Vapour–liquid-equilibria (VLE) models of CO2–amine–H2O systems are critical to understand the
behaviour of these systems as a function of composition and temperature. Such models are a crucial
component of any process simulation and the prediction of process performance. VLE models consist of
chemical reactions and an activity model to describe non-ideal chemical behaviour at high ionic strength.
For CO2–amine–H2O systems complex and highly dimensional activity models are used such as Pitzer,
UNIQUAC and eNRTL equations. These require many tens of parameters to be known or determined,
and in many cases this leads to over-fitting. The resulting parameter values then have little physical
meaning and extrapolation to different conditions is unreliable. In this work a much simpler activity
model, specific ion interaction theory (SIT), has been used and its performance assessed using published
VLE data of CO2 absorption in aqueous MEA, AMP, PZ and mixtures of AMP and PZ. This simple
activity model, which requires the determination of few if any parameters, coupled to an appropriate
chemical model, was found to yield results as good as or better than that found for the more complex
models. Extrapolation using this simpler model will be much more reliable than a complex model
that contains many ill-defined parameter values and its inclusion in process simulation is much more
straightforward.