Mass Transfer Of Bubbles In A Turbulent Flow Chamber At Elevated Temperature

Oxidation,hydrogenation and chlorination processes are gas-liquid reaction processes which commonly occur in chemical engineering processes.Mass transfer characteristic is a key factor to influence the efficiency of these processes which usually occur at elevated temperature according to their reaction kinetics.However,with temperature increasing,the vapor partial pressure in gas phase at elevated temperature is different from which at ambient temperature.Therefore,the effect of vapor partial pressure due to changing temperature on mass transfer cannot be neglected,which makes quantitative analysis of mass transfer at elevated temperature more complex.In our work,mass transfer of single bubble at elevated temperature is studied in a controllable chamber flow developed and used at ambient temperature.The liquids used in the study of single bubbles mass transfer are Newtonian liquid and non-Newtonian liquid,respectively.The effect of temperature on multiple bubbles mass transfer is explored preliminarily.In order to quantitatively analyze the effect of temperature on single bubble mass transfer at elevated temperature,high-speed camera and image recognition methods are used to capture bubble behaviour and bubble motion characteristics.Particle image velocimetry method is used to measure liquid flow fields in chamber.Mass transfer coefficients of single bubbles are measured at different temperatures.It is found that the mass transfer coefficient of single bubbles increases by 5.4 times when temperature increase from 25℃to 85℃.Changes of diffusivity of oxygen in liquid film with temperature is the main reason for the enhancement of mass transfer,compared with changes of liquid flow fields induced by physical properties changing with temperature.Based on the experimental results,a semi-empirical correlation to describe the relationship between the mass transfer coefficient k _Land gas partial pressure is proposed.The mass transfer coefficients in the chamber are compared with those in stirred tanks at elevated temperatures.The effect of temperature on mass transfer coefficient in chamber is similar with which in stirred tanks.The simulated results of liquid flow fields by k-ε,RSM and LES models are compared with experimental measurements.The comparison shows that LES model can describe the liquid flow characteristics more accurately than the other two models.The bubble motion characteristics in chamber flow are simulated by the coupled LES-VOF method.The mass transfer of single bubbles is simulated by different mass transfer models using user defined function coupled with LES-VOF method.It is found that Brauer mass transfer model can predict the mass transfer coefficient of single bubbles more accurately than other models.The deviation between experiment values and simulation is within 10%.Combined the coupled LES-VOF method and the mass transfer correlation proposed in our work,the mass transfer characteristics of single bubbles is simulated at elevated temperature.It is found that the effect of temperature on mass transfer coefficient of single bubbles can be properly predicted.The effects of liquid flow rate in the channel above flow chamber and sodium polyacrylate concentration on liquid dynamic characteristics and gas-liquid mass transfer characteristics are experimentally investigated,respectively,in non-Newtonian liquid with sodium polyacrylate.It is found that the effect of liquid flow rate on mass transfer process is not significant,while the effect of sodium polyacrylate on mass transfer is negative.With the concentration of sodium polyacrylate changing from 0 wt%to 0.01 wt%,the mass transfer coefficient of single bubble with equivalent diameter of 3.0±0.3mm decreases by 25.85%.From 25℃to 85℃,the mass transfer coefficients of single bubble in non-Newtonian fluids with same concentration and with approximately same rheological behavior increases by 2.7 and 0.82 times,respectively.In the preliminary research of mass transfer of multiple bubbles at elevated temperature,the overall average mass transfer coefficient is used to evaluate 2 to 4 bubbles mass transfer characteristics.The effect of bubble number,gas holdup and bubble diameter ratio on multiple bubbles motion and mass transfer characteristics are preliminarily explored.The data distribution of overall average mass transfer coefficient on different factors are obtained.For cases of 2 bubbles,the mass transfer process at elevated temperature can be significantly enhanced by the increase of temperature.From 25℃to 85℃,the overall average mass transfer coefficient increases by nearly 7 times,while the effect of bubble number on overall average mass transfer coefficient is not significant.