Choosing a property method

• Hydrocarbons and light gases, any pressure — Peng-Robinson or SRK. With hydrogen, nitrogen, CO or methane above their critical temperatures, Peng-Robinson (Boston-Mathias).
• Polar liquids: alcohols, water, acids, ketones — an activity model: NRTL, UNIQUAC or Wilson where fitted pairs exist, UNIFAC or UNIFAC (Dortmund) to predict the ones that don't.
• Two liquid phases (water and an organic, a decanter) — NRTL or UNIQUAC. Wilson cannot split a liquid.
• Polar mixtures at high pressure — PSRK.
• Salt solutions — Electrolyte (Pitzer / Davies).
• Close-boiling, near-ideal mixtures at low pressure (benzene–toluene) — Ideal is enough, and the clearest for teaching.

• Ideal (Raoult's law) — K = Psat/P. Exact for an ideal mixture at low pressure, and the baseline to compare the others against.
• Cubic equations of state — Peng-Robinson, SRK and Peng-Robinson (Boston-Mathias) treat both phases with one equation, with a built-in library of interaction parameters for gas and hydrocarbon pairs. Right for pressure; weak on strongly polar liquids.
• Activity models — NRTL, UNIQUAC and Wilson are fitted to data for each pair; UNIFAC and UNIFAC (Dortmund) predict from the functional groups in each molecule. Each can take an ideal-gas vapour or a Peng-Robinson (Boston-Mathias) one for higher pressure. You can enter your own binary parameters from a paper.
• PSRK — an equation of state whose mixing is predicted by UNIFAC: no fitted pairs needed, and it covers methane through alcohols.
• Electrolyte — dissociates dissolved salts into ions and sets the water's activity from them: the boiling-point rise of a brine, an evaporator's duty, a crystalliser's temperature.

Ideal, Peng-Robinson, Peng-Robinson (Boston-Mathias) and SRK are on every plan; the activity models, PSRK and Electrolyte come with Pro.

• The binary phase diagram in Properties draws any two fluid components with the method chosen — an azeotrope or a liquid split shows up there before it surprises you in a column.
• Where a method has no parameters for a pair it needs, the panel says so rather than quietly using zero.
• Switch methods and run again: how far the duties and K-values move is how much the choice matters for your process.