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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 l.jpg

Pulping and BleachingPSE 476

Lecture #8

Kraft Pulping: Early Reactions and

Kraft Pulping Lignin Reactions


Agenda l.jpg
Agenda

  • 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 l.jpg
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.


Pulping l.jpg
Pulping

  • 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 l.jpg
The Goal of Lignin Reactions in Kraft Pulping

During kraft pulping, the

large insoluble lignin

molecules are converted

into small alkali soluble

fragments.

Kraft Pulping

Soluble

Fragments

Carbohydrates are also

degraded during pulping


Yield of wood components after kraft pulping l.jpg
Yield of Wood Components After Kraft Pulping

Notes

* Yields = % of wood (pulp) components


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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 l.jpg
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.


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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

charge.


Acidic extractive species l.jpg
Acidic Extractive Species

Lignans

Monoterpenoids

Resin Acids


Consumption of alkali l.jpg
Consumption of Alkali

Impregnation

zone


Where does all the alkali go l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
Kraft Pulping:Reaction Phases of Lignin Removal

70°C

70°C

Initial Phase

Impregnation zone

137°C

170° C

Bulk Phase

Residual Phase

Notes



Dissolution of lignin l.jpg
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.


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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 l.jpg

Nucleophillic

attack

site!

Formation of Quinone Methide

Quinone Methide

(very reactive)

These arrows indicate that a pair

of electrons are moving


Formation of nucleophilic attack sites l.jpg
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 l.jpg
Two Additional Examples of Nucleophilic Addition Sites

Notes

This structure contains an a-keto

group. Notice that a free phenolic

hydroxyl groups is not needed!

Coniferaldehyde type structures


Important issues l.jpg
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 l.jpg
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 l.jpg
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 l.jpg
Kraft Reactions of b-O-4 Linkage (Free Phenolic Hydroxyl)

Formaldehyde

Notice that the

b-O-4 bond is

not cleaved.

Vinyl Ether

Notes


Appendix l.jpg

Appendix

Basic Wood Chemistry


What is the chemical makeup of wood l.jpg
What is the Chemical Makeup of Wood?

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


Cellulose l.jpg
Cellulose

  • 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.


Hemicelluloses l.jpg
Hemicelluloses

  • 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 l.jpg
Xylan Structure

4--D-Xly-14--D-Xly-14--D-Xly-14--D-Xly4--D-Xly

4-O-Me--D-Glc 

-L-Araf


Glucomannan structure l.jpg

14--D-Glc-14--D-Man-14--D-Man-14--D-Man-1

6

2,3

1

Acetyl

-D-Gal

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.


Lignin l.jpg
Lignin

  • 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 l.jpg
Lignin Nomenclature

Side Chain

Notes

}

Phenylpropane Unit

C9

Common Names


Lignin reactions linkage frequencies l.jpg
Lignin Reactions:Linkage Frequencies

Notes


Extractives l.jpg
Extractives

  • 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.


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