One of the known catalysts can be used as the catalyst, for example aluminium oxide, silica-alumina, silicon oxide, titanium oxide, zirconium oxide, boron phosphate or a mixture of two or more of these catalysts. The expression catalysts'is here understood to be any material that promotes the conversion of urea into melamine under the employed reaction conditions.

In addition to 1-3 vol. % melamine, the melamine-containing gas mixture that is discharged in the process contains, for example, 3-10 vol. % C02 and 87-96 vol. % NH3. Usually the melamine is separated from such a hot melamine-containing gas mixture in a cooling zone, in which the melamine is released by bringing the gas mixture into direct contact with a circulating amount of cooling liquid, which is known as quenching.

The process is then preferably carried out with the aid of two gas scrubbers placed in series, in which the hot gas mixture is brought into contact with the circulating amount of cooling agent.

Such a process is for example described in

NL-B-126,892, in which the hot melamine-containing gas mixture is cooled in two steps. A first step in which the hot gas mixture is in a first cooling zone passed in co-current contact with the cooling agent flowing along the inside of the cooling zone, causing a large portion of the melamine to be separated, after which the separation is completed by passing the now precooled gas in counter-current contact with the cooling agent in a second cooling zone. In this way almost complete collection of the melamine is realised. The gas discharged from the second cooling zone contains less than 0.05% of the amount of melamine originally present.

NL-B-126892 mentions as an advantage that the hot gas mixture is cooled less quickly as a result of this cooling in two steps. This less rapid cooling results in the separation of coarser melamine particles.

It has now been found that a disadvantage of this process on a commercial scale is that the slow cooling leads to a large amount of by-products in the cooling zone and scaling of the walls in the cooling zone. The consequence of this is that the cooling zone must be regularly cleaned, for which the plant must be temporarily shut down.

It has now surprisingly been found that it is possible to minimise the amount of by-products formed during the quenching on a commercial scale by designing the cooling zone so as to ensure intensive contact between the cooling liquid and the reactor gas, to effect rapid cooling.

This can be realised by choosing a combination of a high gas impulse (= gas density times gas velocity squared) based on the cross-section of the column in the cooling zone, a high specific area of the

cooling medium (gas-liquid interface) and a short residence time of the gas in the cooling zone to arrive below 210°C, preferably below 170°C.

It has been found that, in separating melamine from hot melamine-containing gas mixtures by cooling the gas mixture in a cooling zone through direct contact with an evaporating medium, this evaporating medium is sprayed so that the specific area is greater than 600 m2/m3 of liquid, preferably between 800 and 8000 m2/m3 of liquid, and the gas has in at least a part of the cooling zone an impulse (= rhogas* (Vgas) 2) of more than 0.2 kg/ (m. s2) and the residence time of the gas in the cooling zone, in which the gas is cooled to a temperature below 210°C, is less than 11 sec., preferably less than 8 sec. Preferably the gas impulse is greater than 0.5 kg/(m. s2), in particular greater than 1.0 kg/ (m. S2). The cooling liquid is preferably sprayed via a nozzle, the rate at which this liquid leaves the nozzle being greater than 2 m/s. The cooling liquid preferably contains at least one of the components water, ammonia or ammonium carbamate and can be sprayed with a one-or two-phase sprayer.

The drop diameter is for practical reasons chosen to be not too small because a high energy consumption and a large number of sprays are required to obtain small drop diameters. It moreover becomes increasingly difficult to separate the droplets from the gas phase in the event of too small a drop diameter.

The ratio in the cooling zone of the mass flow rates of the cooling medium and the gas from the reactor is between 1 and 10, preferably between 1 and

5.

A portion of the cooling medium evaporates in the cooling zone. This portion amounts to less than 60%.

A portion of the reactor gas will dissolve in the cooling medium. The amount that absorbs, excluding melamine, will usually be less than 10 wt. %.

The quenching section can be designed so that a slurry of melamine crystals is obtained or that a solution without crystals is formed.

The cooling time of the gas can be calculated according to the following formula: Cooling time = (Q"melamine + dTgas* rhogas* Cp, gas)/ (a*h* (Tgas-Tcooling)) (in sec.) where: Q"melamine = desublimation energy of melamine from the gas phase per m3 of gas (j/M3 gas) dTgas = cooling of the gas (°C) rhogas = density of the gas (kg/m3) Cp, gas = specific heat of the gas (J/ (kg. C) a = contact area between the cooling medium and the gas per gas h = heat transfer coefficient between the cooling medium and the gas (W/ (m. °C) Tgas-Tcooling = temperature difference between the gas and the cooling medium (°C).

The above equation must be mathematically integrated over the cooling zone to calculate the time required for cooling from the ingoing gas temperature to 210°C and 170°C, respectively. In the above equation 1000 kJ per kg of melamine must be used for the desublimation energy of melamine.

The contact area a between the droplets and the cooling medium per m3 of gas is determined by the average (Sauter) drop diameter and the drop concentration in the gas phase and can be approximately calculated using the following formula: a= 6*pcooling/d*pgas where: d = average (Sauter) drop diameter in m (pcooling = flow rate of the cooling liquid in m3/hour (pgas = average volumetric flow rate of the gas in the cooling zone in m3/hour.

The average (Sauter) diameter of the sprayer is determined under atmospheric conditions using water as the liquid at the same volumetric flow rate as under the process conditions.

An advantage of this process is that a smaller amount of by-products is formed during the quenching.

The invention will now be elucidated with reference to the following examples, but is not limited thereto.

Example I The melamine is produced in a cylindrical fluidised bed with an internal diameter of 1 metre and a height of 15 m at a pressure of 0.78 MPa and a temperature of 390°C. The urea is dosed at a rate of 1200 kg/hour, with 700 kg/hour ammonia via two-phase sprayers. The flow rate of the ammonia supplied via the fluidisation plate is 1000 kg/hour. The gas stream leaving the reactor is quenched with 8000 kg/hour carbamate solution at 0.75 MPa. The carbamate solution contains 12 wt. % NH3 and 4 wt. % CO2, the rest being water, and has a temperature of 80°C. The quenching liquid is sprayed, the Sauter diameter of the droplets being 3.6 mm. 1 ton/hour water is caused to flow along the walls of the column to prevent the risk of scaling of the wall. The quenching column has an internal diameter of 0.5 metre and a height of 6 metres. The impulse of the gas in the cooling zone is more than 4 kg/(m. s2).(m. s2). The time required for the gas to cool to 210°C is less than 3 sec. and the cooling time to 170°C is less than 4 sec. The amounts of ammelide (Ade) and cyanuric acid (CA) relative to melamine after the quenching are 0.24 kg of Ade and 0.11 kg of CA per ton of melamine, respectively.

Example II The melamine is produced in the same set-up as described in Example I. The reactor pressure is 1.8 MPa. The urea is dosed at a rate of 1200 kg/hour, with 700 kg/hour ammonia via the two-phase sprayers. The flow rate of the ammonia supplied via the fluidisation plate is 1300 kg/hour. The gas stream leaving the

reactor is quenched with 8000 kg/hour carbamate solution (having the same composition as in Example I) at 1.76 MPa. The quenching column and other quenching conditions are the same as those in Example I, except for the average drop diameter. The average drop diameter is 1.4 mm. The impulse of the gas in the cooling zone is more than 2 kg/ (m. s2). The amount of time required for the gas to cool to 210°C is less than 2 s and the cooling time to 170°C is less than 3 s. The amounts of ammelide (Ade) and cyanuric acid (CA) relative to melamine after the quenching are 0.21 kg of Ade and 0.09 kg of CA per ton of melamine, respectively.

Example III The melamine is produced in the same set-up as described in Example I. The reactor pressure is 1.8 MPa. The urea is dosed at a rate of 600 kg/hour, with 600 kg/hour ammonia via the two-phase sprayers. The flow rate of the ammonia supplied via the fluidisation plate is 600 kg/hour. The gas stream leaving the reactor is quenched with 2000 kg/hour carbamate solution (having the same composition as in Example 1) at 1.76 MPa. The quenching column and other quenching conditions are the same as those in Example 1, except for the average drop diameter. The average drop diameter is 2.1 mm. The impulse of the gas in the cooling zone is more than 0.6 kg/ (m. s2). The time required for the gas to cool to 210°C is less than 5 s and the cooling time to 170°C is less than 8 s. The amounts of ammelide (Ade) and cyanuric acid (CA) relative to melamine after the quenching are 0.28 kg of

Ade and 0.13 kg of CA per ton of melamine, respectively.

Comparative Example A The melamine is produced under the same conditions as in Example I. The urea is dosed at a rate of 1200 kg/hour, with 700 kg/hour ammonia via the two- phase sprayers. The flow rate of the ammonia supplied via the fluidisation plate is 1000 kg/hour. 1000 kg/hour water is caused to flow along the walls of the column to prevent the risk of scaling of the wall. In the column are 3 vertical plates set at intervals of 0.1 m, along which flows 8000 kg/hour carbamate solution. The carbamate solution has the same composition as in Example I. The amounts of ammelide (Ade) and cyanuric acid (CA) relative to melamine after the quenching are 0.41 kg of Ade and 0.18 kg of CA per ton of melamine, respectively. Scaling was observed on the plates after 5 days'operation.