| For processes to maximize low carbon olefin,such as two-stage riser catalytic process of heavy oil for maximizing propylene yield(TMP),final product distribution depends on not only uniform flow structure in risers,but also solids circulating intensity.Under given conditions,to improve particles lifting effects in bottom pre-lifting section,overcome the bottleneck of catalysts circulation encountered in current apparatus and achieve high-flux and high-density operations has been the important technology development direction of circulating fluidized bed(CFB)in recent years.All experiments were carried out in a cold-model apparatus,using air and FCC catalysts as research objects.Effects of operating conditions and the position of pre-lifting outlet on solid fluxes as well as solids concentration distribution inside this section were investigated.Meanwhile,the causes were also analyzed.The results showed that the solids circulation rate increased with increasing operating gas velocity and reduced flow pressure drop.The pressure supporting gas-solids flow in riser was from storage vessel and butterfly valve.Increasing particle storage and decreasing butterfly valve pressure drop also helped boost solids circulation.Solids distribution in this experimental CFB apparatus was characterized by axial monotonic exponential function distribution and radial core-annulus structure.when the solids flux provided by storage vessel was high enough,elevation of pre-lifting outlet position had changed gas-solids flow structure in the gas-solids confluent zone by preventing solids downwards and pushing solids upwards.Detailed analysis of pressure balance during pneumatic conveying,and effects of gas back-mixing as well as gas-solid flow pattern at the bottom of riser on solids circulation rate were given in this study,with the flow structure inside the pre-lifting section described.Especially,this paper made a comparative study of the solids fluxes as well as the difference of gas-solids flow behavior in pre-lifting section and region adjacent to nozzles under different height of pre-lifting section,with the gas flow structure in pre-lifting section first predicted.The results showed that pre-lifting gas and jet injection were two important driving forces in pushing particles upward in riser.Multiple gas countercurrent contact,along with internal circulation flow as well as regional vortex effect,had effectively promoted radial particle mixing and solids circulation rate in bottom region.Lower pre-lifting height could help achieve better performance in overall solids fluxes,solids holdup as well as gas-solid contact in nozzles region.Combined with industrial production application,effects of two different pre-lifting structure on solids circulation rate were investigated,as well as the gas-solids flow behavior compared both macroscopically and microscopically.The results showed that there was a remarkable difference in terms of solids circulation rate between standpipe-type riser and showerhead-type riser.Due to more particles flow areas and reduced entrance resistance,solids circulation rate in standpipe-type riser as a whole was higher than showerhead-type riser,especially in high particle feeding intensity.Under same operating condition,solids holdup at the bottom in showerhead-type riser was higher than standpipe-type riser.Higher radial non-uniformity index and intermittency index in showerhead-type riser indicated that while showerhead distributor caused intense particle turbulence,it also gave rise to more radially non-uniform profiles of gas-particle interaction and particle-particle interaction.Instantaneous solids concentration and probability density distribution reflected that although showerhead distributor could partly relieve gas-solids segregations in transition region,but did not bring fundamental change to general non-uniform flow structure. |
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