Extension of Pitzer CSP models for vapor pressures and heats of vaporization to long-chain hydrocarbons

Abstract

Morgan, D.L. and Kobayashi, R., 1994. Extension of Pitzer CSP model for vapor pressures and heats of vaporization to long-chain hydrocarbons. Fluid Phase Equilibria, 94: 51-87. The three-parameter (Tc, Pc,ω) Corresponding States Principle (CSP) of Pitzer et al. (Pitzer, K.S. et al., 1955, J. Am. Chem. Soc., 77: 3427-3440) has been applied to long-chain n-alkanes in two ways using new vapor pressure measurements and heat of vaporization results from Morgan (Morgan D.L., 1990. Ph.D. thesis. Rice University, TX), and the critical point correlation of Twu (Twu C.H., 1984. Fluid Phase Equilibria, 16: 137-150). The first model, PERT2, is a direct extension of Pitzer's CSP in which a second order perturbation term has been included in the Taylor series expansion about ω = 0 for a given thermodynamic property, Ψ = Ψ(Tr,ω). In the second approach, C1/C8, the two real-fluid reference equation method of Ambrose and Patel (Ambrose, D. and Patel, N.C., 1984. J. Chem. Thermodyn., 16: 459-468) has been applied. A pair of reference fluids, methane and octane, are forced to predict vapor pressures of long-chain n-alkanes by regression of P'c and ω' parameters. While both models depend upon the same Tc, P'c and ω' differ from Pc and ω because of shape factors. Parameters (Tc, Pc, ω, P'c, ω') for the n-alkanes, C1 to C50, are presented. The two methods are compared with selected literature correlations and data and are shown to be of high accuracy. Older literature CSP correlations are extendible to long n-alkanes by use of the C1/C8 parameters. When boiling point and specific gravity inputs are used to predict vapor pressures of model coal-liquid compounds, PERT2 appears to be somewhat more accurate than LK/MB, the Lee and Kesler (Lee, B.I. and Kesler, M.G., 1980. Hydrocarbon Process., 59(7): 163-167), Maxwell and Bonnell (Maxwell, J.B. and Bonnell, L.S., 1957. Ind. Eng. Chem., 49: 1187-1196) correlation. Use of more accurately known parameters offers a considerable improvement over LK/MB.