Chapter 4. Evaluation of guest-dependent hydrate-based desalination
4.1. Thermodynamic properties of gas hydrates in saline water
4.1.2. Thermodynamic properties of R152a hydrate
The accurate thermodynamic properties of gas hydrates are important prerequisites for the application of gas hydrates to various industrial processes because they can be fundamental criterion for
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determining the optimal operating conditions of the processes. To evaluate the thermodynamic stability of R152a hydrate in the saline system, the three-phase equilibria of the R152a + NaCl (0 wt%, 3.5 wt%, and 8.0 wt%) + water mixtures were measured using an isochoric method and the results were shown in Fig. 4.1.1 and Table. 4.1.1. The three-phase equilibrium data of pure R152a hydrate were in good agreement with those from the literature107. For saline systems, the equilibrium curves of R152a hydrate were shifted to harsher regions (represented by lower temperature and higher pressure) by the inhibition effect of salt ions. As seen in Fig. 4.1.1, the extent of the equilibrium curve shift increased with an increase in the NaCl concentration, indicating that an additional and larger subcooling for gas hydrate formation is required for saline water with a higher NaCl concentration.
Fig. 4.1.1. Three-phase equilibria of R152a + NaCl (0 wt%, 3.5 wt%, and 8.0 wt%) + water mixtures.
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Table. 4.1.1. Three-phase equilibria of R152a + NaCl (0 wt%, 3.5 wt%, and 8.0 wt%) + water mixtures Pure water NaCl (3.5 wt%) solution NaCl (8.0 wt%) solution T (K) P (MPa) T (K) P (MPa) T (K) P (MPa)
277.5 0.10 277.3 0.13 275.7 0.13
281.7 0.18 280.4 0.19 277.7 0.17
283.7 0.24 282.1 0.24 279.1 0.21
285.7 0.32 284.0 0.31 281.7 0.29
287.0 0.38 285.3 0.37 283.4 0.36
The crystallographic information about gas hydrates enabled us to estimate the amount of gas that could be captured in gas hydrates because the cage-to-water ratio in a unit cell of gas hydrates is an intrinsic characteristic of each hydrate structure. To examine the crystalline structure of R152a hydrate, PXRD was employed and the measured diffraction patterns of pure R152a hydrate were provided, as shown in Fig. 4.1.2. There has been a controversy over the crystalline structure of R152a hydrate (sI or sII)108. The PXRD patterns in Fig. 4.1.2 clearly demonstrate that the R152a hydrate is sI with a space group of cubic Pm3̅n and that it has a lattice parameter of 11.9888(5) Å (Rwp = 10.7% and χ2 = 8.08). The sI hydrate consists of 6 large (51262) cages and 2 small (512) cages with 46 water molecules, and its ideal unit cell formula is 6(51262)·2(512)·46(H2O). Due to the molecular size of R152a, the Rietveld refinement of the PXRD patterns showed that R152a molecules were captured only in the large (51262) cages of sI and the cage occupancy of R152a molecules in the large (51262) cages was 0.99, indicating that the hydration number, n, which refers to the ratio of the number of host water molecules to the number of entrapped guest molecules in a unit cell, is 7.74 (R152a·7.74H2O). As noted earlier, ions and salts are not included in hydrate crystals, so it is reasonable to say that the crystallographic properties of R152a hydrate are not affected by the surrounding saline environment.
89 Fig. 4.1.2. PXRD patterns of R152a hydrate.
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Because the continuous HBD process requires the repeated formation and dissociation of gas hydrates, the heat transfer (absorption or liberation) during the phase changes of the gas hydrates is a critical factor in establishing an energy balance of the process. In this study, the heat flows during the dissociation of R152a hydrate formed with pure water and NaCl (3.5 wt%) solution were observed using the HP μ-DSC, as depicted in Fig. 4.1.3 (a) and (b). The R152a + NaCl (3.5 wt%) hydrate showed a much broader endothermic heat-flow curve than the pure R152a hydrate. Because salt is excluded from hydrate crystallization, the NaCl concentration in the residual solution after R152a hydrate formation was expected to be much higher than 3.5 wt%. Therefore, as can be seen in Fig. 3, the temperature at which the R152a hydrate formed with NaCl (3.5 wt%) solution started to dissociate was much lower than that of the pure R152a hydrate, and even lower than the expected dissociation temperature for the NaCl (3.5 wt%) solution. As the dissociation of R152a hydrate progressed, the NaCl concentration in the residual solution gradually decreased due to the dilution caused by the water supply from dissociated hydrates; it finally approached the initial NaCl concentration. Because of the gradual and progressive dissociation of gas hydrates in the salt-containing systems, it was very difficult to measure the accurate onset temperature of R152a + NaCl hydrate using the dynamic DSC method.
However, the peak temperatures of the endothermic heat-flow curves in Fig. 4.1.3 (a) and (b) indicated that the presence of NaCl inhibited the hydrate formation and accordingly, the dissociation temperature of the gas hydrate was shifted to the lower one for saline water.
Since the values of cage occupancy and the hydration number of R152a hydrate were quantitatively given by the Rietveld refinement of the PXRD patterns, the dissociation enthalpy (ΔHd) value of the R152a hydrate was obtained by integrating the endothermic heat-flow curve measured by the HP μ- DSC. The ΔHd values of the R152a hydrates formed with pure water and NaCl (3.5 wt%) solution were found to be 82.2 ± 0.2 kJ/mol-gas and 81.9 ± 0.5 kJ/mol-gas, respectively, which were consistent with the literature value (85.5 kJ/mol-gas) calculated using the Clausius-Clapeyron equation109. A slightly larger error in the ΔHd value for the NaCl (3.5 wt%) solution was attributable to a slightly larger uncertainty in integrating the broader and more progressive heat-flow curve for the NaCl (3.5 wt%) solution, which might contain R152a hydrate as well as a small amount of solid NaCl and NaCl 2H2O at low temperatures91.
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Fig. 4.1.3. Heat flows during dissociation of R152a hydrates formed with pure water and NaCl (3.5 wt%) solution.
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