4 th class supplement bc it

8/22/2019 4 Th Class Supplement Bc It

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4th

Class

Power Engineering(Supplementary Information)

Pulp and Paper Mill Operations

Crude Oil Refining


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Supplementary

Information

Crude Oil Refining

Introduction Although not required specifically by the 4th class

Syllabus, this section has been included in this workbookto provide additional information on crude oil refining.This is intended for those Power Engineers who areworking in the oil and gas industry or are interested inthese processes and desire more information than isprovided in the Learning Modules.

Distillation

As noted earlier in your modules, the many substances incrude oil boil and condense at a different temperature.The process of boiling and then condensing is calleddistillation. Gasoline, for example, starts to boil intovapor at 32oC but furnace fuel oil will not start to boil untilit reaches 200oC and its vapors will condense attemperatures below 200oC. Crude oil refiners make useof these characteristics of the substances in crude oil to

separate them from each other. The refineries begin byheating the crude oil inside a furnace to about 345oC. Atthis temperature, about 65 percent of the crude oilchanges to vapor. The vapors go to what is called anatmospheric distillation tower. As the vapors swirl upinside the tower they begin to cool. Those that condenseat higher temperatures condense first and collect in trayslower down. Those that condense at the lowesttemperatures become liquid in trays near the top.

Supplementary Information Crude Oil Refining Unit B1-10 Workbook

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HVAC 6012 Psychrometric Properties of Air Unit B1-9 Instructor Guide


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The figure shows a distillation tower. Crude oil is heatedin a furnace to about 345oC causing most of it tovaporize. The hot crude is discharged into the bottom ofthe distillation tower where the gases begin to passupward through the tower and cool. As the gases pass up

through the tower they are routed through holes in trayspositioned inside the tower. Each of the holes is fittedwith a bubble cap. These trays are covered with a layer ofliquid hydrocarbon. The bubble caps force the vapors tobubble through this layer of liquid. As the vapors cool, thefractions begin to condense at the appropriatetemperature. The condensate builds up on the trays andmaintains the level of liquid. Excess liquid overflows intocollections headers and is drawn off to the appropriatetanks. The heavier oils condense first and are drawn off.Then the lighter oils and gasolines are drawn off toward

the top. Gases which are not condensed are used inother processes.

The unvaporized heavy residue at thebottom is pumped to the vacuumdistillation unit. The vacuum permitsthe residue to boil at lower

temperatures so that it may beseparated into stocks for light,intermediate, and heavy lubricating oil,vacuum gas oil, and pitch. Thevacuum gas oil is further processedinto fuels and the pitch is used to makeasphalt.

SOPEEC Topic BN; BO Boiler Maintenance; Types of Plants Unit B1-10 - Workbook


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Conversion

There are a variety of ways of converting oil. In cracking, one of the main conversionprocesses, an oil fraction such as gas oil is heated under pressure. This causes itsmolecules to break down and

form smaller molecules ofgasoline and lightfuel oils.The process not only makesmore gasoline from crude oil,it also produces a gasolinewith a higher octane than thatobtained from distillation.High-octane gasoline isnecessary for high-compression engines.Cracking operations are

carried out with the aid of acatalyst, a substance thathelps a chemical processtake place without beingaffected itself.

Another conversion process isalkylation. In the alkylation process, some of these gasespass over a catalyst and their molecules join to make molecules of gasoline. The

gasoline so formed is of higher qualitythan that secured from the distillationprocess.reforming gets its name from

the fact that the process changes theform or shape of gasoline molecules.This is necessary because gasolinefrom the distillation process is notpowerful enough for modern cars; theyneed higher octanes. One way to dothis is to change the molecularstructure of the gasoline from thedistillation process by reacting to it inthe presence of a catalyst containingplatinum.



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Treating

Nearly all the products that come from distillation,cracking, and other major manufacturing processescontain small amounts of impurities. Some of these givethe product a bad odor and others may cause rust orunnecessary wear. Treating removes these impurities. Themethod of treating used will depend on the product and theimpurities that are in it. Some treating methods are simple,such as passing the product through a solution of lye andwater, while others are complicated.

Blending

Blending combines various compounds to achieve aproduct with all the qualities it needs to do its job properly.Gasoline is a good example of a product that must becarefully blended. The gasolines made for use in carshave different qualities than those made for tractors oraircraft. Gasolines are also blended to have differentqualities in different seasons. In winter, for example,gasoline is blended to evaporate more readily. One way todo this is by adding butane. This helps the car engine to

start quickly in cold weather. Such quick starting would notbe possible with the gasoline that is sold in summer.



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When gasoline is blended, other substances are added toimprove its performance. A special oil is sometimes addedto help protect the engine from wear. Other compoundskeep the engine clean and prevent the formation ofdeposits. A substance may be added so that the gasoline

will keep well in storage.

Fuels

The oil refiners chief job is making gasoline. Other fuels,however, are important products of most refineries.Kerosene and stove oil are burned in space heaters, cookstoves, and other small, oil-fired appliances. Domesticheating oil is used in oil furnaces in houses. Light industrialfuels are burned in the furnaces of office buildings,apartment houses, and factories. Bunker fuels, which are

dark, heavy mixtures, are usually burned in specializedboilers in ships, power-generating stations, and largeindustrial plants. Diesel fuels cover a wide range of uses.The relatively small diesel engines used in buses operatingin the city may run on a product similar to stove oil.Transport trucks, inter-city buses, and cars with dieselengines usually burn a product much like domestic heatingoil.

Big diesel engines on trains, in power stations, and onships use a variety of industrial diesel fuels. Aviation fuels

are of two types: specially blended gasoline and jet fuel.J et fuels, used in jet and turbo-prop engines, are made thesame way as kerosene; by distillation, treating, andblending. Gases are another important class of fuels. Theyare produced in most refinery processes and are used invarious ways. Some fuel refinery furnaces. Others areliquefied under pressure, put in steel bottles, and sold asLPG (liquefied petroleum gas) to householders as fuel forgas appliances and space heaters.

Lubricating oi ls, greases, waxes, asphalt

None of the cars, trucks, or machines on farms or infactories would run very long without oil and grease.However, compared with gasoline and fuel oils, lubricatingoils and greases are used in small quantities and only afew of the refineries in Canada make them in addition togasolines and fuel oils.



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Refineries throughout the world have different ways ofseparating andtreating lubricating oils to get the productsrequired. These methods include treating with chemicals toseparate the various types of oils, to remove wax, and toget rid of impurities. The largest lubricating-oil plant in

Canada, which is located in the Imperial Oil refinery atSarnia, Ont., makes 11 different lubricating oils as basesfrom which about 550 different products can be blended.Other substances have to be added to the oils to enablethem to do special jobs such as preventing rust, sticking tofast-moving machinery, or bearingthe weight of heavyequipment.

Greases

It is not always possible to lubricate a machine with a

liquid, particularly where the liquid may run off the part it issupposed to lubricate. A common solution to this problemis to use a solid lubricant, usually a grease. Most greasesare made by mixing lubricating oil and special soap. Thesoap holds the oil the way a sponge holds water.

Asphalts

Asphalts for roads, shingles, and other purposes are madefrom the residues left after the other fractions have beenremoved. Mixed with sand, gravel, and crushed stone this

residue can be used on roads without further processing.Harder asphalt that will not flow is needed for shingles andthis is made by blowing air through the hot residue,causing it to harden gradually. The degree of hardness willdepend on how much air is blown through.

Inspection and Research

Guarding the quality of the products turned out by arefinery or gas plant is the job of the men and women inthe inspection laboratories. Day and night, seven days aweek, people are on duty testing samples. They test forcolor, odor, boiling point, sulphur content, burning andquality. They also perform dozens of other necessarychecks. If the inspectors find the quality of a product is notup to standard, they pass this information quickly to theoperators who trace the cause and correct it.



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Refiners also depend on researchers to find ways ofimproving the products the refinery is making, to developnew products, and to find easier and cheaper ways of

refining oil. All the processes used in the refinery and mostof the products that come out of it were developedbeforehand in research laboratories.

Supplementary Information Crude Oil Refining Unit B1-10 Workbook 9


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Supplementary

Information

Pulp and Paper Mill Operations

Introduction Although not required by the 4th class Syllabus, this

section is included in this workbook to provide additionalinformation on the processes used on pulp and papermills. This is intended for those Power Engineers who areworking in the paper mill industry or are interested in theseprocesses and desire more information than is provided in

the Learning Modules.

Pulp and paper Mill Processes

The information provided in the module is limited and amore detailed discussion is provided in this supplement.

Most pulp that is used in making paper is manufacturedfrom wood or from old papers (recycle). Wood is made upof cellulose fibers which are bonded together with lignin.Both the cellulose and lignin are organic hydrocarbonshaving different properties. The cellulose fibers can be

compared to a drinking straw consisting of a long tubularcell wall with a hollow center. This cellulose is the basethat paper is made from. When paper is produced on apaper machine, the fibers are mixed with water to form a

pulp slurry. When this pulp slurry is spread across the wireof the paper machine, the water passes through leaving amat of fiber that intermingles to form a bond. This is thefirst stage in the formation of a sheet of paper.

Since the sheet consists mostly of cellulose fibers which

mesh together, the longer these fibers are, the strongerthe sheet. The method of producing the pulp from wood isdetermined by how long the fibers need to be. Severalmethods are used to break wood down into cellulose fibersfor the production of paper.

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Although largely produced for the manufacture of paper,wood pulp is also used for many other things. Dissolvinggrade pulp is used to manufacture cellophane (fromcellulose), as well as rayon and other clothing fabrics.Cellulose fibers are also added to foods to act as a binding

agent holding foodstuffs together.

Mechanical Pulping

The simplest method of producing pulp is to press short logs against a grinding wheel,to grind the wood up and separate the cellulose fiber mechanically

This process is illustrated in Figure 1. Shortlogs which have had the bark removed are

loaded into a magazine where a hydraulic rampresses these logs against a rotating grindingwheel. This causes the wood to be ground updislodging the fiber. Showers wash and coolthe grinding wheel, flushing the fibers off thewheel and down into the basin located belowthe grinding wheel.

The fiber and water slurry is the drawn off thissump and pumped to the paper mill for themanufacture of paper. The pulp produced bythis process is very stiff and includes a largeamount of lignin. In addition, the cellulose isbroken down during the grinding process andvery low grade fiber is produced.

Figure 1 Mechanical Pulping

The paper produced using this process is calledmechanical or ground wood paper and is of very lowquality. These papers are used for low grade containerssuch as paper egg cartons. Ground wood pulping is veryinexpensive, with the major cost being in the fiber costs(raw logs), and energy cost to run the grinders.

In order to improve the quality of the fiber produced bymechanical pulping, the logs are often chipped and thenstored in a steaming vessel, where steam is used to heatthe chips for several minutes to several hours. Thesteam penetrates the wood chips, and softens the ligninso that the cellulose fibers more easily detach from each

Supplementary Information Pulp and Paper Mill Operations Unit B1-10 Workbook 11


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other. This reduces the energy costs and allows thechips to be broken down, using a refiner or defibratorinstead of the grind stone. Refiners cause less damageto the fiber, and produce longer cellulose fibers that canbe used to produce higher quality papers.

Figure 2 Thermo Mechanical Defibrator

The defribrators used in this process consist of circularstationary and rotating plates as shown in the figure. The

rotating plate is positioned very close to the stationaryplate and both plates are fitted with corrugate surfaces.The softened chips are forced between the rotating andstationary plates by the feed screw. Attrition between theplates and the chips breaks them up into fiber, which iscombined with water used for cooling, and dischargedfrom the defibrator as a pulp slurry.



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Interestingly, the friction produced in this pulping processcauses the water which is used in the refiner, to boilforming a large quantity of steam. This steam is dirty, butcan be used in a heat exchanger, to boiler clean water toproduce enough steam to supply the chip steaming

vessels. An auxiliary fired boiler is normally used toproduce any extra steam required and to start up thesystem.

Although thermo mechanical pulping produces betterquality fibers and better paper than mechanical pulping,the quality is still very low because the fibers are mixedwith lignin and are badly broken up during the process.This paper is used to produce low quality corrugatedboard, building papers and used for the production ofroofing papers.

Chemical Pulping

In chemical pulping, a digester is used. The digesterconsists of a pressure vessel into which the wood chipsare placed. Cooking liquor is added to the digester alongwith the chips. The cooking liquor contains chemicalswhich dissolved the lignin, causing the wood chips tobecome softer than with heating alone. Once filled withwood and liquor, the digester is heated and pressurized byaddition of steam. The pressure in the digester forces the

liquor to penetrate into the wood chips more quickly, andthe high temperature increases the reaction between theliquor and lignin.

There are many types of digesters. Batch digesters arefilled with liquor and chips and then closed up andpressurized with steam for the cooking process. Once thecooking is completed, the bottom valves on the digesterare opened, and the pressure in the vessel forces the pulpslurry and liquor out into a blow tank. Another processuses continuous digesters like the one shown in figure 3.The digester consists of a series of horizontal tubes whichare fitted with internal screw conveyors. The chips are fedinto the top tube by a pressurized feeder.



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Figure 3 Continuous Digester

Cooking liquor is also added at the top and the mixture isheated by direct injection of steam. The screw conveyormoves the chip plug along the tube until it reaches theopposite end where it drops into the second tube. Thescrew in this tube moves the chips along to the other end,and drops into the next tube, and so on, until finally thechips are dropped from the last tube to be discharged fromthe unit into a defibrator.



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In another design, chips are fed into a vertical tower at thetop and moved downward through the tower as the stockis drawn off the bottom. Heated cooking liquor is added,along with the chips, and steam is injected at variouslevels to maintain the temperature and pressure in the

vessel. This is the most common type of digester used inmodern chemical pulping and is called a Kamyr digester.

Chemical pulping consists of several different processes initself. The process varies according to the type of chemicalused and the pressure, temperature and retention time inthe digester. Some common pulping processes that aredependant on the cooking chemical used are:

Kraft process

sulfite process

bi-sulfite process

soda process

This document will concentrate on the Kraft process, whichis by far the most common in Canada. The Kraft process isan improvement over the earlier soda process which is stillin use in some mills. The soda process uses sodiumhydroxide to dissolve the lignin in the wood chips to breakthem down. Sodium hydroxide is very active in attackingorganic material and tends to attack the cellulose as well

as the lignin. To reduce the degradation of the cellulose,the cooking times are usually short being from 5 to 20minutes in the digester. To further protect the cellulosefibers, soda ash is added to the cooking liquor. Soda ashis sodium carbonate.

Later research determined that sodium sulphide was betterat protecting the cellulose fibers from attack by the sodiumcarbonate. Addition of sodium sulphide allows cookingtimes of up to 8 hours, to ensure that the lignin is welldissolved and allow the wood chips to fall apart easily.



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Kraft Pulping

In Kraft pulping, the digester is filled with wood chips andcooking liquor which is called white liquor. White Kraftcooking liquor is made up of water, cooking chemicals and

a number of dead load chemicals, such as salt and spentliquor solids. For this discussion, only the active chemicalswill be considered. As mentioned earlier, these activechemicals in the Kraft pulping process are sodiumhydroxide (NaOH) (also called caustic soda) and sodiumsulphide (Na2S). In the digester, the cooking chemicals areoxidized and lignin is dissolved from the wood. As a result,the white liquor used in the digester is changed as thedigester reaction continues.

The cooking takes place under pressure and at high

temperature. This causes the liquor to soak into the chipsin a process called impregnation. After the cooking time iscompleted, the mixture of spent cooking liquor and chips isdischarged from the digester into a blow tank and nodefibrator is required. Since the liquor is at saturationtemperature in the digester, when the mixture isdischarged and the pressure dropped, some of the waterin the liquor flashes into steam. This process causes thefibers to be blasted apart as they pass from the digester tothe blow tank. The water which is flashed off the liquorforms a stream contaminated steam. This steam has

heating value and is often recovered and used in someheating process.

In the pulping process, toxic gases are formed which arevented off with the flash steam, or dissolved in the spentliquor. These are called non-condensable gases. Thesenon-condensable gases produce pollution and must becontained. This is discussed in more detail later in thissupplement.

The slurry discharged from the digester is pumped to thebrown stock washers. This consists of the wood cellulosethat is to be used for making pulp or paper and the spentliquor used in the digesters. The liquor contains thedissolved lignin from the wood which gives it a dark brown



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color. At this point, the liquor is called black liquor. Thebrown stock washers consist of screens, where the liquoris drained from the pulp, and the pulp is washed to removeresidual chemicals. The black liquor is then pumped to therecovery plant (usually part of the power plant), where the

chemicals are recovered. Since the liquor is only about 10to 15% solids (lignin and chemical) and 85 to 90% water, itis called weak black liquor.

Paper Making

The chemical recovery process is the major topic in thissupplement and will be discussed in detail. But first a briefdescription of the paper making process is included. Paper(or market pulp) is made by processing the pulp fiberswhich are washed at the brown stock washers.

Figure 4 Paper Making Process



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The figure shows a typical paper making process for ableached paper product made from 100% virgin fiber.Virgin fiber is cellulose, that has been supplied by chippinglogs and breaking down the chips in digesters (or bymechanical means) as discussed earlier. Much of the

paper produced today is a mixture of virgin fiber andrecycled fiber. Recycled fiber is obtained from paper that isre-pulped using a large pulper which operates on aprinciple similar to a blender. This process breaks thepaper down into pulp once again and new paper isproduced from this pulp. Paper is produced with between 5and 100% recycled fiber.

As shown in the figure, raw logs are chipped and stored inlarge chip bins before going to the digesters. The treesthat are used to produce the logs can affect the paper

since the cellulose of trees differs from one species toanother. Often various species are blended to produce thedesired paper qualities.

In the digesters, white liquor is mixed with the chips, andthe mixture is cooked under pressure by direct injection ofsteam. In addition, heat exchangers are often used to heatthe liquor that is circulated through the digesters. This isimportant to the Power Engineers because the steam issupplied to the digesters from the power plant. Steam thatis injected into the stock inside the pulpers cannot be

recovered as condensate. As a result, a lot of makeupfeedwater is required for pulp and paper mill boilers. Inaddition, the liquor heaters present a severe contaminationhazard. Usually, the steam used in the liquor heaters isreturned as condensate to the power plant, however, thiscondensate must be constantly monitored to ensure that itis not contaminated by the liquor chemicals. When it is, thecondensate must be dumped immediately.

White liquor, used in Kraft process digesters has sodiumhydroxide and sodium sulphide as the active chemicals.The sodium hydroxide, also called caustic soda is added(or recovered) at the recausticizing plant (recaust).



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The pulp stock produced in the digesters, is dischargedinto the blow tank where the pressure is reduced fromdigester pressure to about atmospheric pressure. Thestock is then pumped to the brown stock washers, wherethe black liquor is separated from the pulp fiber and the

fiber is washed. The weak black liquor produced at thewashers is pumped to the recovery plant.

Washed pulp is then screened in the brown stock screenroom. Screening removes fiber clumps and knots that arenot completely digested in the digester process. In additionto the screens, the pulp is passed through cleaners wheredirt particles and fiber fines (to small to be used for paper)are removed. At this point in the process, the pulp is brownin color. This color is due to the fact that some residuallignin remains on the pulp fibers. Pure cellulose is very

white in color. Cotton from cotton plants is actually purecellulose that has leaked out of the plant.

If the paper mill produces brown paper such as that usedfor the manufacture of boxes and bags, then the pulp isused as is comes from the screen room. If the paper millproduces white paper, however, the stock is passedthrough a bleaching process where the remaining lignin isremoved.

Bleaching is really an extension to the digester reaction

where more lignin is removed using different chemicals.Many chemicals are used for bleaching including oxygen,chlorine, sulphur dioxide, sodium hypochlorite etc. Thebleaching process must be carefully monitored because itcan cause the production of toxic materials resulting inpollution.

The stock is heated in the bleach plant also. This heatingis done by injecting steam into the bleaching towers, andtherefore, the condensate is not recovered. In some cases,heat exchangers may be used for heating bleachingchemicals and some condensate may be returned.

After bleaching, the stock goes to the bleached screenroom, where it is screened again and passed through alarge number of centrifugal cleaners, to remove fines thatare produced, as a result of the bleaching process.



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The pulp stock is now ready to be used in the productionof paper. It is normally screened on thickeners to removeexcess water to reduce storage requirements. Thedewatered stock is then stored in the high density chestuntil required by the paper machine. The best way to

describe how paper is produced is to say that hugequantities of water are added to the pulp fiber in stagesand then the water is removed in stages as the sheet isformed and dried. Water is added as the pulp stockapproaches the paper machine, until the consistency isless than 1% stock fiber in 99% water. This lowconsistence slurry is then pumped through a final set ofpressure screens to the paper machine headbox.

Figure 5 Paper Machine

The paper machine is the heart of the paper makingprocess. This is a huge machine with several majorsections and is operated by highly skilled paper makers. Atypical paper machine is 7.5 meters wide and 100 meterslong. There are many different types of paper machine butthe majority have the same major sections, although eachsection many have various configurations.



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The first section of the paper machine is the formingsection which starts at the headbox. Low consistencystock is pumped to the headbox. The headbox is usually aclosed chamber designed to keep the stock in suspension,and stop the fibers from collecting into lumps (flocking). A

thin opening along the edge of the headbox allows thestock to flow (squirt) out along the entire width of themachine. This opening is called the slice, and isadjustable to ensure the same amount of stock exits theheadbox, along the entire width so that the sheet formed isconsistent thickness (caliper).

The stock is discharged from the headbox onto a formingwire. The forming wire is traveling along the machine andas the stock squirts from the headbox, and lands on theforming wire, the water passes through the wire. This

results in a mat of pulp fiber building up on the wireforming the initial sheet of paper. There are many types ofpaper machine formers. The one shown in this figure iscalled a Fourdrinier, which is the most common but isbeing superseded by new types of formers. Most of thenewer formers use more than one wire, and the headboxdischarges into the nip between two moving wires. Thisforms a more consistent paper sheet.

Often paper is produced in layers, where several smallformers are used to produce thin layers of paper, one on

top of the other until the required thickness is achieved.These layers are formed on separate wires travelingaround cylinders and are called cylinder machines. Few ofthese are made today but many still exist. One advantageto a cylinder machine is that different fibers can besupplied to each layer. For example, the bottom layers canbe cheaper brown fiber, and the top layer can be bleachedstock for better printing characteristics. This results in apaper that is brown on one side and white on the other likemany cereal boxes.

Fourdrinier machines can also produce paper in layerswith one or two wires above the bottom wire, each with aseparate headbox.



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Figure 6: Twin Wire Former

This figure shows a twin wire former which is used toproduce a two ply sheet. A second forming wire ispositioned above the bottom wire, and a thin sheet isproduced on each forming wire. These two sheets thencome together to form a consolidated sheet, whichcontinues along the bottom forming wire until it reachesthe suction roll (couch roll) at the end of the formingsection. Many other configurations using several formingwires are also produced but are beyond the scope of thiscourse.

Note that there are several devices used to assist inremoving water from the sheet as it moves along theforming wires. First the water flows naturally through thewire, but soon a fiber mat builds up which restricts thewater flow. Then, foil boxes are used with foils, which actagainst the bottom of the wire forming a very slightvacuum to assist water flow from the sheet. After the foilboxes are flat boxes. The flat boxes are connected tovacuum pumps. The vacuum in the flat boxes furtherassists in removing water from the sheet. As the sheetproceeds down the wire, the vacuum in each consecutive

set of flat boxes is increased. The greatest vacuum is inthe roll at the end of the forming section called the couchroll.



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As you can see, a lot of water is drained from the stock onthe former. Many paper machines produce 1000 tons ofpaper a day or more. If the consistency at the headbox is1%, it means that 100 tons of stock at the headboxproduces 1 ton of paper. To produce 1000 tonnes of

paper, 100 000 tonnes of stock passes though theheadbox and therefore, 99 000 tonnes of water is removedon the paper machine. About 50% of this is removed in theforming section as liquid water. This means that over 4000cubic meters of water is added to the stock per hour, as itapproaches the paper machine. Most of this water is thendrained off at the former.

If fresh water was used in paper making, a tremendousamount of water would be required, and a huge volume ofeffluent would be produced. To avoid this, the water

drained from the former is recovered and added back intothe stock approaching the headbox. This water is alsofilled with additives and fines, so that recycling the waternot only reduces water usage, but also recovers thechemicals and fines. In a bleached paper mill, the finesgive the recycled water a milky appearance and the wateris called white water. In brown paper mills the recycledwater is brown but is still called white water.

There is a limit as to how much water can be drained fromthe sheet in the forming section, and the sheet still has

about 55% moisture content when it leaves the former.This moisture content must be reduced to about 5% for thefinished paper product. Further, water removal is achievedin the press and dryer sections of the paper machine.



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Figure 7 Paper Machine Press Section

The sheet is picked up off the former and transferred to thepress section. Since the paper machine is running atspeeds of 500 meters per second, or more, the speed ofeach section of the machine must be carefully controlled to

avoid slack or tearing the sheet. Normally the sheet istransferred to a felt on the press. In the example shown inthe figure, the press has two sets of press rolls. The papersheet is sandwiched between two press felts passingthrough each press, and carried by the felts through thepress nips. Hydraulic pressure acts to force the press rollstogether and squeeze water from the sheet. The water istransferred through the press felts and into holes drilled inthe press roll covers. As the rolls rotate the water is thrownclear.



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The press felts are equipped with high pressure showersand vacuum boxes, that clean and remove excess waterfrom the felts on a continuous basis. Pressing removesanother 5% or so of the water from the sheet, and alsohelps to consolidate the sheet by pressing the fibers

together to form a better bond. This makes the sheetstronger and easier to transfer from one section of themachine to the next. After the press section, the remainingwater is removed from the sheet in the dryers.

Figure 8 Paper Machine Dryers

The longest section of the paper machine are the dryers.In most machines, this consists of a series of rotating steeldrums which are heated using steam. The sheet leavesthe press section and passes around the first dryercylinder. In this example, the sheet passes around the

bottom side of the first dryer cylinder. The sheet thenpasses around the top of the second cylinder and bottomof the third and so on. The sheet weaves its way throughthe dryer around all the cylinders, until it reaches the endwhere the moisture content has been reduced to thedesired value.



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Each of the dryer cylinders is heated with steam. As thesheet passes around each cylinder, it is heated by the hotsteel of the cylinder. This causes the water to beevaporated from the sheet as is moves along the dryers.On most of the dryer cylinders a felt is used. The felt

assists in pressing the sheet against the cylinders andhelps transfer heat. As the water evaporates, airsurrounding the dryer cylinders becomes very moist. Asyou know from air conditioning theory, when the humidityreaches 100% no more water will be evaporated.Therefore, the dryer is supplied with warm fresh air andmoist air is continuously vented. Most dryers are enclosedat the top, or completely enclosed by a dryer hood wherethe atmosphere is carefully controlled.

Steam from the power plant is admitted to the rotating

dryer cylinders and condensate formed in the dryer isdrawn off and returned to the power plant. The dryers usea large volume of steam. If the sheet is 50% water when itenters the dryer and 5% water when it leaves, one tonneof paper will have about 950 kg of water evaporated from itin the dryers. If the machine produces 1000 tonnes ofpaper per day, 950 tonnes of water must be evaporated.This is done by transferring heat from steam which iscondensing. Therefore, approximately 950 tonnes ofsteam is required (this various according to steampressures in the dryer cylinders). This paper machine

would consume about 40 000 kg of steam per hour.

The condensate produced by the paper machine dryers isclean, and can normally be returned to the power plant foruse as feedwater.



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Figure 9 Calendar and Reel

After the dryer, the paper is finished if it is a simple papersuch as used for newsprint, boxes, bags etc. or if it is to besold as pulp to other mills. If the machine is used toproduce fine paper such as writing paper, the sheet iscoated with various materials which are used to fill in voidsto make the paper smoother and to give it a white surface.

In any case, after drying is completed, the sheet usuallypasses through a calendar and scanner and is then rolledup onto a large roll at the reel. This roll is called a jumboand is a completed product except that it is too large toship to customers.

The calendar is used to give the sheet the desired surfaceproperties and caliper. There are many types of calendars.Some have heated or cooled rolls, and some usedcontrolled crown rolls that can apply more pressurebetween them, at various points across the machine tocontrol sheet caliper. Some calendars have several rollsand the sheet passes through several nips between theserolls. The type of calendar used depends on the qualityand type of paper to be produced, as well as technologyavailable at the time of production.



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The scanner consists of an analyzing head that movesback and forth across the sheet and scans it continuouslyfor such things as caliper, moisture content, color, orwhatever the desired characteristic of the finished sheetare. The scanner may provide information only for the

operators, or it may control various parts of the machinesuch as the headbox slice, steam to the dryers ormoisturizing steam showers.

The purpose of the reel is to continuously wind up thesheet as it comes out of the dryer. The reel includes amethod of cutting the sheet, and changing the roll core to anew roll, while the machine is operating at full speed. Todo this an empty core is brought up to the speed of thejumbo, and then the sheet is cut and transferred to thenew core. In the figure, a new core has just been started,

and the full jumbo roll is allowed to roll away from the reelto make room for rotating the new core onto the rails.

The finished paper is now ready to be cut into smaller rollsor sheets and sent to the customers. The jumbo roll ishoisted off the machine by a crane and transferred to awinder, if it is to be cut into smaller rolls. If the jumbo is tobe cut into sheets, it is transferred to a sheet cutter calleda layboy.

This discussion has provided a brief description of the

paper making process. Remember that there are manyvariables that have not been discussed, depending on thetype of paper to be produced, and the type of fiberavailable. No two paper mills are the same although manyof the components are similar.

Mill Power Plant

The power plant in a paper mill supplies many of therequirements of the mill. These include:

steam for process

electricity, whether generated on site or purchased

compressed air for process and for instrumentation

water for the process, fire fighting, and for drinking(as well as boiler feedwater)

cooking liquor for the digesters



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In addition to these utilities, the power plant is usuallyresponsible for the effluent treatment plant.

The production of steam, electricity and compressed airhave been discussed in other parts of this course and will

not be discussed here. Also, effluent treatment and waterprocessing is discussed in other areas. The remainder ofthis supplement deals only with the production andrecovery of cooking chemicals in a Kraft paper mill.

Figure 10 Digester



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In the Kraft process wood chips are soaked for severalhours in white liquor. During this period, the digester isheated and pressurized with steam at about 1000 kPa.Chemical reactions which take place in the digester, as thelignin is dissolved, cause the chemicals in the white liquor

to be oxidized forming different compounds. As discussedearlier, the active chemicals in white liquor are sodiumhydroxide (NaOH) also known as caustic and sodiumsulphide (Na2SO4). The sodium hydroxide is oxidized tosodium carbonate (Na2CO3) in the digester, and thesodium sulphide is oxidized to sodium sulfate (Na2SO4).

If a new chemical was used in the digester all the time, thecost of chemicals would make the Kraft process tooexpensive. Also, there would be a huge amount of spentchemical to dispose of. Therefore, it is essential to the

process that the chemicals used in the digester arerecovered and used over and over. In other words, thesodium carbonate and sodium sulfate in the black liquormust be converted back to sodium hydroxide and sodiumsulphide respectively. In order to convert these chemicals,they must first be separated from the other constituents inthe black liquor which are mostly water and lignin. Inaddition, dirt and other contaminants that get into thechemicals must be removed.

There are two major processes which the black liquor is

subjected to in order to convert it back into white liquor forreuse in the digesters. The first of these takes place in therecovery boiler and the second takes place in therecausticizing plant (recaust). The recovery boiler is theresponsibility of the Power Engineers in a paper mill andthe recaust usually also falls under the power houseoperations and may or may not be operated by PowerEngineers. These processes are critical to the operation ofthe paper mill and are also hazardous areas in which towork.



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Evaporation

The first step in converting black liquor to white liquor is toremove the water from the black liquor. This is done invarious types of evaporator and then in the recovery boiler

furnace. The evaporators remove most of the water fromthe liquor to increase the solids concentration to at least60%. At this concentration, the remaining lignin in theblack liquor can be burned in the recovery furnace.Burning the lignin from the black liquor works to removethe lignin, and provides the heat required to evaporate theremaining water from the liquor, and also causes thechemicals to melt. The molten chemical flows from thebottom of the recovery furnace as a smelt.

It is very critical that the solids concentration of the black

liquor be 60% or more before the liquor is burned. If thesolids are lower than this, the excess water causes thefurnace to be cooled too much and the liquor will not burnproperly. More importantly, the water in the liquor must beevaporated in suspension because if any water reachesthe molten smelt at the bottom of the recovery furnace, asmelt-water reaction can occur. Smelt-water reactionsmake recovery boiler operation very hazardous. Asmelt-water reaction is a steam explosion that can becatastrophic. Many early recovery boilers were destroyed,and operators killed by recovery furnace explosions due to

smelt-water reactions.

When a small amount of water leaks onto the molten smeltbed, it can be caused to flash into steam very rapidly. Thisrapid flashing means that suddenly the small volume ofwater is turned into a very large volume of steam. Thesudden increase in volume results in an explosion similarto dynamite. This explosion alone can destroy a recoveryboiler but also often results in failure of the boiler, itselfresulting in a subsequent boiler explosion.

Since the liquor has water in it, and the boiler tubessurrounding the recovery furnace are filled with water,extreme care and maintenance must be exercised withrecovery boilers to ensure that water does not reach thesmelt in the furnace.



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Figure 11 Evaporation



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The weak black liquor is evaporated before firing to formstrong black liquor which is about 50% solidsconcentration. The strong black liquor then passes througha second evaporation process, to produce heavy blackliquor at 60% solids or higher for firing in the recovery

boiler. Evaporating the water from the liquor begins inmultiple effect evaporators. Multiple effect evaporators arelarge heat exchangers (usually shell and tube), which usesteam to evaporate water from the liquor. After the multipleeffect evaporators, the second evaporator stage variesfrom paper mill to paper mill.

Traditionally, hot flue gases from the recovery boiler wereused to concentrate the liquor to 60%. In this process, theliquor is sprayed into the flue gases and heat, in the fluegases, is used to evaporate water from the liquor. This has

the added benefit of scrubbing the flue gases, removingsolids that are carried over form the furnace. However, thehot flue gases also evaporate some volatile matter fromthe liquor which is carried with the flue gases to the stack.This volatile organic matter contributes to the smell of aKraft paper mill and results in added pollution. Therefore,these direct contact evaporators are no longer installedalthough some remain in use.

There are two major types of direct contact evaporator:

cascade evaporator

cyclone evaporator

The cascade evaporator consists of a rotating wheelsimilar in appearance to the paddlewheel on an old ship.The horizontal slats, however, are actually flattened outtubes. This wheel is suspended in a vat filled with strongblack liquor so that the wheel is about 40% submerged. Amotor rotates the wheel slowly. The strong black liquorcoats the tubes, and is lifted from the vat by the rotation ofthe wheel. The liquor then cascades back into the vat.Flue gases from the boiler economizer pass through theupper portion of the cascade evaporator. The flue gasespass around and through the tubes of the wheel, cominginto direct contact with the cascading strong black liquor.



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A large amount of chemical called saltcake carries overfrom the furnace with the flue gases. The saltcake isre-dissolved in the strong black liquor. Heat in the fluegases also causes the water in the strong black liquor tobe evaporated. The combination of evaporation and

re-dissolving chemical increases the concentration of theliquor from about 50% to over 60% thus forming heavyblack liquor. The flow of liquor from the cascadeevaporator is controlled to maintain the desiredconcentration.

From the cascade evaporator, the liquor is pumpedimmediately to the burners. Because of its concentration,the liquor will become very viscous and difficult to removefrom pumps and piping, if it is not kept hot and in motion.When shutting down, the evaporator, pumps and piping

must be flushed out immediately using steam and water.

The second common type of direct contact evaporator isthe cyclone evaporator. Cyclone evaporators are largevertical vessels with conical bottoms. Strong black liquor issupplied to the cyclone evaporator from the multiple effectevaporators. Pumps located at the bottom of the cyclonedraw liquor from the cyclone and pump it to the top. Theliquor is sprayed through nozzles at the top of the cycloneonto the walls and flows down the walls to the bottom.Another set of nozzles is located at the flue gas inlet to the

cyclone where liquor is sprayed into the entering fluegases.

The flue gases from the boiler economizer enter thecyclone evaporator at a tangent and swirl around theinside of the unit. Saltcake particles in the flue gases arethrown out by centrifugal force, and become imbedded inthe liquor flowing down the walls. This re-dissolves thesaltcake. Heat in the flue gases also evaporates waterfrom the liquor, and the solids concentration is increasedto above 60% forming heavy black liquor. The heavy blackliquor is pumped immediately to the burners.



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Both cascade evaporators and cyclone evaporatorsrequire a lot of monitoring to ensure that they are operatingproperly. Remember that the liquor in the evaporators is afuel, and that the hot gases can have embers, which maycause fires in the direct contact evaporators that are very

difficult to extinguish. Heavy black liquor also causes pluggauges in lines and connections such as instrument fittingsand drains. Steam connections are used throughout thesystem to heat and blow lines clear as necessary.

Modern paper mills and refitted mills use low odourrecovery boilers, and concentrate the liquor to 60% insteam heated evaporators called concentrators orcrystallizers. These use heat exchangers similar to themultiple effect evaporators, but are designed to handlehigh liquor solids concentrations that tend to scale up

conventional multiple effect evaporators.

Multiple Effect Evaporators

The first stage of evaporation in the multiple effectevaporators involves several stages of evaporation.Typically five or six evaporator stages or effects areused. Steam is used to heat the liquor in the first effect.The steam causes the liquor in the first effect to boil.Steam produced by boiling liquor in the first effect is thenused to heat the second effect causing the liquor to boil in

this vessel also. The steam produced in the second effectis used to heat the third effect and so on.



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Figure 12 Two Evaporator Effects(Falling Film)

This figure shows the first two evaporator effects from atypical a set of six. Each effect consists of a vertical shelland tube heat exchanger with a chamber at he bottom.Black liquor is stored in the lower chamber of eachevaporator effect. A circulating pump draws black liquorfrom the bottom chamber and pumps it to the top of theevaporator effect. The liquor flows out over the top tube



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sheet and down through the tubes of the heat exchangerback to the bottom storage chamber. This is called afalling film evaporator because the liquor flows downwardas a film coating the inside of the heat exchanger tubes.The tubes are not filled with liquor. The liquor coats the

tubes and an empty space is formed inside the ring offalling liquor.

Referring to the figure, the liquor falling downward throughthe tubes in the first effect is heated by live steam thatsurrounds the tubes. The steam is supplied from the papermill low pressure steam system which is normally viaturbine exhaust or extraction. In this example, the steampressure to the first effect is 200 psi. Note that each effectis actually divided into two sections:

steam side (or vapour side) liquor side

Steam fills the steam side of the first effect on the outsideof the heat exchanger tubes. There is no contact betweenthe liquor and steam. As the steam gives up its heat to theliquor it condenses. The condensate formed is drawn offthe bottom of the heat exchanger section of the evaporatoreffect. The liquor circulating through the tubes is heated bythe steam outside the tubes and boils. Since the center ofthe tubes is empty with this type of evaporator, the steam

collects in the center of the tubes and is drawn downwardto the bottom chamber.

The steam formed inside the evaporator is commonlycalled vapour to differentiate it from the live steam on thesteam side of the first effect. In the evaporator shown here,the vapour is drawn off the top of the storage section in thebottom of the evaporator. This causes the vapours to flowdown through the tubes to the bottom. In the bottomsection of the evaporator, the vapours are separated fromthe remaining liquor. The liquor is circulated through theevaporator again and the vapours are drawn off to the nexteffect.



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Note that the pressure on the liquor side of the first effectis lower than the pressure on the steam side. This is veryimportant. If the steam and liquor in the first effect were atthe same pressure, they would be at the sametemperature because they are both at saturation

temperature (one is condensing and the other is boiling). Ifthe liquor and steam are at the same temperature therewill be no heat transfer because heat only travels from asubstance at a higher temperature to one at a lowertemperature. If there is no heat transfer, the liquor doesnot evaporate and no steam (vapour) is produced in thevapour side of the first effect. Therefore, there is no heatsource for the second effect and the whole evaporatorsystem shuts down.

It is very important that a difference in pressure exists

between the vapour side and the steam side in everyeffect, or the evaporators will stop working. The powerengineer must carefully monitor these pressures andensure that this differential is maintained.

The same process takes place in the second effect of themultiple effect evaporators. Vapour produced in the firsteffect is used to heat the liquor in the second effect. Thepressure on the vapour side of the second effect is thesame as the pressure on the liquor side of the first effect.The pressure on the liquor side of the second effect is

lower to ensure heat transfer. This continues from eacheffect to the next until the last effect of the evaporators.Since the pressure drops in each effect, you can see thatin the third and subsequent effects, the pressure will bebelow atmospheric pressure.

The vapour produced in the last effect of the evaporatorsgoes to a surface condenser. The surface condenser isanother heat exchanger which uses water in the tubes tocondense the vapours from the last effect of theevaporators. This, in turn, heats the water slightly which isnormally stored for use in the paper mill processes.Condensing the large volume of vapour from the last effectinto water in the condenser creates a vacuum. Thisvacuum maintains the low pressure in the last fewevaporator effects. The lowest pressure in the system is inthe condenser which is typically about 680 mm Hg(27Hg).



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The only place that live steam is used for heating in theevaporators is the first effect. The steam is condensed inthe first effect to form clean condensate, which can bereused as boiler feedwater as long as it is notcontaminated. The vapour produced in each effect and

used to heat the following effect is also condensed. Thisvapour is not clean, however, as it contains volatile gasesfrom the liquor, as well as liquor droplets carried over fromthe previous effect. The condensate drawn off all exceptthe first effect is called contaminated condensate, orprocess condensate or combined condensatedepending on the mill. Contaminated condensate is alsodrawn off the condenser, where the vapours from the lastevaporator effect are condensed.

The contaminated condensate is collected and used for

the production of white liquor as discussed later in thissupplement.

The volatile organic material in the liquor which is drivenoff in the evaporators do not condense in the followingeffect. These non-condensable gases would soon fill theevaporator heat exchanger, and cause the pressure toincrease stopping operation of the units. Also, anynon-condensable gases in the steam to the first effectmust be vented. Since the first effect is heated with cleansteam and is above atmospheric pressure, the

non-condensable gases can be vented to atmosphere.However, the remaining effects have toxic non-condensable gases which cannot be vented toatmosphere.

A line from each effect leads to a non-condensableheader. The non-condensable header in turn leads to thecondenser where the lowest pressure exists. Thenon-condensable gases are, therefore, drawn off eacheffect to the condenser. A non-condensable valve isprovided in the line from each effect. This valve can beadjusted to control the volume of non-condensable gasesdrawn off the effect. This valve is used to control thepressure in each effect to some degree.

The non-condensable gases are drawn off the condenserby ejectors, and sent to the power boiler or lime kiln to beburned.



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The second common type of multiple effect evaporatorcalled a rising film evaporator is shown in the figure below.

Figure 13 Two Evaporator Effects (Rising Film)

In these evaporators, the liquor flows upward through thetubes. In rising film evaporators, there is no storagechamber at the bottom, and the liquor is pumped into theheat exchanger at the bottom and rises through the tubesto overflow into a storage chamber at the top. As a resultthe tubes are flooded with liquor. In these units, the liquoronly passes through the evaporator tubes one time.

Although fewer pumps are required, rising film evaporatorsare more difficult to control, and tend to scale at the pointwhere boiling begins inside the tubes. In a falling film unit,all the liquor is at the saturation temperature and boiling ismore uniform.

In a rising film unit, the liquor is heated as it rises upwardthrough the tubes. At some point the liquor begins to boil.



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This level in the tubes tend to become scaled. Allevaporators require cleaning out at some point. The unitsare frequently boiled out by passing weak black liquor theentire set, and returning it to the weak black liquor storagetank. This is done with little or no steam on and the weak

liquor washes out the strong black liquor. Periodically,most evaporators will require a detailed cleaning wherehigh pressure water is used to hydroblast buildup fromthe tubes, piping, and heat exchangers.

Typical Multiple Effect Evaporator Set

This figure shows a complete set of multiple effectevaporators. These are falling film evaporators and thereare six evaporator effects. All effects are similar to theones illustrated in the figure showing two falling film units.

Figure 14 Typical Multiple Effect Evaporator Set



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Evaporators are numbered according to the flow of steam.Live steam enters the first effect. The steam is suppliedfrom a 280 kPa header in this example. The steam flow isregulated by a control valve to maintain the pressure in thefirst effect. The steam used in the evaporators must be at

saturation temperature. Superheated steam can causescaling problems in the evaporators.

Vapour from the first effect goes to heat the second effectand so on. The flow of liquor through the evaporators canvary. In this example, the weak black liquor from the brownstock washers enters the fourth effect. Some water isevaporated from the liquor in the fourth effect. Liquor iscirculated through each effect by a circulating pump. Thefourth effect circulating pump also supplies liquor to thefifth effect to maintain level in the fifth effect. The liquor is

then transferred from the fifth to the sixth effect.

A separate liquor transfer pump is used to pump liquorfrom the sixth effect to the third effect, and then the liquorflow from the third to the second and to the first effects.Recall that the first effect is at a positive pressure. Theliquor from the first effect flows to a flash tank where thepressure is dropped to about atmospheric. Note that theflash tank vents to the inlet of the third effect, wherepressure (in this example) is close to atmospheric. As theliquor is flashed in the product flash tank more water is

removed. The final product is drawn off the flash tank andpumped to the strong black liquor storage tank.

The steam used in the first effect is condensed. The cleancondensate is then pumped back to the power house forre-use in the boilers. The vapour condensed in the secondto sixth effect and in the surface condenser is calledprocess condensate at this mill. Second effect condensateis pumped to the third effect but from there to the sixtheffect, the process condensate flows by differentialpressure. Since the pressure in each effect is lower, nopump is required. The combined condensate from thesurface condenser also flows by gravity but this is assistedby head pressure. The surface condenser is much higherthan the evaporators so that the condensate flows down tothe condensate pot.



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The condensate pot is at a vacuum of the sixth effect, andthe condensate is pumped from the condensate pot to theprocess condensate storage tank.

Non-condensable gases are produced when the steam

and vapours are condensed, in each effect of theevaporators and in the surface condenser. The NCGs fromthe first effect are vented to atmosphere. NCGs from theremaining effects are directed to a header that carriesthem to the surface condenser where the pressure is thelowest. All of the NCGs, therefore, end up in the surfacecondenser. These NCGs are then drawn off the surfacecondenser by steam operated ejectors. The NCGs mixwith the steam and the mixture is discharged into theejector condenser.

The ejector condenser is used to condense the steam fromthe ejectors, and also any steam carried over from thesurface condenser. There are two ejectors to increase thepressure from the vacuum in the surface condenser toabove atmospheric pressure. The first stage ejector drawsNCGs from the surface condenser, and discharges into theejector condenser where the steam is condensed. TheNCGs are then drawn from the first stage ejectorcondenser, and discharged to the second stage ejectorcondenser (both condensers are in the same shell). Thesteam from the second stage ejector is condensed and the

NCGs are now vented to the lime kiln where they areburned.

The strong black liquor produced in the multiple effectevaporators is about 50% solids concentration. Additionalwater must be removed before this liquor can be burned,but it is difficult to increase the solids concentration furtherin standard evaporators. At this point, the liquor is so thickthat it flows very slowly and any more evaporation on thetubes causes scaling problems.



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Concentrator

To reduce scaling and plugging problems, the strong blackliquor is evaporated to heavy black liquor in a directcontact evaporator as discussed earlier or in a

concentrator (more modern). The concentrator uses steamto evaporate water also, but the design is different than themultiple effect evaporators. The concentrator shown in thefigure consists of an elevated liquor vessel with twocirculating pumps and two heat exchangers. Strong blackliquor from the multiple effect evaporators is pumped intothe suction side of one circulating pump at a time.

Figure 15 Typical Concentrator

The entering strong black liquor is circulated through theheat exchanger on the left side in this figure. The steam isshut off to this heat exchanger, and the circulating liquorhelps to clean the tubes and piping on this side. Strongblack liquor is drawn off the left side of the liquor chamberand flows into the pump suction on the right side. This



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liquor is circulated through the right heat exchange whichis in service. Steam entering the heat exchanger heats theliquor, but it does not evaporate any water inside the heatexchanger. This is because the heat exchanger is underpressure due to the pump. The liquor in the line from the

heat exchanger up to the liquor vessel and in the liquorvessel, produces backpressure due to the head of liquor.This pressure keeps the liquor below its saturationtemperature in the heat exchanger because of thisincreased pressure.

The liquor flows from the heat exchanger to the liquorvessel. The pressure in the liquor vessel is lower causingwater in the liquor to flash off. The flash steam is drawn offand used to heat the second effect. This unit is a two effectsystem although some use only one. The vapour from the

last effect is condensed in a surface condenser. Afterseveral hours of operation, the tubes in the heatexchanger tend to become scaled. This is reduced byswitching over the sides, so that the weaker strong blackliquor is admitted to the left side and the product drawn offthe right. The steam is then shut off the right side andopened up to the left side.

Once the liquor has been evaporated to about 60% solids,it is stored in a conical bottom tank with an agitator toensure it is kept hot and in motion at all times. As soon as

possible the liquor is burned in the recovery boiler.

Heavy Black Liquor System

Heavy black liquor is pumped from the heavy black liquorstorage tanks, or from the direct contact evaporator to therecovery boiler furnace, by the heavy black liquor pumpssometimes called nozzle pumps. The liquor is usuallysupplied to several liquor burning nozzles by a ring headerthat passes around the recovery boiler on the firing floor.



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Figure 16 Recovery Furnace

Liquor nozzles are normally located on two sides of therecovery boiler furnace but may be on all four. The liquor issprayed through the liquor nozzles into the furnace. Thenozzles may be stationary or oscillating units. Althoughoscillating nozzles were used almost exclusively in thepast, stationary nozzles have become more commontoday. The nozzles are easily removed by undoing clamps

and hose couplings to allow them to be removed forcleaning. Cleaning liquor nozzles is a constantmaintenance task as they easily become plugged andscaled up with heavy black liquor and carbon at the tips.

Two refractometers are provided on the heavy black liquorline just before the ring header. These units constantlymeasure the liquor solids concentration. If liquor solids is



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below a critical value (typically 56%) the refractometersprovide a low liquor solids alarm and divert the liquor backto the heavy black liquor storage. This is necessary toavoid a smelt-water reaction which could result from lowliquor concentrations. Two refractometers are used with

automatic purging to ensure accurate measurement at alltimes.

The heavy black liquor is sprayed into the furnace wherethe high furnace temperature evaporates the remainingwater. This results in the liquor solids forming porous ballslike popcorn, that tend to float around the furnace as theorganic matter in the liquor burns. As these liquor particlesbecome heavier, they settle to the bottom of the recoveryfurnace, forming a bed of char that continues to burn onthe floor of the recovery boiler. As the carbon is burned off

the char bed, the chemicals in the liquor are reduced andmelt. The molten chemical forms a liquid on the floor of therecovery boiler called smelt.

The smelt formed is a combination of sodium carbonatefrom the black liquor and sodium sufide. The sodiumsulphide is formed in the recovery boiler furnace fromsodium sulfate. Recall that sodium sulphide is one of thechemicals used in the digesters and supplied in the whiteliquor. This chemical is recovered for re-use in therecovery boiler.

Air is supplied to the recovery boiler furnace by forceddraft fans. The air enters the furnace in stages. Primary airis supplied at the bottom of the furnace around the charbed. This air is necessary to supply oxygen forcombustion, and also to ensure that the char bed is keptaway from the walls of the furnace. Primary air ports arelocated along the furnace walls at the bottom. These portspass through the wall between the wall tubes. If the char isallowed to contact the wall, the cooler tubes will cause thechar to freeze forming a solid mass and blocking theprimary air ports. This situation is called a blackout, andmust be attended to immediately, as the loss of primary airwill reduce recovery boiler performance. Blackouts arecleared by using bars and torches, to break loose and meltthe solids from around the primary air ports.



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During operation of the recovery boiler the primary airports must be monitored and cleaned out continuously bythe operators to ensure that they do not become plugged.Plugging of individual primary air ports contributes toblackouts. Modern recovery boilers often have automatic

port rodders which clear the ports continuously.

The primary air added around the smelt bed does notprovide adequate oxygen to burn all the organics in thechar and, as a result, a large amount of carbon monoxideis produced in this area:

C + O2 CO

carbon insufficient oxygen carbon monoxide

The carbon monoxide produced then reacts with sodium

sulfate in the char, reducing it to sodium sulphide:

Na2SO4 + 4 CO Na2S + 4 CO2

Sodiumsulfate

Carbonmonoxide

Sodiumsulphide

Carbondioxide

This produces the sodium sulphide required in white liquorand removes the sodium sulfate from the mix. The sodiumcarbonate in the black liquor is not changed in therecovery boiler, and simply melts and mixes with themolten sodium sulphide. This mixture forms the smeltwhich is drawn off the bottom of the recovery boiler intothe dissolving tank.

Secondary air is added to the furnace above the char bed,and possibly tertiary air above that, to ensure thatadequate air is added in the combustion zone, tocompletely burn the combustible material in the blackliquor. The secondary and tertiary air ports also assist increating turbulence.

In the dissolving tank, the molten smelt is dissolved in

weak wash from the recaust. This weak wash has alsobeen used to wash lime mud and contains residual whiteliquor chemicals. The solution formed in the dissolvingtank is called green liquor because it gets a greenish colorfrom iron compounds in the liquor. The green liquor is theprimary ingredient used to produce white liquor in therecaust. The flow of green liquor from the dissolving tank



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is controlled to maintain green liquor density withinrequired limits. If the green liquor is too weak, the whiteliquor will be too weak also. The green liquor is drawn offslowly enough, to control the amount of smelt dissolved inthe liquor to maintain this density. The flow of weak wash

into the dissolving tank is then controlled to maintain thelevel.

The smelt flowing from the recovery boiler is very hot andwill burn through steel smelt spouts. To avoid this, thesmelt spouts are usually water cooled. The smelt coolingwater system is very critical because if it fails, the smeltwill burn through the spout and come into contact with thewater remaining in the smelt spout. This can result in adisastrous smelt-water explosion. Smelt cooling watersystems always have a backup water supply, and are

usually gravity fed to avoid failure due to the pumpstopping.

The smelt does, however, flow into the water (weak wash)in the dissolving tank. If a free flow of smelt is allowed, thesmelt-water reaction in the dissolving tank can becomedangerous. To avoid this, the smelt flow must be checkedand kept steady through all spouts. In addition, the flow isbroken up by a jet of steam so that only small droplets ofsmelt contact the water. This steam jet is called a shatterspray. Often a nozzle using recirculated green liquor is

also used below the shatter spray. The green liquor nozzleis sometimes called a fishtail. In addition, the smelt isquickly dissolved by agitators in the dissolving tank.

During normal operation a recovery boiler burns only blackliquor. However, an auxiliary fuel is required for startup ofthe boiler and for burning down the char bed when shuttingdown the boiler. In addition, auxiliary fuel burners aresometimes used to assist in producing steam required forthe process, when the power boilers cannot keep up to thedemand. Burners may also be required to keep thefurnace temperature high enough to ensure goodcombustion and reduction of the liquor.



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Often two sets of auxiliary fuel burners are used. Loadburners are located higher in the furnace, to assist inproducing steam and hearth burners are located low in thefurnace, to increase the furnace temperature and helpburn down the char bed. When shutting down a recovery

boiler, it takes several hours to burn down the char bedand then several more hours for the remaining smelt tocool enough, to allow the furnace to be washed out withwater.

Chemical Recovery Boiler

Chemical recovery boilers are similar to other largeindustrial boilers, but have several construction features, toprovide safety and stability for the combustion of blackliquor and handling of smelt and saltcake.

Figure 17 Chemical Recovery Boiler



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This figure shows a modern single drum recovery boiler,which uses strong black liquor from a concentrator (nodirect contact evaporators). Notice that the bottom of thefurnace is sloped toward the smelt spouts to assist the flowof smelt. The furnace is completely surrounded with tubes

like most modern industrial boilers, but the tubes in thelower section around the char bed have short studs,welded to them. Smelt builds up between these studs, andis cooled by the tubes causing this smelt to freeze, forminga refractory type layer on the tubes, which protects themfrom the erosion and chemical reactions in the char bedarea.

Note that down comers from the steam drum are locatedcompletely outside the boiler. These lead to headers at thebottom, below the furnace floor, which feed the furnace

wall tubes. The water and steam flow upward through thewall tubes (risers), rises back to the steam drum where thesteam is separated. The steam then passes through twopasses in the primary superheater, and then through thesecondary superheater before leaving the boiler. Normally,a desuperheater is located between the primary andsecondary superheater. This controls the steamtemperature, leaving the secondary superheater byspraying water into the steam entering. Cooling the steambefore the secondary superheater instead of after, helps toprotect the secondary superheater tubes.

The flue passes through the superheaters through aconvection section boiler bank. After the boiler bank, is alarge economizer to lower the flue gas temperature whilepreheating the feedwater. Older recovery boilers withdirect contact evaporators have small economizers, sincethe gases are cooled in the evaporator.

The electrostatic precipitators used on modern recoveryboilers are very large. This is to remove the saltcake thattends to be carried over with the flue gases. Since there isno direct contact evaporator to remove these, a largerprecipitator is required. The saltcake is mixed back into theblack liquor in a mixing tank.



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A lot of the saltcake is removed from the flue gases, asthey change direction in the boiler bank and economizersalso. The saltcake falls to the bottom hoppers where it isdischarged into conveyors and carried to a mix tank. At themix tank, the saltcake is dissolved in black liquor pumped

up from the heavy black liquor storage tank.

The induced draft fan is usually located on the outlet of theprecipitator to maintain the precipitator below atmosphericpressure and keep saltcake from blowing out openings.This also reduces the saltcake in the flue gases passingthrough the fan and reduces erosion. The induced draft fandischarges to the stack.

Chemical recovery boilers have two different stacks. Onefor the boiler flue gases and one for the dissolving tank.

The molten smelt running into the dissolving tank results inthe generation of a considerable amount of steam in thedissolving tank. This steam must be vented to prevent overpressurizing the dissolving tank. The steam carrieschemicals, which must be removed before it is vented.This is necessary to reduce pollution and recover thechemicals. A scrubber is used on the dissolving tank vent.The scrubber fan draws vent steam off the main stack andforces it through the scrubber. The steam enterstangentially causing it to swirl around inside the scrubber.This action causes the saltcake particles to be thrown out

of the flow. Weak wash is sprayed into the scrubber todissolve the saltcake and wash it back down to thedissolving tank.

Recaust Flows

The final stage of liquor recovery takes place in therecausticizing plant. Green liquor from the recovery boilerdissolving tank is cleaned to remove carbon and othercontaminants from the recovery boiler and then calciumoxide is added to convert the sodium carbonate to sodiumhydroxide.



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Figure 18 Recausticizing Plant

The green liquor is pumped to the recaust where it entersa clarifier. The clarifier allows heavier suspended mattersuch as carbon to settle out. The green liquor has atendency to form crystals which eventually plug up thelines and pumps. To reduce this problem, two green



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liquor lines are used. Weak wash is pumped through oneline from recaust back to the recovery boiler. Green liquoris pumped through the other line from the recovery boilerto recaust. The weak wash flushes out the line while theother green liquor line is in service. The lines are switched

over daily or whenever plugging problems occur. Thiskeeps the lines clear.

The dregs, which settle out in the clarifier are usuallywashed to remove residual chemicals and then discarded.This is often done in another clarifier called a dregswasher. Water, used to wash the dregs is supplied fromthe evaporators as contaminated condensate. Thisrecovers chemicals in the dregs and the condensate isthen used to wash the lime mud also.

After clarifying, the green liquor is pumped to the slaker. Inthe slaker, calcium oxide (quick lime) is added from the hotlime silo. The calcium oxide reacts with water in the greenliquor to form calcium hydroxide. This is called limeslaking.

CaO + H2O Ca(OH) 2

calcium oxide water calcium hydroxide

As you know from water treatment, calcium compoundsare not very soluble in water and, therefore, the slaker and

causticizers contain a slurry of lime mud and liquor.Constant agitation is used to keep the lime mud insuspension. Once the lime is slaked to calcium hydroxide,it begins to react with the sodium carbonate in the greenliquor. This reaction forms sodium hydroxide and calciumcarbonate. Sodium hydroxide stays in solution but thecalcium hydroxide does not. This is a fine powder oflimestone in liquor. Limestone is added to make up forlosses in the system. Some of these losses result fromlimestone rock forming in the kiln, which is removed at theslaker and discarded. The waste removed from the slaker

is called grits.

Na2CO3+ Ca(OH) 2 2 NaOH + CaCO3

sodiumcarbonate

Calciumhydroxide

Sodiumhydroxide

Calciumcarbonate



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The causticizing reaction takes place in the slaker andcausticizers, and then the mud solution produced ispumped to the white liquor filter. This is now a solution ofwhite liquor and limestone. In many plants, the limestonemud is separated by settling in a white liquor clarifier.

These work fairly well because the limestone is so heavythat it settles rapidly. However, some limestone is carriedover with the white liquor. To prevent this carryover, othermills use white liquor filters like those shown in the figure.These may be pressure filters as shown of unpressurizedopen filters.

In either type of white liquor filter, the limestone slurry ispumped into a large vessel and begins settling. The levelrises in the vessel as the slurry is pumped in. Filter socksextend down through a separator plate near the top. The

white liquor must pass through the filter socks so that limecarried with the liquor is filtered out. The filtered whiteliquor then goes to the storage tank before being pumpedback to the digesters for re-use. In pressurized units, thevessel is closed and as the level rises, air trapped abovethe white liquor outlet is compressed in the top, providing aslight pressure in the vessel. In others, the top is open toatmosphere allowing the air to escape and is not underpressure.

The flow through the mud filter is stopped every few

minutes for a short period. This is done by shutting off theliquor flow into the filter. Stopping the flow allows whiteliquor above the separator plate to run backward throughthe filters, and dislodge the lime building up. In a pressurefilter, this is assisted by the air pocket which expands inthe top of the filter. The mud removed from the filter socksis allowed a short settling time, before the flow is startedagain. Mud settles to the bottom of the liquor filter and isscraped out by a slowly rotating rake assembly.

The mud is filter mixed with condensate and pumped to asecond filter (or clarifier). The condensate washes residualwhite liquor chemicals off the mud. The weak washproduced is then recovered and used to produce greenliquor at the recovery boiler as described earlier.



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The lime mud, like the liquor chemicals, is re-used overand over. Once washed, the mud is pumped to a mud filterwhere the water is removed forming a mud cake. This mudcake then goes through a lime kiln. The kiln is a long steeltube which is lined with refractory. The kiln rotates slowly

and is fired at the bottom using an auxiliary fuel burner.

The incline and rotary motion of the kiln causes the limemud to cascade around the sides of the kiln and slowlymove down the kiln from the mud filter toward the kilnburner. As the lime mud passes through the kiln it isheated. The hot gases evaporate the remaining water fromthe lime mud. Once the water is evaporated, the dry limemud (calcium carbonate) is heated. Heating the lime mudresults in a chemical reaction, that breaks down thelimestone driving off carbon dioxide. This converts the

calcium carbonate into calcium oxide (quick lime). The dryquick lime is then transferred to the hot lime silo where it isstored until required for re-use in the slaker.

CaCO3 + CaO + Ca(OH) 2

calcium oxide calcium oxide carbon dioxide

Recovery Boiler and Recaust Hazards

There are several hazards involved with chemical recoveryin a Kraft paper mill. First of all, are the typical hazards of

working around the boilers which include burns from hotsteam, fuel and water lines, hazards of furnace and boilerexplosions, electrical shock hazards, fall hazards etc. Inaddition to this, two hazards are especially applicable torecovery boilers. These are:

smelt-water reactions

chemical burn hazards

As mentioned earlier, smelt-water reactions occur in andaround the chemical recovery boiler when water comes

into contact with hot smelt. Since water can be leaked intothe boiler, if the liquor solids concentration becomes toolow or when a tube leak occurs, these possibilities must be



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monitored continuously and when suspected, appropriateaction must be taken. Low liquor solids requires divertingthe liquor from the recovery boiler.

Leaking from boiler furnace tubes can be catastrophic.

This must be monitored by all personnel in the recoveryboiler department. If leaking is suspected by anypersonnel, the lead operator must be informed and if it isdetermined that the possibility of a smelt-water reactionexists, emergency procedures must be initiated. Thisconsists of carrying out an emergency shutdownprocedure.

4 th class supplement bc it

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