Investigation of theoretical maximum water yield and efficiency-optimized temperature for cyclopentane hydrate-based desalination.

Hydrate-based desalination (HBD) shows promise as a freshwater production technology for saline water. Liquid-phase hydrate formers, with their ability to facilitate hydrate formation at atmospheric pressure, have gained attention for their high energy efficiency in HBD. This study explored cyclopentane (CP) HBD by experimentally measuring the thermodynamic properties of CP hydrate in saline solutions and developing a theoretical framework to estimate the water yield of CP HBD under various operating conditions. The measured dissociation enthalpy of CP hydrate was found to be 12 % and 22 % lower compared to those of propane and R134a hydrates, respectively. The equilibrium dissociation temperatures of CP hydrate at different NaCl concentrations under atmospheric pressure were experimentally measured and then predicted using the Hu-Lee-Sum correlation. The theoretically achievable maximum salinity and water yield for CP HBD were calculated in the temperature range of 268-280 K and the initial salinity range of 0-8 wt.%. Additionally, the concept of HBD heat efficiency, representing the maximum amount of pure water producible per unit of heat, was introduced to identify an optimal operating condition for the HBD process. Efficiency-maximized temperatures, where the HBD heat efficiency reached its peaks, were determined for various initial salinities in the process, for example, 273.4 K for NaCl 3.5 wt.% solution. This novel approach provides invaluable guidance for determining the most energy-efficient operating conditions in the HBD process and establishes a solid foundation for further advancements in this field.

Thermodynamic and structural features of chlorodifluoromethane (a sI-sII dual hydrate former) + external guest (N2 or CH4) hydrates and their significance for greenhouse gas separation.

In this study, a new sI-sII dual hydrate former [chlorodifluoromethane (CHClF2); an important greenhouse gas with a global warming potential of 1810], which forms sI hydrate by itself and forms sII hydrate in the presence of external help guests such as CH4 and N2, was introduced and closely investigated for its potential significance in gas hydrate-based gas separation. The phase equilibria of CHClF2 hydrate, binary CHClF2 (5%) + N2 (95%) hydrate, and binary CHClF2 (5%) + CH4 (95%) hydrate were measured to examine the formation conditions and thermodynamic stability regions of CHClF2 + external guest hydrates. Nuclear magnetic resonance and in situ Raman spectroscopic results confirmed the formation of sII hydrates for CHClF2 + external guest (N2 or CH4) mixtures. Powder X-ray diffraction patterns clearly demonstrated a structural transition of sI to sII hydrates and a preferential incorporation of CHClF2 molecules in the hydrate phase when external guests (N2 or CH4) were involved in CHClF2 hydrate formation. The measured dissociation enthalpy values of CHClF2 hydrate, binary CHClF2 (5%) + N2 (95%) hydrate, and binary CHClF2 (5%) + CH4 (95%) hydrate using a high-pressure micro-differential scanning calorimeter also indicated preferential CHClF2 enclathration. The experimental results provide new insights into the thermodynamic and structural features of the CHClF2 (sI-sII dual hydrate former) + external guest hydrates for understanding and designing the hydrate-based CHClF2 separation process.

Evaluation of kinetic salt-enrichment behavior and separation performance of HFC-152a hydrate-based desalination using an experimental measurement and a thermodynamic correlation.

Hydrate-based desalination (HBD), a type of freezing-based desalination, can concentrate salts of saline water and produce fresh water via hydrate crystal formation. In this study, the thermodynamic stability, crystallographic information, and kinetic growth behavior of HFC-152a hydrate were investigated to estimate the desalination efficiency of HBD. The phase equilibria revealed that the HFC-152a hydrate could be formed at a higher temperature in the presence of NaCl (0 wt%, 3.5 wt%, and 8.0 wt%) than the HFC-134a hydrate at 0.3 MPa. The hydration number of the HFC-152a hydrate (sI) was found to be 7.74 through the Rietveld refinement of the powder X-ray diffraction patterns, and it was also used to determine the dissociation enthalpy of the HFC-152a hydrate. The Hu-Lee-Sum correlation was employed to predict the equilibrium shift and hydrate depression temperature of both HFC-152a and HFC-134a hydrates in the presence of NaCl. Faster hydrate growth kinetics and higher hydrate conversion were observed for the HFC-152a hydrate in saline solutions despite the smaller initial driving force at 0.3 MPa and the subcooling temperature of 3 K. Additionally, to quantify the desalination efficiency of the HFC-152a HBD, the maximum achievable salinity and maximum water yield were examined using the HLS correlation. The salt-enrichment efficiency decreased with an increase in the initial salinity and increased with increasing the subcooling. The overall results indicate that HFC-152a is, potentially, a superior candidate for HBD. The novel approach examined in this study will be useful for assessing the desalination efficiency of the HBD process.

Influence of Competitive Inclusion of CO2 and N2 on sII Hydrate-Flue Gas Replacement for Energy Recovery and CO2 Sequestration.

This study investigated the structural transformation, guest distributions, and the extent of replacement in CH4 + C3H8-flue gas replacement occurring in sII hydrates via gas chromatography, NMR spectroscopy, and powder X-ray diffraction (PXRD). Simulated flue gas (CO2 (20%) + N2 (80%)) was injected into an sII CH4 (90%) + C3H8 (10%) hydrate for guest exchange. The extent of replacement occurring in CH4 + C3H8-flue gas replacement was much lower than that of CH4 + C3H8-CO2 replacement. Furthermore, 13C NMR spectra and PXRD patterns revealed that unlike CH4 + C3H8-CO2 replacement, CH4 + C3H8-flue gas replacement did not undergo any structural transformation during the replacement (i.e., iso-structural replacement in the sII hydrate). Rietveld refinement of PXRD patterns of gas hydrates after replacement using flue gas injection demonstrated that CO2 molecules occupied both the small (512) and large (51264) cages, whereas N2 molecules occupied only the small (512) cages. CO2 and N2 were not complementary but competitive in replacing CH4 in the small (512) cages, which contributed to the maintenance of the cage stability of the initial sII hydrate and thus, resulted in a lower extent of replacement. The experimental results obtained in this study provide valuable insights on the accurate replacement mechanism and cage-specific guest exchange behavior of sII hydrates using flue gas injection for energy recovery and CO2 sequestration.