Pulping and Bleaching PSE 476/Chem E 471 Lecture #19 Oxygen Bleaching and NaOH extraction

Agenda Process Overview Advantages/Disadvantages Reduction of Oxygen: Oxygen species Lignin Reactions Carbohydrate Reactions Effect of Process Variables NaOH extraction

Oxygen Bleaching Process Overview Feed: Unbleached pulp from brown stock washer. Medium consistency 10-14%, High consistency 20-28%. Alkali (NaOH) added in pump to feed tank. Oxygen added in high shear mixer. Pulp (oxygen) pass through up flow reactor (1 hr). Pulp and gases (O2 & other) separated. Pulp thoroughly washed (twice).

Oxygen Bleaching

Sequence from last lecture

Oxygen Bleaching

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 ClO2 used). Chemical costs: Oxygen much cheaper than ClO2. Lower corrosiveness.

Disadvantages of Oxygen Bleaching High capital costs. Low solubility of oxygen (75 times less soluble than Cl2). 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.

Chemistry of Oxygen Bleaching Oxygen used in bleaching is applied as a gas (O2). 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 Species Generated During Bleaching Oxygen is reduced through the reaction with phenolic hydroxyl groups to superoxide radical (-O2•). 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. hydroperoxy radical hydrogen peroxide hydroxide radical

Oxygen Species HO2• : hydroperoxy radical, pKa ~ 4.8 Ionized form (- O2•) : Superoxide radical = weak oxidant. H2O2: hydrogen peroxide, pKa ~ 11.6 ionized form (-HO2): 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.

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 .

Radical Reactions: Generalities Ring Cleavage Ortho Quinones Side Chain Cleavage Ring Substitution

Oxygen Bleaching Carbohydrate Reactions Carbohydrates degraded more in oxygen stage than in ClO2 or extraction stages. Two major degradation pathway: Glycosidic cleavage by hydroxide radicals (OH•)-major reaction. Peeling induced through oxidation. Both pathways accelerated by metals (radical formation). Selectivity improved through the addition of magnesium. Precipitates metals thus reducing radical formation.

Oxygen Bleaching Process Variables

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 Bleaching Effect of Temperature

Oxygen Bleaching Effect of NaOH on Lignin Removal

Oxygen Bleaching Effect of NaOH on Carbohydrates

NaOH Extraction Lignin Reactions Purpose: to dissolve and then to remove compounds made alkali soluble in the proceeding acid delignification. Lignin (2 functions) Removal (solubilization) of reacted (modified) lignin Reactivation of residual lignin (3 theories) Chlorolignin prevents migration of Cl2 or ClO2. Removal by NaOH opens new sites. Remaining lignin in LCC’s. Other oxidants beside chlorine are necessary to cleave. Acid oxidation changes lignin to non reactive form. Alkali converts lignin to a reactive species.

NaOH Extraction Carbohydrate Reactions Peeling Limited amount – somewhat dependent upon previous stage. Stopping With Oxygen or H2O2 Oxygen enhances the effect of extraction. Standard O2 reactions (oxidative peeling, etc).

NaOH Extraction Conditions (1) NaOH Charge The more NaOH used, the more residual lignin removed. Effect levels out at higher NaOH concentration. Optimum charge leaves a residual pH = 10.5 NaOH charge affects the amount of oxidant necessary in the next stage. Consistency Medium (8-10%) most practical. Mixing, capital costs, operating costs, etc.

NaOH Extraction Conditions (2) Temperature Higher temperature = faster rate. Typical: 60-80°C. Time Dependent on other variables. Typical: 60 –90 minutes. pH: >10.5

NaOH Extraction Conditions (3) Oxygen or H2O2 Addition By 1980, it was discovered that addition of O2 or H2O2 to the E stage improved delignification. Driven by the desire to get away from AOX out of plant. Less expensive O2. Reduce chlorinated hydrocarbons in pulp. Activates lignin towards future stages. Alkalinity after Acidic Stage Improves O2/H2O2 Efficiency. Washing after-very important.