COMPUTER SIMULATION WITH TXS? OF POLYOLEFIN EXTRUSION IN A W

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COMPUTER SIMULATION WITH TXS? OF POLYOLEFIN
EXTRUSION IN A WP ZSK 57 TWIN SCREW EXTRUDER
(I) E. Fontán and G. MarinoDepartment of
Chemical Technology, Repsol YPF S. A.c/
Embajadores 183. 28045 Madrid, SPAIN.
INTRODUCTION   n the last years
Repsol YPF is involved in a research project for
non-invasive extrusion monitoring. The virtual
manufacturing permits to save on fine-tuning and
optimisation for new processes,
at RD levels as well as manufacturing level.
Research progression as well as computers
powerfulness make together real unavoidable
advantages with fast return on investment. One of
the objectives of the project is to improve the
quality and economical extruder operation of the
industrial plants using a simulation software.
This is the first part of a work related with the
simulation of the twin-screw extrusion process at
semi-industrial and industrial scale. The
Twin-Screw Extruder Simulator (TXS) is a
personal computer software package for process
simulation and analysis of plastics compounding
operations in corotating, intermeshing, modular
twin-screw extruders. TXS allows to obtain quick
approximate answers to many processing problems
that arise in the compounding plant and the
process development laboratory. TXS simulates all
the steps in the process solids conveying,
melting, mixing, melt conveying, pressurization,
and die flow. The simulations are performed in
realistic settings, with multiple feed and vent
ports. TXS can simulate complex processing
operations, such as the extrusion compounding of
filled compounds, reinforced materials, colour
concentrates, etc.
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EXPERIMENTAL AND RESULTS   The twin-screw
extruders supported by TXS are modular machines.
Their geometry is determined by the specific
sequence of pre-designed extruder components. In
order to perform a simulation after an extruder
configuration has been established, materials
must be selected and a complete set of operating
conditions imposed. The process requires several
steps Set the material properties. Select
the feed state solid or melt. Set values for
the main operating conditions throughput rate,
screw speed, feed temperature and pressure, and
head pressure. Set the barrel wall temperature
profile.   The material properties and related
parameters are Melt Properties Viscosity
Parameters Solid Properties Particulate
Parameters   In this work, we present a
simulation of a HDPE grade in different extrusion
conditions. The properties of the material has
been measured or obtained from the open
literature. The selected viscosity model was the
truncated power law using the Arrhenius equation.
Heat transfer in twin-screw extruders is not a
completely settled issue and several models are
presently available that claim to represent the
heat transfer characteristics of operating
extruders. The simulation has been made with the
JKS model. The polymer side heat transfer
coefficient is computed based on a custom
modification of the classic model developed by
Janeschitz-Kriegl and Schijf (1956) 1 for
single-screw extruders. JKS model is an
optimistic approach to the problem and provides
an upper bound to the heat transfer capacity of
the extruder.   Melting in twin-screw extruders
is the least understood process step. No
satisfactory quantitative model is presently
available. The selected melting model has been
Active Bed. The melting process is simulated
using a custom model based on the ideas developed
by Zehev Tadmor, Costas Gogos, and collaborators
at Polymer Processing Institute (PPI) in past
several years 2. Several energy dissipation
mechanisms are considered heat transfer from the
barrel, plastic deformation of the particulate
solid by compressive and shear stresses,
polymer/wall and polymer/polymer friction, and
viscous dissipation in the partially melt. This
model may be considered the one that best
represents the physics of the melting process,
but the approximate nature of the models used to
estimate the contribution of each melting
mechanism requires the use of material-dependent
empirical coefficients.
The extruder used was a ZSK-57 of Werner
Pfleiderer with a diameter of 57 mm and L/D ratio
of 22. The screw configuration used was a
standard profile to process this kind of
material. The figure 1 shows the grid of the
experimental conditions with three different
temperature profiles, screw speeds and
throughputs.   The results reported by the
software displays a series of global processing
parameters. The software TXS let us temporarily
modify the values of some physical properties,
material parameters and process variables
utilized by the simulator. The feature may help
debugging problem simulations, and should be
used with extreme care. This feature is
controlled by the Debug module using selected
empirical calibration factors in the current
simulation. The calibration factors available
are   1.- Melting Factor affects the active-bed
melting model coefficients. 2.- Log Viscosity
Factor affects the material viscosity. 3.- Log
Shear Rate Factor affects the shear rate (only
for viscosity calculations). 4.- Specific Heat
Factor affects the material specific heat 5.-
Heat of Fusion Factor affects the latent heat of
fusion of semicrystalline resins. 6.- Channel
Energy Factor affects the rate of energy
dissipation in the screw channel. 7.- Tip Energy
Factor affects the rate of energy dissipation in
the flight tip gaps. 8.- Heat Transfer Factor
affects the polymer-side heat transfer
coefficient.   The table 1 shows the parameters
used in the debug tool. The figure 2 shows the
deviations between experimental and computed melt
temperature, torque and specific energy input.
After tune-up the simulations using the debug
module good results have been obtained, with a
maximum deviation of 4.1, 10.3, 8.5 for melt
temperature, torque and specific energy input
respectively.
Table 1 - Calibration factors in the debug tool. Table 1 - Calibration factors in the debug tool. Table 1 - Calibration factors in the debug tool. Table 1 - Calibration factors in the debug tool.
Factor Range Defect value Optimized value
Melting Factor (MF) 0,02 50 1
Log Viscosity Factor (LVF) -5 5 0 -2,56
Log Shear Rate Factor (LRSF) -5 5 0
Specific Heat Factor (SHF) 0,02 50 1
Heat of Fusion Factor (HFF) 0,02 50 1
Channel Energy Factor (CEF) 0,02 50 1
Tip Energy Factor (TEF) 0,02 50 1
Heat Transfer Factor (HTF) 0,02 50 1 3,1
REFERENCES   1. H. Janeschitz-Kriegl, J. Schijf,
A Study of Radial Heat Transfer in Single-Screw
Extruders, Plastics and Polymers, 37, pp.
523-528 (1969)  2. C. C. Gogos, Z. Tadmor, M. H.
Kim, Melting Phenomena and Mechanisms in Polymer
Processing Equipment, Adv. Polymer
Technol., 17, pp. 285-305 (1998)