The process gas exiting the secondary reformer contains approximately 10-15% CO (dry gas base). In the shift conversion phase, CO is converted to CO₂ and H₂. In conventional plants, this conversion takes place in an adiabatic CO reactor. The conversion is conducted at two temperatures; the high temperature conversion (350-380^oC) using a Fe-Cr catalyst and the low temperature conversion (200-220^oC) using a Cu-Zn catalyst. After the conversion, the residual CO content of the process gas is about 0.2-0.4% (IPTS/EC, 2007 p.41).

In an isothermal reactor the conversion can be conducted in a single step at about 250^oC with the use of a special copper-based catalyst (see Figure). The conversion is achieved isothermally by continuously removing heat from the catalyst bed with the use of cooling tubes (IPTS/EC, 2007 p.78). In a single step the isothermal reactor allows conversion to 0.7% CO (dry basis) (UNIDO, 1979 p.179). The heat generated from the reaction is recovered by generating medium pressure steam in a built-in heat exchanger.

This measure is applicable to all feedstock types, can also be implemented as a revamp in existing ammonia plants.