Investigation Of Hydrodynamic Behaviors In A Concurrent Downflow Gas-liquid-solid Three-phase Moving Bed

Catalyst deactivation is an unavoidable and important problem in the chemical industry,so it must be considered in process design and operation to reduce the harm of catalyst deactivation.To deal with the catalyst deactivation in gas-liquid-solid threephase catalytic reactions,some researchers have proposed the concept of concurrent three-phase moving bed reactor to provide a solution for on-line catalyst replacement in three-phase reactions with moderate deactivation rates and requiring continuous regeneration.Unfortunately,no experimental studies on this type of reactor are available in the literature,which seriously hinders the industrialization of this type of reactor.Therefore,based on the concept of concurrent three-phase moving bed,this work designs and builds a series of cold model experimental platforms and devices.The influences of design parameters,operating conditions,and fluid properties on the hydrodynamic behaviors such as pressure drop,residence time distribution of each phase,and flow regime in the concurrent three-phase moving bed are systematically investigated by using a combination of pressure fluctuation,photographic method,and pulse tracing and other multiphase detection methods.A series of prediction models for hydrodynamic parameters are established,and the scale-up rules of the concurrent three-phase moving bed are initially explored.The results of the study are of great significance for understanding and mastering the design method and operating characteristics of the concurrent three-phase moving bed reactor.The main research contents and conclusions of this work are as follows:1.The dynamic behaviors of pressure drop under unsteady-state operations of the concurrent three-phase moving bed are investigated by using a U-tube differential pressure gauge,and the effects of gas and liquid mass flow rates,solid flow rate,and liquid viscosity on the pressure drop are investigated under steady-state conditions.Results show that the initial bed layer formed by the free-falling of particles has lower voidage,and the discharge of the initial bed layer during the start-up process is necessary for the system to reach a steady-state.Under the steady-state conditions,the pressure drop increases with the gas and liquid superficialvelocities,liquid viscosity,and decreases approximately linearly with the solid flow rate.Based on the trickle bed pressure drop prediction model proposed by Clements,an empirical correlation for the prediction of pressure drop in a concurrent three-phase moving bed is established.The average relative deviation between the predicted pressure drop and the experimental data is 9.32%within the operating conditions of this work.2.The influences of operating conditions on the residence time distribution characteristics of liquid and solid are systematically investigated using the pulse tracer method,and the flow patterns of liquid and solid in the concurrent three-phase moving bed reactor are simulated and analyzed by the axial dispersion-exchange model considering internal diffusion and the generalized axial dispersion model,respectively.The experimental results show that the adsorbed liquid in the particle pore channel makes the liquid residence time distribution curve in the concurrent three-phase moving bed show obvious trailing and poor symmetry,and the trailing of the liquid residence time distribution curve decreases and the symmetry increases with the increase of liquid mass flow rate and solid flow rate,and the dispersion coefficient increases.The residence time distribution curve of solid has small trailing and good symmetry,and with the increase of liquid-mass flow rate,the lubricating effect of liquid makes the solid back-mixing less and the flow pattern is closer to the plug flow.With the increase of solid flow rate,the funnel flow and wall effect intensify leading to the increase of solid back-mixing.3.Using pressure fluctuation and visual observation,the flow regimes of the concurrent three-phase moving bed reactor are classified,the typical characteristics of trickle flow and pulse flow are clarified.The influence and mechanism of operating conditions on the trickle-pulse flow regime transition boundary are revealed,the flow regime transition map is constructed,and the quantitative prediction model of the flow regime transition boundary is established.It is found that similar to the trickle bed reactor,two typical flow regimes(trickle flow and pulse flow)also exist in the concurrent three-phase moving bed reactor.At a given gas mass flow rate,as the solid flow rate increases,the bed voidage and the gap between particles increase,the dynamic liquid holdup and the average liquid film thickness decrease,the number and volume of liquid packets between particles decrease,and the liquid mass flow rate required to form local liquid blockage and to initiate the pulse increases.As a result,the tricklepulse flow transition boundary shifts to a higher liquid mass flow rate.The critical dynamic liquid holdup at the transition boundary is correlated with the gas Reynolds number,solid flow rate,and the relative viscosity of the liquid phase to establish a prediction model for the trickle-pulse flow transition boundary in a concurrent threephase moving bed,and the average relative deviation between the model predictions and the experimental data is 4.82%.4.The influences of operating conditions on the dynamic liquid holdup,liquid distribution,and wetting efficiency are systematically investigated by the drainage method,annular liquid collector,and fluorescence tracer photography,respectively.The experimental results show that the dynamic liquid holdup increases with the liquid velocity and liquid viscosity,and decreases with the gas velocity and solid flow rate.The prediction model of dynamic liquid holdup in a concurrent three-phase moving bed is established based on the Clements model for predicting the dynamic liquid holdup in a trickle bed reactor,and the average relative deviation between the predicted and experimental values is 2.06%.Due to the carrying effect of the solid particles on the liquid phase,the uniformity of the outlet radial liquid distribution increases with the increase of liquid mass flow rate,and increases first and then decreases with the increase of solid flow rate.The wetting efficiency increases with the increase of bed height and liquid mass flow rate,and increases first and then decreases with the increase of solid flow rate.5.The hydrodynamic scale-up roles of concurrent three-phase moving bed reactors with different scales are initially explored under the premise of satisfying the geometric similarity and kinematic similarity.The results show that the scale-up effects of pressure drop and liquid holdup exist in concurrent three-phase moving bed reactors of different sizes.The liquid distribution,void distribution,and wall effect are the main causes of the scale-up effects,and there is a synergic competition between the factors,and the scale-up effects are especially obvious at a high liquid mass flow rate.The prediction models of pressure drop and liquid holdup are modified by coupling the size parameters,and new prediction models applicable to different size reactors are established,and the predicted values all fall within±20%of the experimental values.