Rate determining step and kinetics of oxygen delignification

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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
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