Extension of the cubic-plus-association (CPA) equation of state to amines

The cubic-plus-association (CPA) equation of state has been extended to modeling mixtures containing amines. Special focus was given to primary and secondary amines, which are known to self-associate, thus forming hydrogen bonds in mixtures with alkanes. Pure-compound parameters have been determined from vapor pressure and liquid density data, but phase equilibrium data were also used as guidance for selecting the optimum sets among those which best represent the pure-compound properties.

Diethylamine was modeled as a two-site molecule, whereas the two primary amines considered (methylamine, ethylamine) were modeled using both the two-site and three-site schemes. Both schemes perform overall equally well, when care is exercised in the parameter estimation, by giving more weight on the vapor pressures rather than the liquid densities.

Thus, only results using the two-site scheme are shown. Excellent binary vapor-liquid equilibria have been obtained for amine + aliphatic hydrocarbons with a low value of the binary interaction parameter. Satisfactory results are obtained also for cross-associating systems with alcohols but less so with water, using, moreover, a rather high value of the interaction parameter.