MODELLING OXYGEN DELIGNIFICATION IN PULP PROCESSING OPERATIONS JACKY SUSJXO. B.A.Sc, University of Indonesia, 2002

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1 MODELLING OXYGEN DELIGNIFICATION IN PULP PROCESSING OPERATIONS by JACKY SUSJXO B.A.Sc, University of Indonesia, 2002 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS OF THE DEGREE OF MASTER OF APPLIED SCIENCE in THE FACULTY OF GRADUATE STUDIES (CHEMICAL AND BIOLOGICAL ENGINEERING) THE UNIVERSITY OF BRITISH COLUMBIA APRIL 2005 JACKY SUSILO, 2005

2 Abstract A mathematical model of oxygen delignification for pulp processing operations has been developed to predict the kappa number (lignin content) and pulp strength (CED viscosity) as a function of system operation variables. The model incorporates a set of appropriate chemical reaction kinetics chosen from those available in the literature and gas-liquid mass transfer resistances for pulp suspension mixing and pulp suspension flow through retention towers as measured in our laboratory. This thesis reviews the available kinetic and mass transfer data incorporated in the model and discusses several important issues related to model development and interpretation, including kappa number measurement, the molecular size distribution of the lignin remaining in the pulp and lignin leaching from the fibre. The model is compared with laboratory experimental data from several sources as well an industrial oxygen delignification system. ii

3 Table of Contents Abstract ii Table of Contents iii List of Tables vii List of Figures viii Nomenclature xi Acknowledgements. xiv Chapter 1 1 General Introduction Pulping and Bleaching Oxygen Delignification Objectives and Scopes of Thesis 4 Chapter 2 5 Literature Review Oxygen Delignification Process Chemistry of Oxygen Species and Oxygen Delignification Lignin: Structure and Compositions Lignin Model Compounds 15 iii

4 2.3.2 Lignin Reactions Issues in Industrial Oxygen Delignification Systems Lignin Leaching Kappa Number Determination Molecular Weight Distribution Kinetics and Process Variables Time and temperature Alkali Charge Oxygen Pressure Consistency Washing and black liquor solids carry-over Oxygen and Alkali Consumption Mass Transfer in Oxygen Bleaching Gas-Liquid mass transfer in medium consistency mixers Gas-Liquid mass transfer in pulp retention towers Maximum Lignin Removal in Oxygen Delignification 55 Chapter 3 57 Model Development Introduction Model Formulations Delignification Kinetics 63 iv

5 3.4 Model Development Assumptions Governing Equations Oxygen and Alkali Balances Gas-liquid Mass transfer, Tower Pressure & Temperature Carbohydrate Degradation Effect of Black Liquor Solids Carry-Over 85 Chapter 4 S9 Simulations Results and Discussions Simulations Effect of Temperatures Effect of Tower Pressure Effect of Alkali Charge Effect of Oxygen Charge Effect of Suspension Consistency Effect of Black Liquor Solids Carry-Over Effect of Mixing Power Effect of Gas-Liquid Mass Transfer Coefficient in the Retention Tower Model Validation Howe Sound Pulp & Paper Simulations Mill Survey 113 Conclusions 116 v

6 Recommendations (Future Work) 117 Bibliography 118 Appendix A: LMC Delignification Kinetics 126 A.l Stilbene Structures 126 A.2 P-aryl Ether Structures 130 A.3 Vinyl (enol) Ether Structures 131 A.4 Diphenyl-Methane (5-5) Structures 132 Appendix B: Molecular Weight Distribution Experiments 134 Appendix C: Kappa number & Leaching Experiments. 137 AppendixD: MATLAB Programming Codes 142 D. 1 Non-linear Fitting of Lignin Model Compound Kinetics 142 D. 1.1 Stilbene Structure 144 D. 1.2 Vinyl (Enol) Ether Structure 145 D. 1.3 Beta Aryl Ether Structure 146 D.l.4 Dipropylbiguaiacol (5-5) Structure 147 D.2 LMC Oxygen Delignification Model 148 D.3 LMC Model (with Iribarae & Schroeder's Carbohydrate Degradation Kinetic) 152 Appendix E: Gas-liquid Mass Transfer in Tower 156 Appendix F: Simulations Results 162 vi

7 List of Tables Table 2.1 Oxygen species present in oxygen delignification systems 8 Table 2.2 Proportions of common types of linkages in native lignin [Sjostrom, 1993] 13 Table 2.3 Changes in frequency of linkages (%) of residual lignin as it moves from native wood to oxygen bleached [Northey, 2001; Gellerstedt, 2001] 14 Table 2.4 Approximate fractions of softwood residual lignin in pulp [Ala-Kaila, 2001] 24 Table 2.5 Fraction of apparent and actual birch residual lignin as kappa number [Ala-Kaila, 2003] 27 Table 2.6 Summary of delignification kinetic studies, listing the determined exponents and parameters. 34 Table 2.7 Summary of carbohydrate degradation kinetic studies with fibre suspension 38 Table 3.1 Proposed composition of residual lignin going into oxygen delignification stages 62 Table 3.2 Kinetic parameters acquired from lignin model compound studies 66 Table 3.3 Characteristic rate of oxygen consumption and diffusion for all model compounds 76 Table 3.4 The contents of lignin and its high molar mass in black liquors 85 Table 4.1 Range of operating conditions used in the simulations 89 Table 4.2 Oxygen and alkali consumption per unit kappa number reduction during oxygen bleaching of softwood kraft pulp at medium consistency (laboratory and commercial systems) 94 Table 4.3 Input variables for Howe Sound Pulp & Paper Simulations Ill Table 4.4 Over-all industrial process conditions from mill survey of Bennington and Pinault [1999] vii

8 List of Figures Figure 2.1 Typical schematic diagram of industrial oxygen delignification system 6 Figure 2.2 Reactions of oxygen species under alkaline bleaching conditions [Argyropoulos, 2001] 9 Figure 2.3 Steps in the mechanism of oxygen bleaching [McDonough, 1996] 9 Figure 2.4 Substitutedphenylpropane unit [Sjostrom, 1993] 10 Figure 2.5 Proposed lignin structures [Brunow, 1998] 11 Figure 2.6 Common linkages between phenylpropane units [Sjostrom, 1993] 12 Figure 2.7 Relative reactivity of lignin model compounds under alkali-oj conditions [Ljunggren et al, 1994] 16 Figure 2.8 Initial reactions that lead to oxygen delignification [McDonough, 1996] 18 Figure 2.9 Reactions of intermediate hydroperoxides that lead to lignin fragmentation [McDonough, 1996] 19 Figure 2.10 Representation of the tortuous pathway for a lignin macromolecule to diffuse out in the wood cell wall [Goring et al, 1984] 21 Figure 2.11 Theoretical delignification vs delignification efficiency achieved by surveyed mills [Bennington & Pineault, 1999] 28 Figure 2.12 Molecular size distribution of unbleached kraft ( -) and oxygen delignified ( ) residual lignin of Eucalyptus Globulus [Lachenal et al, 2001] 31 Figure 2.13 Effect of successive oxygen stages to molecular weight distribution [Lachenal et al, 2004] 32 Figure 2.14 Two-stage model of oxygen delignification [Olm and Teder, 1979] 35 Figure 2.15 Effect of temperature on oxygen delignification rate [Harder et al, 1970] 36 Figure 2.16 Viscosity of the oxygen bleached pulp versus kappa number [Hsu and Hsieh, 1987] 37 Figure 2.17 Effect of Alkali Charge on oxygen delignification rate [Liebergott et al, 1985] 40 viii

9 Figure 2.18 Effect of alkali charge on viscosity drop [Liebergott et al, 1985] 41 Figure 2.19 Effect of oxygen pressure on delignification rate [Hsu andhsieh, 1987, 1988] 42 Figure 2.20 Effect of cooking carry-over to kappa number and delignification (kappa #16.7 HW kraftpulp, 10% consistency, 2% NaOH, 3% 0 2, 60 minutes reaction, 100 C) [Iijima & Taneda, 1997]. 43 Figure 2.21 Profile of residual alkali and ph during oxygen delignification reaction (kappa #16.7 HW kraft pulp, 10% consistency, 2% NaOH, 3%02, 60 minutes reaction, 100 C) [Iijima & Taneda, 1997] 44 Figure 2.22 Effect of temperature on oxygen consumption [Berry et al, 2002] 45 Figure 2.23 Mass transfer in oxygen delignification [Iribarne & Schroeder, 1997] 47 Figure 2.24 Effect of mixing on oxygen solubilization in water and NaOH solution [Berry et al, 2002]. 48 Figure 2.25 Effect of secondary mixing (mixing after initial high-intensity mixing at 2400rpm) on the degree of delignification: Mode 1, no mixing; Mode 2, mixing at 240 rpm for 4 seconds after 20 minutes; Mode 3, mixing at 400 rpm for 4 seconds every 5 minutes; Mode 4, mixing at 400 rpm for 4 seconds every 20 seconds; Mode 5, mixing at 60 rpm continuously [Berry et al, 2002] 49 Figure 2.26 Mixing intensity vs degree of delignification [Iijima & Taneda, 1997] 50 Figure 2.27 k L a vs pulp consistency (C ) at different mixing rotational speed [Rewatkar & Bennington, 2000] 52 Figure 2.28 k L a vs superficial gas velocity at various pulp consistency C m [Rewatkar & Bennington, 2002] 53 Figure 2.29 Gas residence time distribution in an industrial oxygen tower [Hornsey et al, 1998] 54 Figure 3.1 Model representation of the actual lignin structures [Jurasek, 1995] 61 Figure 3.2 Degradation of stilbene model compound at different ph (left) with first order plot of the model compound reaction rate vs reaction time (right) [Ljunggren & Johansson, 1990a] 64 Figure 3.3 Differentiation of native lignin into four distinct reactive lignin structures 68 Figure 3.4 Schematic flowsheet of the model (model inputs are shown in bold) 69 Figure 3.5 Representation of mixer and retention tower in the model 70 ix

10 Figure 3.6 Medium consistency oxygen tower [left, White and Larsson, 1996] and pulp RTD of an upflow CIO2 bleaching tower [right, Bennington, 2000] 73 Figure 4.1 Effect of temperature on kappa number during oxygen bleaching 92 Figure 4.2 Dissolved oxygen concentration profile at different temperature vs. bleaching time 93 Figure 4.3 Profile of a decreasing alkali concentration as reaction progresses 95 Figure 4.4 Effect of temperature on viscosity drop during oxygen bleaching 96 Figure 4.5 Effect of temperature on viscosity drop during oxygen bleaching (estimated using carbohydrate degradation kinetics of fibre suspension [Iribarne & Schroeder, 1997]) 97 Figure 4.6 Effect of oxygen pressure on kappa number during oxygen bleaching 98 Figure 4.7 Effect of alkali charge (%) on kappa number during oxygen bleaching 99 Figure 4.8 Alkali exhaustion profile that limit the extent of delignification (ymoh = 1.9 grams/ kg pulp, kappa) 101 Figure 4.9 Oxygen exhaustion during oxygen bleaching (yo2 = 0.55 grams/ kg pulp, kappa) 102 Figure 4.10 Dissolved oxygen concentration profile at different oxygen charge vs. bleaching time 103 Figure 4.11 Effect of consistency on kappa number during oxygen bleaching 104 Figure 4.12 Effect of black solids carry-over on kappa number during oxygen bleaching 105 Figure 4.13 Effect of mixer power on delignification during oxygen bleaching 106 Figure 4.14 Effect of mixer power on kappa number during the first 10 minutes reaction 707 Figure 4.15 Effect of mixer power on oxygen concentration in the liquid phase 108 Figure 4.16 Effect ofk L a (tower) on kappa number during oxygen bleaching. 109 Figure 4.17 HSPP simulation result and measured kappa number with 95% confidence intervals shown 112 Figure 4.18 Actual vs. Predicted outgoing kappa number 114 x

11 Nomenclature Symbol a A A c E A C 2 3 specific gas-liquid interfacial area [m Im ] pre-exponential factor constant alkali charge [% on pulp] activation energy [kj/mol] concentration [mol/l] Cm pulp consistency [%] D oxygen diffusivity inside the fibres [m A] D t DP DP 0 ECCSA h H k diameter of oxygen retention tower [m] degree of polymerization of cellulose initial degree of polymerization of cellulose effective capillary cross-sectional area constant height of oxygen retention tower to be evaluated [m] total height of oxygen retention tower [m] delignification kinetic rate constant initial-stage delignification kinetic rate constant k 2 final-stage delignification kinetic rate constant oxygen solubility constant h gas-liquid mass transfer coefficient [m/s] kia volumetric gas-liquid masss transfer coefficient [s' ] ] DP cellulose (carbohydrate) degradation kinetic rate constant L m n Mw o 2, a, lignin model compound substrate or kappa number number-average of moles of cellulose per metric ton of pulp molecular weight [gram/mol] dissolved oxygen concentration in the liquid phase [mol/l] saturated (maximum) dissolved oxygen concentration [mol/l] xi

12 [Off] hydroxide ion concentration [mol/l] [0 2 ] oxygen concentration [mol/l] p p p top p bottom oxygen charge [% on pulp] pulp production rate [ton/day] oxygen partial pressure at top of retention tower [kpa] oxygen partial pressure at bottom of retention tower [kpa] oxygen partial pressure [kpa or atm] r o> R oxygen consumption rate [molllls] ideal gas constant [J/mol.K] R2 coefficient of determination [%] t reaction time [minutes, hours, or seconds] T temperature [ C or K] T. in T out u s V temperature at bottom of retention tower [ C or K] temperature at top of retention tower [ C or K] superfical gas velocity [m/s] pulp suspension velocity [mlmin] V volume [m 3 or L] W fibre wall thickness (m) X lignin model compound composition [%] y 0l oxygen consumed per kg pulp per kappa number drop [glkg.k ynaoh NaOH consumed per kg pulp per kappa number drop [glkg.k] Superscripts/subscripts b i j m n q exponent of the correction in the k L a at elevated temperature at particular lignin model compund total number of lignin model compound in the model reaction rate constant with respect to oxygen concentration reaction rate constant with respect to alkali concentration reaction rate constant with respect to kappa number xii

13 Greek Letter K K K ( K" K o\ K 02 kappa number initial kappa number final kappa number floor kappa number fast kappa number slow kappa number TAPPI/CED pulp viscosity [mpa.s] M Mo ML intrinsic viscosity of pulp [cc/g] initial intrinsic viscosity of pulp [cc/g] liquid water viscosity [N.s/m 2 ] mixer power dissipation [ Wlm 3 ] gas void fraction rate of oxygen diffusion across the fibre walls [g O^kg pulpls] o rxn rate of oxygen reaction (consumption) [g O^kg pulpls] Acronyms AOX BLS BOD CED COD CSTR HMML HW L/D PFR PMS SW absorbable organic halide black liquor solids [kg/ton of pulp] biological oxygen demand cupri-ethylene-diamine chemical oxygen demand continuous stirred tank reactor high molecular mass lignin hardwood length to diameter aspect ratio plug flow reactor peroxymonosuphate softwood xiii

14 Acknowledgements I gratefully acknowledge the following for their contribution to this project: Dr. C.P.J. Bennington, my research supervisor, for his guidance, kind support, suggestion, and helpful discussion throughout the course of this research. Dr. B.D. Bowen and Prof. C.W. Oloman, members of my advisory committee, for their helpful advice. R.M. Berry, for his valuable discussion. G.L. Pageau and members at the Howe Sound Pulp and Paper Ltd., for their support during our mill tests and supplying the pulp samples used in this research. Dr. J.F. Kadla and members of biomaterials chemistry laboratory at the Department of Wood Science, for obtaining the molecular weight distributions of residual lignin through enzymatic isolation procedures. Members of the Pulp and Paper Centre and Chemical and Biological Engineering Department for their help in many aspects of this research. My family, my mother, brother and sister, and all my friends, thank you for your help and never ending support. xiv

15 Chapter 1 General Introduction 1.1 Pulping and Bleaching Pulp and paper mills generally are made up of a wide variety of processes which transform fibrous raw material into pulp and paper products. Complicated physical and chemical phenomena take place in many of the unit operations involved in these processes. There are several different methods of converting wood chips into unbleached or fully bleached pulp. Chemical pulping using the kraft process is a typical example. The most important unit operations involved in the kraft process are digesting for pulping, brown stock washing for chemical recovery, and a series of bleaching towers for brightening the pulp. There are also many other smaller units that play an important role and are an integral part of the pulp and paper mill such as pumping, mixing, stock chests for storage and blending, sensors and control systems, etc. During pulping with chemicals and heat in the digester, lignin and other alkalisoluble constituents of wood are dissolved in the cooking liquor and fibres are liberated. The mixture of pulp and spent cooking liquor after the digestion process, referred as brown stock, is fed to a series of washers, where spent cooking liquor is washed out from the pulp using either fresh or recycled water or some combination of the two. The spent cooking liquor, called weak black liquor, is then delivered to the recovery system where the valuable spent cooking chemicals and the energy content of the liquor are recovered. The washed pulp is transferred to the bleach plant where it is subjected to a series of bleaching stages involving different chemical Chapter 1: Introduction

16 2 treatments to increase pulp brightness while maintaining pulp strength. Fully bleached or 'white' pulps, are then produced. Chemicals that have been used for bleaching include, but are not limited to, oxygen, ozone, chlorine, chlorine dioxide, sodium hydroxide, and hydrogen peroxide. Figure 1.1 shows a rough schematic of a typical kraft pulp mill starting from the wood chip to the fully bleached pulp. Wood Chips 1 Digestion Cooking Liquor I Make-up Chemicals H Z -1 Fresh/Reused water Brown Stock Washing Black liquor Recovery System Energy Alkali Oxygen Oxygen Delignification Fresh/Reused water Post-0 2 Washing Bleaching chemicals Bleach Plant ir Bleached Pulp Figure 1.1 Schematic diagram of kraft pulp process with oxygen delignification 1.2 Oxygen Delignification For over the last decade, there have been quite a number of efforts attempting to optimize the extent of pulp delignification before it enters the bleach plant. One of the processes that has Chapter 1: Introduction

17 3 received considerable attention is oxygen delignification. This is the process that takes place between the cooking and the bleaching processes. The fact that the chemicals applied to oxygen delignification are compatible with the kraft recovery system has made this process desirable and relatively easy to integrate into an existing kraft mill. However, it was not until 1970 in South Africa that the first commercial oxygen delignification system was commissioned. Many mills have incorporated oxygen delignification into their process lines since then. Figure 1.2 shows how the worldwide production capacity of pulp made via oxygen delignification has increase since ? 5 c 2-6 CO o A Single-stags systems Two-stage systems 9 O. (0 U Figure 1.2 Growth of Oxygen Delignification Worldwide (Dence & Reeve, 1996) The main unit operations in the oxygen delignification system are the mixing of chemicals into a pulp slurry, the reaction stage, and the washing of pulp after the reaction stage. Optimization of oxygen delignification must take into account the mass transfer of oxygen to the reactive lignin sites within the fibre and the subsequent chemical reaction kinetics. Also, there Chapter 1: Introduction

18 4 are some other critical issues that are of concern, including accurate kappa number determination, lignin leaching, mass transfer limitations, and the floor kappa number. All these topics have been the subject of research and publication. Any model of an oxygen delignification system needs to consider all of these issues. 1.3 Objectives and Scopes of Thesis Computer simulation can be of considerable help in solving design and, more importantly, operational problems. This has been the reason why there is an interest in developing better computer models that are not only able to predict what could happen in the actual process but also able to underline the significance of the particular unit operation being used in the mill as well as, if possible, to help optimize the system. The industrial oxygen delignification processes can be modeled mathematically based on mass and energy balances and appropriate reaction kinetics. In this project, a set of mathematical ordinary differential equations are developed which incorporate the appropriate reaction kinetics of several lignin model compounds as well as mass transfer and consumption of both oxygen and alkali in the mixer and oxygen tower. The model can be used to study the response of the final kappa number, viscosity, and residual chemicals to changes in various process variables both in the medium-consistency mixer and in the retention tower that follows. The model is compared with data from several operating mills as well as from batch operated laboratory systems. Chapter 1: Introduction

19 5 Chapter 2 Literature Review 2.1 Oxygen Delignification Process Oxygen delignification is a well-known and well-established technology for removing a substantial portion of the residual lignin in unbleached pulp using oxygen and alkali. The dissolved lignin can then go to the recovery furnace instead of to the bleach plant where it would be a potential source of environmental problems. The environmental benefits that oxygen delignification processes offer, together with the more stringent environmental regulations being imposed, has made oxygen delignification one of the most important technologies that can be incorporated in modern pulp and paper mills. In addition to its environmental advantages, the development of oxygen delignification has also been driven by economic considerations such as operational cost savings and the desire to improve pulp properties. Lower chemical costs result from the decreased requirement for delignifying oxidizing chemicals in the bleach plant (e.g., chlorine, chlorine dioxide, hydrogen peroxide, and ozone). This is because oxygen is less expensive, and oxidized white liquor usually provides the necessary alkali for the oxygen stage at low cost. Further savings result from a decrease in the chlorine dioxide charge needed for the final bleaching stages [McDonough, 1996]. Improvements in pulp quality, such as higher brightness [McDonough, 1995], higher delignification without sacrificing pulp selectivity, and higher yield, have all been reported using the proper adjustment of operating conditions [Bokstrom, 1999]. Chapter 2: Literature Review

20 Oxygen delignification is typically carried out under medium-consistency conditions with either softwood or hardwood pulps using sodium hydroxide as an alkali source (1 to 4% NaOH on pulp) and with an oxygen pressure of 400 to 1000 kpa. Three phases are present in the reaction vessel: the solid pulp fibres, the aqueous phase around the fibers and within the fiber pores, and the oxygen gas phase distributed throughout the mixture. The pulp at 8 to 12% consistency is heated to about 80 to 100 C in a steam mixer, oxygen is injected in a high-shear mixer(s), and retention times of 20 to 90 minutes are normally achieved using an up-flow oxygen tower(s). Figure 2.1 illustrates the flowsheet of a typical industrial oxygen delignification system. Figure 2.1 Typical schematic diagram of industrial oxygen delignification system The rate of oxygen bleaching is determined by both physical and chemical phenomena [McDonough, 1986]. Physical factors govern the movement of the reacting species within the Chapter 2: Literature Review

21 pulp (mass transfer), while chemical factors govern the rate at which the pulp and bleaching chemicals react with one another once they are in contact (chemical kinetics). It is therefore obvious that, to understand the oxygen bleaching process, two key obstacles must be overcome: the tendency of the process to be chemically nonselective and the difficulty in achieving efficient mass transfer. The selectivity problem arises from the natural tendency of oxygen to form reactive free radicals that can attack cellulose and other carbohydrates as well as lignin. Limitations in mass transfer derive from the fact that oxygen bleaching processes involve threephase systems, and depending on the conditions, the rate of oxygen transport can limit the rate of the overall process [McDonough, 1996]. Key parameters in any commercial oxygen delignification system are the temperature, alkali charge, oxygen pressure, and the degree of mixing for efficient mass transfer of oxygen [McDonough, 1986; Bennington and Pineault, 1999]. Surveys of industrial oxygen delignification systems show that the overall extent of delignification varies from mill to mill, with an average of 36.0% for softwoods and 36.6% for hardwoods, with system delignifications ranging from a low of 21% to a high of 48% for softwoods and from 26% to 46% for hardwoods [Bennington and Pineault, 1999]. 2.2 Chemistry of Oxygen Species and Oxygen Delignification The fundamental interactions of oxygen in aqueous alkali with lignin moieties have been the subject of various structural and kinetic enquiries. However, the conclusions of such studies cannot fully describe the events that occur when oxygen interacts with lignin macromolecules. It is likely that the multitude of branching and cross-linking variations that occur within lignin may Chapter 2: Literature Review

22 8 cause significant accessibility variations of reactive moieties towards even the same functional group [Argyropoulos, 1997]. Under the conditions typically used in oxygen bleaching, there are a large number of oxygen species present (Table 2.1). Table 2.1 Oxygen species present in oxygen delignification systems Oxygen Species Anionic Form Oxygen 2 Hydroperoxy Radical HO r Superoxide Anion Radical Hydrogen Peroxide H Hydroperoxy Anion HO~ Hydroxyl Radical HO- Oxyl Anion Radical ~0- The reaction of oxygen with lignin moieties under alkaline conditions generates a superoxide radical anion through a one-electron transfer from the lignin active site to oxygen. This is generally the rate-determining step of the oxidation and requires the presence of metal ions or elevated temperatures. This superoxide radical can undergo a metal catalyzed dismutation forming hydroperoxide and a hydroxy radical [Argyropoulos, 2001]. Of the oxygen species listed in Table 2.1, only the hydroxyl radical and its conjugate base are strong oxidants; the oxygen itself is a weak oxidant. Therefore oxygen bleaching is usually run under alkaline conditions so that ionized phenolic hydroxyl groups on the lignin will furnish the high electron density needed to initiate an electron transfer. A listing of some of the interconversion reactions between oxygen species is shown in Figure 2.2. Certain of these reactions are extremely rapid under oxygen bleaching conditions Chapter 2: Literature Review

23 9 while others may require the presence of metals or protons to catalyze the reaction [Argyropoulos, 2001]. 0 2 ~ + HO' -> HO~ H0 2 ~ + HO' -> HO~ + *0 2 HO- + HO' -> H ~ + *0 2 + H 2 0 -> HO~ HO~ '0 2 + H > HO~ + HO' H H0 2 ~ -> H HO- + '0 2 ~ Figure 2.2 Reactions of oxygen species under alkaline bleaching conditions [Argyropoulos, 2001] The complex oxidation processes that occur in the oxygen-bleaching reactor include radical chain reactions involving a variety of organic species derived from both lignin and carbohydrate. Figure 2.3 shows likely initiation, propagation, and termination steps. Initiation RO > RC (1) RH > R. + HO z. (2) Propagation R» > R0 2 (3) R RH -> R0 2 H + R. (4) Termination RO + R«-> ROR (5) Figure 2.3 Steps in the mechanism of oxygen bleaching [McDonough, 1996] In oxygen bleaching, the substrate is activated by providing alkaline conditions to ionize free phenolic hydroxyl groups in the residual lignin. The resulting anionic sites are electron-rich Chapter 2: Literature Review

24 10 and therefore vulnerable to attack by oxygen. An electron is abstracted, forming a superoxide anion and a phenoxy radical (Figure 2.3, reaction 1). An alternate pathway for initiation of the radical chain reaction is abstraction of a hydrogen atom from an unionized phenolic group or other functional group to give the corresponding organic radical (Figure 2.3, reaction 2). Propagation of the chain reaction occurs by reactions such as the one between oxygen and an organic radical to form a peroxy radical which, in turn, may abstract a hydrogen atom to regenerate a new organic radical (reactions 3 and 4). The chain is terminated by coupling reactions. 2.3 Lignin: Structure and Compositions Lignin is a heterogeneous polymer that is made up of substituted phenylpropane, i.e. coniferyl alcohol (softwood) or a mixture of coniferyl and sinapyl alcohol (hardwood) units. Although various studies have been conducted, the exact structure of lignin is still not known. Brunow [1998] proposed a widely accepted model for the structure of softwood lignin (Figure 2.5). Softwood lignin has an average molecular weight of 20,000 while the molecular weight of hardwood lignin is lower [Sjostrom, 1993]. Due to its complexity, most researchers have simplified lignin into a basic substituted phenylpropane unit (Figure 2.4). R R = H.OCH>C- R' = H.C- Figure 2.4 Substituted phenylpropane unit [Sjostrom, 1993] Chapter 2: Literature Review

25 11 Figure 2.5 Proposed lignin structures [Brunow, 1998] The aromatic ring on lignin structures has one or two methoxy substituents (carbon-3 and carbon-5, see position designation in Figure 2.4), a C-C linkage (carbon-5), and an ether or a hydroxy substituent (carbon-4). Different wood species will have different types and numbers of these basic lignin units. These individual units can be linked through the R-groups (Figure 2.4) and the a-p double bond to form both C-C and C-O-C (ether) bonds by radical reactions [Sjostrom, 1993]. The ether linkages dominate approximately two-thirds or more and the rest are Chapter 2: Literature Review

26 12 of the carbon-to-carbon type. Some of the common linkages between the phenylpropane units in lignin are shown in Figure 2.6. The most abundant linkage between these units in native lignin is the ether P-O-4 bond. The list of all common linkages in native lignin with their approximate proportions can be seen in Table P-5 P-1 P-P Figure 2.6 Common linkages between phenylpropane units [Sjostrom, 1993] During alkaline pulping, most of the ether bonds having p-0-4 and a-o-a structures are cleaved by hydroxide ions promoting efficient lignin fragmentation by generating new free phenolic hydroxyl groups. A minor portion of lignin during alkaline pulping can also be ascribed to the cleavage of carbon-carbon bonds forming formaldehyde that may initiate lignin condensation reactions. Chapter 2: Literature Review

27 13 Table 2.2 Proportions of common types of linkages in native lignin [Sjostrom, 1993] Linkage type Dim er Stru ctu re Percent of the total linkages Softwood Hardwood Ary lgly cerol-b^ary 1-ether 50 6o a-o-4 Noncyclic benzyl aryl ether p-5 Phe ny lc ou ma ra n p-1 l, 2-Diary 1 propane lo-ll Biphenyl Diarylether 7 7 P-P Linked through side chains 2 3 In kraft pulping, the majority of condensation reactions occur in the unoccupied C-5 position of the phenolic unit. Therefore, condensation occurs more easily in softwood than hardwood and is part of the reason why hardwood pulps are normally easier to bleach than softwood pulps. Moreover, it has been found that, during conventional kraft pulping, significant structural changes occur in the residual lignin with the progressive enrichment of carboxylic acid and condensed phenolic hydroxyl groups. Toward the end of pulping, residual lignin contains much more C-5 condensed phenolic hydroxyl groups than were present in the native wood. Both C-5 and C-6 condensed phenolic units were found to be relatively stable toward oxygen delignification [Jiang and Argyropoulos, 1999]. The approximate proportions of changes in composition of residual lignin as it moves from native wood to oxygen delignified pulp are given in Table 2.3. As can be seen from Table 2.3, a variety of reactions that take place during a kraft cook lead to the formation of new structures, i.e. stilbene and enol-ether structures. These structures exist in negligible amounts in native lignin; however, the amount of stilbenes following cooking has been estimated using UV analysis to increase to approximately 3-5% of the total linkages Chapter 2: Literature Review

28 14 [Northey, 2001]. An increase in the amount of vinyl (enol) ether structures was also observed which could be up to 0.5-2% of the total linkages. Table 2.3 Changes in frequency of linkages (%) of residual lignin as it moves from native wood to oxygen bleached [Northey, 2001; Gellerstedt, 2001] Linkage Type M m " Kraft lignin 2) 0 2 delignified 3) (% of linkages) (%) (%) a) ;,) 0-O-4 uncondensed condensed uncondensed 7 lo condensed n O-5 condensed ff -O -4 uncondensed fi-p uncondensed Stilbenes highly reactive Negligible Vinyl Ethers reactive Negligible 2 - diphenyl Methane condensed Negligible Total loo milled wood lignin approximate value for a residual kraft lignin from a 30 kappa pulp approximate value after an O-stage kappa 9.3 Another noticeable change in the residual lignin composition shown in Table 2.3 is in the condensed structure of diphenylmethane (DPM). The exact amount of these structures is a matter of debate, as the analysis of residual lignin samples using various methods have generated values from roughly 5% to as high as 60%) of the total linkages [Argyropoulos et al, 1998; Chiang and Funaoka, 1990]. Studies have shown that residual lignin undergoes structural changes creating new functional groups and new compositions following the kraft cooking process. Studies with lignin model compounds were needed to determine how these new functional groups and linkages react under oxygen delignification conditions. Chapter 2: Literature Review

29 Lignin Model Compounds In order to get a better and more complete understanding of the reactivity and mechanisms involved in oxygen delignification, model compounds containing the linkages and functional groups assumed to represent the native lignin structures found in the residual lignin of kraft pulps were studied. The selection of which functional groups and linkages to investigate was based upon the presumed structure of residual lignin. Today, lignin model compounds have been extensively developed using the results of wet chemical procedures such as permanganate oxidation, acidolysis, 1 3 C Nuclear Magnetic Resonance (NMR), and also 31 P NMR [Northey, 2001]. However, since native lignin is comprised of various different functional groups, one lignin model compound can only represent a small portion of the native (actual) lignin present in the fibre wall. Numerous studies with individual model compounds have demonstrated that only those structures containing a free phenolic hydroxyl groups react to any extent under oxygen bleaching conditions. Because of the lack of reactivity of non-phenolic model compounds, the majority of studies on ring cleavage by oxygen have focused on compounds with free phenolic hydroxyl groups. However, a non-phenolic model compound has been degraded by oxygen in the presence of a compound containing a free phenolic hydroxyl group and excess iron. This increase in reactivity of the non-phenolic hydroxyl model compound was caused by the generation of oxygen radical species in the reaction of oxygen with the free phenolic model compound [Argyropoulos, 2001]. The most reactive lignin model compounds are of the stilbene type closely followed by the vinyl (enol) ether structures. Stilbene structures are rapidly degraded by oxygen under Chapter 2: Literature Review

30 16 alkaline condition even at temperatures as low as 45 C [Ljunggren, 1986; Ljunggren & Johansson, 1990]. A series of lignin model compounds are listed in Figure 2.7 in relative order of their susceptibility to oxidation. diguaiacol stilbene /^-hydroxy stilbene >(\ > CHOH i CHaO CHa y=s H OH CHs CHs phenolic p-aryl ether dipropylbiguaiaco! Figure 2.7 Relative reactivity of lignin model compounds under alkali-0 2 conditions [Ljunggren et al., 1994] When subjected to alkaline oxidation conditions, degradation of phenolic stilbenes occurs across the double bond forming phenolic aldehydes [Gierer and Nivebrant, 1986]. Vinyl ether structures also cleave across the double bond. Although still quite reactive, vinyl ethers oxidize over one hundred times slower than their stilbene counterparts. The remaining structures listed in Figure 2.7, other than stilbene and vinyl (enol) ether, are significantly less reactive with oxygen under typical oxygen bleaching conditions. All of these compounds react, by an order of magnitude or more, slower than the vinyl-ether structures. The P-O-4 and P-1 linked compounds react fairly slowly, with the diphenylmethane, 5-5, and Chapter 2: Literature Review

31 17 P-5 linked compounds being the slowest. It is important to note that all of these compounds were reacted alone and so any effects of multiple branching and crosslinking that exist in native lignin were eliminated Lignin Reactions Studies of the mechanisms of lignin removal during oxygen bleaching have been made since the 1960s and still continue up to now [Gierer, 1993]. The reason for this is because reactions of lignin during oxygen delignification are complex and not completely understood. Model compounds have provided considerable insight about the effects of the process and structural features of both residual and dissolved lignin into the physical and chemical aspects of the reaction mechanism involved in the oxygen degradation of lignin [Gierer, 1993; Ljunggren and Johansson, 1990 & 1994; Northey, 2001; Argyropoulos, 2003]. McDonough [1996] summarized studies of lignin reactions under oxygen alkaline conditions. He showed that free phenolic hydroxyl groups play a major role in oxygen delignification. The phenolic group is ionized forming a phenoxy radical under strong alkaline conditions, generating a site with high electron density, which undergoes transfer of a single electron to molecular oxygen or any of the available radical species. The resulting phenoxy radical is a resonance hybrid of structures which, in several ways, could be transformed into the hydroperoxy radical, an intermediate structure which can subsequently undergo an intramolecular nucleophilic reaction at its adjacent site leading to the formation of oxirane, muconic acid, and carbonyl structures. The last step corresponds to breakage of a bond joining two lignin monomeric units and therefore leads to lignin fragmentation. The other step Chapter 2: Literature Review

32 18 corresponds to the introduction of hydrophilic groups, imparting a polar character. Both types of reactions may be expected to enhance the solubility of lignin in an alkaline medium. The degradation products from oxygen delignification are predominantly organic acids and carbon dioxide. The suggested mechanisms of oxygen delignification are illustrated in Figure 2.8 and Figure 2.9. Figure 2.8 Initial reactions that lead to oxygen delignification [McDonough, 1996] Studies employing pulp suspensions have confirmed, to a large extent, the conclusion drawn from model compound studies. Gellerstedt [1986, 1987] and Argyropoulos [1997] Chapter 2: Literature Review

33 19 investigated the structure of the lignin in oxygen-bleached pulp and concluded that oxygen bleaching reduces the content of free phenolic units. Lignin remaining in pulp after oxygen bleaching was enriched with biphenyl-type condensed structures and p-hydroxyphenyl-type lignin. It was further confirmed that the number of phenolic hydroxyl groups in the pulp lignin decreased and the number of carboxylic acid groups increased the lignin solubility. Figure 2.9 Reactions of intermediate hydroperoxides that lead to lignin fragmentation [McDonough, 1996] A final point concerning the chemistry of lignin reactions during oxygen bleaching is the role of covalent linkages between lignin and one or more of the carbohydrate components. Researchers have found that a lignin-carbohydrate complex extracted from pulp after oxygen delignification contained about half of the residual lignin, and that it was extensively degraded [McDonough, 1996]. They concluded that cleavage of a bond between xylan and lignin would allow more extensive oxygen delignification. Chapter 2: Literature Review

34 Issues in Industrial Oxygen Delignification Systems Often, positive results obtained in laboratory systems are not duplicated in commercial operations and hence the expected return of investment is not delivered to the customer by the vendor. Some of this discrepancy could be caused by using laboratory reactors that allow the chemistry of the oxygen delignification reaction to proceed without taking into account some of the physical constraints that exist in a commercial system [Berry et al, 2002]. Some of the differences between commercial systems and many laboratory reactors are: A specific charge of oxygen is applied in a commercial system whereas a large excess of oxygen is used in many laboratory reactors. Commercial systems have variations in pressure because of a changing hydrostatic head in the retention tower while laboratory reactors are commonly used with a constant oxygen pressure. There is generally only one opportunity to mix oxygen with pulp in a commercial system whereas mixing is either intermittent or continuous in laboratory reactors. The pulp volume to reactor surface area is very much larger in a commercial system than in laboratory reactors. The effect of carry-over is normally neglected in laboratory experiments. Depending on the washing efficiency, the carry-over in commercial systems could be quite significant. Despite all of the differences mentioned above, there are some other critical issues that may affect the performance of an industrial oxygen delignification system. Some of the issues are lignin leaching and kappa number determination. Understanding these issues is essential for the modeling of industrial oxygen delignification systems. Chapter 2: Literature Review

35 Lignin Leaching Leaching of lignin can be defined as the transport of the lignin macromolecule out of the fibre wall into the bulk liquid. This process normally takes place in the washing system with the addition of fresh or reused water. During oxygen delignification, residual lignin reacts with oxygen gas under alkaline conditions, after which the degraded lignin must diffuse out of the fibre wall before it can be dissolved in the bulk liquor. As illustrated in Figure 2.10, the degraded lignin macromolecule has to traverse a tortuous path across the fibre wall before it can dissolve in the bulk liquor, which makes the pathway of a particular molecule being leached from the fibre considerably longer than the distance measured directly across the fibre wall [Goring et al., 1984]. Moreover, the distance between adjacent lamellae in the fibre wall may vary so that a lignin macromolecule may then be trapped in a pore of the fibre wall (pore restriction). Another restriction on the leaching of lignin is the possible viscous drag force between the macromolecule and the walls of the pore which will reduce the rate at which the lignin can diffuse out [Goring et al., 1984]. Figure 2.10 Representation of the tortuous pathway for a lignin macromolecule to diffuse out in the wood cell wall [Goring et at., 1984] Chapter 2: Literature Review

36 22 Several publications [Yean et ah, 1976; Favis et ah, 1981; 1983; 1984] on the leaching of lignin during the washing stage have been published and all have described the slow diffusion of lignin macromolecules from the fibre wall into the wash liquid. It was reported that the diffusion coefficient observed during these leaching experiments was several orders of magnitude smaller than that corresponding to free diffusion in water. It is likely that the interaction of lignin with the cellulose hydrogel (mainly hemicellulose) must greatly restrict the diffusion of the lignin macromolecules out of the fibre. Diffusion or leaching of lignin will always take place whenever there is free liquor between the fibres and a concentration difference between this bulk liquid and the immobile liquor inside the fibre wall. Diffusion is retarded at high pulp consistency because of the limited amount of free liquor between the fibres. The speed at which diffusion take place is a strong function of ph and temperature [Favis et ah, 1983]. The size (molecular weight) of the diffusing lignin, type of wood species, free liquor turbulence, and process configuration also affects the diffusion phenomenon [Favis et ah, 1984; Ala-Kaila et ah, 1997]. After a kraft cooking process, the residual lignin in pulp fibres consists of different types of chemical fractions [Northey, 2001]. As in kraft cooking, oxygen delignification also proved to promote certain aromatic structures in residual lignin over some other structures [Gellerstedt et ah, 1986; 1987]. The vast variety of lignin moieties formed during pulping and oxygen bleaching, depending on the process conditions used in the digester and oxygen delignification system, differ in their degree of oxidation. A certain portion of the residual lignin could be partially oxidized or fragmentized, another portion could be fully oxidized, while the remaining residual lignin could be left un-oxidized. Partially oxidized lignin molecules following the cooking and oxygen delignification processes, depending on their size and location within fibre Chapter 2: Literature Review

37 23 wall, are trapped inside the fiber wall and over time could be leached out from the fibre to the surrounding liquor. However, the partially oxidized lignin will consume permanganate during a kappa number test and, hence, contribute to what is called 'transient' material. The term 'transient' for residual materials left in the pulps, i.e. residual lignin, comes from fact that a considerable amount of residual material could be leached out from the fiber to the free liquor surrounding the fiber providing sufficient time is allowed. Consequently, the measurement of how much lignin is actually left inside the fiber wall would depend on when the kappa number test is conducted. In other words, there are situations where the kappa number test does not measure the actual lignin requiring oxidation during the oxygen delignification process. This issue will have a significant impact when examining the performance or overall efficiency of an industrial oxygen delignification system. The transient behavior of the residual lignin components in chemical pulps has been the subject of intensive study [Crotogino, 1987; MacLeod, 1993 & 1996; Ala-Kaila, 1996 & 2001]. Mill-produced kraft pulp fibres hardly ever represent a state of equilibrium with respect to residual alkali-soluble lignin. When examining the overall performance or efficiency of a particular process, this time-dependent behavior of residual lignin components should be taken into consideration in order to get a precise evaluation on the efficiency of the process. The group of Ala-Kaila [1996, 2001, 2003] did extensive research on lignin leaching, particularly during the oxygen delignification stage. From leaching experiments with softwood kraft pulp, they found that residual material in the pulp could be divided into four different fractions: wash loss, easily leachable, slowly leachable, and stagnant. The wash loss fraction is specified as kappa number drop after 5 minutes washing. The easily leachable lignin is the amount of kappa number that was removed after 30 minutes leaching. The third fraction, slowly Chapter 2: Literature Review

38 24 leachable lignin, was removed after 24 hours of leaching. Stagnant lignin was defined as the kappa number that remains following 24 hours of leaching. The sum of the four fractions is equal to the total kappa number of the original pulp sample before leaching. The leaching rate was also found to accelerate with increased pulp temperature, alkali content (ph), and the presence of oxygen gas or air. These results were in agreement with the previous experiments of Goring [1983] and MacLeod [1993; 1996]. The approximate fractions of residual lignin in the pulp samples are shown in Table 2.4. Leaching experiments were performed at low consistency at C under nitrogen bubbled through the suspension to prevent oxidation during leaching. Pulp samples were centrifuged up to 38% consistency before leaching was started to remove any dissolved solids or impurities [Ala-Kaila, 2001]. Table 2.4 Approximate fractions of softwood residual lignin in pulp [Ala-Kaila, 2001] Approximated fractions BSW Oj blow' 0 2 blow' Initial kappa number Wash loss fraction Kappa after.5-min washing 19.7 i Easily leachable fraction Kappa after 30-mm leaching Slowly leachable fraction Kappa after 24-h leaching Stagnant fraction T=85 C, 10% Consistency, 3.5% NaOH, 15% 0 2, 45 minutes retention 2) T=100 C, 10% Consistency, 0.4% NaOH,.l% 0 2, 45 minutes retention From the results of the leaching experiments shown in Table 2.4, most of the leached lignin was present in the wash loss fraction that took place during the first 5 minutes period of washing with de-ionized water at 2% consistency. These results underline the importance of thorough washing in the mill to achieve maximum delignification in an oxygen-alkali Chapter 2: Literature Review

39 25 delignification process. Moreover, despite a decrease in total kappa number, there is a noticeable reduction in the amount of the easily leachable fraction from 1.1 to 0.4 kappa, from the brown stock washer to the 1 st oxygen blowline (Oi blow, Table 2.4). This finding suggested that both leaching phenomena and oxidation reaction are occurring in the 1 st oxygen tower as the pulp suspension moves upward through the tower. Furthermore, when compared with the result following the 2 nd reactor (O2 blow, Table 2.4), lower amounts of easily and slowly leachable fraction are observed. This suggested that in the 2 nd oxygen reactor (tower), the structure of the lignin is becoming less reactive towards oxygen, possibly being more condensed in structure as has been measured by Argyropoulos [1997] and the effect of leaching could be more predominant in the second oxygen retention tower Kappa Number Determination Kappa number is defined as the volume (in milliliters) of 0.1N potassium permanganate solution consumed by one gram of moisture-free pulp under conditions specified in the standard kappa method (TAPPI T-236 cm-85 or PAPTAC G.18). The method can be used for all types and grades of chemical pulp obtained in yields under 60%. The standard kappa number test is widely employed and uses the rapid oxidation of lignin by acid potassium permanganate to estimate the residual lignin content in pulp fibres. The method has been used both in mill operations and in laboratory work since 1934 as a measure of the degree of delignification of chemical pulps in pulping, oxygen delignification, and bleaching stages. However, recent studies [Gellerstedt, 1998a] on the kinetics and mechanisms of the kappa number test have pointed out a drawback and an improper definition given in the standard Chapter 2: Literature Review