Hydrate technology for the concentration of aqueous salt solutions using fluorinated refrigerants.
Ngema, Peterson Thokozani.
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The world is facing a challenge with the scarcity of fresh water, due to an increase in the world population and the large expansion of industrial projects. This results in greater demand for fresh water. In such a scenario, seawater shows the potential as an alternative source of fresh water, if a suitable purification system can be economically incorporated. Desalination is a process used to remove salts from seawater, in so doing, converting into fresh water. In general, there are two desalination processes, which are thermal processes and membrane processes. These processes are used in water industries to treat seawater and wastewater. Seawater and industrial wastewater contain higher concentration of minerals such as NaCl, Na2SO4, MgCl2 and CaCl2, which need to be eliminated from drinking water. However, the thermal and membrane processes consume a large amount of energy. They are costly to operate because of the scaling and membrane damage that is caused by saturated sulphates and the presence of chlorides, respectively. It is important to recover fresh water from concentrated brine solutions at ambient conditions, consequently, desalination using gas hydrate technology has been proposed by several researchers to be an alternative technology. This is simply because gas hydrate technology offers one of the most promising economical alternatives for desalination of seawater and industrial wastewater, especially in the use of fluorinated refrigerants as hydrate former in the presence of a promoter. The use of fluorinated refrigerants to form gas hydrate is attractive because it may facilitate hydrate formation in ambient environments. Then, the dissociation of gas hydrate results in the production of fresh water after all the minerals and contaminants are eliminated. However, there is a lack of research regarding the formation of hydrate using fluorinated refrigerants with single and mixed electrolytes, as well as in the presence of promoter. Consequently, one of the objectives of this study was to conduct the extensive research that is required for the hydrate phase equilibrium data for refrigerants, with electrolyte solution (single and mixed electrolytes), as well as in the presence of the promoter. The generated hydrate dissociation data and the apparent rate constant obtained from the kinetic data were used to design the gas hydrate reactor for the proposed process for the treatment of seawater and industrial wastewater using gas hydrate technology. Another objective of this study was to develop a hydrate electrolyte equation of state to model the generated data as well as to undertake future predictions.Hydrate dissociation data were measured using the non-visual isochoric equilibrium cell designed by author in the MSc programme in 2014to conduct a gas hydrate measurement. In this study, gas hydrate measurements were conducted using the pressure-search method. The synthetic saline solutions were prepared within a typical range for industrial wastewater concentrations,such as those found at Tutuka Eskom,and seawater concentrations. It was found that the hydrate was formed, but phase boundary condition was shifted slightly, to lower dissociation temperatures. In the presence of promoter, it was revealed that dissociation temperatures were closed to ambient conditions. Moreover, higher concentrations of electrolytes above seawater concentration were investigated. It was revealed that as the concentration of electrolytes increases, hydrates dissociation temperatures shifted more toward slower values. In the presence of a promoter the dissociation temperatures increases, but they were below ambient temperatures. The solubility of electrolytes was measured to ensure that no salts are above the solubility limits.Among the studied refrigerants (hydrate former), it was found that hydrate former (R410a) was suitable to be utilised for the desalination gas hydrate technology due to its properties such as being environmentally-friendly, not harmful to human, availability, lower cost and form hydrate at lower pressures near ambient temperatures. The use of R410a as hydrate former shows that hydrates are formed near ambient temperatures, where the main purpose is to design the process to operate at ambient conditions. Cyclopentane was used as a promoter.It shows impressive results on hydrate systems,because it was able to shift dissociation temperatures near to ambient conditions, and can therefore can be used as a promoter in hydrate processes. All gas hydrate measurements were modelled using the developed combinations contributions terms, namely the Hydrate Electrolytes Cubic Plus Association (HE–CPA) equation of state. The results obtained show that the hydrate dissociation data strongly agree with the model results. Consequently, it is recommended that this model can be used for predictions for hydrate systems and can be used in water industry to optimise processes. It was concluded that for the proposed desalination process using gas hydrate technology, the R410acould be used as hydrate former in the hydrate reactor to form hydrates slurry. The proposed process was designed at a high level,where only hydrate reactor, separator and compressor were designed. It is recommended that one has to design and scale up a process in full.