Puu-0.4110: Conventional and Non-Conventional Pulping as a Basis forBiorefinery (7 cr)Lecture 11: Conventional Alkaline Processes II

Washing Operations in Kraft Pulp Mills

11. Alkaline Processes II Pekka Tervola

Learning Objectives

After this lecture the student

•understands the principles of kraft pulp washingsimulation•is able to explain the most important concepts in washingoperations (e.g. washing loss, dilution factor)•can formulate an overall balance for kraft pulp millwashing plant•can explain the importance of adequate brown stockwashing

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11. Alkaline Processes II Pekka Tervola

Kraft pulp mill

11. Alkaline Processes II Pekka Tervola

Fibre line (Example)

D1D /Q0 EOP

ECF Bleaching

Condensate/Filtratefrom bleaching (E )OP

Hot water/CondensateTo POW 0-water

Oxygen delignification Deknotting&Screening

To post screening

Cooking Wa shing

11. Alkaline Processes II Pekka Tervola

Purpose of pulp washing• Pulp washing represents the crossroads between the fiberline and

the chemical recovery• The dissolved organic matter from cooking and reacted cooking

chemicals forms the black liquor, which is pursued to be separatedfrom the fibre material as efficiently and as concentrated as possible

• On the other hand, the post-cooking delignification and bleachingstages imply that cook-based organic matter can be washed away (inorder to reduce additional chemical consumption, to maintain fibrestrength and pulp yield as well as considering the environmentalaspects)

11. Alkaline Processes II Pekka Tervola

Pulp washing in fibre line

•After delignification (cooking).This is called brown stcok washing (BSW)

•After oxygen delignification (O).This is called post oxygen washing (POW)

•Typically after each bleaching stage(D, A, AD, E, EP, EOP, P, PO, Z, Q,…)

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11. Alkaline Processes II Pekka Tervola

Motivation of pulp washing

• Separate fibres and dissolvedinorganic and organic material

• Recovery of cooking chemicals for re-use.• Recovery of dissolved organic material for energyproduction in a recovery boiler.

• Decrease of bleaching chemical consumption.• Decrease of environmental discharge.• Maintain product quality.

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11. Alkaline Processes II Pekka Tervola

Counter-current washing

• Motivation to save water• The clean wash water is introduced into the last washing stage, from

which the filtrate is used as the washing liquid in the previous stageetc.

• The most unclean (highest concentration of the washable material) iswithdrawn from the first washing stage. This filtrate is taken toevaporation. Nowadays in the modern cooking methods the firstwashing stage is arranged inside the cooking process.

11. Alkaline Processes II Pekka Tervola

Wash water and the connectionsof washing

• The wash water is commonly hot water of 70 – 80 oC and is obtained from the hotwater system of the pulp mill. Often also the secondary condensates from theevaporation plant are used but they may be unclean (rather high concentration of COD)which needs to be taken into account

• Some mills are considering the usage of the alkaline filtrates of bleaching as the washwater of pulp washing (due to environmental reason). The COD-content and Cl-contentof such filtrates needs to be taken into account, as well.

• Sometimes the washing line has sidestreams: a sidestream of pulp for eg. producingunbleached pulp. Sometimes there may be sidestreams in the filtrate side (eg.leakages of water from the pump seals). In such situations the complete countercurrentconnection is disturbed and any calculations imply the use of simulation programs.

11. Alkaline Processes II Pekka Tervola

Principles of washing

• The most simple pulp washing principle is the dilution of pulp withthe washing liquid followed by thickening (consistency increase).Commonly used in the washing of mechanical pulps and in smallproduction pulping lines.

• The most important washing principle is displacement washing(cake washing), where the clean washing liquor displaces the dirtierone from the pulp suspension. Displacement commonly takes placeat a consistency of 10 – 12 % which often implies pre-thickening ofpulp to the desired consistency ahead of the displacement.

Often an actual washer is combination of above two main basic operations.

11. Alkaline Processes II Pekka Tervola

Principles of washing

11. Alkaline Processes II Pekka Tervola

What are the substances to be washedaway in pulp washing?

• The total dissolved material in black liquor is the main target to be washedaway (dissolved solids, DS)

• Often the washing of organic and inorganic matter are kept separate from eachother: The organic matter is described with the Chemical Oxygen Demand(COD) and the inorganic matter eg. with sodium sulphate (Na2SO4)

• The washing efficiency in terms of the COD correlates with the performance ofoxygen delignification and bleaching (1 kg COD corresponds 0,6 – 0,8 kgakt. Cl in bleaching) as well as with the COD- ja AOX- discharges frombleaching

• The Na-washing loss was earlier in the general use because the sodium loss inthe washed pulp represented one of the main losses from the chemicalcirculation

11. Alkaline Processes II Pekka Tervola

Terminolgy of pulp washing• Washing loss (carry-over):

Ø The amount of washable material entering oxygen delignification orbleaching per ton of pulp

Ø Can be expressed as COD-loss (kg COD/ ton pulp), dissolved solidsloss (kg DS/ton of pulp) or sodium loss (eg. kg Na2SO4/ton pulp)

Ø Determination of the washing loss is standardized (eg. according toSCAN or Tappi standards). Many mills have their own ”fast methods”that may give different results from the standard metods

11. Alkaline Processes II Pekka Tervola

Washing terminolgy

• Dilution factor (DF):

Ø DF = total wash water amount (t/ton pulp) – the water amount in thewashed pulp (t/ton pulp)

Ø The dilution factor keeps constant in all the equipment in the washingline provided that the filtrates are connected in completecountercurrent (no sidestreams of pulp or the filtrates) and if thewashing line is completely in balance

Ø The dilution factor is the main control parameter of the pulpwashing line

11. Alkaline Processes II Pekka Tervola

Washing terminology• Definitions of the washing efficiency:

Ø E-factor, Nordén number: The most widely used parameter for thewashing efficiency. It expresses how many ideal mass transferstages (dilution-thickening stages) there are in the washing system orin a certain washing apparatus.

Ø Displacement ratio, DR: Traditional measure for washing efficiency.Commonly used in many countries (North America, Asia).

Ø Recovery rate, washing yield: Expresses the recovery rate of thewashable substance in relation to the input from cooking and oxygendelignification

11. Alkaline Processes II Pekka Tervola

Washing stage

Pulp in

Feed consistency (%) C0Liquid with pulp L0Washable material conc. x0

Outlet consistency (%) C1Liquid with pulp L1Washable material conc. x1

Pulp out

Wash water inFiltrate out

Wash water V2Washable material conc. y2

Filtrate V1Concentration of washable material y1

The principal drawing of a washing stage:

11. Alkaline Processes II Pekka Tervola

BalancesAll liquid streams m3/ton pulp or t/pulp

All concentrations of the washable material in the same unit(eg. g/l, w-% etc.)

ton pulp is either ADt or BDt (ODt)

1 ADt is 900 kg fiber (ADt = Air Dry ton)

1 BDt (ODt) is 1000 kg fiber (BDt = Bone Dry ton,ODt = Oven Dry ton)

11. Alkaline Processes II Pekka Tervola

Balances

1120 VLVL +=+

11112200 VyLxVyLx +=+

12 LVDF -=

Total liquid balance

Balance of the washable material

Dilution factor

L0 = (100 – C0)/C0 , L1 = (100 – C1)/C1 (t/BDt, t/ODt)

L0 = 0.9(100 – C0)/C0 , L1 = 0.9(100 – C1)/C1 (t/ADt)

V1, V2 (t/ADt, t/BDt)

DF = V1 – L0 Dilution factor

11. Alkaline Processes II Pekka Tervola

Washing efficiency: Norden E-factor

÷÷ø

öççè

æ

úû

ùêë

é--

=

1

2

211

1

ln

)()(

ln

LV

yxLyxL

E

oo

11. Alkaline Processes II Pekka Tervola

Definition of E-number

Norden E-number

C1 C1 C1

V2, y2

V1, y1

L0, x0

Example: E = 3

L1, x1

Thickening

11. Alkaline Processes II Pekka Tervola

Washing efficiency: Measured valuesdoes necessary fullfill material balance

:

E

L0 + V2 = L1 + V1

L0x0 + V2y2 = L1x1 + V1y1

Inserting DF to solute balance equation one gets

L0x0 + (L1 + DF)y2 = L1x1 + (L0 + DF)y1

Solving different parameters from above equationand inserting to E-factor formula one gets differentmaterial balance closing methods.

11. Alkaline Processes II Pekka Tervola

Washing efficiency:Modification of theE-factor, most common

Basic definintion(material have to fit!, whenapply this):

Correction in the pulp feed(either L0 or x0 is calculated):

úû

ùêë

é+

úû

ùêë

é--

+=

1

211

21

1ln

)()(

1ln

LDF

yxLyyDF

E

E÷÷ø

öççè

æ+

÷÷ø

öççè

æ--

=

1

211

100

1ln

)()(ln

LDF

yxLyxL

E

11. Alkaline Processes II Pekka Tervola

Washing efficiency: Modification of theE-factor (most common)

Correction in DF(DF is calculated):

Correction in the filtrate y1(y1 is calculated):

E

( )

÷÷ø

öççè

æ+

÷÷ø

öççè

æ÷÷ø

öççè

æ--

++

=

1

211

20

0

0

1ln

)(1ln

LDF

yxLyxDF

DFLL

E

÷÷ø

öççè

æ+

÷÷ø

öççè

æ--

=

--+-=

1

211

100

21121100

1ln

)()(ln

)/())()((

LDF

yxLyxL

E

yyxyLyxLDF

11. Alkaline Processes II Pekka Tervola

Washing efficiency: E-factor values

• E-factor is quite constant regardless of dilution factor• E-factor is characteristic of each washing equipment type. It can be

determined experimentally• Ideal displacement: E = infinite• Dilution + thickening: E =1• Non-existing displacement: E = 0• In practice the E-value of industrial washing equipment varies

between 2 - 10

11. Alkaline Processes II Pekka Tervola

Calculating a washing system

11. Alkaline Processes II Pekka Tervola

Washing system

=÷÷ø

öççè

æ--

=÷÷ø

öççè

æ

+

+

)()(lnln1

1001

nnnn

ntot yxL

yxLL

VE =÷÷ø

öççè

æ-

---

--

+

--

)()(...

)()(

)()(ln

1

11

322

211

211

100

nnn

nnn

yxLyxL

yxLyxL

yxLyxL

=÷÷ø

öççè

æ-

-++÷÷

ø

öççè

æ--

+÷÷ø

öççè

æ--

+

--

)()(ln...

)()(ln

)()(ln

1

11

322

211

211

100

nnn

nnn

yxLyxL

yxLyxL

yxLyxL

÷÷ø

öççè

æ++÷÷

ø

öççè

æ+÷÷

ø

öççè

æ +

n

nn L

VELVE

LVE 1

2

32

1

21 ln...lnln

11. Alkaline Processes II Pekka Tervola

Calculating the washing system

The total E-value vs. Individual E-values of equipment

nnntot RERERERE ln...lnlnln 2211 +++=

nnn LVR

LVRLVR

/....

//

1

232

121

+=

==

Important special case:

R1 = R2 = R3 = … = Rn

Etot = E1 + E2 + E3 + … + En

11. Alkaline Processes II Pekka Tervola

Definition of Est-factor (st = standard)

C1, L1, x1

V2, y2

V1, y1

L0, x0 Cst, Lst, x1,st

Lst – L1, y2

V2 + Lst – L1

E

Washing, E

L1x1 + (Lst – L1) = Lstx1,st

11. Alkaline Processes II Pekka Tervola

Relation between E and Est

=

÷÷ø

öççè

æ -+

÷÷ø

öççè

æ

--

=

st

st

ststst

LLLVyxL

yxL

E12

2,1

100

ln

)()(ln

=

÷÷ø

öççè

æ -+

÷÷ø

öççè

æ

--

st

ststst

LLV

yLxLyxL

12

2,1

100

1ln

)(ln

=

÷÷ø

öççè

æ+

÷÷ø

öççè

æ--+

-

st

stst

LDF

yLyLLxLyxL

1ln

)()(ln

22111

100

÷÷ø

öççè

æ+

÷÷ø

öççè

æ--

stLDF

yxLyxL

1ln

)()(ln

211

100

=

÷÷ø

öççè

æ

÷÷ø

öççè

æ--

=

1

2

211

100

ln

)()(ln

LV

yxLyxL

E =

÷÷ø

öççè

æ +

÷÷ø

öççè

æ--

1

1

211

100

ln

)()(ln

LDFL

yxLyxL

÷÷ø

öççè

æ+

÷÷ø

öççè

æ--

1

211

100

1ln

)()(ln

LDF

yxLyxL

÷÷ø

öççè

æ+

÷÷ø

öççè

æ+

=

st

st

LDFL

DF

EE1ln

1ln1

11. Alkaline Processes II Pekka Tervola

Calculating the washing systemE-value in the ”standard consistency”:- Makes it easier to compare different washing equipmentif the discharge consistencies of different equipment vary

- Calculating the total E-value is easy- The commonly used standard consistency is 10 %

BDTmL /910

10100 3=-

=

=>

ntotal EEEE ,102,101,10,10 ... +++=

÷øö

çèæ +

÷÷ø

öççè

æ--

=

0.91ln

)()(ln

211

100

10 DFyxLyxL

E

11. Alkaline Processes II Pekka Tervola

Washing efficiency: Displacement ratio(DR-value)

Pulp in

Feed consistency (%) C0Liquid with pulp L0Washable material conc. x0

Outlet consistency (%) C1Liquid with pulp L1Washable material conc. x1

Pulp out

Wash water inFiltrate out

Wash water V2Washable material conc. y2

Filtrate V1Concentration of washable material y1

20

10

yxxx

DR--

=

11. Alkaline Processes II Pekka Tervola

DR-value

• Often used in pulp mills, since it is easy to calculate• DR-value depeds on the feed amd discharge consistency of

a washer and DF.• Be careful, when DF is changing.

DR is the volume of actually displaced liquor dividedby the volume of the original liquor which is the same as theliquor on the pulp out of the washer.

11. Alkaline Processes II Pekka Tervola

Model washer

11. Alkaline Processes II Pekka Tervola

Equivalent DR-values

)1)(()()(

)1(10,01,00

0,0

,1

1

2,0

,1,0, ,1,0 DRLLLLDFL

DFLLLLDR

yxxx

DRstst

st

stst

ststCC stst ---+

+--=

--

=

,)1)(99()99(

)(99333.7

)1(1010

0112,1 DRLLDFL

DFLLDRDREDR---+

+--==

Equivalent Displacement Ratio: DR of a filter: C0,st = 1% and C1,st = 12%

Equivalent DR-values can be used for comparison of thedifferent washers (same DF)

11. Alkaline Processes II Pekka Tervola

Washing yield

Pulp in

Feed consistency Co

Liuid with pulp L oWashable material conc. x o

Outlet consistency C1Liquid with pulp L1Washable material conc. x 1

Pulp out

Wash water inFiltrate out

Wash water V2Washable material conc. y 2

Filtrate V 1Concentration of washable material y1

The Washing yield expresses how big share of the washable materialin the inlet flow is recovered in the filtrate:

11. Alkaline Processes II Pekka Tervola

Washing yields

)()(1

200

211

yxLyxLY

--

-=)(

))((

200

210

yxLyyDFLY

--+

=

)()()(1

21,0100

211,0 yyLyxL

yxLYst

C st -+--

-= )()())((

21,0100

21,0,0 yyLyxL

yyDFLY

st

stC st -+-

-+=

Equivalent wash yield. The constant feed consistency C0,st

)(0.9)()(1

21100

21110 yyyxL

yxLY-+-

--=

)(0.9)())(0.9(

21100

2110 yyyxL

yyDFY-+-

-+=

Example: Standard feed consistency C0,st = 10% (L0,st = 9.0 t/BDt)

Equivalent wash yield can used for the comparison of thedifferent washers (same DF)

11. Alkaline Processes II Pekka Tervola

Washing yields (Continue)

Mass balance error is put in the leaving filtrate flow

Example: Standard feed consistency C0,st = 10%

))(()()(1

21,0211

211,0 yyDFLyxL

yxLYst

C st -++--

-=

)()()()()()(1

211,00200,0

2110,0 yxLLLyxLDFL

yxLDFLYstst

C st --+-+-+

-=

Mass balance error is put in the pulp feed flow

11. Alkaline Processes II Pekka Tervola

Washing yields (Continue)

Material balance error is put in the leaving filtrate

))(0.9()()(1

21211

21110 yyDFyxL

yxLY-++-

--=

)()0.9()()0.9()()(1

2110200

211010 yxLLyxLDF

yxLDFLY--+-+

-+-=

Example: Standard feed consistency C0,st = 10% (L0,st = 9.0 t/BDt)Fixing the material balanceMaterial balance error is put the pulp fedd flow

11. Alkaline Processes II Pekka Tervola

Washing yield vs. DF and E-value

0,8

0,84

0,88

0,92

0,96

1

0 1 2 3 4 5 6 7Dilution Factor

Was

hing

Yiel

d

48121620

E-values

11. Alkaline Processes II Pekka Tervola

Example: Calculating the washingefficiency and other washing parameters

Washing systemWashable pulp Washed pulp

Wash liquidFiltrate

Co = 10 %xo = 220 g COD/l

Co = 12 %xo = 1 g COD/l

y2 = 0 g COD/ly2 = 171,5 g COD/l

The dilution factor of the system is DF = 2,5 m3/BDT

11. Alkaline Processes II Pekka Tervola

Example…

• Calculate:– Washing loss– E-factor with the different material balance closing methods– Standard E-value (E10)– Displacement Ratio (DR), equivalanet displacement Ratio (EDR)– Washing Yield, equivalent washing yield

11. Alkaline Processes II Pekka Tervola

Example…

• Define first the flows:

BDTmC

CLo

oo /9

1010100100 3=

-=

-=

BDTmC

CL /33,712

12100100 3

1

11 =

-=

-=

BDTmLDFVLVDF /83,933,75,2 31212 =+=+==>-=

From the total flow balance:

BDTmLVLV o /5,1133,783,99 3121 =-+=-+=

11. Alkaline Processes II Pekka Tervola

Example…

• Washing loss:

Washing loss = kg COD/BDT pulp

x1 = 1 g COD/l = 1 kg COD/m3

L1 = 7,33 m3/BDT

=> Washing loss = 7,33 kg COD/BDT

11Lx

11. Alkaline Processes II Pekka Tervola

Example…

• E-value according to Nordén definition

• 9*220 +9.83*0 – 7.33*1 – 11.5*171.5 = 0.42Balance does not match! (lucily balance error is small)BE CAREFUL, WHEN APPLYING DIRECTLY E-FORMULA!!

9,13

33,783,9ln

)01(33,7)5,171220(9ln

ln

)()(ln

1

2

211

1

=

úû

ùêë

é

úû

ùêë

é-

-

=

úû

ùêë

é

úû

ùêë

é--

=

LV

yxLyxL

E

oo

11. Alkaline Processes II Pekka Tervola

Example…

Balance error is put in the pulp feed flow

9,13

33,75,21ln

)01(33,7)05,171(5,21ln

1ln

)()(1ln

1

211

21

=

úû

ùêë

é +

úû

ùêë

é-

-+

=

úû

ùêë

é+

úû

ùêë

é--

+=

LDF

yxLyyDF

E

11. Alkaline Processes II Pekka Tervola

Example…

Balance error is put in the leaving filtrate

( )

=

÷÷ø

öççè

æ+

÷÷ø

öççè

æ÷÷ø

öççè

æ--

++

=

1

211

20

0

0

1ln

)(1ln

LDF

yxLyxDF

DFLL

E

( )

9.13

33,75,21ln

)01(33,702205,21

5,299ln

=÷ø

öçè

æ+

÷÷ø

öççè

æ÷÷ø

öççè

æ--

++

11. Alkaline Processes II Pekka Tervola

Example…

Balance error is in DF

=--+-= )/())()(( 21121100 yyxyLyxLDF

502,2)05,171/())10(33,7)5,171220(9( =--+-

( )( )

=

÷÷ø

öççè

æ+

÷÷ø

öççè

æ--

=

1

211

100

1ln

ln

LDF

yxLyxL

E

( )( ) 9,13

9502,21ln

0133,75,1712209ln

=÷øö

çèæ +

÷÷ø

öççè

æ-

-

11. Alkaline Processes II Pekka Tervola

Example…

E-factor -> E10-factor (cst=10%)

=-

=st

stst c

cL 100 910

10100=

-

=

÷÷ø

öççè

æ+

÷÷ø

öççè

æ+

=

stLDFL

DF

EE1ln

1ln1

10 7,16

95,21ln

33,75,21ln

9,13 =÷øö

çèæ +

÷ø

öçè

æ +

11. Alkaline Processes II Pekka Tervola

E-factor lineWashing efficiency estimation

12

12.5

13

13.5

14

14.5

15

15.5

16

16.5

17

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3DF (t/bdt)

E-va

lue

x0, L0 y1

11. Alkaline Processes II Pekka Tervola

E10-factor lineWashing efficiency estimation

1212.5

1313.5

1414.5

1515.5

1616.5

1717.5

1818.5

1919.5

20

2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3

DF (t/bdt)

E10

-val

ue

x0,L0 y1

11. Alkaline Processes II Pekka Tervola

Example…

• Standard E-value:

7,16

95,21ln

)01(33,7)05,171(5,21ln

91ln

)()(1ln

211

21

10 =

úûù

êëé +

úû

ùêë

é-

-+

=

úûù

êëé +

úû

ùêë

é--

+=

DFyxLyyDF

E

11. Alkaline Processes II Pekka Tervola

Example…

• Displacement Ratio, DR:

• Equivalent Displacement Ratio, EDR:

EDR = DR1,12 = 1 – (1 – 0,995)*(7,33/7,33)*99*(9 + 2,5)/……(9*(2,5 + 99) – 7.33*(99 – 9)*(1 – 0.995)) = 0,994

995,002201220

2

1 =--

=--

=yxxxDR

o

o

11. Alkaline Processes II Pekka Tervola

Example…Washing yield:

Y = 11,5*(171,5 – 0)/(9.0*(220,0 - 0)) = 0,996

Material balance error is put in the feed pulp:

Y10 = 1 – 7,33*(1 – 0)/(7,33*(1 – 0) + (9,0 + 2,5)*(171,5 – 0)) = 0,996

Material balance error is put in the leaving filtrate:

Y10 = 1 – (9,0 + 2,5)*7,33*(1 – 0)/...

…(9,0 + 2,5)*9,0*(220 – 0) + (9,0 – 9,0)*7,33*(1 – 0)) 2,5) = 0,996

11. Alkaline Processes II Pekka Tervola

Calculating washing systems:Simulation programs

• By means of the above mentioned methods, the washing efficiencyand the balances of a washing system can be calculated manually orby using simple Excel-programs if the system is connectedcompletely in the countercurrent way

• If the washing system is complicated with sidestreams or the oxygendelignification is needed to be calculated accurately, a balancesimulation program is needed

• Such simulation programs are Pulpsim, WinGems, MASBAL etc.

11. Alkaline Processes II Pekka Tervola

Snapshot from simulation