Pulping and Bleaching PSE 476

1
Pulping and BleachingPSE 476

• Lecture 8
• Kraft Pulping Early Reactions and
• Kraft Pulping Lignin Reactions

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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

3
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.

4
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.

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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
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Yield of Wood Components After Kraft Pulping
Notes
Yields of wood (pulp) components
7
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.

8
Alkaline HydrolysisExample 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.

9
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.
10
Acidic Extractive Species
Lignans
Monoterpenoids
Resin Acids
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Consumption of Alkali
Impregnation zone
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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).

13
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?

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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)

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Kraft PulpingReaction Phases of Lignin Removal
70C
70C
Initial Phase Impregnation zone
137C
170 C
Bulk Phase
Residual Phase
Notes
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Kraft Pulping Lignin Reactions
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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
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Formation of Quinone Methide
Quinone Methide (very reactive)
These arrows indicate that a pair of electrons
are moving
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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.

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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
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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?

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Reactions of a-O-4 LinkagePhenolic and Etherified

• In kraft pulping, a-O-4 linkages do not react
  with HS-
• Reaction with OH-
• Phenolic Units a -O-4 are very rapidly cleaved
  by alkali. This is the fastest of the lignin
  degradation reactions. (Will occur at low
  temperatures)
• Etherified Units a -O-4 linkages are stable (no
  reaction).
• Please work out reaction mechanism.

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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 ß-O-4 linkage.
• Reaction is very rapid even at lower
  temperatures.

Mechanisms on following pages
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Kraft Reactions of b-O-4 Linkage (Free Phenolic
Hydroxyl)
Formaldehyde
Notice that the b-O-4 bond is not cleaved.
Vinyl Ether
Notes
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Appendix

• Basic Wood Chemistry

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What is the Chemical Makeup of Wood?
Data for Cellulose, Hemicellulose Lignin on
extractive free wood basis
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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.

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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.

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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
?
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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.

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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.

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Lignin Nomenclature
Side Chain
Notes

Phenylpropane Unit C9
Common Names
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Lignin ReactionsLinkage Frequencies
Notes
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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 (lt5).
• 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.