Rate determining step and kinetics of oxygen delignification

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