Production of urea and melamine

Abstract

The present disclosure relates to a process for the production of urea and melamine, wherein off-gas from a high-pressure melamine plant is combined with urea liquid obtained from flashing of a urea synthesis solution. Also disclosed is a system for combined production as well as a retrofit of an existing plant or system.

Description

Technical Field

[0001] This invention relates to the combined production of urea and melamine, wherein waste gas from melamine production is used as feedstock for urea synthesis. Specifically, this invention relates to integrated processes and systems for the production of urea and melamine. Background Technology

[0002] Urea is typically produced from ammonia and carbon dioxide. Thus, carbon dioxide and ammonia react in a high-pressure (HP) urea synthesis section to form a urea synthesis solution. Understanding the underlying reactions is often enlightening in understanding various aspects of the urea process and production facility. Based on an exothermic reaction, ammonia and carbon dioxide first react to form ammonium carbamate:

[0003] 2NH3 + CO2 → H2N–CO–ONH4.

[0004] In the second step, based on the endothermic equilibrium reaction, the formed ammonium carbamate is dehydrated to obtain urea:

[0005]

[0006] The resulting urea synthesis solution is subjected to the recovery of unreacted ammonia and carbon dioxide. This can be achieved in several ways, which are typically applied in various combinations in urea plants. Thus, in a urea stripping plant, the urea synthesis solution is stripped and condensed in the HP section, thereby recovering ammonia and carbon dioxide. For example, the stripped urea synthesis solution is subjected to further recovery of unreacted ammonia and carbon dioxide in one or more parallel and / or series urea recovery sections, operated at medium pressure (MP) and / or low pressure (LP). The recovered ammonia and carbon dioxide are ultimately reused as an ammonium carbamate solution in the synthesis section, thereby completing the urea synthesis loop. The urea solution is typically subjected to evaporation to form a highly concentrated solution commonly referred to as urea melt. Urea melt can be used in connected facilities (such as melamine facilities) and / or can be refined into solid forms, such as pellets or granules.

[0007] The challenge in the aforementioned recovery process is typically to separate unreacted ammonia and carbon dioxide as efficiently as possible and to reuse them with the least amount of water possible. The latter is important because, as seen in the second reaction step described above, the presence of water will unfavorably shift the equilibrium towards the carbamate side. Therefore, in particular, each recovery stage usually involves a treatment step in which the urea solution is subjected to the decomposition (dissociation) of the remaining ammonium carbamate into its components ammonia and carbon dioxide, which are then condensed and further reused. Regarding the reuse of ammonium carbamate, it should be understood that ammonium carbamate requires a minimum amount of water to remain dissolved, and this required amount increases as the pressure decreases.

[0008] Another factor typically considered in various recycling and reuse steps is the efficient use of energy, such as the heat required for the decomposition of carbamates, and the reuse ratio of N (ammonia) and C (carbon dioxide component). Regarding the latter, it should be understood that the urea formation reaction involves a 2:1 N:C ratio, but the overall ratio for the urea synthesis stage typically involves excess ammonia, for example, a 3:1 N:C ratio.

[0009] Melamine is typically produced from urea, which generates a waste gas (hereinafter referred to as "melamine waste gas") containing ammonia and carbon dioxide as byproducts (in a 2:1 molar ratio used in the urea formation reaction described above). There are generally two types of melamine production processes. One is a catalytic "low-pressure" process, which uses atmospheric pressure (from atmospheric pressure to approximately 70 bar). The other is called a "high-pressure" process. In melamine production, high pressure refers to a pressure equal to or higher than approximately 70 bar (7 MPa), and more typically equal to or higher than 80 bar (8 MPa).

[0010] It is well known in the art to combine the production of urea with the production of melamine. Thus, typically, at least some or all of the urea produced at the urea production facility is fed to the melamine facility as starting material. Therefore, the integrated production of urea and melamine usually involves feeding waste gas from the melamine facility directly or indirectly into the synthesis section of the urea facility.

[0011] An ongoing challenge in this field is finding a suitable location for introducing melamine exhaust gas into the urea synthesis loop. A particular problem with exhaust gas from the high-pressure melamine process is that its pressure is mismatched with the operating pressures of any conventional section of a urea production facility. The exhaust gas generated in the high-pressure melamine synthesis process is initially obtained at the synthesis pressure, which is typically at least 70 bar and usually in the range of 80 to 150 bar. In many cases, this pressure is lower than that of the HP urea synthesis section, which typically ranges from 120 to 400 bar and is more typically 140 to 160 bar. Furthermore, in some cases, the exhaust gas from melamine synthesis is not actually released at the synthesis pressure, but at even lower pressures, typically 2-30 bar, such as 20-25 bar. This is typically achieved as a result of downstream processing or inspection (such as washing steps), resulting in exhaust gases released at low pressure containing significant amounts of water in addition to ammonia and carbon dioxide, such as 3% to 40% by weight, and more specifically 5% to 30% by weight. It is known to reuse these exhaust gases in MP sections already present in urea facilities or otherwise in integrated facilities connecting the two production processes.

[0012] This type of method is disclosed in WO2008 / 052640. In this process, the exhaust gas stream from the melamine facility is condensed together with the reused ammonium carbamate solution in the condensation section of the medium-pressure treatment zone of the urea facility. This presents a passive method commonly used to reuse melamine exhaust gas in urea synthesis.

[0013] It is hoped that melamine waste gas can be utilized in different or even better ways as an active component in urethane recovery and / or that the available energy in the waste gas can be better utilized. Summary of the Invention

[0014] To better address one or more of the aforementioned desired outcomes, the present invention provides, in one aspect, a process for the production of urea and melamine, wherein the production of urea includes subjecting ammonia and carbon dioxide to a urea formation reaction in a high-pressure urea synthesis section to obtain a high-pressure urea synthesis solution, wherein the urea synthesis solution is preferably subjected to flash evaporation under medium pressure to produce flash vapor containing ammonia and carbon dioxide and flash-evaporated urea liquid; wherein the flash-evaporated urea liquid is preferably treated in a medium-pressure treatment section to obtain a urea product solution, preferably a medium-pressure urea product. The treatment includes one or more ammonia and carbon dioxide release steps, thereby removing ammonia and carbon dioxide vapors from flash-evaporated urea liquid, wherein the production of melamine includes subjecting urea to a non-catalytic high-pressure melamine-forming reaction to obtain melamine and melamine waste gas containing ammonia and carbon dioxide; and wherein the combined process includes contacting at least a portion of the melamine waste gas with flash-evaporated urea liquid before or during at least one of the ammonia and carbon dioxide release steps.

[0015] On another front, the present invention provides a system for the production of urea and melamine, the system comprising a urea production zone; the urea production zone including a high-pressure urea synthesis section and a urea recovery section located downstream of and in fluid communication with the high-pressure urea synthesis section, the urea recovery section being adapted to separately obtain urea solution and urethane recovery stream; the system further comprising a melamine production zone; the melamine production zone including a high-pressure melamine synthesis section, the production zones being interconnected to allow the transport of melamine waste gas obtained from the melamine synthesis section to the urea production zone, wherein the urea production zone includes a medium- or low-pressure treatment section adapted to receive the melamine waste gas, the treatment section including a flash evaporation unit and at least one ammonia and carbon dioxide release unit. The flash evaporation unit has an inlet for urea synthesis solution connected to the outlet of the high-pressure synthesis section via a pressure reducing device, an outlet for flash vapor, and an outlet for flashed liquid. The ammonia and carbon dioxide release unit has an inlet for flashed liquid connected to the outlet for flashed liquid from the flash evaporation unit, an outlet for urethane recovery vapor, and an outlet for urea product stream. The outlets for flash vapor and urethane recovery vapor are each connected to the urea synthesis section via a condensation and recirculation loop. The outlet for melamine exhaust gas from the melamine synthesis section is in fluid communication with the inlet of at least one ammonia and carbon dioxide release unit, thereby allowing the melamine exhaust gas to contact the flashed liquid in or upstream of the ammonia and carbon dioxide release unit.

[0016] In another aspect, the present invention relates to a method for modifying a pre-existing urea facility, the pre-existing urea facility comprising a high-pressure urea synthesis section and a urea recovery section located downstream of and in fluid communication with the high-pressure urea synthesis section, the urea recovery section being adapted to separately obtain urea solution and carbamate recovery streams, the method comprising connecting the urea facility to a melamine facility comprising a high-pressure melamine synthesis section to allow melamine waste gas to be recycled from a melamine waste gas treatment section back to the urea facility, the method comprising adding a medium-pressure or low-pressure treatment section adapted to receive the melamine waste gas, the treatment section comprising a flash evaporation unit and at least one ammonia and carbon dioxide release unit, the flash evaporation unit having a... The ammonia and carbon dioxide release unit has an inlet for urea synthesis solution, an outlet for flash vapor, and an outlet for flashed liquid connected to the outlet of the high-pressure synthesis section via a pressure reducing device. The ammonia and carbon dioxide release unit also has an inlet for flashed liquid connected to the outlet for flashed liquid from the flash unit, an outlet for urethane recovery vapor, and an outlet for urea product stream. The outlets for flash vapor and urethane recovery vapor are each connected to the urea synthesis section via a condensation and recirculation loop. The outlet for melamine exhaust gas from the melamine synthesis section is in fluid communication with the inlet of the ammonia and carbon dioxide release unit, thereby allowing the melamine exhaust gas to contact the flashed liquid in or upstream of the ammonia and carbon dioxide release unit.

[0017] In another aspect, the present invention provides a method for modifying a pre-existing system for the production of urea and melamine, the pre-existing system comprising a urea production zone and a melamine production zone; the urea production zone comprising a high-pressure urea synthesis zone in fluid communication with a urea recovery zone adapted to separately obtain a urea solution and a urethane recovery stream, the recovery zone comprising a medium-pressure or low-pressure treatment zone comprising a flash evaporation unit and an ammonia and carbon dioxide release unit, the flash evaporation unit having an inlet for a urea synthesis solution connected via a pressure reducing device to the outlet of the high-pressure synthesis zone, an outlet for flash vapor, and an outlet for flashed liquid; the ammonia and carbon dioxide release unit having an inlet for flashed liquid connected to the outlet for flashed liquid from the flash evaporation unit, an outlet for urethane recovery vapor, and an outlet for... At the outlet of the urea product stream, the outlets for flash vapor and urethane recovery vapor are each connected via a condensation and recirculation loop to the urea synthesis section. The melamine production section includes a high-pressure melamine synthesis section and a melamine waste gas treatment section located downstream of and in fluid communication with the high-pressure melamine synthesis section for obtaining depressurized melamine waste gas. The production sections are interconnected to allow the delivery of melamine waste gas from the melamine waste gas treatment section to the urea production section, and optionally also to allow the delivery of urea from the urea production section to the melamine synthesis section. The method includes providing a connection between the outlet of the melamine waste gas from the melamine synthesis section and the inlet of the ammonia and carbon dioxide release unit, thereby allowing the melamine waste gas to contact with flashed liquid in or upstream of the ammonia and carbon dioxide release unit. Attached Figure Description

[0018] Figure 1 , Figure 2 and Figure 3 This is a schematic diagram of an embodiment of a system for the production of urea and melamine;

[0019] Figure 4 and Figure 5 This is a schematic diagram illustrating the application of a single flash evaporation and heat exchange device in a system used for the production of urea and melamine. Detailed Implementation

[0020] This invention is based on the ingenious insight of utilizing the sensible heat from the waste gas produced during high-pressure melamine production to support the decomposition of carbamates present in the urea synthesis solution. For this purpose, the waste gas is combined with liquid urea obtained from the flash evaporation of the urea synthesis solution. This differs significantly from existing processes that combine urea and melamine, in which melamine waste gas is typically fed into a condensation step in the urea production process.

[0021] Flash evaporation of liquids (such as urea synthesis solutions) implies that such liquids expand at pressures below their original pressure. Urea synthesis solutions originate from sections operating at high pressure (HP). In the context of urea production processes and facilities, flash evaporation occurs in sections operating at medium pressure (MP) or low pressure (LP). These terms each have specific meanings in the field of urea production. HP typically indicates a pressure of 12-40 MPa (120 to 400 bar), preferably 140 to 160 bar. MP in urea production typically indicates a pressure in the range of 1 MPa to 8 MPa, preferably 1 MPa to 4 MPa (10-40 bar), more preferably 15 to 35 bar, and most typically 20 to 25 bar. LP in urea production represents a pressure below 1 MPa, preferably 0.3-0.5 MPa (3-5 bar), typically about 4 bar.

[0022] In the context of melamine production, the term "HP" generally indicates 7 MPa (70 bar) or higher, preferably 8 MPa (80 bar) or higher, and more preferably 80 to 150 bar.

[0023] In this process, at least a portion of the melamine waste gas is contacted with flash-evaporated urea liquid in one or more ammonia and carbon dioxide release steps. Urea production processes typically include such steps to optimize the recovery of unreacted ammonia and carbon dioxide. This unreacted ammonia and carbon dioxide are present in the urea synthesis solution in the form of water-soluble ammonium carbamate. Therefore, at various stages of the urea production process, ammonia and carbon dioxide are released from the urea synthesis solution, typically removed as vapors, and ultimately recycled back to urea synthesis.

[0024] Typically, such ammonia and carbon dioxide release steps require a driving force to shift the equilibrium between dissolved ammonium carbamate and ammonia and carbon dioxide vapors to more strongly favor the latter. One way to achieve this is through the thermal decomposition (or in other words, dissociation) of ammonium carbamate, thereby enabling the removal of gaseous ammonia and carbon dioxide. Another method is to apply a stripping gas to the liquid to remove carbon dioxide and ammonia vapors. Typically, heat and the stripping gas are applied simultaneously. This is well known to those skilled in the art and does not need to be elaborated upon here.

[0025] In the process of this invention, melamine waste gas is contacted with flash-evaporated urea liquid during the ammonia and carbon dioxide release steps. This contact can facilitate one or both of the phenomena described above.

[0026] The exhaust gas from the high-pressure melamine process has a higher temperature than the urea solution obtained from the medium- or low-pressure flash evaporation of the urea synthesis solution. Therefore, when introduced into the flash-evaporated urea liquid, the exhaust gas acts as a direct heating agent.

[0027] Flash evaporation is carried out at medium or low pressure, typically between 3 and 80 bar. Medium-pressure flash evaporation (MP flash evaporation) is generally preferred. This preference relates to the general concept that the purpose of processing steps such as flash evaporation is to recover and recycle unreacted ammonia and carbon dioxide (typically as ammonium carbamate) to urea synthesis. To maintain urea synthesis balance, it is generally preferred to recycle as little water as possible to urea synthesis. Condensation of medium-pressure flash vapor requires less water than condensation of low-pressure flash vapor. Flash evaporation is preferably carried out at 10-70 bar, more preferably 10-35 bar, for example 18-25 bar.

[0028] Flash evaporation is preferably carried out at a pressure equal to or higher than the pressure of the ammonia and carbon dioxide release unit, for example, 0-10 bar higher than the pressure of the ammonia and carbon dioxide release unit, for example, 0-7 bar higher, preferably 0-7 bar higher, and more preferably at a pressure equal to the operating pressure of the ammonia and carbon dioxide release unit. The temperature of the flashed urea liquid (i.e., the liquid temperature downstream of the unit in which flash evaporation occurs) is pressure-dependent. Typically, at a flash pressure of 3 bar, the temperature is 100°C, at a flash pressure of 10 bar, the temperature is 127°C, and at a flash pressure of 70 bar, the temperature is 165°C. Preferably, flash evaporation is carried out at 20-30 bar, such as at 25 bar. At 25 bar, the temperature of the flashed liquid is 148°C. Pressure expressed in bar refers to an absolute pressure value (bara).

[0029] Melamine waste gas can also be introduced into flash-evaporated urea liquid in a manner that allows the melamine waste gas to effectively serve as a stripping gas. This can be achieved in units designed as stripping units, such as plate towers or packed towers, or typically in units that include packed beds.

[0030] The contact discussed occurs during the ammonia and carbon dioxide release steps. This typically takes place in corresponding processing units, i.e., ammonia and carbon dioxide release units, such as heating units (typically heat exchange units), packed beds, or combinations thereof. Such units have an inlet for the liquid (i.e., in this case, flashed urea liquid), an outlet for the treated liquid, and an outlet for the gas, thereby enabling the removal of ammonia and carbon dioxide, i.e., urethane recovery vapors. The unit is further designed to provide heat and / or stripping, as described above. In the case where the ammonia and carbon dioxide release unit is a heater, this will typically take the form of a shell-and-tube heater, many different designs of which are known to those skilled in the art. In this document, the medium to be heated is located in one compartment (such as the tube side), while the heat exchange medium is located in another compartment (such as the shell side). Typically, the medium to be heated (in this case, flashed urea liquid) will be passed via the tube side, while an external heating medium (such as hot liquid or steam) will flow on the shell side. The release of ammonia and carbon dioxide necessarily involves the decomposition (dissociation) of urethanes. Therefore, ammonia and carbon dioxide release units are sometimes also referred to as decomposers or dissociators. Unless otherwise stated, the terms “heating unit,” “heat exchange unit,” “heater” (such as “MP heater”), “decomposer” (such as “MP decomposer”), and “dissociator” (such as “MP dissociator”) are used interchangeably with the term “ammonia and carbon dioxide release unit.” In embodiments, the ammonia and carbon dioxide release unit may be a packed bed. Such a packed bed can be operated with heat, with stripping gas for adiabatic stripping (e.g., carbon dioxide), or both. In this invention, melamine waste gas can provide heat and act as the stripping gas in such a packed bed. Packed beds are known in the industry, and their possible structures and materials (e.g., metals, ceramics, glass) need not be described herein.

[0031] Given its high temperature and pressure, the melamine waste gas introduced into the flash-evaporated urea solution directly transfers heat to the solution, thereby facilitating the decomposition of ammonium carbamate. Therefore, the melamine production waste gas is cleverly housed in the same compartment as the solution to be heated (typically the tube side of a shell-and-tube heat exchanger, while steam or hot liquid may be separately located on the shell side). Depending on the precise conditions and heat requirements, this process can be envisioned with the ammonia and carbon dioxide release units, in which hot melamine waste gas is injected, operating with or without an external heating medium.

[0032] Technicians will be able to determine the precise heat requirements based on the heat and pressure of the melamine exhaust gas, as well as the conditions in one or more associated medium- or low-pressure ammonia and carbon dioxide release steps.

[0033] The waste gas generated in the high-pressure melamine synthesis process is initially obtained at a synthesis pressure, typically at least 70 bar, and usually in the range of 80 to 150 bar. When introduced into the medium-pressure (MP) or low-pressure (LP) section of a urea plant, the pressure of the melamine waste gas is reduced by an expansion valve at the inlet of said MP or LP section. Typical conditions for melamine waste gas introduced into the ammonia and carbon dioxide release units are 250°C and 80 bar. In some types of melamine production facilities that are also generally suitable for carrying out the methods disclosed in this invention, the melamine waste gas is discharged at a pressure between 10 and 70 bar, preferably between 20 and 30 bar, for example, 25 bar.

[0034] The temperature of the waste gas obtained from the high-pressure melamine synthesis section can vary considerably. Typically, such melamine waste gas is discharged at temperatures ranging from 140°C to 400°C, such as 150°C to 300°C. Preferably, the temperature is above 160°C, more preferably above 175°C. Preferably, the temperature is below 285°C, more preferably below 275°C. Most preferably, the temperature is in the range of 200°C to 250°C, such as 210°C to 240°C, such as 215°C to 235°C. It should be understood that, in carrying out the method of the present invention, the temperature of the melamine waste gas before contact with the flash-evaporated urea liquid is higher than the temperature of the liquid.

[0035] By introducing melamine exhaust gas into the urea solution during the ammonia and carbon dioxide release steps, the available heat from the HP melamine exhaust gas is well utilized, while less heat is required from the heat exchange medium (typically steam or a hot liquid flow, such as condensate).

[0036] Typically, the ammonia and carbon dioxide release steps (especially those leading to the decomposition of ammonium carbamate) are carried out in an MP or LP heating unit, which is a shell-and-tube heat exchanger, where the medium to be heated is located in one compartment (such as the tube side), while the heat exchange medium is located in another compartment (such as the shell side). However, the melamine production exhaust gas is cleverly located in the same compartment that is in direct contact with the solution to be heated (usually the tube side of a shell-and-tube heat exchanger, while steam or hot liquid may be separately located on the shell side if needed). The melamine exhaust gas can be introduced into the urea liquid in parallel flow or counter-flow.

[0037] It should be understood that in this process, melamine waste gas interacts with flash-evaporated urea liquid during the ammonia and carbon dioxide release steps, particularly the stripping step and / or preferably the decomposition step. These steps produce both liquid and gaseous outputs. The liquid output can be considered a medium- or low-pressure urea product solution. It is also conceivable that the ammonia and carbon dioxide release step is not a single step but involves multiple subsequent steps. This can, for example, be carried out in multiple heating units (such as a medium-pressure dissociation unit), or in a combination of one or more heating units and a unit in which stripping occurs. The latter can refer to stripping upstream of decomposition (described below) or downstream of it (which is a typical existing configuration, e.g., disclosed in WO 02 / 090323).

[0038] In cases involving multiple subsequent ammonia and carbon dioxide release steps, the liquid input to the first step in such steps is a specially flash-distilled urea liquid. This first step produces a liquid derived from the flash-distilled urea liquid but from which some ammonia and carbon dioxide have been removed. For convenience, in this specification, the liquid derived from the flash-distilled urea liquid and subjected to multiple subsequent ammonia and carbon dioxide release steps is indicated as "flash-distilled urea liquid" in each such subsequent step. The liquid output from the final medium-pressure (or low-pressure) ammonia and carbon dioxide release step is indicated as "medium-pressure (correspondingly, low-pressure) urea product solution." This solution will be further processed in accordance with what is commonly done in the art, such as being sent, in particular, to a low-pressure recovery section.

[0039] In the case of multiple subsequent ammonia and carbon dioxide release steps, melamine waste gas may be introduced into flash-evaporated urea liquid in any one or more of these steps. The gaseous output of each of these steps is hereby indicated as urethane recovery vapor.

[0040] It should be understood that the purpose of obtaining urethane recovery vapors is to ultimately recycle the recovered ammonia and carbon dioxide into urea synthesis. Such recycling typically involves condensing the urethane recovery vapors at medium or low pressure to produce an MP or LP urethane solution. According to techniques widely available in the art, the latter is typically recycled via one or more additional processing steps, particularly a low-pressure recovery step.

[0041] The resulting condensate will be further processed so that it can eventually be recycled to the HP synthesis section, which may involve one or more compression steps.

[0042] It is noteworthy that the flash vapor from the flash urea HP synthesis solution thus has a high N / C ratio, attributed to the 2:1 reaction being carried out in the presence of excess ammonia during synthesis, as described above. Therefore, carbon dioxide is typically added to adjust the N / C ratio during recycling when further processing the carbamate recovery vapor. A suitable option for such CO2 addition is via an MP stripper, in which the carbamate recovery vapor is stripped with CO2.

[0043] Typically, in addition to the aforementioned waste gases, melamine production can also generate liquid carbamate streams, for example, by condensing a portion of the waste gases in the melamine facility. Such streams can be reused in urea production, with the preferred location depending on the stream composition (e.g., some melamine processes produce streams containing 40% ammonia, 40% carbon dioxide (i.e., 80% carbamate), and 20% water). It is also conceivable to produce leaner carbamate streams (such as 50% carbamate and 50% water). According to the invention, in addition to the reuse of gaseous effluents from melamine production, such additional liquid effluents can generally also be recycled to urea production, for example, reused in the low-pressure recovery section.

[0044] The characteristic of the melamine and urea production process according to the invention is effectively the flash evaporation of the urea synthesis solution via MP or LP, and preferably MP decomposition. It should be understood that the flash evaporation property of this solution, which removes ammonia and carbon dioxide vapors, allows for the addition of further amounts of ammonia and carbon dioxide obtained from the melamine production process. Therefore, the process is suitable for any urea production zone in which the urea synthesis solution is subjected to MP or LP flash evaporation.

[0045] The process described above combines the production of urea and melamine by using melamine waste gas from urea production. Preferably, further integration of the two processes involves using some or all of the produced urea as starting material for melamine production. This may refer to an integrated facility with urea and melamine production areas whose capacities are mutually adapted. It is also possible that melamine production has a relatively high capacity, for example, by also feeding urea from another urea facility. This would allow for the transfer of larger quantities of melamine waste gas to the urea facility. Considering the heat requirements in one or more ammonia and carbon dioxide release steps in urea production, it is generally more beneficial if a large quantity of waste gas from HP melamine production can be used in combination with flash-distilled urea liquid from urea production.

[0046] The general process of urea production involves subjecting ammonia and carbon dioxide to a urea formation reaction in a reactor in a high-pressure urea synthesis section of a urea production area (such as a urea production facility) to obtain a urea synthesis solution.

[0047] Referring to the above-described connection features, the carbamate recovery in the process according to the invention includes subjecting the urea synthesis solution to flash evaporation at medium pressure (MP). Preferably, in one embodiment, this solution is obtained from HP stripping. This stripping can be carried out using ammonia as the stripping medium, providing only heat (“self-stripping”), or particularly using carbon dioxide or ammonia with heat. The most widely used stripping process today is the Staminacapon stripping process, which uses CO2 as the stripping medium, effectively introducing at least a portion of the CO2 as feed for urea synthesis via an HP stripping tower. HP stripping results in the removal of initial amounts of unreacted ammonia and carbon dioxide, which are subsequently condensed in an HP carbamate condenser. The condensate is then recycled to the reactor. The stripped urea solution, still containing ammonium carbamate, is subjected to further recovery of ammonia and carbon dioxide at medium and low pressures.

[0048] It should be understood that the aforementioned flash steam and urethane recovery steam will ultimately be recycled to urea synthesis. The flash steam and urethane recovery steam will typically be condensed, either individually in separate MP and / or LP condensation sections or in the same MP or LP condensation section. Such condensation sections include at least one condenser, such as two condensers. Preferably, the flash steam and urethane recovery steam are fed to the same condenser, preferably to an MP condenser. The resulting condensate will be further processed for eventual recycling to the HP synthesis section. This can be direct, involving appropriate compression steps, or the condensate can advantageously be mixed with LP urethane recycled from the LP recovery section. As those skilled in the art of urea production will recognize, LP urethane necessarily contains a relatively large amount of water. To desirably reduce the amount of water transferred to urea synthesis, this process makes it possible to increase the concentration of LP urethane by mixing it with the condensate obtained from the urethane recovery steam. In a preferred embodiment of the invention, the urethane recovery steam is condensed together with the LP urethane in a single condenser. Alternatively, the vapor from flash condensation can be condensed separately in the first condenser, and the urethane recovery vapor can be combined with LP urethane and condensed in the second condenser.

[0049] The process of this invention preferably involves using urea produced during urea production as the starting material in melamine synthesis. Depending on the mutual production capacity, this may be all or a portion of the urea produced in the urea production area. For example, the invention can be used in the context of dedicated urea production, where the capacity of the urea production area or facility matches the capacity of the connected melamine production area or facility. In a generally preferred configuration, the capacity of the urea production area or facility exceeds that of the melamine production area or facility. Thus, a portion of the produced urea is transferred to melamine production, and another portion is provided to one or more urea refining processes to produce solid urea, such as pellets or granules, or liquid urea is provided to processes for preparing diesel exhaust fluids, and / or urea is applied to the production of products such as ammonium urea sulfate or ammonium urea nitrate. Depending on the capacity requirements for producing such urea products, it should be understood that the connected melamine production may also be wholly or partially, and possibly temporarily, fed with urea from elsewhere.

[0050] As those skilled in the art will recognize, the urea solution obtained from urea recovery is typically concentrated before being used as a starting material in melamine synthesis. This concentration preferably reaches the point of forming a highly concentrated urea solution (e.g., having more than 90% by weight of urea, such as more than 95% by weight, such as more than 99% by weight of urea) (the term "urea melt" is used in this art for this purpose). It should be understood that such concentration occurs between urea recovery and melamine synthesis, and preferably in the evaporation section of the urea facility. Typically, the urea solution is concentrated in the evaporation section to a urea melt with a final moisture content of 0.2% to 5.0% by weight.

[0051] Where not all of the urea produced is used for melamine production, the surplus urea (especially urea melt) can undergo conventional urea refining, for example, to obtain it in granule or pellet form.

[0052] Furthermore, urea not transferred to melamine synthesis can be obtained in aqueous solution form, such as having 30%-35% urea by weight and sufficient purity to be suitable as so-called diesel exhaust fluid (DEF). These and other refined urea products are known to those skilled in the art and need not be elaborated here.

[0053] The present invention also relates to a system for the production of urea and melamine according to the method described above. The system includes a urea production zone. This can be a separate urea facility or a zone as part of an integrated facility for the production of urea and melamine. The urea production zone includes a high-pressure urea synthesis section and a low-pressure urea recovery section located downstream of and in fluid communication with the high-pressure urea synthesis section, the low-pressure urea recovery section being adapted to separately obtain aqueous solutions of urea and carbamate. The system also includes a melamine production zone, which can also be a separate facility or a zone included in an integrated facility. The melamine production zone includes a high-pressure melamine synthesis section. It should be understood that the term "high pressure" has different meanings for the urea production zone and the melamine production zone, as explained above.

[0054] The production areas are interconnected to allow melamine waste gas from the high-pressure melamine synthesis section to be transported to the urea production area, which includes a medium- or low-pressure treatment section, preferably a medium-pressure treatment section, suitable for receiving the melamine waste gas. According to the invention, and taking into account the aforementioned process, the treatment section includes a flash evaporation unit and at least one ammonia and carbon dioxide release unit.

[0055] The flash evaporation unit has an inlet for the urea synthesis solution, which connects to the outlet of the high-pressure synthesis section. Since this involves a connection from the HP section to the MP or LP section, the HP urea synthesis solution will necessarily leave the HP section via a pressure reducing device (typically a pressure reducing valve). The flash evaporation unit has an outlet for flash vapor and an outlet for the flashed liquid. The ammonia and carbon dioxide release unit has an inlet for the flashed liquid connected to the flash evaporation unit, an outlet for urethane recovery vapor, and an outlet for the urea product stream. As described above, the MP and / or LP processing sections of the urea production area may include more than one ammonia and carbon dioxide release unit.

[0056] The outlets for the flash steam and the urethane recovery steam are each connected to the urea synthesis section via a condensation and recirculation loop. As mentioned with reference to the process described above, this may involve connecting separate condensation sections for the flash steam and the urethane recovery steam, or the respective outlets may be connected to the same condensation section. The condensation section optionally includes more than one condenser, such as two or more condensers connected in series and / or in parallel.

[0057] According to the invention, the outlet for melamine waste gas from the melamine synthesis section is in fluid communication with the inlet of at least one ammonia and carbon dioxide release unit, such as a heating unit. This allows the melamine waste gas to contact with flashed liquid in said unit. It should be understood that one or more ammonia and carbon dioxide release units will have a gaseous output, which is the aforementioned urethane recovery vapor. Therefore, each ammonia and carbon dioxide release unit has an outlet for urethane recovery vapor.

[0058] The ammonia and carbon dioxide release unit can be designed as a stripping unit (preferably, an MP stripping tower), a heating unit (preferably an MP heater), or both. Pressure indications (MP and / or LP) typically refer to the pressure at which the equipment operates. This also places design requirements on those skilled in the art; for example, MP equipment should be able to withstand at least the MP range defined in urea production. It should be understood that such equipment is well-known in the urea industry and can be designed to meet MP requirements without difficulty. Such designs are fully known to those skilled in the art, as are the designs for HP and LP equipment. Preferably, the flash-distilled liquid is conveyed to the ammonia and carbon dioxide release unit via a distillation column. This distillation column receives the carbamate reuse vapor from the ammonia and carbon dioxide release unit countercurrently with the liquid flow.

[0059] In an interesting embodiment, a packed bed is positioned downstream of the flash evaporation unit, typically within a distillation column, and upstream of the heating unit. In this embodiment, the packed bed may have an inlet for at least a portion of the melamine waste gas. This inlet is optionally positioned such that the melamine waste gas comes into countercurrent contact with the flashed urea liquid flowing through the packed bed. Due to the increased surface area provided by the packed bed, the melamine waste gas in this embodiment acts as a stripping gas for the flashed urea liquid, thus providing carbamate recovery vapors generated from stripping.

[0060] Preferably, the flash evaporation unit, the distillation column (optionally also including a packed bed), and the ammonia and carbon dioxide release unit are housed in a single device. This device includes a flash evaporation vessel at its upper end, a distillation column (optionally equipped with a packed bed) below the flash evaporation vessel, and a heating unit at its lower end. The single device may have one or more inlets for melamine exhaust gas (e.g., located in the heating unit and / or at the packed bed) and can provide countercurrent or cocurrent flow of melamine exhaust gas and flash-evaporated urea liquid. In a preferred embodiment, the device has a single inlet for melamine exhaust gas downstream of the heating unit. This has been found to result in a more efficient use of the heat from the melamine exhaust gas. Not wishing to be bound by theory, the inventors believe that the exhaust gas introduced in this manner into the flash-evaporated urea liquid causes turbulence that facilitates the release of ammonia and carbon dioxide. Due to this embodiment, the heating unit can be designed to be smaller, which is advantageous in terms of space and equipment cost.

[0061] It should be understood that the flash vapor and carbamate dissociation vapor, both containing ammonia and carbon dioxide, will be treated so that they can ultimately be returned to the urea synthesis section as reactants. For this purpose, the outlets for the flash vapor and carbamate dissociation vapor are each connected to the urea synthesis section via condensation and recirculation loops. Those skilled in the art of urea production are familiar with various design options for one or more condensers, possibly decomposers, and additional condensers.

[0062] Aside from the specific configuration of the MP or LP section, which includes a flash evaporation unit and ammonia and carbon dioxide release units, the urea production area can be designed according to options known and available to those skilled in the art. Some embodiments of the system of the present invention will be discussed below.

[0063] The HP synthesis section typically includes an HP stripping tower, an HP reactor, and an HP carbamate condenser. The stripping tower is preferably a CO2 stripping tower. The urea production section includes a liquid flow path from the reactor to the HP stripping tower. Optionally, the urea production section is provided in parallel with the MP dissociator by providing a second liquid flow path from the reactor to such an MP dissociator.

[0064] The HP synthesis section includes an inlet for NH3 feed, for example, located at the HP urethane condenser.

[0065] The reactor is configured to form urea from NH3 and CO2 and has an outlet for the urea synthesis solution. In a CO2 stripping facility, the N / C ratio of the urea synthesis solution is, for example, 2.85 to 3.3 (in a conventional non-CO2 stripping facility, the N / C ratio would be, for example, 3-3.4). The reactor operates at a urea synthesis pressure (i.e., HP) and a urea synthesis temperature (e.g., above 100 bar, for example 120 bar to 300 bar, for example 120 bar to 200 bar); and / or at a temperature of, for example, 160°C to 240°C, and preferably at a temperature of 170°C to 220°C. For so-called conventional facilities, an example pressure range is 190°C to 200 bar, and for CO2 stripping facilities, an example pressure range is 183°C to 145 bar.

[0066] The reactor is, for example, a vertical reactor with trays, wherein the feed inlet is located at the bottom, and the urea synthesis solution is taken out from the upper part of the vertical reactor, for example using a downcomer.

[0067] For example, the reactor has a separate gas (so-called inert gas) outlet. The inert gas is supplied directly or indirectly to the preferred AN section, for example, as a gas stream. The inert gas originates from, for example, the feed stream and contains, for example, N2. The gas stream from the reactor's gas outlet also includes, for example, NH3.

[0068] A urea production area may include one or more reactors connected in series, such as a first reactor and a subsequent reactor. A urea production facility may also include one or more reactors connected in parallel.

[0069] The reactor and HP urethane condenser are optionally combined in a single vessel. An example is the integrated reactor / condenser described in US5767313. The example integrated condenser / reactor includes a reaction zone and a condensation zone combined in a single vessel. The condensation zone includes, for example, heat exchange surfaces, such as tube bundles. The reaction zone includes, for example, baffles. The reaction zone is typically arranged downstream of the condensation zone within the vessel. An example integrated condenser / reactor is a submerged condenser (pool condenser) comprising a horizontal vessel and a tube bundle configured to receive cooling fluid in the tubes and process media in the vessel space.

[0070] The HP stripping tower has an inlet for CO2 feed as stripping gas and an outlet for the stripped urea solution and an outlet for the gas stream. The facility is preferably a CO2 stripping type. The stripped urea solution comprises urea, water, carbamate, and ammonia and is supplied to the MP or LP flash evaporator.

[0071] HP carbamate condensers (HPCCs) typically receive at least a portion, preferably all, of the feedstock NH3. HPCCs receive at least a portion, such as all, of the gas stream from an HP stripper. The HP carbamate condenser has an outlet for a stream comprising condensed carbamate, the outlet being connected to the reactor inlet, and in a combined condenser / reactor, the condensation zone is in fluid communication with the reaction zone.

[0072] The urea production area may include a CO2 compressor to compress the CO2 feed stream to the urea synthesis pressure. The compressor may be, for example, a multi-stage compressor. CO2 is available at a relatively low pressure (e.g., less than 20 bar) at the boundary, for example, from a syngas facility. The syngas facility includes, for example, a steam reformer, a water-gas shift reactor, and a CO2 removal unit. The syngas facility may also produce H2 for the ammonia facility. The NH3 feed to the urea facility may originate from the ammonia facility. Other sources of the CO2 and NH3 feed streams are also possible.

[0073] Typically, in various embodiments of the invention, the HP stripping tower includes, for example, a vertical shell-and-tube heat exchanger having an inlet for supplying the urea solution to be stripped into the tubes, the inlet being located at the top of the stripping tower, and an outlet at the bottom for the stripped urea solution and an outlet at the top of the stripping tower for a mixed gas stream. The HP stripping tower is preferably of the CO2 stripping type and has an inlet at the bottom for CO2 feed to be used as the stripping gas. The mixed gas stream is condensed in an HP carbamate condenser into a high-pressure recirculation stream containing carbamate, which is supplied to the reactor. In the case of a combined reactor / condenser in a single container, this may involve conveying the condensate from a condensation section within the container to a reactor section, specifically conveying the carbamate-containing liquid from the condensation section within the container to the reaction zone.

[0074] HP stripping towers are typically devices configured for countercurrent contact between a urea solution and a stripping gas stream. Typically, an HP stripping tower contains a urea solution and a stripping gas stream within tubes, with heat supplied by steam on the shell side. In this invention, the HP stripping tower uses all or part of the feedstock CO2 as the stripping gas. Stripping with CO2 as the stripping gas results in a decrease in the N / C ratio of the urea solution. The N / C ratio of the stripped urea solution is lower than that of the urea synthesis solution, for example, less than 3.0, less than 2.7, or even less than 2.5, such as in the range of 1.5-2.5 or 2.0-2.5.

[0075] In the facilities and methods of the present invention, the HP carbamate condenser (HPCC) is, for example, a shell-and-tube heat exchanger. The HPCC is, for example, a vertical condenser or a horizontal condenser. The shell-and-tube heat exchanger as an HPCC is operated with a process medium (specifically, the gas to be condensed) in the shell-side space and a cooling fluid in the tubes, or with a process medium in the tubes and a cooling fluid in the shell-side space.

[0076] In some embodiments, an HPCC is, for example, a falling film urethane condenser having a cooling fluid in the shell. In other embodiments, an HPCC is, for example, a submerged condenser. An HPCC is, for example, a shell-and-tube condenser having a horizontal U-shaped tube bundle, a process medium in the shell side, and a submerged tube bundle, and is, for example, a pool condenser. An HPCC includes, for example, a U-shaped tube bundle or a straight tube bundle. An HPCC is optionally a pool reactor, which includes a pool condenser zone and a reactor zone. A pool reactor includes, for example, baffles in the shell space.

[0077] HPCCs, for example, also include an inlet for feeding NH3 into the housing space.

[0078] Preferably, the flash unit (preferably the MP flash unit) is configured for (substantially) adiabatic flash evaporation. Preferably, the flash unit is a flash vessel.

[0079] Advantageously, by performing preferred (substantially) adiabatic flash evaporation from HP to MP in an MP flash evaporator (MPF), the resulting MP gas stream (flash vapor) has a low NH3:CO2 molar ratio, such as below 2.0, for example in the range of 0.8-1.2, and / or a lower N / C ratio than the urea solution stripped from HP. The flash vapor can advantageously have a relatively high CO2 concentration, allowing for more favorable N / C ratio operation in the MP condensation section.

[0080] In some embodiments, NH3 and CO2 are removed from the urea solution in the MP flash vessel at a molar ratio of less than 2.0, for example, 0.8-1.2. If CO2 is added to the gas stream, the first MP gas stream may have an even lower molar ratio of NH3 to CO2.

[0081] MP or LP processing units typically include a gas / liquid separation zone, such as in a preferred flash container, for example through a zone or flash container having a top outlet for gas and a bottom outlet for liquid, wherein the liquid is a urea solution.

[0082] The preferred adiabatic properties of flash evaporation in the preferred MP flash vessel advantageously contribute to a low N / C ratio (NH3 to CO2 molar ratio) of the resulting gas stream. Specifically, as the adiabatic pressure decreases, CO2 leaves the urea solution in the CO2 HP stripping tower to a greater extent than NH3.

[0083] The flash vapor has an N / C molar ratio (NH3 to CO2 molar ratio) preferably less than 2.0, for example 0.5-1.5, such as 0.9-1.2, for example about 1.0. The flash vapor contains an NH3 to CO2 molar ratio preferably less than 2.0, for example 0.5-1.5, such as 0.9-1.2, for example about 1.0. The first gas stream contains, for example, about 40%-50% by weight of NH3, about 40%-50% by weight of CO2, and for example 10%-20% by weight of H2O; these ranges also apply to components removed from the liquid phase of the urea solution when CO2 is added to the flash vapor. The flash vapor received by the MP urethane condenser (i.e. at the condenser inlet) preferably has such an N / C molar ratio (NH3 to CO2 molar ratio) and preferably contains, for example, about 40 wt% to 50 wt% of NH3, about 40 wt% to 50 wt% of CO2 and, for example, 10 wt% to 20 wt% of H2O.

[0084] A preferred MP heater (MP dissociator MPD) has an inlet for receiving high-pressure exhaust gas from the melamine synthesis section. The MPD is typically a heat exchanger that generally (and in addition to hot melamine exhaust gas) uses a heating fluid (e.g., steam) for indirect heat exchange to dissociate urethanes included in the urea synthesis solution. Using steam as the heating fluid in the MP dissociator offers the advantage of great flexibility in ensuring adequate removal of urethanes from the urea solution, specifically independent of the N / C ratio at the inlet of the MP dissociator. For example, the MP dissociator is a shell-and-tube heat exchanger, where steam is located in the shell and the urea solution is located in the tubes. The MP dissociator includes, for example, a distillation section arranged upstream of the shell-and-tube heat exchanger section for the urea solution, configured for gas / liquid separation of the urea solution expanded from HP to MP and for countercurrent contact between the urea solution and the gas stream from the heat exchange section. This facilitates good removal of urethanes from the urea solution.

[0085] The preferred medium-pressure urea product stream at the outlet of the MP dissociator, i.e., the MP urea solution, has an N / C ratio of at least 4, for example. The N / C ratio of the second MP gas stream from the MP dissociator is, for example, at least 2.5.

[0086] In some embodiments, the MP dissociator also receives other urea solution streams, such as urea solutions obtained indirectly from the HP stripping tower.

[0087] Carbamate dissociation vapors are supplied from the MP dissociator to the MP condenser section (MPCC). The MP condenser section has a liquid outlet for the carbamate solution.

[0088] The urea production area includes a gas flow path for directly or indirectly (preferably directly) conveying the flash-evaporated vapor to the MP condensation section, such that at least a portion (preferably all) of the gas is delivered as gas to the MP condensation section. Advantageously, the MP condensation section can be used to recover CO2 contained in the stripped urea solution.

[0089] In embodiments with preferred (substantially) adiabatic flash evaporation, at least a portion of the vapor from the flash evaporation can be used to correct (reduce) the N / C ratio in the MP condensation section. The combined urethane condensation of the flash vapor and the urethane dissociation vapor in the MP condensation section provides an optimal N / C ratio close to 2 for the formed condensate (i.e., the urethane solution). This makes it possible to recover CO2 in the form of the urethane solution. The urethane condensation is also advantageously achieved at relatively high temperatures (higher condensation points), which is advantageous for having a relatively low N / C ratio in the MP condensation section.

[0090] With the aid of the MP processing unit, CO2 included in the flash vapor (which originates from the HP stripper) can be used and recovered in the MP condensation section, resulting in lower vapor consumption α in the HP stripper at a constant stripping efficiency. This allows for a relatively larger supply of CO2 to the HP stripper. In a preferred embodiment with an ammonia consumption unit (e.g., an ammonium nitrate section), the HP stripper can advantageously operate at a relatively lower stripping efficiency α compared to a urea production section that only produces urea melt, because the NH3 contained in the stripped urea solution (also as a carbamate) and received by the preferred LP dissociator can be used in the preferred ammonia consumption unit, for example, in the preferred ammonium nitrate section. Those skilled in the art will understand that, in the context of a urea facility, a lower stripping efficiency α, as used in the art, can provide advantages.

[0091] The MP condensation section includes one or more MP urethane condensers and has a liquid outlet for the MP urethane solution, which is connected to a recirculation flow line, for example, leading to the HP synthesis section. The MP condensation section also includes an outlet for uncondensed gas. The gas is supplied to, for example, an absorber or scrubber, or, for example, a neutralization section of the optional ammonium nitrate section.

[0092] The MP condensation section may include an MP urethane condenser operated with a cooling fluid (e.g., a cooling liquid, such as cooling water), optionally as a second MP urethane condenser disposed downstream of a first MP urethane condenser, wherein the first condenser is, for example, thermally integrated with a pre-evaporator. The second condenser is, for example, a shell-and-tube heat exchanger. The second condenser receives both vapor and liquid, for example, from the first MP urethane condenser. The MP condensation section further includes a gas / liquid separator, specifically for separating the urethane solution from the uncondensed gas.

[0093] The MP condensation section preferably receives a water stream, such as ammonia, for example, water from the wastewater treatment section, or, for example, steam condensate. This advantageously prevents the crystallization of carbamates. Optionally, the MP condensation section receives, for example, an LP carbamate solution from an optional LP carbamate condenser (if used).

[0094] The aforementioned systems for the production of urea and melamine can be designed as new (basic) facilities. Systems can also be provided by constructing a new melamine facility connected to a pre-existing urea facility. Alternatively, the system includes a new urea facility to be connected to a pre-existing urea facility. The system of the present invention can also be provided by connecting a pre-existing urea facility and a pre-existing melamine facility. For example, a pre-existing urea facility having sections including a flash evaporation unit and a heating unit for dissociating flashed urea liquid can be provided with melamine exhaust gas connections to said heating unit, preferably an MP heating unit. Pre-existing urea facilities without MP or LP flash evaporation can be modified to include an MP or LP flash evaporation unit for dissociating flashed urea liquid, an ammonia and carbon dioxide release unit (such as an MP heater), and connections for connecting melamine exhaust gas to such an MP heater. Furthermore, the present invention relates to the modification (reorganization) of pre-existing systems for the combined or integrated production of urea and melamine. For example, a pre-existing system including connected HP melamine and urea production areas and having melamine exhaust gas connected to the urea production area (such as the MP condensation section of the urea production area) can be configured with melamine exhaust gas (to replace or possibly supplement the aforementioned connection) connected to the MP or LP section of the urea production area, preferably the ammonia and carbon dioxide release unit in the MP section, and has (or should be provided with) a flash evaporation unit, preferably an MP flash evaporation unit, for conveying flashed urea liquid to the ammonia and carbon dioxide release unit. Furthermore, a pre-existing system including connected HP melamine and urea production areas and having melamine exhaust gas connected to the HP urea synthesis section can be modified by adding a flash evaporation unit with a heater, preferably an MP flash evaporation unit with a heater (as described above), and providing a melamine exhaust gas connection to the heater to replace or possibly supplement the aforementioned connection.

[0095] The invention is further illustrated below with reference to the non-limiting accompanying drawings. In these drawings, numbered reference numerals indicate equipment parts (“units”) and letters indicate process flows (“flows”).

[0096] unit:

[0097] 1 HP urea synthesis section

[0098] 2 HP Melamine Synthesis Section

[0099] 3 flash evaporation units

[0100] 4 Ammonia and Carbon Dioxide Release Units

[0101] 5 LP condensation section

[0102] 6 MPa condensation section

[0103] 7. Filled Bed

[0104] flow:

[0105] a urea synthesis solution

[0106] b HP Melamine exhaust gas

[0107] c Flash steam

[0108] d. Urea liquid after flash evaporation

[0109] e',e” ammonia and carbon dioxide vapor

[0110] f MP urea product solution

[0111] g LP carbamate solution

[0112] h LP urea product flow

[0113] exist Figure 1The diagram shows a urea synthesis solution (a) from the HP urea synthesis section (1) undergoing flash evaporation in the MP flash unit (3). This produces flash vapor (i.e., removed ammonia and carbon dioxide vapor (e')) and flashed urea liquid (d). The flashed urea liquid is then conveyed to the upper inlet of the MP heating unit, i.e., the ammonia and carbon dioxide release unit (4). This unit is typically a shell-and-tube heat exchanger in which heat is typically supplied on the shell side by a heat exchange fluid (such as steam or hot condensate (“HEAT”)) that leaves the unit after exchanging heat (“exHEAT”). According to the invention, exhaust gas (b) from the high-pressure melamine facility (2) is conveyed to the MP heating unit (tube side) and comes into countercurrent contact with the flashed urea liquid therein. Heating the flash-evaporated urea liquid causes the ammonium carbamate present in the flash-evaporated urea liquid to decompose, thereby releasing ammonia and carbon dioxide vapors (e”), which are removed together with the aforementioned flash vapors. The urea liquid from which the decomposed carbamate has been removed is obtained as the MP urea product solution (f). Depending on the sensible heat available from the melamine exhaust gas and the heat requirements in the MP heating unit, the application of the heat exchange fluid can be reduced or abandoned, or the heat exchange fluid can be fully utilized in combination with the heat provided by the melamine exhaust gas. This reflects the general possibilities for carrying out the invention and is further applicable to all figures.

[0114] exist Figure 2 The figure shows a urea synthesis solution (a) from the HP urea synthesis section (1) undergoing flash evaporation in the MP flash unit (3). This produces flash vapor (i.e., removed ammonia and carbon dioxide vapor (e')) and flashed urea liquid (d). The figure further shows the HP melamine synthesis section from which HP melamine exhaust gas (b) is obtained. The flashed urea liquid (d) and melamine exhaust gas (b) are combined and conveyed to the lower inlet of the MP heating unit, i.e., the ammonia and carbon dioxide release unit (4). This unit is typically a shell-and-tube heat exchanger in which heat is typically supplied on the shell side by a heat exchange fluid (such as steam or hot condensate (“HEAT”)) that leaves the unit after exchanging heat (“exHEAT”). According to the invention, the flashed urea liquid (d) is guided through the MP heating unit (tube side). By contacting with hot melamine exhaust gas (b) and optionally with heat from a heat exchange fluid, the ammonium carbamate contained in the flash-evaporated urea liquid undergoes decomposition, and thus undergoes the release of ammonia and carbon dioxide vapors (e”), which are removed by the aforementioned flash vapors. The urea liquid from which the decomposed carbamate has been removed is obtained as MP urea product solution (f).

[0115] Figure 3 It shows the relationship with Figure 2 The same setup applies, with the same description applicable. Furthermore, as shown in the figure, the MP urea product solution (f) is conveyed to the low-pressure recovery section (5). From this section, the LP urea product stream (h) and the LP carbamate solution (g) are obtained. The latter is conveyed (via a pump, not shown) to the MP condensation section (6), where it combines with the condensate of ammonia and carbon dioxide vapors (e' and e'") to form MP carbamate (not shown).

[0116] Figure 4 A single flash evaporation and heating device that can be used in operation of the present invention is shown. A urea synthesis solution (a) is shown entering the device via the top inlet. In the flash evaporation unit (3) of the device, the urea solution is flashed. This produces flash vapor (i.e., ammonia and carbon dioxide vapor (e') removed at the top of the device) and flashed urea liquid (d). The latter is directed through a packed bed (7) and conveyed to the bottom inlet of the device. Further shown is HP melamine exhaust gas (b) combined with the flashed urea liquid, the combined flow then entering the device via the bottom inlet. This is the lower inlet of the MP heating unit (i.e., the ammonia and carbon dioxide release unit (4)). This unit is typically a shell-and-tube heat exchanger, in which heat is typically provided on the shell side by a heat exchange fluid (such as steam or thermal condensate (“HEAT”)) that leaves the unit after exchanging heat (“exHEAT”). According to the present invention, the flashed urea liquid (d) is directed through the MP heating unit (tube side). By contacting the hot melamine exhaust gas (b) and optionally also by heat from the heat exchange fluid, the ammonium carbamate contained in the flashed urea liquid is decomposed and thus subjected to the release of ammonia and carbon dioxide vapors (e”), which are conveyed across the packed bed (7), thereby acting as stripping agents for the flashed urea liquid (d) and being removed together with the flash vapors (e’) at the top of the equipment.

Claims

1. A process for the production of urea and melamine, wherein the production of urea comprises subjecting ammonia and carbon dioxide to a urea formation reaction in a high pressure urea synthesis section to obtain a high pressure urea synthesis solution, subjecting the urea synthesis solution to flashing at medium or low pressure of 3 to 80 bar, thereby producing a flash vapor comprising ammonia and carbon dioxide and a flashed urea liquid; subjecting the flashed urea liquid to a treatment to obtain a urea product solution and a carbamate recovery vapor, the treatment comprising one or more ammonia and carbon dioxide release steps, whereby ammonia and carbon dioxide vapor is removed from the flashed urea liquid, wherein the production of melamine comprises subjecting urea to a non-catalytic high pressure melamine formation reaction to obtain melamine and a melamine off-gas comprising ammonia and carbon dioxide; and wherein the combined process comprises: At least a portion of the melamine off-gas is contacted with the flashed urea liquid prior to or during at least one of the ammonia and carbon dioxide release steps.

2. The process of claim 1, wherein the flashing is conducted at medium pressure and includes subjecting the flashed urea liquid to treatment in a medium pressure treatment section to obtain a medium pressure urea product solution and a medium pressure carbamate recovery vapor.

3. The process of claim 1, wherein at least a portion of the high pressure urea synthesis solution is obtained from subjecting effluent from the urea formation reaction to high pressure stripping.

4. The process of claim 3, wherein the high pressure stripping employs carbon dioxide as a stripping medium.

5. The process of any one of claims 1-3, wherein the flashing vapor and the carbamate recovery vapor are subjected to condensation to form a carbamate solution, and the carbamate solution is subjected to recycle to a high pressure synthesis section.

6. The process of any one of claims 1-3, comprising: subjecting the urea product solution to recovery of ammonia and carbon dioxide still present at low pressure to obtain a low pressure carbamate solution and a low pressure urea product stream.

7. The process of claim 5, wherein the carbamate liquid obtained from melamine synthesis is combined with at least one carbamate solution.

8. The process of any one of claims 1-3, wherein obtaining a medium pressure urea product stream includes medium pressure stripping.

9. The process of claim 8, wherein the medium pressure stripping employs carbon dioxide as a stripping medium.

10. The process of any one of claims 1-3, wherein at least a portion of the urea produced is the urea subjected to the melamine formation reaction.

11. The process of any one of claims 1-3, comprising: subjecting the melamine off-gas to contact with the flashed urea liquid in a counter-current manner.

12. The process of any one of claims 1-3, wherein the melamine off-gas contacted with the flashed urea liquid has a temperature of 150°C to 300°C.

13. The process of claim 12, wherein the melamine off-gas has a temperature of 200°C to 250°C.

14. A system for the production of urea and melamine, the system comprising a urea production zone; the urea production zone comprising a high pressure urea synthesis section and a urea recovery section downstream of and in fluid communication with the high pressure urea synthesis section, the urea recovery section being adapted to separately obtain a urea solution and a carbamate recovery stream; the system further comprising a melamine production zone; the melamine production zone comprising a high pressure melamine synthesis section, the production zones being connected to each other so as to allow the transport of melamine off-gas obtained from the melamine synthesis section to the urea production zone, wherein the urea production zone comprises a medium or low pressure treatment section adapted to receive the melamine off-gas, the treatment section comprising a flash unit having an inlet for urea synthesis solution connected to an outlet of the high pressure synthesis section via a pressure reduction device, an outlet for flash vapor and an outlet for flashed liquid, and at least one ammonia and carbon dioxide release unit having an inlet for flashed liquid connected to the outlet for flashed liquid from the flash unit, an outlet for carbamate recovery vapor and an outlet for a urea product stream, the outlets for flash vapor and carbamate recovery vapor each being connected to the urea synthesis section via a condensation and recycle loop, wherein the outlet for melamine off-gas from the melamine synthesis section is in fluid communication with the inlet of at least one ammonia and carbon dioxide release unit, allowing the melamine off-gas to contact the flashed liquid in or upstream of the ammonia and carbon dioxide release unit.

15. The system according to claim 14, wherein the at least one ammonia and carbon dioxide release unit is a heating unit.

16. The system according to claim 14, wherein the urea production zone comprises an outlet for urea connected to an inlet of the melamine synthesis section.

17. The system according to claim 15, wherein the flash unit and the heating unit are connected via a rectification column.

18. The system according to claim 17, the flash unit, the rectification column and the heating unit being housed in a single apparatus.

19. The system according to claim 17, wherein the rectification column comprises a packed bed, and wherein the outlet for melamine off-gas from the melamine synthesis section is in fluid communication with an inlet of the rectification column, the inlet being positioned to allow the off-gas to flow across the packed bed.

20. The system according to claim 17 or 19, wherein the outlet for melamine off-gas from the melamine synthesis section is in fluid communication with an inlet of the heating unit.

21. The system according to claim 20, wherein the inlet of the heating unit is positioned at a lower end of the heating unit.

22. A process for retrofitting a pre-existing urea plant comprising a high pressure urea synthesis section and a urea recovery section downstream of and in fluid communication with the high pressure urea synthesis section, the urea recovery section adapted to separately obtain a urea solution and a carbamate recovery stream, the process comprising connecting the urea plant to a melamine plant comprising a high pressure melamine synthesis section so as to allow for recycling of melamine offgas from a melamine offgas treatment section to the urea plant, the process comprising adding a medium or low pressure treatment section adapted to receive the melamine offgas, the treatment section comprising a flash unit having an inlet for urea synthesis solution connected via a pressure reduction device to an outlet of the high pressure synthesis section, an outlet for flash vapor and an outlet for flashed liquid, and at least one ammonia and carbon dioxide release unit having an inlet for flashed liquid connected to the outlet for flashed liquid from the flash unit, an outlet for carbamate recovery vapor and an outlet for a urea product stream, the outlets for flash vapor and carbamate recovery vapor each connected via a condensation and recycle loop to the urea synthesis section, wherein an outlet for melamine offgas from the melamine synthesis section is in fluid communication with the inlet of the ammonia and carbon dioxide release unit, allowing for contact of the melamine offgas with the flashed liquid in or upstream of the ammonia and carbon dioxide release unit.

23. The method of claim 22, comprising: providing a connection between an outlet for urea from the urea production section to an inlet of the melamine synthesis section.

24. A method for retrofitting a pre-existing system for the production of urea and melamine, said pre-existing system comprising a urea production zone and a melamine production zone; said urea production zone comprising a high pressure urea synthesis section in fluid communication with a urea recovery section adapted to separately obtain a urea solution and a carbamate recovery stream, said recovery section comprising a medium or low pressure treatment section, said medium or low pressure treatment section comprising a flash unit having an inlet for a urea synthesis solution connected via a pressure reduction device to an outlet of said high pressure synthesis section, an outlet for a flash vapor and an outlet for a flashed liquid, and an ammonia and carbon dioxide release unit having an inlet for a flashed liquid connected to said outlet for a flashed liquid from said flash unit, an outlet for a carbamate recovery vapor and an outlet for a urea product stream, said outlets for a flash vapor and a carbamate recovery vapor each being connected to said urea synthesis section via a condensation and recycle loop, said melamine production zone comprising a high pressure melamine synthesis section and a melamine off-gas treatment section downstream of and in fluid communication with said high pressure melamine synthesis section for obtaining a pressure-reduced melamine off-gas; said production zones being connected to each other so as to allow for the transport of melamine off-gas from said melamine off-gas treatment section to said urea production zone, said method comprising providing a connection between an outlet for melamine off-gas from said melamine synthesis section to an inlet of said ammonia and carbon dioxide release unit, thereby allowing said melamine off-gas to contact the flashed liquid in or upstream of said ammonia and carbon dioxide release unit.

25. The method according to claim 24, said production zones being connected to each other further allowing for the transport of urea from said urea production zone to said melamine synthesis section.

Images