alkaline pulping

Upload: vivekbhuchem

Post on 03-Jun-2018




0 download

Embed Size (px)


  • 8/12/2019 Alkaline Pulping




    Alkaline pulping fundamentals

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


  • 8/12/2019 Alkaline Pulping


    Plot of alkali concentration vs. time

  • 8/12/2019 Alkaline Pulping


    Removal of lignin during pine kraft cooks

  • 8/12/2019 Alkaline Pulping


    Change of alkali concentration, lignin & hemi-

    cellulose removal as a function of cooking time

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    H factor

    It is a useful parameter

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

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    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.

  • 8/12/2019 Alkaline Pulping


    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.

  • 8/12/2019 Alkaline Pulping


    Active alkali=NaOH+Na2S

    Effective alkali=NaOH+1/2Na2



    All quantities expressed as Na2O

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    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


    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.

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    Effect of chemical to wood ratio on

    chemical consumption and pulp yield

  • 8/12/2019 Alkaline Pulping


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

  • 8/12/2019 Alkaline Pulping


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

    of maximum temperature (1700C) in cooking

  • 8/12/2019 Alkaline Pulping


    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.

  • 8/12/2019 Alkaline Pulping


    Effect of sulphidity on pulp composition and yield

  • 8/12/2019 Alkaline Pulping


    Effect of sulphidity during kraft pulping of southern yellow pine

  • 8/12/2019 Alkaline Pulping


    Influence of sulphide charge onstrength-yield

    relationship (constant EA 15% on wood as Na2O)

  • 8/12/2019 Alkaline Pulping


    Influence of effective alkali andsulphide charges on pulp

  • 8/12/2019 Alkaline Pulping


    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%

  • 8/12/2019 Alkaline Pulping


    Mechanism of anthraquinone action

  • 8/12/2019 Alkaline Pulping


    Effect of yield on strength properties

  • 8/12/2019 Alkaline Pulping


    Effect of anthaquinone on mixed southern hardwoods

  • 8/12/2019 Alkaline Pulping


    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.

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    Batch digestion system- stationary,

    cylindrical and indirect heating type

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    Flat bottom blow tank with side entering

  • 8/12/2019 Alkaline Pulping


    Cone bottom blow tank

    Condenser system

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    Surface condenser system

  • 8/12/2019 Alkaline Pulping


    Jet condenser system

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


  • 8/12/2019 Alkaline Pulping


    Low energy cooking sequence

  • 8/12/2019 Alkaline Pulping


    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


    C using airblow

    For the new batch the chips along with hot

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    Wash Loss in a pulp mill

  • 8/12/2019 Alkaline Pulping


    Reducing wash loss by installing a

    press at the end of a washing

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    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


    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

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    Nordon Method

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    E value of different wash equipments

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    Hydraulic load vs. pulp consistency

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

    Increasing pulp consistency decreases flow rate

  • 8/12/2019 Alkaline Pulping


    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.

  • 8/12/2019 Alkaline Pulping


    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%.

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    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


  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    Pressure Screen

  • 8/12/2019 Alkaline Pulping


    Examples of flow patterns in pressure screen

    S i f il k l t

  • 8/12/2019 Alkaline Pulping


    Sweeping foil keeps pressure screen plate open

    Combination of screens

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    Mixing of accepts of different purity

    Cascade co pling Reject screening in

  • 8/12/2019 Alkaline Pulping


    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

  • 8/12/2019 Alkaline Pulping


    Optimum screening system

  • 8/12/2019 Alkaline Pulping



    Centricleaners (centrifugal cleaners)

  • 8/12/2019 Alkaline Pulping


    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