| Fluid catalytic cracking(FCC)is a key process for lightening heavy wax oil.The FCC is achieved by using circulating fluidized bed technology to carry out catalyst fluidization,transportation,distribution,separation and other unit operations for gas-oil cracking reaction and burning off the carbon deposits on the catalyst.The stable circulation operation of catalyst between the reactor and the regenerator is an essential prerequisite for FCC process.However,in the standpipe-valve system,the poor transportation of the catalyst directly affects the cracking reaction depth of gas-oil,yield of light oil product,product distribution and operation stability,which has become a bottleneck in the long-term operation of FCC unit.Therefore,it is not only of great academic value,but also of economic and production significance that study the flow patterns and transformation mechanism of catalyst downward flow in FCC standpipe.The gas-solid two-phase flow in the standpipe-valve system is depending on gravity and against negative pressure gradient,as a result the catalyst flow pattern has the characteristics of diversity and variability.The downflow flow pattern is a combination of one or several types,which are the lean phase flow,dense phase flow,stratified flow,slug flow,bubbling flow and packed bed flow.The actual flow patterns tape and numbers depend on the gas-solid flow parameters,negative pressure difference and valve opening rate,etc.There are many literatures on flow patterns for the standpipe.These standpipe models were usually vertical standpipes with bottom valve constraints,and the flow pattern type numbers in the standpipe were one or two.However,the standpipe-valve system of FCC unit is a combination structure of inclined pipe and vertical pipe.The length of standpipe is more than 20 m length,and the slide valve is installed at the bottom of the standpipe.As a result,the flow pattern type numbers are greater than two,and meanwhile the flow patterns are varying with the gas-solid flow parameters,negative pressure differential and valve opening rate.These factors directly affect the catalyst transport capacity,especially some factors may cause the faults and failure of catalyst transport.However,due to lack of understanding of gas-solid two-phase flow pattern transformation mechanism in the standpipe-valve system,the troubleshooting is mainly based on previous experience and data analysis,and the results of fault treatment is poor reliability.This paper first carried out a research work by the experimental and theoretical methods based on the measurement data that was obtained from laboratory Φ100 mm standpipe-valve system and industrial FCC unit Φ760 mm standpipe-valve system,then analyzed the relationship between flow pattern and gas-solid flow parameters,negative pressure difference and valve opening rate,investigated the theoretical problems of multi-flow patterns coexistence and flow pattern transition mechanism in the standpipe-valve system,and finally established a varying flow pattern model of gas-solid two-phase flow.The practical quantitative identification and adjustment technology of standpipe flow pattern in FCC unit was developed and applied to the fault diagnosis of catalyst transport problems in the industrial FCC unit to verify the practicality and reliability of the technology.The main conclusions are as follows:1.A multiple flow patterns coexistence standpipe model was established.Vertical standpipe and inclined pipe have the different flow patterns due to the effect of negative pressure differential,bend in the standpipe-valve system,particles aggregation and gas escape in the vertical standpipe,which leads to the downward particles flow pattern varying from fluidized flow to transition packed bed flow or packed bed flow.The flow pattern in the inclined standpipe is dense phase fluidized flow or stratified flow.The flow pattern behind the valve depends on the particle mass flow rate and the slide valve pressure drop,which is drip flow or slug flow.The flow pattern type numbers are greater than two in the entire standpipe-valve system.Therefore,A multiple flow patterns coexistence standpipe model was established for the FCC standpipe-valve system.2.The quantitative identification technology of standpipe flow pattern in FCC unit was established.The pressure instability in the standpipe was closely related to the type of flow pattern.Experimental results showed that the pressure pulsation of different gas-solid flow state corresponded to the flow patterns,in which dominant frequency,amplitude and variance varied greatly with different flow patterns.Therefore,the flow pattern phase diagrams containing the flow parameters and the valve opening rate were established based on the experimental measurement data,which could be used for quantitative identification of different flow patterns.The quantitative identification of the flow patterns of the recirculation standpipe in a 1.0 Mt/a FCC unit shows that the slide valve opening rate had a decisive influence on the standpipe flow patterns.When the opening rate of the recirculation slide valve was lower than 70%,the flow patterns in the standpipe were bubbling flow,transition packed bed flow and packed bed flow.When the opening rate of the recirculation slide valve was greater than 85%,the flow patterns in the standpipe were bubbling flow and transition packed bed flow,and gas leakage phenomenon of the slide valve was detected.3.Regulation mechanism of valve adjusting particle mass flow rate was established.The experimental results showed that particle mass flow rate was not only controlled by the valve opening rate,but also was affected by the standpipe flow pattern,negative pressure difference and aeration air.There are two mechanisms of the valve regulating particle mass flow rate.On the one hand,the pressure drop of the valve is changed,which depends on varying the resistance of the valve.On the other hand,the flow patterns above and behind the valve are changed,which depends on the different transport capacity of different flow patterns.Both can also be changed at the same time to vary the particle mass flow rate through the valve.However,the adjustment of particle mass flow rate requires a certain slide valve pressure drop.When the slide valve pressure drop decreases to a certain critical value,the slide valve loses the control function of particle mass flow rate.4.The change of standpipe flow pattern is an adaptive adjustment of gas-solid energy.The conditions of multi-patterns coexistence in the standpipe-valve system depend on gas-solid flow parameters,negative pressure difference,aeration air and valve constraints,etc.In essence,the multi-patterns coexistence is the adaptive coordination result of various energies in gas-solid two-phase flow.That is the conversion,transfer and adjustment of pressure energy,potential energy and kinetic energy to adapt to the change of external constraints.When one of the parameters of gas-solid flow parameters,negative pressure difference and valve constraints changes,the flow pattern will change according to the mechanism of the adaptive adjustment of gas-solid energy.A new flow pattern combination was established to meet the boundary conditions.5.Aeration gas model were established.The experimental results show that there was an optimal layout of aeration nozzles and aeration flow rate in the standpipe-valve system,otherwise the standpipe flow pattern could not be improved effectively,and the stable particle mass flow rate would be affected.The optimal aeration flow rate design methods of the vertical standpipe were proposed.· When the upward velocity of bubbles was greater than the downward flow velocity of particles,aeration air was set at the bottom of the standpipe,and the stable operation condition of the standpipe was as follows:(?)· When the upward velocity of bubble was less than the downward flow velocity of particles,the stable operation condition of the standpipe was as follows:(?)... |
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