Twin Screw Extruder and Continuous Mixer Rate Limitations

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Meredith Hunter
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1 MPC Materials Processing Consultants LLC Twin Screw Extruder and Continuous Mixer Rate Limitations Anthony C. Neubauer SPE Fellow; Dow Fellow (retired) Materials Processing Consultants LLC

2 Why Extruders? Ideal for processing viscous fluids; e.g., polymers Melting Pressure Generation Blending and Compounding Devolatilization Reactive Extrusion Morphology Conversion e.g., blown film blow molding injection molding sheet tubing / pipe granular pellets 2

3 Equipment Description Continuous Mixer Feed Twin Screw Extruder Feed Solids Conveying Melting Melt Conveying Solids Conveying Melting Melt Conveying In essence, a series of forward acting and reverse acting screw pumps with & without throttling systems. 3

4 Assumptions Resin Feed System not Limiting Starved Fed, i.e., not Flood Fed Downstream Processing Systems not Limiting Melt Filtration Die Pressure Etc. 4

5 Rate Limiting Systems Power (Torque) Feed Neck; i.e., no longer Starved Fed Feed Throat Fluidization Solids Conveying Screw Channel Fluidization Melt Conveying Melting Section Other Compounding and Blending Reactive Extrusion Devolatilization 5

6 Power (Torque) Limitation Rate max = Maximum Allowable Power Specific Energy Input (SEI) SEI is a function of: Polymer Type Viscosity; MW and MWD Heat of Fusion Screw / Rotor Geometry Operating Conditions Speed, Feed Rate, Feed Temperature Discharge Pressure Slot / Gate / Throttle Valve Position (if applicable) 6

7 Power (Torque) Limitation Power (Torque) Limit High Viscosity High Heat of Fusion High Discharge Pressure Low Feed Temperature Low Viscosity Low Heat of Fusion Low Discharge Pressure High Feed Temperature High Speed Low Speed Resin Mass Rate 7

8 Feed Throat Fluidization Resin Feed Return Gas is the result of the between solid and melt bulk densities; e.g., Return Gas Bulk Density, kg/m 3 Volumetric Rate, m 3 /hr Velocity, m/s Solid Resin 0.17 Melt Gas 0.09 = 2.0 Basis: 90 mm Ø Port Basis: 1,600 kg/h Resin Feed Rate 8

9 Feed Throat Fluidization Resin Mass Rate Low Bulk Density High APS High Bulk Density Low APS Scale-Up rate D area D 3 2 Machine Size APS = Average Particle Size 9

10 Solids Conveying High Bulk Density Resin Mass Rate Low Bulk Density Machine Size 10

11 Screw Channel Fluidization Ratio = Superficial Gas Velocity Minimum Fluidization Velocity Less than 5: No fluidization in the screw channels. Greater than 8: Sufficient fluidization within the screw channel to result in rate limitation. 11

12 Fluidization Solutions Increase cross-sectional area - free volume rectangular feed throats modify screw cross-section profile Increase screw / rotor speed Increase effective particle size and bulk density co-feed large particles use cohesive additive Design TSE / CM with up or down stream vents Install force feeder (crammer / stuffer) 12

13 Melt Conveying Governing Equation: Q net = = Q drag Q α N β P μ pressure Melt conveying increases when: Free volume increases Speed increases Discharge pressure decreases Viscosity increases α, β = geometric constants 13

14 Melting Section Continuous Mixer Resin Mass Rate High Viscosity - High ΔT Low Viscosity - Low ΔT Machine Diameter 14

15 Why? Forward Reverse Q net = ( ) ( ) Q Q Q Q D P D P α, Ν = constant = β P (μ F -μ μ μ F R R ) Viscosity T µ Q net High High High +++ Low Low Low + 15

16 Melting Section Twin Screw Extruder Difficult to generalize Principles similar 'Infinite' number of combinations Refer to manufacturers Processing Guidelines Basic operating characteristics 16

17 Combined Results Normalized Rate Limitation Screw Channel Limits Rate Power Melt Conveying Resin Mass Rate 17

Rate Limiting Systems - Other Blending and Compounding Distributive mixing Dispersive mixing Reactive Extrusion Injection & mixing of reactants Activation energy, kinetics, etc.

18 Rate Limiting Systems - Other Blending and Compounding Distributive mixing Dispersive mixing Reactive Extrusion Injection & mixing of reactants Activation energy, kinetics, etc. Residence time Devolatilization Diffusion, surface renewal, etc. Mixing; e.g., stripping agent Vent gas flow velocity Vacuum ports vapor-liquid separation Souders-Brown eq. V = k ρl ρv ρ v where k = m/s 18

19 References Neubauer, A.C., Leach, E.A., "Twin Screw Extruder and Continuous Mixer Rate Limitations, SPE ANTEC Technical Papers, 49, 327 (2003) Souders, M., Brown, G.G., Design of Fractionating Columns, Entrainment and Capacity, Industrial and Engineering Chemistry, 26, 98 (1934) Todd, D.B., Polymer Devolatilization, 32 nd SPE ANTEC Technical Papers, 472 (1974) Biesenberger, J.A. (ed.), Devolatilization of Polymers: Fundamentals, Equipment, Application, Hanser (1983) 19

20 Questions??

FEED ENHANCEMENT TECHNOLOGY FOR LOW BULK DENSITY MATERIAL INTO CO-ROTATING TWIN-SCREW COMPOUNDING EXTRUDERS

FEED ENHANCEMENT TECHNOLOGY FOR LOW BULK DENSITY MATERIAL INTO CO-ROTATING TWIN-SCREW COMPOUNDING EXTRUDERS Paul G. Andersen, Coperion Corporation, Ramsey, NJ Maria Hoelzel and Thorsten Stirner, Coperion