Soda-anthraquinone pulping of a softwood mixture: Applying a pseudo-kinetic severity parameter

Bioresourc e Technology 60 (1997) 161-167

© 1997 Elsevier Science Limited All rights reserved. Printed in Great Britain 0960-8524/97 $17.00 ELSEVIER

PII:S0960-8524(97)00010-2

S O D A - A N T H R A Q U I N O N E PULPING OF A SOFTWOOD MIXTURE: APPLYING A PSEUDO-KINETIC SEVERITY PARAMETER J. M. Martfnez, J. Reguant, J. Salvad6 & X. Farriol* Departament d'Enginyeria Quimica, ETS d'Enginyeria Quimica, Universitat Rovira i Virgili, Carretera de Salou S/N, 43006 Tarragona, Catalunya, Spain

(Received 3 April 1996; revised version received 26 November 1996; accepted 20 December 1996)

Abstract

Sref

Experimental and published data about lignin solubilization during the soda-anthraquinone pulping of a softwood mixture were successfully described with the aid of a generalized severity parameter, K'Ron. Based on a first-order decay process and a time-dependent rate constant, the K'Ron concept combines the main operation variables (temperature, time and chemical loads) into a single reaction ordinate which is then used to summarize the changes in chemical composition and physical properties of the resulting pulp. The product K.Ron proved to be a valuable tool which provides an absolute scale that enables different biomass species treated in different conditions to be compared. © 1997 Elsevier Science Ltd.

Y(t) Yref

Reference chemical load (g chemical per kg of slurry) Chemical load (g chemical per kg of slurry), for the second active species Reference chemical load (g chemical per kg of slurry)

Greek letters 6 2 09 ?

Key words: Soda-anthraquinone pulping, delignification, severity parameter, softwood.

Parameter expressing the influence of the second active catalyst Parameter expressing the influence of the first active catalyst Parameter related to the average activation energy of the process, expressing the influence of temperature Parameter introducing a distribution of activation energies

INTRODUCTION NOTATION AQ % DSB E

At K

R2 Ro Roll t T(t)

Tref

X(t)

Most of the world's cellulose pulp is produced by Kraft pulping (chemical sulphate pulping). The sulphur compounds that are used in the process produce reduced sulphur forms which appear in effluent gases. Such compounds, as hydrogen sulphide, mercaptans and organic sulphides, cause an unpleasant 'bad eggs' smell around the mill, even at extremely low concentrations (smell threshold 2-6 ppb) (Sundquist, 1994). Alternative methods are mainly nonsulphur alkaline processes in the presence of the catalyst anthraquinone (AQ) and solvent pulping (organosolv) (Kinstrey, 1993). Anthraquinone pulping, first reported by Holton (1977), generally includes increased delignification rates, selectivity and velocity, as well as reduced alkali charges and yields. The chemical mechanism of alkaline AQ pulping is fairly well known, with AQ being described as a redox catalyst in the liquor system (Kubes et al., 1978; Fleming et al., 1979).

Anthraquinone Percentage of dry solid basis Activation energy (kJ mol- i) Fraction of solubilized lignin Reaction rate in the reference conditions (min-0 Correlation coefficient Severity factor (Overend and Chornet, 1987) Severity factor (Abatzoglou et al., 1992) Time (min) Reaction temperature (°C), which may vary as a function of time in non-isothermal conditions Reference temperature (°C) Chemical load (g chemical per kg of slurry), for the first active species

*Author to whom correspondence should be addressed. 161

162

J.M. Martinez, J. Reguant, J. Salvad6, X. Farriol

Numerous results demonstrate that softwood sodaAQ pulping of bleachable grades becomes comparable to conventional kraft pulping with regard to yields and cooking times. In this way, the use of anthraquinone means, on the one hand, lower air pollution because it reduces or totally eliminates sulphur and, on the other, less waterway pollution with bleach effluent from the plant because of extended pulping at lower kappa numbers. In terms of their catalytic effects on delignification, anthraquinone and the hydrosulphide ion ( H S - ) seem to be equivalent, but anthraquinone is hundreds of times more effective, molecule for molecule (Blain, 1993). By contrast, the pulp obtained in soda-AQ pulping is harder to bleach and the strength properties are generally worse or at least no better than those of kraft pulps (McLeod, 1979). Apart from that, AQ does not dissolve completely during the pulping process, so causing problems in linerboard operations (Blain, 1993). In the present work we have tried to describe the extent of delignification during the pulping of a softwood mixture following the soda/AQ delignification process. In this way, the operation variables involved in the pulping step have been unified with a pseudokinetic severity parameter which has been used to monitor the behaviour of lignin solubilization, as well as the yield of the resulting pulp. Our study has been carried out using experimental and published data sets in an attempt to validate such a severity factor.

METHODS Lignocellulosic substrate The experiments were carried out using a homogeneous batch of ground softwood residues from the north-east of Spain, a mixture of spruce (Abies alba) and pine (Pinus insignis). The ground material was sieved at 100 mesh and had a moisture content of 7% of the total weight. Table 1 shows the average composition and the confidence limits of seven different samples analysed from the batch. Table 1. Average composition and 95% confidence interval for the softwood mixture. Results expressed as % dry solid basis

Fraction Ash Hot water extractives Ethanol/toluene extractives Klason lignin Glucan Xylan Galactan Arabinan Mannan

Softwood mixture

Confidence limit

0.4 7.4 3.3 25.1 38.2 5.9 4.1 6.6 12.0

0.1 1.4 1.4 0.8 0.7 0.3 0.2 0.3 0.8

Analytical procedures Chemical analysis of the original substrate was conducted using the following standard methods: ASTM E-871-82 for moisture content; ASTM D-3516-76 for ash content; ASTM D-1111-84 for hot-water extractives and modified ASTM D-1107-84 for ethanol/toluene extractives. Klason lignin was measured in the extractive-free samples according to ASTM D-1106-84. The carbohydrate analysis was performed by HPLC following quantitative saccharification of the substrates (Saeman et al., 1945). The lignin content in the delignified samples was calculated from the kappa number according to the relationship in eqn (1) (Browning, 1967). The kappa number was measured using the TAPPI T236 os-76 standard.

% residual lignin = 0.15 x kappa no.

(1)

Delignification conditions Soda and soda-anthraquinone delignification experiments were performed in a 300 ml, high-pressure, stirred-batch reactor. Initially, several experiments were carried out to achieve good conditions for the delignification process. Six of the most cited bibliographical variables (alkali charge, liquor to wood ratio, maximum temperature, time to maximum temperature, time at maximum temperature and anthraquinone addition), were sequentially modified in order to decrease the kappa number and so minimize the pulping yield loss if possible. A total of nine experiments were conducted in this way. The experimental plan covered temperatures from 160 to 170°C, residence time at these temperatures from 60 to 150 min, time to these temperatures from 15 to 75 min, alkaline loads (NaOH) of between 20 and 40% DSB (dry solid basis), anthraquinone loads of 0.1% DSB if necessary and a liquor to wood ratio from 20:1 to 10:1. All experiments were performed at a stirrer speed of 430 rpm.

RESULTS AND DISCUSSION The soda and soda-anthraquinone process depends on a great number of different variables (temperature, alkali load, anthraquinone addition, heating rate, cooking time and liquor to wood ratio) which directly affect the yield and properties of the pulp obtained. Due to the high cost of a factorial design, a sequential searching strategy was used to find out which operation conditions were able to generate bleachable pulps, that is to say, which had a kappa number lower than 50. Atmospheric pressure runs were taken into account initially, with a temperature range between 90 and 100°C. Results suggested that high-pressure conditions had to be considered if kappa numbers lower than 200 were to be reached.

Soda-anthraquinone pulping of a softwood mixture

163

ties of the pulp (Lachenal et al., 1980). Moreover, AQ loads above 0.1% DSB exceed the level which the US Food and Drug Administration mandates for the production of food-grade paper.

A first set of experiments used the conventional soda delignification process, while a second set included anthraquinone addition, in order to evaluate its much commented effectiveness (Holmbom et al., 1979; McLeod et al., 1980). Table 2 shows the operating conditions used in the delignification process for the softwood sawdust studied. The same table also lists the kappa number and the pulp yield

Heating rate The heating rate does not seem to significantly modify the kappa number, as can be observed in experiments 5 and 6, where it remains practically constant at 56.

(% o.d. wood). EFFECT OF THE VARIABLES ON THE DELIGNIFICATION PROCESS

Cooking time Cooking time directly affects the delignification rate. Experiments 5 and 7 show how the kappa number varies from 57 at 90 min to 46 at 150 min. There is also a small reduction in the pulp yield in these operation conditions, which implies that higher severities in the delignification process do not cause further solubilizations.

Temperature Experiments 1 and 2, Table 2, show the influence of the reaction temperature and residence time on the yield and chemical composition of the pulp. The kappa number decreases from 156 at 160°C and 60 min to 137 at 170°C and 90 min. Alkali load Experiments 2-5 show the effect of the alkali when it doubles from 20 to 40% DSB. The kappa number decreases from 137 to 93 if no anthraquinone is added and from 105 to 57 if it is. Moreover, pulp yield reduces from 73 to 53% DSB and from 68 to 47% DSB, respectively, which indicates that a considerable amount of the polysaccharide fractions originally present in the untreated softwood is solubilized with the lignin.

Liquor to wood ratio Experiments 7 and 8 show the effect of the liquor to wood ratio when it doubles from 20:1 to 10:1. The kappa number decreases from 46 to 30, while the pulp yield goes down from 45 to 37% (DSB). As can be seen, the lower the liquor to wood ratio, the higher the alkali concentration is, since the alkali to wood ratio does not vary. Table 3 shows some of the most common limits cited in the literature for the operating conditions of a softwood delignification process. Alkali loads remain almost constant at around 20% DSB, while anthraquinone addition varies from 0.04% DSB to 1.0% DSB. Conversely, the maximum operating temperature reaches 180°C with times to this temperature varying from i0 to 150 min and residence time at this temperature from about 30 to 500 rain. The liquor to wood ratio changes between 5:1 and 4:1. The autoclave reactor used in this study was small and it was impossible to reach such a high value of the liquor to wood ratio, 4:1, as there were mixing problems. For this reason, higher alkali loads were used to compensate.

Anthraquinone addition Experiments 2-5 also list the effect of the anthraquinone addition, 0.1% DSB, at different alkali loads. Thus, the kappa number decreases from 137 in experiment 2 to 105 in experiment 4 at an alkali load of 20% DSB for both of the experimental runs. In contrast, the kappa number decreases from 93 in experiment 3 to 57 in experiment 5 at an alkali load of 40% DSB. As can be observed, the combination of high alkali load and anthraquinone improves delignification, although there is a considerable reduction in pulp yield. Higher amounts of anthraquinone do not lead to a substantial decrease in the kappa number but do lower the bleachable proper-

Table 2. Experimental conditions used in this study for the soda-anthraquinone deUgnification of the soltwood mixture. All percentages refer to dry wood

Experiment

%NaOH

Liquor: wood

Tm~x (°C)

t to Tma x (min)

t at Tma x (min)

%AQ

Kappa number

Pulping yield, %

1 2 3 4 5 6 7 8 9

20 20 40 20 40 40 40 40 40

20:1 20:1 20:1 20:1 20:1 20:1 20:1 10:1 10:1

160 170 170 170 170 170 170 170 170

15 25 25 25 25 75 25 25 25

60 90 90 90 90 90 150 150 150

NO NO NO 0.1 0.1 0.1 0.1 0.1 0.1

156.4 137.0 92.7 104.5 57.4 54.4 46.3 30.2 26.7

73.1 72.8 52.5 68.3 47.1 46.5 44.7 40.4 40.9

164

J. M. Martfnez, J. Reguant, J. Salvad6, X. Farriol

Table 3. Experimental conditions used in the different data sets. All percentages refer to dry wood

Authors, date Abbot and Bolker, 1984 Al6n et al., 1984 Andrews and Yethon, 1979 Blain, 1993 Chang et al., 1973 Fleming et al., 1979 Gourang et al., 1979 Holmbom et al., 1979 Holton, 1977 Kubes et al., 1978 Lachenal et al., 1980 Leu et al.,

1980 McLeod et al., 1980 Werthemann, 1982

Species

%NaOH Liquor: wood

Tma x

(°C)

t to Tmax t at Tma x (min) (min)

%AQ

Kappa number

Pulping yield, %

Picea mariana

40

10:1

150

--

180

0.0-0.5

57.6-130.0 56.6-72.0

Pinus sylvestris

8-10

4:1

150

10

30-90

0.0

--

77.2-84.4

Picea mariana

20

4:1

170

90

90

0.25

30.0

50.0

Picea/pinus ~

20-23

4:1

170

--

80-90

Pinus taeda

20

4:1

170

--

180

0

120.0

56.0

Picea mariana

23

4:1

170

100

80

0

93.4

54.4

Picea abies

20

5:1

173

30

60

0.1

81.5

56.5

Picea abies

20

4:1

170

150

90

0.0-0.5

35.6-112.0 50.6-59.0

Softwood mixtureh Picea mariana

20 23

4:1 4:1

170-180 166

60-90 90

50-100 120

0.0-0.1 0

27.5-116.0 48.7-55.3 95.2 56.3

Pinuspinaster

21

4:1

170

90

90

0.3

Tsuga heterophylla

24

5:1

170

135

95-500

0.0-0.1

31.0-46.0 39.5-46.9

Pseudotsuga/Tsuga c

23

4:1

170

90

110-120

0.25

25.5-36.6 41.2-44.2

Picea abies

18

4:1

173

93

120

0.04-0.05

0.15-0.25 29.8-34.1 48.6-50.8

32.0

9.4-10.2

45.7

46.0-50.7

aRuns performed with Picea mariana and Pinus eUiottii. hSoftwood mixture composed of black spruce, balsam fir and a pine. CRuns performed with Pseudotsuga menziesii and Tsuga heterophylla. DESCRIPTION OF THE SOFTWOOD DELIGNIFICATION THROUGH A SEVERITY PARAMETER

Data discussed in the previous section suggest that it is possible to produce very similar changes in the lignin yield of the lignocellulosic substrate at multiple combinations of the main operating conditions previously cited. This behaviour has also been widely observed for the depolymerization and solubilization of the hemicelluloses present in lignocellulosic biomass under dilute acid hydrolysis (Martfnez et ai., 1995) and autohydrolysis (Montan6 et al., 1993). Overend and Chornet (1987) grouped the operating variables in the case of the autohydrolysis of wood into a severity parameter which was then used as a single reaction ordinate, Ro factor. Abatzoglou et al. (1992) defined the Rou severity concept from modelling the solubilization of hemicellulose during the hydrolysis of lignocellulosics assuming first-order kinetics with respect to the reactant substrate. Considering that the overall delignification process also shows, phenomenologically, a first-order dependence with the lignin concentration (Larocque & Maass, 1941), a similar approach was used in this work to model lignin solubilization during the pulping process. In contrast to hemicellulose depolymerization, the delignification process studied here uses a mixture of different chemicals to rupture

the lignin-carbohydrate bonds. The active species are the O H - anion and anthraquinone. Although the severity parameter was originally developed to account for only one catalyst or active chemical species, it can be easily modified for situations in which two active species are present. So, the essential equations of the model with two catalytic species become

Roll = J o exp

to

]~f = 1 - e x p ( - K ' R o H )

(3)

where ~f is the fraction of lignin solubilized by the pulping process, to is a constant dependent on the average activation energy of the reaction process, ;t and 6 are constants which express the effect of the first and second active catalyst on the reaction rate, is a parameter that introduces a distribution of activation energies and K is a model constant which refers to the reaction rate in the reference conditions (ref). As can be observed, this severity parameter proves to be a useful tool for manipulating data obtained in situations in which the reaction

Soda-anthraquinone pulping of a softwood mixture

165

temperature and the catalyst concentrations vary with time during the reaction procedure. Moreover, the chemical load for both catalytic species has b e e n expressed in g of chemical per kg of slurry in order to consider the liquor to wood ratio. The optimal values of the model parameters (K, 09, 4, 6 and y) for the experimental results obtained in this work, Table 2, and for some sets of published data on wood delignification listed in Table 3 (37 published data corresponding to 14 different authors), were obtained following eqn (3) and the corresponding expression of the severity parameter, eqn (2). The experimental and model-predicted values were regressed by using the non-linear regression algorithm proposed by Marquard (Kuester & Mize, 1973). Considering the reference state (Tref, Xref, Yref) as the situation in which no significant reaction occurs (Abatzoglou et al., 1992; Montan6 et al., 1994), values of 20°C and of 0.0 g of chemical per kg of slurry have been used for Tref, Xref and Yref in all the subsequent modelling work. The optimum values of the constants, the standard error and the correlation coefficient are shown in Table 4 for the softwood mixture treated in this study and for the published data which are grouped as a single biomass type. Results show a very high concordance between experimental data and the model predictions for the softwood sawdust used, with a standard deviation of 3.3 x 10 - 2 and a correlation coefficient (R 2) of 0.97. In contrast, a standard deviation of 7.0 x 10 -2 is reached for the grouped published data with a R 2 of 0.93. The 09 value is similar for the softwood mixture and the published data with a difference of + 5 % from a central value of 37. This is what one should have expected since this parameter is associated with the activation energy in the delignification process and, thus, with the lignin-carbohydrate bond strength. The values of 2 and 3 are different for each particular biomass set used as these parameters are associated with the catalytic effects. In fact, if it is assumed for the hypothesis that the activation energy is equally affected by the two chemical species for both the two runs studied, it can be concluded that differences in 2 and 6 are due to dissimilar structural effects in the lignocellulosics considered. It is important to notice the value of 0.21 found for the 7 parameter ( 0 < ~ < 1 ) in the case of the softwood mixture, which denotes a wide distribution

of the activation energy values for the delignification reaction (Montan6 et al., 1994). The existence of this distribution of activation energies reflects a physical situation in which the reacting substrate is composed of many similar chemical species with different lifetimes. The 7 value increases as the species studied becomes more homogeneous. The delignification runs for the grouped published data have a ~ value close to 0.6, which denotes a lower heterogeneity of the energy reaction barrier when compared with the previous case, possibly due to a compensation effect between the different raw material used (spruce, pine and hemlock) and the cooking operations carried out. Further information about the approach to the continuous distribution of activation energies can be found in Plonka (1986). Figure 1 shows the fraction of solubilized lignin for the two data sets reported in Tables 2 and 3 as a function of the pretreatment severity expressed as the product K'RoH. The use of this combined parameter as a reaction ordinate furnishes an absolute scale that enables different biomass species to be compared. Both data sets behave very similarly and there is good agreement between the model prediction and the experimental results, although the model does not quite satisfactorily predict the lignin solubilization profile at high severity, giving lower values than the ones observed. Chemical pulping removes sufficient lignin to permit easy fiberization but leaves a small percentage of residual lignin on dry matter. Pulping chemicals are not sufficiently selective to enable more lignin to be removed without excessively damaging the carbohydrate fraction (Reeve, 1989). Table 5 shows the average composition of the delignified softwood pulp obtained at the highest severity studied. The same table also shows the composition yield of every fraction with respect to the original biomass. As can be seen, there is a considerable decrease both in the hemicellulose content and lignin solubilization. Glucan content also undergoes a significant reduction as there is almost a 68% solubilization with respect to the original composition in the untreated lignocellulosic. Table 5 also shows how additional steps are necessary not only to obtain a highly delignified pulp, but a highly purified cellulose with low hemicellulose and lignin content. In this respect, initial pretreatments must be considered prior to delignification if hemicellulose removal is desired. A

K'RoH concept.

Optimization results for the different data sets used in this work

Table 4. Deligniflcation kinetics using the

Biomass Softwood sawdust Published data

K

~

•

(L)

~Y

0"n-- 1 (%)

R2

3.9 x 10-4 2.0 x 10-4

12.28 48.45

0.11 0.68

38.92 35.18

0.21 0.59

3.3 7.0

0.98 0.93

J. M. Martinez, J. Reguant, J. Salvad6, X. Farriol

166 100 80 "d

cation rate of different biomass species. To this effect, the delignification process of a softwood mixture composed of pine and spruce lignocellulosic was successfully simulated together with some published data.

<><>

60 40

ACKNOWLEDGEMENTS O

20 ©

0 0.1

10.0

1.0

100.0

K'RoH

Fig. 1. Evolution of the lignin removal with the generalized severity parameter, K'RoH for the soda-anthraquinone pulping of a softwood sawdust and some published data. Softwood sawdust composed of spruce (Abies alba) and pine (Pinus insignis), e. Published data: softwood mixture o, spruce/fir D, pine % hemlock zx. The continuous line is the model prediction. bleaching sequence must be the last step if the kappa number is to be reduced as much as possible. CONCLUSIONS The phenomenological kinetics of a soda-anthraquinone delignification process was studied with the aid of a generalized severity parameter based on the Roll concept. This parameter was originally developed to simulate the solubilization of hemicelluloses during the hydrolysis of lignocellulosics using a first decay process and a time-dependent constant and including an active chemical species to describe the chemical action. This severity parameter was modified in order to account for systems in which two active chemical species are presented. In the case of the soda/AQ pulping process, these catalysts are alkali and anthraquinone. The Roll parameter is an easy way of predicting the extent of delignification for a given set of operational conditions (maximum temperature, time to maximum temperature, time at maximum temperature, liquor to wood ratio, alkali charge and anthraquinone addition). The K'RoH combination is also a valuable tool to show, under a single reaction ordinate, the delignifiTable 5. Average composition of the delignified softwood mixture at the highest severity studied. Results expressed as % dry solid basis

Fraction Glucan Xilan Mannan Klason lignin Total

Delignified pulp

Pulp yield (% original)

79.5 6.2 12.1 4.4 102.2

32.4 2.5 4.9 1.8 41.6

The authors are indebted to the CICYT and the Generalitat de Catahrfaya (Catalan Regional Government, SPAIN) for financial support, project number QFN95-4720.

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constant and a generalized severity parameter to pulping and correlation of pulp properties. Wood Sci. Technol., 28(6), 387-402. Overend, R. P. & Chornet, E. (1987). Fractionation of lignocellulosics by steam-aqueous pretreatments. Phil. Trans. Royal Soc. London, A321, 523-536. Plonka, A. (1986). Time-dependent reactivity of species in condensed media. Lecture Notes in Chemistry, 40, Springer, Berlin. Reeve, D. W. (1989). Bleaching technology. Pulp and Paper Manufacture, ed. M. J. Kocurek, Vol. 5, pp. 391-424. Alkaline Pulping. The Joint Textbook Committee of the Paper Industry, Tappi, Atlanta. Saeman, J. F., Bubl, J. L. & Harris, E. E. (1945). Quantitative saccharification of wood and cellulose. Ind. Engng Chem., 17(1), 35-37. Sundquist, J. (1994). Towards cleaner technologies in pulp and paper manufacture. Paperija Puu, 76(1-2), 22-26. Werthemann, D. P. (1982). Sulfide and anthraquinonelike catalysts delignify wood via different chemical mechanisms in alkaline pulping. Tappi J., 65(7), 98-101.