Puu-0.4110: Conventional and Non- Conventional …...Puu-0.4110: Conventional and Non-Conventional...

Puu-0.4110: Conventional and Non-Conventional Pulping as a Basis forBiorefinery (7 cr)Lecture 12: Conventional Alkaline Processes III

Washing Equipment in Kraft Pulp Mills

12. Alkaline Processes III Pekka Tervola

Learning Objectives

After this lecture the student*understands, and is able to explain, the phenomenataking place during pulp washing•recognises and is able to describe equipment used in kraftpulp washing•can explain how the theoretical washing principles applyto the described equipment•understands the advantages and disadvantages of thedifferent washing equipment and can make conclusions onwhy certain equipment is used at a certain point of thefibreline

Puu-0.4110

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Modelling the mass transfer of washingdisplacement

• The above presented E-value concept is a kind of ”blackbox” model that defines the washing process as anumber of completely mixed ideal stages

• Washing can be considered by means of dispersionmass transfer

• Washing can be considered by semi-batch model,where the cake is divided severallayers (thickness direction).


Mass transfer in pulp washing• Dispersion mass transfer in pulp washing: modelling the displacement

phenomena

– In the displacement washing, the cleaner washing liquid pushes (displaces) themore unclean liquid from the pulp mat

– This is the main washing mechanism in practically all pulp washing equipment– During the displacement, there is always some mixing of the washing liquid and

the unclean washable liquid (axial mixing)– The axial mixing makes the washing response curve look like a skiing hill instead

of the a sharp step-wise change– The displacement washing can be modelled either by means of the E-factor

concept or by using dispersion models


Mass transfer in pulp washing• Dispersion mass transfer in pulp washing: modelling the displacement

phenomena


Mass transfer in pulp washing• The basic dispersion model of pulp washing:

– c = concentration of the washable component– DL = dispersion coefficient– u = velocity of the displacing flow– x = spatial coordinate– t = time coordinate

Time-dependent termAxial dispersion

Convection term

xcu

xcD

tc

L ¶¶

-¶¶

=¶¶

2

2


Mass transfer in pulp washing

• Dispersion models:– Introducing dimensionless parameters

• Dimensionless concentration C:

• Dimensionless length Z:

• Dimensionless time, :

• Peclet number, Pe:

LzZ =

Lut

=Q

LDuLPe =

Thickness of the pulp mat

Initial conc.

Conc. of wash liquid

C=


Mass transfer in pulp washing• Dimensionless dispersion model where the mass transfer between the liquid and the

fibers has been omitted

– Initial condition:

– Boundary conditions:

ZC

ZC

PeC

¶¶

-¶¶

=Q¶

¶2

21

0,1 =Q=C

0,0 ==¶¶

- ZZCCPe (On the mat surface)

1,0 ==¶¶ Z

ZC (At the bottom of the mat from

where the filtrate escapes)


Mass transfer in pulp washing• Breakthrough curves describe the solution of the dispersion model:

0

0,2

0,4

0,6

0,8

1

0 0,5 1 1,5 2 2,5 3 3,5Dimensionless time, Q

Dimensionlessconcentration, C

Pe = 2Pe = 6Pe =10Pe = 20


Segregated filtrate circulation:2-stage, 3-sections (example)


Solute from the cake, 2-stage washing

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00

Wash ratio

Frac

tion

ofso

lute

rem

oved

from

cake

P=0.0

P=7.5

P=2.0P=1.0


Change, 2-stage, P = 1

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

0.018

0.020

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00

Wash ratio

Cha

nge

infr

actio

nof

solu

tere

mov

edfr

omca

ke Case 2Case 3


W = 1.2

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

2 3 4 5 6 7

Number of washing stages

Frac

tion

ofso

lute

inth

eca

keaf

ter

was

hing

C1, P = 1 C2, P = 1 C3, P = 1 C1, P = 2 C2, P = 2 C3, P = 2


Observation of segregation

• Segregation of filtrates intensify displacement washing• Improvement is the largest, when wash ratio is around

one (DF=0)• With segregations of filtrates less washing stages is

needed in some cases



• The significance of the Peclet number:

– The smaller the Peclet number, the more axial mixing and thelower washing efficiency

– The higher the Peclet number, the more sharp displacement andthe higher washing efficiency

– The E-factor and the Peclet number are related to each other



• The higher the wash liquid velocity => the higher dispersion coefficient =>the lower Peclet number => the lower washing efficiency


Other mass transfer issues: Sorption

• Sorption, for instance for sodiumLangmuir isotherm A*B*c/(1 + B*c)A unit kgNa/bdt (for instance A= 4.04)B unit l/mg (for instance B = 0.0168)

c unit mg/l

A is maximum amount of sorped sodium.Parameters depends on pH, wood species, Kappa-number etc.


Other mass transfer issues: Leaching

Organic material is leached out from a fiber phase to external liquid phase

Rapid in the beginning, but can take several hours

Depends on temperature, ionic strength of the liquid,wood species, kappa-number etc.


Properties of fibers and the fiber cake

Charge of fibers apK = 3.1 uronic acids 1HA = 118 mmol kg-1 dry fiber

apK = 5.6 carboxyls in lignin and in extractives 2HA = 21 mmol kg-1 dry fiber

apK = 9.0 phenolic hydroxyls in lignin 3HA = 20 mmol kg-1 dry fiberFiber Saturation Point (FSP) fV = 1.45 dm3 kg-1 dry fiberCake dimension area A = 0.00139 m2

height L = 0.10 m

Table 1. Pulp and cake properties (Räsänen, 2003).


Donnan equilibrium (idea)

M+

M2+

M+M2+

M+

I-

M+ I-

M+

M2+

OH-

I-

I-H+

f s


Donnan equilibrium (Basic concept)

))1((

)(

,,

,,

,,.

-+=

=

+=

+=

+« -+

zfTs

Tkeqe

eqez

eqf

feT

sfkfekeTk

jj

VVmn

c

cc

VVVmcVcVn

AHHA

l

l


Washing and Donnan equilibrium

• Advection-dispersion equation is applied to all solute components inthe external liquid phase of the cake

• Fiber charges and acid groups and volume of the fiber phase (FSP)

• Electro neutrality in the fiber phase and in the external phase

• Donnan equilibrium included between the fiber phase and theexternal phase

• A time-dependent mass transfer included between the fiber phaseand the external phase


Leaching and displacement washing test

• Leaching test: Oxygen delignified hardwood pulp.Sulfuric acid was added and the filtrate samples weretaken in the different time.

• Displacement test: Oxygen delignified hardwood pulp.Wash liquid (pH = 1.8) was applied. The samples fromthe leaving filtrate were taken in the different time.


Leaching test

0

10

20

30

40

50

60

0 5 10 15 20 25 30 35 40 45 50 55 60

Time (min)

Sodi

um/M

agne

sium

/Cal

cium

(mg/

l)

0.0

0.1

0.2

0.3

0.4

Man

gane

se(m

g/l)

Na

Mg

Ca

Mn


Displacement washing: pH, filtrate

0

1

2

3

4

5

6

7

8

9

10

11

0 1 2 3 4 5 6

Dimensionless volume (VE/VE0)

pH

Calculated Measured


Displacement washing: Na, filtrate

0

1

2

3

4

5

6

7

8

9

0 1 2 3 4 5 6

Dimensionless volume (V/VE)

Na

(mm

ol/l)

Calculated Measured


Mg, filtrate

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 1 2 3 4 5 6

Dimensionless volume (V/VE)

Mg

(mm

ol/l)

Calculated Measured


pH inside the cake (simulation)

1

2

3

4

5

6

7

8

9

10

11

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Dimensionless thickness (x/L)

pH

V/VE=0.5 V/VE=1.0 V/VE=1.5 V/VE=2.0


Washing equipment and theiroperational principles

• Digester washing:– Continuous cooking Hi-Heat washing– Displacement batch cooking (SuperBatch, RDH) terminal displacement

• Diffuser washing:– Single stage atmospheric diffuser– Two stage atmospheric diffuser– Pressure diffuser


Washing equipment and theiroperational principles

• Drum filters:– Vacuum filters– Pressure filters

• Multistage displacement washers:– DD-washers– Flat wire washers

• Wash presses:– Dilution-thickener presses (screw or roll presses)– Displacement presses(Twinroll-, Compact Presses, AWP)


Continuous cooking Hi Heat washing

- Wash temperature 130 – 160 oC, washing time 2 – 4 h,- E10-value at best > 10, typically 6 – 8, high capacity 3 - 5

> 120 oC


The terminal displacement of thedisplacemen batch cooking

• Wash temperature 95 – 160oC• Cooling of the wash liquid• Washing time 40 – 50 min• E10-value = 2 – 3 for the dissolved dry solids• The main purpose of the terminal displacement is the

heat recovery which is clearly more efficient thanthe actual washing phenomenon (E10=5-6)


Diffuser washers: atmospheric diffuser

- Feed consistency 8 – 12 %; ouletconsistency about the same

- The washing is pure displacement- Stepwise operation:

- Screens up =washing stage

-Fast return down when thescreens are cleaned

- E10-values:- One stage 4 – 6- Two stages 6 – 8,5


Construction of the atmosphericdiffuser: screen assembly


Continuous digester and a diffuser washer


Pressure diffuser

- Washing can be performed in120 – 130 oC

- Displacement in overpressure- Small footprint, stands outside- Otherwise the same operational

principle as with atmosphericdiffusers


Pressure diffuser: construction


Drum washers

- Traditonal pulp washer type,still used in older pulp mills

- Washing is based on thickeningand displacment

- Outlet consistency 12 – 14 %- The required pressuredifference either with vacuumor overpressure

- E10-values typically 2 - 4


Multistage washing: DD-washer

In the same washerthere can be 1 – 4Washing stages


Multistage washing: DD-washer

• In the same washing apparatus 1 – 4 countercurrentwashing stages can be performed

• Feed consistency either 3 – 4 % (LCDD) tai8 – 10 % (MCDD)

• Max. capacity in the range of 6000 ton/d (one stage)• E10-value= 12 - 16 (4-stages)

= 5 – 10 (1 – 2-stages)• Washing efficiency can be improved with the so calledsegregated washing


Typical layout of a DD-washer

DeknottingWasher

Filtrate tanks

Pulp storage tower

Feed pump

Dilution line


Wash presses

• Twin roll presses (Andritz, Valmet, GL&V)• Screw presses (eg. Andritz,Thune, Krima)• Belt presses (Andritz)


TwinRollTM Family

Feed pulp Feed pulp

Wash liquidFiltrates out

Washed pulp


TwinRoll-press

Feed pulp

Wash liquid Filtrate

Washed pulp


Compact Press (GL&V)


TwinRoll Evolution (Valmet)


AWP (Andritz Wash Press)


Wash presses

• One-stage displacement-thickening• Feed consistency typically 3 – 7 %• Outlet consistency 30 – 35 %• E10-value = 3.5 – 5• A press is well applicable especially at the end of thewashing line where the clean washing water is introduced.The demand of washing water is small due to the highdischarge consistency.•Water lock.


Summary of the washing equipmentWasher type Feed cons., % Outlet cons., % E10-value Supplier

Continuous cooking Hi-Heat washing

9 - 10 9 -10 4 – 9 Andritz, Valmet

Displacement batchTerminal displacement

7 - 10 7 - 10 1.5 – 2.5 GL&V, Valmet

Atmospheric diffuser(1-stage)

9 - 13 9 - 13 4 - 5 Andritz, Valmet

Atmospheric diffuser(2-stage)

9 - 13 9 - 13 7 – 9 Andritz, Valmet

Pressure diffuser 9 - 13 9 - 13 4,5 – 5,5 Andritz, Valmet

DD-washer(4-stage)

4 – 4,5 12 - 14 12 - 16 Andritz

DD-washer(1,0 - 2-stage)

4 – 9 12 - 14 5 - 10 Andritz

Twinroll-press.Compact press

3.5 – 7 30 - 35 4 - 5 GL&V, Valmet

Dewatering Press 4 – 7 30 - 35 3 – 3.5 Andritz, Valmet

Drum filter1 - 2 10 - 14 2 – 4 Many suppliers


Washing systems• Suggested values for the Total washing efficiency:ØE10-value = 14 – 19 before the O2-stage (depends on wood)ØE10-value = 6 - 8 after the O2-stage

• COD-wash loss suggested values:Ø Before the oxygen stage < 100 kg/ton pulpØ Into bleaching < 10 kg COD/ton pulp

• Dilution factor: 2 – 3 m3/ton pulp

• The selection of the washing equipment is based on many factors:investment costs vs. washing performance, capacity, layout,operational availability etc.

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