Theoretical Foundation And Technology Of Desalinizing Seawater Via Gas Hydrate

Gas hydrate is one kind of clathrate crystal compounds.It is with specific themodynamics characteristics and special features of gas storage and salt exclusion.A novel proposal of desalinizing seawater is based on the formation,separation and dissociation of gas hydrate.At present,the gas hydrate based desalination technology still lacks systematic researches,In particularly,the thermodynamic phase equilibrium conditions,the gas,water consumptions and their quantitative relationships,the separation of hydrate slurry and desalination efficiency as well as the integrated system in hydrate based desalination process were not complete.This paper focused on the theoretical foundation and technology of desalinizing seawater via gas hydrate.The experimental and technical studies on gas hydrate based desalination have been conducted.This paper provided theoretical foundation for the application of hydrate based technology on desalination.In order to guide the guest choice and formation condition setting,the formation and dissociation processes of CO2-propane hydrate,CO2-cyclopentane hydrate and CO2-methylcyclopentane hydrate in porous media were investigated experimentally in this paper.The phase equilibrium conditions of these hydrates under different guest ratios were determined.The results indicated that the phase equilibrium conditions of structure ? hydrate is more moderate.Meanwhile,the relative gas uptakes of hydrate formation per unit volume water were compared.It is found that the hydrate of structure H,containing propane or cyclopentane,decreased the relative gas uptake,while the hydrate of structure ?,containing methylcyclopentane,increased the relative gas uptake.It is an inner requirement for hydrate based desalination to obtain a high hydrate yield under a short formation period.A low-field NMR system was employed to measure the real-time quantitative gas and water relationship of CO2 hydrate formation under constant pressure rapidly.Two stages of cage shaping and cage occupancy were distinguished by different consumption rates.The real hydration number of CO2 hydrate stable at 2.7,3.0 and 3.3 MPa were obtained.In order to reveal the temporal and spatial variation of liquid water during hydrate formation furtherly,an MRI system was used to observe the formation and dissociation process of CO2-propane hydrate in porous media.With the presence of propane,the migration of water induced by hydrate formation occurred.The evlolution process of drawing water,growing towards gas and keeping moveing was analyzed.It is general that the nucleation randomly and grow regularly.Decreasing the water drawn by the hydrates can improve the hydrate based desalination efficiency.A separation method by gas purging hydrate was developed and a hydrate based desalination experimental system with continuous separation under high pressure was designed and built.The separation methods of warming,squeezing and purging were employed to compare the desalination effects.The removal efficiencies of different ions in natural seawater using hydrate-purging method were analzed.By conducting multiple injection,formation and separation operations,the desalination efficiency exceeded 83%and the freshwater recovery reached 30%in the continuous desalination process.Based on the main experimental results above in this study,an integrated hydrate-based desalination system,coupled with three processes of Cv2 capture from 3000 m3/h flue gas,desalination of 1 t/h seawaer and utilization of cold energy from liquefied natural gas,was simulated in Aspen Hysys.The simple separator model and 30%conversion model were used in CO2 capture process and desalination process,respectively.After the utilization of cold energy from LNG,the enegy consumption of the two processes decreased 52%and 97%,respectively.