How Nitrous Oxide Plant Works

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.

Decomposition:

NH4NO3 ? N2O   +   2H2O   ?H  =  -59 KJ/g-mol (-315 BTU/lb)

Dissociation:

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.

 THE PROCESS

Ammonium Nitrate is decomposed in the Melters  to give a mixture of Nitrous Oxide & super heated steam. At the same time impurities are produced including Ammonium Nitrate fumes, Nitrogen & other oxides of Nitrogen. The steam & impurities are removed by scrubbing with water, caustic soda and sulphuric acid in sequence Nitrogen present in traces is removed by bleeding from the top of storage vessels where Nitrous Oxide is stored, after compression by a compressor.

Ammonium nitrate (even technical grade) is melted in a melter. The molten ammonium nitrate flows into a combustion pot where ammonium nitrate is decomposed according to the following reaction.

The reaction is ticklish and has to be controlled properly. If temperature of ammonium nitrate is allowed to go beyond a certain point then N2o is not formed. In our plant we almost get Efficency of reaction equal to 95% or even more. In practice, however, with N2O some other impurities are also obtained as products of combustion. There is some un decomposed ammonium nitrate, with its fumes. These are recovery in the primary scrubber. The steam in the hot gas is condensed in the first water scrubber and the entrained moisture and traces of ammonium nitrate get separated in the mist separator. The residual traces of ammonium nitrate, carbon dioxide and other oxides of nitrogen are removed in the caustic scrubber.

The gas containing some nitrogen is then stored in the gas balloon. The gas is compressed with a compressor, dried in a drier and then refrigerate when it becomes liquid. Liquid nitrous oxide is stored in high-pressure storage vessel into cylinders and the gas is filled by weight. In a low pressure plant the gas after refrigeration is stored in a storage tank and from there it is filled in cylinders with a pump. Each batch is analyzed in the laboratory to ensure quality of the product and proper record is maintained.

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