Effect of Isomeric Structures of Branched Cyclic Hydrocarbons on Densities and Equation of State Predictions at Elevated Temperatures and Pressures

The <i>cis</i> and <i>trans</i> conformation of a branched cyclic hydrocarbon affects the packing and, hence, the density, exhibited by that compound. Reported here are density data for branched cyclohexane (C6) compounds including methylcyclohexane, ethylcyclohexane (ethylcC6), <i>cis</i>-1,2-dimethylcyclohexane (<i>cis</i>-1,2), <i>cis</i>-1,4-dimethylcyclohexane (<i>cis</i>-1,4), and <i>trans</i>-1,4-dimethylcyclohexane (<i>trans</i>-1,4) determined at temperatures up to 525 K and pressures up to 275 MPa. Of the four branched C6 isomers, <i>cis</i>-1,2 exhibits the largest densities and the smallest densities are exhibited by <i>trans</i>-1,4. The densities are modeled with the Peng–Robinson (PR) equation of state (EoS), the high-temperature, high-pressure, volume-translated (HTHP VT) PREoS, and the perturbed chain, statistical associating fluid theory (PC-SAFT) EoS. Model calculations highlight the capability of these equations to account for the different densities observed for the four isomers investigated in this study. The HTHP VT-PREoS provides modest improvements over the PREoS, but neither cubic EoS is capable of accounting for the effect of isomer structural differences on the observed densities. The PC-SAFT EoS, with pure component parameters from the literature or from a group contribution method, provides improved density predictions relative to those obtained with the PREoS or HTHP VT-PREoS. However, the PC-SAFT EoS, with either set of parameters, also cannot fully account for the effect of the C6 isomer structure on the resultant density