Patent Application
20250100978
The present invention relates to a process for the production of melamine and the relative plant.
There are essentially two types of processes for the production of melamine currently applied industrially, low-pressure processes (2-10 bar) with catalytic synthesis and non-catalytic high-pressure processes (>70 bar).
In both processes, melamine is produced starting from urea according to the following overall reaction:
NH3 and CO2 are a product of the conversion reaction of urea into melamine and this stream leaving the reactor in gaseous form is called off-gas.
In so-called integrated processes for the synthesis of melamine and urea, the off-gas stream is recycled to the process for the production of urea wherein NH3 and CO2 are converted into urea according to the following two reactions in series:
One of the most widespread industrial processes for the production of melamine is based on the pyrolysis of urea at high pressure and is described in the state of the art by U.S. Pat. No. 7,125,992 and EP2385043 in the name of the same Applicant.
The state of the art of the processes that produce urea from NH3 and CO2 is described in literature in various publications, among the most authoritative publications, volume 37, chapter: “Urea” pages. 658-683 of Ullmann's Encyclopedia of Industria Chemistry, 7th edition revision, should be considered.
The integration of the urea process with the melamine process is widely described in the prior art, such as, for example, in EP2094652.
As described in the prior art, when the urea process is integrated with the melamine process, part of the urea produced by the urea process is used for the production of melamine and the off-gas stream generated by the melamine synthesis is recycled to the urea process for reconverting the ammonia and carbon dioxide contained in the off-gas stream back into urea.
According to the state of the art, the integration of the melamine process with the urea process mainly consists in identifying a section or apparatus, existing or new, of the urea plant, into which the off-gas stream coming from the melamine process is to be injected, with the aim of minimizing or cancelling the negative effects due to the recycling of the off-gas stream.
According to the state of the art, the integration is advantageous as, by reconverting the off-gas into urea, it allows the production of melamine by consuming urea in stoichiometric quantities according to reaction (1), i.e. producing melamine with a consumption of three moles of urea per mole of melamine, which equals 1.4 tonU/tonM.
This integration, however, involves a series of technical problems: a first problem is that, when the two processes are integrated, the melamine process influences the operating capacity of the urea process with an increase in recycled streams and consequently an increase in energy consumption.
This problem is due to the fact that the off-gas stream represents an additional supply stream of the reagents (NH3 and CO2) to the urea process, and as such, the off-gas stream influences the load of the urea plant and the quantity of urea produced. When the melamine process is integrated with the urea process, the off-gas stream from the melamine process is, in fact, added as a supply stream to the two typical supply streams of the urea process which are a stream of liquid NH3 and a gaseous stream of CO2. A variation in terms of flow-rate, pressure or temperature of the off-gas stream coming from the melamine process consequently leads to a variation in the operating conditions of the urea process.
If, for example, the off-gas stream recycled to the urea process has a change in the flow-rate due to the trend of the melamine process, the urea process must compensate by modifying the flow-rate of the other two supply streams NH3 and CO2, in order to keep the production of urea constant. Furthermore, if the pressure of the off-gas stream decreases, the section of the urea plant receiving the off-gas stream must decrease its operating pressure to allow the off-gas stream to enter the urea process. If, on the other hand, it is the temperature of the off-gas stream from the melamine process that varies, the urea process must vary the other streams entering the section where the off-gas stream is recycled in order to maintain the required operating conditions. Any operational variation in the melamine plant is consequently reflected in an operational variation in the urea plant with the problem of running the urea plant in a non-optimal manner compared to the operating capacity guaranteed by feeding only the two streams of liquid NH3 and gaseous CO2 and consequently with an increase in the recycled streams and energy consumption.
Based on what has already been specified, the addition of a third reagent supply stream (i.e. the off-gas stream) inevitably leads to a reduction in gaseous CO2 and liquid NH3 fed to the urea process to counterbalance the CO2 and NH3 fed as off-gas and maintain the production capacity of the urea process.
Furthermore, in urea processes that use CO2 as a stripping agent in the high-pressure decomposer (CO2 stripping process), a reduction in the quantity of gaseous CO2 in the feed leads to a reduction in the stripping efficiency which must be compensated by increasing the steam and energy consumption.
A second problem is the loss of efficiency of the urea process when it is integrated with a melamine process, particularly when the off-gas stream is recycled in the form of liquid carbamate. The water used for the condensation of the off-gas stream to carbamate has a negative effect on the conversion of the reagents into urea inside the reactor, with a consequent increase in the recycled streams and therefore in the energy consumption per ton of urea produced.
A further problem is that when a urea process already exists and is integrated with a melamine process, the integration may involve stopping the urea process to allow, for example, the off-gas stream to be connected with the urea reactor or with one of the sections downstream of the urea synthesis section. If, in fact, the integration also requires an increase in the production capacity (capacitive revamping) of the urea plant, the integration cannot be carried out during the scheduled shutdowns of the urea plant. In this case, without any doubt, stopping the urea process for the time necessary for carrying out the integration with the new melamine process represents a significant economic loss due to the lack of urea production.
The integration of an existing urea process with a new melamine process may in fact require the need for modifying the urea process in order to increase its production capacity (capacitive revamping), when the same is not sufficient for reconverting the additional stream of reagents recycled with the off-gas stream into urea, and reducing the flow-rate of the gaseous CO2 stream and the liquid NH3 stream and consequently reducing the urea produced, is not required. Capacitive revamping involves a higher cost than simply connecting the two processes as it requires the addition or replacement of equipment and therefore longer stoppage times. Capacitive revamping due to the integration of the existing urea process with the melamine process can lead to a significant economic loss due to the lack of urea production and, in some cases, the cost of lost production could make the implementation of a new melamine process unprofitable.
In any case, an integrated urea/melamine process, such as those described in the state of the art, is simply a process wherein the urea synthesis process substantially continues to operate as a non-integrated urea synthesis process and the melamine synthesis process continues to operate as a non-integrated melamine synthesis process: so-called “integration” means that a part of the urea synthesized in the urea process is sent to the “integrated” melamine process which uses it as a raw material; the off-gas deriving from the melamine synthesis process is sent to the “integrated” urea synthesis process as raw material.
For the rest, the two processes/plants essentially continue to function independently of each other.
The present invention aims to overcome the disadvantages of the known art previously indicated. The objective of the present invention is in fact an innovative process scheme for the production of melamine (Stand-Alone Melamine Process) which does not need to be integrated with the urea process and which allows the problems listed above to be overcome.
The present invention relates to a process for the production of melamine by means of a high-pressure urea pyrolysis reaction, comprising the following operating steps:
f) a urea concentration step CON to which the fourth stream 32 comprising urea and H2O obtained at the end of the low-pressure decomposition step e) LPD is fed, obtaining a stream of concentrated urea 33 which is fed to the off-gas scrubbing step OGQ with molten urea and a stream of H2O 34 which is fed to the process-water treatment step PWT.
The stream 23, comprising NH3, CO2 and melamine vapours, fed to the scrubbing step with urea OGQ, can comprise the gaseous stream 21 alone, comprising NH3, CO2 and melamine vapours, obtained together with a liquid stream 3 of raw melamine comprising melamine, unreacted urea, oxidized intermediate products of OAT pyrolysis and deammoniation and condensation products of melamine (polycondensates), from the separation of a biphasic liquid-gaseous effluent produced by the urea pyrolysis reaction R.
Alternatively, the stream 23, comprising NH3, CO2 and melamine vapours, fed to the scrubbing step with urea OGQ, can comprise said first stream of anhydrous off-gas 21 and a second stream 22 of anhydrous off-gas comprising NH3, CO2 and melamine vapour, obtained by putting said liquid stream 3 of raw melamine in contact, in a post-reaction and stripping with ammonia step PR, with a stream of gaseous NH3, anhydrous and superheated under pressure 16P, to form a liquid stream 4 of raw melamine depleted of CO2 and said second stream 22 of anhydrous off-gas.
Off-gas streams 27, 29 and 31 are mainly composed of NH3, CO2 and H2O and may further comprise passivation air.
In the process according to the present invention, said liquid stream 4 of raw melamine depleted of CO2, before being subjected to a purification step P and a crystallization step C by cooling, is subjected to a quench-ammonolysis treatment QAL through contact with at least one aqueous ammonia stream 12, with the formation of an aqueous ammonia stream 5 comprising purified melamine substantially free of polycondensates, said quench-ammonolysis treatment QAL being preferably carried out by contact with said aqueous ammonia stream 12 coming from the process-water treatment step PWT and with a pure ammonia stream 15 coming from the ammonia recovery step AR.
Said aqueous ammonia stream (5), comprising purified melamine substantially free of polycondensates, is fed to the purification step P from which a solution of purified melamine 6 is obtained, fed to the cold crystallization step C, from which a suspension of melamine 7 is obtained, then fed to a separation step S from which a stream of pure melamine 200 and a stream of mother liquor 9 are obtained, said mother liquor stream 9 comprising H2O, traces of melamine and OATs being partly fed to the process-water treatment step PWT, where it is put in contact with the aqueous stream 34 coming from step f) and with the off-gas stream 31 comprising NH3, CO2 and water coming from the low-pressure decomposition LPD step e), obtaining at the end of the process-water treatment step PWT, a stream of pure water 300, the liquid stream 12 comprising ammonia and water which is sent to the quench-ammonolysis step QAL, a liquid stream 13 comprising water, ammonia and CO2 which is sent to the ammonia recovery step AR and the gaseous stream 14 comprising water, ammonia and CO2 which is sent to the off-gas condensation section OGC.
More specifically, in the process-water treatment step PWT, the stream of mother liquor 9, comprising H2O, traces of melamine and OATs is subjected to a decomposition treatment by hydrolysis at a temperature ranging from 270° C. to 300° C., with the formation of a gaseous stream 14, comprising NH3, CO2 and H2O, and a liquid stream which is subjected to a stripping treatment in order to obtain a liquid stream 300 of pure water.
Said ammonia recovery step AR preferably removes from the liquid stream 13 comprising water, ammonia and CO2 coming from the process-water treatment step PWT and from the stream of off-gas 29 coming from the medium-pressure decomposition step MPD, at least a part of the ammonia contained therein, forming a liquid stream 17 comprising NH3, CO2 and water which is sent to step a) for the condensation of the off-gas OGC and a stream of pure ammonia which is sent partly 15 to the quench-ammonolysis step QAL, partly 16R to the pyrolysis reaction R and partly 16P, to the post-reaction and stripping with ammonia step PR. The ammonia that is sent to the pyrolysis reaction R and to the post-reaction and stripping step PR is preferably gaseous ammonia, anhydrous and superheated under pressure.
In an embodiment of the process according to the present invention, (represented in
In a different embodiment of the process according to the present invention, (represented in
In a further embodiment of the process according to the present invention, (represented in
The three embodiments represented in
In the present description, when the definition “high pressure” is used, it refers to a pressure range that varies from 70 to 220 bar, preferably from 80 to 150 bar; the definition “medium pressure” refers to a pressure range that varies from 8 to 30 bar, the definition “low pressure” refers to a pressure range that varies from 1 to 8 bar.
The present invention further relates to a plant for the production of melamine as defined in claims 9-14.
In the plant according to the present invention, there are suitable pumps for correctly moving the fluid streams when necessary based on pressure differences, and there are also suitable heat exchangers when necessary based on temperature differences. These devices are not shown in the figures or specified in the description of the various lines as they are of common knowledge to skilled persons in the field.
In the plant according to the present invention the sections OGC and OGQ operate at the same pressure as the section R and, as the section OGC is connected to the section HPD, the section HPD operates at the same pressure as the section R: consequently stream 14 fed to the section OGC is brought to the same pressure.
The present invention is described hereunder with reference to the following schematic figures which are illustrative and non-limiting with respect to the protection scope defined by the enclosed claims.
In the figures attached to the present invention,
In the following description, for illustrating the figures, identical reference numbers are used for indicating elements with the same function. Furthermore, for clarity of illustration, some numerical references may not be repeated in all the figures.
The different characteristics in the different embodiments can be combined together as desired according to the previous description, should the advantages resulting specifically from a particular combination be used.
A first embodiment of the Stand-Alone Melamine Process, object of the present invention, is illustrated in the block diagram of
a scrubbing section of the off-gas OGQ for recovering the melamine vapour contained in the anhydrous off-gas coming from the Reaction section R and the Post-Reaction section PR. The anhydrous off-gas streams 21 and 22 coming from the sections R and PR are subjected, together or separately, to a melamine recovery treatment by scrubbing with urea, in a single off-gas scrubbing section OGQ or in separate scrubbing sections (not shown in
The second stream 22 of anhydrous off-gas coming from the Post-Reaction section is preferably combined with the first stream 21 of anhydrous off-gas coming from the reaction section, forming a single stream 23 of anhydrous off-gas which is subjected to the same treatment in a single scrubbing section of the off-gas with urea OGQ. The section OGQ operates at approximately the same pressure as the Reaction R and Post-Reaction PR sections, whereas the scrubbing temperature varies within the range of 140° C. to 250° C. In the scrubbing with urea, the melamine contained in the anhydrous off-gas is removed and brought into solution/suspension in the urea. The stream of urea 1 fed to the off-gas scrubbing section OGQ is obtained from the combination of the molten urea stream from the battery limits 100 and the recycled concentrated urea stream 33, coming from the urea concentration section CON. The urea stream 2 exiting from the off-gas scrubbing section OGQ is fed to the Reaction section R, whereas the anhydrous off-gas stream 24 is fed to the off-gas condensation section OGC.
The Reaction section R comprises a pyrolysis reactor that operates continuously and in which a suitable heating system provides the reagent system with the necessary reaction calories, maintaining the temperature within the range of 360° C. to 420° C. The reaction pressure is kept at a value higher than 70 bar, preferably within the range of 70 to 220 bar, more preferably within the range of 80 to 150 bar. A stream of gaseous NH3, anhydrous and superheated under pressure 16R, coming from the ammonia recovery section AR is also preferably introduced into the reactor together, with the urea.
The product of the urea pyrolysis reaction, consisting of a two-phase effluent comprising a gaseous phase 21 and a liquid phase 3, is subjected to subsequent treatments. The liquid phase 3, exiting from the Pyrolysis Reactor R, containing melamine, unreacted urea, oxidized intermediate products of OAT pyrolysis and melamine deammoniation and condensation products (polycondensates), is sent to a subsequent reaction step in a Post-Reactor PR which operates under temperature and pressure conditions substantially the same as those of the Pyrolysis Reactor R and into which a stream of gaseous, anhydrous and superheated NH3 under pressure 16P is fed, in order to eliminate the dissolved CO2 and complete the urea pyrolysis reaction and transform most of the OATs into melamine, obtaining a liquid phase of melamine 4 and a second gaseous phase 22.
The liquid phase of melamine exiting from the Post-Reactor 4 is brought into aqueous solution in a quench-ammonolysis section QAL, in the presence of NH3, fed as pure NH3 by means of a stream 15 coming from the ammonia recovery section AR and as ammonia in solution by means of a stream 12 coming from the water treatment section PWT, the whole system being kept under controlled conditions of temperature and residence time, preferably at a temperature ranging from 160° C. to 180° C., more preferably from 170° C. to 172° C., at a pressure ranging from 25 to 45 bar, more preferably equal to 30 bar, for a time ranging from 30 to 60 minutes, more preferably equal to 45 minutes, obtaining a solution substantially free of polycondensates 5 which is brought to a purification section P to obtain a solution free of impurities 6 which is then subjected to crystallization in the crystallization section C, to obtain a melamine suspension 7 which is then fed to the separation section S to give high-purity melamine 200. A part of the mother liquor 10 containing melamine and small quantities of OATs is recycled from the section S, without any treatment, to the quench-ammonolysis section QAL.
The remaining part of the mother liquor 9 is treated in a process-water treatment section PWT. Said section PWT also receives an off gas stream 31 from the low-pressure decomposition section LPD and a liquid stream 34 of process water coming from the urea concentration section CON. A liquid stream containing water and ammonia 12 is obtained, through decomposition and distillation, from the process-water treatment section PWT, which is sent to the quench-ammonolysis section QAL, together with a liquid stream 13 containing water, NH3 and CO2 which is sent to the ammonia recovery AR section, a gas stream 14 containing NH3, CO2 and water sent to the off-gas condensation section OGC and a stream of purified process water 300 sent to the battery limits.
The ammonia recovery section AR, in addition to receiving the liquid stream 13 from the process-water treatment section PWT, also receives the off-gas gaseous stream 29 from the medium-pressure decomposition section MPD. In the ammonia recovery section AR, a stream of pure ammonia is obtained, by distillation, partly recycled to the quench-ammonolysis section QAL as stream 15, partly recycled to the reaction section R as stream 16R and partly recycled to the Post-reactor RP as stream 16P. A second liquid stream 17 is obtained from the ammonia recovery section AR, at the outlet, containing NH3, CO2 and water sent to the off-gas condensation section OGC.
Step a) of the process according to the present invention is carried out in the off-gas condensation section OGC which has an operating pressure equal to or slightly lower than the off-gas scrubbing section OGQ, which in turn has an operating pressure equal to or slightly lower than the reaction section R, and has an operating temperature such as to obtain the total condensation of the incoming gaseous streams. The off-gas condensation section OGC receives three gaseous streams, the off-gas stream 24 from the off-gas scrubbing section OGQ, the stream 14, comprising NH3, CO2 and water, from the process-water treatment section PWT and the off-gas stream 27 from the high-pressure decomposition section. Furthermore, the section OGC receives a liquid stream 17, comprising NH3, CO2 and water, from the ammonia recovery section AR. The condensation heat of the three gaseous streams is recovered by generating steam which is used within the Stand-Alone Melamine Process. The liquid stream 25 exiting from the section OGC comprising NH3, CO2 and water is pumped to the conversion section OGR, where step b) of the process according to the present invention takes place. The liquid stream 25 can be possibly preheated to improve the conversion of the off-gas in step b) of the process according to the present invention.
The off-gas conversion section operates at a pressure within the range of 120 bar to 250 bar, preferably at a pressure within the range of 140 bar to 220 bar. The operating temperature of the section OGR varies within the range of 150° C. to 250° C., preferably within the range of 170° C. to 220° C. and the molar ratio NH3/CO2 is equal to 2.2-5, preferably equal to 2.5-4.5 mole/mole.
The carbamate fed with stream 25, comprising NH3, CO2 and water, is partially converted into urea inside the section OGR. The stream 26 exiting from the section OGR contains urea, ammonia, unreacted carbamate and water. This stream 26 is sent to step c) of the process according to the present invention in the high-pressure decomposition section HPD where the liquid carbamate is decomposed into NH3 and CO2 in an exchanger to which heat is supplied by steam. The section HPD operates at the same pressure as the section OGC or slightly higher, and at an operating temperature within the range of 150 to 250° C., preferably 170° C. to 220° C. A gaseous stream 27 is formed at the outlet of the section HPD, which is recycled to the section OGC, together with a liquid stream 28 containing urea, water and also a part of non-decomposed carbamate, which is sent to step d) of the process according to the present invention in the medium-pressure decomposition section MPD. The section MPD operates at a pressure ranging from 8 to 30 bar, preferably at a pressure within the range of 12 to 24 bar and at an operating temperature within the range of 140° C. to 180° C., preferably 150° C. to 170° C. In the section MPD, part of the carbamate contained in stream 28 is decomposed, the necessary heat is supplied by steam, and a liquid stream 30 containing urea, water and a part of carbamate is obtained, which is sent to step e) of the process according to the present invention in the low-pressure decomposition section LPD together with an off-gas gaseous stream 29 sent to the ammonia recovery section AR. The section LPD operates at a pressure within the range of 1 to 8 bar, preferably at a pressure within the range of 2 to 6 bar and at an operating temperature within the range of 130° C. to 170° C., preferably 140° C. to 160° C. In the section LPD, the carbamate contained in the stream 30 is decomposed, the necessary heat is supplied by steam, a liquid stream 32 is obtained, mainly comprising urea and water, sent to step f) of the process according to the present invention in the urea concentration section CON and a gaseous stream 31 sent to the process-water treatment section PWT. The urea solution is concentrated in the section CON, obtaining a urea stream 33 which is recycled to the section OGQ and a process-water stream 34, mainly comprising water and a low concentration of urea, NH3 and CO2 which is sent and treated in the section PWT.
In the above-mentioned liquid stream 32, comprising mainly urea and water, the term “prevalently” means that the stream contains approximately 98-99% by weight of urea and water, the remaining percentage being composed of traces of ammonia and CO2. In the process-water stream 34, comprising mainly water and a low concentration of urea, NH3 and CO2, the term “prevalently” means that the stream contains approximately 98-99% by weight of water.
A second embodiment of the Stand-Alone Melamine Process, object of the present invention, is illustrated in the block diagram of
In a third embodiment of the process according to the present invention, (represented in
The three embodiments illustrated in the block diagrams of
The present invention as described above relates to a non-catalytic high-pressure melamine production process which does not need to be integrated with a urea process, as the off-gas produced from the synthesis of urea to melamine are converted back into urea within the same process for the production of melamine, thus solving the problems dependent on integration with a urea process.
The process for the production of melamine, object of the present invention, has an innovative element compared to the known art of being a scheme which uses some sections in common both in the steps of the melamine production process relating to the production, treatment and purification of the melamine streams and in the steps of the melamine production process according to the present invention relating to the treatment of the off-gas: these common sections have revealed surprising positive effects by solving the problems listed above.
In the process scheme according to the present invention, in fact, the process-water treatment section PWT, the ammonia recovery section AR and the off-gas condensation section OGC are common sections, i.e. they receive and return streams to the process sections for the production of melamine and the sections of the process for the treatment of off-gas and the production of urea.
The off-gas condensation section OGC has the purpose of condensing the high-pressure off-gas gaseous streams coming from the different steps of the process for the production of melamine according to the present invention and using the condensation heat for generating steam, in turn used in the melamine production process. The section OGC receives the anhydrous off-gas from the section OGQ, it receives the off-gas generated by the decomposition of the carbamate (a stream containing NH3, CO2 and H2O) from the section HPD, it receives an off-gas stream 14 from the common section PWT and a liquid stream of carbamate 17 from the common section AR. The section OGC returns a liquid stream of carbamate fed to the conversion section OGR to convert some of the carbamate back to urea.
The ammonia recovery section AR has the purpose of recovering pure ammonia and recycling it to the appropriate sections. The section AR receives the off-gas from the section MPD, it receives a liquid carbamate stream 13 from the section PWT and returns a liquid carbamate stream 17 to the section OGC and a stream of pure ammonia sent partly as stream 15 to the section QAL and partly as streams 16R, 16P to the reaction section R and to the post-reaction and ammonia stripping section PR.
The process-water treatment section PWT has the purpose of treating all the process water and recycling it to the appropriate sections. The section PWT receives the mother liquor from the section S with stream 9, the process water from the urea concentration CON with stream 34, the off-gas from the low-pressure decomposition section with stream 31 and returns a high-pressure off-gas stream 14 to the section OGC, a liquid stream 12 of NH3 and water to the section QAL and a liquid stream of carbamate 13 to the section AR.
It should also be pointed out that, unlike the sections HPD, MPD and LPD of the integrated melamine/urea processes of the state of the art, each of which comprises a decomposer (heater) and a condenser, the sections HPD, MPD and LPD of the process for the production of melamine according to the present invention require only the presence of a heater with considerable advantages in terms of installation costs and consumption. Condensers are not necessary thanks to the particular process scheme according to the present invention which involves the use of sections OGC, AR and PWT in common for different steps of the process.
A further advantage of the process according to the present invention and the relative plant is that the entire system is managed by a single operating control unit.
The following example represents a non-limiting embodiment of the present invention.
In a melamine plant with a nominal capacity of 40,000 t/y, 16.0 t/h of molten urea are sent to the off-gas scrubbing section OGQ to effect the scrubbing of the off-gas stream of 13.0 t/h, coming from the reaction sections R and PR, in order to obtain a stream of anhydrous off-gas, free of melamine; the section OGQ operates at the same pressure as the reaction sections R and PR, 110 bar and at a temperature of 215° C.
The molten urea containing the melamine recovered from the section OGQ and a stream of gaseous NH3 of 1.0 t/h are fed to the melamine reactor R; the reactor operates at 380° C. and 110 bar.
11.8 t/h of anhydrous off-gas comprising melamine steam and 5.7 t/h of raw liquid melamine are separated from the reactor. This raw liquid melamine is treated under the same reactor conditions in a post reactor PR with 1.1 t/h of anhydrous gaseous NH3 and superheated under pressure, obtaining 5.6 t/h of liquid melamine comprising only 0.02% by mass of dissolved CO2; this stream also contains 3.4% by mass of polycondensates, 0.6% by mass of OATs and is substantially free of unreacted urea.
1.2 t/h of anhydrous off-gas comprising melamine steam is separated from the post-reactor PR, which combined with the off-gas exiting from the reactor R form a stream of 13.0 t/h comprising 3.8% by mass of melamine steam which, as indicated above, is sent to the off-gas scrubbing section OGQ.
The liquid melamine exiting from the PR is sent to the quench-ammonolyser QAL for purification in aqueous solution with NH3, under the following operating conditions: 172° C., 30 bar and NH3 concentration 14% by mass. The aqueous solution present in the quench-ammonolyser is obtained by recycling to QAL an aqueous stream coming from the section PWT of 53.9 t/h having a concentration of NH3 at 15% by mass and a stream coming from the section AR of 0.2 t/h of pure ammonia.
67.1 t/h of an aqueous solution containing 8% by mass of melamine, 14% by mass of NH3 and comprising approximately 4,000 ppm by mass of OATs and approximately 500 ppm by mass of polycondensates, leave the QAL quench-ammonolyser.
This solution is fed to the purification section P, comprising filters, from which a solution containing less than 100 ppm by mass of polycondensates exits; the purification section P operates under the same conditions as the section QAL (pressure 30 bar and temperature 172° C.). The solution exiting from the section P is then sent to the crystallizer C, which operates at a temperature of 45° C., a pressure of 0.5 bar and a pH of approximately 11.5.
The suspension exiting from the crystallizer C is sent to a solid/liquid separator (centrifuge) S, which separates the crystallization mother liquor from the high-purity melamine (titre over 99.9% by mass on dry matter).
The approximately 62.1 t/h of mother liquor exiting from S is divided into two streams. A stream of 7.4 t/h is recycled directly to the quench-ammonolysis section QAL, without undergoing any treatment; the remaining 54.7 t/h are sent to the process-water treatment section PWT.
The section PWT, in addition to receiving the stream of mother liquor, receives a stream of process water from the urea concentration section CON and an off-gas stream from the low-pressure decomposition section LPD. The purpose of the section PWT is to purify the process water by thermally decomposing urea, OATs and melamine contained in the incoming streams obtaining NH3 and CO2, together with pure water to be sent to the battery limit. A stream of purified process water of 2.1 t/h sent to the battery limit, a liquid stream of water and ammonia of 53.9 t/h recycled to the section QAL, a liquid stream of carbamate (NH3, CO2 and H2O) of 3.7 t/h recycled to the section AR and a stream of gaseous off-gas of 0.9 t/h recycled to the section OGC, exit from the section PWT.
The section PWT comprises equipment for the thermal decomposition of urea, for the thermal decomposition of OATs and melamine at a temperature of 290° C. and equipment for the distillation and recovery of ammonia which is thus recycled internally.
The ammonia recovery section AR has the purpose of distilling pure ammonia from the incoming streams and recycling it within the process. The ammonia recovery section AR, in addition to the carbamate stream from the section PWT, also receives an off-gas stream of 4.9 t/h from the medium-pressure decomposition section MPD. At the outlet of the section AR, there is a liquid stream of 2.3 t/h of pure ammonia and a liquid stream of 6.3 t/h of carbamate recycled to the section OGC.
The off-gas condensation section OGC has the purpose of condensing the incoming gaseous off-gas streams and using the condensation heat for generating steam and therefore recovering energy. The section OGC, in addition to receiving the off-gas stream from the section PWT and the liquid carbamate stream from the section AR, receives the gaseous off-gas stream of 10.1 t/h from the high-pressure decomposition section HPD and receives the gaseous off-gas stream of 12.5 t/h from the section OGQ. The section OGC has an operating pressure slightly lower than the sections OGQ and HPD from which it receives the off-gas, its operating conditions are 105 bar and 155° C. At the outlet of the section OGC, a stream of liquid carbamate of 29.8 t/h is obtained which is preheated and fed to the conversion section OGR where the carbamate is converted into urea according to reaction (3).
The section OGR operates at a pressure of 200 bar and a temperature of 190° C. and a liquid stream is obtained at the outlet, having a concentration of 30% by mass of urea, containing unreacted carbamate and water. This stream is treated in the section HPD where part of the carbamate contained in the stream exiting from the section OGR is thermally decomposed at a pressure of 105 bar and at a temperature of 200° C. obtaining a liquid stream of 19.7 t/h having a concentration of 45% urea by mass. The stream is further concentrated in the section MPD at a pressure of 18 bar and a temperature of 160° C., obtaining a liquid stream of 14.8 t/h at the outlet, containing 60% urea by mass. A further decomposition step at low pressure at 4 bar and a temperature of 145° C. allows a liquid solution of 13.6 t/h to be obtained, containing water and urea with a mass concentration of 65% urea. This stream is concentrated in the concentration section CON operating at a pressure lower than atmospheric pressure and at a temperature of 136° C., from which a stream of concentrated urea of 8.9 t/h and a stream of process water of 4.7 t/h are obtained, sent to the section PWT. The stream of concentrated urea from the section CON is combined with a stream of molten urea of 7.1 t/h from the battery limit to form a stream of urea of 16 t/h fed to the off-gas scrubber section OGQ.
The example describes a melamine plant with a nominal capacity of 40,000 t/y which receives from the battery limit only a stream of molten urea of 7.1 t/h and which has, at the outlet, a stream of pure melamine of 5 t/h and a stream of pure water of 2.1 t/h.
The mass percentages indicated in this example are calculated with respect to the total mass of the stream or solution considered.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102023000019557 | Sep 2023 | IT | national |
