Studies Of Flow Behavior And Mass Transfer Performance In A Disk-distributor Rotating Packed Bed

Rotating packed bed(RPB)improves the liquid dispersion and surface renewal rate inside the bed through the high-speed rotation of the rotor and its internal packing,thereby enhancing the mass transfer and mixing processes.RPBs are widely used in processes such as wastewater and exhaust gas treatment,nano-particle preparation,and rectification.The working fluids in these processes are often water or aqueous solutions,of which the viscosities are relatively low,in the range of 1 to 100 m Pa·s.However,there still exists many moderate-and high-viscosity processes in chemical engineering,such as heavy oil hydrogenation,biocatalysis,polymer devolatilization and ionic liquid decarbonization.In these processes,the viscosity of the working fluid is as high as thousands of times that of water.In order to expand the application of RPB in moderate-and high-viscosity processes,it is significant to solve the issues of initial distribution,flow behavior and mass transfer performance of moderate-and high-viscosity fluid in the RPB.In this work,a disk-distributor rotating packed bed(DRPB)was constructed for moderate-and high-viscosity systems.The disk distributor was designed to replace the traditional nozzle distributor.The liquid flow behavior and mass transfer performance in the DRPB were studied.Through systematic scientific experiments and computational fluid dynamics(CFD)simulations,the liquid flow characteristics of different viscosities over the surface of the disk distributor and in the packing were revealed.Based on the flow research,a mass transfer model for the moderate-and high-viscosity system was established in the DRPB.The main research contents are as follows:1.A three-dimensional VOF simulation method of liquid film flow over the surface of the disk distributor was developed.The typical wave patterns were proposed,including smooth film,concentric wave,and spiral wave in the spreading direction,and sine-like wave and pulse-like wave in the fluctuating direction.The phenomenon of relative movement between fluid layers inside the liquid film was discovered,and the formation mechanism and dynamic process of different wave patterns were revealed accordingly.The wave amplitude and the liquid film thickness over the disk surface were quantified.A correlation of the average thickness of liquid film was established,which was accessible for moderate-and high-viscosity systems.The process intensification by the disk distributor was contributed to two aspects:the surface wave could strengthen the disturbance in the liquid film,and the reduction of liquid thickness could shorten the diffusion distance.2.A three-dimensional Eulerian simulation method of liquid holdup in the packing was developed with a geometric model based on the real packing structure.The experiment of the gravimetric measurement was performed to validate the simulation.As the viscosity increased,the aggregation state of liquid in the packing changed from dispersive to continuous modes,and the liquid streamline altered from reverse-curled arcs to radially diverging straight lines.Liquid holdup and wetting efficiency were positively correlated with viscosity and liquid flow rate,and negatively correlated with rotational speed.The maximum liquid holdup and wetting efficiency of packing were 0.25 and0.416,respectively.The correlations of liquid holdup and wetting efficiency applicable to the moderate-and high-viscosity systems were established through non-dimensional analysis.Based on the assumption of plug flow,the average residence time of liquid in the packing was analyzed.Results indicated that the residence time of low-viscosity liquid was less than 0.5 s,the residence time of moderate-viscosity liquid was between 0.5 and 5 s,and the residence time of high-viscosity liquid was greater than 5 s.3.High-speed camera technology was employed to investigate the liquid flow pattern and micro-element characteristics in the packing.It was found that the liquid flow in the packing presented four typical patterns:droplet flow,short-ligament flow,continuous ligament flow,and liquid film flow.The transition criteria between different flow patterns were established.With the increase of Oh _P number and Re _PWe _P~n,the liquid flow pattern altered from droplet flow to film flow.On the basis of the mass conservation of liquid in the packing,a semiempirical model of the characteristics of the micro-element was developed.Analyzed by the model and experiment,the liquid characteristics in the packing were clarified including the equivalent spreading radius of liquid film and the average thickness of liquid film,as well as the number of liquid ligament and the average diameter of liquid ligament.Based on the dimensionless analysis,correlations for the average thickness of liquid film and the average diameter of liquid ligament were established for the moderate-and high-viscosity systems.4.Based on the research results of fluid flow,combined with the classical mass transfer theory,the liquid-side volumetric mass transfer coefficient model in the DRPB was developed with the verification by the devolatilization experiment of viscous liquid.The surface area and volume of different liquid flow patterns were analyzed to quantify the gas-liquid specific surface area in the packing.Applying the two-film theory to the disk zone and the surface renewal theory to the packing zone,the mass transfer coefficient was obtained,and a corresponding correlation was establish.For the effective conditions of N<800 r/min,the error between the predicted value and the experimental value of the acetone removal efficiency was limited within±20%,indicating that the mass transfer model of this study could provide guidance for the application of RPBs in moderate-and high-viscosity systems.