The Effect Of Gas Leakage On Performance Of FCC Cyclone Separator

Flapper valve is usually installed at the bottom outlet of the secondary cyclone dipleg in the FCC disengager.The function of flapper valve is to prevent gas bypassing and discharge catalyst particles.However,in the actual operating process of FCC unit,the gas around the bottom of flapper valve can get into the cyclone dipleg,resulting in the gas leakage and decreasing the separation efficiency of cyclone separator,and then influences operation of the whole FCC unit.Therefore,through two aspects of experiment and numercial simulation,this work studies the influences of gas leakage on pressure drop and flow field in a single cyclone separator and the multi-parallel cyclone separators built in a FCC disengager.The main conclusions are as follows.In the experiment,the main cylindrical diameter of the cyclone is 300 mm,the effect of gas leakage on pressure drop of cyclone at different inlet velocities and particle concentrations is studied.Experimental results show that in pure gas flow,the pressure drop of cyclone decreases with increasing amount of gas leakage,when inlet velocity is lower,the influence of gas leakage is more obvious.In gas-solid flow,the pressure drop decreases first,then increases and finally decreases largely with increasing amount of gas leakage.With the inlet velocity increases,the influence of particle concentration on pressure drop increases.The influence of gas leakage on each experimental point of pressure fluctuations is great,with gas leakage increases,each point of pressure fluctuations has a high extent of increasing,especially fluctuations of gas-solid flow are more unstable.The influence of gas leakage on the performance of the single cyclone and disengager is simulated with Fluent,the flow models are the Reynolds stress model(RSM)and the discrete phase model(DPM).Simulation results show that in pure gas flow,the pressure drop decreases with amount of gas leakage,gas leakage weakens the swirling intensity of the cyclone,causing that pressure increases and velocity decreases in the centre.Swirling flow of the tangential velocity distribution is no longer symmetrical,upward flow velocity increases and down flow velocity mainly remains unchanged of axial velocity distribution.Along down the axial direction of cyclone,the influence of gas leakage on the tangential and axial velocity distribution is more obvious.In the gas-solid flow,particles are mainly concentrated in the side walls when the rate of gas leakage is zero.Gas leakage weakens the swirling intensity of the cyclone,causing of particles in the dust appears backmixing and upward into the conic section,with the rate of gas leakage increases,distribution of static pressure is no longer symmetrical,when the particle concentration is higher,the influence of gas leakage on static pressure and tangential velocity is greater.Simulation results of the disengager show that it presents fine swirling flow state in the first and secondary cyclone while all the diplegs of the secondary are closed.Oil gas whirls at high speed in the cyclone but whirls slowly in the disengager space.When the bottom of diplegs of the secondary cyclone are open,gas leakage leads to the disorder of the whirling flow and increase of the pressure,in addition,the pressure of the secondary cyclone whose diplegs are closed increases at different degrees because of gas leakage.