Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/002—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by condensation
• • 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
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
  • C08G12/00—Condensation polymers of aldehydes or ketones with only compounds containing hydrogen attached to nitrogen
  • C08G12/02—Condensation polymers of aldehydes or ketones with only compounds containing hydrogen attached to nitrogen of aldehydes
  • C08G12/26—Condensation polymers of aldehydes or ketones with only compounds containing hydrogen attached to nitrogen of aldehydes with heterocyclic compounds
  • C08G12/30—Condensation polymers of aldehydes or ketones with only compounds containing hydrogen attached to nitrogen of aldehydes with heterocyclic compounds with substituted triazines
  • C08G12/32—Melamines

Definitions

• the present invention relates to a process for the production of melamine, with particular reference to the separation phase and treatment of the reaction product.
• melamine is normally produced by the pyrolysis of urea, according to the general reaction:
• the carbon dioxide and ammonia which develop from this pyrolysis are commonly called off-gas.
• the overall pyrolysis reaction requires a heat supply and is carried out, depending on the technologies available, both in liquid phase, at a high pressure and without catalysts, and in vapour phase, at lower pressures and with heterogeneous catalysts .
• the selection of the operat- ing parameters allows the reaction to be specifically shifted towards the exhaustion of the urea fed.
• 234 kg of off-gas are formed, consisting of a mixture of C0 2 and NH 3 , . with a gas/melamine weight ratio equal to 1.85.
• the technical problem which arises therefore relates to the separation of the melamine within specification as well as the recovery and re-use of the considerable amount of off-gas which derives from the main reaction.
• the efficiency and economy of the separation and treatment of the effluents of the reaction step are crucial for the commercial success of the production process of melamine. It is specifically in the separation and recov- ery steps that there are the major differences between the various competing processes.
• the production plants of melamine from urea are coupled, or even integrated, with the production plants of urea, so that the off-gas can be re-used in the overall cycle.
• the pyrolysis reactor of urea to melamine operates at temperatures between 380 and 450°C, at pressures within the range of 80-150 bar.
• the off-gas can be absorbed in water, forming ammonium carbamate or carbonate, it can be condensed and fractionated to separate the ammonia from the carbon dioxide, or it can be used for the production of ammonium nitrate or sulfate, which can be used as fertilizers.
• the above method allows this part of the melamine to be recovered and to recycle the off-gas in the anhydrous state to the urea plant section which has a pressure compatible with that of the off-gas thus made available.
• the molten urea, which contains the recovered melamine is then fed to same melamine synthesis reactor.
• the efficiency itself of the pyrolysis unit is substantially diminished by the recycling of both the recovered melamine which, under regime conditions, recirculates on itself for about 15-20%, and also of the off-gas which recirculates for about 40-50% of that produced by the "useful" pyrolysis reaction.
• This recirculation not only requires an increase in the reaction volumes, but also disturbs the circulation inside the reactor 'and diminishes the efficiency of the thermal exchange due to the unfavourable change in the gas/liquid ratio in the reactor. A substantial increase in the thermal exchange surfaces is therefore necessary for the same net production of melamine.
• the known art also proposes a system for separating off-gas from the raw effluent of the pyrolysis reaction - melamine and off-gas still in gaseous phase - in a rapid cooling column - normally called a quench column - by separation with water or cold aqueous ammonia or ammonia solutions.
• This separation system is illustrated in the scheme of figure 1.
• the reaction is carried out in the reactor A fed with molten urea through line 1 and with additional ammonia through line 2.
• the raw reaction effluent produced in step A is discharged from the top through line 3 and is rapidly cooled in the quench column B to a temperature at which there is the complete recovery of melamine, which passes in solution or in suspension in the aqueous separation stream.
• the column B is fed with the aqueous stream of the bottom of column C in which the gases pro- Jerusalem in the stripping column D are separated.
• Condensed melamine is obtained in the aqueous liquid phase leaving column B, together with ammonia and saturation carbon dioxide.
• the remaining off-gas is released, rising up the column, with a certain water vapour content but substan- tially free of melamine.
• the column B can operate within an extremely wide pressure range, from the reaction pressure up to 1-2 bar, depending on the requirements of the subsequent system which receives the off-gas.
• the operating temperatures correspond to the pressure and composition of the gaseous phase.
• the quenching operation is generally carried out at 20-30 bar (about 2-3 10 6 Pascal) and at 155-165°C.
• This system also has its disadvantages. It leads to the formation of an aqueous solution of melamine containing however an unacceptable quantity of C0 2 in the steps downstream.
• the product at the bottom of column D contains melamine and is subsequently sent for purification, indicated as E for short. It can be seen how the removal of C0 2 re- quires an additional stripping and separation step with an increase in investment costs and energy consumption.
• the vapour consumption alone in the stripping column D is in the order of 1 kg of vapour per kg of NH 3 and C0 2 stripped.
• An objective of the present invention is to provide a treatment process of off-gas which overcomes the disadvantages of the processes according to the known art in a simple and economic way.
• the process according to the invention is aimed at recovering the content of melamine, still in vapour phase, . from off-gas, after it has been directly separated in the upper part of the same reactor or in the separator immediately downstream.
• Figure 1 represents the known art according to what is described above.
• Figure 2 illustrates an embodiment of the invention, whereas figures 3, 4 and 5 show its possible variations.
• the pyrolysis reaction takes place in the reactor A, consisting of a vertical vessel into which the molten urea is fed from the bottom through line 1, together with a stream of anhydrous ammonia, preferably in gas phase, fed through line 2.
• the pyrolysis reaction for the transformation of urea to melamine takes place in the reactor A, operating at 380-450°C and at 80-150 bar, and provid- ing the heat necessary for sustaining the pyrolysis reaction.
• a mixed phase is formed in the pyrolysis reactor A, substantially consisting of the liquid melamine formed, the off-gas produced in the reaction and the excess am o- nia injected from the bottom through line 2.
• This mixed phase is maintained in constant circulation due to the hydraulic pressure of the gases in formation and to the particular internal geometry of the pyrolysis reactor.
• reaction gases containing melamine in vapour phase in a ratio of 6-8% by weight are separated by the upper part of the reactor A as illustrated in the scheme of figure 2, or by a gas/liquid separator A' immediately downstream as shown in the scheme of figure 3, and are sent through line 3 to the gas washing apparatus B.
• the molten raw melamine which contains a small part of dissolved gases in a ratio of 2-3% by weight of NH 3 and C0 2 with respect to the total, is sent to the dissolution and quench equipment C through line 4.
• this molten raw melamine can be subjected to enhancing purification treatment, as described in U.S. patent 6,252,074 and Italian patent application MI 01A001216 filed by the same Applicant.
• a stream of aqueous ammonia is fed into the apparatus C from the bottom through line 6, as illus- trated in the schemes of figures 2, 3, 4 and 5.
• a special feature of the invention consists in the fact that - with respect to the schemes of the known art as shown in figure 1 - the quench column C operates in liquid phase and in the presence of a reduced concentration of C0 2 , due to its low solubility in the mixture of raw melamine fed through line 4.
• the treatment consisting of the injection of ammonia through line 6, operating according to what is described in pat- ent application WO 001/36397A1, allows an effective elimination of the polycondensates, without having to resort to the preventive stripping of C0 2 , as is required, on the contrary, in the scheme of figure 1.
• a stream of purified melamine is extracted from the upper part of the apparatus C, which is sent to the crys- tallizer through line 7 together with the melamine separated in the apparatus B, discharged through line 9.
• the total separation of the melamine takes place in the apparatus B, with a modest quantity of reintegration water, generally called make-up, fed from the top through line 8, together with a recirculation stream of the aqueous solution of melamine through line 9' , taken from the stream of aqueous solution of melamine separated in the column B which is discharged through line 9.
• the reinte- gration or make-up stream can consist of demineralized water, supplied to the plant from an external source, or it can be taken from a suitable stream inside the plant, having a composition compatible with its use in column B.
• the stream 9' is cooled with an exchanger F before being recycled to the upper part of the column B, which allows the quantity of make-up to be limited, with the same separation efficiency.
• the exchanger F substantially supplies the cooling necessary in column B for separating the condensed components from the gaseous reaction effluent, and which are discharged through line 9.
• the quantity of make-up water necessary is normally extremely reduced, in a weight ratio with the off-gas treated in column B within the range of 0.2-1.5, preferably ranging from 0.3 to 0.5, whereas the recirculation of the melamine solution through line 9' is in a weight ratio with the off-gas treated within the range of 4-40, preferably ranging from 10 to 15.
• the separation of the melamine in column B is generally carried out at tem- peratures within the range of 120-180°C, preferably 155- 165°C, at pressure within the range of 2-30 bar, preferably around 25 bar.
• the melamine recovered in the gaseous phase with the stream of line 9 has a very high purity, as contains nei- ther oxyaminotriazines, generally called OAT, nor poly- condensates. It consists of a suspension/solution which can be sent directly to the crystallization section.
• This alternative embodiment of the invention ensures a significant improvement in the constancy and qualitative homogeneity of the melamine produced. As there is no longer a significant content of C0 2 in the solution, the apparatus C can act as an effective purifier of the product from the polycondensates .
• the melamine is produced with a non-catalytic pyrolysis process of molten urea in a reactor with internal recirculation operating at 390°C and 80 relative bar (about 8-10 6 Pascal) .
• the reaction product consists of a liquid phase (a) and a gaseous phase (b) with the following weight compo- sitions :
• phase (b) is cooled and washed with water, which can be recycled water, in order to de- sublimate and dissolve the melamine contained therein.
• Two streams consequently leave the cooling and washing apparatus of phase (b) , which operates at 25 relative bar and 160°C: a liquid phase (c) and a gaseous phase (d) with the following weight compositions:
• phase (c) contains pure desublimated melamine and is sent directly for crystallization.
• Phase (a) mainly consists of liquid melamine which is sent for quenching where it is cooled and dissolved with water.
• the lat- ter is fed with a quantity of ammonia which is such as to have a phase (e) in solution with an NH 3 content of at least 13% by weight, with the following composition:
• the operating conditions are 25 bar and 172°C and correspond to those necessary for the purification as indicated in WO 01/36397A1.
• the resulting liquid phase contains 3.8% by weight of C0 2 and must therefore be subjected to stripping in the column D to remove it before being sent for subsequent purification with ammonia.

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• 2002
  • 2002-05-14 IT IT2002MI001026A patent/ITMI20021026A1/it unknown
• 2003
  • 2003-04-29 CN CNB038132893A patent/CN1300122C/zh not_active Expired - Lifetime
  • 2003-04-29 WO PCT/EP2003/004556 patent/WO2003095516A1/en not_active Application Discontinuation
  • 2003-04-29 AU AU2003232234A patent/AU2003232234A1/en not_active Abandoned
  • 2003-04-29 EP EP03749861A patent/EP1504045A1/en not_active Withdrawn

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