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.

In the prevalent and most updated industrial prac- tice, melamine is normally produced by the pyrolysis of urea, according to the general reaction: 6 moles of urea 1 mole of melamine + 3 C02 + 6NH3 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 hetero- geneous catalysts.

In these technologies, the selection of the operat- ing parameters allows the reaction to be specifically

shifted towards the exhaustion of the urea fed. On the basis of the molecular weights of the chemical species involved, for every 126 kg of melamine produced in the reaction, 234 kg of off-gas are formed, consisting of a mixture of C02 and NH3,. 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 treat- ment 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. In the current indus- trial practice prevalently adopted, 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.

In the description of the present invention, refer- ence is made to the high pressure pyrolysis technology, which operates in liquid phase by feeding a heated reac- tor with molten urea, in the presence of an excess of am- monia and without the use of catalysts. In general, the

pyrolysis reactor of urea to melamine operates at tem- peratures between 380 and 450°C, at pressures within the range of 80-150 bar.

Several methods have been proposed in the known art for the treatment of off-gas, after the separation and recovery of the melamine still contained therein in va- pour phase. For example, 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 fer- tilizers. These methods on the whole have considerable drawbacks due to both the high investments and high en- ergy costs.

A more recent system for the treatment of off-gas, relating to the synthesis of melamine at high pressure, in liquid phase, resorts to a washing of the gas phase, which is separated from the reaction effluent, with mol- ten urea, in order to separate the melamine present in the off-gas. Its content is not negligible, as it can reach 10-20% of the total melamine produced, depending on the operating conditions. 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 synthe- sis reactor.

These recovery systems with molten urea are de- scribed, for example, in US patents 3,700, 672 and 4,565, 867. This washing method with molten urea operates on the gaseous phase separated from the reaction efflu- ent, at the same synthesis pressure and at temperatures normally around 180°C. The selection of the washing tem- perature represents a compromise between the necessity of preventing the condensation of the ammonium carbamate and limiting the degradation of the urea to undesired prod- ucts, such as biuret, triuret or cyanuric acid, which cause great drawbacks in the operating units. Under the operating conditions of said adsorption with molten urea, the condensable substances in the off-gas separated from the liquid phase, essentially melamine, are condensed and solidified, remaining dispersed in the molten urea, which is then fed to the reaction. Under these conditions, how- ever, there is also the parallel phenomenon of a signifi- cant dissolution of the same off-gas in the molten urea.

Data available in the state of the art, show, for exam- ple, that the molten urea coming from the separation ap- paratus of melamine, contains around 5% by weight of dis- persed melamine, and about 20% by weight of off-gas in

solution, thus causing an undesired recycling of the off- gas which should instead be separated and sent for subse- quent treatment.

The recovery treatment of melamine from off-gas with molten urea, making the. anhydrous off-gas available for possible re-use, appears to be relatively interesting, but is not without complications and inconveniences. The washing section of the molten urea requires machinery and equipment which operate almost under the same pressure conditions as the pyrolysis reactors of urea to melamine, and with substances which cause significant corrosion phenomena, even in the case of stainless steel materials ; it is therefore necessary to resort to the use of valu- able materials, such as alloys called INCONEL and HASTEL- LOY, and extremely costly constructions.

The efficiency itself of the pyrolysis unit is sub- stantially diminished by the recycling of both the recov- ered melamine which, under regime conditions, recircu- lates 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 di- minishes 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 produc- tion of melamine.

There are also considerable drawbacks with respect to the reduced reliability of the whole system, in which the pyrolysis section is integrated with the washing sec- tion with molten urea, which forms the pyrolysis feed. A malfunctioning in the washing section-which operates under extremely delicate conditions-generally causes the immediate stoppage of the primary pyrolysis section and subsequent interruption in production. Furthermore, stoppages in the primary section of the pyrolysis, whether programmed or accidental, involve complex proce- dures such as emptying the reactor, normally effected by sublimating the melamine through an injection of ammonia at high temperature. When the washing unit with molten urea is integrated with the reaction step, this method of emptying by sublimation can no longer be adopted, as the molten urea used for the washing cannot be disposed of by feeding it to the reactor, which is not operating: it is therefore necessary to install an additional specific section which can receive the sublimation products and which is activated only in such occurrences.

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 ammo- nia 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 sepa- ration stream. The column B is fed with the aqueous stream of the bottom of column C in which the gases pro- duced 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 di- oxide. 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 pres- sure up to 1-2 bar, depending on the requirements of the subsequent system which receives the off-gas. The operat- ing temperatures, in turn, correspond to the pressure and composition of the gaseous phase. The quenching operation

is generally carried out at 20-30 bar (about 2-3 106 Pas- cal) and at 155-165°C. This system, however, also has its disadvantages. It leads to the formation of an aqueous solution of melamine containing however an unacceptable quantity of C02 in the steps downstream. It must be re- moved before going to the purification phase to obtain a product with a low content of polycondensates (among which those generally called melam, melem and melom are prevalent), as described for example in WO 01/36397A1, causing a removal of the COs in a stripping column D. The solution/suspension of melamine is subjected therein to stripping and produces a vapour phase, containing NH3 and C02, which is sent through line 7 for separation carried out in the absorption column C with recycled water coming from line 8. The product at the bottom contains stripped C02 and is recycled through line 5 to the quench column B. The product at the bottom of column D contains mela- mine and is subsequently sent for purification, indicated as E for short. It can be seen how the removal of C02 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 NH3 and C02 stripped.

An objective of the present invention is to provide

a treatment process of off-gas which overcomes the disad- vantages of the processes according to the known art in a simple and economic way. The process according to the in- vention 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.

This objective is achieved with the process accord- ing to the present invention of which claim 1 forms the most general definition; its preferential embodiments or possible variations are defined in the dependant claims.

The characteristics and advantages of the process according to the present invention for the treatment of off-gas for the recovery of the melamine contained therein and for sending it to the subsequent sections of the plant, will appear more evident from the following description, which is illustrative and non-limiting, re- ferring to the schemes of the figures.

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 possi- ble 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 trans- formation 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 reac- tion.

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 ammo- 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.

The 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 dis- solved gases in a ratio of 2-3% by weight of NH3 and C02 with respect to the total, is sent to the dissolution and quench equipment C through line 4. Before being charged into the dissolution and quench apparatus C, 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, as illustrated in the schemes of figures 2-5, 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 C02, due to its low solubility in the mixture of raw melamine fed through line 4. Under these conditions, 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 re- sort to the preventive stripping of CO2, 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 sepa- rated 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 aque- ous 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.

According to a preferred embodiment of the inven- tion, 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. In general practice, the ex- changer 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 recircu- lation 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, prefera- bly 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.

In the embodiments illustrated in figures 4 and 5, the part of the stream of line 9, which is not recircu- lated with line 9'to the separation of column B, is fed to the bottom of the purification column C together with the ammonia solution coming with line 6. This alternative embodiment of the invention ensures a significant im- provement in the constancy and qualitative homogeneity of the melamine produced. As there is no longer a signifi- cant content of C02 in the solution, the apparatus C can act as an effective purifier of the product from the polycondensates.

EXAMPLE With reference to the scheme of figure 2, the mela- mine 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"Pas- cal). The reaction product consists of a liquid phase (a) and a gaseous phase (b) with the following weight compo-

sitions: Phase (a) Phase (b) melamine 92. 5% 6. 9% NH3 1. 5% 44. 5% C02 1.. 0% 48. 6% Urea 5. 0%---- The gaseous phase (b) is cooled and washed with wa- ter, which can be recycled water, in order to de- sublimate and dissolve the melamine contained therein.

Two streams consequently leave the cooling and wash- ing 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) Phase (d) melamine 20. 0%---- NH3 15. 0% 36. 3% C02 4. 0% 43. 5% H2O 61. 0% 20. 2% The solution of 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. As there is not a significant quantity of carbon dioxide present in the quench apparatus, the lat- ter is fed with a quantity of ammonia which is such as to

have a phase (e) in solution with an NH3 content of at least 13% by weight, with the following composition: Phase (e) melamine 10. 10% urea 0. 55% NH3 13. 10% C°2 0. 11% H20 76. 14% The operating conditions are 25 bar and 172°C and correspond to those necessary for the purification as in- dicated in WO 01/36397A1.

As a comparison, if the same reaction product is fed, according to the scheme of figure 1, as mixed liq- uid/gas phase directly to the quench apparatus B, with the other conditions remaining unvaried, the resulting liquid phase contains 3. 8% by weight of C02 and must therefore be subjected to stripping in the column D to remove it before being sent for subsequent purification with ammonia.

A comparison of the two schemes of figure 1, which illustrates the known art, and figure 2, which on the other hand, illustrates an embodiment of the invention, shows that the investments necessary are much lower for the scheme according to the invention, due to the fewer number of apparatuses operating at high pressures and re-

quiring valuable materials ; the energy consumption and relative costs are greatly reduced as a result of the elimination of the consumption for the stripping of C02 and NH3 in the stripping column D of the scheme of figure 1, which is no longer. necessary, thus avoiding a high consumption of both vapour for the stripping and cooling water for the absorption in column C of the scheme of figure 1.