Research On Fluidization Decomposition And Multiphase Flow Characteristics Of Natural Gas Hydrate

With the increasing demand for energy resources,natural gas hydrate as a clean alternative energy has attracted wide attention.Solid-state mining method of natural gas hydrate provides a new idea for exploitation of natural gas hydrate,but the stability of long-distance multi-phase flow transportation of hydrate slurry has restricted its further application.Therefore,the fluidized bed is proposed,which is used as decomposition unit of natural gas hydrate in solid-state mining.After decomposition of natural gas hydrate,only methane is transported to sea level.This use of fluidized bed optimizes and improves solid-state mining process of natural gas hydrate,which is of great significance to utilization of natural gas hydrate resources.The study of multi-phase flow and reaction process in fluidized bed is significant for the practical application of fluidized bed.Therefore,the multiphase flow and decomposition of hydrate system are studied by numerical simulation in this work.Firstly,the three-phase fluidization process of methane-water-hydrate particles in fluidized bed is studied.The results show that gas holdup is mainly affected by the fluidized gas velocity.The axial gas holdup increases with the fluidized gas velocity.The radial gas holdup has a maximum in the middle of the bed and the maximum increases with the fluidized gas velocity.At the height of 0.3 m,the maximum value is 0.32 at 0.20 m/s.Axial solid holdup is mainly affected by fluidized gas velocity,solid density and initial solid volume fraction.When the fluidized gas velocity is greater than 0.10 m/s,solid density is less than 2200 kg/m3 and initial solid volume fraction is less than 0.1,the particles do not deposite at the bottom of the and the fluidization quality is better.Besides,the radial solid holdup fluctuates with the increase of fluidized gas velocity and initial solid volume fraction.The pressure drop is mostly affected by the initial volume fraction of solid phase.When the initial volume fraction is 0.4,the maximum pressure drop is 5794 Pa in all simulated conditions.Secondly,based on three-phase fluidization of methane-water-hydrate,the decomposition of hydrate particle in the fluidized bed has been studied.Since the decomposition of hydrate particle is a process coupled with heat and mass transfer,the heat and mass transfer between water flow and hydrate particle in the decomposition process has also been highlighted.The results show that the decomposition rate is affected by water temperature,water velocity and initial particle size.But the main controlling factor is water temperature.When the water temperature reaches 298 K,the hydrate particle size decreases from 2 mm to 0.644 mm in 60 s,and the conversion rate reaches 96.65%.The heat and mass transfer between hydrate particle and water flow is controlled by water velocity and initial particle size.Increasing the water velocity and decreasing the particle size can accelerate the heating rate of hydrate pariticle and reduce the methane concentration outside the particle.With the flow velocity reaching 0.40 m/s,the surface temperature of particles rises from 274 K to 293 K within 2 s.As the initial particle size decreases to 1.0 mm,the surface temperature of particles rises from 274 K to 293 K in 1s.Finally,due to the study of multi-phase flow of methane-water-hydrate particle and decomposition of single hydrate particle in fluidized bed,an axial model of hydrate decomposition coupled with flow in fluidized bed was established on the scale of reactor.The enlargement of the process was analyzed by increasing the bed width.The simulation results show that the decomposition rate of hydrate increases with the increase of bed temperature and the decrease of particle size obviously.The instantaneous conversion rate increases from 1.72%to 47.44%as the temperature increases from 283 K to 298 K.The instantaneous conversion rate of hydrate increases from 11.91%to 94.93%as the particle size decreases from 4.0 mm to 0.5 mm.In addition,the flow in the bed is greatly influenced by fluidized gas velocity.Increasing fluidized gas velocity is helpful to the uniform distribution of solid phase in the axial direction.Besides,the time for the bed to enter a stable state is shorter with the increase of fluidized gas velocity.Moreover,it is found that with the increase of the fluidized bed width,the instantaneous conversion of hydrate decreases slightly.