Rate determining step and kinetics of oxygen delignification

oxygen delignificationT30Rate determining step andkinetics of oxygen delignificationWINNER OF THEHOWARD RAPSONAWARDBy Y. Ji, E. Vanska and A. van HeiningenAbstract: A differentially operated, continuous stirred tank reactor (CSTR) was used to study the kineticsof oxygen delignification. The delignification kinetics and reaction rate were determined at differenttemperatures, oxygen pressures and caustic concentrations on softwood kraft pulps. The kinetics are firstorder in residual lignin content (HexA corrected). The kinetics of phenolic delignification can be describedby assuming that the decomposition of the hydroperoxide anion at carbon 3 of the aromatic ring is the ratedetermining step. The cellulose degradation kinetics were described by two contributions: one due to radicalsproduced by phenolic delignification, and the other due to alkaline hydrolysis.Many studies have been performedto determine the kineticsof oxygen delignification 1-5 . Almostall kinetic equations were derivedusing experiments performed in laboratory batchreactors. However it is difficult to determine thereaction orders in alkali concentration and residuallignin content when operating a batch reactor atmedium consistency (MC) because these variablesare continuously changing with time. Also masstransfer of oxygen from the gas to the liquid phasemust be efficient enough so that the oxygen concentrationin the liquid phase is close to saturationthroughout the experiment. Another difficulty isthat pulp properties can only be determined afteran experiment so that many experiments at differentreaction times must be run, thereby increasingexperimental error.In the present study, a constant flow of an oxygenatedcaustic solution is fed to a MC pulp bed.The pulp bed is retained inside a basket placedinside a pressured reactor. The caustic solution circulatesrapidly through the pulp, while fresh oxygenatedcaustic solution is added to the reactor andspent liquor is removed simultaneously at the samerate. In other words, the device is a differentiallyoperated continuous stirred tank reactor (CSTR)in which the concentrations of the dissolved oxygenand NaOH are close to that of the feed.Oxygen delignification kinetics previouslyreported have mathematical forms which weremostly chosen to obtain a good fit with theexperimental data rather than derived based onfundamental chemical mechanisms. This leadsto kinetic equations with high reaction orders inresidual lignin content or to arbitrary identificationof so called “fast reacting” and “slow reacting”lignin. Another aspect of the reported delignificationequations is that the lignin content is gener-ally derived from the kappa number, i.e. they donot distinguish between real lignin, hexenuronicacids (HexA) and non-lignin based oxidizablestructures as identified by Li and Gellerstedt 6 .Since it is known that hexenuronic acids are stableduring oxygen delignification, the lignin contentin the present study is corrected for the presenceof HexA 7 . The delignification data are then fittedto kinetics which are derived based on elementaryreactions identified in a chemical mechanism ofoxygen delignification. The cellulose degradationkinetics are described by two contributions: onedue to radicals produced by phenolic delignification,and the other due to alkaline hydrolysis.ExperimentalReactor setupA flow diagram of the CSTR system is shown inFigure 1. Oxygen is bubbled overnight into a causticsolution in a pressurized (0.9 MPa maximum)heated stainless steel container of 11 liter. A 280mL Berty reactor (Autoclave Engineers) with a100 mL stationary basket holds the pulp bed. Arotor underneath the basket induces flow throughthe pulp mat. Any gas inside the reactor is ventedat the top of the reactor so that the entire reactor isfilled with liquid during operation. The reaction isstarted by feeding the oxygenated caustic solutionat constant flow rate and oxygen pressure. Thereactor pressure and temperature, and flow rateand UV-VIS absorption of the outflow stream arecontinuously recorded.Pulps and analysesUnbleached southern pine kraft pulps were used.The dissolved lignin concentration was determinedfrom UV absorption at 280 nm using IndulinAT lignin (MeadWestvaco) for calibration 8 .The HexA content of pulp was determined by theY. Ji,National Renewable EnergyLaboratoryGolden, CO, USAE. Vanska,Helsinki University ofTechnologyEspoo, FinlandA. vanHeiningen,University of MaineOrono, ME, USApulpandpapercanada.com Pulp & Paper Canada March 2009 29

T31oxygen delignificationFig. 1. Flow diagram of the continuous stirred tank reactor(CSTR) system.Fig. 2. Delignification rate vs. residual lignin at different O 2pressures.where the constant K c= K HL*·[HL *total]·C.The reaction rate constant, k, is obtainedfrom the slope of the delignification rateversus L ccurve shown in Figure 2 fora commercial unbleached southern pinekraft pulp (Kappa 26). The linear decreasein delignification rate with HexA-freeresidual lignin content, L C, can be intermethodof Tenkanen et al. scaled down to200 mg pulp 9 . The residual lignin contentin pulp, L c, was calculated asHexA mg ligninL c= ( Kappa – ––––– ) 31.5 ( –—––––– ) (1)10 g pulpwhere the kappa number is calculated as theinitial pulp kappa number minus the amountof lignin dissolved as measured by UV. Thekappa number is corrected for the HexAcontent (in µmol/g pulp). Since 10 µmol/g ofHexA is equivalent to one kappa unit 10 , theHexA content is divided by 10. The factor1.5 is the standard conversion of kappa to mglignin per gram of pulp. Ji has shown that theHexA content of pulp does not change duringoxygen delignification except when a severeyield loss occurs so that HexA groups areremoved together with the carbohydrates 11 .The phenolic hydroxyl group content of theliquor and the pulp was determined from thedifference in UV absorbance of neutral andalkaline solutions at 300 and 350 nm 11-12 . Toavoid lignin precipitation, 100 µl of dioxanewas added to the samples.Data analysis procedureDetails about the data analysis procedurecan be found in earlier publications 8, 11 .Development of rate expression anddetermination of rate constantsIt is well accepted that the first step involvingoxygen delignification is dissociation ofphenolic groups forming an active ligninanion site 3, 5, . L *– . Oxygen delignificationkinetics can be interpreted as the resultof the rate determining reaction betweenadsorbed oxygen and the active ligninanion site, L *– , as:L *– + O 2 $ L *. + O 2.–(2)forming a superoxide anion radical, O .– 2,and a lignin radical L *. . Therefore the rateexpression of oxygen delignification maybe written as:dL – ––––– C = k [L* – ]·[O 2] ads(3)dtwhere [L *– ] is the reactive lignin site concentrationand [O 2] adsis the adsorbed oxygenconcentration on the reactive ligninsite. The equilibrium constant K HL*of theprotonation of the lignin active site is:[L *– ][H + ]K HL*= –––––––– (4)[HL * ]When the total number of active ligninsites is defined as HL * = total L*– + HL * , it canbe derived that:dL CK HL*[HL * ][OH – ][Ototal 2] – –––– = k ––––––––––––––––––––– ads (5)dt K water+ K HL*[OH – ]where K water= [H + ] 3 [OH - ]. If it is nowassumed that the active sites are uniformlydistributed throughout the residual ligninL c, then [HL * ] = C·L , where C is atotal Cproportionality constant. Thus, equation(5) becomes:dL C[OH – ][O 2] – ––––ads/LC = k = K C= ––––––––––––– (6)dt K water+K HL*[OH – ]preted as a first order reaction in lignin.The wavy character of the data is causedby slow small temperature fluctuations(± 1°C) in the CSTR due to imperfectcontrol.Equation 6 predicts that when [OH – ]is very high, the reaction order in [OH – ]approaches zero, and that the reactionorder in [OH – ] should approach first orderwhen [OH – ] is close to zero. By taking theinverse of equation 6, we obtain1 K HL*K water1– = ––––––– + –––––––– · –––––– (7)k K C[O 2] adsK C[O 2] ads[OH – ]The terms K HL*/K c[O 2] adsand K water/K C[O 2] adsare constant when the temperatureand oxygen pressure are kept constant.Thus, by plotting 1/k versus 1/[OH – ] atconstant temperature and oxygen pressure,but at variable [OH – ], a linear relationshipshould obtained. This is indeed the caseas shown in Figure 3. From the abscissaand slope of Figure 3 one obtains thatK HL*/K c[O 2] ads= 29.54 and K water/K C[O 2] ads= 3.28 or K * = 9.01·K . Since the pK HL water wof water, –log(K water), at 90°C is 12.40(Palmer et al. 2004), the pKa of the activelignin sites at 90°C is calculated as 11.5.This value is almost two units higher than9.8 measured for Indulin AT at this temperature13 , indicating that the lignin activesite is not a phenolic group.To include the effect of oxygen pressurein equation 6, a relationship was derivedbetween [O 2] adsand P O2based on the followingassumptions:1. Oxygen adsorption follows a Langmuirtype adsorption isotherm,2. The total number of adsorption sites isconstant,30 March 2009 Pulp & Paper Canada pulpandpapercanada.com

peer reviewedT32FIG. 3. 1/k (min) vs. 1/[OH]. FIG. 4. 1/k (min) vs. 1/P O2.3. The adsorption/desorption equilibrium can be described byequation 8.L*– +O 2, disO 2, ads(8)where O 2,disis oxygen dissolved in the caustic solution.As the adsorption and desorption of oxygen from the activelignin sites is fast, [O 2] adsis at quasi-equilibrium, and the adsorptionrate, r a= k a[L *– ][O 2,dis] is equal to the desorption rate, r d=k d[L *– ][O 2,] ads. The total number of active sites is constant, i.e.[HL *- ] + [O 2] ads= C t, thus it may be derived thatK eC t[O 2] dis[O2 ] ads= –––––––––– (9)1 + K e[O 2] disk where a Ke = –––kdEquation 9 shows that when [O 2] disis very large, the reaction iszero order in dissolved oxygen concentration, while it is first orderin dissolved oxygen concentration at very low oxygen concentration.The dependence of [O 2] disin water on the NaOH concentrationand temperature has been reported by Tromans 14 . Thedata in the quoted paper show that for the present experimentalconditions (0.5M [NaOH], 0.55 MPa, and 110°C) the saturatedoxygen concentration at oxygen delignification conditions is:[O 2] dis= 6.4310 –3 3P O2(10)where [O 2] disis expressed in mol/l and PO 2in MPa.Table 1 shows the saturated oxygen concentration at differentoxygen partial pressures for the present experiments performed at90°C. P H2Ois the saturated steam pressure at 90°C.Inserting equations 9 and 10 into equation 6 givesdL C[OH – ] P O2– –––– = C 1––––––––––––––– · ––––––––– · L C(11)dt K water+ K HL*[OH – ] 1 +K eP O2where C 1= k · K HL*C · K eC tAt constant temperature and [OH – ], equation 11 can be rearrangedas1 1 K e1–––––––––– = ––– = –––– + ––––– (12)(– ––––)/L dLc k C 2C 2P O2dt Ctable i. Oxygen concentrations at different partial pressuresat 90°C. P O2 = P total -P H2O + 0.1.P totalat 363K P O2at 363K O 2Conc. [O 2]dis(MPa) (MPa) (mol l –1 )0.24 0.27 0.0017480.38 0.41 0.0026320.52 0.55 0.0035170.66 0.69 0.004401where C 2is a constant. For the four conditions listed in Table1, the slope of plots of – dL C/dt versus Lc is equal to k. A plotof the inverse of the slope (or 1/k) versus 1/P O2is presented inFigure 4. It can be seen that a relative straight line behavior isobtained, supporting the assumption that [O 2] adsis governed by aLangmuir-type adsorption. The value of K eobtained from Figure4 is K e= 47.91/14.13 = 3.39 (1/MPa).The constant C 1in equation 11 was calculated using all theCSTR experiments performed at 90°C. The value of C 1is 0.175with a standard derivation of 0.0127. Thus, the final oxygendelignification kinetic equation at 90°C is:dL C[OH – ] P – ––– = –––––––––– · ––––––––O2· LC (13)dt 0.111+[OH – ] 1 + 3.39P O2with –dL C/dt expressed in mg lignin/g pulp/min, [OH – ] in mol/land P O2in MPa.Based on equation 13, the first order reaction rate constant, k,can be defined as[OH – ] P k = 0.175 –––––––––––– · ––––––––O2(14)0.111 + [OH – ] 1+3.39P O2Figure 5 illustrates that k calculated with equation (14) compareswell with the measured reaction rate constant (slope of theexperimental curves of –dL C/dt versus L c).Finally, the activation energy was determined from the temperaturedependence of the rate constant, k, in the present studyas 53 kJ/mol 15 . This value is in agreement with that of a reactioncontrolled process.To confirm the first order is behavior in L C, –dL C/dt was measuredfor three Loblolly Pine kraft pulps of different initial kappanumbers; 23, 26, and 34 (see Figure 6) 16 . The initial higher fasterdelignification may be due to additional peeling delignificationpulpandpapercanada.com Pulp & Paper Canada March 2009 31

T33oxygen delignificationFIG. 5. Slope (1/min) vs. predicted reaction rate constant k.FIG. 6. Oxygen delignification rate of Loblolly pine kraftpulps at different kappa no. levels.FIG. 7. Phenolic group content of liquor and pulp at differentreaction times.liquor was also measured. Figure 7 showsthese values and that of the pulps at differentreaction times as well as the sumof the phenolic group content in the pulpand liquor based on original pulp. As canbe seen, the reduction in the amount ofphenolic groups in the pulp is nearly equalto the amount of phenolic groups removedwith the liquor since the sum of phenolicgroups does not change significantly duringoxygen delignification. This suggeststhat the dissolved lignin is still mostlyaromatic in nature.Finally, we found that the ratio(MeOH formation rate/32) / (delignificationrate/185), as determined from themethanol and lignin content of the liquorsamples, remains mostly constant duringdelignification at about 0.9 ± 0.2. Thismeans that methanol is released essentiallyquantitatively from lignin monomer unitswhen they are solubilized 11The delignification rate versus residuallignin of three brown stock pulps cookedto different kappa numbers from the sameLoblolly Pine (Figure 6) shows an initialfast rate. However, after oxygen delignifiashas been discussed by van Heiningenet al 17 .The first order behavior in L Ccan beinterpreted that the lignin active sites, L *– ,are uniformly distributed throughout thelignin, and have the same reactivity duringthe entire oxygen delignification process.It is generally accepted that the first stepduring oxygen delignification is the dissociationof the phenolic groups. This isthe reason why the phenolic group contentof residual lignin was measured before andafter oxygen delignification. In agreementwith literature data 7 we obtained values forthe phenolic fraction of lignin in the originalpulp of around 0.41 and after 60 min at90°C, 0.52 MPa oxygen pressure and 3.3g/l of NaOH of around 0.27. After 3 h ofoxygen delignification, the phenolic fractionof the residual lignin in pulp decreasesfurther to 0.22, i.e. to half the initial value.Based on this and on the high pKa value ofthe lignin active sites, L *– we suggests thatthe rate determining step of oxygen delignificationis not a direct attack of oxygenon the dissociated phenolic groups.The phenolic lignin content of theFIG. 8. Three experiments; (1) experiment in Berty CSTRduring 3 h; (2) 60 min batch reactor + 80 min Berty CSTR;(3) 20 min batch reactor + 75 min Berty CSTR.cation of the fresh 24.4 kappa brownstockpulp for different times (20 and 60 minutes)in a batch reactor, continued delignificationin the CSTR does not displaythe initial fast rate (Figure 8). The absenceof the initial fast rate for the latter twopulps has been explained by the absenceof hemicelluloses with reducing ends andattached lignin fragments in these pulps 17 .Figure 8 also shows that the oxygen delignifiedpulp obtained in a batch reactor hasa similar delignification rate as the originalpulp. This implies that lignin condensationis insignificant in a batch reactor since thepulps are exposed to high concentrations ofdissolved lignin at the end of the delignificationprocess.Delignification mechanismBased on the presented data we proposethat the active lignin site is not the phenolicgroup, but another less acidic sitewhich is uniformly distributed throughoutthe residual lignin to satisfy the first orderin lignin rate behavior. Because hydroperoxideshave a pKa of 12 – 13 comparedto about 10.5 for Indulin AT at room32 March 2009 Pulp & Paper Canada pulpandpapercanada.com

peer reviewedT34a)R 1OH -R 1R 1R 2 OCH 3OHpKa 10-11R 2 OCH 3 R 2OO 2OOCH3R 1O 2-R 1O-OR 1O-OHR 2 OCH 3b)OR 2 OCH 3OpKa 12-13R 1R 2 OCH 3OR 1CO 2CO 2+ CH 3 OHR 2 OCH 3OO-OR 2R 1R 2temperature 13 , a mechanism proposed by Chang and Gratzl 18was adopted to explain the present kinetics. Figure 9 shows theformation of cyclohexadienone hydroperoxide by attack of oxygenat alkaline conditions on phenolic lignin.As illustrated, a hydroperoxide anion is formed by reactionof superoxide radical anion with the phenolate radical located atthe carbon 3 position. Chemical computational modeling 19 tocalculate the enthalpies of reactions of lignin model shows thatthe reaction pathway in Figure 19 9a is possible. Other phenolateradical intermediates have been proposed (McDonough 1996)where the radical is located on carbon 1 and the beta carbon inaddition to the carbon 3 shown in Figure a. All these three structuresare part of the same resonance structure in which the oddelectron formally resides at each of the three carbon positions.The explanation, why the coupling of the superoxide anion radicalwith the radical at the carbon 1 and beta carbon is not effectivefor delignification, is likely that the formation of the respectiveoxirane and carbonyl structures is much more difficult than theformation of the muconic acid and quinone structures shown inFigure 9b when the hydroperoxide anion is located at the carbon3 position.Figure 9b shows that the hydroperoxide anion forms either amuconic acid structure and MeOH or an orthoquinone structureand MeOH. Since the hydroperoxide anion is the dominant speciesat pH > 12~13, based on the present kinetics is probable thatthe carbon at the 3 position of the lignin aromatic ring is the activesite for oxygen delignification. Thus the present kinetics may beinterpreted that the rate determining step is the unimoleculardecomposition of the formed hydroperoxide anion, i.e. the rateis determined by the dissociated hydroperoxide concentration.The latter is determined by the concentration of adsorbed oxygenon methoxyl groups at the carbon 3 (the lignin active site, L*),as well as by the pKa of the hydroperoxide. Further support thatthe carbon 3 with a methoxyl group is the lignin active site, L*,O+ CH 3 OHOFigure 9 Reaction FIG. 9. mechanism Reaction of lignin mechanism with oxygen of (adopted lignin with from Chang oxygen and (adopted Gratzl, 1980 18 ). a)Formation of cyclohexadienon hydroperoxides by attack of oxygen. b) Homolytic and heterolyticfrom Chang and Gratzl, 1980 18). a) Formation of cyclo-fragmentation of para-cyclohexadienone hydroperoxidaseshexadienon hydroperoxides by attack of oxygen. b) Homolyticand heterolytic fragmentation of para-cyclohexadienonehydroperoxidases.FIG. 10. Cellulose degradation in CSTR and batch reactors;Experimental and predicted data.is that almost all residual softwood lignin monomer units containthis moiety, thus explaining the uniform reactivity and first orderkinetics in residual lignin for oxygen delignification.The pH dependence of the kinetics implies that the protonatedhydroperoxide does not fragment into muconic acid orquinone structures. Also, the fact that dissolved lignin mostlyretains its aromatic nature, and that MeOH is released essentiallyquantitatively during delignification, further support the hypothesisthat quinone formation is the dominant pathway.Kinetics of cellulose degradationThe cleavage of cellulose was modeled by Iribarne and Schroeder19 as the increase in number-average moles of cellulose per gramof pulp (m n). Similarly one can describe the cellulose degradationby the number of cellulose chain scissions during oxygen delignification.Violette and van Heiningen 20 calculate the number ofcellulose chain scissions from the average degree of depolymerizationof cellulose (DP) in the pulp at time t=0 and time t=t, as 1/DP t-1/DP 0. DP can be obtained from the intrinsic viscosity [h]by equation (15). This formula considers the actual weight ofcellulose rather than the pulp weight being responsible for theviscosity, and makes a correction for the small contribution of thehemicelluloses to the pulp intrinsic viscosity 21 .1.65[h] – 116H1.111DP of Celluose = ( ——————— ) (15)Gwhere [h] is intrinsic viscosity of the pulp in cm 3 /g, and G and Hare the mass fractions of cellulose and hemicellulose in the pulp(see Table 2). This formula considers the actual weight of celluloserather than the pulp weight being responsible for the pulpviscosity, and makes a correction for the small contribution of thehemicelluloses to the intrinsic viscosity.The number of moles of cellulose per gram of pulp, mn, canbe calculated by equation (16) as 22 :1 1 Moles162DP n+18 _ 162DP nGram Pulpm n= ––––––––– . ––––––– (–––––––––– )(16)The content of cellulose (G) and hemicellulose (H) in the pulpwas measured by high pressure anion exchange chromatography(HPAEC) on double hydrolyzed pulp samples 23 . The results forthe pulp samples are listed in Table 2.pulpandpapercanada.com Pulp & Paper Canada March 2009 33

T35oxygen delignificationtable Ii. Properties of CSTR oxygen delignified pulps (at 3.3g/l NaOH, 90°C and0.52MPa).Time Kappa Viscosity Cellulose, G Hemicellulose, H DP of(min) (ml/g) (g/g pulp) (g/g pulp) cellulose0 24.4 1189 0.714 0.142 656110 20.1 1079 0.732 0.139 572420 18.5 1033 0.734 0.142 543540 14.3 877 0.746 0.140 444560 12.7 828 0.750 0.138 4144180 7.6 592 0.763 0.135 2784table iII. Properties of Batch oxygen delignified pulps (3% NaOH, 90°C, 0.52MPa, 10% Cons.).Time Kappa Viscosity Cellulose, G Hemicellulose, H DP of(min) (ml/g) (g/g pulp) (g/g pulp) cellulose0 24.4 1189 0.714 0.142 656110 20.8 1058 0.723 0.139 567420 18.5 1011 0.723 0.138 539440 15.0 924 0.729 0.139 483160 13.5 898 0.736 0.140 4629180 10.1 829 0.736 0.141 4230Based on these results the cellulosedegradation was modeled by two contributions:one due to radicals produced byphenolic delignification, and the other dueto alkaline hydrolysis. The model can bedescribed as equation (17):dm ndK–––– = –k c–––– + k h[OH – ] (17)dt dtwhere k cis the rate constant for radical attack,and k his the alkaline hydrolysis rate constant.[OH – ] is the alkali concentration in g/L.Integration of equation (17) gives:m n= m 0+ k c(K – K 0) + k h[OH – ]t (18)Comparable results for the same brownstock pulp oxygen delignified in a batchreactor samples are given in Table 3. SinceNaOH is rapidly consumed during theinitial phase of oxygen delignification ina batch reactor, the influence of the termk h[OH – ]t in equation (18) may be neglectedfor t ≥ 20 minutes. This allows calculationof kc by fitting the batch reactor as3.60 x 10 -8 (moles/g pulp·kappa). Usingthis value in the analysis of the CSTR datagives a value for k hof 1.07 x 10 -9 (liter·molcellulose/g pulp·g NaOH·minute). Thesetwo values provide a good fit of the cellulosedegradation in the CSTR and batchreactor as can be seen in Figure 10.ConclusionsThe kinetics of oxygen delignification arefirst order in residual lignin content (HexAcorrected), and follow a Langmuir-typebehavior for adsorption of oxygen on theactive aromatic lignin sites. The reactionorder in NaOH of the kinetics implies thatthe rate determining step involves an acidiclignin active site with a pKa almost 2 unitshigher than that of phenol in lignin. Basedon these results it is proposed that thelignin active site is the 3 carbon of the aromaticring where oxygen reacts to form ahydroperoxide. The rate determining stepis identified as the unimolecular decompositionof the formed hydroperoxide anion.The almost uniform presence of aromaticmethoxyl groups in residual lignin furthersupports the first order in lignin kinetics.Further supporting evidence is the closerelationship between delignification anddemethoxylation. The significant reductionin phenolic group content of the ligninduring oxygen delignification is consistentwith the hypothesis that the rate determiningstep of oxygen delignification is not adirect attack of oxygen on the dissociatedphenolic groups. The cellulose degradationduring oxygen delignification was modeledby two contributions: one due to radicalsproduced by phenolic delignification, andthe other due to alkaline hydrolysis.AcknowledgementFinancial support by the TechnologyDevelopment Agency of Finland(TEKES), Helsinki University of Technologyand the Ober Chair is gratefullyacknowledged. The discussions with Dr.Raymond Fort Jr. at the University ofMaine were essential for our understandingof the mechanism, and are greatlyappreciated.Literature:1. Edwards L. and Nordberg S-E. (1973) AlkalineDelignification Kinetics, a General Model Applied toOxygen Bleaching and Kraft Pulping, TAPPI, 56(11),108-1112. Teder A. and Olm L. (1981) Extended delignificationby combination of modified kraft pulp and oxygenbleaching, Paperi Puu, 63(4a), 315-3263. Ljunggren, S.C.H and Johansson ,E.C. (1987)Reaction Kinetics of Lignin Structures during OxygenBleaching-Effects of Solvent, Oxygen Pressure andpH, In: International Oxygen Delignification Conference,Tappi Proceedings, 1254. Kovasin K., P. Uusitalo P., M. Viilo, (1987) Dimensioningof oxygen delignification reactors, InternationalOxygen Delignification Conference, June 7-12,San Diego, USA, pp. 223-2305. Johansson, E., Ljunggren, S., (1994) The Kineticsof Lignin Reaction During Oxygen Delignification,Part 4. The Reactivates of Different Lignin ModelCompounds and the Influence of Metal Ions on theRate of Degradation, J. Wood Chem. Technol. 14(4),507-5256. Li, J. and Gellerstedt, G., (2002) Oxymercuration– Demercuration Kappa Number: an Accurate Estimationof the Lignin Content in Chemical Pulps, NordicPulp Paper Res. J. 17(4), 410-4147. Rööst, C., Lawoko, M., Gellerstedt, G. (2003) StructuralChanges in Residual Kraft Pulp Lignins. Effectsof Kappa Number and Degree of Oxygen Delignification,Nordic Pulp Paper Res. J. 18(4), 395-3998. Ji Y. and van Heiningen A. (2007) A new CSTRfor Oxygen Delignification Mechanism and KineticsStudy, Pulp Paper Canada, 108(5), 38-429. Tenkanen, M.,G. Gellerstedt, T. Vuorinen, A.Teleman, M. Perttula, J. Li, and J. Buchert (1999)Determination of Hexenuronic Acid in SoftwoodKraft Pulps by Three Different Methods, J. Pulp PaperSci. 25(9), 306-31110. Vuorinen, T. et al., (1999) Selective Hydrolysis ofHexenuronic Acid Groups and its Application in ECFand TCF Bleaching of Kraft Pulps, J. Pulp Paper Sci.25(5), 155-162.11. Ji, Y. (2007) Kinetics and Mechanism of OxygenDelignification, PhD thesis, University of Maine,Orono, USA12. Gartner, A., Gellerstedt G. and Tamminen T.(1999) Determination of Phenolic Hydroxyl Groupsin Residual Lignin using a Modified UV-Method.Nordic Pulp Paper Res. J. 14(2), 163-17013. Norgren, M., and Lindstrom B., (2000) Dissociationof Phenolic Groups in Kraft Lignin at ElevatedTemperatures Holzforschung, 54, 519-52714. Tromans D., (1998) Oxygen Solubility Modelingin Inorganic Solutions: Concentration, Temperatureand Pressure Effects, Hydrometallurgy, (50), 279-29615. Ji, Y., Wheeler M. C. and van Heiningen A. (2007)Oxygen Delignification Kinetics: CSTR and BatchReactor Comparison, AIChE J. 53(10), 2681-268716. Vanska, E., (2007) Kinetics of Oxygen Delignificationin a Continuous Stirred Tank Reactor, M. Sc.Thesis, Helsinki University of Technology, Espoo,Finland17. Van Heiningen, A., Ji, Y., Vänskä, E., (2008) Newkinetics and mechanism of oxygen delignification:International Pulp Bleaching Conference, June 2-5,Quebec City, Canada. 91-9818. Chang H. and Gratzl J. S. (1980) Ring CleavageReactions of Lignin Models with Oxygen and Alkali,In. Chemistry of Delignification with Oxygen, Ozoneand Peroxide, Uni Pub. Co. Ltd, Tokyo, Japan, pp.151-16319. Hausman, M.C., Elder, T.R. and Fort, R.C. Jr.(2003) How Do Phenoxyl Radicals Form During OxygenDelignification? 12 th International Symposium onWood and Chemistry (ISWPC), June 9-12, Madison,WI, USA, Poster presentations pp. 59-6220. McDonough, T. J. (1996) Chapter IV 1: OxygenDelignification, in Pulp Bleaching – Principles and34 March 2009 Pulp & Paper Canada pulpandpapercanada.com

peer reviewedT36Practice, Dence, C. W. and Reeve, D. W. Editors,Tappi Press, Atlanta, GA., 220-22321. Iribarne J. and Schroder L.R. (1997) High-PressureOxygen Delignification of Kraft Pulps, TAPPI,80(10), 241-25022. Van Heiningen A. and Violette S., (2003) SelectivityImprovement during Oxygen Delignification byAdsorption of a Sugar-based Polymer, J. Pulp PaperSci. 29(2), 48-5323. Da Silva Perez, D. and van Heiningen A.R.P.(2002) Determination of Cellulose Degree of Polymerizationin Chemical Pulps by Viscosimetry, 7thEuropean Workshop on Lignocellulosics and Pulp(EWLP) Conference, Turku, Finland 393-39624. Davis, M.W. (1998) A Rapid Modified Method forCompositional Carbohydrate Analysis of Lignocellulosicsby High pH Anion-Exchange Chromatographywith Pulsed Amperometric Detection (HPAEC/PAD),J. Wood Chemistry and Technology, 18(2), 235-252Résumé: Un réacteur à fonctionnement continu et à commande différentielle a été utilisé pourétudier la cinétique de la délignification à l’oxygène. La cinétique de la délignification et la vitessede réaction ont été déterminées à des températures, des pressions d’oxygène et des concentrationscaustiques différentes avec des pâtes kraft de résineux. La cinétique joue un rôle de premierordre dans la teneur en lignine résiduaire (HexA corrigé). On peut décrire la cinétique de ladélignification phénolique en partant de l’hypothèse que la décomposition de l’anion de peroxyded’hydrogène à carbone 3 du noyau aromatique constitue l’étape cinétiquement déterminante.Deux contributions expliquent la cinétique de la dégradation de la cellulose : l’une attribuable auxradicaux produits par la délignification phénolique et l’autre, à l’hydrolyse alcaline.Reference: JI, Y., VANSKA, E., VAN HEININGEN, A. Rate determining step and kinetics ofoxygen delignification. Pulp & Paper Canada March 2009:T30-36. Paper presented at the 2008International Pulp Bleaching Conference in Quebec City, QC Canada, June 2-5, 2008. Not to bereproduced without permission of PAPTAC. Manuscript received August 1, 2008. Revised manuscriptapproved for publication by the Review Panel December 29, 2008.Keywords: oxygen delignification kinetics; rate determining step, cellulosedegradation kinetics.pulpandpapercanada.com Pulp & Paper Canada March 2009 35