Gas Conversion

Gas Conversion

 

the processing of gases to change the composition of the initial gas mixture. Gaseous hydrocarbons (methane and its homologs) and carbon monoxide are usually converted to produce hydrogen or mixtures of hydrogen with carbon monoxide. Such mixtures are used in the synthesis of organic products and in metallurgy as reducing gases, or they may be further refined to produce hydrogen. Conversion is accomplished by using various reagents (oxygen, water vapor, carbon dioxide, and their mixtures) as oxidizing agents. Metal oxides may also be used for this purpose.

Methane (natural gas) is the most economical raw material for conversion. The conversion of methane by different oxidizing agents can be described by the following equations:

CH4 + H2O ⇄ CO + 3H2 ˗ 206.6 kJ (49.3 kcal)

CH4 + CO2 ⇄ 2CO + 2H2 ˗ 247.6 kJ (59.1 kcal)

CH4 + 0.502 ⇄ CO + 2H2 + 358.2 kJ (8.5 kcal)

CH + H2O ⇄ CO2 + H2 + 410.6 kJ (9.8 kcal)

The oxidation reactions of the homologous methane series occur in a similar manner.

A distinction is made between catalytic and high-temperature gas conversion. Catalytic conversion of methane is performed with water vapor in tube furnaces with external heating (steam conversion) or with a steam-oxygen mixture in shaft-type units under low pressure (1.5–2.0 kilograms-force per sq cm [kgf/ cm2], or 0.15–0.2 meganewtons per sq m [MN/m2]; see auto-thermal conversion in Table 1) and under high pressure (20–30 kgf/cm2, or 2–3 MN/m2). The best catalyst is nickel with various additives.

High-temperature conversion is performed in the absence of catalysts at temperatures of 1350°-1450°C and pressures of 30–35 kgf/cm2, or 3.0–3.5 MN/m2; thus, methane and other hydrocarbons are almost completely oxidized by oxygen to CO and H2. The approximate composition of the gas produced during noncatalytic high-temperature oxygen conversion of methane is 3–4 percent CO2, 36–38 percent CO, 57–59 percent H2, 0.2–0.4 percent CH4, 2 percent N2. The advantage of this method is the absence of a catalyst and the simplicity of the equipment required; a disadvantage is the high consumption of oxygen.

Carbon monoxide conversion is used primarily to produce hydrogen. The use of catalysts provides the necessary reaction rate. Ferric oxide catalysts with different additives are most efficient. Carbon monoxide conversion is usually carried out at 400°-450°C at low or high pressure, with supply of a triple or larger (as opposed to stoichiometric) excess of water vapor.

REFERENCES

Spravochnik azotchika, vol. 1. Moscow, 1967. Section 2.
Termodinamika protsessov polucheniia gazov zadannogo sostava iz goriuchikh iskopaemykh. Moscow, 1969.

V. S. AL’TSHULER

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