a low-pressure (LP) dissociator (8). This dissociator (e.g., a heat exchanger) has an outlet for the purified urea solution (9) and an outlet for the low-pressure gas stream (10). The device includes a low-pressure (LP) carbamate condensation zone (11) configured to subject the low-pressure gas stream together with a second part of the gaseous CO2 feed stream to condensation to form a carbamate solution (12). The device 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 device includes a gas flow connection (13), such as a gas flow line, from the CO2 supply unit (14) directly or indirectly to the LP carbamate condensation zone (11); this gas flow connection can 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 device, the CO2 is sourced from a synthesis gas plant, i.e., a plant that produces hydrogen for an upstream co-production ammonia plant, which produces NH3 feed for the urea plant, and also includes a CO2 generation unit, which is connected to the LP carbamate condensation section. For example, the device includes a gas flow connection for the CO2 from the CO2 removal unit (especially a desorber) of the synthesis gas plant to the LP carbamate condensation zone (11). However, other sources of CO2 are possible, such as from a waste incinerator, or generally from a flue gas stream.

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

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

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

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

[0075] Preferably, the apparatus includes a contacting 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 contacting unit having a first outlet for a liquid stream and a second outlet for a gaseous stream, wherein the second outlet is connected to the 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 contacting, wherein the gas inlet of the second portion (13) of the gaseous CO2 feed stream is at the bottom, the gas outlet is at the top, the liquid inlet of the purified urea solution (9) is at the top, and the liquid outlet is at the bottom, and for example employs a packed bed or a 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 partly provided as a first compartment for indirect heat exchange with a second compartment of the urea solution to be heated. Preferably, the apparatus includes a liquid flow line for the urea solution from the LP dissociator (8) to the second compartment. Preferably, the first compartment and the second compartment are provided as a shell and tube heat exchanger. Preferably, the second compartment is provided by a tube bundle (i.e., tubes), and the first compartment is provided by the shell side space. Preferably, the first compartment operates as a pre-evaporator. Preferably, the apparatus includes a gas / liquid separator downstream of the tube outlet to separate the 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 can lead from a gas inlet, through a unit (such as a gas / liquid contacting 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. A unit provides a liquid flow connection from a liquid inlet to a liquid outlet. For example, a decomposer and a condenser connected by a gas flow connection from the decomposer to the condenser do not provide a liquid flow connection from the liquid inlet of the decomposer to the liquid outlet of the condenser. However, the condenser provides a liquid flow connection from the liquid inlet of the unit to the liquid outlet of the unit.

[0078] 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 species. The amounts of NH3 and CO2 in the water stream include the amounts present as carbonate species.

[0079] As used herein, for the process streams of a urea plant (i.e., not for steam pipelines nor for melamine plants), 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), in particular 15 bar to 30 bar, and low pressure (LP) is for example 0 bar to 10 bar, in particular 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 in bar absolute (bar).

[0080] The terms "typical", "suitable" and "in particular" and derivative forms 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 herein, the term'melamine offgas' refers to the offgas from the melamine production section and refers to a gas stream mainly containing NH3, CO2 and possibly H2O.

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

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

[0084] The N / C ratio represents the molar ratio of NH3 to CO2 in a gas stream based on NH3 and CO2; for a carbamate solution, based on NH3, CO2 and carbamate, and for the synthesis section, based on the theoretical initial reaction mixture consisting of H2O, NH3 and CO2. The N / C ratio of the carbamate condenser refers to the N / C ratio at the liquid outlet. The free N / C ratio of a urea solution 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 the present invention, is effected through a heat exchange wall that contacts the urea solution to be heated on a first side and a gas containing CO2 and NH3 (which condenses to form a carbamate solution) on a second side of the wall.

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

[0087] Example 1

[0088] A urea plant is considered and simulated, in which 30 wt.% of the urea melt is supplied to the melamine production section and a corresponding amount of melamine off-gas is recycled to the urea synthesis section. Tons are metric tons, and the values are approximate.

[0089] The overall design of the plant is as Figure 1 shown; the urea solution undergoes MP adiabatic flash between the HP stripper and the LP dissociator.

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

[0091] Under optimal operating conditions, the first LP condensation unit operates at a liquid outlet temperature of 93 °C, and the liquid outlet temperature of the second LP condensation unit is 66 °C, at approximately 5.4 bar. The N / C ratio of the urea solution at the outlet of the HP stripper 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 condensation 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 recycled from this condenser to the HP urea synthesis section. The MP carbamate condenser also receives the gas from the MP flash.

[0092] Case 2, less preferred: The CO2 feed rate to the LP section is reduced to 3 tons per hour; the amount of CO2 feed to the HP section is 93 tons per hour. At the optimal operating conditions, the first LP condensation unit operates at a liquid outlet temperature of 91 °C, and the liquid outlet temperature of the second LP condensation unit is 57 °C, at approximately 5.4 bar. The N / C ratio of the urea solution at the outlet of the HP stripper is 2.7 (free N / C). The N / C ratio of the carbamate solution at the liquid outlet of the second LP condensation unit is 3.2. This carbamate solution is supplied to the MP carbamate condenser, the liquid outlet N / C ratio of which is 2.55, and is recycled 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 load on the WWT also increases. Additionally, the reflux condenser temperature decreases from 68 °C in Case 1 to 50 °C in Case 2; a larger reflux condenser is required in the WWT. Due to the increase in the CO2 feed stream, the reduction in the total steam consumption of the HP stripper is only marginal. Therefore, the performance of the co-production plant and method in Case 1 is better compared to Case 2. Due to the higher N / C ratios in the MP and LP, the WWT load in Case 2 is higher, which results in a higher absorber load for the uncondensed gas from the corresponding condenser, and the aqueous solution from the absorber will be treated in the WWT section.

[0094] Case 3 (comparative): The CO2 feed rate to the LP section is reduced to 0 tons per hour; the amount of CO2 feed to the HP section is 96 tons per hour. The aim is to achieve 30% melamine integration and maintain MP flashing. The simulation results show that in Case 3, a separate NH3 condenser is required in the recovery section, and the condensed NH3 is recovered separately, otherwise there will be a large amount of ammonia loss to the atmosphere in Case 3.

[0095] Example 2

[0096] Case 1A: Case 1A is the same as Case 1, with the following details added. In the first LP carbamate condenser, a total steam feed is supplied at 78 ton / h (where 65 ton / h of this steam comes from LP rectification / dissociation and 13 ton / h comes from LP CO2 stripping). Approximately 57 ton / h of aqueous carbamate is added to the first carbamate condenser, provided by 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 the single outlet at the exit of the first condenser to the inlet of the second condenser, the stream is provided by approximately 105 ton / h of liquid (N / C approximately 2.4) at about 95 °C and approximately 30 ton / h of steam at 95 °C. At the exit of the second condenser, the total liquid is approximately 132 ton / h (N / C approximately 2.8) at about 69 °C; the uncondensed gas is approximately 3 ton / h at 69 °C. This steam is supplied to the atmospheric condenser.

Claims

1. A co-production method for producing urea and melamine, the method comprising: - Producing urea in a high-pressure urea synthesis section (1) including a high-pressure (HP) CO2 stripper (2), generating a stripped urea solution (3), wherein the HP CO2 stripper uses a first portion (4) of a gaseous CO2 feed stream as a stripping agent; - Producing melamine (6) in a melamine production section (5), thereby also releasing a melamine tail gas (7); - Optionally supplying the melamine tail gas (7) directly or indirectly as a condensate to the high-pressure urea synthesis section (1); - Supplying the stripped urea solution (3) directly or indirectly via a liquid flow connection to a low-pressure (LP) dissociator (8), generating a purified urea solution (9) and a low-pressure gas stream (10); - Subjecting the low-pressure gas stream to condensation together with a second portion (13) of the gaseous CO2 feed stream in a low-pressure (LP) carbamate condensation zone (11) to form a carbamate solution (12).

2. The method according to claim 1, wherein the purified urea solution (9) is in countercurrent contact (15) with the second portion (13) of the gaseous CO2 feed stream in a contact unit, generating a liquid stream and a gaseous stream (16), and supplying the gaseous stream (16) to the LP 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, wherein the melamine tail gas (7) is condensed into a first carbamate stream, and the first carbamate stream is directly or indirectly supplied to a urea reaction zone, wherein preferably a) the first carbamate stream is supplied to the high-pressure urea synthesis section, or b) the melamine tail gas (7) is supplied as a gaseous stream to the high-pressure urea synthesis section (1) and is condensed into the first carbamate stream in a carbamate condenser included in the high-pressure urea synthesis section (1).

5. The method according to any one of the preceding claims, wherein the condensation in the LP carbamate condensation zone (11) is at least partially in indirect heat exchange with a urea solution to be heated.

6. The method according to any one of the preceding claims, the method comprising performing a medium-pressure treatment on the stripped urea solution upstream of the LP dissociator.

7. The method according to claim 6, wherein the medium-pressure treatment comprises or consists of an adiabatic flash.

8. The method according to claim 6, wherein the medium-pressure treatment comprises heating the stripped urea solution.

9. The method according to any one of the preceding claims, wherein the second portion (13) of the gaseous CO2 feed stream is 2 mol% to 20 mol% of the total CO2 feed stream.

10. The method according to any one of the preceding claims, wherein the amount of CO2 in kg / hr in the melamine off-gas stream supplied to the HP urea synthesis section is at least 5%, preferably at least 10% of the amount of CO2 in kg / hr converted to urea in the HP urea synthesis section.

11. The method according to any one of the preceding claims, wherein the purified urea solution is optionally expanded to atmospheric pressure with the formation of a gas stream after counter-current contact with the second part (13) of the gaseous CO2 feed stream, and wherein the gas stream is condensed at atmospheric pressure.

12. The method according to any one of the preceding claims, wherein the LP carbamate condensation zone (11) operates at a pressure of at least 3.0 bar, and wherein the LP carbamate condensation zone receives the second part (13) of the gaseous CO2 feed stream as a gas stream at a pressure of at least 3.0 bar.

13. The method according to any one of the preceding claims, wherein LP is at least 2 bar, preferably at least 4 bar.

14. The method according to any one of the preceding claims, wherein the second part (13) of the gaseous CO2 feed stream is at least 5 mol.% and less than 25 mol.% of the total CO2 feed stream based on CO2; and wherein preferably the amount of CO2 in the gas stream at the gas outlet of the contact unit (15) is at least 5 mol.%.

15. A combined production plant for producing urea and melamine, the plant comprising: - a high-pressure urea synthesis section (1) for producing urea, which includes a high-pressure (HP) CO2 stripper (2), the stripper having an outlet for the stripped urea solution (3) and an inlet for a first part (4) of the gaseous CO2 feed stream used as a stripping agent; - a melamine production section (5) having an outlet for melamine (6) and an outlet for melamine off-gas (7); - a fluid flow connection for supplying the melamine off-gas (7) optionally directly or indirectly as a condensate to the high-pressure urea synthesis section (1); - a liquid flow connection for supplying the stripped urea solution (3) directly or indirectly to a low-pressure (LP) dissociator (8), the low-pressure dissociator having an outlet for the purified urea solution (9) and an outlet for a low-pressure gas stream (10); - a low-pressure (LP) carbamate condensation zone (11) configured to subject the low-pressure gas stream to condensation together with the second part of the gaseous CO2 feed stream to form a carbamate solution (12), the plant including a gas flow connection (13) from a CO2 supply unit (14) to the LP carbamate condensation zone (11), and a gas flow pipeline for the low-pressure gas stream (10) from the low-pressure (LP) dissociator (8) to the low-pressure (LP) carbamate condensation zone (11).

16. The co-production device according to claim 15, wherein the gas flow connection part (13) is from the CO2 removal unit to the LP carbamate condensation zone (11).

17. The co-production device according to claim 15 or 16, wherein the gas flow connection part (13) is from the hydrogen removal unit to the LP carbamate condensation zone (11), and the unit is configured to remove hydrogen from the CO2 gas stream by catalytic combustion.

18. The co-production device according to any one of claims 15 to 17, wherein the gas flow connection part (13) is from the CO2 compressor, particularly an intermediate stage of the compressor, to the LP carbamate condensation zone (11).

19. The co-production device according to any one of claims 15 to 18, the co-production device includes a contact unit (15), preferably an LP stripping column, for bringing the purified urea solution (9) into contact with the second part (13) of the gaseous CO2 feed stream, the contact unit having a first outlet for a liquid stream and a second outlet for a gaseous stream, wherein the second outlet is connected to the LP carbamate condensation zone (11).

20. The co-production device according to any one of claims 15 to 19, wherein the LP carbamate condensation zone (11) is at least partially provided as a first compartment for indirect heat exchange with a second compartment for the urea solution to be heated, and wherein the device preferably includes a liquid flow line for the urea solution from the LP dissociator (8) to the second compartment, and wherein the first compartment and the second compartment are preferably provided as a shell and tube heat exchanger.

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