Modeling acid gas scrubbing
Modeling acid gas scrubbing requires the use of fairly specialized physical property models and, in many cases, one cannot assume equilibrium between phases.
Using an appropriate physical property model is key to successfully modeling any unit operation. General purpose simulators like Aspen Plus, PRO II, etc. have dozens of physical property models to choose from. These can be divided into two classes: equation of state models (e.g. ideal gas, Redlich-Kwong-Soave) and activity coefficient models (e.g. UNIFAC, UNIQUAC, NRTL).
Acid gas scrubbing involves species that act as molecules in the vapor space but behave as dissociated ions in the aqueous phase. As far as I know, no equation of state model will properly represent that. All the simulators capable of modeling acid gas scrubbing do so with some type of activity coefficient model.
For systems where they are appropriate, equation of state properties will work with only the most basic information about the molecules in the mixture. They generally do not need information about the interactions between the specific molecules in the mixture.
For acid gas scrubbing systems and, in fact, any electrolyte system; the activity coefficient models need species-to-species interaction parameters. These need to be regressed against laboratory data (from data libraries such as DeChema). With the Aspen Plus simulator, the activity coefficient model of choice was eNRTL and there were various physical property packages that provided pre-regressed parameters for common systems. For example, one would model a Benfield system using the “HOTDEA” package of parameters.
One also needs to keep in mind that many acid gas scrubbing systems do not operate at equilibrium. In other words, there are mass-transfer limitations between the vapor phase and the liquid phase. A common example of that is nitric acid scrubbing.
We used to model nitric acid absorption columns with a column model called RATEFRAC which could represent non-equilibrium mass-transfer. That was several years ago and I imagine that other general purpose simulators now have a similar non-equilibrium tower models (and there have long been specialized simulators for representing non-equilibrium acid-gas systems).