(225g) Binary Adsorption of Pentafluoroethane (HFC-125) and Difluoromethane (HFC-32): Thermodynamic Modeling of Pure and Binary Adsorption for Process Design

A 2021 report published by the United Nations (UN) highlighted the severity of anthropogenic global climate change and the pressing need to reduce greenhouse gas (GHG) emissions. Although CO₂ contributes the most toward climate change due to the amount released, other GHGs such as CH₄, NO₂, and fluorocarbons are additionally problematic and have much higher global warming potentials (GWPs) on a per mass basis (e.g., the 20-year GWP of CO₂ is 1, whereas that for NO₂ and CH₄ is 280 and 56, respectively). If GHG emissions are not reduced, the UN projects a global temperature increase of 1.5-2.0 K, which will further increase the intensity of severe weather patterns such as flooding and drought. Hydrofluorocarbons (HFCs) are among the most potent GHGs with GWPs up to 10,000 times that of CO₂; therefore, a global phaseout of many commonly used HFCs including 1,1,1,2-tetrafluoroethane (HFC-134a, CH₂FCF₃, GWP 1,430), pentafluoroethane (HFC-125, CHF₂CF₃, GWP 3,500), and 1,1,1-trifluoroethane (HFC-143a, CH₃CF₃, GWP 4,470) is currently taking place. The U.S. American Innovation and Manufacturing (AIM) act in 2020 called for an 85% reduction in HFCs by the year 2035, whereas many European countries are starting to ban some HFCs from further use. HFC restrictions are expected to increase as the next-generation, low GWP hydrofluoroolefin (HFO) refrigerants are phased-in.

As HFOs are phased-in, the new refrigerants will displace an estimated 2,800 ktonnes of HFCs that are in global circulation. Rather than venting or incinerating the unused HFCs, it is desired to reclaim, recycle, and repurpose them as a more sustainable option; however, recycling HFC refrigerants is challenging since many are azeotropic or near-azeotropic mixtures that must first be separated. Since traditional distillation cannot separate the HFC mixtures other techniques including extractive distillation and both membrane and adsorption technology are being studied to perform the separations. Our group previously studied the use of zeolites 4A and 5A for separating refrigerant R-410A (50/50 wt% HFC-125/HFC-32) and found that both adsorbents are commercially viable for the application. Other studies have been conducted with similar conclusions using additional zeolites, carbons, and MOFs with other binary and ternary HFC mixtures. Adsorbent-based separation is an established technology that can be used to perform the challenging separations needed for recycling HFCs.

In order to design commercial-scale separation process (e.g., pressure swing adsorption and temperature swing adsorption systems), reliable and accurate thermodynamic models must be established from thermodynamically consistent multicomponent adsorption data. The present poster will present the continued study of R-410A separation with an emphasis on thermodynamic modeling for process design. Pure adsorption isotherms have been measured for various zeolites using a Hiden Isochema XEMIS gravimetric microbalance. Binary adsorption measurements have been conducted for the same adsorbents using a separate XEMIS that operates in dynamic mode and uses the Integral Mass Balance (IMB) method. The data is used to build thermodynamic models based on both Ideal Adsorbed Solution Theory (IAST) and Real Adsorbed Solution Theory (RAST) and the accuracy of the models are assessed. Nonidealities in adsorption behavior will also be elucidated from comparing model results. The use of RAST models for modeling fixed-bed reactors will additionally be presented.