White liquor composition and calculations · 7. Fractionation: Part 3 – Batch and continuous digesters Kyösti Ruuttunen White liquor composition and calculations Puu-0.4110 1 H

7. Fractionation: Part 3 –Batch and continuousdigesters

Kyösti Ruuttunen

White liquor composition andcalculations

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H. Sixta, A. Potthast and A. W. Krotschek, Chemical PulpingProcesses in H. Sixta (ed.) Handbook of Pulp.Wiley-VCH Verlag GmbH, 2006, p. 113

Puu-0.4110: Conventional and Non-Conventional Pulping as a Basis forBiorefinery (7 cr)Lecture 7:Biomass fractionation. Part 3.Batch and continuous digesters.


Kyösti Ruuttunen

After this lecture the student

• is familiar with the concepts batch digester andcontinuous digester and can describe the technicalprinciples, as well as the advantages anddisadvantages, of each (e.g. in the case of kraft pulping)

• can describe what is meant with modified cookingconcepts

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Kyösti Ruuttunen

Kraft cooking was invented 135 yearsago!• The main chemical cooking methods were invented

between 1850–1900– 1850 Soda cooking (England, USA)– 1865 Sulfite cooking– 1879 Sulfate (kraft) cooking (Germany)

• Until 1950’s all cooking processes utilized batchdigesters– Rotating spherical digesters– Conical bottom vertical digesters

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Kyösti Ruuttunen

At the beginning…and still today…used insmall production pulp mills: Rotatingspherical digesters

• Diameters 2–4 m allowed pulpamounts of 0.2–2 tons/batch

• Rotation for better contactbetween cooking liquid andchips

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Kyösti Ruuttunen

Papelera Mercedes (PAMER), UruguayCapacity 30 tons pulp/d

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Kari Kovasin


Kyösti Ruuttunen

Batch digester

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- Batch digesters can be heated either by direct steam or circulationequipped with a heat exchanger

- Having external circulation improved the uniformity of cookingconsiderably compared to rotating or direct steam heated digesters


Kyösti Ruuttunen

Process steps of conventional batchcooking

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Kyösti Ruuttunen

Cooking conditions in conventionalbatch cooking

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Kyösti Ruuttunen

Typical features of conventional batchcooking• Hot digester discharge (”hot blow”) is mechanically very

aggressive to fibers: major strength loss (30–35%measured as tear index at constant tensile)

• Although external circulation is performed, the uniformityof cooking is not acceptable (too much reject)

• Heat economy is bad (steam demand 4–5 GJ/ton pulp is2–3 times that of continuous cooking)

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Kyösti Ruuttunen

Development of continuous digesters

• The development of continuous digesters was carriedout in the 1940’s by the Norwegian-Swedish companyKamyr

• The first commercial Kamyr digester was started in 1952• Since then, several steps of development have taken

place…

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Kyösti Ruuttunen

Evolution of continuous digesters2140 ADt/day

3060 m3

100 ADt/day100 m3

Joutseno -52

21m

Dia 2´500

Dia 12’500

60’0

00

3000 ADt/day3300 m3

Fray Bentos - 07

SW Eucaca. 1000ADt/day

SW

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Kyösti Ruuttunen

The main driving forces for the furtherdevelopment of continuous digesters• Hot digester discharge was very aggressive to fibers;

the pulp quality was inferior to batch cooked pulp– Dilution of the bottom of the digester by wash filtrate (cooling

effect)– Washing in counter-current before the cooling and digester

discharge (”Hi-heat” washing)– Very long washing times made it necessary to carry out the

impregnation stage in a separate vessel

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Kyösti Ruuttunen

Modern continuous digester

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KnowPulp 7.0, 2008


Kyösti Ruuttunen

Process steps of conventional kraftcooking

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Chipsteaming

Feedingthe chips

intodigester

Feedingthe chips

intodigester

Chipsteaming

Impregnationof chips with

cooking liquor

Heating tocooking

temperatureCookingphase

Washing ofchips in the

digester

Digesterdischarge

Continuous cooking

Batch cooking

Chipsteaming

Heating tocooking

temperatureCookingphase

• Conventional cooking means that the entire amount of thecooking liquor is charged at the beginning of the cooking cycle• No profiling of chemical concentrations during the cook


Kyösti Ruuttunen

Development of modern cookingtechnologies• Modifying the chemistry

– From conventional cooking to modified cooking– Cooking additives– Several commercial cooking processes and new trade / brand

names

• Coping with the increasing production rate is achallenge for the digester design

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Kyösti Ruuttunen

Modified cooking – Why to do it?

• Lower steam consumption• Environmental reasons

extended cooking to lower lignin content lower bleaching chemical consumption

• Pulp quality• Efficient use of alkali• Often essential due to higher production rates

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Kyösti Ruuttunen

Important steps of development in moderncooking technology took place at thebeginning of 1980’s• Hartler and Teder (Sweden) introduced the ”four rules of

cooking modification”– modified continuous cooking (MCC of Kamyr AB)

• Displacement batch cooking was developed:– The main driving force was improved heat economy– Also features of the modified cooking chemistry– First commercial process was the RDH-process of Rader (Rapid

Displacement Heating)

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Kyösti Ruuttunen

Cooking modification ”rules”

1. Alkali concentration must be leveled out– Decreased in the beginning– Increased at the end

2. The concentration of hydrogen sulfide ions should beas high as possible– Especially at the beginning of the bulk delignification phase

3. The concentration of dissolved lignin and sodium ionsin the liquor should be as low as possible– Especially in the final cooking phase

4. Temperature should be low– Especially at the beginning and at the end of the cook

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Modified Continuous Cooking

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Automation Applications, Inc (AAI)


Kyösti Ruuttunen

Continuous flows in continuous cooking

• Inlet and outlet streams steady– Chip feed– Liquor feed– Pulp discharge– Black liquor extraction

• Chip flow downward• Liquor flow co-current or countercurrent

(e.g. Hi-Heat washing zone)

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Kyösti Ruuttunen

Conventional Continuous CookingOne-vessel digester (hydraulic)

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BL extraction

WL

Chips

Pulp

Washer Filtrate


Kyösti Ruuttunen

The effect of alkali profiling (”Rule 1”)

• First trials of Continuous Cooking Modification 1983 atVarkaus mill

• White liquor was added at three points in the digester– Impregnation– Transfer circulation– Countercurrent cooking

• A recirculation line for liquor around impregnation vesselwas installed

• The final stage of the cook was carried out incountercurrent mode

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Kyösti Ruuttunen

The effect of black liquor HS- (”Rule 2”)

• A high HS- ion concentration in the early stage ofcooking has turned effective particularly in cooking ofsoftwood (faster cooking, better fiber quality)

• Practical implementation to increase HS- ionconcentration at the beginning of cooking black liquorre-circulation

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Kyösti Ruuttunen

The effect of dissolved solids (”Rule 3”)

• Flow-through type laboratory pulping simulations haveshown:– High dissolved solids have a detrimental impact on pulp

strength and on bleaching response– Dissolved wood solids consume alkali in secondary reactions

during pulping especially during initial part of bulk delignification

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Kyösti Ruuttunen

Continuous cooking processes

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ConventionalContinuous

Cooking

EMCCMCC

Lo-SolidsEAPC

DownflowLo-SolidsLo-Solids

BLIITC ConcurrentCompact Cooking

CompactCooking

Kamyr AB splitsat the beginning of1990’s

Ahlstrom/Kamyr Inc. -> Andritz (currently)

Kvaerner Pulping -> Metso Paper (currently)


Kyösti Ruuttunen

Lo-SolidsTM cooking (Andritz)

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Pulp

Washer FiltrateWL

BL extraction 1

WL

Chips

Washer Filtrate

BL extraction 2

Washer FiltrateWL


Kyösti Ruuttunen

Compact cooking (Kvaerner)

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Chips

WL

BLextraction 1

WasherFiltrate

Pulp

WL

WL

BL extraction 2


Kyösti Ruuttunen-29-

Alkali profiles of continuous cookingImpregnation

Cooking 1

Cooking 2Hi-Heat

0

10

20

30

40

50

0 30 60 90 120 150 180 210 240 270 300 330

Time (min)

EA(g

/l)Conv.MCCEMCC/ITC

ExtractionMCC

Transfercirculation

Extractionconventional



0

10

20

30

40

50

0 30 60 90 120 150 180 210 240 270 300 330

Time (min)

EA(g

/l)Conv.Lo-Solids

Extraction 1Extraction 2

WLaddition Extraction

conventional

Alkali profiles of continuous cooking


Kyösti Ruuttunen

From countercurrent to downflowcooking• Problems of countercurrent stages:

– Non-uniformity of flows and temperature– Unstable runnability

• Benefits of concurrent / downflow cooking:– Smoother movement of the chip column– Increased total capacity of the digester

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Development of continuous cooking flowdirection

,

Lo-SolidsDownflowLo-Solids

110°C

150°C

150°C

150°C

110°C

150°C

150°C

150°C



Downflow Lo-Solids® Cooking, two-vessel system

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Kyösti Ruuttunen

Two loops arecommonlyused toachievethe desiredcookingtemperature;upper cookingand lowercooking.

Liquor travels from the screens, through the circulation pump, heat exchanger, to the centerpipewhere is travels radially through the chip mass, heating the chips as it passes through them,returning to the screens to be recirculated. (source: www.metso.com)

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Kyösti Ruuttunen

Summary of continuous cooking

• Continuous cooking is the leading cooking technologytoday

• The latest major installations for cooking eucalyptusand/or acacia in South America and Asia are based oncontinuous digesters

• Introducing the Four Rules of modified cookingchemistry has brought a sequence of new continuouscooking technologies

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Kyösti Ruuttunen

Batch cooking modifications

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Kyösti Ruuttunen

Features of batch cooking

• “Primitive but effective”• Easy to maintain• Production is very safe:

– easy to take one digester out at a time for maintenance

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Kyösti Ruuttunen

Features of batch cooking

• Digester house is an assembly of individually loadeddigesters– 4 minimum to maintain reasonable sequencing and steam

loading– 6 - 8 maximum to use same header pipe lines– Several lines can use same tank farm

• Less sensitive to chip quality (than continuous cooking)• Possibility to produce different grades of pulp in the

same cooking plant

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Kyösti Ruuttunen

Modern batch cooking processes

• Based on liquor displacement technology• Pressurized liquor accumulators• Improved heat economy• Low temperature impregnation possible• Liquor concentration profiling during impregnation and

cooking• Gentle pump discharge• So far, the highest measured pulp strength delivery with

softwood… pulp equals laboratory made pulp

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Kyösti RuuttunenPuu-0.4110

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Batch digester houseKraft pulping (Super Batch)

KnowPulp 7.0, 2008



SuperBatch digester 400 m3

Directsteam

Circulationscreens

Circulationscreens

Chip feed screw

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Kyösti Ruuttunen

The main cooking modification rules indisplacement cooking (SuperBatch)1. Alkali concentration must be leveled out

– Very low HO- at the beginning– High residual EA at the residual phase, back-end of cooking

2. The concentration of hydrogen sulfide ions should beas high as possible– Very much HS- at the beginning of bulk delignification phase

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ConventionalBatch

Cooking

EKONOWLI EnerBatch

RDH

SuperBatch SuperBatch-K

SundsCold Blow

CBC

Displacement Batch ProcessesRDH and SB: Black liquor started displacement batchWLI: White liquor started displacement batch

ContinuousBatch

Lenzing Technology

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Kyösti Ruuttunen

SuperBatch cooking process



SuperBatch references

SW mills

HW mills

SW/HW mills

Campaign mills

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Kyösti Ruuttunen

Summary of modern batch cookingprocesses• Main references from softwood cooking from the 90’s

(better pulp quality; nowadays no major difference withcontinuous cooking)

• Varying and ”bad” quality chips can be processed• Not competitive in very large installations (complicated,

need of very good control systems)

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Kyösti Ruuttunen

Additional material

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Kyösti Ruuttunen

Digesters for other raw material,such as cereal straws, annual

grasses and bagassePuu-0.4110

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Kyösti Ruuttunen

Flow diagram for wheat straw pulping usingthe Pandia continuous digester system

Source: Kocurek 1983Puu-0.4110

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Kyösti Ruuttunen

Pandia continuous digester system

• Main references at pulp mills utilizing non woodfibers, such as bagasse and cereal straw

• Installed in over 200 pulp mills

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Kyösti Ruuttunen

PANDIA digester systems

GL&V

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Kyösti Ruuttunen

AGRO’s Continuous Digester CD-600

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Documents

White liquor composition and calculations · 7. Fractionation: Part 3 – Batch and continuous digesters Kyösti Ruuttunen White liquor composition and calculations Puu-0.4110 1 H