DE1226112B - Process for the continuous production of melamine - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY GERMAN Wed PATENT OFFICE Int. Cl .:

C07d

EDITORIAL

German class: 12 ρ - 10/05

Number: 1226 112

File number: S 80520 IV d / 12 ρ

Filing date: July 20, 1962

Opening day: October 6, 1966

The invention relates to an improved method for the continuous production of melamine by heating urea at substantially atmospheric pressure in the presence of ammonia and a catalyst.

From US Pat. No. 2,760,961 it has become known that melamine can be obtained from urea or its pyrolysis products in a good yield by adding the starting products together with a silica gel with a large internal surface area as a catalyst in the presence of ammonia to a temperature of 220 to 400 ^{Heated to 0} C; the catalyst is brought into the state of a fluidized bed by ammonia and the starting material is introduced into this fluidized bed in a suitable manner. The melamine formed leaves the fluidized bed, ie the catalyst, together with ammonia. A fundamental disadvantage of this process is the movement of the entire catalyst in the fluidized bed, with the consequence of strong abrasion, so that considerable amounts of catalyst per unit amount of melamine produced must always be renewed; A further disadvantage is the short residence time of the reaction starting materials or intermediates on the catalyst, as a result of which they break through the fluidized bed in part unreacted, which accordingly reduces the melamine yield and makes expensive separation operations necessary when the melamine is condensed or on the condensed sublimate.

The above-mentioned process has been improved, as was set out in German Auslegeschrift 1102 165, to the effect that, as part of the conversion of urea to melamine, which takes place in several steps, the conversion of urea to cyanamide takes place in a fluidized catalyst bed while the heat is supplied Reaction cyanamide - * ■ Melamine runs off in a downstream catalyst bed, to which the cyanamide-melamine vapor mixture produced in the fluidized bed is fed together with the ammonia serving as the carrier gas for the fluidized bed. The melamine that forms in the catalyst rest layer sublimes with the carrier gas ammonia from the catalyst and is obtained by condensation at a correspondingly low temperature.

The latter process has the advantage over the former that the residence time of the reaction starting materials and intermediates on the catalyst in the downstream catalyst rest layer, which is necessary for a useful conversion, can be adjusted by appropriately dimensioning the two units. Furthermore, through the on-process for the continuous production of
melamine

Applicant:

South German calcium cyanamide works

Corporation,

Trostberg (Obb.)

Named as inventor:

Dr. Ernst Doehlemann, Tacherting;

Dr. Franz Kaess, Traunstein;

Dr. Leo Reitter,

Dr. Kurt Scheinost, Tacherting

given measures, the most favorable apparatus for heat transport can be selected.

The process, urea in a catalyst fluidized bed with a downstream catalyst rest layer in Converting melamine, however, is still flawed. Most importantly is the use of Catalysts, as they are suitable for melamine synthesis, are problematic at all in the fluidized bed, because these catalysts are so poorly abrasion resistant that they rubbed into dust in a very short time will. This requires a great deal of catalyst material and the need to use the resulting To separate catalyst dust from the gas stream before it enters the catalyst resting layer, otherwise blockages are inevitable there. Those that still behave most favorably in terms of abrasion Silica gels have the unpleasant property that they shatter when liquid urea is absorbed are, with which a rapid wear of the catalyst material is given again. The same Objections also apply to a greater extent to the process which only works in the fluidized bed. It should also be noted that when urea is introduced into a fluidized catalyst bed For example, from silica gel with a large internal surface, only a part is converted into cyanamide in a short time becomes, which evaporates; the rest of the urea converts to melamine in the catalyst, which is bound quite tightly by the catalyst and only released again slowly. With heavy impact the fluidized bed with urea in continuous operation can now happen that in the Time unit more melamine is formed on the catalyst

609 669/417

is, as sublimated, d. i.e., the catalyst material in the fluidized bed gradually saturates itself completely with Melamine. As a result, urea evaporation in the direction of cyanamide is no longer possible is and incrustations from urea decomposition to cyanuric acid and other difficult to evaporate Intermediate products occur, which paralyze the fluidized bed in a very short time.

Apart from these difficulties, there is a series connection of two after different Principles of working catalyst layers have another disadvantage: due to fluctuations in the catalytic effectiveness of the fluidized bed, because of the short residence time of the reaction medium is very sensitive to all influences, the stress on the fixed bed catalyst fluctuates likewise seTir considerably what the observance of a constant temperature in the fixed bed, especially when carried out on an industrial scale difficult.

After all, it is the catalysts that are probably not very abrasion-resistant, but still hard, cause in the fluidized bed severe erosion, so that the service life of the apparatus must be estimated as low.

The invention is based on the object in the continuous production of melamine by heating urea with ammonia. in a fluidized bed of inert material to at least 250 ⁰ C and passing the so. generated ammonium cyanate vapor via a downstream catalyst bed at 260 to 450 ⁰ C, with the lowest consumption of catalyst, optimal. . To achieve melamine yields.

The object set is achieved according to the invention in that the reactions mentioned are carried out in a device made of copper and aluminum oxide or silicon dioxide, lithium dioxide, zirconium dioxide, vanadium pentoxide, chromium oxide, iron oxide or carbon is used as the catalyst resting layer, with the proviso , -that these substances with phosphoric acid, boric acid, arsenic acid ,. Sulfuric acid or with a salt of these acids are activated.
. In the process according to the invention, melamine is obtained in accordance with the equation

6 urea - »1 melamine -f 3 CO ₂ + 6 NH ₃

45

up to 96 ° / ₀ obtained in a yield.

From the German patent specification 1 028 126 it is known that one. obtained by passing vapors of cyanuric acid along with added ammonia over silica gel as a catalyst in a quartz tube melamine with 73 ° / _o yield. However, it could not be foreseen that when amniotic cyanate vapor is passed over the catalyst together with ammonia, melamine is then formed with a yield of 96% if a phosphate, borate, proposed by the inventor is used as a catalyst; Arsenate- or sulfate-containing aluminum oxide, silicon oxide, titanium dioxide, zirconium dioxide, vanadium pentoxide, chromium oxide, iron oxide or carbon catalyst used (cf. Belgian patents 604 973 and 607 305).

To evaporate the urea, a means can Ammonia produced fluidized bed from at temperatures Above 250 ° C, resistant to urea and ammonium cyanate or cyanic acid, abrasion-resistant and if possible non-porous inert material ver ^ are used, in which the urea is entered in a suitable manner, as per se from the 'US A. Patent 2 712 491 is known. It has been found that particularly useful as an inert material for the fluidized bed Copper grains are well suited, especially because they are soft and therefore the material of the apparatus not, erode. "=

The main difference in using an inert material instead of a catalyst in the The fluidized bed consists in the fact that in the former case the urea is merely rearranged to form ammonium cyanate learns which evaporates, while in the latter case A conversion in the direction of melamine already takes place in the fluidized bed.

By using inert materials in the fluidized bed, which only play the role of a heat carrier have to take over and are much more abrasion-resistant and generally cheaper than those for the pressureless conversion of urea in melamine suitable catalysts, is achieved that the expensive Catalyst only has to be used in the form of a resting layer. Accordingly, come in handy there are no catalyst losses and there are no operating costs for the catalyst. Furthermore .. it is not necessary between the fluid bed evaporator and Fixed bed reactor, a filter or a dust collector to be built in.

The use of inert material instead of a catalyst in the fluidized bed has, in addition to the already mentioned, but also the following essential advantage: the paralyzing of the fluidized bed by intermediate products that are difficult to evaporate, which becomes possible with the already mentioned blocking of a catalyst fluidized bed by melamine, including melamine in general no: more residue-free vaporizing compounds biuret, cyanuric acid, ammelide and ammeline: are to be added, is completely avoided when using an inert material, as only highly volatile ammon cyanate is formed. : ..-

After the urea evaporation to ammonium cyanate is very heat consuming, a fluidized bed with its high heat transfer is a suitable one Apparatus for this process step; on the other hand, the implementation of ammon cyanate vapor is too Melamine is slightly exothermic and can therefore be easily controlled in terms of heat in a fixed catalyst bed. · ■ -

Any reactor is suitable for the fixed catalyst bed, which allows the reaction of ammonium cyanate vapor or cyanide vapor to melamine dissipate heat released at the catalyst, d. H. each reactor in which the catalyst room is designed as a heat exchanger. The following therefore come into question: tube reactors with accommodation the catalyst in the tubes; Catalyst beds with embedded cooling coils, storage of the catalyst on several trays arranged one above the other, the trays at the same time Represent heat exchange surfaces, and other known constructions.

The temperatures in the catalyst layer are rest to maintain 260-450 ° C, preferably between 320 and 400 ⁰ C,.

The amount of catalyst is expediently chosen so that the weight ratio of catalyst to abandoned The amount of urea per hour is greater than 1, preferably greater than 5. -

The amount of ammonia required for the process (to operate the fluidized bed, as possible

Reaction partner and for subliming off the melamine formed on the catalyst) depends on the amount of urea to be enforced, namely should · the weight ratio of ammonia to urea at the inlet in the catalyst bed is more than 1, preferably more than 1.5.

In order to obtain melamine yields of '96 ° / ₀ by the process according to the invention, the special catalysts are to be used; Namely substances containing phosphate, botate, arsenate or sulfate, otherwise consisting entirely or predominantly of aluminum oxide in free or chemically bound form; or catalysts containing phosphoric acid, boric acid, arsenic acid or sulfuric acid based on finely divided silicon dioxide, titanium dioxide, zirconium dioxide, vanadium pentoxide, chromium oxide, iron oxide or carbon, which have no internal surface. Prepare by the method of the invention, but with silica gel as catalyst, the yield is only 88.2 ° melamine / _0th

The ammonium cyanate vapor occurring as an intermediate in the process according to the invention is highly corrosive and can be very strong due to the catalytic influence of metals or alloys Measures are caused to undesirable, yield-reducing side reactions. About the same Conditions are given in the pressure synthesis of melamine from urea, where ammonium cyanate is also used Intermediate occurs. For this pressure synthesis there are already different materials than to some extent been called useful. Most of them will, however, still corrode and catalyze Side reactions of ammonium cyanate or cyanic acid; are the really suitable materials, such as B. the titanium, very expensive.

Due to the established very good usability of copper grains as inert material in the fluidized bed copper was also used as an apparatus material for the Tested method according to the invention and surprisingly found that it is neither corroded nor catalyzes possible side reactions of the ammonium cyanate. It is therefore essential to the invention in which Carrying out the process, all parts of the apparatus which contain ammonium cyanate, ammonia and melamine come into contact to make of copper. Essentially including the fluidized bed, the Understand the reactor and the connecting pipes. About the same melamine yields as in a copper apparatus could only be obtained in an apparatus made of fused silica. Made of fused silica but do not manufacture equipment on an industrial scale. When using other common materials the melamine yield drops considerably, as the comparative experiments show.

example

The apparatus consists of an externally heated fluidized bed evaporator, which is connected via a pipeline to a vertically positioned reaction tube which can be heated and cooled from the outside and in which 900 parts by weight of catalyst are accommodated. The reaction tube which is flowed through from top to bottom is the gas outlet side with a maintained at 200 ⁰ C by means of steam Melaminabscheider. With the exception of the melamine separator made of aluminum, copper is used as the material. The fluidized bed is filled with sea sand with a grain size of 0.1 to 0.3 mm. Finely divided silicon dioxide (diameter of the primary particles 20 mm), which _{is activated by applying 10 ° / 0} H ₃ PO ₄ , is used as the catalyst. The temperature in the fluidized bed is 375 ° C, in the catalyst bed 340 to 360 ° C. to fluidize the lake Andes and simultaneously as the carrier gas is blown hourly Nl 300 to 350 ⁰ C preheated ammonia in the fluidized bed evaporator. 150 parts by weight of molten urea are injected every hour into the fluidized bed created in this way. Within 72 hours reach this example 10 800 parts by weight of urea to implementation. "In Melaminabscheider fall within this time 3647 parts by weight sublimate with a melamine content of 99.5 ° / ₀ in. This corresponds to a melamine yield of 96% based on the introduced urea.

Comparative experiments

Attempt 1

The experiment described in the previous example was carried out using silica gel as the catalyst. In 65 hours 3074 parts by weight sublimate were obtained with a melamine content of 97.9 ° / ₀ obtained from 9750 parts by weight of urea. This corresponds to a yield of 88.2 ° melamine / _0th This experiment shows that a significantly better melamine yield can be obtained by the process according to the invention than by that described in German patent 1,028,126.

Attempt 2

The experiment described in the example was repeated using other equipment materials, the copper was replaced by ordinary steel, Remanit HC, V4A or Rotosil (Fused silica). The melamine yields obtained in the individual experiments are shown below Table juxtaposed with the melamine content of the sublimates.

Material used

copper

V4A

Ordinary steel

Remanit HC

Rotosil (fused silica).

Melamine yield
in %

96.0
72.4
56.6
40.4
94.5

Melamine content of
Sublimates

99.5
85.7
77.2
61.8
99.6

As can be seen from the table, the method according to the invention is very beneficial in a copper apparatus good melamine yields, while using other materials - with the exception of the large-scale Rotosils that cannot be used - · the melamine yield drops sharply.

The comparison of the comparative tests with the example shows the technical progress of the process according to the invention for the pressureless production of melamine from urea compared to the previously known method.

Claims (2)

Patent claims: 1. A process for the continuous preparation of melamine by heating urea with ammonia in a fluidized bed of inert material to at least 250 ⁰ C and passing the so Ammoniumcyanatdampfes generated via a downstream catalyst bed rest at 260 to 45O ⁰ C, characterized in that the device in which these conversions are carried out is made of copper and the catalyst rest layer is made of aluminum oxide or from silicon dioxide, titanium dioxide, zirconium dioxide, vanadium pentoxide, which has no inner surface, Chromium oxide, iron oxide or carbon, with the proviso that these substances activated with phosphoric acid, boric acid, arsenic acid, sulfuric acid or with a salt of these acids are. 2. The method according to claim 1, characterized in that in the fluidized bed as an inert material Copper grains can be used. Considered publications: U.S. Patent No. 2,712,491; French Patent No. 1261271; Gmelins Handbuch der inorganic Chemie, Kupfer, Part A, 8th edition (1955), p. 1221, last Unit volume. 609 669/417 9.66 ® Bundesdruckerei Berlin