Classifications

• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07D—HETEROCYCLIC COMPOUNDS
  • C07D251/00—Heterocyclic compounds containing 1,3,5-triazine rings
  • C07D251/02—Heterocyclic compounds containing 1,3,5-triazine rings not condensed with other rings
  • C07D251/12—Heterocyclic compounds containing 1,3,5-triazine rings not condensed with other rings having three double bonds between ring members or between ring members and non-ring members
  • C07D251/26—Heterocyclic compounds containing 1,3,5-triazine rings not condensed with other rings having three double bonds between ring members or between ring members and non-ring members with only hetero atoms directly attached to ring carbon atoms
  • C07D251/40—Nitrogen atoms
  • C07D251/54—Three nitrogen atoms
  • C07D251/56—Preparation of melamine
  • C07D251/60—Preparation of melamine from urea or from carbon dioxide and ammonia

Definitions

• the present invention relates to a high-pressure process for the preparation of highly pure melamine from urea.
• melamine can be prepared from urea at a temperature of 390-410 °C according to the following reaction formula:
• the reaction is strongly endothermic.
• the heat requirement is 649 kJ per melamine mole, when the heating af the urea from 135 °C (melting point of urea) to the reaction temperature is included.
• a fluid-bed reactor In typical low-pressure processes, a fluid-bed reactor is used in which the catalyst is fluidized with gaseous ammonia or a mixture of ammoma and carbon dioxide. The melamine emerges in gaseous state from the reactor.
• the fact that corrosion is less than in high-pressure processes is regarded as one of the advantages of low-pressure processes.
• the best known users of low-pressure processes are BASF (Hydrocarbon Processing, September 1969, p. 184), Chemie Linz (Hydrocarbon Processing, November 1966, p. 146), and DSM/Stamicarbon (Chem. Eng., May 20, 1968, p. 124), each of which has developed its own process version.
• the reaction takes place in a liquid phase.
• the reactor is full of molten melamine mixed to some degree with molten raw material, i.e. urea, and intermediate reaction products.
• gas bubbles consisting of ammonia and carbon dioxide and a small amount of gaseous melamine.
• the required high amount of reaction heat is usually generated by intra-reactor heating elements, in which the heat is generated by means of electricity or, for example, a circulating hot salt melt.
• the Montedison process (Ausind) is a typical high-pressure melamine production process (Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, Vol. A 16, p. 177).
• urea melt and hot ammoma are introduced into a reactor.
• the reactor conditions are 70 bar and 370 °C.
• From the reactor the mixture of melamine melt and product gases is directed to a quencher, into which water containing ammonia and carbon oxide is also introduced.
• the temperature of the quencher is 160 °C and its pressure 25 bar. From this quencher the reactor off- gases are fed for further use, for example for the production of urea or fertilizers.
• the melamine is recovered from the slurry by a highly multiple-stage further treatment, which includes the removal of ammonia and carbon dioxide, the dissolving of the melamine in a large amount alkaline water, removal of color with activated carbon, crystallization, filtration, drying, and packaging.
• the Montedison process has two significant disadvantages. First, the product gas is obtained at a relatively low pressure, only 25 bar. Second, the number of process stages is very high, since the impure product obtained from the reactor requires a thorough purification treatment. Furthermore, the purification apparatus is relatively large in size, since the pressures in it are already low.
• the product gas is thus obtained at a pressure of 100 bar and in anhydrous state, which may be a considerable advantage in terms of its further use.
• the urea melt to be used as raw material is fed in via the scrubbing tower. There it heats up and water is removed from it (reacts with urea).
• the melamine melt is dissolved in an aqueous ammonia solution. This solution is maintained under ammoniacal pressure at 180 °C for a certain period, during which the impurities are said to be eliminated. Thereafter follows a further treatment with numerous apparatus, including filtration and crystallization. Thus the number of unit operations and apparatus is really high, which raises the process costs.
• the melamine melt is directed to a quencher unit, in which it is cooled rapidly by means of, for example, liquid ammoma or water. Crystalline melamine is obtained, which is withdrawn via the bottom of the quencher unit and taken to drying.
• the quantity of apparatus in the process is really small as compared with the processes described above. However, the purity of the product is only 96-99.5%. In this respect the process is not competitive with the processes described above. Achieving a competitive purity by the Melamine Chemicals process would require that a purification process, for example one similar to those in any of the above processes, be installed after the process. Thereby the advantages of the process, i.e. small quantity of apparatus and consequent cost efficiency, would be eliminated.
• melamine preparation patents include an example in which approx. 9 kg of ammonia per one kilogram of melamine product is fed into a reactor which operates at the temperature of 400 °C and under a pressure of 40-80 bar.
• the amount of ammonia is in this case so high that all of the produced melamine is vaporized into the gas phase.
• the promoting effect of ammonia on melamine vaporization is based on the fact that it reduces the partial pressure of melamine in the gas phase.
• the ammoma amount required could be decreased by lowering the reactor pressure or by raising the temperature.
• the melamine would vaporize more easily into the gas phase, and ammonia would not be needed in such a large amount to lower its partial pressure.
• the reactor pressure is lowered to below 50 bar, there will form in the reactor a solid which will complicate and ultimately hinder the operation of the reactor.
• all of the melamine can be caused to vaporize into the gas phase.
• the concentration of melamine in the leaving gas phase will be approx. 7% by volume, calculated stoichiometrically.
• the feed of ammonia is 1 g per one gram of urea
• the melamine content in the gas will be approx. 3.2% by volume.
• the first-mentioned case (melamine 7% by volume) requires a pressure of approx. 72 bar and a temperature of 480 °C or, for example, a pressure of 40 bar and a temperature of 455 °C.
• the prior-art melamine production processes can be divided into two categories. In most low-pressure processes and the Nissan (meaning the first-mentioned of the two Nissan processes referred to) and Montedison high-pressure processes, evidently a competitive purity is achieved, but the processes involve a multiple-stage purification section with a large quantity of apparatus, which clearly increases both the operating costs and the investment.
• the processes described in patents US 4,565,867 and US 3,484,440 constitute the second category. They are in the same category in terms of the quantity of apparatus, and they do not have the multiple- stage purification section mentioned above. However, the purity of the product obtained from them is not competitive.
• this previous process disclosed in WO 95/01345 comprises the steps of: i) feeding molten urea and hot ammonia gas into a reactor having a pressure within the range 50-150 bar and a temperature within the range 360-430 °C, to obtain a reaction product containing a liquid melamine melt and a gas mixture; ii) separating the gas mixture from the liquid melamine melt; iii) evaporating the liquid melamine melt thus obtained in an evaporator; and iv) cooling the melamine-containing gas obtained from the evaporator, whereby the melamine is crystallized in a very pure state.
• the melamine melt obtained from the melamine reactor can be evaporated by lowering the pressure or by raising the temperature or by feeding ammonia gas into the evaporator or by using two or all of these three methods.
• pressures are within the range 20-90 bar and temperatures are within the range 420-470 °C.
• at a pressure of 50 bar and a temperature of 450 °C at minimum approx. 2.4 kg of ammonia per one kilogram of melamine is needed in the evaporator, meaning that the molar ratio of ammonia to melamine is at least 17.6: 1.
• the cooling of the melamine-containing gas is carried out by contacting said gas directly with liquid ammonia in a cooler at a temperature below 130 °C and at a pressure below 40 bar.
• the aim of the present invention is to further improve the process disclosed in WO 95/01345 especially in respect of simplicity of performance and energy consumption or heat recovery in order to make the process more economic.
• the present invention provides a process for the preparation of melamine from urea comprising the steps of:
• Steps a) and b) are preferably carried out as explained in WO 95/01345.
• the pressure is within the range 1-15 bar, preferably within the range 5- 15 bar, and the temperature is within the range 290-520 °C, preferably within the range 310-520 °C and more preferably within the range 360-480 °C.
• the temperature is preferably within the range 300-350 °C and at a pressure of 10 bar the temperature is preferably within the range 340-425 °C.
• the molar ratio of ammonia to melamine is preferably between 1: 1 and 17: 1, more preferably between 7: 1 and 17: 1.
• superheated ammonia gas is introduced into the evaporator.
• the temperature of superheated ammoma is typically about 540 °C.
• the melamine retention time in the evaporator is preferably less than half an hour, and especially preferably less than 10 minutes.
• the pressure is within the range 5-15 bar
• the temperature is within the range 360 °C - 480 °C
• the molar ratio of ammonia to melamine is 7: 1 to 17: 1.
• the temperature, pressure and molar ratio of ammoma to melamine are dependent on each other. For example at a pressure of 10 bar and a molar ratio of 7: 1, the temperature required to evaporate all melamine is about 440 °C, and at a pressure of 10 bar and a molar ratio of 17: 1, the temperature required to evaporate all melamine is about 400 °C.
• cooling step d) the pressure is within the range 1-15 bar, preferably within the range 5-15 bar, and the temperature is within the range 0-250 °C, preferably within the range 30-230 °C. Preferably the pressure is the same as in the evaporator.
• a cooling medium is preferably introduced into the cooler and contacted directly with the melamine-containing gas.
• said cooling medium is liquid ammonia wherein the cooling is effected by evaporating ammoma.
• the cooling is effected by evaporating ammoma.
• cooler and crystal separation can be combined into one apparatus for example of cyclone-type.
• Evaporating ammonia and ammonia gas originating from the evaporator are withdrawn from the cooler and at least a portion thereof is condensed to form liquid ammonia to be recycled into the cooler as cooling medium.
• the heat recovered from the ammonia cooling can be utilized for heating ammonia e.g. fresh liquid ammonia, to be introduced into the evaporator.
• said cooling medium is gaseous ammonia.
• cooler and crystal separation can be combined into one apparatus for example of cyclone-type.
• Gaseous ammonia whereof a part originates from the cooling medium and a part from the evaporator is withdrawn from the cooler, and at least a portion thereof is cooled to form gaseous ammoma to be recycled into the cooler as cooling medium.
• the heat recovered from the ammonia cooling can be utilized for heating ammonia e.g. fresh liquid ammonia, to be introduced into the melamine reactor.
• the off-gases separated from the melamine melt can be introduced into an absorption device for recovering the small amount of melamine present in the off-gases.
• the off-gas treatment is carried out by absorbing melamine from the off-gases in a counter-current direct contact system wherein urea is the absorbent.
• the absorption device comprises an absorption column.
• a condenser is provided for condensation of urea-absorbent. This apparatus recovers most of the urea present in the off-gas of the absorption column.
• a one or two stage absorber can be used.
• the two stage absorber is preferred.
• the heat recovered from the urea condensor can be used for heating ammonia to be introduced into the melamine reactor.
• the heat recovered from the urea condenser can be used for steam production.
• the cooled off-gases containing mainly ammonia and carbon dioxide can be recycled to a urea plant.
• the absorption device preferably operates at essentially the same pressure as the melamine reactor.
• the operating temperature of the absorption device has to be above 180 °C to avoid carbamate formation and below 260 °C in order to suppress the formation biureth and triureth.
• the temperature is preferably within the range 230-240 °C.
• Fig. 1 illustrates a process diagram of a system which can be used for carrying out a first embodiment of the process of the present invention
• Fig. 2 illustrates an other process diagram of a system which can be used for carrying out a second embodiment of the process of the present invention.
• Both process diagrams of Fig. 1 and Fig. 2 comprise from left to right four sections, namely Off-gas treatment - Melamine reactor - Evaporator - Cooler.
• urea melt containing small amounts of absorbed melamine from the absorption device 4 and ammonia, preferably hot ammonia are fed into the melamine reactor 1 via different pipelines.
• the conditions prevailing in the reactor are those of a typical high-pressure process, i.e. the temperature is 360- 470 °C and the pressure within the range 50-150 bar.
• the temperature is about 400 °C and the pressure about 100 bar.
• the heating is effected by internal heating elements.
• the reactor off-gases typically containing up to about 1% melamine, the balance being ammonia, carbon dioxide and unreacted urea, are separated from the melamine melt in the upper section of reactor 1 and directed to the absorption device 4 wherein melamine is recovered from the off-gases by using molten urea introduced from a urea plant as the absorbent.
• melamine is absorbed from the off-gases by using a counter- current direct contact system followed by condensation of the urea absorbent in a separate condenser.
• a one or two stage absorber can be used. However, to make sure that all off-gases are cooled sufficiently for recovery of melamine the two stage absorber is preferred.
• the main purpose of the off-gas treatment is recovery of melamine. Other purposes are preheating the urea melt, cooling the off-gases and dehydration of urea.
• the advantages of the above described off-gas treatment as compared to previously known systems are that the present system is simple and does not need any pumps or mixers, has a lower residence time (typically only few minutes), results in better dehydration of urea and higher preheat of urea, and needs lower heat input in the melamine reactor.
• the operating pressure of the absorption device is preferably the same as for the melamine reactor, i.e. within the range 50- 150 bar, preferably about 100 bar.
• the operating temperature of the absorption device is within the range 180-260 °C, preferably within the range 230-240 °C.
• the heat recovered from the urea condenser of the off-gas treatment can be used for evaporating fresh liquid ammonia. As shown in Fig. 1 the evaporated ammonia is further subjected to hot salt superheating to obtain hot ammonia to be fed into reactor 1.
• the melamine melt obtained in reactor 1 is directed to the evaporator 2.
• the temperature is within the range 290-520 °C and can for example be about 400 °C, and the pressure is within the range 1-15 bar, preferably within the range 5-15 bar.
• a preferred pressure for the evaporator of the system shown in Fig. 1 is about 10 bar.
• the evaporator can be heated by, for example, internal heating elements.
• Ammonia gas is fed into the evaporator in order to evaporate the melamine.
• the ammonia gas fed into the evaporator can be superheated ammonia gas having a temperature of about 540 °C.
• the molar ratio of ammonia to melamine in the evaporator is preferably between 1: 1 and 17: 1, more preferably between 7: 1 and 17: 1.
• the evaporator of the present invention operates at a substantially lower pressure, and with a lower molar ammonia/melamine ratio.
• the gas mixture containing melamine and ammoma is directed from the evaporator 2 to the cooler 3. Simultaneously a cooling medium is introduced into the cooler and contacted directly with the gas mixture.
• the cooling medium is liquid ammonia.
• the liquid ammonia when evaporating, will bind the heat released in the cooling and in the crystallization of the melamine.
• the pressure of the cooler is within the range 1-15 bar, preferably within the range 5- 15 bar, and the temperature is within the range 0-250 °C, preferably within the range 30-230 °C.
• a preferred pressure for the cooler of the system shown in Fig. 1 is about 10 bar.
• no liquid ammonia is present, and the cooler and crystal separation can be combined into one apparatus for example of cyclone-type.
• a portion of the gaseous ammoma withdrawn from the cooler is condensed to form liquid ammonia for reuse as cooling medium in the cooler. Heat recovered from the ammonia cooling can be used for heating ammoma to be introduced into the evaporator.
• the melamine will be recovered in crystalline form having the same high purity as the melamine produced according to the Applicants WO 95/01345.
• Fig. 2 shows a second embodiment of the process of the present invention.
• the heat recovered from the urea condenser is used for steam production.
• the operating pressure is about 10 bar.
• the cooling medium is gaseous ammonia. A portion of the gaseous ammonia withdrawn from the cooler is cooled to form gaseous ammonia for reuse as cooling medium in the cooler.
• the heat recovered from the ammonia cooling can be utilized for preheating fresh liquid ammonia which subsequently is subjected to superheating, for example hot salt superheating, and then fed into the melamine reactor 1.
• the remainder of the gaseous ammonia withdrawn from the cooler is subjected to superheating, for example hot salt superheating, and fed into evaporator 2.
• melamine > 91 wt-%) containing melam (concentration below the detection limit 100 ppm), melem 8.0 wt-%, ammeline 0.03 wt-% and ureidomelamine 0J4 wt-% as impurities determined by high pressure liquid chromatography (HPLC) was introduced into the evaporator together with ammonia gas.
• HPLC high pressure liquid chromatography
• the molar ratio of ammonia to melamine in the feed mixture to evaporator was 15: 1 and the total pressure was 10 bar, thus the temperature needed to evaporate melamine totally was about 400 °C (406 °C, measured).
• the residence time of melamine in the evaporator was one hour to make sure that all melamine was totally evaporated and to allow certainly enough time for the purifying reactions to take place.
• the impurities of melamine like melem were converted into melamine resulting in purification of melamine which evidently showed that the used amount of excess ammoma and the used pressure were sufficient.
• the gas mixture was cooled rapidly below 100 °C by contacting it with liquid ammoma. The resulting solid crystalline melamine was analysed for impurities.
• concentrations of impurities after the evaporation were melam (concentration below the detection limit 100 ppm), melem 0.02 wt-%, ammeline 0.04 wt-% and ureidomelamine 0.01 wt-% resulting in > 99.9 wt-% melamine.

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Citations (4)

• 1998
  • 1998-12-31 FI FI982847A patent/FI107257B/fi active
• 1999
  • 1999-12-29 AU AU21118/00A patent/AU2111800A/en not_active Abandoned
  • 1999-12-29 WO PCT/FI1999/001090 patent/WO2000040567A1/en not_active Application Discontinuation
  • 1999-12-29 EP EP99965593A patent/EP1140869A1/en not_active Withdrawn

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