Azeotropic Refrigerant Mixtures Separation using Ionic Liquid Entrainers

Abstract

Hydrofluoroolefins (HFOs) represent the next-generation in refrigerant technology, poised to revolutionize the industry due to having substantially lower global warming potential (GWP) and zero ozone-depleting potential (ODP). These characteristics position HFOs as the ideal replacements for hydrofluorocarbons (HFCs), which have been widely used in air-conditioning and refrigeration systems since the early 1990’s but are now being phased out due to concerns over having a high GWP. As the industry transitions to HFO refrigerants, there will remain millions of systems that will continue to have useful life and require servicing. Many HFOs are not drop-in replacements for HFCs, so refrigerant producers have developed HFO/HFC blends to meet the demands of the service industry and in some cases for new equipment. This strategy aims to reduce the overall GWP of the blend by using lower GWP HFC refrigerants mixed with HFOs while ensuring that the thermophysical properties are a close match with the HFC being replaced and compatible with existing equipment. This is especially critical for equipment that might not perform optimally or may even be incompatible when using pure HFOs. However, blending HFCs with HFOs introduces its own set of challenges, most notably the formation of azeotropic mixtures. These mixtures present difficulties during the recycling and reclamation processes of the refrigerants. Consequently, there is a pressing need for the development of highly efficient separation techniques to address this issue. One promising solution to this problem is extractive distillation, wherein an ionic liquid (IL) is employed as an entrainer. This method has shown significant promise in effectively separating azeotropic mixtures. The suitability of an IL as an entrainer is determined based on its selectivity and strong affinity towards specific components within the mixture. The selection of the most appropriate IL for a given gas mixture is influenced by several factors, including the desired purity levels of the separated products and the associated energy and cost implications of the separation process. To evaluate the performance of various ILs in this context, comprehensive simulations were conducted using an equilibrium-based and rate-based models in ASPEN Plus, as well as vapor pressure estimations and thermal stability analysis were carried out. Different imidazolium and phosphonium-based ILs were modeled to determine the effectiveness of separating azeotropic refrigerant mixtures including, R-410A, R-450A, R-456A, R-513A, R-515B, and R-516A, which are commonly used in refrigeration systems. The results of the simulation revealed notable differences among the ILs in terms of overall heat duties, the number of stages required for separation (NT), operating pressure (P), solvent-to-feed ratio (S/F), and reflux ratio (RR). A rate-based model simulation, incorporating experimentally measured mixture properties, was developed to replicate the performance of the pilot extractive distillation column for separating R-410A. In addition, the vapor pressure of ILs were estimated using non-isothermal TGA at temperatures above 298.15 K. Thermal stability of the ILs were analyzed and discussed to identify and avoid degradation conditions. These findings underscore the importance of carefully selecting the appropriate solvent for optimizing energy efficiency and economic feasibility for a more sustainable separation process.