What should be the appropriate hydrogen-to-carbon ratio in methanol synthesis?

Reply to 1# yangzhi5114: When the feed gas contains hydrogen, carbon monoxide, and carbon dioxide, the theoretical molecular ratio of the various gases involved in methanol synthesis is (H2-CO2)/(CO+CO2) = 2； Maintaining a small amount of carbon dioxide in the feed gas results in the consumption of one additional molecule of hydrogen and the production of one additional molecule of water; however, due to the thermodynamic principles of the synthesis reaction, this has a positive effect on maintaining stability in the heat released during the synthesis as well as on controlling the temperature. Too much carbon dioxide in the syngas not only consumes a large amount of hydrogen, reducing production capacity, but also leads to water formation. This reduces the concentration of crude methanol. Therefore, in order to maintain the hydrogen balance in the syngas and produce crude methanol at a certain concentration, excess carbon dioxide in the feed gas must be removed before it enters the synthesis process, so that the hydrogen-to-carbon ratio and the carbon dioxide content in the syngas meet the required standards. In actual production, depending on the processes and equipment available in each company, the hydrogen-to-carbon ratio (H2-CO2/CO+CO2) is generally kept between 2.05 and 2.20. Processes for producing methanol from hydrogen and pure carbon dioxide already exist; these include methods that utilize carbon dioxide from flue gases or cement plant exhausts, as well as hydrogen from ion-exchange membranes in chlor-alkali plants to produce methanol. The concentration of crude methanol produced in such processes is around 55%, but these methods are still in the pilot stage at present.

Reply to 2# sjxx30: The hydrogen-to-carbon ratio you mentioned should refer to that in the fresh gas. So what should be the hydrogen-to-carbon ratio of the syngas at the inlet of the synthesis tower? In other words, what is the hydrogen-to-carbon ratio after adding recycled gas to the fresh gas?

Reply to 3# yangzhi5114: Hydrogen-to-carbon ratio control generally refers to the fresh gas； The control of the hydrogen-to-carbon ratio in the methanol synthesis system can only be achieved through venting gas. However, what is referred to here is generally the control of the inert gas levels in the synthesis system, rather than the control of the hydrogen-to-carbon ratio of the circulating gas. Inert gases are substances that are useless within the system; they include components such as CH4, N2, and Ar. Their presence affects both the operation of the synthesis tower and the power consumption of the compressors. Therefore, it is necessary to keep the amount of inert components in the system at the lowest possible level. Achieving this is done by venting a portion of the circulating gas. Excessive gas venting represents a significant waste of raw material gas. Therefore, the emission of purge gas should be controlled based on the conditions of the different raw materials; generally, the inert gas content should be kept at around 16% (V). At the beginning of catalyst use, its activity is high, and the content of inert gases can be kept at around 20% (V); as the catalyst’s activity declines over time, the content of inert gases can be controlled at around 12% (V). If high yield is required, the content of inert gas can be appropriately reduced; if low consumption is desired, the content of inert gas can be appropriately increased. The decision should be based on the actual conditions of production.

This post was last edited and replied to by renwucungao on 2010-7-5 00:06. Reply to 2# sjxx30: According to what is stated in the text, it should refer to a hydrogen-to-carbon ratio of 2.05~2.15 for the gas entering the tower; but generally, isn’t the hydrogen-to-carbon ratio meant to refer to the ratio of components in the fresh gas? Which one is the correct one? :L

I think it’s better in practice to control the H/C value for the cycling phase; 4-6 should be sufficient, while for CO2, 3.0-3.5 is my personal opinion

The choice of H/C depends not only on synthesis but also on its impact on distillation; there are numerous side reactions during synthesis, and these need to be considered in comparison with the catalyst selected. Just based on H/c, I think 4-6 is a reasonable range

Reply to 5# renwucungao: The hydrogen-to-carbon ratio of the synthesized fresh gas should be 2.05~2.15

The hydrogen-to-carbon ratio of the fresh gas is 2.05–2.15; based on practical production experience, a hydrogen-to-carbon ratio of 3–6 for the gas entering the tower is the most suitable.

Our factory maintains a hydrogen-to-carbon ratio of 3.5–6, with carbon dioxide levels kept below 15%.

The hydrogen-to-carbon ratio of the fresh gas should be maintained between 2.05 and 2.15. However, the hydrogen-to-carbon ratio of the gas entering the tower should be (4-5)/1