Processo Lavagem - 7. Pulp Washing 12 Session 7

UDELARSchool of EngineeringDepartment of Forest Products Technology

Pulp Mill Fiber Line Processes Olavi Pikka -1-

Pulp Mill Fiber Line Processes

Lecture 7: Fiberline unit operations: pulp washing and screening, pumping,

agitation & mixing



Pulp washing and screening1. Pulp mill fiberline after cooking2. Purpose of pulp washing3. Terminology of pulp washing4. Washing calculations

Mass balances and washing efficiency calculations

5. Industrial pulp washing practices Washing equipment Washing systems/Fray Bentos

6. Purpose of pulp screening7. Theoretical aspects and terminology of screening8. Screening equipment and screening systems;

location in the process9. General unit operations of pulp suspensions



Process stages pulp mill fiberline

Woodyard Cooking Washing & screening Oxygen delignification Bleaching Pulp drying Pulp baling

Unit operations with:- Fiber suspensions- Fiber beds

Unit operations with:- Chips- Chip beds

Unit operations with:- Fiber suspensions- Fiber beds- Fiber sheets



Brown stock fiberline

Woodyard Cooking Washing & screening Oxygen delignification Bleaching Pulp drying

Connected to each other by:- Counter-current washing- Black liquor system



Brown stock fiberline of a pulp mill



Pulp washing

Objective of pulp washing:Removing black liquor from the cooked pulp by

using as little fresh water as possibleThe removed black liquor needs to be as

concentrated as possible in order to save steam in evaporation (therefore as little fresh wash water as possible!)

The washed pulp has to be as clean (of the dissolved organic material of cooking) as required in order to save chemicals in oxygen delignification and bleaching



Connecting pulp washing with the fibreline

Pulp washing is a multi-stage and countercurrentlycombined separation process

Cooking

Chips

Black liquor to evaporation

White liquor

Screening

Rejects

Pulpwashing O2-

delig.

Pulpwashing

Wash water

Bleaching

DryingBaling

Pumping to the paper mill (integrated production)

Baled market pulp

Brown stock fiberline



Pulp washing is an optimization task where different cost factors are compared. However, the limiting factor is

certain demand for the washing lossOperationalcosts, €/ton pulp

Wash water, m3/ton pulp

Chemical costs(bleaching, make-up chemicals)

Evaporation costs

Optimum operational costs

Demand for the washing loss(Bleaching, environmental load, Pulp quality factors)



What are the substances to be washed away in pulp washing?

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

Often the result of washing the organic and inorganic matter is measured separately from each other: The organic matter is described with the Chemical Oxygen Demand (COD) and the inorganic matter eg. with the amount of sodium, often calculated as sodium sulphate (Na2SO4)

The washing efficiency in terms of the COD correlates with the performance of oxygen delignification and bleaching (1 kg COD corresponds 0,6 – 0,8 kg act. Cl in bleaching) as well as with the COD- ja organochlorine- discharges from bleaching

The Na-washing loss was earlier in the general use because the sodium loss in the washed pulp represented one of the main losses from the chemical circulation (in terms of the cost effect)



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

Describes the amount of washable material entering oxygen delignification or bleaching per ton of pulp

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

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



Washing terminolgy

Dilution factor (DF):

DF = total wash water amount (m3/ton pulp) – the water amount in the washed pulp (m3/ton pulp)

The dilution factor keeps constant in all the equipment in the washing line provided that the filtrates are connected in complete countercurrent and if the washing line is completely in balance

The dilution factor is the main control parameter of the pulp washing line



Liquor amount in pulp

Assume: C = fiber consistency in the pulp suspension, as % of fibers Assume 100 kg of fiber suspension => we have C kg fibers we have 100 – C kg liquor

Liquid amount in the pulp suspension as kg liquid/kg of pulp or m3/ton of pulp => L = (100 –C)/C



Liquor and Water amount in pulp:

Pulp consistency C as %Pulp amount P as ODT or ADT (0,9 ODT)

Pulp suspension amount tons, SP = (P/C)*100

Water content in pulp suspension (m3) ton/ODT pulp L = SP - P

L = (P/C)*100 – P

L = P* (100 – C)C

If P is 1 ODT, liquor L m3 or ton/ODT

ccL

100



Washing terminology Definitions of the washing efficiency:

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

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

Recovery rate, washing yield: Expresses the recovery rate of the washable substance in relation to the input from cooking and oxygen delignification



Principles of washing The most simple pulp washing principle is the

dilution of pulp with the washing liquid followed by thickening (consistency increase). Commonly used in the washing of mechanical pulps and in small production pulping lines

The most important washing principle is displacement washing where the clean washing liquor displaces the dirtier one from the pulp suspension. Displacement commonly takes place at a consistency of 10 – 12 % which often means that there has to pre-thickening of pulp to the desired consistency ahead of the displacement.

The third principle is pressing (> 30 % consistency). The pressing may take place after displacement or can be just directly thickening by means of pressing



Washing?



Washing principles



Principles washing

Dilution washing Displacement washing



Wash water used in pulp washing

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

In unbleached mills producing unbleached pulp and further eg. brown paper or carton board, the circulation water of the paper/board machine may be used as wash water.



Washing efficiency

Pulp in

Feed consistency Co

Liquid with pulp LoWashable material conc. x o

Outlet consistency C1Liquid with pulp L1Washable material conc. x1

Pulp out

Wash water inFiltrate out

Wash water V2

Washable material conc. y 2Filtrate V 1Concentration of washable material y1

The principal drawing of a washing stage:

- All liquid streams m3/ton pulp- All concentrations of the washable material in the same unit

(eg. g/l, w-% etc.)



Washing efficiency:Balances

1120 VLVL

11112200 VyLxVyLx

12 LVDF

Total liquid balance

Balance of the washable material

Dilution factor

Pulp in

Feed consistency Co



Pulp out


Wash water V2




Washing efficiency:E-factor

1

2

211

1

ln

)()(

ln

LV

yxLyxL

E

oo

ccL

100

Pulp in

Feed consistency Co



Pulp out


Wash water V2




y,x-diagram of pulp washing

0

1

2

3

4

5

6

7

8

0 1 2 3 4 5 6 7 8

x

y

Equilibrium curve y = x

Operational line y = (L/V)x + A

Washable pulp

Washed pulp

The number of equilibrium stages = E-value



Washing efficiency:E-factor values

E-factor is practically constant regardless of dilution factor

E-factor is characteristic of each washing equipment type. It can be determined experimentally

Ideal displacement: E = indefinite Dilution + thickening: E =1 In practice the E-value of washing equipment

varies between 2 - 10



Washing efficiency:Displacement ratio, DR

20

10

yxxx

DR

Pulp in

Feed consistency Co



Pulp out


Wash water V2




Washing equipment and their operational principles

Digester washing: Continuous cooking Hi-Heat washing Displacement batch cooking (SuperBatch) terminal

displacement Diffuser washing:

Single or two phase atmospheric diffuser Pressure diffuser

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- E-value at best > 10, typically 6 – 8-- In modern digesters washing zone shorter, E-value 3-4 - The washing stage is sensitive to the channeling of the wash liquid, which can be emerged when running the digesterover the nominal capacity

> 120 oC



The terminal displacement of the SuperBatch cooking

• Wash temperature 95 – 160 oC• No heating of the wash liquid• Washing time 40 – 50 min• E-value = 2 - 3• The main purpose of the terminal displacement of batch cooking is the heat recovery which is clearly more efficient than the actual washing phenomenon (mass transfer).



Diffuser washers

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

- The washing is based on pure displacement

- Stepwise operation:- Screens up =washing stage

-Fast return down when thescreens are cleaned

- E-values:- One stage 4 – 6- Two stages 7 – 8,5



Diffuser washers: the connection of an atmospheric diffuser with the continuous

cooking digester washing



Pressure diffuser

- Washing can be performed in120 – 130 oC

- Displacement in overpressure- Otherwise the same operational

principle as with atmosphericdiffusers



Drum washers

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

- Washing is based on thickening and displacment

- Outlet consistency 12 – 14 %

- The required pressuredifference either with vacuumor overpressure

- E-values typically 2 - 4



Wire washing



Drum filter washers



Drum Displacer Washers



Multistage washing: DD-washer

In the same washerthere can be 1 – 4Washing stages



Operation principle of 4-stage DD Washer



1. web formation section2. first washing stage3. second washing stage4. pulp discharging

1

23

4

1 2 3 4

Operation principle of 2-stage DD Washer



Multistage washing: DD-washer

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

• Feed consistency either 3 – 4 % (LCDD) or8 – 10 % (MCDD)

• Max. capacity in the range of 5000 ton/d

• E-value = 12 -15 (4-stages)= 5 – 10 (1 – 2-stages)



Wash presses

Twinroll-press:

Feed pulp

Wash liquid Filtrate

Washed pulp



Wash Press (Metso)



Compact Press (GL&V)



AWP Wash press (Andritz)



Principle of the wash press - animation



Wash presses (Metso Paper)



Wash presses• One-stage displacement-thickening• Feed consistency typically 3 – 5 %; also MC-feeding applications available• Outlet consistency 30 – 35 % • Max. capacity up to 4500 ton/d• E-value = 4 – 5• A press is very well applicable especially at the end of the washing line where the clean washing water is introduced. The demand of washing water is small due to the high discharge consistency:

BDTmL

c

/33,230

30100%30

31

1

If DF = 2,5 m3/BDT =>Wash water = 2,33+3,5 =4,83 m3/BDT



The discharge consistency of the last washing stage determines the washing water

amount:

BDTmL

c

/33,712

12100%12

31

1

Discharge consistency 12 %:

Liquid in the pulp: If DF = 2,5 m3/BDT =>Wash water demand = 7,33+2,5 =9,83 m3/BDT

Discharge consistency 30 %:

Liquid in the pulp:

BDTmL

c

/33,230

30100%30

31

1

If DF = 2,5 m3/BDT =>Wash water demand = 2,33+2,5 =4,83 m3/BDT



Washing systems• Suggested values for the Total washing efficiency:E-value = 12 – 16 before the O2-stageE-value = 6 - 8 after the O2-stage

• COD-wash loss suggested values: Before the oxygen stage < 100 kg/ton pulp Into bleaching ca. 6 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, operational availability etc.



Washer Feed consistency,%

Discharge consistency,%

Typical E10 for COD at DF = 2.78 t/bdmt

Continuous digester 10 10 3 -- 6

Modified batch digester 7 -- 10 7 -- 10 1 -- 2

Pressure diffuser 10 10 4 -- 6

1-stage diffuser (AD) 10 10 3 -- 5

2-stage diffuser (AD) 10 10 7 -- 8

Wash press 3 -- 9 28 -- 35 3 -- 5

Pressurized filters 3 -- 4 12 -- 14 3 -- 5

Vacuum filters 1 -- 2 12 -- 14 2 -- 4

1-stage DD washer 4 -- 10 12 -- 14 4 -- 5

2-stage DD washer 4 -- 10 12 -- 14 7 -- 9

3-stage DD washer 4 -- 10 12 -- 14 9 -- 11

4-stage DD washer 4 -- 10 12 -- 14 12 -- 15

Typical feed consistencies, discharge consistencies and washing efficiencies of different washing devices.



Washing systems: traditional 4-stage drum filter washing



Washing systems: modern fiberline with SuperBatch cooking (Botnia/Rauma)

SB-keittämö

Digester displacement

Wash press Wash press

DD-washer DD-washerO2-stage

Screening

SB-cooking



Uruguay_fiberline_flowsheets_EN.ppt/8.7.2007KKn

Fiberline DivisionSlide No. 1

Fiberline

Washing systems: modern fiberline with continuous cooking (Botnia/Fray Bentos)



Washing and Oxygen Delignification



Washing



Summary The purpose of brown stock washing is the recovery of the

dissolved organic matter from cooking and oxygen delignification as well as the inorganic delignification chemicals and on the other hand to clean the pulp suspension well enough before the oxygen stage and bleaching so that not too much additional chemicals are needed in addition to the actual bleaching chemicals

Well performed pulp washing reduces the environmental load of COD and AOX from bleaching

A washing plant is a multistage operation that is connected in countercurrent in terms of the filtrate connections. The excess amount of wash water, i.e. the dilution factor is mainly determined by the available capacity of the evaporation plant and the target washing loss. The commonly used value for the dilution factor is 2 – 3 m3/ton pulp



Summary.. The measure for the washing efficiency is the E-factor. A well

operating brown stock washing plant shall reach an E-value of 12 – 16 before the oxygen stage and 6 – 8 after the oxygen stage

The industrial washing equipment are based on the displacement washing or the combination of displacement and pressing

The first washing stage is carried out in the connection of the cooking process (Hi-Heat washing or the terminal displacement of SuperBatch cooking). In a modern washing plant, different kinds of washing equipment can be used: diffuser washers, DD-washers, press washers.

Processo Lavagem - 7. Pulp Washing 12 Session 7

Documents

Transcript of Processo Lavagem - 7. Pulp Washing 12 Session 7