Brownstock Washing


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Brownstock Washing

  1. 1. Pulp and Paper IndustryBrown Stock Washing
  2. 2. Is a mill’s Soda Loss equal to its Soda Make-up ? YES !! Focus on Soda Loss, not Soda Make-up !
  3. 3. Anatomy of Soda Loss100 #Na2SO4/ADT (total mill loss/make-up) 80 #/ADT (BSW loss) 20 #/ADT (spills/purges) Washable Bound Pulp and Power sewers 50#/ADT 30#/ADT- Paper sewer - paper sewer 10#/ADT – World Class- Paper sheet - paper sheet 15#/ADT – Best Practice 20#/ADT – Go find the low hanging fruit
  4. 4. Pulp and Power Sewer LossesGood sewer flow and conductivity system w/Alarms on PI or DCS (awareness/training)Food for thought:Establish conductivity round in smaller sewers to find sourcesDregs filter (<= 2#/ADT)Slaker grit leachate (<=2#/ADT)Digester sand separator trapWeak wash balanceSalt cake purgingUse of sumps in recaust and evaps
  5. 5. Monthly Saltcake Reconciliation• Properly establishes true loss/make-up with inventory changes taken into account• Matches make-up to measured total mill sewer loss to insure good measurement• Prattville Example
  6. 6. Daily Focus Big Picture• BSW Carryover Soda Loss• Pulp/Power Sewer Soda Loss• Total Mill Sewer Soda Loss
  7. 7. BSW Optimization & Economics• Visual audit• Evaluate washing efficiencies• Audit equipment condition during outage• Audit controls• Determine benefit of washing versus cost of evaporation• Employ dilution factor control to minimize over washing and under washingApril 2005 Pulp Washing Training 8 of 152
  8. 8. BSW Assessment• Separate digester washing from external washing• Assess washers against OEM DF/DR curves• Assess digester against OEM DF/DR curves• Water/Dissolved solids balance
  9. 9. Dilution Factor 1st Stage BSW 0.86 0.84Displacement Ratio 0.82 0.80 0.78 0.76 0.74 1 2 3 4 Dilution Factor Dilution Factor April 2005 Pulp Washing Training 10 of 152
  10. 10. Dilution Factor• Shower Liquor In Excess Of The Liquor InThe Discharging Pulp Mat• DF = (lb liq/lb pulp)shower - (lb liq/lb pulp)mat• May Be Positive Or Negative
  11. 11. Displacement Ratio• The Ratio of DS Removed to the MaximumPossible DS Removable• DR = (DS vat – DS mat) / (DS vat – DS shower)• 0.0 < DR < 1.0
  12. 12. Washer Stage Data Collection- Inlet vat DS- Outlet mat filtrate DS- Outlet mat consistency* (high sensitivity/biggest chance for error)- Shower DS- Production rate- Shower flow- Inlet vat consistency** ( needed for DS and water balance)- Washer filtrate DS** ( needed for DS and water balance)
  13. 13. DR FormulaDR = (Sv - Sm) / (Sv - Ss)
  14. 14. DF FormulaDF = Shower flow(gpm) x 6 - 100-mat const Production (odt/d) mat const
  15. 15. Example ProblemIs this first stage drum washer performing to OEM standards ?Production rate : 1154 ODT/dayDrum specific loading: 0.80 ODT/ft2Shower flow: 1440 gpmDischarge consistency: 14%Vat DS: 6.41%Mat DS: 2.88%Shower DS: 1.76%
  16. 16. ResultsDF = 1.35OEM DR = 0.802Tested DR = 0.759No ! Washer is not performing to OEM standards.Result of getting this one drum to OEM standards: 96#/ton -> 90 #/ton Net savings is over $500M/yr
  17. 17. Drum Washer Operation CYLINDER ROTATION WASHING PULP MAT Key issues SHOWERS1. Uniform shower distribution TAKE-OFF DOCTOR INLET2. Maximize Vat dilution BOX INLET WEIR 8-12”3. Proper Vat level CASCADE DISPLACEMENT TO VAT ATMOSPHERIC ZONE4. Minimize air entrainment LEVEL TAKE-OFF ZONE5. Operator training EXTRACTION GRAVITY ZONES6. Keep equipment in good condition FORMATION ZONE7. Uniform sheet formation VACUUM FORMATION8. Mat thickness 1 to 2” ZONE9. Proper vacuum10. Good sheet take-off REPULPER11. Temperature12. Run Automated Controls PULP DISTRIBUTION13. Apply defoamer at the point of the BAFFLES problem DILUTION CD0024-002
  18. 18. Washing Basics to Audit• Showering – Maximize displacement washing• Vat consistency – Maximize dilution washing• Vat level control – Reduce air entrainment• Inspect Vacuum Valves - Reliability• Vacuum leaks - Reliability• Face wire – Reliability• Drum Integrity – Reliability
  19. 19. Purpose of Wash ShowersGently & uniformly apply wash water to the pulp mat for optimal displacement washing.Distribute the washer water flow appropriately over the washing arc.
  20. 20. Washer Showers BasicsUniform distribution – Keep showers clean and washed off – Keep nozzles unplugged – Soda losses can vary 30% or better across a washing stage due to poor shower distribution – Consider feeding water to both sides of the shower headers on washer larger than 18’. – Shower pipes should be free to rotate for finite adjustment
  21. 21. Washer Showers BasicsLow turbulence – Apply gently / tangentially to mat, do not disturb the mat – Blasting the sheet with shower water tends to aerate the stock – Minimize roll-back, balance the shower flows to each shower. Run back grooves the pulp mat, causing uneven mat formation. Uneven mat means an uneven wash.
  22. 22. Washer Showers Basics– Shower type / arrangement  High volume / low velocity on top  Low volume / high velocity on bottom  Bottom shower as low as possible to avoid air entrainment into sheet  Top shower in position such that filtrate is pulled into mat before vacuum cut-off  The velocity of the shower water should be close to the speed of the drum. Shower water should lay on the mat gently and evenly
  23. 23. OperationsMaximize Inlet Vat Dilution• Vacuum increases with increased dilution. More flow down the drop leg produces more vacuum.• Typically as mills increase production, the vat dilution pump is a constraint – Consider upgrading pump• Mills should consider a separate shower pump which will improve washing by reducing variability• Maximize blow tank dilution – dilution washing starts here
  24. 24. OperationsMaximize Inlet Vat Dilution• Ideal consistency as a result of adding dilution is 1% to 1 ½%. The surface area of the inlet vat will appear “velvety” with little or no flocculation – More vat dilution will reduce entrained air, improve mat formation, induce good vat turbulence, no dead zones, good deflocculation – Thinner sheet for better displacement, minimizes “fingering” in mat, ideal mat thickness is 1” – 2” – Dilution should be as much as the washer can hydraulically handle, while keeping the washer drum speed at an average of 2 to 2½ RPM – If drum speed is too high, then rewetting of sheet discharge mat could reduce washing effectiveness
  25. 25. Operations• Vat level control – Reduce air entrainment  4-8” cascade over weir  Need some drop over weir to shear/deflock the pulp  Higher levels allow more vat dilution – it increases the drainage area in the vat  Ensures gravity fill of drum – looser sheet, better formation, liquor can move through sheet easier  Higher levels reduce air entry into mat  Good level indication and control tuning required
  26. 26. Consequences of Low Vat Level Operation Total surface area = 3.14 x 11.5’=36.11’ Total Surface area lost = 11% Submergence zone = 16’ BUT actually 25% of the submergence zone is lost. 24” This allows air to enter the pulp mat which affects April 2005 Pulp Washing Training efficiency by 5 to 10%. 28 of 152
  27. 27. Air is the Enemy• Air bubbles interfere with wash water drainage• Use defoamers to control air at the point of the problem• Do not overuse defoamers – carryover will harm paper machine runnability – it reduces surface tension which cause an increase in machine breaks – Measure defoamer carryover - carryover should be: • Less than 30 PPM out of the brownstock wash system • Less than 10 PPM out of the bleach plant• Recommend using low molecular weight silicon defoamer• Check to make sure defoamer is effective in the temperature range that the washers operate• If use increases, operators should check places where air can enter system
  28. 28. Operations Potential Sources of Air • Low level in blow tank • Low level in filtrate tanks • Dilution injection points  Make sure dilution points are submerged. Free falling dilution will cause foaming • Soap build-up in first stage service tank • Vortexing in dilution and stock pump suctions • Poor air removal from filtrate tanks • Improper drum washer operation  Low vat levels  Poor shower distributionApril 2005 Pulp Washing Training 30 of 152
  29. 29. Brownstock Seal Tank Design Air is released DROP LEG VERTICAL BAFFLES Filtrate DROP LEG ENTRY Overflows Air & Filtrate Separate DROP LEG INSIDE OUTSIDE VERTICAL SIZE DIAMETER DIAMETER OPENINGS BAFFLES Up to 12” dia. 36” 60” 6” x 12” 300 900 1500 150 x 300 14” & 16” dia. 48” 72” 6” x 18” SIX OPENINGS 350 - 400 1200 1800 150 x 450 18” dia. 60” 84” 8” x 24” 450 1500 2150 200 x 600 20” & 22” dia. 72” 102” 10” x 24” 500 - 550 1800 2600 250 x 600 24” & 26” dia. 84” 120” 10” x 30” 600 - 700 2150 3050 250 x 750April 2005 Pulp Washing Training 31 of 152
  30. 30. Washer Seal Tank/Dropleg Design Dropleg Design: • Sized for > 12 to 16 fps Drum superficial velocity Washer • Straight, vertical, drop no jogs Dropleg Pulp Recommended Vent Feed ht. > 30 ft. Filtrate Tank Seal Baffle Chamber Vat To Shower Dilution or Effluent EKA of 152 32 CHEMICALSCourtesy April 2005 of Beloit Pulping Pulp Washing TrainingCONFIDENTIAL & TRADE SECRET Technical Marketing & Mill Services
  31. 31. Diffuser Efficiency Keys• Screen and backflush cycle• Pulp and wash water distribution (orifice sizing)• Pulp consistency and bed integrity (>10% inlet, <14% outlet)• Washing temperature (>160 F)• Production Load (5-6 ADMT/d m2)
  32. 32. Screen and Backflush Cycle *Only washing when your extracting• Upstroke 10% faster than pulp (proportional valve)• Quick closing time of extraction valve and wash valve to minimum position (2-3 sec) (mech stops for wash)• Short backflush (<= 1sec)• Proper pressure and volume of backflush
  33. 33. Atmospheric Diffuser
  34. 34. Pressure Diffuser
  35. 35. April 2005 Pulp Washing Training 38 of 152
  36. 36. Cost of Brown Stock Washing Typical Mill Economic Relationship 4500.00 ClO2 $M/year 4000.00 NaOH $M/year Steam $M/year Total $M/year 3500.00 3000.00Net Cost ($1000/year) 2500.00 2000.00 1500.00 $$ Sweet Spot $$ 1000.00 500.00 0.00 April 2005 Decreasing Washing DF Pulp Washing Training 39 of 152 Soda Loss lbs/ton (% Weak Liquor Solids)
  37. 37. Drum Wash Line Controls Control Objectives- Reduce soda carryover variability- Optimize use of DF- Wash line operating stability- Reduce mundane low valve activities of operator
  38. 38. Drum Wash Line Controls Minimum Control Expectations- Vat level control with drum speed- Seal take levels w/cascade shower flows & digester bypass- Dilution factor, open loop- On-line measurement of conductivity
  39. 39. Pulp Wash Water In Dirty Pulp Washer Decker In Knotter Washed Pulp Screens Out Filtrate Tanks Liquor Liquor out toApril Evaps 2005 Pulp Washing Training 42 of 152
  40. 40. BSW Improve Projects• New high performance showers to replace old whistles, spoons, and weirs  LaValley, Comarco Uniflows, Oramac• Split Washing - Hot water to last washer top showers to replace high COD condensate• Dilution factor controls – Course control of showers• Shower control using conductivity – Fine control of showers• Closing decker filtrate systems – Reduced soda loss, energy• Anti-rewet drum decks• Training operators on the principles of drum washers April 2005 Pulp Washing Training 43 of 152
  41. 41. Digester Washing AssessmentCompare to OEM DF/DR curve:- Vat DS are calculated end of cook DS or upper extraction DS- Shower flow and DS are cold blow flow and DS- Mat consistency and DS are blow consistency and DS- Filtrate flow and DS are total extraction flow and DS