|Methanol synthesis||CO + 2 H2 ↔ CH3OH|||
|Water-Gas Shift reaction||CO + H2O ↔ CO2 + H2|||
The change in enthalpy in these reactions is DHr = - 90.84 kJ/mole and 41.27 kJ/mole respectively. The methanol reaction  is exothermic: higher methanol yields are obtained at lower temperatures and higher pressures. The conversions are limited by the chemical equilibrium Keq,MeOH.
Usually, CO is preferred over CO2 as a reactant by all copper catalysts, and one aims at producing syngas with the highest possible CO and lowest possible CO2 content, securing the theoretically optimal stoichiometric number SN:
SN = [H2] - [CO2] = 2.0
[CO] + [CO2]
This is the stoichiometric amount of hydrogen required for methanol synthesis. For kinetic reasons, however, a certain minimum quantity of CO2 (2.5 to 3.5 vol.%) must be present in practice to attain a high CO conversion. As a function of the CO2 content in the syngas, the CO conversion rises rapidly to a maximum, where after it drops slightly up to a CO2 concentration of about 12 vol.%. Above 12 vol.% CO2 it drops more steeply.
Using several commercial copper-based catalysts, it could be proven that no methanol can be produced using syngas without CO2, and from which all water was withdrawn. In addition, istopic labelling proved that CO2 is the source of C in the methanol (Spath and Dayton, 2003). CO2 is also believed to keep the catalysts in an intermediate oxidation state (Cu0/Cu+), preventing ZnO reduction followed by brass formation. At high CO2 concentrations, it reduces the catalyst activity, however, by inhibiting methanol synthesis.
The conversion of CO and CO2 to methanol is thus limited by the pertinent chemical equilibrium (see Table below).
Table 1. The conversion of CO and CO2 at equilibrium conditions (syngas: 3 vol.% CO2, 27 vol.% CO, 64 vol.% H2, 6 vol.% CH4 + N2)
Two measures are essential in methanol synthesis:
The temperature rise inherent to reaction must be minimised in order to operate at good equilibrium values. Besides, this is important for retaining catalyst activity as well;
CO and CO2 must be converted as much as possible to attain the highest possible methanol yield.
A copper catalyst appears to be only slightly active at temperatures below 230 oC, and accelerated re-crystallization of the copper must be expected at temperatures exceeding 270 oC, there is only little tolerance in selecting the most favourable reaction temperature. Rising pressures favours CO and CO2 conversions: however, costs for equipment in the synthesis loop and syngas compressor move in opposite directions on the pressure in the synthesis reaction.
In general practice, un-reacted gas from the methanol synthesis reactor is recycled back to the reactor, thereby acting as a syngas quench cooler. The recycle ratio (recycle gas : syngas feed gas) is 3 : 1 up to 7 : 1 in normal practice.