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Refining Soft Models 9-24-16

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Ronald Musselman
Ronald Musselman

This document discusses soft models for visualizing and optimizing the pulp refining process. It presents conceptual soft models for high and low consistency refining as well as counter-rotating refiners. The models divide refiners into rotor, stator and plate gap regions and describe fiber movement and forces in each region. Key factors influencing refining like consistency, plate design, steam flow and pressure are discussed. The goal of the soft models is to help operators and non-experts visualize refining mechanisms.

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Refining Soft Models METHOD TO VISUALIZE & OPTIMIZE THE PULP REFINING MECHANISM
Why do we need Soft Models?  Mathematical models are so complex with so many assumptions needed that practical application to operating refiners is limited.  Actual refining zone measurements of pressure, temperature, plate bar forces along with high speed photography of steam & fiber flows in plates & pulp samples extracted from multiple radial locations in the plates give added insight into the refining mechanisms, but math models based upon this data are still extremely complex which limits practical use.  A less complex and more practical modelling method is needed to allow a broader range of individuals like refiner operators and non-mathematically inclined persons to visualize what is happening in the refining zone.
My High Consistency Single Disc Soft Model
Three Distinct Regions of the Single Disc Conceptualization Model  Rotor  Stator  Plate Gap
Rotor Region  Fiber movement dominated by centrifugal force due to rotation.  Rotor plate less filled & plate bars less covered with fiber than stator where no rotation & centrifugal force exists.  Can result in more rapid rotor plate wear as bar tops more exposed to fibers sliding across metal.  Wear differences more obvious at lower plate gaps associated with lower energy plate concepts and refining processes.  Plate gap changes due to refining consistency changes dominated by changes in rotor plate fiber filling, as fiber mass variation changes centrifugal forces, varying fiber retention time.
Pulp Displacement Rotor Stator Rotor feeds forward due to centrifugal force. The steam flowing back in the stator grooves will carry the pulp, and make it move backwards towards the plate inlet. This results in a large accumulation of pulp in the inner part of the plate (thick pulp pad). In the outer zone, the forward flow of steam carries the pulp forward and out of the refining zone.
Stator Region  Without rotation, fiber flow dominated by steam flow conveying forces.  Fiber surface area affects ease of conveying.  Fiber mass(consistency) also affects ease of conveying but much less than rotor centrifugal forces.  Temperature & pressure measurements indicate a steam pressure peak exists at some radial location in the refining zone.  Outboard of peak, steam & fiber flow forward  Inboard of peak steam & fiber flow back to the inlet  Rotor must feed production rate + fiber returning in stator  Results in higher inlet zone filling; can completely fill inlet and feeding screw.
Temperature and pressure profile 110 120 130 140 150 160 170 180 190 Primary Secondary Temperature°C Note : More coarse chips & fiber bundles absorb energy even at larger gaps @ plate inlet
Stator Region, cont’d  Publications have shown that the differential between refiner inlet and outlet steam pressures impacts the retention time of fiber in the refining zone and thus the plate gap.  Higher outlet than inlet pressure(pressure boost) increase fiber retention & plate gap.  Lower outlet than inlet pressures decrease fiber retention & gap.  I would suggest that stator plate fill rate(fiber retention) changes dominate the differential pressure gap changes, as stator filling is dominated by steam flow conveying effects.
Plate Gap Region – Fiber trapped between rotor & stator plate bar tops.  Conveying influenced by rotor and stator bar crossing angle(sliding direction) & stator frictional forces.  Bi-directional plates evenly alternate inward and outward sliding direction.  Directional feeding plates increase the portion of outward vs inward sliding forces.  Directional holding plates do the opposite.  Typically need feeding inboard of pressure peak to reduce inlet filling, and holding outboard of peak to retain fiber & increase filling.
Directional Refiner Plate Feed Hold
Plate Gap Region, cont’d  Higher stator bar frictional forces tend to hold fiber, promoting sliding of fiber across the rotor bar tops, increasing rotor bar edge rounding & wear.  Fiber tends to slide on bar surface with the lowest frictional forces.  Bar angle and bar edge condition influence holding forces on fibers.  Newly manufactured plates exhibit break-in period with higher plate gaps and higher energy consumption.  I would suggest effect caused by the grinding burr on bar edge giving increased roughness and holding = larger plate gap  Once burr worn away reach normal holding level until excessive bar edge rounding begins to reduce plate gap.
My Counter-rotating Refiner Soft Model
Counter-rotating vs Single Disc  2 Rotors & no Stator  Must feed through holes in 1 disc(Feed End)  Feeding challenge w/chips, even more issues with fiber  Use added dilution water to help feed(lower refining consistency)  Use steam to help feed(discharge pressure lower than inlet)  Add steam to inlet to heat chips(not self pressurized)  Steam flow effects upon fiber flow more limited, centrifugal forces dominate feeding  At 60 hz(1200 rpm) should be lower intensity than single disc as higher retention time & more bar crossings(2400 vs 1800)  Lower refining consistencies & plate designs without surface dams, both feeding requirements, actually make intensity higher  Non-feeding disc adjusts axially to control gap & load(Control End)
Counter-rotating vs Single Disc  Although a pressure peak & steam seal may still be formed between the plates, backflowing steam must be limited & the seal ring behind feed disc allows steam flow around disc.  Limited steam pressure boost possible for retention time increase  Pressure boost would cause more steam flowing back to the inlet across seal ring
Plate Gap  Fiber in the plate gap stops as it is not rotating with either disc & dragging forces cancel out  Limits how large gap can be before feeding stops(Pinch-off)  Reason for sub-surface not surface dams @1200 rpm?  Keep fiber in Feed End disc in the inlet area to assure feeding.  No bars on Control End opposite feed openings to prevent stoppage of Feed Disc rotational motion to maintain feeding  Cavitation(high pressure collapse) on any Control End bars opposite the feed openings in the Feed disc
My Low Consistency Refining Soft Model
Model Similarities – Lo-Co vs Hi-Co  3 Zones – Rotor, Stator & Gap  Shear & Compression forces on fibers  Plate Gap impact upon energy transfer & efficiency  Fiber retention time impacts probability of treatment  Stator fiber recirculation
Model Differences – Lo-co vs Hi-co  Fiber retention function of flowrate & internal recirculation, not centrifugal force  Fiber holding affect of surface dam only at dam, not region before dam  Dams increase groove pressure drop significantly thus reducing flow capacity  Inlet & outlet pressures function of flow, feed pump operation, plate geometry, refiner rotational speed and plate diameter, not independently adjustable
Lo-co Fundamentals  Rotor plate increases pressure from inlet to outlet similar to a pump impeller  Grooves in rotor and stator cause pressure drop as a series of small pipes in parallel  Pressure increase from inlet to outlet is the net result of rotor plate pumping less groove pressure drop  As plate gap closes, initially pumping efficiency increases as pump impeller clearance decreases thus more pressure increase  Further gap closing can decrease pressure increase as less gap & more groove flow increasing pressure drop  Larger refining gaps give a higher proportion of shear forces, promoting more external fibrillation – potentially higher energy to a given pulp quality  Smaller refining gaps give a higher proportion of compressive forces, promoting more internal fibrillation(fiber splitting) – potentially lower energy  Limit in reducing gap is fiber cutting(length reduction)
Lo-co Fundamentals, cont’d  Lower plate bar height with wear reduces rotor pressure increase & increases groove pressure drop thus reducing pressure increase across refiner & potentially causes a pressure drop from inlet to outlet  As long as discharge flow control valve is < 100% open, flow control is still possible  If valve 100% open, refiner plates are the flow control valve
Rotor Region  Strongest groove vortex to load fibers onto bar edge  Potential for forward flow regime below vortex  Influenced by fiber type, consistency, groove geometry  Refiner plate geometry dominates pumping characteristics:  Bar angle, & height  Plate diameter  Presence of dams, surface(full bar height) or sub- surface(< full bar height)
Tracing of fluid particle at the top and bottom of the rotor groove. Reference: Numerical simulation of the flow in a disc refiner, Gohar.M.Khokhar, Master’s Thesis 2011 Outlet
Stator Region  Weaker vortex rotationally but less vortex pitch radially vs rotor  Internal recirculation due to higher discharge pressure than inlet displaces main flow volume  Unrefined fiber velocity increases/retention time decreases  Groove flow below vortex, if present, radially inward if typical higher discharge than inlet pressure  Groove volume influences pressure drop not pumping
Fluid particle tracing at the top and bottom of stator groove going in the negative Y-direction Reference: Numerical simulation of the flow in a disc refiner, Gohar.M.Khokhar, Master’s Thesis 2011 Outlet
Plate Gap Region  Fiber flocs compressed & locally de-watered  Compressive and shear forces applied to flocs  Groove vortices bring flocs to gap region for possible trapping  Flocs trapped then released  Plate bar geometry(width and crossing angle) impact treatment probability & severity as well as closing force  Bar surface sliding on fiber wears while floc holding surface is protected from wear
Lo-Co Refining Evaluation & Optimization  Energy consumption comparison must include feed pump energy  Less refiner no-load(pumping) but more pump energy reduces savings  Feed pump VFD needs to be part of any energy reduction plan  Replacing multiple refiners with a single refiner reduces no-load energy even with the same refiner concept  Therefore the true energy reduction of any new refiner concept must be single refiner vs single refiner  Refiner filling design determines actual forces on fibers, therefore filling selection for optimization will impact long term peak performance.  Single source of fillings can limit long term optimization  Any increased filling cost per ton produced needs to be deducted from energy cost savings  Filling supply competition can speed optimization & reduce filling cost

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Refining Soft Models 9-24-16

  • 1. Refining Soft Models METHOD TO VISUALIZE & OPTIMIZE THE PULP REFINING MECHANISM
  • 2. Why do we need Soft Models?  Mathematical models are so complex with so many assumptions needed that practical application to operating refiners is limited.  Actual refining zone measurements of pressure, temperature, plate bar forces along with high speed photography of steam & fiber flows in plates & pulp samples extracted from multiple radial locations in the plates give added insight into the refining mechanisms, but math models based upon this data are still extremely complex which limits practical use.  A less complex and more practical modelling method is needed to allow a broader range of individuals like refiner operators and non-mathematically inclined persons to visualize what is happening in the refining zone.
  • 3. My High Consistency Single Disc Soft Model
  • 4. Three Distinct Regions of the Single Disc Conceptualization Model  Rotor  Stator  Plate Gap
  • 5. Rotor Region  Fiber movement dominated by centrifugal force due to rotation.  Rotor plate less filled & plate bars less covered with fiber than stator where no rotation & centrifugal force exists.  Can result in more rapid rotor plate wear as bar tops more exposed to fibers sliding across metal.  Wear differences more obvious at lower plate gaps associated with lower energy plate concepts and refining processes.  Plate gap changes due to refining consistency changes dominated by changes in rotor plate fiber filling, as fiber mass variation changes centrifugal forces, varying fiber retention time.
  • 6. Pulp Displacement Rotor Stator Rotor feeds forward due to centrifugal force. The steam flowing back in the stator grooves will carry the pulp, and make it move backwards towards the plate inlet. This results in a large accumulation of pulp in the inner part of the plate (thick pulp pad). In the outer zone, the forward flow of steam carries the pulp forward and out of the refining zone.
  • 7. Stator Region  Without rotation, fiber flow dominated by steam flow conveying forces.  Fiber surface area affects ease of conveying.  Fiber mass(consistency) also affects ease of conveying but much less than rotor centrifugal forces.  Temperature & pressure measurements indicate a steam pressure peak exists at some radial location in the refining zone.  Outboard of peak, steam & fiber flow forward  Inboard of peak steam & fiber flow back to the inlet  Rotor must feed production rate + fiber returning in stator  Results in higher inlet zone filling; can completely fill inlet and feeding screw.
  • 8. Temperature and pressure profile 110 120 130 140 150 160 170 180 190 Primary Secondary Temperature°C Note : More coarse chips & fiber bundles absorb energy even at larger gaps @ plate inlet
  • 9. Stator Region, cont’d  Publications have shown that the differential between refiner inlet and outlet steam pressures impacts the retention time of fiber in the refining zone and thus the plate gap.  Higher outlet than inlet pressure(pressure boost) increase fiber retention & plate gap.  Lower outlet than inlet pressures decrease fiber retention & gap.  I would suggest that stator plate fill rate(fiber retention) changes dominate the differential pressure gap changes, as stator filling is dominated by steam flow conveying effects.
  • 10. Plate Gap Region – Fiber trapped between rotor & stator plate bar tops.  Conveying influenced by rotor and stator bar crossing angle(sliding direction) & stator frictional forces.  Bi-directional plates evenly alternate inward and outward sliding direction.  Directional feeding plates increase the portion of outward vs inward sliding forces.  Directional holding plates do the opposite.  Typically need feeding inboard of pressure peak to reduce inlet filling, and holding outboard of peak to retain fiber & increase filling.
  • 12. Plate Gap Region, cont’d  Higher stator bar frictional forces tend to hold fiber, promoting sliding of fiber across the rotor bar tops, increasing rotor bar edge rounding & wear.  Fiber tends to slide on bar surface with the lowest frictional forces.  Bar angle and bar edge condition influence holding forces on fibers.  Newly manufactured plates exhibit break-in period with higher plate gaps and higher energy consumption.  I would suggest effect caused by the grinding burr on bar edge giving increased roughness and holding = larger plate gap  Once burr worn away reach normal holding level until excessive bar edge rounding begins to reduce plate gap.
  • 14. Counter-rotating vs Single Disc  2 Rotors & no Stator  Must feed through holes in 1 disc(Feed End)  Feeding challenge w/chips, even more issues with fiber  Use added dilution water to help feed(lower refining consistency)  Use steam to help feed(discharge pressure lower than inlet)  Add steam to inlet to heat chips(not self pressurized)  Steam flow effects upon fiber flow more limited, centrifugal forces dominate feeding  At 60 hz(1200 rpm) should be lower intensity than single disc as higher retention time & more bar crossings(2400 vs 1800)  Lower refining consistencies & plate designs without surface dams, both feeding requirements, actually make intensity higher  Non-feeding disc adjusts axially to control gap & load(Control End)
  • 15. Counter-rotating vs Single Disc  Although a pressure peak & steam seal may still be formed between the plates, backflowing steam must be limited & the seal ring behind feed disc allows steam flow around disc.  Limited steam pressure boost possible for retention time increase  Pressure boost would cause more steam flowing back to the inlet across seal ring
  • 16. Plate Gap  Fiber in the plate gap stops as it is not rotating with either disc & dragging forces cancel out  Limits how large gap can be before feeding stops(Pinch-off)  Reason for sub-surface not surface dams @1200 rpm?  Keep fiber in Feed End disc in the inlet area to assure feeding.  No bars on Control End opposite feed openings to prevent stoppage of Feed Disc rotational motion to maintain feeding  Cavitation(high pressure collapse) on any Control End bars opposite the feed openings in the Feed disc
  • 18. Model Similarities – Lo-Co vs Hi-Co  3 Zones – Rotor, Stator & Gap  Shear & Compression forces on fibers  Plate Gap impact upon energy transfer & efficiency  Fiber retention time impacts probability of treatment  Stator fiber recirculation
  • 19. Model Differences – Lo-co vs Hi-co  Fiber retention function of flowrate & internal recirculation, not centrifugal force  Fiber holding affect of surface dam only at dam, not region before dam  Dams increase groove pressure drop significantly thus reducing flow capacity  Inlet & outlet pressures function of flow, feed pump operation, plate geometry, refiner rotational speed and plate diameter, not independently adjustable
  • 20. Lo-co Fundamentals  Rotor plate increases pressure from inlet to outlet similar to a pump impeller  Grooves in rotor and stator cause pressure drop as a series of small pipes in parallel  Pressure increase from inlet to outlet is the net result of rotor plate pumping less groove pressure drop  As plate gap closes, initially pumping efficiency increases as pump impeller clearance decreases thus more pressure increase  Further gap closing can decrease pressure increase as less gap & more groove flow increasing pressure drop  Larger refining gaps give a higher proportion of shear forces, promoting more external fibrillation – potentially higher energy to a given pulp quality  Smaller refining gaps give a higher proportion of compressive forces, promoting more internal fibrillation(fiber splitting) – potentially lower energy  Limit in reducing gap is fiber cutting(length reduction)
  • 21. Lo-co Fundamentals, cont’d  Lower plate bar height with wear reduces rotor pressure increase & increases groove pressure drop thus reducing pressure increase across refiner & potentially causes a pressure drop from inlet to outlet  As long as discharge flow control valve is < 100% open, flow control is still possible  If valve 100% open, refiner plates are the flow control valve
  • 22. Rotor Region  Strongest groove vortex to load fibers onto bar edge  Potential for forward flow regime below vortex  Influenced by fiber type, consistency, groove geometry  Refiner plate geometry dominates pumping characteristics:  Bar angle, & height  Plate diameter  Presence of dams, surface(full bar height) or sub- surface(< full bar height)
  • 23. Tracing of fluid particle at the top and bottom of the rotor groove. Reference: Numerical simulation of the flow in a disc refiner, Gohar.M.Khokhar, Master’s Thesis 2011 Outlet
  • 24. Stator Region  Weaker vortex rotationally but less vortex pitch radially vs rotor  Internal recirculation due to higher discharge pressure than inlet displaces main flow volume  Unrefined fiber velocity increases/retention time decreases  Groove flow below vortex, if present, radially inward if typical higher discharge than inlet pressure  Groove volume influences pressure drop not pumping
  • 25. Fluid particle tracing at the top and bottom of stator groove going in the negative Y-direction Reference: Numerical simulation of the flow in a disc refiner, Gohar.M.Khokhar, Master’s Thesis 2011 Outlet
  • 26. Plate Gap Region  Fiber flocs compressed & locally de-watered  Compressive and shear forces applied to flocs  Groove vortices bring flocs to gap region for possible trapping  Flocs trapped then released  Plate bar geometry(width and crossing angle) impact treatment probability & severity as well as closing force  Bar surface sliding on fiber wears while floc holding surface is protected from wear
  • 27. Lo-Co Refining Evaluation & Optimization  Energy consumption comparison must include feed pump energy  Less refiner no-load(pumping) but more pump energy reduces savings  Feed pump VFD needs to be part of any energy reduction plan  Replacing multiple refiners with a single refiner reduces no-load energy even with the same refiner concept  Therefore the true energy reduction of any new refiner concept must be single refiner vs single refiner  Refiner filling design determines actual forces on fibers, therefore filling selection for optimization will impact long term peak performance.  Single source of fillings can limit long term optimization  Any increased filling cost per ton produced needs to be deducted from energy cost savings  Filling supply competition can speed optimization & reduce filling cost