Residence time distribution in twin-screw extruders

For the twin-screw extruders used in the food industry at short time high temperature processes the knowledge of their reactor properties is incomplete for mass- and heat flow. Therefore each process change such as: scale-up or product development requires a great number of measurements before an acceptable product quality can be made with a stable process. The number of measurements is considerable by the great number of possible variables in the extrusion process and the interactions between process conditions such as: temperatures, viscous dissipation and distributive mixing.From these constraints the objective of this thesis is expressed as the measurement and characterisation of the mass flow in a twin-screw extruder, possibly without laborious methods. As the direct measurement of these mass flows is quite time consuming, it is chosen to describe the relation between the mass flow and the residence time distribution. This relation is described in a residence time model which translates the axial mass flow in a twin-screw extruder into a residence time distribution. At the start of this project an appropriate measuring method for the average residence time was already available from a collaboration with ITAL. Its accuracy was calculated during this project.The influence of the adjustable variables of the twin-screw extruder on the degree of fill and the shape of the residence time distribution is dependent of the properties of the extruded material as well as of the extruders geometry. A standard behaviour of the residence time distribution has been found for biopolymers. For non-food applications a considerable number of exceptions can be found in the literature. The mechanisms behind these exceptions are not appointed in the literature.In this work a method is given to search for statistical interesting hypotheses which describe possible relations between the multitude of possible variables in extrusion research. By this method a steam reflux was found in a counter-rotating, twin-screw extruder.The developed residence time distribution model describes the extruder as a machine in which chambers are formed and conveyed by the revolutions of the screws. During there residence in the extruder these chambers lose leakage flows which give a substantial part of the axial mixing. The mixing of these leakage flows and the contents of the chamber is described.The model can be used for co- and counter- rotating, twin-screw extruders in food and non-food applications. The residence time distribution of the compression zone of a counter-rotating, twin-screw extruder can be simulated with a complete mixing of the leakage flows and chamber content. Incomplete mixing has been used for most of the measurements on a corotating, twin-screw extruder and on the feed zone of a counter- rotating type.In all comparable logarithmical F-curves from the literature a bend was found. This bend can increase the fraction with the longest residence time considerably, while the characteristic variables for the curve width are hardly changed. This phenomenon can be simulated by the developed model with a time dependent change in mixing properties.

Bibliographic Details

Main Author:	Jager, T.
Other Authors:	van 't Riet, K.
Format:	Doctoral thesis biblioteca
Language:	English
Subjects:	cast iron, extrusion, gietijzer, uitpersing,
Online Access:	https://research.wur.nl/en/publications/residence-time-distribution-in-twin-screw-extruders
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