CO₂ Removal

In this process step, the CO₂ is removed from the process gas with physical or chemical absorption. The residual CO₂ content typically ranges between 50 and 3000 ppmv. In chemical absorption, the solvents mostly used are aqueous amine solutions (MEA and aMDEA), or hot potassium carbonate solutions. In physical absorption, typical solvents are glycol dimethylethers (Selexol), propylene carbonates and others (IPTS/EC, 2007). The heat requirements in a modern chemical absorption plant range between 30 and 60 MJ.kmol^-1 CO₂. A physical absorption plant can be designed to have zero heat requirements. However, mechanical energy requirements will have to be taken into account (EFMA, 2000). 

A promising development, pressure swing absorption (PSA), can combine the CO₂ removal step and the methanation step into one. When a pure stream of CO₂ is required, solvent scrubbing can be used for CO₂ recovery (IPTS/EC, 2007).

Methanation

After CO₂ removal there are still small amounts of CO and CO₂ that can poison the ammonia synthesis catalyst and need to be removed. In this process CO and CO₂ are transformed into methane in a reactor filled with a nickel containing catalyst. The reactions are the following:

CO + 3H₂ → CH₄ + H₂O,     ΔΗ = - 206 kJ/mol

CO₂ + 4H₂ → CH₄ + 2H₂O,   ΔH = - 165 kJ/mol

The water produced needs to be removed before entering the converter. This is done by cooling and condensation downstream of the methanator (IPTS/EC, 2007 p.42).