pulp and paper
DESCRIPTION
handoutsTRANSCRIPT
![Page 1: Pulp and Paper](https://reader035.vdocuments.mx/reader035/viewer/2022081822/5695d2291a28ab9b02995918/html5/thumbnails/1.jpg)
PULP AND PAPER
The pulp and paper industry comprises companies that use wood as raw material and produce pulp, paper, board and other cellulose-based products.
PulpPulp consists of wood or other lignocellulosic materials that have been broken down physically and/or chemically such that (more or less) discrete fibers are liberated and can be dispersed in water and reformed into a web.
History
• 2nd Century BC - oldest archeological fragment of paper was discovered in China.
• 2nd century AD - pulp making process is said to have been developed in China as early as the year 105 A.D.
• 13th century - Europe make use of cotton and linen fibers in making paper.; 1st water powered paper mills were built.
• 19th century - fiber crops such as flax, which provided linen fibers were still the primary material source in making paper.
• 1844 - the use of wood to make pulp for paper began with the development of mechanical pulping in Germany by Friedrich Gottlob Keller and the Canadian inventor Charles Fenerty.
• 1867 - J. Roth use sulfurous acid(H2SO4) to treat wood.- Benjamin Tilghman use
calcium bisulfite(Ca(HSO3)2 ) to pulp wood.
• 1879 - the sulfate or kraft process was developed by Carl F. Dahl.
• 1890 – 1st kraft mill started in Sweden
• 1900’s - sulfite pulping had become the dominant means of producing wood pulp.
• 1960 - 1st commercial sulfite pulp mill was built in Sweden.
• Early 1930‘s - G. H. Tomlinson invented recovery boilers which allowed kraft mills to recycle almost all of their pulping chemicals.
Pulp and Paper Process
I. Harvesting treesII. Preparation for pulpingIII. Pulping
• Chemical pulping• Mechanical pulping• Recycled pulp (de-inking)• Organosolv pulping• Alternative pulping methods
IV. BleachingV. AdditivesVI. Producing paperVII. Finishing
I. PULPINGTypes of Pulping Processes
• Chemical• Semi-Chemical• Chemi-Mechanical• Mechanical
Screening
• Screening of pulp after pulping is a process whereby the pulp is separated from large shives, knots, dirt, and other debris.
• Accepts consist of the pulp that has passed through the screens. The accept yield is the yield of accepts.
• Rejects or screenings are the larger shives, knots, large dirt particles, and other debris removed by the screens after the pulping process.
Mechanical Pulping
• Mechanical pulp is pulp produced by using only mechanical attrition to pulp lignocellulosic materials; no chemicals (other than water or steam) are used.
• Light colored, non-resinous softwoods and some hardwoods are often the fiber source. The total yield is about 90-98%. Lignin is retained in the pulp; therefore, high yields of pulp are obtained from wood.
• Mechanical pulps are characterized by high yield, high bulk, high stiffness, and low cost. They have low strength since the lignin interferes with hydrogen bonding between fibers when paper is made and also causes the pulp to turn yellow with exposure to air and light.
• The use of mechanical pulps is confined mainly to non-permanent papers like newsprint and catalog paper. Mechanical pulps constitute 20- 25% of the world production and this is increasing due to the high yield of the process and increasing competition for fiber resources.
• Chemi-thermomechanical pulping is a process where woodchips are pretreated with sodium carbonate,sodium hydroxide, sodium sulfite and other chemicals prior to refining with equipment similar to a mechanical mill.
Chemical pulping
Chemical pulping is a method of producing pulp by combining wood chips and chemicals in large vessels known as digesters where heat and chemicals break down the lignin, which binds the cellulose fibers together, without seriously degrading the cellulose fibers.
THREE TYPES OF CHEMICAL PULPING
•KRAFT PROCESS
•SULFITE PROCESS
•SODA PULPING
KRAFT PROCESS (SULFATE PROCESS)
• Kraft process is a process for conversion of wood into wood pulp consisting of almost pure cellulose fibers.
1. IMPREGNATION
• Common wood chips used in pulp production are 12–25 millimetres (0.47–0.98 in) long and 2–10 millimetres (0.079–0.394 in) thick. The chips normally first enter the presteaming where they are wetted and preheated with steam. Cavities inside fresh wood chips are partly filled with liquid and partly with air. The steam treatment causes the air to expand and about 25% of the air to be expelled from the chips. The next step is to saturate the chips with black and white liquor. Air remaining in chips at the beginning of liquor impregnation is trapped within the chips. The impregnation can be done before or after the chips enters the digester and is normally done below 100 °C (212 °F).
2. COOKING
The wood chips are then cooked in pressurized vessels called digesters. Some digesters operate in a batch manner and some in a continuous process. There are several variations of the cooking processes both for the batch and the continuous digesters. Digesters producing 1,000 tonnes or more of pulp per day are common, with the largest producing more than 3,500 tonnes per day. In a continuous digester, the materials are fed at a rate which allows the pulping reaction to be complete by the time the
![Page 2: Pulp and Paper](https://reader035.vdocuments.mx/reader035/viewer/2022081822/5695d2291a28ab9b02995918/html5/thumbnails/2.jpg)
materials exit the reactor. Typically, delignification requires several hours at 170 to 176 °C (338 to 349 °F). Under these conditions lignin and hemicellulose degrade to give fragments that are soluble in the strongly basic liquid. The solid pulp (about 50% by weight of the dry wood chips) is collected and washed. At this point the pulp is known as brown stock because of its color.
3. RECOVERY PROCESS
• The excess black liquor contains about 15% solids and is concentrated in a multiple effect evaporator. After the first step the black liquor has about 20 - 30% solids. At this concentration the rosin soap rises to the surface and is skimmed off. The collected soap is further processed to tall oil. Removal of the soap improves the evaporation operation of the later effects.
• The weak black liquor is further evaporated to 65% or even 80% solids ("heavy black liquor") and burned in the recovery boiler to recover the inorganic chemicals for reuse in the pulping process. Higher solids in the concentrated black liquor increases the energy and chemical efficiency of the recovery cycle, but also gives higher viscosity and precipitation of solids (plugging and fouling of equipment). During combustion sodium sulfate is reduced to sodium sulfide by the organic carbon in the mixture:
1. Na2SO4 + 2 C → Na2S + 2 CO2
The molten salts ("smelt") from the recovery boiler are dissolved in a process water known as "weak wash". This process water, also known as "weak white liquor" is composed of all liquors used to wash lime mud and green liquor precipitates. The resulting solution of sodium carbonate and sodium sulfide is known as "green liquor", although it is not known exactly what causes the liquor to be green. This liquid is mixed with calcium oxide, which becomes calcium hydroxide in solution, to regenerate the white liquor used in the pulping process through an equilibrium reaction (Na2S is shown since it is part of the green liquor, but does not participate in the reaction):
2. Na2S + Na2CO3 + Ca(OH)2 ←→ Na2S + 2 NaOH + CaCO3
Calcium carbonate precipitates from the white liquor and is recovered and heated in a lime kiln where it is converted to calcium oxide.
3. CaCO3 → CaO + CO2
Calcium oxide (lime) is reacted with water to regenerate the calcium hydroxide used in Reaction 2:
4. CaO + H2O → Ca(OH)2
4. BLOWING
• The finished cooked wood chips are blown by reducing the pressure to atmospheric pressure. This releases a lot of steam and volatiles. The steam produced can then be used to heat the pulp mill and any excess used in district heating schemes or to drive a steam turbine to generate electrical power. The volatiles are condensed and collected; in the case of northernsoftwoods this consists mainly of raw turpentine.
5. SCREENING
• Screening of the pulp after pulping is a process whereby the pulp is separated from large shives, knots, dirt and other debris. The accept is the pulp. The material separated from the pulp is called reject.
• The screening section consists of different types of sieves (screens) and centrifugal cleaning. The sieves are normally set up in a multistage cascade operation because considerable amounts of good fibres can go to the reject
stream when trying to achieve maximum purity in the accept flow.
• The fiber containing shives and knots are separated from the rest of the reject and reprocessed either in a refiner and/or is sent back to the digester. The content of knots is typically 0.5 - 3.0% of the digester output, while the shives content is about 0.1- 1.0%.
6. WASHING
• The brownstock from the blowing goes to the washing stages where the used cooking liquors are separated from the cellulose fibers. Normally a pulp mill has 3-5 washing stages in series. Washing stages are also placed after oxygen delignification and between the bleaching stages as well. Pulp washers use counter current flow between the stages such that the pulp moves in the opposite direction to the flow of washing waters.
Bleaching
• In a modern mill, brownstock (cellulose fibers containing approximately 5% residual lignin) produced by the pulping is first washed to remove some of the dissolved organic material and then further delignified by a variety of bleaching stages.
• In the case of a plant designed to produce pulp to make brown sack paper or linerboard for boxes and packaging, the pulp does not always need to be bleached to a high brightness. Bleaching decreases the mass of pulp produced by about 5%, decreases the strength of the fibers and adds to the cost of manufacture.
SULFITE PROCESS
• Sulfite process produces wood pulp which is almost pure cellulose fibers by using various salts of sulfurous acid to extract the lignin from wood chips in large pressure vessels called digesters.
1. PULPING LIQUOR PREPARATION
2. PULPING
3. CHEMICAL RECOVERY
• Calcium Based
• Ammonia Based
• Magnesium Based
• Sodium Based
Pulping liquor preparation
The pulping liquor for most sulfite mills is made by burning sulfur with the correct amount of oxygen to give sulfur dioxide, which is then absorbed into water to give sulfurous acid. Care must be taken to avoid the formation of sulfur trioxide since it gives undesired sulfuric acid when it is dissolved in water. Sulfuric acid is undesirable since it promotes hydrolysis of cellulose without contributing to delignification.
Delignification – removal of lignin from woody tissue (as by natural enzymatic or industrial chemical processes)
Pulping
• Sulfite pulping is carried out between pH 1.5 and 5, depending on the counterion to sulfite (bisulfite) and the ratio of base to sulfurous acid. The pulp is in contact with the pulping chemicals for 4 to 14 hours and at temperatures ranging from 130 to 160 °C (266 to 320 °F), again depending on the chemicals used.
• Most of the intermediates involved in delignification in sulfite pulping are resonance-stabilized carbocations
![Page 3: Pulp and Paper](https://reader035.vdocuments.mx/reader035/viewer/2022081822/5695d2291a28ab9b02995918/html5/thumbnails/3.jpg)
formed either by protonation of carbon-carbon double bonds or acidic cleavage of ether bonds which connect many of the constituents of lignin. It is the latter reaction which is responsible for most lignin degradation in the sulfite process.
• The sulfite process does not degrade lignin to the same extent that the kraft process does and the lignosulfonates from the sulfite process are useful byproducts.
Chemical recovery
• The spent cooking liquor from sulfite pulping is usually called brown liquor, but the terms red liquor, thick liquor and sulfite liquor are also used (compared to black liquor in the kraft process). Pulp washers, using countercurrent flow, remove the spent cooking chemicals and degraded lignin and hemicellulose. The extracted brown liquor is concentrated, in multiple effect evaporators. The concentrated brown liquor can be burned in the recovery boiler to generate steam and recover the inorganic chemicals for reuse in the pulping process or it can be neutralized to recover the useful byproducts of pulping. The sulfite process can use calcium, ammonium, magnesium or sodium as a base.
Calcium-based
• The earliest process used calcium, obtained as inexpensive calcium carbonate and there was little incentive to recover the inorganic materials.
Ammonia-based
• Ammonia-based processes do not allow recovery of the pulping chemicals since ammonia or ammonium salts are oxidized to nitrogen and nitrogen oxides when burned.
Magnesium-based
• The recovery process used in magnesium-based sulfite pulping the "Magnefite" process is well developed.[8] The concentrated brown liquor is burned in a recovery boiler, producing magnesium oxide and sulfur dioxide, both of which are recovered from the flue gases. Magnesium oxide is recovered in a wet scrubber to give a slurry of magnesium hydroxide.
• This magnesium hydroxide slurry is then used in another scrubber to absorb sulfur dioxide from the flue gases producing a magnesium bisulfite solution that is clarified, filtered and used as the pulping liquor.
Sodium-based
• Sodium-based processes use a recovery system similar to that used in the kraft recovery process, except that there is no "lime cycle".
SODA PULPING
• Soda pulping is a chemical process for making wood pulp with sodium hydroxide as the cooking chemical.
• In the Soda-AQ process, anthraquinone (AQ) maybe used as a pulping additive to reduce the carbohydrate degradation.
• It is a solution for silicate scaling.
DE-INKED PULPING (RECYCLING PROCESS)
• De-inked pulping is a process where used papers are recycled by removing the printing links and other unwanted elements and freed the paper fibers.
• DIP is used as raw material in papermaking. (newsprint, toilet paper and facial tissues grades)
II. BLEACHING
• Bleaching of wood pulp is the chemical processing carried out on various types of wood pulp to decrease the color of the pulp, so that it becomes whiter.
• Bleaching differs depending on the pulping method.
BLEACHING FOR CHEMICAL PULPING METHOD
BLEACHING FOR MECHANICAL PULPING METHOD
BLEACHING FOR RECYCLING METHOD (DE-INKED METHOD)
BLEACHING FOR CHEMICAL PULPING METHOD
• Chlorine and hypochlorite
Chlorine replaces hydrogen on the aromatic rings of lignin via aromatic substitution, oxidizes pendant groups to carboxylic acids and adds across carbon carbon double bonds in the lignin sidechains. At pH >8 the dominant species is hypochlorite, ClO−, which is also useful for lignin removal. The main objection to the use of chlorine for bleaching pulp is the large amounts of soluble organochlorine compounds produced and released into the environment.
• Chlorine dioxide
Chlorine dioxide is sometimes used in combination with chlorine, but it is used alone in ECF (elemental chlorine-free) bleaching sequences. It is used at moderately acidic pH (3.5 to 6). The use of chlorine dioxide minimizes the amount of organochlorine compounds produced. Chlorine dioxide (ECF technology) currently is the most important bleaching method world wide. About 95% of all bleached Kraft pulp is made using chlorine dioxide in ECF bleaching sequences.
• Other bleaching agents
A variety of more exotic bleaching agents have been used on chemical pulps. They include peroxyacetic acid, peroxyformic acid, potassium peroxymonosulfate (Oxone), dimethyldioxirane
Types of Pulps
• Air dry pulp - is the most common form to sell pulp. This is pulp dried to about 10 percent moisture content. It is normally delivered as sheeted bales of 250 kg. The reason to leave 10 percent moisture in the pulp is that this minimizes the fiber to fiber bonding and makes it easier to disperse the pulp in water for further processing to paper.
• Roll pulp or reel pulp - is the most common delivery form of pulp to non traditional pulp markets. Fluff pulp is normally shipped on rolls (reels). This pulp is dried to 5–6 percent moisture content.
• Flash Dried Pulp - This is done by pressing the pulp to about 50 percent moisture content and then let it fall through silos that are 15–17 m high.
III. ADDITIVES
• Besides the fibers, pulps may contain fillers such as chalk or china clay, which improve its characteristics for printing or writing.
• Additives for sizing purposes may be mixed with it and/or applied to the paper web later in the manufacturing process; the purpose of such sizing is to establish the correct level of surface absorbency to suit ink or paint.
![Page 4: Pulp and Paper](https://reader035.vdocuments.mx/reader035/viewer/2022081822/5695d2291a28ab9b02995918/html5/thumbnails/4.jpg)
IV. PRODUCING PAPER (PAPER MAKING)
• The pulp is fed to a paper machine where it is formed as a paper web and the water is removed from it by pressing and drying.
• Pressing the sheet removes the water by force; once the water is forced from the sheet, a special kind of felt, which is not to be confused with the traditional one, is used to collect the water; where when making paper by hand, a blotter sheet is used instead.
• Drying involves using air and/or heat to remove water from the paper sheets; in the earliest days of paper making this was done by hanging the sheets like laundry; in more modern times various forms of heated drying mechanisms are used.
• On the paper machine the most common is the steam heated can dryer. These can reach temperatures above 200 °F (93 °C) and are used in long sequences of more than 40 cans; where the heat produced by these can easily dry the paper to less than 6% moisture.
V. FINISHING
• The paper may then undergo sizing to alter its physical properties for use in various applications.
CALCULATIONS OF WOOD, PAPER, AND OTHER MATERIALS
Wood Moisture Content
Measure of the water content relative to either the total wet weight of material (the green weight of wood) or to the weight of ovendried wood material (the oven-dry basis)
Weight of water in wood = wet weight of wood – ovendry weight of wood
Specific gravity and density
Specific gravity (sp gr) of wood is the oven dry weight of wood divided by the weight of displaced volume of water
Density of a material is defined as the mass per unit volume, mass/volume. For wood, it is customary to take the total mass (or weight) divided by the volume both at the same moisture content.
Pulpwood measurement
Measurements of wood can be based on weight (with moisture content correction to determine a reliable estimate of oven-dry weight), by solid wood volume (for example, the unit), by gross, stacked volume (for example, the cord), or as volume of chips (for example, the unit).
![Page 5: Pulp and Paper](https://reader035.vdocuments.mx/reader035/viewer/2022081822/5695d2291a28ab9b02995918/html5/thumbnails/5.jpg)
Breaking Length
Breaking length, L, is a measure of tensile strength by calculating the length of a piece of material such that it breaks under its own weight
Chemical Concentration
Chemical charge (to a process), percent chemical (on wood or pulp).: The chemical charge is a measure of the weight of chemical used to process (i.e., pulp or bleach) a material.
Kraft Liquor Chemical Calculations
1) Total chemical or total alkali (TA)
The total alkali is the sum of all of the sodium salts in the liquors (as Na2O) that contribute to AA or are capable of being converted to AA in the kraft cycle, specifically NaOH, Na2S, Na2CO3, and Na2SxOy (as Na2O).
2) Total titratable alkali (TTA)
TTA is the sum of all of the bases in the white liquor that can be titrated with strong acid. Generally, it is considered as NaOH, Na2S, and Na2C03 (as Na2O), although small amounts of Na2S03 and other acids might be present.
3) Active alkali (AA)The sum of the active ingredients in the pulping process is known as active alkali
4) Effective alkali (EA)EA is the sum of sodium chemicals that will produce OH during kraft pulping.
5) SulfidityThe ratio of Na2S to the active alkali, expressed as a percent; Increases the rate of delignification, which occurs by nucleophilic action of the hydrosulfide anion (HS) and appears to protect cellulose against degradation
6) Causticity
![Page 6: Pulp and Paper](https://reader035.vdocuments.mx/reader035/viewer/2022081822/5695d2291a28ab9b02995918/html5/thumbnails/6.jpg)
The ratio of NaOH to active alkali, expressed as a percentage; therefore causticity + sulfidity = 100%
7) Causticizing EfficiencyThe causticizing efficiency is the ratio of NaOH to NaOH and Na2CO3. This is a measure of how efficient causticizing is; it represents the percentage of the Na2CO3 from the recovery boiler that is converted back into useful NaOH cooking chemical. A value of 77-80% is typical.
8) Reduction EfficiencyThe reduction efficiency is the ratio of Na2S to the sum of Na^S and Na2S04 in green liquor expressed as a percentage. This is a measure of the reduction efficiency in the recovery boiler.
Calcining Equations
Two equations are used to characterize calcining of lime mud to produce fresh lime.
1) specific energy consumption is an indication of how much fuel is required to process the lime mud and is often reported as Btu per ton of lime.
2) lime availability is an indication of the purity of the lime in terms of available CaO divided by the amount of lime product.
SAMPLE SOLVED PROBLEM:
Given:
• 50 Tons Chips
• 50% Moisture Content
• Liquor Charge to Digester:
![Page 7: Pulp and Paper](https://reader035.vdocuments.mx/reader035/viewer/2022081822/5695d2291a28ab9b02995918/html5/thumbnails/7.jpg)
» 1200 ft3 white liquor
- EA = 13% (alkali charge on OD wood as Na2O)
- Sulfidity = 25.2%
» 1300 ft3 black liquor
• Question: How many lbs./ft3 of NaOH and Na2S were charged to the digester in the white liquor? (assume no chemical contribution from black liquor)
Step 1: Calculate the amount of oven dry wood
50 tons chips • 2000 lbs./ton • 0.5 (m.c.) = 50,000 lbs. o.d. wood
Step 2: Calculate the amount of NaOH and Na2S as Na2O in the white liquor using the EA and Sulfidity numbers
EA = NaOH + 1/2 Na2S = 13% on od wood.
NaOH+ 12Na2S=0.13×50,000= 6500 lbs.
NaOH=6500lbs .−12Na2S
Sulfidity= Na2SNa2S+NaOH
x 100=25.2%
Na2S
Na2S+(6500−12 Na2 S)=0.252= Na2S
0.5Na2 S+6500 lbs
Na2S=0.126Na2S+1638 lbs .
0.874Na2S=1638 lbs .
Na2S=1874 lbs(Na2O)
NaOH=6500lbs−(0.5 ) (1874 lbs )=5563 lbs(Na¿¿2O)¿
Step 3: Convert NaOH and Na2S values from Na2O
Na2O = 62 g/mole or lbs./mole for this exercise
NaOH = 40 g/mole
Na2S = 78.1 g/mole
*these calculations are based on an equivalence in sodium (Na). This means that Na2S and NaOH are equivalents but that NaOH is equal to 1/2 Na2O.
Na2S = 1874 lbs. (Na2O) • 1mole/62 lbs. • 78.1 lbs./mole = 2360.6 lbs.
NaOH = 5563 lbs. • 1 mole/62 lbs. • 2.0 • 40 lbs./mole = 7178 lbs.
So: Na2S = 2360.6/1200 ft3 = 1.97 lbs./ft3
NaOH = 7178/1200 ft3 = 5.98 lbs./ft3
PROBLEMS:
1) A sample of wood with green dimensions of 3 cm x 5 cm x 10 cm weighs 105 g green (wet) and 62 g when oven-dry. Calculate:1. Specific gravity2. MCOD
3. MCGR
4. Oven-dry weight in kg/w?Answers: 0.413; 69.4%; 41.0%; 413 kg/w?
![Page 8: Pulp and Paper](https://reader035.vdocuments.mx/reader035/viewer/2022081822/5695d2291a28ab9b02995918/html5/thumbnails/8.jpg)
2) A sawmill produces 87.3 BDU (one bone dry unit is 2400 pounds of oven-dry wood chips per day with 50% moisture content green basis. (The solid wood specific gravity is 0.44.) The chip bulk density is 10 pounds dry wood per cubic foot. These rail cars have a rated capacity of 30 units. (One unit is 200 cubic feet.) How many rail cars are needed per day. (Give the final result to 3 significant digits.) Answer: 3.49 rail cars.