(388c) Pilot-Scale Extractive Distillation System to Separate the Azeotropic Mixtures of HFC-32 and HFC-125, from Recycled Refrigerant, R-410A Using Ionic Liquid [C1C2im] [Tf2n]

Hydrofluorocarbons (HFCs) are potent greenhouse gases that accelerate anthropogenic global climate change. Though the U.S. Environmental Protection Agency (EPA) proposed a rule under the American Innovation and Manufacturing (AIM) Act of 2020 to significantly reduce the production and consumption of these climate-damaging pollutants, today, an estimated 2,800 kilotons global refrigerant supply will need to be effectively and responsibly disposed of. This investigation presents the optimization of a pioneer, cutting-edge, in-house-built pilot-scale extractive distillation system fully instrumented and automatized to separate the azeotropic mixture of R-410A. This technology uses recycled refrigerant, R-410A (50/50 wt% of HFC-32 and HFC-125). The HFC-32/HFC-125 azeotropic blend is separated using 1-Ethyl-3-methylimidazolium bis (trifluoromethyl sulfonyl) imide [C₁C₂im][Tf₂N] as an ionic liquid entrainer.

Methods/Procedures/Process

A state-of-the-art extractive distillation system will be discussed that comprises a tower with structured packing, reboiler, flash tank, condenser, heat exchanger, reflux drum, pumps, and other minor equipment and instrumentation. Likewise, a gas chromatograph with a thermal conductivity detector and helium carrier gas is used to verify the tested refrigerant products' compositions. Initially, 3 kg/h of refrigerant R-410A (50/50 wt% of HFC-32 and HFC-125) is fed into the column as either a vapor or liquid at 20 °C. The column operates at 0.9 MPa, and its stripping and rectifying sections are split into four symmetric structured packings with 12 embedded RTDs. A camera and LED light are placed in each sight glass of the column to observe the liquid distribution entering the packing. Each packing has pressure drop sensors to determine the column hydraulics and flooding regions. Above the column packing sections, a single pass shell-tube vertical condenser collects the condensate into a reflux drum.

The facility chilling water is the cooling fluid that exchanges heat with the light product HFC-125 in the condenser. HFC-125 production rate is fixed, and the rest of the accumulated condensate is returned to the column; the proportionality of the flow meters calculates the reflux ratio. In the bottom part of the column, a partial reboiler is installed where the solvent [C₁C₂im][Tf₂N] and the heavy product HFC-32 leaves to feed a flash tank for ionic liquid recovery. A proportional valve or a diaphragm pump, aligned to the outlet stream, controls the reboiler liquid level. The flash tank operates at vacuum conditions of 0.01 MPa and 100 °C drawn by a vacuum pump. A demister is attached to the flash tank vapor stream to prevent solvent in the recovered HFC-32. In the flash tank, the liquid level is controlled by the outlet liquid flow through a diaphragm pump back to the column. A cooling water heat exchanger is used before the solvent returns to the column. Since the light product obtained from the flash tank condenses at low temperatures, a dry-ice bath is used before collecting the product in a refrigerant cylinder. All digital readings are recorded with the National Instruments LabVIEW software. All the vessels and internals are constructed with 304/304L stainless steel, which offers the most significant resistance to ionic liquid corrosion.

Results/Observations/Conclusion

Heavy and light refrigerant products are obtained from the extractive distillation system presented in this investigation work. The light product HFC-125 is produced with a purity of 99.5 wt%; the rest is HFC-32 in 0.5 wt%. On the other hand, the heavy product HFC-32 is recovered with 95.5 wt% purity, and the rest corresponds to HFC-125 in 0.5 wt%. The HFC-125 recovered refrigerant meets the technical specifications (AHRI 700) for the industrial commercialization and future reuse as a fluorinated feedstock for producing new low global warming potential products such as fluoropolymers, agriculture chemicals, and electrolytes for batteries. Similarly, the produced HFC-32 recovered refrigerant meets the technical specifications and can be repurposed into future refrigerant products such as R-454B (i.e., 68.9 wt.% HFC-32 and 31.1 wt% 2,3,3,3-tetrafluoropropene, HFO-1234yf) due to the low global warming potential.

Application/Significance/Novelty

This experimental work is the first attempt to separate HFC-32 and HFC-125 refrigerants by using the ionic liquid [C₁C₂im] [Tf₂N] in a pilot-scale extractive distillation system. Likewise, the technology presented corresponds to the first pressurized extractive distillation tower designed for using ionic liquid [C₁C₂im] [Tf₂N] as an entrainer. This innovation includes a continuous ionic liquid solvent recovery system with a vacuumed flash vessel. The results from this effort provide significant support to the industrial application of azeotropic separation with ionic liquid entrainer in extractive distillation for recycling HFC refrigerants.

Figure 1

Process flow diagram