EA042169B1 - COMBINED PRODUCTION OF UREA AND MELAMINE - Google Patents

Description

The field of technology to which the invention belongs

The invention relates to the combination of the production of urea and melamine, in particular urea and melamine, the production of which is carried out according to the technology for producing carbamide at high pressure in the liquid phase. The invention also relates to methods for retrofitting existing urea and melamine plants.

State of the art

Combining the production of carbamide and melamine has long been known. In this case, melamine is obtained from carbamide according to the following reaction:

(NH ₂ ) ₂ CO C ₃ H ₆ N ₆ + 6 NH ₃ + 3 CO ₂

Interestingly, ammonia and carbon dioxide are obtained from this process in exactly the stoichiometric ratio in which these substances form urea.

The production of carbamide is essentially presented in the form of two successive reaction steps. At the first stage, during the exothermic reaction, ammonium carbamate is formed:

2NH ₃ + CO ₂ -a H ₂ N - CO - ΟΝΗ,ι

The formed ammonium carbamate is then dehydrated in the second step to form urea as a result of an endothermic equilibrium reaction:

H ₂ N - CO - ONH4 o H ₂ N - CO - NH ₂ + H ₂ O

In this field, plants for the production of urea are usually filled with raw materials on the principle of excess ammonia (ie, above the stoichiometric ratio of 2: 1). In the zone of urea synthesis, these reagents are subjected to pressure from 12 to 40 MPa and temperature from 150°C to 250°C.

In the combined production of carbamide and melamine, the urea obtained in the urea synthesis zone is sent to the melamine synthesis zone. Carbon dioxide and off-gas ammonia from melamine production are in turn recycled as feedstock for use in the urea synthesis zone.

It should be understood that, regardless of the excess of supplied ammonia, the reagents are removed from the synthesis zone in a ratio of no more than 2: 1, therefore, the exhaust gas during the melamine synthesis introduces back the urea-forming reagents in the same ratio. Thus, the ratio of ammonia and carbon dioxide during the entire synthesis of urea and melamine and in the recirculation circuit remains unchanged.

Combined production can be carried out at a combined plant with urea and melamine synthesis zones. More typically, however, the melamine and urea synthesis zones are contained in separate urea and melamine production plants, and these plants are connected via respective flow lines so as to realize the combination described above.

A typical urea plant is a urea stripping plant (stripping - a process for obtaining urea). In it, the decomposition of ammonium carbamate that has not been converted to urea and the removal of the usual excess ammonia generally occurs at a pressure substantially at or below the pressure in the synthesis reactor. This decomposition and removal takes place in one or more strippers installed downstream of the reactor, possibly with the aid of a stripping gas such as, for example, carbon dioxide or ammonia, and heating. The gas stream leaving the stripper contains ammonia and carbon dioxide, which are condensed in a high pressure condenser operating at a pressure substantially equal to the stripper pressure and then returned to the urea synthesis zone.

In a urea stripper, the synthesis zone operates at a temperature of 160-240°C, and preferably at a temperature of 170-220°C. The pressure in the synthesis reactor is 12-21 MPa, preferably 12.5-20 MPa. The molar ratio of ammonia and carbon dioxide (N/C ratio) in the urea synthesis zone of the stripper is usually in the range of 2.2 to 5, preferably 2.5 to 4.5 mol/mol. The synthesis zone may be located in a single reactor or in multiple reactors arranged in parallel or in series.

After the stripping treatment, the pressure of the distilled urea solution is reduced in the urea regeneration section (or in the recirculation section, since the carbamide is recycled from this section). In the regeneration section, the unconverted ammonia and carbon dioxide in the urea solution are separated from the urea and water solution. The regeneration section typically includes a heater, one or more gas/liquid separation sections, and one or more condenser sections. The carbamide solution entering the regeneration section is heated to evaporate the volatile components, ammonia and carbon dioxide, from the solution. The heat carrier in the heater is usually steam. The resulting vapors in said heater are separated from the aqueous urea solution in a liquid/gas separator, after which said vapors are condensed in a condenser to form a carbamate solution. The heat generated during condensation is usually dissipated in the cooling water. The resulting carbamate solution in this regeneration section operating at a lower pressure than the pressure in the synthesis section is preferably returned to the carbamate synthesis section operating at synthesis pressure. The regeneration section is essentially a single section or may consist of a plurality of successive regeneration sections.

In a urea stripper using carbon dioxide as the stripping gas, it is generally advantageous to introduce substantially all of the carbon dioxide into the synthesis loop through the stripper. However, in the case of co-production of urea and melamine, the feed part of the carbon dioxide is determined by the recirculation of the off-gas in the production of melamine. Since the off-gas contains both carbon dioxide and ammonia, it is less suitable for use as a stripping medium, since its use would not reduce the partial pressure of only one of the components of the liquid phase.

Since part of the carbon dioxide reactants is thus introduced elsewhere in the urea synthesis section, the carbon dioxide supply to the stripper is necessarily reduced compared to a stand-alone urea plant, i. not combined with the production of melamine. This leads to less efficient operation of the stripper. Although this can be compensated for by increasing the stripping temperature, the latter results in an increase in the overall energy consumption of the plant, usually in the form of an increased demand for steam (steam is used in the annulus of the stripper to supply heat). Moreover, supplying more heat to the stripper can only be realized by raising the temperature of the stripper. However, there is a limitation in this case, since elevated temperatures reduce the corrosion resistance of the materials from which stripper devices are usually made. Accordingly, an increase in the temperature of the stripper leads to an increase in corrosion, which can damage said stripper and thus reduce its life.

The invention is directed to providing co-production of urea and ammonia, which will optimize the efficiency of stripping, steam consumption, or both.

This issue has not yet received an adequate solution in this area, despite the many descriptions regarding various ways to combine the production of urea and melamine. Reference is made to the following documents as examples reflecting the state of the art.

In WO 98/08808 A1, the block diagram shown in FIG. 2 illustrates a process for co-production of urea and melamine. In the known process, the gas stream resulting from the high pressure treatment to produce melamine is fed directly into the high pressure section of the stripping process to produce urea. The disadvantage of the known method is that the stable operation of the obtained combined method for producing melamine and urea is difficult: pressure fluctuations in one of the processes can easily affect the other process through the gas flow and thus lead to unstable operation and disruption of the process. Another disadvantage is that the best performance of the known process is achieved provided that the melamine production process is operated at a higher pressure than the pressure in said high pressure section of the urea production process. As noted, the energy consumption of the urea plant processing off-gas from the combined melamine production process is higher than that of the stand-alone urea plant. In particular, as explained above, the energy consumption increases if the urea plant is a stripper using carbon dioxide or ammonia.

Another method is described in US 7,893,298 B2 and is illustrated by the block diagram shown in FIG. 4. In the known process, the gas stream resulting from the high pressure treatment to produce melamine is condensed into an aqueous ammonium carbamate solution stream formed in a CO ₂ stripping process to produce urea.

Another method is described in WO 2008/052640 A1. In this known method, the urea plant contains a medium pressure processing section which includes a decomposer. The gas stream from the production of melamine is fed along with the steam generated in said decomposer and together with the aqueous ammonium carbamate solution generated in the downstream urea recovery section to a medium pressure processing section condenser. This makes it possible to obtain a concentrated aqueous solution of carbamate, which is recycled to the urea synthesis section at high pressure.

The invention is aimed at reducing the consumption of steam in a urea production plant after being combined with a melamine production plant. Alternatively, the invention is directed to keeping the steam flow rate in the urea production section of a combined urea and melamine plant at least the same. Thus, the invention ultimately aims at improving the economy and functionality of a urea/melamine co-production plant.

Disclosure of invention

In order to better achieve one or more of the aforementioned desired results, the invention in one aspect provides a process for co-production of urea and melamine, wherein:

(i) carbon dioxide and ammonia in the urea production zone are exposed to conditions

- 2 042169 urea synthesis, in which a urea synthesis stream is formed containing urea, water and ammonium carbamate, and the specified urea production zone contains a urea synthesis section, which contains a reactor and a stripper;

(ii) treating the urea synthesis stream to form an aqueous urea and steam stream containing ammonia, carbon dioxide and water, said treatment comprising stripping in said stripper;

(iii) supplying the urea to the melamine production zone;

(iv) subjecting the urea fed to the melamine production zone to the conditions under which melamine is formed to produce melamine and a melamine production off-gas containing carbon dioxide and ammonia;

(v) supplying said off-gas to a urea production zone;

(vi) condensing said steam and off-gas from melamine production in indirect heat exchange with said aqueous urea solution stream, thereby forming a combined steam condensate stream from said treatment from step (ii) and said off-gas condensate to form a dilute melamine off-gas condensate ; and (vii) using said diluted melamine off-gas condensate as the indirect heat exchange fluid.

The invention also proposes a plant for the combined production of urea and melamine, containing a zone for the production of urea and a zone for the production of melamine; wherein the urea production zone comprises a urea synthesis section and a carbamate recycling section containing a liquid/gas separation section; the melamine production zone comprises a melamine synthesis section and an off-gas separation section; moreover, the specified installation contains a line for transporting a fluid medium from the urea production zone to the melamine production zone, while the specified line is configured to supply the resulting urea to the melamine synthesis section, the gas flow line from the off-gas separation section of the melamine production zone to the urea synthesis section, while said gas flow line is in fluid communication with the gas inlet of the melamine off-gas condenser, and said condenser comprises a liquid outlet for the condensed gas, and a gas flow line from the liquid/gas separation section in fluid communication with the condenser for vapor containing ammonia, carbon dioxide and water, wherein said condenser contains a liquid outlet for condensed gas, wherein said condenser for melamine production off-gas and said condenser for said vapor are part of a condensation system, wherein said liquid outlets are combined into a single transport system fluid sorting.

In yet another aspect, the invention provides a method for reducing energy consumption in a urea production zone of an existing urea-melamine co-production plant comprising a urea production zone and a melamine production zone; wherein the urea production zone comprises a urea synthesis section and a carbamate recycling section containing a liquid/gas separation section; the melamine production zone comprises a melamine synthesis section and an off-gas separation section, wherein the method involves adding a concentrator to the urea production zone, said concentrator comprising a vaporization chamber for a liquid to be heated and vaporised, said vaporizer chamber comprising a gas outlet for the vaporized liquid and is in heat exchange communication with the indirect heat exchange chamber; providing a conveyance line for supplying liquid from the liquid/gas separation section of the urea plant to said flash chamber; providing a gas flow line from a liquid/gas separation section of the urea plant to a condensation system that includes a liquid outlet in fluid communication with the indirect heat exchange chamber; and providing a gas flow line from the off-gas separation section of the melamine plant to the condensing system.

In a further aspect, the invention provides a method for improving the productivity of a melamine production zone equipment of an existing urea/melamine co-production plant comprising a urea production zone and a melamine production zone; wherein the urea production zone comprises a urea synthesis section and a carbamate recycling section comprising a liquid/gas separation section; the melamine production zone comprises a melamine synthesis section and an off-gas separation section, wherein the process increases the melamine synthesis capacity in the melamine synthesis section; adding a concentrator to the urea production zone, said concentrator comprising a flash chamber for the liquid to be heated and vaporized, and said flash chamber containing a gas outlet for the vaporized fluid and in heat exchange communication with the indirect heat exchange chamber; providing a conveyance line for supplying liquid from the liquid/gas separation section of the urea plant to said flash chamber; providing a gas flow line from a liquid/gas separation section of the urea plant to a condensing system, said condensing system comprising a fluid outlet in fluid communication with the indirect heat exchange chamber; and provide a gas flow line from the off-gas separation section of the plant along

- 3 042169 production of melamine in the condensation system.

In yet another aspect, the invention proposes the use of the aforementioned methods to convert a urea/melamine co-production plant.

Brief description of the drawings

In FIG. 1 is a schematic drawing of a conventional urea stripper.

In FIG. 2 is a schematic drawing of a urea/melamine co-production plant according to an embodiment known in the art.

In FIG. 3 is a graph showing typical energy consumption, expressed in kilograms of water vapor per tonne of urea produced, versus the throughput of an attached melamine plant at a constant urea throughput for the embodiment shown in FIG. 2.

In FIG. 4 is a schematic drawing of a urea/melamine co-production plant according to another embodiment known in the art.

In FIG. 5 is a graph such as that shown in FIG. 3 for the embodiment shown in FIG. 4.

In FIG. 6 is a schematic drawing of a urea/melamine co-production plant according to an embodiment of the invention.

In FIG. 7 is a graph such as that shown in FIG. 3 for the embodiment shown in FIG. 6.

In FIG. 8 is a schematic drawing of a urea/melamine co-production plant in accordance with another embodiment of the invention.

In FIG. 9 is a graph such as that shown in FIG. 3 for the embodiment shown in FIG. 8.

In FIG. 10 is a schematic drawing of a urea/melamine co-production plant in accordance with yet another embodiment of the invention.

In FIG. 11 is a graph such as that shown in FIG. 3 for the embodiment shown in FIG. 10.

Implementation of the invention

In a general sense, the invention is based on an unexpected insight on how to use the thermal energy generated by the flow of off-gas in the melamine production zone (hereinafter also referred to as melamine off-gas) co-located with the urea production zone. This insight led to the combination of said effluent gas condensate, which contains relatively little water, and dissociated carbamate vapor condensate (a gaseous stream containing ammonia, carbon dioxide and water) from urea production, which has a high water content. The resultant combined condensate has a water content higher than that of the off-gas from the melamine production zone. By diluting the off-gas from melamine production in this way, after condensation, the resulting dilute condensate can be further used by directing it elsewhere in the co-production plant, in particular by recycling it for use in the urea synthesis.

In order to obtain a combined condensate, the off-gas from the melamine production zone and the dissociated carbamate vapor from the urea production zone are sent to a condensation system suitable for receiving and combining the condensates of both said gas streams.

As a rule, this is the name of a system containing a condenser for off-gas, melamine production, and said condenser has a liquid outlet for condensed gas, and a condenser for dissociated carbamate vapor containing ammonia, carbon dioxide and water, while said condenser has a liquid outlet for condensed gas, while these liquid outlets are combined into a single fluid transportation system. Depending on the relative pressures of the gas streams, as described below, the condensing system may be a single condenser that receives both gas streams. If it is preferable not to take steps to equalize the pressures of the two gas streams, then the condensation system preferably comprises a plurality of condensers in fluid communication with each other.

Essentially, the liquid at the outlet of the condensation system is the combined condensate of the off-gas from the melamine production zone and the dissociated carbamate vapor from the urea production zone. If the condensation of both streams does not occur approximately at the same time or in the same vessel, but sequentially, then preferably the condensation of the off-gas from the production of melamine occurs downstream of the condensation of the dissociated carbamate vapor. Thus, condensate with a low water content (i.e., melamine off-gas condensate) will immediately be taken into the water stream and diluted, which will help prevent the melamine off-gas condensate from curing. Preferably, the dilute condensate contains 20 to 35% water by weight.

Preferably, the condensing system is in the form of an annulus of a shell-and-tube heat exchanger. In any case, the condensing system is located in the heat exchanger

- 4 042169 communication with a device that uses the heat released during condensation.

In a preferred embodiment of the process according to the invention, the combined condensation of dissociated carbamate vapor and melamine production off-gas is carried out in indirect heat exchange with the aqueous carbamate solution stream.

Preferably, indirect heat exchange is carried out in a shell-and-tube heat exchanger so that dissociated carbamate vapor and off-gas from melamine production are supplied to the annular space of the heat exchanger, and an aqueous solution of carbamate is supplied to its pipe part.

To concentrate the specified flow, heat exchange with the flow of an aqueous solution of urea is used. This results in a liquid stream, which is a concentrated solution of urea, and a gas stream, which is evaporated water containing ammonia and carbon dioxide.

As a result of the invention, the energy efficiency of the combined production plant is increased. More specifically, if the urea production zone is a urea stripping plant, the increase in specific steam flow rate in the urea production plant is slowed down, despite the fact that due to the lower stripping efficiency described above in the urea plant, which processes off-gas from melamine production plant, additional steam is required and, accordingly, the amount of carbon dioxide entering directly into the stripper is reduced. The inventors have also unexpectedly found that the specific energy consumption even drops as melamine production increases.

It should be noted that, as a rule, the off-gas in the production of melamine has a pressure in the range, as a rule, 1.0-25 MPa, preferably 1.0-3.0 MPa, for example, 1.5-2.5 MPa, usually approximately 2 MPa. In the context of urea production, this should be considered medium pressure. Urea is usually synthesized at high pressure, for example 10-25 MPa, preferably 12-22 MPa. This implies that if the dissociated carbamate vapor containing ammonia from the high pressure stripper in the urea production zone is combined with the off-gas from the melamine production zone into a combined condensate, then the combined condensate is formed only after the two gas streams condense.

However, in one interesting embodiment, the dissociated carbamate vapor has a pressure substantially equal to that of the off-gas from the melamine production, allowing for gas phase pooling. In this case, the steam and off-gas can be co-condensed. This can be done if the dissociated carbamate vapor is at approximately the correct pressure or brought to the correct pressure.

This can be realized, for example, in a urea production zone with a medium pressure stage. Such urea production zones are known to those skilled in the art. In the medium pressure stage, all or part of the urea synthesis solution obtained from the high pressure synthesis section is distilled, the medium pressure stage effluent being a distilled aqueous urea solution and dissociated medium pressure carbamate vapor. The person skilled in the art is aware of urea production zones including a medium pressure stage, typically comprising a medium pressure stripper and a medium pressure carbamate condenser. See, for example, Ullmann's Encyclopedia of Industrial Chemistry, Vol 37, (2012) p 657-693.

Medium pressure dissociated carbamate steam can also be obtained by flashing. In a preferred embodiment, the above treatment of the urea synthesis stream to separate the aqueous urea solution stream from the residual dissociated carbamate vapor containing ammonia, carbon dioxide and water comprises adiabatically flashing the aqueous urea solution stream, typically at a pressure of 1.0 to 8.0 MPa, and more specifically, from 1.5 to 5.0 MPa. This results in an expansion which causes a significant portion of the ammonia, carbon dioxide and water to separate from the urea solution as vapor. This separated steam, i.e. essentially dissociated carbamate vapor, is at medium pressure and can be sent to the same condensation system, which is also sent to the off-gas from the melamine production. In the system, steam and off-gas can be combined in the gas phase and then co-condensed. The remaining carbamide solution is generally still subjected to the usual recirculation treatment to separate the rest of the dissociated carbamate vapor from the urea solution. The last dissociated carbamate vapor is preferably also sent to the aforementioned condensation system. The advantage of the adiabatic flash stage is that the heat generated during the synthesis of urea can be recovered by condensing the vapor produced by the flash and used in indirect heat exchange.

The invention finds useful application in a method for the combined production of urea and melamine and in a production plant designed for this. It also finds useful application in a process for reducing the energy consumption of a urea production zone in a urea/melamine co-production plant. The most useful application relates to increasing the productivity of a melamine plant integrated with a urea plant. In general, the invention proposes a method for converting a plant for combined

- 5 042169 production of urea and melamine.

The process for co-production of urea and melamine in accordance with the invention includes exposing carbon dioxide and ammonia to urea formation conditions in a urea synthesis section to form a urea synthesis stream containing urea, water and ammonium carbamate.

This can be done with standard urea production technology used in the art.

The urea synthesis stream is treated to separate the urea from the residual dissociated carbamate vapor containing ammonia, carbon dioxide and water. Ways to accomplish this are well known to the skilled person. Said dissociated carbamate vapor is typically condensed into a carbamate solution which is recycled to the urea synthesis section. This can also be done using standard urea production techniques known to those skilled in the art.

Preferably, treatment of the urea synthesis stream comprises distillation of said stream to obtain a distilled aqueous urea solution. Stripping can be carried out by applying heat (thermal stripping) or using a stripping medium, usually a countercurrent flow of stripping gas. In the production of carbamide, ammonia or carbon dioxide is usually used as a stripping medium.

The distilled aqueous solution of urea is subjected to a recirculation step, where the remaining ammonium carbamate is separated from the urea to obtain a solution of urea and a dissociated carbamate vapor containing ammonia, carbon dioxide and water; said dissociated carbamate vapor is condensed into a carbamate solution, which is recycled to the urea synthesis section.

In the invention, in one embodiment, the condensation of the dissociated carbamate vapor is carried out in combination with the condensation of off-gas obtained from the melamine production section and recycled to the urea production section as part of the resulting combined condensate. In another embodiment, the dissociated carbamate vapor is condensed first. The condensate is then brought to a suitable pressure, mixed with the melamine production off-gas, and then the melamine production off-gas is condensed. The condensed fluid is used for heat exchange.

In the co-production of urea and melamine, the urea generated in the urea production section is sent as a raw material to the melamine production section, where it is subjected to the conditions under which melamine is formed. These conditions and associated melamine synthesis equipment are generally available to those skilled in the art.

Another part of the combined production of urea and melamine relates to the recirculation of off-gas from the melamine synthesis section to the urea synthesis section. It should be understood that in the invention this recirculation takes place, in whole or in part, after the co-condensation described above. In other words, after condensation, the off-gas from melamine production is recycled to the urea synthesis section as a combined condensate of said off-gas and dissociated carbamate vapor from urea production, as explained above.

In addition to the established requirements set forth above for the use of melamine off-gas in condensation, the combination of urea and melamine production can generally be carried out in a manner known to the person skilled in the art. In this regard, there are many systems in the field for combining melamine and urea plants.

Co-production of urea and melamine requires that the urea production zone and the melamine production zone are connected in the manner described above. Production areas may be part of a single combined installation. More typically, production areas are separate installations, usually located in sufficient proximity to each other, which makes the combination economically and practically feasible.

Separate urea and melamine plants may be built as new plants, or one or both may already exist. If both installations exist and are located in sufficient proximity to each other, then in this area they are traditionally already combined. However, it can also be imagined that such a combination has not yet been realized, in particular due to the disadvantages associated with the stripping efficiency of the urea plant described above. The invention eliminates these disadvantages.

If the plants already exist in combination, it can be imagined that the connection to the urea plant does not actually allow the specialist to increase the productivity of the melamine plant due to the problem of lower stripping efficiency and/or higher steam consumption due to increased dioxide supply. carbon and ammonia to a urea plant. The invention solves this problem.

With respect to the urea production zone, a frequently used method for producing urea according to a stripping process is a carbon dioxide stripping method, for example, as described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333350. In this process, a synthesis section is followed by one or more regeneration sections. The synthesis section contains a reactor, a stripper, a condenser and a scrubber, in which the operating pressure is in the range from 12 to 20 MPa, and preferably in the range from 13 to 18 MPa. In the synthesis section, the urea solution exiting the urea reactor is fed into a stripper, in which a large amount of unconverted ammonia and carbon dioxide is separated from the aqueous urea solution. Such a stripper may be a shell-and-tube heat exchanger in which the urea solution is fed into the top side of the tubes, and the carbon dioxide supplied for synthesis is introduced into the bottom of the stripper. Water vapor is introduced into the annular space to heat the solution. The urea solution exits the heat exchanger at the bottom while the vapor phase exits the stripper at the top. The steam leaving said stripper contains ammonia, carbon dioxide and a small amount of water. Said vapor is condensed in a falling film heat exchanger or in an immersion type condenser, which may be of a horizontal type or a vertical type. The horizontal type immersion heat exchanger is described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. The heat released by the exothermic condensation reaction of the carbamate in said condenser is typically used to generate steam used in the downstream urea processing section to heat and concentrate the urea solution. Since the liquid remains in the immersion type condenser for some time, part of the urea formation reaction takes place already in the said condenser. The resulting solution containing condensed ammonia, carbon dioxide, water and urea, together with non-condensed ammonia, carbon dioxide and inert steam, is sent to the reactor. In the reactor, the aforementioned reaction of converting carbamate to urea reaches an equilibrium point. The molar ratio of ammonia and carbon dioxide in the urea solution leaving the reactor is essentially in the range of 2.5 to 4 mol/mol. It is also possible to combine the condenser and reactor in one set of equipment. An example of such a set of equipment is described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. The resulting carbamide solution leaving the urea reactor is fed to a stripping device, and inert steam containing non-condensed ammonia and carbon dioxide is sent to the purification section operating at the same pressure as the reactor. In this purification section, inert steam is removed from ammonia and carbon dioxide. The absorbent used in this treatment section is a carbamate solution from the downstream regeneration system. In the invention, the resulting carbamate in the combined condensate can be sent to the aforementioned scrubber as an absorbent and/or to the condensation zone in the synthesis section.

The invention is not limited to any particular process for the production of urea. Other methods and apparatuses include those based on technology such as the HEC method developed by Urea Casale, the ACES method developed by Toyo Engineering Corporation, and the method developed by Snamprogetti.

As for the melamine production zone, it is preferably a melamine production plant using the so-called high pressure treatment. Methods for the production of melamine are described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A16, fifth ed., pp. 174-179.

The invention also relates to a plant for the combined production of urea and melamine. Said plant comprises a urea production zone and a melamine production zone, which may take the form of a combined plant or two associated separate plants, as essentially explained above. The urea production zone contains a urea synthesis section and a carbamate recycling section containing a liquid/gas separation system; the melamine production zone contains a melamine synthesis section and an off-gas separation section. In order for the production of urea and melamine to be combined, the plant includes a line for transporting a fluid medium from the urea production zone to the melamine production zone, wherein said line is configured to supply the formed carbamide to the melamine synthesis section, and a gas flow line from the off-gas separation section of the melamine production zone to urea synthesis section. In accordance with the invention, the plant further comprises a gas flow line from the liquid/gas separation system to the condenser.

The aforementioned gas flow line from the melamine off-gas separation section is directed to the gas inlet of the condenser of said melamine off-gas. The condenser contains a liquid outlet for the condensed gas. The gas flow line from the liquid/gas separation section of the urea production zone is fed to the gas inlet of a steam condenser containing ammonia, carbon dioxide and water, said condenser also containing a liquid outlet for the condensed gas. According to the invention, the melamine off-gas condenser and the vapor condenser are part of a condensation system in which said liquid outlets are combined into a single fluid transport system. As will be understood from the foregoing, the condensing system may comprise a single condenser (in which the gas inlets of both streams may be different, or may be the same if the gas streams are combined at some location upstream of the condenser), or it may contain two or more condensers to separately condense the gas streams and then combine the condensates.

Said condensation system preferably includes an annulus one

- 7 042169 or more shell-and-tube heat exchangers.

In a preferred embodiment, the urea production zone is based on a stripping technology, i.e. a urea stripping plant. The synthesis section of such an installation contains a reactor and a stripper. In a further preferred embodiment, preferably combined with the stripping technology described above, the condensation zone is in a state of indirect heat exchange with the aqueous urea solution stream.

Preferably, indirect heat exchange is carried out in a shell-and-tube heat exchanger, wherein the dissociated carbamate vapor and off-gas from melamine production are fed into the annular space of the heat exchanger, and the aqueous carbamate solution is fed into its tube part.

Thus, it should be understood that in the preferred embodiment, the condensing system is part of the concentrator. From the aqueous urea solution stream fed to said concentrator, water will evaporate, resulting in the formation of a concentrated urea solution and water vapor.

In view of its preferred use, namely for concentrating an aqueous solution of urea, the condensation system preferably comprises one or more liquid supply lines that are in fluid communication with the urea synthesis section.

Preferably, at least a portion of said fluid communication passes through the liquid/gas separation system. In yet another preferred embodiment, the urea production zone further comprises a flashing system between the urea synthesis section and the carbamate recycle section. The flashing system is in fluid communication with the urea synthesis section, which allows the urea synthesis stream to enter the fluid inlet of the flashing system. The flashing system has a gas outlet allowing dissociated carbamate vapor separated from the urea synthesis stream by flashing to exit the flashing system. Preferably, said gas outlet is or comprises a gas flow line to the same condensation system as the melamine production off-gas. The flashing system has a liquid outlet in fluid communication with the carbamate recirculation section, which allows the urea solution remaining after flashing to be directed to the recirculation section, in particular to its liquid/gas separation system.

Flash evaporation systems are known to the skilled person.

The invention also provides a method for reducing the energy consumption of the urea production area of an existing urea/melamine co-production plant. The plant contains a carbamide production zone and a melamine production zone; the urea production zone includes a urea synthesis section and a carbamate recycling section containing a liquid/gas separation section; the melamine production zone contains a melamine synthesis section and an off-gas separation section. According to the invention, in one embodiment, the method includes adding a concentrator to a urea production zone, said concentrator comprising an indirect heat exchange system with a gas stream. The concentrator typically includes a vaporization chamber for heating a liquid, said vaporization chamber being in heat exchange communication with an indirect heating chamber for heating the fluid. The method of the invention further includes providing a conveyance line for supplying liquid from the liquid/gas separation section of the urea plant to said flash chamber. Additionally, a gas flow line is provided from the liquid/gas separation section of the urea plant to the condensation system, said condensation system having a liquid outlet in fluid communication with the indirect heating chamber. Finally, the invention includes providing a gas flow line from the off-gas separation section of the melamine production zone to the condenser system.

The most advantageous application of the invention is to implement a process for improving the productivity of a melamine production area of an existing urea/melamine co-production plant. It should be understood that said plant comprises a urea production zone and a melamine production zone; the urea production zone includes a urea synthesis section and a carbamate recycling section containing a liquid/gas separation section; the melamine production zone contains a melamine synthesis section and an off-gas separation section. The process of the present invention includes improving the melamine synthesis capacity of the melamine synthesis section. This can be done in known ways, for example by increasing the size of the melamine reactor and downstream equipment, or changing the operating conditions so as to provide more urea for processing per unit time. The invention includes adding a concentrator to a urea production zone, said concentrator comprising a flash chamber for heating a liquid, said flash chamber being in heat exchange communication with an indirect heat chamber for heating the fluid. The invention further includes providing a transfer line for supplying liquid from the liquid/gas separation section of the urea plant to said flash chamber and providing a gas flow line from the liquid/gas separation section of the urea plant to the condenser system. The condensation system includes a liquid outlet in fluid communication with the indirect heating chamber. According to the invention, a gas flow line is further provided from the off-gas separation section of the melamine production zone to the condensing system.

As a result, the increased productivity of the melamine production zone, which occurs together with the increase in the output of off-gas containing carbon dioxide and ammonia, can be used to beneficially use the heat of condensation contained in said off-gas, as essentially described above herein. Preferably, the urea production zone contains a urea stripper. It should be understood that the particular benefits of increasing the productivity of melamine according to the invention are achieved when the urea production zone is based on stripping technology. Reference is made to the aforementioned problem of reduced stripping efficiency due to reduced carbon dioxide or ammonia supply to the stripper due to increased reactant supply from melamine production off-gas. The invention aims to overcome this problem by providing an additional source of heating, namely from the co-condensation of melamine production off-gas and dissociated carbamate vapour, as described above.

In general, any or both of the methods mentioned above, ie. the reduction in energy consumption in the urea production area and the increase in melamine production in the melamine production area can be used to convert the plant to co-production of urea and melamine.

In the following, the invention will be further illustrated with reference to the following non-limiting examples. All examples refer to a 1900 t/d urea plant.

Example 1 (comparative).

Fig. 1 illustrates a urea plant according to a typical CO2 stripping technology as described in Ullmann's Encyclopaedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. In this process, the carbon dioxide used as feedstock enters the high pressure stripper HPSTR via line b. In the HPSTR stripper, the effluent from the high pressure HPREA reactor is contacted in countercurrent with carbon dioxide, resulting in a drop in the partial pressure of ammonia and decomposition of the carbamate in this solution. The heat required to complete the decomposition of the carbamate with the desired efficiency is supplied by passing high pressure saturated steam around the tubes of the HPSTR high pressure stripper. The vapor leaving the HPSTR stripper, containing ammonia, carbon dioxide and a small proportion of water, is sent via line c to the high pressure carbamate HPCC condenser. The high pressure carbamate HPCC condenser can be a falling film condenser, an evaporative type condenser, or an immersion type condenser. In the high pressure carbamate HPCC condenser, the released heat of condensation is used to produce saturated steam at a pressure of 0.2 to 0.6 MPa, which is used, for example, for heating in downstream processing in a urea plant. Ammonia as a feedstock is fed into the high pressure carbamate HPCC condenser through line a before the high pressure ejector HPEJ, in which said ammonia is used as a driving force to transfer the resulting carbamate in the high pressure scrubber HPSCR through line g to said high pressure carbamate HPCC condenser through line h.

The resulting carbamate in the high pressure carbamate HPCC containing ammonia, carbon dioxide, water and optionally urea, together with the non-condensed vapor leaving said high pressure carbamate condenser containing ammonia, carbon dioxide, water, nitrogen, oxygen and low concentrations of other inert components , enters the high pressure HPREA reactor under the action of gravity along line d. In the high pressure reactor, the remaining ammonia and carbon dioxide are converted to urea until equilibrium is reached. The resulting urea solution exiting the high pressure HPREA reactor, containing urea, ammonia, carbon dioxide, water and small amounts of other components, flows by gravity into said HPSTR stripper. The inert vapor exiting the high pressure HPREA via line e, containing uncondensed ammonia, carbon dioxide, nitrogen, oxygen and small amounts of other inert components, is fed to the high pressure HPSCR scrubber. In the high pressure HPSCR scrubber, most of the non-condensed ammonia and carbon dioxide are separated from the inert vapor containing mainly nitrogen and oxygen components by contacting the vapor leaving the high pressure HPREA reactor with the carbamate solution along the ac line formed during the following in the processing stream at the urea plant. Optionally, the carbamate solution exiting the high pressure HPREA reactor first passes through a condenser where, after the release of inert vapor, it comes into contact with said carbamate solution from further processing. The carbamate formed in this HPSCR scrubber is sent via line g to the high pressure ejector HPEJ. The inert vapor leaving the HPSCR high pressure scrubber via line f is released to the atmosphere or further processed, for example in an absorber, and then vented to the atmosphere.

The urea solution exiting the high pressure stripper HPSTR is sent to the recirculation section LPREC via line aa. The recirculation section LPREC operates at a lower pressure than the pressure in the synthesis section. In this section of the LPREC recirculation, a separation takes place between the unconverted ammonia and carbon dioxide and the aqueous urea solution. The separated ammonia and carbon dioxide are condensed to form a carbamate solution, which is returned via line ac to the HPSCR high pressure scrubber in the synthesis section. The urea solution, containing urea, water and small amounts of ammonia and carbon dioxide, is collected in the UVV storage tank, where the ad line solution is then concentrated by evaporating the EVAP to the desired concentration needed to finish the ae line product to a final solid state. Energy consumption, expressed in kilograms of steam per ton of urea produced, typically ranges from 870 to 950 kg per ton, depending on the quality of the steam and the type of finish used to produce the final product. In this example, the energy consumption is 920 kg/ton.

Example 2 (comparative).

Fig. 2 illustrates the method presented in the prior art and described in WO 98/08808 A1.

This method includes the urea plant described for FIG. 1 (capacity 1900 t/d) and VAPMEL steam from a melamine plant containing ammonia, carbon dioxide and optionally water and an inert material is added to the high pressure carbamate HPCC condenser in the synthesis section.

Fig. 3 is a graph in which typical energy consumption in kilograms of water vapor per ton of urea produced is plotted against the throughput of an attached melamine plant at constant urea production.

As expected, the energy consumption of the urea plant increases if the productivity of the associated melamine plant is increased by increasing the amount of steam from said melamine plant to be processed by said urea plant.

Example 3 (comparative).

Fig. 4 illustrates an exemplary embodiment in accordance with the prior art described in US 7,893,298 B2. This method includes the urea plant described for FIG. 1 (capacity 1900 t/d), and the VAPME vapor from the melamine plant, containing ammonia, carbon dioxide and optionally water and inert material L, is condensed in a CARBCOND condenser, after which the resulting carbamate solution is added to the synthesis section of said urea plant . In this example, said nascent carbamate is added via line af to the high pressure HPSCR scrubber of the urea synthesis section, however it is also possible to add this carbamate solution or part of the solution elsewhere in the urea synthesis section, such as the high pressure carbamate HPCC condenser of the urea plant. In the CARBCOND condenser, the normally formed carbamate in the downstream of the urea plant is added as a solvent through line ac, and typically the water concentration in said carbamate solution from the LPREC recycle section is higher than the water concentration in the resulting carbamate solution leaving the CARBCOND condenser. Typically, the pressure in the CARBCOND condenser is higher than the pressure in the LPREC recirculation section from which carbamate is added, and lower than the pressure in the section of the melamine plant where the VAPMEL vapor from the melamine plant is exhausted. In the CARBCOND condenser, the generated heat of condensation is dissipated in cooling water or used to generate water vapor that can be used in downstream processing in a urea plant.

Fig. 5 is a graph in which typical energy consumption in kilograms of water vapor per ton of urea produced is plotted against the throughput of an attached melamine plant at constant urea production. The energy consumption of the combined urea plant and melamine plant at constant production of the urea plant shows that there is a slight increase relative to the original urea plant not combined with melamine production.

Example 4

In FIG. 6 shows the plant and method according to the invention. This method includes a urea plant as described in FIG. 1 (1900 tons/day). The urea solution leaving the HPSTR stripper in the synthesis section of the urea plant via line aa is added to the low pressure recirculation system. Steam from the melamine plant, containing ammonia, carbon dioxide and optionally water, is added to the PREEVAP concentrator, in which the urea solution leaving the LPREC recirculation system through line bd,

- 10 042169 concentrate, after which the concentrated solution is collected in the storage UVV line bb.

The PREEVAP concentrator is preferably a shell-and-tube heat exchanger in which vapors leaving the melamine plant are preferably added to the annular space of this concentrator and the urea solution leaving the recirculation section is preferably added to the tubular part of said concentrator. In addition, the carbamate solution formed in the recirculation section is also added via the ac line to the annulus of the PREEVAP concentrator. The resulting carbamate in said PREEVAP concentrator is preferably transferred by pump via line af to the synthesis section. In this example, this carbamate solution is added to the HPSCR scrubber in the synthesis section, but it is possible to add it elsewhere in the synthesis section, such as the high pressure carbamate HPCC condenser. The non-condensed steam leaving the annular space of this concentrator is usually sent to an absorber located in the urea plant. Typically, the concentration of water in said added carbamate solution from the LPREC recirculation section is higher than the concentration of water in the resulting carbamate solution exiting the PREEVAP concentrator. Typically, the shell side pressure of the PREEVAP condenser concentrator is equal to or lower than the operating pressure of the VAPMEL vapors added from the melamine plant, but higher than the pressure in the low pressure LPREC recirculation section.

Fig. 7 is a graph in which typical energy consumption in kilograms of water vapor per ton of urea produced is plotted against the throughput of an attached melamine plant at constant urea production.

This graph shows that the energy consumption of a combined urea and melamine plant at constant urea production is about 20-50 kg of steam per tonne of urea produced less compared to a typical urea plant not combined with melamine production, as shown in fig. 1. In addition, the steam consumption of the urea plant at a constant production rate decreases as the productivity of the melamine plant increases.

Example 5

Fig. 8 illustrates another embodiment of the invention. This method includes a urea plant as described in FIG. 1 (capacity 1900 tons/day). The urea solution leaving the high pressure stripper HPSTR in the synthesis section of the urea plant via line aa is expanded adiabatically by FLASH to a pressure of 1.0 to 8.0 MPa, more specifically 1.5 to 6.0 MPa. Upon expansion, a significant part of the ammonia, carbon dioxide and water is separated from the urea solution, which contains mainly urea and water as components, as well as some residual amount of ammonia and carbon dioxide dissolved in the urea solution. Said urea solution is discharged via line be to the LPREC recirculation section of the urea plant. The flashing vapor from the FLASH adiabatic flashing stage, containing ammonia, carbon dioxide and water, is discharged via line ba to the PREEVAP concentrator, in which the urea solution leaving the LPREC recirculation system via line bd is concentrated, after which the concentrated solution is collected in UVV storage along the bb line.

The steam leaving the melamine plant, containing ammonia, carbon dioxide and optionally water, is also added to the PREEVAP concentrator. In addition, the PREEVAP concentrator is preferably a shell-and-tube heat exchanger, in which the vapors leaving after the adiabatic expansion of FLASH and the vapors leaving the melamine plant are preferably added together to the annulus of this concentrator, and the urea solution leaving the recirculation section is preferably added to the pipe part of the specified concentrator. Moreover, the carbamate solution formed in the LPREC recirculation section is also added via line ac to the annular space of the PREEVAP concentrator. The resulting carbamate in said PREEVAP concentrator is preferably transferred by pump via line af to the synthesis section. In this example, this carbamate solution is added to the HPSCR scrubber in the synthesis section, but it is possible to add it elsewhere in the synthesis section, such as the high pressure carbamate HPCC condenser. The non-condensed steam leaving the annular space of this concentrator is usually sent to an absorber located in the urea plant. Typically, the concentration of water in said added carbamate solution from the LPREC recirculation section is higher than the concentration of water in the resulting carbamate solution exiting the PREEVAP concentrator. Typically, the shell side pressure of the PREEVAP condenser concentrator is equal to or lower than the operating pressure at which FLASH adiabatically expands, although this pressure is higher than the pressure in the LPREC recirculation section from which the carbamate is added.

Fig. 9 is a graph in which typical energy consumption in kilograms of water vapor per ton of urea produced is plotted against the throughput of an attached melamine plant at constant urea production.

This graph shows that the energy consumption of a combined urea and melamine plant at a constant urea production for approximately 50-150 kg of steam

- 11 042169 less per tonne of urea produced compared to a typical urea plant not combined with melamine production, as shown in FIG. 1. In addition, the steam consumption of the urea plant at a constant rate decreases with the increase in the production rate of the melamine plant.

Example 6

This example relates to the embodiment of the invention shown in FIG. 10. In several aspects, this embodiment is similar to the embodiment described in Example 5. However, in the present example, a PREEVAP concentrator is located downstream of the UVV storage and upstream of the EVAP evaporation section where the urea solution is concentrated to the desired concentration required for final processing of the resulting carbamide melt to the final product. The off-gas leaving the high pressure reactor in the synthesis section is sent via line e to flash steam from the FLASH adiabatic flash process. This embodiment is more preferable than the embodiment shown in FIG. 5, because this embodiment avoids the formation of ammonia vapor in the UVV storage, and due to the large volumes of melamine production in the plant, for example, it becomes possible in this embodiment to concentrate the urea solution in the PREEVAP concentrator more deeply without the formation of ammonia vapor in the stored solution urea UVV compared to the embodiment of Example 5 shown in FIG. 8.

Fig. 11 is a graph in which typical energy consumption in kilograms of water vapor per ton of urea produced is plotted against the throughput of an attached melamine plant at constant urea production.

This graph shows that the energy consumption of a combined urea and melamine plant at constant urea production is about 50-150 kg of steam per tonne of urea produced less compared to a typical urea plant not combined with melamine production, as shown in fig. 1. In addition, the steam consumption of the urea plant at a constant rate decreases slightly with the increase in the production rate of the melamine plant.

Claims (10)

CLAIM 1. A method for the combined production of urea and melamine, in which: (i) carbon dioxide and ammonia in the urea production zone are subjected to urea formation conditions that form a urea synthesis stream containing urea, water and ammonium carbamate, said urea production zone comprising a urea synthesis section that includes a reactor and a stripper; (ii) treating the urea synthesis stream to form an aqueous solution of urea and steam containing ammonia, carbon dioxide and water, said treatment comprising stripping in said stripper to obtain a stripped urea solution and processing said stripped urea solution in a carbamate recycling section, comprising a liquid/gas separation section; (iii) supplying the urea to the melamine production zone; (iv) subjecting the urea supplied to the melamine production zone to the conditions under which melamine is formed to produce melamine and a melamine production off-gas containing carbon dioxide and ammonia; (v) supplying said off-gas to a urea production zone; (vi) condensing said vapor from step (ii) to form a first carbamate solution, and condensing said melamine production off-gas in the presence of said first carbamate solution, thereby providing a dilute melamine production off-gas condensate, wherein the melamine production off-gas is condensed in indirect heat exchange with said urea aqueous solution stream, thereby concentrating said urea aqueous solution stream by thermal evaporation; and (vii) recycling said diluted melamine off-gas condensate to the urea synthesis section. 2. The method according to claim 1, in which indirect heat exchange is carried out in a shell-and-tube heat exchanger, wherein said steam and off-gas from melamine production are fed into the annular space of the heat exchanger, and an aqueous solution of carbamate is fed into its pipe part. 3. A process according to claim 1 or 2, wherein said steam is brought to a pressure sufficiently equal to that of the off-gas from the melamine production to ensure gas phase pooling. 4. The method according to any one of claims 1 to 3, wherein the urea synthesis section further comprises a condenser and a scrubber, wherein the operating pressure in the urea synthesis section is from 12 to 20 MPa. 5. Process according to any one of claims 1 to 4, wherein the dilute condensate contains 20 to 35 wt% water. 6. The method according to any one of claims 1 to 5, in which said distillation is carried out using a pro- - 12 042169 counter-current flow of stripping gas selected from ammonia and carbon dioxide. 7. Installation for the combined production of urea and melamine in accordance with the method according to p. 1, containing the zone of production of urea and the zone of production of melamine; the urea production zone comprising a urea synthesis section and a carbamate recycle section comprising a liquid/gas separation section providing a flow (bd) of an aqueous urea solution; wherein said urea synthesis section comprises a reactor (HPREA) and a stripper (HPSTR), wherein the urea production zone comprises a concentrator (PREEVAP), wherein said concentrator contains an evaporation chamber for a stream (bd) of an aqueous solution of urea to be heated and evaporated, wherein said the evaporation chamber contains a gas outlet for the evaporated liquid; the melamine production zone comprises a melamine synthesis section and an off-gas separation section; moreover, the specified installation contains a line for transporting a fluid medium from the urea production zone to the melamine production zone, while the specified line is configured to supply the resulting urea to the melamine synthesis section, the gas flow line (VAPMEL) from the off-gas separation section of the melamine production zone to the urea synthesis section wherein said gas flow line is in fluid communication with the gas inlet of the melamine off-gas condenser, and said condenser comprises a liquid outlet (af) for the carbamate solution, and a gas flow line from the liquid/gas separation section in fluid communication with the vapor condenser, containing ammonia, carbon dioxide and water, and said condenser contains a liquid outlet (ac) for carbamate solution, and said condenser for melamine production off-gas and said condenser for said vapor are part of the condensation system, moreover, said liquid outlets are combined into a single system the subject of conveying a fluid, wherein said condensing system is configured to subject said melamine production off-gas to condensation in indirect heat exchange with said aqueous urea solution stream (bd) contained in said evaporator chamber; wherein one or more liquid outlet lines of the condensation system are in fluid communication with the urea synthesis section. 8. Plant according to claim 7, wherein the condensing system comprises an annulus of one or more shell and tube heat exchangers. 9. The plant according to claim 7 or 8, comprising a flash section with a liquid inlet in fluid communication with the liquid outlet of the urea synthesis section, a liquid outlet in fluid communication with the liquid inlet of the carbamate recirculation section, and a flow line gas to the condensing system. 10. A method for converting an existing urea-melamine co-production plant into a urea-melamine co-production plant according to claim 7, said existing plant comprising a urea production zone and a melamine production zone; wherein the urea production zone comprises a urea synthesis section and a carbamate recycling section comprising a liquid/gas separation section; the melamine production zone comprises a melamine synthesis section and an off-gas separation section, wherein the process involves adding a concentrator (PREEVAP) to the urea production zone, said concentrator comprising a flash chamber for a stream of an aqueous urea solution to be heated and vaporized, said flash chamber contains a gas outlet for the evaporated liquid and is in heat exchange communication with the indirect heat exchange chamber; both provide a conveyance line for supplying a stream (bd) of an aqueous solution of urea from the liquid/gas separation section of the urea plant to said flash chamber; providing a gas flow line from the liquid/gas separation section of the urea plant to a condenser for steam containing ammonia, carbon dioxide and water, which contains a liquid outlet (ac) in fluid communication with the indirect heat exchange chamber; and providing a gas flow line (VAPMEL) from the off-gas separation section of the melamine plant to the indirect heat exchange chamber.