| The storage and transportation technology of natural gas hydrate(NGH Technology),Utilizing the excellent gas storage capacity of hydrates,converts natural gas from gaseous to solid hydrate for easy transportation.lt is an totally new and intrinsically safe storage and transportation technology of natural gas hydrate and expected to become an effective supplement and alternative technology for LNG or PNG technology,therefore,it has attracted wide attention and in-depth researches of all parties.But most of these studies are still concentrated on laboratory research and theoretical calculations,and the studies on the transfer characteristics of hydrate production process and the multiphase flow transfer characteristics of reaction system are relatively weak.The understandings of the multiphase flow and interphase transfer characteristics of natural gas hydrate production process are mostly empirical and there are only a few theoretical models.In view of the complexity of hydrate preparation process and the importance of industrial application of hydrate technology,it is necessary to carry out scale-up experimental researches and relevant numerical simulation.Meanwhile,the magnification of the hydrate production equipment faced two issues:High efficiency mixing of gas and liquid and The removal of reaction heat.So around these two issues,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,kinetic theory of granular flow,Population Balance Module,heat and mass transfer theory,hydrate formation kinetics theory,phase equilibrium theory and multiphase flow experimental research are combined.It is not only using experiment to measure directly,but also using the simplified models based on mechanisms and numerical simulations to characterize multiphase flow systems of natural gas hydrate production process.It also uses reliable experimental data to guide and verify theoretieal analysis and numerical simulation studies.The main research contents and innovations are as follows:1.An idea of a hydrate preparation system under an endogenous force field is innovatively proposed.A new 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 a new internal spiral-grooved tube is carried out.The effects of superficial velocity,particle size,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,solid particle and gas,solid and gas accumulate to the center of tube and the concentration decrease near the wall.The existence of secondary flow makes the rapid refreshment of each interface and the effective separation of the reaction mixture,and promotes the heat and mass transfer process of the hydrate formation system.3.CFD method combining with population balance model(PBM),Considering the effect of coalescence and breaking on the flow field and mass transfer in the flow process and,based on the solute permeation model and Kolmogorov isotropic turbulence theory,was utilized to simulate gas-liquid mass transfer coefficient under different temperatrues and pressures,apparent velocities,and gas composition in the internal spiral-grooved tube.Under certain temperature,the pressure has no effect on the gas-liquid mass transfer coefficient kl.Meanwhile,the mass transfer coefficient kl decreases with the decrease of the temperature under certain temperature.With the increase of the apparent velocity of gas and liquid,the gas-liquid mass transfer coefficient kl increases,the mass transfer coefficient near the tube wall is the largest,while the mass transfer coefficient at the center of the tube is the smallest,and the order of magnitude is between 10-5 and 10-4.Due to the difference of diffusivity rate of gases in water,the gas-liquid mass transfer coefficients for different gases are also different.4.The spiral inner tube has the effect of strengthening the heat transfer and the Nu number in the tube under different conditions.With the increase of Reynolds number,the Nu number of the gas-liquid two-phase flow in the spiral inner grooved tube is the largest.The Nu number of the single-phase flow is the second and the Nu number of the single-phase flow is the smallest.Originality propose the use of a hybrid heat transfer method in the NGH production reactor:Falling liquid film outside the tube combined with the intermal 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 enhanced heat transfer simulation of single-phase flow(liquid phase)and gas-liquid two-phase flow in the intermal spiral-grooved tube was carried out by using CFD method.Because of the obvious secondary flow phenomenon in the internal spiral-grooved tube,the turbulence of the fluid is improved.Meanwhile,according to the field synergy theory,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 Nu number in the internal spiral-grooved tube under different conditions increases with the increase of the Reynolds number.And the Nu 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 calculated.Compared with the traditional shell and tube heat exchanger,it was proved that the heat transfer efficiency of the falling film flow is much higher.Heater;Simultaneously simulated the falling film flow and heat transfer process by VOF method and verified each other.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.5.The hydrate formation kinetics model was modelled after extending the classical hydrate nucleation and growth theory proposed by Kashchiev and Firoozabadi for a single component gas-water system to the multi-component gas(natural gas)-water-sodium dodecyl sulfate system and according to the classical crystallization theory,the hydrate growth model was modified by the simulated gas-liquid mass transfer coefficient.The effective surface energy of the reaction system was used as the only model parameter,and its effect on the nucleation and growth rate of natural gas hydrate was investigated.The larger the effective surface energy,the lower the nucleation rate,and has no effect on the driving force and growth rate of hydrate formation.6.An NGH production pilot reactor consisting of a multi-tube(the intermal 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.By matching with the experimental data,the effective surface energy under different conditions was optimized.The hydrate kinetics model was used to calculate the total gas consumption,average gas consumption rate,and gas consumption rate curve during the reaction time.The average gas consumption rate results show that the model agrees well with the experimental data.Overall,the preparation of hydrates in the reactor is a very complicated process.The research in this paper only makes a preliminary analysis of this process.However,the combination of CFD and other technologies and experiments can provide theoretical guidance for the further design and construct reasonable reaction crystallizers,and provide possibilities for the large-scale production of hydrates. |
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