Separation of Azeotropic Hydrofluorocarbon Refrigerant Mixtures: Thermodynamic and Kinetic Modeling for Binary Adsorption of HFC-32 and HFC-125 on Zeolite 5A

Abstract

Hydrofluorocarbons (HFCs) have been used extensively as refrigerants over the past four decades; however, HFCs are currently being phased out due to large global warming potentials. As the next generation of hydrofluoroolefin refrigerants are phased in, action must be taken to responsibly and sustainably deal with the HFCs currently in circulation. Ideally, unused HFCs can be reclaimed and recycled; however, many HFCs in circulation are azeotropic or near-azeotropic mixtures and must be separated before recycling. Previously, pure gas isotherm data were presented for both HFC-125 (pentafluoroethane) and HFC-32 (difluoromethane) with zeolite 5A, and it was concluded that this zeolite could separate refrigerant R-410A (50/50 wt % HFC-125/HFC-32). To further investigate the separation capabilities of zeolite 5A, binary adsorption was measured for the same system using the Integral Mass Balance method. Zeolite 5A showed a selectivity of 9.6–10.9 for HFC-32 over the composition range of 25–75 mol % HFC-125. Adsorbed phase activity coefficients were calculated from binary adsorption data. The Spreading Pressure Dependent, modified nonrandom two-liquid, and modified Wilson activity coefficient models were fit to experimental data, and the resulting activity coefficient models were used in Real Adsorbed Solution Theory (RAST). RAST binary adsorption model predictions were compared with Ideal Adsorbed Solution Theory (IAST) predictions made using the Dual-Site Langmuir, Tóth, and Jensen and Seaton pure gas isotherm models. Both IAST and RAST yielded qualitatively accurate predictions; however, quantitative accuracy was greatly improved using RAST models. Diffusion behavior of HFC-125 and HFC-32 was also investigated by fitting the isothermal Fickian diffusion model to kinetic data. Molecular-level phenomena were investigated to understand both thermodynamic and kinetic behaviors.

Investigation on Absorption and Separation Performance of R-32, R-125, R-134a, and R-1234yf Refrigerants Using EMIM-Based Ionic Liquids

“…Yancey et al stated that Zeolite 5A was an excellent candidate for the adsorption separation of R-410A with a selectivity of approximately 9.7. 6 Ribeiro et al successfully recovered R-32 from R-410A by a four-step vacuum swing adsorption process using activated carbon. 7 Moreover, Pardo et al tested membrane separation on a pilot scale with polymer/ionic liquid composite membranes based on polyether block-amide.…”

“…The present research focused on three different advanced separation technologies: adsorption separation, membrane separation, and extractive distillation. Yancey et al stated that Zeolite 5A was an excellent candidate for the adsorption separation of R-410A with a selectivity of approximately 9.7 . Ribeiro et al successfully recovered R-32 from R-410A by a four-step vacuum swing adsorption process using activated carbon .…”

Understanding the Molecular Features Controlling the Solubility Differences of R-134a, R-1234ze(E), and R-1234yf in 1-Alkyl-3-methylimidazolium Tricyanomethanide Ionic Liquids

“…In this sense, new F-gas legislation established a new paradigm in the RAC sector by including the term “reclamation”, defined as the reprocessing of a fluorinated greenhouse gas recovered during maintenance or prior to disposal, to match the equivalent performance of a virgin substance . At present, different technologies have been proposed for the separation of close-boiling and azeotropic refrigerant mixtures, such as membrane separation, − adsorption on porous materials, − or absorption-based extractive distillation processes. ,− This last application is where ionic liquids (ILs) have attracted the most attention because of their unique properties, among which their nonvolatility and the ability to solubilize large amounts of F-gases stand out, related to the objective of the present study. , …”