6.1 Water-Gas-Shift
To increase the H2/CO ratio or to (essentially completely) convert CO into H2 the water-gas-shift reaction is used:
CO + H2O => CO2 + H2
The water gas shift equilibrium among carbon monoxide, carbon dioxide, hydrogen, and steam depends on temperature but is almost independent of pressure in the industrial range (elevated pressures up to 70 bar). Above 950 to 1000 ̊C, the equilibrium is established rapidly enough without a catalyst, but typically the shift is carried out at lower temperatures. Temperature ranges used in commercial carbon monoxide shift conversions are:
High-temperature shift (HTS) at about 300 to 510 ̊C with copper-promoted catalysts
Low-temperature shift (LTS) at about 180 to 270 ̊C with copper-zinc-aluminum oxide-based catalysts.
Depending on the required H2/CO ratio the water-gas-shift reaction is carried out in two steps. In the first high temperature (HTS) step the bulk of the CO is converted; the remaining CO concentration is minimum 1-2 vol%. If hydrogen production is aimed at, a second shift LTS step is carried out to reduce the CO content to 0.2-0.5 vol%.