4 replies to this topic

[chemengisfun]

I realised that the stoichiometric number of the syngas required for methanol synthesis is often defined as (H2-CO2)/(CO+CO2), and preferably at 2. Please may I know how to arrive to this definition? Thank you.

[daryon]

methanol can produced by the following reactions:

CO + 2H₂ <=> CH₃OH ............(1)
CO₂ + 3H₂ <=> CH₃OH + H₂O........(2)

The optimum syngas will have the right number of moles of each component to allow these reactions to take place to form the product. If the syngas is stoichiometric it has exactly the right number of moles of the reactant components to give a balanced reaction to form the product(s). The stoichiometric ratio for syngas can be written:

R = moles H₂ / (2 x moles CO + 3 x moles CO₂).

When R=1 the syngas is balanced or stoichiometric i.e it has the correct number of reactant components to form the desired product (s). Sometimes the degree of stoichiometry fot syngas is defined as the stiochiometric number instead;

S = (moles x H₂ - moles x CO₂)/ (moles x CO + moles x CO₂)

now the number needs to be 2 to get a balanced or stoichiometric gas. If you rearrange the Stoichiometric number you can see it is the same as the stoichiometric ratio which is simply derived from the number of moles of each component required for the two methanol forming reactions to balance.

Hope this helps

[kkala]

I realised that the stoichiometric number of the syngas required for methanol synthesis is often defined as (H2-CO2)/(CO+CO2), and preferably at 2. Please may I know how to arrive to this definition?

Having dealt with syn gas as a student, following interpretation is not totally satisfactory, though it might help.

Reaction of methanol synthesis is : CO + 2H2 --> CH3OH . So a molar ratio H2/CO = 2 is justified as stoichiometric number. Mentioned one indicates that CO2 content in syn gas has a detrimental effect on H2 and a favorable effect on CO, probably through a reaction that occurs.
Consider the reverse of water gas shift conversion (a reversible reaction): CO2 + H2 --> CO + H2O, which could explain the mentioned stoichiometric number.
Wikipedia indicates that only direct shift conversion is industrially significant, but probably methanol synthesis catalyst favors this reverse reaction? Or some other by side reaction to have same effects?

I have looked into wikipedia for some historic background of mentioned stoichiometric number, without apparent result.

[kkala]

methanol can produced by the following reactions:
CO + 2H₂ <=> CH₃OH ............(1)
CO₂ + 3H₂ <=> CH₃OH + H₂O........(2)

Thanks, daryon, above seems to support mentioned stoichiometric number of syn gas even by kkala's view; for reaction (2) could be (at least in theory) broken down as follows:
CO2 + H2 --> CO + H2O (α) (reverse of water gas shift conversion)
CO + 2H2 --> CH3OH (β) (same reaction as (1)).

So methanol synthesis can be considered (at least from stoichiometric viewpoint)as:
CO + 2H2 --> CH3OH (1) (main reaction)
CO2 + H2 --> CO + H2O (2') (reaction that is also realized)

Stoichiometric number would be H2/CO = 2 (molar basis) if (1) occurred alone. But reaction (2') consumes 1 mole H2 and produces 1 mole CO per mole of existing CO2, so mentioned number should be corrected to (H2-CO2)/(CO+CO2) referring to reaction (1).

Note: I was not aware of post by daryon, when sending the previous post.

Edited by kkala, 04 April 2010 - 02:33 PM.

[cloud]

Thanks both for the explanation.