Measurement Of Particle Velocity And Mathematical Simulation For Gas-solid Fluidized Bed Of Fischer-tropsch Synthesis

The current situation of high dependence on foreign crude oil in China has brought severe pressure to our country’s economic development.At the same time,as a strategic resource,requires the development of non-oil routes technology.The medium and long term programme for the development of science and technology in our country list the"development of coal liquefaction and gasification and chemical conversion of coal technologies" as a priority development theme.Coal indirect liquefaction technology is one of the two main approaches for getting liquid fuels from coal,in which Fischer-Tropsch synthesis technology is the most critical core technology.The key to achieve Fischer-Tropsch synthesis is the preparation of catalysts and the reactor technology,in which gas-solid fluidized bed reactor is still a hot research topic.A gas solid fluidized bed cold model experiment device is set up in this paper,which consists of three parts:gas supply system,gas-solid fluidized bed,laser Doppler velocimetry and computer analysis system.Under the condition of good fluidization,the flow behavior of the particles in the fluidized bed is measured by using branched pipe distributor and the circle pipe distributor.When using branched pipe distributor,the dimensionless radial coordinate is in range of 0.8-1,the kinetic energy of the particles is consumed due to the wall effect,so that the particle velocity is small at this moment.However,dimensionless radial coordinate is 0-0.6,the velocity of the particles is significantly increased.When using circle pipe distributor,the particle velocity is almost the same while the dimensionless radial coordinates 0-0.8,compared with the branched pipe distributor,it shows that it has a certain effect to overcome the wall effect.By changing the superficial gas velocity in the fluidized bed,it can be concluded that as the superficial gas velocity increases,the particle velocity increases.When using circle pipe distributor,at the same axial height,the velocity is greater than using the branched pipe distributor.Under the action of the force,the velocity of the particle is from rising to descending,then descending to rising,there is a moving direction of the particle in this process.The absolute value of the amplitude of tangential movement statistics is defined to describe the level of particle fluctuations in the fluidized bed.With the increase of axial height,the absolute value of the amplitude of tangential movement statistics decreases.when the fluidized state is good,the absolute value of the amplitude of tangential movement statistics do not change significantly with the superficial gas velocity increase.When the fluidization state is not good,the absolute value of the amplitude of tangential movement statistics of the particle changes greatly and there is a significant fluctuation of the absolute value of the amplitude of tangential movement statistics absolute value and dimensionless radial coordinates.The ratio of upside and downside particles is defined by accurately measuring the number of upside and downside particles to quantitatively describe the flow state in the fluidized bed.In the complete fluidized state,the number of upside particles is equal to the number of downside particles.When the axial height is high,the superficial gas velocity is at 0.43m/s and 0.39m/s,the ratio of upside and downside particles rises considerably,the fluidized state of the fluidized bed is not good at this time.While the superficial gas velocity is at 0.47m/s-0.59m/s,the fluidization state is good,and the curve of the ratio of upside and downside particles is closer to 1 with the increase of superficial gas velocity.Two-fluid model is selected by using FLUENT and 2D non-steady state algorithm is selected for numerical simulation.At the same condition,when using circle pipe distributor,the particle velocity is greater than using the branched pipe distributor.At different axial heights,the phenomenon that the superficial gas velocity decreases with the increase of the axial height.This is consistent with the experimental phenomenon that the direction of particle motion can be observed in the simulation.In order to describe the Fischer-Tropsch synthesis reaction in the fluidized bed reactor better,a 3D mathematical model was established by using the open source code MFIX.The model considers momentum,heat transfer,mass transfer,and incorporates Fischer-Tropsch synthesis kinetics.The Fischer-Tropsch synthesis in the fluidized bed was simulated according to the mechanism described by Marvast et al.The simulation results show that the selectivity of C5+ decreases sharply with the increase of the wall temperature but is less affected by other reaction parameters.According to the lumped dynamics of Fischer Tropsch synthesis,the branched pipe distributor and circle pipe distributor are considered in the simulation.When the superficial gas velocity is in the range of 5 to 15 times of the minimum fluidization velocity,the production rate of all products are slightly higher than those using the branched pipe distributor when using circle pipe distributor.