Agenda • Chemical Description of Wood • Carbohydrates • Extractives • Lignin • Loss of Components During Kraft Pulping • Reactions in the Early Portion of the Cook
What is the Chemical Makeup of Wood? * Data for Cellulose, Hemicellulose & Lignin on extractive free wood basis
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 • 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 4--D-Xly-14--D-Xly-14--D-Xly-14--D-Xly4--D-Xly 4-O-Me--D-Glc -L-Araf
14--D-Glc-14--D-Man-14--D-Man-14--D-Man-114--D-Glc-14--D-Man-14--D-Man-14--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 • 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.
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.
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.
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
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.
Saponification:Example 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.
Consumption of Alkali Rapid initial drop in alkali concentration that must be replenished by diffusion of chemicals into the chip