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Pulping and Bleaching PSE 476. Lecture #8 Kraft Pulping: Early Reactions and Kraft Pulping Lignin Reactions. Agenda. Basic Chemical Pulping Discussion Loss of Components During Kraft Pulping Reactions in the Early Portion of the Cook Saponification Neutralization of Extractives

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pulping and bleaching pse 476

Pulping and BleachingPSE 476

Lecture #8

Kraft Pulping: Early Reactions and

Kraft Pulping Lignin Reactions

  • Basic Chemical Pulping Discussion
  • Loss of Components During Kraft Pulping
  • Reactions in the Early Portion of the Cook
    • Saponification
    • Neutralization of Extractives
  • Initial Lignin Discussion
  • Kraft Pulping Lignin Reactions
wood chemistry
Wood Chemistry
  • For the students who do not recognize this molecule (did not take PSE 406), there is a short appendix at the end of this lecture to help you. Additionally, the class notes are available for review.
  • The goal of kraft pulping is to remove the majority of lignin from chips (or other biomass) while minimizing carbohydrate loss and degradation.
  • Removal of lignin is accomplished through treatment of raw material with NaOH and Na2S at elevated temperatures.
the goal of lignin reactions in kraft pulping
The Goal of Lignin Reactions in Kraft Pulping

During kraft pulping, the

large insoluble lignin

molecules are converted

into small alkali soluble


Kraft Pulping



Carbohydrates are also

degraded during pulping

yield of wood components after kraft pulping
Yield of Wood Components After Kraft Pulping


* Yields = % of wood (pulp) components

initial reactions low temperature
Initial Reactions: Low Temperature
  • Carbohydrates
    • Alkaline hydrolysis of acetyl groups on xylan (see next slide).
    • Removal of certain soluble carbohydrates.
      • Certain galactoglucomannans.
      • Arabinogalactans.
  • Extractives
    • Alkaline hydrolysis of fats (saponification), waxes, and other esters.
    • Neutralization of extractives.
      • There are a number of acidic extractives which consume NaOH.
alkaline hydrolysis example using acetyl groups
Alkaline Hydrolysis:Example Using Acetyl Groups
  • Esters are cleaved in alkaline solutions through hydrolysis reactions forming carboxylic acids and alcohols.
  • Hydrolysis of acetyl groups occurs readily in alkaline solutions.
    • Reaction occurs rapidly even at room temperature.
  • Reaction consumes alkali.
saponification of fats review slide from pse 406
Saponification of Fats(Review slide from PSE 406)
  • Treatment of fats with alkali converts them to fatty acids and glycerol through saponification.

Once again this reaction

consumes part of the alkali


acidic extractive species
Acidic Extractive Species



Resin Acids

consumption of alkali
Consumption of Alkali



where does all the alkali go
Where Does All the Alkali Go?
  • Spruce wood was soda pulped at a NaOH concentration of 19% (as Na2O).
  • 12.5% (or 66% of alkali) consumed to lower lignin content of wood to 2.8%.
    • 2.3-3% used in dissolution of lignin.
    • 1.3% for hydrolysis of acetyl and formyl groups.
    • 8.2-8.9% for neutralization of acidic products
      • Some extractives
      • Mostly carbohydrate degradation products (discussed later).
lignin removal during kraft pulping
Lignin Removal during Kraft Pulping
  • This chart shows the lignin removal rate during a kraft cook. It is important to note that the rate of lignin removal is temperature dependent. What does this fact tell us about of lignin removal in this slide?
lignin removal
Lignin Removal
  • In the last slide, the rate of lignin removal appears to be linear over a large portion of the cook; even as the temperature increases.
  • This means that lignin removal in the first portion of the cook is easier than as the cook proceeds.
  • Lignin removal has been broken into three sections:
    • Initial Phase (fast lignin removal reactions)
    • Bulk Phase (slow lignin removal reactions)
    • Residual Phase (really slow lignin removal)
kraft pulping reaction phases of lignin removal
Kraft Pulping:Reaction Phases of Lignin Removal



Initial Phase

Impregnation zone


170° C

Bulk Phase

Residual Phase


dissolution of lignin
Dissolution of Lignin
  • In review the goal in kraft pulping is the cleavage of lignin into alkali soluble fragments.
  • Cleavage is affected by the following factors:
    • Type of linkage
    • Presence of free phenolic hydroxyl group
    • Functional groups (benzyl hydroxyl, carbonyl)
    • Type and amount of nucleophiles (OH-, HS-)
    • Reaction temperature
  • We are going to first look at the chemical mechanisms of the reactions and then the kinetics.
sites for nucleophilic attack
Sites for Nucleophilic Attack
  • The cooking chemicals used in kraft cooking (NaOH and Na2S: OH- and HS-) both act as nucleophiles* because of their free pair of electrons.
  • Sites for nucelophilic attack in lignin are those areas of reduced electron density (partially positive sites).

* Notes

formation of quinone methide




Formation of Quinone Methide

Quinone Methide

(very reactive)

These arrows indicate that a pair

of electrons are moving

formation of nucleophilic attack sites
Formation of Nucleophilic Attack Sites
  • A free phenolic hydroxyl group is needed for the formation of a quinone methide.
  • The oxygen of the quinone group (carbonyl) attracts the electron density on the double bond thus making the carbon more positive. This in turn shifts the electron densities of the other bonds on this conjugated system.
two additional examples of nucleophilic addition sites
Two Additional Examples of Nucleophilic Addition Sites


This structure contains an a-keto

group. Notice that a free phenolic

hydroxyl groups is not needed!

Coniferaldehyde type structures

important issues
Important Issues!!!!
  • When learning about alkaline pulping mechanisms, remember to ask yourselves these questions!
    • Which reactant are we concerned with: OH- or HS-?
    • Does the lignin structure have a free phenolic hydroxyl group or is it etherified?
    • Which linkage are you hoping to cleave?
    • Is there an a-carbonyl or benzyl hydroxyl?
reactions of o 4 linkage phenolic and etherified
Reactions of α-O-4 LinkagePhenolic and Etherified
  • In kraft pulping, α-O-4 linkages do not react with HS-
  • Reaction with OH-
    • Phenolic Units: α -O-4 are very rapidly cleaved by alkali. This is the fastest of the lignin degradation reactions. (Will occur at low temperatures)
    • Etherified Units: α -O-4 linkages are stable (no reaction).
    • Please work out reaction mechanism.
reactions of b o 4 linkages free phenolic hydroxyl benzyl hydroxyl
Reactions of b-O-4 Linkages:Free Phenolic Hydroxyl/Benzyl Hydroxyl
  • Reaction with OH- alone
    • The ether linkage is not cleaved; a vinyl ether structures is formed.
    • Vinyl ether linkages are difficult to cleave.
  • Reaction with HS- (OH- present)
    • HS- is a very strong nucleophile which cleaves the β-O-4 linkage.
    • Reaction is very rapid even at lower temperatures.

* Mechanisms on following pages

kraft reactions of b o 4 linkage free phenolic hydroxyl
Kraft Reactions of b-O-4 Linkage (Free Phenolic Hydroxyl)


Notice that the

b-O-4 bond is

not cleaved.

Vinyl Ether




Basic Wood Chemistry

what is the chemical makeup of wood
What is the Chemical Makeup of Wood?

* Data for Cellulose, Hemicellulose & Lignin on extractive free wood basis

  • Very long straight chain polymer of glucose (a sugar): approximately 10,000 in a row in wood. Cotton is nearly pure cellulose.
    • Think about a very long string of beads with each bead being a glucose molecule.
  • Cellulose molecules link up in bundles and bundles of bundles and bundles of bundles of bundles to make fibers.
  • Uncolored polymer.
  • Branched little uncolored sugar polymers (~ 50 to 300 sugar units)
    • Composition varies between wood species.
      • 5 carbon sugars: xylose, arabinose.
      • 6 carbon sugars: mannose, galactose, glucose.
      • Uronic Acids: galacturonic acid, glucuronic acid.
      • Acetyl and methoxyl groups (acetic acid & methanol).
  • Major hemicelluloses:
    • Xylans - big in hardwoods
    • Glucomannans: big in softwoods
  • Minor hemicelluloses: pectins, others.
xylan structure
Xylan Structure


4-O-Me--D-Glc 


glucomannan structure







Glucomannan Structure
  • There are different structured glucomannans in hardwoods and softwoods (and within softwoods)
  • Glucomannans are mostly straight chained polymers with a slight amount of branching. The higher the branching, the higher the water solubility.
  • Phenolic polymer - the glue that holds the fibers together.
  • Lignin is a very complex polymer which is connected through a variety of different types of linkages.
  • Colored material.
lignin nomenclature
Lignin Nomenclature

Side Chain



Phenylpropane Unit


Common Names

  • The term extractives refers to a group of unique chemical compounds which can be removed from plant materials through extraction with various solvents.
  • Typically these chemicals constitute only a small portion of the tree (<5%).
    • In some tropical species this can be as high as 25%.
  • Extractives are produced by plants for a variety of uses.
    • The most common use by plants is protection.
  • Extractives can cause serious problems for processing.
  • Pitch is a term which is often used when describing some groups of extractives.
  • Extractives are responsible for the characteristic color and odor of wood.