2. raw materials for pulp and paper

8/13/2019 2. Raw Materials for pulp and paper

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Bark• Outermost layer of tree trunks and branches that protects the tree

from its environment.

• Bark is a contaminant in the wood supply used for making pulpbecause it decreases the quality of pulp proportional to its level ofcontamination coz there is very little usable fiber in bark (mostly dueto the very small size of the fibers in bark), and it consumes chemicalsduring pulping and bleaching stages.

• lso causes dark specks in the final paper product.

• !he relatively high level of nonprocess elements (impurities) such assilica and calcium interfere with chemical recovery processes.

• "or the pulp industry, typical bark tolerances in wood chips are #.$ to#.%&

• 'hole tree chipping in the forest (a practice some argue will becomeimportant in the future as it gives a higher yield of wood chips)requires cleaning of the chips before pulping to remove bark, dirt,needles or leaves, twigs, etc.

• Bark typically consists of about #$#& e*tractives, %+%& cellulose,%+#& lignin, and the remainder is other carbohydrates and tannins

(condensed polyphenolic compounds).• !annins are used commerciall for d es, astrin ents, and leather



'ood

• !echnically, wood is *ylem tissue,which arises from the cambium(inner bark) of trees and shrubs andconsists of cellulose, hemicellulose,lignin, and e*tractives, hence alignocellulosic material.

• function is to support the tree andconduction of water and mineralsfrom the roots to the leaves of a tree



Gross structure of a softwood stem

cross section.

the outer part ofthe trunk and

contains some livingcells

found in the centerof older trees,

containsonly dead cells,and is generally

drier than

sapwood.

ach annual growth

ring containsearlywood

(sometimes calledspringwood), which

is laiddown in the early

part of the growingseason

(spring and summer)and is characterized

by largecells with thin cell

walls, and latewood(sometimes

calledsummerwood), whichis characterized by

small cells and thickcell walls



• !he inner portion of wood in a treeeventually dies with a corresponding

deposition of e*tractives, the processof heartwood formation

• -eartwood is more difficult to pulp

than sapwood• !he moisture content of sapwood is

high as the wood is normally saturated

with water, but with heartwoodformation, the moisture contentdecreases, with air replacing some ofthe water.



The function and characteristics of various

tree stem tissues



ome of the important pulping variables of

wood and wood chips are

– /oisture contentpercentage of water reportedrelative to dry or wet wood weight.

– pecific gravity0the density of wood materialrelative to the density of water.

–

!ension and compression strength properties. – Bark content. – 1hemical compositioncellulose, hemicellulose,

lignin, and e*tractives. –

2ength of storage0amount of decay ande*tractives content. – 1hip dimensions. – 'ood species



• !he moisture content of wood is an important factor, since one pays for wood onan ovendry basis, which represents the actual amount of wood material present.

•

Often it is desirable to have a low moisture content to reduce the energyrequirements to chemically pulp the wood and reduce transportation costs, butthere is little that can be done to control the moisture content of wood sources.

• !he wood density and wood chip bulk density (including air between the woodchips) are also important in determining the amount of wood material onepurchases and in determining digester charge levels

• !he properties of wood (or chips from wood) depends on growth factors such as

the location of the tree from which it came and its location within the tree – !he average fiber length typically increases gradually as the first %# inner growth rings are

formed – fibers are longest in wood #3# ft above the ground – !rees growing on wet, warm, and sunny locations are fast growing and have coarse, stiff

fibers, whereas trees growing on dry, cold, and less sunny locations are slow growing andhave fine, dense fibers.

• oftwood4s 5 -ardwood4s structure , chemical composition 5 fiber characteristics(ssignment)



/oisture content

• /oisture content (wet or green basis), /167

– 7epresents the amount of water in wood as a fraction of thewet weight of wood.

– !he weight of water is determined by weighing the woodbefore and after drying at #%81.

– !he greenbasis moisture content of freshly cut wood istypically %#&, but can vary from about $#9#&



• /oisture content (dry basis), /1O:

– measure of the moisture content of wood based on theovendry (an obsolete term is bone dry) weight of wood

–

represents the amount of water in a wood sample divided bythe ovendry weight of wood material.

– !he ovendry weight is obtained by drying the wood toconstant weight at #$#%81

– "reshly cut wood has an ovendry basis moisture content onthe order of ##&, although it varies from about +%%#&



• olid 1ontent – !he solids content is a measure of the

solid material in wet samples such aswood, pulp, and paper.

– !he term consistency is used instead of

solids content in pulp slurries.



• olid 'ood :ensity – measure of the dry weight of wood per unit volume of

green wood. – ince wood contracts about ;%& on a volume basis as it

is dried below $#& moisture,

– o it is important to specify the moisture content at whichthe volume was measured

•

pecific gravity – pecific gravity is the ratio of the solid wood density to the

density of water at the same temperature.

– !he solid wood density may be determined using the greenvolume, the ovendry volume, or intermediate volumes.

– as wood shrinks about ;%& as it dries. !he basic specificgravity always uses the green volume.

– oftwoods have typical specific gravities of #.$%#.%# g<ccon a green volume basis, but can vary from #.3=#.9#

– -ardwoods have typical specific gravities of #.$%#.9# on a

green volume basis, but can vary from #.$##.=#



'ood decay and other deterioration

• t ambient temperatures and suitable p- conditions, wood can decay in thepresence of moisture and o*ygen

• specially when above 3#& /1O: and e*posed to air.

• >n temperate environments, decay leads to much more loss of structural woodthan is caused by termites.

• 'ood in the form of logs kept under a water sprinkler or immersed in water

tends to decay slowly

• ?eeping wood in any form below 3#& moisture content prevents decay, but it isgenerally impractical to dry wood for pulping, as dry wood usually does not pulpwell since liquor penetration is more difficult

• 'ood chip piles, as a rule of thumb, lose & per month due to decay andchemical o*idation

•

'ood deterioration begins by respiration of living parenchyma cells of the woodrays which increases the temperature and accelerates bacterial and fungaldecay. – -eating above +%%% 81 is mostly due to chemical o*idation (especially of e*tractives),

as most bacteria and fungi will not grow well at these temperatures.

– !emperatures above %%81 lead to severe losses upon pulping.



aw :ust

• residue generated by saw teeth whenwood is cut into lumber

• gives a pulp with short fibers that is

suitable as part of the furnish for tissueand writing papers

• But, since the =@#s, saw blades havebecome thinner with more teeth, which, inmany cases, makes the sawdust too smallto be used as a fiber source for pulp.



'ood 1hips• /echanically disintegrated wood, traditionally in pieces

– 33% mm (<3 to in.) along the grain, – variable in width, and $9 mm (<; to <+ in.) thick

• Aniform chip size is very important in chemical pulping because large chipsundercook, leaving large amounts of shives ( small bundle of incompletelycooked wood fibers in the chemical pulp)

• while small chips clog the liquor circulation system 5 use large amounts ofchemical, and give a low yield of weak pulp.

• !ypically the content of shives in ?raft pulp is #. to .#&. n e*cess of shives isa sign of poor impregnation of the wood chips.

• Bark, dirt and other materials should always be kept to a minimum (#.%& orless), especially in mechanical pulps where they give a dark pulp that cannot bebrightened

• hort chips will give paper that is slightly weaker due to fiber cutting.

• oftwood chips less than 3 mm long (since this is the a*is parallel to fiberorientation) will have reduced average fiber lengths since many of the fibers willbe cut. "or e*ample, – 3% mm long chips from :ouglasfir will have an average fiber length of $.% mmC

– 3 mm long chips will have an average fiber length of $.# mm

– 9 mm long chips will have an average fiber length of only 3 mm

One key factor in obtainingthe highest quality pulp possible

with the most

efficient use of pulping andbleaching chemicals

and the least environmentalimpact is to have as

uniform a chip as possible.



1hip ilos

• "or the sake of efficiency, afterscreening,

• wood chips must be metered into thedigesters

• without constant supervision. Onemethod of

• accomplishing this is with the use ofchip silos that

• hold %#$## tons of chips.



'OO: 1->D D7D7!>OE

>t can be basically classified under $ separatesection i.e.

– :ebarking● :rum :ebarker●

7otary :ebarker● 7ing :ebarker

– 1hipping● :isc 1hipper● :rum 1hipper

– creening● /echanical creening● 'ind creening● ir :ensity eparator



1hemical 1omposition Of'ood

• n organic material i.e. containsmainly 1

• 'ith very little variation btw differentspecies, woody matter is mainlycomposed of $ main elements 1, O3 5

E3 and some inorganic elements like

Ea, ?, 1a, /g 5 i• !o a lesser e*tent some low /'

substances (e*tractives 5 inorganic

substances, ash) can also be found

/acromolecular substances of wood cell wa



1hemical 1omparison of various woodspecies (dry basis)



1ellulose• !he most abundant renewable polymer source available worldwide

• bout #F to #F3 tons are synthesized annually in its pure form by photosynthesis but then getscombined with lignin 5 other polysaccharides (hemicellulose) in cell wall of the woody plants

• 1hemically, a linear polymer of anhydro:glucose connected by G(+)linkages• :D is above #,### in unaltered wood, but less than ### in highly bleached ?raft pulps

• Dhysically, cellulose is a white solid material that may e*ist in crystalline or amorphous states

• bout %#@#& cellulose in wood iscrystalline 5 forms the HbackboneH

structure of a wood fiber• !he crystalline form of cellulose is

particularly resistant to chemicalattack and degradation.

• -ydrogen bonding between cellulosemolecules results in the highstrength of cellulose fibers



2ignin



*tractives

2. raw materials for pulp and paper

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