Gas–solid flow in a fluid catalytic cracking （FCC） riser exhibits poor mixing in the form of a core–annulus flow pattern and a dense bottom/dilute top distribution of solids. To enhance gas–solid mixing, studies ...Gas–solid flow in a fluid catalytic cracking （FCC） riser exhibits poor mixing in the form of a core–annulus flow pattern and a dense bottom/dilute top distribution of solids. To enhance gas–solid mixing, studies on dense fluidized beds have suggested using a pulsating flow of gas. The present study investigates the effect of pulsating flow on gas–solid hydrodynamics inside the FCC riser employing computational fluid dynamics. Two flow conditions are investigated： a cold flow of air-FCC catalyst in a pilot-scale riser and a reactive flow in an industrial-scale FCC riser. In the cold-flow riser, pulsating flows cause the slug flow of solids and thus increase the average solid accumulation in the flow domain and solid segregation towards the wall. In the industrial FCC riser, pulsating flows produce radial profiles that are more homogeneous. Pulsating flows further improve the conversion and yield in the initial few metres of height. At 7 m, the conversion from pulsating flow is 59%, compared with 44% in without pulsating flow. The results and analysis presented here will help optimize flow conditions in the circulating fluidized bed riser, in not only FCC but also applications such as fast pyrolysis and combustion.展开更多
Two modes of gas-solid riser operation, i.e., fluid catalytic cracking （FCC） and circulating fluidized bed combustor （CFBC）, have been recognized in literature; particularly in the understanding of choking phenome...Two modes of gas-solid riser operation, i.e., fluid catalytic cracking （FCC） and circulating fluidized bed combustor （CFBC）, have been recognized in literature; particularly in the understanding of choking phenomena. This work compares these two modes of operation through computational fluid dynamics （CFD） simulation. In CFD simulations, the different operations are represented by fixing appropriate boundary conditions： solids flux or solids inventory. It is found that the FCC and CFBC modes generally have the same dependence of solids flux on the mean solids volume fraction or solids inventory. However, during the choking transition, the FCC mode of operation needs more time to reach a steady state; thus the FCC system may have insufficient time to respond to valve adjustments or flow state change, leading to the choking. The difference between FCC and CFBC systems is more pronounced for the systems with longer risers. A more detailed investigation of these two modes of riser operation may require a three-dimensional full loop simulation with dynamic valve adjustment.展开更多
文摘Gas–solid flow in a fluid catalytic cracking （FCC） riser exhibits poor mixing in the form of a core–annulus flow pattern and a dense bottom/dilute top distribution of solids. To enhance gas–solid mixing, studies on dense fluidized beds have suggested using a pulsating flow of gas. The present study investigates the effect of pulsating flow on gas–solid hydrodynamics inside the FCC riser employing computational fluid dynamics. Two flow conditions are investigated： a cold flow of air-FCC catalyst in a pilot-scale riser and a reactive flow in an industrial-scale FCC riser. In the cold-flow riser, pulsating flows cause the slug flow of solids and thus increase the average solid accumulation in the flow domain and solid segregation towards the wall. In the industrial FCC riser, pulsating flows produce radial profiles that are more homogeneous. Pulsating flows further improve the conversion and yield in the initial few metres of height. At 7 m, the conversion from pulsating flow is 59%, compared with 44% in without pulsating flow. The results and analysis presented here will help optimize flow conditions in the circulating fluidized bed riser, in not only FCC but also applications such as fast pyrolysis and combustion.
基金This work is financially supported by the National Natural Science Foundation of China under Grant Nos. 91334204 and 21576263, the Chinese Academy of Sciences under Grant No. XDA07080100, and the Ministry of Science and Technology of the People＇s Republic of China under Grant No. 2012CB215003.
文摘Two modes of gas-solid riser operation, i.e., fluid catalytic cracking （FCC） and circulating fluidized bed combustor （CFBC）, have been recognized in literature; particularly in the understanding of choking phenomena. This work compares these two modes of operation through computational fluid dynamics （CFD） simulation. In CFD simulations, the different operations are represented by fixing appropriate boundary conditions： solids flux or solids inventory. It is found that the FCC and CFBC modes generally have the same dependence of solids flux on the mean solids volume fraction or solids inventory. However, during the choking transition, the FCC mode of operation needs more time to reach a steady state; thus the FCC system may have insufficient time to respond to valve adjustments or flow state change, leading to the choking. The difference between FCC and CFBC systems is more pronounced for the systems with longer risers. A more detailed investigation of these two modes of riser operation may require a three-dimensional full loop simulation with dynamic valve adjustment.
