Thermodynamic modelling of VLE behaviour at the high purification of R717 refrigerant by high-pressure batch distillation

Abstract

The feasibility of using thermodynamic simulation models for describing the VLE behaviour of dilute ammonia-impurity solutions during high purification of R717 refrigerant by distillation is discussed. A comprehensive evaluation of different thermodynamic models (EoS-, LACT-based, combined EoS-LACT, predictive) has been done by comparing the simulation results with published experimental VLE data for ammonia-based dilute solutions in the pressure/temperature range from the boiling point of ammonia up to the critical point. The behaviour of the three important impurity groups (permanent gases, hydrocarbons, and water) in high-purity ammonia is considered in the dilute region. The studied systems are characterised by non-ideality, combine components from different chemical classes and with different polarity, contain subcritical and supercritical components, and can form associates in liquid and vapour phase. The results show that, depending on the system and the operating pressure, various modelling approaches can be successfully employed in the simulation of distillation-based separations involving VLE of non-ideal highly diluted ammonia-based mixtures. Equally important is the type of mixing rules, as well as the quality of the binary interaction parameters used in the relations. The usage of some of the predictive methods is shown to be a sensible option for a preliminary design.