Alternative Desalination Technology for Saturated Salinity Water
최종 수정일: 2019년 4월 10일
For water security, the hydrate method can be economically used for highly contaminated wastewater or saturated salinity water.
The shortage of fresh water is among the most serious issues in the world. A representative technology to overcome the problem is desalination, but most conventional methods (RO membrane or thermal distillation) have been focused on the treatment of relatively low salinity water, such as seawater or brackish water.
Limitation of Conventional Technologies for Desalination
Presently, the conventional technologies most used for desalination are (i) thermal distillation processes (multi-stage flash, multiple-effect, vapor-compression evaporation, cogeneration, and solar water desalination) and (ii) membrane processes (reverse osmosis, electrodialysis, and membrane distillation). These technologies produce high-purity water at relatively low cost compared to other methods (e.g., direct/indirect freezing method or ion exchange). However, each of the conventional methods has its own intrinsic limitation; for example, multi-stage flash is a massively energy-intensive process, requiring both thermal and mechanical energy sources, and with reverse osmosis membranes, there is a salt concentration limit (total dissolved solids, ~60,000 mg/L) due to fouling effects.
No Concentration Limit, Hydrate-Based Desalination
To strengthen water security, in this study, we introduce a possibly economic technology for desalination of high salinity water (over-saturated concentration, in this study, a 30 wt% NaCl system) via gas hydrate formation by coupling LNG waste cold energy. First, the thermodynamic effects of NaCl on CH4 (methane), SF6 (sulfur hexafluoride), and HFC-134a hydrates were investigated. Based on the phase equilibrium of each hydrate, experimental pressures for kinetic experiments were selected under vapor pressure boundaries as follows: 4.5 MPa for CH4, 0.75 MPa for SF6, and 0.16 MPa for HFC-134a at 258.15 K (assuming the use of LNG waste cold energy). The results of the formation kinetics on the basis of gas moles consumed for hydrates showed the order CH4 > HFC-134a > SF6; however, after considering the hydration numbers and structures for each hydrate, surprisingly, the conversion rate of water to gas hydrates showed the order HFC-134a > CH4 > SF6, even though the experimental pressure condition for HFC-134a was very mild (0.16 MPa) compared to CH4 (4.5 MPa). For this interesting phenomenon, we suggest a possible mechanism through visual observations during hydrate formation. We believe these thermodynamic, kinetic, and morphological results show potential as an alternative desalination technology, especially for saturated salinity water, with lower energy consumption.
Is the hydrate-based desalination or water purification really practical? Is it possible to achieve high salt removal efficiency with only one-step process?
Even though there were many publications about hydrate-based desalination, it is true that this technology needs more practical and realistic approach for desalination
The hydrate-based desalination/water purification was proposed 70 years ago, however, its application on an industrial scale has still been unsuccessful due to its (i) high energy consumption for achieving the required low temperature and high pressure condition, (ii) relatively slow formation kinetics, and (iii) difficulty of hydrates separation from solution.
We're Making This Technology Applicable and Realistic.
We are currently working on another follow-up manuscript that will discuss a scale-up process and demonstrate how the fundamental data from this study is applied. Briefly, we achieved very high salt removal efficiency and large amount of hydrates with only one-step process (experimental duration = 1 h).
Publication
Title: Thermodynamic and kinetic analysis of gas hydrates for desalination of saturated salinity water
Journal: Chemical Engineering Journal
Volume: 370
Pages: 980-987