Oxygen Delignification Two factors that dictate process design of o2 delignification AOXygen reacts only with units having free- phenolic hydroxyl group only at temperature above70° A Solubility of oxygen in water is very low, especially at temperature above 70.C ■Note Process condition for oxygen delignification South China University of Technology
South China University of Technology Oxygen Delignification Two factors that dictate process design of O2 delignification △Oxygen reacts only with units having freephenolic hydroxyl group only at temperature above 70℃ △Solubility of oxygen in water is very low, especially at temperature above 70℃ Note Process condition for oxygen delignification
Stepwise Reduction of Oxygen +2e,2H+ teH +e,H+ +e,H+ +e.H+ O2—*HOO.—HOOH+HO+OH=·2HOH H++o H++OoH H++·O pKa 4.8 11.8 11.9 South China University of Technology
South China University of Technology O2 HOO· HOOH HO·+-OH 2HOH +e - ,H+ +e - ,H+ +e - ,H+ +e - ,H+ +2e - ,2H+ Stepwise Reduction of Oxygen H++·O2 - pKa 4.8 H++ -OOH 11.8 H++·O- 11.9
a Reactive Species in Oxygen Bleaching Electrophiles:○2=·O2,HOO,HO,(O2)(·O) Nucleophiles: HOO, ( HOOH) a Reactivity of transients in oxygen/alkali delignification systems South China University of Technology
South China University of Technology Reactive Species in Oxygen Bleaching Electrophiles: O2≡·O2·, HOO·, HO·, (·O2 - )(·O- ) Nucleophiles: HOO- , (HOOH) Reactivity of transients in oxygen/alkali delignification systems
1 OXygen Bleaching System A Complex interplay between radical and ionic reactions ARadical (electrophilic) reactions Fast Mailnly responsible for lignin fragmentation(delignification) Alonic(nucleophilic)reactions Slow Destruction of chromophoric structures (brightness improvement) O-stage combines delignification and bleaching A Chemistry of oxygen and hydrogen peroxide bleaching interrelated AThe main challenge in bleaching with oxygen is to generate enough hydroxyl radicals to maintain acceptable rates of oxidative processes without initiating excessive damage to carbohydrates South China University of Technology
South China University of Technology Oxygen Bleaching System △Complex interplay between radical and ionic reactions △Radical (electrophilic) reactions Fast Mailnly responsible for lignin fragmentation (delignification) △Ionic (nucleophilic) reactions Slow Destruction of chromophoric structures (brightness improvement) O-stage combines delignification and bleaching △Chemistry of oxygen and hydrogen peroxide bleaching interrelated △The main challenge in bleaching with oxygen is to generate enough hydroxyl radicals to maintain acceptable rates of oxidative processes without initiating excessive damage to carbohydrates
a Comparison of MC HC systems △ MC systems △ HC systems ■ Process variab|es alkali charge oxygen pressure time& temp consistency South China University of Technology
South China University of Technology Comparison of MC & HC systems △MC systems △HC systems Process variables alkali charge oxygen pressure time & temp consistency