Agenda

1
Agenda

• Chemical Description of Wood
• Carbohydrates
• Extractives
• Lignin
• Loss of Components During Kraft Pulping
• Reactions in the Early Portion of the Cook

2
What is the Chemical Makeup of Wood?
Data for Cellulose, Hemicellulose Lignin on
extractive free wood basis
3
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.

4
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.

5
Xylan Structure
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4-O-Me-?-D-Glc ??
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6
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.

7
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.

8
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.

9
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.

10
Five Steps in Kraft Pulping Process

• Transport of ions from the liquor to the exterior
  surface of the chip
• Diffusion of the ions to the interior of the chip
• Chemical reactions between the ions and the wood
  components
• Diffusion of the reaction components to the the
  chip exterior
• Transport of the reaction products into the liquor

11
Yield of Wood Components After Kraft Pulping
12
Initial Reactions Low Temperature

• Carbohydrates
• Saponification of acetyl groups on xylan (see
  next slide).
• Reactions with easily removable carbohydrates.
• Galactoglucomannans.
• Arabinogalactans.
• Extractives
• Saponification of fats.
• Neutralization of extractives.
• There are a number of acidic extractives which
  consume NaOH.

13
SaponificationExample Using Acetyl Groups

• Saponification is the basic hydrolysis of esters.
• Saponification of acetyl happens readily in
  alkaline solutions.
• Reaction occurs rapidly even at room temperature.

14
Acidic Extractive Species/Saponifiables
15
Consumption of Alkali
Rapid initial drop in alkali concentration that
must be replenished by diffusion of chemicals
into the chip