| Polymer devolatilization is the process of removing residual volatile components,such as unreacted monomers and solvents,from the polymer product.This process enhances the quality and performance of polymer,but it requires specialized devolatilization equipment,most of which have complex structures and high energy consumption.The falling film devolatilizer is a type of equipment that has a simple structure,easy operation,large specific surface area and low energy consumption.It can satisfy the demands of high efficiency,energy saving and environmental protection in polymer devolatilization.The key component of falling film devolatilizer is falling film element.Falling film element improves the surface renewal frequency and the mass transfer efficiency by altering fluid flow state.The object of study in this paper is a new type of falling film devolatiler with cone element.When the fluid flows over the surface of the cone element,the Coanda effect occurs and the suspended film is formed.Experimental and computational fluid dynamics(CFD)methods were used to systematically study the falling film flow,film formation,and devolatilization processes of the falling film devolatilizer with a cone element.The findings provide a basis for the engineering development and application of this new devolatilizer.Firstly,the experimental approach was used to investigate the hydrodynamics and film formation of highly viscous fluids.The effects of distributor clearance,fluid viscosity and liquid level on falling film flow were analyzed.The experimental results were fitted to obtain the semi-empirical correlation for calculating the flow rate of falling film.The film formation characteristics were studied using visual experimental techniques.The results showed that liquid film flow could be classified into three flow regimes:dripping,stream,and suspended film.The formation law of suspended film based on Coanda effect is revealed and the phase diagram of film formation is drawn.It is found that with the increase of flow rate,the overall surface velocity increases,the film thickness increases,and the stretching effect of the film surface enhances.Compared with the attached film,the suspended film can increase the liquid film surface velocity,decrease the film thickness,and enhance the stretching effect,which is conducive to the surface renewal.Based on the symmetrical structure of the conical falling film element,the twodimensional CFD simulation was carried out using the volume of fluid(VOF)and laminar flow model.The velocity distribution,film thickness distribution,surface stretching ratio and surface renewal frequency of high viscosity fluid during vertical falling film flow were studied.The simulation results were verified with the data of visualization experiment and particle image velocimetry(PIV)measurement.After the falling film flow reached a steady state,the fluid accumulates gradually near the upper end of the cone element.At this time,the surface flow rate decreased,the film thickness increased,and the surface renewal rate reached a peak.A strong stretching effect was generated on the membrane surface at the lower end of the cone element,the surface flow rate reached the maximum,the membrane thickness was the thinnest,and the surface turnover frequency reached the maximum.Acetone was used as volatile to investigate experimentally the mass transfer under vacuum conditions.Orthogonal tests were designed to analyze the effect of vacuum level,film flow rate,viscosity,and the initial concentration of acetone on the devolatilization ratio.The phase diagram of suspended film formation for three cone elements was obtained by visualization experiment technique.It was more difficult for three cone elements to form films than a single cone element,and the flow rate of film formation for three elements forming suspended film was larger.According to the range analysis,the influence degree of each factor on the devolatilization ratio from large to small was:vacuum degree,viscosity,flow rate,initial concentration.It was known from quantitative analysis of variance that vacuum was highly significant,viscosity and flow were significant,and initial concentration of acetone was not significant within the tested level range.The mass transfer test results of falling film devolatiler with cone element and without falling film element show that the introduction of cone element can double the devolatilization ratio when the vacuum degree is 60 kPa,which reflects high efficiency devolazation performance of the new devolatiler against high viscosity fluid. |
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