pse201 - for exam 2

advantages of the kraft process (5)

adaptability to all wood species, efficient chemical recovery, high quality pulp, lower manufacturing cost, relatively simple

disadvantages of the kraft process (3)

higher capital cost, total reduced sulfur (odor/smell), higher bleaching cost

chemicals used in the kraft process? (6, 3 main ones to know)

NaOH, Na₂S, Na₂CO₃, Na₂SO₄, Na₂SO₃, Na₂S₂O₃

what is total alkali?

total of all viable sodium alkali compounds

what is total titratable alkali (TTA)?

total of NaOH + Na₂S, + Na₂CO₃

what is active alkali (AA)?

total of NaOH + Na₂S

what is effective alkali?

total of NaOH + ½ Na₂S (only ½ the chemical actually dissolves for pulping)

what is sulfidity?

ratio of Na₂S to AA as a %

% of the AA that is due to Na₂S

what are the things that make up the “pulping triangle?”

NaOH concentration, reaction time, reaction temperature

how much lignin is lost in kraft pulping (bleachable-grade)?

92%

how much hemicellulose is lost in kraft pulping (bleachable-grade)?

90%

how much cellulose is lost in kraft pulping (bleachable-grade)?

10%

how much extractives are lost in kraft pulping (bleachable-grade)?

100%

benefits of sulfidity (up to 30%)?

faster rate of reaction, lower cooking temperature, lower %AA, higher yield, better strength properties

variables measure before cooking? (6)

chip size, presteaming/chemical impregnation, wood charge, chemical charge, % sulfidity, liquor-to-wood ratio

variables controlled during cooking?

ramp up time to maximum temperature, maximum temperature (“hold” temp), time at maximum temp, H-factor

what is H-factor?

way to adjust a cook if time and/or temperature is changed

variables measured/controlled after pulping?

kappa number, viscosity, % rejects

what is kappa number?

test that measures residual lignin in the pulp after cooking, based on oxidation, uses potassium permanganate (KMnO₄)

what is viscosity?

measures the DP (degree of polymerization) in the pulp, damage done to cellulose during pulping

% rejects definition

pulp put through screen that separates individual fibers from fiber bundles

brownstock processing goals

maximize quality: remove unfiberized material, remove materials that cause dirt, remove black liquor, remove contaminants

minimize cost: maximize production of on-spec product, maximize yield, maximize black liquor recovery, minimize excessive consumption of bleaching chemicals

types of contaminants and brownstock (and their definition)

natural (things found in wood, can’t be avoided), foreign (things introduced accidentally or by external forces)

steps of decontamination of brownstock (3)

remove undercooked materials, remove black liquor, remove shives

things considered as unfiberized materials

cooked chips, undercooked chips, knots

what is a hot stock refiner?

defiberizing in hot residual liquor, low consistency, reduces shives, placed after the blow tank

what are deshiving refiners?

disk refiner, up to four in series with decreasing plate gap

what is a knotter?

screen that breaks up knots, usually applies pressure and uses water

machines that remove knots (3)

knotter, vibratory drainer, vertical screws

what is a vibratory drainer?

drains/washes off black liquor and fiber before disposal of knots

what is a brownstock washer?

separates the solid and liquid phases of brownstock after the digester

brownstock washer steps

removes black liquor in slurry, use fresh(er) filtrate to wash out any liquor trapped in fiber walls

what are other things that the brownstock washer affects?

mill energy balance, chemical recovery, pulp cleanliness/bleaching, environment

2 main washing mechanisms?

dilution and thickening, displacement

what is dilution and thickening?

mix dilution water with slurry, hold for some time, drain free liquid from slurry

what is displacement?

3x more efficient, drains free black liquor from slurry, push/pull dilution water through resulting pulp mat

types of washing equipment? (6)

rotary vacuum washer, atmospheric diffuser, pressure diffuser, horizontal belt washer, twin-roll press, DD washer

what is a rotary drum vacuum filter? (type of washing and what is does)

displacement style washing

pulp mat formed on drum covered with fine mesh wire, vacuum formed inside drum by vacuum pumps or drop-leg

what is counter-current washing?

uses several stages, feed cleanest filtrate to last washing stage then take filtrate to previous stages (displacement shower water only has to be cleaner than filtrate in the mat)

what is a twin-roll wash press?

uses both washing types, intense displacement on suction rolls → intense dewatering (30%+ consistency)

what is a horizontal belt washer?

brownstock on wire moving around rolls, washing filtrate moves counter-current to pulp (similar to paper machine)

what is a drum displacer (DD) washer?

drum washer with internal piping that permits 2-4 counter-current washing stages, pulp sits in trays and filtrate from later stages is fed to earlier stages

washing can be evaluated by taking a sample and measuring… (4)

BOD (biological oxygen demand), COD (chemical oxygen demand), dissolved solids, conductivity

what is dilution factor?

how much excess water is added to a system by showers on the washer (usually value of 2-3)

what is displacement ratio?

the fraction of incoming dissolved solids that is removed (value of 0-1)

what is norden efficiency factor (E-factor)?

number of perfect dilution and thickening stafes equivalent to the washer being evaluated (higher is better)

other things that affect brownstock washing (5)

feed rate, mat coverage, air entrainment, shower water cleanliness, shower water temperature

how to pressure screens work?

feed goes inside screen basket, accepts go to outside chamber and accept nozzle, rejects move down to reject chamber, rotating foil keeps a mat from forming, shives are either too large or too stiff to pass through

what do the gatekeeper and stock recovery stages do?

primary stage (stock passing through is clean and mostly shive-free, rejects 20-30% good fiber), next stages recover any good fiber initially missed

what is dewatering?

used to store brownstock at 12% consistency or higher, deckers are used (rotary filter w/out washing showers)

what is bleaching?

chemical process applied to cellulosic materials to increase brightness

what is brightness?

ability to reflect light at blue wavelength in comparison to a known standard, measure of reflectivity

what causes color in paper? (2)

chromphoric groups on lignin, heavy metal ions and extractives

chromphoric group definition

functional groups that absorb visible light

2 types of bleaching?

lignin-removing, lignin-preserving

what type of pulp is lignin-preserving bleaching used on?

mechanical pulps

what does lignin-preserving bleaching do?

selectively destroys chromphoric groups, doesn’t remove lignin, low brightness target, lower brightness stability

what type of pulp is lignin-removing bleaching used on?

chemical pulps

what does lignin-removing bleaching do?

removes residual lignin and chromophores, includes delignifying and brightening stages, high brightness and brightness stability

2 main types of bleaching chemicals?

lignin-removing chemicals, chromophore attacking (brightening) chemicals

what do lignin-removing chemicals do?

attack, degrade, and remove lignin (lower yield, higher brightness)

what do chromophore attacking (brightening) chemicals do?

leave lignin along but destroy chromophores (higher yield, lower brightness)

what are chemical mixers?

high-shear devices that fluidize medium-consistency pulp and inject chemicals

what is a upflow bleach tower?

simplest design but pumping against gravity (high energy), must have uniform consistency, must be full to operate (low reliability)

what is a downflow bleach tower?

pulp pumped to top and sinks by gravity, doesn’t have to be full to operate (higher reliability)

what is a upflow/downflow bleach tower?

combine advantages/disadvantages of upflow and downflow configurations, upflow “pre-tube” allows for volatile chemicals to pre-react

what is ClO2 used for and what does it do?

used for bleaching chemical pulps to high brightness, attacks lignin and mostly leaves cellulose alone, toxic and unstable

what does a ClO2 generator do?

prodcues ClO2 which is immediately absorbed in chilled water, ClO2 water is applied to pulp

what is ECF bleaching?

elemental chlorine-free bleaching, no chlorine or hypochlorite

what is TCF bleaching?

totally chlorine-free bleaching, no chlorine, hypochlorite, or chlorine dioxide, alternative bleaching agents only

what is oxygen delignifiaction?

digester followed by pressurized reactor (with oxygen), pulp is mixed with alkali then pumped into this reactor, removes 50% lignin left in pulp after cooking

what are the contents of black liquor?

dissolved wood (most lignin ~10% cellulose, almost all hemicelluloses), spent cooking cehmicals (Na2CO3 and Na2SO4)

what is an evaporator?

industrial heat exchanger that pumps liquid inside metal tubes (or plates) and condenses steam on the outside of tubes, condensing steam gives up heat of evaporation, which moves through tube wall to heat up liquid flowing inside

steam economy equation?

steam economy = lbs water evaporated / lbs steam used