Pulping and Bleaching PSE 476: Lecture 191 Pulping and Bleaching PSE 476/Chem E 471 Lecture #19 Oxygen Bleaching Lecture #19 Oxygen Bleaching

Pulping and Bleaching PSE 476: Lecture 192 Agenda Process Overview Advantages/Disadvantages Reduction of Oxygen: Oxygen species Lignin Reactions Carbohydrate Reactions Effect of Process Variables Process Overview Advantages/Disadvantages Reduction of Oxygen: Oxygen species Lignin Reactions Carbohydrate Reactions Effect of Process Variables

Pulping and Bleaching PSE 476: Lecture 193 Oxygen Bleaching Process Overview Feed: Unbleached pulp from brown stock washer. »Medium consistency 10-14%, High consistency 20-28%. Alkali (NaOH or white liquor) added in pump to feed tank. Oxygen added in high shear mixer. Pulp (oxygen) pass through up flow reactor (1 hr). Pulp and gases (O 2 & other) separated. Pulp thoroughly washed (twice). Feed: Unbleached pulp from brown stock washer. »Medium consistency 10-14%, High consistency 20-28%. Alkali (NaOH or white liquor) added in pump to feed tank. Oxygen added in high shear mixer. Pulp (oxygen) pass through up flow reactor (1 hr). Pulp and gases (O 2 & other) separated. Pulp thoroughly washed (twice).

Pulping and Bleaching PSE 476: Lecture 194 Advantages of Oxygen Bleaching Environmental: »Less chlorinated organics in discharge (AOX). »Significantly lower BOD, COD, and color in effluent. –This is because the effluent from oxygen bleaching can be evaporated and burned in the recovery system (if capacity available). This means that the oxygen bleaching stage must be the first stage (before any ClO 2 used). Chemical costs: -Oxygen much cheaper than ClO 2. Lower corrosiveness. Environmental: »Less chlorinated organics in discharge (AOX). »Significantly lower BOD, COD, and color in effluent. –This is because the effluent from oxygen bleaching can be evaporated and burned in the recovery system (if capacity available). This means that the oxygen bleaching stage must be the first stage (before any ClO 2 used). Chemical costs: -Oxygen much cheaper than ClO 2. Lower corrosiveness.

Pulping and Bleaching PSE 476: Lecture 195 Disadvantages of Oxygen Bleaching High capital costs. »Low solubility of oxygen (75 times less soluble than Cl 2 ). »Need equipment that can generate good oxygen gas/fiber contact. Economics dictate that this is done at a medium to high consistency. Loss of selectivity when delignification above 50%. »Oxygen bleaching is used to remove lignin. »Approximately 50+% lignin can be removed using oxygen - no more. High capital costs. »Low solubility of oxygen (75 times less soluble than Cl 2 ). »Need equipment that can generate good oxygen gas/fiber contact. Economics dictate that this is done at a medium to high consistency. Loss of selectivity when delignification above 50%. »Oxygen bleaching is used to remove lignin. »Approximately 50+% lignin can be removed using oxygen - no more.

Pulping and Bleaching PSE 476: Lecture 196 Chemistry of Oxygen Bleaching Oxygen used in bleaching is applied as a gas (O 2 ). In this state, most of the oxygen is in the triplet state which means there are 2 unpaired electrons in the outer shell with parallel spin. »Oxygen can also exist in the singlet state: 2 paired or unpaired electrons with antiparallel spin  excited state. Oxygen is not extremely reactive. It reacts in the triplet state with ionized phenolic hydroxyl groups generating phenolic radicals. Therefore, the bleaching must be carried out under alkaline conditions (to generate phenolic hydroxyls) »Metals are needed to drive this reaction Oxygen used in bleaching is applied as a gas (O 2 ). In this state, most of the oxygen is in the triplet state which means there are 2 unpaired electrons in the outer shell with parallel spin. »Oxygen can also exist in the singlet state: 2 paired or unpaired electrons with antiparallel spin  excited state. Oxygen is not extremely reactive. It reacts in the triplet state with ionized phenolic hydroxyl groups generating phenolic radicals. Therefore, the bleaching must be carried out under alkaline conditions (to generate phenolic hydroxyls) »Metals are needed to drive this reaction

Pulping and Bleaching PSE 476: Lecture 197 Oxygen Species Generated During Bleaching Oxygen is reduced through the reaction with phenolic hydroxyl groups to superoxide radical (-O 2 ). A simplified version of what happens next is that through a variety of oxidation/reduction and interconversion reactions, a number of different oxygen species are generated (pH dependent). All of these different species have different degrees of reactivity. The scheme below shows the reduction steps of oxygen on the acid side.

Pulping and Bleaching PSE 476: Lecture 198 Oxygen Species HO 2 : hydroperoxy radical, pKa ~ 4.8 »Ionized form (- O 2 ) : Superoxide radical = weak oxidant. H 2 O 2 : hydrogen peroxide, pKa ~ 11.6 » ionized form (-HO 2 ): hydroperoxy anion = weak oxidant HO. : Hydroxide radical (strong oxidizer), pKa ~ 11.9 »Ionized form (-O): oxyl anion radical Radical species are very reactive: Unfortunately they are not selective and react with carbohydrates. HO 2 : hydroperoxy radical, pKa ~ 4.8 »Ionized form (- O 2 ) : Superoxide radical = weak oxidant. H 2 O 2 : hydrogen peroxide, pKa ~ 11.6 » ionized form (-HO 2 ): hydroperoxy anion = weak oxidant HO. : Hydroxide radical (strong oxidizer), pKa ~ 11.9 »Ionized form (-O): oxyl anion radical Radical species are very reactive: Unfortunately they are not selective and react with carbohydrates.

Pulping and Bleaching PSE 476: Lecture 199 Oxygen Bleaching Lignin Reactions Bleaching conducted under alkaline conditions. »Requires free phenolic hydroxyls on lignin. »Ionized form of oxygen species typically more reactive. All oxygen species involved in process. Reaction Mechanisms. »Ring structures are cleaved and/or substituted with oxygen. »Some cleavage of side chains/linkages. »Lignin/carbohydrate cleaved. Bleaching conducted under alkaline conditions. »Requires free phenolic hydroxyls on lignin. »Ionized form of oxygen species typically more reactive. All oxygen species involved in process. Reaction Mechanisms. »Ring structures are cleaved and/or substituted with oxygen. »Some cleavage of side chains/linkages. »Lignin/carbohydrate cleaved.

Pulping and Bleaching PSE 476: Lecture 1910 Oxygen Bleaching Carbohydrate Reactions Carbohydrates degraded more in oxygen stage than in ClO 2 or extraction stages. Two major degradation pathway: »Glycosidic cleavage by radicals (OH). »Peeling induced through oxidation. Both pathways accelerated by metals (radical formation). »Selectivity improved through the addition of magnesium. -Precipitates metals thus reducing radical formation. Carbohydrates degraded more in oxygen stage than in ClO 2 or extraction stages. Two major degradation pathway: »Glycosidic cleavage by radicals (OH). »Peeling induced through oxidation. Both pathways accelerated by metals (radical formation). »Selectivity improved through the addition of magnesium. -Precipitates metals thus reducing radical formation.

Pulping and Bleaching PSE 476: Lecture 1911 Oxygen Bleaching Process Variables

Pulping and Bleaching PSE 476: Lecture 1912 Oxygen Bleaching Consistency Oxygen has a low solubility in alkaline solutions. »In order to obtain reasonable rates of delignification it is necessary to have good distribution of bubbles in solution. »Originally this was accomplished by dewatering the pulp to very high consistency and fluffing it. This creates a slurry of fibers in a continuous gas phase. »The development of shear mixing devices in the 1970s made it possible to produce very small gas bubbles in medium consistency pulp. Oxygen has a low solubility in alkaline solutions. »In order to obtain reasonable rates of delignification it is necessary to have good distribution of bubbles in solution. »Originally this was accomplished by dewatering the pulp to very high consistency and fluffing it. This creates a slurry of fibers in a continuous gas phase. »The development of shear mixing devices in the 1970s made it possible to produce very small gas bubbles in medium consistency pulp.

Pulping and Bleaching PSE 476: Lecture 1913 Oxygen Bleaching Effect of Temperature

Pulping and Bleaching PSE 476: Lecture 1914 Oxygen Bleaching Effect of NaOH on Lignin Removal

Pulping and Bleaching PSE 476: Lecture 1915 Oxygen Bleaching Effect of NaOH on Carbohydrates