A generalized vapor pressure equation for heavy hydrocarbons

Abstract

Twu, C.H., Coon, J.E. and Cunningham, J.R., 1994. A generalized vapor pressure equation for heavy hydrocarbons. Fluid Phase Equilibria, 96: 19-31. Pitzer's acentric factor is widely used in physical property estimations; one important area is that of generalized vapor pressure correlations. Since Pitzer's acentric factor is defined in terms of the vapor pressure at a reduced temperature equal to 0.7, using this approach to generalize a vapor pressure equation gives excellent accuracy at reduced temperatures between 0.7 and 1.0, but predicts vapor pressures less accurately at lower reduced temperatures. To improve the vapor pressure prediction of hydrocarbons at low reduced temperatures, the definition of the acentric factor is re-examined and a modified acentric factor is proposed. Two sets of generalized vapor pressure equations are presented, with Pitzer's acentric factor and the modified acentric factor, respectively, as the third corresponding states parameter. These two equations are applied from the triple point to the critical point. Both correlations give more accurate vapor pressure predictions than the Lee-Kesler (1975) 2 2 Lee, B.I. and Kesler, M.G., 1975. AIChE J., 21: 510-527. correlation, especially at low reduced temperatures, with the correlation using the modified acentric factor as the third parameter being the most accurate. Additionally, an internally consistent approach is proposed for estimating the normal boiling point, critical temperature and critical pressure of petroleum fractions using a single low temperature vapor pressure data point. The procedure proposed uses either correlation from this work and gives an accurate estimation of the normal boiling point and acentric factor for heavy petroleum fractions for use in equations of state.