Experiment And CFD Simulation Of Devolatilization Characteristics For Falling Film With Highly Viscous Fluids

Polymeric devolatilization is a process for separating low molecular weight components from polymer systems.The development of highly efficient polymer devolatilization equipment is essential for improving the performance of polymer products,simplifying process and reducing energy consumption.The falling film flow along the vertical pipe has been used as a devolatilizer in polymer devolatilization process because of its large contact area between gas and liquid phases,simple structure,high devolatilization efficiency,no external stirring,low energy consumption.Taking the development and application of tube-type devolatilizer as the background,using CFD simulation technology and cold model experiment,the relationship among fluid flow,viscosity,structural parameters and film-forming properties was studied,and the relationship between film forming and devolatilization characteristics was established.A two-dimensional CFD model of gas-liquid flow was developed using the volume of fluid(VOF)model,integrating two momentum source items,i.e.the surface tension momentum source and the interface shear stress.Considering the volatilization of small molecular component on the interface,the mass transfer source was also added.The influence of flow rate,viscosity and structure parameters on the film forming performance was studied.Compared with the flow of low viscosity fluid,the flow rate of highly viscous fluid film on the surface is slow,and the film surface stretch increases with the increase of viscosity.The velocity distribution curve at the peak and valley is the traditional parabolic shape.Combining experiment with CFD simulation,the falling film and devolatilization characteristics of highly viscous fluid along the smooth tube were studied.The characteristics of fluid flow,film formation and surface renewal under different slit width of annular distributors and liquid height,were studied with visualization technique and UV spectroscopy.The results show that the flow pattern along the smooth tube is close to the plug flow,and the velocity gradient of the liquid film is obvious in the direction of wall to film thickness.The empirical relationship formula among the falling film flow rate and the fluid viscosity,the liquid height and the distributor slit width is set up.The relative standard deviation between the experimental and the calculated value is within ±8%.Three different shaped tubes,diamond,single cone and double cone were designed.The formation conditions,film forming efficiency,velocity distribution and surface renewal frequency under different flow conditions were studied by means of experiment and CFD simulation.The results show that the film formation efficiency of the three different shaped tubes is 1.3 times that of smooth tube,which effectively improves the surface devolatilization efficiency.Diamond shaped tubes have better film forming stability and are suitable for complex fluids with a wide range of viscosities.The single cone shaped tube has higher film forming efficiency,especially in highly viscous system,while its devolatilization performance is better than that of diamond shaped tube.The removal efficiency of the double cone shaped tube is the highest,and the efficiency of the removal is three times higher than that of the smooth tube under the same conditions.