Enclathration of 2,3,3,3-tetrafluoropropene (HFO-1234yf) in structure II hydrate in the presence of 1,1,1,2-tetrafluoroethane (HFC-134a)

Some refrigerant blend mixtures are very difficult to separate, because they are designed to have nearly similar physical properties. This study was conducted to examine the practicability of the hydrate-based gas separation process for separating the HFO-1234yf from its mixture with HFC-134a, as the simple HFO-1234yf hydrate does not form and HFC-134a readily form structure II hydrate. The investigation concentrated on the phase equilibria for determining the operating condition and the structural and compositional features of HFC-134a and HFO-1234yf hydrates. Phase equilibrium measurement showed a monotonic increase in pressure with increasing temperature, and the phase equilibrium line shifted toward higher pressure and lower temperature regions as the HFC-134a concentration decreased. Powder X-ray diffraction analysis revealed that the HFC-134a+HFO-1234yf mixture formed structure II hydrates with Fd3̅m space group. The ¹⁹F and ¹³C MAS NMR spectra showed no detectable signals for the inclusion of R-1234yf to gas hydrate structure, which appears to be attributed to its low occupancy. The ¹³C CP NMR spectrum clearly showed peaks assigned to R-1234yf, and its enclathration was further confirmed by Raman analysis. Results confirmed that HFO-1234yf molecules occupied the large cages of structure II hydrate in the presence of HFC-134a molecules, and their cage occupancy was quite small (below 3 %). This indicates that the hydrate-based gas separation method is applicable to this azeotropic HFO and HFC mixture. A hybrid process consisting of hydrate-based gas separation and adsorption was proposed to enhance separation efficiency.