Pulping And Bleaching (Part - 2) | Chemical Technology - Chemical Engineering PDF Download

DEVELOPMENT IN PULPING AND BLEACHING:  

Driving force in pulping and bleaching technology has been to produce pulp with lower possible lignin content while preserving the yield and strength of pulp. Lowering the bleaching requirements has been instrumental in making it possible to find alternatives to the traditional chlorine based bleaching [Pulliam,1997]. Some of the development has in pulping has been Preimpregnation of chips with lower caustic, high sulphidity liquor, split addition of alkali charge as cook progresses, longer milder, lower temperature cooking. Another development has been oxygen delignification prior to bleaching to produce stronger pulp with higher strength and low bleaching requirements. Use of additives like anthraquinone and polysulphide has been also implemented. Major development in bleaching has been to go for either elemental chlorine or chlorine from bleaching. Oxygen delignification and ozone bleaching has been introduced chlorine free bleaching. Other development has been use of peroxide, peroxy acids, enzyme etc. Process flow diagram of Kraft puling and bleaching process is shown in Figure M-III 2.1.

Year                                   Bleaching Sequences  

1950-1975                      CEH, CEHH  
1975-1990                      CE(DC),O(ED) EOP) (EP), DEDED 
1990 onwards                 (DZ)DED;

                                        (ZD)EDED 
                                        APDED 
                                        QPDED 
                                        D (DC)CO(EO)(EOP) 
                                        P(PO)OZPAAPXX 

C: chlorination, E: alkaline extraction, H: hyochlorite, D: chlorine dioxide, DC; Combination of chlorine dioxide and chlorine, O: oxygen, EO: alkaline extraction under mild condition, EOP alkaline extraction under both under hydrogen peroxide a, P: peroxide, PO: peroxide under oxygen pressure, Q: chelating, Z: ozone, PAA: peroxy acetic acid, Px: peroxymono sulohonic acid X: enzymes, AQ; Antroquinone pulping, PS : poloysulphide Source:  

Figure M-III 2.1: Kraft Puling and Bleaching Process

PULPING OF RICE STRAW, WHEAT STRAW, GRASSES JUTE STICKS 

Pulping of agricultural residues require mild cooking condition as compared to hard wood and soft wood pulping. Various steps involved are.

• Cutting and dusting.
• Cooking batch or rotary spherical or tumbler digester.
  • Bulk density: 130-150 kg/m3
  • Bath ratio: 1:6, % alkali: 8-10%
  • Cooking temp: 135-140 oC.
  • Temp to rise: 135 oC 1hr.
  • Time at: 135-140 oC 2hr.
• Blowing
• Washing and screening, centricleaning,
• Bleaching
• Recovery of chemicals 

Problems in Utilization of Agricultural Residues

• Higher % of silica
• Problem in washing due to slow drainage ,high dilution
• Higher % silica create problem in evaporator
• Longer wire parts due to poor drainage
• Lower strength of pulp
• Low bulk density, seasonal availability, higher pollution load.  

BAGASSE PULPLING  

Due to availability of bagasse from sugar mills number of agro based paper mills are using bagasse. About 10% of the bagasse is available from sugar mills.

Bagasse 60% fibre and 30% pith, 60% good quality fibre, 30% pitch, 10% water

• 0.25 ton of bagasse (dry) per tonne of sugarcane.
• 3 tons of bagasse per ton of unbleached pulp

Bagasse with 50% moisture & about 2-2.5% sucrose & water soluble material 

PROCESS: Process of bagasse pulping involves depithing and pulping of depethed bagasse.

Depithing-Removal pith is necessary before pulping as pith has high ash, difficult to bleach and has little value in paper making. Various depithing processes are dry depthing, wet depthing, and combined depthing. Figure M-III 2.2 illustrates the agro residue, pulping and bleaching process. Pulping washing and screening: cooking of bagasse can be done in batch or continuous digester. Typical cooking conditions for bagasse pulping are: Batch cooking10-12% NaOH, 160 ^oC, 6-7 kg/ash,2 hrs , bleaching requirement 5-6%3- stage Brown stock washing is done which is followed by screening and centri-cleaning.

Figure M-III 2.2: Agro Residue, Pulping and Bleaching 

Detail pulp digesting process is illustrated in Figure M-III 2.3.

Figure M-III 2.3: Pulp Digesting Process \

MECHANICAL PULPING

Mechanical pulping has significant importance in appear making because of the high yield and its requirement in News print. Conventional method of mechanical pulping was ground wood stone pulping which is being still used by many mills. However many new generation mills has gone for refiner mechanical pulping or thermo mechanical pulping. Process flow diagram for mechanical pulping is shown in Figure M-III 2.4.

STONEGROUND WOOD (SGW) 

Primary action in grinding zone involves compression and release of wood as the grit projecting from the pulp stone surface passes of the underneath of the wood. The function of wood as wood, as it is deformed, friction of wood and pulp stone and friction of fluid generates heat which causes softening of the lignins, and resins binding the fibres and also generate vapour pressure which assist in separating the fibre from the wood wall. The act of compression and release of wood fibre flushes fluid in and out of the wood wall thus dividing some of the binding agents and assisting in the release of the fibre from wood. Pulp stone manufacture supply their basic abrasives, silicon carbide, aluminum oxide and modified aluminum oxide. The grinding surface of the pulp stone will be maintained as near constants possible to ensure uniform quality of the pulp.

Motor 5000 h/s – 10000 h/s, Speed of the pulp stone is 35-40 m/sec, 360 rpm Purpose of grinding is to fibre the wood with a minimum damage to fibres and to develop quality required for the end wood, thereby lowest consumption of energy and grinding material and higher production for grinding and desired to minimize the products as the cost the following variables are important.

Wood-species, shape of bolts manufacture of wood.  Stone-Type of abrasives, size of grit, type of pattern, depth of grooves, extent of wear.
Grinder-Type of grinder, specific grinding pressure which is inter related to the load and the rate of wood advance, length of grinding zone, width of stone-grinding area, tangential speed of stone, pulp temp, pulp consistency, shower temperature, depth of emersion of stone in pil. 5-10 ton/day 50-1000 h/sec 20 ton/day 5000 h/sec Silicon carbide, Aluminium oxide sintered together.

Figure M-III 2.4: Mechanical Pulping 

REFINERY MECHANICAL PULPING 

This involves mechanical reduction of chips in a double rotating disc refiner with the use of two refiners in series and gave pulp much stronger than SGW in tensile burst and tear strength. Although soft wood does need any treatment but hardwood need light chemical soak with about 1-1.5%NaOH+ 0.7-1.0 Na2SO3 at a temp of 75-80 0c. 

Refining 

Fiberising of chips was earlier done by rod mill. The chips were fed in a rotating drum at one end, the defilation was achieved by the crushing action of a number of steel rods and the fibres were collected at the opposite end of the drum. The variables in refining: are Speed of rotation, Disk inter distance, load, temperature and consistency 

Refiner Mechanical Pulp (RMP): RMP involves mechanical reduction of chips in a precision double rotating disc refiner with use of two refiners in series. Stronger pulp than SWD in tensile, burst and tear strength and permitted reduction of chemical pulp.

Thermo Mechanical Pulping (TMP): Thermo mechanical pulping consists of refining of chips in pressurized environment with steam and chemicals in gas phase, following by washing, screening, refining, bleaching, Short steaming period with a duration2-3 min at about 110-120 oc. at refining pressure of 10-25 psi in the first stage with second stage refining under atmospheric conditions. Refining is done at temperature of 100-130 oC improved pulp quality long fibre fraction is increased because of softening of the lignin at elevated temperature which losses the bond between the fibres in the wood structure. The usage of small amount of chemical either swelling (NaOH) and or brighter NaHSO3 increase giving high strength and brighter pulp, During TMP less damage to pulp. Various variables are chip quality chip washing, pre-steaming, solute pattern, plate size, speed, consistency.

Power 100-200 HPD/T

SWPG-65-85 HPD/T 

Some of the advantage of thermo-mechanical pulping are

• Complete utilization of the trees.
• Greater flexibility of raw material utilization enabling utilization of cheaper raw materials, as wood stacks, edgings, sawdust, shaving, and other refuse.
• No or very little consumption of chemicals.
• Pulp with good bonding properties and high content of long fibres, improved strength characteristics, improved paper machine runnability, higher production, lower costs.
• Lower shive content and simplified cleaning.
• Good bulk, improved quality and printability.
• Minimum labour cost.
• High yield and high strength resulting in either reduced or eliminated costly chemical pulp, improved quality and lower cost/ton.
• More and better utilization of the available raw material. 
• No air-atmospheric pollution, little water pollution and reduced environmental damage.
• Adaptability to easier control automatic and computer control.
• Good bulk, improved quality and printability.  
  • 1300 KWH/TON    :  stone ground
  • 1475 KWH/TON    : RMP 
  • 1475 KWH/TON    : TMP 

In chemical pulping substances if the middle lamellas chemically dissolved to an extent that makes fiberising possible as that mechanical treatment is more elaborate machinery.

Mechanical pulping give nearly quantitative yield but causes rupture of the fiber walls on fiberising and gives pulp which contain pulping by pressing wood against a revolving grindstone in the presence of water to cool the stone, which become heated by friction.

Stone surface is burred to a certain pattern of grooves before the stone is engaged in production.

Production refiner mechanical pulp involves two basic steps.

• Fiberisation (defibration)- center original wood structure to single structure.
• Fibrillation- reduces the portion 

 Chemi Thermo Mechanical Pulping: This involves short cooking with 1-2 % caustic and sodium sulphate which facilitate the separation maintaining maximum strength characteristic. It gives better strength and bright pulp.

Advantage: Power Shaving, Superior quality of Pulp and Finished Product, permits utilization higher % of mechanical pulp, power shires, minimum labor, high yield, less pollution.

Power Consumptions

1300-1400 kWh/Ton SWW

1475   kWh/Tin RMP

1475  kWh/Ton TMP