Coupled Simulation Of Flow And Reaction In Reaction And Regeneration System Of FCC

In the present study, based on the data from a bench scale riser (H=3.0m, D=0.012m, air and FCC catalyst are used as fluidizing agent, superficial velocity U0=2.95~3.44m/s, solid flux Gs=23~40kg/m2s). Firstly, a predictive mathematical model is proposed. The model calculation and experimental data shown: (a) Axial voidage increases with superficial velocity and decreases with the solid flux; (b) Higher solid volume fraction along riser in small scale riser than that in large scale one in the similar condition, especially in the bottom zone; (c) With the same superficial gas velocity, pressure gradient along the riser increases with the solid flux.The proposed model assumes the CFB riser to be axially composed of two regions: an acceleration zone at the riser base, where solids re-injected from a standpipe are accelerated to a constant upward velocity, and a fully-developed region, where the flow characteristics are invariant with height, extending from the end of the acceleration region to the rise exit. The model postulates the existence of core-annulus type of flow structure and is based on both fundamental principles and empirical relationships. The input parameters to the model including the riser operate conditions (solids circulation flux and gas superficial velocity), riser geometry and gas and solids physical properties. What's more, it can be easily coupled to kinetic models for process simulation.Secondly, in order to couple the model with gas expanded reaction, a"variable-superficial velocity"hydrodynamic model was established for CFB riser, and applied the model to predict normal experimental data, it further confirmed the reliability of the proposed model. Finally, coupling the variable-superficial velocity hydrodynamic model with four lumps kinetic model, a hydro-kinetic model was established for FCC riser. The simulation proposed a riser reactor with 3 meter in height and 0.012 meter in diameter. The predictive results shown: (a) The conversion of the feedstock (VGO) is 93% through the 3-meter riser reactor, and most reaction take place in the initial third height of the riser. Intermediate product gasoline reach a peak (about 57%) at 1m, then gradually decreased for the secondary reaction. Final yield is 43% at the exit. As the final product, C1-C4 and coke final yield are 40% and 10%, respectively. (b) Two phase temperature distribution: at the inlet, the catalyst temperature declined for the fierce absorbs heat of catalytic reaction. And for feedstock (VGO), it fast increased to the balance with catalyst at one-third length of riser. (c) It shown that catalyst particles clustered in the bottom, but soon be accelerated and a constant average voidage is obtained in the upper section. (d) From the entrance to the exit, absolute pressure is decline, the whole riser pressure drop about 8.6kPa.Besides, a conventional single-stage reverse-type regenerator is considered in the present thesis. The proposed model assumes the regenerator to be axially composed of two regions: dense bed, where majority of the catalyst are located and coke burning reactions take place, and freeboard, where a few ejected catalyst are captived by gravity and cyclones and after-burning take place. The model is used to simulation the coke concentration on catalyst against residence time, the coke concentration on regenerated catalyst against the height of regenerator, and the molar ratio of oxygen, carbon monoxide, carbon dioxide to regenerate air and/or to total stack gas along the regenerator.