A MIXING RULE TO INCORPORATE SOLUTION MODEL INTO EQUATION OF STATE

A mixing rule is developed to incorporate solution model in equation o f state (EOS) to extend the EOS to highly nonideal mixtures. To obtain the mixture parameter a of the EOS, the excess free energy of the sol ution model is matched to that of the EOS at a molar volume equal to a fixed multiple of the excluded volume b such that all of the mixed li quids are at a low pressure (by the EOS). The mixing rule is applicabl e to cubic EOS as well. as to noncubic perturbation type EOS, for whic h the zero-pressure-based or infinite pressure-based mixing rules are not applicable. A wealth of solution model parameters that have been f itted to low-pressure data are made useful by means of the mixing rule for phase equilibrium calculations at elevated pressures. Examples ar e presented of the use of the UNIFAC model with previously reported pa rameters and the Wang-Chao-Wilson model using newly fitted parameters incorporated into the Peng-Robinson EOS and the chain-of-rotators grou p contribution EOS.