PROCESS FOR THE PREPARATION OF MELAMINE

The present invention relates to a process for the production of melamine starting from methane and ammonia as educts. It is well known that melamine can be prepared from urea at elevated temperature according to the following reaction equation:

6H ₂ N-CO-NH ₂ → C ₃ N ₃ (NHz) ₃ + 6NH ₃ + 3CO ₂

This reaction is strongly endothermic.

There are two basic types of industrial melamine production processes using urea as the raw material, namely, catalytic low-pressure processes and high-pressure processes in which no catalyst is used. In the former, the reactor pressure is approx. 1 MPa or lower, in the latter reactor pressure is usually higher than 8 MPa (Ullmann's Encyclopedia of Industrial Chemistry, 5th edition. Vol. A 16, p. 174-179).

It is a drawback of the so far commercially used urea route for production of melamine that considerably high amounts of carbon dioxide are produced and have to be reused to make the process economically feasible. One way to overcome this problem is to recycle the ammonia and carbon dioxide evolved in the production of melamine into a urea plant (Ulmann's Encyclopaedia of Industrial Chemisty, 5 ^th edition, volume A16).

But there is still the need to have an economically feasible and more flexible process for making melamine where possible by-products obtained in the reaction to make melamine can be used in different ways depending on the commercial needs.

An alternative route for the preparation of melamine uses hydrogen cyanide and ammonia as starting material. Compared to the processes employing urea as starting material a totally different chemistry is involved. Such a process is known for example from GB-A 803,195, where melamine is produced from a vapor mixture of hydrogen cyanide and ammonia, at substantially atmospheric pressure in the presence of a dehydrogenation catalyst which comprises an oxide of chromium and/or a metal of Groups 8-10 of the Periodic

Table of the Elements. Examples given of the Group 8-10 metals are platinum, palladium, nickel and cobalt. A disadvantage of a process such as described in GB-803.195 is the low yield that can maximally be obtained.

When reacting hydrogen cyanide with ammonia besides melamine hydrogen is produced as a by-product:

3 HCN + 3 NH ₃ => C ₃ N ₃ (NHz) ₃ + 3 H ₂

Hydrogen can be easily separated from the melamine product and is a valuable compound that may be used either for energy production or in all kinds of different hydrogenation reactions. But the drawback of the hydrogen cyanide route is that the educt hydrogen cyanide is less readily available and due to its toxicity difficult to handle.

For the production of hydrogen cyanide in principle three industrial productions methods are available, as summarized in Ullmann's Encyclopaedia of Industrial Chemistry (Online version December 12 ^th , 2008) Cyano Compounds, Inorganic; 1. Hydrogen Cyanide, DOI:

10.1002/14356007.a08_159.pub2, Article Online Posting Date: January 15, 2004).

According to the Andrussow process methane is reacted with ammonia and an oxygen-containing gas, preferably compressed air to produce hydrogen cyanide. The reaction is strongly endotherm and is conducted at temperatures above 1000 ⁰ C at atmospheric pressure with a relatively high gas velocity through the catalyst and quenching of the effluent gas in a heat boiler to avoid decomposition of hydrogen cyanide. It is a disadvantage of the process that a high quality of the methane starting material is essential and the yield based on methane and ammonia is low. Furthermore, the low hydrogen cyanide concentration of the product gas requires large gas volumes to be handled which is expensive in terms of investment and operation costs. The low yield may be attributed to the fact that due to the present oxygen in the process large amounts of compounds that cannot be reused are formed like molecular nitrogen, water and carbon dioxide. Furthermore, the reaction mixture contains considerable amounts of ammonia that have to be removed in order to prevent polymerization of hydrogen cyanide. This is done by washing with aqueous sulphuric acid producing ammonium sulfate which is to be considered as a waste product.

An alternative route is the Degussa BMA process wherein methane is reacted with ammonia at temperatures above 1 ,200 ⁰ C in externally heated alumina tube bundles coated with a special platinum catalyst. To avoid deposition of carbon black on the platinum catalyst the reaction is run with a slight stoichiometric excess of ammonia. The advantage of the Degussa BMA process compared to the Andrussow process is that the yield of hydrogen cyanide based on methane and ammonia is considerably higher, especially no oxygen-containing compounds are produced and the concentration of hydrogen cyanide in the educt gas stream is considerably higher, thus lower gas volumes have to be processed. As a dominant side product molecular hydrogen is produced that can be effectively reused as energy source or in hydrogenation processes. But due to the required excess of ammonia, the educt gas contains considerable amounts of ammonia that have to be removed in order to avoid polymerization of hydrogen cyanide. Similar methods as described above with respect to the Andrussow process may be used with the same drawbacks.

A further advantage of the BMA Degussa process is that natural or refined gas with a content of 50 to 100 vol% methane can be used so that the process is more flexible with respect to the hydrogen carbon educt.

But similar to the Andrussow process separation of hydrogen cyanide and purification of hydrogen cyanide is complex requiring different process units, like ammonia absorber, hydrogen cyanide absorber, hydrogen cyanide stripper and hydrogen cyanide fractionator which is expensive in terms of investment and operation costs.

A third industrial process for making hydrogen cyanide is the Shawinigan process wherein ammonia is reacted with a hydrocarbon by passing the gas through a fluidized bed of coke heated by electrodes immersed in the bed. No catalyst is required, but the temperatures are kept above 1 ,500°C. The advantage of that process is that the reactor effluent gas contains very low amounts of ammonia, but the process is only attractive for commercial reasons where low-cost electricity is available. According to all processes for production of hydrogen cyanide the product is obtained in liquid form.

Thus, due to the deficiency of the processes for making hydrogen cyanide, the alternative route for making melamine from hydrogen cyanide which

is well known since the 1950s (GB-A-803195) was not able to compete with the production of melamine from urea on an industrial level.

Thus, the object of the present invention is to provide an alternative process to the present industrial processes for making melamine that is economically feasible and overcomes the problems discussed above. It is particularly an object of the present invention to provide a process for production of melamine using readily available and inexpensive starting materials wherein possible by-products of the process besides the melamine product can be economically and flexibly used that is economically attractive in terms of investment and operation costs. Especially the energy consumption should be minimized.

SUMMARY OF THE INVENTION

These objects have been attained by a process for the preparation of melamine, which process comprises at least the following steps: i) feeding methane and ammonia to a pre- reactor which pre- reactor is maintained at a temperature between 700 and 1500 °C whereby a gaseous mixture (i) is formed comprising at least hydrogen cyanide, ii) optionally, purifying mixture (i) thereby forming a mixture (ii) that contains at least gaseous hydrogen cyanide, a. feeding hydrogen cyanide from mixture (i) or (ii) and ammonia, either separately or combined, to a reactor that contains a catalyst and which reactor is maintained at a temperature of 300-500 °C, at a space velocity lying between 10 ² and 10 ⁶ ml/(g.hr), whereby a gaseous mixture (a) is formed comprising at least melamine, b. optionally, purifying mixture (a) producing a melamine containing mixture (b), c. separating at least a part of the formed melamine thereby forming a mixture (c) and separated melamine. The inventors have surprisingly found that in a two-step process in a pre-reactor a readily available starting material methane is reacted with ammonia to produce a gaseous effluent comprising hydrogen cyanide. The gaseous effluent from the pre-reactor may be optionally purified, preferably to remove hydrogen from the reaction mixture, as will be discussed later. The

resultant gaseous hydrogen cyanide-containing mixture together with ammonia is fed to a reactor and reacted to produce melamine and hydrogen.

According to a preferred embodiment of the present invention, the hydrogen produced in step (a) in the melamine reactor is at least partially removed either by chemical conversion or by physical separation methods, as will be discussed later. Since in the melamine reactor in step (a) hydrogen may be anyway removed, the process is very flexible with respect to the hydrogen content of the gaseous effluent leaving the hydrogen cyanide pre-reactor. Thus, when looking at the overall process, hydrogen might be removed from the reaction mixture either after step (i) from the effluent leaving the hydrogen cyanide pre- reactor or in step (a) in the melamine reactor or at both locations. Besides the optional purification of the gaseous effluent from the hydrogen cyanide pre- reactor, particularly to limit the amount of hydrogen, no further purification or treatment of the effluent from the hydrogen cyanide reactor is desired or necessary. Especially the gaseous effluent does not have to be liquefied since it can be directly fed into the melamine reactor of the process of the present invention. Thus, no further purification besides the at least partial removal of hydrogen is performed on the gaseous effluent from the hydrogen cyanide pre- reactor prior to entry into the melamine reactor. Thereby, a highly economical process in terms of investment and energy costs using a readily available starting material for the production of melamine is provided whereby hydrogen is produced as by-product which is a valuable chemical that can be flexibly used either for energy production or in all kinds of different hydrogenation processes. This is a considerable advantage compared to the urea route so far used in industrial practice.

DETAILED DESCRIPTION OF THE INVENTION

The temperature in the hydrogen cyanide pre-reactor is generally between 700 and 1500 ⁰ C. A preferred range for the temperature is 900-1400 ⁰ C. The pressure in the pre-reactor is generally kept at moderate pressures. A suitable pressure range is 0.1-0.5 MPa, preferably 0.1-0.3 MPa. Here and hereinafter the pressure is used as the absolute pressure. Generally a catalyst will be present. Suitable catalysts for this process step are known to the man skilled in

the art. An example is a platinum based catalyst. Methane and ammonia in the feed are converted to hydrogen cyanide and one or more byproducts. The gaseous mixture (i) that is formed in step i) of the process comprises at least hydrogen cyanide and possibly in addition to hydrogen cyanide hydrogen gas, nitrogen, unreacted methane and/ or ammonia.

In the process according to the invention the gaseous mixture (i) is optionally purified so as to make it possible to use part of the constituents of the mixture in subsequent steps or to isolate the constituents for further use. Preferably hydrogen is at least partially removed from the reaction mixture (i). In one embodiment after purifying mixture (i) a gaseous mixture (ii) is formed that contains at least hydrogen cyanide and ammonia. Mixture (ii) is enriched in hydrogen cyanide compared to mixture (i). The hydrogen cyanide from mixture (ii) is now fed, either separately or combined with additional ammonia to the melamine reactor. It has been found advantageous to add a cooling step after step

(i), whereby reaction mixture (i) is cooled down to a temperature between 300- 500 ⁰ C, preferably to a temperature between 350 and 450 ⁰ C. The advantage of this additional step is that it becomes possible to separate hydrogen that is produced in the hydrogen cyanide reaction, from the other constituents in the mixture. Preferably, hydrogen is removed by using a selective membrane technology. Lower temperatures as obtained after the above described cooling step allow a broader range of suitable membranes and increase the effective operating live time of the membrane. The separated hydrogen can now be used in all kinds of applications, such as for example in other synthesis routes or as fuel gas for a furnace. By separating the hydrogen and using it in other processes the overall costs for the production of melamine are reduced. Additionally it has been found that by removal of hydrogen the yield to melamine is increased.

It has been found advantageous to include in the process a compression step as an additional step after step (i). whereby reaction mixture (i) is compressed to a pressure between 0.5 and 5 MPa. This step can be used separately from or combined with the additional step of cooling down reaction mixture (i).

The catalyst that is present in the melamine reactor, in step (a), can be any catalyst that is able to catalyze the reaction between ammonia and hydrogen cyanide.

The melamine reactor in step (a) of the process of the present invention can be operated at atmospheric or higher pressure. Preferably, the pressure in the reactor is at least 0.2 MPa or higher. It is preferred to use a pressure in the reactor of at least 0.5 MPa, more preferably the pressure is at least 1 MPa. It has been found that at high pressures the yield of melamine is much higher than at low pressures, such as for example atmospheric pressure. The pressure in the reactor is preferably kept below 20 MPa, more preferably below 10 MPa. At very high pressures the construction of the reactor and other apparatus tends to be very heavy, increasing the investments costs considerably. Therefore it is preferred to use lower pressures. A suitable range for the pressure in the reactor is 0.2-20 MPa. A preferred pressure in the reactor lies within the range of 0.5-10 MPa, more preferably 0.5-5 MPa, most preferred 1-5 MPa.

The temperature in the reactor is maintained between 300 and 500 ⁰ C. Preferably the temperature is kept between 350 and 450 ⁰ C (borders included), because it has surprisingly been found that within this range the yield and selectivity to melamine are most favorable. The reaction is carried out at a space velocity lying between 10 ² and 10 ⁶ millilitre of reaction mixture per gram of catalyst per hour (ml/(g.h)) (borders included). It was found that the occurrence of undesired side -reactions was minimised at higher space velocities. The space velocity is preferably at least 3.10 ² ml/(g.h), more preferably at least 1.1O ³ ml/(g.h). It was found that the melamine yield was maximised when the space velocity is at most 10 ⁶ ml/(g.h). Preferably the space velocity is below 3.10 ⁵ ml/(g.h), even more preferably the space velocity is below 10 ⁵ ml/(g.h).

The catalyst that is present in the melamine reactor in step (a) can be any catalyst that is able to catalyse the reaction between ammonia and hydrogen cyanide. A catalyst that is preferably used in step (a) of the process of the present invention comprises at least one group 11 element.

With "group 11 element" is meant an element from group 11 of the Periodic Table of the Elements. A current Internet reference for the IUPAC Periodic Table of Elements is www.iupac.org/reports/periodic table; the version

as used here and hereinafter is dated June 22, 2007. Within the context of the present invention, the term "group 11 element" is understood to mean the element itself, a compound comprising the element or a mixture of the element and/or compound comprising the element. The catalyst will generally comprise a support material and at least one catalytic active element. The support material is used to increase the catalyst surface area and also for reasons of ease of handling and sturdiness. It is preferred to use the group 11 element supported on at least one heat resistant inorganic material including carbon containing supports, more preferably the support is made from a heat resistant inorganic material with a high surface area.

Within the context of the present invention, the term 'support' is understood to mean one heat resistant inorganic material or a mixture of two or more heat resistant inorganic material. Examples of suitable heat-resistant inorganic materials are alumina, silicon carbide or other carbon-containing supports, silica, silica-alumina, titanium oxide, zirconium oxide, chromium oxide, silica magnesia, magnesium oxide, diatomaceous earth, zeolite, pumice, zirconium oxide, cerium oxide, calcium sulphate, titanium phosphate, silicon phosphate, activated carbon and mixtures of any of them. A preferred support material is chosen from the group comprising silica, alumina, silica-alumina, titanium oxide, zirconium oxide, chromium oxide, magnesium oxide, activated carbon and mixtures of any of them.

When one group 11 element is present in the catalyst, the amount of it generally lies within the range 0.5-50 wt%. Preferably the amount of group 11 element lies within the range 1-35 wt%, more preferably 1-25 wt%. The preferred group 11 element is copper. Preferably copper is present in an amount of between 1 and 25 wt%.

When more than one group 11 element is present in the catalyst, the different group 11 elements can be present on or in the same support or on or in different supports. The amount of these active components ( i.e. components showing catalytic activity, to the total weight of the catalyst) varies, depending amongst others upon the support used, method of catalyst preparation and atom ratio of the active components, but is generally at least 0.1 and generally at most 99 wt%. More preferably the amount of active components is at least 1 wt%. Preferably the amount of active components is at most 95 wt%, more preferably at

most 90 wt%, even more preferably at most 80 wt% and most preferably at most 70 wt%.

Another preferred catalyst for use in the process according to the invention comprises in addition to the at least one group 11 element, at least one other element from the transition metals, the alkali metals, the alkaline earth metals and/or the lanthanide group. The transition metals here referred to are considered to be the elements in the groups 3-12 of the Periodic Table of the Elements. The alkali metals are considered to be the elements in group 1 of the Periodic Table of the Elements. The alkaline earth metals are considered to be the elements in group 2 of the Periodic Table of the Elements. The elements from the lanthanide group are considered to be the elements with atomic number 57 (Lanthanide) up to and including 71 (Lutetium).

If in addition to the group 11 element(s), at least one other of transition metal, alkali metal, alkaline earth metal or element from the lanthanide group is present in the catalyst, they can be present on or in the same support or on or in different supports. The amount of these active components ( i.e. components showing catalytic activity, to the total weight of the catalyst) varies, depending amongst others upon the support used, method of catalyst preparation and atom ratio of the active components, but is generally at least 0.1 and generally at most 99 wt%. Preferably the amount of these active components is at least 1 wt%, Preferably the amount of these active components is at most 95 wt%, more preferably at most 90 wt%, even more preferably at most 80 wt% and most preferably at most 70 wt%.

Examples of suitable transition metals are titanium, vanadium, chromium, molybdenum, manganese, ruthenium, cobalt, nickel, palladium and platinum. Preferred transition metals are vanadium, ruthenium, cobalt, nickel, palladium, most preferred transition metals are vanadium, cobalt. Examples of suitable elements from the alkali metals are lithium, potassium, rubidium and cesium, preferred elements are lithium and cesium, most preferred is lithium. A suitable element from the alkaline earth metals is calcium. Examples of suitable elements from the lanthanide group are lanthanum, cerium, praseodymium, ytterbium and lutetium. Preferred elements are cerium and praseodymium, most preferred is praseodymium.

A specially preferred catalyst comprises in addition to copper another element from group 11 , a transition metal and/or an alkali metal and/or an element from the lanthanide group. A suitable catalyst was found to be a catalyst that comprises in addition to the support: copper, vanadium, silver, praseodymium, lithium and cobalt.

The catalyst for use in the process according to the invention preferably contains the group 11 element in an amount higher than the individual amounts of the other components showing catalytic activity. The at least one group 11 element is thus preferably the major (by weight) component showing catalytic activity.

The catalyst to be used in the process according to the invention may be prepared by methods known as such to the skilled person. Reference can for example be made to the "Encyclopedia of Catalysis", John Wiley & Sons, ed. Istvan Horvath, 2007. It has been found advantageous to pre-treat the catalyst before the reactants are fed to the catalyst. A suitable method is to pre-treat the catalyst in a flow containing hydrogen at an elevated temperature during an extended period of time.

The catalyst as used in the process according to the invention can have various shapes, such as for example small particles, granules, wires or gauzes. If the catalyst comprises or consists essentially of particles- either as such or in agglomerated or sintered form- it is preferred that the said particles are in size between 100 nm and 5 mm. The term size is defined herein as the average value of the largest and smallest dimension of a particle. Since the catalyst is essentially in the solid phase and the reaction mixture is essentially in the gaseous or supercritical phase, it follows that the catalyzed reaction step in the process according to the invention falls into the category of heterogeneous catalytic reactions.

When performing the process of the present invention it has been surprisingly discovered that especially in runs where the melamine yield was particularly high in the effluent from reaction step (a) in the melamine reactor contained an increased amount of methane. Without wanting to be bound to a theory, this may be attributed to a side reaction wherein the hydrogen formed in the melamine reactor reacts with unreacted hydrogen cyanide to form methane

and ammonia. The correlation between the increased amount of methane and the increased yield in the reactor may be attributed to the fact that by the side reaction hydrogen is removed from the system and, thus, the equilibrium of the reaction is shifted towards melamine. Thus, according to a preferred embodiment, as already discussed above, hydrogen is removed from the reaction mixture in step (a) in the melamine reactor. This can be done by a chemical conversion as described above, for example by reaction with hydrogen cyanide present in the reactor or by adding other reactants into the melamine reactor in step (a) that do not negatively influence the reaction between hydrogen cyanide and ammonia. Alternatively, hydrogen may be removed from step (a) by means of physical separation, for example membrane technology, as already discussed above with respect to the purification step (ii).

Thus, according to one embodiment, removal of hydrogen in step (a) can be achieved by applying a catalyst that promotes the reaction of hydrogen cyanide with ammonia to melamine and hydrogen as well as the reaction of hydrogen cyanide with hydrogen to form methane an ammonia. Suitable catalysts that have these properties may contain in addition to the group 11 element a group 10 element, especially palladium or platinum. Generally, one would assume that it is detrimental to a process to promote a side reaction wherein one of the educts, in the present case hydrogen cyanide, is consumed. But in the process according to the present invention, the overall balance is positive since, first of all, the equilibrium shifted towards the desired product melamine and the side products obtained by the above-described side reaction are methane and ammonia that may be easily recycled into step (i) in the hydrogen cyanide pre- reactor. Thus, no valuable product is lost and, in addition, the total amount of gas to be recycled into the process is considerably reduced due to the shift of equilibrium into the direction of melamine which is the desired product. This is an additional advantage of the process of the present invention that results in a further improved process efficiency.

Furthermore, the mixture (c) obtained after separation of melamine according to the process of the present invention may be recycled either into the hydrogen cyanide pre-reactor (i) in step (i), which is preferred,

and/or into the melamine reactor in step (a) of the process of the present invention.

It has been found that it is advantageous to perform the process according to the invention in the absence of added oxygen. When no stream of oxygen-containing gas is fed to the hydrogen cyanide pre-reactor or to the melamine reactor the selectivity is better, purification of the products is easier and the process is economically more advantageous as no valuable feedstock is lost in the conversion to unwanted, oxygen-containing, products. With "no stream of oxygen-containing gas is fed to the reactor" is meant that oxygen is not intentionally fed to the reactor, however low amounts of oxygen can, unintentionally, enter the reactor for example as contamination in the feed stream or by tiny leakages in the apparatus. These low amounts of oxygen will not negatively influence the reactions taking place. It is preferred to keep the level of oxygen in the reactor below 2 vol%, more preferably below 1 vol%, most preferably below 0.5 vol%. It is even more preferred to perform the process according to the invention in the absence of oxygen.

The present invention will now be elucidated by the following non- limiting examples.

Figure 1 is a flowchart of one embodiment of the process according to the present invention.

Figure 2 is a flowchart of another embodiment of the process according to the present invention.

Figure 3 is a flowchart of a process wherein the BMA Degussa process is used for making hydrogen cyanide and the thus obtained hydrogen cyanide is reacted with ammonia to produce melamine.

Example 1

Example 1 is described herein with reference to Figure 1. Methane and ammonia are fed via line 1 into the hydrogen cyanide pre-reactor 2

operating at 1 ,300 ⁰ C and 0.2 MPa. The process unit 2 depicted in Figure 1 includes the hydrogen cyanide pre-reactor as well as a separation unit wherein hydrogen is removed from the reactor effluent. A gaseous mixture comprising hydrogen cyanide, ammonia and less than 2 vol% of hydrogen are fed into the melamine reactor 4. Additional ammonia is fed via line 22 into the reactor 4. A gaseous stream of hydrogen is removed from the separation unit via line 19. The melamine reactor 4 is operated at 400 ⁰ C and 0.22 MPa and at a space velocity of 1200 ml/(g.hr) . The reaction in the melamine reactor is performed in presence of a combined catalyst promoting the conversion of hydrogen cyanide into melamine and the conversion of hydrogen. The reaction product is fed via line 5 into a cooler/separator crystallizer unit 6 maintained at a temperature of 200 ⁰ C and 0.2 MPa. Cooling is achieved by contact with a recycle gaseous mixture comprising ammonia and hydrogen that is fed into the cooler/separator crystallizing unit 6 via line 18 at a temperature of 120 ⁰ C. By-products are removed from the cooler/separator crystallizer unit 6 via line 20. A mixture of ammonia, methane and hydrogen gas with melamine crystals is fed via line 7 into a gas/solid separation unit 8 which is operated at 200 ⁰ C and 0.18 MPa. In this gas/solid separation unit 8 the majority of ammonia, methane and hydrogen gas is separated from the melamine crystals and is fed via line 12 to a recycle blower 14. The melamine crystals obtained from the gas/solid separation unit 8 are fed via line 9 to a depressurize cooling unit 10 wherein the melamine crystals are cooled down to 5O ⁰ C and are depressurized to atmospheric pressure. Melamine product crystals are removed via line 11. The separated gas mixture from the depressurize cooling unit 10 is fed via line 13 using a blower to increase the pressure to the recycle blower 14. Then, one gaseous stream comprising ammonia, methane and hydrogen is fed via line 16 as recycle stream to the hydrogen cyanide pre-reactor 2 and the melamine reactor 4. Another part of the recycle gas stream is fed via line 15 to a gas cooler wherein the gaseous stream is cooled to 120 ⁰ C at a pressure of 0.22 MPa. This cooled gas stream is fed via line 18 as cooling medium into the cooler/separator crystallizer unit 6.

Example 2

The process according to example 2 is described with reference to Figure 2. The same numbers refer to the same lines and process units as described above for example 1 with reference to Figure 1. The difference between the process according to example 2 compared to example 1 is that the pressure in the melamine reactor 4 is adjusted to 1.2 MPa. Furthermore, the produced hydrogen is removed from the reactor by means of physical separation via line 21. The pressure in the cooler/separator crystallizer unit 6 is 1.1 MPa and in the gas/solid separation unit 8 1.05 MPa. Another difference between the process according to example 2 compared to example 1 is that the recycle gas stream comprising ammonia and hydrogen from the gas/solid separation unit 8 is recycled as cooling gas to the cooler/separator crystallizer unit 6 and as recycled gas to the melamine reactor 4 only. The gas stream obtained in the depressurize cooling unit 10 is due to the considerably lower pressure recycled to the hydrogen cyanide pre-reactor 2.

Example 3 (not according to the present invention)

Example 3 is explained with reference to Figure 3 whereby the same number depict the same lines and process units as in Figures 1 and 2. In Figure 3 process unit 2 refers to a BMA hydrogen cyanide process unit comprising a reactor that is operated at 1300 ⁰ C and 0.1 MPa, an ammonia absorber, a hydrogen cyanide absorber, a hydrogen cyanide stripper and a hydrogen cyanide fractionator as is well known fro the BMA process. Into this reaction system water is fed via line 24 and sulphuric acid via line 23 into the system and ammonia is removed as ammonium sulfate via line 25, and hydrogen is removed via line 19. In the BMA process hydrogen cyanide is obtained in liquid form and is evaporated in the evaporator 26. The hydrogen cyanide gas is fed into the melamine reactor 4 wherein it is reacted with ammonia that is fed via line 22 into the reactor. The reaction is conducted at 400 ⁰ C and 0.22 MPa and all other conditions and workup stages are the same as in example 1.

In table 1 the energy consumption per ton of produced melamine is given for examples 1 to 3.

Table 1

As can be seen from the examples, the embodiments according to the present invention result in a considerably reduced energy consumption per ton of produced melamine compared to example 3 wherein liquid hydrogen cyanide as produced by the BMA process according to the prior art is used in a process for reacting hydrogen cyanide with ammonia in the gas phase to produce melamine.