Extension of Cubic-Plus-Chain Equation of State: Incorporating Short-Range Soft Repulsion for Nonassociating Mixtures

The cubic-plus-chain (CPC) equation of state hybridizes the classical cubic equation of state with the chain term from the statistical associating fluid theory. The addition of the chain term allows the model to account for the physics of both short-chain and long-chain compounds within the same framework. The CPC framework is not restricted to a specific monomer formulation and radial distribution function, instead numerous modifications can be applied to the model. In this work, the three pure-component parameters are correlated with vapor pressures and liquid densities for 53 nonassociating components from different chemical families, including n-alkanes, alkenes, benzene derivatives, branched alkanes, cycloalkanes, ethers, and gases. The CPC-SRK-b(T) model proposed in this work uses a temperature-dependent segment covolume parameter (b) based on perturbation theory to describe short-range soft repulsion between molecules, whereas the previous version of CPC uses a temperature-invariant b. This CPC formulation is compared with two CPC versions for selected n-alkanes. The addition of the temperature-dependent function in b allows the CPC-SRK-b(T) model to outperform the other two forms in modeling the vapor pressure and liquid density. Furthermore, the vapor-liquid and liquid-liquid equilibria of various binary mixtures are simulated with the CPC-SRK-b(T) model showing excellent agreement with experimental data obtained from the literature.

Automated summary

The cubic-plus-chain (CPC) equation of state combines the classical cubic equation of state with the chain term from statistical associating fluid theory, allowing it to model both short-chain and long-chain compounds. The model can be modified and has been successfully used to simulate the phase equilibria of various mixtures.