DE1138785B - Process for the production of melamine. - Google Patents

Description

The conversion of urea and its pyrolysis products into melamine can be known under Pressure, mostly without the use of catalysts, or without pressure on catalysts with large internal Surface, such as silica gel or clay. The transformation takes place after equation

6 CO (NH ₂ ) ₂ - * * melamine +

6 NH ₃ + 3 CO ₂ .

In the procedure without the application of pressure, urea, e.g. B. with silica gel, to temperatures Heated above 300 ° C, forming melamine, which is isolated by extracting the silica gel with water can be.

Another method of obtaining the melamine is to use e.g. B. on heated. Silica gel melted Let urea drip on, while simultaneously preheated ammonia through the catalyst flows and discharges the melamine that forms in vapor form. By cooling the vapors Melamine and ammonium carbamate are deposited as a sublimate.

It has also been proposed to first heat urea or one of its pyrolysis products without any flushing gas with silica gel until NH ₃ and CO ₂ are released and in a second operation to sublimate the _{melamine formed from this reaction mixture with NH 3.}

If you work according to one of these rules, it turns out that you get a melamine product, which, apart from ammonium carbamate, has significant amounts of water-soluble and water-insoluble by-products contains. Correspondingly, the yield of melamine, based on urea, is also included at most 75 to 80% of theory, usually even considerably less.

The melamine obtained must therefore be subjected to further purification, for example by recrystallization will. The resulting by-products are either lost or have to be dried and be reacted again with a catalyst.

Finally, a method is also known for converting urea into melamine without the application of pressure, in which the urea is first heated to 130 to 150 ° C. in a rapid stream of ammonia, thereby splitting off water; the intermediate product is then heated again in a rapid stream of ammonia at 220 to 300 ⁰ C, which is converted to melamine, which is removed by sublimation with the ammonia. A major disadvantage of this process is that very large amounts of ammonia are required to use this in processes for the production of melamine

Applicant:

South German calcium cyanamide works
Aktiengesellschaft, Trostberg (Obb.)

Dr. Ernst Doehlemann, Tacherting,
Dr.-Ing. Franz Kaess, Traunstein,

and Dr. Kurt Scheinost, Tacherting,
have been named as inventors

the water released in the first reaction stage, which is removed as soon as it is formed must dissipate. The ammonia requirement is 70 to 100 mol of anhydrous ammonia per 1 mol of reacted Urea. This makes this process, although there are good yields of melamine and quite a To deliver a pure product is very costly.

The process according to the invention for the preparation of melamine consists in that urea and / or its pyrolysis products in the presence of a catalyst with a large internal surface and in the presence heated by supplied ammonia, and is characterized in that in a first stage Urea is heated at 200 to 400 ° C in the presence of only so much added ammonia that the Melamine formed is not discharged, and that one then in a second stage at 250 to 400.degree removes the melamine located on the catalyst by means of a reinforced Ammomaksitromes and carries it through Cooling the gas separates as a sublimate, with the amount of ammonia used in the second stage is preferably 50 to 150 times greater than the amount used in the first stage.

The advantage of this two-step process is apparently based on the fact that the urea and the Synthesis of various intermediate products, such as biuret, cyanuric acid, ammonium cyanate inter alia, due to the fact that the small amount of ammonia added practically no flow in the reaction material caused, opportunity is given, in the first stage in intimate contact with the catalyst, for example as an adsorbate, largely in melamine

209 679 / 2Ϊ7

to convert, which is stripped from the catalyst only in the second stage at mostly higher temperatures and greater NH _{3 speed.}

In the case of the known one-step process, on the other hand, parts of the intermediate connections are found not enough time to react to the melamine because it is a result of being economical Sublimation speed high gas flow are discharged together with the melamine. Accordingly, it should be most expedient be able to work in the first stage - as already known - without any gas supply, what however, as the following examples also show, this is not the case.

This is because it has also been found that melamine already formed and also its precursors at the reaction temperatures in question are reversed _{by the NH 3} : CO _{a mixture formed as a by-product during the reaction}

Melamine was 93% of theory.

Example 2

a) That also in the continuous, one-step process A series of tests showed that a high purity of the melamine and a large yield cannot be achieved in a vertical, heatable quartz tube

drove from NH ₃ or other gases ^{heated to 320 0} C within 3 hours and left at this temperature for 30 minutes. The material treated in this way was subjected in the same glass tube apparatus as described above to a treatment _{for 17 hours with 50 liters of NH 3} per hour at 320 ° C. to sublimate the melamine. An average of 96% melamine was obtained with a yield of 69%.

c) The repetition of the test series b) with a nitrogen supply of 0.51 per hour in the The first hour resulted in an abortion in the glass apparatus under otherwise identical conditions 94% melamine with a melamine yield of 65% of theory.

d) A further series of tests was carried out under the same conditions as in test series b) driven in two stages, but according to the invention in the first stage 0.51NH per hour through the with

Formation equation to be split again in part, 20 urea-laden catalyst was passed. That which resulted in a corresponding reduction in the yield of the melamine removed was 99%, which is the result of the yield. This reverse reaction is prevented by adding small amounts of ammonia during the melamine formation in the first stage, because that
resulting NH ₃ : CO ₂ mixture continuously from the 25th
Reaction zone, ie is removed from the catalyst bed.

The advantage of adding ammonia, however, can be
don't exhaust in this flushing effect; because at
a replacement of ammonia by nitrogen in the 30 with 60 mm diameter, in which a silica gel first stage, the melamine yields were housed even worse layer. Urea was applied continuously from above when carrying out the first stage without gassing. The temperature is supplied from the outside. Possibly acts therefore carried 350 ° C. 501 NH ₃ per hour were introduced from above ammonia itself as a catalyst for the conversion, the vapors exiting below condense urea into melamine, or it attacks in up to 35 siert. The best experiment resulted in 106 hours, now unknown, directly as the reaction component duration and an hourly addition of 8 to 9 g nents in one of the simultaneous reactions taking place urea 84% melamine with 79% yield,
a. b) In contrast to the preceding, lets

Another advantage of the process according to the invention after the two-stage process according to the invention is that the 4 ° drive is advantageously carried out as follows.
Formation of melamine and the separation of the same A horizontally arranged rotary tube, which with

Porcelain or earthenware is lined and heated from the inside to 300 ° C by means of a stationary rod heater, carries a closed device at one end for continuous charging with a urea-catalyst mixture. On this side there is also a heated exhaust pipe with a condenser chamber for receiving the NH ₃ and CO ₂ gases formed, which are precipitated in the chamber as ammonium carbamate. Excess NH ₃ can escape from the condensate chamber. At the other end of the rotary tube there is a closed discharge device through which the

A small layer of alumina without urine- 55 melamine-laden catalyst is continuously filled in on the entrance side. The pipe is deployed within an hour. At the same time, ammonia enters the rotary kiln on this side with the introduction of 501 NH ₃ per hour at 320 ° C. The loaded catalyst, heated and with the same NH ₃ -StTOm at this temperature, then reaches the upper part of a tube temperature kept for 14 hours. Is obtained in the middle-shaped, heated to 330 ° C sublimation, of sixteen experiments after removal of the medium-60 where it travels against a 84% sodium MEI from top to bottom a NH ₃ stream entabgeschiedenen Ammoncarbamates which all or only a non-amine with a melamine yield of 75% of the
Theory.

b) In a second series of experiments, the

The same catalyst as in Example 1 with the 65 furnace is the catalyst freed from melamine the same amount of urea in an inclined, discharged and fed back to the rotary kiln;

can drain from the catalyst under different temperature conditions. One is in that Able to maintain the optimal conditions for these partial processes of the reaction.

The following comparative examples provide the best overview of the situation.

example 1

a) In a glass tube with a tube width of 45 mm, which is located in a horizontally positioned furnace, 150 g of alumina, onto which 45 g of urea have been melted, and additionally on the

Absorbs part of the melamine in vapor form and in emits a downstream condensation chamber as a sublimate. At the bottom of the sublimation

elongated glass flask rotating slowly around its axis, which was immersed in an oil bath, without

its loading with urea can take place in the rotary kiln or outside the rotary kiln.

Claims (1)

5 6 Test data area and of added ammonia, as a result (mean values from 48-hour test) ^ ΤΤίΤοηηΐ · Tno ^ ^ ^^ ^ ^ 6 Urea at 200 to 400 ° C in the presence of Drenron mare only heats so much added ammonia that the urea used 1350 g 5 per hour the melamine formed is not discharged, and silica gel used 3000 g per hour that in a second stage at 250 to mean residence time of the solids 180 minutes 4000 C the melamine on the catalyst ammonia stream 50 Nl per minute by means of an increased ammonia stream away - Temperature leads 300 ° C and it separates by cooling the gas as a sublimation stage 10 mat, the amount of ammonia used in the second stage ammonia flow 3700 Nl per hour preferably 50- Average residence time of the solids 180 minutes to 150 mf f ° ßer than the amount in the first stage temperature 300 ° C ^ f%. v2. Process according to Claim 1, characterized in that at least one stage of raw melamine 442.7 g per hour is operated continuously and the catalyst is circulated with the purity of raw melamine .. 99.8%. Yield of pure melamine 441.8 g per hour. 3. Process according to claims 1 and 2, da- = 93.5 of theory, characterized in that a catalyst with melted urea and / or its pyrolysis products is used as the starting material. 1. Process for the production of melamine by Heating of urea and / or its pyrolysis- Documents considered: products under normal pressure in counter-25 German Patent No. 944490; War of catalysts with a large inner upper German Auslegeschrift No. 1028 126. © 209 679/297 10. 62