Andrussow process

The Andrussow process is a process for the industrial production of hydrocyanic acid (HCN) from methane, ammonia and oxygen, and is named after its inventor, Leonid Andrussow named.

Thermodynamics

The reaction of a mixture of methane and ammonia platinum - networks, with simultaneous injection of pure oxygen is highly exothermic and leads to HCN at elevated temperatures.

The reaction enthalpies are calculated from the enthalpies of formation of the gaseous reactants methane ( -74.8 ), ammonia ( -46.1 ), hydrogen cyanide ( 135.1 ) and water ( -241.8 kJ / mol).

Andrussow pointed here by detailed that in the first step ammonia is oxidized to a as " nitroxyl " (ENT) designated intermediate and this reacts with methane in subsequent steps with elimination of hydrogen. Because less side reactions (such as formation of carbon monoxide ) has an excess of oxygen of 0.3-0.4 moles be used, but the oxidation reaction product, conversely, the hydrogen to water.

The process temperature is about 1100 ° C, instead of oxygen is used industrially air as carrier gas for methane and ammonia. At the overall reaction numerous upstream equilibrium reactions are involved with high activation energies.

Process design

Liquid ammonia is first evaporated and then mixed with the gaseous methane and air feed. The mixture is preheated and then introduced into the reactor, where a portion of the methane with oxygen and burns, heat is released during the formation of hydrogen cyanide from methane and ammonia consumed heat. The residence time is very short. The hot gas from the reactor is immediately passed through a heat exchanger for cooling is produced in the steam for energy recovery. The cooled gas is then moved to an acid wash, is removed in the excess ammonia. After the removal of ammonia, the hydrocyanic acid is removed by washing with cold, slightly acidic water. The hydrocyanic acid dissolved in the water is thereafter separated by rectification from the water.

Catalyst

The catalyst is composed of networks of platinum, which is doped with rhodium. Platinum is the actual active component, while rhodium ensures long-term stability. The catalyst has reached its optimal activity only after a longer period of activation under the reaction conditions. During this period, the catalyst surface area is increased by Verrauhung.

Reaction mechanism on the catalyst surface

Grabow et al. postulated the following reaction mechanism for the formation reaction of hydrogen cyanide. In their computational model mean x, y = 0,1, or 2 X * represents an adsorbed on the catalyst species and * for a vacancy on the catalyst surface:

In this reaction scheme, the decomposition of CH4 is fast, so that the active board surface has a high degree of coverage on carbon. The decomposition of NH3 is obstructed by carbon on the surface and is therefore slow. As a result the rate of formation of N2 and HCN by the decomposition of NH3 is limited.

Application

Cyanide is currently produced on a large scale for the most part by the Andrussow process and mainly used as a precursor for the production of polyamide 66 (nylon) and polymethyl methacrylate (Plexiglas ). Further use on an industrial scale is carried out for the production of NaCN, which is used in large quantities for the cyanide in the extraction of precious metals from ores with low metal contents.

In addition, HCN is produced by the BMA process and Degussa process after the cleavage of formamide from BASF.

When ammoxidation those procedures are implemented instead of methane, other hydrocarbons to nitriles.