Development of a Group Contribution Method Based on UNIFAC Groups for the Estimation of Vapor Pressures of Pure Hydrocarbon Compounds

A group contribution method based on UNIFAC groups was developed to estimate the vapor pressures of pure hydrocarbons at reduced temperatures (0.450.95). The maximum pressure for the correlation is 35atm. Experimental vapor pressures for 456 hydrocarbons were collected and used to calculate model parameters. The developed model utilizes the combinatorial and the residual UNIFAC terms and the fugacity of pure hydrocarbons. The model parameters were fitted in two modes, using contributions from first-order groups. In the first case, the Gibbs free energy was considered only as an explicit function of temperature, and in the second one, in addition to the temperature effect, the effect of the molecular structure was also considered. Taking the molecular structure into account significantly decreases the prediction error. Hence, a uniform linear relationship to structure is considered. In the modified models, the error in prediction of the vapor pressure in the temperature range discussed is less than 15%.