Development Trends in Kraft Pulping (2)

8/10/2019 Development Trends in Kraft Pulping (2)

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DEVELOPMENT TRENDS IN

KRAFT PULPINGDevelopments in chemical pulping processes are aimedto:

Savings in fibrous raw materialsSavings in bleaching chemicalsSavings in energy inputs

Savings in water consumptionReduced amounts of liquid effluents and solid wastesReduced pollution loads / toxicity with liquid effluents,

solid wastes, and gaseous emissions


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The approaches to achieve these objectives include:

Pulp and paper industry is under pressure to reduce the AOXdischarges by combining new developments in pulping and

bleaching processes (Preventive strategy) and/or effluenttreatment technologies (Curative strategy). As it is better to avoid the formation of AOX than to cleanup the

effluent, most of the efforts of the pulp and paper industry areheading towards process improvements.

Since chlorine-containing compounds (e.g. Cl2, ClO2) andlignin fragments are the main reactants during the formation of AOX, a wise reduction strategy must be based on the reductionor elimination of these two groups of compounds, as well asother precursors (some oil-based defoamers, pitch controllers)added during the process.

The most important preventive strategy is the elimination ofelemental chlorine in bleaching; instead, using ECF/TCFbleaching methods.

The greatly desired ECF/ TCF pulps can be economicallyobtained only if very low kappa numbers are achieved duringpulping.


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The driving force of present pulping technologies is toremove as much lignin as possible so that less amountschemicals must be required during bleaching.

This approach will permit to lower the amount of chlorine orincrease its substitution for ClO2 to bleach pulp atequivalent quality standards.

Lowering of kappa number can be achieved by processmodifications that enhance pulp yield without deterioratingstrength properties extended delignification, cold blow,displacement heating.

Alternatively, some chemicals may be added to the digesterthat helps in increasing yield or lowering kappa number.

After pulping, the kappa number may be lowered further by

oxygen delignification and/or enzymatic treatments of thepulps. Pulps must be effectively washing before bleaching and

between bleaching stages.


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EXTENDED DELIGNIFICATION

For continuous digesters, the processes for extended delignificationavailable at present include Modified Continuous Cooking (MCC) andExtended Modified Continuous Cooking (EMCC).

For batch cooking, processes such as Rapid Displacement Heating (RDH),

SuperBatch, and EnerBatch have allowed extended delignification.

A key part of the batch processes is liquor displacement, which wasintroduced initially for heat economy in the mid-1980s. The manipulation of

conditions made possible by liquor displacement allows improved pulpstrength and extended delignification.

Unfortunately, large capital investment is required to convert a conventionalbatch digester to the liquor displacement type of system, and this hasprevented many conventional batch mills from up-grading their digesters.

In theory, extended delignification in kraft pulping can be obtained if thefollowing conditions are applied in the digester operation:

(1) an even distribution of effective alkali throughout cooking; (2) high sulfide ion concentration at the beginning of cooking; (3) low dissolved lignin concentration toward the end of cooking; (4) low cooking temperature.


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The major challenge is to achieve these conditions without increasing operating costsand decreasing production rate or, more specifically, without increasing chemicalconsumption and cooking time.

In conventional pulping, there is a significant drop in both hydroxide and sulfideconcentrations at the very beginning of cooking.

Delignification during the major part of cooking proceeds under a relatively lowchemical driving force.

In liquor displacement batch cooking, significant amounts of hydroxide and sulfide areadsorbed by wood chips from the warm and hot black liquors during the initial blackliquor displacements.

Hence, the consumption of hydroxide and sulfide just after white liquor has beenadded to the digester will be much lower than in the conventional process

. Thus, the cooking phase is performed at higher hydroxide and sulfide concentrations. To a given kappa number, the dissolved lignin concentration at the end of cooking is

the same in both the conventional and liquor displacement processes.

Therefore, extended delignification mainly comes from the higher rate ofdelignification at higher hydroxide and sulfide concentrations.


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PULPING WITH ADDITIVES

The extended delignification processes require extensive retrofitting ordigester replacements limiting their usefulness if capital expenditure is aconstraint.

On the other hand, pulping processes with digester additives offer lessextensive and/or less expensive alternatives to digester modifications.

Fortunately, digester modifications and pulping additives workcomplementary to each other.

A list of these additives is as follows: Sodium sulfide (Na2S) Amines (R-NH2), Polysulfides (Na2Sx+1), Anthraquinone (AQ) and Surfactants (formula variable) . Other pulping additives: sodium borohydride (NaBH4), hydrazine,

peroxymonosulfuric acid, seleniun, lignin derived quinones, trisodium tetra-4-sulfonatophthalocyanineiron(III) (Fe(TSPc)), and trisodium meso - tetra-4-sulfonatophenylporphinerhodium(III) (Rh(TSPP)).


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Sodium Sulfide Sodium sulfide (Na2S) is introduced in the white liquor

as Na2S or is formed by reduction of sodium sulfate in

the recovery system. Sodium sulfide does not participate directly in thedelignification reaction. It reacts with water to generatesodium hydrosulfide (NaHS) and sodium hydroxide

(NaOH). NaHS is the active chemical that participatesin delignification by reducing the active sites from thelignin fragments generated after the alkalinehydrolysis.

There is a tendency to reducing Na2S dosage(sulfidity) because of corrosion problems and theenvironmental issues related to sulfur emissions(TRS). The attempt is reduce sulfidities to between 10

and 15.


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Amines The use of amines as pulping additives was first

studied in 1936.

Different aliphatic and aromatic amines havebeen used in both soda and kraft pulping:methylamine, ethylenediamine (EDA),monoethanolamine (MEA), propanediamine

(PDA), butanediamine (BDA), hexanediamine(HAD), or phenylenediamine (PhDA).

Amines do not function as pulping agent, ratherthey alter the nature and rate of delignification inthe presence of strong alkali. The presence ofamines in alkaline cooking liquors tends toprotect polysaccharides, preferentially cellulose,against degradation and subsequent dissolution.

Amines have not been used at industrial scale.


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Polysulfides Polysulfides were initially used in the 1940's and resurfaced in the

mid 80's. Traditionally, polysulfides into the pulping liquor were introduction by

methods like: (a) dissolution of sulfur in white liquor, (b) addition of

polysulfide to the white liquor, (c) air oxidation of a mixture of whiteand black liquors and (d) use of specific inorganic oxidant. Present technologies are based on polysulfide production from the

sulfides in the white liquor. Polysulfide is generated according to equation: Na2S + XS Na2Sx+1

Polysulfide itself has no effect on the rate of delignification. It is supposed to stabilize the carbohydrates at the beginning of the

cook by inhibiting the "peeling reaction", and thereby increasing theyield as more carbohydrates, particularly hemicelluloses, are left inthe pulp.

Alternatively, this will allow cooking to a lower kappa number without

a detrimental effect on the yield. Furthermore, there is a reduction of the organic content, viscosity

and heat value of the black liquor. Finally, there are no increases in sulfur emissions and the

polysulfide does not change the sulfidity of the liquor.


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Development of an effective regeneration process like the Moxy MT hasmade polysulfide pulping a very attractive alternative to be used in the pulpand paper industry.

Polysulfides dosages are in the range 1-to-3% with yield benefits of theorder of 2 to 3%, varying from mill to mill. A yield gain of 6.3% at 2.5%addition of polysulfide with bamboo as raw material has been reported.

Anthraquinone and its derivatives

Anthraquinone (AQ), naphthoquinone (NQ), and other quinone derivatives

have been investigated as pulping additives since 1970. Anthraquinone is a polycyclic aromatic hydrocarbon containing two opposite

carbonyl groups (C=O) at 9,10 position. It is yellow or light gray to gray-green crystal powder and insoluble in water.

Anthraquinone and its disodium salt (DDA or SAQ) have been usedindustrially as pulping additives. Some of the other AQ-derivatives are evenbetter than AQ, but they are very expensive.

AQ based pulping additives are effective in increasing the delignificationrate, pulp yield and pulp quality.


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They stabilize carbohydrates during cooking to help increase in pulp yield. AQ facilitates -ether cleavage producing lower molecular weight ligninfragments, thereby increasing the delignification rate.

Lower molecular weight lignin fragments lead to the better bleachability ofkraft-AQ pulps.

The dosage of AQ depends in the type of wood. AQ dosage can bebetween 0.03-0.05% in hardwoods (lower lignin content) and between 0.05-0.1 for softwoods.

This 0.1% is the upper limit for AQ application regulated by the Food andDrugs Administration in the USA.

It is presumed that the AQ participates as a catalyst through a reduction-oxidation reaction. The AQ oxidizes cellulose thereby protecting it from

alkaline degradation (peeling). The AQ is reduced to 9, 10-dihydroxyanthracene which then can react with lignin. The lignin getsdegraded and becomes more water soluble and thereby more easy to washaway from the pulp, while the AQ is regenerated.

However, AQ cannot be recovered and reused. Another view is that the AQgets consumed forming aromatic compounds with lignin, rather than actingas true catalyst. Presence of AQ by-products in cooking liquor improves theselectivity of the cooking liquor.

Drawbacks of AQ include formation of AQ deposits in B/L evaporators andincreased AQ residuals in crude tall-oil.

AQ is delivered as a 50 % waterborne dispersion, containing small amountsof emulsifier and stabiliser that give easier handling properties.

AQ can be used in alkaline or neutral cooking processes like kraft, soda andNSSC.

The AQ can be used to lower the kappa number, lower the H-factor, lowerchemical demand and also to increase yield.


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Surfactants

The term "surfactant" or "surface active agent" refers to a general description for anymolecule with 'activity' or that modifies the properties at liquid-gas, liquid-liquid andliquid-solid interfaces by reducing the interfacial (surface) tension.

A 'dispersant' is more specific and refers only to a molecule that can stabilize a

dispersion of minute solid particles in a liquid. Likewise, an "emulsifier" designates only a molecule that can stabilize an emulsion oftwo immiscible liquids.

Some surfactants can be quite effective as both dispersants and emulsifiers. Otheruses of surfactants are: wetting agents, detergents, foaming agents, solubilizers, etc.

Work on use of surfactants as pulping additives started in 1975. Surfactants reduce the strong cohesive forces found on and between molecular

surfaces. Surfactant-based additives function by improving penetration of pulping reagent into

wood (by reducing surface tension between the liquor and chip) allowing for morethorough wetting of the chip surface.

Better penetration of cooking liquor into the wood chips can result in faster defiberingof chips, improved pulp deresination, reduction in kappa number, redection in rejects,and overall improvement in pulp quality.

Additional benefits of surfactants are their ability to keep both lignin and woodextractives in solution in the black liquor.

The surfactants may help the deresination in the hot alkaline extraction stage. Surfactants are very effective at very low concentrations; usually 0.025 to 0.06%

based on od. wood. The dosage depends on the degree of delignification.


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The effects of sodium sulfide, polysulfides, AQ, and surfactants during kraft pulping aswell as their interactions are given in thefollowing Figure.


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COMBINATIONS OF PULPING ADDITIVES

Sodium sulfide-AQ

It has been proved that anthraquinone losses its activity as the sulfidity ofcooking liquor increases; its activity is almost zero when sulfidity is around40%.

The best effect of AQ as a delignifying agent is achieved in soda process.

AQ-Surfactants

It seems that this combination is a promising alternative to enhance thedelignification, to protect the carbohydrates and to reduce the rejects.

Ideally, the industry would like to use a surfactant-based digester additive inconjunction with a lower dosage of AQ below the scaling threshold.

The synergy between AQ and surfactants generates an additional yieldgain.

AQ P l lfid


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AQ-Polysulfide

Some of the shortcomings of use of polysulfide include:

Requirement of process modification; white liquor is catalyticallyoxidized to yield the orange liquor (polysulfide).

Compared with AQ, it requires more capital expenditure, continuingcost of electricity, and other operating costs.

Unlike AQ, polysulfide has detrimental impact on softwood pulpstrength reduction of 5-15% in tear strength.

It is needed to add conventional pulp into polysulfide pulp tomaintain tear strength.

Sodium thiosulfate is produced with the manufacture of polysulfide.It increases burdens on a recovery limited mill as the dead loadincreases. It can also lead to increased corrosion.

AQ with polysulfide make a good combination. There is a synergisticeffect when AQ is used together with polysulfide cooking liquor.

Combinations of AQ-polysulfide give more yield gains than the sumof respective yield increase.

This synergistic effect decreases at low kappa number. Strategies for implementing AQ and AQ-Polysulfide extended

digester delignification are site-specific.

HEXENEURONIC ACID (H A)


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HEXENEURONIC ACID (HexA) The bleaching chemicals used in ECF sequences - chlorine dioxide, ozone and

peroxide - have their own characteristics and selectivity. Often, increasing scale in equipment and yellowing of pulp are observed compared

with conventional bleaching. The residual Hexeneuronic Acid (HexA) in the bleachedpulp is considered the cause for yellowing.

The HexA react with permanganate; therefore the residual HexA in unbleached pulpwill contribute to its kappa number (in addition to the lignin).

The hexenuronic acid groups are unreactive in alkaline oxygen and peroxidebleaching stages. HexA reacts with all electrophilic bleaching chemicals, includingchlorine, chlorine dioxide, ozone, and peracids, thus consuming these chemicals.

The formation and stability of HexA during pulping is known to be strongly affected bythe type of raw material and the pulping conditions, i.e. temperature and alkalinity.For softwoods, the HexA content of kraft pulp decreases gradually with decrease ofthe kappa number. For hardwoods, the HexA content has a peak to kappa number.

HexA in hardwood bleached kraft pulp (HBKP) bleached with mainly chlorine dioxidetends to remain more than chlorine or ozone bleached pulp because of the selectivityof chlorine dioxide. Chlorine dioxide degrades less HexA than lignin in a fixed ratio.

Superbatch cooking with high residual alkali results in nearly complete degradation ofHexA during the cooking.

It has been suggested that HexA can be selectively removed through a mild acidhydrolysis of kraft pulps. During the selective hydrolysis HexA gets converted to furan

derivatives. The selective hydrolysis is claimed to significantly improve the brightnessstability of oxygen and peroxide bleached pulps without profound effects on otherpaper properties.


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DEPOSIT CONTROL


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DEPOSIT CONTROL Deposit control is used to reduce the need for

maintenance stops by controlling deposit and/or scaleforming substances in pulping processes.

Anti scaling is used in digester and bleaching; chelantsare used in O2 delignification and bleaching; dispersingagents are used throughout the entire process.

Anti-scaling agents are chemicals that are used toprevent the build up of or to remove scale and fouling,and are often targeted at specific problems such aslimescale, pyrolusite, barite, etc.

These chemicals often contain mixes of differentchelants and dispersing agents.

Anti-scaling agents may be used in the digester orbleaching plant to give longer run times and reduce the

need for maintenance and cleaning stops.

C l i d h i l


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Complexing agents used as process chemicals arenormally chelants that make multiple bondings withtroublesome ions, hence preventing them from reactingwith other substances.

In pulping, these are used in peroxide bleaching toprevent metal ions from consuming the peroxide.

Dispersing agents are a wide range of surface activechemicals used to keep different substances insuspension.

In pulping these are normally used to prevent rosin,pitch or other wood extractives from forming deposits bykeeping them finely dispersed.

Felt conditioners are different chemicals that aresprayed on felts to protect the wire from deposits,keeping fine structures open.

This increases runnability and reduces the need forwashing.


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CALCIUM PRECIPITATION IN KRAFTPULPING

Scaling is a significant problem in kraft process.

Scaling on heat transfer surfaces diminishescapacity and increases steam consumption andvariability in product quality.

Scale also deposits on screens, pumps, andpipes carrying process liquors. CaCO3 depositsin kraft process are common.

Chemical species present in process streamscan influence scaling.

These chemicals can be additives to controldeposit problem or species released into theprocess liquors by delignification reactions.

Increasing the Na2CO3 in cooking liquordecreases the maximum calcium concentration

at precipitation.


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Even when no Na2CO3 is initially present in the white liquor,CaCO3 precipitation occurs, which is assumed to be driven bycarbonate produced in the carboxylation of uronic acids groups fromsolubilised hemicelluloses.

Aminocarboxylic acids are well known sequesterants. EDTA areeffective at controlling CaCO3 scaling process , but the requiredconcentration under typical kraft pulping conditions is likely to makethis treatment prohibitively expensive.

Effect of carboxylic acid containing polymers such as polyacrylic

acids and poly maleic acids in controlling CaCO3 precipitation iswell documented. However, at kraft pulping temperatures, much ofthe influence of these treatments appears lost. Their effects increasewith increasing dosages, but the concentration required tosignificantly inhibit the calcium precipitation are similar to those usedin the sequesterant treatment.

Many of the organophosphoric acids are reported to have significantefficiencies in control of carbonate scales with relatively lowconcentration by delaying the onset and slowing the rate ofprecipitation.

BROWN STOCK WASHING


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BROWN STOCK WASHING The washing operation serves to separate pulp fibres

from spent cooking liquor that contains inorganic cookingchemicals and the organic substances dissolved from

the fibrous raw material.

An efficient washing operation is essential for thefollowing reasons:.

Process economy has a great bearing on recovery ofinorganic cooking chemicals and thermal energy of theorganic materials.Carryover of black liquor to bleaching plant consumesmore bleaching chemicals and adds to the bleachingcosts.Carryover of organic substance to bleaching will result inincreased discharge of pollutants/toxic /chlorinatedorganic compounds, which are slowly degradable in therecipient of the discharge and may be toxic.

WASHING LOSS (Washing efficiency)


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WASHING LOSS (Washing efficiency) The washing loss is the quantity of dissolved

washable substance that is not removed from thepulp in the washing operation. It is defined in anumber of ways:

As inorganic substance expressed as kg Na2SO4 perton pulp.

As organic dissolved substance expressed as kg CODper ton pulp. Other measures of dissolved organics arechlorine demand, iodine demand, and permanganatedemand.

As total dissolved substance expressed as kg TDS perton pulp.

Originally, the washing loss was commonly expressed interms of equivalent make-up of sodium sulphate, as theloss of cooking chemical was of great economicimportance.

Nowadays, the washing loss has increasingly become ameasure of environmental problems and bleachchemical consumption.

For addressing these issues a method capable of


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For addressing these issues, a method capable ofdetermining oxidizable organic material would be helpfulin improving and controlling washing relative to thebleach plant.

The two types of measures are different since the ratio ofinorganic to organic dissolved substances in black liquoris not a constant. It largely depends on the pulpingvariables rather than on those of washing.

Washing loss in terms of percent solids (total dissolvedsolids) takes into account both inorganic and organiccomponents.

Nevertheless, it would be a poor choice for a routinemethod because of the difficulty of accurately measuringthe solids concentration in dilute filtrates.

COD is used but it also is time consuming. One testtakes about two hours and a spectrophotometer isrequired.

Permanganate test is recommended because thechemicals used are the same as used in k-number orkappa number testing.

The test is fast as it takes about 10 minutes to complete.

DILUTION FACTOR


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DILUTION FACTOR The cleanliness of the washed pulp can be

rather easily improved by using an excess

amount of wash water. However, this excess water consumes asubstantial amount of energy to evaporate in therecovery cycle besides its own cost.

Therefore, it is desired to wash the pulp cleanwith the low dilution. By definition the dilution factor is equal to the

mass of liquor going to the recovery section

minus the mass of the original liquor,expressed in kg per kg of dry pulp . Dilution factor is also equal to the total wash

water added minus that leaving the washing

equipment with the pulp.


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BASIC WASHING METHODS Various washing equipment have different ways

of operation. They combine several unit operations such as

dilution, mixing, dewatering, displacement,diffusion etc.

Quite often, more than one unit operation isinvolved concomitantly in an industrial washingprocess.

The basic washing methods can be divided into

the following types: 1 Dilution agitation extraction washing 2 Displacement washing 3 Combination of dilution extraction

displacement washing

WASHING IN STAGES


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WASHING IN STAGES The washing operation is usually carried out in stages for efficient

washing of pulp with minimum amount of wash water used. Two types of pulp and liquor flow arrangements in multistage

washing are commonly discussed.

In cross flow washing the clean water is used in each stage. Thisrequires large quantities of wash water. Moreover, the liquors obtained after washing from different stages

have different concentrations and it will be a bad practice to mix verydilute liquor from the last stage with the concentrated liquorextracted from the first stage. This arrangement is not used for pulpwashing.

In countercurrent washing , the wash liquor and the pulp flow inopposite directions.

The cleanest wash water is used to wash the cleanest pulp in thefinal washing stage.

The filtrate from any one stage is then used as wash liquor in thepreceding stage. This is the most commonly employed flow arrangement in the pulp

washing systems.


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DISPLACEMENT RATIO The performance of a single stage in the washing system is often expressed as

displacement ratio, which is the ratio between the actual displacement and thetheoretically achievable displacement when all the incoming liquor is displacedby wash liquor.

The displacement ratio attains values between zero and one. In the ideal

displacement, the concentration of dissolved solids in the liquor with outgoing pulp isthe same as the concentration in the wash liquor. In rotary drum filter washing systems, the displacement ratio however, does not

account for the actual washing that takes place due to dilution and extraction stages. There will be some washing even if the displacement ratio values were zero. The

practical range of DR appears to be between 0.5 and 0.8 for most washingsystems.

The displacement ratio largely depends on the type of washing equipment. It varies,however, slightly with the following conditions:

Dilution, Pulp properties Production load Presence of air (foam) in pulp, Spray nozzle arrangement on a filter, General condition of the filter.

The value of displacement ratio has traditionally been used from experience orconcentration measurements in practical operations.


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Operational Control Strategy A good operational strategy and reliable control of

the chosen operational parameters are necessarytools to achieve a good washing result and low

polluting discharges . One must, however, be aware of the difficultiesencountered in most existing filter and diffuser washerplants with their counter current washing system andlong time lag due to large buffer volumes

Equipment like belt washers and wash presses, thatallow a shorter time lag in the system are off coursebetter in this respect.

The best possible control of pulp and liquor flow as wellas the concentration of dissolved substance in various

liquors is very essential. The pulp flow and the dilution factor should be kept asconstant as possible.

Consequently, the washing plant should never beallowed to serve as a buffer between the cooking and

the bleaching department (or the paper mill)

Closed or Open Screening System


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Closed or Open Screening System

The entire system of washing, screening and thickening must be considered whenseeking to improve the removal of organics.

In older mills, screening of pulps was performed after the washing. The water usedfor dilution in the screens was discharged to sewer together with fiber losses anddissolved substance.

In the modern mills the screening, or at least part of it, is often integrated withthe washing. This means that the last washing stage lies after the screeningand a substantial part of the dilution water needed in the screens is used aswashing liquor. By this integration most of the screening discharge has beeneliminated.

With the increasing attention on environmental concerns, the option of closing thescreen room or keeping it open must be considered carefully at each mill. Adisadvantage with closed screening is that the carry over of dissolved organicsubstance to the bleach plant or to the paper mill is increased, unless thewashing efficiency can be improved correspondingly . During the last few yearthe need to eliminate the occurrence of toxic chlorinated compounds in the dissolvedorganic substance, has led to alternatives, which mean a partly reversion to sometype of open screening .In these case the very last washing stage is made with warmwater that is directly sewered or led to an aerated lagoon.

An important consequence of closed screening is that the temperature will behigher than that is usual in open screening. This requires a special design of theequipment, as an increased concentration of dissolved substance in thecirculating liquor is likely to increase the risk of foaming. The apparatus mayhave to be furnished with hoods and ventilators.

Air, Foam and Soap Considerations


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, p Air enters the system through the pulp cake on the washer face. The amount of air entering the cake must be adequate for driving out

the water or liquor, but at the same time it must be minimised to avoidfoaming which can create operating problems .

The air that comes down the washer drop leg is separated from the liquor in

the filtrate tank. The air separation is important because entrained air will interfere withformation and drainage of the mat in the subsequent stages.

Higher drainage gives better washing efficiency and increases effectivewasher capacity.

Foam generated from entrained air collects on top of the liquor in the filtratetanks.

The filtrate tanks are vented to a foam tank. Moving foam through thesystem requires a considerable amount of energy.

Pressures that are developed in the filtrate tanks to deliver form to the foamtank naturally take away from the vacuum and the capacity of the washer orfilter.

Foam lines are generally quite large, however, build-up can occur in foam

lines over long operating periods. Foam build-up can be related to many things, including ray cells andfiber fines from the pulp, as well as soap and resinous materials

Foam Build-up is difficult to clean out and usually requires cuttinghole in the line and using a high-pressure water hose.

SCREENING AND CLEANING


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SC G C G

Screening and cleaning systems are used to remove particulateimpurities

Screens separate unwanted impurities from good fibres based on

the difference in the size and shape of the particles of the two types. This operation is based upon some form of perforated barrier thatlets the acceptable fibres pass through while withholding theimpurities.

The size of the holes or slots in the screening plate determines theminimum size of contaminants removed.

Cleaners separate impurity particles from good fibres mainly due tothe difference in their specific weights

In gravity settling equipment such as rifflers or sand traps, theheavier particles settle in the fluid faster than the lighter particles.

The speed of separation can be greatly increased by utilisingcentrifugal force in hydrocyclones.

Apart from the density, the size and shape of the particles will alsoaffect the separation.

Long and thick shives may be separated from good fibres eventhough the specific weight of the two types of particles is the same.

UNDESIRABLE


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UNDESIRABLECONSTITUENTS IN PULP

In general, contaminants may benonfibrous or fibrous.

Nonfibrous contaminants, such as sand,

grit and bark particles, rust, slime, foam,etc., must be separated and removed fromthe system.

Fibrous contaminants, such as uncookedchips, knots, and shives should beremoved, treated and returned to the pulp.

It m st considered ho the debris ill infl ence:


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It must considered how the debris will influence:

Pulp saleability and price The appearance of the final paper

Paper strength Paper machine runnability The influence upon subsequent converting machinery. The wear on other equipment. A negative influence upon these factors makes it necessary to

remove the debris. To avoid too much mutual interaction between the particles in the

pulp the screening operation must be performed at lowconsistencies (0.22%).

This implies high water consumption, high energy demand for

pumping, and large area demand (building volume). Altogether, this makes screening a relatively expensive operation.

EFFICIENCY OF SCREENING / CLEANING


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The ideal screen would separate all of the debris from the unwantedparticles without the loss of good fibre. Such a screen does notexist.

In actual practice, some particles are large enough in all of theirdimensions to be positively removed

Only to a limited extent do screens work by `positive screening' i.e.by the debris being too large to pass through the perforations.

Except in a few cases of coarse screening (for instance knotterscreens placed early in the pulp flow from a digester in order toremove uncooked chips),

Most debris particles will have such a size and shape that they maypass the screen plate perforations if they are given the chance toapproach the perforations in the correct position

The screening process must therefore be considered as


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The screening process must therefore be considered asa statistical process.

There is a certain possibility of rejection of unwantedparticles in the screen.

The screening efficiency denotes the fraction of theunwanted particles in the injects that are separated and

rejected by the screen. Normally E will be in the range of 70 to 90%.

The amount of overscreening (or reject ratio) tells

how much fraction of the total injects (good fibresplus debris) will be separated as rejects. The amount of overscreening is normally kept in the

range of 5 to 30%.

THE MECHANISM OF SCREENING


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THE MECHANISM OF SCREENING

The stock flowing in a screen has two major velocitycomponents parallel to screen plate (transverse) andperpendicular to screen plate (radial).

These velocities in the screen produce forces thatdetermine the degree of rejects separation.

To flow through the screen plate, the stock mustconverge and accelerate in the radial direction.

This radial acceleration produces a fluid drag force whichtends to sweep particles towards the screen plate and toalign them so they will be able to pass through theopenings.

A flexible fibre flows easily with the radial flow but a longrigid particle cannot pass through due to high shear forceof the transverse velocity component.

GOALS OF SCREENING AND


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GOALS OF SCREENING ANDCLEANING

Ideally, all debris should be removed fromthe system.

This is physically impossible and

economically unviable. Different paper types may have widely

differing demands regarding cleanliness.

What cannot be tolerated in one type ofpaper may be fully acceptable in another


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Particle properties affecting rejection


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Flexibility: The greater the flexibility of the particle the greater is thechances of it being accepted

.Length: Fibres of equal flexibility are rejected primarily based onlength. The rejection increases with increase in fibre length.

.Width: The rejection of shives of same length increases withincrease in width or thickness.

.Specific Surface: The drag forces act more on the fibres withhigher specific surface, so that they are more readily accepted.

Sp. Gravity: Not an important parameter affecting separation in ascreen.

Kappa Number: Soft cooks (low Kappa number) reduce the i d i h i i j


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screens capacity and increase the minimum reject rate.

On the other hand there is a limitation to the maximum kappanumber which can be handled in a screen for bleachable operation,

Because at some point the pulp contain too much debris to bescreened without some mechanical treatment prior to screening

.Feed Debris: As the level of feed debris increases, the minimumreject rate at which the screen will operate increases.

The effect of Kappa number and feed debris is much morepronounced with softwood than with hardwood

Temperature: Screen capacity increases and minimum reject ratedecreases as the stock temperature increases.

Effect is more important with sulphite pulping than with kraft.

C l f i


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Control parameters for screening:

Screen Plate Mass Reject Rate Feed Consistency

Internal Dilution Rotor Speed Pressure drop across the Screen Plate Pulp Freeness

SCREENING SYSTEMS DESIGN


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Both screens and centrifugal cleaners must be operated with anoverscreening of 10-30% to achieve a good screening or cleaningefficiency.

To avoid too heavy losses of good fibres the reject will have to berescreened, one or more times.

This is called to screen in more stages with primary, secondary, andpossibly also tertiary screens.

It is found that an increase in overscreening beyond 30% gives avery limited increase in screening efficiency.

This means that if a higher screening efficiency is needed there islittle to be gained by increasing the amount of overscreening.

Instead the accepts may be screened once more. .

This is called double Screening (and not screening in two stages).


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g ( g g )Double screening yields a clean accepts.

The drawback is that large amounts of rejects are created and have

to be rescreened.

Modern primary screens are so effective that double screening isnot very much used.

If double screening is used it is of course necessary to make surethat the accepts is not contaminated with the accepts from asecondary or tertiary screen.

It should be noted that more stages of screens or cleaners in thepurification of the rejects inevitably will lead to a less clean acceptfrom the primary screen.

There is a compromise between final cleanliness and the fibre lossfrom the system


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COMMON SCREENING PROBLEMS


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Problem: Blinding of the screen plate Usual causes: Reject rate too low Inject consistency too high Screen plate opening too small Screen plate open area too large Throughput too high Internal dilution too low

Problem: Low capacity

Usual causes: Air in stock Feed pressure too low Feed consistency too low (or too high) Reject rate too low Screen plate opening too small Internal dilution too high Low kappa number

Accepts valve not opened enough or a restriction downstream Screen incompletely filled or under vacuum Unstable feed conditions



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Problem: Accept quality too low Usual causes: Feed quality too low Reject rate too low Screen plate opening too large Screen plate broken Screen plate improperly seated Internal dilution too high Feed consistency too low Volumetric throughput too high Feed conditions unstable Accept control unstable

The list of problems and causes is far from complete. Other causes exist for these problems and more than one factor may

contribute. Most screen operating difficulties are due to system problems rather than

problems with individual screens, and often there are several problemscombined

CENTRIFUGAL CLEANERS


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CENTRIFUGAL CLEANERS Operation of a centrifugal cleaner is based upon the formation of a

free vortex when the pulp is pumped tangentially into the upperportion of a conical tube.

The Inject consistency is approximately 0.6 -0.8% and the pressuredrop over the cleaner is typically 2- 4 bar.

The cleaner may have a cylindrical upper part connected to aconical lower part.

The pulp gets its rotary motion as an effect of the inlet velocity.

The pulp stream moves spirally downwards, and the tangentialvelocity will increase as the radius decreases.

Because of the large centrifugal forces, the heavy particles getthrown outwards against the tube wall, follow the wall downwardsand leave the cleaner through the reject outlet at the bottom.

As an effect of the rotation, an air column is formed in the centre ofthe cleaner


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the cleaner

The pulp will move spirally downwards but also inwards towards thecentre as the radius decreases.

At a certain diameter, near the air column, the downward movementis reversed, and the pulp will flow spirally upwards and out throughthe accept outlet at the top of the cleaner.

Hence, there is a conical layer where the pulp moves neither

downwards nor upwards.

A sand particle will be rejected as long as its outwards velocity islarger than the inward velocity of the pulp flow.

Small particles may stop before they reach the tube wall but maystill be separated if they stop in the downwards moving pulp.

A sand particle will be rejected as long as its outwards velocity is larger thanthe inward velocity of the pulp flow


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the inward velocity of the pulp flow . Small particles may stop before they reach the tube wall but may still be

separated if they stop in the downwards moving pulp.

Fibres without entrapped air have a specific weight around 1.5 and onemay ask why these fibres will not be thrown outwards and rejected.

The explanation why this does not happen is that the fibres are long (have ahigh length to diameter ratio), get caught in the rapidly moving water nearthe centre of the cleaner and pulled into the upward moving pulp stream.

This is analogous to logs being caught in the fastest moving water in astream.

The drag exerted by the water on a particle is proportional to the surfacearea of the particle

This means that a particle with a large specific surface will be pulled intothe faster moving stream with a larger force than a particle with a smallerspecific surface.

Thus, the drag on a fibre will be larger than the drag on a sand or bark

particle.

VARIABLE AFFECTING CENTRIFUGAL CLEANING


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Pressure drop across the cleaner The cleaner is constructed and built to work at a certain

flow velocity and normally with a certain, continuous flowof reject.

This gives an optimal flows pattern and the maximumcleaning efficiency.

The pressure drop may vary from 1.5 bar in small unitsup to 3 bar. In special units with large diameter, designed to remove

large, heavy debris, the pressure drop may be as low as0.3 1.0 bar.

The pressure drop affects the cleaning efficiency as wellas the hydraulic capacity of the cleaner.

Feed consistency


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Feed consistency The consistency has a small influence upon the pressure

drop but affects the cleaning efficiency.

Higher consistency normally leads to a lower cleaningefficiency.

The importance of the feed consistency depends upon

the type of debris.

Heavy debris like nuts, bolts, etc. may be separated inhydrocyclones at a relatively high consistency, up to 5%

Bark, fly ash and other types of light debris will be morestrongly influenced and the consistency should not bemuch higher than 0.5%.

Reject rate


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This overscreening may be controlled by the size ofthe reject orifice and the back pressure in theaccepts pipeline.

If the back pressure increases, the reject orifice must bereduced in order to keep the same amount ofoverscreening.

If the back pressure is too high there is a danger ofplugging the cleaner.

Just as for ordinary screens, the cleaning efficiency of acentrifugal cleaner is determined by the amount ofoverscreening (reject rate).

Some centrifugal cleaners are made with a reject

chamber for discontinuous removal of reject. This is typical for big hydrocyclones for the removal of

coarse, heavy debris.

Operating problems with centrifugal cleaners


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p g p g

Changes in feed consistency or in the back pressure must be controlledbecause they may cause plugging of the reject orifice.

A deflocculating screen is beneficial a head of small centrifugal cleaners . When centrifugal cleaning is performed in more steps, one must be aware

of the fact that the rejects will be somewhat concentrated compared to theinject consistency.

Some dilution is therefore normally needed before the reject is fed to thesecond stage cleaners.

The consistency of the rejects will normally be approximately three timeshigher than the inject consistency.

There is a certain danger of air being sucked into the reject orifice.

The reject opening is therefore normally submerged in a reject channel, orconnected to a vacuum chamber.

One has to make sure that these arrangements do not lower the cleaningefficiency, for instance by changing the pressure drop across the cleaner.

In a centrifugal cleaner sand and other types of abrasive particles are


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In a centrifugal cleaner, sand and other types of abrasive particles arepressed against the inner surface of the apparatus with high velocity andlarge forces.

This results in a high wear of the equipment and the conical part must be

replaced before the flow pattern is disrupted by grooves in the inner surface.

The conical part of centrifugal cleaners is most often made of a ceramic,water resistant material to minimise the rate of wear.

However, the necessary control and adjustments of a screen or centrifugal

cleaner system is very time consuming, and thus often neglected inpractical operation.

Reverse Cleaners It should also be mentioned that centrifugal cleaners may be operated in

reverse manner for the removal of light debris. This is of particular interest for the removal of small, sticky contaminants

from waste paper stock .

Development Trends in Kraft Pulping (2)

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