FI109021B - Reactor and process for the production of melamine - Google Patents

Description

109021

Reactor and process for producing melamine

The present invention relates to a multi-tube gas stove reactor for the production of melamine from urea under high pressure. The present invention further relates to a process 5 for the production of high purity melamine using a multi-tube gas stove reactor.

Melamine is produced strongly from urea by an endothermic reaction at a temperature in the range of 400 ° C, where urea reacts with melamine, ammonia and carbon dioxide, usually in the presence of excess ammonia. There are two basic types of production processes, low pressure catalytic processes with pressures typically close to 1 MPa and high pressure processes without the need for a catalyst at pressures greater than 8 MPa. Low pressure methods have the advantage of lower corrosion of the reactor internals but require complicated post-treatment unit steps. High-pressure methods are much more un-15 single, but reactors are expensive because they require thicker walls.

In the high pressure process, the reaction from urea to melamine occurs in the liquid phase. The continuous reactor is filled with molten melamine containing urine melt, reaction intermediates or by-products such as melam or melamine, and. . ·. 20 reaction products, ammonia and carbon dioxide, and some gaseous: melamine. A large amount of heat for this endothermic reaction is typically provided by internal electrical heating elements or by heat transfer systems using molten salt.

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Melamine users typically require very high purity of the product, over 99.8 25%. The production methods therefore, in most cases, include additional drive units .... and devices for separating the various types of noxious gases and for purifying the melamine.

Gas jet reactors, which generally comprise one ascending and one ascending: Y portion, provided, for example, with bayonet heater elements within the reactor, are used to synthesize melamine from urea. Traditional structure conception ·. 30 is the reactor shell through which the reaction mixture flows as it is heated by projecting heating elements such as electric heaters or tubes circulating, for example, molten '··· ’salt. In such a configuration, the entire reaction vessel must be designed for a pressure of about 8 MPa. Such high pressure requires a thick reactor shell which is expensive to fabricate and susceptible to corrosion. Thanks to its simple construction, 109021 gas gas reactors are increasingly used in the chemical process industry, metallurgical processes and biological treatment of waste water.

An alternative to the conventional reactor in the high-pressure melamine-5 process is to carry out the reaction in a gas-jet reactor, which comprises an internal multi-tube construction for the transport of reactants and heaters flowing on the side of the shell. In this type of reactor, only the risers and reactor ends are under high pressure, which lowers the cost of the vessel. Problems that may occur in a multi-tube air aero reactor include flow shutdown formation and uneven fluid distribution in the riser tubes.

Multi-tube air aero reactors are particularly suited for processes requiring high heat transfer rates. Such processes are either highly exothermic, such as aerobic fermentation, or endothermic, such as urea or melamine synthesis. However, there are very few publications on multi-tube air aero reactors. No publications were found on industrial scale applications in the chemical field. Only the hydrodynamics of a multi-tube pilot-scale air steroid reactor with triple risers and descents has been studied by Majeed, JG et al., Gas Separation and Purification, 9 (1995) 2 pp. 101-109, and oxygen transfer from distilled water to a multi-scale laboratory . In 20 lift reactors, Bekassy-Molnar, E. et al., Chem. Eng. J., 68 (1997): γ: pp. 29-33.

: U.S. Pat. No. 2,927,923 to Madison et al. Describes a continuous tube; ; structural high-speed reactor melamine-forming agent such as a stream ···. to convert melamine to melamine under high pressure in the presence of ammonia. <''. 25 widows. The patent provides a horizontal box-like reactor containing a long, multiple, U-shaped inner diameter small tubing in which the reaction mixture is circulated and converted to melamine. With this type of reactor, the need for excess ammonia is reduced, with a tendency for clogging and corrosion. decrease and heat transfer is satisfactory.

:.; There are several publications on melamine reactors in which the heating medium circulates inside a reactor shell in a tubular structure.

, ···. Eurotecnica's patent WO 99/00374 describes a high pressure melamine production process utilizing a normal type tank reactor in which at least one plug flow type tubular reactor is connected downstream with respect to a nor 3 109021 paint reactor. Liquid melamine is fed continuously with fresh NH3 to this tubular reactor, where substantially all of the volume is in the liquid phase without any mixing of the reaction product with the reactants or intermediates. This reactor is maintained at a pressure in the range of 360 ° C to 450 ° C:> 5 ° C and above 7 kPa. In this solution, the tubular reactor is not used for the actual urea-to-melamine reaction, but rather to enhance conversion.

The traditional melamine reactor is difficult to operate when it comes to start-up and shut-down. The requirement for high pressure increases the risk of corrosion, clogging, and leakage of materials, leading to loss of productivity, degradation of product quality, and general safety considerations. Thick vessel walls that increase investment costs are needed.

Conventional melamine reactors are also difficult to grow for commercial size use or to shrink an existing reactor for a smaller capacity. This process of scaling up or down involves manipulation of a number of parameters, which entails uncertainty and thus potentially increases errors. Typical difficult factors are the differences in cross sectional area of the reactor zone, the lower or differently heated reactor surface per cross sectional area that affect the reactor. 20 current flow characteristics.

Particularly during start-up or shut-down, heat transfer to the reaction mixture is an important issue. The urea conversion reaction to melamine is not ·: ··: complete before reaching 400 ° C. At lower temperatures, corrosive by-products tend to form. The operating temperature should therefore be »· ·. · ·. 25 as quickly as possible to minimize the formation of by-products.

For example, when heating bars are used, full heating efficiency cannot be used. before the rods are completely covered by the reaction mixture. Otherwise, the tanks are likely to become warped or even broken when the temperature above the liquid surface becomes too high. Local temperature differences may also cause: Y: 30 formation of by-products. Unexpected failures can lead to some of the warm up; '': Detachment from the fuel elements, which causes colder spots inside the reactor and Λ leads to the accumulation of a viscous melamine-containing substance in their vicinity.

It is an object of the present invention to overcome some of the weaknesses and disadvantages described above in the following detailed description of the invention.

s 4 109021

Accordingly, according to one aspect of the present invention, there is provided a multi-tube gas steroid reactor for producing melamine from urea under high pressure, said reactor comprising one substantially cylindrical body: a first zone at the bottom of the reactor comprising a urine feed port and gaseous am? moniakille; a second zone in the central portion of the reactor connected to the first zone, comprising at least five risers, wherein the lower density feeds) flow upwardly and wherein the melamine synthesis reaction takes place, from ; and a third zone coupled to the second zone at the top of the reactor, comprising an exhaust gas outlet for removing gaseous reaction products and a product outlet for liquid melamine product, said reactor further comprising at least one downstream high-frequency reaction mixture 20 molten urea and gaseous ammonia fed to the first reaction zone, said lower density feed mixture flowing upwardly through the risers.

Said at least one downpipe may be located within another zone or! outside of it. The number of downpipes is preferably 1-15, and more preferably 25 1-4.

Preferably, the first zone further comprises a flow divider to provide a uniform distribution of the starting materials as they enter the reaction zone.

The "second zone" may include at least three heating fluid discharge pipes ... located at different heights of the wall surrounding the second zone, preferably one at: 30: 30, one in the middle and one at the top.

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'; The working zone preferably comprises a single baffle disposed in said space for controlling the heating medium and enhancing the heat transfer.

The diameter of the risers is preferably 10 to 100 mm, and more preferably about 20 mm.

5 109021

According to a preferred embodiment, the total cross-sectional area of the risers is substantially equal to the cross-sectional area of the riser or the cross-section of the risers.

The number of risers is preferably 5 to 1000.

In another aspect of the invention, there is provided a method of producing high purity melamine from urea under high pressure in a substantially vertical, cylindrical, multi-tube gas inert reactor, comprising the steps of: supplying liquid urea and gaseous ammonia to the reactor; feeding a low-density feed mixture from a first zone to a second zone in the center of the reactor comprising at least five riser tubes through which said feed mixture flows upward and wherein the melamine synthesis reaction occurs, said tubes being externally heated to the second zone; feeding the reaction mixture from risers to a third zone at the top of the reactor separating the exhaust gases from the liquid reaction mixture and removing some of the liquid reaction product as a liquid melamine product and a portion of the reaction product having a density greater than that of the riser mixture. 20 recycling from the third zone to the first zone with a high · · · “Y ampoule reaction mixture together with the melt fed to the first zone * · ·, · :; 'With urea and gaseous ammonia forms an inner mixture of said lower density.

I I · 4 · • ·: '' '. Typically, 60-98%, preferably 85-95% of the higher density reaction product .1 ". 25 is recycled from the third zone to the first zone.

The process of the invention may further comprise the step of introducing the separated ':' ': gases into an adsorption device for the recovery and / or recycling of the ammonia.

'; The process of the invention may also comprise the additional step of introducing liquid: ... * 30 melamine from the reactor with ammonia to a vaporizer where the liquid melamine is evaporated to form a melamine-containing gas mixture. Melamine-rich. ' ". the gas mixture may be introduced into a condenser to convert the gaseous melamine to a solid high purity melamine.

6 109021

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

Thus, the present invention provides a cylindrical vertical melamine reactor having an as-5 containing a multi-tube reaction zone, i.e., said second zone. Recirculation of the reaction mixture inside the tubes in the reaction zone can be accomplished using the gas lift principle by adding steam to the reaction mixture at the inlet of the tubes below the reaction zone. The tubes are heated externally with a heating medium that flows freely within the residual region of the reaction zone inside the reaction vessel. This reactor is used to produce pure melamine from urea under high pressure. The reactor may be combined with other unit operations for further purification, waste gas recovery and product refinement to produce high purity melamine.

To further clarify the principles of the present invention, details of preferred embodiments of the reactor vessel, and the natural method of using it, the present invention is illustrated in the accompanying drawings, in which:

Figure 1 is a schematic diagram of a preferred reaction vessel showing various zones and portions thereof; and

Figure 2 is a cross-sectional view of the central portion of the reactor vessel along line A-A showing the design of the tubes within the reaction zone.

The reactor of the present invention comprises a vertical cylindrical reactor vessel as shown in Fig. 1. The reactor vessel is constructed or can be divided into three zones or chambers. When in operation, these zones or chambers are ... closed together to form one reactor body.

* · * ,,, · * The first zone is the urea feed zone 1. The molten urea used as the melamine feedstock 25 is introduced into the reactor via the urea feed port 2, ·; ·; located substantially on the rounded bottom of the container. Ammonia gas is conducted to the urea feed zone through the ammonia feed flange 3. Since the urine is a viscous liquid and this reactor operates in accordance with the principle of a multipurpose gas pump, it is necessary to include a flow divider in the feed urea and inlet belt. The flow divider may include various means for evenly distributing the flow of urea and ammonia and for selecting operating parameters such as gas delay. A preferred alternative is to use separate urea and ammonia feed nozzles to ensure uniformity of the reaction mixture and liquid. to distribute evenly across the cross sectional 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 series of riser pipes 5, at least one drain pipe 6 and a heating medium inlet 7 and at least one outlet 8.

The risers 5 are evenly distributed over the cross-sectional area of the reaction zone, as shown in Figure 2. The number of risers is from 5 to 1000, with the preferred number being dependent on the diameter of the tubes and the desired reactor capacity. The diameter of the risers is dependent on the diameter of the risers. The cross-sections of the downpipes (s) and risers 10 are interconnected so that it is possible to operate the reactor as a closed rigid system. The cross-sectional areas shall be close together. The risers may have a diameter of 10 mm to 100 mm, preferably about 20 mm. The smaller the tube diameters, the easier the heat exchange to the reaction mixture and the smaller the local temperature variations. The melamine yield can thus be optimized and device clogging minimized.

The downpipe 6 is located either inside the reaction zone or outside the wall of the reaction vessel (not shown in the drawings). If it is internal, it is easier to use multiple downpipes and thus better circulation, thereby improving the quality of the melamine product. This type of plan is more compact, which reduces the risk of leakage.

: ·: · * 20 On the other hand, the external drain is easier to replace and the reactor structure is mechanically simpler, for example, the reactor diameter may be smaller, which is ·,:; The cost advantage. The downer is preferably located within the reaction zone. The number of downpipes may be from 1 to 15, preferably from 1 to 4, and especially one.

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The heating medium is led to a closed space 14 in the reaction zone through the heating medium inlet 7 at the bottom of the reaction zone. In continuous operation, the upper heater outlet pipe 8 is used for recycling. During start-up and shut-off, the central heater outlet 9 and lower heater / outlet 10 are used to maintain the heater surface at a readable level to ensure proper conversion to melamine and to continue operation for as long as possible. The number of heater outlet openings and their locations may vary as needed.

. '' '. The third zone of the melamine reactor is the settling zone 11. It comprises the exhaust gas outlet 12 at the rounded top of the landing zone and the product outlet 13 at the side of the landing zone.

8 109021

Upon reactor start-up or similarly shut-down of the reactor, the conversion reaction can be started even though the reactor is not fully loaded with the reaction mixture by gradually filling the reaction zone with heating pad and circulating it through the lower heating medium outlet. Thus, the heat transfer to the reaction mixture is effected efficiently, no additional heating time is required and overheating causing the formation of corrosive by-products can be avoided. This increases production capacity and allows for a very late extension of production in the event of closure. In addition, the quality of the melamine product increases.

The reactor according to the invention is operated continuously. The reactor needs to be shut down for about 10 servicing only about once a year. In the event of a malfunction, it is occasionally necessary to disable one or more of the risers or downpipes. When using the reactor of this invention, partial removal of risers does not cause any additional interference in heating of the reaction mixture, as is the case in a conventional reactor if one of the heating rods is removed. This could give rise to a cold spot, possibly leading to the formation of a very viscous melamine / urea melt and formation of by-products.

The reactor exhaust gas outlet can be connected to an absorber to recover small amounts of melamine in the exhaust gases. For example, in a direct-contact countercurrent system, urea can be used as an adsorbent monohydrate. 20 na. The heat recovered to cool the exhaust gases can be used to heat the ammonia introduced into the reaction. For example, the recovered ammonia and carbon dioxide can be recycled to the urea plant.

•: · ·: The melamine product from the reactor product outlet is preferably fed to a vaporizer where the melt can be evaporated by increasing the amount of ammonia gas. 25 by lowering or raising the temperature. The melamine gas mixture is then cooled in a crystallizer. Alternatively, the melamine melt product may be derived from crystals. timeen and cool to the final product.

By using the reactor described above in combination with an exhaust gas separator, a vapor separator. ·. ·. timen, i.e.. with the evaporator, and the crystallizer, it is possible to obtain high purity melamine, preferably at least 99.9%.

The reactor described above has the advantage of efficient heat transfer to the reaction mixture:. of the heating agent and a uniform temperature profile through the reaction mixture. The shell or wall - • · · material thickness for this high pressure process can be reduced compared to conventional reactor designs, which is a significant economic advantage. The conversion of the mela 9 109021 mine is enhanced and thus its quality is increased and corrosion of the reactor core is reduced, especially during start-up, shut-down or malfunction.

This type of reactor is more versatile than increasing capacity scale! in the sense of reduction, since geometric changes in important parts such as risers are not necessary, but can be better replaced by changing the number of tubes.

The following is a preferred method description and some preferred operating conditions.

The urea melt is continuously fed to the reactor urea feed belt 10 through a urine feed valve at a flow rate of 10,000 to 15,000 kg / h, preferably 13,800 kg / h at 200-230 ° C. At the same time, gaseous ammonia is passed through an ammonia feed valve to the ammonia feed zone at a flow rate of 2000-3000 kg / h, preferably 2700 kg / h at 350-500 ° C. The ammonia gas is mixed with the urea melt 15 in the dispenser and this reaction mixture is introduced into the soot tubes in the reaction zone. This lower density reaction mixture flows upward inside the risers and the conversion to melamine occurs. A heating medium, such as molten salt, is circulated around the risers through the heating medium inlet and outlet flanges and externally heated to the desired temperature of 350-500 ° C. Due to the small diameter of the riser - :::: the temperature range of the flowing reaction mixture can be kept within 30 ° C. In this structure, the pipe wall thickness and the shell wall thickness:: ': may be significantly less than that of the structure where the reaction mixture is in contact with the ·: ··: shell walls. When this lower density reaction mixture is discharged from the risers to the landing zone, the gaseous components of the reaction mixture, .f * ·. 25 such as ammonia, carbon dioxide and some gaseous melamine are separated and fed into the exhaust gas outlet. The density of the reaction mixture increases and this. the higher density reaction mixture is led into a downer for further recycling.

... * A portion of the product is led to a product discharge tube for further processing. Melamine •; * '85-95% is recycled through the drain. Typical production is 90: Y: 30 t / day.

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Claims (16)

A gas lift reactor with multiple tubes for producing high pressure melamine from urea, comprising a single piece of substantially vertical cylinder shape: a first zone (1) located in the lower part of the reactor, which zone (1) comprises a Y opening (2) for feeding molten urea and an opening (3) for feeding gaseous ammonia; : ·:: A second zone (4) located in the center of the reactor and connected to it:: first zone, said second zone (4) comprising at least five riser pipes (5) in which a lower density feed mixture flows and in which the melamine synthesis reaction takes place, a space (14) for a heating medium, an opening (7) for feeding heating medium into said space, and at least one heating medium for opening the heating medium (8) for removal of heating medium from. · '. said space; and a third zone (11) located in the upper part of the reactor and connected to the second zone, said third zone (11) comprising an exhaust gas removal aperture (12) for the removal of gaseous reaction products, and a product removal opening (13): y; for removing the melamine product in liquid form,. * ·. said reactor further comprising at least one outlet tube (6) for circulating a higher density reaction mixture from the third zone (11) to the first zone (1): wherein said higher density reaction mixture together with the urea in molten form and The gaseous ammonia fed to the first reaction zone forms said lower density feed mixture which flows upstream through the riser (5). The reactor of claim 1, wherein said at least one outlet tube (6) is located within the second zone (4). The reactor of claim 1, wherein said at least one outlet tube is located outside the other zone. The reactor of claim 1, wherein the number of outlet tubes (6) is 1-15, preferably 1-4. The reactor of claim 1, wherein the first zone (1) further comprises a flow distributor for providing an even distribution of the starting materials. The reactor of claim 1, wherein the second zone (4) comprises at least three heating medium removal openings (8, 9, 10) located at different heights in the second zone, preferably one on the bottom, one on the middle and one on the upper. the- 15 One. A reactor according to claim 1 or 6, wherein the second zone (4) further comprises at least one intermediate disk located in said space (14) for controlling the ::: heating medium and for effecting the heat transfer. The reactor of claim 1, wherein the diameter of the riser (5) is 10-100 mm, preferably about 20 mm. 9. Reactor according to claim 1, in which the calculated diameter of the riser tubes: '(5) is substantially equal to the diameter of the outlet tubes (6) or the calculated diameter of the outlet tubes. The reactor of claim 1, wherein the number of riser pipes (5) is 5-1000. , · '., 25 A process for producing very pure melamine from urea under high pressure in a gas pipeline reactor with many tubes of substantially vertical cylinder shape, comprising: * 'the following stages: molten urea and gaseous ammonia are measured into a first zone located in Iraq; '' ': Tom's lower part; A low density feed mixture is measured from the first zone to a second zone located in the center portion of the reactor which comprises at least five riser pipes through which said feed mixture flows upstream and in which the melamine synthesis reaction takes place, said tubes being extremely heated by means of a heating medium conducted to the second zone; the reaction mixture is measured from the riser into a third zone located in the upper part of the reactor in which the exhaust gases are separated from the reaction mixture in liquid form and a portion of the reaction product in liquid form is removed as a melamine product in liquid form; and a portion of the reaction mixture, the density of which is higher than the density of the mixture contained in the riser, is circulated from the third zone to the first zone, where the higher density reaction mixture together with the urea in molten form and the gaseous ammonia fed to it the first reaction zone forms the lower density feed mixture. The method of claim 11, wherein 60-98%, preferably 85-95%, of the higher density reaction product is circulated from the third zone to the first zone. The method of claim 11, further comprising a stage in which the separated exhaust gases are measured into an adsorption device for collecting and / or re-circulating ammonia. The process of claim 11, further comprising a stage in which the liquid melanin coming from the reactor is conducted together with the ammonia gas to an extinguisher in which the liquid melanin is displaced to a melamine containing gas mixture. • * · «* The method of claim 14, further comprising a stage in which the> melamine-containing gas mixture is passed to a cooler to convert the melanin into:; 25 gaseous to solid, very pure melamine. ....: 16. The method of claim 11, further comprising a stage in which it · · ·. from the reactor coming in liquid form the melanin is led to a cooler to convert the liquid melanin into solid, very pure melamine.