Study On Bubble Hydrodynamics Characteristics In An Airlift External Loop Reactor

The external loop airlift reactor is the main equipment for ammoniation,and has the advantages of uniform material mixing,turbulent flow,scattered bubbles,and large mass transfer areas.The most important factors affecting the performance of the external loop airlift reactor are local hydrodynamics and bubble size distribution(BSD).The flow behaviors and BSD in the external loop airlift reactor can help to understand the mass transfer and reaction mechanism,and guide the selection of operation parameters and optimization of the reactor.It is important for energy saving and emission reduction in the process of ammonia industriy.In this paper,PIV technique,digital image analysis(DIA)and simulation methods were used to study the hydrodynamic behaviors and BSD in the laboratory scale external loop airlift reactor.It mainly includes the following aspects:The flow characteristics of the external loop ariflift reactor were experimentally studied by PIV technique.The results show that when the superficial gas velocity is 30mm/s,the flow pattern enters the transition flow from the homogeneous flow,and then the superficial gas velocity is greater than 80mm/s,the flow pattern changes from a transitional flow to a hetergeneous flow;as the superficial gas velocity increases,the circulating fluid velocity first increases to a maximum value,then rapidly decreases and eventually becomes flat.When the superficial gas velocity is 30mm/s,the circulating fluid velocity reaches the maximum value of 0.41m/s;the sparger hole size has a certain influence on the gas holdup of the riser.In homogeneous flow and transition flow,the average gas holdup decreases with increasing sparger hole size;in heterogeneous flow,the sparger hole size has little effect on the gas holdup of the average gas holdup of the riser.The Euler-Euler model and the interphase force between the gas and liquid phases and the turbulence correction are discussed in detail.Comparing the effects of different drag models on the simulated average gas holdup and axial fluid velocity,the results show that the Tomiyama model underestimates the axial fluid velocity,and overestimates the average gas holdup;the Karamanev model is the opposite;the estimated value of the DBS-local model is more consistent with the experimental value.Lift and turbulent diffusion forces have a large impact on the simulation results,so the effects of lift and turbulent diffusion forces need to be considered in the simulation.BSD in laboratory-scale ELALR at different superficial gas velocities was analyzed using DIA technique.The experimental results show that when the superficial gas velocity is less than 20 mm/s,the gas holdup calculated by DIA agrees well with the experimental data,indicating that the BSD can be calculated by the DIA method.As the superficial gas velocity increases,the PDF curve becomes flatter,changing from unimodal to the bimodal.For the bimodal PDF curve,the peak of the small bubble is about 1.5 mm.As the superficial gas velocity increases,the probability of a small peak increases.On the other hand,the peak value of the larger bubble changes from 4.5 mm to 5.5 mm,and as the superficial gas velocity increases,the probability of a large peak decreases.With the increase of circulating fluid velocity at the same superficial gas velocity,the bubble becomes smaller,the rising velocity of the bubble increases,and the velocity field tends to be symmetrical and stable.A PBM model for analyzing BSD was established.The mechanism of different bubble breakup and coalescence was analyzed.The model was modified and a CFD-PBM model was developed to simulate the hydrodynamic characteristics and BSD of gas-liquid system.The results show that,with the increase of turbulent dissipation rate and gas holdup rate,the bubble coalescence rate also increases;in the range of homogeneous flow and transitional flow,PIV experimental value and CFD-PBM model agree well,and the liquid velocity distribution is parabolic.The CFD-PBM model has good predictive ability for flow behavior and BSD.The highest gas holdup is located at the axis of the riser and some high gas holdup spots exist near the wall.Gas-liquid flow changes into the fully developed state at about 300 mm.As the superficial gas velocity increases,the PDF curve becomes flatter,changing from unimodal to the bimodal,which is in agreement with experimental observations.In chemical production operation,the circulating fluid velocity can be accelerated and internal components such as stirring or mesh can be added to increase the bubble breakup rate,thereby reducing the average bubble size and improving the gas-liquid mass transfer efficiency.