BSE 201

BSE 201 Winter 2013

LECTURE 5

Pulping processes

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Today

• What is the purpose of pulping? • Compare mechanical and chemical

pulping • What types of chemical pulping are

commonly used and how do they compare to each other?

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Pulping processes

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Biomass Debarking chipping

screening Pulping

Pulp Spent

pulping liquor

Bleaching

Screening Refining

Paper making Converting Paper

products

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Definition of pulping • The process of

rupturing the bond between individual plant fibers – Primarily by disrupting

the lignin in the middle lamella

• Goal is to get a suspension of fibers in water that can be formed into paper and other products

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Middle lamella

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Imagine…

• Imagine that the wood is a bundle of uncooked spaghetti held together by glue

• You want to separate the strands of spaghetti without breaking them

• How to do this?

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Two types of pulping:

1. Mechanical Separates fibers by

mechanical shearing force using large amounts of energy

2. Chemical Separates fibers using

chemicals that dissolve the lignin “gluing” the fibers together

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1. Mechanical Pulping • Utilizes mechanical energy to produce usable fiber from whole

logs and wood chips • Much higher yields than chemical pulps

– BUT lower quality and longevity – Example: newspaper

• Mechanical pulping “loosens” the fibers and separates them, leaving lignin intact

• Extremely energy intensive – huge economic issue: NORPAC newsprint mill in Longview WA

is the State’s LARGEST consumer of electricity!

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Two types of mechanical pulping:

• Using whole logs and a stone: a) Stone groundwood (SGW) b) Pressurized Groundwood – PGW

• Using chips and a refiner:

c) Refiner mechanical pulping (RMP) d) Thermomechanical pulping (TMP) e) Chemi-thermomechanical pulping (CTMP)

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A. Stone Groundwood Process • Abrasion of fibers from small log

(bolt) by grinding stone – Oldest mechanical pulping process – High yield, but poor quality fiber

• Almost gone in US – (Blandin Paper Mill, Grand Rapids, MI-

UPM Kymmene) – New Zealand, Whakatane mill

• Pulp flushed away from stone with water

• Modern “stones” are made of metal • Highest yield of all pulping processes

– 95%

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Modern “stones”

Original stone

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Stone Groundwood Process

magazines

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Stone groundwood

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B. Pressurized Groundwood Pulping

• Same as stone ground wood, but magazine is pressurized with low pressure steam – 220 – 255 °F (~ 18 – 32 psi saturated steam)

• Heat softens chips/lignin – Better quality fiber – Higher brightness (some lignin removed)

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C. Refiner Mechanical Pulping (RMP)

• Utilizes mechanical refiner for fiberization • Starting feed is wood chips

– Higher quality pulp than SGW, thus stronger sheet

• Excellent method for pulping of plantation cottonwood and poplar

• G-P Wauna, OR

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Refiner Mechanical Pulping (RMP) • Refiners have two

grooved, circular plates – One rotates against the

other • Chips enter refiner in the

center of the plates and move outward

• As they exit the refiner, the fibers become smaller and smaller

• Plates can have different patterns for different fiber properties.

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Refiner operation

(a) Double-disc refiner. (b) Example of the disc pattern. (c) Loading caused by the moving

refiner discs. As chips enter and exit the grooves they are decompressed and compressed, loosening the fibers

Chips in

Fiber out

Chips in

Fiber out

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D. Thermomechanical Pulping (TMP)

• RMP with pre-steaming step added – Pre-steaming softens lignin before grinding – Significantly higher pulp quality

• Major applications: newsprint and LWC basesheet

• Dominant mechanical pulping process • NORPAC, Longview, WA • Ponderay Newsprint, Usk, WA • Publishers Paper, Spokane, WA

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E. Chemi-Thermomechanical Pulping (CTMP) • Combines mechanical, thermal and chemical

treatments – Thermal pre-treatment includes:

• Na2SO3 • NaOH

• Highest quality “mechanical pulp” – Pulp yields 80-85%

• Mill effluents are environmental concern – lignin and extractives – Popular in Canada

• Whitecourt Alberta: Millar Western – Paper towel production

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Mechanical pulping comparison Feedstock Apparatus Heat? Chemicals? Still used?

Stone ground wood (SGW)

Whole logs Large abrasive

wheel

None None

No

Pressurized ground wood (PGW)

Whole logs Large abrasive

wheel

Steam None

No

Refiner mechanical (RMP)

Chips Refiner None None

No

Thermo mechanical (TMP)

Chips Refiner Yes None

YES

Chemi-thermo mechanical (CTMP)

Chips Refiner Yes Yes YES

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Mechanical pulp benefits

• Relatively cheap – low capital costs – High yields (85-95% of wood recovered as

fiber) • High opacity product

– Means that thinner sheets can be made and printed on both sides (newsprint)

• Good surface for printing

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Mechanical pulp drawbacks

• Relatively weak • Limited brightness

– No lignin removal – Can be bleached

• Photoyellowing – High lignin causes formation of chromophores

upon reaction of light and oxygen with lignin

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How do you measure effectiveness of mechanical pulping? • Can’t measure lignin because no lignin is

removed • Measure fiber properties instead • Well pulped fibers will retain water well –

this can be measured as “Freeness” – Freeness is the ease with which water leaves

the fiber mat formed on a wire mesh from a dilute slurry.

• High freeness = low water retention by fibers, unrefined, poor fiber separation

• Low freeness = high water retention, well-refined

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Canadian standard freeness test 1. Pour pulp suspension into

apparatus with screen at the bottom

2. Allow water to drain from the pulp 3. Measure the amount of water

that drains More water = higher freeness

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Chemical pulping • Use acidic or basic chemical solutions

(liquors) to dissolve the lignin between fibers – Chemicals can also dissolve cellulose – Need a balance between lignin removal and

cellulose degradation • Produces highest quality fiber, but at lowest

yields • Normally requires some type of chemical

recovery process for economic and environmental viability → Kraft recovery process

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Steps in chemical pulping

1. Chip harvest, chipping, storage and screening

2. Cooking (pulping) – Using pulping chemicals – In a digester (batch or continuous)

3. Fiber recovery – Also recover spent cooking liquor

4. Fiber washing, bleaching, refining 5. papermaking

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Important chip considerations for chemical pulping • Uniform chip size

– Large chips undercook (give shives/rejects) – Small chips overcook (give fines, weak pulp)

• Wood deterioration – Don’t use decayed chips – Chip piles can accelerate degradation by

harboring microorganisms and getting hot – Use oldest chips first

• Debris – Bark, foliage, plastic, metals, dirt, decayed wood – Dirt specks, loss of strength, wear on equipment

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How to improve chip quality?

Screen chips – Remove large

chips (“overs”) and debris

– Remove small chips (“fines”)

– “Accepts” are chips in the correct size distribution

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Chemical Pulping • Major Chemical Pulping Processes:

1. Kraft process (sulphate) alkaline 2. Sulfite (sulphite) process acid 3. Soda process alkaline 4. Neutral sulfite semi-chemical neutral

• While sulfite pulping was developed earlier,

Kraft pulping has dominated chemical pulping for the past 60-70 years. (kraft = “strong”) – Strongest fiber – Greatest versatility – Efficient recovery process

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1. Kraft pulping process • Alkaline-based = high pH cooking liquor • Basic cooking chemicals:

– Sodium hydroxide (NaOH) – Sodium sulfide (Na2S)

• Sulfur gives rise to odor, but dramatically accelerates cooking reactions – Major improvements in Kraft odor control over

past 10-15 years • Pulping chemicals are recycled

– Kraft recovery cycle – Makes process economical

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Kraft pulp

• Yield: 40-60% • Preferred wood species

– All types of wood and non-wood • Used to produce most paper products

– Unbleached pulp is darker than other wood pulps, but it can be bleached to make very white pulp

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Kraft chemicals • Solution of chemicals = liquor • Types of kraft liquors:

– White liquor: cooking solution of NaOH and Na2S used for start of pulping process

– Black liquor: spent cooking liquor from digester and washers; contains lignin fragments and other compounds from wood dissolution

– Green liquor: raw liquor after dissolution of recovered “smelt” - primarily solution of Na2CO3 and Na2S

• All are HIGHLY alkaline and caustic

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Kraft Recovery Cycle • Main key to the

dominance of the Kraft pulping process: – Recovers sodium (Na) and

sulfur (S), and regenerates the white cooking liquor

– Produces high temperature steam for process heating needs throughout the mill

– Can potentially also generate electricity via the installation of a cogeneration process

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Kraft Recovery Boiler or Furnace • Single most expensive process unit on pulp

mill - > $100 million! • Fire + high pressure steam = dangerous • Burns black liquor, generates smelt,

generates steam all on a 24/7 schedule • Complicated, expensive, and potentially

dangerous, BUT absolutely essential to the Kraft pulping process

• Temperature: can be up to 2500° F! • Capacity: up to allowing 3000 tons per day of

pulp production – Most mills “recovery limited”

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2. Sulfite pulping process • Oldest chemical pulping process

– In 1900, surpassed mechanical pulping as dominant process

• Acid-based cooking liquor (low pH) • Active species are bisulfite ion (HSO3

-1) and sulfite ion (SO3

-2) • Needs to be “buffered” by a base - 5 options:

– Calcium (Ca) – Magnesium (Mg) – Ammonium (NH4) – Sodium (Na) – Potassium (K)

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Sulfite pulping • Yield: 42 - 52% • Preferred wood species

– Hemlock – Spruce – Balsam fir

• Sulfite pulping currently fills a specialty niche: – Dissolving pulps (high purity for rayon, cellulose

acetate, cellulose nitrate feedstock) – Tissue and towel grades (softness) – Fine paper grades (smoothness)

• However, sulfite pulping is slowly disappearing – only makes up 10% of chemical pulping now

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Sulfite process

• No chemical recovery – Sugars remain intact in spent liquor and can

be fermented to ethanol or other bioproducts – Lignosulfonates used for various chemical

byproducts: road binders, drilling mud additives, colloidal stabilizers, etc.

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Kraft vs. Sulfite Pulping • Similar yields – 40-50% • Advantages of Kraft

– Produces high strength pulp – Utilizes effective chemical recovery

method – Handles wide variety of wood species

• Advantages of sulfite – Produces brighter unbleached pulp – Pulp is easier to bleach to full

brightness – Produces higher yield of bleached

pulp – Pulp is easier to refine – Spent liquor contains

monosaccharides and lignosulfonates

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Kraft Acid sulfite

Time 2-4 h 4-20 h

pH 13+ 1-2

Temp 170-180°C

120-135°C

Chemi-cals

NaOH, Na2S

H2SO3

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Other pulping methods 3. Soda Pulping • Very first chemical pulping process (1851) • Soda refers to caustic soda = NaOH

– Alkaline cooking liquor like Kraft – Eliminates sulfur (plus) – Slower and less robust than Kraft process (minus)

• Generally not used for wood chips (cooking time) • Effective for nonwood pulping:

– Straw, rice, bagasse (sugar cane residue), kenaf, other non-wood plants

• Can be used at atmospheric pressure (less expensive equipment)

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Chemical Pulping

4. Neutral sulfite semi-chemical (NSSC) • Neutral (or slightly alkaline) cooking liquor • Much higher yields • Utilized chiefly for corrugating medium

pulp – Very stiff fibers from hardwoods

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Chemical Pulping Equipment

• Cooking vessel is called a digester • Two basic types:

– Batch – Continuous

• Batch digesters are older process - very flexible

• Continuous digesters are newer (1960) and more energy efficient – Allow continuous pulp production

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Batch digester • Large pressure reactors

– 2500 to 12,500 ft3

• Typical mill will have banks of 6-8 digesters

• Filled with chips and pulping liquor • Heating of system is either through

direct heat (steam) or though indirect heating of the cooking liquor with forced circulation

• At the end of the cook, the contents are discharged to a blow tank

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Continuous Digester

• Developed in 1940’s • Chips travel through from

top to bottom continuously through different zones

• Most common is the Kamyr digester – 60-70 m tall – Produces 1000 tons pulp per

day

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Continuous Reactor

Chips

Steam

Steam

S c r e

e n s

Chips and Liquor

Liquor

Liquor

Cooking Zone

Washing Zone

Blow Line

Impregnation

High Pressure Feeder

Chips Silo Zone

White Liquor

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Continuous principle • Chips start at a low density • As they are digested, density is increased

and they move downwards • Screens at different points in the digester

remove liquor – Can be used to wash chips

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Batch versus continuous

Batch • More Flexible:

– Grade Changes – Fiber Source

• Maintenance Issues – Less down time,

production loss • Production Flexibility

– Add another digester

Continuous • Lower Energy Costs • Easier to control non-

condensable gases: odor – Lower environmental

impact • Compact (space

efficient) • Steady-state flow rate

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Lecture 6 summary

• Pulping is like grinding or pulverizing wood – T/F

• Kraft pulping generates large amounts of chemical waste – T/F

• Sulfite pulping produces more products than just pulp – T/F

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