Prediction of solid formation conditions in mixed refrigerants with iso-pentane and methane at high pressures and cryogenic temperatures

Abstract

High boiling-point components in mixed refrigerants can improve the performance of natural gas and hydrogen liquefaction facilities. However, such heavy compounds can freeze out from the refrigerant mixture, posing blockage and plant shutdown risks for cryogenic heat exchangers. To improve the predictions of these conditions, freezing and melting temperatures of pure iso-pentane and (methane + iso-pentane) binary systems were measured at temperatures down to 87.5 K and pressures up to 13 MPa. The iso-pentane melting data are compared with predictions of a thermodynamic model embedded in the ThermoFAST software package. Adjusting the model's fusion molar volume change parameter to force agreement with the measurements reduced the deviations of the experimental data from the model by over 90% relative to the default parameter value. The measured melting data for binary mixtures were used to confirm solubility predictions for iso-pentane in mixed refrigerants. Adding 20 mol% iso-pentane to a methane-rich refrigerant increases the available duty for cooling natural gas or hydrogen from (313–123) K by a factor of three. This improvement outweighs the risk of freeze-out in this refrigerant with the melting temperature being 98 K, which is 15 K lower than the minimum temperature needed for LNG production.