Syngas, Hydrogen, and Carbon Monoxide Separation

Raw synthesis gas is a mixture of hydrogen, carbon monoxide, carbon dioxide, water vapor and residual unconverted hydrocarbons. Hydrogen and carbon-monoxide-rich offgas streams from petrochemical and refinery processes contain these and other contaminents such as nitrogen, argon and sulfur compounds. Separation and purification is necessary to produce hydrogen and carbon monoxide suitable for use as feedstock in petrochemical processes and a variety of methods have been developed to achieve these separations. 

The desired products are synthesis gas mixtures of various hydrogen to carbon monoxide ratios, pure hydrogen, pure carbon monoxide and carbon dioxide. Although carbon dioxide is sometimes recovered as a byproduct and sold on the merchant market, it is not currently used as a feedstock for petrochemical production. The others, however, are important petrochemical feedstocks. 

Pure carbon monoxide is sometimes required as the sole product for petrochemical feedstock. In this case, raw syngas is first recovered from carbon dioxide and then hydrogen and carbon monoxide are separated. Hydrogen is always a by-product in the manufacture of syngas. It can be minimized but not eliminated. The hydrogen and carbon monoxide are separated and the hydrogen is either sold separately or burned as fuel. 

Hydrogen can be recovered from many refinery and petrochemical offgas streams. Recovered hydrogen is either recycled to the process from which it is recovered, utilized downstream in another process or sold for distribution by pipeline or liquefaction and distribution on the merchant market. Pro- 

Styrene monomer offgas 90-95 CH4, CO2, C2H4, benzene, ethylbenzene 

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If only hydrogen is required, the plant becomes a hydrogen plant. If only CO is required, the plant becomes a carbon monoxide plant. If both hydrogen and carbon monoxide are required as separate streams, the plant is typically known as a HYCO plant. If only a hydrogen/carbon monoxide mixture is required, the plant is typically known as a synthesis gas (or syngas) plant. If all three products are required, the plant is considered a combination (hydrogen/carbon monoxide/syngas) plant. 

Petrochemical applications of syngas require a ratio of hydrogen to carbon monoxide of either 1 1 or 2 1. Commercial processes for syngas yield ratios much higher therefore, separation technology, by-product credits and production techniques which can adjust the hydrogen to carbon monoxide ratio are important aspects of syngas production. 

The process begins with a gasification process that converts coal into carbon monoxide and hydrogen. Part of this gas is sent to a water-gas shift reactor to increase its hydrogen content. The purified syngas is then cryogenically separated into a carbon monoxide feed for the acetic anhydride plant and a hydrogen-rich stream for the synthesis of methanol. 

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