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Ind. Eng. Chem. Res. 2003, 42, 4156-4161
KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetics of the First Chlorine Dioxide Bleaching Stage (D1) of a Hardwood Kraft Pulp Maria J. M. C. Barroca* and Jose´ Almiro A. M. Castro† Department of Chemical Engineering, University of Coimbra, Po´ lo II - Pinhal de Marrocos, 3030-290 Coimbra, Portugal
In the present work, kinetic studies were performed to investigate the first chlorine dioxide bleaching stage of a Eucalyptus globulus kraft pulp. The bleaching reaction can be described by an initial short but very fast step, followed by a slower step. The proposed model for the D1 stage is based on both the light absorption coefficient and the chlorine dioxide concentration. A set of two depletion factors for the chlorine dioxide concentration and for the light absorption coefficient was described as a function of temperature for the fast and short steps of the reaction. The longer and slower periods were described by a homogeneous model. A nonlinear relationship between the chlorine dioxide consumption and the decrease in the light absorption coefficient, which is dependent on the extent of delignification but independent of temperature, was also established. Moreover, a linear relationship between the upper brightness limit and the temperature was observed. The fit of the experimental results obtained for different temperatures, initial chlorine dioxide concentrations, and initial light absorption coefficients is very good, revealing the ability of the model to predict typical mill operating conditions. Introduction In recent years, the technology of bleached chemical pulp manufacture has entered a period of extremely rapid change driven by a growing concern for the environment. In this context, many oxygen-based reagents have emerged as part of this important process technology. However, the main drawback of these oxidants is their low selectivity, as they lead to severe polysaccharides in the degradation of pulps with very low lignin contents. As a consequence, they have been used only under carefully controlled operating conditions, almost limited to the earlier stages of bleaching, and/or they have been used with expensive additives such as chelants and stabilizers. To obtain bleached pulp by removing or modifying the color compounds that remain in the delignified fibers, the pulp has to be submitted to bleaching stages. However, to preserve the chemical and physical properties of the final pulp, it is essential to use selective reagents in the later stages of the bleaching sequences. Thus, one of the most important chemicals used in the final stages is chlorine dioxide, because it does not react to any significant extent with carbohydrates.1-3 In fact, the D1 stage is extremely important from a quality perspective because it enables a significant increase in pulp brightness without a significant loss in pulp strength. In addition, the low amount of organic material removed and the low charge of chlorine dioxide used in this stage virtually eliminate potentially problematic * To whom correspondence should be addressed. Tel.: +351 232 480 600. Fax: +351 239 798 703. E-mail:
[email protected]. † Deceased.
compounds from the liquid effluents, such as polychlorinated dibenzo-p-dioxins and -furans (PCDD/Fs).4,5 Of the pulp properties measured after the D1 stage, the pulp brightness is an important control target for the final product brightening, which can be predicted by kinetics equations. On the basis of experiments carried out at low consistency and at constant chlorine dioxide concentration, some researchers6-9 have developed kinetic models for the chlorine dioxide bleaching of softwood kraft pulps. Furthermore, to represent the overall reactions taking place during bleaching, Saltin10 proposed a complex model that requires knowledge of appropriate values for a large number of parameters. However, Saltin’s study also focused on softwood pulps, and consequently, little attention has been paid to hardwood pulps. Moreover, the chemical components of hardwoods are different from those of softwoods with respect to quantity and especially to quality. As is wellknown, hardwood lignins are more reactive than softwood lignins because of their higher S/G ratio and perhaps their less condensed structure. Thus, the kinetic models for softwoods and hardwoods are not necessarily similar. Recently, we11,12 proposed a simple methodology in which two sequential reaction stages were considered to take place during the chlorine dioxide delignification of a hardwood kraft pulp (Eucalyptus globulus). This strategy led to a robust model with excellent prediction capabilities for hardwood pulps with different incoming kappa numbers, varying between 12 and 18. The first period of the process, which is very fast, was assumed to last for a short period of t ) θ ≈ 15 s, where the kappa number decrease and the corresponding consumption of chlorine dioxide were both shown to be exclusive
10.1021/ie030126f CCC: $25.00 © 2003 American Chemical Society Published on Web 08/01/2003
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functions of the reaction temperature. In the second period of the process, which is characterized by a low velocity, a typical chemical reaction was considered to be the dominant step. This slow regime was best described by a power-order model, whose state variables are the kappa number after D0 and the concentration of chlorine dioxide in the liquid phase. In addition, the integrated form of the stoichiometry was described by a nonlinear relationship, and the floor lignin by a single linear function of temperature. Employing the modeling strategy described above for the chlorine dioxide delignification (D0 stage), we selected as the major objective of the present work the establishment of a mathematical model to describe the kinetics of pulp bleaching with chlorine dioxide in the D1 stage using the same hardwood E. globulus kraft pulp. Materials and Methods The unbleached kraft pulps used in this work were produced in a laboratory digester using E. globulus chips. The unbleached pulps were then predelignified in two conventional stages comprising a chlorine dioxide oxidation followed by an alkaline extraction with sodium hydroxide, the D0E1 sequence, in accordance with standard industrial operating conditions listed in Table 1. Table 1. Operating Conditions Used in the Reference Bleaching D0E1 Sequence operating conditions
D0 stage
E1 stage
chlorine dioxide (% as active Cl2) sodium hydroxide (%) temperature (°C) reaction time (min) pH (final)
3 50 30 2.5-3
70 120 >11
Between the D0 and E1 stages and also before all D1 bleaching experiments, the pulps were thoroughly washed with cold water, followed by extra washing with an excess of warm water. The predelignified pulps used in this work had an ISO brightness between 61.6 and 69.9%. The bleaching experiments with chlorine dioxide were carried out in a 2.6-L thermostatic glass reactor at low pulp consistency (0.7%), with a chlorine dioxide charge of 1-2% (o.d. pulp), and at a stirring rate of 600 rpm. After disintegration of the pulp suspension for 10 min, sulfuric acid was added just before chlorine dioxide, to impose a pH around the desired value at the beginning of the reaction (pH ≈ 4). The pH was kept constant during the experiment by successive additions of sodium hydroxide, and the temperature was maintained by appropriate automatic control of a heating circulating fluid. In each experiment, liquid samples were taken with a syringe equipped with a special glass filter and immediately analyzed after thermal conditioning. With regard to the pulp samples, rapid washing with cold water was performed to stop the bleaching reaction. The pulp was then further washed with an excess of warm water for later characterization. Each run consisted of a set of experiments in which the chlorine dioxide concentration and the pulp properties were measured as functions of time between 15 s and 3 h. To determine the maximum of chromophores that could be removed from the pulp in a single stage, a set of several
independent experiments was conducted under the same conditions, at different temperatures, using a time of 12 h. The light absorption coefficient (k value) was determined by measuring the reflectance of handsheets at a wavelength of 457 nm (ISO 3688-77) over bright and black backgrounds using an Elrepho 2000 spectrophotometer. Results and Discussion General. The behavior of the brightness during the bleaching experiments in the D1 stage showed clear similarities to earlier results in terms of the evolution of the kappa number upon predelignification in the D0 stage using the same oxidant. Thus, the modeling strategy adopted to describe the D1 bleaching stage was the same as used previously for chlorine dioxide predelignification in the D0 stage.11,12 In the first chlorine dioxide bleaching stage, D1, the elimination of colored material is usually observed as an increase in brightness. However, the pulp brightness (B) is not a simple function of the concentration of colored material in the pulp as it is a function of the ratio between the light absorption (k) and the light scattering (s) coefficients, as described in the KubelkaMunk equation13,14
x2(ks) + (ks)
B k )1+ 100 s
2
(1)
As shown by Teder and Tormund,9 the light absorption coefficient of the pulp (k in the Kubelka-Munk equation) is directly proportional to the absorptivity of the material and, consequently, to the concentration of chromophores in the pulp. Thus, in the present work, this coefficient is assumed to be a direct measure of the total concentration of colored material in the pulp fibers and was calculated from brightness data collected for E. globulus pulps assuming a specific value for the light scattering coefficient. The latter parameter is determined by the fiber dimensions and the degree of interfiber bonding, and it is specific to each pulp species. Furthermore, it can be assumed as a constant value during the chemical cooking and bleaching process.13,14 The light scattering coefficient for E. globulus was determined from reflectance measurements on a set of pulps obtained from 80 D1 bleaching experiments with D0E1 delignified pulps and conducted over large ranges of temperature (50-85 °C), pH (3-6), and time (0-3 h). A value of the light scattering coefficient of 35.37 m2/kg was determined. By assuming this value as typical of a chemical in E. globulus pulp and measuring the reflectance of handsheets, B, the light absorption coefficient (k value) of the D1 pulps was determined from eq 1. Chlorine Dioxide Bleaching. As shown in the chlorine dioxide predelignification stage, D0, with respect to kappa number, the elimination of chromophores during D1-stage bleaching can also be described as occurring in two distinct steps: a very fast and short decrease, followed by a much longer and slower period of reaction where the rate of decrease of the chromophore concentration almost approaches zero. To describe the first period of reaction, which can reasonably be considered to last for t ) θ ≈ 15 s, a set of experiments was carried out at several temperatures with different predelignified pulps.
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Figure 1. Normalized light absorption coefficient, (k0 - k)/k0, after 15 s of reaction time at different temperatures for various incoming values of the light absorption coefficient (k0).
Figure 3. Chlorine dioxide consumption as a function of the decrease in the light absorption coefficient for two D0E1 predelignified pulps (b, temperatures of 20, 40, 65, and 90 °C; [, timevarying temperature, ranging from 18 to 90 °C). Table 2. Optimal Estimates for the Normalized Depletion Factors (Chlorine Dioxide Concentration and and Light Absorption Coefficient) in the Fast Regime of the D1 Bleaching Stage (Eq 2) parameter
([ClO2]0 - [ClO2]θ)/k0
100 × (k0 - kθ)/k0
β1 β2 β3
1.01 0.01 0.07
35.64 6.48 0.017
correlated with temperature for the D1 stage by means of the following equation
y )β1 + β2eβ3(T-273.15) Figure 2. Normalized chlorine dioxide consumption, ([ClO2]0 [ClO2])/k0, after 15 s of reaction time at different temperatures for various incoming values of the light absorption coefficient (k0).
As expected, the pulps exhibited a similar behaviors during both the D0 and D1 stages with respect to the initial chlorine dioxide consumption and the decrease in the light absorption coefficient. For a given temperature, the initial bleaching and the chlorine dioxide consumption rate increased with the light absorption coefficient of D0E1 pulps, which corresponds to a higher concentration of colored material. That is, the initial rate of chromophores elimination during chlorine dioxide bleaching is dependent on the extension of the pulp predelignification. However, as highlighted in Figure 1, the behaviors of the pulps are very similar and independent of their initial light absorption coefficient values when the decrease in this property is normalized with respect to the light absorption coefficient of the predelignified pulp (k0). Thus, it can be concluded that a predelignified pulp always exhibits the same degree of elimination of chromophores (normalized), regardless of its incoming light absorption coefficient. Regarding the initial consumption of chlorine dioxide, a similar procedure was adopted in which the depletion factor was also normalized with respect to the light absorption coefficient of the predelignified pulp. This pattern is illustrated in Figure 2, where, again, a common behavior for all pulps can be seen. In accordance with the approach proposed by Barroca et al.11 for chlorine dioxide predelignification in the D0 stage, these two normalized depletion factors were
(2)
where y is an initial depletion factor that represents either the decrease in the light absorption coefficient or the chlorine dioxide consumption at t ) θ ≈ 15 s and T is the temperature in Kelvin. The best estimates of the corresponding parameter values for both normalized chlorine dioxide consumption and light absorption coefficient in this fast regime are shown in Table 2. To model the second and much slower regime of the D1 bleaching process, it is essential to know the stoichiometry of the reaction and the maximum concentration of chromophores that can be removed in a single bleaching stage. According to different researchers7-9,11 the stoichiometry of the reaction of pulp with chlorine dioxide is not constant. In fact, this phenomenon was also confirmed experimentally for the D1 bleaching of E. globulus pulps by the fact that the chlorine dioxide consumption and the decrease in the light absorption coefficient of pulps are not directly proportional, as shown in Figure 3. Moreover, in this figure, the data points for the predelignified pulp (D0E1) having a light absorption coefficient of 2.29 m2/kg correspond to bleaching experiments carried out over a wide range of temperatures, between 20 and 90 °C, which reveals that temperature does not affect the stoichiometry. The experimental results, i.e., the integrated form of the stoichiometry, are better described by a similar relationship proposed by Teder and Tormund9
∆[ClO2] )
a (k 1-n - k1-n) 1-n 0
for n * 1
(3)
where ∆[ClO2] represents the consumption of chlorine
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Figure 4. Chlorine dioxide consumption as a function of the normalized light absorption coefficient for different D0E1 predelignified pulps.
Figure 5. Brightness limit, B∞, at different temperatures for a D0E1 pulp having an initial light absorption coefficient of 2.29 m2/ kg.
dioxide in mg/g; a and n are constants; and k and k0 correspond to the light absorption coefficient at time t and to the initial light absorption coefficient of the predelignified pulp, respectively. Figure 3 shows that the relationship between the chlorine dioxide consumption and the removal of colored material in the pulp (decrease in light absorption coefficient) is dependent on the extent of delignification, which affects k0; consequently, the constants a and n vary with the predelignification sequence. In contrast to the work of Teder and Tormund,9 where the constants a and n were determined for each sequence of bleaching pulps, our objective is to define a global expression that can be applied to different pulps independently of the extension of their predelignification. Thus, if the experimental results obtained for different pulps are normalized with respect to the light absorption coefficient of the predelignified pulp, the cumulative stoichiometry is independent of the extension of delignification, as indicated in Figure 4. The nonlinear relationship shown in this figure represents the best fit of the data and is described by the following expression with a correlation coefficient of 0.99
Table 3. Optimal Estimates of the Kinetic Parameters Proposed in This Work for the Slow Regime of the D1 Bleaching Stage and Their 95% Confidence Intervals
[ () ]
0.911 k ∆[ClO2] ) 11 - 2.02 k0
1-2.02
(4)
From this equation, it is possible to conclude that the corresponding differential stoichiometry, after some algebraic manipulation, can be expressed as
R)
d[ClO2] ) 0.911k01.02k-2.02 dk
(5)
In contrast to all previous studies, where the floor lignin content of pulp was considered as a constant parameter, we12 identified a clear relationship between the floor lignin content of the pulp (K∞) and the temperature of the predelignified stage D0. To determine whether this pattern holds in the D1 bleaching stage with regard to the content of chromophores that can be removed, a set of experiments was performed for 12 h at different temperatures, between 20 and 80 °C. The results are presented in Figure 5 and show a clear relationship between the upper brightness limit obtained after the D1 stage and the temperature of the reaction. This correlation can be described by the following equation with the temperature expressed in
parameter A Ea (J/mol) nD nk
optimal estimate 4.47 × 39 908 1.4 2.5
102
95% confidence interval 9.9-8.84 × 102 33 625-47 364 1.1-1.7 2.3-2.7
Kelvin and with a correlation coefficient of 0.995
B∞ ) 81.47 + 0.137(T - 273.15)
(6)
To apply this useful information in a mathematical model, the upper brightness limit (B∞) can easily be expressed in terms of a residual absorption coefficient (k∞) through the Kubelka-Munk equation (eq 1). Using the modeling approach we proposed previously11 for the slow regime of reaction of the D0 stage, similar equations can be defined for the D1 bleaching stage for the main state variables, light absorption coefficient and chlorine dioxide concentration.
dk ) -Ae-Ea/RT[ClO2]nD[k - k∞]nk dt
(7)
mpulp -Ea/RT d[ClO2] ) -R Ae [ClO2]nD[k - k∞]nk (8) dt V Thus, the kinetic parameters of the slow regime were estimated simultaneously by combining the above kinetic eqs 7 and 8 with the stoichiometry (R), the residual absorption coefficient (k∞), and the depletion factors for light absorption coefficient and chlorine dioxide concentration in the fast regime (with the values shown in Table 2). The model nonlinear ordinary differential equations were solved numerically using an implicit algorithm (DDASAC), and the optimum set of parameters was found by a nonlinear least-squares technique based on an orthogonal distance regression method (ODRPACK). All available experimental data were used, and these included runs performed at different temperatures (2090 °C) and concentrations of chlorine dioxide (80 and 140 mg/L). The values and the corresponding 95% confidence intervals of the optimal estimation of the parameters are shown in Table 3 for the preexponential factor, A; activation energy, Ea; and partial reaction orders with
4160 Ind. Eng. Chem. Res., Vol. 42, No. 18, 2003
Figure 6. Experimental and model predictions of the light absorption coefficient (k) at different temperatures in the D1 stage for a D0E1 pulp having an initial light absorption coefficient of 2.29 m2/kg.
Figure 8. Experimental vs predicted chlorine dioxide concentrations at different temperatures for D0E1 pulps having initial light absorption coefficients of 2.29 and 3.74 m2/kg.
analysis of the profiles illustrated in Figures 6 and 7 reveals that the errors are within the experimental uncertainty, even at short reaction times. Figure 8 also shows good agreement between the experimental and predicted results for the chlorine dioxide concentration under different operating conditions. Thus, the fitting of the profiles obtained for different temperatures, initial chlorine dioxide concentrations, or initial light absorption coefficients are a good indication of the ability of the model to predict typical mill operating conditions. Conclusions
Figure 7. Experimental and model predictions of the light absorption coefficient at 65 °C for pulps having different initial light absorption coefficients after the D0E1 sequence.
respect to the chlorine dioxide concentration (nD) and the light absorption coefficient (nk). Taking into account the fact that all parameters are simultaneously estimated, the quality of the optimal estimates can be seen as very satisfactory, with the exception of the preexponential factor, A. However, in an attempt to analyze the effect of the estimate of this factor on the estimation of the others, the model was used to fit the same data but with the preexponential factor fixed at the optimal value of Table 3, and the parameters were re-estimated. Although the number of degrees of freedom had decreased, the optimal estimates of the remaining parameters were maintained but with narrower 95% confidence intervals. Thus, the optimal estimates of the kinetic parameters proposed in Table 3 can be considered satisfactory for describing the slow regime of the D1 bleaching stage. The fitting of the experimental data for the pulp having a light absorption coefficient of 2.29 m2/kg for temperatures between 20 and 90 °C (Figure 6) reveals the ability of the model to predict chlorine dioxide concentration profiles for three different temperatures. The same remark can be made concerning the light absorption coefficient, whose profiles are shown in Figure 7 for experiments carried out at 65 °C for two different initial chlorine dioxide concentrations (∼140 and 80 mg/L) and two initial pulp light absorption coefficients (k0 ) 2.29 and 3.74 m2/kg). In fact, a residual
The kinetics of chlorine dioxide bleaching (D1 stage) of an E. globulus kraft pulp was investigated. The total consumption of chlorine dioxide in this bleaching stage can be expressed in terms of a nonlinear relationship with the decrease in the light absorption coefficient, which is dependent on the extent of delignification, but it is not affected by temperature in the range of industrial interest. Furthermore, the upper brightness limit was found to be a linear function of temperature. By using two initial depletion factors, both functions of temperature, to characterize the sharp changes taking place in the first period of reaction, it was possible to describe the overall behavior of this heterogeneous process with the help of a homogeneous kinetic model for the slow period. The proposed modeling strategy enables the description of the rates of both bleaching and chlorine dioxide consumption in the D1 stage in a quite satisfactory manner for the two sequential stages. The fit of the experimental results is excellent, and the deviations are within the experimental error. Acknowledgment The authors are thankful to the Ministry of Science and Technology, through Program PRAXIS, Project 3/3.2/PAPEL/2326/95, for the financial support of this project and for the scholarship granted to M.J.M.C.B.. Special thanks are also due to RAIZ, Instituto de Investigac¸ a˜o da Floresta e do Papel, for carrying out the bleaching experiments to estimate the light scattering coefficient of E. globulus pulp. M.J.M.C.B. is indebted to Professor Maria da Grac¸ a Carvalho for valuable comments and for reviewing the manuscript.
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Literature Cited (1) Teder, A.; Tormund, D. Carbohydrate Degradation in Chlorine Dioxide Bleaching. Tappi J. 1978, 61 (12), 59. (2) Rapson, W.; Anderson, C. Bleaching in Five Stages to the Asymptotic Limit Using Only One Oxidant and Sodium Hydroxide. In Proceedings of the 1985 International Bleaching Conference; Canadian Pulp and Paper Association (CPPA): Ottawa, Ontario, Canada, 1985; p 227. (3) Barroca, M. J. M. C.; Marques, P. J. T. S.; Seco, I. M.; Castro J. A. A. M. Selectivity Studies of Oxygen and Chlorine Dioxide in the Pre-Delignification Stages of a Hardwood Pulp Bleaching Plant. Ind. Eng. Chem. Res. 2001, 40 (24), 2680. (4) Solomon, K.; Bergman, H.; Higget, R.; Mackay, D.; McKague, B. A Review and Assessment of the Ecological Risks Associated with the Use of Chlorine Dioxide for the Bleaching of Pulp. Pulp Pap. Can. 1996, 97 (10), T345. (5) Weigand, P.; Thacker, W.; Miner, R. Effluent Quality at Kraft Mills that Use Complete Substitution Bleaching. Tappi J. 1999, 82 (4), 135. (6) Edwards, L.; Norrstrom, H. Bleaching Kinetics: a General Model. Sven. Papperstidn. 1973, 76 (3), 123. (7) Germgard, U.; Teder, A.; Tormund, D. Mathematical Models for Simulation and Control of Bleaching Stages. Nord. Pulp Pap. Res. J. 1987, 2 (1), 16.
(8) Teder, A.; Tormund, D. Kinetics of Chlorine Dioxide Bleaching. CPPA Trans. Sect. 1977, 3, 2, TR41. (9) Teder, A.; Tormund, D. Mathematical Model for Chlorine Dioxide Bleaching and its Applications. AIChE Symp. Ser. 1980, 200 (75), 133. (10) Saltin, G. E. Kraft Pulp Bleaching: Kinetics Models of Conventional systems and Laboratory Investigations of Chlorine Free Sequences. Ph.D. Dissertation, University of Idaho, Moscow, ID, 1993. (11) Barroca, M. J. M. C.; Simo˜es, R. M. S.; Castro J. A. A. M. Kinetics of Chlorine Dioxide Delignification of a Hardwood Kraft Pulp. Appita J. 2001, a54 (2), 190. (12) Barroca, M. J. M. C.; Simo˜es, R. M. S.; Castro, J. A. A. M. Effect of Unbleached Pulp Kappa Number on the Kinetics of Chlorine Dioxide Delignification. Appita J. 2001, b54 (6), 532. (13) Jordan, B.; O’Neill, M. The Kubelka-Munk Absorption Coefficients of Several Carbon Blacks and Water-Based Printing Inks. J. Pulp Pap. Sci. 1994, 20 (12), J371. (14) Dence, C. W., Reeve, D. W., Eds. Pulp Bleachings Principles and Practice; Tappi Press: Atlanta, GA, 1996.
Received for review February 12, 2003 Revised manuscript received May 27, 2003 Accepted June 10, 2003 IE030126F
