FI107257B - Process for the preparation of melamine - Google Patents

Description

107257

Process for the production of melamine - Förfarande for framställning av mela-min

The present invention relates to a high pressure process for the preparation of high purity melamine from urea.

It is known that melamine can be prepared from urea at 390-410 ° C according to the following reaction formula: 6H2N-CO-NH2 -> C3N3 (NH2) 3 + 6NH3 + 3CO2

The reaction is strongly endothermic. The heat requirement is 649 kJ per mole of melamine, including heat from 135 ° C (mean melting point) to the reaction temperature.

Users expect melamine to be very pure; 99.8% and 99.9% of product labels are typical purity grades. For this reason, its manufacturing processes often include a complex cleaning department with a large amount of equipment.

There are two basic types of melamine production processes that use honey as a raw material, i. low-pressure catalytic and high-pressure catalytic processes. In the former processes, the reactor pressure is not more than about 1 MPa, in the latter, more than 8 MPa (Ullmanris Encyclopedia of Industrial Chemistry, 5th edition, vol. A 16, p. 174).

Typical low pressure processes use a fluidized bed reactor in which the catalyst is fluidized with ammonia gas or a mixture of ammonia and carbon dioxide. The me-laminate leaves the reactor as gas. One advantage of low pressure processes is that less corrosion occurs than in high pressure processes. The most well-known users of low-pressure processes are BASF (Hydrocarbon Processing, September 1969, p. 184), 25 Chemie Linz (Hydrocarbon Processing, November 1966, p. 146) and DSM / Stami-carbon (Chem. Eng., May 20, 1968, p. 18). 124), each of which has developed its own process version.

In typical high pressure processes, the reaction occurs in the liquid phase. The reactor is then filled with molten melamine mixed with some molten feedstock, i. honey, and reaction intermediates. The mixture also contains gas bubbles consisting of ammonia, carbon dioxide and a small amount of gaseous melamine. The required large amount of reaction heat is generally generated by heating elements on the inside of the reactor 2 107257, where the heat is generated by electricity or, for example, by circulating hot salt melt.

A smaller device size can be considered as one of the advantages of high pressure processes over low pressure processes. The reaction in the liquid phase takes up significantly less space.

In addition, the process device in which the gas is treated is a reasonably sized quantity i due to pressure. One advantage is the high pressure of the resulting product gas, a mixture of ammonia and carbon dioxide. This gas is often used for the production of urea and, when pressurized, is more suitable for this purpose as such.

The Montedison process (Ausind) is a typical high pressure melamine production process (Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, vol. A 16, p. 177). As with other melamine processes, urea melt and hot ammonia are suddenly introduced into the reactor. The reactor conditions are 7 MPa and 370 ° C. From the reactor, a mixture of melamine melt and product gases is led to a cooling tank, which is also fed with water containing ammonia and carbon dioxide. The temperature of the cooling tank is 150 ° C 15 and the pressure is 2.5 MPa. From this tank, the reactor effluent gases are fed for further use, for example in the production of urea or fertilizers. The melamine is recovered from the slurry by a rather multiple step treatment which includes removal of ammonia and carbon dioxide, dissolution of melamine in a large amount of alkaline water, decolorization with activated carbon, crystallization, filtration, drying and packaging.

20 There are two major drawbacks to the Montedison process. First, the product gas is obtained under relatively low pressure, only 2.5 MPa. Second, the process step is numerous because the impure product obtained from the reactor requires thorough cleaning. treatment. In addition, the purification apparatus is relatively large, since its pressures are already small.

25 The Nissan high pressure process (Chemical Economy & Engineering Review, Vol. 8, (1976), no. 1, 2, p. 35) has gained advantages over the Montedison process, or as regards exhaust gases, as follows. Also in the Nissan process, the reactor is fed with melamine melt and hot ammonia. The temperature is 400 ° C and the pressure is 10 MPa. The melamine melt and the gases are separated at the top of the reactor. Gas: Drain 30 into a washer, where they are washed with urea melt. The melamine in the gas dissolves in the urea melt. At the same time, the gases cool to about 200 ° C. Thus, the north-west gas is obtained at 10 MPa and anhydrous, which can be a significant advantage for further use. The urea melt used as the raw material is fed to the washing mill. There, it heats up and drains water (it reacts with urea). Melamine: ulate 35 is dissolved in aqueous ammonia. This solution is maintained under an ammonia pressure of 3 107257 at 180 ° C for a period of time during which impurities are shown to be removed. It is then further processed in many installations, including filtration and crystallization. Thus, there are many unit operations and equipment, which increases the cost of the process.

5 Melamme Chemicals has developed a process (U.S. Patent No. 4,565,867) where the amount of equipment is relatively small compared to the Montedison and Nissan processes. The reactor fed urea melt and hot ammonia operates at a temperature of 370-425 ° C and a pressure of 11-15 MPa. The mixture of melamine melt and product gases is fed to a gas separation tank where the product gases are separated. The gas separation 10 tank operates under the same conditions as the reactor. After urea washing the product gases are recycled. As with the Nissan process, product gases are obtained at a pressure of about 10 MPa and anhydrous. As in the Nissan process, urea melt is fed into the reactor via a urea scrubber. From the gas separation tank, the melamine melt is led to a cooling unit, where it is rapidly cooled, for example, by liquefied ammonia or water. Crystalline melamine is obtained, which is removed for drying through the base of the cooling unit. There are no actual cleaning steps. The amount of process equipment is really small compared to the processes described above. However, the purity of the product is only 96-99.5%. In this respect, the process is not competitive with the processes described above. In order to achieve competitive purity with the Melamine Chemicals 20 process, the process should be followed by a purification process similar to one of the processes described above. Then the advantages of the process, i. the small amount of equipment and the resulting cost-effectiveness would be eliminated.

Purification by evaporation of the product has been described previously. One of the earliest melamine-making patents (GB Patent No. 800,722) is an example in which about 9 kg of ammonia per kilogram of melamine product is fed to a reactor operating at 400 ° C and 4 to 8 MPa. The amount of ammonia in this case is so large that the melamine produced is completely evaporated into the gas phase. The effect of ammonia on the evaporation of melamine is based on the fact that it reduces the partial pressure of melamine in the gas phase. Taking into account both the above reaction equation and additional ammonia, the stoichiometric calculation shows that the volume of gas exiting the reaction is by volume: 94.8% ammonia, 3.9% carbon dioxide and 1.3% melamine. Thus, the melamine must be recovered from a very large amount of gas. In addition, a large amount of ammonia requires the removal of carbon dioxide before it can be recycled to a reactor. Such a process could probably produce quite 4,107,257 pure melanin. In fact, the said patent states that the only impurity present in the product was unreacted urea. However, the process is uneconomical due to the large amounts of gas and the associated separation operations, which are the separation of melamine from very high gas and carbon dioxide from very high ammonia.

The amount of ammonia required in the process described above could be reduced by lowering the reactor pressure or raising the temperature. This would allow the melimine to evaporate more easily into the gas phase and would not require so much ammonia to lower its partial pressure. If the reactor pressure is reduced to less than 5 MPa, a solid is formed in the reactor io, which complicates and ultimately prevents the reactor from operating. This has been found, for example, in U.S. Pat. No. 3,484,440, column 1, line 65. Jo; while the temperature is raised above 400 ° C, corrosion increases, which weakens the reactor structural materials.

Nissan has investigated a process (U.S. Patent No. 3,484,440) which resembles the process described above, but which can be carried out at a lower ammonia rate, whereby the amount of melamine in the exhaust gas is several times higher. In this process, hot ammonia is charged to the reactor at a rate of 0.2 to 1.0 g per gram of urea. Reactor conditions are 360-400 ° C and 5-15 MPa. The liquid melamine melt and, among them, the product gases (ammonia and carbon dioxide) are introduced from the reactor through a heat meter to an evaporator. The evaporator maintains a weight of 4-10 MPa and a temperature of 420-480 ° C. When the temperature of the pressures is suitably selected within the ranges shown, the melamine can be completely evaporated into the gas phase. For example, when ammonia is introduced into the reactor at a rate of 0.2 g urea per gram, the melamine concentration in the effluent gas phase is about 7% by volume, stoichiometrically 1 stroke. When ammonia is fed per gram of urea, the melamine content of the gas is about 3.2% by volume. In the former case (melamine? 7% v / v), the pressure should be, for example, about 7.2 MPa and a temperature of 40 ° C, or a pressure of, for example, 4 MPa and a temperature of 455 ° C. These pressure and temperature values are derived from the patent diagram. However, before evaporation, the melamine present in the evaporator must remain in the liquid melt for at least one hour. The impurities formed in the reaction are then converted back to melamine. The melamine is removed from the evaporator, where it is cooled off with water to crystallize at 150 ° C and approximately atmospheric pressure. According to the patent text, the product thus obtained has a purity of 99% and the product obtained in Example 35 of the patent has a purity of 99.2%. Such a product is not competitive in purity. Users generally demand that the purity of melamine should be 5,107,257 99.9% to be mentioned in a well known artisan (Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, vol. A 16, p. 179). Achieving this purity by the process described above would require a separate purification process, for example leaching and recrystallization, as in most melamine processes. This would be a costly addition and would question the whole meaning of evaporation; after all, purification could begin directly from the reactor melt product without wasting energy on evaporation.

The prior art melamine production processes can be divided into two groups. Most low-pressure processes and Nissan (the first of the two io Nissan processes mentioned) and Montedison's high-pressure processes all result in competitive cleanliness, but the process includes a multi-stage cleaning department with a large amount of equipment, which significantly increases operating and investment costs. The processes described in US 4,565,867 and US 3,484,440 form another group. They have about the same number of Law-15 theists as the first group and do not have the aforementioned multi-stage purification department. However, the product obtained by them is not competitive in purity.

Applicants have previously developed a high pressure process for the production of melamine with substantially the same equipment as the aforementioned Melamine 20 Chemicals process (US 4,565,867) but with a purity of at least 99.9%. This process is based on e.g. evaporation of the melamine melt from the melamine reactor and subsequent crystallization of the melamine from the gas phase. More specifically, this prior method, which is described in the patent publication; EN 96028, comprises the following steps: i) supplying molten urea and hot ammonia gas to a reactor at a pressure of 5 to 15 MPa and a temperature of 360 to 430 ° C to obtain a reaction product containing a liquid melamine melt and a gas mixture; (iii) evaporating the liquid melamine melt thus obtained in an evaporator; and (iv) cooling the melamine-containing gas obtained from the evaporator to crystallize the melamine in high purity.

According to FI 96028, melamine melt obtained from a melamine reactor can be evaporated by lowering the pressure or raising the temperature or by feeding ammonia gas into the evaporator or by using two of these three methods; 35 or all three. Specifically, pressures of 2 to 9 MPa and temperatures of 420 to 470 ° C are mentioned. It is stated in the specification that at a pressure of 5 MPa and a temperature of 450 X '6 107257 the evaporator requires at least about 2.4 kg of ammonia per kilogram of melamine, which means that the molar ratio of ammonia to melamine is at least 17.6: 1.

In addition, according to a preferred embodiment of FI 96028, the quenching of a gas containing melamine is effected by direct contacting said gas with liquefied ammonia in the condenser at a temperature below 130 ° C and a pressure below 4 MPa.

The object of the present invention is to further improve the process described in FI 9602 $, in particular with regard to simple operation and energy consumption or heat recovery, in order to make the process more economical.

Thus, the present invention provides a process for the preparation of melamine from urea comprising the steps of: a) introducing a urea melt and hot ammonia gas into a melamine reactor at a pressure of 5-15 MPa and a temperature of 360-470 ° C to obtain melamine melt and piston gas; separating the exhaust gases from said melamine melt, c) introducing said melamine melt and ammonia gas into an evaporator at a pressure of 0.1-1.5 MPa and a temperature of 290-410 ° C to obtain a melamine-containing gas mixture; and 20 d) introducing said melamine-containing gas mixture into a pressure of 0.1-1.5 MPa and a temperature of 0-250 ° C to convert gaseous melamine to solid, high purity melamine.

Steps a) and b) are preferably carried out in the manner described in FI 96028.

In step c), the pressure is 0.1 to 1.5 MPa, preferably 0.2 to 1 MPa, and the temperature is 290 to 25410 ° C, preferably 300 to 400 ° C. At a pressure of 0.2 MPa, the temperature is preferably from 00 to 335 ° C, and at a pressure of 1 MPa, the temperature is preferably from 340 to 400 ° C. In the evaporator, the molar ratio of ammonia to melamine is preferably from 1: 1 to 10: 1, more preferably from 3: 1 to 7: 1. Preferably, the superheated ammonia gas is introduced into the evaporator. The temperature of the superheated ammonia is typically about 540 ° C. The retention time of IVa-30 mines in the evaporator is preferably less than half an hour and particularly preferably less than 10 minutes.

107257 In cooling step d), the pressure is 0.1-1.5 MPa, preferably 0.2-1 MPa, and the temperature is 0-250 ° C, preferably 30-230 ° C. Preferably, the pressure is the same as that of the evaporator. The coolant is preferably introduced into the cooler and brought into direct contact with the melamine-containing gas.

According to the first embodiment, said refrigerant is liquefied ammonia, wherein cooling is effected by evaporation of the ammonia. The condenser does not contain liquefied ammonia, and the separation of the condenser and the crystals can be combined with the same equipment, for example cyclone type. The volatile ammonia and ammonia gas from the evaporator are removed from the condenser and ai-10 portions thereof are condensed to form liquefied ammonia which is recycled to the condenser as a refrigerant. The heat recovered by cooling the ammonia can be utilized to heat the ammonia fed to the evaporator, e.g., fresh liquefied ammonia.

According to another embodiment of the invention, said refrigerant is ammonia-15 gas. As in the first embodiment, the condenser does not contain liquefied ammonia, and the separation of the condenser and the crystals can be combined into one apparatus, for example, cyclone type. Ammonia gas, some of which is derived from the refrigerant and some from the evaporator, is removed from the condenser, and at least a portion thereof is cooled to form ammonia gas for recycling to the condenser as a refrigerant. The heat recovered from the ammonia cooling can be utilized to heat the ammonia fed to the melamine reactor, e.g., fresh liquefied ammonia.

According to the invention, the exhaust gases separated from the melamine melt can be introduced into an absorber to recover small amounts of melamine in the exhaust gases. The treatment of the off-gases is preferably accomplished by absorbing melamine in a counter-current direct contact system in which the urea is absorbent. Typically, the absorption device has an absorption column. To reduce the loss of urea in the absorption column, the exhaust gas is provided with a condenser for condensing the urea: absorbent. This equipment can recover most of the urea contained in the ab- • 30 sorption column exhaust gas. One or two-phase absorbers can be used. However, to ensure that all the exhaust gases are sufficiently cooled for melamine recovery, a two-stage absorber is preferred. The heat recovered from the urea condenser can be used to heat the ammonia fed to the melamine reactor. Alternatively, the heat recovered from the urea condenser 35 may be used for steam generation. The cooled exhaust gases, which mainly contain ammonia and carbon dioxide, can be recycled to urea plant. Preferably, the absorption device is operated at substantially the same pressure as the melamine reactor. The absorption device operating temperature should be above 180 ° C to avoid carbamate formation and below 260 ° C to suppress the formation of the burette and the triuret. The temperature is preferably 230-240 ° C.

The invention will be described in more detail below with reference to the accompanying drawings, in which Fig. 1 illustrates a process diagram of a system that can be used to implement a first embodiment of the process of the present invention; .

The flow chart of both Fig. 1 and Fig. 2 includes four compartments, from left to right, which are exhaust gas treatment - melamine reactor - evaporator - condenser.

According to Fig. 1, a urea melt containing small amounts of absorbed melamine from an absorber 154 and ammonia, preferably hot ammonia, is fed to the melamine reactor 1 through different tubes. Reactor conditions: conditions of a typical high pressure process, e.g. the temperature is 360-470 ° C and the pressure is 5-15 MPa. The temperature is preferably about 400 ° C and the pressure about 10 MPa. Heating is achieved by means of internal heating elements. The reactor effluent gases, typically containing up to about 1% melamine, equilibrated with ammonia, carbon dioxide, and unreacted urea, are separated from the melamine melt at the top of reactor 1 and fed to an absorber 4 where the melamine is ethane. recovered from the off-gases using melted urea derived from a urea plant ». .

absorbent.

In the absorber 4, the melamine is absorbed in a direct contact system upstream of the exhaust gases, whereupon the urea absorber is condensed in a separate condenser. One or two stage absorbers may be used. In order. it is ensured that all the exhaust gases are sufficiently cooled for the absorption of melamine, but a two-phase absorber is preferred. The main purpose of the exhaust gas after-30 is to recover melamine. Other purposes include preheating the urea melt, cooling the exhaust gases and dewatering the soil. The advantages of the above-described exhaust gas treatment over previously known systems include the simplicity of the present rigid system, the fact that no pump and / or mixing is required, its lower (typically only a few minutes) delayed; time, 35 to achieve better dewatering of urea and preheat of urea to a higher temperature of 9,107,257 and the need to deliver less heat to the melamine reactor. The operating pressure of the absorption device is preferably the same as that of the melamine reactor, i.e. 5 to 15 MPa, preferably about 10 MPa. The operating temperature of the absorption device is 180-260 ° C, preferably 230-240 ° C. The heat recovered from the exhaust gas treatment urea condenser can be used to evaporate the fresh liquefied ammonia. Further, as shown in Figure 1, the evaporated ammonia is overheated with hot salt to provide hot ammonia for feeding to reactor 1.

The melamine melt from Reactor 1 is fed to evaporator 2. The temperature of the evaporator is 290-410 ° C and may be, for example, about 350 ° C, and the pressure is from 0, Ι-ΙΟ 1.5 MPa, preferably from 0.2 to 1 MPa. The evaporator pressure of the system shown in Figure 1 is preferably about 0.7 MPa. The evaporator can be heated, for example, by internal heating elements. Ammonia gas is fed to the evaporator to evaporate the melamine. The ammonia gas supplied to the evaporator may be an overheated ammonia gas having a temperature of about 540 ° C. The molar ratio of Ammonium-15 to niacin in the evaporator is preferably 1: 1 to 10: 1, more preferably 3: 1 to 7: 1. Compared to Applicants' earlier patent publication FI 96028, the evaporator of the present invention is used at significantly lower pressure and temperature and has a lower ammonia / melamine molar ratio. Lower pressure and reduced excess ammonia promote polymerization reactions 20 by forcing ammonia out of the melamine and shifting the reaction equilibrium back toward the melamine.

A gas mixture containing melamine and ammonia is passed from evaporator 2 to condenser 3. At the same time, coolant is introduced into the condenser, which is brought into direct contact with the gas mixture. In the embodiment shown in Figure 1, the refrigerant is liquefied ammonia. When controlled, the liquefied ammonia binds the heat released by the melamine during cooling and crystallization. The ammonia gas introduced into the condenser together with the melamine and evaporated refrigerant ammonia is discharged from the top of the condenser. The cooler pressure is 0.1-1.5 MPa, preferably 0.2-1 MPa, and the temperature is 0-250 ° C, preferably 30- “··· 30 230 ° C. The pressure of the radiator of the system shown in Figure 1 is preferably about 0.7 MPa. In this evaporative ammonia cooling, no liquid ammonia is present, and the separation of the condenser and the crystals can be combined, for example, into a cyclone-type apparatus. .

10 107257

The melamine is recovered as crystals of the same purity as the melamine prepared according to the applicant's patent publication FI96028.

Figure 2 illustrates another embodiment of the process of the present invention. To avoid repetition, only features other than those described above are shown. 5 In the waste gas treatment section, the heat recovered from the urea condenser is used for steam production. The operating pressure of the evaporator 2 is about 0.5 MPa. The operating pressure of the cooler 3 is about 0.5 MPa. In addition, according to the embodiment shown in Figure 2, the refrigerant is ammonia gas. The ammonia gas portion removed from the condenser is cooled to form ammonia gas for use as a refrigerant in the condenser. The heat recovered from the anunion cooling can be utilized in preheating fresh liquefied ammonia, after which the ammonia is superheated, for example with hot salt, and then fed to the melamine reactor 1. The ammonia gas residue removed from the condenser is superheated, e.g.

«· J

Claims (18)

107257 A process for the production of melamine from urea, characterized in that it comprises the following steps: a) Melted urea and hot ammonia gas are introduced into a melamine reactor at a pressure of 5-15 MPa and a temperature of 360-470 ° C; for obtaining exhaust gases; b) separating the exhaust gases from said molten melamine; c) introducing said molten melamine and ammonia gas into an evaporator at a pressure between 0.1 and 1.5 MPa and a temperature between 290 and 410 ° C to obtain a gas mixture containing melamine; and d) introducing said melamine-containing gas mixture into a condenser at a pressure between 0.1 and 1.5 MPa and a temperature between 0 and 250 ° C to convert the gaseous melamine to a solid, high purity melamine. Process according to Claim 1, characterized in that the evaporating pressure 15 is between 0.2 and 1.0 MPa. Process according to Claim 2, characterized in that the evaporating pressure is between 0.4 and 0.8 MPa. Method according to one of Claims 1 to 3, characterized in that the evaporation temperature is between 300 and 400 ° C. Process according to one of Claims 1 to 4, characterized in that the molar ratio of ammonia to melamine in the evaporator is 1: 1 to 10: 1. Process according to claim 5, characterized in that said molar ratio is from 3: 1 to 7: 1. Process according to one of Claims 1 to 6, characterized in that more than 25 heated ammonia are introduced into the evaporator. • · Process according to one of Claims 1 to 7, characterized in that the refrigerant is introduced into the condenser and the refrigerant is brought into direct contact with the melamine-containing gas. Process according to Claim 8, characterized in that said refrigerant is liquefied ammonia. 107257 Process according to Claim 9, characterized in that the gaseous ammonia is removed from the condenser and at least part of it is condensed to form liquefied ammonia for re-use as a refrigerant in the condenser. A process according to claim 10, characterized in that the heat recovered by cooling the ammonia is used to heat the ammonia fed to said evaporator. A process according to claim 8, characterized in that said refrigerant is gaseous ammonia. Process according to Claim 12, characterized in that the gaseous ammonia is removed from the condenser and at least a portion thereof is cooled to form gaseous ammonia for use as a refrigeration vessel in the condenser. Process according to claim 13, characterized in that the heat recovered by cooling the ammonia 15 is used to heat the ammonia fed to said melamine reactor. A method according to any one of the preceding claims, characterized in that said exhaust gases separated from the molten melamine are led to an absorber unit to recover the melamine present in the exhaust gases. ; 16. A method according to claim 15, characterized in that the melanin is absorbed in a direct-contact system operating upstream of the exhaust gases, wherein the urea is in the form of an absorbent followed by condensation of the absorbent in the condenser. Process according to claim 16, characterized in that the heat recovered from the urea condenser is used to heat the ammonia fed to said melamine reactor. • · - * Process according to one of Claims 15 to 17, characterized in that the absorber operates at substantially the same pressure as the melamine reactor and at a temperature between 180 and 260 ° C. 13 107257