Study On The Instability Behavior Of The Catalyst Downflow System In The Particle Circulation Loop Of FCC Unit

An important characteristic of the circulating fluidized bed process is that the solid particles flow in a set closed loop,which forms a particle circulation system.Particle is carried out for operation of gas solid reaction,heat transfer and particle recovery during circulation flow.Gas is one-way flow to complete the process.The fluid catalytic cracking process(FCC)performs the cracking reaction of the catalyst and the oil,the regeneration of the catalyst and gas by means of gas solid circulating fluidized bed technology.In FCC,the reaction of catalyst and oil in the reactor,regeneration of catalyst and gas in the regenerator are conducted in the particle circulating loop.The necessary condition for the FCC operation is the stability circulation of the catalyst.Therefore,any faults that affect to the catalyst circulation can lead to the non-normal operation of FCC,or even unscheduled shutdown of the unit.The particle circulation system in FCC is composed by the upward and downward flow system.The upward flow system has consisted of the riser and the fluidized bed,which has been the main content of fluidization research field and has the rich literature.The main units in the downward flow system are consisted of the cyclone,the dipleg,the trickle valve,the inclined pipe,etc.Although the catalyst in these units does not participate in chemical reaction process,the operation of these units has an important influence on the circulation of the catalyst particles.The upward flow system in the particles circulation loop is characteristics of movement of the gas carrying particles.The particle flowing in the downward flow system is characteristics of movement of the particles under the condition of the gravity,and the flow pattern is more complicated and has strong unstable behavior.It is shown that the fault probability of the downward flow system is more liable to accident according to the statistics of the particle circulation system in FCC unit.For example,cyclone performance deteriorating,cyclone dipleg blocking,trickle valve wear and leakage,inclined pipe particle flow instability,etc.Now,there are a lot of attention on the problems in the process of riser reactor and fluidized bed reactor involved catalytic cracking process,therefore which are paid more attention to the upward flow system.There are still many subjects that are conducted around the riser and fluidized bed.However,there is not enough attention to the downward flow system in the catalyst particle circulation loop.So,there are still many phenomenon and characteristics that are not understanding clearly,especially about the instability behavior.This research is carried out to focus on the downflow system in the catalyst circulation flow loop in FCC.The static parameters and dynamic parameters of the gas-solid two-phase flow are measured in the experiment setup and analyzed based on gas-solid two-phase flow theory.Furthermore,the date is contrasted with that in the industrial FCC to verify reliability.The flow pattern in the downflow system,which are varying with operation parameters,is investigated,also the flow instability,the instability mechanism.Some faults in downard flow system in the FCC are analyzed according the experiment results and the diagnosis model.We can further understand the catalyst downward flow instability in FCC through the study,and improve the unsteady operation of catalyst downflow.The model of calculating the downflow pressure and pressure balance relation are put forward applying for cyclone,dipleg and trickle valve.And a new idea of fault diagnosis for cyclone system is proposed.The work is helpful to guide the design and operation of FCC.The main conclusions are as follows:1.Pressure distribution in the catalyst particles circulating loopThe pressure in the circulating loop is the driving force for the particle circulation flow,and is shown in the form of the pressure accumulator and pressure drop.The pressure gradient in downflow system is smaller than that of upflow system.The former is negative differential pressure flow,while the latter is a positive differential pressure flow,both intersect in a certain height.The overall pressure balance profiles form an ? shape distribution.Particle circulation rate determines the starting point pressure of upflow system and affects much on the end point pressure of downflow system,also determines the pressure of downward flow system.The flow instability in the particle circulating loop manifests in the form of fluctuation pressure originated mainly from the downward flow system.The fluctuating pressure are characteristic of low frequency and high amplitude.2.The instability of gas solid separating process in the cycloneThe swirl flow in the cyclone is unstable under the condition of gas phase,which is shown as the rotation center of the swirl flow deviating the center of the cyclone geometric center.The eccentric rotation induces the pressure pulsation in the cyclone,which is a kind of low frequency and high amplitude pressure pulsation.Under the condition of gas solid two phase flow,there exists a rotating flow deviates from the geometric center also.Addition,the particles,which are separated by the centrifugal force,accumulate on the wall of cyclone,and form the non-uniform dust strands.These strands flow leads to the discharge fluctuation and the cyclone body vibration.3.The dipleg negative pressure and the fluctuation of dischargeThe dipleg of the cyclone is a negative pressure standpipe.The negative pressure difference is about equal the pressure drop of cyclone.Discharge condition of dipleg is that the accumulating pressure is greater than that of external negative pressure difference.There are two kinds of fluid patterns in the secondary cyclone dipleg.One is dense phase conveying when mass flux rate is high,and another is coexistence of dilute and dense phases when the mass flux rate is low.The particle flow down produces the pressure fluctuation,which results from compressing gas and particle agglomeration as dense phase conveying.The particles entrain gas downflowing.The pressure fluctuation results from the rising bubbles as coexistence of dilute and dense phases.The gas flows upward.4.Discharge process unstable of trickle valveThe trickle valve is an important component of the cyclone,which keeps particles one-way flowing and prevent from leakage.The discharge form is determined by the negative pressure gradient and particle mass flux rate.There are two kind of discharge forms,one is continuous trickle discharge,another is intermittent periodic discharge.The discharge form can interchange each other by adjusting negative pressure gradient or particle mass flux rate.The discharge process has very strong instability.The pressure fluctuation from the discharge process can be used for recognition model of the discharge form for identifying discharging flow regime.5.The instability of the particle conveying process in the inclined pipeInclined tube is a particle conveying pipeline between two containers.The open rate of the control valve in inclined pipe decides the flow pattern and fluctuating pressure of valve front and valve back.With the increasing open rate of the valve in the inclined pipe,the flow pattern at valve front changes from the moving bed,gas solid stratification to fluid flowing,at valve back from the particle strands,gas solid stratification to fluid flowing.Gas-solid stratification discharge produces a great fluctuating pressure,which forms an inclined tube vibration excitation source.When the flow pattern is fluid flow,the pressure fluctuation is stable and the pressure difference of valve front and valve back changes from the positive pressure difference into negative pressure difference.6.The fault diagnosis of cyclone systemThe cyclone system fault generates from two aspects of process and of machinery.The process faults are mainly over flow parameter,the mechanical faults have trickle valve leakage,body wear,dipleg blocking,etc.The faults lead to much catalyst loss.The pressure signal and catalyst particle size distribution can be used to diagnose the faults of cyclone,such as catalyst loss,dipleg blockage,valve erosion etc.A new fault diagnosis method is proposed for the cyclone system in FCC unit based on the experimental results and analysis results.The catalyst particles diagnosis technology has the characteristics of accurate,reliable for FCC cyclone separator system.Also,the cyclone separator system vibration produced by exciting force induced from fluctuation pressure is discussed.