Residence time distribution in twin‐screw extruders

Abstract

Twin screw extruders are finding increased usage in reacting and devolatilizing applications. Using self‐wiping profiles, the twin screws fulfill the requirement that there be no “dead” or “unmixed” zones. Agitator design must be chosen with care so that a reasonable balance can be obtained between forwarding rate, surface‐generation rate, vapor passageway, power, and axial mixing. Techniques have been developed for measuring residence time distributions and characterizing axial flow behavior. The method also permits direct determination of the holdup in starved barrel applications. Data on residence time distribution are presented for 4‐in. diameter twin screw equipment with a variety of rotor configurations.

Factorial design to quantify the influence of extrusion parameters in the mean residence time

“…It is also important to notice that feeding rate and screw speed coefficients present a negative value, which means that t n increase with their decrease. This corroborates with the previous explanation that the pressure inside the barrel increases with throughput, the extruder pumping the material faster [4,7] , whereas material is carried faster through the extruder if the screw speed is increased. Adding kneading elements to the screw profile increases the mean residence time because its mass transport characteristic is less efficient than conveying elements [4] .…”

“…The effect of the operational parameters in the mean residence time observed so far was readily expected, according to the literature [7] . However, it is important to understand their individual Extrusion RTD measurements were performed in a twin-screw extruder (ZSK-30, Werner-Pfleiderer) equipped with a gravimetric K-Tron feeder, operating at steady state.…”

“…This results in a distribution of residence times (RTD) around a mean value [6][7][8][9] . The knowledge of the RTD is very important for polymer extrusion because it is related, among other things, to the level of polymer degradation [10][11][12][13] and special processes like reactive extrusion [14][15][16][17] .…”

“…One of the ways to determine the RTD curve of a given processing condition is by a pulse response technique, where an amount of a material, different from the flowing polymer, called tracer or marker, is fed into the extruder and its concentration measured at a point downstream as a function of time [7] . The tracer concentration can be quantified by several ways: the extruded material is collected periodically and quantified by analytical or spectroscopic techniques.…”

Sistema portátil para medida on-line da distribuição de tempo de residência na extrusão

“…Como fluido utilizaram uma solução de glicose e a detecção quantitativa era feita por medidas da condutividade de soluções aquosas das frações coletadas na saída da extrusora. Em 1975 Todd [7] propôs a utilização de corantes insolúveis (azul de metileno em polibuteno) que após extração das frações era quantificado por análise colorimétrica. Aproximadamente no mesmo período, Wolf & White [3] propuseram a utilização de traçadores radioativos, usando um concentrado com dióxido de manganês, que poderia ser irradiado em reator nuclear tanto antes da adição na extrusora como após a coleta das frações.…”