Chemicals - Pharmaceuticals > Prussic acid production

Hydrogen cyanide is produced on a large-scale predominantly according to the Andrussow process based on an exothermic reaction of CH4, NH3 and O2. Relative to pure methane, the yield is around 7 vol% HCN.

With a yield of around 23 vol% HCN, the BMA process developed by Degussa / Evonik, which is based on direct conversion of CH4 and NH3 to HCN, is much more efficient in this respect. Because of the strongly endothermic character of the reaction with 251 kJ/mol, however, this process requires the permanent supply of heat and therefore a medium to transfer heat into the reaction chamber.

In real BMA processing plants, shaft furnaces with tubes made of high-purity, dense-sintered Al2O3 ceramic, which have a diameter measuring in the order of 25 mm and a length of around 2.5 m. On their inside surface, these tubes are coated with platinum as a catalyst.

The production of HCN begins with the introduction of educts into the ceramic tube, which is heated to approx. 2000 °C at the beginning of the reaction zone. The temperature at the end of the zone is around 1100 °C. This gradient lies over around 65 % of the tube length. The reaction itself takes place above 1200 °C.

Al2O3 ceramic tubes suitable for the BMA process exhibit the following properties:

  • Purity of the material >99.5 %
  • Minimum density of the material of 95 % of the theoretical density
  • High mechanical strength
  • High thermal conductivity
  • Thermal resistance with around 2000 °C
  • Resistance with high thermal gradients
  • Thermal shock resistance
  • Corrosion-resistant microstructure
  • No or at most low particle size growth at high temperature
  • Low creep tendency
  • Uniformly thin walls
  • High degree of roundness in the tube cross-section
  • High degree of straightness
  • Dimensional stability at high temperature
  • Suitability of the inner tube surface for the reliably bonded coating with platinum

The Al2O3 ceramic tubes available for the BMA process on the market today have been extensively optimized with regard to their microstructural properties. Service lifetimes of several years in continuous operation are therefore not unusual in these tough application conditions.

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