Anatomy of Soda Loss
100 #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
Pulp and Power Sewer Losses
Good sewer flow and conductivity system w/Alarms on PI or DCS (awareness/training)
Food for thought:Establish conductivity round in smaller sewers to find sources
Dregs filter (<= 2#/ADT)
Slaker grit leachate (<=2#/ADT)
Digester sand separator trap
Weak wash balance
Salt cake purging
Use of sumps in recaust and evaps
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
Daily Focus Big Picture
• BSW Carryover Soda Loss
• Pulp/Power Sewer Soda Loss
• Total Mill Sewer Soda Loss
April 2005 Pulp Washing Training 8 of 152
• 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 washing
BSW Optimization & Economics
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
April 2005 Pulp Washing Training 10 of 152
Dilution Factor
1st Stage BSW
Dilution Factor
0.74
0.76
0.78
0.80
0.82
0.84
0.86
1 2 3 4
Dis
pla
cem
en
t R
ati
o
Dilution Factor
Dilution Factor
• Shower Liquor In Excess Of The Liquor In
The Discharging Pulp Mat
• DF = (lb liq/lb pulp)shower - (lb liq/lb pulp)mat
• May Be Positive Or Negative
Displacement Ratio
• The Ratio of DS Removed to the Maximum
Possible DS Removable
• DR = (DS vat – DS mat) / (DS vat – DS shower)
• 0.0 < DR < 1.0
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)
Example Problem
Is this first stage drum washer performing to OEM standards ?
Production rate : 1154 ODT/day
Drum specific loading: 0.80 ODT/ft2
Shower flow: 1440 gpm
Discharge consistency: 14%
Vat DS: 6.41%
Mat DS: 2.88%
Shower DS: 1.76%
Results
DF = 1.35
OEM DR = 0.802
Tested DR = 0.759
No ! 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
CD0024-002
Drum Washer Operation
WASHING
SHOWERS PULP MAT
TAKE-OFF
DOCTOR
REPULPER
CYLINDER ROTATION
DISTRIBUTION
BAFFLES
PULP
DILUTION
INLET BOX INLET WEIR
8-12” CASCADE TO VAT LEVEL
DISPLACEMENT
ZONE
EXTRACTION
ZONES
VACUUM
FORMATION
ZONE
GRAVITY
FORMATION
ZONE
ATMOSPHERIC
TAKE-OFF ZONE
Key issues1. Uniform shower distribution2. Maximize Vat dilution3. Proper Vat level4. Minimize air entrainment 5. Operator training 6. Keep equipment in good condition 7. Uniform sheet formation8. Mat thickness 1 to 2”9. Proper vacuum10. Good sheet take-off11. Temperature12. Run Automated Controls13. Apply defoamer at the point of the
problem
• 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
Washing Basics to Audit
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.
Washer Showers Basics
Uniform 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
Washer Showers Basics
Low 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.
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
Operations
Maximize 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
Operations
Maximize 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
• 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
Operations
April 2005 Pulp Washing Training 28 of 152
Consequences of Low Vat Level OperationTotal 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 efficiency by 5 to 10%.
• 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
Air is the Enemy
April 2005 Pulp Washing Training 30 of 152
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 distribution
Operations
April 2005 Pulp Washing Training 31 of 152
GL&V Pulp Group Inc.
Brownstock Seal Tank Design
VERTICAL
BAFFLESDROP LEG ENTRY
DROP LEG
DROP LEG
SIZE
INSIDE
DIAMETER
OUTSIDE
DIAMETER OPENINGS
Up to 12” dia.
300
36”
900
60”
1500
6” x 12”
150 x 300
14” & 16” dia.
350 - 400
48”
1200
72”
1800
6” x 18”
150 x 450
18” dia.
450
60”
1500
84”
2150
8” x 24”
200 x 600
20” & 22” dia.
500 - 550
72”
1800
102”
2600
10” x 24”
250 x 600
24” & 26” dia.
600 - 700
84”
2150
120”
3050
10” x 30”
250 x 750
SIX OPENINGS
VERTICAL
BAFFLES
Air is released
FiltrateOverflows
Air & FiltrateSeparate
April 2005 Pulp Washing Training 32 of 152EKA CHEMICALS
CONFIDENTIAL & TRADE SECRET Technical Marketing & Mill Services
Washer Seal Tank/Dropleg Design
Recommended
ht. > 30 ft.
Pulp
Feed
To Shower
or Effluent
Dropleg
Seal
ChamberBaffle
Vat
Dilution
Filtrate
Tank
Drum
Washer
Dropleg Design:
• Sized for > 12 to 16 fps
superficial velocity
• Straight, vertical, drop
no jogs
Vent
Courtesy of Beloit Pulping
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)
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
April 2005 Pulp Washing Training 39 of 152
Cost of Brown Stock Washing
Typical Mill Economic Relationship
0.00
500.00
1000.00
1500.00
2000.00
2500.00
3000.00
3500.00
4000.00
4500.00
Soda Loss lbs/ton (% Weak Liquor Solids)
Net
Co
st
($1
00
0/y
ea
r)
ClO2 $M/year
NaOH $M/year
Steam $M/year
Total $M/year
Decreasing Washing DF
$$ Sweet Spot $$
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
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
April 2005 Pulp Washing Training 42 of 152Liquor
Wash Water In
Pulp
Washed Pulp
Out
Dirty Pulp
In
Liquor out to
Evaps
Knotter Screens
Washer
Decker
Filtrate Tanks
April 2005 Pulp Washing Training 43 of 152
• 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
BSW Improve Projects