دانلود رایگان مقاله انگلیسی تشکیل هیدرات گازی در آب نمک دارای NaCl و MgCl2 در فشار کم - الزویر 2019

ABSTRACT

Hydrate-based desalination (HBD) has been developed to obtain fresh water from seawater in an economic and environmental sustainable manner. As a low-pressure hydrate former for the HBD process, chlorodifluoromethane (R22) hydrates were formed in the pressure range of 1–6 bar under brine environments, aqueous NaCl and MgCl2 solutions. Synchrotron X-ray diffraction and Raman spectroscopy measurements revealed that the R22 hydrates formed with NaCl and MgCl2 show structure I and enclathration of R22 molecules into the large cages. These results also confirmed that the salt ions cannot be encaged in the gas hydrate framework, reflecting the ion exclusion behavior for HBD above the eutectic of water and hydrated salts. The formation kinetics of R22 clathrate hydrate in the presence of salts show that both the initial growth rate and pressure drop of the R22 hydrates heavily depend on the salinity of aqueous solutions. A new theoretical approach adopting the transient time-dependent apparent rate constant of hydrate formation with salts was proposed to predict the formation kinetics of R22 hydrates under brine environments, which was in good agreements with the experimental results. These results provide good information for separating ionic compounds from aqueous solutions by hydrate-based separation processes.

4. Conclusions

In this study, we identified the crystal structure and guest inclusion behavior of R22 hydrates in the presence of salts, NaCl and MgCl2. Even though R22 hydrates were formed in the aqueous salt solutions, there were no significant changes in the crystal structure and guest inclusion behavior of the resulting R22 hydrates. This clearly indicates that salts such as NaCl and MgCl2 do not participate in the gas hydrate framework. To monitor the hydrate formation kinetics in various salt environments, we investigated the formation kinetics of R22 hydrates in the presence of 5 wt% NaCl, 10 wt% NaCl, 5 wt% MgCl2, and 10 wt% MgCl2. We confirmed that the hydrate formation kinetics can be significantly affected by the concentration of the salt solutions. Finally, the multi-stage formation of R22 hydrates in the presence of 10 wt% NaCl solution at 274 K revealed that the equilibrium pressure of the R22 hydrates in the presence of salts gradually increases with each progressive stage, leading to high hydrate yields and increased salt concentration in the brine solutions. This study provides key parameters for developing the HBD technology.