Production Process of Methanol

Methanol production process description table, which gives some brief about the chemical reactions and catalysts used in each method, established in modern  production plants:

Feedstocks	Process technology and main reactions	Catalyst
Syngas	Methanol synthesis  CO + 2H2  ↔  CH3OH  CO + 3H2     ↔    CH3OH + H2O	Cu/ ZnO/Al2O3 Cu/ ZnO/Cr2O3
Syngas	Two-step methanol synthesis  CH3OH + CO  ↔  HCOOCH3  HCOOCH3 + 2H2 ↔   2CH3OH	Potassium methoxide, Cu chromite
Methane	Direct oxidation CH4 + ½ O2       →       CH3OH	Metal oxides,  (eg MoO3 based)
Methane	Bioprocessing	Enzymes eg., cytochrome P450, methanotrophs

Production of Syngas (synthesis gas) from natural gas:

Synthesis gas (Syngas) is obtained from natural gas. Natural gas is desulphurised by passing over activated carbon, preheated and mixed with steam and CO₂ under 2 atm pressures. CO₂ required for the reaction is obtained by burning natural gas in excess of air.

CH₄ + 2O₂ (air)    → CO₂ + H₂O

3CH₄ + CO₂ + 2H₂O → 4CO + 8H₂

 

Mass balance block diagram of methanol produced from carbon monoxide and hydrogen, units in Kg/h

The hydrocarbon feed is mixed with steam and fed to the reforming furnace, nickel catalyst is packed in its vertical tubes having 3-4 inches diameter and 20-25 feet long. Heat for the endothermic reaction is supplied by burners by combustion of the fuel gas. The reaction temperature is maintained in the range of 700 -1000^oC; high-temperature alloy steel is used for the tubes and whole steel wall of the furnace is refractory lined. A space velocity of 500-600 Kg/hr is maintained in reformer tubes. The effluent reformer gas is cooled to 35^oC and pumped to a hot potassium carbonate scrubbing system to remove CO₂.

Methanol from Syngas:

 A simplified flowsheet for production of methanol from Synthesis gas is shown below. Hydrogen and Carbon monoxide in a mole ratio of 2.25(12% greater than theoretical) is compressed to 3000-5000 psi, mixed with recycle gas, and fed to a high-pressure converter. Internal preheat is usually employed. The reactor is copper lined and contains a mixed catalyst of Zinc, Chromium, Manganese or Aluminum oxides. The temperature is maintained at 300-375^oc by proper space velocity and heat exchange design.

The reaction is highly exothermic and takes place with a decrease in volume. The enhanced pressure would, therefore, result in a more favourable equilibrium (Le Chatlier’s principle). The exit gases are cooled by heat exchange with reactants, then with water to 0-20^oC and then condensed in high-pressure condensers, where methanol is condensed at 3000-4000 psi. The liquid methanol is drawn off and the residual gases are recycled. The methanol condenses under full operating pressure to maximize yields (50%conversion per pass). The liquid methanol is depressurized, purified by permanganate to remove traces of ketones, aldehydes, and other such impurities, sent to a stripper to remove light ends such as dimethyl ether, and to fractionators to separate methanol from higher molecular weight compounds. The methanol from a stripper on distillation gives 99% pure methanol. The yield is around 98% with recycling. Dimethyl-ether (1-2%) and higher alcohols, such as n-propanol and isopropanol (0.3-0.5%) are obtained as by-products.

Methanol Production by Synthesis Gas