| A large amount of natural gas hydrate resources are found in deep sea.The natural gas produced will make the pipeline exhibit gas(natural gas)liquid(liquid water)solid(hydrate particles)three-phase feature during transportation.The hydrate particles in the pipelines may grow or decompose due to the changing temperature and pressure conditions.Under the condition of gas-liquid-solid three-phase flow,parameters such as temperature,pressure,flow rate and ratio of gas and liquid phase will affect the phase transition behavior of hydrate particles.In turn,the phase transition of hydrate particles will affect the distribution of hydraulic and thermal parameters of pipelines.The dynamic coupling of the hydrate particle phase transition with the flow parameters of the pipelines makes it very difficult to predict and analyze the hydrate kinetic parameters and pipeline flow parameters.Aiming at the above problems,method combined theory,numerical simulations and experiments was used to study the gas solubility rule under the equilibrium of liquid-hydrate two phases,the characteristics and mechanisms of hydrate particle growth and decomposition kinetics as well as the theory of gas-liquid-solid three-phase flow with hydrate particles,based on the theory of fluid mechanics,thermodynamics,kinetics,heat transfer and mass transfer.Models were established including a water-hydrate two-phase equilibrium prediction model,a hydrate growth kinetic model,a hydrate decomposition kinetic model,and a gas-liquid-solid three-phase flow mathematical model considering the phase transition of hydrate particles.The goal was achieved which is descripting the thermodynamic and hydraulic characteristics and laws of gas-liquid-solid three-phase pipeline flow coupling hydrate particle growth or decomposition.This work provides theoretical and technical support for the design,operation and management of deepwater natural gas hydrate transport pipeline.The specific research content and main achievements are as follows:(1)The predictive reliability of gas solubility in the aqueous phase under the equilibrium state of water-hydrate two-phase has an important influence on the prediction of hydrate growth driving force and growth rate.Considering the characteristics of liquid-hydrate two-phase equilibrium,the van der Waals-Platteeuw theoretical model was used to describe the hydrate phase.The methods of Henry's law,PR EoS,VPT EoS and TB EoS were used to calculate the gas fugacity and describe the aqueous phase with dissolved gas,respectively.As a result,four water-hydrate two-phase equilibrium prediction models named of v-HL,v-PR,v-VPT and v-TB were established to predict the gas solubility in this state.Using the experimental data in the literature and the commercial software Multiflash 4.4,the advantages and disadvantages of the four models were evaluated.Among them,the v-VPT model has higher precision.However,when describing the asymmetric interaction between water molecules and methane gas molecules,the model does not fully consider the nature of the polarity enhancement of the supercooled water,which leads to some deviation.In this regard,based on the experimental data,the methane-water binary interaction coefficient of the asymmetric contribution term under the non-density dependent mixing rule in the v-VPT model was re-fitted.So,the improved v-VPT water-hydrate two-phase equilibrium prediction model was formed..The improved model error has been reduced from the previous 13.7%to 5.2%and is superior to the simulation accuracy of the commercial software Multiflash 4.4.(2)Using the full transparent high pressure autoclave,16 groups of experiments were conducted to study the growth and decomposition kinetics of methane hydrate with mixing rate varying from 200 to 1000 PRM.Through the combination of experimental parameter measurement and phenomenon recording,the complete characteristics of hydrate growth and decomposition kinetics were obtained.The controlling mechanism of hydrate phase transition was explained.The experimental process can be divided into four stages:induction period,hydrate rapid growth period,hydrate slow growth period and hydrate decomposition period.Moreover,the influence of the stirring rate and hydrate volume fraction on the hydrate slurry flow characteristics was analyzed,according to the variation law of the stirring motor torque.Results showed that the critical volume fraction of hydrates that can be carried in the system was 20%when the Reynolds number was in the range of 4800 to24000.The flow resistance of the hydrate slurry increased sharply when the volume fraction higher than that.When the volume fraction exceeded 30%,the hydrate accumulation phenomenon was serious in some part of the cell,the fluidity of the slurry was drastically weakened,and the risk of plugging was faced.(3)Based on the theory of gas mass transfer in single film,taken the Skovborg&Rasmussen mass transfer restriction model as the basic form,the prediction methods of key parameters such as mass coefficient,gas-liquid interface area and concentration difference driving force were established,forming a comprehensive hydrate growth kinetic prediction model.Factors like the effect of the hydrate film coating on the gas-liquid interface and the viscosity of the hydrate slurry on the mass transfer were considered.The reliability of the model was verified by autoclave and flowloop experimental data.By correcting the maximum carrying volume fraction of solid phase to the maximum effective carrying solid volume fraction in the Graham viscosity model,the accuracy of the hydrate slurry viscosity model was improved,and the error of the growth kinetic prediction model was reduced from 53%to 37%,much better than the Boxall intrinsic growth kinetic model and other mass transfer limiting models.(4)Based on the hydrate decomposition "three-step" mechanism,a hydrate decomposition prediction model was established considering hydrate intrinsic decomposition kinetics and gas mass transfer theory.An empirical function of the undecomposed hydrate molar ratio was introduced into the model to indirectly characterize the effect of the number density of gas bubbles generated by the hydrate decomposition on the mass transfer coefficient.By comparing the model predicted values with the experimental values,the average deviation of the decomposition rate was 16.9%,and the average deviation of the decomposition mole percentage was 6.9%,which both had high accuracy.(5)Considering the coupling of the growth and decomposition of hydrate particles with the state parameters of the transportation pipeline,a mathematical model of gas-liquid-solid three-phase flow with the phase transformation of hydrate particles was established,based on the conservation principles.The model was numerically solved based on CFD technology through combining with hydrate particle growth and decomposition kinetic prediction models.The reliability of the model was verified by means of literature experimental data.Through the simulation of horizontal and vertical hydrate transportation pipelines,the development process of the hydrate particles and the three-phase flow laws in the pipelines were quantitatively described.The results showed that parameters,such as gas phase volume fraction,hydrate concentration and flow rate had a great influence on the pressure drop of the horizontal hydrate transportation pipelines.The phase transition of hydrate particles had effects on the mass flow rate,flow rate,phase distribution of each phase,temperature and pressure drop in the vertical pipelines.Among them,the decomposition of hydrate particles had a great influence on the flow velocity and little effect on the pressure drop.Based on the temperature distribution of seawater in the South China Sea,the flow safety of the deepwater natural gas hydrate transportation pipeline was analyzed. |
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