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Improving Brown Stock Washing by

On-line Measurements – Mill

Investigations

Contents

1. The purpose of brown stock washing

2. Refractometer measurements

3. Real-time refractometer measurement results for washer performance optimization

4. Implications of process and mill levels

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Target

Series of investigations to find solutions, which will minimize

washing loss and consume less energy, water and chemicals.

Pulp Washing in

Bleaching

Brownstock Washing • Digester

• Post-digester

• Post O2-stage

• Post screening

Rationale

1. There is a need to improve brown

stock washing and its monitoring, for

economic and environmental reasons.

2. Conventional on-line and traditional

lab sampling methods do not reveal

all process variations.

→ Better real time monitoring tools

are needed.

Basic Concepts

• The reason for brown stock washing

• Washing before and after Oxygen

delignification

• The parameters, which describe the

washing performance and its

effectiveness: wash loss and

dilution factor

• Wash loss evaluation

Refractometer

• Indicates total dissolved solids in solution, suitable for all concentrations

• Measures the washable liquid substances concentration exactly

• Detects organic materials with large molecular size, like lignin

• Not influenced by COD caused by methanol

Refractometer

• Not influenced by suspended solids,

bubbles or fibres

• In-line real-time measurement

• Short response time → suitable for

control

• Mounts directly in pulp line

Principle

Light

source

Optical image

Process medium

Critical angle

Prism

Arrangement

Process Analyses

• A software tool Wedge was used for washing

result analyses and for calculating the

different factors, which define the washing

line performance

• The Wedge was used to calculate liquor

balances and dilution factors in the washing

line, as well as to calculate E-value

91ln

1ln211

21

10DF

yxL

yyDF

E

Experiments

Experiments were divided into three parts:

1. Washing in the digester

2. Washing before oxygen delignification

3. Effects of washing loss on oxygen

delignification

Digester washing

Bypass was decreased and the amount of

wash liquor fed into the digester was

increased (into the central distribution

chamber). The same amount of filtrate from

the wash circulation was extracted.

700,0

720,0

740,0

760,0

780,0

800,0

820,0

840,0

860,0

880,0

900,0

24.7.2009 11:30 24.7.2009 19:45 25.7.2009 4:00

Wash

ing

lo

ss,

(kg

DS

/BD

t)

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

Extr

acti

on

fro

m w

ash

cir

cu

lati

on

, (t

/BD

t)

Washing loss from digester Extraction from wash circulation

Digester washing

The digester’s washing result can be increased by utilising radial displacement washing.

6,50

7,50

8,50

9,50

10,50

11,50

24.7.2009 11:31 24.7.2009 19:46 25.7.2009 4:01

DS

-co

nte

nt,

(%

)

Liquor in blow Wash liquor to digester Wash liquor to PDs

Pressure Diffuser’s Feed

Consistency Effect

With a higher feed consistency,

the washing efficiency is increased.

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

9,0 9,5 10,0 10,5 11,0 11,5 12,0

Feed consistency, (%)

E10

Effect of the Downward Velocity

of the Pressure Diffuser Screen

• The downward velocity of the screen notably affects the

pressure diffuser’s washing result

• In literature: “The optimum velocity ratio should be

slightly higher than one being from 1.1 to 1.3.”

0

1

2

3

4

5

6

0,110 0,115 0,120 0,125 0,130 0,135

Downward velocity of the screen, (m/s)

E10

640

660

680

700

720

740

760

0,110 0,115 0,120 0,125 0,130 0,135

Downward velocity of the screen, (m/s)

Wash

ing

lo

ss,

(kg

DS

/BD

t)

Optimum range Poor wash results Optimum range

Effect of Dilution Ratio on E10

The dilution ratio notably affects the

pressure diffuser’s washing efficiency

0

1

2

3

4

5

6

7

0,90 0,92 0,94 0,96 0,98 1,00 1,02

Dilution ratio

E10

slinliquorwash

sloutfiltratewashratioDilution

/),(

/),(

Effects of Washing Loss in

Oxygen Delignification

• The oxygen delignification response was highly affected

by the amount of washing loss

• This is due to the oxygen being consumed in the

oxidation reactions of the washing loss

18

20

22

24

26

28

30

32

34

100 150 200 250 300 350 400 450

Washing loss to oxygen delignification, (kgDS/BDt)

Kap

pa r

ed

ucti

on

in

oxyg

en

deli

gn

ific

ati

on

, (%

)

Optimum range Process difficulties

Effect of Washing Loss in

Oxygen Reactor’s Temperature

88,0

89,0

90,0

91,0

92,0

93,0

94,0

95,0

96,0

100 200 300 400 500

Washing loss to oxygen delignification, (kgDS/BDt)

Te

mp

era

ture

in

ox

yg

en

rea

cto

r, (

ºC)

Temperature in the oxygen reactor correlates

with the amount of washing loss

Conclusions

• By using refractometer measurements, TDS changes in the incoming washer flows (liquid and pulp suspension) can be detected immediately and reliably

• TDS changes also had a clear effect on washing loss in the brown stock line’s oxygen delignification performance

• By using refractometers and data-analyses tools, it is possible to discover the black spots of the washing line and to evaluate the washing result continuously, thus improving the washing efficiency

Implications

• Digester and diffuser washing can be improved by utilising refractometer measurement results

• Real-time measurements provide whole new possibilities for implementing higher level process optimization, e.g. throughout the entire washing department

Refractometer

Thank You

Thank you for your attention!

Any questions?