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Ind. Eng. Chem. Res. 1999, 38, 4608-4614
Ozone Delignification of Pine and Eucalyptus Kraft Pulps. 2. Selectivity Roge´ rio M. S. Simo˜ es and Jose´ A. A. M. e Castro* Department of Paper Engineering, University of Beira Interior, Rua Marqueˆ s D’AÄ vila e Bolama, 6200 Covilha˜ , Portugal, and Department of Chemical Engineering, University of Coimbra, Largo Marqueˆ s de Pombal, 3000 Coimbra, Portugal
The selectivity of ozone in the delignification of unbleached pine and eucalyptus kraft pulps is studied at ultralow consistency in a stirred reactor under closely controlled experimental conditions. The effect of several operating variables is analyzed, but special attention is paid to the depolymerization rate of polysaccharides with the particular goal of evaluating the influence of the lignin contents on its kinetics. By using substantially different ozone concentrations in the pulp suspension and different reaction temperatures, it is possible to show that ozone selectivity can only be slightly improved by manipulating these operating variables. Furthermore, for the same type of material, it was observed that the initial rate of delignification plays the most important role on selectivity. In fact, for a given pulp, selectivity decreases with a decrease of the initial lignin contents, and such results can be well justified by the corresponding reduction of the initial rates of delignification. To further investigate the effect of lignin on pulp degradation, experiments were carried out at 4 °C between ozone and holocellulose, which represent the polysaccharides of the unbleached pulps. The results suggest that molecular ozone can be responsible for an important part of the polysaccharides depolymerization during the delignification process. Moreover, the comparison of the kinetic behavior of holocellulose and of the corresponding unbleached pulp also reveals that the presence of lignin in the pulp enhances both the depolymerization and the degradation rates of polysaccharides. Introduction Environmental concerns in the pulp industry about the mill effluents containing organo-chlorine compounds grew significantly in the last decade. As a result, the pulp industry has completely removed elemental chlorine from the bleaching stages and oxygen compounds arose as very attractive alternatives. Ozone is a powerful delignification agent, but its very high oxidation potential makes it also prone to depolymerize and to degrade pulp polysaccharides. In fact, its selectivity is lower than that exhibited by conventional chlorinebased chemicals, such as chlorine and chlorine dioxide. The prevalent view attributes this lack of selectivity to the generation of highly reactive and nonselective hydroxyl radicals during the bleaching process. This radical generation is usually ascribed to ozone selfdecomposition, to ozone decomposition catalyzed by transition metals, and mostly to lignin-ozone reactions. To minimize the first mechanism, ozone bleaching is usually carried out under acidic medium. The metal contents in the pulp also plays a role on selectivity, but there is no current agreement about its relative importance. Some investigators claim that the reaction between ozone and a metal-free unbleached pulp should be very selective while others showed that under acidic conditions the improvements are not relevant. Colodette et al.1 with an intense DTPA (diethylenetetraminepentaacetic acid) chelation pretreatment, which efficiently removes metals from the pulp, reported selectivity gains of only 14%. Concerning the effect of lignin, the prevailing explanation points to ozone-lignin reactions as the most important sources of radicals, which have been considered as responsible for most of the degradation
of polysaccharides.2-6 Ragnar et al.5 demonstrated that all model lignin compounds studied led to radical generation and that their yield is also a function of lignin structure and in particular of the relative contents of its phenolic and non-phenolic compounds. The former are known to exhibit a higher radical yield than the nonphenolic ones. These differences and the effect of ozone concentration on polysaccharides degradation rate rationalize published data for pulps with low and high lignin contents.7,8 Despite these progresses, the role of molecular ozone (direct ionic reaction) on the polysaccharides depolymerization remains unclear. While Lind et al.9 suggest that molecular ozone contributes to approximately 50% of the polysaccharides degradation for a pulp with a low κ number (K ≈ 2), Ragnar et al.5 claim that molecular ozone is very selective. This unresolved question is a consequence of the difficulty in eradicating radicals from the reaction medium. Most papers on ozone selectivity are based on experiments carried out at high consistency, where it is difficult to measure both the ozone concentration and reaction temperature in a clear way. Experimental work at low and ultralow consistencies with a close control of the above operating parameters is very scarce in the literature. The same happens with most published results concerning the kinetics of both delignification and polysaccharides degradation by ozone. The main purpose of this study is to investigate the selectivity of ozone in the delignification process of eucalyptus and pine pulps. This is accomplished with experimental conditions that eliminate the external resistance to mass transfer, i.e., outside the fiber (ultralow consistency and high mixing intensity). Moreover, these conditions enable a correct monitoring of the ozone
10.1021/ie980807o CCC: $18.00 © 1999 American Chemical Society Published on Web 11/11/1999
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Figure 1. Effect of temperature on the selectivity of delignification for pine (a, left) and eucalyptus (b, right) pulps.
concentration in the pulp suspension and a more effective control of the temperature. Special emphasis will be given to the effects of reacting conditions and of pulp lignin contents on the selectivity. Lignin-free materials produced from the corresponding unbleached pulps (holocellulose) will be used as carbohydrate models, in order to isolate the role played by molecular ozone in the depolymerization of polysaccharides. The results will be analyzed not only in the perspective of selectivity as it is common practice but also from a kinetic point of view. The rate of depolymerization of polysaccharides will be looked at with the purpose of evaluating the importance of pulp lignin contents. Experimental Arrangement The experimental arrangement is described in part 1 of this series.10 The materials investigated in this work are unbleached kraft pulps from eucalyptus and pine woods, the corresponding holocelluloses, and pulps partially delignified. The κ numbers (ISO 302) and the viscosities (ISO 5351/1) of the unbleached pulps are 15.5 and 31.7 and 1270 cm3/g and 971 cm3/g, respectively, for eucalyptus and pine. Holocelluloses were produced from these unbleached pulps by the chlorite (pH ) 4.9) method.11 The residual lignin in these materials is very low (κ numbers 2 and 3), and the loss in viscosity is negligible, which is very important for their use as models of pulp polysaccharides. The holocellulose viscosities are 1185 and 906 cm3/g for eucalyptus and pine pulps, respectively. All materials were also submitted to an acidic pretreatment before the reaction with ozone in order to minimize their metal contents. The reaction with ozone was carried out at ultralow consistency in accordance with two different experimental procedures described in part 1 of this series10 and named as conventional and presaturation methodologies. Results and Discussion In the present work, the selectivity is defined by
S ) rL/rC
(1)
where rL and rC are the delignification and the polysaccharides depolymerization rates, respectively, characterized by the following:
rL ) - dK/dt
(2)
rC ) - dVisc/dt
(3)
Therefore,
S ) dK/dVisc
(4)
The effect of temperature on selectivity can be appreciated in Figure 1, where pulp viscosity is plotted against κ number. In this figure, selectivity is the inverse of the local slope of the line passing through the points representing a particular set of experiments. For the case of pine pulp, selectivity exhibits a low sensitivity with respect to temperature in the 4-43 °C range. In fact, between 4 and 23 °C the difference is within the range of the experimental error. On the other hand, when the results are compared between 4 (solid line) and 43 °C (dashed line), a lower selectivity for the higher temperature becomes noticeable. For a given final κ number, the difference in pulp viscosity can reach 80 units. However, this is small, regarding the experimental error associated with the determination of pulp viscosity and with the corresponding absolute values. For the unbleached eucalyptus pulp, which has a higher viscosity and a lower lignin content, the delignification results for 4 and 23 °C show, as before, no detectable effect of temperature. These observations for both pine and eucalyptus pulps are in good agreement with those from other researchers.12-14 Griffin et al.,13 working at high consistency (30%) and at +30 °C and -20 °C, with pulp fibers impregnated with an aqueous solution of 81% acetic acid, detected small improvements in selectivity with a temperature decrease. This insensitive behavior of selectivity with respect to temperature is, probably, a consequence of similar activation energies for the delignification and for the depolymerization of polysaccharides. It has already been shown3,15 that both ozone and the radicals arising from its secondary reactions contribute to the two competing processes. However, it is not possible to impute such a small difference between the activation energies only to any of such species. Two further important observations from Figure 1 are, first, selectivity remains almost constant in the high range of κ number down to around 4 and, second, it decreases sharply thereafter, as also reported by Lind et al.9 The first observation suggests that rL and rC present a parallel pattern along the reaction for a κ number above 4. While the decrease of rL with time is naturally associated with the decrease of lignin contents in fibers, the decrease of rC cannot only be imputed to the concentration of polysaccharides which remains practically invariant along the reaction time; in this case the reactivity of cellulose and the ozone accessibility
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Figure 2. κ number (a, left) and viscosity (b, right) profiles for eucalyptus and pine unbleached pulps, at 4 °C.
Figure 3. Experimental ozone profiles (a, left) and selectivity results (b, right) for unbleached pine pulp, at 4 °C.
must be considered to justify such behavior. The second observation can be explained by the kinetic approach. Figure 2, where the κ number and viscosity profiles are illustrated along the reaction time, shows that below the range 3-4 of κ number the rate of degradation of polysaccharides, although smaller than that in the beginning of the reaction, is still significant in opposition to what happens with the rate of delignification. For the case of eucalyptus, this almost stops after a period of around 2 min, while the polysaccharides degradation continues thereafter although at a slower rate. Thus, for both pulps, selectivity (dK/dVisc) is smaller in this low κ number range than in the higher κ number zone which is an expectable consequence of the very low delignification rate. This, in turn, is probably the result of the conjugated effects of very low lignin contents and reactivity and, eventually, low accessibility of the remaining traces of lignin. To investigate the effect of the ozone concentration in the liquid phase on the selectivity of the delignification process, two sets of experimental runs were planned. In the first set, the liquid inside the reactor is previously contacted with a stream of ozone/oxygen for a given period of time to achieve a prespecified ozone concentration. Then the pulp is added, and the ozone supply is maintained during the entire reaction period (Figure 3a). Despite the fact that the concentration in the reaction medium differs by a factor of ≈5 (LOW, low ozone; HIGH, high ozone series), there is no distinction between the corresponding profiles of viscosity versus κ number. For similar experiments carried out at 23 °C, it was, also, not possible to disclose any differences in selectivity. Pulps with a higher viscosity and a lower lignin content, such as the one from eucalyptus, exhibit analogous behavior. In Figure 4 one can compare the
results from experiments carried out with (W.S.) and without (W-O.S.) ozone presaturation of the liquid phase (second set of experiments). Despite the large differences in the ozone concentration profiles between these runs (Figure 5), the selectivities are similar. It is also noteworthy that these distinct concentration profiles lead to significantly different reaction times for the same delignification extent. The above results clearly demonstrate that there is no apparent way to improve selectivity in the delignification process by just manipulating the two main operation variables, i.e., temperature and concentration. In part 1 of this series10 it was shown that the rate of delignification increases with both variables. Combining this information with the results reported by Ragnar et al.5 for radical yield, one can conclude that the generation rate of hydroxyl radicals must also increase with temperature and concentration, in a way parallel with that of the delignification rate. Therefore, it is expectable that the rate of depolymerization of polysaccharides is related to that of delignification. Moreover, even in the case of lignin-free materials, both the temperature and ozone concentration enhance polysaccharides depolymerization.16 However, this is once again the conjugated effects of the molecular ozone and the hydroxyl radicals. Another important conclusion that can be drawn from the results in Figures 1 and 4 is that, independently of the experimental reaction conditions, selectivity is higher for pine than for eucalyptus pulp. Figure 6a highlights the viscosity versus κ number profiles for eucalyptus and pine unbleached pulps from tests carried out in equivalent operating conditions. The differences between the selectivities in these pulps are large and cannot be ascribed to the distinct ozone profiles il-
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Figure 4. Effect of ozone concentration on the selectivity of the delignification process for pine (a, left) and eucalyptus (b, right) pulps, at 4 °C. Table 1. Effect of Initial K Number (K0) on the Initial Delignification Rate (rL) at 4 °C, for Pine and Eucalyptus Pulpsa pulp
K0
tr, min
[O3]l, mg/L
K
∆K
rLCE, ∆K/min
rL, ∆K/min
pinea pine pine pine pine eucalyptusa eucalyptus
31.7 21.5 18.1 9.4 5.2 15.5 4.4
0.80 0.85 0.85 0.85 0.85 0.90 0.90
4.2 4.9 4.7 5.4 6.1 5.3 6.2
20.7 13.5 11.6 5.4 2.7 6.8 2.4
11.0 8.0 6.5 4.0 2.5 8.7 2.0
13.8a 9.4 7.6 4.7 2.9 9.7a 2.2
** 6.0 4.4 1.6 0.5 ** 0.7
a These pulps were not submitted to a CE treatment and therefore there is no interest in estimating the value referred to as **.
Figure 5. Liquid-phase ozone profile, with and without presaturation.
lustrated in Figure 6b, as previously demonstrated. These profiles can be seen as a consequence of the higher rate of ozone consumption of the pine pulp which is certainly related to its higher lignin contents. From a chemical point of view, the most important differences between these pulps are their lignin contents and the reactivity of the respective lignins. According to the results presented in part 1,10 lignin content exerts a strong influence on the delignification rate of both pulps. Regarding reactivity, rL is clearly higher for the eucalyptus pulp, for the same lignin contents. Thus, as a first attempt to further investigate the behavior of eucalyptus and pine pulps with respect to selectivity, a set of experiments was performed with samples of both pulps having similar κ numbers. A CE sequence (chlorine and NaOH extraction) was used to produce the sample of pine pulp, which resulted in a κ number of 18.1. The
selectivities for this and for the eucalyptus pulp (κ number of 15.5) are illustrated in Figure 7. Despite their similar lignin contents and the expected higher rL for eucalyptus, the later exhibits a smaller selectivity. In fact, this pulp suffers a higher initial delignification (rL, 9.7 against 7.6, Table 1), that should lead to a higher selectivity if the depolymerization rates (rC) were alike for the two pulps. However, this is not the case, and the more extensive initial degradation of polysaccharides observed in the eucalyptus pulp (Figure 2b) can justify, at least partially, this behavior of selectivity. To confirm this hypothesis, holocellulose samples obtained from both pulps were also submitted to ozone. The outcome is presented in Figure 8, and independently of the viscosity considered (standard or reduced with sodium borohydride), ozone exerts a much more intensive depolymerization on the material with higher initial viscosity, i.e., the eucalyptus holocellulose. If rC for the eucalyptus pulp is higher than that for pine, it is
Figure 6. Selectivity (a, left) and ozone profiles (b, right) for eucalyptus and pine unbleached pulps, at 4 °C.
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Figure 7. Ozone selectivity for eucalyptus and pine pulps with similar initial lignin contents, at 4 °C.
expectable that the selectivity of the former is smaller, even if its rL is slightly higher.
The role of the lignin contents on the selectivity has been a matter of much discussion and controversy in recent investigations.7,8,17 To aid in the discussion, a comparative study of the action of ozone during depolymerization is performed with the following three materials: unbleached pulps, partially delignified pulps, and holocelluloses (Figures 9 and 10 for eucalyptus and pine, respectively). The unbleached pulps were submitted to various CE treatments in order to prepare samples with different lignin contents but similar viscosities. According to Lachenal et al.,18 the contents of phenolic units in the residual lignin increases with the CE treatment, which simultaneously increases the reaction with ozone and the generation of radicals.5 Concerning the delignification rate, the observed values for these CE pulps (Table 1, column 7) are significantly higher than those estimated (eqs 2 and 3, part 1) for pulps without CE treatment but with the same initial
Figure 8. Standard (S.V.) and reduced (R.V.) viscosity profiles of eucalyptus (a, left), and pine (b, right) holocelluloses, at 23 °C.
Figure 9. Depolymerization of eucalyptus pulps with different lignin contents (a, left) and selectivities (b, right), at 4 °C.
Figure 10. Depolymerization of pine pulps with different lignin contents (a, left) and selectivities (b, right), at 4 °C.
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 4613 Table 2. Metal Contents after Acidic Treatment material
Fe, ppm
Mn, ppm
Cu, ppm
Co, ppm
unbleached pine holocellulose pine unbleached eucalyptus partially delignified eucalyptus holocellulose eucalyptus
21 18 16 14 15
1 1
