1 / 15

Agenda

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.

jelani-holloway
Download Presentation

Agenda

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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 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 

  6. 14--D-Glc-14--D-Man-14--D-Man-14--D-Man-1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.

  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 (<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.

  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. 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.

  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

More Related