alkaline pulping

8/12/2019 Alkaline Pulping

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ALKALINE PULPING,

WASHING, SCREENING ANDCLEANING PRINCIPLES

Alkaline pulping fundamentals


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Alkaline Pulping is the separation of fibers from rawmaterials by dissolution of lignin

Alkali attacks cellulose/hemicellulose & lignin.

The attack is faster on lignin than carbohydrates.

As pulping reaction proceeds lignin concentrationdecrease

More carbohydrate fraction is attacked leading topulp degradation & reduction in strength properties.

Complete removal of lignin during pulping is notattempted

It would give a highly degraded pulp unsuitable forpaper making.

Pulping to a target kappa number is attempted wherethe attack on carbohydrate function is minimal.

Further removal of lignin is attempted duringbleaching with more selective bleaching agents


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Carbohydrates in the alkaline liquor areattacked by peeling reaction.

Removal of end glucose unit by breakdownof 1,4 glucoside/xyloside linkage

This is due to active aldehydic grouppresent in one end unit of cellulose chain.

Removal of one unit, generates anotheractive end unit and removal processcontinues till such time a stopping reactiontake place in which the active aldehydicgroup is converted to non reactive group.

This take place when about 300 glucoseunits have been removed


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Plot of alkali concentration vs. time


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Removal of lignin during pine kraft cooks


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Change of alkali concentration, lignin & hemi-

cellulose removal as a function of cooking time


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Effect of Temperature

Rate of reaction varies with temperature and time.

The extent of reaction - kdt = H

The rate of reaction follows Arrehenious equation

k = Ae-Ea/RT

ln k = ln A Ea/RT

ln k100= ln A = Ea/373R

ln k/k100= ln kr= Ea/R[1/373 1/T ]

At Ea= 32 kcal mole, kr = relative rate constant

ln kr= - 16113/T + 43.2

The values of krhas been evaluated at differenttemperatures. H factor has been defined as

H = krdt = Extent of reaction


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H factor

It is a useful parameter

It can indicate the change in reaction time if there isa small change in the cooking temperature


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Chip Impregnation

The pulping liquor must be uniformlydistributed in the chip before cookingtemperature is reached.

The transport of the liquid is throughdiffusion, because the chips are pre steamed

& the cavities are full with water. Diffusion is a slow process

Diffusion improves by raising thetemperature, or increasing impregnation time

or increasing hydraulic pressure At pH 13 or above, the rate of diffusion is

same in all the directions, longitudinal, radialor tangential directions


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So chip thickness is the controlling factor forchip impregnation.

But as the cooking liquor enters the chips, itreacts and pH drops.

If the pH drops below 12 the rate of diffusiondrops sharply and liquor may not be able to

reach the center of the chips. So thinner (3-6 mm) chips are preferred.

Increase of temperature by 10oC nearlydoubles (2.2) the rate of delignification butrate of impregnation increases by 2%

(dependent upon temperature in oK). Hard wood require 4mm and soft woods

7mm thick chips.


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Chip Damage Chipping damages the fibers at the ends.

Many fibers are cut and are shortened bychipper knife.

But most damaging effect is due to wedgeshaped blade causing localized distortions of

the wood structure. The damaged parts are more susceptible to

acid hydrolyses than alkaline liquors.

Sulphite pulps give 15% reduction in

strength where as in kraft process thereduction is minimal.

Crushing parallel to fiber axis is moredetrimental than perpendicular to the grain.


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Active alkali=NaOH+Na2S

Effective alkali=NaOH+1/2Na2

S

Sulphidity=NaOH/(NaOH+Na2S)d

All quantities expressed as Na2O


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Pulping Variables

Alkali (NaOH + Na2S) dose : 16% in case of softwoods and 12% in

case of hardwood expressed as Na2O

Unbleached screened yield 50-55%

However pulp yield depends upon the

chemical composition of raw material

i.e. hollocellulose content


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Pattern of chemical consumption

The results are of a study with soda pulping

of spruce with 19% alkali as Na2O. 12.5% was consumed in pulping wood to

give lignin content of 2.8% based on woodand at a yield of 44%.

Out of 12.5%, 2.3-3.0% was consumed fordissolution of lignin

1.3% was consumed for hydrolysis of formyland acetyl groups.

Of the remaining 8.2-8.9% most of it wasneutralized by acidic degradation products ofcarbohydrates but a small part was retainedon the pulp by absorption


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About 20% is used for attacking lignin

70-75% for attacking carbohydrates and

neutralization of degradation products 3-4% residual alkali is maintained to

prevent re-precipitation of dissolvedlignin on fibers.

Since the alkali concentration is highand reduces with reaction time.

So sufficient alkali must be added so

that a minimum residual alkaliconcentration of 6-8 gpl is maintainedto prevent precipitation of lignin


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During the initial part of the cook, i.e. raisingof temperature carbohydrates are attacked.

Lignin starts getting attacked at 1300C Rate of reaction increases with increase in

temperature.

By splitting alkali charge i.e

lower alkali charge at the beginning

second dose added near the cookingtemperature

some alkali charge at the end of the cook Pulp of lower kappa number, high viscosity

and improved yield can be obtained.


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Wood to liquor ratio

Wood liquor ratio of about 3.5-4.1 for soft

woods, 2.8 for hard woods is maintained

A lower bath ratio is preferred

It reduces steam demand in evaporation

section of the chemical recovery Lowering the bath ratio increases

alkali concentration

rate of delignificationdegradation of cellulose


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Effect of chemical to wood ratio on

chemical consumption and pulp yield


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The brightness of bleached pulp (CEHDED) asa function of the effective alkali during the cook


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Kappa number and yield for the differentcharges of effective alkali and different times

of maximum temperature (1700C) in cooking


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Summary of impact of increased effective

alkali charge on pulp quality

Xylan content is reduced

Glucomannan content is increased.

Brightness of unbleached pulp increased.

Longer beating time is required to reach

given breaking length or drainage resistance

Maximum breaking length is reduced.

Tear factor at a given breaking length isincreased.


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Effect of sulphidity on pulp composition and yield


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Effect of sulphidity during kraft pulping of southern yellow pine


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Influence of sulphide charge onstrength-yield

relationship (constant EA 15% on wood as Na2O)


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Influence of effective alkali andsulphide charges on pulp


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Kraft Pulp Modification Modifications for improving pulp yield.

Major reduction is loss of hemicelluloses and somedissolution of cellulose.

This loss is by peeling reaction involving the activeend unit containing aldehyde group.

Modification of aldehydic group by

oxidation to carboxylic

or reduction to hydroxy OH

the peeling reaction will stop

dissolution of carbohydrate fraction reduces, giving

higher pulp yield. This is done by using polysulphide or anthraquinine

where the CHO group is partially converted to COOHreducing peeling reaction giving an improvement inyield of 3-5%


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Mechanism of anthraquinone action


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Effect of yield on strength properties


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Effect of anthaquinone on mixed southern hardwoods


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Alkaline Digester Systems Two types of digesters :Batch & Continuous.

Batch digesters arelow cost, easy to operate and maintain

responds well to raw material variation

gives a pulp with high quality variations

have low energy efficiency,give a weaker pulp due to hot blow

emits higher amount of odourous mercaptan

Continuous digesters give pulp of

consistent & superior quality (stronger pulp)

Give low emissions and has high energyefficiency but are expensive.


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Batch digestor Batch digesters are spherical or cylindrical

shape, direct heating or in directly heating

type, stationary or tumbling type.

A stationary cylindrical digester withindirectly heating is used at many places.

In indirect heating, heating of liquor is doneoutside the digester using heat exchangers

Heated liquor is circulated in digester givinguniform temperature and liquor composition

In direct heating type steam gets condenseddiluting the pulping liquor. Tumbling actiondo not give uniform temperature profile inthe digester giving a non uniform cooking


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Batch digestion system- stationary,

cylindrical and indirect heating type


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Chips are added from top along with hotblack liquor

When full, lid is closed, heating is done bycirculating hot liquor through heatexchangers

Before reaching to a cooking temperature of165-170 oC, digester is vented for a short

time to allow air in the chips to escape Air acts as a barrier to heat transfer and also

gives false pressure.

The temperature (165-1700C) in the digester

corresponds to saturated steam pressure 6-7kg/cm2

If air is there in the digester the temperatureis lower


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After cooking the chips, the digester is blown

by connecting to blow tank at 1 atm pressure

The feed is tangentially at the top, which

feeds the cooked chips into the blow tank

and gives circular motion. Because of centrifugal forces the gases are

separated from chips & liquor, which falls

under gravity and collects in the blow tank

Blow Tank


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Flat bottom blow tank with side entering


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Cone bottom blow tank

Condenser system


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Condenser system

Hot vapors pass through a condenser wherethese are cooled giving condensate

containing dissolved organics imparting highBOD

Non condensable gasses escape to theatmosphere which contains mercaptansresponsible for odor.

The hot gases are cooled in the condensersystem giving only hot water.

This heat is not recovered.

Energy requirements are high or energyefficiency is low.

Blowing is done at 1650C, the fibers are softand some rupture in the internal structuretake place, lowering the strength properties


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Surface condenser system


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Jet condenser system


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Cold blow in a batch digester In this process, after the cooking is over, the heat of

the black liquor is used to heat the white liquor andraising the temperature of the digester

Some steam is needed to raise temperature ofdigestor to cooking temperature and maintainingcooking temperature during cooking period

Cold black liquor from the previous cook is pumpedin to the digestor to displace the hot black liquor

Energy (steam) requirements are lower by 40%

Temperature of the chips goes down below 1000C,atmospheric blowing using air compressor is done

Damage to the chips is reduced,vapours are greatlyreduced requiring small condenser systemsignificantly reduced emissions of non condensablegases mercaptans, sulphides etc


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Low energy cooking sequence


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RDH Process

After the cooking is over, the black liquor at

130-140oC is pumped in to the digester Displaced black liquor is collected in the tank

Cold black liquor at 70-80oC is fed to thedigester, hot black liquor of 130-140oC is

displaced Displaced liquor is collected in another tank

which is used for heating of chips and liquor

Temperature of chips now falls to 80-90oC

Digester is now blown at 80-90

o

C using airblow

For the new batch the chips along with hot


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p gblack liquor is added

This liquor is displaced by hot black liquor at130 140oC to which white liquor is added

This raises temperature of chips to130- 140oC This liquor is then replaced by black liquor at

150-160oC along with steam to raisetemperature of the digester to cooking

temperature The process has high steam economy,

low TRS emissions and gives a stronger pulpwith a lower kappa number

The pulp kappa number is lower due to indigester washing (displacement) whichreduces the bleach chemical demand


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Wash Loss in a pulp mill


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Reducing wash loss by installing a

press at the end of a washing


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Pulp Washing Blown pulp is screened to remove uncooked/

undercooked chips, knots etc. from pulpsuspension

Pulp is washed to remove dissolved solidsparticularly black liquor which containsdissolved lignin and will contribute towardsresidual lignin content kappa number of pulp

This will enhance the bleach chemicalrequirements in bleaching, increasing bleachchemical costs, degrade pulp and increase

environment pollution Direct counter current washing (4-5 stages)

is generally used


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Washing Principles The principle of washing is displacement

A pulp mat is formed on the washing equipment The mat contains black liquor. This liquor is

displaced by wash liquor and washes pulp

Ideally wash liquor required for washing must be

equal to liquor present with pulp In real system because of turbulence and

movement of organics due to diffusion mixing atinterface take place, more wash liquor is neededto wash the pulp than present with washed pulp

DF is equal to difference in outgoing liquor andincoming liquor with pulp or difference in washliquor used and liquor with outgoing pulp


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Increasing DF will improve the washing butthis additional water will have to beevaporated in the chemical recovery

Low values of DF are preferred 2.2-2.4 Washing improved by increasing number of

wash equipment

Washing efficiency is ratio of dissolved

solids removed to actually present with pulp. It is expressed as ratio or as percent

Dissolved solids contain two components:inorganic sodium and organics -

carbohydrates, lignin etc expressed as COD Both quantities are used in estimating

washing efficiency

I th fi t b ti ti di


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In the first case by estimating sodiumcontent in washed and unwashed pulp

This gives good results when washing

efficiencies are moderate Sodium is present in two forms free and

bound

Free sodium can be removed by washing ,

bound sodium remain attached with pulp (0.51.0 meq/g)

Other approach is to measure COD of liquoraccompanying washed and unwashed pulp

COD is a measure of organic content

Washing efficiency of an equipment/systemcan be expressed by two methods


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Washing means adding wash liquor and removing/displacing equal

volume of the dirty liquor with the unwashed pulp

If the pulp suspension is diluted by adding wash liquor, concentration

of dissolved solids in the pulp is reduced indicating washing.

In this case no dissolved has been removed and thus no washing

DR gives correct results only when inlet and outlet pulp consistencies

in wash equipment s same

Nordon Method


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Nordon Method


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W = Liquor weight of inlet pulp / Liquor weight of outgoing liquor

R = Wash Liquor used / Liquor with outgoing washed

pulp Values of both W and R are always greater than 1

If more than one wash equipments are used in series(Direct counter current washing) each equipment willhave a value of R represented as R1, R2, R3etc

These values may be same or different. Each wash equipment has E value as E1, E2EN

Identical wash equipments E1, E2ENwill be same

Wash Yield = Y = 1 - (W-1)/[ WR1E1R2

E2 .RnEn1]

This equation can be used to know what will be theeffect on washing if the number of wash equipmentsare changed, a change in DF, change of inlet/out CYor a wash equipment is replaced by other washequipment or a different wash equipment is addedto the existing set of wash equipments

A i l hi t ill i l d ddi h


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A single washing step will include adding wash

liquor in the pulp mixing it and removing the

equivalent volume of liquor from pulp

The same process is repeated a number of times A curve is plotted between dissolved solids in the

pulp as a function of number of washing stages

This curve is used to evaluate the E value of

particular equipment E value is defined as the number of washing steps

needed to give the same wash result as obtained

from a given wash equipment

The E value depends upon inlet and outlet pulpconsistencies

E values of some of equipments are given in Table


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E value of different wash equipments


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Filter equivalent Filter equivalent = E value of equipment/

E value of filter Filter equivalent indicates that the washing

efficiency of a equipment will be equal to thatobtained from such number of filters

Filter equivalent of radial washer is 2indicating that it will give the same washresult as obtained from 2 vacuum fillers

Examples -- Single/double stage vacuum

filter, Radial washer (Atmospheric diffuser),Pressurized diffuser, Wash press, Beltwasher, Displacement washing in Kamyrdigester or in batch digesters etc


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Screening & Cleaning Principles Pulp contains two types of impurities dissolved

solids which are removed by washing and the othersuspended solids shives, sand etc which areremoved by screening & cleaning

For screening of the pulp, screens with circular/slotted perforations are used

If there is a sizeable difference in the sizes of fibers& impurities, these can be separated by screening

Fibers being small are able to pass through screenopening and bigger under cooked material like knotsare retained as done in case of Johnson screen

Johnson screen is an example of positive screeningwhen the separation is taking place based ondifference in size of fibers and impurities.

Fibers & shives have similar fiber dimension


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Fibers & shives have similar fiber dimension

Both pass through or retained on screenplate

Major difference is to pass or retain onscreen plate

Screening is a statistical process.

A high probability for fibers to pass through

screen Impurity has a low probability to pass

through and it is retained on the screen plateand is rejected.

Pulp slurry is divided into accepts andrejects

In accepts there is lower amount ofimpurities in comparison to inlet pulp

So the pulp has become clean

One of major parameter influencing


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One of major parameter influencing

screening operation is difference in flexibility

of fiber & impurity

A shive is a group or bundle of fiber, has

higher stiffness

It can not easily bend, flexibility is small

A screen plate is a cylinder shell with holes The pulp stock is rotated

Fib d hi li th l ith


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Fibers and shives align themselves with

rotary flow

Water flow through the perforations,fibers being flexible, bend easily and

pass through perforations where as

shives are stiff, do not bend easily andare not able to pass through the

perforations of screen plate and are thus

retained on screen plate.

So accept fraction will have lower

proportion of shives, giving a clean pulp.

T t d t d ib


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Two parameters are used to describe ascreen

Screened Efficiency(%) = Impuritiesremoved/ Impurities originally present

Over screening reject rate(%) =Weight of Pulp rejected/ Initial weight of pulp

Usual values of screening efficiency are 70-80% and over screening is 10-30%.

Screening efficiency increases withdecreasing size of the perforations andcleanliness of pulp

Value is higher at higher value of Reject ratebut total amount of pulp rejected is increased


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Hydraulic load vs. pulp consistency

Capacity of the screen depends flow rate (hydraulicload) across the screen plate

Increasing pulp consistency decreases flow rate


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Kraft pulp fluidization energy vs. pulp consistency

For screening, pulp fibers must be present as individual fibresor as separated fibers at the time it reaches screen plate

Above 0.5% Cy, fibers start forming net works, strength ofnetworks increasing with Cy

So these net works are to be broken by providing agitationalenergy fiberizing energy.


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For screening, pulp fibers must be present asindividual fibres or as separated fibers when thesereach screen plate

Above 0.5% Cy, fibers start forming net works,strength of networks increasing with Cy

So these net works are to be broken by providingagitational energy fiberizing energy

This energy increases with Cy. Above 4% Cy fiberizing energy starts rising sharply

and agitational energy may not be sufficient to breaknetworks.

Above 4% Cy, the flow rate becomes small, so

capacity becomes small This indicates operating consistency range of

screens as 1-4%.


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Time for reflocculation vs. pulp consistency

Time of re-flocculation decreases with pulp consistency.

This time should be greater than the time required to reach thescreen platefrom the point of agitation of pulp

I t f ti l t


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Impact of operational parameters

Raising the pulp temperature may make the impurity

more flexible and reduce the screening efficiency Screens work with screening efficiency on a

percentage basis

The purity of the accept will be directly influenced by

the amount of debris in the inject If pressure drop increases, a large portion of small

impurities will be forced through the openings of

screen plate thus lowering the screening efficiency

If the inject flow rate is constant and the pressuredrop across the screen starts to increase, indicating

that the inlet consistency has increased or screen

plate has started to plug

Types


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Types Flat screens

Rotary Screens

Centrifugal Screens

Pressure Screens commonly used

MC Screens becoming popular

Pressure screens are compact (small size), require

unattended operation and have high capacity Screens work at lower consistency and hence

require pumping of large volumes of water

Pressure screens are used as coarse and fine screenas well as knotter screens depending upon size and

type of perforation MC screens work at medium consistency, do not

require lowering of consistency and require muchwater handling


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Pressure Screen


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Examples of flow patterns in pressure screen

S i f il k l t


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Sweeping foil keeps pressure screen plate open

Combination of screens


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Combination of screensScreening in more stages Double screening

Double screening give high cleanliness but yield is quite low.

Reject screening give high pulp yield but cleanliness has decreased

as compared to a single screen


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Mixing of accepts of different purity

Cascade co pling Reject screening in


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Cascade coupling Reject screening in

in two stages two stages

Cascading of screens, the accept from a secondary screen is mixedwith inject of the first screen,

This will improve the cleanliness of the pulp from screening system

Optimum screening system


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Optimum screening system


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Centricleaners

Centricleaners (centrifugal cleaners)


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Centricleaners (centrifugal cleaners)

are used to remove heavier matter like

sand from pulp suspension Inverted centricleaners are used to

remove lighter impurities.

The use of centricleaners has declinedas these works at very low consistency

0.5-1.0% and is considered to be

expensive in operation due to pumpingof very large quantities of water

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