Further Insight into Carbohydrate Degradation and Dissolution

Article pubs.acs.org/IECR

Further Insight into Carbohydrate Degradation and Dissolution Behavior during Kraft Cooking under Elevated Alkalinity without and in the Presence of Anthraquinone Markus Paananen,*,† Tiina Liitia,̈ ‡ and Herbert Sixta*,† †

Aalto University, School of Chemical Technology, Department of Forest Products Technology, Vuorimiehentie 1, FI-00076 AALTO, Finland ‡ VTT Technical Research Centre of Finland, P.O. Box 1000, FI-02044 VTT, Finland S Supporting Information *

ABSTRACT: The polysaccharide degradation and dissolution behavior during high liquor-to-wood ratio (200:1) kraft cooking of Scots pine wood meal was studied at high (1.55 M) and moderate (0.50 M) hydroxide ion concentrations at a constant sulfidity of 33%. Both alkalinity levels were studied in and without the presence of anthraquinone (AQ) (0.05, 0.15, and 0.25 g AQ/L). High alkalinity experiments without AQ at 130−160 °C clearly confirmed significant galactoglucomannan stabilization (in respect to lignin content) throughout initial and bulk delignification phases. Additionally, at high alkali compared to moderate alkali concentration, lower amounts of low molecular weight carboxylic acids originating from the degradation of carbohydrates were detected in spent black liquor. The presence of AQ provided significant hemicellulose stabilization against endwise degradation reactions, being more pronounced at moderate 0.50 M concentration than at 1.55 M hydroxyl ion concentration. In all cases, higher alkalinity promoted carbohydrate removal via dissolution, and the addition of AQ reduced the degradation of the dissolved carbohydrate fraction, thus further increasing the amount of dissolved polysaccharides found in black liquor.

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INTRODUCTION In the course of kraft pulping, substantial pulp yield losses occur, owing to carbohydrate removal via dissolution and degradation reactions. Kraft pulping of pine to bleachable grades generally results in glucomannan, xylan, and cellulose losses of 75%, 38%, and 10%, respectively.1 Under kraft conditions, already at lower temperatures, stepwise carbohydrate removal occurs due to the peeling-off reaction, in which chain-end monomer units are eliminated from the reducing end-groups and converted mainly into isosaccharinic acids and lactic acid.2 The immediate glucomannan vulnerability to peeling reactions at alkaline conditions has been explained by unrestricted access to the β-alkoxyelimination reaction, which, in the case of arabinoxylan, is greatly hindered by the substituents at C-2 and C-3, thus eventually diverting the reaction pathway to the formation of an alkali-stable metasaccharinic acid end-group.3 Under alkaline conditions, peeling and stopping reaction rates are highly dependent on the ambient alkali concentration. At a moderate temperature of 100 °C, the peeling reaction rate of amylose was found to reach a maximum at a hydroxyl ion concentration of 0.1 M, whereas the stopping reaction rate continued to increase to approximately 1.5 M alkalinity.4 Similar results were obtained in our previous kraft pulping study with Scots pine wood meal, where the peeling reaction rate showed a strong leveling-off tendency during the whole studied hydroxyl ion concentration range of 0.31 to 1.55 M, whereas the stopping reaction rate continuously increased with alkalinity.5 At a temperature of 130 °C, the ratio of peeling-to-stopping reaction rates (kp/ks) was decreased from 55 to 20 when the alkalinity was elevated from 0.31 to 1.55 M [OH−]. It has been proposed that, unlike the peeling © 2013 American Chemical Society

reaction, the stopping reaction takes place only if sufficiently high alkali concentration is provided, thus favoring the formation of dianionic intermediates, contributing in both peeling and stopping reaction routes.6 Besides the primary peeling-off reaction, random base-catalyzed cleavage of glycosidic bonds constitutes an important degradation pathway under alkaline conditions at temperatures above 140 °C, occurring after the heating-up phase and resulting in the formation of new reducing end-groups susceptible to further peeling (secondary peeling). During kraft pulping of pine, polysaccharides undergo degradative reactions to various acidic degradation products such as gluco- and xyloisosaccharinic (Gisa, Xisa), 2hydroxybutyric, 3,4-dideoxypentonic, lactic, formic, and glycolic acids.1,7 The Supporting Information contains a scheme of the generation of the most important hydroxy acids during the alkaline treatment of polysaccharides. The formation of acidic degradation products is already extensive at the beginning of the cook (especially isosaccharinic acids originating from endwise degradation reactions), and it continues to increase as the cooking proceeds.8 A large number of polysaccharide derived hydroxy carboxylic acids found in kraft and sodaanthraquinone black liquors have been identified by Alén et al. (1985), who reported a total amount of hydroxy acids of 16− 18% on the original wood weight.9 It was concluded that at the end of the cook, 75−80% of the removed carbohydrates had Received: Revised: Accepted: Published: 12777

June 7, 2013 August 8, 2013 August 21, 2013 September 3, 2013 dx.doi.org/10.1021/ie4018012 | Ind. Eng. Chem. Res. 2013, 52, 12777−12784

Industrial & Engineering Chemistry Research

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been converted to monomeric aliphatic acids.9 After the addition of anthraquinone (AQ), the formation of glucomannan-derived degradation products such as lactic, 3,4dideoxypentonic, and glucoisosaccharinic acids was diminished, whereas the formation of glycolic, 3-deoxypentonic, and 3,4dideoxyhexonic acids was promoted.7,10 Additionally to the pulping conditions, even minor variations in the chemical composition of the feedstock have an important effect on the formation of various degradation products.11 Not all the carbohydrates that are removed from the wood matrix during the pulping process are degraded into acidic products. Hemicelluloses, particularly xylan, are dissolved in polymeric form after some depolymerization. Whether polysaccharide removal proceeds via degradation reaction or dissolution is strongly dependent on the cooking conditions. Xylan, especially, is known to dissolve at rather high concentrations into alkaline solution, reaching a maximum concentration well before the final cooking temperature is obtained.12 Xylan, in particular, is known to adsorb on cellulosic surfaces. In general, the extent of adsorption is dependent on the type of cellulosic fiber and the alkali concentration13 as well as on the size and substitution degree of the dissolved polysaccharide.14 The increase in chain linearity by the cleavage of the side chains of the hemicelluloses is known to be an effective means of enhancing both xylan and glucomannan adsorption efficiency.15,16 Lignin in wood matrix forms a complex structure, as the lignin C9 precursor units are interlinked to each other with various aryl−ether, aryl−alkyl, and alkyl−alkyl type bonds.17 During the initial delignification phase, diverse lignin structure is fragmented, as the most dominant interlinks between lignin phenylpropanoid units, β−O−4 bonds,18 are cleaved from phenolic lignin moieties according to zero order with respect to both effective alkali and sulfidity.19,20 More drastic conditions are needed to render also the nonphenolic lignin units susceptible to fragmentation, producing new phenolic structures. The formation of lignin-derived degradation products such as guaiacol, vanillin, acetovanillone, and dihydroconiferyl alcohol progressively increases toward the end of the cook.21 Since the pioneering work of Bach and Fiehn (1972),22 various quinone derivatives have been recognized as selectivity enhancing additives in the field of chemical pulping. Owing to its effectiveness and feasibility compared to other quinonic derivatives,23 the most studied compound has been 9,10anthraquinone (AQ), suitable to be combined with a wide range of pulping applications.24 During alkaline pulping, AQ acts as an electron carrier between carbohydrates and lignin, according to the widely accepted anthraquinone/anthrahydroquinone (AQ/AHQ) redox cycle. Pulping selectivity is enhanced as AQ oxidizes the reducing end-groups of carbohydrate chains into aldonic acid groups, being relatively stable toward alkali-induced endwise degradation reactions. Additionally, the lignin structure is fragmented as the readily formed AHQ anion reacts with the quinone methide intermediates while regenerating AQ.25 Analogously to hydrogen sulfide, the AHQ anion promotes the fragmentation of the β-aryl ether bond after addition to the quinone methide and subsequent intramolecular attack at the neighboring β-carbon. Thus, AQ addition significantly contributes to an increase in the delignification rate, especially during soda pulping.26 Even though it is highly effective in improving the selectivity during alkaline cooking, an elevated AQ charge (5% w/w AQ on amylose) has also been reported to promote polysaccharide

degradation due to random AQ-induced chain scission reactions occurring at low temperatures.27 In any case, high costs prevent the use of higher amounts of AQ in kraft or soda cooking. Recently, this has led to rather promising studies about recyclable anthraquinone copolymers that are able to preserve their catalytic properties after recovery.28 In addition to the active cooking anions (OH− and HS−), the so-called inactive ions also affect the degradation kinetics during kraft cooking. The ionic strength of industrial black liquor is usually investigated with [Na+], which can be adjusted by the addition of various sodium salts. Along with an increasing [Na+], a gradual decrease in the delignification rate occurs. In recent studies, the effect of ionic strength during alkaline pulping has been adjusted by the addition of NaCl, as it is easy to apply in laboratory conditions.29−31 The retarding effect of inactive anions originating from various sodium salts on the delignification rate during kraft pulping has recently been reported by Bogren et al. (2009).32 They showed that the anions of sodium salts diminish the delignification rate to different extents, which has been explained by the Hofmeister effect. Based on their findings, the greatest effect on decelerating delignification during kraft cooking was observed in the presence of Cl− anions. In this study, high alkalinity was used to enhance the stopping reactions over the endwise peeling reactions during kraft cooking. Additionally, the combined effect of AQ and high alkalinity was evaluated. The present study provides new insights into the degradation and dissolution behavior of wood carbohydrates during kraft pulping in the absence and presence of AQ, which constitutes a good basis for further improvements of the kraft process on an industrial scale.

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EXPERIMENTAL SECTION Scots pine (Pinus sylvestris) raw material from southern Finland was chipped and screened according to SCAN-CM 40:01. Screened chips were milled with a Wiley mill to pass through a 1 mm slot screen, after which the wood meal was frozen. (The chemical composition of the wood raw material is presented in Table 1.) Table 1. Chemical Composition of Scots Pine Wood Raw Material, Calculated According to Janson (1970)35 wood component

% on wood

cellulose glucomannan (acetyl groups excluded) xylan other carbohydrates uronic acids (not in xylan) Klason and acid soluble lignin extractives

41.7 16.9 8.2 1.4 2.1 26.6 3.2

Cooking experiments were conducted in a 10-L batch reactor at a liquor-to-wood ratio of 200:1, at high (1.55 M [OH−], 130−160 °C) and conventional (0.50 M [OH−], 160 °C) hydroxide ion concentrations, both with a sulfidity of 33%. The high liquor-to-wood ratio of 200:1 was chosen to maintain relatively constant cooking chemical concentrations throughout experiments. Additionally, both alkalinity levels were studied with the addition of 1, 3 and 5% AQ on wood at 160 °C temperature (0.05, 0.15, and 0.25 g AQ/L, respectively). The chosen AQ concentrations corresponded to those typical in industrial applications, which, in combination with the high 12778

dx.doi.org/10.1021/ie4018012 | Ind. Eng. Chem. Res. 2013, 52, 12777−12784


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liquor-to-wood ratio, resulted in relatively high AQ charges. NaCl was used to adjust the ionic strength to a constant level of 2.00 M [Na+]. Air-dried wood meal (40 g of absolutely dry weight) was charged together with 8 L of cooking liquor into the reactor. The reactor was first flushed with nitrogen gas and then pressurized to 0.5 MPa for 5 min. Pressure was released, and the reactor was heated to the preset temperatures. Heating was obtained within 20 min ± 1 min, despite the target temperature. After the desired time at temperature, the cooling phase lasted around 20 min until the reactor pressure was close to atmospheric. Spent liquor was collected as the pulp suspension was filtered with a tissue filter exhibiting a 6 μm pore diameter. Wood residue was further washed with at least 5 L of deionized water. The filter cake was collected and air-dried at room temperature. Wood residue was first analyzed for gravimetric yield and acetone extractives (SCAN 49:03). The extracted wood residue sample was analyzed for carbohydrates and gravimetric and acid soluble lignin, according to analytical methods issued by the National Renewable Energy Laboratory.33 Acid soluble lignin was determined using a Shimadzu UV-2550 spectrophotometer at a wavelength of 205 nm and an absorption coefficient of 110 L*(g cm)−1 according to Swan (1965).34 Monosaccharide composition was determined using high performance anion exchange chromatography with pulsed amperometric detection (Dionex HPAEC-PAD). The polysaccharide composition was calculated from the monosaccharide composition using published equations corresponding to the polysaccharide structures in wood.35 The dissolution and degradation of hemicelluloses was evaluated according to the carbohydrate composition of black liquors (BL), also determined after acid hydrolysis by HPAEC-PAD, according to SCAN-CM 71:09. The short-chain carboxylic acids were detected by capillary electrophoresis after the removal of excess −SO42− and Cl− ions, to evaluate the polysaccharide losses through peeling reactions.36,37 For salt removal, Dionex Onguard II Ba/Ag/H cartridges were used. The molar mass measurements of BL polysaccharides were performed by size exclusion chromatography (SEC) after precipitation, according to Engström et al. (1995).38 The Waters HPLC system with PSS MCX 1000 and 10000 columns, 0.1 M NaOH eluent (0.5 mL/min flow rate) and refractive index (RI) detector was used. The average molar masses were calculated relative to pullulan standards, using Waters Empower 2 software.

Figure 1. Effect of alkalinity and temperature on delignification rate at constant 33% sulfidity and hydroxyl ion concentration of 0.50 M (160 °C) and 1.55 M (130−160 °C) during kraft pulping of pine.

In an earlier study with a softwood kraft pulp, a yield loss of 0.5−2% was observed when at constant sulfidity, a liquor-towood ratio of 4:1, and a temperature of 170 °C the effective alkali charge was increased from 17.5% to 25%.40 Interestingly, the observed increase in glucomannan yield was more than compensated for by higher combined yield losses of cellulose and xylan. However, as shown in Figure 2, we could not observe any significant effect of hydroxyl ion concentration and temperature on the yield of the wood residue throughout the entire delignification phases.

Figure 2. Effect of alkalinity and temperature on wood residue yield as a function of lignin content at constant 33% sulfidity and hydroxyl ion concentration of 0.50 M (160 °C) and 1.55 M (130−160 °C) during kraft pulping of pine.

In a recent study, a significant stabilization of galactoglucomannan (GGM) was observed at high hydroxyl ion concentration in the initial phase of the kraft cooking of Scots Pine in the temperature range between 100 and 130 °C.5 The present study revealed that the advantage of elevated [OH−] on the GGM yield in the wood residue increased with temperature but was more pronounced at lower delignification degrees. During initial delignification at a Klason lignin content of 21% on pulp, 1.55 M [OH−] provided a 1.3%-unit (140 °C) and 2.4%-unit (160 °C) higher glucomannan content on wood, respectively, as compared to a 0.50 M [OH−] cook at 160 °C (Figure 3). With proceeding delignification, higher alkalinity remains to provide enhanced glucomannan content, but the temperature dependency diminishes to almost negligible at a Klason lignin content of around 10% on pulp. At this point, 1.55 M [OH−] results in a 1.4%-unit higher glucomannan content compared to 0.50 M [OH−], regardless of temperature. Higher alkalinity strongly accelerates the arabinoxylan (AX) removal, whereas a decrease in temperature was found to improve xylan preservation rather well (Figure 4). Thus, the

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RESULTS AND DISCUSSION Effect of Alkalinity on Carbohydrate Degradation and Dissolution. Wood Residue. The wood residue was analyzed with the main emphasis on evaluating the effect of cooking conditions on delignification and carbohydrate retention. The increase of alkali concentration from 0.50 to 1.55 M [OH−] was connected with a substantial acceleration of the delignification rate, thus allowing a decrease in cooking temperature without a substantial prolongation of pulping time (Figure 1). In order to obtain 80% lignin removal in kraft pulping to a κ-number of 60 (κ-60), the H-factor39 could be decreased from 1480 to only 300 when the hydroxyl ion concentration was increased from 0.50 to 1.55 M at the given high liquor-to-wood ratio of 200:1. In obtaining a κ-number of 30 (κ-30), which corresponds to the conventional production of bleachable softwood kraft pulps, the H-factor can be decreased from 2370 to only 480, owing to higher alkalinity. 12779



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dissolution or degradation reactions. As shown in Figure 6, the total amount of carbohydrates in the black liquor was

Figure 3. Effect of alkalinity and temperature on galactoglucomannan retention at constant 33% sulfidity and hydroxyl ion concentration of 0.50 M (160 °C) and 1.55 M (140 and 160 °C) during kraft pulping of pine.

Figure 6. Effect of alkalinity on galactoglucomannan, xylan, and cellulose dissolution as a function of pulp lignin content during kraft pulping of pine at constant 33% sulfidity, 160 °C temperature, and hydroxyl ion concentration of 0.50 and 1.55 M.

significantly increased due to dissolution promoted by elevated cooking liquor alkalinity. While the increased amount of dissolved hemicelluloses at high alkalinity may be particularly attributed to AX, the effect of temperature on the extent of dissolution was more pronounced for GGM. Prolonged cooking times at higher alkalinity resulted in a decrease in the amount of dissolved xylan, indicating an increase in competitive degradation pathways of the dissolved xylan (Figure 6).41 The maximum amount of dissolved polysaccharides was achieved at Klason lignin contents of 15% and 6% on pulp for 0.50 and 1.55 M [OH−], respectively. For both studied alkali concentrations, the decrease in the amount of dissolved xylan is accompanied by an enhanced formation of 2-hydroxybutyric acid and lactic acid. The former is formed in peeling reactions of xylan via formic acid elimination from the 3-keto intermediate followed by benzilic acid rearrangement, and the latter via β-alkoxy elimination and reversed aldol condensation reaction. Xyloisosaccharinic acid formation via β-alkoxy elimination, followed by keto−enoltautomerisation and benzilic acid rearrangement, was also clearly enhanced at lignin contents below 6%, supporting the xylan degradation (Supporting Information). However, the formation of glucoisosaccharinic acid originating from cellulose and glucomannan was also significantly enhanced at κ levels below 4%. At high alkalinity, AX degradation clearly starts at lower lignin content than previously reported42 for conventional cooking conditions. Therefore, maintaining an elevated alkali concentration and shorter cooking times is proposed in achieving a higher concentration of dissolved high molecular weight hemicellulose at the same κ levels, thus providing a means of enhancing the redeposition of dissolved hemicelluloses on fibers through liquor recirculation.43,44 The effect of alkali concentration on the extent of carbohydrate endwise degradation was also evaluated by analyzing the main organic acids formed as a result of peeling reactions. Clearly, less lactic and 2-hydroxybutyric acids were formed at the same pulp yield or degree of delignification for all black liquors derived from 1.55 M [OH−], as compared to 0.50 M [OH−] cooking conditions. Some reduction was also evident in the formation of xyloisosaccharinic acid and to some extent also in formic acid, but no clear reduction in the formation of other polysaccharide-derived acids was detected. Overall, lower

Figure 4. Effect of alkalinity and temperature on xylan retention at constant 33% sulfidity and hydroxyl ion concentration of 0.50 M (160 °C) and 1.55 M (140 and 160 °C) during kraft pulping of pine.

results confirm the findings reported by Aurell and Hartler (1965).40 Even though the overall pulp yield is hardly affected, the ratio of GGM to AX is substantially altered by kraft cooking at high alkali concentration (Figures 2−4). This is accomplished by substantial GGM stabilization, while AX is eventually lost through dissolution. Additionally, a minor improvement in cellulose retention at lower delignification degrees was found at 0.50 M [OH−], but the advantage was progressively lost as the delignification proceeded (Figure 5). Black Liquor. Black liquor samples throughout cooking were analyzed to allocate the polysaccharide losses to either

Figure 5. Effect of alkalinity and temperature on cellulose retention at constant 33% sulfidity and hydroxyl ion concentration of 0.50 M (160 °C) and 1.55 M (140 and 160 °C) during kraft pulping of pine. 12780



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0.50 M, respectively. Similarly, the H-factor was decreased to 90% (90%), 77% (78%), and 63% (66%) of the original Hfactor value at 1.55 M [OH−]. These results indicate that even a small AQ addition provides a substantial delignification rate promotion at moderate hydroxyl ion concentration, whereas at elevated alkalinity, higher AQ concentrations are required to recognize a substantial additional H-factor reduction (Table 2). The effect of AQ on carbohydrate stabilization was already evident at the lowest studied charge of 0.05 g AQ/L, but obviously, the major differences occurred at the highest charge of 0.25 g AQ/L. There, at moderate alkalinity, the maximum GGM stabilization was observed (Figure 8). Any addition of AQ was found to result only in a minor enhancement in xylan retention, regardless of alkalinity, whereas cellulose retention was considerably better in the presence of AQ. As a result of the 0.25 g AQ/L anthraquinone addition, cellulose yield in κ-30 pulp was increased from 84% to 89% at 0.50 M [OH−] and from 85% to 90% at 1.55 M [OH−]. Black Liquor. It was also clear that any addition of anthraquinone increased the amount of dissolved polysaccharides and decreased the amount of hydroxy acids found in the black liquor at both studied hydroxyl ion concentrations. This suggests that AQ is also able to hinder the alkali-induced degradation reactions in the black liquor. Even though anthraquinone addition markedly stabilized the dissolved polysaccharides, at 160 °C temperature and 1.55 M [OH−] without anthraquinone, a higher total recoverable hemicellulose content in black liquor was obtained than at 0.50 M [OH−] with anthraquinone (Supporting Information). Additionally, in the presence of anthraquinone, the degradation of dissolved xylan was transferred into a lower pulp lignin content than without AQ. Owing to this, the maximum xylan concentration in black liquor was detected at 4−6% pulp lignin content, as was also the case at high alkalinity without any addition of anthraquinone (Supporting Information). Regardless of the alkalinity, the maximum molar mass of 35 kDa was detected at a pulp lignin content of 7%, after which the molar mass of the precipitated xylan decreased significantly (Supporting Information). As already mentioned, the very efficient xylan stabilizing effect of anthraquinone in black liquor was supported by the lower amount of detected acidic degradation products. The formation of 2-hydroxybutyric acid originating from xylan was significantly reduced in all experiments including anthraquinone (Table 2), and no 2-hydroxybutyric acid could be detected when anthraquinone and high alkalinity were combined. The beneficial effect of anthraquinone was also evident according to the degradation products originating from cellulose and galactoglucomannan. No 2,5-dihydroxypentanoic acid could be detected in any AQ black liquors. This suggests especially that the endwise degradation reactions via formic acid elimination from the 3-keto intermediate, followed by benzilic acid rearrangement generating the 2-hydroxybutyric and 2,5dihydroxypentanoic acids,46 are reduced owing to anthraquinone. In general, anthraquinone addition also reduced the total formation of lactic and isosaccharinic acids. However, it was difficult to derive direct conclusions about the synergistic effect of hydroxyl ion concentration and various AQ charges (Table 2). It seems that at 0.50 M [OH−], the isosaccharinic acids-tolactic acid ratio is significantly lower than at 1.55 M [OH−], suggesting that the increased alkalinity promotes the end-group fragmentation reaction through benzilic acid rearrangement to

amounts of hydroxy acids were formed at high alkalinity between 140 and 160 °C at a given degree of delignification (Supporting Information). This suggests that the degradation of polysaccharides occurs to a lower extent, supporting our hypothesis that stopping reactions are enhanced over peeling reactions at higher alkali concentration. Also, the ratio of isosaccharinic acids to lactic acid was higher, suggesting a lower extent of peeling reactions. This ratio has previously been shown to correlate with pulp yield, showing increased ratio value for higher pulp yield.45 Effect of Inactive Anions. As previously discussed, the presence of inactive Cl− anions is a direct consequence of the ionic strength adjustment (expressed as [Na+]) with NaCl. Therefore, a number of experiments at high and conventional alkali concentration was executed without the addition of NaCl to reveal the effect of ionic strength and chloride anions. With the experimental setup used, it was not possible to assess the influence of [Na+] and [Cl−] separately. Results indicate that the rather minor NaCl addition needed to adjust the [Na+] into 2.00 M for the cooking liquor containing 1.55 M [OH−] and 0.31 M [HS−] has practically no effect on the delignification rate or on carbohydrate removal. On the other hand, at 0.50 M [OH−], the decreased delignification rate was undoubtedly a consequence of NaCl addition. When no additional NaCl was added to the white liquor, the delignification proceeded more efficiently. This was demonstrated in a 200 min cook at 160 °C, 0.50 M [OH−] and 33% sulfidity where the residual lignin content decreased from 9.6% to 4.8% in the case where no NaCl was added. Despite the considerably reformed delignification kinetics, the content of GGM, AX, and cellulose at certain Klason lignin content on pulp was virtually identical, with or without NaCl addition. Effect of Anthraquinone on Carbohydrate Degradation and Dissolution. Wood Residue. The effect of AQ on carbohydrate degradation and dissolution behavior was evaluated at constant 33% sulfidity and 160 °C temperature and at 0.50 M [OH−] and 1.55 M [OH−]. The delignification rate was greatly accelerated using AQ in combination with 0.50 M [OH−]. This enabled significant H-factor reduction to reach a targeted delignification degree (Figure 7). However, not even the highest AQ concentration of 0.25 g/L reached the delignification rate as obtained at elevated base concentration without AQ addition at 160 °C. The results in Table 2 reveal that the H-factor for reaching κ60 (κ-30) was decreased to 62% (71%), 45% (54%), and 37% (43%) of the initial value by the addition of AQ in concentrations of 0.05, 0.15, and 0.25 g/L at a [OH−] of

Figure 7. Effect of anthraquinone on delignification at 160 °C temperature. 12781



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Table 2. Mass Balance Table with Wood Residue and Black Liquor Results, Together with Overall Mass Balance Calculations 0.50 M [OH−]

κ-60 (-30) AQ charge (gL) H-factor

1480 (2370)

cellulose galactoglucomannan xylan Klason lignin total polysaccharides Wood residue yield

37.0 (35.2) 5.2 (4.6) 3.4 (2.9) 4.6 (2.0) 45.6 (42.7) 50.2 (44.7)

cellulose galactoglucomannan xylan xyloisosaccharinic acid glucoisosaccharinic acid lactic acid 2-hydroxybutyric acid 2,5-dihydroxypentanoic acid ISAs/lactic acid, ratio total dissolved polysaccharides total acidsa

0.1 (0.1) 0.7 (0.9) 1.4 (1.1) 1.4 (1.9) 4.6 (4.8) 1.9 (2.4) 1.1 (1.3) 0.4 (0.5) 3.2 (2.8) 2.2 (2.1) 14.4 (16.8)

total polysaccharides in pulp and BL

47.8 (44.8)

a

1.55 M [OH−]

0.05 920 (1690)

0.15 0.25 660 (1270) 550 (1030) 300 (480) wood residue (% on wood) 37.0 (36.0) 37.7 (36.8) 38.1 (37.3) 36.4 (35.4) 6.8 (5.9) 9.5 (8.2) 10.8 (9.5) 6.4 (5.5) 3.6 (2.9) 3.7 (2.8) 3.8 (2.9) 1.9 (1.1) 4.7 (2.1) 5.1 (2.2) 5.3 (2.3) 4.5 (2.0) 47.4 (44.8) 50.9 (47.8) 52.7 (49.7) 44.7 (42.0) 52.1 (46.9) 56.0 (50.0) 58.0 (52.0) 49.2 (44.0) black liquor (% on wood) 0.1 (0.1) 0.1 (0.1) 0.1 (0.2) 0.2 (0.3) 1.0 (1.2) 1.4 (1.7) 1.6 (1.9) 2.5 (2.9) 1.9 (1.6) 2.6 (2.3) 3.0 (3.1) 3.9 (3.8) 0.6 (1.6) 0.6 (0.7) 0.9 (1.0) 0.7 (0.8) 1.0 (1.7) 2.9 (1.5) 2.3 (2.6) 4.5 (5.0) 2.6 (3.0) 1.0 (2.0) 0.6 (1.3) 1.4 (1.5) 0.2 (1.0) 0.6 (0.7) 0.3 (0.4) 0.6 (0.7) n.d. n.d. n.d. 0.2 (0.4) 0.6 (1.1) 3.5 (1.1) 3.7 (2.8) 3.7 (3.9) 3.0 (2.9) 4.1 (4.1) 4.7 (5.2) 6.6 (7.0) 10.2 (14.9) 6.3 (12.4) 5.9 (12.5) 13.0 (13.0) Total Polysaccharides in Pulp and BL (% on wood) 50.4 (47.7) 55.0 (51.9) 57.4 (54.9) 51.3 (49.0)

0.05 270 (450)

0.15 230 (390)

0.25 190 (330)

37.1 (36.3) 7.3 (6.4) 2.0 (1.3) 4.6 (2.0) 46.4 (44.0) 51.0 (46.0)

38.0 (37.3) 8.6 (7.5) 2.3 (1.4) 5.0 (2.2) 48.9 (46.2) 53.9 (48.4)

38.3 (37.7) 9.5 (8.0) 2.5 (1.5) 5.1 (2.3) 50.2 (47.2) 55.4 (49.5)

0.2 (0.2) 2.6 (3.0) 4.0 (4.1) 0.9 (1.2) 3.3 (3.5) 0.7 (1.0) n.d. n.d. 6.0 (4.7) 6.8 (7.3) 9.9 (13.2)

0.3 (0.3) 3.3 (4.4) 4.0 (4.5) 0.4 (1.3) 2.3 (2.6) 0.3 (0.7) n.d. n.d. 9.0 (5.6) 7.6 (9.2) 5.7 (7.8)

0.3 (0.3) 3.8 (4.7) 4.4 (4.7) 0.1 (0.3) 2.3 (2.3) 0.4 (0.6) n.d. n.d. 6.0 (4.3) 8.5 (9.7) 7.5 (8.2)

53.2 (51.3)

56.5 (55.4)

58.7 (56.9)

Total acids include gluco- and xyloisosaccharinic acids, lactic, formic, acetic, glycolic, 2-hydroxybutyric and 2,5-dihydroxypentanoic acids.

thus increasing the amount of dissolved polysaccharides found in spent black liquor. Extending the delignification from κ-60 to κ-30 further increased the amount of dissolved polysaccharides found in the black liquor under elevated alkalinity conditions, whereas at 0.50 M [OH−], practically no difference between the two κ-numbers was observed (Table 2). An increase in AQ charge further increases the retention of GGM in pulp; however, the effect is highly pronounced at the 0.50 M hydroxyl ion concentration. This suggests that using even higher AQ charges to obtain further GGM stabilization might be justified, especially when combined with a lower alkali concentration. Additionally, cellulose and xylan retention is also expected to improve as the AQ charge increases, even though to a lesser extent than in the case of GGM (Table 2). Total acid formation is significantly reduced in the presence of AQ; for example, 2,5-dihydroxypentanoic acid could not be detected from AQ black liquors at all (Supporting Information). Additionally, the formation of 2-hydroxybutyric acid originating from xylan was significantly reduced in all experiments including anthraquinone, but especially when combined with high alkalinity. At a conventional alkalinity of 0.50 M [OH−], only the higher 0.15 and 0.25 g AQ/L charges significantly reduced the lactic acid formation originating from fragmentation reactions via reversed aldol condensation, suggesting that these reactions are inhibited to a noticeable extent only if the anthraquinone amount is sufficient (Table 2). Overall Mass Balance. Over the entire studied κ-number range, higher alkalinity conditions during kraft cooking revealed a substantial net carbohydrate yield increase (pulp + black liquor) compared to conventional kraft cooking conditions (Figure 9). As the elevated alkalinity without AQ mainly increased galactoglucomannan and decreased xylan content in pulp, the wood residue yield was virtually equal at both studied alkali concentrations. Therefore, the net yield increase seen in Figure 9 was mainly due to the significant enhancement in the

Figure 8. Effect of anthraquinone on galactoglucomannan retention at 0.50 and 1.55 M [OH−] at 160 °C.

form isosaccharinic acids instead of promoting the reversed aldol condensation reaction pathway to form lactic acid. Under the used conditions, the obtained experimental results are well within agreement with previously published data47 concerning alkaline degradation of polysaccharides. In the presence of anthraquinone, alkaline endwise degradation reactions are predominantly prevented, as the reducing endgroup is oxidized to form the alkali-stable aldonic acid endgroup. This might also explain why glucomannan stabilization through direct oxidation in the presence of AQ is enhanced under moderate alkalinity, where “competing” high alkaliinduced stopping reactions take place to a lesser extent. Yet today, an inadequate amount of published data is available on the anthraquinone derivatives in black liquors, despite the availability of several useful analytical methods.48−50 Effect of Anthraquinone Charge. At both studied hydroxyl ion concentrations, any addition of anthraquinone improved the polysaccharide yield in pulp and additionally impeded the degradation of the alkali extracted polysaccharides 12782



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Article

ASSOCIATED CONTENT

S Supporting Information *

General reaction scheme describing the endwise degradation of softwood carbohydrates and the formation of the main hydroxy acids. Figures presenting the isosaccharinic and lactic acid formation; the total content of small molecular weight acids; xylan and glucomannan dissolution without and in the presence of AQ; molecular weight of the dissolved polysaccharides; the effect of AQ charge on glucomannan retention and total hydroxy acid formation; total identified components. Tables presenting the comprehensive experimental data from the wood residue and black liquor analyses. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 9. Total recoverable polysaccharide fraction from wood residue and black liquor in the presence of anthraquinone (0.50 and 1.55 M [OH−], without and in the presence of 0.05 and 0.25 g AQ/L at 160 °C).

AUTHOR INFORMATION

Corresponding Authors

*E-mail: markus.paananen@aalto.fi. *E-mail: herbert.sixta@aalto.fi.

dissolved hemicellulose fraction. At a temperature of 160 °C and a pulp lignin content of 9% (corresponding κ-60), the studied hydroxyl anion concentration of 1.55 M had a 3.4%unit higher net carbohydrate yield than conventional 0.50 M [OH−]. At κ-30, the advantage of elevated alkalinity was even further emphasized, as the net carbohydrate yield describing the amount of recoverable polysaccharides was 4.1% units higher compared to the amount at 0.50 M hydroxyl ion concentration. An increased amount of dissolved polymeric sugars found in the black liquor offers an improvement in overall carbohydrate yield. Enhanced utilization of the dissolved polysaccharide fraction would require recycling of the carbohydrate-enriched lye into the later pulping stages to initiate a partial reprecipitation or to collect the dissolved polysaccharides by other means, for example, membrane filtration. Additionally, owing to elevated alkalinity, a significantly accelerated delignification rate offers further possibilities for process economy optimization through reduction of the pulping Hfactor. The total amount of identified components suggests that the polysaccharide fragmentation reactions forming various acidic degradation products are rather well identified in this study (Supporting Information).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Finnish Funding Agency for Technology and Innovation (Tekes) and Finnish Bioeconomy Cluster Oy (FIBIC) are acknowledged for financial support. Stella Rovio from VTT is commended for skilled black liquor analyses.

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REFERENCES

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CONCLUSIONS Increasing kraft pulping liquor alkalinity resulted in a significant acceleration of the delignification rate, while pulp yield was practically unaffected. The observed stabilization of pulp glucomannan was largely compensated for by an additional loss of xylan, owing to enhanced dissolution. A notable improvement in the total amount of recoverable polysaccharides was obtained with higher alkalinity, owing to increased dissolution of all polysaccharides without pulp yield loss. The formation of acids originating from carbohydrate degradation reactions during kraft pulping was slightly reduced with increasing liquor alkali concentration, and it was greatly reduced in the presence of AQ. The influence of AQ was even more distinctive once it was charged into high alkalinity white liquor, thus effectively reducing the formation of isosaccharinic and lactic acids and completely preventing the formation of 2-hydrobutyric and 2,5-dihydroxypentanoic acids. Additionally, regardless of alkalinity, AQ addition increased the amount of dissolved polysaccharides found in black liquor, further increasing the amount with increasing AQ charge. 12783



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