WO2002002535A1

WO2002002535A1 - Reactor and method for producing melamine

Info

Publication number: WO2002002535A1
Application number: PCT/FI2001/000625
Authority: WO
Inventors: Ernest Groensmit; Jarno Ilme; Lauri MÄÄTTÄ; Rune Strahl; Suvi Sundquist; Ilkka Turunen; Hans Vrijenhoef; Matti Vuokko
Original assignee: Kemira Agro Oy
Priority date: 2000-06-30
Filing date: 2001-06-29
Publication date: 2002-01-10
Also published as: FI20001565A; FI109021B; FI20001565A0; PL360504A1; EP1296963A1

Abstract

The present invention relates to a vertical multitubular gaslift reactor for the production of melamine from urea at high pressure, said reactor comprising: a first lower zone (1) comprising a urea inlet (2) for molten urea and an ammonia inlet (3) for gaseous ammonia; a second middle zone (4) comprising at least five riser tubes (5) wherein a lower density feed mixture is flowing upwards and wherein the reaction of melamine synthesis occurs, a space (14) for a heating medium, a heating medium inlet (7) for supplying heating medium into said space, and at least one heating medium outlet (8) for removing heating medium from said space; and a third upper zone (11) comprising an off-gas outlet (12) for the removal of gaseous reaction products and a product outlet (13) for the removal of liquid melamine product, said reactor further comprising at least one downcomer (6) for circulating a higher density reaction mixture from the third zone (11) to the first zone (1). The invention also relates to a method for the production of high purity melamine.

Description

Reactor and method for producing melamine

The present invention relates to a multitubular gaslift reactor for the production of melamine from urea at high pressure. In addition this invention relates to a method for producing high purity melamine using a multitubular gaslift reactor.

Melamine is produced from urea by a strongly endothermic reaction at a temperature in the range of 400°C where urea reacts into melamine, ammonia and carbon dioxide usually in the presence of excess ammonia. There are two basic types of production processes, low pressure catalytic processes in which the pressures are typically near 1 MPa and high pressure processes without a need for catalysts in which the pressures are over 8 MPa. Low pressure processes have the advantage of less corrosion of the reactor interiors but require complex downstream unit operations. High pressure processes are far more simple but the reactors are expensive due to required thicker vessel walls.

In a high pressure process the reaction from urea to melamine takes place in liquid phase. A continuously operating reactor is filled with molten melamine including urea melt, reaction intermediates or by-products such as mela or melem, and reaction products, ammonia and carbon dioxide gas and some gaseous melamine. The high amount of heat for this endothermic reaction is typically supplied by internal electric heater elements or by molten salt heat-transfer systems.

Melamine users require typically very high product purity, over 99.8%. Therefore, the production processes in most cases comprise additional operation units and apparatus for various types of off-gas separation and melamine purification.

Gaslift type reactors which generally include one riser and one downcomer section equipped with, for example, bayonet heating elements inside the reactor have been used to synthesise melamine from urea. A traditional construction includes a reactor shell through which the reaction mixture is flowing heated with emerged heating elements such as electrical heaters or tubes wherein for example molten salt is circulating. In this kind of a configuration the whole reactor vessel has to be designed for the pressure of approximately 8 MPa. Such a high pressure requires a thick reactor shell which is expensive to manufacture and susceptible for corrosion. Gaslift reactors are increasingly used in chemical process industry, metallurgical processes and biological waste water treatment due to the simple structure. An alternative solution for the conventional reactor in a high pressure melamine process is to carry out the reaction in a gaslift reactor comprising a multitubular construction inside for transportation of reacting media, and heating media flowing on the shell side. In this type of reactor only the riser tubes and the heads of the reactor are under high pressure which lowers the cost of the vessel. Problems that may arise in a multitubular airlift reactor are the formation of a slug flow regime and uneven distribution of fluids in the riser tubes.

Multitubular airlift reactors are especially suitable for processes where high heat transfer rates are required. Such processes are either very exothermic like aerobic fermentation or endothermic like synthesis of urea or melamine. However, publications on multitubular airlift reactors are very scarce. No publications on industrial scale applications in the field of chemistry could be found. Only the hydrodynamics of a multitubular pilot scale airlift reactor that had triple riser and downcomer tubes has been studied by Majeed, J. G. at al., Gas Separation and Purification, 9 (1995) 2 pp. 101 — 109, and the oxygen transfer from air to distilled water in a multitubular laboratory scale airlift reactor has been investigated by Bekassy-Molnar, E. at al., Chem. Eng. J., 68 (1997) pp. 29—33.

A continuous tubular high velocity reactor for converting a melamme-forming substance such as urea, to melamine at high pressure in the presence of ammonia is described in US 2,927,923 patent pubhcation by MaUison et al. The patent provides a horizontal box-like reactor containing a long multiple times U-shaped pipeline of small internal diameter wherein the reaction mixture circulates and the conversion to melamine occurs. With this type of reactor the need for excess ammonia is decreased, the clogging and corrosion tendencies are reduced and the heat transfer is satisfactory.

There are several publications on melamine reactors were the heating medium is circulating in a tube type construction inside the reaction shell.

Eurotecnica's patent WO 99/00374 describes a high pressure melamine manufactoring process utilising a standard type tank reactor where at least one plug flow type tubular reactor is connected downstream of the standard reactor. Liquid melamine is continuously fed together with fresh NH₃ to this tubular reactor in which the essentially whole volume is occupied by the liquid phase without any mixing of the reaction product with reactants nor the intermediate products. This reactor is kept at 360 — 450°C and under a pressure higher than 7 10³ kPa. In this solution the tubular reactor is not used for the actual reaction from urea to melamine but rather for enhancing the conversion.

A traditional melamine reactor is difficult to operate in terms of start-up and shutdown. The high pressure requirement increases the risk of corrosion of materials, plugging and leakages leading to productivity losses, product quality deterioration and general safety concerns. Thick vessel walls are needed which increase the investment costs.

Conventional melamine reactors are also difficult to scale up to commercial size operation or to scale down when multiplying an existing one for smaller capacity. This scaling up or down process consists of manipulating several parameters which introduces uncertainty and consequently possible scale up errors. Typical difficult factors are differences in the cross sectional area of the reactor zone, lower or different heated reactor surface per cross sectional area influencing the flow regime in the reactor.

Especially, at start-up or shut-down the heat transfer to the reaction mixture is an issue. The conversion reaction from urea to melamine is not perfect untill the temperature of 400°C is reached. Corrosive by-products tend to form at lower temperatures. Thus, the operation temperature should be reached as fast as possible to minimise the formation of by-products. When using heating rods for example, the full heating efficiency cannot be used untill the rods are totally submerged in the reaction mixture. Otherwise the rods are likely to get twisted or even brake as the temperature above the liquid surface gets too high. Local temperature differences may cause formation of by-products, as well. Unexpected malfunctions may lead to disconnection of part of the heating elements which creates colder spots inside the reactor leading to accumulation of viscous melamine-containing material in the vicinity.

The purpose of the present invention is to overcome some of these defects and inconveniences as described above in the following detailed description of the invention.

Thus, in one aspect of the present invention there is provided a multitubular gaslift reactor for the production of melamine from urea at high pressure, said reactor comprising, in one single body having an essentially vertical cylindrical shape: a first zone in the bottom portion of the reactor comprising a urea inlet for molten urea and an ammonia inlet for gaseous ammonia; a second zone in the middle portion of the reactor being connected to the first zone and comprising at least five riser tubes wherein a lower density feed mixture is flowing upwards and wherein the reaction of melamine synthesis occurs, a space for a heating medium, a heating medium inlet for supplying heating medium into said space, and at least one heating medium outlet for removing heating medium from said space; and a third zone in the upper portion of the reactor being connected to the second zone and comprising an off-gas outlet for the removal of gaseous reaction products and a product outlet for the removal of liquid melamine product, said reactor further comprising at least one downcomer for circulating a higher density reaction mixture from the third zone to the first zone, said higher density reaction mixture forming together with the molten urea and the gaseous ammonia fed into the first zone said lower density feed mixture flowing upwards through the riser tubes.

Said at least one downcomer can be positioned inside or outside the second zone. The number of downcomers is preferably from 1 to 15, and more preferably from 1 to 4. ϊ Preferably the first zone additionally comprises a flow distributor for obtaining an even distribution of the starting materials when entering the reaction zone.

The second zone can include at least three heating medium outlets positioned at varying heights of the wall surrounding the second zone, preferably one in the bottom, one in the middle and one in the top.

Preferably the second zone additionally comprises at least one baffle positioned in said space for guiding the heating medium and for enhancing the heat transfer.

Preferably the diameter of the riser tubes is from 10 to 100 mm, and more preferably about 20 mm.

According to a preferred embodiment the addend cross-sectional area of the riser tubes is essentially equal to the cross-sectional area of the downcomer or the addend cross-sectional area of the downcomers.

Preferably the number of the riser tubes is from 5 to 1000. In a second aspect of the present invention there is provided a method for the production of high purity melamine from urea at high pressure in an essentially vertical cylindrical multitubular gaslift reactor, comprising the steps of: feeding liquid urea and gaseous ammonia into a first zone in the bottom portion of the reactor; feeding a low density feed mixture from the first zone into a second zone in the middle portion of the reactor comprising at least five riser tubes through which said feed mixture is flowing upwards and in which the reaction of melamine synthesis occurs, said tubes being heated externally by means of a heating medium being supplied into the second zone; feeding the reaction mixture from the riser tubes into a third zone in the upper portion of the reactor, wherein off-gases are separated from the liquid reaction mixture and a portion of the liquid reaction product is removed as liquid melamine product; and circulating a portion of the reaction mixture having a higher density than the mixture in the riser tubes from the third zone into the first zone wherein the higher density reaction mixture together with the molten urea and the gaseous ammonia fed into the first zone forms said lower density feed mixture.

Typically from 60% to 98%, preferably from 85% to 95% of the higher density reaction product is circulated from the third zone into the first zone.

The method of the invention can further comprise the step of feeding the separated off-gases into an adsorption device for ammonia recovery and/or recirculation.

The method of the invention can also comprise the additional step of introducing the liquid melamine from the reactor into a vaporiser with ammonia gas in which the liquid melamine is vaporised into a melamme-contaimng gas mixture. The melamme-containing gas mixture can be introduced into a cooler to convert the gaseous melamine to solid high purity melamine.

It is also possible to introduce the liquid melamine from the reactor into a cooler to convert the liquid melamine to solid high purity melamine.

Thus, this invention presents a cylindrical vertical melamine reactor with a vessel containing a multitubular reaction zone i.e. said second zone. Circulation of the reaction mixture inside the tubes in the reaction zone can be achieved using the gaslift principle by adding vapour into the reaction mixture at the inlet of the tubes below the reaction zone. The tubes are externally heated by a heating medium flowing freely inside the residual reaction zone space inside the reaction vessel. This reactor is used for producing pure melamine from urea at high pressure. The reactor can be connected to other unit operations for further purification, off-gas recovery and product refining in order to produce very pure melamine.

To more clearly define the principles of this invention, details of the preferred embodiments of the reactor vessel and of the process inherent in its operation, the invention will be illustrated by the accompanying drawings, in which

Figure 1 is a schematic view of a preferred reactor vessel showing the different zones and parts therein, Figure 2 is a cross-sectional view from the middle of the reactor vessel taken along line A-A showing the layout of the tubing inside the reaction zone, and Figure 3 is a perspective view of an experimental apparatus used in model experiments.

The reactor according to this invention comprises a vertical cylindrical reaction vessel shown in figure 1. The reaction vessel is constructed from or can be divided into three zones or chambers. In operation these zones or chambers are attached to each other foπning a single reactor body.

The first zone is a urea feed zone 1. The molten urea used as a starting material for melamine is supplied into the reactor through a urea inlet 2 located essentially in the rounded bottom of the vessel. Ammonia gas is supplied to the urea feed zone through ammonia feed 3 flange. As urea is a viscous fluid and this reactor operates according to the gaslift principle comprising multiple reaction tubes it is necessary to include a flow distributor into the urea feed zone to ensure even distribution of the starting materials when entering the reaction zone. The flow distributor may comprise varying means for dividing the urea and ammonia flow evenly as well as depending on the choice of operating parameters such as gas hold-up. A preferred alternative is to use separate urea and ammonia feed nozzles to ensure a reaction mixture of uniform quality and to distribute the fluids evenly across the cross- section area of the urea feed zone of the reaction vessel.

The second zone of the melamine reactor is the reaction zone 4. It comprises a set of riser tubes 5, at least one downcomer 6 and a heating medium inlet 7 and at least one outlet 8. The riser tubes 5 are evenly distributed across the cross-sectional area of the reaction zone as shown in figure 2. The amount of riser tubes is from 5 to 1000, the preferred amount depending on the diameter of the tubes and the desired capacity of the reactor. The diameter of the riser tubes depends on the diameter of the down- comer(s). The cross sectional areas of the downcomer(s) and riser tubes are linked together in a way that it is possible to operate the reactor as a closed system. Thus, the addend cross sectional areas must be close to each other. The diameter of the riser tubes can be between 10 mm and 100 mm, preferably around 20 mm. As the diameters of the tubes are getting smaller the easier is the heat transfer to the reaction mixture and the minor are the local temperature fluctuations. Melamine yield can be thus optimised and clogging of the apparatus minimised.

The downcomer 6 is situated either inside the reaction zone or outside of the reaction vessel wall (not shown in the drawings). If internal, it is easier to use multiple downcomers and the circulation is better enhancing thus the quality of the melamine product. This type of layout is more compact reducing the probabihty of leaks. On the other hand an external downcomer is easier to replace and the reactor construction is mechanically simpler, for example the reactor diameter can be smaller, which is a cost advantage. Preferably the downcomer is situated inside the reaction zone. The amount of downcomers can be from 1 to 15, preferably from 1 to 4 and especially one.

Heating medium is supplied to the closed space 14 in the reaction zone through a heating medium inlet 7 at the bottom of the reaction zone. At continuous operation, the upper heating medium outlet 8 is used for circulation. During start-up of shutdown the heating medium middle outlet 9 and heating medium lower outlet 10 are used to maintain the surface of the heating medium at a desired level to ensure proper conversion into melamine and to continue the operation as long as possible. The amount of heating medium outlets and their locations may vary according to the need.

The third zone of the melamine reactor is the settling zone 11. It comprises an off- gas outlet 12 at the rounded top of the settling zone and a product outlet 13 at the side of the settling zone.

At the reactor start-up or similarly at the reactor shut-down the conversion reaction can be initiated even though the reactor is not fully loaded with the reaction mixture by filling up the reaction zone gradually with the heating medium and circulating it through a lower heating medium outlet. Thus heat transfer to the reaction mixture occurs efficiently, no additional heat up time is necessary and overheating causing corrosive byproduct formation can be avoided. This enhances the production capacity and enables in shut-down situation to prolong the production to a very late stage. Further, the quality of the melamine product is enhanced.

The reactor according to the invention is operated continuously. The reactor needs to be shut down for maintenance only about once a year. At a malfunction situation it is occasionally necessary to take one or part of the riser tubes or downcomers out of service. Using a reactor according to this invention partial removal of riser tubes does not cause any further disturbances in heating the reaction mixture as is the case in a conventional reactor if one heating rod has been removed. This might create a cold spot leading to a possible accumulation of very viscous melamine/urea melt and by product formation.

The off-gas outlet of the reactor can be attached to an absorption device for recovering small amounts of melamine present in the off-gases. For example in a counter current direct contact system urea can be used as an absorbent. The heat recovered in cooling the off-gases can be used for heating ammonia to be introduced into the reactor. The recovered ammonia and carbon dioxide can be for example recycled to a urea plant.

The melamine product from the reactor product outlet is preferably directed to a vaporiser where the melt can be evaporated by increasing the amount of ammonia gas, lowering the pressure or elevating the temperature. Subsequently, the melamine gas mixture is cooled in a crystalhser. Alternatively, the melamine melt product can be directed to a crystalhser and cooled for final product.

Using the above described reactor together with an off-gas separator, vaporiser, i.e. evaporator, and crystalhser, it is possible to obtain very pure melamine, preferably at least 99.9%.

The reactor described above has the advantage of efficient heat transfer to the reaction mixture from the heating medium and a uniform temperature profile across the reaction mixture. The thickness of the shell or wall materials for this high pressure process can be reduced compared to conventional reactor configurations which is an essential economical advantage. Conversion and thus quality of melamine is enhanced and corrosion of the reactor interiors is reduced, especially during start-up, shut down or malfunction situations. This type of reactor is more versatile in terms of scaling up or down for capacity changes as geometrical changes of the important parts such as riser tubes are not necessary but can rather be replaced by changing the amount of tubing.

As mentioned above, high pressure melamine reactors which are operated by gaslift principle are known, and these reactors have been successfully used in commercial melamine production for a long time.

The present invention also utilises gaslift principle to maintain favourable flow conditions in the reactor. The present invention is based on the use of several riser tubes surrounded by a common heat transfer jacket. This new feature brings several advantages to the process but may also cause certain operational problems.

The advantages reached by the invention, when compared to the traditional technology, include at least the following features:

- Due to several small size (preferably from 1 to 10 cm) riser tubes, the heat transfer surface per reaction mass is high.

- Only the riser tubes and the heads of the reactor have to be built to stand high pressure, and not the whole shell, as in the conventional construction. This reduces the price substantially.

- If one of the riser tubes is clogged the operation of the reactor can be continued. In conventional technology, if one of the heating elements is out of order, there might be cold spots in the reactor and the smooth operation is questionable.

- The new reactor brings advantages in start-up and shut-down procedures.

- The conditions (temperature and concentration distributions) are probably better then in a conventional reactor.

Problems that may arise in this new multitubular airlift reactor are the formation of a slug flow regime and uneven distribution of fluids in the riser tubes.

Bubble flow is the preferred flow pattern in the riser tubes. Occurrence of slug flow is undesirable because of smaller gas-liquid interfacial area and in this case especially because of lower heat transfer rate between the reaction medium and the heat transfer fluid. Slug flow does not completely prohibit the operation of the reactor but makes it worse. To maintain uniform conditions and to minimize the by-product formation, equal flow of fluids should occur in each riser tube. Therefore the distribution of both hquid and gas to the tubes should be uniform.

Because both the advantages and possible disadvantages, mentioned above, result from fluid dynamical phenomena, it is sufficient to study them with fluid dynamical experiments. Experiments were done to study the advantages and especially to demonstrate that the disadvantages mentioned above either do not appear or can be controlled.

Fluid dynamical experiments can be done without real process fluids. In the experiments, air was used as gas and water as liquid. In some experiments, xanthan gum was added to water to increase the viscosity. The experimental apparatus is shown in Fig. 3.

As shown in Fig. 3, the test reactor had five riser tubes 15 and one external downcomer tube 16. The riser tubes were surrounded by a heat exchanging jacket 17 where hot water was circulated. Air was brought through a horizontal tube 18 and fed into the risers through five nozzles, as shown in the figure. Recycled hquid was fed back into risers through single-tube or 3-branched feed pipes. The inner diameter of the risers was 19 mm and length 2 m. The riser tubes, as well as the heating jacket, were made of transparent material to allow visual observation of flow phenomena in the risers.

Several experiments were carried out to demonstrate the fluid dynamical operation of the reactor. In the experiments typical range of linear velocity in the tubes were 0.3-1.0 m/s and gas hold-up 3-20 vol-%.

The main purpose of the experiments was to study the occurrence of slug flow and the Hquid distribution into the five riser tubes. The occurrence of slug flow depends on the physical properties of the fluids (liquid viscosity, interfacial tension), tube diameter and gas hold-up.

The experiments were made in the tubes with 19mm inner diameter. In larger tubes, less slug flow would appear. The effect of gas hold-up and hquid viscosity was studied. Increasing the viscosity of the liquid phase promotes bubble coalescence and thus the formation of slug flow. With xanthan gum it was possible to modify the viscosity. With pure water, the formation of slug flow started with gas hold-up values above 14%. If the viscosity is two and half times the viscosity of water, the slugs appear with about 12% gas hold-up, and with viscosity of ten times water viscosity, slugs were formed if 7% gas hold-up was exceeded.

Experiments were made by using both 3-branched and single-tube feed systems at 25°C and 1 arm. The hquid was water, sparger size 0.4 mm, and the overall gas flow rate in to the reactor 215 N 1/h. The superficial hquid velocity for each tube was plotted against the gas hold-up. Some difference in the velocities were observed. However, the difference is not so large that it would be a problem. The difference is mainly due to small difference in the feed nozzles.

The situation in real melamine reactor is, of course, different because different fluids are used. However, these model fluids give good approximation of the fluid dynamical behaviour of the melamine reactor of this type. Because these fluid dynamical conditions are the main difference between the conventional and this new reactor type, these experiments are sufficient to show that the reactor would work in practice.

In the following a preferred process description with some preferred operational conditions are set forth.

Under continuous operation urea melt is supphed into the urea feed zone of the reactor through a urea inlet valve at a flow rate of 10 000 — 15 000 kg h, preferably 13 800 kg/h at a temperature of 200 — 230°C. Simultaneously, gaseous ammoma is supphed through an ammonia feed valve into the ammoma feed zone at a flow rate of 2000—3000 kg/h, preferably 2700 kg/h at a temperature of 350— 500°C. Ammonia gas is mixed with urea melt in the distributor and this reaction mixture is guided into the riser tubes at the reaction zone. This lower density reaction mixture is flowing upwards inside the riser tubes and conversion to melamine takes place. Heating medium such as molten salt is circulated around the riser tubes through heating medium inlet and outlet flanges and is externally heated for desired temperature which is between 350 and 500°C. Due to the small diameters of the riser tubes the temperature range of the flowing reaction mixture can be maintained within 30°C. In this construction the thickness of the tube walls and the thickness of the shell wall can be considerably less than in the construction where the reaction mixture is in contact with the shell wall. After eruption of this lower density reaction mixture from the riser tubes into the settling zone gaseous components from the reaction mixture such as ammoma, carbon dioxide and some gaseous¹ melamine, are separating and directed to the off-gas outlet. The density of the reaction mixture increases and this higher density reaction mixture is directed to the downcomer for further circulation. Part of the product is directed to product outlet for further refining. Between 85 and 95% of the melamine is circulated back via the downcomer tube. A typical production volume is 90 tn per day.

Claims

1. A multitubular gaslift reactor for the production of melamine from urea at high pressure, said reactor comprising, in one single body having an essentially vertical cylindrical shape: a first zone (1) in the bottom portion of the reactor comprising a urea inlet (2) for molten urea and an ammonia inlet (3) for gaseous ammoma; a second zone (4) in the middle portion of the reactor being connected to the first zone and comprising at least five riser tubes (5) wherein a lower density feed mixture is flowing upwards and wherein the reaction of melamine synthesis occurs, a space (14) for a heating medium, a heating medium inlet (7) for supplying heating medium into said space, and at least one heating medium outlet (8) for removing heating medium from said space; and a third zone (11) in the upper portion of the reactor being connected to the second zone and comprising an off-gas outlet (12) for the removal of gaseous reaction products and a product outlet (13) for the removal of liquid melamine product, said reactor further comprising at least one downcomer (6) for circulating a higher density reaction mixture from the third zone (11) to the first zone (1), said higher density reaction mixture forming together with the molten urea and the gaseous ammonia fed into the first zone said lower density feed mixture flowing upwards through the riser tubes (5).

2. A reactor according to claim 1 wherein said at least one downcomer (6) is positioned inside the second zone (4).

3. A reactor according to claim 1 wherein said at least one downcomer is positioned outside the second zone.

4. A reactor according to claim 1 wherein the number of downcomers (6) is from 1 to 15, preferably from 1 to 4.

5. A reactor according to claim 1 wherein the first zone (1) additionally comprises a flow distributor for obtaining an even distribution of the starting materials.

6. A reactor according to claim 1 wherein the second zone (4) includes at least three heating medium outlets (8, 9, 10) positioned at varying heights of the second zone, preferably one in the bottom, one in the middle and one in the top.

7. A reactor according to claim 1 or 6 wherein the second zone (4) additionally comprises at least one baffle positioned in said space (14) for guiding the heating medium and for enhancing the heat transfer.

8. A reactor according to claim 1 wherein the diameter of the riser tubes (5) is from 10 to 100 mm, preferably about 20 mm.

9. A reactor according to claim 1 wherein the addend cross-sectional area of the riser tubes (5) is essentially equal to the cross-sectional area of the downcomer (6) or the addend cross-sectional area of the downcomers.

10. A reactor according to claim 1 wherein the number of the riser tubes (5) is

11. A method for the production of high purity melamine from urea at high pressure in an essentially vertical cylindrical multitubular gaslift reactor, comprising the steps of: feeding hquid urea and gaseous ammonia into a first zone in the bottom portion of the reactor; feeding a low density feed mixture from the first zone into a second zone in the middle portion of the reactor comprising at least five riser tubes through which said feed mixture is flowing upwards and in which the reaction of melamine synthesis occurs, said tubes being heated externally by means of a heating medium being supphed into the second zone; feeding the reaction mixture from the riser tubes into a third zone in the upper portion of the reactor, wherein off-gases are separated from the hquid reaction mixture and a portion of the hquid reaction product is removed as hquid melamine product; and circulating a portion of the reaction mixture having a higher density than the mixture in the riser tubes from the third zone into the first zone wherein the higher density reaction mixture together with the molten urea and the gaseous ammonia fed into the first zone forms said lower density feed mixture.

12. A method according to claim 11 wherein from 60% to 98%, preferably from 85% to 95% of the higher density reaction product is circulated from the third zone into the first zone.

13. A method according to claim 11 further comprising the step of feeding the separated off-gases into an adsorption device for ammonia recovery and/or recirculation.

14. A method according to claim 11 further comprising the step of introducing the hquid melamine from the reactor into a vaporiser with ammonia gas in which the hquid melamine is vaporised into a melamme-containing gas mixture.

15. A method according to claim 14 further comprising the step of introducing the melantine-containing gas mixture into a cooler to convert the gaseous melamine to sohd high purity melamine.

16. A method according to claim 11 further comprising the step of introducing the hquid melamine from the reactor into a cooler to convert the hquid melamine to sohd high purity melamine.