By Glenn Combs and Lynn McGuire
last twenty years, or so, MDEA (Methyldiethanolamine) based solvents have become increasingly popular as a preferred
means to recover CO2 from process gases employed in Hydrogen, Methanol, Ammonia and Synthesis Gas production. The
technology for amine based solvents use in CO2 removal from synthesis gas streams is very old, and has steadily improved.
Early amine based processes used MEA (Methylethanolamine, or 2-Aminoethanol) in low concentrations of about 20 Wt%
mixed with Water (H2O). Improvements resulted in energy savings, by raising the concentration of MEA to about 30%, and
adding proprietary corrosion inhibitors to the amine solvent, permitting the use of carbon steel equipment in low temperature
sections of the process. Newer formulations of amine based CO2 removal solvent use MDEA as the main chemical
component, with additions of other amines to improve the solvent retention and efficiency in a CO2 removal plant. One of the
most popular solvents in Chemical Industry uses Piperazine (C4H10N2, or Pz) as a component in the mixture, further
combined with Water. A typical composition for the solvent on a CO2 Free basis would be 3-5 Wt% Pz, 35-37 Wt% MDEA
and 60 Wt% H2O. A higher concentration of Pz improves the solvent’s removal of CO2 from Synthesis Gas streams in
chemical plants. Higher overall strength of “Total Amine” (Pz plus MDEA) also improves CO2 removal from Synthesis Gas
(Syn-Gas), but also can result in higher equipment corrosion in the cool lean solvent sections of plant equipment, where
carbon steel is often used. Thus, there are trade-off’s in solvent composition, that result in optimal Pz and MDEA component
strength that can achieve high process performance, while controlling equipment cost. A flow diagram for a typical CO2
Removal System commonly used in an Ammonia plant is shown in Figure 1. Ammonia plants generate tremendous amounts of
CO2 to be removed in the manufacture of Syn-Gas, bearing Hydrogen and Nitrogen. The Syn-Gas in Ammonia processes
typically contains about 18 volume (mole) percent CO2, which must be removed to product gas purity generally less than 500
parts per million by volume (ppm). Many such plants prefer to purify the Syn-Gas to less than 100 ppm CO2 in the product
gas from the CO2 Absorber.
