Andrussow method

Late 1930, semi-technical facility for tests according to the Andrussow method in Herne , operated by Leonid Andrussow .

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

thermodynamics

The reaction of a mixture of ammonia and methane on platinum gauzes with the simultaneous injection of pure oxygen is strongly exothermic and leads to hydrogen cyanide at high temperatures.

${\displaystyle \mathrm {CH_{4}\ +\ NH_{3}\ +\ 1,0\ O_{2}\longrightarrow \ HCN\ +\ 2\ H_{2}O\ +\ H_{2},\ \Delta _{R}\ =\ -228\ kJ/mol} }$

${\displaystyle \mathrm {CH_{4}\ +\ NH_{3}\ +\ 1,5\ O_{2}\longrightarrow \ HCN\ +\ 3\ H_{2}O,\ \Delta _{R}\ =\ -469\ kJ/mol} }$

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

Andrussow demonstrated in detail that in the first step ammonia is oxidized to an intermediate product known as “nitroxyl” (HNO) and that this reacts in subsequent steps with methane to split off hydrogen. Because of minor side reactions (e.g. formation of CO), an excess of oxygen of 0.3-0.4 mol must be used, which conversely also oxidizes the reaction product hydrogen to water.

${\displaystyle \mathrm {O_{2}\ +\ NH_{3}\longrightarrow \ O=NH\ +\ H_{2}O,\ \Delta _{R}\ =\ -111\ kJ/mol} }$

${\displaystyle \mathrm {CH_{4}\ +\ O=NH\longrightarrow \ H_{2}C=NH\ +\ H_{2}O,\ \Delta _{R}\ =\ -152+/-30\ kJ/mol} }$

${\displaystyle \mathrm {H_{2}C=NH\longrightarrow \ HCN\ +\ H_{2},\ \Delta _{R}\ =\ +35-/+30\ kJ/mol} }$

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

Procedure structure

Diagram from 1931 with the Andrussow method

Liquid ammonia is first evaporated and then mixed with the gaseous methane and the air inlet. The mixture is preheated and then fed into the reactor, where part of the methane burns with oxygen and heat is released, while the formation of hydrogen cyanide from methane and ammonia consumes heat. The dwell time is very short. The hot gas from the reactor is immediately passed through a heat exchanger for cooling, in which steam is generated for energy recovery. The cooled gas is then run into an acid scrub, in which excess ammonia is separated off. After the ammonia has been separated off, the hydrocyanic acid is separated off by washing with cold, slightly acidic water. The hydrogen cyanide dissolved in the water is then separated from the water by rectification .

catalyst

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

Reaction mechanism on the catalyst surface

Grabow et al. postulated the following reaction mechanism for the formation reaction of hydrogen cyanide. In your computer model, x, y = 0.1, or 2. X * stands for a species adsorbed on the catalyst and * for a vacancy on the catalyst surface :

${\displaystyle \mathrm {CH_{4}(g)+\ 2*\longrightarrow \ CH_{3}*\ +\ H*} }$

${\displaystyle \mathrm {CH_{3}*+\ *\longrightarrow \ CH_{2}*\ +\ H*} }$

${\displaystyle \mathrm {CH_{2}*+\ *\longrightarrow \ CH*\ +\ H*} }$

${\displaystyle \mathrm {CH*+\ *\longrightarrow \ C*\ +\ H*} }$

${\displaystyle \mathrm {NH_{3}(g)+\ 2*\longrightarrow \ NH_{2}*\ +\ H*} }$

${\displaystyle \mathrm {NH_{2}*+\ *\longrightarrow \ NH*\ +\ H*} }$

${\displaystyle \mathrm {NH*+\ *\longrightarrow \ N*\ +\ H*} }$

${\displaystyle \mathrm {CH_{x}*+\ NH_{y}*\longrightarrow \ CH_{x}NH_{y}*\ +\ *} }$

${\displaystyle \mathrm {CH_{x}NH_{y}*+\ (x+y-1)*\longrightarrow \ HCN*\ +\ (x+y-1)H*} }$

${\displaystyle \mathrm {HCN*\longrightarrow \ HCN(g)\ +\ *} }$

${\displaystyle \mathrm {2H*\longrightarrow \ H_{2}(g)\ +\ 2*} }$

${\displaystyle \mathrm {2N*\longrightarrow \ N_{2}(g)\ +\ 2*} }$

In this reaction scheme, the decomposition of CH _{4 is} rapid, so the active platinum surface has a high degree of carbon coverage. The decomposition of NH ₃ is hindered by carbon on the surface and is therefore slow. As a result, the rate of formation of N ₂ and HCN is limited by the decomposition of NH ₃ .

application

Hydrocyanic acid is nowadays mainly produced on an industrial scale using the Andrussow process and is mainly used as a preliminary product for the production of polyamide 66 ( nylon ) and polymethyl methacrylate (plexiglass). Further use on an industrial scale is for the production of NaCN , which is used in large quantities for cyanide leaching in the extraction of precious metals from ores with low metal contents.

In addition, HCN is made by the BMA process of Degussa and after the formamide -Spaltverfahren the BASF prepared.

${\displaystyle \mathrm {CH_{4}\ +\ NH_{3}\longrightarrow \ HCN\ +\ 3\ H_{2},\ \Delta _{R}\ =\ +251\ kJ/mol} }$

In the process called ammoxidation , other hydrocarbons are converted to nitriles instead of methane.

literature

• Manfred Baerns , Arno Behr , Axel Brehm, Jürgen Gmehling, Hanns Hofmann , Ulfert Onken: Technical Chemistry Textbook. 480 figures, 190 tables. Wiley-VCH Verlag GmbH September 2006 - bound - XXIV733
• About the kinetics and mechanism of ammonia / hydrocarbon reactions on platinum-alumina catalysts (dissertation by Rudolf Heinrich Häusler)
• About the production of methylamines by catalytic methods (dissertation by Theo Meyer)

Individual evidence

1. ↑ ^{a b} Leonid Andrussow: About the rapid catalytic processes in flowing gases and ammonia oxidation (V). In: Reports of the German Chemical Society (A and B Series). 60, 1927, p. 2005, doi: 10.1002 / cber.19270600857 .
2. ↑ ^{a b} L. Andrussow: About the catalytic oxidation of ammonia-methane mixtures to hydrocyanic acid. In: Angewandte Chemie. 48, 1935, p. 593, doi: 10.1002 / ange.19350483702 .
3. ^ Enthalpies of education . - By comparing the enthalpies of formation of various gaseous substances (methanol –201, methylamine –23, ethane –85, formaldehyde –109, ethylene –52, carbon monoxide –111, hydrogen cyanide +135, acetylene +227, nitrogen monoxide +90, nitrous oxide +82 kJ / mol) one can estimate the following values : methylenimine + 100 ± 30 kJ / mol and “nitroxyl” HNO + 85 ± 3 kJ / mol. - Using the Benson method , +108 kJ / mol can be calculated for methyleneimine .
4. ↑ It is known that ketone or olefin-forming dehydrogenations are endothermic with + 21 ± 7 kJ / mol .
5. ↑ Transfer of chemical processes from the laboratory to the production scale ( Memento from January 23, 2014 in the Internet Archive ) Lecture Technical Chemistry, University of Frankfurt (PDF; 1.2 MB).
6. ↑ ^{a b} Grabow, LC, Studt, F., Abild-Pedersen, F., Petzold, V., Kleis, J., Bilgaard, Th., Norskov, JK; Descriptor-Based Analysis Applied to HCN Synthesis from NH ₃ and CH ₄ , Angew. Chem. Int. Ed., 2011 , 50, 4601-4605, doi: 10.1002 / anie.201100353 .