Study On The Dissolutions Of CH₄ And CO₂ In Three N-alkanes And Their Hydration Rates Under Phase Change Of Ice Granules

Hydrates are formed by small gaseous molecules and water molecules at moderate temperature and pressure conditions.They have many applications such as natural gas storage,carbon dioxide capture and sequestration,desalination,refrigeration,and so on.However,large exothermic heat of hydration and lower solubility of the gas in water cause slower hydration rate and poor gas uptake during hydration.To enhance the formation rate,enlarge the water conversion and accelerate the nucleation during the hydration of methane or carbon dioxide,this work employs an experimental setup whereas a pressure-volume-temperature（PVT）unit is used to study the solubility of methane in three n-alkanes near the hydration conditions and hydrations of methane or carbon dioxide through pressure drop method.The following work is included in this dissertation:1)In this work,we established an experimental setup for researching the methane hydrate formation in the slurry system,and carried out experiments on methane hydrate in three different n-alkanes,including decane,heptane,and octane.The heat of methane hydration is removed by using a phase change of ice granules.First of all,n-alkanes are chosen to make emulsions of the water-in-oil in terms of their lower fusion points and higher methane solubilities,and the ice particles from the supercooled emulsions provided the nucleation center and removed the hydration heat promptly through their reversed phase change.Meanwhile,the solubility differences of methane in the paraffins and water enlarged the driving force of mass transfer.Finally,the methane hydration rates were significantly enhanced in a batch autoclave at initial conditions of temperature from-1 to-5.7℃,constant pressure of 5.90 MPa,water-cuts 10–30 vol.%,and stirrer speed of 700 rpm.The main work includes:studying the effects of water-cut,paraffins,and experimental operating temperature on induction time,hydrate formation rate,water conversion,and total hydrate formation,and optimizing operating conditions.The rate of hydration was significantly enhanced by 1–16 times greater than those achieved under indirect heat removal.It was observed that 20 Vol.%water-cut in hexane emulsion system at a temperature of-5.7℃ have maximum hydration rate of 158.22 kmol hr^-1m^-3and 95%of total hydration was completed in 1.75 min because of higher relative solubility in water and lower surface tension of hexane than octane and decane.It was also observed that the time taken by hydrates nucleation was shorter than one minute in most of the experiment because supercooled water changed phase from water to ice,whose fine particles provide nucleation center for hydration.2)An isothermal pressure-volume-temperature apparatus was used to determine the solubility of methane in those n-alkanes that were used in our previous work for methane hydrate formation.All the experiments were taken place at three different temperatures of267.15 K,269.15 K,and 271.15 K,which are close to the methane hydration conditions of the previous work.A pressure range of 1.5 to 6 MPa was used to measure methane solubility in the n-alkanes.The obtained equilibrium data were correlated thermodynamically with Peng-Robinson（PR）and Soave-Redlich-Kwong（SRK）equations of state（EOS）combined with the temperature-dependent one-parameter mixing rules.The two kinds of EOSs fitted the experimental data satisfactorily.3)A phase change heat removal method was used by making slurry of ice in oil to study the CO₂ hydrate formation process.Normal alkanes including decane,heptane,and hexane were used as a continuous phase in three different W/O emulsions.An autoclave was used to form hydrates by using ice granules as an immediate refrigerant and nucleation center and the surrounded normal alkanes as an absorbent of high solubility.The slurries of ice in normal alkanes from cooling three different W/O emulsions were prepared to perform all the hydration experiments in a batch autoclave at a constant temperature of 267.15 K and pressure range of 1.9-2.5 MPa with a stirring speed of 600rpm.Kinetics of CO₂ hydrate formation such as induction time,hydration duration,molar gas uptake,and hydrate growth rate were determined using a mole balance（PVT）model.Compared to earlier investigations of hydrate formation,in the present work,the hydrate growth rate increased by 7-39 times and enlarged to 352 times as compared to pure water while gas uptake per mole of water increased by 1.6-10 times.4)A shrinking core model was developed to correlate the experimental data during methane hydration in the presence of Sodium Dodecyl Sulfate（SDS）as a surfactant.The experimental data obtained in the form of time-dependent pressures in a batch hydration vessel.The trends of different SDS amounts and experimental pressures were also obtained and correlated with the developed model.The diffusion-based shrinking core model satisfactorily correlated the experimental data.