DISTRIBUTION OF SPECIFIC ENERGY IN TWIN-SCREW
COMPOUNDING EXTRUDERS USING ONE-DIMENSIONAL
PROCESS SIMULATION
Adam Dreiblatt, Century Extrusion, Traverse City, MI
Eduardo Canedo, PolyTech, Campina Grande, Brazil
Copyright 2016, Century Extrusion, all rights reserved
Presentation outline
 Role of specific energy
 1D simulation
 Energy distribution along screw axis
 Summary
Specific (mechanical) energy
Single value representing the cumulative mechanical
energy for melting, mixing, conveying, pressurization
 Does NOT capture or differentiate between
dispersive/distributive mixing
 Does NOT capture residence time or RTD
 Primary factor used for scale-up
Specific (mechanical) energy – cont.
 Formulation specific
 Machine independent
 Influenced by operating conditions
Can we reduce specific energy (e.g. energy savings) ?
Why 1D simulation
1D simulation provides ‘insight’ into the compounding
process yielding information not otherwise obtainable
 Specific energy predictions easy to validate
 Incremental energy consumption along screw axis
can be validated (not so easy, but possible)
 Identify opportunities to reduce specific energy
1D Simulation example*
Extruder: 70mm, L/D = 32, 1.55 D/d
Operating conditions: 800 to 1600 kg/hr, 400 to 800 rpm
Material: PP, MFR = 5 to 30
*PolyTech WinTXS™ Simulation Software
Specific energy increases
(0.108 to 0.124 kWh/kg) when
increasing screw speed
PP (MFR=5) @ 1200 kg/hr
Although total kW increases,
specific energy decreases
(0.124 to 0.112 kWh/kg)
when increasing feed rate
PP (MFR=5) @ 600 rpm
Barrel temperature has
virtually no influence on
specific energy
PP (MFR=5) @ 1200 kg/hr, 600 rpm
The influence of polymer viscosity on specific energy
only appears AFTER polymer is melted…lower
viscosity requires less mechanical energy to convey
PP resins @ 1200 kg/hr, 600 rpm
PS
LDPE
PA6
Specific energy is primarily a
function of polymer enthalpy
All resins run @1200 kg/hr, 600 rpm
Essentially zero kW required
in solids conveying
PP resins @ 1200 kg/hr, 600 rpm
The slope of this line indicates the large
amount of power consumed to raise the
temperature of solid polymer in the first
kneading elements…up to 80% of the
total kW (SME) is applied in these first
kneading elements…which is WHY
these elements wear so quickly!
PP resins @ 1200 kg/hr, 600 rpm
Extruder energy requirement
What is pellet temperature approaching first kneading elements?
Assumptions:
-
PP pellets enter machine at 25°C
-
PP is heated by conduction in barrel #2 (only conveying elements)
-
Using 70mm extruder as example, installed heater power on closed barrel = 7 kW
-
Heat capacity of PP, solid-state = 2.34 kJ/kg °C
-
Conservative estimate: all heater power conducted into polymer
At 1200 kg/hr, barrel heaters only have enough
power to increase temperature 9°C !!
Extruder energy requirement
Material: PP, Melt Flow = 5 to 30 dg/min (@230°C, 2.16 kg)
Thermal properties: 2.34 kJ/kg °C (solid)
1.61 kJ/kg °C (melt @200°C)
Energy to raise 1 kg PP from [25+9°C] to melting temp (160°C) = 0.082 kWh
Heat of fusion (28% crystallinity) = 60 kJ/kg (0.016 kWh)
Total theoretical energy for melting 1 kg: 0.098 kWh
Energy to raise melt to discharge temperature = 0.021 kWh (208°C) - 0.031 kWh (230°C)
Theoretical specific energy = 0.119 to 0.129 kWh/kg
Assuming 95% motor and gearbox efficiency, this is 0.125 to 0.135 kWh/kg
MELTING REPRESENTS 70-80 PERCENT OF TOTAL ENERGY INPUT !
Melting in kneading elements
FLOW
PLASTIC
DEFORMATION
Definition: frictional heating of SOLID polymer to raise the material temperature
Melting in kneading elements
FLOW
RESIN
MELTING
Definition: adding heat of fusion for crystalline and semi-crystalline polymers
Melting in kneading elements
FLOW
VISCOUS
DISSIPATION
Definition: frictional heating of MELT…dispersion starts after polymer is melted
Energy distribution in melting
Plastic Deformation
Resin Melting
Viscous Dissipation
PS
LDPE
PA6
Melting position depends on
crystallinity and enthalpy of polymer
All resins @ 1200 kg/hr, 600 rpm
The influence of polymer viscosity (on melting)
is only seen once polymer is melted…affecting
the viscous dissipation within the melting section
PP resins @ 1200 kg/hr, 600 rpm
The slope of these lines indicates the
amount of power consumed to convey,
mix and pressurize the melt. Notice slope
does not change dramatically between
conveying and mixing elements.
PP resins @ 1200 kg/hr, 600 rpm
Viscous dissipation
represents the mechanical
energy expended into the
MELT (e.g. total kW minus
melting kW)
PP resins @ 1200 kg/hr, 600 rpm
Replacing all mixing
elements with
conveying screws –
virtually no change in
specific energy !
PP resins @ 1200 kg/hr, 600 rpm
Conclusions
Up to 80% of specific energy is applied to the solid polymer to raise the
temperature up to it’s melting range within the first few kneading elements.
The remaining 20% of specific energy (aka viscous dissipation) is distributed
along the remaining screw length
- Changes in screw design after melting do not significantly affect specific energy
Machine operating conditions can be optimized to minimize specific energy while
maintaining compound physical properties
- Minimum screw speed, maximum feed rate
Can we reduce specific energy ?
We cannot reduce melting energy requirement (without preheating polymer feed)
There is a minimum energy required to move the melt through the machine; the
only way to reduce this is to shorten the ‘wetted’ length of the extruder
Do not believe claims “…this new ______ can reduce specific energy and lead to
significant energy savings…”
CPM Extrusion Group is a global leader in the supply of high performance
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parts and services
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