The most common industrial process for the manufacture of nitrous oxide is based upon thermal decomposition of ammonium nitrat). There are a number of other nitrous oxide production processes e.g. direct oxidation of ammonia or purification of off-gas from adipic acid production (polyamide chain) etc.
Nitrous oxide is produced by thermally decomposing a hot solution of ammonium nitrate and water at concentrations varying from between 80 to 93% at a temperature of approximately 250°C to 255°C, (482°F to 491°F). Thermal decomposition of ammonium nitrate is complex and can follow different routes.
The main and desired reaction is NH4NO3 ? N2O +2 H2O.
This reaction is exothermic, generating 59 kJ / mole at approximately 250°C,(482°F) and it is a first order reaction with an estimated energy of activation of between 150 – 200 kJ / mole at standard conditions (273 K, 1013 mbar).
The reaction kinetics of decomposition doubles for every 10°C (18°F) increase in temperature (or the rate of decomposition multiplies by a factor of 1.07 for each°C. As an order of magnitude, a mass of molten ammonium nitrate producing 200 kg/h of nitrous oxide in a reactor at 250°C (482°F) develops a thermal power of about 70 kW; at 255 °C (491°F) the same reactor would produce 280 kg/h (40% more), with a heat production of 98 kW.
A variety of reactions take place in an ammonium nitrate reactor being operated to produce nitrous oxide. The pure ammonium nitrate salt melts at 169°C (337°F), and begins decomposing at 190°C (375° F). At temperatures up to 250°C (482°F), two reactions predominate and are of primary interest to the production of nitrous oxide by thermal decomposition of ammonium nitrate.
NH4NO3 ? N2O + 2H2O ?H = -59 KJ/g-mol (-315 BTU/lb)
NH4NO3 ? NH3 + HNO3 ?H = +159.9 KJ/g-mol (+860 BTU/lb)
Note that the decomposition reaction is exothermic and the dissociation reaction is endothermic.
The decomposition reaction is the desired reaction, producing nitrous oxide. The dissociation reaction becomes appreciable at 210°C (410°F) and continues to become more predominant with increasing temperature. Increasing pressure suppresses the dissociation reaction. If adequate venting is provided for the reactor, in the event of loss of control of the reactor, with rapidly rising temperature, the dissociation reaction eventually checks the temperature.