A more extensive utilization of forest resources and the development of biorefinery processes aiming at the complete and more sophisticated utilization of wood in the course of pulping are facing several challenges. For example, not all forest biomass is available for harvesting. The cost of wood is high, amounting to 30–50% of the total production cost of pulp for paper grades. The wood cost can be expected to increase in the case of more intensive utilization. One ton of softwood contains approximately 700 kg of polysaccharides, yet the average pulp yield in kraft pulping is merely 470 kg. Combustion of carbohydrates dissolved in the black liquor is a misuse of wood bioresources. Polysaccharide yield losses in chemical pulping are due to inhomogeneous and unselective delignification of wood chips due to a chain of events, which begins with inhomogeneous impregnation of the chips. In the worst case, the chip core will contain no or an insufficient amount of alkali, and this is the main source of rejects formation (Gullichsen et al. 1992, 1995; Enqvist 2006). Other effects are nonuniform carbohydrate reactions with more hemicelluloses dissolution and extensive cellulose degradation at the chip surface compared to the chip center (Li et al. 2000; Grénman et al. 2010). Improved impregnation results in higher pulp yield (Tikka and Kovasin 1990; Gullichsen et al. 1992; MacLeod 2007; Wedin et al. 2010; Brännvall and Bäckström 2016), higher pulp viscosity (Gullichsen et al. 1992; Wedin et al. 2010, 2013), which means higher molecular mass of the obtained cellulose (Wedin et al. 2013; Tavast and Brännvall 2017). Measures for better impregnation are chip thickness control (Akhtaruzzaman and Virkola 1979; Gullichsen et al. 1995; Dang and Nguyen 2008) or prolonged impregnation (Karlström 2009; Wedin et al. 2010; Tavast and Brännvall 2017). Elevated alkali concentration also has a favorable effect on impregnation homogeneity (Gullichsen et al. 1992, 1995; Brännvall and Bäckström 2016). Impregnation of wood chips is still a matter of basic investigation (Kolavali and Theliander 2013; Kuitunen et al. 2013; Kolavali and Hasani 2017).
Impregnation of wood with alkali can be described by the shrinking core model with respect to the acetyl groups cleaved in an advancing reaction front through the chip from surface to core (Zanuttini et al. 2000, 2003) as illustrated in Figure 1. In the course of hydroxide ions diffusion into the chip and their reaction with wood constituents, a swollen outer shell is formed and an unreacted core remains. The core and shell are separated by a reaction front, where reactions between alkali and acetyl groups take place. It would be desirable that hydroxide and hydrogen sulfide ions diffuse to all parts of the chips evenly (blue line in Figure 1). The red line in Figure 1 shows the amount of acetyl groups on hemicelluloses. The alkali cleavage of acetyl groups accompanied by acetic acid formation is also desired as it increases the paths in the wood chips available for ion transport (Sjöström et al. 1965; Zanuttini et al. 1998; Inalbon et al. 2009a, 2013). This reaction occurs only in the reaction zone. Saponification of extractives and dissociation of free phenolic groups (OHphen) in lignin and carboxylic groups in pectin and hemicelluloses are reactions that can take place during impregnation and lead to alkali decrement for the subsequent delignification stage. All these reactions are initiated in the reaction zone, when [OH−] is high enough for deprotonation. Dissolution of lignin is an advantageous reaction but not the aim of impregnation. Disadvantageous reactions are carbohydrate degradation via peeling and alkaline hydrolysis and dissolving of carbohydrates, mainly that of xylan. These reactions begin early and proceed in the alkali swollen shell.
The present study is devoted to the optimization of impregnation and observation of the desired and undesired effects of impregnation with modified parameters. The impregnation efficiency will be evaluated by analyzing the concentration of alkali in the bound liquor in chips and the amount of acetic acid formed. The unavoidable and undesired reactions will be assessed by the amount of degradation products formed and the amount of dissolved wood during impregnation. The essential impregnation parameters are: 105°C and 130°C for 30 and 120 min, while the initial effective alkali (EA) concentration will be 1.3 and 1.7 M.
Materials and methods
Dried softwood chips (70% pine and 30% spruce) with a moisture content (MC) of 8%, were screened to obtain a 4–8-mm chip thickness fraction. Bark and knots were removed. Stock solution of NaOH was prepared by dissolving pastilles of puriss grade (VWR International AB, Radnor, PA, USA) in deionized water, and a stock solution of Na2S was prepared by dissolving technical grade flakes of Na2S (VWR International AB). Impregnation was performed in steel autoclaves with a volume of 2.5 dm3 with batches of 150.0 g oven dry (o.d.) chips. The chips were deaerated under vacuum for 30 min. Cooking liquor was prepared from the stock solutions of NaOH and Na2S to obtain [HS−]=0.35 M and initial EA concentration either 1.30 M or 1.70 M. The liquor was sucked into the autoclaves; the L/W ratio was 3.5 l kg−1 wood. The autoclaves were placed in a steam-heated glycol bath at either 105.0°C or 130.0°C. The heating time to required temperature was 10 min, after which the actual impregnation time started. After finishing the impregnation, the autoclaves were cooled in a water bath. The spent impregnation liquor was drained off the chips and the volume, Vdrained, and weight, mdrained, noted. The chips were centrifuged for approximately 1 min to remove liquid from the chip surface and the weight loss caused by centrifugation was noted, Δmcentr.. The total volume of free liquor, Vfree, and the volume of bound liquor, Vbound, were calculated:
To determine the EA concentration in bound liquor, [OH−]bound, deionized water was added, Vadded, to the centrifuged chips, 2.00 l in the case with an initial EA concentration of 1.3 M and 3.00 l for 1.7 M and the entrapped liquid was leached out for 48 h, after which the alkali concentration was determined, cleach. The concentration of the entrapped liquor was calculated:
Alkali consumption was calculated:
Residual alkali was determined according to SCAN-N 33:94 and hydrogen sulfide ion concentration according to SCAN N 31:94 in duplicate. The carbohydrate composition was determined at RISE Bioeconomy (Stockholm, Sweden) according to SCAN-CM 71 by acid hydrolysis followed by ion chromatography; the measurement uncertainty was ±2 mg g−1 for monosaccharides at the level of 1–10 mg g−1 and ±20% for monosaccharides at a level >10 mg g−1. The xylan content was calculated as arabinose+xylose, the glucomannan content as galactose +(1+1/3.5)× mannose and the cellulose content as glucose-(1/3.5)× mannose.
Glycolic, lactic, acetic, formic and oxalic acids are formed in reactions between alkali and wood components. Quantification is difficult as some of the acids coelute during chromatography. Ion chromatography (Käkölä et al. 2008) and high performance liquid chromatography-mass spectrometry (HPLC-MS) (Käkölä et al. 2007) were used for analysis of low molecular acids. In the present study, an improved ion chromatography was applied. A solution was prepared from certified standards of five different acids (glycolic, lactic, acetic, formic and oxalic acid (Accustandard, New Haven, USA)) to a concentration of 1 ppm of each acid. A Dionex IonPac AS15 analytical column was applied in combination with an AG15 guard column and a Dionex IonPac AS11-HC in combination with an AS11-HC guard column. The eluent concentration was 1 mM KOH (flow rate 1.2 ml min−1). (KOH, ion chromatograph and column AS11 were obtained from Thermo Fisher Scientific, MA, USA).
The hydroxy acids formed in the peeling reaction were quantified by analytical pyrolysis coupled with mass spectrometry (Py-GC/MS). The technique “simultaneous pyrolysis and methylation” was applied with tetramethylammonium hydroxide (TMAH), (Merck, Darmstadt, Germany) methylation agent. The following hydroxy acids were successfully identified:
α-glucoisosaccharinic acid (3-methoxy-2-methoxymethyl-2,4,5-tri-O-methylribonic acid methyl ester)
β-glucoisosaccharinic acid (3-methoxy-2-methoxymethyl-2,4,5-tri-O-methylribonic acid methyl ester)
xylo-isosaccharinic acid (3-deoxy-2-C-hydroxymethyl-tetronic acid)
dihydroxy acid (2,5-dihydoxypentanoic acid)
In Figure 2, a pyrogram from pyrolysis of a softwood black liquor (BL) is presented. Two internal standards (IS), hexanoic acid and heptanoic acids (Thermofisher Gmbh (Alfa Aesar) Karlsruhe, Germany), were added to the liquor for quantification of the hydroxy acids. These standards are not present in BL and do not overlap with other components, but they have similarities in chemical structure with the hydroxy acids of interests. The response factor was set to 1 for all compounds. A calibration curve was created by diluting BL to obtain various concentrations of β-isosaccharinic acid. As the conditions during pyrolysis are highly alkaline, degradation by the same mechanism as in peeling can occur (Fabbri and Helleur 1999). However, it has been shown that the amount of polymeric carbohydrates dissolved is small and very constant with increasing time and temperature while the amount of hydroxy acids increases (Nieminen et al. 2014; Lehto and Alén 2015). Thus, the results from Py-GC/MS analysis are safe concerning the impregnation results. Table 1 shows the quantitative results (including the SD and CV values) for repeated measurements of the hydroxy acids (2,5-dihydoxypentanoic acid and α- and β-isosaccharinic acid). The BL samples were kept frozen prior to analysis.
Table 2 shows the essential data concerning the yield of wood and concentration of alkali and hydrogen sulfide ions in free liquor and alkali in bound liquor after impregnation of softwood as a function of the impregnation parameters. Expectedly, longer time and higher temperature increase the wood dissolution. At 105°C/30 min, the yield was approximately 84.5%, decreasing to 78–78.5% at 130°C/30 min or for 105°C/120 min. More alkali was consumed during impregnation at higher temperature, 130 kg t−1 wood at 130°C compared to 112 kg t−1 at 105°C for 30 min and 120 kg t−1 for 2 h. In Table 3, the concentration of low molecular acids and in Table 4 the concentration of hydroxy acids in the spent liquor after impregnation are presented. Glycolic, lactic, formic and hydroxy acids are formed in the peeling reaction and will be referred to as degradation acids (Adegr). Also here, longer time and higher temperature resulted in higher amounts of Adegr. The instability of glucomannan in alkali is seen as a large decrease in mannose content after impregnation compared to originally in wood (Table 5).
In the course of cooking, chemicals migrate into the chips, cleaving acetyl groups along the impregnation path and forming acetic acid (AA). The deacetylation index, i.e. the proportion of acetyl groups cleaved, has been proposed as a measure to determine the extent of alkaline impregnation (Costanza and Zanuttini 2004), thus the AA amount indicates the impregnation progress. As seen in Figure 3, prolonging impregnation time resulted in a slightly higher concentration of AA in the free liquor as well as a higher average EA concentration in the bound liquor (comparison of triangles and circles). Because higher temperature elevates the diffusion rate of cooking chemicals, an improved degree of impregnation would be expected in the case of 130°C instead of 105°C. However, according to Figure 3, the AA in the free liquor was very similar or even slightly lower at 130°C (squares) compared to 105°C (triangles). Moreover, higher temperature also increases the diffusion rate of dissolved components from the bound liquor to the free liquor as demonstrated by Pakkanen and Alén (2013). The concentration of AA in the free liquor at 130°C in the present study probably reflects well the concentration in the bound liquor, whereas at 105°C the concentration of AA would be higher in the bound liquor than in the free liquor. This implies that impregnation progressed further at 105°C compared to at 130°C, which is confirmed by the higher [OH−]bound obtained at the lower temperature. Impregnating chips with alkali is a balance between diffusion and reaction, while higher temperature promotes the latter more. The detrimental effect of elevated temperature on the impregnation progress was investigated by Egas et al. (2002), who obtained a higher alkali concentration in the bound liquor at 80°C compared to 165°C. At 165°C, the quoted authors noted a rapid decrease in [OH−] in the free liquor but not an analogous increase in [OH−] in the bound liquor as alkali was rapidly consumed in reactions with wood. An enlightening example of the effect of temperature on the balance between diffusion and reaction rate was presented by Määttänen and Tikka (2012): the impregnation of chips with alkali was faster at 90°C than at both 60 and 120°C. At 60°C, the diffusion rate is lower than at 90°C, whereas at 120°C the rate of alkali consuming reactions is higher. On the other hand, Montagna et al. (2016) found that impregnation was faster at 130°C than at 110°C. After a given time, the alkali penetrated deeper into the chip at 130°C compared to 110°C and at a given distance from the chip surface the [OH−] was higher at 130°C than at 110°C. However, Montagna et al. (2016) worked with a very high L/W ratio (1000:1), which allows the [OH−] of the free liquor to be constant, whereas in the present study consumption of alkali leads to a decreased [OH−]free, and thereby to a lower diffusion rate of OH− into the chip. In the present study, an increase of [OH−]initial from 1.3 M (unfilled symbols) to 1.7 M (filled symbols) led to a significantly higher [OH−]bound (Figure 3), in accordance with earlier studies (Inalbon et al. 2009b; Määttänen and Tikka 2012; Montagna et al. 2016; Brännvall and Bäckström 2016). High initial EA makes it possible to rapidly transport sufficient amounts of alkali to the reaction sites within the fiber wall, enabling an improved impregnation without decreasing the overall pulp production rate.
There is no general correlation between the amount of AA in the free liquor and the average EA concentration in the bound liquor (Figure 3).
Figure 4 shows the ratio between the alkali concentration in the fiber wall and in the bulk liquor as a function of [OH−]initial. The EA concentration in the free liquor can be very different from the concentration of the bound liquor. It is thus not advisable to determine the alkali availability for reactions from the analysis of the free liquor, as the alkali at the reaction site can be much lower as demonstrated by Egas et al. (2002). Increasing temperature during impregnation accelerates diffusion rate but as reaction rates also increase, no improvement on the ratio [OH−]bound/[OH−]free was obtained. Prolonged impregnation resulted in a more even alkali profile between free and bound liquor as did a higher [OH−]initial. This is in accordance with previous studies, which have claimed that alkali concentration in bound liquor is strongly influenced by the initial alkali concentration and to lesser extent by time (Costa et al. 2008) and temperature (Enqvist 2006; Costa et al. 2008).
At 130°C, the rate of the peeling reaction is increased, which can be quantified by the amount of degradation products formed. In Figure 5a, the amount of Adegr comprising low molecular acids (formic, glycolic and lactic acid) and hydroxy acids (α- and β-glucoisosaccharinic acid and 2,5-dihydrohypentanoic acid) in the free liquor are presented. At higher temperature (squares), more carbohydrate degradation takes place, manifested by the significantly higher amount of Adegr. According to Pakkanen and Alén (2013), the concentration of hydroxy acids is much higher in the bound liquor after a given time at 100°C than at 140°C. This can be an explanation for the lower [OH−]bound at 130°C compared to 105°C in Figure 3. Prolonged impregnation time also resulted in more carbohydrate degradation. According to Figure 5a, impregnating for 0.5 h at 130°C was as detrimental to yield as 2 h at 105°C. Performing the impregnation with an elevated alkali concentration did not affect the amount of Adegr. This can be attributed to an increase of the rate of the stopping reaction at elevated alkali concentrations, whereas the rate of the peeling reaction is not affected by alkali concentration increment, when [OH−]>0.5 M (Paananen et al. 2010, 2013).
From Figure 5b, it is obvious that a higher temperature resulted in a significantly higher consumption of alkali, in accordance with Määttänen and Tikka (2012). Also prolonged impregnation resulted in a higher consumption of alkali. The initial alkali concentration does not have a significant impact on the alkali consumption.
In Figure 6a, the yield after impregnation is illustrated. As can be seen, increased charge of EA did not affect the amount of dissolved wood in accordance with Brännvall and Bäckström (2016). However, earlier studies demonstrated that an increased alkali concentration results in a decreased yield (Al-Dajani and Tschirner 2008; Lehto and Alén 2015). The contradictory results can be explained by the extent of impregnation obtained in the earlier studies. Lehto and Alén (2015) performed the alkaline treatment at very low initial hydroxide ion concentration (0.05–0.4 M), so complete impregnation was likely not achieved and the increase in dissolved material at higher alkali charge was due to the progress of alkali into the chips. Al-Dajani and Tschirner (2008) performed the alkaline extraction at similar concentrations, 1.0–2.1 M, as applied in the present study, but temperature was lower (32–90°C).
Both prolonged time and higher temperature decreased the yield. Yield did not correlate to alkali consumption, but Figure 6b indicates that yield correlates with the amount of degradation products formed during impregnation. Although only three types of hydroxy acids have been analyzed, these represent around 85% of the total amount of hydroxy acids usually found in BLs (Alfredsson and Samuelson 1968; Malinen and Sjöström 1975; Löwendahl et al. 1976).
The effect of increased rate of the stopping reaction at elevated alkali concentration is not conclusive (Table 6). At 105°C/30 min the same amount of glucomannan was lost independently of [OH−]initial with 1.3 M or 1.7 M. However, after prolonging the impregnation time the glucomannan yield was higher at [OH−]initial=1.7 M. At 130°C, elevated [OH−]intial also resulted in increased glucomannan yield. It has been shown that increased temperature affects the rate of the stopping more than the rate of peeling (Haas et al. 1967; Paananen et al. 2010). High [OH−]intial resulted in more dissolved xylan promoted by increased temperature. Cellulose yield was only affected by prolonged time.
Bulk delignification reactions start at 130–140°C (Chiang et al. 1987). Increasing temperature from 105°C to 130°C facilitated lignin dissolution (Table 6) in accordance with Pakkanen and Alén (2013). Increased EA did not significantly affect the amount of dissolved lignin, as seen in previous studies (Pakkanen and Alén 2013; Brännvall and Bäckström 2016). As noted in Figure 5b, the EA consumption was higher at 130°C compared to 105°C. The higher consumption could not be explained entirely by more peeling, as the amount of Adegr was at the same level as for the prolonged impregnation, which had lower EA consumption. Neither could it be explained by the amount of AA formed as this was similar or lower. It can thus be concluded that a substantial amount of EA was consumed in delignification reactions. As pointed out in the introduction, delignification is not the purpose of impregnation, but this is, in principle, not disadvantageous. However, the results of the present study show that delignification during impregnation has a negative effect on the amount of EA transported into chips.
The selectivity of the impregnation could be evaluated as the ratio between AA and Adegr. The higher the amount of AA, the more thorough the impregnation and the lower the amount of Adegr, thus the less is the amount of carbohydrates degraded. This is the reason why a high ratio AA/Adegr and sufficiently high alkali concentration in the fiber wall is indicative for more favorable impregnation conditions.
A good impregnation of chips is achieved under conditions that improve the diffusion of alkali and complete the deacetylation, while avoids the undesirable reactions (mainly peeling) as much as possible. According to Figure 7, the most favorable conditions would be impregnation at low temperature, short time and high initial EA concentration. This is in accordance with Inalbon et al. (2009a,b), who found that increasing alkali concentration is more favorable than increasing temperature, when the alkali diffusion into wood should be improved. Impregnation at elevated EA concentration and low temperature is utilized in the EnerBatch concept in which chips are impregnated with white liquor at temperatures below 100°C (Wizani et al. 1992; Uusitalo and Svedman 2000). It is claimed that higher yield and lower reject contents are achieved by the concept. The effect of elevated temperature during impregnation is seen in the RDH batch process. In this concept, chips are impregnated with black BL at 125–130°C (Uusitalo and Svedman 2000). Alkali consuming reactions are fast at the elevated temperature and because the initial EA concentration is low, as BL is used, alkali depletion can occur (Pu et al. 1991).
The aim of the study was to establish the optimal conditions of softwood chips in the course of kraft pulping. Impregnation at high initial EA concentration results in a high EA concentration in the bound liquor within chips, ensuring a sufficiently high alkali level at the onset of cooking. Pulp yield was not affected by an elevated initial EA concentration, but both increased temperature and longer time lead to more degradation of carbohydrates during impregnation. Impregnation at 130°C resulted in the highest extent of peeling, measured as the amount of degradation acids formed. If high temperature was combined with low initial EA, the EA concentration in the bound liquor is very low, which is due to an increased alkali consumption caused not only by more peeling but also by delignification reactions. No universal correlation exists between the amount of AA deliberated and the EA concentration within the chip liquor, thus the amount of AA in the free liquor is useless as a measure of the impregnation progress. Analysis of alkali in the free liquor can be misleading, as it is a measure of alkali available within the chips after impregnation. At low initial alkali concentration and short time, the EA concentration in the bound liquor can be less than half of the free liquor.
The Södra Foundation for Research is gratefully acknowledged for funding the study. We thank Dr Anna Jacobs and Marie Bäckström for valuable comments on the manuscript and Gonzalo Soler for skilful experimental work.
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About the article
Published Online: 2017-11-15
Published in Print: 2018-02-23
Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.
Research funding: The Södra Foundation for Research.
Employment or leadership: None declared.
Honorarium: None declared.
Citation Information: Holzforschung, Volume 72, Issue 3, Pages 169–178, ISSN (Online) 1437-434X, ISSN (Print) 0018-3830, DOI: https://doi.org/10.1515/hf-2017-0078.
©2018 Elisabet Brännvall, published by De Gruyter, Berlin/Boston. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License. BY-NC-ND 4.0