MXPA01009270A - Method for producing solid melamine - Google Patents

Abstract

The invention relates to a method for producing solid melamine by expanding liquid, ammoniacal melamine which is mixed with excess ammonia, whereby a dispersion is produced. Said dispersion is expanded, optionally after it has been held under ammonia pressure, whereby solid melamine is deposited. The solid melamine is optionally held under ammonia pressure. Expansion is then optionally carried out in any particular order, along with cooling to room temperature and the pure melamine is isolated.

Description

PROCESS FOR PREPARING SOLID MELAMINE Field of the Invention The present invention relates to a process for preparing solid melamine, by depressurizing a dispersion of ammonia and liquid melamine, after which the solid melamine is precipitated.

Background of the Invention Melamine is preferably prepared by pyrolyzing urea, using either low pressure processes or high pressure processes, for example those described in "Ullmann's Encyclopedia of Industrial Chemistry, Vol. A 16, 5th edition (1990), pages 171-185". Depending on the preparation process, the melamine synthesized comprises from about 94 to 98% by weight of melamipa, and also in particular "melam", "mele", "ureidomelamine", "ameline" and "amelide" as significant impurities, and it has to be further purified through particular process steps for the most demanding application sectors. To obtain the solid melamine, the liquid melamine melt can be cooled, for example by using water, using solutions or suspensions containing aqueous melamine or using cold inert solids or solid melamine, as in AT 159/98, for example in REF: 132522 a fluidized bed. A particularly advantageous method is to inject and depressurize an ammonia-containing melamine melt, for example as in O97 / 20826, in a cooling vessel, in which an ammonia atmosphere is present, after which a pure solid is precipitated . However, this process does not give ideal results under all conditions of pressure and temperature.

Description of the Invention Therefore, the objective was to find a process which, without taking into account the temperature used and the pressure used, produces a solid melamine of good quality by means of depressurization, and in a wide range of temperature and of pressure, and in particular at low melting pressures. It has now been found that this objective can be achieved in that the melt of melamine to be depressurized comprises excess ammonia in addition to the dissolved ammonia, which gives a mixture of two phases in the form of a dispersion of ammonia and liquid melamine. Therefore, the invention provides a process for preparing solid melamine by depressurizing the ammonia-containing liquid melamine, characterized in that a) the ammonia-containing liquid melamine is mixed with excess ammonia, after which an ammonia dispersion is formed and liquid melamine, b) where appropriate, the dispersion is allowed to stand under the pressure generated by the ammonia, c) the dispersion is depressurized, after which the solid melamine is precipitated, d) where appropriate, the solid melamine is left rest under the pressure generated by the ammonia, e) and then, where appropriate and in any desired sequence, there is an additional depressurization at atmospheric pressure, and a cooling at room temperature, and the melamine is isolated. The ammonia (gas phase) is preferably supercritical and is preferably in finely dispersed form in the liquid melamine melt (liquid phase) to produce a very finely dispersed "melamma foam". The mixing process forms a dispersion of melamma and ammonia, the liquid melamma becomes saturated with ammonia. It is preferable that the liquid phase melamine has been saturated with ammonia.

According to the invention, it is possible either that the ammonia has been dispersed in the liquid elamma or that the liquid melamma has been dispersed in the ammonia. It is important that the complete amount of ammonia (dissolved ammonia and ammonia present in the gas phase) be sufficiently large so that the amount of heat dissipated during depressurization is that necessary to solidify the melamine. Therefore, a particular advantage of the invention is that with the help of the excess ammonia dispersed in the melt it is possible to obtain sufficient heat dissipation to solidify the melamine during the depressurization even at relatively low pressures., and relatively high temperatures of the melamine melts where relatively little ammonia has dissolved in the melamine melt. The amount of the excess ammonia in the melamine melt depends in particular on the temperature level of, the pressure on, and the ammonia saturation of the melt prior to depressurization, and the degree to which the melamine must be cooled down. its melting point once it has been solidified. Therefore, the high temperatures of, and the low pressures on, the melt require larger amounts of excess ammonia than temperatures which are scarcely above the melting point of melamine, which depends on the ammonia pressure used. . On the other hand, high pressures require smaller amounts of excess ammonia. Therefore, the amount of excess ammonia can vary within wide limits. Since the melting point of melamine at lower pressures after depressurization is higher than at high pressures, it is also possible according to the invention - in particular if the temperature of the melt is not too high fusion, which depends on the pressure, and the amount of excess ammonia is not very large - that the temperature during solidification remains the same or even rises. According to the invention, it is preferable that the pressures at which the ammonia-containing liquid melamine is mixed with ammonia are about 50 to 1000 bar, and that the liquid ammonia-containing melamine is then depressurized at a pressure of about 1 to 200 bar, after which the solid melamine is precipitated. Depending on the selected procedure, the pressure both before and after depressurization can vary over a wide range. The upper pressure limit before depressurization is preferably about 600 bar, preferably about 350 bar or about 250 bar. However, the upper limit can also be around 150 bars or around 130 bars. The lower pressure limit before depressurization is preferably about 60 to 80 bar. The pressure after depressurization can vary in the same way within a wide range. If a softening process follows immediately, depressurization takes place at relatively high pressures, otherwise depressurization to atmospheric pressure is possible. The pressure after depressurization is therefore preferably about 1 to 100 or 150 bar, particularly preferably about 1 to 60 bar. However, it can also be around 10 to 20 bars. During mixing with ammonia, or before depressurization, the temperature of the ammonia-containing liquid melamine is preferably in the range of about 60 ° C above the melting point of melamine, which depends on the ammonia pressure used , just above the melting point of the melamine, which depends on the pressure of the ammonia used, particularly preferably at temperatures between about 1 and 40 ° C, very particularly preferable between 1 and 20 ° C, above the melting point of melamine, which depends on the ammonia pressure used. The most useful temperature is only very slightly above the melting point of melamine, which depends on the pressure of the ammonia used. The desired depressurization temperature is more particularly preferable below about 350 ° C. It is preferable that liquid melamine containing ammonia has been saturated with ammonia. Suitable mixing equipment can be used to mix the liquid melamine containing ammonia with excess ammonia, forming a dispersion, for example mixers, stirrers, reactors with natural suction agitators or static mixers, injectors, ejectors or other suitable mixing equipment. The melamine melt can be mixed with either gaseous or liquid ammonia, but if liquid ammonia is used the melt should not solidify. The temperature of the melt preferably decreases during this process and is thus brought close to the desired temperature, or to the desired temperature. Before depressurization, either the pressure or the temperature can be, where appropriate, increased, decreased or maintained constant, using any desired method, either before or also after mixing the melamine with ammonia, but the mass Melt should not solidify during this process. In order to achieve a particularly good quality of the melamine, it is advantageous if the liquid melamine melt is aged under the pressure generated by the ammonia before the depressurization, either before or after the mixing of the liquid melamine and the ammonia. This preferably takes place for about 1 minute to 1 hour depending on the selected process conditions in the temperature range of about 350 ° C to just above the melting point of the melamine, which depends on the ammonia pressure used , and preferably at temperatures above the melting point of the melamine, which depends on the ammonia pressure used, from about 1 to 60 ° C, particularly preferably from about 1 to 40 ° C, more particularly preferably from about 1 to 20 ° C. It is advantageous here to lower the temperature of the liquid melamine, for example by introducing liquid or gaseous ammonia. The pressure during the aging process in the present is in the range of about 50 to 1000 bar, preferably about 80 to 600 bar, particularly preferably about 130 to 400 bar. If aging takes place after the mixture of melamine and ammonia, care must be taken that the dispersion is retained during the aging process. The temperature at which the dispersion of the ammonia and the liquid melamine is depressurized is preferably above the melting point of the melamine, which depends on the ammonia pressure used, of about 1 to 60 ° C, particularly preferably about 10 to 60 ° C. 1 to 40 ° C, more particularly preferably from about 1 to 20 ° C, after which the solid melamine is deposited, its melting point is now higher at a lower pressure. In a first embodiment of the invention, the depressurization of the melamine-ammonia dispersion follows the mixing process. Additional ammonia can also be introduced at this stage. It is preferable that the dispersion be depressurized within a separate vessel, and where appropriate heated, in which an ammonia atmosphere is present. The melamine dispersion can be sprayed into the container here by means of nozzles, for example individual fluid nozzles, double fluid nozzles or Venturi nozzles, or for example by means of injectors or ejectors. In another embodiment of the invention, it is also possible that the liquid melamine or a dispersion of melamine and ammonia is mixed with the excess ammonia during depressurization. This preferably takes place in specific mixing and conveying apparatuses, for example injectors or ejectors, in which the ammonia, as the blowing gas, is mixed with the liquid melamine, or the liquid melamine, as the blowing medium, mixed with ammonia, and the The resulting dispersion is transported simultaneously, with a pressure drop, inside the depressurization vessel. After depressurization, it is also possible for the solid melamine to be agitated, for example by stirring or depressurizing the melamine melt in a rotating drum or in a fluidized bed, for example, with solidification. The melamine temperature may become either lower or may become higher, or also remain the same through depressurization. Especially when a relatively large excess of ammonia is used, the temperature of the solid melamine after the depressurization is mainly lower than the temperature of the dispersion before the depressurization. However, according to the invention and due to the relatively high melting point of the melamine at relatively low pressures, it is also possible and particularly advantageous that the temperature of the solid melamine after depressurization remains the same or even rises, due to the heat released from crystallization. It has also proven advantageous that the melamine is also aged after the solidification process, under the pressure generated by the ammonia. During this process, after depressurization, the solid melamine is aged in an ammonia atmosphere, where appropriate with mechanical agitation or in a rotating drum or through pneumatic agitation, for example in a fluidized bed, for example during 10 seconds to 20 hours, preferably about 1 minute to 2 hours, in the temperature range of about 150 ° C to the melting point of melamine, which depends on the pressure of the ammonia used. The residence time in the softening process may become shorter as the temperature and pressure become higher. This temperature is preferably very slightly below the melting point, which depends on the pressure of the ammonia used, advantageously up to 10 ° C, particularly preferably up to 5 ° C. The pressure after depressurization can vary over a wide range. If the softening follows directly, depressurization takes place at a relatively high pressure, but otherwise depressurization to atmospheric pressure is possible. The pressure after depressurization is therefore preferably about 1 to 150 or 100 bar, particularly preferably about 1 to 60 bar. However, it can also be around 10 to 20 bars. If the temperature during depressurization and solidification remains the same or rises, it is advantageous that the solid melamine which is formed during this process from the melamine melt is allowed to stand (softened) under the conditions of temperature and pressure in which precipitates after depressurization. The process of the invention can give a melamine of purity above 99% by weight. Depending on the selected temperature and pressure conditions before and during the depressurization, it is also possible to obtain a melamine with a purity of up to 99.9 * by weight, in some cases above 99.99% by weight, and it is particularly advantageous here to use pressures high ammonia, and also temperatures near the melting point of melamine. The process of the invention can be carried out either discontinuously or continuously. The process is preferably suitable for following a process for the preparation of melamine, in particular for following any desired high pressure process for the preparation of melamine from urea in which the melamine is first produced in liquid form, as a melt . High pressure processes usually give the melamma in liquid form, such as a melt, at pressures of about 70 to 800 bar and at temperatures -depending on the selected pressure-of at least about 360 ° C. Exhaust gases that arise during the synthesis of melamine, in particular, NH3, CO2 and melamine gas, are usually purified by being passed through a urea melt. During this process, the urea melt is heated by the hot exhaust gases and is advantageously passed into a melamine reactor for the synthesis of melamine, while the purified exhaust gases are advantageously passed into a urea reactor. The exhaust gases can either be passed directly into the urea reactor or condensed, for example with the help of solutions of ammonium carbonate or solutions of ammonium carbamate, for example those produced in the melamine plant or in the urea plant. One way to use the heat that arises is to preheat the ammonia used in the urea plant, or to produce steam. Once the exhaust gases have been removed, the melt of melamine can be advantageously distilled, for example using NH3, this process mainly removes the residual C02. It is also advantageous to age the melamine melt in an aging vessel. However, it is also possible that the melt of melamine arising from the reactor is directly mixed with the excess ammonia once the exhaust gases have been removed.

The advantage of the process of the invention is mainly that, whatever the degree of saturation of the melt of melamine with dissolved ammonia, this depending on the pressure used and the temperature used, it is also possible to introduce a variable amount of ammonia inside. of the melamine melt. This allows a simple control, over a wide range, of the temperature used during the depressurization of the melamine melt, as required by the requirements related to the process and related to the product, and depending on the amount of excess ammonia in the melt. The process also allows, for example, that solid melamine of good quality be produced, even at low pressures. An additional advantage is the versatility to produce melamine in the purity required for the relevant applications. If the cooling does not take place during the solidification process, there is the additional advantage that any softening carried out does not require, or only requires little, the introduction of additional heat.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (14)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1. A process for preparing solid melamine by depressurizing ammonia-containing liquid melamma, characterized in that a) the liquid ammonia-containing melamine is mixed with excess ammonia , after which a dispersion of ammonia and liquid melamma is formed, b) where appropriate, the dispersion is aged under the pressure generated by the ammonia, c) the dispersion is depressurized, after which the solid melamine is precipitated, d) where appropriate, the solid melamma is allowed to stand under the pressure generated by the ammonia, e) and then, where appropriate and in any desired sequence, there is an additional depressurization at atmospheric pressure, and a cooling at room temperature , and the melamine is isolated.
2. 2. The process according to claim 1, characterized in that the liquid ammonia-containing melamine is mixed with excess ammonia and depressurizes simultaneously, after which the solid melamine is precipitated.
3. 3. The process according to claim 2, characterized in that the mixture of liquid melamine containing ammonia with excess ammonia takes place with simultaneous depressurization with the aid of injectors or ejectors.
4. 4. The process according to any of claims 1 to 3, characterized in that the solid melamine is precipitated during depressurization at the same temperature, or with an increase in temperature.
5. 5. The process according to any of claims 1 to 4, characterized in that the liquid melamine containing ammonia has been saturated with ammonia.
6. The process according to any of claims 1 to 5, characterized in that the pressures in which the ammonia-containing liquid melamine is mixed with excess ammonia are about 60 to 600 bar, preferably about 80 to 350 bar. bars, and that the liquid ammonia-containing melamine is then depressurized at a pressure of about 1 to 60 bar, preferably about 1 to 20 bar, after which the solid melamine is precipitated.
7. The process according to any of claims 1 to 6, characterized in that the temperature range within which the ammonia-containing liquid melamine is mixed with excess ammonia is about 60 ° C above the melting point of the melamine, which depends on the pressure of the ammonia used, precisely above the melting point of melamine, which depends on the ammonia pressure used, preferably from 1 to about 20 ° C above the melting point of melamine , which depends on the pressure of the ammonia used.
8. 8. The process according to any of claims 1 to 7, characterized in that it is current under a continuous high pressure process to prepare melamine from urea.
9. 9. The process according to any of claims 1 to 8, characterized in that the depressurization takes place in a separate vessel in which an ammonia atmosphere is present.
10. 10. The process according to any of claims 1 to 9, characterized in that the additional ammonia is introduced during depressurization.
11. 11. The process according to any of claims 1 to 10, characterized in that the liquid melamine containing ammonia is allowed to stand under the pressure generated by the ammonia before depressurization, preferably for about 1 minute to 10 hours, preferably at a temperature which is about 1 to 20 ° C above the melting point of the elamma, which depends on the pressure of the ammonia used.
12. 12. The process according to any of claims 1 to 11, characterized in that the dispersion of ammonia and liquid melamma is depressurized at a temperature which is around 1 to 20 ° C above the melting point of the melamma., which depends on the ammonia pressure used.
13. The process according to any of claims 1 to 12, characterized in that, after depressurization, the solid melamine is aged in an ammonia atmosphere for about 1 minute to 2 hours, in a temperature range of about 150 ° C to the melting point of the melamma, which depends on the ammonia pressure used.
14. 14. The process according to any of claims 1 to 13, characterized in that the mixture of the liquid melamine containing ammonia with excess ammonia, which forms a dispersion, takes place with the aid of stirrers, static mixers, injectors or ejectors.