Production of urea and melamine

Abstract

The present disclosure relates to a process for the production of urea and melamine in which off-gas from a high pressure melamine plant is combined with a urea liquid obtained from the flash evaporation of a urea synthesis solution. A system for combined production and retrofit to existing facilities or systems are also disclosed.

Description

Technical Field

[0001] The present invention relates to the combined production of urea and melamine, wherein off-gases from the production of melamine are used as feed for the urea synthesis. In particular, the present invention relates to an integrated process and system for the production of urea and melamine. Background Art

[0002] Urea is typically produced from ammonia and carbon dioxide. From there, carbon dioxide and ammonia react in a high pressure (HP) urea synthesis section to form a urea synthesis solution. In understanding various aspects of a urea process and production facility, it is often instructive to understand the underlying reactions. Ammonia and carbon dioxide first react to form ammonium carbamate according to an exothermic reaction:

[0003] 2NH3+CO2→H2N–CO–ONH4.

[0004] In the second step, the ammonium carbamate formed is dehydrated to give urea according to an endothermic equilibrium reaction:

[0005]

[0006] The formed urea synthesis solution is subjected to the recovery of unreacted ammonia and carbon dioxide. This can be achieved in several ways, which are usually applied in various combinations in urea facilities. Therefore, in a urea stripping facility, the urea synthesis solution is subjected to stripping and condensation in the HP section, thereby recovering ammonia and carbon dioxide. 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 and operated at medium pressure (MP) and / or low pressure (LP). The recovered ammonia and carbon dioxide are finally reused in the synthesis section as ammonium carbamate solution, thereby completing the urea synthesis loop. The urea solution is usually subjected to evaporation to form a highly concentrated solution commonly referred to as a urea melt. The urea melt can be used in connected facilities (such as melamine facilities) and / or can be subjected to fine processing into solid forms, such as pellets or granules.

[0007] The challenge in the above-mentioned recovery processes is generally to separate off the unreacted ammonia and carbon dioxide as efficiently as possible and to reuse them with as little water as possible. The latter is important because, as can be seen from the above-mentioned second reaction step, the presence of water will shift the equilibrium unfavorably towards the carbamate side. To this end, in particular, the various recovery stages generally involve a treatment step in which the urea solution is subjected to a decomposition (dissociation) of the ammonium carbamate still present into its components, ammonia and carbon dioxide, which are subsequently condensed and further reused. As far as the reuse of ammonium carbamate is concerned, it will be appreciated that ammonium carbamate requires a minimum amount of water to remain dissolved, which amount increases as the pressure decreases.

[0008] Additional factors that are typically considered in the various recovery and recycling steps are efficient use of energy, for example with reference to the heat required for carbamate decomposition, and the recycling ratio of N (ammonia) and C (carbon dioxide component). With respect to the latter, it will be appreciated that the urea formation reaction involves a 2:1 N:C ratio, but the overall ratio for carrying out the urea synthesis section typically involves excess ammonia, for example a 3:1 N:C ratio.

[0009] Melamine is typically produced from urea, thereby producing an off-gas (hereinafter also referred to as "melamine off-gas") comprising ammonia and carbon dioxide as by-products (in the 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 applies atmospheric pressure (from atmospheric pressure to about 70 bar). The other is known as a "high pressure" process. In melamine production, high pressure refers to pressures equal to or higher than about 70 bar (7 MPa), more typically equal to or higher than 80 bar (8 MPa).

[0010] It is well known in the art to integrate the production of urea with the production of melamine. Thus, typically, at least a portion or all of the urea produced by a urea production facility is sent to a melamine facility as starting material. Thus, the integrated production of urea and melamine typically involves feeding the off-gas obtained from the melamine facility directly or indirectly to the synthesis section of the urea facility.

[0011] A persistent challenge in this area is to find a suitable location for the melamine offgases to enter the urea synthesis loop. A particular problem with the offgases from the high pressure melamine process is that the pressure of the offgases does not match the operating pressure of any conventional section of a urea production facility. The offgases produced in the high pressure melamine synthesis process are initially obtained at a synthesis pressure, which is typically at least 70 bar, and typically in the range of 80 to 150 bar. In many cases, this pressure is lower than the pressure of the HP urea synthesis section, which is typically in the range of 120 to 400 bar, and more typically 140 to 160 bar. Further, in some cases, the offgases from the melamine synthesis are not actually released at the synthesis pressure, but at a lower pressure, typically 2-30 bar, such as 20-25 bar. This is usually achieved as a result of downstream processing or work-up, such as a washing step, as a result of which the offgas released at low pressure contains, besides ammonia and carbon dioxide, a large amount of water, such as 3 to 40% by weight and more particularly 5 to 30% by weight. It is known to recycle these offgases to an MP section, which is already present in a urea plant or otherwise in an integrated plant connecting the two production processes.

[0012] A process of this type is disclosed in WO2008 / 052640. In this process, the off-gas gas stream from a melamine plant is condensed together with the recycled ammonium carbamate solution in a condensation section of the medium-pressure treatment section of a urea plant. This presents a passive method generally used for recycling melamine off-gases to urea synthesis.

[0013] It would be desirable to utilize the melamine off-gas in a different or even better way as an active component in carbamate recovery and / or to make better use of the available energy of the off-gas. Summary of the invention

[0014] In order to better address one or more of the aforementioned expectations, the present invention provides, in one aspect, a process for the production of urea and melamine, wherein the production of urea comprises subjecting ammonia and carbon dioxide to a urea forming reaction in a high-pressure urea synthesis section so as to obtain a high-pressure urea synthesis solution, subjecting the urea synthesis solution to flash evaporation, preferably at medium pressure, thereby producing flash vapor comprising ammonia and carbon dioxide and a flashed urea liquid; subjecting the flashed urea liquid to treatment, preferably in a medium-pressure treatment section, so as to obtain a urea product solution, preferably a medium-pressure urea product solution, and carbamate recovery vapours, preferably medium pressure carbamate recovery vapours, the treatment comprising one or more ammonia and carbon dioxide release steps, thereby removing ammonia and carbon dioxide vapours from the flashed urea liquid, wherein the production of melamine comprises subjecting urea to a non-catalytic high pressure melamine forming reaction so as to obtain melamine and a melamine off-gas comprising ammonia and carbon dioxide; and wherein the combined process comprises: contacting at least a portion of the melamine off-gas with the flashed urea liquid before or during at least one of the ammonia and carbon dioxide release steps.

[0015] In another aspect, the present invention provides a system for the production of urea and melamine, said system comprising a urea production zone; said urea production zone comprising a high-pressure urea synthesis section and a urea recovery section located downstream of the high-pressure urea synthesis section and in fluid communication with the high-pressure urea synthesis section, said urea recovery section being suitable for separately obtaining a urea solution and a carbamate recovery stream; said system further comprising a melamine production zone; said melamine production zone comprising a high-pressure melamine synthesis section, said production zones being connected to each other so as to allow the melamine off-gas obtained from the melamine synthesis section to be conveyed to the urea production zone, wherein the urea production zone comprises a medium-pressure or low-pressure treatment section suitable for receiving said melamine off-gas, said treatment section comprising a flash unit and at least one ammonia and carbon dioxide release unit 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 vapour and an outlet for a urea product stream, said outlets for flash vapour and carbamate recovery vapour being each connected to the urea synthesis section via a condensation and recirculation loop, wherein an outlet for melamine offgases from the melamine synthesis section is in fluid communication with an inlet of at least one ammonia and carbon dioxide release unit, thereby allowing contact of the melamine offgases with the flashed liquid in or upstream of the ammonia and carbon dioxide release unit.

[0016] In yet another aspect, the present invention relates to a method for retrofitting 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 the high-pressure urea synthesis section 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 method comprising connecting the urea facility to a melamine facility comprising a high-pressure melamine synthesis section so as to allow recirculation of melamine off-gas from the melamine off-gas treatment section to the urea facility, the method comprising adding a medium-pressure or low-pressure treatment section adapted to receive said melamine off-gas, the treatment section comprising a flash unit and at least one ammonia and carbon dioxide release unit, the flash unit having a urea recovery stream and a melamine off-gas treatment section adapted to receive said melamine off-gas. an inlet for urea synthesis solution, an outlet for flash vapour and an outlet for flashed liquid connected to the outlet of the high pressure synthesis section by a pressure reducing device, said 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 vapour and an outlet for a urea product stream, said outlets for flash vapour and carbamate recovery vapour each being connected to the urea synthesis section via a condensation and recirculation loop, wherein an outlet for melamine off gases from the melamine synthesis section is in fluid communication with the inlet of the ammonia and carbon dioxide release unit, thereby allowing contact of the melamine off gases with the flashed liquid in or upstream of the ammonia and carbon dioxide release unit.

[0017] In still another aspect, the present invention provides a method for retrofitting 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 section in fluid communication with a urea recovery section, the urea recovery section being adapted to separately obtain a urea solution and a carbamate recovery stream, the recovery section comprising a medium-pressure or low-pressure processing section comprising a flash unit and an ammonia and carbon dioxide release unit, the flash unit having 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 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 flashed liquid at the outlet of the urea product stream, said outlets for flash vapour and carbamate recovery vapour each being connected to a urea synthesis section via a condensation and recirculation loop, said melamine production zone comprising a high-pressure melamine synthesis section and a melamine off-gas treatment section located downstream of the high-pressure melamine synthesis section and in fluid communication with the high-pressure melamine synthesis section, said melamine off-gas treatment section being used to obtain reduced-pressure melamine off-gas; said production zones being connected to each other so as to allow the melamine off-gas from the melamine off-gas treatment section to be conveyed to the urea production zone, and optionally also to allow the urea from the urea production zone to be conveyed to the melamine synthesis section, the method comprising providing a connection between the outlet for melamine off-gas from the melamine synthesis section to the inlet of an ammonia and carbon dioxide release unit, thereby allowing the melamine off-gas to be contacted with the flashed liquid in or upstream of the ammonia and carbon dioxide release unit. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0019] Figure 4 and Figure 5 is a schematic diagram showing the use of a single flash evaporation and heat exchange apparatus in a system for the production of urea and melamine. DETAILED DESCRIPTION

[0020] The present invention is based on the ingenious insight of utilizing the sensible heat of the offgases from high-pressure melamine production to support the decomposition of carbamates present in the urea synthesis solution. To this end, the offgases are combined with the urea liquid obtained from the flash distillation of the urea synthesis solution. This is significantly different from existing processes for the combined production of urea and melamine, in which the melamine offgases are usually conveyed to a condensation step in the urea production process.

[0021] Flash evaporation of a liquid such as a urea synthesis solution implies that such liquid expands at a pressure lower than the original pressure. Urea synthesis solution originates from a section operating at high pressure (HP). In the context of urea production processes and facilities, flash evaporation occurs in a section operating at medium pressure (MP) or low pressure (LP). These terms each have a clear meaning in the field of urea production. HP generally indicates a pressure of 12-40 MPa (120 to 400 bar), preferably 140 to 160 bar. MP in urea production generally 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 lower than 1 MPa, preferably 0.3-0.5 MPa (3-5 bar), typically about 4 bar.

[0022] In relation to melamine production, the term "HP" generally indicates a pressure at or above 7 MPa (70 bar), preferably at or above 8 MPa (80 bar), and more preferably from 80 to 150 bar.

[0023] In the present process, at least a portion of the melamine offgas is contacted with the flashed urea liquid in one or more ammonia and carbon dioxide release steps. Urea production processes usually include such steps in order to optimize the recovery of unreacted ammonia and carbon dioxide. Such 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, usually removed as vapor and eventually recycled to the 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 vapor to more strongly favor the latter. One way to achieve this is thermal decomposition (or in other words, dissociation) of the ammonium carbamate, thereby enabling the removal of gaseous ammonia and carbon dioxide. Another way is to apply a stripping gas to the liquid in order to remove carbon dioxide and ammonia vapor. Typically, heat and stripping gas are applied simultaneously. This is well known to the skilled person and does not need to be explained here.

[0025] In the process of the present invention, melamine waste gas is contacted with flashed urea liquid in the ammonia and carbon dioxide release step. This contact may promote one or both of the above-mentioned phenomena.

[0026] The off-gas from the high pressure melamine process has a higher temperature than the urea solution obtained by flashing a urea synthesis solution at medium or low pressure. Therefore, the off-gas will act as a direct heating agent when introduced into the flashed urea liquid.

[0027] Flashing is carried out at medium pressure or low pressure, usually at a pressure of 3 to 80 bar. Medium pressure flashing (MP flashing) is usually preferred. This preference relates to the general concept that the purpose of a process step such as flashing is to recover and recycle unreacted ammonia and carbon dioxide (usually as ammonium carbamate) to urea synthesis. Taking into account the urea synthesis balance, it is usually preferred to recycle as little water as possible to urea synthesis. The condensation of medium pressure flash vapor requires less water than the condensation of low pressure flash vapor. Flashing is preferably carried out at 10-70 bar, preferably 10-35 bar, for example 18-25 bar.

[0028] The flash is preferably operated at a pressure equal to or higher than the pressure of the ammonia and carbon dioxide release unit, for example a pressure 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 operated at a pressure equal to the pressure at which the ammonia and carbon dioxide release unit is operated. The temperature of the flashed urea liquid (i.e. the temperature of the liquid downstream of the unit in which the flash occurs) is related to the pressure. 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, the flash is carried out at 20-30 bar, such as at 25 bar. At 25 bar, the temperature of the flashed liquid is 148°C. Pressures expressed in bar refer to absolute pressure values ​​(bara).

[0029] Melamine offgas can also be introduced into the flashed urea liquid in such a way that it effectively acts as stripping gas. This can be achieved in an apparatus designed as a stripping unit, such as a plate tower or a packed tower, or generally in a unit comprising a packed bed.

[0030] The contact discussed occurs in the ammonia and carbon dioxide release step. This is usually carried out in the corresponding treatment unit, i.e., ammonia and carbon dioxide release unit, such as a heating unit (usually a heat exchange unit), a packed bed or a combination thereof. Such units have an inlet for liquid (i.e., in this case, urea liquid through flashing), an outlet for treated liquid and an outlet for gas, so that ammonia and carbon dioxide can be removed, i.e., carbamate recovery steam. 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 usually be in the form of a shell and tube heater, and its many different designs are known to technicians. In this article, the medium to be heated is located in a compartment (such as a tube side), and the heat exchange medium is arranged in another compartment (such as a shell side). Generally, the medium to be heated (in this case, urea liquid through flashing) will be transferred via the tube side, and an external heating medium (such as a hot liquid or steam) will flow on the shell side. The release of ammonia and carbon dioxide necessarily involves the decomposition (dissociation) of carbamate. Therefore, the ammonia and carbon dioxide release unit is sometimes also referred to as a decomposer or dissociator. Unless otherwise stated, in this specification the terms "heating unit", "heat exchange unit", "heater" (such as "MP heater"), "decomposer" (such as "MP decomposer") and "dissociator" (such as "MP dissociator") are all used interchangeably with the term "ammonia and carbon dioxide release unit". In an embodiment, the ammonia and carbon dioxide release unit may be a packed bed. Such a packed bed may be operated with heat, with a stripping gas (such as carbon dioxide) for adiabatic stripping, or both. In the present invention, the melamine waste gas may provide heat and serve as a stripping gas on such a packed bed. Packed beds are known in the industry, and their possible structures and materials (such as metal, ceramic, glass) need not be elucidated here.

[0031] On account of its higher temperature and pressure, the melamine offgases introduced into the flashed urea solution will directly transfer heat to the solution, thereby contributing to the decomposition of the ammonium carbamate. The offgases of melamine production are therefore advantageously present in the same compartment (usually the tube side of a shell and tube heat exchanger, while steam or hot liquid may additionally be provided on the shell side) in direct contact with the solution to be heated. Depending on the precise conditions and heat requirements, it is envisaged in the present process that the ammonia and carbon dioxide release units into which the hot melamine offgases are injected are operated with or without an external heating medium.

[0032] The skilled person will be able to determine the precise heat requirement based on the heat and pressure of the melamine off-gas and the conditions in the associated medium or low pressure ammonia and carbon dioxide release step(s).

[0033] The off-gas produced in the high-pressure melamine synthesis process is initially obtained at a synthesis pressure, which is generally at least 70 bar and is generally in the range of 80 to 150 bar. When introduced into the medium-pressure (MP) or low-pressure (LP) section of the urea plant, the pressure of the melamine off-gas is reduced by an expansion valve at the inlet of said MP or LP section. Typical conditions of the melamine off-gas introduced into the ammonia and carbon dioxide release unit are 250° C., 80 bar. In some types of melamine production plants, which are also generally suitable for carrying out the process disclosed in the present invention, the melamine off-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 off-gas obtained from the high-pressure melamine synthesis section can vary greatly. Typically, such melamine off-gas is discharged at a temperature of 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 when carrying out the process of the present invention, the temperature of the melamine off-gas before contact with the flashed urea liquid is higher than the temperature of said liquid.

[0035] By introducing the melamine off-gas into the urea solution during the ammonia and carbon dioxide release step, the available heat from the HP melamine off-gas is well used while less heat needs to be provided by a heat exchange medium (typically steam or a hot liquid stream such as condensate).

[0036] Typically, the ammonia and carbon dioxide release step (particularly the step leading to the decomposition of ammonium carbamate) is carried out in an MP or LP heating unit, which is a shell and tube heat exchanger, in which the medium to be heated is located in one compartment (such as the tube side) and the heat exchange medium is arranged in another compartment (such as the shell side). However, the waste gases of melamine production are advantageously present in the same compartment (usually the tube side of the shell and tube heat exchanger, while steam or hot liquid can be additionally arranged on the shell side as required) in direct contact with the solution to be heated. The introduction of the melamine waste gases into the urea liquid can be carried out co-currently with the flow of the urea liquid, or counter-currently.

[0037] It should be understood that in the present process, the melamine waste gas interacts with the flashed urea liquid during the ammonia and carbon dioxide release step, in particular the stripping step and / or preferably the decomposition step. The steps produce a liquid output and a gaseous output. The liquid output can be considered as a medium-pressure 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 be carried out, for example, 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 the decomposition (as described below) or downstream thereof (which is a typical existing configuration, for example, as disclosed in WO 02 / 090323).

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

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

[0040] It should be understood that the purpose of obtaining carbamate recovery vapor is to eventually recycle the recovered ammonia and carbon dioxide to urea synthesis. Such recycling usually involves condensing the carbamate recovery vapor at medium or low pressure to produce MP or LP carbamate solution. The latter is usually recycled via one or more additional processing steps, in particular a low-pressure recovery step, according to the technology widely available in the art.

[0041] The resulting condensate will be further processed in order to be finally recycled to the HP synthesis section, which optionally involves one or more compression steps.

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

[0043] Typically, in addition to the above-mentioned off-gases, melamine production may also produce a liquid carbamate stream, for example by condensing part of the off-gases in the melamine plant. Such a stream may be recycled to urea production, the preferred location of which depends on the composition of the stream (for example, some melamine processes produce a stream comprising 40% ammonia, 40% carbon dioxide (i.e. 80% carbamate) and 20% water). It is also conceivable to produce a leaner carbamate stream (such as 50% carbamate and 50% water). According to the invention, in addition to the recycling of the gaseous effluent of the melamine production, such additional liquid effluent may also typically be recycled to urea production, for example to the low-pressure recovery section.

[0044] The feature connecting the melamine and urea production process according to the invention is effectively the MP or LP of the flashed urea synthesis solution, and preferably the MP decomposition. It will be appreciated that the flashed nature of this solution, with the removal of ammonia and carbon dioxide vapours, enables the addition of further quantities of ammonia and carbon dioxide obtained from the melamine production process. Thus, the process is applicable to any urea production zone in which the urea synthesis solution is subjected to MP or LP flashing.

[0045] The process as described above combines the production of urea and melamine by using the melamine waste gas in the urea production. Preferably, the further integration of the two processes involves using some or all of the produced urea as the starting material for melamine production. This may refer to an integrated facility with a urea production area and a melamine production area, and their mutual production capacity is adapted to each other. It is also possible that the melamine production has a relatively high production capacity, for example, by also feeding urea from another urea facility. This will enable a larger amount of melamine waste gas to be transmitted to the urea facility. Considering the heat demand in one or more ammonia and carbon dioxide release steps in the urea production, if a large amount of waste gas from the HP melamine production can be used to combine with the flashed urea liquid in the urea production, it will generally be of greater benefit.

[0046] In general terms for a urea production process, the process comprises subjecting ammonia and carbon dioxide to a urea forming reaction in a reactor of a high pressure urea synthesis section of a urea production area, such as a urea production facility, in order to obtain a urea synthesis solution.

[0047] With reference to the above-mentioned connection features, the carbamate recovery in the process according to the invention comprises subjecting the urea synthesis solution to flash evaporation at medium pressure (MP). Preferably, in one embodiment, this solution is a solution obtained from HP stripping. This stripping can be carried out using ammonia as stripping medium, providing heat only ("self-stripping"), or in particular with carbon dioxide or ammonia and with heat. The most widely used stripping process today is the Stamicarbon stripping process, using CO2 as stripping medium, effectively introducing at least a portion of CO2 as feed for urea synthesis via an HP stripping tower. HP stripping results in the removal of an initial amount of unreacted ammonia and carbon dioxide, which are subsequently subjected to condensation in the HP carbamate condenser. Thereafter, the condensate is recycled to the reactor. The stripped urea solution, which also contains ammonium carbamate, is subjected to further recovery of ammonia and carbon dioxide at medium and low pressures.

[0048] It should be understood that the above-mentioned flash steam and carbamate recovery steam will eventually be recycled to urea synthesis. Flash steam and carbamate recovery steam will usually be subjected to condensation in separate MP and / or LP condensation sections or in the same MP or LP condensation section. Such condensation section includes at least one condenser, such as two condensers. Preferably, flash steam and carbamate recovery steam are sent to the same condenser, preferably to the MP condenser. The resulting condensate will be further processed so as to be eventually recycled to the HP synthesis section. This can be direct, involving appropriate compression steps, or the condensate can be advantageously mixed with LP carbamate recycled from the LP recovery section. As will be appreciated by those skilled in the art of urea production, LP carbamate necessarily contains a relatively large amount of water. In order to desirably reduce the amount of water transferred to urea synthesis, the present process enables the concentration of LP carbamate to be increased by mixing LP carbamate with the condensate obtained from carbamate recovery steam. In a preferred embodiment of the present invention, carbamate recovery steam is condensed in a single condenser together with LP carbamate. Alternatively, the vapors from the flash may be condensed alone in a first condenser, and the carbamate recovery vapors may be combined with the LP carbamate and condensed in a second condenser.

[0049] The process of the present invention preferably involves using urea produced in a urea production process as a starting material in the synthesis of melamine. Depending on the mutual production capacities, this can be all of the urea produced in the urea production area, or a portion thereof. For example, the present 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 arrangement, the capacity of the urea production area or facility exceeds the capacity of the melamine production area or facility. Thus, a portion of the produced urea is conveyed to melamine production, and a portion is provided to one or more urea finishing processes to produce solid urea, such as pellets or granules, or liquid urea is provided to a process for preparing diesel exhaust treatment fluid, and / or urea is applied to the production of products such as urea ammonium sulfate or urea ammonium nitrate. Depending on the capacity requirements for the production of such urea products, it will be understood that the connected melamine production may also be fed in whole or in part and possibly temporarily with urea from elsewhere.

[0050] As the skilled person will appreciate, the urea solution obtained from urea recovery is usually subjected to concentration before being used as a starting material in melamine synthesis. This concentration is preferably to the extent that a highly concentrated urea solution is formed (e.g. having more than 90% by weight of urea, such as more than 95% by weight of urea, such as more than 99% by weight of urea) (for which the term "urea melt" is used in the art). It should be understood that such concentration takes place between urea recovery and melamine synthesis, and preferably in the evaporation section of a urea plant. Typically, the urea solution is concentrated in the evaporation section to a urea melt having a final moisture content of 0.2%-5.0% by weight.

[0051] In case not all of the produced urea is used for melamine production, the remaining urea, in particular urea melt, can be subjected to conventional urea finishing, for example to be obtained in the form of granules or pellets.

[0052] Furthermore, urea not conveyed to the melamine synthesis is available in the form of an aqueous solution, e.g. with 30-35% by weight of urea and with sufficient purity to be suitable as a so-called diesel exhaust fluid (DEF). These and other finished urea products are known to the skilled person and do not need to be elucidated here.

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

[0054] The production zones are connected to each other so as to allow the transfer of melamine off-gases obtained from the high-pressure melamine synthesis section to the urea production zone, wherein the urea production zone comprises a medium-pressure or low-pressure treatment section, preferably a medium-pressure treatment section, suitable for receiving said melamine off-gases. According to the present invention, and in view of the aforementioned process, said treatment section comprises a flash unit and at least one ammonia and carbon dioxide release unit.

[0055] The flash unit has an inlet for urea synthesis solution, which is connected 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 (usually a pressure reducing valve). The flash unit has 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 unit, an outlet for carbamate recovery vapor, and an outlet for urea product stream. As described above, the MP and / or LP processing section of the urea production area may include more than one ammonia and carbon dioxide release unit.

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

[0057] According to the present 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 the flashed liquid in the unit. It should be understood that one or more ammonia and carbon dioxide release units will have a gaseous output, which is the aforementioned carbamate recovery vapor. Therefore, each ammonia and carbon dioxide release unit has an outlet for carbamate recovery vapor.

[0058] The ammonia and carbon dioxide release unit can be designed as a stripping unit (preferably, an MP stripper), a heating unit (preferably, an MP heater) or both. The indication of pressure (MP and / or LP) generally refers to the pressure at which the device operates. At the same time, this also imposes design requirements on the technician, for example, the MP device should at least be able to withstand the MP range defined in urea production. It should be understood that such equipment is well known in the urea field and can be designed to meet the MP requirements without difficulty. Such designs are completely known to the technician, as are the designs of HP and LP devices. Preferably, the flashed liquid is conveyed to the ammonia and carbon dioxide release unit via a distillation column. Such a distillation column receives carbamate reuse vapor from the ammonia and carbon dioxide release unit in countercurrent to the liquid flow.

[0059] In an interesting embodiment, the packed bed is arranged downstream of the flash unit, typically in 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 off-gas. This inlet is optionally positioned so that the melamine off-gas is in countercurrent contact with the flashed urea liquid flowing through the packed bed. By virtue of the increased surface area provided by the packed bed, the melamine off-gas in this embodiment acts as a stripping gas for the flashed urea liquid, thus providing a carbamate recovery vapor produced by the stripping.

[0060] Preferably, the flash unit, the rectifying tower (optionally also comprising a packed bed) and the ammonia and carbon dioxide release unit are contained in a single device. This device includes a flash vessel located at its upper end, a rectifying tower located below the flash vessel (optionally provided with a packed bed) and a heating unit located at its lower end. A single device may have one or more inlets for melamine waste gas (e.g., located in the heating unit and / or at the packed bed), and may provide a countercurrent or cocurrent flow of melamine waste gas and flashed urea liquid. In a preferred embodiment, the device has a single inlet for melamine waste gas on the downstream side of the heating unit. It has been found that this results in a more efficient use of the heat from the melamine waste gas. Without wishing to be bound by theory, the inventor believes that the waste gas introduced into the flashed urea liquid in this way causes turbulence that contributes to 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 steam and carbamate dissociation steam, both containing ammonia and carbon dioxide, will be processed so as to eventually return to the urea synthesis section as reactants. For this reason, the outlets for the flash steam and carbamate dissociation steam are each connected to the urea synthesis section via a condensation and recirculation loop. Various design options for one or more condensers, possible decomposers and additional condensers are known to those skilled in the art of urea production.

[0062] Apart from the specific setup of the MP or LP section comprising a flash unit and an ammonia and carbon dioxide release unit, the urea production zone may be designed according to options known and available to the skilled person.Some embodiments of the system of the invention will be discussed below.

[0063] The HP synthesis section typically comprises an HP stripper, an HP reactor and an HP carbamate condenser. The stripper is preferably a CO2 stripper. The urea production zone comprises a liquid flow line from the reactor to the HP stripper. Optionally, the urea production zone provides a parallel connection with the MP decoupler by providing a second liquid flow line from the reactor to such an MP decoupler.

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

[0065] The reactor is configured for forming urea from NH3 and CO2, and has an outlet for a urea synthesis solution. In a CO2 stripping plant, the N / C ratio of the urea synthesis solution is, for example, 2.85 to 3.3 (in a conventional non-CO2 stripping plant, the N / C ratio will be, for example, 3-3.4). The reactor is operated at a urea synthesis pressure (i.e., HP) and a urea synthesis temperature (e.g., above 100 bar, such as 120 bar to 300 bar, such as 120 bar to 200 bar); and / or, for example, at a temperature of 160°C-240°C, and preferably at a temperature of 170°C-220°C. For so-called conventional plants, an exemplary pressure range is 190°C-200 bar, and for a CO2 stripping plant, an exemplary pressure range is 183°C-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 withdrawn 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, for example, from the feed stream and comprises, for example, N2. The gas stream from the gas outlet of the reactor also comprises, for example, NH3.

[0068] The urea production zone may comprise one or more reactors connected in series, such as a first reactor and a post reactor. The urea production facility may also comprise one or more reactors connected in parallel.

[0069] The reactor and the HP carbamate condenser are optionally combined in a single container. An example is an integrated reactor / condenser described in US5767313. An example integrated condenser / reactor includes a reaction zone and a condensation zone combined in a single container. The condensation zone includes, for example, a heat exchange surface, such as a tube bundle. The reaction zone includes, for example, baffles. The reaction zone is typically arranged downstream of the condensation zone in the container. An example integrated condenser / reactor is an immersed condenser (pool condenser) comprising a horizontal container and a tube bundle, which is configured to receive a cooling fluid in the tubes and a process medium in the container space.

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

[0071] The HP carbamate condenser (HPCC) generally receives at least a portion, preferably all of the raw NH3 feed. The HPCC receives at least a portion, such as all of the gas stream from the HP stripper. The HP carbamate condenser has an outlet for a stream comprising condensed carbamate, which is connected to the inlet of the reactor, and in a combined condenser / reactor, the condensing 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 is, for example, a multistage compressor. CO2 is, for example, available at a relatively low pressure (e.g., less than 20 bar) at the boundary, for example, from a synthesis gas facility. The synthesis gas facility includes, for example, a steam reformer, a water-gas shift reactor, and a CO2 removal unit. The synthesis gas facility may also produce H2 for an ammonia facility. The NH3 feed of the urea facility may come from the ammonia facility. Other sources of CO2 feed stream and NH3 feed stream are also possible.

[0073] Typically, in various embodiments of the present invention, the HP stripper comprises, for example, a vertical shell and tube heat exchanger having an inlet for supplying the urea solution to be stripped into the tube, the inlet being arranged at the top of the stripper, and an outlet for the stripped urea solution at the bottom and an outlet for the mixed gas stream at the top of the stripper. The HP stripper is preferably of the CO2 stripping type and has an inlet at the bottom for a CO2 feed used as a stripping gas. The mixed gas stream is condensed in the HP carbamate condenser into a high-pressure recycle 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 the condensation section in the container to the reactor section, specifically conveying the carbamate-containing liquid from the condensation zone in the container to the reaction zone.

[0074] The HP stripper is generally a device configured for countercurrent contact of a urea solution with a stripping gas stream. Typically, the HP stripper is configured with a urea solution and a stripping gas stream in a tube, while heat is supplied by steam on the shell side. In the present invention, the HP stripper uses all or part of the raw CO2 feed as a stripping gas. The stripping action with CO2 as a stripping gas results in a reduction in the N / C ratio of the urea solution. The stripped urea solution has a lower N / C ratio than the urea synthesis solution, for example an N / C ratio of 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 in the range of 2.0-2.5.

[0075] In the plant and method of the 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 HPCC operates with a process medium (in particular, 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, the HPCC is, for example, a falling film urethane condenser with a cooling fluid in the shell. In other embodiments, the HPCC is, for example, an immersed condenser. The HPCC is, for example, a shell and tube condenser with a horizontal U-shaped tube bundle, a process medium in the shell side, and an immersed tube bundle, and is, for example, a pool condenser. The HPCC comprises, for example, a U-shaped tube bundle or a straight tube bundle. The HPCC is optionally a pool reactor, which comprises a pool condenser zone and a reactor zone. The pool reactor comprises, for example, baffles in the shell space.

[0077] The HPCC also comprises, for example, an inlet for feeding NH 3 into the shell space.

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

[0079] Advantageously, by the preferably (substantially) adiabatic flash from HP to MP in the MP flash vessel (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 HP stripped urea solution. The flash vapor may advantageously have a relatively high CO2 concentration, so that the MP condensation section can be operated at a more favorable N / C ratio.

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

[0081] The MP or LP treatment unit typically comprises a gas / liquid separation zone, for example in a preferably flash vessel, for example by means of a zone or flash vessel having a top outlet for the gas and a bottom outlet for the liquid, wherein the liquid is a urea solution.

[0082] The preferred adiabatic nature of the flash in the preferred MP flash vessel advantageously contributes to a low N / C ratio (NH3 to CO2 molar ratio) of the resulting gas stream. Specifically, upon adiabatic pressure reduction, CO2 leaves the urea solution of the CO2 HP stripper to a higher degree than NH3.

[0083] The flash steam has a N / C molar ratio (NH3 to CO2 molar ratio) of preferably less than 2.0, for example 0.5-1.5, such as 0.9-1.2, for example about 1.0. The flash steam contains NH3 and CO2 in a molar ratio of 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 wt%-50 wt% NH3, about 40 wt%-50 wt% CO2 and, for example, 10 wt%-20 wt% H2O; in the case where CO2 is added to the flash steam, these ranges may also apply to the components removed from the liquid phase of the urea solution. The flash vapor received by the MP carbamate 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%-50 wt% NH3, about 40 wt%-50 wt% CO2 and, for example, 10 wt%-20 wt% H2O.

[0084] The preferred MP heater (MP dissociator MPD) has an inlet for the high pressure offgas received from the melamine synthesis section. The MPD is typically a heat exchanger that uses a heating fluid (e.g., steam) for indirect heat exchange to dissociate carbamates included in the urea synthesis solution in general (and in addition to the hot melamine offgases). The use of steam as a heating fluid in the MP dissociator provides the advantage of great flexibility to ensure that carbamates are adequately removed 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, wherein the steam is located in the shell and the urea solution is located in the tubes. The MP dissociator includes, for example, a rectification section arranged upstream of the shell and tube heat exchanger portion for the urea solution, the rectification section being configured for gas / liquid separation of the urea solution expanded from the HP to the MP and countercurrent contact between the urea solution and the gas stream from the heat exchange portion. This promotes good removal of carbamates from the urea solution.

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

[0086] In some embodiments, the MP desplitter also receives other urea solution streams, such as urea solution obtained indirectly from the HP stripper.

[0087] The carbamate dissociation vapor is supplied from the MP dissociator to the MP condensation section (MPCC). The MP condensation section has a liquid outlet for the carbamate solution.

[0088] The urea production zone comprises a gas flow line for conveying the flashed steam directly or indirectly (preferably directly) to the MP condensation section, so that at least a portion (preferably all) of the gas is conveyed as gas to the MP condensation section. In this way, advantageously, the MP condensation section can be used to recover the CO contained in the stripped urea solution.

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

[0090] By means of the MP treatment unit, the CO2 included in the flash vapor (which CO2 originates from the HP stripper) can be used and recovered in the MP condensation section, so that the steam consumption of the HP stripper can be lower at a constant stripping efficiency α. Relatively more CO2 can thus be advantageously supplied to the HP stripper. In a preferred embodiment with an ammonia consumption unit (e.g., an ammonium nitrate section), the HP stripper can advantageously be operated at a relatively low stripping efficiency α compared to a urea production section that only produces a urea melt, because the NH3 contained in the stripped urea solution (also as carbamate) and received by the preferred LP dissociator can be used in the preferred ammonia consumption unit, for example, to react in the preferred ammonium nitrate section. The skilled person understands that in the context of a urea facility, a lower stripping efficiency α, as the term is used in the art, can provide advantages.

[0091] The MP condensation section comprises one or more MP carbamate condensers and has a liquid outlet for the MP carbamate solution, which is connected to, for example, a recycle flow line leading to the HP synthesis section. The MP condensation section also comprises an outlet for uncondensed gases. The gases are supplied to, for example, an absorber or scrubber, or to a neutralization section such as an optional ammonium nitrate section.

[0092] The MP condensation section may include an MP carbamate condenser operated with a cooling fluid (e.g., a cooling liquid, such as cooling water), optionally as a second MP carbamate condenser arranged downstream of the first MP carbamate condenser, wherein the first condenser is, for example, thermally integrated with the 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 carbamate condenser. The MP condensation section further includes a gas / liquid separator, specifically for separating the carbamate solution from the uncondensed gas.

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

[0094] The above-mentioned system for the production of urea and melamine can be designed as a new (grassroots) facility. The system can also be provided by building a new melamine facility, which will be 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 a section including a flash unit and a heating unit for dissociating a flashed urea liquid can be provided with a melamine waste gas connection with the heating unit, preferably an MP heating unit. A pre-existing urea facility without MP or LP flash can be transformed so as to be provided with an MP or LP flash unit for dissociating a flashed urea liquid and an ammonia and carbon dioxide release unit (such as an MP heater) and a connection for connecting the melamine waste gas to such an MP heater. In addition, the present invention relates to transforming (reorganizing) a pre-existing system for the combined or integrated production of urea and melamine. For example, a pre-existing system comprising connected HP melamine and urea production zones and having a connection of melamine off-gases to the urea production zone, such as to the MP condensation section of the urea production zone, may be provided with a connection of the melamine off-gases to the MP or LP section of the urea production zone, preferably to an ammonia and carbon dioxide release unit in the MP section, in place of or possibly in addition to the aforementioned connection, a flash unit, preferably an MP flash unit, for conveying flashed urea liquid to said ammonia and carbon dioxide release unit is provided (or is to be provided) . Also, a pre-existing system comprising connected HP melamine and urea production zones and having a connection of melamine off-gases to the HP urea synthesis section may be modified by adding a flash unit plus a heater, preferably an MP flash plus a heater (as described above) and providing a melamine off-gas connection to said heater, in place of or possibly in addition to the aforementioned connection.

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

[0096] unit:

[0097] 1 HP urea synthesis section

[0098] 2 HP melamine synthesis section

[0099] 3 Flash unit

[0100] 4 Ammonia and carbon dioxide release unit

[0101] 5 LP condensation section

[0102] 6 MP Condensation Section

[0103] 7 Packed Bed

[0104] flow:

[0105] a Urea synthesis solution

[0106] b HP melamine waste gas

[0107] c Flash steam

[0108] d Flashed urea liquid

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

[0110] f MP urea product solution

[0111] g LP urethane solution

[0112] h LP urea product stream

[0113] exist Figure 1In the figure, it is shown that the urea synthesis solution (a) from the HP urea synthesis section (1) is subjected to flash evaporation in the MP flash unit (3). This produces flash vapours, i.e. ammonia and carbon dioxide vapours (e') which are removed, and flashed urea liquid (d). The flashed urea liquid is 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, on the shell side, heat can typically be provided by a heat exchange fluid, such as steam or hot condensate ("HEAT"), which leaves the unit after exchanging excess heat ("exHEAT"). According to the invention, the offgas (b) from the high-pressure melamine plant (2) is conveyed to the MP heating unit (tube side) and is contacted therein in countercurrent with the flashed urea liquid. Heating of the flashed urea liquid causes the decomposition of the ammonium carbamate present in the flashed urea liquid and thereby releases ammonia and carbon dioxide vapor (e") which are removed together with the aforementioned flash vapor. The urea liquid from which the decomposed carbamate has been removed is obtained as MP urea product solution (f). Depending on the sensible heat available from the melamine waste gas and the heat demand in the MP heating unit, the use of a 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 waste gas. This reflects the general possibility of implementing the invention and further applies to all figures.

[0114] exist Figure 2 In the figure, it is shown that the urea synthesis solution (a) from the HP urea synthesis section (1) is subjected to flash evaporation in the MP flash unit (3). This produces flash vapours, i.e. ammonia and carbon dioxide vapours (e') which are removed, and flashed urea liquid (d). The figure further shows the HP melamine synthesis section from which the HP melamine off-gas (b) is obtained. The flashed urea liquid (d) and the melamine off-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, on the shell side, heat can typically be provided by a heat exchange fluid, such as steam or hot condensate ("HEAT"), which leaves the unit after exchanging superheat ("exHEAT"). According to the invention, the flashed urea liquid (d) is conducted through the MP heating unit (tube side). By contact with the hot melamine offgas (b), and optionally also with heat from a heat exchange fluid, the ammonium carbamate contained in the flashed urea liquid undergoes decomposition and, as a result, release of ammonia and carbon dioxide vapors (e") which are removed with the above-mentioned flash vapors. The urea liquid from which the decomposed carbamate has been removed is obtained as MP urea product solution (f).

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

[0116] Figure 4 A single flashing and heating apparatus is shown which can be used when operating the present invention. Shown is a urea synthesis solution (a) entering the apparatus via an upper inlet. In the flash unit (3) of the apparatus, the urea solution is subjected to flashing. This produces flash vapour, i.e. ammonia and carbon dioxide vapour (e') which are removed at the top of the apparatus, and flashed urea liquid (d). The latter is conducted through a packed bed (7) and conveyed to the bottom inlet of the apparatus. Further shown is that HP melamine waste gas (b) is combined with the flashed urea liquid, after which the combined stream enters the apparatus 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, on the shell side, heat can typically be provided by a heat exchange fluid, such as steam or hot condensate ("HEAT"), which leaves the unit after exchanging excess heat ("exHEAT"). According to the present invention, the flashed urea liquid (d) is conducted through the MP heating unit (tube side). By contact with hot melamine offgases (b), and optionally also with heat from a heat exchange fluid, the ammonium carbamate contained in the flashed urea liquid is subjected to decomposition and thus to release of ammonia and carbon dioxide vapor (e") which are conveyed across the packed bed (7), thereby acting as stripping agents for the flashed urea liquid (d) and are removed at the top of the plant together with the flash vapor (e').

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 forming reaction in a high pressure urea synthesis section so as to obtain a high pressure urea synthesis solution, subjecting the urea synthesis section to flash distillation, thereby producing 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, thereby removing ammonia and carbon dioxide vapor from the flashed urea liquid, wherein the production of melamine comprises subjecting urea to a non-catalytic high pressure melamine forming reaction so as 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 before or during at least one of the ammonia and carbon dioxide release steps.

2. The process according to claim 1, wherein the flashing is carried out at medium pressure and comprises subjecting the flashed urea liquid to treatment in a medium pressure treatment section so as to obtain a medium pressure urea product solution and a medium pressure carbamate recovery vapor.

3. Process according to claim 1 or 2, wherein at least part of the high-pressure urea synthesis solution is obtained from subjecting the effluent from the urea forming reaction to high-pressure stripping, preferably using carbon dioxide as stripping medium.

4. Process according to any one of the preceding claims, wherein the flash vapors and the carbamate recovery vapors are subjected to condensation in order to form a carbamate solution, and the carbamate solution is subjected to recycle to the high-pressure synthesis section.

5. A process according to any one of the preceding claims, comprising: The urea product solution is subjected at low pressure to recovery of the ammonia and carbon dioxide still present, thereby obtaining a low-pressure carbamate solution and a low-pressure urea product stream.

6. Process according to claim 4 or 5, wherein a carbamate liquid obtained from melamine synthesis is combined with at least one carbamate solution.

7. Process according to any one of the preceding claims, wherein obtaining the medium pressure urea product stream comprises medium pressure stripping, preferably using carbon dioxide as stripping medium.

8. A process according to any one of the preceding claims, wherein at least part of the urea produced is the urea subjected to the melamine forming reaction.

9. A process according to any one of the preceding claims, comprising: The melamine off-gas is contacted in counter-current with the flashed urea liquid.

10. Process according to any one of the preceding claims, wherein the melamine off-gas contacted with the flashed urea liquid has a temperature of 150 to 300°C, preferably 200 to 250°C.

11. A system for the production of urea and melamine, said system comprising a urea production zone; said urea production zone comprising a high-pressure urea synthesis section and a urea recovery section located downstream of said high-pressure urea synthesis section and in fluid communication with said high-pressure urea synthesis section, said urea recovery section being suitable for separately obtaining a urea solution and a carbamate recovery stream; said system further comprising a melamine production zone; said melamine production zone comprising a high-pressure melamine synthesis section, said production zones being connected to each other so as to allow conveying melamine off-gas obtained from said melamine synthesis section to said urea production zone, wherein said urea production zone comprises a medium-pressure or low-pressure treatment section suitable for receiving said melamine off-gas, said treatment section comprising a flash unit and at least one ammonia and carbon dioxide release unit, in particular a heating unit, said The flash unit has an inlet for urea synthesis solution, an outlet for flash vapour 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 has an inlet for flashed liquid connected to the outlet for flashed liquid from the flash unit, an outlet for carbamate recovery vapour and an outlet for a urea product stream, the outlets for flash vapour and carbamate recovery vapour each being connected to the urea synthesis section via a condensation and recirculation loop, wherein the outlet for melamine offgas 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 offgas to contact with the flashed liquid in or upstream of the ammonia and carbon dioxide release unit.

12. The system according to claim 11, wherein the urea production zone comprises an outlet for urea connected to an inlet of the melamine synthesis section.

13. The system according to claim 11 or 12, wherein the flash unit and the heating unit are connected via a distillation column and are preferably accommodated in a single device.

14. The system of claim 13, wherein the distillation column comprises a packed bed, and wherein an outlet for melamine off-gas from the melamine synthesis section is in fluid communication with an inlet of the distillation column, the inlet being positioned to allow the off-gas to flow across the packed bed.

15. System according to claim 13 or 14, wherein an outlet for melamine off-gases from the melamine synthesis section is in fluid communication with an inlet of the heating unit, preferably located at the lower end of the heating unit.

16. A method for retrofitting a pre-existing urea facility, said pre-existing urea facility comprising a high-pressure urea synthesis section and a urea recovery section located downstream of said high-pressure urea synthesis section and in fluid communication with said high-pressure urea synthesis section, said urea recovery section being adapted to separately obtain a urea solution and a carbamate recovery stream, said method comprising connecting said urea facility to a melamine facility comprising a high-pressure melamine synthesis section so as to allow recirculation of melamine off-gas from a melamine off-gas treatment section to said urea facility, said method comprising adding a medium-pressure or low-pressure treatment section adapted to receive said melamine off-gas, said treatment section comprising a flash unit and at least one ammonia and carbon dioxide release unit, said flash unit having a pressure reducing device connected to said melamine off-gas treatment section via a pressure reducing device an inlet for urea synthesis solution, an outlet for flash vapour and an outlet for flashed liquid at the outlet of the high pressure synthesis section, the 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 vapour and an outlet for a urea product stream, the outlets for flash vapour and carbamate recovery vapour each being connected to the urea synthesis section via a condensation and recirculation loop, wherein an outlet for melamine off gases from the melamine synthesis section is in fluid communication with the inlet of the ammonia and carbon dioxide release unit, thereby allowing contact of the melamine off gases with the flashed liquid in or upstream of the ammonia and carbon dioxide release unit.

17. The method according to claim 16, comprising: A connection is provided between an outlet for urea coming from the urea production zone to an inlet of the melamine synthesis section.

18. A method for retrofitting 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 section in fluid communication with a urea recovery section, the urea recovery section being adapted to separately obtain a urea solution and a carbamate recovery stream, the recovery section comprising a medium-pressure or low-pressure processing section, the medium-pressure or low-pressure processing section comprising a flash unit and an ammonia and carbon dioxide release unit, the flash unit having 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 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 flash ... The outlets for steam and carbamate recovery steam are each connected to the urea synthesis section via a condensation and recirculation loop, the melamine production zone comprises a high-pressure melamine synthesis section and a melamine off-gas treatment section located downstream of the high-pressure melamine synthesis section and in fluid communication with the high-pressure melamine synthesis section, the melamine off-gas treatment section being used to obtain reduced-pressure melamine off-gas; the production zones are connected to each other so as to allow melamine off-gas from the melamine off-gas treatment section to be conveyed to the urea production zone, and optionally also to allow urea from the urea production zone to be conveyed to the melamine synthesis section, the method comprising providing a connection between the outlet for melamine off-gas from the melamine synthesis section to the inlet of the ammonia and carbon dioxide release unit, thereby allowing the melamine off-gas to be contacted with flashed liquid in or upstream of the ammonia and carbon dioxide release unit.

Images