Prediction of Critical Temperature and Pressure of Hydrocarbons Using Simple Molecular Properties

Abstract

Four hundred and seventy (470) hydrocarbons (C_nH_m) were utilized to fit their critical temperature (T_c) and critical pressure (P_c) as functions of molecular weight and carbon atomic fraction. The proposed model is of the form: T_c=a*(C_frac)^b*(MW)^c and P_c= a*(C_frac)^1/3+b*(MW)^1/3+c where a, b, and c are the non-linear regressed parameters for the given model; C_frac is the carbon atomic fraction in a molecule, which is equal to n/(n+m) for a hydrocarbon compound; and MW is the molecular weight, which is calculated as (12n + m). The model was found to predict both T_c and P_c with an adequate accuracy, manifested via the associated percent relative error (PRE) of the curve-fitted T_c and P_c. Out of the examined 470 hydrocarbons, low MW compounds were found to have PRE values higher than 10% for the predicted T_c values. On the other hand, for Pc prediction, higher PRE values were found for higher molecular weight compounds, with C₂₆ and above.

Although the proposed model does not strictly differentiate among isomers having the same molecular weight and chemical formula, nevertheless, the difference in Tc and Pc among isomers is not significant to be picked up by a simple, straight forward model. A more rigorous model will work hard to offset such small differences in Tc and Pc among isomers, nevertheless, at the expense of model simplicity.