Study On Local Residence Time Distribution In Twin Screw Extruder

Twin screw extruder (TSE) was widely used in polymer processing. Residence time distribution (RTD) was an important parameter for characterizing the mixing performance of a reactor. It reflected the history of the heat, shear or chemical conditions for the materials under processing. The total RTD (TRTD) charactered the mixing and transporting abilities of the whole twin-screw extruders, while the local RTD (LRTD) reflected the mixing and transporting of materials in the certain area of TSE.Using a fluorescence monitoring device, the partial RTD of a co-rotating TSE was measured online. A deconvolution method was constructed to deal with two different RTD curves detected to obtain the local RTD of kneading blocks with different stagger angles(KB) and turbine mixing elements(TME) between two detecting points. Meanwhile the partial and local RTD of TME were transformed to residence volume distribution (RVD) and residence revolution distribution (RRD).The full factorial design was introduced to investigate the factors including rotating speed of the screw, the stagger angles of the kneading blocks and the specific throughput that have significant effects on delay time and the mean residence time of PRTD in twin screw extruder. The interaction effects between any two factors were also studied. The results showed that the delay-time and the mean residence time were most significantly influenced by rotating speed, the angles of kneading blocks and the specific throughput have similar effects on the responses. The delay time is influenced by the interaction between two factors, and the sequence is angle with specific throughput first, rotating speed with specific throughput second and angle with rotating speed third;But for mean residence time ,the order is specific throughput with rotating speed first, angle with specific throughput second, angle with rotating speed third .The LRTD results of KB and TME reflected that the minimum residence time(delay-time), mean residence time and axial mixing extent decreased as the screw speed and feeding rate increased. Also it was found that the normalized LRTD were overlapped with the same specific throughput (Q/N), and the RTD which reflected the inbeing of mixing abilities of kneading blocks were obtained.The RTD of TME was transformed to RVD and RRD. It was detected that the RVD and RRD superposed with same Q/N for a given screw configuration. Subtracting the delay-volume, all the RVD of different operating conditions overlapped each other which indicated that the shape of the RVD was depending on the screw configuration.By comparing the LRTD of different elements at same operating condition, it was found that the delay-time of TME90 was longer than other elements, and the curves was widest indicating that TME90 had the best mixing abilities. And the LRTD of the kneading block with stagger angle 30 had shortest delay-time and minimum width indicating poorest mixing abilities. The mixing abilities of FAMME was better than KB30 and KB60, while the delay-time was almost the same.