A GENERALIZED VAPOR-PRESSURE EQUATION FOR HEAVY HYDROCARBONS

Pitzer's acentric factor is widely used in physical property estimatio ns; one important area is that of generalized vapor pressure correlati ons. Since Pitzer's acentric factor is defined in terms of the vapor p ressure at a reduced temperature equal to 0.7, using this approach to generalize a vapor pressure equation gives excellent accuracy at reduc ed temperatures between 0.7 and 1.0, but predicts vapor pressures less accurately at lower reduced temperatures. To improve the vapor pressu re prediction of hydrocarbons at low reduced temperatures, the definit ion of the acentric factor is re-examined and a modified acentric fact or is proposed. Two sets of generalized vapor pressure equations are p resented, with Pitzer's acentric factor and the modified acentric fact or, respectively, as the third corresponding states parameter. These t wo equations are applied from the triple point to the critical point. Both correlations give more accurate vapor pressure predictions than t he Lee-Kesler (1975) (a) correlation, especially at low reduced temper atures, 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 poin t, critical temperature and critical pressure of petroleum fractions u sing 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.