Mech 450 Pulping and Papermaking Topic 3 Mechanical Pulping. James A. Olson

Transcription

1 Mech 450 Pulping and Papermaking Topic 3 Mechanical Pulping James A. Olson Pulp and Paper Centre, Department of Mechanical Engineering, University of British Columbia Mechanical Pulping Comparison of Mechanical and Chemical Pulps Debarking Stone Groundwood Refiner Mechanical Pulp Thermo mechanical pulping (TMP) Chemi thermo mechanical pulping (CTMP) Brightening 1

2 Mechanical Pulping Fibres mechanically removed from wood matrix Chemical Pulping Lignin holding fibres together is dissolved Lignin Fibres In addition to fibre removal, fibres are broken and fines (fibres < 0.5mm) are created About 1/3 of pulp mass is in form of fines 2

3 In contrast, chemical pulping produces intact fibres General Parameters Chemical Mechanical Yield Fibre/Wood - Low 40-70% - High 90-98% Cellulose Purity - High - lignin - Low - lignin dissolved remains End Uses - Dissolving pulp - Low quality - High quality paper - High volume paper (e.g. book) (e.g. newsprint) - Reinforcement pkg. - Molded products Raw Material Sensitivity - Low - High 3

4 Quality Parameters Chemical Mechanical Strength - High - fibres intact - Low - fibres damaged Bulk - Low - more and - High - few and less flexible fibres flexible fibres Optical - Dark but bleachable - Bright but hard - Poor light scattering to bleach high - Good light scattering Drainability - Good - long fibres, - Poor - short fibres, few fines many fines Permanence - Good - Poor (optical) Cost Parameters Chemical Mechanical Raw Material - High - low yield - Low - high yield Capital - High - Low Operating - High - Low - becoming (chemicals, energy etc) lower - high for electrical energy Auxiliary - High - Low - for slush pulp (pollution recovery etc) Mechanical pulps are generally used for short-life, inexpensive products, e.g. newsprint 4

5 History Pre-mid 1800 s paper made of rags. 1841, Friedrick Keller inventor 1848 Johan Voith in Heidenheim made first commercial grinder Voith developed Raffineur to break up any course material not properly ground. First success Full plant powered by steam. Paper made with 70% wood (Worlds fair Paris) 1868 Tampella (finish company) started making grinders. 5

6 Debarking Drum Ring Debarking 6

7 Debarking resistance Factors: Species Moisture content Felling season Storage duration Temperature Debarking resistance N/cm^ Jan May Sept Dec Stone Groundwood (SGW) Pulp produced by pressing logs against rotating grindstone Unchanged for 150 years. 7

8 Action of grinder Circumferential speed 30 m/s Grinding pressure 250kPa Grits deform fibre-lignin matrix Repeated visco-elastic deformation creates heat Increased heat in wood Heat softens lignin that s found in between fibres and helps to release the fibres Action of grinder Fibres are peeled back in layers Grits pass over partially removed fibres Develops surface and flexibility of fibres paper strength. Fibres are released Next layer peeled off 8

9 Operating Parameters Species and property of wood Amount of spray water Temperature of spray water Rate of wood feed Pressure applied Speed of grinder Structure of stone Pulp Constituents Shives: fibre bundles (3%) Long, intact fibres (20%) Short, broken fibres (35%) Fines (45%) Flour 30x30 Fibrils 30x1 Dust 1x1 9

10 Pulp Properties Higher strength as more energy applied CSF drops ml as energy applied Brightest of unbleached pulps up to 65 ISO Stone Sharpening Stones wear due to constant highspeed abrasion Ceramic stones Sharpening every 6-14 days Sharpness affects energy and production grits 2.5mm 10

11 Continuous Grinding Pressure Ground Wood (PGW) Higher pressure leads to higher temperatures Softer lignin, easier to detach whole fibres Stronger pulp 11

12 Example The quality of the pulp produced during grinding is dependent on the temperature in the grinding zone. Fibre can be liberated largely intact if the lignin has been softened by temperature, however, if the temperature is too low the fibres will be largely broken or if the temperature is too high the wood will start to darken. Since virtually all of the grinding power is dissipated as heat in the grinding zone, it follows that temperature in that zone is controlled by the addition of shower water. For a given grinding operation, wood, F, and dilution, D, (kg/s) enter the grinder at Tin degrees C. The suspension leaving the grinder at T out and at a consistency, C. Assume that the steam is not formed. Determine the electrical energy applied, E (J/kg), to maintain these outlet conditions. E F D C 12

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14 Mech 450 Pulping and Papermaking Topic 3b Refiner Mechanical Pulping James A. Olson Pulp and Paper Centre, Department of Mechanical Engineering, University of British Columbia Refiner Mechanical Pulp (RMP) Wood chips are comminuted into fibres by bars on rotating and stationary discs 14

15 History 1957 Stora (Sweden) installed a Defibrator raffinator. Bauer shortly after 1963 Both companies modified to operate under pressure to make Thermo-mechanical pulp 1970 s First 100% TMP newsprint 1980 s 2-stage refining and heat recovery 1985 Large refiners 15MW. Chemicals added to further soften lignin (CTMP). Mechanical pulps are replacing chemical pulps 15

16 Chip Handling Wood is typically chipped in a disc chipper Goal is to have a high proportion of acceptable chips 3-16 knives on a disc 4 m diameter 450 m^3 / hr of solid wood Low cutting speed (20 m/s) as pin chips increase with speed Effect of chip size Over size chips Uneven feed in refiner Reduces quality Over thick fraction Contains most of the knots Decreases fibre length and long fibre portion Decreases strength and brightness Fines Fraction Lowers energy consumption Decreases strength, sheet density, brightness and light scattering Creates linting problems and increases shive content 16

17 Chip washing Immersed in a tank fed by a paddle wheel (Sunds). Removes: Rocks, metal, sawdust, bark Adds moisture Raises temperature Chip Screening Chips are passed through a series of screens Oversize: left on screen with 45 mm holes Overthick: left on screen with 7 mm slots Accept: left on screen with 7 mm holes Pin chips: left on screen with 3 mm holes Fines: pass through last screen Overthick chips don t react well to pre-treatments, lower yield Fines and pin chips produce too many shives (not refined) 17

18 Chip Steaming/Preheating Atmospheric type Steam to C Most are pressurized (50kPa to 110kPa over pressure) Objective is to warm chip and equalize the moisture content Can optimize a bit: Higher temperature gives longer fibres, higher tensile Lower temperatures give better optical properties Chip impregnation systems Used in CTMP Processes Compresses chips Water is removed and is high in extractives fed to effluent 4:1 compression ratio or higher Passes chips into a pool liquor containing chemicals Increase moisture content by 6-7% Refining Equipment Disc Refiner 18

19 Self Pressurization Refining imposes cyclic compression of visco-elastic material Generates tremendous amount of heat and steam Dilution required to maintain approx 30% consistency Steam pressure reaches max and flows both ways Can cause blow-back Types of Refiners Single disc, Moving rotor staionary stator 1.7m Dia. 15 MW Double Disc Two counter-rotating discs More power delivered Less energy required per ton Higher shives, less long fibres, (similar to SGW) Twin refiner One rotor, two stators more refining surface Low intensity refining possible 19

20 Refiner size over time Conical Disc Refiners Flat disc section and conical section Increases grinding surface without increasing diameter Power: CD70, 76, 82 uses 15, MW 20

21 Refining Action Chips are preheated to soften lignin Chips hit breaker bars and undergo a series of normal and shear forces Rapid Breakdown in screw feeder, entrance zone and breaker bars section. Fractures along grains, mostly along fracture planes initiated in chipping Match stick size fragments accumulate in refining zone with major axis along tangential direction Match sticks defibred by longitudinal grinding and brooming Fibres form flocs and flow out by steam drag and inertial forces Flocs caught on bar edges and repeatedly compresssed by passing bars. Breakerbars Refining action Fibre development step Fibres undergo cyclic compressions between bars Internally and externally delaminates the fibres Increases flexibility and surface area 21

22 Refiner Segment Design Parameters Width of Grooves and Bars Traditionally the main parameter Wide grooves - narrow bars reduce specific energy consumption in refiner Open volume allows gap to be narrower and can result in lower pulp quality Wide bars / narrower grooves Increase specific energy consumption and improve quality When Volume in groove is reduced steam flow is impeded and axial load is higher and infeed of fibres is more difficult. This can lead to unstable feed Height of the bars Higher the more open the groove volume, the better steam removal Low bar height forces fibres to the plate gap an pulp quality improves. Dam number, height, and placement Forces pulp from the grooves to the plate gap Residence time increases. Hinders steam removal Bar taper and angle When bars form a pumping angle fibre are forced through, lower residence time which reduces energy consumption Thermo-mechanical Pulp (TMP) Pulping carried out in two refiners in tandem First refiner - pressurized with steam (along with pre-steamer) Second refiner is atmospheric Produces longer fibre (stronger paper) and fewer shives (small bundles of fibres) 22

23 Theory Specific Energy Intensity: Number of impacts Intensity of each impact: Specific energy per impact P P E No Load QC E e E A High Intensity I Low Intensity E B N How do we calculate residence time? Force balance on element of pulp F C F F bs r1 r2 4 rp ( r) c( r) () dr v m 2 v 2 dv r r m b 2 c r U () r Cf Ap() r r 2 r 1 dr v 23

24 Operating parameters Refiner speed (increase) Increase intensity at same power Lower energy at same freeness, lower length, and tear Inlet Consistency (increase) Increase moisture content and fibre length Production rate (increase) Reduce energy and lower length and strength Preheating and steaming temperature Not too critical Plate Gap Increases intensity Lead to pad collapse Effect of refining on coarseness Coarseness: Decreasing coarseness support delamination theory Lower coarseness of small fraction indicate they are created from fragments of cell wall Not always evident if we measure coarseness of whole pulp Difficult to measure coarseness of pulp with fines 24

25 Effect of refining on long fibres Effect of refining on fibre width Refining reduces fibre width by removing outer wall material. 25

26 Effect of refining on wall thickness High intensity refining reduces wall thickness more at same energy Outer part of fibre wall is being peeled away Effect of refining on fibre collapse X-section measured by CLSM Collapse index is an indication of fibres ability to form ribbons High intensity process creates more collapsed fibres at same energy Wall stiffness about the same Therefore wall thickness is less for high intensity 26

27 Effect of refining on fibre flexibility Effect of increasing energy plateaus at moderate energies Fibre development is mostly through removal of outer wall Not through internal delamination Comparison of Pulp Properties SGW RMP TMP Energy required (GJ/ton) Freeness Burst index Tear index Breaking length (km) Shive content (%) Long fibre content (R48) Fines content (P100) Brightness (unbleached)

28 Miscellaneous Other Data Typical Production Rate (of one refiner) Typical gap between plates Typical Specific Energy Typical Power to Refiners 300 Bdt/d 800 Bdt/d - modern mm 7 GJ/t MW (27,000 42,000 horsepower train diesel locomotive) Latency Removal After refining fibres are kinked and curled and not suitable for papermaking Lignin cools and holds kinked shape Latency removal straightens fibres Low consistency 30 minutes 90 degrees C 28

29 Latency removal Latency removal result in: Chemi Thermo Mechanical Pulping (CTMP) 29

30 Chemi-Mechanical Pulps To decrease energy cost or to improve pulp quality, chemical treatments are often added to mechanical pulping Pretreatment of chips to lower energy Interstage treatment lower energy, fibre flexibilization Post-treatment fibre flexibilization Sulphonation reactions 30

31 Usual means is sulphonation using sodium sulphite or sodium bisulphite Increased long fibre content 1. increase in tear index 2. increase in freeness Low sulphonate content (0-1%) Softening of middle lamella lignin Improved fibre separation Decreased shive content High sulphonate content (1-2%) Softening of fibre wall lignin Increase in fibre flexibility and conformability Decrease in freeness Increase in breaking length Decrease in specific scattering Pulp Properties RMP fibres broken TMP separated at primary wall, some fibre broken CTMP Middle lamella very soft, almost all fibres separated at M.L. 31

32 Pulp Properties Light scattering reflects fines content Tensile reflects surface area and flexibility of long fibres. Pulp Properties Changes during Refining Strength increase Corresponds to energy increase without cutting 32

33 Alphabet Pulps Many combinations of treatment and pulping processes are possible REINFORCEMENT PULPS PRINTING PULPS MONO PULPS SGW PGW RMP TRMP PRMP TMP LFCMP CTLF TCMP CRMP CTMP OPCO SCMP BCMP UHYBS UHYS HEAVY FRACTIONAL LIGHT HEAVY PURE MECHANICAL CHEMICALLY MODIFIED Effect of sulphonation on Lignin softening temperature 33

34 Effect of yield on fibre stiffness Effect of sulphonation on fibre length 34

35 Effect of sulphonation on tensile Effect of Sulphonation Energy required 35

36 Effect of sulphonation on light scattering Scattering vs Energy 36

37 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 Problem: If lignin not removed, brightness not permanent (reversion, yellowing) Example: BCTMP (Bleached Chemi-Thermo-Mechanical Pulp) 37

38 Screening and Cleaning Pulping process imperfect Small bundles of fibres (shives) remain These must be removed and further refined Mechanical pulping is therefore follows by an elaborate screening system Subject of next lecture (after LC-refining) TMP System 38

39 Process may also include cleaners (hydrocyclones) 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 39

40 Energy balance About 65% of electrical energy can be recovered in this manner RTS results Retention: Short retention in preheater (10-20s). The short time at elevated temperature reduces the brightness losses Temperature: increase pressure to bar Speed: Increase speed to RPM. Decreases specific energy to get same quality of pulp. 15% energy reduction. 40

41 Conclusions Refining characterized by specific energy and intensity Refining removes outer wall material Thin wall, collapsible fibres Smoother, stronger paper Heat softens lignin More long fibres and less fines CTMP softens lignin in fibre wall Even more long fibres, less fines Makes fibres more collapsible at same wall thickness Less fines The end 41