Studies On Residence Time Distribution In Twin-screw Extruders Using Experimental Method And Numerical Simulation

The development of new materials with improved properties seems to rely nowadays more on blending and compounding than on the synthesis of chemically new polymers.The mixing directly affects morphology and structure of multi-component polymers,therefore, the selection of mixing devices and optimization of processing parameters are two important issues in polymer processing.The twin-screw extruders(TSE)have a modular geometry,which allows one to adjust the screw profile to control the axial,dispersive and distributive mixing.TSE are widely used as mixers/reactors for blending,compounding, and reactive processing.However,the study on mixing in twin-screw extruders has been one of difficulties in theory researchs due to the complex geometry configuration and transient flow pattern.The flow visualization provides the qualitative analysis for shearing, stretching,and tearing motions of polymers.Morphological analysis of sample obtained from stopping extruders didn't characterize the dynamic feature in extrusion processing. The in-line measurement with numerical simulation is the main trend to study the mixing in TSE.The polymer processing is an interdisciplinary field with many unsolved,challenging fundamental research topics and practical applications related to polymer rheology,polymer chemistry and physics,instrument science,chemical mechanical science and CFD.This work aimed at developing a new instrument to measure in real time the RTD which characterized the axial mixing and transport abilities of different screw elements based on analysis of the transient flow pattern and systematic evaluation of mixing theory in TSE. Distributive mixing of polymer melts is characterized by generation of interfacial area, which is much more difficult to experimentally measure.The numerical simulation based on computational fluid dynamics(CFD)is a key tool to solve this difficulty.Most relevant results obtained in this work are summarized as follows.1.Firstly,we developed a new instrument to measure in real time the RTD in screw extruders based on the tracer fluorescent characteristic.The device had to be evaluated.It was done in terms of the reproducibility,on the one hand,the linearity between the amplitude of the response signal and the amount of the tracer,on the other hand,and the comparison between the off-line and in-line methods finally.Increasing screw speed shifted the RTD curve toward the shorter time domain,whatever the probe location.However,their width seemed to be only slightly affected.This implies that under the specified conditions, increasing screw speed did not improve much the quality of the axial mixing.Rather,it simply conveyed the material toward the die exit in a more rapid manner.Increasing feed rate shifted the RTD curve toward the shorter time domain,whatever the probe location. This is similar to the effect of increasing screw speed.However,the RTD curves significantly narrowed,which was different from increasing screw speed.This infers that increasing feed rate tended to decrease the quality of the axial mixing.2.In order to research the effect of different kneading discs and one special mixing element on local RTD,the screw configurations were designed to match the in-line measurement.The local RTD can provide the experimental validation for 3-D numerical simulation.The local RTD of a kneading zone depended very much on the staggering angle of the kneading discs.The mean residence time and the axial mixing quality characterized by the width of the RTD following the order 30°＜45°≤60°＜90°＜gear discs,indicating gear discs had the best axial mixing performance.3.It was confirmed theoretically and experimentally that specific throughput Q/N, defined as a ratio of throughput(Q)over screw speed(N),was indeed a key process parameter for controlling the dimensionless time distribution,RRD and RVD.For a given value of Q/N,the overall,partial and local RTD were different when Q and N varied. However the corresponding dimensionless RTD as well as the RRD and RVD all fell on single master curves,respectively.This is because the mean degree of fill and complete fill length were the same for a given value of Q/N.4.The performance of two different macromolecular tracers has been evaluated in co-rotating twin-screw extruder using PS and PP as polymeric models.As for global RTD, no difference is observed in the measured RTD.A good dispersion of the tracers in polymeric models is a necessa.ry condition for that.When the length of screw decreased,the mixing ability also decreased,the influence of different tracers on RTD is present.The temperature has little influence on RTD.5.The other objective of this thesis is to analyze the flow mechanisms and distributive mixing in the kneading disc domain of co-rotating twin screw extruders by the 3-D finite element method.Mesh superposition technique(MST)was introduced to model intermeshing twin-screw extruders without calling upon remeshing.Experimental validation of simulated results is big challenge.Initially,these particles are randomly distributed in an inlet vertical plane and their trajectories between the inlet and outlet are calculated from the velocity profiles.Along each trajectory,the residence time is obtained.The residence time distribution is then obtained based on the residence time of each of those particles. Simulated results are compared with experimental ones obtained by an in-line measuring instrument.The distributive mixing parameters such as the area stretch ratio of material surfaceη,instantaneous mixing efficiency e_ηand time-averaged mixing efficiency are calculated using the interface tracking techniques.These parameters are then used to compare the distributive mixing performance and efficiency of different kneading discs.