Dynamic Analysis of Growth of Ice and Hydrate Crystals by In Situ Raman and Their Significance in Freezing Desalination

Abstract

In this study, the thermodynamic properties and dynamic behavior of ice and hydrate crystal growth in the freezing desalination process were studied. Carbon dioxide (CO 2 ) and 1,1,1,2-tetrafluoroethane (HFC134a) were chosen for this study from various hydrate formers reported in our previous study. A novel approach for calculating the water conversion to ice using the volume change of a gas phase in a reactor was proposed. Under the same subcooling condition, ice and hydrate crystal growth experiments were conducted and compared in pure water and 3.5 wt % NaCl solution. With the experimental conditions of this work, the crystal growth rate results were as follows: In the pure water system, ice crystal > HFC134a hydrate > CO 2 hydrate. However, in the presence of 3.5 wt % NaCl, the order changed as follows: HFC134a hydrate > ice crystal > CO 2 hydrate. These results were consistent with the visual morphology observations and in situ Raman analysis. The variations of subcooling temperature and NaCl concentration were studied using the Hu-Lee-Sum correlation and freezing point depression equation to evaluate the desalination efficiency of freeze desalination. Based on the Raman spectral analysis, we could identify specific hydrate structures and distinguish between ice and hydrate crystals. In situ Raman analysis revealed that dynamic O−H band spectral trends of ice and CO 2 hydrates (structure I) are similar and contrasting with those of the HFC134a hydrate (structure II). These results were first observed during the crystal formation process and can be useful in the design of freezing processes in industrial applications.

Molecular Study on Carbon Dioxide Hydrate Formation in Salty Water

“…In light of the fact that 97.5% of the earth’s water resources are seawater, desalination of seawater is considered to be one of the most effective methods for alleviating the freshwater shortages exacerbated by the growing global population. , Desalination is a process of the removal of salts and other chemicals from seawater. Except for the typical desalination technologies, for example, thermal desalination and reverse osmosis technology, there are several novel techniques proposed, such as geothermal energy or solar energy-based desalination, , water freezing desalination, − and clathrate hydrate-based desalination. − Clathrate hydrate-based seawater desalination has the potential to be a low-energy, environmentally friendly method to achieve desalination of saturated salinity water …”

Thermodynamic and kinetic study of fluorinated gas hydrates for water purification

“…The details of this calculation can be found in our previous work. [24] Figure 4 shows that the salinity of the residual solution gradually increases during hydrate formation. The salinity enrichment was significantly high in the HFC134a hydrate.…”

“…Therefore, the investigation of the salinity in the residual solution is essential for optimizing the operation and design of the water purification process. Previous studies [15,24] considered that the salt components (Na + and Cl À ions) are not contained in the hydrate crystals but are only accumulated in the residual solution. As a result, we can calculate the salinity in the residual solution.…”

“…The hydrate phase equilibria of each guest molecule in pure water and NaCl solutions (3.5 and 8.0 wt.%) were predicted by Hu-Lee-Sum (HLS) correlations and compared with experimental values in our previous studies. [2,24] To evaluate the effect of salinity on hydrate formation, the growth behaviour of hydrate was investigated. In situ Raman spectrometry was used to identify the structural information and cavity filling of each gas molecule.…”

Simulated and experimental thermodynamic investigations for the selection of suitable hydrate formers for water treatment

“…These studies were conducted to evaluate the potential of gas hydrates for water purification, which are reported in several publications. [2,10,11] They focused on the performance of the hydrate-based water treatment process. However, the criteria for the selection of guest molecules for pilot plants before starting the design process have not been adequately studied.…”