Measurement and modeling of hydrocarbon dew points for five synthetic natural gas mixtures

The dew points of five synthetic natural gas (SNG) mixtures were measured using a custom made chilled mirror apparatus. The chilled mirror apparatus was designed to detect hydrocarbon dew points from low pressures up to the cricondenbar. The experimental temperature range was from 235 to 280 K and the pressure range from 0.3 to 10 MPa. The synthetic natural gases were comprised of methane and gravimetrically prepared fractions of ethane, propane, i-butane, n-butane and n-pentane. The experimental data were compared to calculations with the Soave–Redlich–Kwong (SRK) equation of state with classical mixing rule. However, considerable and increasing deviations between calculated and experimental dew points were observed as the pressure approached the cricondenbar. Therefore, a model was utilized based on the Redlich–Kwong (RK) equation of state. The Mathias and Copeman (MC) function was used to express the temperature dependence of the attractive term for all components. For methane, different sets of MC coefficients were used below and above the critical point. An optimization procedure was employed to fit the coefficients of supercritical methane to both pure component fugacity and experimental dew points. There is good agreement between experimental data and modeling results. For pressures higher than the pressure corresponding to the cricondentherm, the proposed model is better than the standard SRK equation of state. Good predictions with the model were obtained when comparing to bubble and dew points data from literature.