Vapour–liquid equilibrium of carboxylic acid systems: Propionic acid + valeric acid and isobutyric acid + valeric acid

The vapour–liquid equilibrium behaviour of two carboxylic acid systems was studied; viz. propionic acid (1)–valeric acid (2) and isobutyric acid (1)–valeric acid (2). For each system, a single isobar at 20 kPa and three isotherms at 393.15, 403.15 and 413.15 K, respectively, were measured. Liquid-phase activity coefficients were determined using a procedure based on the chemical theory for the vapour phase similar to those of Tamir and Wisniak [A. Tamir, J. Wisniak, Chem. Eng. Sci. 30 (1975) 335–342], Marek [J. Marek, Coll. Czech. Chem. Commun. 20 (1955) 1490–1502], and Marek and Standart [J. Marek, G. Standart, Coll. Czech. Chem. Commun. 19 (1954) 1074–1084]. The experimental activity coefficients, γA and γB, proved difficult to correlate using the commonly utilized Gibbs excess energy models, with best results obtained from the T–K Wilson equation. Area tests for thermodynamic consistency (strongly dependent on the chemical equilibrium constants, K) showed greater apparent consistency for the isobutyric acid–valeric acid system than for the propionic acid–valeric acid system. This is attributed to the possible formation of small amounts of propionic acid trimer. The vapour-phase correction factors, Φi, showed considerable departure from unity, confirming the highly non-ideal nature of the vapour-phase for the mixtures studied. An efficient bubble-point calculation routine is proposed that yields system pressure and vapour-phase compositions (at any arbitrary liquid composition and temperature) of sufficient accuracy for distillation equipment design.