Formation And Dissociation Of Methane Hydrate Under Intensified Heat Transfer By Phase Change Of N-tetradecane

Gas hydrate formation is an exothermic process and gas hydrate dissociation is an endothermic process.Hydrate formation or dissociation will be hindered if heat is not removed or supplied,timely and effectively.In this thesis,heat transfer was intensified by the phase change of n-tetradecane,and the work has been carried out as following:(1)An experimental instrument for measurement of hydrate phase equiribrium data was established.n-Tetradecane was chosen as the phase change material.It was mixed with Span 80,Tween 60 and deionized water according to a proportion.Phase change emulsion was prepared by emulsifying the mixture using a disperser.The stability of the emulsion was studied.The sizes,size distribution and the temperature for solidification in the hydrator and a beaker of the n-tetradecane droplets in the emulsion were measured.The fusion point of the solid particles of n-tetradecane in the slurry was measured using DSC.Considering that the methane-water-hydrate three-phase equilibrium pressure was included in the kinetic model equation,it was measured in the prepared emulsion at temperatures between 276.55~282.25 K.And the measured three-phase equilibrium pressures were compared with those in pure water calculated by the software of CSMGem.(2)An experimental instrument for hydrate formation was established.The methane hydrate growth in aqueous slurries containing 25~45 wt% of solid n-tetradecane were investigated in a stirred semi-continuous hydrator at pressures between 4.70 and 6.46 MPa(gauge)and near the fusion point of solid n-tetradecane.A mass balance and a heat balance were established to quantify the methane enclathrated in hydrates during hydrate growth.The rule of hydrate growth rate was analyzed.The influences of pressure and the initial mass fraction of n-tetradecane in the slurry on the hydrate growth rate and the hydrate product were investigated.Considering the influences of mass transfer and crystal growth on the hydrate growth rate,kinetics models were developed to describe the experimental data.The induction time,hydrate growth rate and the hydrate product in the present of solid particles of n-tetradecane were compared with those without solid particles of n-tetradecane.(3)An experimental instrument for hydrate dissociation was established.The dissociation of methane hydrates,which were distributed in slurries containing 45 wt% of liquid n-tetradecane,in a stirred batch hydrator was researched at initial pressures between 2.06 and 3.72 MPa(gauge)and near the solidification temperature of n-tetradecane.A mass balance and a heat balance were established to quantify the methane released from hydrates.The influences of initial pressure and temperature on the hydrate dissociation rate were analyzed.Two equations were established to describe the hydrate dissociation rate in terms of mass transfer and reversible process,respectively.Combining the mass balance,heat balance and the two equations,two equations were developed to predict the hydrate dissociation rates under different initial pressures and temperatures and they were named as mass transfer model equation and reversible model equation,respectively.The system pressure and the hydrate dissociation proportion under dynamic equilibrium were calculated according to the reversible model equation.The relationships between the actual hydrate growth kinetic constant,the actual hydrate dissociation kinetic constant and the mass transfer coefficient for hydrate dissociation,and the temperature were investigated,respectively.