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papr7 - Pulping Overview Lecture 7& 8 PAPR 1000...

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Unformatted text preview: Pulping Overview Lecture 7 & 8 PAPR 1000 Introduction to Pulp and Paper Manufacturer 1 What is Pulping? Pulping is the process by which wood is reduced to a fibrous mass It is the means of rupturing bonds within the wood structure Separation of fiber 2 Pulping Methods Pulping processes are of three principal types: Mechanical SGW RMP TMP Chemical Sulfate or Kraft Sulfite Hybrid Many combinations of both 3 Pulping Methods Mechanical Pulping Fibers are mechanically removed from the wood matrix Chemical Pulping Lignin holding fibers together is dissolved 4 Mechanical Pulping Mechanical action breaks the ML which is more brittle than the fiber wall The fiber wall is left intact, but substantial fiber cutting occurs Chemical composition is not altered very much 5 Mechanical Pulp 6 Mechanical Pulping Most of the constituents of wood retained Composed of Fiber bundles Fiber fragments Some whole fibers Yield high but pulp unsuitable for many uses due to lignin content which stiffens fibers Lignin also results in yellowing of pulp with time Week sheet. Long fiber need to be mixed High energy usage 7 Stone Groundwood Pulp produced by pressing logs against rotating grindstone Fibers are compressed, and loosened Friction creates heat to soften the lignin 8 Refiner Mechanical Pulping Wood chips are broken into fibers by bars on two rotating or one rotating and one stationary discs 9 Therm0mechanical Pulping Pulping carried out in two refines First refiner-pressurized with steam Second refiner is atmospheric pressure Heat softens the lignin makes the fibers easier to separate Produces longer fiber (stronger paper) and fewer shives (small bundles of fibers) 10 Comparison of Pulp Properties Properties Energy required (mJ/kg Burst Index (kPa m^2/g) Tear Index (mNm^2/g) B. Length (km) Shive Content (%) Long Fiber Content Fines Content Brightness (unbleached) SGW 5.0 1.4 4.1 3.2 3 28 50 61.5 RMP 6.4 1.9 7.5 3.5 2 50 38 59 TMP 7.0 2.3 9.0 3.9-4.3 0.5 55 35 58.5 11 Screening and Cleaning Pulping process imperfect Small bundles of fibers (shives) remain These must be removed and further refined Mechanical pulping is therefore followed by an elaborate screening system 12 Uses of Mechanical Pulps Newsprint Magazine Book Tissue Board Specialty 13 Advantages as Compared with Chemical Pulping High yield >90 % High opacity and good printing property Lower capital cost Lower operational cost 14 Disadvantages as Compared with Chemical Pulping Lower pulp strength Lower brightness Lower brightness stability Higher energy consumption 15 Mechanical Pulp Brightening Often desirable to make pulp brighter (whiter) Do not want to remove lignin to keep yield high Use "brightening" chemicals e.g. hydrogen peroxide 16 Energy Recovery Enormous volume of steam produced from heat created in mechanical pulping This steam can be recovered and used for mill process steam e.g. for paper drying About 65% of electrical energy can be recovered in this manner 17 Chemithermomechanical Pulps (CTMP) To decrease energy cost or to improve pulp quality, chemical treatments (alkaline sulfite, sodium sulfite) are often added to mechanical pulping Pretreatment of chips (to lower energy) Inter-stage treatment (lower energy, increase fiber flexibility) Post-treatment (fiber flexibility) 18 Chemical Pulping Chemicals degrade and dissolve the lignin Hemicellulose also removed Mainly secondary wall material remains 19 Chemical Pulping 20 Chemical Pulping Sulfate (Kraft) NaOH +Na2S (white liquor) pH- 13+ Produces highest strength pulp Proven technology for efficient chemical recovery Handles a variety of wood Tolerates bark Sulfite H2SO3 + bisulfite pH-1.5 to 5.0 Produces brighter unbleached pulp Easy to bleach to full brightness Higher yield bleached pulp Easier to refine 21 Kraft Pulping-Cooking Chemicals NaOH (sodium hydroxide) function 1. 2. Swells the lignin De-polymerize the lignin Na2S (sodium sulfide) function 1. Helps in depolymerization of lignin 2. Stops recondesation of lignin 3. Protect cellulose from degradation 22 Kraft Pulping- Digester Batch Digester 1. More reliable operation 2. Greater operation flexibility 3. More efficient recovery Continuous Digester 1. Less energy used -lower steam consumption 2. More compact size 3. Includes diffusion washing 4. Lower ancillary equipment capacity required due to continuous operation 23 Kraft Pulping Cooking Conditions Cooking conditions 1. 2. 3. 4. Temp- 170-180 0C Pressure 8-10 kg/cm2 Time ~3 hrs Liquor to wood (chips) ratio~ 3-4 :1 24 Pulping Terminology Delignification Delignification is the process of breaking down the chemical structure of lignin and rendering it soluble in a liquid Three phase of delignification (1) extraction (2) bulk (3) residual (Fig 7.5) H factor H-factor is related to the rate of delignification Lower H factor indicates an increase in delignification and thus shorter cooking time and increased digester output Function of reaction rate and temperature (Fig 7.4) 25 Pulping Terminology Cont'd Kappa Number The kappa number is a measure of the lignin of pulp; higher kappa numbers indicate higher lignin content Pulp Viscosity The pulp viscosity is a measure of the average chain length (degree of polymerization, DP) of cellulose Dissolving Pulp Dissolving pulp is a low yield (30-35%) bleached chemical pulp that has a high cellulose content (95% or higher) suitable for use in cellulose derivatives such as rayon, cellulose acetate, and cellophane 26 Chemical vs. Mechanical Pulping Chemical Yield Cellulose Purity End uses Raw material sensitivity Low (~45%) High (Lignin removed) Dissolved pulp, high quality paper Low Mechanical High (>90%) Low (Lignin remains) Low quality, High volume paper High 27 Chemical vs. Mechanical Pulping Quality Parameters Chemical Strength Bulk Optical Drainability High (fibers intact) Low (more flexible fibers) Dark but bleachable Good (long fibers, few fines) Mechanical Low (fibers damages) High (less flexible fibers) Bright but hard to bleach Poor (short fibers, many fines) 28 ...
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