Weekly Topic 5.3, Issue 25: Why is CO2 needed for methanol synthesis?

Positive effects: (1) CO2 itself can also participate in the synthesis reaction, and its presence inhibits the formation of dimethyl ether to a certain extent. (2) It prevents CO from being converted into CO2, a reaction that occurs in the presence of H2O. (3) It helps to regulate temperature, prevent overheating, protect the activity of copper-based catalysts, and extend their service life. (4) It can prevent catalyst carbonization. Disadvantages: (1) Compared to methanol synthesis from CO, 0.7 m3 more H2 is consumed per 1 kg of methanol produced. (2) The water content in crude methanol increases, while the methanol concentration decreases. It is mainly the defects in current methanol synthesis processes that dictate the necessity of including CO2

What level of CO2 concentration is needed to effectively reduce the formation of impurities? Does anyone have any relevant data they can share?

Some literature suggests that CO2 can reduce the formation of the impurity ethanol, but data on this aspect is also lacking!

Reply to 1# wawjia: http://bbs.hcbbs.com/thread-423163-1-1.html The discussion on this link is as follows: In what ways does the presence of a small amount of CO2 in the raw gas used for methanol synthesis have a positive impact on its production? Answer: (1) From the reaction equation, CO2 can also participate in the reaction to produce methanol. The synthesis of methanol from CO2 requires 1 more molecule of H2 compared to using CO, and 1 molecule of H2O is produced as a byproduct. Therefore, when the hydrogen content in the feed gas is low, it is necessary to use more H2 and CO to produce methanol. 8 k1 Q7 w+ K6 }8 h’ S: r6 @+ c (2) The presence of CO2 inhibits the formation of dimethyl ether to a certain extent. Since dimethyl ether is the product of the dehydration reaction of 2 molecules of methanol, the reaction between CO2 and H2 to produce methanol generates 1 molecule of H2O, and the presence of H2O plays a positive role in suppressing the dehydration reaction of methanol. 1 `+ @% w+ u4 d- p2 a% ~8 w7 v9 P (3) It prevents CO from being converted into CO2, a reaction that occurs in the presence of H2O. 4 |9 {4 b" L8 c/ w (4) is more effective at regulating temperature, preventing overheating, preserving the activity of copper-based catalysts, and extending their service life. 5 d$ `# b! g X( v4 K （5）can prevent catalyst carbonization. ) ^- d, e1 Q) V+ ?/ o; y& l Therefore, the CO2 content in the feed gas is generally kept at around 3%. What is the adverse effect of CO2 presence on the coupling reaction? Compared to methanol synthesis from CO, 0.7 m3 more of H2 is required per kilogram of methanol produced ； ②An increase in water content in crude methanol and a decrease in methanol concentration lead to higher steam consumption. ( s7 s( g3 H2 f) I

Methanol synthesis was initially considered to be a hydrogenation reaction of CO and CO2. However, in the late 1970s, isotope tracing experiments revealed that the carbon in methanol molecules comes from CO2 molecules. Subsequently, the reaction mechanisms for the hydrogenation of CO2 to form CH3OH and the gas-phase conversion of CO to CO2 were proposed. (1) CO2 + X ⇌ X•CO2 (2) H2 + X ⇌ X•H2 (3) X•CO2 + H2 ⇌ X•CO2•H2 (4) X•H2 + CO2 ⇌ X•CO2•H2 (5) X•CO2 + H2 + X ⇌ X•O + X•CO•H2 (6) X•CO•H2 + H2 ⇌ CH3OH + X (7) CH3OH + 2X•O ⇌ H2 + H2O + CO2 + 2X. (⇌ represents a double arrow; it cannot be displayed here.) So, what the people mentioned earlier likely referred to are macroscopic phenomena, ones that are well-known to everyone

By understanding the methanation reaction, one can see why the presence of CO2 is more conducive to temperature control. I learned from catalyst manufacturers that the presence of CO2 helps to suppress ethanol, probably for the same reason as it does in the formation of dimethyl ether

(1) From the reaction equation, CO2 can also participate in the reaction to produce methanol. The synthesis of methanol from CO2 requires one more molecule of H2 compared to using CO, and one molecule of H2O is produced as a byproduct. Therefore, when the hydrogen content in the feed gas is low, it is necessary to use more H2 and CO to produce methanol. (2) The presence of CO2 inhibits the formation of dimethyl ether to a certain extent. Since dimethyl ether is the product of the dehydration reaction of 2 molecules of methanol, the reaction between CO2 and H2 to produce methanol generates 1 molecule of H2O, and the presence of H2O plays a positive role in suppressing the dehydration reaction of methanol. (3) It prevents CO from being converted into CO2, a reaction that occurs in the presence of H2O. (4) It is more effective at regulating temperature, preventing overheating, protecting the activity of copper-based catalysts, and extending their service life. (5) It can prevent carbon deposition on the catalyst. Therefore, the CO2 content in the feed gas is generally kept at around 3%.

The presence of CO2 results in a lower heat of reaction when it is used with H2 to synthesize methanol compared to CO; this makes it easier to control the temperature of the catalyst bed, which is beneficial for extending the catalyst’s service life. However, too high a CO2 concentration leads to increased consumption of H2, higher water content in the crude methanol, reduced compressor capacity, and increased energy consumption for gas compression and the distillation of crude methanol. Therefore, there is an optimal level of CO2 in the feed gas (around 3%).

This post was last edited and replied to by wawjia on 2011-5-4 at 13:15. Reply 5# 3968668: Thank you for the reminder; similar posts do exist on the forum. I posted similar content in the hope that someone would share data related to CO2 concentrations! I talked with people from a catalyst manufacturer some time ago, and they asked about the minimum concentration of CO2. I’m not sure; I haven’t seen any information on that in the materials I’ve come across! So, I hope those who know about this will kindly share it!

I would like to ask everyone: if the CO2 content in the feed gas is 3%, what is the CO2 level in the circulation loop?