uses of pulp

7/29/2019 uses of pulp

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Background

Paper making 2000 years old, started in

China

World Consumption 300 million tons/year

Highest consumption in the US -

700 lbs/person, followed by Finland

500 paper mills in operation in the US,

10,000 worldwide

Paper Manufacturing

Made from the natural fibers in wood -cellulose

Fibers from both hard and soft woods Softwoods have long fibers, hard woods

short fibers

Long fibers add strength -used for grocerybags and boxes

Short fibers used to make paper smoother

Fibers are often blended to combine

physical properties

Hardwood fibers blended with softwood to

make paper strong yet smooth

Plants other than trees can be utilized in

paper making -straw, cotton, bamboo,

eucalyptus, kenaf have all been used

Over 45% of paper in the US is recovered

for use in the paper industry


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Composition of wood

Wood is about 50% cellulose and 30-40% lignins.

The fibers and xylem tracheids in the wood are almost

pure cellulose.


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The mechanical processIn this process, the wood pulp is ground and the fibers still are mixed with pectinsand lignin. The fibers are short and weak. The resultant paper is not of good qualityand yellows quickly. Today newspapers, catalogs, paper towels, cardboard, cheap

magazines, building boards, etc. are made by this process. None of these is expectedto last indefinitely.The yield of paper compared to the amount of wood used is relatively high, oftenabout 90%. About one fourth of all paper is made by the mechanical process.Made from spruce, fir, some pine and hemlock. These trees have light colored

woods and long fibers.


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The soda processIn 1851, a process in which wood was treated with sodium hydroxide (NaOH) was devised

which dissolved away the lignins from the wood fibers. This process produces a relativelyweak paper. Blotters are usually made by this process. Yields are about 48%. A number ofhardwoods are usually used.Usually uses hardwoods. Aspen, cottonwood, basswood, beech, birch, maple and gum.

The sulfite processThis process was developed in 1857. Solutions of sulfites and sulfur dioxide (sulfurous acid)

are used to dissolve the lignins. The fiber has good strength properties and can be bleachedwith chlorine and calcium hypochlorite. Unfortunately, paper made by the sulfite process hasa high acid content and gradually the paper becomes brittle and disingegrates.The acid doesn't come from the digestion itself, but from acid added later in the processing.Most books printed since 1850 have been printed on this kind of paper. Yields are less than 50%.Mostly softwoods that are low in resin. Spruce, fir, hemlock, tamarack, some pines, but also birchand aspen.

Used for books, bond paper, wrapping paper, tissue, rayon, and to mix with other pulps.


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The sulfate or Kraft processIn this case, the wood is treated with sodium hydroxide and sodium sulfide.This process was developed about 1885. Just about any kind of wood can

be used. Normally the pulp is not bleached. Yields still less than 50%.Conifer woods are commonly employed. The xylem tracheids of thesetrees are longer than the xylem vessels of hardwoods (2.0-4.0 to 0.5-1.5 mm).Tall oil is a by product of pulping. This material contains resins and fatty acids.


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A pulp digester converts wood chips into pulp by cooking a mixture of woodchips and white liquor to a targeted K or Kappa number. The quality of thepulp (high brightness and lower screen rejects) and overall higher pulp yieldare determined by cooking time, cooking temperature, liquor to wood ratio,sulphidity, and wood chip quality.

The Effective Alkali concentration, which is measured in the digester, is usedto control the cooking and white liquor addition processes. Precise control ofEffective Alkali results in higher levels of sulphidity being maintained in theWhite Liquor (25-35%), which in turn ensures that optimal levels of active

chemical concentration for kraft cooking are obtained.

The data for this application note came from a continuous digester, one ofthree digesters in a 1,350 tons/day bleached kraft pulp mill.The Control ObjectiveThe objective is to achieve maximum pulp production at a specified K or

Kappa number with a minimum input of chemicals and energy and aminimum contribution to effluent. Controlling the digester is difficultbecause:

long residence time of pulp in the digester results in a dead time of 2to 4 hours; andchip moisture content can vary by 30% during a day's production.

The Effective Alkali was measured every thirty minutes using an on-lineanalyzer.


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The Control Objective

The control objective is to keep the brightness of the pulp after thechlorination tower at a constant target level, with a minimum ofresidual chemical at that point. This minimizes the use of bleaching

chemicals while obtaining the desired pulp brightness.

Controlling this bleaching process is very difficult because:

the chemical reaction of chlorine with lignin takes 20 to 40 minutesto progress through the chlorination tower; the reaction rate depends on many factors including temperature,

pH, cleanliness and K number of the incoming stock, and the speciesof wood; and the reactivity and thus gain of the process loop is quite different foreach bleaching agent(elemental chlorine or various substitution levels of chlorine dioxide),and plants increasingly employ these variations.

Conventional control for this system consists of monitoring the stockflow and calculating an approximate dose of chlorine. The dosage setpoint is then controlled from the combined brightness and chlorineresidual signals before the chlorination tower.


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The same paper machine line can be used to produce a hundred differentkinds of paper, depending on the kind of pulp, thickness and line speed.

Paper can vary from very thin, high quality "Bible Paper" to very heavy,low quality box or kraft paper used to make 'cardboard boxes'. Typicallymills are broadly separated according to "High Quality Papers" or "Kraft

Mills".Kraft mills make the same paper all the time, with small variations for

basis weight or coatings. Quality Paper mills tend to make a variety ofpapers that mostly serve the printing industry. Newsprint falls someplacein the middle

General Schematic of Paper Making


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The Fourdrinier is the most common paper making machineThe picture shows all of the major sections, but issimplified when compared to most machines in use today.Today's machines are much larger (longer) to allow for fasterline speeds. The Dryer Section may include over 100 steam

cans (drums). Line speed is almost always above 500 feetper minute, and may exceed 1,000 feet per minute.

Machines have also become wider. The average widthexceeds 100 inches
http://www.energysolutionscenter.org/GasIRPaper/Images/PaperFoourdrinierMach.jpg


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Head Box - receives the liquid pulp, with all ingredients ready

to make the paperFlow Spreader - controls the pulp distribution on the TableFourdrinier Table - a perforated conveyor belt that supportsthe pulp solids and allows the water to drain through

Press Section - rollers that squeeze water from the pulpDryer Section - typically steam drums (cans) that progressively

dry the pulp as it turns to paper; the drying section may also includeelectric or gas infrared heaters and convection heating hoods.

Calendar Stack- metal rollers that compress the paper to formuniform thickness; may be smooth or include some sort of patternor texture.

Reel - takes up the paper as it is finished

.

When the paper sheet enters the paper machine Dryer Section it is about 50% water It must be


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When the paper sheet enters the paper machine Dryer Section, it is about 50% water. It must bedried to less than 10% water for a finished product. The most common Dryer Sections includeSteam Cylinders and Air Dyers. Infrared Dryers are most commonly used on coating lines. Dryingoccurs in Three Phases or Zones.

In the Heating Phase the sheet enters at about 100F and is brought up to about 180F. Generally,about the first 5 cylinders are considered the 'Heating Phase'. In the Constant Rate Zone, heat isadded at about the same rate that evaporation is removing the heat. Most of the water is removedin the Constant Rate Zone. The Falling Rate Zone is the most difficult as it must remove the last10% or so of moisture without causing problems related to un-even or over-drying.

Dryer Types


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Steam cylinders are 4 - 5 feet in diameter and slightly longer than the width of thepaper sheet. A typical paper machine has 40 to over 100 steam cylinders, depending onthe line speed; the faster the line speed, the longer the drying section. Typical machinesare as long as a football field and 3 to 5 stories tall. They require 1,275 to 1,575 BTUssteam input per pound of water dried from the sheet.

Steam Cylinders

Dryer Types

Steam Cylinders/Cans/Drums (Conduction)

PaperBoard Paper

Air Dryers (Convection)

Tissue - Yankee DryerMarket Pulp - Flakt Dryer

Air Dryers


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Air DryersAir dryers are direct fired or use steam-to-air heat exchangers to produce a hot airstream that is forced over the surface of the paper. Hoods or 'caps' are used to containand direct the air flow. Air dryers tend to be used on lighter weights of paper, such astissues, and to supplement the drying of steam cylinders.

A Yankee Dryer is a specific kind of dryer used most commonly for Tissue and Towelingmanufacturing that combines a large steam cylinder and an air hood.The Flakt Pulp Dryer is also a special kind of air dryer that is used to dry "market pulp"(Pulp that is made in one location and sold for final paper manufacturing at anotherlocation). A Flakt Dryer is constructed in decks that the pulp serpentines through as hotair is blasted through it by a series of jets.

IR Dryers produce infrared radiation from a hot surface. Different wave lengths areproduced from different kinds of heaters and varies with temperature. How receptive amaterial is to IR energy is a function of its surface reflectance and the IR wavelength.The more receptive the material, the higher the IR system efficiency


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Natural Gas IR Heaters operating at about 1,800F (1,000C) produce an IR wave-lengthbetween 2 and 3.5 microns. This is also the segment that both paper and water have ahigh receptiveness. When the IR energy is absorbed by the surface water, itstemperature rapidly increases and it evaporates. When the paper absorbs the IR, itstemperature also increases and drives out the moisture from within the sheet.

The net effect is a very efficient system that uses over 50% of the available energy to dry thesheet. In some applications the exhaust gas can be recovered to pre-heat incoming air and furtherincrease efficiency.

Locating IR Heaters


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Locating IR Heaters

The diagram shows potential locations of IR Heaters to

accomplish required results and require the minimum amount of

retro-fit work:

The IR AdvantageGas IR heaters produce more drying, in a smaller space, more efficiently, than any other

drying system available
http://www.energysolutionscenter.org/GasIRPaper/Images/IR_HeaterLocations.jpghttp://www.energysolutionscenter.org/GasIRPaper/Images/IR_HeaterLocations.jpg


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Kappa Number: A term used to define the degree of delignification.

Kraft Pulp: The predominant fiber used by the paper industry. It is obtained bycooking wood chips with the chemicals sodium sulfate and sodium hydroxide. Thesodium sulfate is converted to sodium sulfide in the process. It is the sodium sulfidethat is actually the effective cooking agent, but the word sulfate is still used as thetitle.Lignin: A complex constituent of the wood that cement the cellulose fiberstogether.

Pulp: A suspension of cellulose fibers in water.

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uses of pulp