DE102016112944A1 - Process for the production of melamine with separation and recovery of CO2 and NH3 of high purity - Google Patents

Abstract

The present invention relates to a process for the production of melamine from urea, comprising the separation and recovery of high purity CO 2 and NH 3, comprising the following operations: a) synthesizing melamine in a main reactor by pyrolysis of a molten urea jet (301) in the presence of NH3 (319), obtaining a jet of molten crude melamine (303) and a jet of exhaust gas with NH3 and CO2 (302); b) optionally treating the jet of molten crude melamine (303) exiting the main reactor in a post reactor in the presence of gaseous ammonia (304) to complete the pyrolysis of urea reaction, obtaining a jet of molten crude melamine (306 ), essentially free of unconverted urea and CO2 and a jet of exhaust gas (305) consisting mainly of NH3 and containing all the CO2 formed in the reaction; c) subjecting the jet of exhaust gas (302) from step a) and, if present, the jet of waste gas (305) from step b) to the distillation in the presence of an aqueous extractive solvent (329, 218), obtaining a gaseous jet of CO2 high purity (314) and an aqueous ammonia jet (315) with CO2; d) exposing the aqueous ammonia jet from step c) (315) to stripping to remove any remaining CO2; e) cooling and solubilizing the molten raw melamine jet (303) obtained from step a) and (306) step b), if present, by contacting with the aqueous ammonia jet (315) from step c), obtaining an aqueous ammonia solution (330 ) with dissolved melamine, impurities of polycondensates and OATs; f) purifying the aqueous ammonia solution from melamine (330) from step e) by remaining in the presence of NH3, converting polycondensate to melamine; g) crystallizing the melamine present in the purified aqueous solution (307) from step f) and separating melamine crystals from the solution, obtaining a moist cake (309) of melamine and a jet of crystallization mother liquor (310), only OATs, dissolved melamine residues and Containing NH3; h) distilling the mother liquor leaving step g), obtaining a jet of NH3 of high purity (317) and an aqueous jet free of ammonia (322) containing OATs and melamine residues in solution, followed by specific downstream processes, for Example in a decomposition / stripping section, recovered and / or eliminated; i) drying the moist melamine cake (309) from step g) with hot air, obtaining dried melamine crystals having a high degree of purity and an exhaust air stream with ammonia; wherein the method requires that step c) for the extractive distillation of the exhaust gas stream (302) from step a) and, if present, the exhaust gas stream (305) from step b) in the presence of an aqueous extraction solvent at a pressure of 55 to 250 bar, preferably 70 to 170 bar, is performed.

Description

The present invention relates to a process for the production of melamine with separation and recovery of CO ₂ and NH ₃ of high purity.

In particular, the present invention relates to a process for the production of high purity melamine according to the synthesis process based on high pressure pyrolysis of urea, with the simultaneous separate and complete recovery of high purity NH ₃ and CO ₂ produced in the various phases of the process.

The production of melamine from urea is effected in industrial practice at a high temperature in the presence of NH ₃ , both at low pressure, in the presence of a suitable catalyst, and at high pressure in the absence of the catalyst. In both cases, the conversion of urea to melamine takes place according to the following overall reaction.

In addition to the major product (melamine), NH ₃ and CO _{2 are} also formed along with minor but non-negligible amounts of by-products well-known to those skilled in the art, including each of oxy-amino-triazines (OATs) and polycondensates, intermediates of Pyrolysis and subsequent reactions of melamine.

From the stoichiometry of the overall reaction (1), it can be deduced that 50% of moles of urea fed to the melamine synthesis process are theoretically converted to NH ₃ and CO ₂ . As a result, an amount of CO ₂ and NH ₃ corresponding to at least 1.86 kg per 1 kg of melamine produced is present in the effluent leaving the synthesis reactor along with the melamine.

Due to the high economic value of NH _3, there is a problem with the recovery of this reaction by-product in all melamine production processes.

One of the most commonly used processes for the production of melamine is based on the high pressure pyrolysis reaction of urea, with the recovery and purification of melamine in an aqueous ammonia solution. In this process, the effluent leaving the synthesis reactor consists of a liquid phase containing most of the melamine produced and a gaseous phase containing most of the NH ₃ and CO _{2 by-} products.

The entire effluent exiting the reactor (gas and liquid) is contacted in a specific apparatus, referred to herein as a "quench", in contact with an aqueous ammonia jet coming from the downstream sections containing CO ₂ . A liquid jet containing all the melamine produced leaves the quench bottom, which after removal of the CO ₂ present is treated with ammonia to remove the polycondensates, reaction by-products, and to keep the OATs in solution. Melamine of a high degree of purity with a titer higher than 99.8% is obtained from the above purified solution by crystallizing and then drying the separated solid.

A water-saturated gaseous jet of NH ₃ and CO ₂ (wet exhaust gas) is extracted from the quenching head. This gaseous jet is normally recycled to the urea synthesis plant (urea plant) to recover the NH ₃ and CO ₂ contained therein, allowing them to be reused immediately by their (re) conversion to urea. An indispensable condition for ensuring that the recovery and reuse of the above gases is economically feasible, the melamine plant is in close proximity to the urea plant.

However, the presence of water in the wet flue gas leaving the quench (usually more than 20% by weight) makes the reuse of NH ₃ and CO ₂ in the urea plant difficult and adds to the overall cost of melamine production especially if the urea plant is small compared to the melamine plant.

The above prior art production process of melamine will be described below with reference to the appended drawings 1 and 2 showing a simplified block diagram of the main operating phases of the method.

In 1 becomes the jet of molten urea 101 from the adjacent urea plant at a temperature of 135-140 ° C in a pyrolysis reactor (reactor) together with the gaseous jet 114 fed to anhydrous NH ₃ . The reactor operates continuously and is equipped with a heating system which supplies the reagent mixture with the heat necessary for the synthesis of melamine, keeping the reaction environment at a temperature of 360-420 ° C. The reaction pressure is maintained at a value of more than 70 bar. The reactor is a single-step reactor and the reagent mixture is maintained in high circulation by the gas formed during the pyrolysis of urea.

Under the above operating conditions, the pyrolysis reaction produces a biphasic effluent consisting of a liquid phase containing crude molten melamine containing reduced amounts of unconverted urea and other by-products (among which the most significant are the already mentioned polycondensates and oxy-amino-triazines ( OATs) and a gas-containing phase, consisting essentially of NH ₃ and CO ₂ , consists. This biphasic outflow 102 is sent to the quench which is positioned directly after the reactor for melamine recovery. In the quench becomes the biphasic beam 102 in contact with the CO ₂ -containing aqueous ammonia jet 103 brought from other sections of the plant, and its temperature is lowered to about 160 ° C. Under these conditions, all of the melamine, unreacted urea and impurities formed during the process come in aqueous solution or remain in suspension. The watery jet 104 Leaving the quench is directed towards the subsequent stages of the process, whereas the wet exhaust 105 (at a pressure of about 25 bar), consisting essentially of a jet of NH ₃ and CO ₂ , saturated with moisture, recycled to the adjacent urea plant.

The watery jet 104 which leaves the quench contains, in addition to crude melamine, a consistent amount of NH ₃ and CO _{2 dissolved} in the following step of the process by stripping with steam (CO ₂ stripper section). The removal of CO ₂ is necessary to help achieve a high degree of purity of melamine in the subsequent purification step.

A gaseous jet 106 leaves the head of the CO ₂ -stripper, which together with the recovery beams 113 and 124 to an absorption column (NH ₃ -CO ₂ absorber) is sent. The NH ₃ -CO ₂ absorber allows complete recovery of the above rays in a single aqueous jet 103 being recycled to the quench.

An aqueous jet 107 without CO ₂ (and without NH ₃ ) containing melamine in solution at a concentration of 6 to 12% by weight leaves the bottom of the CO ₂ stripper, along with non-negligible amounts of OATs and polycondensates. Due to their low solubility, the polycondensates must be eliminated before exposing the aqueous jet of melamine to the subsequent crystallization phase.

The elimination of the polycondensates takes place in the purification section where the jet 107 is heated to about 170 ° C in the presence of ammonia which is added (jet 116 ) until a concentration of 12-15% by weight has been reached. Under the above conditions, the polycondensates are almost completely converted to melamine.

The aqueous ammonia jet 108 Melamine, substantially free of polycondensates, leaves the purification section and is fed to the crystallization section, where the temperature of the jet 108 is lowered to 40-50 ° C, whereby the crystallization of most of the melamine is made possible.

The presence of NH ₃ in the jet 108 keeps the OATs in solution, allowing the precipitation of high purity melamine crystals. The beam 109 in suspension with the melamine crystals is sent to the solid / liquid separation section, in which a moist cake 110 made of melamine and a beam 111 be separated from crystallization mother liquor. The beam 111 is saturated with ammonia and contains all OATs, both those leaving the reactor and those formed by the hydrolysis of melamine in the subsequent steps of the manufacturing process. The formation of OATs is particularly preferred in the process steps carried out at a high temperature in the presence of water.

The wet cake 110 is finally dried by means of hot air in the drying section, whereby a product of high purity with a titer of melamine of more than 99.8% is obtained. The exhaust air flow used for drying 112 is purified by washing with water in the NH ₃ scrubber from the NH ₃ present therein. The thus purified air is released into the environment.

The aqueous wash jet 113 containing absorbed ammonia is recycled to the process by the NH ₃ -CO ₂ absorber.

The jet consisting of the crystallization mother liquor 111 containing melamine in a concentration in the range of 0.8-1.0 wt% is sent to the ammonia separation section. In this section, the following products are separated by distillation: a jet 115 of anhydrous NH _3, which is recycled with the addition of further anhydrous NH ₃ to the cleaning section; a cleaning jet 123 that may be in the mother liquor 111 contains existing CO ₂ ; an aqueous jet 117 without NH ₃ , essentially containing only melamine and OATs.

The beam 117 is cooled to a temperature of 60-70 ° C and neutralized to pH 7 by adding CO ₂ to effect the precipitation of the OATs. The beam 118 containing OATs in a colloidal suspension is subjected to a separation process of the OATs by ultrafiltration on ceramic candles according to what is disclosed in the patent application WO 01/46159 is described. In the OAT ultrafiltration section the OATs are used as retentate (jet 119 ) from the corresponding jet of permeate 120 separated. The latter jet is virtually free of OATs and is recycled to quench for recovery of the residual melamine contained therein.

• – einem gereinigten wässrigen Strahl (121), der in die Umwelt abgelassen werden kann:
• – einem nassen Strahl, reich an NH₃ und CO₂ (122), der zum NH₃-CO₂-Absorber recycelt wird.

The beam of retentate 119 containing virtually all OATs is subjected to heat treatment in the disassembling stripper section where the OATs, residual melamine and any other organic substances present therein are almost completely converted to NH ₃ and CO ₂ by hydrolysis, with formation from:

• A purified aqueous jet ( 121 ) that can be released into the environment:
• - a wet jet, rich in NH ₃ and CO ₂ ( 122 ), which is recycled to the NH ₃ -CO ₂ absorber.

The process outlined above, although currently used in numerous chemical plants, is characterized by high steam consumption and a conversion yield of urea to melamine much lower than the stoichiometric value. In addition, the process is characterized by a heavy recovery of the gaseous reaction products NH ₃ and CO ₂ due to the considerable amount of water in the jet 105 the wet exhaust gas is present, endangered.

The water present in the wet exhaust gas recycled to the adjacent urea plant actually causes an increase in urea production costs, which of course are borne by the management of the melamine manufacturing plant.

This problem has already been addressed in the past, with attempts to find separation processes of NH ₃ and CO ₂ from mixtures of exhaust gas and by-products derived from the melamine synthesis process to obtain pure gases that can be recycled to the urea plant , stored and / or released to the environment (CO ₂ ).

In contrast to what is known in the art, which also describes already distillations of CO ₂ , to obtain gaseous jets of CO ₂ of high purity (IT 1387832, whose process scheme in the attached 2 Now, the Applicant has surprisingly found that certain operating conditions make it possible to obtain a process for the production of melamine from urea, with the separation and recovery of high purity CO ₂ and NH ₃ improved and optimized and with reduced amounts of water works, with surprising results in terms of energy consumption.

The object of the present invention is therefore to overcome the disadvantages resulting from the prior art.

• a) Synthetisieren von Melamin in einem Hauptreaktor durch Pyrolyse eines Strahls von geschmolzenem Harnstoff in der Gegenwart von NH₃, Erhalten eines Strahls von geschmolzenem Rohmelamin und eines Strahls von Abgas mit NH₃ und CO₂;
• b) optional Behandeln des Strahls von geschmolzenem Rohmelamin, der den Hauptreaktor verlässt, in einem Postreaktor in der Gegenwart von gasförmigem Ammoniak, für die Fertigstellung der Pyrolysereaktion von Harnstoff, Erhalten eines Strahls von geschmolzenem Rohmelamin, im Wesentlichen frei von nicht konvertiertem Harnstoff und CO₂ und eines Strahl von Abgas, der hauptsächlich aus NH₃ besteht und das gesamte in der Reaktion gebildete CO₂ enthält;
• c) Aussetzen des Strahls von Abgas aus Schritt a) und, falls vorhanden, des Strahls von Abgas aus Schritt b) der Destillation in der Gegenwart eines wässrigen Extraktionslösungsmittels, Erhalten eines gasförmigen Strahls von CO₂ von hoher Reinheit und eines wässrigen Ammoniakstrahls mit CO₂;
• d) Aussetzen des wässrigen Ammoniakstrahls aus Schritt c) dem Stripping, um das noch vorhandene CO₂ zu entfernen;
• e) Abkühlen und Solubilisieren des aus Schritt a) und Schritt b), falls vorhanden, erhaltenen geschmolzenen Rohmelaminstrahls, indem sie mit dem wässrigen Ammoniakstrahl aus Schritt c) in Kontakt gebracht werden, Erhalten einer wässrigen Ammoniaklösung mit aufgelöstem Melamin, Unreinheiten von Polykondensaten und OATs;
• f) Reinigen der wässrigen Ammoniaklösung aus Melamin aus Schritt e) durch Verbleiben in Gegenwart von NH₃, Umwandeln der Polykondensate zu Melamin;
• g) Kristallisieren des in der gereinigten wässrigen Lösung aus Schritt f) vorhandenen Melamins und Trennen von Melaminkristallen von der Lösung, Erhalten eines feuchten Kuchens aus Melamin und eines Strahls von Kristallisationsmutterlauge, nur OATs, aufgelöste Melaminrückstände und NH₃ enthaltend;
• h) Destillieren der aus Schritt g) austretenden Mutterlauge, Erhalten eines Strahls von NH₃ von hoher Reinheit und eines wässrigen Strahls frei von Ammoniak, OATs und Melaminrückstände enthaltend, im Anschluss in spezifischen dem Verfahren nachgelagerten Abschnitten, zum Beispiel in einem Zerlegungs-/Stripping-Abschnitt, zurückgewonnen und/oder eliminiert;
• i) Trocknen des feuchten Melaminkuchens aus Schritt g) mit heißer Luft, Erhalten von getrockneten Melaminkristallen mit einem hohen Grad an Reinheit und einem Abluftstrom mit Ammoniak; wobei das Verfahren erfordert, dass Schritt c) des extraktiven Destillierens des Abgasstroms aus Schritt a) und, falls vorhanden, des Abgasstroms aus Schritt b) in der Gegenwart eines wässrigen Extraktionslösungsmittels bei einem Druck von 55 bis 250 bar, bevorzugt 70 bis 170 bar, ausgeführt wird.

Indeed, an object of the present process relates to a process for the production of melamine from urea, with the separation and recovery of high purity CO ₂ and NH ₃ , comprising the following operations:

• a) synthesizing melamine in a main reactor by pyrolysis of a jet of molten urea in the presence of NH ₃ , obtaining a jet of molten crude melamine and a jet of exhaust gas with NH ₃ and CO ₂ ;
• b) optionally treating the jet of molten crude melamine exiting the main reactor in a post reactor in the presence of gaseous ammonia to complete the pyrolysis reaction of urea, obtaining a jet of molten crude melamine substantially free of unconverted urea and CO ₂ and a jet of exhaust gas consisting mainly of NH ₃ and containing all the CO ₂ formed in the reaction;
• c) subjecting the jet of exhaust gas from step a) and, if present, the jet of exhaust gas from step b) to distillation in the presence of an aqueous extraction solvent, obtaining a gaseous jet of high purity CO ₂ and an aqueous ammonia jet with CO ₂ ;
• d) exposing the aqueous ammonia jet from step c) to stripping to remove the remaining CO ₂ ;
• e) cooling and solubilizing the molten crude melamine jet obtained from step a) and step b), if present, by contacting it with the aqueous ammonia jet from step c), obtaining an aqueous ammonia solution with dissolved melamine, impurities from polycondensates and OATs ;
• f) purifying the aqueous ammonia solution from melamine from step e) by remaining in the presence of NH ₃ , converting the polycondensates to melamine;
• g) crystallizing the melamine present in the purified aqueous solution from step f) and separating melamine crystals from the solution, obtaining a moist cake of melamine and a jet of crystallization mother liquor, containing only OATs, dissolved melamine residues and NH ₃ ;
• h) distilling the mother liquor leaving step g), obtaining a jet of NH ₃ of high purity and an aqueous jet free of ammonia, containing OATs and melamine residues, subsequently in specific downstream processes, for example in a separation / stripping process Section, recovered and / or eliminated;
• i) drying the moist melamine cake from step g) with hot air, obtaining dried melamine crystals having a high degree of purity and an exhaust air stream with ammonia; wherein the process requires that step c) of the extractive distillation of the exhaust gas stream from step a) and, if present, the exhaust gas stream from step b) in the presence of an aqueous extraction solvent at a pressure of 55 to 250 bar, preferably 70 to 170 bar, is performed.

The aqueous extraction solvent is preferably water or an aqueous ammonia-free solution, more preferably a recycled aqueous ammonia-free solution.

The aqueous extraction solvent is present in an amount of 3 to 6, preferably 4 to 5 tons of solvent per 1 ton of melamine produced.

In addition, extractive distillation step c) is preferably carried out in separation equipment (distillation column or Distex) made of materials suitable to withstand the required operating conditions, such as an alloy consisting essentially of a combination of nickel, chromium and molybdenum or metals such as zirconium, titanium or suitable combinations of these materials.

• – die gleichzeitige Trennung und Rückgewinnung von CO₂ und NH₃ von hoher Reinheit, erzeugt/verwendet im Herstellungszyklus und bei der Reinigung von Melamin;
• – eine Reduzierung der Gesamtmenge an verwendetem Wasser und dessen vollständiges Recyceln im Melaminherstellungsverfahren, mit der anschließenden Eliminierung von Flüssigkeitsabläufen;
• – eine extraktive Destillation von CO₂ ausgeführt bei hohen Drucken, wobei die Verwendung der Drucke die direkte Destillation von Abgas, das den Reaktor und möglicherweise den Postreaktor des Syntheseverfahrens von Hochdruckmelamin verlässt, ermöglicht, ohne Erfordernis einer Zwischenbehandlung, um sie auf die Betriebsdrucke einer Extraktionssäule zu bringen, die bei Drucken von weniger als 50 bar arbeitet, wie in den Verfahren nach dem Stand der Technik beschrieben;
• – eine extraktive Destillation von CO₂, die, auch wenn bei einem hohen Druck ausgeführt, reduzierte Energieverbrauche aufweist, die in Bezug auf den Betrieb bei niedrigeren Drucken sogar halbiert werden können, ermöglicht.

The process according to the present invention therefore consists of a process for the production of melamine, improved and optimized with respect to the prior art, which in addition to enabling the production of melamine of high purity with a considerable increase in yield and at a reduced cost also

• The simultaneous separation and recovery of high purity CO ₂ and NH ₃ , produced / used in the manufacturing cycle and in the purification of melamine;
• - a reduction in the total amount of water used and its complete recycling in the melamine production process, with the subsequent elimination of liquid effluents;
• An extractive distillation of CO ₂ carried out at high pressures, the use of which enables the direct distillation of off-gas leaving the reactor and possibly the post-reactor of the high pressure melamine synthesis process, without the need for intermediate treatment, to the operating pressures of an extraction column which operates at pressures of less than 50 bar as described in the prior art processes;
• An extractive distillation of CO ₂ which, even when run at high pressure, has reduced energy consumption, which can even be halved in terms of operation at lower pressures.

An essential aspect of the method according to the present invention is also the provision of a treatment of exhaust gas with CO ₂ and NH ₃ to obtain CO ₂ and NH ₃ of high purity, wherein a gaseous jet of CO ₂ of high purity in a first step is separated and a jet of high purity NH _{3 is} separated in a subsequent step.

In addition, the exhaust gases with CO ₂ and NH ₃ generated / used in the production and purification cycle of melamine treated according to the process of the present invention may be produced in the same plant in which the extractive distillation column is present or may be come from single or multiple combinations at other melamine manufacturing plants, conventionally referred to as high or low pressure equipment, which are not equipped with an extractive column or Distex.

Another object of the present invention relates to a specific group for the separation and recovery of high purity CO ₂ and NH ₃ from waste gas coming from a plant for the production of melamine, the group comprising the following elements: a first distillation column for the extraction of CO ₂ , suitable for operation with an aqueous extraction solvent such as water; a second separation column suitable for recovering the CO ₂ and NH ₃ from the head present in the jet leaving the bottom of the first extractive distillation column; a third separation column suitable for the recovery of the NH ₃ from the head present in the jet leaving the head of the second column, the elements being suitable for carrying out the separation and recovery steps of the CO ₂ and NH ₃ beams of high Purity of the exhaust effect.

The group may be used in cases of conversion of existing plants for the production of melamine and / or for the integration thereof in plants for the production of urea.

The group according to the present invention can be used, for example, for the separation and recovery of high purity CO ₂ and NH ₃ produced in a process for the production of melamine from urea comprising the operations a) -i) described above.

The method proposed by the present invention applies simple but substantial changes with respect to the methods known in the art.

The synthesis process of melamine from urea according to the present invention and the advantages derived therefrom can be described with reference to the following description 3 , which is a simplified schematic of the most important operational steps of a preferred embodiment, will be better understood. The description and the relative process scheme should be considered to constitute the present invention and are in no way intended to limit the scope of the appended claims.

The following characteristic feature of the present invention should first be noted, namely, that the proximity of the urea plant to the melamine production unit is no longer binding because the process according to the present invention no longer produces exhaust gas mixture containing NH ₃ and CO ₂ (either wet exhaust gas or anhydrous Exhaust gas), whose recovery and transport to the urea plant is economically viable only because of this proximity. In addition, the specific pressure conditions in the extractive distillation step of the process according to the present invention allow to obtain an optimized process which works with reduced amounts of water and with surprising results in terms of energy consumption.

These specific features make the process of the present invention unique with respect to the high pressure technologies previously used and / or proposed for the production of melamine from urea.

A stream of molten urea 301 is fed to a first reactor (main reactor) in which the high-pressure pyrolysis process takes place under standard conditions, ie in the presence of anhydrous ammonia (jet 319 ) at a temperature in the range of 360 to 420 ° and at a pressure of more than 70 bar. Unlike in 2 The prior art melamine production processes shown are the two components of the effluent produced by the pyrolysis reaction, ie, the anhydrous exhaust stream 302 consisting of NH ₃ and CO ₂ , and the liquid jet 303 consisting of molten raw melamine, separate from the main reactor or from a downstream suitable phase separator, 3 not shown, extracted and treated separately.

In the in 3 illustrated embodiment of the present invention, the beam 303 to a second reaction step (post reactor) in which, when operated under the same temperature and pressure conditions as the main reactor and in the presence of a continuous flow of anhydrous ammonia (jet 304 ), the transformation of the still in the beam 303 Leaving the main reactor, existing unreacted urea is completed along with the intermediate reactions leading to the formation of melamine. At the same time, the conditions present in the post reactor in which the partial ammonia pressure is much higher relative to that of the main reactor also result in the reduction of polycondensates and OATs in the molten crude melamine produced by the main reactor.

Another beneficial effect of the presence of the post reactor is a reduction in the amount of CO ₂ present in the liquid phase, ie in the stream of molten crude melamine, also due to the stripping effect of the gaseous jet of NH ₃ which is continuously entering. However, step b) of the process is optional. The process according to the present invention actually allows the production of high purity melamine with the separate recovery of NH ₃ and CO ₂ even without the aid of the post reactor.

Two separate jets also leave the post reactor, both free of water: an exhaust jet 305 consisting mainly of NH ₃ and a liquid jet 306 consisting of molten raw melamine, virtually free of urea and CO ₂ , containing very limited amounts of high-boiling by-products such as OATs and polycondensates.

The exhaust gas jet 305 leaving the post reactor, or from a downstream positioned suitable separator, in 3 not shown, with the exhaust jet 302 which leaves the main reactor to be treated in a specific extractive distillation column (hereinafter "Distex") as shown below. The jet of molten raw melamine 306 which leaves the post reactor is in turn sent to the recovery step of the melamine by dissolution in an aqueous ammonia solution. This step is carried out in a specific device (hereinafter "quench") in which the jet of molten crude melamine 306 at a temperature in the range of 110 to 180 ° C, preferably from 150 to 170 ° C comes into contact with the aqueous ammonia jet, which consists predominantly of the aqueous ammonia jet leaving the CO ₂ -tripper (jet 315 ), containing a substantial concentration of NH ₃ , but almost free of CO ₂ . Under these conditions, a preliminary demolition of the polycondensates takes place in the quenching apparatus. The melamine solution exiting the quench is then added to the cleaning section (jet) for complete demolition of the polycondensates 330 ) cleverly.

Purification of the aqueous solution 330 with melamine exiting the quench is carried out under pressure and concentration conditions known in the art but at a lower temperature. The aqueous ammonia solution of purified melamine 307 which leaves the cleaning section is then supplied to the crystallization section, in analogy to the method of the prior art 2 the temperature of the solution is lowered to 40-50 ° C, whereby the separation of extremely pure melamine crystals is obtained. The beam 308 with the melamine crystals in suspension is sent to the solid / liquid separation section, with the separation of a moist cake of melamine 309 and an aqueous jet 310 which consists of crystallization mother liquor. The anhydrous crystallized melamine having a high degree of purity (titer higher than 99.8% by weight) is recovered from the drying section.

The jet of crystallization mother liquor 310 containing ammonia is partially recycled directly to the quench (Ray 310a ) and partially fed to the ammonia separation section (jet 310b ) whose main function is to recover all the NH ₃ with a high degree of purity. The separation of NH ₃ in this section is effected by distillation, analogous to the prior art melamine production processes.

The gaseous jet 312 , from the combination of the two anhydrous exhaust gases 302 and 305 originating from the main reactor and the post reactor and saturated with melamine fume, is subjected to extractive distillation in the specific distillation column (Distex) in the presence of an aqueous extraction solvent selected from water and an aqueous ammonia-free solution extending from top to bottom in the Pillar moved, in countercurrent with respect to the ascending steam jet.

The aqueous extraction solvent is preferably a recycled aqueous ammonia-free solution, which is from the section of the Appendix (blasting 313 and 325 ) from the disassembly / stripper section.

The gaseous jet 314 , which consists of high purity CO ₂ (NH ₃ content less than 10 ppm) is discharged from the head of the Distex, at the operating pressure of the column, whereas an aqueous ammonia jet 315 , with a low content of CO ₂ , that in the beam 312 Contains existing melamine recovered with it being discharged from the bottom of the Distex. The entire aqueous jet 315 leaving the distex is sent to the CO ₂ -tripping column to completely remove the CO ₂ from the bottom solution, and the bottom solution is then sent to the quench as a solvent for the recovery of melamine exiting the post reactor cleverly. The beam 316 which comes from the NH ₃ scrubber containing the ammonia which has developed as a result of the drying of the wet melamine cake is fed to the quench as an aqueous solvent. A beam of pure NH ₃ 320 is also fed to the quench to obtain the concentration of NH ₃ required for the purification step. Consequently, the aqueous ammonia jet contains 307 flowing after the purification section, all of the NH ₃ produced by the process, ie, that resulting from the pyrolysis of urea (see reaction (1)) and that resulting from the hydrolysis reactions (especially from the disassembling / stripping Section). The total amount in the beam 310b contained ammonia is at a high purity ( 317 ) separated by distillation in the ammonia separation section.

The aqueous ammonia-free jet 322 containing OATs and residues of dissolved melamine in solution is also separated from the ammonia separation section.

The process according to the invention therefore also enables complete recovery, and at a high degree of purity, of the total ammonia produced in the melamine production section after complete recovery and with a high degree of purity of the total CO ₂ produced in the melamine production process. The pure NH ₃ in the liquid state, which is the beam 317 is partially used at various points in the manufacturing process, such as the main reactor (jet 319 ), Post reactor (beam 304 ) and the cleaning section (jet 320 ), whereas the excess NH ₃ , which is the most relevant part, is sent to the plant boundary of the plant (Beam 321 ) and can be transferred to the urea plant or recovered as a commercial high quality by-product that can be used as a raw material in other industrial processes.

The aqueous ammonia-free jet 322 discharged as a bottom product of the ammonia separation section is sent to the decomposition / stripper section where the OATs and the residual melamine are decomposed into ammonia and carbon dioxide by high-temperature thermal hydrolysis in addition to the other existing organic molecules. The NH ₃ and CO ₂ are separated by stripping in the same section as top product and recycled to Distex (Strahl 318 ), whereas a jet of almost free water is recovered from the bottom and either as a solvent of the extractive distillation ( 325 ) is recycled to the Distex or for washing the exhaust air from the dryer (jet 313 ) or drying section is sent to the scrubber.

The drying step may in fact be carried out with hot air or other means known to those skilled in the art.

The small amounts of CO ₂ that may be in the beam 310 (Crystallization mother liquor) are separated in the ammonia separation section in the form of an aqueous ammonia mixture of CO ₂ , as is known in the art, and transported to Distex for their recovery (purifying jet 329 ). The entire CO ₂ formed in the production cycle (by pyrolysis and hydrolysis) is then separated at a high purity in Distex, as top product, and at the plant boundary of the plant at the preferred pressure of 70 to 170 bar (jet 314 ) made available. Similar to NH ₃ , CO _{2 may} also be transferred to the urea plant or recovered as a raw material for use in other industrial processes.

• – Aussetzen des Abgasstrahls, der den Melaminsynthesereaktor verlässt, der Destillation in einer extraktiven Destillationssäule unter Verwendung von Wasser als Extraktionslösungsmittel oder einer wässrigen Lösung, und bevorzugt eines wässrigen Rückgewinnungsstrahls in einer Menge von 3 bis 6, bevorzugt von 4 bis 5 Tonnen an Lösungsmittel pro Tonne an Melamin, und Betreiben bei einem Druck von 55 bis 250 bar, bevorzugt von 70 bis 170 bar, wodurch ein gasförmiger Strahl an CO₂ von hoher Reinheit und ein wässriger Ammoniakstrahl, der für die Rückgewinnung und Reinigung des in der Reaktion hergestellten Melamins erhalten wird, mit besonders reduziertem Energieverbrauch.

According to the embodiment presented herein, the process for the production of melamine from urea with separate recovery and with a high purity of the by-products ammonia and carbon dioxide comprises the following basic operation:

• Exposing the exhaust jet leaving the melamine synthesis reactor to distillation in an extractive distillation column using water as the extraction solvent or aqueous solution, and preferably an aqueous recovery jet in an amount of from 3 to 6, preferably from 4 to 5 tons of solvent per ton to melamine, and operating at a pressure of 55 to 250 bar, preferably from 70 to 170 bar, whereby a gaseous jet of high purity CO ₂ and an aqueous ammonia jet obtained for the recovery and purification of the melamine produced in the reaction , with particularly reduced energy consumption.

The aqueous ammonia jet resulting from the solid / liquid separation step, after crystallization, is then distilled to produce a high purity ammonia jet and an aqueous ammonia-free jet of residual melamine, OATs and other minor organic impurities for recovery Elimination is obtained.

In a preferred embodiment of the process according to the present invention, the recovered high purity NH ₃ and CO ₂ streams are supplied substantially separately to the urea plant, with particular advantages with respect to the feed jet of these substances contained in the exhaust jet, not separated ,

Alternatively, one or both of the above-mentioned rays are suitably used as raw materials in other industrial production processes.

• – einen Reaktionsabschnitt, der aus einem Hauptreaktor und einem Postreaktor besteht, in dem die Pyrolyse mit einer hohen Konversion von Harnstoff zu Melamin mit der Trennung eines Strahls von anhydrischem Abgas aus NH₃, CO₂ und Melamindämpfen bewirkt wird;
• – eine extraktive Destilliersäule (Distex), die mit dem Reaktionsbereich verbunden ist, von dem sie den Abgasstrahl erhält, wobei der Distex ein Kopfprodukt, das aus CO₂ von hoher Reinheit besteht und einen wässrigen Bodenstrahl mit NH₃ und restlichem Melamin trennt, unter Verwendung von bevorzugt wässrigen ammoniakfreien Lösungen aus dem Rückgewinnungs- und Reinigungszyklus von Melamin als Extraktionslösungsmittel; wobei der CO₂-Strahl von hoher Reinheit bei dem bevorzugten Druck von 70–170 bar destilliert wird, d. h. bei dem gleichen Druck, bei dem er den Reaktor verlässt, ohne dass eine Zwischenbehandlung erforderlich ist; einen Stripping-Abschnitt zum Entfernen des restlichen CO₂ aus der ammoniakfreien wässrigen Lösung;
• – einen Quench-Abschnitt und einen Reinigungsabschnitt für die Rückgewinnung und Reinigung von Melamin mittels wässriger Ammoniakstrahlen, die vom Herstellungszyklus kommen, eventuell mit der Hinzufügung von NH₃;
• – einen Präzipitationsbereich durch Kristallisation des gereinigten Melamins, verbunden mit den Trenn- und Trocknungsabschnitten des Produkts;
• – einen Ammoniaktrennabschnitt von der Kristallisationsmutterlauge mit der Herstellung, durch Destillation, von Ammoniak von hoher Reinheit, das für die internen Erfordernisse des Zyklus verwendet wird, wobei das überschüssige NH₃ bei einer hohen Reinheit an die Anlagengrenze zur Übertragung an eine Harnstoffanlage oder zum Verkauf auf dem Markt geliefert wird;
• – einen Abschnitt für die Behandlung der ammoniakfreien Mutterlauge, die den Ammoniaktrennabschnitt verlässt, wobei eine kongruente Rückgewinnung des darin enthaltenen Melamins und/oder die thermische Demolierung aller anderen vorhandenen organischen Substanzen mit der vollständigen Rückgewinnung des sich ergebenden NH₃ und CO₂ bewirkt wird; wobei der aus dieser Betriebsserie stammende wässrige Strahl das meiste der Extraktionslösungsmittelanforderungen des Distex bildet.

The present invention therefore relates to an innovative cycle organized to obtain the separation and recovery of high purity NH ₃ and CO ₂ in a melamine production process by the pyrolysis of urea, the cycle comprising:

• A reaction section consisting of a main reactor and a post reactor in which pyrolysis with a high conversion of urea to melamine is effected with the separation of a jet of anhydrous exhaust gas from NH ₃ , CO ₂ and melamine vapors;
• An extractive distillation column (Distex) connected to the reaction zone from which it receives the exhaust jet, the Distex using an overhead product consisting of high purity CO ₂ and separating an aqueous soil jet with NH ₃ and residual melamine preferably aqueous ammonia-free solutions from the recovery and purification cycle of melamine as extraction solvent; wherein the CO ₂ jet of high purity is distilled at the preferred pressure of 70-170 bar, ie at the same pressure at which it exits the reactor without the need for intermediate treatment; a stripping section for removing residual CO ₂ from the ammonia-free aqueous solution;
• A quench section and a purification section for the recovery and purification of melamine by means of aqueous ammonia jets coming from the production cycle, possibly with the addition of NH ₃ ;
• A precipitation zone by crystallization of the purified melamine associated with the separation and drying sections of the product;
• An ammonia separation section from the crystallization mother liquor with the preparation, by distillation, of high purity ammonia used for the internal requirements of the cycle, the excess NH _{3 being} at a high purity at the plant boundary for transfer to a urea plant or for sale delivered to the market;
• A section for the treatment of the ammonia-free mother liquor leaving the ammonia separation section, wherein a congruent recovery of the melamine contained therein and / or the thermal demolition of all other organic substances present with the complete recovery of the resulting NH ₃ and CO _{2 is} effected; the aqueous jet from this series of operations forms most of the Distex extraction solvent requirements.

• 1) die vollständige Rückgewinnung, bei einer hohen Reinheit, und separat, der gasförmigen Nebenprodukte CO₂ und NH₃, die in der Synthesereaktion von Melamin (Reaktion (1)) erzeugt oder im Rückgewinnungs- und Reinigungszyklus von Melamin gebildet werden. Der innovative Rückgewinnungsmodus von CO₂ und NH₃, Gegenstand der vorliegenden Erfindung, in Bezug auf die Recyclingpraktiken von Abgas (nass oder anhydrisch) an die Harnstoffanlage, bekannt aus dem Stand der Technik, ermöglicht die Überwindung aller Probleme in Bezug auf diesen letzten Vorgang.

The production process of high purity melamine by the high pressure pyrolysis of urea, object of the present invention, makes it possible to obtain the following advantages with respect to the processes known in the art:

• 1) the complete recovery, at a high purity, and separately, the gaseous by-products CO ₂ and NH ₃ generated in the synthesis reaction of melamine (reaction (1)) or formed in the recovery and purification cycle of melamine. The innovative recovery mode of CO ₂ and NH ₃ object of the present invention, in relation to the recycling practices of exhaust gas (wet or anhydrous) to the urea plant known in the prior art, allows to overcome all problems related to this last operation.

• a) einem erheblichen Anstieg der Umwandlung für jeden Durchgang von CO₂ zu Harnstoff, mit einem daraus folgenden Anstieg der Produktionskapazität, bei gleicher Investition, und einer Reduzierung der Herstellungskosten von Harnstoff;
• b) einer erheblichen Reduzierung des Energieverbrauchs wie Dampf und/oder Elektrizität, die für den Betrieb der Harnstoffanlage notwendig ist, zusätzlich zu einer erheblichen Verbesserung der Energierückgewinnungsmodi aus derselben Harnstoffanlage;
• c) einer Reduzierung der Kompressionskosten des Zuführstrahls an die Harnstoffanlage, insbesondere des CO₂, das an der Anlagengrenze der Melaminanlage beim bevorzugten Druck von 70–170 bar zur Verfügung gestellt wird;
• d) einer Reduzierung der Kosten für die Zwischenbehandlung, die das Abgas, das den Melaminreaktor verlässt, auf Drucke von weniger als 50 bar bringt, bei dem die Destilliersäulen gemäß dem Stand der Technik arbeiten;
• e) weniger Korrosionsproblemen der Ausrüstung und Linien, die am Recycling beteiligt sind;
• f) einer insgesamt verbesserten Betriebszuverlässigkeit und Verfügbarkeit des Syntheseverfahrens von Harnstoff;

• 2) das Herstellungsverfahren von Melamin gemäß der Erfindung erfordert keine Zufuhr von zusätzlichen NH₃- und CO₂-Strahlen. Die Verfahrensanforderung wird tatsächlich ganzheitlich und ausschließlich durch die/das partielle Wiederverwendung/Recyceln des nebenbei hergestellten und in den verschiedenen Schritten des Verfahrens zurückgewonnenen NH₃ befriedigt;
• 3) einer Reduzierung der Bildung von OATs durch Hydrolyse des Melamins, nicht nur aufgrund der Wirkung der Reduzierung der Anzahl und des Volumens der bei einer hohen Temperatur tätigen Ausstattung, sondern auch als Ergebnis der Reduzierung der Temperatur selbst. Die Reinigung kann tatsächlich bei einer Temperatur von 5 bis 10 Grad weniger als derjenigen nach dem Stand der Technik ausgeführt werden. Dies führt zu einer weiteren Erhöhung des Gesamtertrags des Verfahrens zusätzlich zu demjenigen, der als Folge der höheren Umwandlung von Harnstoff dank der Einfügung des Postreaktors erhalten wird;
• 4) der Möglichkeit der Verwendung sowohl von NH₃ als auch von CO₂ in anderen industriellen Verfahren als denjenigen für die Herstellung von Harnstoff; da beide Verbindungen mit einer hohen Reinheit und separat zurückgewonnen werden, sind sie für verschiedene Verwendungen und mit einem möglichen höheren Mehrwert verfügbar;
• 5) einer Reduzierung der Gesamtmenge an verwendetem Wasser und dessen vollständiges Recyceln im Herstellungsverfahren von Melamin, mit der anschließenden Eliminierung von Flüssigkeitsabläufen;
• 6) einer extraktiven Destillation von CO₂, die, obwohl sie bei einem hohen Druck durchgeführt wird, einen reduzierten Energieverbrauch aufweist;
• 7) das Melaminherstellungsverfahren gemäß der vorliegenden Erfindung kann an Verfahren für die Herstellung von Harnstoff, die in vorhandenen Harnstoffanlagen einer beliebigen bekannten Technologie durchgeführt werden, gekoppelt werden, wodurch die Zeiten der Abschaltung und invasiven technologischen Modifikationen, die für die Modernisierung und Anpassung der Anlagen erforderlich sind, drastisch reduziert werden.

The separate recycling of NH ₃ and CO ₂ to the urea plant as an alternative to the traditional recycling of the waste gas jet in turn leads to:

• a) a significant increase in conversion for each CO ₂ to urea pass, with a consequent increase in production capacity, with the same investment, and a reduction in the cost of urea;
• b) a significant reduction in energy consumption, such as steam and / or electricity, necessary for the operation of the urea plant, in addition to a significant improvement in the energy recovery modes from the same urea plant;
• c) a reduction of the compression costs of the feed jet to the urea plant, in particular the CO ₂ , which is provided at the plant boundary of the melamine plant at the preferred pressure of 70-170 bar;
• d) a reduction in the cost of the intermediate treatment which brings the exhaust gas exiting the melamine reactor to pressures of less than 50 bar at which the distillation columns operate according to the prior art;
• e) less corrosion problems of equipment and lines involved in recycling;
• f) overall improved operational reliability and availability of the synthesis process of urea;

• 2) The melamine production process of the invention does not require the addition of additional NH ₃ and CO ₂ jets. The process requirement is indeed satisfied holistically and exclusively by the partial reuse / recycling of the by-produced NH ₃ recovered in the various steps of the process;
• 3) a reduction in the formation of OATs by hydrolysis of the melamine, not only due to the effect of reducing the number and volume of equipment operating at a high temperature, but also as a result of reducing the temperature itself. The cleaning may actually be at a temperature from 5 to 10 degrees less than those of the prior art. This leads to a further increase in the total yield of the process in addition to that obtained as a result of the higher conversion of urea thanks to the insertion of the post reactor;
• 4) the possibility of using both NH ₃ and CO ₂ in other industrial processes than those for the production of urea; since both compounds are recovered with a high purity and separately, they are available for different uses and with a possible higher added value;
• 5) a reduction in the total amount of water used and its complete recycling in the production process of melamine, with the subsequent elimination of liquid effluents;
• 6) an extractive distillation of CO ₂ which, although carried out at a high pressure, has a reduced energy consumption;
• 7) The melamine production process according to the present invention can be coupled to processes for the production of urea which are carried out in existing urea plants of any known technology, thereby reducing the times of shutdown and invasive technological modifications required for the modernization and adaptation of the plants are drastically reduced.

An embodiment of the method according to the present invention is provided below for illustrative and non-limiting purposes of the present invention.

EXAMPLE 1

In a plant for the production of melamine of high purity, designed according to the present invention, as in 3 shown was a jet of molten urea 301 sent to the main reactor at a flow rate of 22,650 kg / h, together with a stream of anhydrous ammonia 319 in an amount of 1,500 kg / h.

The liquid jet 303 which leaves the main reactor was then sent to the subsequent post reactor, where the synthesis reactions of melamine were completed in a virtually CO ₂ -free environment, also thanks to the stripping effect caused by the jet 304 of 1,600 kg / h of gaseous NH ₃ , which is continuously introduced to the soil.

Then a liquid effluent was obtained from the post reactor (Strahl 306 ), corresponding to 7,600 kg / h of melamine, containing reduced amounts of OATs and polycondensates (0.6 and 1.0 wt .-%, respectively). Both reactors were operated at a temperature of 380 ° C and a pressure of 80 bar.

The gaseous phases consisting of NH ₃ and CO ₂ 302 and 305 , saturated with melamine vapors, were separated from the two reactors and placed in a single stream 312 which was sent to the extractive distillation column (Distex) for melamine recovery and separation of high purity CO ₂ .

The Distex was used at a pressure of 75 bar and took as extraction solvent 30,000 kg / h of the coming of the subsequent operating phases aqueous jet 325 on.

In particular, the amount of aqueous jet used as the extraction solvent in the process according to the present invention is less than 280% with respect to an identical process in which the distillation in Distex was carried out at a pressure of 35 bar.

8,950 kg / h of extremely pure and essentially anhydrous CO ₂ with an NH ₃ content of less than 10 ppm were separated from the head of the Distex, whereby the gaseous jet 314 was formed at the plant boundary at a pressure of 75 bar.

The watery jet 315 Discharged from the bottom of the Distex, contains all the ammonia and melamine, that with the rays 318 . 325 and 329 enters the process and has a minimum amount of residual CO ₂ . The watery jet 315 was sent to the CO ₂ stripper for complete removal of CO ₂ from the aqueous jet containing ammonia and melamine. The beam at the head of the column ( 315b ), which is the entire CO ₂ and part of the ammonia of the jet 315 is sent to the separating section of ammonia; the ground spray 315 is used to recover the raw melamine coming from the post reactor (jet 306 ) sent to the quench.

The entire product leaving the quench (Ray 330 ), was sent to the cleaning section, which operates at a pressure of 25 bar and a temperature of 165 ° C. Another amount of ammonia equal to 9,000 kg / h (jet 320 ) was added in the purification section to obtain complete conversion of the polycondensates to melamine.

The watery jet 307 of purified melamine that is less than 100 contains ppm of polycondensates was cooled to 40-50 ° C and decompressed to atmospheric pressure in the crystallization section, in which the precipitation of melamine crystals was obtained.

The suspension of melamine crystals in the mother liquor (jet 308 ) was then sent from the crystallizer to a centrifugal decanter, whereby a cake of wet crystals was separated from the crystallization mother liquor. The cake was then dried in the specific drying section, producing 7,500 kg / h of anhydrous melamine with a high degree of purity (titer higher than 99.8% by weight and moisture lower than 0.1% by weight).

The beam 310 the crystallization mother liquor leaving the centrifugal decanter was then sent to the ammonia separation section, where 18,800 kg / hr of extremely pure ammonia was recovered at 20 bar pressure and partially in the same melamine (blast 304 . 319 and 320 ) and mostly at the plant boundary (beam 321 ) were provided as a by-product of the plant (6,700 kg / h of NH ₃ in the liquid state).

The beam 322 , produced at the bottom of the ammonia separation section, consists of 71,000 kg / h of ammonia-free mother liquor. The beam 322 contains all the OATs prepared in the pre-plant sections and is saturated with melamine under the crystallization conditions (about 0.8-1.0 wt%). The beam 322 was sent to the final sections of the plant (burster / stripper) to destroy the residual melamine and other organic by-products by the high temperature hydrolysis of OATs.

The beam 322 In fact, it was first heated to 280 ° C and then sent to the disassembly / stripper section, where all organic material (OATs, melamine, other by-products) was hydrolysed and converted to NH ₃ and CO ₂ resulting in a liquid residue after separation of these gases in the stripper an almost pure aqueous jet (jet 313 containing less than 100 ppm of total solid products and less than 10 ppm of free NH ₃ used to wash the exhaust air stream in the scrubber.

The gases formed in the burster and those at the head of the stripper of the same section (beam 318 ) were subjected to extraction distillation in Distex for the recovery of CO ₂ and then NH ₃ .

Portions of the ammonia-free aqueous jet leaving the burster are used as the extraction solvent in the Distex.

The total yield of the manufacturing process was found to be equal to 0.33 kg of melamine per 1 kg of urea, corresponding to a specific consumption of 3.02 kg of urea per 1 kg of high purity melamine produced.

• – ein Strahl aus CO₂ mit einer Reinheit von mehr als 99,9 %, auf einer feuchten Basis, und bei einem Druck von 75 bar, in einer Menge gleich 0,395 kg pro 1 kg verbrauchtem Harnstoff;
• – ein Strahl aus anhydrischem und flüssigem NH₃ mit einem Titer von 99,9 %, in einer Menge von 0,296 kg pro 1 kg verbrauchtem Harnstoff.

The following rays were also obtained in the process:

• - a jet of CO ₂ with a purity of more than 99,9%, on a wet basis, and at a pressure of 75 bar, in an amount equal to 0,395 kg per 1 kg of urea consumed;
• A jet of anhydrous and liquid NH ₃ having a titer of 99.9%, in an amount of 0.296 kg per 1 kg of spent urea.

Further, the energy balance of the process according to the present invention is as follows with respect to an identical process in which the distillation in Distex was carried out at a pressure of 35 bar and with an amount of extraction solvent equal to 85,000 kg / h. print 35 75 bar absorption of water 85,000 30,000 kg / h Distexdampf 23,000 9900 kg / h CO ₂ stripper steam 12,000 10,700 kg / h total steam 35,000 20,600 kg / h (-14400)

EXAMPLE 2

• – 55 bar
• – 170 bar.

The procedure described in Example 1 was repeated with the following operating pressures of Distex:

• - 55 bar
• - 170 bar.

In the following Table 1, the values measured at these pressures are also compared with the corresponding values measured during the test, which at 35 bar according to the teaching of WO 01/46159 and at 75 bar according to Example 1 were performed. Table 1 print 35 55 75 170 bar Water flow rate of absorption to distex (jet 325 ) 85000 57500 30000 11250 kg / h Flow rate of liquid jet 318 4200 4200 4200 4200 kg / h Hourly flow rate of steam demanded by Distex 23000 16500 9900 13000 kg / h Hourly flow rate of steam required by CO ₂ stripper 12000 11500 10700 4,700 kg / h

The results given in the table above show the following reductions in total steam consumption in the case of the process carried out with the Distex at 55 bar, 75 bar and 170 bar with respect to the Distex at 35 bar according to the teaching of WO 01/46159 is performed:
55 bar = -4,000 kg / h
75 bar = -14,400 kg / h
170 bar = -17,300 kg / h

Further, the values given in Table 2 were measured to account for the energy balance for the introduction of the solvent and the recycled streams into the extractive distillation column at a higher pressure. Table 2 print 35 55 75 170 bar For the introduction of the solvent ( 325 ) and the recycled ( 318 ) liquid energy required in the Distex column 140 152 115 120 kw Difference between the total driving force required for the introduction of the solvent ( 325 ) and the recycled ( 318 ) in the Distex column, calculated in relation to the value for the example carried out at 35 bar 0 95 -207 -164 Difference expressed as hourly equivalent amount of water vapor (8 kg driving steam / kilowatt-hour) kg / h Total difference in hourly amount of water vapor calculated in relation to the value of the example carried out at 35 bar 0 -6,905 -14,607 -17,464 kg / h

The contents of Table 2 show that, even taking into account the energy consumed to achieve a higher pressure in the extractive distillation column, substantial reductions in the total consumption of steam are achieved when carrying out the method according to the present invention with the operation of Distex at 55 bar, 75 bar and 170 bar, in relation to the Distex, which according to the doctrine WO 01/46159 is carried out at 35 bar.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 01/46159 [0023, 0087, 0088, 0090]

Claims (17)

Process for the preparation of melamine from urea, comprising separation and recovery of high purity CO ₂ and high purity NH ₃ , comprising the following steps: a) Synthesizing melamine in a main reactor by pyrolysis of a molten urea jet ( 301 ), in the presence of NH ₃ ( 319 ), Obtaining a jet of molten raw melamine ( 303 ) and a jet of exhaust gas with NH ₃ and CO ₂ ( 302 ); b) optionally treating the jet of molten raw melamine ( 303 ) exiting the main reactor in a post-reactor in the presence of gaseous ammonia ( 304 ), for the completion of the reaction of pyrolysis of urea, obtaining a jet of molten crude melamine ( 306 ), essentially free of unconverted urea and CO ₂ and a jet of exhaust gas ( 305 ) consisting mainly of NH ₃ and containing all the CO ₂ formed in the reaction; c) exposing the jet of exhaust gas ( 302 ) from step a) and, if present, the jet of exhaust gas ( 305 ) from step b) of a distillation in the presence of an aqueous extractive solvent ( 329 . 318 ), Obtaining a gaseous jet of CO ₂ of high purity ( 314 ) and an aqueous ammonia jet ( 315 ) with CO ₂ ; d) exposing the aqueous ammonia jet from step c) ( 315 ) Stripping to remove any remaining CO ₂ ; e) cooling and solubilizing the from step a) and ( 306 ) Step b), if present, obtained molten raw melamine jet ( 303 ) by mixing with the aqueous ammonia jet ( 315 ) from step c), obtaining an aqueous ammonia solution ( 330 ) with dissolved melamine, impurities of polycondensates and OAT; f) purifying the aqueous ammonia solution from melamine ( 330 from step e) by remaining in the presence of NH ₃ , converting polycondensate to melamine; g) crystallizing the in the purified aqueous solution ( 307 ) from step f) of melamine and separation of melamine crystals from the solution, obtaining a moist cake ( 309 ) of melamine and a jet of crystallization mother liquor ( 310 ), containing only OAT, dissolved melamine residues and NH ₃ ; h) distilling the mother liquor leaving step g), obtaining a jet of NH ₃ of high purity ( 317 ) and an aqueous jet free of ammonia ( 322 ), OAT and melamine residues, subsequently recovered and / or eliminated in associated downstream process sections, for example in a decomposition / stripping section; i) drying the moist melamine cake ( 309 from step g) with hot air, obtaining dried melamine crystals having a high degree of purity and an exhaust air stream with ammonia; the process requires that step c) of the extractive distillation of the exhaust gas stream ( 302 ) from step a) and, if present, the exhaust air flow ( 305 ) from step b) in the presence of an aqueous extractive solvent at a pressure of 55 to 250 bar, preferably 70 to 170 bar, is performed. The method of claim 1, wherein the aqueous extractive solvent is water or an aqueous solution free of ammonia, more preferably a recycled aqueous solution free of ammonia ( 329 . 318 ). Process according to one or more of the preceding claims, wherein the aqueous extractive solvent is present in an amount of from 3 to 6, preferably from 4 to 5 tons of solvents per 1 tonne of melamine produced. Process according to one or more of the preceding claims, wherein the step c) of extractive distillation is carried out in a separator, preferably in a distillation column or Distex, made of materials suitable to withstand the required operating conditions, such as an alloy consisting essentially of a combination of nickel, chromium and molybdenum or metals such as zirconium, titanium or compounds of such materials. Process according to one or more of the preceding claims, wherein the NH ₃ jet of high purity ( 317 ) from step h) the synthesis reactor in step a) ( 319 ), the post-reactor in step b) ( 304 ) and the purification step e) ( 320 ) is supplied. Process according to one or more of the preceding claims, wherein the NH ₃ jet of high purity ( 317 ) from step h) and the CO ₂ jet of high purity ( 314 ) from step c) are fed to a process for the synthesis of urea. Process according to one or more of the preceding claims, wherein the NH ₃ jet of high purity ( 317 ) from step h) and the CO ₂ jet of high purity ( 314 ) from step c) as raw materials for use in other industrial manufacturing processes. Method according to one or more of the preceding claims, wherein a part of the high-purity gaseous CO ₂ jet ( 314 ) from step c) is used to free the aqueous jet from ammonia ( 322 ) from step h). Method according to one or more of the preceding claims, wherein step h) further comprises the separation of a cleaning jet ( 329 ) with CO ₂ from the crystallization mother liquor jet, the purge jet being subjected to extractive distillation in step c). Method according to one or more of the preceding claims, wherein the exhaust gas air stream is contacted with ammonia from step i) with an aqueous washing jet in a scrubber, whereby an aqueous ammonia jet ( 316 ) and a purified air jet forms. Process according to claim 10, wherein the aqueous ammonia jet ( 316 ) is added from the scrubber to the quench section of step e) to cool and solubilize the molten raw melamine stream. Process according to one or more of the preceding claims, wherein CO ₂ possibly present in the crystallization mother liquor is present as an aqueous ammonia mixture ( 329 ) is separated in the ammonia separation section of CO ₂ and is supplied to the extraction of extractive distillation from step c), together with the exhaust gas jet ( 302 ) from step a) and the exhaust gas possibly coming from step b) ( 305 ). Process according to one or more of the preceding claims, wherein the distillation from step c) is fed with jet of exhaust gases from the processes for the production of melamine carried out in other melamine production plants. Group for separation and recovery of high purity CO ₂ and high purity NH ₃ from exhaust gases from a plant for the production of melamine, the group being characterized in that it comprises the following elements: a first distillation column, CO ₂ extracted, suitable for operation with an aqueous extractive solvent, for example water; a second column for separation, for the recovery of the CO ₂ and NH ₃ in the jet, leaving the bottom of the first column for extractive distillation, suitable from the head; a third column for separation, suitable for the recovery of the NH ₃ in the jet coming from the head of the second column in the head, the elements for performing the steps of separation and recovery of high purity CO ₂ and high purity NH ₃ are suitable from the exhaust gases. A group according to claim 14, for transforming existing plants for the production of melamine and / or for the integration of melamine production plants in urea production plants. Cycle for separating and recovering high purity CO ₂ and high purity exhaust gas NH ₃ from exhaust jets coming from a plant for the production of melamine, by urea pyrolysis, the cycle comprising: a reaction section consisting of a main reactor and a Post reactor exists, wherein the pyrolysis with a high conversion of urea to melamine takes place with the separation of an anhydrous exhaust jet of NH ₃ , CO ₂ and melamine vapors; An extractive distillation column or a Distex connected to the reaction which sends and receives the anhydrous off-gas jet, the Distex being a top product consisting of high purity CO ₂ by distillation at a preferred pressure of 70 to 170 bar and a bottom water jet consisting of NH ₃ and residual melamine, preferably using aqueous solutions free of ammonia from the cycle of purification and recovery of melamine as extraction solvent; a stripping section for removing residual CO ₂ from the aqueous solution free of ammonia; A quench section and a purification section for the recovery and purification of melamine fed with aqueous ammonia jets coming from the production cycle, possibly with the addition of NH ₃ ; A precipitation zone by crystallization of purified melamine associated with the separation and drying of the melamine product; An ammonia separation section for the production of ammonia at high purity by distillation from the crystallization mother liquors; - a section for the treatment of mother liquors free of ammonia, which emerge from the section of ammonia separation. A cycle according to claim 16, wherein the extractive distillation column or the Distex is the first column of the group according to claim 14, intended for the separation and recovery of high purity CO ₂ and high purity NH ₃ from off-gas jets obtained from a plant for manufacture come from melamine.

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