Hydrodynamics In Gas-Liauid Jet-Loop Reactors

Jet-loop reactor(JLR)is a novel gas-liquid and gas-hquid-solid contacting apparatus.JLR is widely utilized in biological fermentation,wastewater and residue oil hydrocracking,due to their simple structure and high efficiency of mixing and solids suspension.The key feature for JLR is the strong internal liquid circulation originated from the jet energy and density difference between the riser and downcomer,which is beneficial to mixing and solids suspension,thus avoiding the particle deposition and hot spot during the hydrogenation and oxidation process.However,most of the open literatures were limited to the experimental conditions of small column size and gas-liquid system.Experiments and simulations on hydrodynamics in the large scale and gas-liquid-solid system were adequate,in particular,the scale-up effect on JLRs was still absent.Thus,in this thesis,two sets of cold apparatus((?)200 and(?)500)were established to comprehensively investigate the hydrodynamics and scale-up effects in the JLR.Both experiments and CFD simulations were performed to systematically investigate the effects of superficial gas velocity,injection mode and solid loading on overall gas holdup,liquid circulation velocity and distributions of local gas holdup,liquid velocity and solid holdup.A large number of basic experimental data were obtained.The scale-up effect in a JLR was discussed and novel hydrodynamic models were developed under various conditions,in order to provide benchmark data for JLRs' industrial design and scale-up.This thesis covers the following aspects:1.Experimental investigation in a(?)200 gas-liquid jet-loop reactor.Conductivity probe and Pavlov tube were utilized to measure the overall gas holdup and radial distributions of local gas holdup and axial liquid velocity under different superficial gas velocities and injection modes in a(?)200 JLR.Experiments show that,the gas-liquid co-injection mode results in larger gas holdup and liquid circulation velocity than those of the gas jet alone mode.Liquid velocity distribution is more prone to reach the fully developed regime than the gas holdup,and the gas-liquid co-injection promotes the flow development.Comparison of JLR with the airlift reactor and conventional bubble column shows that,though the JLR has a lower gas holdup,it has the largest liquid circulation velocity,which is suitable for the gas-liquid-solid three phase reaction processes that need intensification of the solid suspension and mixing.2.CFD simulation in a(?)200 gas-liquid jet-loop reactor.A bi-dispersed bubble model incorporating lateral forces and bubble-induced turbulence was developed to simulate hydrodynamics in an up flow jet-loop reactor operated in gas-liquid co-injection mode.In the model the wake acceleration and bubble hindrance effects were considered in the drag force of the large and small bubble phase,respectively.The simulated radial profiles of gas holdup and axial liquid velocity agree with the cold experiments,which shows that the bi-dispersed bubble model is reasonable for correctly simulating hydrodynamics in a JLR(both in the riser and downcomer)at vanous superficial gas velocities.The bi-dispersed bubble model also reveals the distribution rule of large and small bubbles.In addition,the effects of the draft tube position and its diameter on the overall gas holdup and liquid circulation velocity were discussed by CFD method,and the optimal structure parameters were provided.3.Scale-up effect for jet-loop reactors.The experiments were conducted in a 0500 mm JLR to investigate the hydrodynamics including overall gas holdup and radial distributions of local gas holdup and axial liquid velocity at different superficial gas velocities and injection modes.A modified drift-flux model was developed to predict the overall gas holdup at various superficial gas velocities.The CFD results reveal that the bi-dispersed bubble model can be utilized in large scale JLRs.The comparison between different column sizes shows that,the scale-up effect in the JLR is distinct,which means the critical column diameter,namely its negligible effect on overall gas holdup,is larger than that in the bubble column.The curve of overall gas holdup vs.gas velocity in(?)200 mm JLR is steeper than that in 0500 mm JLR,revealing that the overall gas holdup in 0200 mm JLR at low superficial gas velocities is smaller than that in(?)500 mm JLR,while it is inverse at high gas velocities.(?)200 mm JLR prevails a pseudo-homogeneous flow regime under the present superficial gas velocity range.However,in(?)500 mm JLR,the flow regime is transferred from the pseudo-homogeneous flow to turbulent flow with the increase of gas velocity.4.Experimental and CFD investigation in a jet-loop reactor containing small particles.Global and loeal properties in a(?)200 mm gas-liquid-solid three phase jet-loop reactor containing small particles(Stokes number(St)<1)operated at gas-liquid co?injection mode were experimentally investigated.A large bubble-small bubble-slurry three phase model was developed to predict the hydrodynamics in a three phase jet-loop reactor,which had a good agreement with the cold experiments,especially for the prediction of the flow in the downcomer.Under the considered solid loading range(Cs?15 vol.%),with the increase of solid loading,the gas holdup decreases,while the liquid circulation velocity gradually increases.5.Experimental and CFD investigation in a jet-loop reactor containing large particles.The effects of the superficial gas velocity,liquid velocity and solid loading on gas holdup,axial liquid velocity and bubble properties were investigated by multiple measuring methods in a 0200 mm gas-liquid-solid three phase jet-loop reactor containing large particles(Stokes number(St)>1)operated at gas-liquid co-injection mode.A large bubble-small bubble-liquid phase-solid phase four phase model was developed,which had a good agreement with the cold experiments.Results show that,JLRs have a good solid suspension?ability.Compared with the gas-liquid JLRs,solids decrease the gas holdup and liquid circulation,reduces the bubble velocity and delays the flow development due to the enhanced interaction between bubbles and particles.