Accurate measurement of phase equilibria and dissociation enthalpies of HFC-134a hydrates in the presence of NaCl for potential application in desalination

Abstract

Phase equilibria, structure identification, and dissociation enthalpies of HFC-134a hydrates in the presence of NaCl are investigated for potential application in desalination. To verify the influence of NaCl on the thermodynamic hydrate stability of the HFC-134a hydrate, the three-phase (hydrate (H) -liquid water (L W ) -vapor (V)) equilibria of the HFC-134a+NaCl (0, 3.5, and 8.0 wt%)+water systems are measured by both a conventional isochoric (pVT) method and a stepwise differential scanning calorimeter (DSC) method. Both pVT and DSC methods demonstrate reliable and consistent hydrate phase equilibrium points of the HFC-134a hydrates in the presence of NaCl. The HFC134a hydrate is identified as sII via powder X-ray diffraction. The dissociation enthalpies (ΔH d ) of the HFC-134a hydrates in the presence of NaCl are also measured with a high pressure micro-differential scanning calorimeter. The salinity results in significant thermodynamic inhibition of the HFC-134a hydrates, whereas it has little effect on the dissociation enthalpy of the HFC-134a hydrates. The experimental results obtained in this study can be utilized as foundational data for the hydrate-based desalination process.

Effects of Salinity on Hydrate Phase Equilibrium and Kinetics of SF6, HFC134a, and Their Mixture

“…In recent years, researchers have reported the use of SF 6 mixture gas to improve the hydrate formation rate and thermodynamics, such as SF 6 /CO 2 , and SF 6 /N 2 . However, the formation of SF 6 /HFC134a hydrate has not been studied in term of its potential application to water desalination, despite having confirmed the thermodynamic and kinetic feasibility of pure 1,1,1,2-tetrafluoroethane (HFC134a) and SF 6 hydrates through laboratory-scale experiments. ,,, Considering the results of previous studies, the HFC134a/SF 6 mixture is expected to significantly lower the hydrate equilibrium compared with pure SF 6 hydrate.…”

“…Gas hydrates are nonstoichiometric crystalline compounds formed by the reaction of water (host) and gas molecules (guest) under conditions of relatively low temperature and high pressure . Recently, gas hydrates have been applied to gas hydrate-based desalination (GHBD), because of the increasing need for desalination given the scarcity of fresh water and the fact that sources for desalination are no longer limited to seawater. , In the GHBD process, water molecules form cages around a guest gas component, thereby effectively separating themselves from the brine solution even at temperatures higher than the normal freezing temperature of water . These hydrate crystals when melted are essentially fresh water and the guest component can be reused for desalination …”

“…HFC134a is an alternative to chlorofluorocarbons (CFCs) and chlorohydrofluorocarbons (CHFCs) in air-conditioning systems . Nowadays, SF 6 gas is being replaced by alternative protection gases with a low global warming potential such as HFC134a or SF 6 /HFC134a mixture for magnesium melt protection .…”

Hydrates for Cold Storage: Formation Characteristics, Stability, and Promoters

“…Therefore, hydrochlorofluorocarbon (HCFC) and hydrofluorocarbon (HFC) refrigerants (such as R134a and R141b), which are alternative working substances, have been extensively investigated [3]. Lee et al [17] and Hong et al [18] used stepwise differential scanning calorimetry and different heating methods to determine the operating conditions for R134a. Liang et al [19] analyzed the crystal structure and crystallization characteristics of R141b gas hydrates and their results confirm that only structure II clathrate hydrate crystal was formed.…”

Thermodynamic and Kinetic Investigation of Desalination by Refrigerant Clathrate Hydrate Formation

“…Jeong et al focused on their experimental study to design a hydrate formation reactor to produce R134a hydrate slurry for economic application in desalination of seawater by applying gas hydrate technology [17]. Lee et al investigated the effect of presence NaCl in the system forming to hydrate on the thermodynamic hydrate condition to the R-134a hydrate, the systems consisting of the R-134a+ (0, 3.5, and 8.0 wt%) of NaCl+ water were measured by using isochoric (pVT) method and a stepwise differential scanning calorimeter (DSC) method [18]. Advantages of clathrate hydrate methodare: 1.…”