JPH0369346B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Urea or its pyrolysis products are processed at atmospheric pressure or at elevated pressures up to about 10 bar as catalysts and added ammonia or gas mixtures containing ammonia, such as ammonia/carbon dioxide mixtures, such as the reaction exhaust gas itself of melamine synthesis. It is known to produce melamine by heating to a temperature of 350-450°C in the presence of melamine.

According to German Patent Application No. 1204697, vaporous melamine is separated from the reaction gas by fractional condensation by cooling the gas to a temperature of 150 DEG to 250 DEG C. The gas from which the melamine has been removed under the steam tension still contains unreacted urea, which, as is also known, is a melt of urea or a melt of a mixture of urea and biuret (which can optionally be further removed). It can be scrubbed with urea decomposition products (which may contain urea decomposition products such as cyanuric acid), in which case the gas is further cooled. The urea required for this treatment is preferably circulated and brought into direct contact with the reaction gas. Subsequently, the reaction gas thus cooled is removed from the urea melt by the components recovered from the gas by the melt or by the decomposition products of urea (melamine, isocyanic acid, cyanuric acid, etc.) formed in the melt by heating. acids, biurets, etc.). The reaction gas purified and cooled in this way is used as a fluidizing gas for the catalyst in the synthesis reactor and/or as a cooling gas for separating the melamine.

By removing the urea required in the synthesis reactor from this circuit, the by-products and decomposition products recovered from this circuit during the scrubbing of the reaction gas are returned again, thereby increasing the yield. increase Furthermore, the level of by-products in the urea circuit remains constant and low.

The heat absorbed by the urea conducted in the circuit during cooling and purification of the reaction gas can be used, for example, for melting the fresh urea required for the synthesis and for removing excess (or even total) heat) can be extracted by indirect cooling in a cooler.

Federal Republic of Germany Patent No. 2525781 states:
A method for extracting heat from a urea melt is described, which method provides for the cooling of the melt, which previously took place exclusively in a cooler located outside the urea washer, to take place at least in part in the washer itself. consists of things. The cooling of the melt is therefore carried out in a cooling element arranged in the urea scrubber, in which case the temperature of the refrigerant is at least 3° C. lower than the temperature of the melt and the melt removed from the urea scrubber. should have at most a temperature 4° C. higher than it had when entering the urea scrubber.

The exhaust gas leaving this scrubber consists primarily of the gaseous components ammonia and carbon dioxide formed simultaneously during the conversion of urea to melamine. This gas should be used because it contains an important component, ammonia.

This mixture can be used, for example, by refluxing the NH ₂ /CO ₂ mixture through a urea synthesis unit, if one is present. Another possibility for utilizing the ammonia contained in the exhaust gas is to react the ammonia with acids or acidic solutions such as nitric acid or sulfuric acid to obtain ammonium salts. However, such an application necessarily involves urea synthesis with the production of ammonium salts, for example in the field of fertilizer construction, or with urea production, which is often undesirable or generally impossible.

In order to free the ammonia from such bonds, it is desirable to separate the ammonia from the gas mixture and recover it in pure, preferably liquid form.

Such a method for separating gas mixtures produced, for example, in melamine synthesis was described in German Patent Nos. 2317603 and 2646804. The gist of the method is to firstly mix the gas mixture at least partially in a solvent and at most
The aim is to obtain a solution enriched in ammonia with respect to carbon dioxide by absorption with a contact time of 0.1 seconds. When this absorption is carried out in two steps, the second
Significant ammonia-free carbon dioxide is purged from the process. The work-up of the now ammonia-enriched solution expels the ammonia present in the first desorption step in excess of at most a molar ratio of 3 parts ammonia to 1 part carbon dioxide and entrains it from the expelled ammonia. This is done in the form of washing away the carbon dioxide. The residual solution is removed from the first desorption column and introduced into a second desorption column operating at a temperature of 87-98°C. In this desorption step, the solution is substantially freed of ammonia and carbon dioxide and the expelled gas mixture is purified with fresh gas mixture fed to the absorption step for further work-up. The bottoms taken out from the second desorption step is again supplied to the absorption step as an absorbent.

Excess water can be drained from the process by stripping a portion of the bottoms of the second desorption step on its way back to the absorption step in order to achieve volume balance within the entire system. This bottoms still contains some ammonia, but of course this concentration does not exceed 4%.

Such use of waste water or further treatment does not present any significant drawbacks in industrial complexes consisting of a large number of different manufacturing plants. This problem is of course important for melamine plants that are not located in locations that meet the above prerequisites.

It is therefore an object of the present invention to carry out a complete process of the type mentioned at the outset for the production of melamine by catalytic reaction of urea and for the after-treatment of synthetic waste gas from which melamine has been removed. is substantially ammonia-free, and the synthesis gas decomposed into its constituents ammonia and carbon dioxide is in each case substantially free of other constituents, without any additional components necessary to increase its purity. The aim was to construct the system in such a way that energy did not need to be supplied externally to the entire process, but could be procured from the process itself.

The task was to introduce the urea-containing melt necessary for cooling the melamine-free gas above and below the cooling member, using the bottoms from the second desorption step as the refrigerant and using the cooling member The refrigerant that has flowed through the
Heat the refrigerant to a temperature of 115 to 135°C and
Introduced into the desorption process, the bottoms of this process are heated to 105-120℃.
temperature, the heated refrigerant having a temperature at least 10°C higher than the respective can temperature, and before using the removed substantially ammonia-free bottoms as a solvent in the absorption process. It has been found that a solution can be achieved by using the bottoms after cooling to scrub the gas mixture consisting mainly of carbon dioxide which has not been absorbed.

By dividing the urea melt into two streams according to the invention, one of which is introduced into the urea scrubber from above the heat exchanger and the other from below, the upper part of the urea melt contains all the urea melt. The material is heated more intensely than when it is fed from above the cooling member. Thereby, the refrigerant flowing through the heat exchanger can be heated to a temperature of 115 to 135°C without expanding the heat exchanger area. Urea melt splitting is 150-250℃
This is done on the basis of the temperature of the exhaust gas entering the urea scrubber, which may be , and the respective desired temperature that the refrigerant is to obtain. The division of the urea melt can be varied within wide limits. However, the range is limited by the fact that a minimum amount of the cooling element should be introduced from above so as to ensure a sufficient distribution of the cooling element for heat transfer. On the other hand, if too much is injected from above the cooling member, the temperature rise of the refrigerant will be small.

The amount of heat of the refrigerant thus heated is used to raise the can temperature to 105 to 120°C in the second desorption step. In order to be able to maintain this temperature, the heated refrigerant should have a temperature at least 10° C. higher than the desired can temperature in each case of the desorption step.

In this case, by increasing the can temperature in the second desorption step, a solution substantially free of ammonia and carbon dioxide can be taken out.

After cooling this bottoms to a temperature of 15 to 30 °C and optionally removing excess water, and according to the invention, in order to remove ammonia from the gas consisting mainly of carbon dioxide that was not absorbed in the absorption step. The solvent, now containing ammonia, is then introduced as new absorbent into the absorption process.

Next, the present invention will be explained in detail with reference to the drawings.

Urea is introduced into a vessel 1, where it melts and is introduced by means of a conduit 2 and a pump 3 into a fluidized reactor 4 in which a catalyst, such as aluminum oxide, is fluidized. As a fluidizing gas, it is generated during a reaction,
A gas consisting of ammonia and carbon dioxide from which melamine is separated is used. The gas is supplied via line 5 to the fluidized reactor 4 by means of a gas compressor 6 after being preheated in a heat exchanger 7 . The additional heat required for the reaction is supplied to the catalyst fluidized bed by a heating coil 8 into which the heated salt melt is introduced.
The temperature in the fluidized bed is 350-450°C. The vaporous melamine formed in the fluidized reactor 4 leaves the reactor via the line 9 together with the fluidized gas and the ammonia and carbon dioxide newly formed during the reaction. In a gas cooler 10, the gas mixture is cooled until by-products, such as melene (but without melamine), are condensed, which are removed together with the catalyst dust from the reaction gas in a gas filter 11. . The gas purified in this way is introduced via conduit 12 into a condenser 13, where it is cooled to a temperature of 150 DEG to 250 DEG C. by mixing with cold gas, the origin of which will be explained in more detail below. The gas is fed to the condenser via conduit 14. This cooling of the reaction gas causes the melamine to condense. The condensed melamine is sent together with gaseous ammonia and gaseous carbon dioxide via line 15 to separator 16, where it is separated and removed via line 17. The exhaust gas from which solid melamine has been removed is sent to blower 1.
8 via conduit 19 to washing tower 20 . In the scrubber column the gas is washed with a urea melt having a temperature of 120 DEG -140 DEG C. and cooled at the same time.

The urea melt is removed from vessel 1 and passed through conduits 2 and 21a onto heat exchanger 47 and into conduit 2.
1b and is introduced into the heat exchanger. The urea melt heated by up to 4° C. is removed from the lower part of the column 20 and is separated into melt and gas in a separator 23. The melt is returned to the vessel 1 via conduit 24, where it uses some of the heat recovered from it to melt the urea newly introduced into the vessel, and the rest to convert it into steam in a heat exchanger 22. The steam is discharged to generate, and the vapor serves by means of a heat exchanger 38 to heat the column 35, which will be further explained below.

The gas separated in separator 23 is supplied via line 25 for three different purposes: (a) via line 5 to flow reactor 4; (b) via line 14 to condenser 13; (c) It is supplied via conduit 26 to the exhaust gas after-treatment described below.

The gas withdrawn from conduit 26 is circulated in absorber 27 with a residence time of up to 0.1 seconds via cooler 28 and brought into contact with a solution whose temperature is between -5 and 50°C, preferably between 10 and 40°C. In this case, ammonia is preferentially absorbed. The unabsorbed gas is fed via conduit 29 to a second absorber 30 where it is mixed with a solution circulating via a cooler 31 and absorber 27.
The contact is made under conditions similar to those in . The solution exiting the absorber 30 is fed to the first absorber 27 in a conduit 32 . The ammonia- and carbon dioxide-containing solution removed from the absorber 27 via conduit 33 is heated in a heat exchanger 34 to a temperature of 60 DEG to 90 DEG C. and introduced into a desorption column 35. The upper part of the tower is cooled by a cooler 36. Ammonia is removed from the top of the column via line 37. The can section of the column is heated by a vaporizer 38 to a temperature of 75-90°C. The heat required for this is generated in the heat exchanger 22 and extracted from the steam which is led in by the conduit 39.

The aqueous solution taken out from the can of the column 35 and containing ammonia and carbon dioxide is transferred to the heat exchanger 42.
Column 41 heated by the bottoms flowing out from column 41
is supplied to The can section of column 41 has a temperature of 105-120° C. at a pressure of 1.2-2.0 bar (absolute). The heat required to maintain this temperature is extracted from the circulating bottoms of the scrubbing tower 20 through conduit 43 and heat exchanger 47. Additionally, any heat required can be supplied by a heat exchanger 45.

Ammonia and carbon dioxide are virtually completely desorbed under the operating conditions of column 41 and are incorporated via line 44 into the gas stream fed via line 26 to absorption column 27 . The hot solution, which now contains less than 0.01% by weight of ammonia, is removed via conduit 46, cooled in heat exchangers 42, 34 and 53 to a temperature of 15 to 30°C and sent to absorber 30 as absorption solution in gas washer 49. This serves to remove residual ammonia from the CO 2 gas stream taken off via conduit 48 from the CO ₂ gas stream. conduit 5
After 0, substantially ammonia-free carbon dioxide is removed from the top of the column.

The solution removed from the can section of the column 49 via the conduit 51 is supplied to the absorber 30 as an absorption solution.

Substantially NH ₃ introduced into the gas after-treatment section
Free water can be removed via conduit 52.

EXAMPLE 9.7 t of urea per hour are melted in vessel 1 and fed via line 2 and pump 3 to fluidized bed reactor 4. The conversion to melamine is carried out at 390° C. and a pressure of 1.8 bar (absolute). A mixture of NH ₃ and CO ₂ produced during the reaction is used as a fluidizing gas to maintain the catalyst in a fluidized state. The flowing gas is preheated to 400° C. via line 5 and with the aid of a gas compressor 6 in a heat exchanger 7 and then transferred to the reactor 4.
supply to. The additional heat required for the reaction reaches the catalyst bed via a heating coil 8 into which the heated salt melt is introduced.

The gaseous melamine formed is discharged from the reactor and cooled to 320°C. Condensed melene and catalyst dust are removed by a gas filter 11. by mixing the gas in the condenser 13 with a gas heated to 140° C. which enters the crystallizer via the conduit 14.
Cool to 210°C and crystallize the melamine. Via line 15, the crystalline melamine is sent together with a gas consisting of ammonia and carbon dioxide to a separator 16, where 3.2 t of melamine are separated from the gas via line 17.

The gas is washed with 1900 t of urea melt having a temperature of 130 °C and flowing into the urea scrubber. In this case, 950t will be installed above the heat exchanger and the rest will be installed below the heat exchanger.
The melt heated to 134° C. at the bottom of the washer is collected and sent to a new circuit. In a heat exchanger 47, the bottoms from column 41, which serves as refrigerant, are heated to 128° C. and are used to maintain the vessel part of the column at 115° C. (pressure: 1.7 bar absolute).

Gas stream 44 and exhaust gas exiting from the synthesis section are
NH ₃ 3000Kg, CO ₂ 3500Kg and H ₂ O steam (temperature 140
C.) and the exhaust gas is brought into contact with the absorption solution via line 26 in separator 27 at a temperature of 38.degree. C. with a contact time of approximately 0.02 seconds. The unabsorbed gas mixture is brought into contact with the circulating solution again under similar conditions in the second absorption column 30. A gas mixture is withdrawn from the second absorption column, which in addition to CO ₂ also contains
Contains 5.8% by volume of _NH3 . This gas mixture is washed in scrubber 49 with the bottoms, which is taken from column 41 and is essentially pure water. 3500 Kg of carbon dioxide are taken off from the top of the washer, the gas still containing only 50 ppm by volume of ammonia and 110 Kg of water vapor.

The aqueous solution taken out from the absorption tower 27 contains 13.2% NH ₃ and 4.1% by weight CO ₂ . The solution is heated to 71°C in a heat exchanger 34 and introduced into a desorption tower 35, where the vessel section of the tower is maintained at 85°C. The steam required for this purpose is taken off from the heat exchanger 22 via a line 39. 3000 kg of ammonia essentially free of CO ₂ are removed from the top of the column.

From the can section of column 35, 5.2% by weight of NH ₃ and
A solution containing 4.2% by weight of CO ₂ is removed. This solution is heated to 110° C. in a heat exchanger 42 and sent from there to column 41. The vessel section of this column is maintained at 115° C. via the heat exchanger 47 of the washer 20 at a pressure of 1.7 bar (absolute). The NH ₃ and CO ₂ completely removed from the solution in the column are fed back into the circulation via line 44. A solution containing <0.1% by weight of NH ₃ is taken off from the tank part of the column and is passed through heat exchanger 4.
2, 34 and 53 to 20° C. and is used as a washing solution in column 49 to wash out the carbon dioxide. 90 kg of this solution is removed from the system via conduit 52.

[Brief explanation of drawings]

The drawing is a system diagram of an apparatus for carrying out the method of the invention. 1... Solver, 4... Fluidized bed reactor, 6... Gas compressor, 11... Gas filter, 13... Condenser,
16... Separator, 18... Blower, 20... Washing tower, 23... Separator, 27... First absorber, 30
... second absorber, 35 ... desorption tower, 41 ... tower,
49...Gas cleaning tower.

Claims (1)

[Claims] 1. Catalytic reaction of urea at high temperature, fractional condensation of melamine from the synthesis gas, washing and cooling of the melamine-free synthesis gas with a urea-containing melt whose temperature is kept slightly above the melting point, and removing at least the heat. A portion is removed by means of a cooling element arranged in the urea scrubber, the temperature of the refrigerant flowing through the cooling element being at least 3 °C lower than the temperature of the melt, mainly ammonia and A gas mixture consisting of carbon is heated while intimately mixed in a solvent.
Partial absorption of the gas mixture with a contact time of 0.1 seconds, separation of the resulting solution from the unabsorbed gas, which consists primarily of carbon dioxide, and heating of the solution containing excess ammonia in two steps. In the first desorption step, ammonia present in a molar ratio of at most 3 parts of ammonia to 1 part of carbon dioxide is expelled by heating, and together with the expelled ammonia. The carbon dioxide entrained in the first
The solution removed from the desorption step is further heated in a second desorption step to remove ammonia and carbon dioxide, and the solvent-containing gas produced in this process is subjected to a new desorption step together with a new gas mixture to be worked up. and a urea-containing melt necessary for cooling melamine-free synthesis gas in a method for producing melamine in which the bottoms extracted from the second desorption step are used as a solvent during absorption after removing excess water. is introduced above and below the cooling member, using the bottom liquid from the second desorption process as the refrigerant, and heating the refrigerant flowing through the cooling member to a temperature of 115 to 135°C, and the heated refrigerant is The refrigerant is introduced into the second desorption step, and the can temperature in this step is maintained at 105 to 120°C, and the heated refrigerant is at least lower than the can temperature in each case.
Before using the withdrawn bottoms, which have a temperature 10°C higher and are substantially ammonia-free, as a solvent in the absorption process, the gases mainly consisting of carbon dioxide that have not been absorbed after cooling the bottoms are removed. A method for producing melamine characterized by its use for cleaning.