HUIDPHAS[ [OglUgglA ELSEVIER Fluid Phase Equilibria 135 (1997) 149-167 Role of azeotropy in characterization of complex hydrocarbon mixtures by true-boiling-point distillation D. Glindemann, T. Maskow, D. Browarzik, H. Kehlen *, J. Kutscha Institute of Physical Chemistry, Martin Luther University Halle- Wittenberg, Geusaer StraJ3e, D06217 Merseburg, Germany Received 22 January 1997; accepted 17 March 1997 Abstract True-boiling-point distillation is an efficient batch distillation being one of the most commonly applied techniques to study the polydispersity of complex hydrocarbon mixtures. True-boiling-point curves are obtained by plotting the temperature at the top of the column versus the fraction of mass distilled. The name expresses the assumption that the complex mixture is separated into its components according to their boiling points. However, in the case of azeotropy (e.g. between aliphatics and aromatics) this assumption is not correct. Therefore, based on continuous thermodynamics, a method is developed for describing quantitatively the azeotropic effect. The method presumes distillation of pure compounds or of azeotropic mixtures (if the efficiency of the column is sufficiently large). It is demonstrated for mixtures consisting of a petroleum fraction (low in aromatics) and of alcohols. © 1997 Elsevier Science B.V. Keywords: Vapour-liquid equilibria; True-boiling-point distillation; Influence of azeotropy; Complex hydrocarbon mixtures; Method of calculation; Continuous thermodynamics 1. Introduction Most industrial separation operations for petroleum fractions require quantitative predictions of vapour-liquid equilibria of such complex hydrocarbon mixtures. Once a suitable thermodynamic model has been chosen the most important problem is an adequate description of the composition of the complex mixture To get information about the polydispersity of petroleum and its fractions the classical way is the true-boiling-point analysis (TBP analysis). In spite of the existence of other modem methods TBP analysis is still one of the most commonly applied techniques since this method has two important * Corresponding author. 0378-3812/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S0378-3812(97)00079-4