Development Characteristics Of Particle Flow Structure In A High Density Circulating Fluidized Bed

Fluid catalytic cracking as one of the core processes for heavy crude oil refining,has played a very important role in processing heavy crude in our country.At present,the fluid catalytic cracking technology is developing to the larger catalyst-oil ratio operation.Therefore,the basic research of the high density CFB riser is very urgent.In order to fully understand the gas-solid flow characteristics in the riser reactor,an integrated circulating fluidized bed cold model with a height of 18 m was built.The high density operation conditions were reached with the superficial gas velocity of 5~9m/s and the particle circulation rate of 100~600 kg/m~2s.The characteristics of particle flow in a high density CFB riser were studied in detail by pressure sensors and optical fiber probes.There are lots of particles in the riser under high density operations.When the particle circulation rate reaches 300 kg/m~2s,the particle concentration is above 10%.Particle concentration presents the multi-segment distribution including the inlet control region,the bottom region with high particle concentration,the transition region,the full development region and the outlet region in the axial direction.It can be divided into two regions along the radial direction which is the dense phase region near the wall and the dilute phase region near the center.It is found that the flow development in radial direction at different axial postions along the riser is not synchronous.Particle flow in the central region of the radial direction develops rapidly while it develops gradually from the central region to the wall region.In the wall region,the particle concentration distribution is more sensitive to the operating conditions.The increase of G_s or the decrease of U_g will obviously slow down the development of the particle flow,especially the effect of the particle circulation rate G_s is greater.In addition,the radial inhomogeneous index of the particle concentration shows that with the increase of the particle circulation rate,the increase of radial nonuniformity index of particle concentration is very obvious.The radial inhomogeneity is more intense and the gas-solid interaction is stronger under high density conditions.By comparing the axial distribution of the particle concentration in risers of different heights,it is found that higher riser provides sufficient space for the particle flow to reach the development,which is beneficial to the full development state of particle flow.Besides,in risers of different heights with the same operating conditions,the particle concentration is slightly smaller and the particle flow is more uniform in a higher riser.Under high density conditions,a riser with high height can be used to observe the development state of the particle flow in a comprehensive way.Particle velocity shows the two-segment distribution giving the bottom acceleration region and the outlet region in the axial direction under high density conditions.Compared with low density operations,the development of particle flow is slower under high density conditions.The outlet effect of the riser is serious,which greatly slows down the particle velocity.The increase of the superficial gas velocity or the decrease of the particle circulation rate will increase particle velocity.Particle velocity varies slightly with superficial gas velocity or particle circulation rate under high density conditions.Particle velocity in the center area of the riser is very large,and the particles have reached the constant speed region under 10 m of the riser.Particle velocity in the wall area is small,and it is difficult to reach the constant speed region.There is a strong correlation between the axial distribution of particle velocity and particle concentration in the riser,which shows the good relationship between particle acceleration region and transition region,constant speed region and full developing region in the axial position.Flow development is totally different in high and low density operating conditions which is due to different interaction between gas-solids.In this study,using the pressure signals in the riser,the gas-solid interaction under different operating conditions is preliminarily obtained by using the power spectral density.The power spectrum energy under high density operations is 2 orders of magnitude higher than that of low density operations,which indicates that gas-solid interaction is more intense than the low density operations.