CFD Simulation Of Flow And Mass Transfer Characteristics In An Ozone Oxidation Reactor

Coal chemical wastewater has complex composition,poor biodegradability and is liable to cause serious pollution to the environment.In order to ensure zero discharge of coal chemical wastewater,it is necessary to add advanced treatment unit after pretreatment and biochemical treatment.Ozone oxidation is one of the advanced oxidation technologies.It has the advantages of strong oxidation ability,no secondary pollution and high salinity,so that it is an effective advanced treatment technology.Focusing on improving ozone utilization rate,thesis investigates ozone advanced treatment process for coal chemical wastewater from the perspective of reactor internals optimization.By means of computational fluid dynamics(CFD)numerical simulation method,the flow and mass transfer characteristics in the reactor are thoroughly understood in order to provide theoretical guidance for the enlargement,optimization and engineering design of the process.In this paper,the flow and mass transfer characteristics of electrochemical-ozone combined oxidation reactor and ozone bubble column reactor are investigated.The main research contents and achievements are as follows:(1)Simulation of electrochemical-ozone combined oxidation reactor.In order to enhance the utilization of ozone,eight internal structures are designed.The effects of various internal components on the flow,mass transfer and reaction of the reactor are investigated.It is found that the addition of internal components promoted the flow,mass transfer and reaction in the reactor.The effect of square internal component is the most obvious,which makes the velocity distribution of liquid near the cathode uniform and improves the situation of large velocity gradient when no internal components are added to the reactor.The mass transfer coefficient can be increased by 27%-53% and the conversion rate of ozone in the reactor can be increased by 12%-20% under five flow rates.It is the optimum design scheme of internal components.(2)CFD simulation method for ozone bubble column reactor.In order to obtain reasonable simulation results,the prediction ability of bubble size model,drag force model and turbulence model are investigated,which have great influence on the simulation results of gas-liquid two-phase flow.It is found that the single bubble size model could not accurately describe the wide range of bubble size in the reactor at higher apparent gas velocity,which results in the low simulated gas holdup.The predicted value of Schiller-Naumann model differs greatly from the simulated value in the drag model,while the predicted value of Grace drag model is the most consistent with the experimental value.Combining RNG k-ε turbulence model with Grace drag model in turbulence model,the results of gas holdup simulation are low.The combination of Standard k-ε mixture turbulence model and Grace drag model can achieve high prediction ability for gas holdup and liquid velocity in the tower.(3)Internal component optimization of ozone bubbling column reactor.It is found that the average bubble size increases with the increase of the axial height in the bubble column reactor,resulting in the decrease of the gas-liquid interface area,which is not conducive to mass transfer and reaction.The bubbles can be broken up in the process of rising and redistributed afterwards.The inner components play a similar role as distributors,which can promote the increase of volume mass transfer coefficient and effectively improve the phenomena of high flow rate and high gas holdup in the central region of the bubble column reactor.It provides a basis for the design and optimization of the reactor.