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Benefits of online porosity measurement feb 2018


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Benefits of online porosity measurement feb 2018

  1. 1. Pele Oy Paper Porosity and the Art of Papermaking Pekka Komulainen February, 2018
  2. 2. Pele Oy Porosity and air permeability  Porosity is measured by air flow through paper web over standard pressure difference.  All papers have voids between fibers. Scientific porosity means void volume compared to total volume. That is different than air permeability.  Air permeability is called porosity in the following presentation.  Continuous porosity measurement tells what has been made in papermaking and predicts what will happen in printing or converting.  Online porosity measurement is different compared to laboratory measurement. The main difference is that paper sheet is not moving in the laboratory and the web speed of online measurement can vary very much. 2
  3. 3. Pele Oy What can be optimized with porosity measurement  Pulps and pulping processes  Filler types and filler amount  Broke types and amount  Chemicals and wet end chemistry  Fiber refining and swelling  Fines and filler retention  Air content and foaming  Headbox consistency and formation  Slice and jet controls  Wire table dewatering  Wet pressing and calendering  Sheet release and draws  Drying type, free or restrained  Surface sizing and coating 3
  4. 4. Pele Oy Porosity for total process control  Fibers , refining, fines, fillers and wet end chemistry have effect on drainage, flocculation and retention. All these effect on porosity, which is a good measure of paper quality.  Formation is a measure of small scale variations. Porosity depends on these variations as well as the level of paper quality. 4
  5. 5. Pele Oy Porosity control before headbox Porosity decreases with:  Less softwood kraft pulp  More fine mechanical or hardwood pulp  More coated broke or DIP (normally)  More refining  More fiber fines  Less anionic trash (less hydrophobic material)  More wet end starch and other dry strength chemicals  Less fillers or more platelike fillers (talc, delaminated clay)  Less air in stock 5
  6. 6. Pele Oy Refining and porosity  Conventional control of refining is to control Specific Refining Energy  SRE control of refiners cannot decrease pulp quality variations  With online porosity measurement this can be corrected 6 SRE = Specific Refining Energy Pe = Effective Power, Pt = Total Power, Po = Idle Power m = Fibre flow (Bone Dry)
  7. 7. Pele Oy Online porosity control and refining  In series refining fibers are stiffer in the first refiner. If the load is too high, fiber cutting and fines formation will increase. This decreases freeness and drainage on the wire.  In the second (and third) refiner fibers are more flexible and specific edge load can be higher without fiber cutting and fines formation. However, internal fibrillation advances and more and more flexible fibers compensate the lower binding power of fiber cutting.  Same porosity can be achieved with higher CSF when using increasing loading order. Total energy can be saved, drainage on the wire is increased and lower headbox consistency can be used to get better formation even if the fibers are longer and paper strength is better.  Loading shares between refiners is only one example how online porosity can be utilised to optimise refining. This would not be possible by using freeness measurement, which mostly reflects fines content. 7
  8. 8. Pele Oy Press draw and porosity  A high press draw is not only question of lower runnability but also paper quality. Higher press draw means higher porosity and lower strength. Most of the papers require low porosity but high bulk and strength. Porosity measurement is a good tool to control press draw.  Wet pressing itself always reduces porosity and increases strength. Wet strain after nip has a very negative effect, increasing porosity and decreasing strength. High press draw also increases paper roughness and decreases printability. 8 Picture: Valmet
  9. 9. Pele Oy Porosity control on paper machine Porosity decreases with:  Higher basis weight  Better formation  Good retention  Slow and gentle water removal  More wet pressing  Less press draw  Lower drying shrinkage  More surface size  More film forming size (PVA, CMC)  Less surface size penetration  Pigmented surface size  More calendering 9
  10. 10. Pele Oy Total process control with porosity 10 High internal bond Sheet compaction, Low press to dryer draw Wet Pressing Good formation Low air content Good refining Good bonding Good strength Low oil absorption No pinholes Good runnability No print through Dense sheet Rotogravure printability Offset printability Low silicone pick-up Limit: Set-off in CSWO Low porosity correlates with: Root cause: Final benefit of low porosity:
  11. 11. Pele Oy Total process control with porosity  Constant porosity – Consistent quality  Papermakers know that final porosity of cigarette, filter and sack papers is very important but also printability and runnability properties of printing papers correlate well with final paper porosity.  Most on-line instruments measure and control only one paper property. In addition to final paper porosity, which is an important property for some paper grades, online porosity measurement is a universal analyse of paper quality for most paper grades due to the fact that almost all process parameters have a clear effect on porosity.  On the other hand, porosity correlates well with almost all physical paper properties. If porosity is constant, paper process performs well and paper quality is consistent. 11 Paper making Paper porosity Paper quality
  12. 12. Pele Oy Total process control with porosity 12 Decreased bulk Better formation Lower caliper Lower stiffness Better ply bond Better smoothness Less print- through Less pinholes Higher opacity Better print gloss Lower ink absorption Increased basis weight Increased moisture Decreased Porosity More fiber bonding Higher strength Lower tear strength Lower light scattering Lower opacity and brightness Less dusting Lower ink demand Less fluting in offset Green Red = positive = negative
  13. 13. Pele Oy Process parameters increasing porosity 13 Higher Porosity Lower retention Less broke Softer winding Decreased wet pressing Coarser fiber type Less surface size Carbonate fillers Less platey fillers Less wet end starch Less reject in screening Reduced refining Poor wet end chemistry Pulsating drainage More shrinkage at web edges Reduced calendering Higher press draw More flocculation Higher air content More pin holes Lower fiber bonding Lower fiber fines content
  14. 14. Pele Oy Total process control with online porosity  Porosity is an overall measure of process performance. Porosity measurement is like freeness in stock preparation. When porosity is stable process is performing well.  Porosity measurement is an excellent analyse before surface sizing and coating. Measurement not only shows the history of the paper web but also predicts how good coverage of size or coating can be achieved in the following process.  Film coating and cases where the positive effects should be made with a minimum coat weight are well suited.  When porosity is low, size or coating penetration is reduced and the effect of coating on the paper surface is better. This is important when basis weight is low or coating colour cost is high.  Especially when low brightness base paper is coated to get white surface low porosity is important (coated kraftliner). 14
  15. 15. Pele Oy Process optimization and paper quality  Fast online porosity measurement is the best link between process performance and total paper quality.  Stable porosity is a good guarantee of well working process and consistent paper quality. 15 Tensile Bulk Formation Caliper Stiffness Scott Bond Roughness Print throughPin holes Opacity Print gloss Ink demand Basis weight Moisture Drainage Fillers Wet pressing Retention Fibers Press draw Air content Chemicals Flocculation Refining Screening Web shrinkage Calendering Winding Porosity measurement
  16. 16. Pele Oy MD and CD variation in porosity  CD variation of porosity is very stable. It can easily be measured in laboratory.  Machine direction variation seems to be in this case about ± 15 ml/min. This could be corrected by using online porosity measurement. 16 MD var. Picture: ACA Systems
  17. 17. Pele Oy Calibration with laboratory measurement  Paper is moving in online measurement and there is always some leakage air included in the measurement. This air depends on two parameters: machine speed and paper roughness.  Most paper grades are so sensitive that for runnability reasons it is not possible to increase the tightening suction high enough to totally eliminate the leakage air.  This means that the calibration curve with laboratory measurement is not totally linear even if the paper quality is same.  Normally there are several basis weights of each paper grade and these are run with different speeds. With lower basis weight and higher speed the share of leakage air is higher. The relation to speed is not even. Normally surface air film starts to build when speed is > 10 m/s. 17 Leakage air increases porosity when speed is over 10 m/s Paper web Measured air Air through
  18. 18. Pele Oy Challenges of online porosity compared to lab  Different paper grades normally have different roughness. Higher roughness means that there is more leakage air and the calibration curve of different paper grades is slightly different. This causes problems especially in high speed multigrade paper production.  The other thing is dwell time over the measurement orifice. Dwell time depends on machine speed. It is well known that dwell time has effect on the air flow.  With higher dwell time in the laboratory paper structure will compress more and porosity is lower compared to online measurement.  Last but not least, air temperature and humidity are not constant in online measurement. 18 Online web speed 10 m/s and 0.01 m orifice mean that measurement time is one ms. Paper is viscous and not compressed – porosity is higher. Laboratory measurement time is several seconds – paper is slightly compressed and porosity is lower.
  19. 19. Pele Oy Other challenges  To avoid leakage it is very important to install the porosity measurement head completely aligned with paper web. Each supplier can give instructions for the installation.  Normally suppliers can give guarantees for the workability of the measurement. However, recycled fibers with stickies and dust can be so difficult that the contacting head is not working continuously.  In any case, maintenance of the equipment and changing of filters must be done so often that the system is working as planned. 19
  20. 20. Pele Oy Some suppliers of online porosity sensors  ABB Porolog products  MCS Porosity Measurement System  Microperm Smart  Permi  In addition, Honeywell, MCS Engineering and ACA Systems supply scanning porosity systems. 20
  21. 21. Pele Oy 21 Thank You for your attention Picture: Pekka Komulainen