1 / 18

Pulping and Bleaching PSE 476/Chem E 471

Pulping and Bleaching PSE 476/Chem E 471. Lecture #10 Kraft Pulping Carbohydrate Reactions. Agenda. Carbohydrate Reaction Mechanisms Glycosidic Cleavage Peeling Stopping Glucomannan Reactions Xylan Reactions Other Hemicellulose Reactions Cellulose Reactions. Carbohydrate Reactions.

greta
Download Presentation

Pulping and Bleaching PSE 476/Chem E 471

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. Pulping and BleachingPSE 476/Chem E 471 Lecture #10 Kraft Pulping Carbohydrate Reactions PSE 476: Lecture 10

  2. Agenda • Carbohydrate Reaction Mechanisms • Glycosidic Cleavage • Peeling • Stopping • Glucomannan Reactions • Xylan Reactions • Other Hemicellulose Reactions • Cellulose Reactions PSE 476: Lecture 10

  3. Carbohydrate Reactions • Carbohydrates react slower than lignin under alkaline conditions. Overall, however, just as much carbohydrates react as does lignin. • The main alkaline reactions of carbohydrates are: • Glycosidic cleavage. • Peeling. • Stopping. PSE 476: Lecture 10

  4. Alkaline Pulping : CarbohydratesGeneral Aspects • There are considerable carbohydrate losses during kraft pulping due to alkaline degradation reactions. • Acetyl groups are very quickly cleaved. • Carbohydrates undergo “peeling” • Peeling is the process in which sugars are removed one by one from the reducing end of the polymer. • Hemicelluloses highly degraded through “peeling” • Monosaccharide fragments from peeling are highly degraded to acidic compounds. • This reaction is stopped by “stopping” reactions. • Glycosidic linkages in carbohydrates are cleaved through hydrolysis reducing overall molecular weight and creating new reducing ends. PSE 476: Lecture 10

  5. “Peeling Reaction” Formation of new reducing end group (C) (D) (B) (E) (A) A. Isomerization B enediol formation C. b-alkoxy elimination D. Tautomerization E. Benzilic acid rearrangement Notes PSE 476: Lecture 10

  6. Benzilic acid rearrangement Bond rotation Nuclephilic addition α-hydroxy-carboxylic acid Proton transfer PSE 476: Lecture 10

  7. “Stopping Reaction” (B) (C) (D) (A) A. 1,2 Enediol formation B. b-hydroxy elimination C. Tautomerization D. Benzilic acid rearrangement (will not “peel”) Notes PSE 476: Lecture 10

  8. Cleavage of Glycosidic Bonds (B) (A) (C) A. Inversion of ring confirmation, C2 OH ionized B. Ionized hydroxyl groups attacks C1 eliminating methoxyl group forming 3 membered epoxide (oxirane) • Opening of epoxide forms new reducing end or if steric conditions are correct a 1,6 anhydride. This compound is opened by alkali. This reaction requires elevated temperatures Notes PSE 476: Lecture 10

  9. Cleavage of Glycosidic Bonds Notes PSE 476: Lecture 10

  10. Alkaline Reactions of Glucomanans/Cellulose • Glucomannans: • Very unstable to peeling reactions • Galactose side chains fairly resistant • Cellulose • Large Dp means that glycosidic cleavage more important than peeling • Cellulose loss small but viscosity (Mw) significantly reduced PSE 476: Lecture 10

  11. Loss of Glucomannans During Kraft Pulping Notes PSE 476: Lecture 10

  12. Loss of Xylans During Kraft Pulping Notes PSE 476: Lecture 10

  13. Xylan peeling Reactions • Xylans much more resistant to peeling than glucomannans. • Large % of xylans are dissolved during pulping instead of peeled. Much of these xylans precipitate on the fiber surface at the end of the cook as alkali is consumed. • There appears to be two different temperature dependent mechanisms responsible for the protection from peeling. PSE 476: Lecture 10

  14. Resistance of Xylan to Peeling Below 100°C • Low temperature stability (below 100°C) is due to the resistance of galacturonic acid to peeling at this temperature. • Galacturonic acid groups are found in unique end group. (see below) • Above 100°C, galacturonic acid groups undergo peeling. --Xly-14---D-Xly3--L-Rha-12--D-GalA-14-D-Xly PSE 476: Lecture 10

  15. 4--D-Xly-14--D-Xly-14--D-Xly-14--D-Xly4--D-Xly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  Resistance of Xylan to Peeling above 100°C • A theory has been presented that above 100°C glucuronic acid side chains on xylans slow the peeling reaction. • Above 120°C, glucuronic acids are somewhat converted to hexenuronic acids which are much more stable to peeling. Hexenuronic acid formation is discussed in the next slide. PSE 476: Lecture 10

  16. Formation of Hexenuronic Acids • Hexenuronic acids are formed from uronic acids under alkaline conditions. • Method for identifying these compounds in pulps just developed. • Interfere with Kappa (lignin concentration) determination. • Attract metals (color problem/cause problems in bleaching) PSE 476: Lecture 10

  17. Loss of Other Hemicelluloses During Kraft Pulping • The minor hemicelluloses such as pectins, starches, etc. are supposedly completely destroyed during kraft pulping. • This happens through dissolution and peeling. • Many of these carbohydrates are water soluble so removal is easy. Once they are in the hot alkali solution they are easily destroyed. PSE 476: Lecture 10

  18. Cellulose Reactions During Kraft Pulping • Cellulose undergoes peeling and glycosidic cleavage reactions during kraft pulping. • Because cellulose molecules are so long, peeling reactions only cause small yield losses. • Glycosidic cleavage is more of a problem because of molecular weight losses that cause strength problems. This reaction also increases the rate of peeling through generation of new reducing end groups. • Because cellulose molecules are so large dissolution is not an issue. PSE 476: Lecture 10

More Related