Kraft ProcessKraft Process

Jennifer PrattJennifer PrattChemical Reaction EngineeringChemical Reaction Engineering

5/1/065/1/06

OutlineOutline IntroIntro OverviewOverview TheoryTheory ApplicationsApplications ConclusionsConclusions

IntroductionIntroduction

Paper making began in China 2000 years agoPaper making began in China 2000 years ago-originally used old rags -originally used old rags fibers fibers

Today: trees/ recycled materials Today: trees/ recycled materials fibers by pulping fibers by pulping Kraft process is main chemical method for tree Kraft process is main chemical method for tree pulp pulp ““Kraft”=strong in German. Strong paper is made from Kraft”=strong in German. Strong paper is made from

Kraft pulped paperKraft pulped paper Also “Sulfate Process” because sulfates are produced Also “Sulfate Process” because sulfates are produced

as a byproductas a byproduct Goal = remove lignin from the wood and obtain Goal = remove lignin from the wood and obtain

individual fibersindividual fibers Lignin=strong polymer in plant cell walls for support and Lignin=strong polymer in plant cell walls for support and

structurestructure

Paper Making Process

Source: http://www.epa.gov/ttn/chief/ap42/ch10/final/c10s02.pdf

OverviewOverview Begins in DigesterBegins in Digester

White liquor= NaWhite liquor= Na22s+NaOHs+NaOH Chips=logs chopped up into cornflake sized piecesChips=logs chopped up into cornflake sized pieces

Reaction of White liquor and chipsReaction of White liquor and chips Yields Black Liquor/Pulp mixtureYields Black Liquor/Pulp mixture

Blow Tank then FilterBlow Tank then Filter Contents separated into Black Liquor and Pulp SlurryContents separated into Black Liquor and Pulp Slurry Pulp eventually becomes paperPulp eventually becomes paper

Chemical RecoveryChemical Recovery Black Liquor diluted and burned Black Liquor diluted and burned energy and smelt energy and smelt Smelt diluted Smelt diluted Green Liquor Green Liquor Green Liquor converted back to White Liquor with Calcium Green Liquor converted back to White Liquor with Calcium

OxideOxide

TheoryTheory In the DigesterIn the Digester

NaNa22S + NaOH + wood chips —>NaS + NaOH + wood chips —>Na22SOSO44 + Na + Na22COCO33 + pulp + pulp Five steps in DigesterFive steps in Digester

transportation of liquor ions to wood surfacetransportation of liquor ions to wood surface diffusion of the ions into the wood fibersdiffusion of the ions into the wood fibers chemical reaction between the wood and the ionschemical reaction between the wood and the ions diffusion of the products back to the surface of the wood diffusion of the products back to the surface of the wood

fibersfibers movement of the products into the bulk liquor. movement of the products into the bulk liquor.

TheoryTheory Residence time is importantResidence time is important

More time=more lignin removalMore time=more lignin removal Too much time=cellulose in fibers breaks Too much time=cellulose in fibers breaks

downdown Kappa NumberKappa Number

Degree of LignificationDegree of Lignification Goal=maximize Kappa Number with least Goal=maximize Kappa Number with least

amount of chemicalsamount of chemicals

TheoryTheory Rate of DelignificationRate of Delignification

Initial-extraction of low MW lignin, turpentine Initial-extraction of low MW lignin, turpentine and tall oiland tall oil

Bulk-most lignin removedBulk-most lignin removed Residual-last remnants of lignin removedResidual-last remnants of lignin removed

Bulk Delignification Bulk Delignification KineticsKinetics

-dL/dt=k-dL/dt=k11(OH(OH--)(L) )(L) Constant white liquor inputConstant white liquor inputln(Lo/L)=(kln(Lo/L)=(koo)(t))(t) koo=Ae(-Ea/RT)

Activation energy for each wood is ultimately determined from experiment

ConclusionConclusion Industry needs to know activation energy for Industry needs to know activation energy for

pulp reaction to optimize processpulp reaction to optimize process Different parts of tree have different lignin Different parts of tree have different lignin

content and usescontent and uses Trees from different geographical regions have Trees from different geographical regions have

different lignin contentdifferent lignin content Wood type chosen based on paper product Wood type chosen based on paper product

produced, wood’s resistivity to delignification, & produced, wood’s resistivity to delignification, & integrity of wood fibers produced in pulpingintegrity of wood fibers produced in pulping

ReferencesReferences Andersson, N., Wilson, D., and Germgard, U. (2002). Andersson, N., Wilson, D., and Germgard, U. (2002).

Validating Continuous Kraft Digester Kinetic Validating Continuous Kraft Digester Kinetic Models Models with Online NIR Measurements.with Online NIR Measurements. Anchorage, AK. Anchorage, AK. Proceedings for the American Proceedings for the American Control Conference.Control Conference.

Helm, R. (2000). Helm, R. (2000). Kraft PulpingKraft Pulping. Retrieved April 25, . Retrieved April 25, 2006, from http://dwb.unl.edu/Teacher/NSF/C06 2006, from http://dwb.unl.edu/Teacher/NSF/C06 /C06Links /www.chem.vt.edu/chem-/C06Links /www.chem.vt.edu/chem-dept/helm/3434WOOD/notes2/kraft.html.dept/helm/3434WOOD/notes2/kraft.html.

Wong, B.M., Deka, G.C., and Roy, D.N. (1995). Wong, B.M., Deka, G.C., and Roy, D.N. (1995). Bulk Bulk Delignification Kinetics at Selected Vertical Heights Delignification Kinetics at Selected Vertical Heights With Jack Pine Plus Trees.With Jack Pine Plus Trees. Wood Science and Wood Science and Technology, 29, 11-18.Technology, 29, 11-18.

Questions?Questions?