Pilot Scale Extractive Distillation for Separation of High Global Warming Potential Refrigerants Using Ionic Liquids

Growing environmental restrictions on hydrofluorocarbon (HFC) refrigerants because of their high global warming potential (GWP) have driven the transition to lower-GWP alternatives. However, next-generation refrigerant mixtures comprising azeotropic blends are difficult to separate and recover because conventional distillation techniques are not effective. Ionic liquids (ILs) have been proven to facilitate the energy-efficient separation of refrigerant mixtures, enabling their recycling and reuse in various applications.

This research focuses on optimizing extractive distillation with IL (EDIL) to separate azeotropic refrigerant mixtures. The separation purity of refrigerants from EDIL, such as R-410A, R-507, R-404A, R-407C, and R-513A were studied using gas chromatography (GC). The single refrigerant components, difluoromethane HFC-32 (CH₂F₂), pentafluoroethane HFC-125 (CHF₂CF₃), 1,1,1,2-tetrafluoroethane FC-134a (C₂H₂F₄), 1,1,1-trifluoroethane HFC-143a (CH₃CF₃), and 2,3,3,3-tetrafluoropropene HFO-1234yf (CH₂=CFCF₃) were used for preparing calibration curves. Two GC columns, the HayeSep D and the Carbopack B, were used and compared. Results showed that HayeSep D can quantify all refrigerant mixtures' purities with standard deviations <0.64% by weight and 95% confidence level error of ±0.08 to ±0.8 wt%. The column had lower GC operating temperatures and less expensive supplies than the conventional Carbopack B column, which are advantages of this methodology.

The interfacial tension (IFT) of two refrigerants, HFC-32 and HFC-125, in a mixture with ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C₂C₁im][Tf₂N], as a possible entrainer, was experimentally investigated. Based on the findings, the IFT of HFC-32 in [C₂C₁im][Tf₂N] and HFC-125 in [C₂C₁im][Tf₂N] mixtures increases as the temperature decreases. The IFT of HFC-32 in [C₂C₁im][Tf₂N] shows higher values with respect to HFC-125 in [C₂C₁im][Tf₂N] at the same HFC’s compositions. The IFT of HFC-32 and HFC-125 in binary mixtures with [C₂C₁im][Tf₂N] shows a non-linear relationship, indicating that they will not follow the ideal mixing rule.

Binary interaction parameters based on vapor-liquid equilibrium for ten ILs with HFC-32 and HFC-125 were used with the Peng-Robinson (PR) equation of state to model ternary system solubilities. Ten different ILs were evaluated on a selectivity-solubility front, finding that 1-ethyl-3-methylimidazolium tricyanomethanide [C₂C₁im][tcm] may offer the best overall performance as an entrainer. The ionic liquid 1-ethyl-3-methylimidazolium dicyanamide [C₂C₁im][dca] was also selected to compare the effect of a different anion in the IL. [C₂C₁im][tcm] showed a superior performance compared to [C₂C₁im][tcm] for the separation of R-410A in an EDIL process simulation. Optimal operational parameters for achieving 99.5 wt% distillate and bottom product purities were 20 feed stages, a solvent-to-feed ratio of 6, and a reflux ratio of 3.

A variable-volume view cell was utilized to determine HFC solubilities for ternary systems of HFC-32, HFC-125, and [C₂C₁im][Tf₂N] at three isotherms: 298.15 K, 323.15 K, and 348.15 K, with pressures from 0.1 to 3 MPa. GC analysis was used in line to determine the mass compositions of both the vapor and liquid phases. The densities of the vapor and liquid phases were measured for the ternary system. The experimental solubility data are compared to the theoretical values predicted using the PR equation of state. This work contributes to a comprehensive understanding and optimization of HFC refrigerant mixture separation using EDIL.