What is the impact of controlling the hydrogen-to-carbon ratio on methanol synthesis?
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What is the impact of controlling the hydrogen-to-carbon ratio on methanol synthesis?
The molecular ratio of hydrogen to carbon monoxide in synthetic methanol production is 2:1. However, due to adsorption of the reaction gases on the catalyst surface and other factors, it is necessary to have a higher hydrogen content in the reaction gases than the theoretical amount in order to increase the reaction rate. The hydrogen content in the feed gas obtained through different processes using various raw materials does not necessarily follow this ratio; feed gases with a high hydrogen content require the addition of carbon dioxide before or after conversion to adjust the hydrogen-to-carbon ratio. In the crude feed gas produced from heavy oil or coal, the hydrogen-to-carbon ratio is too low; therefore, a carbon monoxide conversion unit is required to adjust the ratio of carbon monoxide to hydrogen, followed by the removal of carbon dioxide. According to the material balance of chemical reactions, the composition of the gas entering the methanol synthesis tower meets the requirements for methanol synthesis as well as the theoretical ratio of the feed gases; that is, (H2–CO2)/(CO+CO2) = 2.15. During operation, a slightly higher hydrogen content should generally be maintained. An excess of hydrogen not only helps to increase the reaction rate but also reduces the formation of iron carbonyl and higher alcohols, thereby contributing to an extended service life of the catalyst. Therefore, proper control of the hydrocarbon ratio has a significant impact on both the utilization rate of raw materials and the purity of the product.
Practice has shown that the composition at the maximum reaction rate is not the stoichiometric composition, but rather the optimal hydrogen-to-carbon ratio determined by the kinetic equation. The hydrogen-to-carbon ratio of the fresh gas is determined based on the material balance of the entire synthesis system. To meet the requirements of material reactions and maintain stable production of the system. H2-CO2／CO+CO2=2.0～2.2