MXPA98007746A - Method for the preparation of melam - Google Patents

Abstract

The present invention relates to a method for the preparation of melamine from urea, in which the melamine is recovered from a reaction product produced by the pyrolysis of urea, by cooling with a cooling medium in a supecritic state to obtain, without further washing or purification, a solid melamine product commercially usable with a purity of 99.5% by weight or greater, the preferred coolant medium is supercritical ammonia

Description

K.

METHOD FOR THE PREPARATION OF MELAMINE BACKGROUND PE THE INVENTION 1. FIELD OF THE INVENTION This invention relates to a method for the preparation of melamine from urea in which the melamine is recovered from a reaction product containing melamine by cooling the reaction product with a cooling medium in a supercritical state. to obtain "without further washing or purification" a commercially usable product of high purity of solid melamine. Very particularly The invention relates to a method for the preparation of melamine from urea by means of a continuous »anhydrous» high pressure process in which the melamine is recovered from a melted melamine by cooling the molten melamine through direct contact with a cooling medium in a supercritical state to obtain a high purity solid melamine product without further washing or purification. The product thus obtained is commercially valuable. 2. - Description of the Related Art A continuous, anhydrous, non-catalytic, high-pressure process for the production of melamine from urea is described in the U.S. patent. Mo. 4.5S5, T67, incorporated herein by reference. That patent describes the pyrolysis of urea in a reactor at a pressure of about 10.3 MPa to 17.a MPa and at a temperature of about 354 ° C at 427 ° C to produce a reaction product which contains liquid melamine, C0-. and NH3 and is transferred under pressure, as a mixed stream, to a gas-liquid separation unit. In the gas-liquid separator, which is maintained at virtually the same pressure and temperature of the reactor, the reaction product is separated into gaseous and liquid streams. The gas streams contain gases released from C0ß and l \ IH3 and also melamine vapor while the liquid stream consists substantially of liquid melamine. The gaseous product is sent to a melamine separator, while the liquid melamine is transferred to a product cooler. In the melamine separator, the aforementioned gases of C0a and NHa and the melamine vapors are separated, at virtually the same pressure as the reactor pressure, in order to preheat the urea, cool the evolved gases and remove the melamine. The pre-heated urea. which contains the removed melamine »is then fed to the reactor, meanwhile, in the product cooler» the pressure and temperature of the liquid melamine of the gas-liquid separator are reduced by means of a liquid coolant medium, preferably liquid ammonia , in order to produce a solid melamine product. The disadvantage of this method is that the purity of the obtained melamine is typically within the range of 96 to 99.5%. In addition to melamine, contaminants such as urea, C0"" related compounds of elina and other organic solids (e.g., melema mela) are present. The resulting purity of such a product is not high enough for some critical melamine applications such as coatings. There is a need for a method to obtain directly, without further purification steps, »melamine at this purity.

BRIEF DESCRIPTION OF THE INVENTION An object of the present invention is an improved method for the preparation of melamine of very high purity, generally from 99.5 to 99.95J4 by weight, from urea in which the melamine is recovered from the reaction product of urea pyrolysis as a dry powder. A particular object is an improved, continuous, high pressure, anhydrous process for the production of melamine from urea in which the high purity melamine is obtained directly as a dry powder from melted melamine through cooling. This and other objects can be achieved by cooling a reaction product comprising melamine by direct contact with a cooling medium which is in a supercritical state thus obtaining a solid purity melamine product of high purity. The aforementioned and other objects of the present invention are achieved by a continuous process, anhydrous, high pressure to recover solid melamine from the urea pyrolysis reaction product. This process can use a melamine separator unit »a reactor vessel» a gas-liquid separator unit, and a product chiller unit. This method can also optionally use either a post-reactor vessel or an evaporator unit or both. In such a process: (1) The melted liquid urea is fed into the melamine separating unit at a pressure of about 5 to 25 MPa, preferably 8 to 20 MPa and at a temperature above the melting point of the urea. In the separator, the liquid urea makes contact with the gases released from the gas-liquid separating unit. These detached gases are mainly composed of carbon dioxide and ammonia with small amounts of melamine vapor. The molten urea bath separates the melamine vapor from the evolved gas. The detached gases also preheat the molten urea bath prior to its transfer to the reactor vessel. The carbon dioxide and ammonia gases are removed from the separator and are preferably sent to a urea plant for conversion to urea. The preheated molten urea bath, which now contains small amounts of melamine, is removed from the separator and fed to the reactor vessel.

The separator may be provided with a cooling jacket to ensure additional cooling. The separator may also be provided with internal cooling bodies. (2) The urea and molten melamine is fed to the reactor vessel using, for example, a high pressure pump. The ammonia »either in the form of a liquid or hot steam» is fed into the lower part of the reactor vessel in order to promote mixing in the container »and prevent clogging of the lower part of the vessel and avoid the formation of melamine condensate products. for example, melama, melema and melona. (3) The reactor vessel containing the molten urea, melamine and ammonia is heated to a temperature of about 325 ° C to 450 ° C. preferably from almost 350 ° C to 425 ° C, and pressurized to a pressure of almost 5 to 25 MPa »preferably from almost? at 20 MPa. Under these conditions »liquid melamine» gaseous carbon dioxide, and more gaseous ammonia are produced from the pyrolysis of the molten urea bath. Melamine, carbon dioxide and ammonia are fed as a mixed stream to the gas separator. (.4) In the gas-liquid separator, the liquid melamine is separated from the gaseous ammonia and carbon dioxide and sent, for example, to a post-reactor vessel, an evaporator unit or a product cooler. Gaseous ammonia and carbon dioxide that also contain a small amount of melt vapor are sent to the separating unit. The gas-liquid separator is preferably at the same temperature and pressure as the reactor vessel. (5) In the post-reactor vessel the liquid melamine is further reacted with more ammonia under the same conditions of temperature and pressure that are present in the reactor vessel. This second reaction with ammonia further reduces the level of impurity in the melamine. After completing this post-reaction the liquid melamine is sent to either an evaporator unit or a product cooler. (S) In the evaporator unit, the liquid melamine is converted to gaseous melamine. Any impurity remains in the evaporator unit while the gaseous melamine is transferred to a product cooler. (7) In the product cooler »the liquid or gaseous melamine is cooled and solidified by contact with a cooling medium in a supercritical state. The pressure during cooling of the liquid or gaseous melamine is preferably such that the reduced pressure is between about 0.9 and 2.5, most preferably between a reduced pressure of almost 1 and the reactor pressure. The temperature during cooling is preferably such that the reduced temperature is between about 0.9 and 2, most preferably between about 1 and 1.5. The highly pure solid melon is removed from the bottom of the chiller unit.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a flow chart of an embodiment according to the present invention of a system for the production of melamine from urea which incorporates a post-reaction step before the cooling step. Figure 2 is a flow chart of an embodiment according to the present invention of a system for the production of melamine from urea which incorporates an evaporation step prior to the cooling step.

DETAILED DESCRIPTION OF THE INVENTION The present inventors have now discovered that the purity of the melamine can be substantially increased by directly cooling the reaction product of urea pyrolysis with a supercritical cooling medium. More particularly, they have discovered that substantial increases in the purity of melamine can be obtained in the preparation of melamine from urea by a continuous, high pressure anhydrous process in which melamine is recovered from the melted melamine leaves the urea pyrolysis reactor »by cooling the melted melamine by direct contact with a cooling medium which is in a supercritical state. Preferably supercritical ammonia is used as the cooling medium. This cooling can be effected at a pressure that is virtually the same as, or less than, the pressure of the urea pyrolysis reactor, but is generally higher than the critical pressure of the cooling medium. This method produces melamine having a purity, without further washing or purification, greater than 99% by weight and more specifically having a purity of 99.5 to 99.95% by weight. The pressure and temperature of the cooling medium should be such that both the reduced pressure and the temperature are about 0.9 or more. The reduced pressure is defined as the ratio <; P / Pa) between the actual pressure < P in MPa) and the critical pressure (Pß in MPa), and the reduced temperature is defined as the ratio (T / Tß) between the actual temperature (T in ° K) and the critical temperature (Tt, in ° K). The critical pressure is defined as the pressure of the liquid-vapor critical point. The critical temperature is defined as the temperature of the liquid-vapor critical point. For ammonia the critical pressure and temperature are 11.15 MPa and 40B ° K (135 ° C), respectively. Urea, preferably in the form of a molten mixture, is the preferred starting material for the production of melamine. Ammonia and carbon dioxide are byproducts obtained during the preparation of melamine, which proceeds according to the following reaction equation: 6 C0 (NHß) z J-i ^ H, 6 NH. 3 CO.

The preparation of the melamine can be carried out at low pressure, preferably between about 0.1 and 2.5 MPa, in the presence of a catalyst such as aluminum oxide, or at high pressure, preferably between about 5 and 25 MPa, without a catalyst. The temperature of the reaction varies from approximately 325 ° C to 450 ° C, and is preferably between about 350 ° C and 425 ° C. The byproducts of ammonia and carbon dioxide are usually returned to the attached urea plant. The results reported herein can be obtained in a plant suitable for the preparation of melamine from urea. A plant suitable for the present invention can consist of a melamine separator, a reactor vessel, optionally in combination with a gas-liquid separator or with a separate gas-liquid separator, optionally a post-reactor final vessel and a cooler of product. A general description of a plant like this is presented in the patent of E.U.A. No. 4.5S5.8S7, the complete description of which is incorporated herein by reference. The reactor may be a high pressure reactor, such as is described in the U.S.A. No. 3,271,116. 3,470,163 or 3,432,274. Preferably, in the method according to the invention, a post-reactor vessel is used in which the molten melamine is contacted with ammonia, after which the melamine is transferred after its reaction to the product cooler, for cooling by means of direct contact with a cooling medium in a supercritical state. The advantage of incorporating a post-reaction step is that even small amounts of byproducts such as eleme and / or mela are obtained, which results in a high purity melamine product. Suitable post-reactor vessels are described in No. Trogen No. 139, September / October 1982. Fig. 3. The flow diagram of Fig. 1 illustrates one embodiment of the method. As shown, the melamine can be prepared from urea in a plant consisting of melamine separator 1. a reactor vessel 2 containing an internal gas-liquid separation unit 3. a post-reactor vessel 4, and a cooler 5. The molten urea is fed to the gas separator 1 at a pressure of about 5 to 25 MPa, preferably of B at 20 MPa, and at a temperature above the melting point of the urea. The gas separator 1 can be provided with a chiller jacket in such a way as to ensure extra cooling. The melamine separator 1 can also be provided with internal cooling bodies. In the melamine separator 1. the liquid urea comes into contact with the gases released from the gas-liquid separation unit 3 of the reactor vessel 2. These detached gases consist mainly of carbon dioxide and ammonia with small amounts of melamine vapor . The molten urea cleans the melamine vapor out of the detached gases and carries this melamine to the reactor vessel 2. In the separation process the detached gases are cooled to almost the temperature of the reactor 2, that is, between approximately 350 ° C to 425 ° C. and about 175 ° C to 235 ° C, while the molten urea is heated to approximately 175 ° C to 235 ° C. Below the minimum temperatures mentioned above, ammonia and carbon dioxide can condense in the lower part of the melamine separator 1 which can lead to the formation of ammonium carbamate, which can adversely affect the process. In order to avoid the harmful formation of urea decomposition and / or condensation products, the temperature of the melamine separator 1 should, in general, not exceed approximately 275 ° C. The waste gases of carbon dioxide and ammonia are removed from the top of the melamine separator 1 and are preferably returned to a urea plant to be used as a starting material. The urea and the molten melamine can then be removed from the melamine 1 separator and fed, for example via a high-pressure pump, to the reactor vessel 2 »which is at a pressure of approximately 5 to 25 MPa, and preferably of approximately 8 to 20 MPa. Gravity transfer of the molten urea can also be used by placing the melamine separator 1 above the reactor vessel 2. In the reactor vessel 2 the molten urea is converted to melamine, carbon dioxide and ammonia by heating at a temperature of about 325 °. at 450 ° C. preferably about 350 ° to 425 ° C, at a pressure of about 5 to 25 MPa, preferably about 8 to 20 MPa. The ammonia can be fed to the reactor vessel 2 in the form of a liquid or a hot vapor. The ammonia fed can serve as a purification agent to prevent clogging of the bottom of the reactor vessel 2. or to prevent the formation of melamine condensation products such as melama. melema and melon. or to promote mixing of the reactor vessel 2. The amount of ammonia fed to the reactor is about 0.01 to 10 moles per mole of urea. preferably, about 0.1 to 5 moles per mole of urea »and in particular about 0.2 to 2 moles per mole of urea. The carbon dioxide and the urea formed in the reaction "as well as any extra ammonia fed into the reactor vessel 2" can be brought together in an internal gas-liquid separation unit 3, for example in the upper part of the reactor vessel 2, and can be separated »in gaseous form» from liquid melamine. As described above, the resulting gas mixture of carbon dioxide, ammonia and melamine is sent to the melamine separator 1 for removal of the melamine vapor and to preheat the molten urea. The liquid melamine is removed from the reactor vessel 2 and transferred to a post-reactor vessel 4.

In the post-reactor vessel 4, the liquid melamine can again be contacted with about 0.01 to 10 moles of ammonia per mole of melamine and preferably about 0.1 to 2 moles of ammonia per mole of melamine. The contact time in the post-reactor vessel 4 is between about 1 minute and 3 hours, preferably between about 2 minutes and one hour. The temperature and pressure in the post-reactor vessel 4 is virtually the same as in the reactor vessel 2. After completing the post-reaction step, the liquid melamine is discharged and transferred to a product cooler 5. In the cooler of product 5 »The liquid melamine is cooled by contacting it directly with a cooling medium at a temperature and pressure at which the cooling medium is in a supercritical state. Preferably the cooling medium is supercritical ammonia. The pressure of the cooling medium during cooling of the liquid melamine is preferably such that the reduced pressure of the cooling medium is between about 0.9 and 2.5"more preferably between a reduced pressure of about 1 and the reactor pressure. The temperature of the cooling medium during cooling is preferably such that the reduced temperature of the cooling medium is between about 0.9 and 2, more preferably between about 1 and 1.5. The melamine is converted to powder and removed from the bottom of the product cooler 5. The melamine powder has a melamine content of more than 99% by weight »and more specifically a melamine content of 99.5 to 99.95% by weight and it can be used, generally, without further washing or purification, in applications which require high purity melamine such as coatings. In another embodiment of the method, a separate gas-liquid separation unit can be installed after the reactor vessel "instead of internally to the reactor vessel. In another preferred embodiment of the method, as shown in the diagram in figure 2, an evaporation step 6 is included between the reactor vessel 4 and the product cooler 5. In the evaporation step, the molten liquid melamine is converted into gaseous melamine, any byproduct remaining behind in the evaporator 6. According to this embodiment, the gaseous melamine is then cooled in the product cooler 5 using a cooling medium in a supercritical state. This step of evaporation has the advantage of further reducing the amount of polluting byproducts and thus obtaining high purity melamine. This can be advantageous especially in applications that demand a high degree of purity. In another embodiment of the method, an evaporation step can be included between the reactor vessel and the product cooler, and the post-reaction step can be avoided.

A method for the preparation of high purity melamine has been described in patent application No. 1002696 filed in the Netherlands on March 21, 1996, the complete description of which is incorporated herein by reference. The following non-limiting examples further describe the present invention.

EXAMPLES EXAMPLE The following example was carried out in a lotus plant equipped with a melamine separator - a reactor vessel incorporating a gas-liquid separator, a post-reactor vessel and a product cooler. The lower part of the post-reactor vessel was fed with 10 Kg / h of ammonia at a temperature of 400 ° C and at a pressure of 15 MPa. This ammonia flowed from the post-reactor vessel to the reactor vessel and from there along with the ammonia and carbon dioxide formed during the synthesis of melamine, to the melamine separator. The gas separator was fed with 100 Kg / h of liquid urea at a temperature of 140 ° C and at a pressure of 15 MPa. The liquid urea was heated to approximately 200 ° C in the melamine separator by means of the gases evolved from CO-j. I \ IH and melamine from the reactor vessel, which was at a temperature of approximately 400 ° C. The melamine vapor entrained by the detached gases leaving the reactor vessel was removed from the gases evolved by the molten urea and transferred, together with the molten urea, to the reactor vessel. The urea and molten swept melamine were reacted at a temperature of about 400 ° C and a pressure of about 15 MPa in the reactor vessel. The product of the reaction was separated into a gas stream and a liquid stream. The gaseous stream that was comprised of CO2, and melamine vapor was sent to the melamine separator while the liquid melamine stream was sent to the post-reactor vessel. In the post-reactor vessel »the liquid melamine was contacted with ammonia for about 25 minutes at a temperature of about 400 ° C and a pressure of about 15 MPa. From the post-reactor, the liquid melamine was transferred to the product cooler, where it was cooled by mixing it with supercritical ammonia at a temperature of approximately 150 ° C and a pressure of approximately 14 MPa. A solid melamine material of 99.7% by weight was obtained.

Claims (9)

NQVEPAP pE A I V? NC PN qEiviNpt? Ag3¡PNES

1. - A method for the preparation of melamine from a reaction product of urea composed of melamine, consisting of the steps of: cooling said reaction product by direct contact with a cooling medium in a supercritical state to obtain a melamine product high purity solid.

2. A method according to claim 1, wherein said cooling medium is ammonia.

3. A method according to the rei indication 2, wherein said ammonia is at a pressure, during said cooling step, which results in a reduced pressure of about 0.9 to 2.5. A. A method according to claim 3 wherein said ammonia is at a pressure during said cooling step which results in a reduced pressure of about 1 to the reactor pressure. 5. A method according to the indication 2 »in which said ammonia is at a temperature, during said cooling step, which results in a reduced temperature of approximately 0.9 to 2. 6.- A method of agreement to the rei indication 5, in which said ammonia is at a temperature, during said cooling step, which results in a reduced temperature of about 1 to 1.5. 7. A method according to claim 1, wherein said method further comprises the steps of: reacting said reaction product with ammonia in a post-reaction step, prior to said cooling step "to produce a product of post-reaction comprised of mel ami a. 8. A method according to claim 7, wherein said method further comprises the steps of: evaporating said post-reaction product to form a gaseous product of post-reaction melamine compound. after said post-reaction step and prior to said cooling step. 9. A method according to claim 7. wherein said post-reaction step uses approximately 0.01 to 10 moles of ammonia per mole of melamine. 10. A method according to claim 9, wherein said post-reaction step uses approximately 0.1 to 2 moles of ammonia per mole of melamine. 11. A method according to re-indication 7. in which said post-reaction step lasts from approximately 1 minute to 3 hours. 12. A method according to claim 11 »wherein said post-reaction step lasts from about 2 minutes to 1 hour. 13. A method according to claim 1 wherein said method further comprises the steps of: evaporating said reaction product to form a gaseous reaction product composed of melamine, prior to said cooling step. 1

4. A method according to claim 1, wherein said melamine product has a purity greater than 99% by weight. 1

5. A method according to claim 14, wherein said melamine product has a purity of about 99.5 to 99.95% by weight. 16.- A continuous process to produce melamine from urea, which consists of the steps of: pyro! raising urea in a reactor at a pressure of approximately 5 to 25 MPa and at a temperature of approximately 325 ° C to 450 ° C, to produce a reaction product containing liquid melamine, gaseous carbon dioxide and gaseous ammonia; transferring said reaction product as a mixed stream to a gas-liquid separation unit; separating said reaction product in said gas-liquid separation unit into gases released from carbon dioxide and ammonia containing melamine vapor, and liquid melamine; simultaneously transfer (a) said carbon dioxide and ammonia-containing gases containing melamine vapor to a melamine separating unit and separate said stripped gases with molten urea to preheat said molten urea and cool said stripped gases and remove therefrom melamine vapor, and then removing the carbon dioxide and ammonia gases from said melamine separating unit, and transferring said preheated molten urea containing said melamine to said reactor, and (b) said liquid melamine to a post-melamine container. reactor; reacting said liquid melamine with ammonia in said post-reactor vessel to produce liquid melamine subjected to post-reaction »transferring said liquid melamine subjected to post-reaction to a product cooler; cooling said melamine liquid subjected to post-reaction »by contacting it directly with a cooling medium which is in a supercritical state in said product cooler, to produce a solid product of high purity melamine; recovering said high purity solid melamine product. 17.- In a continuous high pressure process to produce pyro! raising urea to produce carbon dioxide. ammonia and melamine "the improvement, wherein the elamine is in the liquid phase, comprising contacting said liquid melamine with a cooling medium that is in a supercritical state to obtain a solid melamine product and recovering said melamine product without further washing or purification as a solid containing approximately 99.5 to 99.95% by weight of melamine. 18. A process according to the rei indication 17, wherein said cooling medium is ammonia. 19. A continuous process for producing melamine from urea, comprising the steps of: pyro! zar urea in a reaction zone at a high pressure and high temperature to produce a reaction product containing liquid melamine »gaseous carbon dioxide and gaseous ammonia» to react said liquid melamine with ammonia in a post-reaction zone to produce melamine liquid subjected to post-reaction; cooling said liquid melamine subjected to postreaction by contacting it directly with a cooling medium that is in a supercritical state "in a product cooler to produce a high purity solid melamine product.