Experimental Study On Hydrodynamics And Gas Back-mixing Characteristics Of Fluidized Beds With Vertical And Packed Internals

The stripper is an important part of fluid catalytic cracking(FCC)unit for oil refining.The stripping effect of the spent catalyst not only affects the yield of light oil and economic benefits but also affects a load of burning coke in the regenerator.In previous experimental studies,they often assume that the gas-solids flow is axisymmetric in cylindrical strippers,so a linear arrangement of measurement points located at radial direction are often used to reflect the gas-solids distribution in the entire bed cross-section.However,they ignored extreme heterogeneity of gas-solids in the circumferential direction in industrial stripper.In addition,hydrogen or helium gas with a relatively low molecular weight was often used to investigate gas back-mixing characteristics in the early studies.Actually,the oil and gas adsorbed and entrained in the catalyst in industrial device was a mixture of hydrocarbons with different molecular weights.Therefore,a single light traced gas does not accurately reflect the competitive adsorption and desorption behavior between different gases inside the stripper.In the large-scale cold model experimental device,high-precision pressure sensor,multi-channel fiber probe,and three different tracer gases are used to investigate the three types of strippers(i.e.empty stripper,vertical tube bundle stripper,packed stripper).By comparing the bed expansion,bed average density,pressure pulsation,solid phase fraction and bubble frequency in three types of strippers,the influence of different internals on the gas-solids flow in the strippers was systematically analyzed.The gas sampling points are traced at three different circumferential positions at the same axial height to analyze the uniformity of circumferential distribution of the tracer gas,and the gas tracer results are processed using a one-dimensional axial diffusion model.The different strippers and tracer gas have significant effects on gas back-mixing characteristics.In addition,the two kinds of distributors with different distribution uniformity are used to investigate the gas distribution uniformity effects on the stripping performance of the stripper.The research shows that under the same operating conditions,according to the order of the empty stripper,the vertical tube bundle member stripper and the packing member stripper,the bed expansion ratio is gradually increased,and the average bed density and pressure pulsation value are sequentially decreased;In terms of the uniformity of the radial distribution of the gas-solid two-phase,the filler member stripper is superior to the vertical tube bundle member stripper,and the empty tube stripper is the worst.For three different tracer gases,the degree of gas back-mixing is the most serious when using R22 as the tracer gas.When helium is used as the tracer gas,the degree of back-mixing is the lightest,and the adsorption of the tracer gas is stronger.The more serious the back-mixing.For the stripping process of the highly absorbing oil and gas components,it is necessary to further extend the gas-solid two-phase contact time,and further optimize the structure of the internal components to achieve better gas-solid contact effect.The axial diffusion coefficient measured by the filler member stripper is the smallest,and the axial diffusion coefficient of the vertical tube bundle stripper is smaller than that of the empty stripper.The packing member can effectively break large bubbles and play a role of diversion and redistribution,which results in obvious effects for improving gas-solid two-phase contact and reducing gas back-mixing.When the dendritic tube distributor is used in the same stripper,the tracer gas distribution,stripping efficiency and gas back-mixing degree are better than those of the loop tube distributor.A gas distributor with uniform distribution has a positive effect on improving stripping performance.