Research On Parameter Estimation Method Of CPA Equation Of State Model

CPA equation of state（Eo S）was proposed by combining the association terms of the classical cubic SRK equation of state and SAFT equations of state.CPA is suitable for the prediction the thermodynamic properties of associated substances.The prediction ability of CPA Eo S and classic cubic Eo S was evaluated for the prediction of thermodynamic properties.The judgment method of the association ability and the fitting method for CPA model parameters were carried out.The method for the estimated the parameters of CPA model by elements and chemical bond was established.In this paper,the prediction ability of CPA model was compared with SRK,PR and PR-Sun Eo S for vapor pressure,liquid volume,and enthalpy of evaporation.The relative deviations of the predictions of CPA Eo S and experimental values of vapor pressure and enthalpy of evaporation are 0.46%and 8.30%respectively,and the results are similar to these of PR-Sun Eo S.The relative deviation of CPA Eo S for liquid volume is 11.47%,which is better than the other three classical cubic Eo S.The parameter A₂ of ln P^sat-T correlation for each substance was regressed with vapor pressures data of 10 kinds of compounds,and the associating strength was judged according to the deviation from the normal alkanes.The results showed that isomeric alkanes,halogenated hydrocarbons,ethers,esters,mercaptans and anhydrides were inert substances,while alcohol,acid,amine and ketone were strongly associated substances,ketones were weakly associated substances.The associating strength of substances gradually decreases with the increase of molecular weights.The methods for the regression of the parameters of the CPA model using saturated vapor pressure data,or saturated vapor pressure and liquid volume were evaluated and were marked as CPA-1and CPA-2 Eo S.The predictive ability of the CPA-1 and CPA-2 Eo S for the vapor pressure,liquid volume,and evaporation enthalpy was evaluated.The results showed that the CPA parameters regressed with saturated vapor pressure and liquid volume can significantly improve the predictions for liquid volume.The regression method for CPA parameters with the saturated vapor pressure and liquid volume is better than that with saturated vapor pressure.A method for estimating the parameters of CPA model based on elements and chemical bond was established.The contribution values of 11 elements and chemical bonds（C,H,O,N,C-H,C-C,C-O,C-N,O-H,N-H,C=O）were obtained by regressing the association parameters of the substances.The contribution values of elements and chemical bond are used to calculate the thermodynamic properties of the substances involved and not involved in fitting parameters.The relative deviations between the calculated results of the vapor pressure and the experimental values for the materials involved in the fitting and those not involved in the fitting are 6.61%and 8.15%,respectively,and the relative deviations between the calculated results of the liquid density and the experimental values are 9.70%and 8.49%.The estimated CPA model parameters with elements and chemical bonds can accurately predict the thermodynamic properties of substances.By establishing the method for justifying the association strength of compound,evaluating the prediction ability of CPA model with different regressing methods,and estimating the parameters of CPA Eo S with elements and chemical bonds,the application scope of CPA model is extended.The research results have important theoretical and practical values.