paper recycling technology
TRANSCRIPT
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Paper Recycling Technology
Dr. Richard A. Venditti
Dept. of Wood and Paper ScienceNorth Carolina State University
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Dr. Richard Venditti
Faculty member in the Wood and Paper Science Dept at NCSUPhD in Chemical Engineering, BS in Pulp and Paper Science and Chemical EngineeringResearch areas:
Paper recyclingUtilization of forest/agricultural materials for new applications
Classes Taught:Process ControlUnit Operations for Pulp and PaperPaper Recycling (Distance)
Director of Hands On Workshop for Pulp and Paper Basics, co-sponsored through TAPPI, on-campus and in-millTechnical services projects for over 20 companies
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Dr. Richard Venditti: Research Projects in Paper Recycling
The detection of adhesive contaminants, tracking through millsThe changes in fibers upon recyclingAutomatic sorting of recovered papersFlotation deinking surfactantsAgglomeration deinkingScreening phenomena and pressure sensitive adhesivesDeposition of adhesive contaminantsAlternate recycling processes
Ultrasonic deinkingSupercritical carbon dioxide to extract wax from OCC
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Course Outline
The Paper Recycling US IndustryContaminantsEffect of Recycling on Fibers/PaperUnit Operations
Pulping, Cleaning, Screening, Washing, Flotation, Dispersion, Bleaching,
Image Analysis, Deinking ChemicalsSystem Design
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Course Activities
Viewing of the Videos of LecturesBase lectures by VendittiGuest lectures from industry leaders
Reading assignments from Recycled Fiber and Deinking, Book 7 of the Series: Papermaking Science and TechnologyReading assignments: selected research papersHomeworks: 6 assignmentsFinal Project: Literature Review and Research Proposal
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Course Objectives
Broad understanding of paper recycling science and technologyDevelop an expertise in a selected research topic in paper recycling
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Critical Issues in Recycling:
Going deeper into the wastepaper stream => poor quality materialMeeting paper specifications, which are getting more stringentIncreased demands for paper from emerging countriesEnvironmentally benign operations at the same time both of the above are being satisfied
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Recovered fiber, not “Wastepaper”
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Recovered Fiber, not “Wastepaper”
Learning objectivesUnderstand the trends of paper recycling in the industryIdentify the major grades of recovered fiberIdentify the major types of contaminants in recovered fiber
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Global Paper and Board DemandOverall Positive Growth
Slide by: Richard B. Phillips, PhDAdjunct Professor WPS NCSU Source: RISI
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GDP Growth Major World EconomiesChina, Eastern Europe Dynamic Growth Regions
China
EasternEurope
Source: RISISlide by: Richard B. Phillips, PhDAdjunct Professor WPS NCSU
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Paper & Paperboard Consumption vs GDPIncreases with Wealth … Levels Off
Source: 2007 FAO Forest Resource AssessmentSlide by: Richard B. Phillips, PhDAdjunct Professor WPS NCSU
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Asian Supply has Grown to Match DemandLittle Incentive for others to develop export strategy
Source: RISISlide by: Richard B. Phillips, PhDAdjunct Professor WPS NCSU
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Use of Recovered PaperGrowing in China, Europe … flat in USA
Source: RISISlide by: Richard B. Phillips, PhDAdjunct Professor WPS NCSU
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Use of Recovered fiber Flat in USAMoving increasingly to Asia
Source: RISISlide by: Richard B. Phillips, PhDAdjunct Professor WPS NCSU
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Recycled Fiber DefinitionsSecondary Fiber: fibers that have previously been used in a manufacturing process and have been reclaimed as raw material for another process.Pre-consumer waste: any waste, printed or unprinted, generated in the fabrication or conversion of finished paper. Before use by a consumer as a final end product. Post-consumer waste: Paper that has passed through the end usage as a consumer product.Internal broke: off-specification paper that is repulped and used at the same site, not considered secondary fiber.
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Recycled Fiber DefinitionsRecovery Rate (RR)
how much paper is diverted from landfill
Utilization Rate (UR)fraction of recycled fibers contained in paper
Consumed Paper of TonsCollected Wastepaper of Tons%100RR =
duced Paper Proof Tonsat Mills Consumed Wastepaper of Tons%100UR =
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Supply1
(000 tons)
Recovered (000 tons)
Recovery Rate2
1993 91,538 35,460 38.70%
1994 95,718 39,691 41.50%
1995 95,971 42,189 44.00%
1996 94,529 43,076 45.60%
1997 99,557 43,989 44.20%
1998 101,183 45,076 44.60%
1999 105,316 46,818 44.50%
2000 102,810 47,311 46.00%
2001 97,395 46,996 48.30%
2002 98,949 47,645 48.20%
2003 98,016 49,255 50.30%
2004 101,882 50,287 49.40%
2005 99,618 51,272 51.50%
2006 100,198 53,488 53.40%
Source: AF&PA, 2006 RecoveredPaper Annual Statistics
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Source: AF&PA, 2006 RecoveredPaper Annual Statistics
Utilization of Recovered Paper in the US
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Paper Recovered (000 tons)
Paper Landfilled(000 tons)
1993 35,460 38,399
1994 39,691 38,118
1995 42,189 36,118
1996 43,076 34,331
1997 43,989 37,174
1998 45,077 38,329
1999 46,818 41,255
2000 47,311 39,568
2001 46,996 35,894
2002 47,645 37,601
2003 49,255 36,032
2004 50,287 39,107
2005 51,272 36,846
2006 53,488 35,660
Source: AF&PA, 2006 RecoveredPaper Annual Statistics
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Pkging Material Recovered, 2006
Paper 22.9Glass 2.8Metal 2.2Plastic 1.3Other 1.3Total 30.5
(000,000 tons)
Source: AF&PA, 2006 RecoveredPaper Annual Statistics
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Source: AF&PA, 2006 RecoveredPaper Annual Statistics
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Recovered Paper Statistical Highlights, 2005 Edition, AF&PA
Exports and Imports of Recovered Paper
0
2
4
6
8
10
12
14
16
18
1990 1992 1994 1996 1998 2000 2002 2004 2006
Year
Mill
ions
of T
ons
ExportsImports
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Major Recovered Paper Grades
Mixed Papers: mixed papers, low quality office waste, magazines, catalogs, telephone directories, recycled boxboard cuttings, tissue paper converting scraps if mainly composed of recycled fiber, mill wrappers, specialty grades, all other grades not specifiedNewspapers: old newspapers, special news, groundwood computer printout, coated groundwood sections, publication blanks, mixed groundwood and flyleaf shavings
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Major Recovered Paper Grades
Corrugated: old corrugated containers, container cuttings, kraft paper and bags, old solid fiber containers, kraft bag clippings, carrier stock and its clippings Pulp Substitutes and high grade deinking: bleached chemical pulped office papers and CPO suitable for deinking, or as a pulp substitute if unprinted, bleached sulfite and sulfate cuttings including tissue paper converting scrap if predominantly composed of bleached chemical pulp fiber, coated book stock
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Grades of Recovered PaperMixed PaperSuper Mixed PaperBoxboard CuttingsMill WrappersNewsSpecial NewsSpecial News De-ink QualityOver-Issue News]MagazinesCorrugated ContainersDouble Sorted Corrugated New Double-Lined Kraft Corrugated CuttingsUsed Brown KraftMixed Kraft CuttingsCarrier Stock New Colored KraftGrocery Bag ScrapKraft Multi-Wall Bag ScrapNew Brown Kraft Envelope CuttingsMixed Groundwood ShavingsTelephone DirectoriesWhite Blank NewsGroundwood Computer Printout
Publication BlanksFlyleaf ShavingsCoated Soft White ShavingsHard White ShavingsHard White Envelope CuttingsNew Colored Envelope CuttingsSemi Bleached CuttingsManila Tabulating CardsSorted Office PaperSorted Colored LedgerManifold Colored LedgerSorted White LedgerManifold White LedgerComputer PrintoutCoated Book StockCoated Groundwood Sections Printed Bleached Board CuttingsMisprinted Bleached BoardUnprinted Bleached Board#1 Bleached Cup Stock#2 Printed Bleached Cup StockUnprinted Bleached Plate StockPrinted Bleached Stock
Scrap Specifications Circular, Guidelines for ….Paper Stock…By: Institute of Scrap Recycling Industries, Inc.
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DefinitionsOutthrow:
“all papers that are so manufactured or treated or are in such a form as to be unsuitable for consumption as the grade specified”
Prohibitive Material: material in excess of specified maximum make the recovered paper unusable as the grade specified Any materials that may be damaging to the equipment
News, De-ink Quality #7. Consists of sorted, fresh newspapers, not sunburned, containing not more than the normal amount of rotogravure and colored sections.
Prohibitive Materials None PermittedTotal Outhrows may not exceed ¼ of 1%
Scrap Specifications Circular, Guidelines for ….Paper Stock…By: Institute of Scrap Recycling Industries, Inc.
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Grade-Year At mills
Export/Molded Prod
Total Recov
Total Supply
RR%
OCC-1993 13.5 2.3/0 15.9 26.9 59.1OCC-2006 20.0 5.2/0 25.2 33.0 76.4ONP-1990 3.8 0.9/0.7 5.5 15.9 35.0ONP-2006 5.5 2.5/1.5 9.6 13.2 72.3Mixed-1993 5.3Mixed-2006 12.7 49Pulp Sub-1993 3.3 100Pulp Sub-2006 2.4 100HG DI-1993 3.7 100HG DI-2006 3.6 100
Recovery of Paper and Paperboard (000 tons)
Source: AF&PA, 2006 RecoveredPaper Annual Statistics
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16.4 million t/yr
3.5 million4.2 million
7.0 million
1.6 million
Source: AF&PA, 2006 RecoveredPaper Annual Statistics
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Ctn Bd 15,153NP 0Packg 398Ind CvtPaper Bd 3,946Tissue 167Export 5,218Other 303Total 25,185
(000 tons)
Source: AF&PA, 2006 RecoveredPaper Annual Statistics
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Ctn Bd 1RecPBd 1,172Tissue 698News 3,213P&W 353Other 1,615Export 2,513Total 9,565
(000 tons)
Source: AF&PA, 2006 RecoveredPaper Annual Statistics
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Newsprint 380Tissue 2,680P&W 1,200Other 1,000Export 7,300Total 15,690
(000 tons)
Source: AF&PA, 2006 RecoveredPaper Annual Statistics
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Recovered Paper Prices, $/tonUS Paper Spot Market Prices May 24, 2007Corrugated Container 105Old Newsprint 85Boxboard 71Old Magazines 19Mixed Paper 28Sorted Office Paper 104White Ledger 266White Envelope 370 ScrapIndex.com
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Recovered Paper Prices
0
50
100
150
200
250
300
350
400
450
1970 1975 1980 1985 1990 1995 2000 2005
Year
$/to
n
Sorted White LedgerONPOCCMixed Paper
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Cost of DIP
SystemMOW to deinked, bleached pulp (DIP)Flotation (2), washing, oxidative bleaching, complementary processesProduce 200 ODTPDYield = 67%
Total Capital Cost Installed= $42 MM$6 MM/yr depreciation for 7 yearsMOW Cost =$220/ton delivered
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Cost of DIP: Variable CostsDescription Quantity Unit Unit Cost, $ Contribution,
$/tonFiber 1.5 ton 220 330NaOH 0.024 ton 400 9.6Peroxide 0.024 ton 600 14.4Sodium Silicate 0.022 ton 500 11.0Flotation Aid 0.0027 ton 1000 2.7Sludge Dewater Polymer 0.00115 ton 1200 1.4Clarifier Flocculant 0.00014 ton 1600 0.2Clarifier Bentonite 0.00085 ton 1600 1.4Electricity 310 kWhr 0.045 14.0Steam 790 kPa 0.25 mton 6 1.5Waste Water Treatment 1.236 m3 0.53 0.7Waste Disposal 0.984 mton 36 35.4Process Water 4.8 m3 0.53 2.5Total Variable Prod. Cost 424.8
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Cost of DIP: Fixed Costs
Fixed Costs $ Per Year $ / ton
Maintenance 4,000,000 57
Labor 2,000,000 29
Operating Materials 1,700,000 24Depreciation (7 yr straight line) 6,000,000 86
Business Overhead 1,100,000 16
Total Fixed Costs, $/ton 211
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Cost of DIP: ROI
Total Costs, $/ton produced 636Total Costs per year, $ 44,532,716Selling Price of ton DIP, $/ton 600Income, $/yr 42,000,000Profit before taxes, $ -2,532,716Taxes (25%) -399,537Profit after taxes, $ -2,133,179
ROI (%) -1
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Paper Recycling ReviewOf the about 100 million tons of paper consumed in the US, ___ % is recycled
Name 3 contaminants in recovered paper: _________________ _________________ _______________
Name 4 major categories of recovered paper ____________ ___________ ___________ ____________
____________ is a recycling operation used to remove large contaminants
____________ is a recycling operation used to remove heavy contaminants
Name the two major de-inking operations: _________ and _________
The largest variable production cost in paper recycling is typically _____________True or False. Basically, all recycle systems have the same # and type of unit operations.The yield of useable fiber
A. Is about 100% for all systemsB. Ranges from 20-80% depending on the systemC. Ranges from 60-94% depending on the systemD. Is 50% for all systems
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Common Contaminants in Waste Paper
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Common Contaminants in Wastepaper
Large Junkmetals: nuts, screws, foil, cansplastics: films, bags, envelopesdirtcloth, yard waste, leather, etc.,
Inks & tonersStickiesCoatingsFillersPapermaking additives
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Inks & Toners
Type Component Drying System Ink Resin Film Particle Size(microns)
End Products
Simple LetterPress
Pigment +Mineral Oil
Absorptioninto Web
Weak 1-15 LetterpressEarlyNewsprint
Newsprint andOffset
Pirgment +Soft Resin &Mineral Oil
Penetration ofVehicle intoWeb + ResinOxidation
Soft Film
Hard Film
2-30 Newsprint,Books
We Offset,Letter Press
Rotogravure Pigment+Hard Resin &Solvent
SolventEvaporation
Hard Film 2-250 Magazines,Catalogues
Flexographic Pigment +Resin & WaterEmulsification
AmineAbsorption,Evaporation
WaterResistant Film
N/A Newsprintinserts,Corrugated
UV Cured Pigment+Monomer
UVPhotopolymerization
Non swelling,Non saponif.Hard Film
50-100 High SpeedCoatedPapers
Specialty VariousPgiments andRosins
Heat set orOther
Hard,CoherentFilms
40+ XerographyLaser PrintersElectronicForms
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StickiesCurrently the most challenging problem in paper recyclingStickies: contaminants in pulp that have the potential to deposit on solid surfacesTypically organic materials:
Man made stickies: adhesives, coatings…Natural stickies: pitch, resins..
May deposit on papermachine wires, press felts, dryer fabrics, calendar rolls and cause significant down-time on machineAre hard to remove in recycling due to often having a neutral density, and an ability to flow and change shape
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Coatings as a Contaminant
Coatings typically contain inorganic fillers and polymeric binders
Coating binders can behave as stickies, sometimes termed white pitchCoating fillers are washed out of the pulp and lower the overall yield of the recycling process
Board is often coated with waxWax can cause recycled paper to be weak and slipperyWax can deposit on machinery
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Fillers as a Contaminant
Fillers are washed out of the pulp and lower the overall yield of the recycling process
Common fillers:– clay– CaCO3– TiO2
Fillers are not desirable in some paper products such as tissue paper
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Papermaking Additives
Additives such as starch, gums, retention aids, etc., are lost during recycling and lower yieldAdditives such as dyes can cause the fibers to not make color specifications, this is also a problem with fluorescent dyesWet strength additives cause the paper to be unpulpable in many cases, making the entire paper product unusable and thus, a contaminant
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Contaminant Removal
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Contaminant Size vs. Removal Efficiency
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Recovered fiber, not “Wastepaper”: Review-Quiz
True/False: The percentage of paper that is recycled overall is about 50% The four major grades of recovered paper are:
True/False: Basically contaminants are similar in shape, strength, density, etc.,
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Effect of Recycling on Fiber Properties
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Effect of Recycling on Fiber Properties
Learning objectivesUnderstand how recycling affects chemically pulped fiberUnderstand how recycling affects mechanically pulped (lignin containing) fibers
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Effects of Recycling on Chemical Pulps
Mechanical damageLoss of finesHardening and stiffening (hornification)Weakening of bonding
less fiber conformabilityless absorptiveloss of hemicellulose
Decreased cleanliness
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Effect of Chemical Pulping
Fiber wall structure as in the tree. Chemical pulping removes lignin from cell wall
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Hornification: Irreversible
A Wet kraft fiber before dryingB 30% consistency (Irreversible processes begin to occur)C 30-75% consistencyD >75% consistency
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Hornification: Irreversible
Never DriedFiber
Fiber SwollenWith Water
Dried CollapsedFiber - Hornified
Rewetting Does not Re-swell
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Effects of Recycling on Chemical Pulps
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Effect of Recycling on Chemical Pulps
Chemically pulped fibers that have never been dried: have the ability to swell with water which makes the fibers flexiblecan be mechanically treated (refining) to increase the flexibility
The flexible never dried fibers are able to conform in the papersheet which increases the fiber bonded area making strong paperWhen dried, the fibers become rigid, termed hornification
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Effect of Recycling on Chemical Pulps
Previously dried fibers, upon exposure to water, do not swell and do not become flexibleThese rigid fibers break upon refining: causing fines When paper is made, the rigid fibers do not conform in the papersheet and the resulting low fiber bonded area produces a weak sheet
Flexible Fibers Stiff fibers
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Effect of Recycling on Mechanical Pulps
1st Use
80 %
20%
2nd Use
70 %
30%
3rd Use
60 %
40%
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Effects of Recycling on Mechanical Pulps
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Effect of Recycling on Mechanical Pulps
Wood containing pulps from mechanical pulping processes have lignin (a three-dimensional crosslinked polymer) which makes fiber walls stiff and deters water swellingThe stiff, somewhat round cross section fibers make paper with low fiber bonded area and weak fiber bondsTherefore, the strength of never dried mechanical pulps is generally lower than of never dried chemical pulpsRecycling may improve properties of mechanical pulps by flatening and flexiblizing the fibers“Different behavior than chemical pulps on recycling”
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Effects of Recycled Fibers on the Papermaking Process
Lower freeness: decrease machine speeds or add drainage aid Lower paper strength: more sheet breaksLow efficiency of chemical additives (fines and anionic trash)Increased depositsDecreased cleanliness
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Paper Strength vs. Number of Times Recycled(Howard and Bichard, 1st Res Forum on Recycling, Oct 1991, CPPA)
0
1
2
3
4
5
6
7
8
9
10
0 1 2 3 4 5
Number of Recycles
Bur
st In
dex
(kPa
m^2
/g)
Bleached Kraft (Chemical Pulp)
Groundwood (Mechanical Pulp)
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Effect of Recycling on Fiber Properties: Review Quiz
True or False: A pulp produced for newsprint using a CTMP process should have almost equal or better strength properties after recycling.
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Paper Recycling Operations
An Example: OCC Recycling
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Paper Recycling OperationsAn Example: OCC Recycling
Learning objectivesIdentify major sub-operations in a typical OCC recycle millUnderstand the purpose and how each sub-operation works
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Why is contaminant removal so difficult?
Contaminates vary bysizedensityshapesurface propertiessolubilitystrength
No single separation device can remove all of the different types of contaminantsThus, recycling processes consist of many sub-operations that complement each other
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Major Recycling StepsPulpingCleaningScreeningDeinking --- used to produce printing or tissue
WashingFlotation
Dispersion and KneadingBleaching --- used to produce printing or tissueWater TreatmentSolid Waste Handling
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Example: An OCC Recycling Process
In the next section, we will take a look at the sub-operations in an OCC recycling processThe process has the following operations:
PulpingHigh Density CleaningScreeningForward CleaningThrough Flow CleaningThickeningDispersionWater Clarification
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Pulping of Recovered Paper
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Pulping of recovered paper
Definition
Pulper : A device whose main objective is to
convert recovered paper into a slurry of well
separated fibers and other waste paper
components.
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Pulping of recovered paper
The pulping operation is the first and probably themost critical operation in paper recycling.
Proper pulping is a requirement if unit operationsdownstream (cleaning, screening, flotation…..) areto be effective.
Incorrect pulping conditions can irreversibly damagefibers making them inappropriate for papermakinguses.
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PulpingMain Function : Disperse recovered paper into separated fibers.
Several sub-objectives that are also important:1. Detach contaminants from fibers.
2. Mix paper with water and chemicals at the correct ratios.
3. Maintain contaminants as large as possible to aid subsequent removal processes.
4. Avoid damage to the fibers (fiber cutting).
5. Removal of large debris from system.
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Basic Pulping Categories : Batch vs. Continuos Pulping
Batch Pulping : The feed recovered paper, water and chemicals are all charged at the beginning of the process and are removed all at once at the end of the process. The batch process is repeated.
Continuos Pulping : The feed recovered paper, water and chemicals are continuously added to the pulper and at the same time, the pulped product is also being continuously removed.
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Basic Pulping Categories : Low vs High Consistency
Low Consistency Pulping: Typicallyfrom 3-6 % K. Produces a relatively easily pumpable fluid. The fluid is“pourable”.
High Consistency Pulping: Typicallyfrom 8 - 18 % K. Produces a thick, slurry that will not flow under the influence of gravity alone.
Consistency (solids) 100 * solids wtsolids wt + liquid wt
= % K
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General Parts of a Pulper
1. Wastepaper feed method (conveyor).2. Pulper tub.3. Rotor - spinning device for agitation, mechanical energy input
to the system.4. Baffles - protrusions to assist in mixing and prevent swirling.5. Dilution water.6. Pulper exit.
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Forces in a Pulper
Mechanical ForcesThese are caused when the fast moving rotor impacts material in the relatively slower body of pulp stock around it.Faster rotor speeds cause more intense mechanical forces in the pulper.
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Pulper Forces
Hydraulic Forces : These are caused by the motion of fluid that is caused by the spinning rotor (not by the direct impact of the rotors).When two adjacent portions of a fluid are moving in different directions ( or at different speeds) a shear force is present. An example in the picture would be at point A.
A
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Forces in a PulperAttritionMechanical shearing forces that occurs between the moving rotor and a static extraction plate near the rotor.The rotor forces fiber bundles between the rotor and extraction plate. Intense hydraulic forces act to cut the fiber bundles and fibers. This can cause significant damage to fibers.Used only for low % K pulping because the pulp must be screenable.
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Low Consistency Pulping
Consistency from 3 -6 %.Low profile rotor that rotates at high speeds.Motion of rotor causes a vortex of pulp stock. The baffles are used to improve mixing.High mechanical force due to impacts of rotor can damage fiber and break contaminants.
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Pulper Types: High Consistency
Typically 8-18 %.High profile rotor used. The helical screw type rotor is needed to “pull down” the non-fluid like high % K stock, from the top to the bottom of the pulper.At the high % K, fiber-fiber (solid-solid) rubbing dominates the forces experienced in the pulper.
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Comparison of Low vs High Consistency Pulping
Rotor/ tank volume is much higher for high % K pulping. This is needed to maintain proper motion of non - fluid pulp stock at high % K. Specific power is higher for high % K due to the higher viscosity pulp stock that must be pulped. However, the specific power consumption per ton of paper is significantly lower for high % K pulping.This is due to high % K pulping having more tons of fiber for the amount of same pulping volume as a low % K pulper. Also, the relatively less amount of water at high % K pulping causes less energy to be expended on moving water.
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Comparison of Low vs High Consistency Pulping
Rotor speed is slower for high % K, causing less damage to fibers via mechanical forces of rotor.Attrition forces are not used for high % K pulping. This decreases fiber cutting and contaminant breakage.RESULTS of above: higher tensile, burst and tear strength for high %K pulpingHigh consistency pulping includes more fiber to fiber rubbing. RESULTS of above: This action increases detachment of contaminants from fiber surfaces. The detachment of ink from fibers is especially important for washing and flotation deinking.
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Comparison of Low vs High Consistency Pulping
Printing and writing grades consist of a high content of fully bleached hardwood and softwood fibers that are susceptible to damage => gentle high consistency pulping is preferredFurther printing and writing grades need ink detachment => high consistency pulping with lots of fiber-fiber rubbing is preferredOCC recycling, a historically older technology, typically has low consistency pulping because unbleached fibers are less susceptible to damage
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Screening and Junk Removal in Pulper
One of the sub- objectives of pulping is to remove large debris that enters the system.
Examples of large debris :wood wet-strength paperplastics baling wirenails and bolts
The removal of debris serves two important functions.Protects equipment downstream from damage.Prevents plugging of downstream equipment.
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Examples of Debris Removal Methods
Different pulpers have different methods to remove debris, examples of common methods follow.
High Consistency Batch PulpingHC pulper with Dilution ZoneHC pulper with Detrasher
Low consistency PulpingContinuos Low consistency pulper with Ragger and junk tower.Continuos Low consistency pulper with a de-trashing system
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High Consistency Batch Pulper with Dilution Zone
The pulper is designed so that during pulping at 15 -18% K the pulper volume is only partially full.At the end of the pulping cycle, dilution water is added to achieve a 5 - 6 % K.After dilution, accepted stock passes through an extraction plate with holes about 3/4 - 1 inch diameter.Finally, large debris is flushed from the pulper through a large rejects opening on the side.
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High Consistency Batch Pulper with External Detrasher
The pulper is “full” at high consistency during pulping.At the end of the pulping dilution water is added at the bottom of the pulper diluting the pulp in the bottom to less than 6 %.A large opening on the bottom/side of the pulper is used as the exit for the pulper contents.The pulp and debris are separated by an external detrasher.Note : There is no extraction plate in the pulper.
Note: drawing not to scale.
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Continuos Low Consistency Pulper with Ragger and Junk Tower
Low consistency continuos pulper typically have an extraction plate that accepts pulped fibers and rejects debris and unpulped flakes.The extraction plate/ rotor can cause attrition, resulting in fiber cutting.A junker is used to collect unpulpables such as bolts or rocks. This debris is thrown out of the pulper into a junk tower where it is removed.A ragger is also used in many cases to remove bale wire, strings, plastics, etc. The ragger is a continuos “rope” formed by entangled debris. The “rope” is continously pulled out of the pulper and cut into sections and disposed. Common in OCC mills.
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Recovered OCC bale storage
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Loading OCC bales on pulper conveyor
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Wire bale cutter
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Pulper conveyor
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Bale falling into pulper
Pulper
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Ragger removing debrisfrom the pulper surface
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Ragger pulling rejectsout of pulper
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Junker Claw
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Continuos Low Consistency Pulper with Ragger and Junk Tower
Low consistency continuos pulper typically have an extraction plate that accepts pulped fibers and rejects debris and unpulped flakes.The extraction plate/ rotor can cause attrition, resulting in fiber cutting.A junker is used to collect unpulpables such as bolts or rocks. This debris is thrown out of the pulper into a junk tower where it is removed.A ragger is also used in many cases to remove bale wire, strings, plastics, etc. The ragger is a continuos “rope” formed by entangled debris. The “rope” is continously pulled out of the pulper and cut into sections and disposed. Common in OCC mills.
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Example Detrashing Process
Accepts
Light rejects
recycle
Heavyrejects
Rotor with screen plate
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Drum PulpingA continuos, high
consistency pulping method.Most often used for pulping old newsprint.Consists of an inclined rotating drum 11 -17 rpm through which the paper/ pulp travel down. The drum is very large approximately 10 feet high and 100 feet long.
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High Consistency Drum Pulper
High % KZone Screening
ZoneRejects
AcceptPulp
Conveyor
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Drum PulpingTwo Zones
High Consistency pulping zoneLow consistency pulping zone.
High Consistency Pulping ZonePaper ,water and chemicals added to ^ 15% K.Baffles on the walls of the drum lift the paper and drop causing defibering in a gentle manner.
Low Consistency Screening zoneWater is added to dilute stock 3-4% K.Pulped fibers pass through 6 mm holes and are accepted from the pulper.Large rejects continue through the pulper and are4 discharged at the end.
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Drum Pulping
Two ZonesHigh Consistency pulping zoneLow consistency pulping zone.
High Consistency Pulping ZonePaper ,water and chemicals added to ^ 15% K.Baffles on the walls of the drum lift the paper and drop causingdefibering in a gentle manner.
Low Consistency Screening zoneWater is added to dilute stock 3-4% K.Pulped fibers pass through 6 mm holes and are accepted from the pulper.Large rejects continue through the pulper and are4 discharged at the end.
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Overview
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Rejects
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Drum Pulping
AdvantagesGentle pulping keeps contaminants large and minimizes fiber degradation.Simple operation that includes screening.
DisadvantagesHigh capital cost.Not an aggressive pulping method (example: cannot pulp wet strength papers).
Paper Feed
rejects exit
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Pulping SummarySeveral methods to pulpMain objective: defiberizeSecondary Objectives:
Remove Large DebrisDetach contaminantsNot destroy fibersMix
Final Thought: If pulping is not done properly, subsequent processing steps will be ineffective and product quality will be unacceptable
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High density cleaners
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High Density Cleaner: removes large heavy rejects from pulp
Objective: separate large heavy contaminants from fibers to protect downstream equipment from damage and pluggage
How it works: centrifugal forces separate materials mainly due to density/size
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Detrasher
Accepts
Feed
Light weightRejects
HeavyRejects out
the back
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Example detrashing unit
Top View
Objective: pulp unpulpedpieces of paper/board and separate contaminants using a screen or centrifugal forces to protect downstream equipment from damage and pluggage
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Deflakers
Objective: impart mechanical energy to break up flakes of unpulped material.
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Screening
Screening separates contaminants based mainly on size, but also on shape and deformabilityPerformed by presenting a barrier for large contaminants (slots or holes) that allow fibers to pass through
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Screening
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Modes of Removal1. Stiff particles with all 3 dimensions larger than width of
slot or diameter of hole are rejected
2. Stiff particles with one or two dimensions smaller than width/diameter have a probability of rejection.
Number of screen contacts
Rejection Probability, %
1 2 3 4 5
100%
50%
0%
1-dimension (rod)< slot size
2-dimension (plate) < slot size
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Screening Types and ConditionsConsistency RangeScreen
TypeScreen
Openings, mm
Rotor circumference
speed,m/sMC<6%
MC<4.5%
LC<1.5%
Disk Hole2-3
20-30 Yes
Cylindrical Hole0.8-1.5
10-30 Yes
Cylindrical Slot0.1-0.4
10-30 Yes Yes
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Screening
Types of perforationscoarse holes .110 in or 2.7 mmfine holes .060 in or 1.52 mmcoarse slots .010 in or .254 mmfine slots .006 in or .152 mm
Also, the fibers offer a resistance to passage, related to the consistency
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Example of Disk Screen
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Pressure screen
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Pressure Screen Principle to Prevent Blinding of Screen
The leading edge of the rotating foil accelerates the stock.
The negative pulse under thesweeping foil momentarily reverses the flow, effectivelypurging the screen openings.
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Pressure Screen Flow Configurations
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Screen Plates
Holes
Slots
Contoured
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Effect of Reject Rate & Plate Opening on Screen Cleanliness
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Screening Factors
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Screen Performance VariablesStock characteristics
fiber type, debris characteristics, debris levelScreen design
flow configuration, plate cleaning mechanism, perforation type (holes or slots), rotor speed
Operating variablesstock flow rate (pressure drop across screen), feed consistency, reject rate, screen plate perforation size, stock temperature, dilution flow to screen
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Screening
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Screen Layout:Always have cascaded screens to save fiber.
Primary Screen
Secondary Screen
Tertiary Screen
Accepts
Rejects
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Open Gravity Screen
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Summary Pressure Screen:
Objective: separate large contaminants from fibers
Can act as barrier screen or probability screen
Typically cascaded to save fiber
Typical conditions to promote increased throughput can have negative impact on cleanliness efficiency.
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Cleaning
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Centrifugal Cleaning
Remove impurities from the pulp stream Centrifugal cleaners remove
metalsinkssandbarkdirtetc.,
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Centrifugal Cleaning
Principles of operationCentrifugal cleaner uses fluid pressure to create rotational fluid motion in a tapered cylinderRotational movement causes denser particles to move to the outside faster than lighter particlesGood fibers carried inward and upward to the accepted stock inletDirt held in the downward current and removed from the bottom
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Three Basic Cleaner Types:
High Density Cleaner: separates very large, heavycontaminants such as rocks, staples, glass. Used after pulping (early in the process) to protect downstream equipment. Diameter = 300-700 mm.Forward Cleaners: separates fine, heavycontaminants such as a sand and inks. Also called cyclones, hydrocyclones, or cleaners. (Described above) Diameter = 70-400 mmThrough Flow Cleaner: separates fine, lightcontaminants such as glues, adhesives, plastics, foam. Also called light-weight cleaners or reverse cleaners. Diameter = 100-400 mm
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Types of Cleaners: Functional Differences
ConsistencyHC cleaner: 2-4.5% K, MC: 1-2, LC: 0.5-1.5
Centrifugal Acceleration (acceleration due to gravity =9.8 m/s2)
HC cleaner: <60 g, MC: <100g, LC: <1000 g
Reject Rate by mass/stageHC cleaner: 0.1-1% , MC: 0.1-1, LC: 3-30
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Centrifugal Cleaner: Features and Flow
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“Bank Arrangement” of Cleaners
Several cleaners are piped in parallel fashion. A single cleaner is not capable of providing enough
through put for typical industrial flows.
Accepts Header
Feed Header
Rejects Header
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Forward Cleaners
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Cleaners
Canister
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Cleaners Pump
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Cascade Arrangement of Cleaners
REJECTS
Dilution Water
ACCEPTSDilution Water
Dilution Water
FEEDPrimary Cleaners
Tertiary Cleaners
Secondary Cleaners
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Typical Cleaner “Curve”
Reject Ratio: OD mass flow reject / OD mass flow inlet
Separation Ratio:
m(in) – m(acc) / m(in)
m= mass flow contaminant
EqualDistribution
EffectiveSeparation
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High Density Cleaner
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High density cleaners
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Through Flow Cleaner
AcceptsRejects
Feed
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Thru-flow cleaners
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Centrifugal Cleaner Performance Variables
Stock Characteristicsfiber typecontaminant characteristics (size, shape, density), dirt level
Cleaner Designbody diameter, feed inlet configuration, accept diameter, cylindrical section height, cone angle, spiral grooves application, reject rate control method (fixed orifice and back pressure)
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Parameters Affecting HydrocycloneCleanliness Efficiency
OperatingVariable increase in:
Cleanliness Efficiency
Sensitivity to Variable
Pressure Difference Incr/Decr HighVolumetric Flow Incr/Decr MediumCyclone Diameter Decrease HighConsistency Decrease HighFlake Content Decrease MediumTemperature Increase LowReject Rate Increase MediumFlushing Flow Decrease Medium
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Effect of Particle Properties on Separation
Particles with large density differences wrtwater are removed more effectivelyParticles with density near 1 g/cm3 may separate from fibersLarger particle at same density will be removed more effectively than smaller particleParticles of the same density but with favorable hydrodynamic shape (cwAp) separate more effectively, eg, a sphere is better than a flat plate
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radial
axial
Fg
Fd
Fc
Fd
Force balance on a single particle in a hydrocyclone
Radial direction: – Net centrifugal force ↔Drag force
Axial direction: – Net gravitational force ↔Drag force
Tangential direction: – Assume: particles move along with fluid
Fundamentals
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One dimension analysis of single particle
Fc (or Fg)Fd
( )
0 ( )
( )
sc g d
c g d
c g d
or
or
or
dUm F F Fdt
F F F
F F F
⋅ = −
= −
=
Assume the time for particle to reach its terminal velocity is very brief
Fundamentals
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Particle slip velocity, Us = velocity of water –velocity of particle
2 22
2
12
12
t tp l p l s p d
p p lts
p l d
u uV U A C
r r
VuU
r A C
ρ ρ ρ
ρ ρρ
⎛ ⎞− =⎜ ⎟⎜ ⎟
⎝ ⎠
−⎛ ⎞= ⎜ ⎟
⎝ ⎠
Net centrifugal force Drag force
for radial direction
dragcoefficient , densityof particleand fluid
radialposition tangential velocityof fluidvolumeof particle projectedarea of particle
d p l
t
p p
C
r uV A
ρ ρ= =
= == =
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Why is the slip velocity important ?
Us“Reject stream”
Us“Accept stream”
For high Us
For low Us
High ut (tangential velocity)
High ρp (particle density)
Low Cd (drag coefficient)
2 12 p p lts
p l d
VuU
r A Cρ ρρ−⎛ ⎞
= ⎜ ⎟⎝ ⎠
Fiber tends to be rejected
Fundamentals
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Cleaner Summary
Several types of cleanersObjective: remove high/low density contaminantsMust reject material to operate effectivelySeveral forces/operational variables/particle characteristics that combine to determine effectiveness in removal
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Paper Recycling OperationsAn Example: OCC Recycling Process: Review Quiz
Match the following:
high density cleaner removes light weight contaminants pulper tries to hide contaminantsscreens thickens pulp stockdisperser breaks paper/board into fibersthru flow cleaner uses size differencesvacuum disk filter uses centrifugal forcesclarifier allows us to re-use process water