CH644331A5 - Process for preparing hydrogen cyanide - Google Patents

Description

The invention is illustrated by the following examples:

The tests were carried out in a conventional BMA apparatus - which consists of a device for gas metering and gas mixing as well as a gas-heated reaction furnace, burner, burner chamber and the ceramic tube that opens into a tulip.

Comparative attempt

A gas of the following molar composition is introduced into the described BMA apparatus:

Methane to ammonia = 1: 1.1, and a short distance to 1300 ° C at about 1 bar abs. heated.

After passing through the reaction tube, the product gas mixture formed was cooled in a known manner to temperatures below 400 ° C. and above 30 ° C. in the furnace head. The yield was 82.7 mol% of hydrogen cyanide, based on the ammonia used, and 91 mol%, based on the methane used. After the absorption of unreacted Am-5 moniak in sulfuric acid and hydrogen cyanide in aqueous sodium hydroxide solution, the residual gas had a composition of 96% by volume of hydrogen, 1% by volume of nitrogen and 3% by volume determined by gas chromatography. Methane.

Example 1

The gas mixture used there is also used in an apparatus which is basically the same as in the comparative experiment, with the only difference that the reaction tube is filled to a height of about 140 cm with a bed of aluminum oxide bodies, some of which are coated with catalyst (platinum). The pressure in front of the pipe is approx. 1.3 bar.

With otherwise identical ratios and the same molar composition of the starting gases «methane to ammonia» of 1: 1.1, the yield was 94.2%, based on the methane used, and 85.6%, based on the ammonia used.

The composition of the residual gas was in volume after the absorption carried out analogously to the comparative experiment

96.7V% H2 1.9V% CH4 1.3V% N2

Example 2

Analogously to Example 1, the tube is replaced with 50 ml of aluminum oxide balls with a diameter of up to max. 3 mm filled and the reaction was carried out at a pipe pressure of approx. 1.1 bar.

The yield was 95.3%, based on the methane used, and 86.6%, based on the ammonia used. The residual gas analysis showed volume

1.2 V% N2 97.2 V% H2 1.54V% CH4

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Claims (3)

644331 1. A process for the preparation of hydrogen cyanide or hydrocyanic acid from hydrocarbon and ammonia by the hydrocyanic acid / methane / ammonia process, characterized in that a gas mixture of ammonia and at least one aliphatic, short-chain hydrocarbon is passed through a reaction tube in a non-laminar flow, which begins at 10 d from the start of the pipe, where d means the pipe diameter, and which continues as far as possible to the end of the pipe, whereupon the product gas obtained is worked up. 2. The method according to claim 1, characterized in that one prevents the formation of a laminar flow through stationary internals or packing in the reaction tube. 3. The method according to claim 2, characterized in that one uses stationary internals, packing or fillings, which are completely or partially covered with catalyst on their surface. 4. The method according to claim 1, characterized in that one prevents the formation of a laminar flow by using the gas mixture itself to produce and maintain a fluidized bed made of heat-resistant material, preferably the material of the reaction tube, inside the tube. As is known, the so-called BMA process for the production of hydrogen cyanide or hydrocyanic acid starts from methane and ammonia and works without the presence of oxygen or air. (BMA = hydrocyanic acid / methane / ammonia). The reaction itself is carried out in hanging reaction tubes made of sintered aluminum oxide, which are coated with a platinum catalyst on the inside. The connection between these ceramic reaction tubes and the gas distribution is expediently made via a copper tulip, see. e.g. Fig. 4 and DE-PS 959 364. Since the reaction is endothermic, the reaction tubes are heated and reaction temperatures around 1300 ° C are obtained. In order to prevent the back reaction from occurring, the resulting gas mixture containing hydrogen cyanide must be rapidly cooled to temperatures below 400 to 300 ° C, which takes place in a water-cooled aluminum chamber in the furnace head itself, see. Ulimann Encyclopedia of Technical Chemistry, 4th Edition, Vol. 9, Page 659; Dechema - Monografie, 1959, issue 33, pages 28 to 46 and also DE-PS 959 364. According to DE patent application 29 13 925.1-41, it has also been proposed to use liquid gases instead of methane. When methane is used, the yields of hydrogen cyanide are around 85% by volume of theory, based on the ammonia used, and around 90% by volume of theory, based on the methane used, see FIG. Ullmann loc. cit. Without significantly reducing the throughput per tube and hour, higher yields seemed no longer achievable with the BMA process. However, they are already much higher than the corresponding yields in the technically important Andrussow process, see. Ullmann loc. cit. The unreacted ammonia cannot be recovered and must therefore always be in some form, e.g. with dilute sulfuric acid as ammonium sulfate, are removed from the product gas stream. This requires additional effort and also additional costs that can only be partially covered by the proceeds from the ammonium sulfate. It is an object of the present invention to provide a process which makes it possible to increase the yield of hydrogen cyanide in the hydrocyanic acid / methane / ammonia process and thus to reduce the proportion of ammonia in the product gas. It has now been found that the yield of hydrogen cyanide in the so-called hydrocyanic acid-methane-ammonia process can be increased if a gas mixture of ammonia and aliphatic, short-chain hydrocarbon, preferably methane, is passed through the tube in a non-laminar flow, which leads to 10 d, preferably 15 d, begins from the start of the pipe, where d means the pipe diameter, and which lasts as far as possible to the end of the pipe, whereupon the product gas obtained is worked up in a conventional manner. In addition to methane, aliphatic hydrocarbons with 3 and 4 carbon atoms, that is, the liquid gases propane, n- and i-butane, are also suitable as hydrocarbons, with the use of the liquid gases additionally requiring hydrogen, in an atomic ratio of C: N : H from 1: 1: 7.1 to 1: 1.33: 13, s. DE patent application P 29 13 925.1-41. Technically used for. At the moment, preference is given to methane, which is both pure and in commercially available form, e.g. can be used as natural gas or refinery gas. When using natural gas and refinery gas, gases with a total methane content of 60 to almost 100% by volume are preferred which contain only small amounts of higher hydrocarbons, in particular aromatics, and whose secondary constituents are mainly nitrogen and / or hydrogen. The reaction gas mixtures of the hydrocarbon and ammonia can be prepared in a known manner. In order to be able to estimate the heat transfer from the pipe wall to a fluid, one uses the known dimensionless key figures, see. Ullmann, Encyclopedia of Techical Chemistry, Vol. 2, p. 455 ff., Where the Reynolds number describes the flow, while the Nussel number is a measure of the heat transfer. Flow (Reynold) and heat transfer (Nusselt) are linked with each other via the Prantl number. The Reynolds number 2300 is known to be the critical number in the case of an irregular pipe inlet, in which the laminar flow changes into a turbulent one, see Ullmann, Vol. 1, 3rd edition, 1951, p. 19). If the Reynolds number is below the critical point, the fluid flows in a laminar manner. The heat transfer between the fluid and the wall is worse in the case of a laminar flow than in the case of turbulence, which is why the aim is to generate as strong a turbulent flow as possible within the BMA pipe, thus shortening the heating distance by improving the heat transfer from the pipe wall to the gas but the reaction distance is lengthened and thus the yield of hydrogen cyanide is significantly improved compared to that in the laminar flow-through empty tube, see. «I & EC Proc. Of. & Developm. » Vol. 12, No. 4, 1973. It is particularly important that the laminar flow form is counteracted, which so far has occurred in the ceramic reaction tube of the BMA process from the end of the short start-up section due to the increase in the viscosity of the gas mixture as a result of the heating after passing through a short tube section. The non-laminar flow, which according to the invention begins at 10 d, preferably 15 d, from the beginning of the reaction tube and continues as far as possible until the end of the tube, can be generated by stationary internals made of heat-resistant material, which can be in any desired geometry. 5 10th 15 20th 25th 30th 35 40 45 50 55 60 65 2nd PATENT CLAIMS 3rd 644331 Filler layers or fluidized beds made of grains of a heat-resistant and abrasion-resistant material, the latter being generated by the gas mixture itself, are also suitable. The surface of the fillings or internals can be completely or partially covered with catalyst. According to DE-PS 1 008 719, 1 058 485, 1 068 681 and 1 161 870 it is known to carry out the reaction of ammonia and hydrocarbon to hydrogen cyanide in a fluidized bed. However, according to the purpose of the method according to the invention, it was important to achieve an increase in yield while maintaining the essential plant parts of the BMA method, such as furnaces, cooling heads and tube dimensions. It is now possible to use the starting materials and reaction gas mixtures customary hitherto, and the reaction tubes customary per se, and above all while maintaining the throughputs of more than 20 moles of hydrocarbon, preferably methane, per tube and hour, to obtain a yield of up to 95 wt .-%, based on the hydrocarbon in question, in a technically simple manner. Such an increase in the unchanged yields obtained over decades was completely unpredictable.