Platinum Metals Rev., 1962, 6, (1), 9
A High-Temperature Reactor for the Synthesis of Hydrogen Cyanide
The Andrussow process for the synthesis of hydrogen cyanide has become well established by now. It employs as catalyst platinum alloy gauzes through which a mixture of air, ammonia and methane is passed, enabling the following reaction to take place on the metal surface:
Yields of 60 to 70 per cent based on methane and of 60 to 65 per cent based on ammonia have been reported; approximately a third of the ammonia fed into the process is recovered as an aqueous solution of ammonium sulphate or bisulphate.
Higher yields of product may be obtained by the use of only ammonia and methane which react according to the following equation:
In the presence of a platinum catalyst the reaction proceeds rapidly at 1100 to 1200c0, yielding hydrogen as a by-product. The energy required by this highly endothermic reaction must be supplied by transfer through the walls of the reaction chamber, and the development of a special tubular furnace enabled the process to be translated to a commercial scale. The reaction takes place in alumina tubes, approximately 2 metres long, 15 mm internal diameter, with thirteen such tubes bundled together and enclosed in a gas-fired furnace compartment.
One furnace comprises eight such compartments. The tubes are mounted in a header located in the top of the furnace which also incorporates a highly efficient water-cooled heat exchanger to cool the individual hot gas streams rapidly to a temperature below 300°C. The gas streams are combined into a manifold after cooling. The lower ends of the reactor tubes leave the furnace compartment through its floor and, to compensate for thermal expansion during heating and cooling, are connected via flexible couplings to a gas distributor.
Coke-oven or similar gas with pre-heated air is fed to three pairs of diametrically opposed burners in the furnace compartment. Air pre-heaters are placed in the flue ducts which conduct the waste gases at 1000°C to a waste-heat boiler.
The catalyst completely coats the inner wall of each reactor tube, and although complex in nature, can be readily produced and applied to the tubes in a single procedure and requires no periodic regeneration. It contains 70 per cent platinum. Almost complete recovery of platinum from spent reactor tubes is obtained after a life of about twelve months.
A furnace with eight chambers achieves an output of 50 tons of hydrogen cyanide per month with an efficiency of 87 to 90 per cent based on methane, and 82 to 85 per cent based on ammonia. Approximately 10 per cent of the ammonia feed stock is recovered as ammonium sulphate.
Product gas leaving the reactor tubes has the following typical volume composition:
Unreacted ammonia is removed by washing with sulphuric acid. Hydrogen cyanide is obtained either as an aqueous solution by washing with water and subsequent distillation, or by direct reaction to form further products, such as alkyl cyanides and cyan-hydrins.
For a given output of hydrogen cyanide this process employs less than one-third of the volume of reactant gases used by the Andrussow process. This leads to a corresponding reduction in the size of the absorption equipment.
The tail-gas of the process consists largely of hydrogen (97 per cent V/V). If carbon monoxide is present in the methane employed as feedstock, it will still be present in the tail-gas and must be removed by conversion to methane if the hydrogen is to be used in hydrogenation processes or other applications which are sensitive to its presence.