Pulp & Paper 1
INTRODUCTION There are about 6.5 billion people living on planet Earth. Worldwide paper consumption in this century has increased 4 times faster than population. Paper and paperboard worldwide will reach 400 million tons in 2011 and 640 million tons in 2020 with world population of 8000 million people and per capita consumption of paper and paperboard of 80 kg. There are about 500 Kraft mills and many thousands of other types of pulp and paper mills in the world. India is fastest growing pulp and paper market in the world with growth rate of 10% over one year in per capita consumption, which is expected to grow in future. 2
The Indian paper industry is among the top 15 global players today, with an output of more than six millions tones annually with an estimated turnover of Rs 150,000 million. Projected demand of 13 million tons by 2020. The growth rate of pulp & paper Industry is around CAGR of 7-8 percent. Total Installed capacity of pulp and paper industry is around 9.18 million tones and is expected to increase to 11.5 million tons by 2010-12. Early 1970 s the share of wood, agricultural residues and wastepaper was 84%, 9% and 7% and presently it constitutes 31%, 22% and 47% in comparison to globally 57%, 39% and 4% respectively. 3
Profile of Indian Paper Mills & Global Competitiveness 4
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Technological turning point in the Indian paper industry is given below: Efficient management of depleting resources with sustainable forest productivity through improved resource planning, better plantation technique, good harvest management Conservation of natural resources Effective waste utilization and recycling of fibre Improved cleaning efficiency Better environmental management for reducing or eliminating toxicity Better fibre properties Improved runnability Improved product appeal (Higher Brightness). 6 Odorless mill
PROFILE OF PAPER INDUSTRY Worldwide paper consumption in this century has increased 4 times faster than population. Paper and paperboard worldwide will reach 400 million tones in 2010 and 640 million tones in 2020 with world population of 8000 million people and per capita consumption of paper and paperboard of 80 kg. There are about 500 kraft mills and many thousands of other types of pulp and paper mills in the world. There are about 500 mills in India with 7percent integrated (capacity more than 100 TPD), 30percent medium (capacity between than 50 to 100 TPD), and 70percent small paper mills with (capacity less than 50 TPD). The average capacity is as less as 14 tonnes per day. 7
Advances in Pulp and Paper Industry Mechanical: Thermo mechanical Pulping, Refiner technology Biotechnology Application: Bio-pulping, Bio--bleaching, Enzymatic Deinking, Enzymatic Refining Washing And Screening: Drum Displacer, Pressure Diffuser, Displacement Presses, Combined Deknotting and Fine Screening, High Temperature Screening Before Washing, Reverse Cleaners Bleaching: Instead Of Dioxin Free Pulp, Now, Pulp Are Classified As Chlorine Free (CGF), Chlorine Chemical Free (CCF), Molecular Chlorine Gas Free (MCGF), Non Chlorine Compound ((NCC), Active Chlorine Free (ACF), Absolutely Chlorine Free (ACF), Almost Chlorine Free (ACF), Elemental Chlorine Free Modified (ECFM), Totally Chlorine Free (TCF). 8
Evaporation and Recovery: Agro based paper mills require more energy to recovery of chemicals since it has high silica content. Black liquor losses from mils without recovery pose a serious environmental problem and result in energy inefficiencies. Fibre Modification: Enhancing the opacity of fibre through pigment particles, better fibre loading, pressurized sensitive adhesives (PSAS) Paper Making: High Speed Machine with Sophisticated Instrumentation and control; closed hood and insulation of dry cylinder end; installation of trinip press. 9
INTRODUCTION (India) The Indian pulp and paper industry is over a hundred years old. First mill in the country was commissioned in 1812 in Serampur (West Bengal). Over the years, the installed capacity has grown from a insignificant 0.15 million tones in the early fifties to the present level of 8.3 million tones. 10
Based on the raw material utilized, the paper units can be classified into three broad categories as: Wood based (Bamboo, hardwood etc.) Agro-based (Bagasse, jute, rice & wheat straw) Waste paper based 11
Manufacturing Process A variety of processes are in use in the paper industry depending on the type of raw material used and the end product desired. Kraft (Sulphate) process, Semi-Mechanical process and Sulphite process are the most popular ones. In the Indian pulp and paper industry, the Kraft process dominates the wood/bamboo pulping. Paper making essentially consists of following stages: Preparation of pulp Stock preparation Sheet formation & water removal 12
Introduction India was also the first country in the world to use bamboo as a basic raw material for making paper. Due to limited forest resources, other raw materials like bagasse, straw, jute, etc. were identified and are now extensively used. Waste paper is also being widely used for paper making. Almost all varieties of paper are produced using there materials. 13
The Indian paper industry can be classified into the following six groups of manufacturing units according to Chemical Weekly magazine: 1. Large integrated paper and paperboard units based primarily on conventional raw materials like bamboo, wood, etc., with inhouse pulping facilities. 2. Small paper units based either exclusively on non-conventional raw materials like bagasse, wheat and rice straws, alone or in combination with imported pulp. 14
3. Small paper units based on waste paper. 4. Paper units set up as a part of large integrated sugar complexes for exploiting the waste products like bagasse. 5.The large integrated newsprint manufacturing units. 6. About 400 hand paper producing units, using cotton rags, jute waste, cotton linters, etc. These units are under control of Khadi & Village Industries Commission (KVIC) and produced about 4,000 tons of paper in 1989-90. 15
PULP Pulp is a commercial cellulose derived from bamboo, bagasse, wood, etc. by mechanical or chemical methods. 16
METHODS OF PRODUCTION 1. Groundwood method: debarked wood is mechanically shredded to form fibers suitable for the production of newsprint, toweling, toilet tissues, and cheap paperback books where strength and ease of bleaching are not important. 17
2. Chemical-the cellulose from wood is freed from lignins and other noncellulose ingredients by reaction with chemical reagents. The two important processes are sulfate (Kraft), and sulfite. The non-cellulosic fraction is solubilized with the insoluble pulp consisting of strong fibers of soft texture. These can be bleached to a white or near white. The yield is only about half that from the mechanical groundwood process (40-65 %) but the pulp is the only type suitable for chemical grade cellulose (rayon and cellulose derivatives) and for paper of high strength 18 and/or fine texture.
3. Semichemical-wood chips are given a mild chemical treatment with dilute mixture: sulfite, sulfate, caustic soda, and/or soda ash reagents. The wood is softened sufficiently to allow mechanical separation of fibers without excess power. A high yield of 65-90% with a somewhat better quality than groundwood pulp is the reason for theincreasing interest in semichemical pulp as a substitute for groundwood pulp. The lower yield results from moredrastic chemical treatment, but a better grade of fiber is produced 19
PRODUCTION PATTERN The production pattern for India is not certain as plants are being changed or modified to yield various types of pulp, depending on the raw material and finished production situation. The lack of sufficient raw material is forcing the production towards a pattern of higher yield, poorer quality pulp via mechanical or semichemical. 20
SULFATE (KRAFT) PULP PROCESS 21
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Process Description Digestion of wood-base materials: The newer continuous process will be presented. Logs with bark are debarked by tumbling and rubbing action, then conveyed to chippers where large rotary disks with many heavy knives reduce the wood to 2-5 cm flat chips. The chips are metered via star valve to a deaerator-preheater. After several minutes, the chips are discharged through a rotating tapered plug into the lift line where recirculating digestion liquor at 12 atms transfers chips to the upper soaking zone of the 25-30 m tall 23 digestor tower.
Chips flow down past a series of circumferential screen plates. Cooking liquor is withdrawn as side streams and circulated through external heat exchangers to reheat and control the digestion temperature within the tower. The digestion time and temperature is adjusted so that maximum lignin removal is accomplished with a minimum cellulose hydrolysis and consequent loss of bulk yield. The digested chips are cooled at the base of the tower by injection of cold black liquor. 24
This is to avoid mechanical weakening of fibers from steam explosion of the hot liquor when passed through the blow-down valve. The pulp liquor slurry is passed through the valve to a blow tank where residual heat is recovered in the form of steam which passes overhead with turpentine vapor to the chip preheater. The pulp is filtered to separate black liquor and screened to remove wood knots and other undigested residue. The brown pulp goes either to product finishing operations or to the bleaching plant. 25
Modified Process For Bagasse The sulfate process described for wood-base materials must be modified for bagasse raw material which contains dirt and pith. This latter is the thinwalled, short cells which make poor paper fiber and must be removed. Depithing methods are based on the fact that the fibrous portion of bagasse is much more difficult to break up by mechanical action than the pith. Thus, exposure of bagasse to strong mechanical shredding-grinding action reduces pith to a fine powder while the desired fiber bundles are reduced in size. 26
Wet grinding in a hammer mill, with the water washing pith through the screens, is the preferred process. The other major modification is in chemical pulping. The finely divided structure and chemical make-up of bagasse allows rapid penetration and reaction by alkaline pulping materials. Thus, 5 mins in the digestor are sufficient, providing a basis for high throughput continuous digestors. 27
Bleaching of Pulp Removal of color residue or bleaching may be accomplished by use of one of various oxidizing agents which must be cheap and have a minimum degrading action on cellulose. Traditionally, chlorine-type oxidizing agents were used in a stage-wise operation. However, chlorine bleaching has been found to produce dioxins and other undesirable components of the bleacher effluent. The use of chlorine as a paper mill bleach has steadily been decreasing. Chlorine has been replaced to a substantial extent by hydrogen peroxide. In the modem and improved bleaching stage, hydrogen peroxide is added together with sodium hydroxide, which activates the peroxide. 28
A stabilizer is necessary, however, to maintain the peroxide concentration at effective levels, and sodium silicate is currently the most commonly used agent. A resulting scale that forms on the internal surfaces of the pulp mill equipment, especially on heat transfer surfaces must be removed in periodic downtime Replacing the sodium silicate with an organic agent developed by Dow Chemical Co., can eliminate the scale formation and the resultant downtime for cleaning. 29
Finishing Operations of Pulp If the plant is a captive paper type, the wet pulp is conveyed to the beater operations which is the first step in the production of paper. Dewatering is necessary if the pulp is shipped any distance for use in paper or as chemical cellulose. This can be done in several ways : 1. Hydraulically pressing pulp at 200-300 atms. to form wet lap sheets which can be dried further. 2. Vacuum flash drying of mechanically dewatered pulp to produce a dry fluffy material which can be dried further. 3. Extrusion in form of easily-handled noodles or pellets containing 30-40% solids for conveying short distances or dried to 90% solids for longer range shipping. 30
Paper Products: Definition Paper is defined as matted or felted sheets of fibers, usually cellulosic and generally formed on a fine wire screen from a water suspension. 31
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Types of Paper Products Paper products are classified by use requirements or by characteristics of the paper. Examples are: Wrapping paper- bag paper, grease-proof paper Tissue paper- cigarette, carbon, toilet, towel, napkin papers Book paper- coated or uncoated, lithograph offset, textbook paper Writing paper- bond weight, linen papers Groundwood printing paper- catalog, newsprint, tablet, poster, wall papers Paperboard-heavier, less flexible, laminated paper stock 33
Raw Materials Fibro s Raw Materials Paper pulp-groundwood, bleached and unbleached sulfite and sulfate, semichemical pulps. Choice depends on end uses and blending of various pulps is frequently required to impart proper specifications to end-products with maximum yield from pulping materials. 34
Reuse pulp-paper products, such as newspapers and paperboard, are repulped and mixed with new pulp for paper mill feedstock. This source accounts for 64 % of fibrous starting materials. In view of raw material sources, increased attention should be given to collection and reuse of waste paper. Miscellaneous cellulose pulp-straw, linen, cotton and rags. Specialty pulp-inorganic fibers such as asbestos and glass. 35
Non-fibrous Raw Materials The paper industry is a good customer of the chemical industry. In addition to chemicals used in producing the pulp, a large variety of materials for fillers, sizing, and coatings are required. Inorganic raw materials-clay, talc, titanium dioxide, zinc sulfide, calcium carbonate, calcium sulfate, barium sulfate, alum Organic raw materials-rosin, glue, casein, waxes, glycerol, dyestuffs 36
Methods of Production The commercial method for paper making is to prepare a suitable fiber suspension in water which is fed to a paper machine where the fibers are matted and dried. 37
Preparation of Fiber Suspensions Pulps are water slurried to ½ - ¾ % fiber content by mechanical disintegrators of various designs. These usually consist of rotating drums with knife attachments or rotating stationary disks to produce viscous shear. Such operations are called beating and refining. 38
Fillers are added to the slurry to increase brightness, bulk, flexibility, softness, and weight of finished paper. Coloring agents are also mixed into the slurry before processing. These are generally synthetic organic dyestuffs, but some coloring is done by water-insoluble pigments. 39
Formation of Paper Conversion of fiber suspension into paper sheet incorporates three principal steps : Random arrangement of fibers into a wet web Removal of free water from wet web by wet pressing and compaction of the sheet Progressive removal of additional water by heated rolls 40
Forming a wet web A wet sheet is formed by running 99 ½ % water-fiber slurry evenly onto a moving endless belt of wire cloth at speeds of 50 m/minute for fine paper to 500 m/minute for newsprint. Water drains by gravity, a part is next removed by a pressure roll and then by a suction roll. The screen also has a side-wise shaking motion to give better interlocking of fibers on the mat. 41
The water collected in this section of the machine is known as white water and is reused to obtain maximum recovery of fiber, to conserve water and additives, and to prevent stream pollution. Pressing the wet sheet The wet paper sheet containing about 80% water is fed via felt roll to the press section where water is removed by mild pressure to reduce content to 60-65% water. Bond or water mark, if needed, is formed on the sheet during the pressing. 42
Drying the sheet The sheet from the press section has sufficient strength to carry its own weight. It is passed through smoothing rolls, then a series of steam-heated metal cylinders where heat and moisture are transferred to a felting or canvas belt running on top of the paper. Paperboard is dried directly without a felt. 43
As the sheet leaves the last drying roll with 5-6% H 2 O, it is passed through a final series of pressure or calendering rolls to produce a smooth, well-finished paper. It is wound on large rolls and transferred to the finishing department where it may be cut, coated, and packaged. 44
In designing the roll speed in dryer section, allowance must be made for a progressive stretching as the paper is pulled through the dryer. The tendency of paper to shrink on drying, coupled with the machine tendency to stretch, creates a delicate force balance in machine design. 45
Recent Improvements In Paper Making Increase inwet-strength of Paper Products The sizing of paper with dialdehyde starch and the use of newer types of resin and plastics such as polyethylene, particularly in Kraft-type paperboard, is responsible for the increased use of paper products under varying moisture conditions. 46
Modifications in Paper Machines to Improve Properties of Paper Research is constantly being done to improve the tensile strength of paper. The KIupack process developed by West Virginia Pulp and Paper Company in the USA is one such improvement. It incorporates a rubber belt before the pressing rolls which can be varied in degree of elongation. By changing to a lessstretched condition at the end of the operation, the fibers on the wet web can be compressed and intertwined to give vastly improved strength on the final paper. 47
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Recovery of chemicals Black liquor from the blow tank contains noitsegid eht fo % 98-99 ria dna retaw diova ot derevocer eb tsum hcihw slacimehc fo ymonoce decnalab a edivorp ot dna smelborp noitullop noitarepo. Multiple effect evaporation using 5-6 stages followed by disk evaporators concentrates the liquor from 15-18% solids to the point where combustion can be sustained in a smelting waste heat boiler. 49
This concentration is around 60% solids. Organic carbon burns in the smelting furnace, supplying the necessary heat and CO 2 to produce an inorganic molten slag or smelt. Make-up alkali is supplied via Na 2 SO 4 chemical reaction. The molten smelt falls into a dissolver where it contacts cold H 2 O to yield green liquor solution. The insoluble impurities, such as unburned carbon, are settled out and the clear liquor causticized by adding lime. Filtration removes the calcium carbonate sludge while the filtrate (white liquor) is returned to the digestor. The carbonate sludge is calcined to lime for recycle. 50
Method ph Range Base Acid 1-2 Ca 2+, Mg 2+, Na +, NH + 4 Active Reagent Max Temp ( C) Time @ Temp (hrs) Pulp Yield (%) H +, HSO 3-125-145 3-7 45-55 Long cooking time is necessary to prevent diffusion of SO 2 ahead of the base (avoid lignin condensation, black cook ); weak pulp; acid hydrolysis; tissuepaper Bisulfite 3-5 Mg 2+, Na +, NH + 4 H +, HSO 3-150-170 1-3 50-65 Medium grades paper: newsprint and writing paper Neutral (NSSC) 6-9 Na +, NH 4 + HSO 3-, SO 3 2-160-180 0.25-3 75-90 Good stiffness pulp, high yield, corrugating medium Alkaline 9-13 Na + SO 3 2-, HO - 160-180 3-5 45-60 Very similar pulps to kraft pulping in terms of yield, brightness, bleachability and strength 51