Multiphase Transport Phenomena In The Process For The Preparation Of Natural Gas Hydrate(NGH)

The storage and transportation technology of natural gas hydrate(NGH Technology),utilizing the excellent gas storage capacity of hydrates,converts natural gas to solid hydrate for easy and safer transportation,which is expected to become an effective supplement and alternative technology for LNG or PNG technology.It is of importance to pursue scale-up experimental researches and relevant numerical simulation.The two key issues,high efficiency mixing of gas and liquid and the removal of reaction heat are required to clarify.However,the understandings of the multiphase flow and interphase transfer characteristics of natural gas hydrate production are mostly empirical and there are only a few theoretical models.This paper focuses on promoting heat and mass transfer in heterogeneous reaction systems.For a novel hydrate reaction unit tube and a hybrid heat transfer method,CFD technology,heat and mass transfer theory and multiphase flow experimental research are combined.The main research contents and innovations are as follows:1.Anew concept of a hydrate preparation system with an endogenous force field is innovatively proposed,in whichan internal spiral-grooved tube was used as the hydrate reaction unit tube,and a hybrid heat transfer method using falling-film heat exchanger combined with spiral inner fins in the tube was used,which can promote the heat and mass transfer process of the system.2.A computational fluid dynamics(CFD)modeling of gas(natural gas)-liquid(water)-solid(hydrate)flow in the new internal spiral-grooved tube is carried out.The effects of superficial velocity,particle size,and bubble size on the flow features of the new spiral-grooved tube using the Eularian-Eularian-Eularian method with the kinetic theory of granular flow(KTGF)are systematically studied.Numerical simulation results show that because of the internal spiral grooves and the density difference among liquid water and solid particle,the solid phase accumulates to the center zone of the internal grooved tube and the solid concentration decrease near the wall.The existence of secondary flow makes the rapid refreshment of the interface and the effective separation of the reaction mixture,which promotes the heat and mass transfer process of the hydrate formation system.3.The spiral inner tube has the effect of strengthening the heat transfer and the Nusselt number under different conditions.With the increase of Reynolds number,the Nusselt number of the gas-liquid two-phase flow in the spiral inner grooved tube is the largest.The Nusselt number of the single-phase flow is the second and the Nusselt number of the single-phase flow is the smallest.The application of a hybrid heat transfer method is proposed in the NGH production reactor:Falling liquid film outside the tube combined with the internal spiral-grooved tube,which create a cold trap condition for the formation of NGH and promote the heat and mass transfer process in the system.The numerical simulation of enhanced heat transfer process of single-phase flow(liquid phase)and gas-liquid two-phase flow in the internal spiral-grooved tube was carried out by CFD method.Due to the obvious existence of the secondary flow in the internal spiral-grooved tube,the turbulence of the fluid is improved.Meanwhile,the production of secondary flow phenomenon increases the synergistic degree between the velocity field and the temperature field,and greatly improves the heat transfer efficiency of the fluid.The Nusselt number in the internal spiral-grooved tube under different conditions increases with the increase of the Reynolds number.And the Nusselt number of the gas-liquid two-phase flow in the internal spiral-grooved is the larger than that of the single-phase flow.The falling-film heat exchanger outside the reaction tube was designed and numerically simulated.It was proved that the heat transfer efficiency of the falling film flow is much higher.Meanwhile,the falling film flow and heat transfer process were simulated by VOF method,and the simulation results were verified with the results of the empirical formula.4.A NGH production pilot reactor consisting of a multi-tube(the internal spiral-grooved tube)gas-liquid bubble reaction crystallizer was set up.The hydrate formation experiment was conducted using the designed reactor,and the gas storage capacity,gas consumption and average gas consumption rate at different temperatures,pressures,and pump circulation rates were obtained.From the results,it can be seen that the average gas consumption rate increases as the pressure increases and the temperature decreases.At the same time,increasing the circulation rate of the pump can increase the gas consumption rate accordingly.