In the most updated industrial practice, melamine is normally produced through the pyrolysis of urea, accord- ing 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 reac- tion requires a heat supply and is articulated into a complex series of reactions which can be carried out in one or more steps-depending on the technology used- both in liquid phase, at high pressure, without cata- lysts, and in vapour phase, at lower pressures and with heterogeneous catalysts.

The reaction strongly moves towards the right by suitably selecting the operating parameters of the reac- tors, with the substantial exhaustion of the urea intro-

duced into the reactor, which means with an almost total conversion of the urea fed. On the basis of the molecular weights of the chemical species involved in the reaction, for every 126 kg of melamine produced, 234 kg of off-gas is formed, consisting of a mixture of C02 and NH3, with a gas/melamine weight ratio equal to 1.85. The technical problem which arises is therefore the separation of the melamine within specification as well as the recovery and re-use of the considerable amount of off-gas which de- rives from the main reaction. The efficiency and economic treatment of the effluents of the reaction step is cru- cial for the commercial success of the whole process, and in these steps there are the major differences between the various competing processes. In normal industrial practice, the production plants of melamine from urea are coupled with the production plants of urea, so that the off-gas can be returned and re-used for the production of urea.

In the description of the present invention, refer- ence is made to the high pressure reaction technology, which operates in liquid phase by feeding a reactor with molten urea in the presence of an excess of ammonia and without the use of catalysts, supplying the necessary heat and controlling the reaction through thermal ex- change surfaces situated in the reactor. 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, with the separation and re- covery of the melamine still contained therein in vapour phase. For example, the off-gas can be absorbed in water, forming ammonium carbamate or carbonate, they can be con- densed and fractionated to separate the ammonia from the carbon dioxide, or they can be used for the production of ammonium nitrate or sulfate, which can be used as fertil- izers.

These methods on the whole have considerable draw- backs due to both the high investments and high energy costs. It should also be noted that their re-use through recycling in water solution to the urea plant, decreases the yield from the urea synthesis, lowering the energy efficiency.

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.

Recovery systems with molten urea are described, 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 effluent, at the same synthesis pressure and at temperatures normally around 180°C. The selection of the temperature represents a com- promise between the necessity of preventing the condensa- tion of the ammonium carbamate and limiting the degrada- tion of the urea to undesired products, such as biuret, triuret or cyanuric acid. Both phenomena cause various drawbacks in the operating units downstream.

In 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, however, there is also the parallel phenomenon of a significant dissolution of the same off-

gas in the molten urea. Data available in the state of the art, show, for example, that the molten urea coming from the separation apparatus of melamine, contains around 5% by weight of dispersed melamine, and about 20% by weight of off-gas in solution, thus causing an unde- sired recycling of the off-gas which should instead be separated and sent for subsequent 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 system requires ma- chinery 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 valuable materials, such as alloys called INCONEL and HASTELLOY, 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 those 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 different gas/liquid ratio in the reactor. Larger thermal exchange surfaces are therefore necessary for the same net production of melamine.

There are also considerable drawbacks with respect to the reliability of the whole system, in which the py- rolysis section integrates with the washing section with molten urea, which constitutes the pyrolysis feed. A mal- functioning in the washing section-which operates under extremely delicate conditions-causes the immediate stoppage of the primary pyrolysis section and the subse- quent interruption in production. Furthermore, stoppages in the primary section of the pyrolysis, whether pro- grammed or accidental, require the emptying of the reac- tor, normally by sublimating the melamine through an in- jection 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 wash- ing cannot be disposed of by feeding it to the reactor which is not operating: it is therefore necessary to in- stall an additional section which can receive the subli- mation products and which is activated in such occur-

rences.

An objective of the present invention is to set up a system for the treatment of off-gas, which recovers therefrom the melamine content in vapour phase and makes them available for subsequent treatment and re-use, avoiding the drawbacks of the known art.

This objective, according to the present invention, is achieved by means of the process according to the most general definition of claim 1, and by the preferred em- bodiments and possible variations defined in the depend- ent 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 them 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 scheme of figure 1.

The pyrolysis reaction takes place in 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 and overheated, fed through line 2. The pyrolysis reac- tion for the transformation of urea to melamine takes place in reactor A, with an upward flow, providing the

heat necessary for sustaining the reaction through heat exchange surfaces, with known techniques, for example by heat supply with a hot fluid, such as molten salts, or through heating with electric resistances.

A mixed phase is then formed in the pyrolysis reac- tor A, which rises up through the reactor and at its up- per outlet substantially consists of the liquid melamine formed, the off-gas produced by the reaction and the ex- cess ammonia injected at the bottom through line 2 ; this corresponds to the known techniques for directing and controlling the reaction towards the maximum production of melamine, at the same time limiting the production of possible by-products.

Line 3 discharges the mixed phase, from the upper part of the reactor A, which rises through the reactor A, and sends it to the separator vessel B, which operates at the same pressure as reactor A. The mixed phase coming from the reactor de-mixes in this separator into two phases: a gas phase containing the off-gas produced in the reaction, and the excess ammonia, in which there is a significant content of melamine in vapour phase. Thanks to the fact that there is no recycling to the reaction of the off-gas, as happens, on the contrary, in the process of the known technique discussed above, the flow-rate of the gas phase is lower and has a melamine content of

around 6% by weight, whereas the remaining 94% consists of ammonia and carbon dioxide.

The separated liquid phase, mainly consisting of raw melamine, remains in the lower part of the separator B, is discharged from the bottom through level control, by line 4, and is sent for subsequent treatment, known from the art, until the product is obtained according to specification. As a variant of the process, it is also possible to effect the stripping of the residual COs, still dissolved in the raw melamine, in the separator B, in order to enhance the characteristics of the product, by reducing the formation of hydroxylated by-products. In the known technique, for example, this stripping is per- formed by injecting gaseous ammonia to the bottom of the separator.

According to an alternative embodiment, separator B can be designed as an upper volume of the reactor A it- self.

An important feature of the present invention is the treatment of the gas stream separated in the separator B.

The gas stream, discharged from the upper part through line 5, is first expanded by lamination in valve 6, which maintains the operating pressure in the reaction system comprising the reactor A and separator B, and is sent to the mixer C. This mixer receives the stream of the gas

flow laminated in valve 6, to which a coolant is added and intimately mixed-a cold gas or a low-boiling liquid which evaporates instantaneously-which rapidly cools the mixture thus formed to a temperature below the de- sublimation temperature of the melamine carried by the off-gas. According to an illustrative embodiment of the invention, the cooling fluid consists of ammonia and, preferentially, a stream of liquid ammonia coming from line 7 and which is injected, finely dispersed, into the off-gas stream, at the beginning of its axial flow in the mixer.

In the mixer C, preferably tubular-shaped, there is therefore an axial flow wherein both the previous lamina- tion of the gaseous stream and the effect of the cooling fluid, such as the rapid evaporation of the liquid ammo- nia introduced through line 7, which is finely dispersed by means of suitable nozzles, cause a sudden lowering of the temperature, from the reaction value (380-450°C) to a value lower than 350°C, preferably between 200 and 250°C, at which temperature the melamine immediately passes from the vapour state to the solid state, finely dispersed in the high rate gaseous stream, during its flow in the mixer. Crusts and deposits of the equipment are avoided by maintaining a suitable rate; the gas/solid fluid phase is sent through line 8 to a separator of the solid por-

tion, for example the cyclone D, in which a considerable centrifugal effect is obtained due to the tangential flow rate which allows the substantial separation of the mela- mine in powder, which settles on its conical bottom and the outflow of the purified gaseous phase from the upper axial duct through line 9. The cyclone D is equipped with extraction devices E1/E2 of the melamine powder which has settled on the bottom, for example with rotating septa valves, fluid sealed, known in the art, normally called rotocells.

The separation is perfected by feeding the off-gas for a second separation in a filter, for example a sleeve filter F, in which the finest fraction of the melamine powder, which has not been retained by the cyclone D, is collected. The sleeve filter is also equipped with dis- charge, fluid-sealed apparatuses analogous to those indi- cated with E1/E2, with which the additionally recovered melamine is extracted. The off-gas purified from the melamine, is sent downstream, through valve 10 and line 11, for subsequent treatment and re-use.

The product is sent from the discharge apparatuses of the de-sublimated melamine powder, obtained from the cyclone separator D and from the sleeve filter F, to an apparatus G, for example a thermostat Archimedes'screw, where a further cooling and the last separation from the

gas are effected.

The use of liquid ammonia as a sudden cooling method represents a preferential embodiment of the invention, especially if it is used when the off-gas is recycled to the synthesis of urea. Equivalent fluids can also be used, which allow an analogous quenching effect, espe- cially if the off-gas is re-used in other ways; for exam- ple, the cooling medium can be cold or liquid nitrogen when the off-gas is sent to an ammonia recovery plant.

With respect to the pressure of the separation sys- tem consisting of the mixer C and separators D and F of the melamine solidified from the off-gas, said pressure is not decisive for the purposes of the separation, and can be selected on the basis of the treatment system and re-use of the off-gas. A typical example is when the off- gas is recycled to the urea production plant, condensing them in its carbamate recovery section: in this case, a pressure of 25-30 bar can be appropriate. Another illus- trative case can be when the pyrolysis reactor of mela- mine operates at 150 bar, in this case the off-gas could be separated at this pressure and sent directly to the most suitable section of a synthesis step of the urea plant, with an energy content of a higher value.

The outflow temperature of the fluid from the sepa- rator C is however substantially lower than that of the

de-sublimation temperature of the melamine, preferably maintained at 200 to 250°C. It should be noted that it is advisable to remain above, and with a certain margin, the temperature at which the condensation of carbamate takes place, in order to remove all the off-gas without the formation of carbamate. As far as the flow-rate of the off-gas treated is concerned, the flow-rate of ammonia to be sent to the mixer C, proves to be within the range of 0.1-0. 2 kg of liquid NH3 for each kg of off-gas.

The off-gas treatment process, with the separation of melamine by de-sublimation and the full availability of the anhydrous off-gas, has considerable advantages with respect to the treatment of the known art. Among these, the following characteristics are worth mention- ing.

The plant costs are reduced, as there is no need for complex equipment and valuable materials, apart from inox steels of standard use. There is no recycling to the melamine and off-gas synthesis section, which means a re- duction in the flow-rates, reaction volumes and thermal exchange surfaces. Encrusting phenomena and clogging of the equipment are reduced.

The melamine separated from the off-gas by de- sublimation and obtained by separation in the separators D and F, requires cooling and degassing only in the appa-

ratus G ; it represents an amount of 15-20% of the total melamine, has a high purity and is of the highest qual- ity, without further treatment.

The de-sublimation process of melamine can be car- ried out within a wide range of pressure and temperature values, in order to obtain the most economical condi- tions, the highest recycling efficiency and re-use of the off-gas. In the commonest case, in which the melamine production plant is coupled with an urea plant, the use of ammonia as cooling fluid is particularly suitable, as it is a raw material for the urea plant and there are no significant restrictions for the ratio between the cool- ing ammonia and off-gas treated.