Prediction of carbon dioxide frost point for natural gas and LNG model systems

Many applications dealing with carbon dioxide are limited because of carbon dioxide frosting. Therefore, it has been important to accurately predict the temperature where carbon dioxide solidifies. In this work, using solid – vapor (SV) equilibria, the frost points of binary and ternary model systems resembling natural gases – that are, methane + carbon dioxide, nitrogen + methane + carbon dioxide and methane + ethane + carbon dioxide are predicted by the predictive Peng-Robinson 1978 (PPR78), Redlich-Kwong-Soave (RKS), Nasrifar-Bolland (NB) equations of state and a solid fugacity relation. The equations of state are used to describe the vapor phase in SV equilibria. Accordingly, the equations of state are coupled with a group-contribution temperature-dependent binary interaction parameter that was previously developed from vapor – liquid (VL) equilibria. The solid fugacity relation was developed in this work for describing solid carbon dioxide. The solid fugacity relation expands around the triple point temperature of carbon dioxide. It is explicit in temperature and pressure and needs triple point temperature and sublimation enthalpy of carbon dioxide as input parameters. The model is fully predictive and does not require any adjustable parameter. When the NB equation of state with the solid fugacity relation was applied to natural gas model systems (NGMS), the agreement with experiments was found satisfactory. The NGMS contained 0.1%–54.2% carbon dioxide, temperature and pressure ranged from 140 K to 216 K and 100 kPa–3500 kPa, respectively. The effects of nitrogen and ethane on the frost point were also evaluated and the model accuracy was found adequate. Average absolute deviation (AAD) in predicting 291 experimental frost points for binary and ternary mixtures containing carbon dioxide was found to be 1.58 K.