Study On The Gas-solid Flow Characteristics In The Dual-loop Circulating Fluidized Bed Reactor In PGFM Process

Propylene and Gasoline from Methanol(PGFM)process was proposed in order to relieve the imbalance between supply and demand of propylene and meet the environmental requirement for clean gasoline from the non-oil route in C hina.ZSM-11 catalyst was used in PGFM process in that it is resistant and insensitive to coke.Therefore,a novel dual-loop circulating fluidized bed reactor would be required in order to realize the continuous reaction-regeneration of the process.The operating mechanism of the novel reactor is described as follows.O n one hand,most of the spent catalysts directly flow into the bottom of the riser through the sub-loop with the coke continuously accumulating on them.O n the other hand,a small amount of spent catalysts which have met the requirement for burning flow into the regenerator to regenerate.Reconstructions of the cold-model circulating fluidized bed apparatus in the lab were needed to enable the solids circulating flow to operate successfully in both main-loop and sub-loop systems.Steady flow of the solids in standpipe was necessary to ensure the smooth operation of the dual-loop system.Experimental results indicated that a moderate aeration rate,whose optimum is 0.15m3/h,would help to eliminate the vibration and adjust the regimes in the standpipe.Aeration rate which was deviated far from the optimum would adversely affect the stabilization of the solids circulating rates of the standpipe.It was recommended that the regime in the standpipe should be maintained close to dense phase fluidized solids flow so as to enable sufficient driving force of the sub-loop circulating.The operation method and interaction mechanism of the dual-loop solids circulating fluidized bed were explored.Experimental results showed that the two circulating loops were not independently adjustable due to the restriction of system pressure balance relationship,whose specific behavior is that the increase of solids circulating rate of one loop leads to the decrease of the other.Superficial gas velocity in the riser and fluidizing gas rate at the bottom of the disengager were both the effective parameters to adjust the solids circulating rates of the two loops.The solids circulating rates of the two loops both increased with the increas ing of superficial gas velocity in the riser and fluidizing gas rate in the disengager would b e helpful to control the ratio of the solids circulating rates.The pressure drops of solids circulating rate control valves in both loops were positive,which would guarantee the normal operation of the whole circulating system.Flow characteristics in pre-lifting zone and bottom reaction zone of the riser were studied under the condition of one solids feed or two.Besides,the definition of the gas-solid phase contact efficiency was proposed.Experimental results showed that the difference between one and two solids feed were mainly focused on the radial non-uniformity and axial symmetry of the solids concentration distribution.C loser solids circulating rates of the two loops would contribute to better radial uniformity and axial symmetry of solids concentration profiles with higher gas-solid contact efficiency.The axial symmetry of the solids concentration profiles would be promoted with the increase of both superficial gas velocity and total solids circulating rate.Gas-solid phase contact efficiency would be decreased with the increasing superficial gas velocity while increased with the rising solids circulating rate.