brownstock washing

Brown Stock Washing

Pulp and Paper Industry

Is a mill’s Soda Loss equal to its Soda Make-up ?

YES !!

Focus on Soda Loss, not Soda Make-up !

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


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)

DR Formula

DR = (Sv - Sm) / (Sv - Ss)

DF Formula

DF = Shower flow(gpm) x 6 - 100-mat const

Production (odt/d) mat const

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


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.


– 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


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


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


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

Atmospheric Diffuser

Pressure Diffuser


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


• 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

Digester Washing Assessment

Compare 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

brownstock washing

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