Bioresource Technology 193 (2015) 97–102
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
High-alkali low-temperature polysulfide pulping (HALT) of Scots pine Markus Paananen ⇑, Herbert Sixta Aalto University, School of Chemical Technology, Department of Forest Products Technology, Vuorimiehentie 1, FI-00076, Finland
h i g h l i g h t s Significantly improved galactoglucomannan and cellulose preservation. Pulp yield increase maintained at bleachable grade pulp. Purification of black liquor and recovery of residual alkali with ultrafiltration.
a r t i c l e
i n f o
Article history: Received 18 May 2015 Received in revised form 15 June 2015 Accepted 16 June 2015 Available online 23 June 2015 Keywords: Alkali recovery High-alkali pulping Polysulfide Softwood Ultrafiltration
a b s t r a c t High-alkali low-temperature polysulfide pulping (HALT) was effectively utilised to prevent major polysaccharide losses while maintaining the delignification rate. A yield increase of 6.7 wt% on wood was observed for a HALT pulp compared to a conventionally produced kappa number 60 pulp with comparable viscosity. Approximately 70% of the yield increase was attributed to improved galactoglucomannan preservation and 30% to cellulose. A two-stage oxygen delignification sequence with inter-stage peroxymonosulphuric acid treatment was used to ensure delignification to a bleachable grade. In a comparison to conventional pulp, HALT pulp effectively maintained its yield advantage. Diafiltration trials indicate that purified black liquor can be directly recycled, as large lignin fractions and basically all dissolved polysaccharides were separated from the alkali-rich BL. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Pulp production is no longer merely considered a process for papermaking fibres and energy, but as a fully operational biorefinery to produce numerous products of added value. Despite efficient fractionation techniques have been successfully demonstrated and utilised with various pulping concepts and types of biomass (FitzPatrick et al., 2010), implementation of fractionation techniques as a part of the kraft process has been slow owing to its highly established chemical recovery and more than self-sustained energy economy. Softwood kraft pulping will maintain its status as one of the most important pulping processes due to its superior wood-based pulp fibres, however, the evolution of kraft pulping has been rather stagnant since the adaptation of the modified continuous cooking (MCC) process in the late 70 s and early 80 s. Since raw material is the major contributor in the overall softwood kraft pulping production costs, improvements in the utilisation of raw material in terms of increased pulp yield and the recovery of valuable side product streams are of importance. ⇑ Corresponding author. E-mail address: markus.paananen@aalto.fi (M. Paananen). http://dx.doi.org/10.1016/j.biortech.2015.06.075 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
At kraft conditions, the losses of softwood polysaccharides via end-wise degradation (primary peeling) at alkaline conditions is already initiated in the heating-up phase. Even though all polysaccharides are subjected to peeling, most losses are caused by galactoglucomannan (GGM) having no stabilizing side groups or ordered structure like arabinoxylan (AX) or cellulose, respectively. In addition, all polysaccharides suffer from losses initiated by chain scission (alkaline hydrolysis) and are materialised in the subsequent secondary peeling reactions and dissolution of short-chain polysaccharides. Peeling reactions can be terminated by the formation of metasaccharinic acid end-groups (stopping reaction, Young and Liss (1978)). Termination reactions are favored by elevated hydroxide ion concentration ([OH ]); thus, provided that the alkalinity is maintained sufficiently high during the pulping process, GGM is stabilized to a certain extent (Paananen et al., 2010). Parallel to the GGM stabilization, a significant amount of acidic degradation products are formed. A large amount of alkali is consumed in these side reactions, further decreasing the rate of stopping reactions. In a pulping process based on the principles of MCC and especially the concept of levelled alkalinity (Johansson et al., 1984), the pulping liquor hydroxide ion [OH ] concentration is, however, not adequately high for optimal GGM stabilization (Paananen et al.,
98
M. Paananen, H. Sixta / Bioresource Technology 193 (2015) 97–102
2010, 2013). On the other hand, alkali overload might result in accelerated cellulose depolymerisation through alkaline hydrolysis especially at higher temperatures, decreasing the fibre strength properties. The principles of high-alkali polysulfide (PS) liquor pretreatment have been successfully demonstrated to yield a pulp with higher viscosity and effective polysaccharide stabilization at high and conventional liquor-to-wood ratios (Gustafsson et al., 2004a,b). Also, no detriment is expected in subsequent fibre processing, as another study reported that pulps pretreated with high-alkali PS liquor had bleachability and strength properties comparable to those of the conventional kraft process (Brännvall et al., 2003). The decomposition of PS is accompanied by a consumption of hydroxide ions, and promoted by increasing temperature. PS ions are shown to exist at conventional cooking temperatures above 160 °C; however, due to accelerated decomposition rates, the concentration is much lower (Lindgren and Lindström, 1995). Lindgren and Lindström also concluded that liquor [OH ] had no influence on the decomposition rate of PS; conversely, the oxidation of the polysaccharide reducing end-groups (stabilization) is favored by increasing [OH ] (Teder, 1969). One of the key challenges in implementing a high-alkali-profile pulping process is the management of unused alkali. Conventionally, the kraft process does not use excess alkali, and the residual black liquor (BL) alkalinity is well below 10 g NaOH/L, which is recoverable via incineration. In a high-alkali pulping, diverting BL directly into the recovery boiler is practically impossible due to the high residual alkalinity. Additional BL processing is, therefore, required. Little attention has been paid to the recovery of residual alkalinity as a part of effective lignin and carbohydrate separation process. For feasible process economy, residual alkali should be recovered from the BL at the process temperature without cooling. This requirement sets additional demands on the separation process. Certain polymeric membranes can withstand elevated alkalinity, however, are limited to lower operation temperatures (Schlesinger et al., 2006). On the other hand, ceramic membranes have been successfully used even at 145 °C to separate the higher-molecular-weight dissolved lignin and carbohydrates of mill softwood BL (Wallberg and Jönsson, 2006). Previously, a high-yield pulp was produced by utilising a high-alkali PS pulping liquor and low temperature in a diluted cooking system with wood meal (Paananen et al., 2015). This paper demonstrates the same cooking concept with chips and low liquor-to-wood ratio. Followed by a reinforced two-stage oxygen delignification, the composition and properties of a high-yield pulp are compared to a pulp produced via conventional kraft pulping methods. Further, results on the recycling of unused sodium hydroxide and the concomitant separation of dissolved lignin and carbohydrate fractions by means of ultrafiltration and diafiltration are presented.
2. Methods
Paananen et al. (2015). The polysulfide charge was set to 6 g S(0)/L (3.6% odw, on dry wood). The concentration was considered comparable to industrial practice when polysulfide liquor is generated via white liquor oxidation. Polysulfide liquor preparation and experiments were conducted in a nitrogen atmosphere. High-alkali low-temperature polysulfide (HALT) pulping experiments were studied at effective alkali (EA) amounts from 50% to 80% odw (2.08–3.33 M [OH ]) and at 130 °C. In addition, several experiments were carried out at 140–160 °C with an EA charge of 60% odw (focus was on 130 °C experiments, experiments at higher temperatures are specifically referred to). All HALT experiments used a 40% initial sulfidity (before sulphur addition). Kraft reference conditions (REF) were 160 °C, L/W 6, 22% EA charge (0.92 M [OH ]), and 40% sulfidity. Heating and cooling were obtained within 20 min ± 1 min, regardless of the target temperature. Experimental work was conducted in a reactor with liquor circulation to ensure similar liquor temperature and properties inside the chip bed. Due to the reactor geometry and chip bed packing, no lower liquor-to-wood ratio than 6 could be utilised while keeping all chips under the liquor level during the experiment. 2.2. Pulp and black liquor characterisation Pulp yield and the content of acetone extractives (SCAN 49:03) were determined gravimetrically. Pulp was decomposed in a two-stage acid hydrolysis stage process according to SCAN-CM 71:09. Acid hydrolysate was filtered with a glass filter to determine the amount of acid insoluble lignin (Klason lignin, KL). Monosaccharide composition was determined from the filtrate by HPAEC-PAD according to SCAN-CM 71:09, and acid-soluble lignin by UV spectroscopy (Shimadzu UV-2550; 205 nm and an absorption coefficient of 110 L * (g cm) 1 according to Swan (1965)). Based on the pulp Klason lignin content, the pulp kappa number (KN) was calculated according to Tasman (1959) (lignin on pulp (%) = KN * 0.15). The polysaccharide composition was calculated from the relative amount of monosaccharides according to Janson (1970). The dissolution of polysaccharides was evaluated according to the monosaccharide composition of BLs after acid hydrolysis according to SCAN-CM 71:09. Owing to the relatively high kappa numbers of all pulps, further lignin removal was necessary prior to viscosity measurement according to SCAN-CM 15:99. Accordingly, 5 min (70 °C) chlorite delignification pretreatment was used to remove excess lignin: 3 g pulp (dry basis), 200 mL water, 20 mL chlorine dioxide water, 2 mL of 100% acetic acid. 2.3. Ultrafiltration Amicon 8200 (200 mL cell) was used in lab-scale filtration experiments (Supplementary material). The operating temperature was limited to room temperature and the pressure to 5 bar (regulated with nitrogen gas). Black liquor feed volume was 180 mL, while the magnetic stirrer was set to 300 rpm. Polyethersulfone ultrafiltration membranes were used, with 5 (UP005) and 10 kDa (UP010) cut-offs (Microdyn-Nadir). Diafiltration was simulated by dilution of the primary retentate with water, followed by a subsequent ultrafiltration.
2.1. Raw material and experimental set-up 2.4. Chelation, oxygen delignification, and inter-stage treatment Scots pine (Pinus sylvestris L.) from southern Finland was chipped and screened according to SCAN-CM 40:01. Chips were frozen until further use. The wood raw material comprised of cellulose (41.7% odw), galactoglucomannan (16.9%), arabinoxylan (8.2%), other carbohydrates (1.4%), acid-insoluble lignin (Klason lignin, 25.9%), acid-soluble lignin (0.7%) and extractives (3.2%) (Paananen et al., 2015). Synthetic orange liquor was prepared from analytical grade NaOH, Na2S, and elemental sulphur, according to
Prior to oxygen delignification, pulp chelation with ethylenediaminetetraacetic acid (EDTA) was performed at 0.2% odw charge at 3% pulp consistency, at pH 5.5 and 50 °C for 30 min. Oxygen delignification (OD) experiments were carried out in a rotating air-bath digester. All experiments were conducted with 30 g odw pulp, 10% consistency, 0.05% MgSO4 (as Mg2+), and 7 bar oxygen pressure in a 2.5 dm3 vessel. As a reference, one-stage oxygen delignification
M. Paananen, H. Sixta / Bioresource Technology 193 (2015) 97–102
99
was carried out with REF pulp (45 min at 85 °C, 3% NaOH). HALT concept pulp was oxygen delignified in a two-stage oxygen delignification process (1st O: 45 min at 85 °C, 4% NaOH; 2nd O: 60 min at 95 °C, 2% NaOH). Inter-stage peroxymonosulphuric acid (Px) treatment was conducted at a 2% active oxygen amount, 10% pulp consistency, and pH of 11 for 10 min at 50 °C. The source of Px was OxoneÒ (Sigma–Aldrich, Switzerland, CAS:70693-62-8, potassium hydrogen peroxymonosulphate sulphate). The active oxygen content in OxoneÒ was determined according to a method by DuPont (DuPont Oxone technical attributes). 3. Results and discussion 3.1. HALT pulping 3.1.1. General considerations At a given liquor-to-wood ratio (L/W), the amount of added PS strongly determines the obtainable pulp yield increase (Sanyer and Laundrie, 1964). However, as the results from the present study are mainly convergent with the previous study at highly diluted pulping systems (identical raw material and g S(0)/L addition, L/W 200) (Paananen et al., 2015), the effect of PS is not merely determined by the PS load per se but by the concentration in the pulping liquor. The concentration of PS generated from elemental sulphur was adjusted to 6 g S(0)/L, thus comparable to that prepared via white liquor oxidation (mill practice). 3.1.2. Pulping at 130 °C Alkali charges ranging from 50% to 80% EA odw were studied at 130 °C. According to the HALT concept presented in this paper, maintaining elevated alkali concentrations is critical in order to maintain the delignification rate. Nevertheless, rather rapid decreases in alkali concentration are expected during the initial part of the cooking, where the main alkali-consuming reactions occur. As shown in Fig. 1a, the initial EA charge strongly affects the delignification rate. Owing to the increase in initial EA charge from EA 50% to 60%, a prominent acceleration in the delignification rate was obtained, and the cooking time to a certain delignification degree was shortened by approximately 20%. The delignification rate was further promoted at 70% and 80% EA charges, but the effect was less pronounced. In respect to the yield of galactoglucomannan (GGM), the main hemicellulose of Scots pine, the main advantage in high-alkali PS pulping is the less extensive end-wise degradation (Paananen et al., 2015). Already without PS, GGM stabilization reactions are promoted by elevated alkalinity (Young and Liss, 1978), though they require temperatures and hydroxide ion concentrations higher than 100 °C and 1.0 M, respectively (Paananen et al., 2010). In the present study, higher than 50% EA charge was needed to level off pulp GGM content and prevent further losses (Fig. 2a), indicating approximately a 50% GGM yield. The residual BL alkalinity with the initial 50% EA charge was around 0.8 M [OH ], and in the case of the 60% initial EA charge, approximately 1.1 M [OH ]. Therefore, it can be concluded that the findings in this study are well in line with the previous observations from diluted pulping systems. Whereas the GGM yield at elevated alkalinity was not significantly influenced by the temperature in a range of 130–160 °C, AX retention in particular was greatly affected by both alkalinity and temperature (Paananen et al., 2013, 2015). Rather surprisingly, the low temperature used in HALT concept pulping practically compensated for the effect of elevated alkalinity, so AX retention in HALT was almost comparable to that in the REF pulp (Fig. 2b). Although the differences in pulp AX content in respect to used alkali charges are rather insignificant in a kappa number
Fig. 1. (a) Kappa number as a function of cooking time, as well as (b) pulp yield, and (c) pulp viscosity as a function of kappa number.
range of 70–40 (Fig. 2b), the highest AX yield was obtained at 60% EA charge and not at 50% EA charge, as expected. In a highly diluted pulping system, it was demonstrated that, without any clear indication of redeposition to the fibre wall, during extended pulping, the decrease in the dissolved AX fraction was coupled with a parallel increase in the detected BL hydroxy acid amount (Paananen et al., 2013). Conversely, at industrial liquor-to-wood ratios around 4–6, xylan in particular is shown to redeposit on fibre surfaces (Li Jansson and Brännvall, 2014) and also locate on the outer fibre wall after pulping (Vaaler, 2008). Therefore, it seems that certain AX adsorption occurred during pulping at 60% EA charge, but not until the point of fibre liberation and dissolution of the middle lamella lignin at around K-60. Consisting mainly of AX, the amount of total dissolved polysaccharides was roughly 3% odw in the HALT concept and 2% odw in REF BLs (Supplementary material). As shown in Fig. 2c, cellulose was found to be rather stable in HALT conditions. Regardless of the used EA charge, the cellulose yield was approximately 92% in a kappa number range of 70–45. In a comparison to REF pulps, the yield advantage of HALT pulps is emphasised at kappa numbers of 60–50, roughly indicating the point of optimum selectivity toward lignin. At a given kappa number of 55, the best HALT concept pulp (EA 60%) had 6.7 wt% on wood higher pulp yield compared to REF pulp (Fig. 1b). Even though cellulose retention was also greatly improved in HALT conditions compared to REF, a major part of the overall pulp yield increase was attributed to improved GGM preservation. While
100
M. Paananen, H. Sixta / Bioresource Technology 193 (2015) 97–102
3.1.3. The effect of pulping temperature (140–160 °C) The effect of pulping temperature was studied only at an 60% EA charge which displayed the most selective high-kappa pulping at 130 °C, however, it should be noted that the given EA charge might not be optimal in combination with higher temperature. Owing to the temperature increase from 130 to 140 °C, GGM and AX suffered from approximately 1% odw additional loss at a given kappa number of 55. Further, during extended pulping to K-25, additional losses for both GGM and AX were approximately 2.5% odw at 160 °C compared to K-55 at 130 °C. However, considering the overall AX retention (in pulp and dissolved in BL), it is clear that pulp AX losses at higher temperature are a direct consequence of enhanced dissolution, most likely enabled by even a single chain scission. In case of GGM, no significant increase in the amount of dissolved GGM was observed. Also, cellulose losses became more distinct as the delignification proceeded. However, different than AX, losses are not caused directly by chain depolymerisation and subsequent dissolution, but rather by the chain scission followed by progressive end-wise degradation. Despite the rather significant cellulose losses during extended pulping to K-25, pulp viscosity was maintained at the level comparable to REF at 140 and 150 °C (Fig. 1c). Nevertheless, it should be emphasised that the data at 150 and 160 °C come from isolated experiments, so strong conclusions should be avoided. 3.2. Black liquor fractionation
Fig. 2. (a) Galactoglucomannan (GGM) (b) arabinoxylan, and (c) cellulose as a function of kappa number.
Table 1 Pulp composition of acetone extracted high-kappa pulps. Cooked according to conventional and HALT concepts. The polysulfide charge in HALT was set to 6 g S(0)/L (3.6% odw, on dry wood). Temperature (°C) Effective alkali charge (% odw) Kappa number H-factor Viscosity Pulp yield, (% odw) Cellulose, (% odw) GGM, (% odw) AX, (% odw)
160 22 61 1540 1225 50.7 36.4 4.8 4.5
130 50 68 211 1290 57.0 38.3 8.1 4.4
130 60 65 168 1200 57.1 38.1 8.7 4.5
130 60 55 190 1250 56.6 38.6 8.8 4.2
130 70 64 152 1165 55.9 37.5 8.5 4.1
130 80 58 139 1065 55.9 38.1 8.7 3.9
the EA charge had a rather minor effect on the pulp yield, the effect on pulp viscosity was more pronounced, indicating additional cellulose depolymerisation at higher alkali charges (Fig. 1c). Even though pulp viscosity is often used to define the cellulose degree of polymerisation, a direct analogy should be avoided. According to the previously reported correlation between measured pulp viscosity and cellulose degree of polymerisation, pulp hemicellulose fractions should be noted as well (da Silva Perez and van Heiningen, 2002). The pulp composition at various pulping modifications around K-60 is presented in Table 1.
Emphasis of the black liquor fractionation was to produce purified alkali rich liquor fraction by removing dissolved polysaccharides and lignin. Depending on the initial alkali load, HALT concept pulping results in BL with a large amount of unused alkali, ranging from 0.8 to 1.5 M [OH ] (Supplementary material), corresponding to initial hydroxide ion concentrations of 2.08 (50% EA) to 3.33 M (80% EA), respectively. Although fewer hydroxy acids are formed in HALT conditions (Paananen et al., 2015) and thus less alkali is required to neutralize these acids, the total alkali consumption was found to be higher compared to REF ([OH ]: residual 0.1 M; initial 0.92 M). This can be partly explained by the PS decomposition reactions and the end-group stabilization reactions, both related to alkali consumption. In addition, higher initial alkali charges lead to increased alkali consumption in HALT conditions. This might indicate more extensive fragmentation reactions of dissolved wood components. In a simple proof of concept lab-scale ultrafiltration set-up, maximum obtainable volume reduction (VR) was mainly determined by the cut-off. While using a membrane with 5 kDa cut-off, the transmembrane flux at a given operating pressure of 5 bar was too low to reach over 70% VR within reasonable time frame. Even moderate increase in the ambient filtration temperature would have positively affected the transmembrane flux (Keyoumu et al., 2004), but this was not possible with the given set-up. Therefore, in diafiltration experiments (BL sample withdrawn from the experiment at 60% EA and 130 °C), membrane with 10 kDa cut-off was used. Owing to the low-molar-mass, cooking chemicals are not fractionated in a membrane separation process, regardless of the cut-off (Wallberg and Jönsson, 2003). Therefore, diafiltration was used to retain dissolved polysaccharides and lignin while decreasing the BL alkalinity to a level tolerable by the recovery boiler. Accordingly, four liquor fractions were prepared, namely primary and secondary retentate (R1 and R2), as well as primary and secondary permeate (P1 and P2) (Supplementary material). With a 10 kDa membrane and VR of 70%, R1 contained 68% and 91% of the initial lignin and carbohydrates, respectively (Table 2). According to the discussion, practically no difference between P1 and R1 was observed in respect to alkali concentration.
101
M. Paananen, H. Sixta / Bioresource Technology 193 (2015) 97–102 Table 2 Lignin, carbohydrate, and residual alkali mass balance during diafiltration with a 10 kDa membrane. Black liquor extracted from HALT cooking with 60% EA at 130 °C. Vol-frac
Initial black liquor Primary retentate (R1) Primary permeate (P1) Diafiltration Secondary retentate (R2) Secondary permeate (P2) Total identification* (%) *
Lignin
1.00 0.24 0.70 1.80 0.36 1.44
Carbohydrates
Residual EA
g/L
% of all
g/L
% of all
g/L
% of all
39.8 112.3 19.7
100.0 67.8 34.5
5.6 21.1 0.4
100.0 90.6 5.1
52.1 53.1 55.7
100 24.5 74.5
50.4 6.7
45.6 24.2 104
12.5 0.2
80.6 6.3 92
7.7 7.0
5.3 19.4 99
Total identification from P1, R2 and P2.
Table 3 Pulp characterisation before and after oxygen delignification sequence. HALT pulp cooked at 60% EA and 130 °C.
REF pulp REF-O HALT pulp HALT-O HALT-OPx HALT-OPxO
Kappa number
Viscosity
Yield (% odw)
CELL (% odw)
GGM (% odw)
AX (% odw)
26.2 13.5 54.7 37.3 21.3 18.7
1145 1106 1250 1260 985 945
46.6 45.5 56.9 54.9 53.5 52.7
36.1 36.1 38.6 38.7 38.8 38.6
4.2 4.2 8.8 8.7 8.5 8.5
4.1 4.0 4.2 4.0 3.8 3.5
Therefore, based on the VR, P1 contained roughly 75% of the overall residual alkali. Dilution of R1 and subsequent filtration resulted in a secondary retentate R2, considered suitable for the recovery boiler owing to the low residual alkali content. Volume of R2 was decreased to 36% of the initial volume, while it contained approximately 45% of the lignin, 81% of the polysaccharides and only 5% of the residual alkali amount in respect to initial BL. Due to the small obtainable sample size, additional processing of R2, P1 and P2 could not be performed. However, subsequent ultra- and nanofiltration of the primary and secondary permeates would even further purify the liquor fractions especially from lignin; consequently enabling the recirculation of P1 and P2 back into alkali intensive process steps, e.g. pulping or oxygen delignification stage, respectively. 3.3. Oxygen delignification of high-kappa pulp In order to remove residual lignin before bleaching, oxygen delignification (OD) is preferred over extended pulping. Transformation from pulping to OD is optimally defined by the distinctive decrease in pulping stage selectivity. In HALT conditions, delignification selectivity decreased at around K-60, far from the conventional practice of approximately K-25. Conventionally used two-stage oxygen delignification processes are suitable for kappa reductions from 25 to 10, though they generally lack the performance to obtain over 70% kappa reduction from a high-kappa pulp. To promote delignification during extended two-stage oxygen delignification processes, a wide range of activators or catalysts was listed by Suchy and Argyropoulos (2002). To produce a starting pulp for oxygen delignification study according to REF and HALT concepts, similar cooking times could be used (REF: K-26, 440 min; HALT: EA 60%, K-55, 450 min). In this study, a two-stage OD process with inter-stage peroxymonosulphuric acid (Px) treatment was utilised for a HALT pulp. In a recent study with high-kappa number softwood pulp (Jafari et al., 2014), a 70% kappa reduction was obtained in a two-stage oxygen delignification sequence, including an inter-stage Px treatment (OPxO). Both REF and HALT pulps were chelated prior to the first O-stage. With the high-yield HALT pulp used in this study (K-55), a kappa reduction of 66% was obtained with a given OPxO sequence. Pulp properties before and after oxygen delignification are presented in Table 3.
Oxygen bleaching was found to be selective toward lignin for high-yield HALT pulps. Minor polysaccharide losses occurred, mainly attributed to AX and, to a lesser extent, to GGM. The cellulose yield was practically unaffected. Surprisingly, extensive kappa reduction was obtained in a 10 min Px treatment, almost equal to the first oxygen stage. As a downside, pulp viscosity decreased greatly during Px, indicating insufficient chelation prior to the OPxO sequence. To prevent certain cellulose depolymerisation and pulp viscosity losses during Px treatment, as was shown previously by Allison and McGrouther (1995), the effective removal of metal ions is critical. The main motivation of the presented oxygen delignification study was not to optimise the oxygen bleaching sequence, but to demonstrate the preservation of pulp yield throughout extended oxygen delignification. HALT pulp maintained its yield advantage over REF pulp even after OD. Lignin-free pulp yield was found to be 6.3%/unit higher; approximately two-thirds attributed to the improved GGM yield and the rest to cellulose (Table 3). 3.4. The HALT process as a base of a biorefinery: possibilities and challenges The economic benefit that reflects from the significant increase in pulp yield is obvious. Especially in Nordic countries, wood raw material comprises up to 90% of the variable costs per produced air dry tonne of pulp and over half of the total production costs in a modern pulp mill (Kangas et al., 2014). Thus, savings from improved raw material utilisation and additional revenue from increased pulp production would be expected. Nevertheless, before commercial realisation of the proposed HALT concept, improvements and modifications in respect to prevailing oxygen delignification practice and BL handling are required. Even though it is acknowledged that delignification selectivity can be maintained in extended oxygen delignification stages and not in extended pulping, industrial solutions for high-kappa oxygen delignification processes are missing. Also, owing to high residual alkalinity, additional BL processing and certain liquor circulation outside the recovery boiler is mandatory. Described liquor circulation would enable accumulative enrichment of other low-molar-mass compounds, such as hydroxy acids, in the liquor. However, a recent study successfully utilised
102
M. Paananen, H. Sixta / Bioresource Technology 193 (2015) 97–102
size-exclusion chromatography to separate hydroxy acids from alkaline pulping liquor (Hellstén et al., 2013). Therefore, in order to recover the unused alkali by removing the dissolved lignin and hemicelluloses, ultrafiltration seems viable. Separation of lignin and hemicelluloses as well as hydroxy acids could create side streams with added value, e.g., via (bio)chemical conversion to biofuels or other chemicals (Mesfun et al., 2014). 4. Conclusions High-alkali low-temperature polysulfide (HALT) pulping was used to obtain maximal polysaccharide stability and effective delignification; at around K-60, the pulp yield increased up to 6.7 wt% on wood compared to conventional practice. At best, the proposed pulping concept significantly improves mill economy through decreased energy consumption owing to lower pulping temperature and enhanced pulp production in respect to raw material utilisation. The novel concept of HALT pulping reveals a substantial potential for yield increase and the recovery of high value-added side products, which, however, cannot be exploited without modifications to the conventional kraft pulping practice. Acknowledgements For their financial support, Andritz, Kemira, Metsä Fibre, Stora Enso, and UPM are highly acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2015.06. 075. References Allison, R., McGrouther, K., 1995. Improved oxygen delignification with interstage peroxymonosulfuric acid treatment. Tappi J. 78, 134–142. Brännvall, E., Gustafsson, R., Teder, A., 2003. Properties of hyperalkaline polysulphide pulps. Nordic Pulp Pap. Res. J. 18, 436–440. da Silva Perez, D., van Heiningen, A., 2002. Determination of cellulose degree of polymerization in chemical pulps by viscometry. In: Proceedings in 7th European Workshop on Lignocellulosics and Pulp, Turku, Finland. pp. 393–396. DuPont™ OxoneÒ Monopersulfate compound – Technical attributes. (accessed March 30, 2015). FitzPatrick, M., Champagne, P., Cunningham, M., Whitney, R., 2010. A biorefinery processing perspective: treatment of lignocellulosic materials for the production of value-added products. Bioresour. Technol. 101, 8915–8922. Gustafsson, R., Ek, M., Teder, A., 2004a. Polysulfide pretreatment of softwood for increased delignification and higher pulp viscosity. J. Pulp Pap. Sci. 30, 129–135. Gustafsson, R., Freysoldt, J., Teder, A., 2004b. Influence of the alkalinity in polysulfide pretreatment on results of cooking with normal liquor-to-wood ratios. Pap. Puu. 86, 169–173.
Hellstén, S., Heinonen, J., Sainio, T., 2013. Size-exclusion chromatographic separation of hydroxy acids and sodium hydroxide in spent pulping liquor. Sep. Purif. Technol. 118, 234–241. Jafari, V., Sixta, H., van Heiningen, A., 2014. Multistage oxygen delignification of high-kappa pine kraft pulp with peroxymonosulfuric acid (Px). Holzforschung 68, 497–504. Janson, J., 1970. Calculation of polysaccharide composition of wood and pulp. Pap. Puu. 52, 323–329. Johansson, B., Mjöberg, J., Sandström, P., Teder, A., 1984. Modified continuous kraft pulping: now a reality. Svensk Papperstidn. 87, 30–35. Kangas, P., Kaijaluoto, S., Määttänen, M., 2014. Evaluation of future pulp mill concepts: reference model of a modern Nordic kraft pulp mill. Nordic Pulp Pap. Res. J. 29, 620–634. Keyoumu, A., Sjödahl, R., Henriksson, G., Ek, M., Gellerstedt, G., Lindström, M., 2004. Continuous nano- and ultrafiltration of kraft pulping black liquor with ceramic filters: a method for lowering the load on the recovery boiler while generating valuable side-products. Ind. Crops Prod. 20, 143–150. Li Jansson, Z., Brännvall, E., 2014. Effect of kraft cooking conditions on the chemical composition of the surface and bulk of spruce fibers. J. Wood Chem. Technol. 34, 291–300. Lindgren, C., Lindström, M., 1995. Thermal decomposition of inorganic polysulfides at kraft cooking conditions. Nordic Pulp Pap. Res. J. 10, 41–45. Mesfun, S., Lundgren, J., Grip, C.-E., Toffolo, A., Nilsson, R., Rova, U., 2014. Black liquor fractionation for biofuels production: a techno-economic assessment. Bioresour. Technol. 166, 508–517. Paananen, M., Tamminen, T., Nieminen, K., Sixta, H., 2010. Galactoglucomannan stabilization during the initial kraft cooking of Scots pine. Holzforschung 64, 683–692. Paananen, M., Liitiä, T., Sixta, H., 2013. Further insight into carbohydrate degradation and dissolution behavior during kraft cooking under elevated alkalinity without and in the presence of anthraquinone. Ind. Eng. Chem. Res. J. 52, 12777–12784. Paananen, M., Rovio, S., Liitiä, T., Sixta, H., 2015. Stabilization, degradation, and dissolution behavior of Scots pine polysaccharides during polysulfide (K-PS) and polysulfide anthraquinone (K-PSAQ) pulping. Holzforschung. http:// dx.doi.org/10.1515/hf-2014-0220. Sanyer, N., Laundrie, J.F., 1964. Factors affecting yield increase and fiber quality in polysulfide pulping of loblolly pine, other softwoods, and red oak. Tappi 47, 640–652. Schlesinger, R., Götzinger, G., Sixta, H., Friedl, A., Harasek, M., 2006. Evaluation of alkali resistant nanofiltration membranes for the separation of hemicellulose from concentrated alkaline process liquors. Desalination 192, 303–314. Suchy, M., Argyropoulos, D., 2002. Catalysis and activation of oxygen and peroxide delignification of chemical pulps: a review. Tappi J. 1, 9–26. Swan, B., 1965. Isolation of acid-soluble lignin from the Klason lignin determination. Svensk Papperstidn. 68, 791–795. Tasman, J.E., 1959. The permanganate consumption of pulp materials. IV. The KAPPA correction coefficient. Pulp Pap. Can. 60, 231–235. Teder, A., 1969. Some aspects of the chemistry of polysulfide pulping. Svensk Papperstidn. 72, 294–303. Vaaler, D., 2008. Yield-Increasing Additives in Kraft Pulping: Effect on Carbohydrate Retention, Composition and Handsheet Properties (Ph.D. thesis). Norwegian University of Science and Technology. Wallberg, O., Jönsson, A.-S., 2003. Influence of the membrane cut-off during ultrafiltration of kraft black liquor with ceramic membranes. Chem. Eng. Res. Des. 81, 1379–1384. Wallberg, O., Jönsson, A.-S., 2006. Separation of lignin in kraft cooking liquor from a continuous digester by ultrafiltration at temperatures above 100 °C. Desalination 195, 187–200. Young, R.A., Liss, L., 1978. A kinetic study of the alkaline endwise degradation of gluco- and galactomannans. Cellul. Chem. Technol. 12, 399–411.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
High-alkali low-temperature polysulfide pulping (HALT) of Scots pine Markus Paananen ⇑, Herbert Sixta Aalto University, School of Chemical Technology, Department of Forest Products Technology, Vuorimiehentie 1, FI-00076, Finland
h i g h l i g h t s Significantly improved galactoglucomannan and cellulose preservation. Pulp yield increase maintained at bleachable grade pulp. Purification of black liquor and recovery of residual alkali with ultrafiltration.
a r t i c l e
i n f o
Article history: Received 18 May 2015 Received in revised form 15 June 2015 Accepted 16 June 2015 Available online 23 June 2015 Keywords: Alkali recovery High-alkali pulping Polysulfide Softwood Ultrafiltration
a b s t r a c t High-alkali low-temperature polysulfide pulping (HALT) was effectively utilised to prevent major polysaccharide losses while maintaining the delignification rate. A yield increase of 6.7 wt% on wood was observed for a HALT pulp compared to a conventionally produced kappa number 60 pulp with comparable viscosity. Approximately 70% of the yield increase was attributed to improved galactoglucomannan preservation and 30% to cellulose. A two-stage oxygen delignification sequence with inter-stage peroxymonosulphuric acid treatment was used to ensure delignification to a bleachable grade. In a comparison to conventional pulp, HALT pulp effectively maintained its yield advantage. Diafiltration trials indicate that purified black liquor can be directly recycled, as large lignin fractions and basically all dissolved polysaccharides were separated from the alkali-rich BL. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Pulp production is no longer merely considered a process for papermaking fibres and energy, but as a fully operational biorefinery to produce numerous products of added value. Despite efficient fractionation techniques have been successfully demonstrated and utilised with various pulping concepts and types of biomass (FitzPatrick et al., 2010), implementation of fractionation techniques as a part of the kraft process has been slow owing to its highly established chemical recovery and more than self-sustained energy economy. Softwood kraft pulping will maintain its status as one of the most important pulping processes due to its superior wood-based pulp fibres, however, the evolution of kraft pulping has been rather stagnant since the adaptation of the modified continuous cooking (MCC) process in the late 70 s and early 80 s. Since raw material is the major contributor in the overall softwood kraft pulping production costs, improvements in the utilisation of raw material in terms of increased pulp yield and the recovery of valuable side product streams are of importance. ⇑ Corresponding author. E-mail address: markus.paananen@aalto.fi (M. Paananen). http://dx.doi.org/10.1016/j.biortech.2015.06.075 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.
At kraft conditions, the losses of softwood polysaccharides via end-wise degradation (primary peeling) at alkaline conditions is already initiated in the heating-up phase. Even though all polysaccharides are subjected to peeling, most losses are caused by galactoglucomannan (GGM) having no stabilizing side groups or ordered structure like arabinoxylan (AX) or cellulose, respectively. In addition, all polysaccharides suffer from losses initiated by chain scission (alkaline hydrolysis) and are materialised in the subsequent secondary peeling reactions and dissolution of short-chain polysaccharides. Peeling reactions can be terminated by the formation of metasaccharinic acid end-groups (stopping reaction, Young and Liss (1978)). Termination reactions are favored by elevated hydroxide ion concentration ([OH ]); thus, provided that the alkalinity is maintained sufficiently high during the pulping process, GGM is stabilized to a certain extent (Paananen et al., 2010). Parallel to the GGM stabilization, a significant amount of acidic degradation products are formed. A large amount of alkali is consumed in these side reactions, further decreasing the rate of stopping reactions. In a pulping process based on the principles of MCC and especially the concept of levelled alkalinity (Johansson et al., 1984), the pulping liquor hydroxide ion [OH ] concentration is, however, not adequately high for optimal GGM stabilization (Paananen et al.,
98
M. Paananen, H. Sixta / Bioresource Technology 193 (2015) 97–102
2010, 2013). On the other hand, alkali overload might result in accelerated cellulose depolymerisation through alkaline hydrolysis especially at higher temperatures, decreasing the fibre strength properties. The principles of high-alkali polysulfide (PS) liquor pretreatment have been successfully demonstrated to yield a pulp with higher viscosity and effective polysaccharide stabilization at high and conventional liquor-to-wood ratios (Gustafsson et al., 2004a,b). Also, no detriment is expected in subsequent fibre processing, as another study reported that pulps pretreated with high-alkali PS liquor had bleachability and strength properties comparable to those of the conventional kraft process (Brännvall et al., 2003). The decomposition of PS is accompanied by a consumption of hydroxide ions, and promoted by increasing temperature. PS ions are shown to exist at conventional cooking temperatures above 160 °C; however, due to accelerated decomposition rates, the concentration is much lower (Lindgren and Lindström, 1995). Lindgren and Lindström also concluded that liquor [OH ] had no influence on the decomposition rate of PS; conversely, the oxidation of the polysaccharide reducing end-groups (stabilization) is favored by increasing [OH ] (Teder, 1969). One of the key challenges in implementing a high-alkali-profile pulping process is the management of unused alkali. Conventionally, the kraft process does not use excess alkali, and the residual black liquor (BL) alkalinity is well below 10 g NaOH/L, which is recoverable via incineration. In a high-alkali pulping, diverting BL directly into the recovery boiler is practically impossible due to the high residual alkalinity. Additional BL processing is, therefore, required. Little attention has been paid to the recovery of residual alkalinity as a part of effective lignin and carbohydrate separation process. For feasible process economy, residual alkali should be recovered from the BL at the process temperature without cooling. This requirement sets additional demands on the separation process. Certain polymeric membranes can withstand elevated alkalinity, however, are limited to lower operation temperatures (Schlesinger et al., 2006). On the other hand, ceramic membranes have been successfully used even at 145 °C to separate the higher-molecular-weight dissolved lignin and carbohydrates of mill softwood BL (Wallberg and Jönsson, 2006). Previously, a high-yield pulp was produced by utilising a high-alkali PS pulping liquor and low temperature in a diluted cooking system with wood meal (Paananen et al., 2015). This paper demonstrates the same cooking concept with chips and low liquor-to-wood ratio. Followed by a reinforced two-stage oxygen delignification, the composition and properties of a high-yield pulp are compared to a pulp produced via conventional kraft pulping methods. Further, results on the recycling of unused sodium hydroxide and the concomitant separation of dissolved lignin and carbohydrate fractions by means of ultrafiltration and diafiltration are presented.
2. Methods
Paananen et al. (2015). The polysulfide charge was set to 6 g S(0)/L (3.6% odw, on dry wood). The concentration was considered comparable to industrial practice when polysulfide liquor is generated via white liquor oxidation. Polysulfide liquor preparation and experiments were conducted in a nitrogen atmosphere. High-alkali low-temperature polysulfide (HALT) pulping experiments were studied at effective alkali (EA) amounts from 50% to 80% odw (2.08–3.33 M [OH ]) and at 130 °C. In addition, several experiments were carried out at 140–160 °C with an EA charge of 60% odw (focus was on 130 °C experiments, experiments at higher temperatures are specifically referred to). All HALT experiments used a 40% initial sulfidity (before sulphur addition). Kraft reference conditions (REF) were 160 °C, L/W 6, 22% EA charge (0.92 M [OH ]), and 40% sulfidity. Heating and cooling were obtained within 20 min ± 1 min, regardless of the target temperature. Experimental work was conducted in a reactor with liquor circulation to ensure similar liquor temperature and properties inside the chip bed. Due to the reactor geometry and chip bed packing, no lower liquor-to-wood ratio than 6 could be utilised while keeping all chips under the liquor level during the experiment. 2.2. Pulp and black liquor characterisation Pulp yield and the content of acetone extractives (SCAN 49:03) were determined gravimetrically. Pulp was decomposed in a two-stage acid hydrolysis stage process according to SCAN-CM 71:09. Acid hydrolysate was filtered with a glass filter to determine the amount of acid insoluble lignin (Klason lignin, KL). Monosaccharide composition was determined from the filtrate by HPAEC-PAD according to SCAN-CM 71:09, and acid-soluble lignin by UV spectroscopy (Shimadzu UV-2550; 205 nm and an absorption coefficient of 110 L * (g cm) 1 according to Swan (1965)). Based on the pulp Klason lignin content, the pulp kappa number (KN) was calculated according to Tasman (1959) (lignin on pulp (%) = KN * 0.15). The polysaccharide composition was calculated from the relative amount of monosaccharides according to Janson (1970). The dissolution of polysaccharides was evaluated according to the monosaccharide composition of BLs after acid hydrolysis according to SCAN-CM 71:09. Owing to the relatively high kappa numbers of all pulps, further lignin removal was necessary prior to viscosity measurement according to SCAN-CM 15:99. Accordingly, 5 min (70 °C) chlorite delignification pretreatment was used to remove excess lignin: 3 g pulp (dry basis), 200 mL water, 20 mL chlorine dioxide water, 2 mL of 100% acetic acid. 2.3. Ultrafiltration Amicon 8200 (200 mL cell) was used in lab-scale filtration experiments (Supplementary material). The operating temperature was limited to room temperature and the pressure to 5 bar (regulated with nitrogen gas). Black liquor feed volume was 180 mL, while the magnetic stirrer was set to 300 rpm. Polyethersulfone ultrafiltration membranes were used, with 5 (UP005) and 10 kDa (UP010) cut-offs (Microdyn-Nadir). Diafiltration was simulated by dilution of the primary retentate with water, followed by a subsequent ultrafiltration.
2.1. Raw material and experimental set-up 2.4. Chelation, oxygen delignification, and inter-stage treatment Scots pine (Pinus sylvestris L.) from southern Finland was chipped and screened according to SCAN-CM 40:01. Chips were frozen until further use. The wood raw material comprised of cellulose (41.7% odw), galactoglucomannan (16.9%), arabinoxylan (8.2%), other carbohydrates (1.4%), acid-insoluble lignin (Klason lignin, 25.9%), acid-soluble lignin (0.7%) and extractives (3.2%) (Paananen et al., 2015). Synthetic orange liquor was prepared from analytical grade NaOH, Na2S, and elemental sulphur, according to
Prior to oxygen delignification, pulp chelation with ethylenediaminetetraacetic acid (EDTA) was performed at 0.2% odw charge at 3% pulp consistency, at pH 5.5 and 50 °C for 30 min. Oxygen delignification (OD) experiments were carried out in a rotating air-bath digester. All experiments were conducted with 30 g odw pulp, 10% consistency, 0.05% MgSO4 (as Mg2+), and 7 bar oxygen pressure in a 2.5 dm3 vessel. As a reference, one-stage oxygen delignification
M. Paananen, H. Sixta / Bioresource Technology 193 (2015) 97–102
99
was carried out with REF pulp (45 min at 85 °C, 3% NaOH). HALT concept pulp was oxygen delignified in a two-stage oxygen delignification process (1st O: 45 min at 85 °C, 4% NaOH; 2nd O: 60 min at 95 °C, 2% NaOH). Inter-stage peroxymonosulphuric acid (Px) treatment was conducted at a 2% active oxygen amount, 10% pulp consistency, and pH of 11 for 10 min at 50 °C. The source of Px was OxoneÒ (Sigma–Aldrich, Switzerland, CAS:70693-62-8, potassium hydrogen peroxymonosulphate sulphate). The active oxygen content in OxoneÒ was determined according to a method by DuPont (DuPont Oxone technical attributes). 3. Results and discussion 3.1. HALT pulping 3.1.1. General considerations At a given liquor-to-wood ratio (L/W), the amount of added PS strongly determines the obtainable pulp yield increase (Sanyer and Laundrie, 1964). However, as the results from the present study are mainly convergent with the previous study at highly diluted pulping systems (identical raw material and g S(0)/L addition, L/W 200) (Paananen et al., 2015), the effect of PS is not merely determined by the PS load per se but by the concentration in the pulping liquor. The concentration of PS generated from elemental sulphur was adjusted to 6 g S(0)/L, thus comparable to that prepared via white liquor oxidation (mill practice). 3.1.2. Pulping at 130 °C Alkali charges ranging from 50% to 80% EA odw were studied at 130 °C. According to the HALT concept presented in this paper, maintaining elevated alkali concentrations is critical in order to maintain the delignification rate. Nevertheless, rather rapid decreases in alkali concentration are expected during the initial part of the cooking, where the main alkali-consuming reactions occur. As shown in Fig. 1a, the initial EA charge strongly affects the delignification rate. Owing to the increase in initial EA charge from EA 50% to 60%, a prominent acceleration in the delignification rate was obtained, and the cooking time to a certain delignification degree was shortened by approximately 20%. The delignification rate was further promoted at 70% and 80% EA charges, but the effect was less pronounced. In respect to the yield of galactoglucomannan (GGM), the main hemicellulose of Scots pine, the main advantage in high-alkali PS pulping is the less extensive end-wise degradation (Paananen et al., 2015). Already without PS, GGM stabilization reactions are promoted by elevated alkalinity (Young and Liss, 1978), though they require temperatures and hydroxide ion concentrations higher than 100 °C and 1.0 M, respectively (Paananen et al., 2010). In the present study, higher than 50% EA charge was needed to level off pulp GGM content and prevent further losses (Fig. 2a), indicating approximately a 50% GGM yield. The residual BL alkalinity with the initial 50% EA charge was around 0.8 M [OH ], and in the case of the 60% initial EA charge, approximately 1.1 M [OH ]. Therefore, it can be concluded that the findings in this study are well in line with the previous observations from diluted pulping systems. Whereas the GGM yield at elevated alkalinity was not significantly influenced by the temperature in a range of 130–160 °C, AX retention in particular was greatly affected by both alkalinity and temperature (Paananen et al., 2013, 2015). Rather surprisingly, the low temperature used in HALT concept pulping practically compensated for the effect of elevated alkalinity, so AX retention in HALT was almost comparable to that in the REF pulp (Fig. 2b). Although the differences in pulp AX content in respect to used alkali charges are rather insignificant in a kappa number
Fig. 1. (a) Kappa number as a function of cooking time, as well as (b) pulp yield, and (c) pulp viscosity as a function of kappa number.
range of 70–40 (Fig. 2b), the highest AX yield was obtained at 60% EA charge and not at 50% EA charge, as expected. In a highly diluted pulping system, it was demonstrated that, without any clear indication of redeposition to the fibre wall, during extended pulping, the decrease in the dissolved AX fraction was coupled with a parallel increase in the detected BL hydroxy acid amount (Paananen et al., 2013). Conversely, at industrial liquor-to-wood ratios around 4–6, xylan in particular is shown to redeposit on fibre surfaces (Li Jansson and Brännvall, 2014) and also locate on the outer fibre wall after pulping (Vaaler, 2008). Therefore, it seems that certain AX adsorption occurred during pulping at 60% EA charge, but not until the point of fibre liberation and dissolution of the middle lamella lignin at around K-60. Consisting mainly of AX, the amount of total dissolved polysaccharides was roughly 3% odw in the HALT concept and 2% odw in REF BLs (Supplementary material). As shown in Fig. 2c, cellulose was found to be rather stable in HALT conditions. Regardless of the used EA charge, the cellulose yield was approximately 92% in a kappa number range of 70–45. In a comparison to REF pulps, the yield advantage of HALT pulps is emphasised at kappa numbers of 60–50, roughly indicating the point of optimum selectivity toward lignin. At a given kappa number of 55, the best HALT concept pulp (EA 60%) had 6.7 wt% on wood higher pulp yield compared to REF pulp (Fig. 1b). Even though cellulose retention was also greatly improved in HALT conditions compared to REF, a major part of the overall pulp yield increase was attributed to improved GGM preservation. While
100
M. Paananen, H. Sixta / Bioresource Technology 193 (2015) 97–102
3.1.3. The effect of pulping temperature (140–160 °C) The effect of pulping temperature was studied only at an 60% EA charge which displayed the most selective high-kappa pulping at 130 °C, however, it should be noted that the given EA charge might not be optimal in combination with higher temperature. Owing to the temperature increase from 130 to 140 °C, GGM and AX suffered from approximately 1% odw additional loss at a given kappa number of 55. Further, during extended pulping to K-25, additional losses for both GGM and AX were approximately 2.5% odw at 160 °C compared to K-55 at 130 °C. However, considering the overall AX retention (in pulp and dissolved in BL), it is clear that pulp AX losses at higher temperature are a direct consequence of enhanced dissolution, most likely enabled by even a single chain scission. In case of GGM, no significant increase in the amount of dissolved GGM was observed. Also, cellulose losses became more distinct as the delignification proceeded. However, different than AX, losses are not caused directly by chain depolymerisation and subsequent dissolution, but rather by the chain scission followed by progressive end-wise degradation. Despite the rather significant cellulose losses during extended pulping to K-25, pulp viscosity was maintained at the level comparable to REF at 140 and 150 °C (Fig. 1c). Nevertheless, it should be emphasised that the data at 150 and 160 °C come from isolated experiments, so strong conclusions should be avoided. 3.2. Black liquor fractionation
Fig. 2. (a) Galactoglucomannan (GGM) (b) arabinoxylan, and (c) cellulose as a function of kappa number.
Table 1 Pulp composition of acetone extracted high-kappa pulps. Cooked according to conventional and HALT concepts. The polysulfide charge in HALT was set to 6 g S(0)/L (3.6% odw, on dry wood). Temperature (°C) Effective alkali charge (% odw) Kappa number H-factor Viscosity Pulp yield, (% odw) Cellulose, (% odw) GGM, (% odw) AX, (% odw)
160 22 61 1540 1225 50.7 36.4 4.8 4.5
130 50 68 211 1290 57.0 38.3 8.1 4.4
130 60 65 168 1200 57.1 38.1 8.7 4.5
130 60 55 190 1250 56.6 38.6 8.8 4.2
130 70 64 152 1165 55.9 37.5 8.5 4.1
130 80 58 139 1065 55.9 38.1 8.7 3.9
the EA charge had a rather minor effect on the pulp yield, the effect on pulp viscosity was more pronounced, indicating additional cellulose depolymerisation at higher alkali charges (Fig. 1c). Even though pulp viscosity is often used to define the cellulose degree of polymerisation, a direct analogy should be avoided. According to the previously reported correlation between measured pulp viscosity and cellulose degree of polymerisation, pulp hemicellulose fractions should be noted as well (da Silva Perez and van Heiningen, 2002). The pulp composition at various pulping modifications around K-60 is presented in Table 1.
Emphasis of the black liquor fractionation was to produce purified alkali rich liquor fraction by removing dissolved polysaccharides and lignin. Depending on the initial alkali load, HALT concept pulping results in BL with a large amount of unused alkali, ranging from 0.8 to 1.5 M [OH ] (Supplementary material), corresponding to initial hydroxide ion concentrations of 2.08 (50% EA) to 3.33 M (80% EA), respectively. Although fewer hydroxy acids are formed in HALT conditions (Paananen et al., 2015) and thus less alkali is required to neutralize these acids, the total alkali consumption was found to be higher compared to REF ([OH ]: residual 0.1 M; initial 0.92 M). This can be partly explained by the PS decomposition reactions and the end-group stabilization reactions, both related to alkali consumption. In addition, higher initial alkali charges lead to increased alkali consumption in HALT conditions. This might indicate more extensive fragmentation reactions of dissolved wood components. In a simple proof of concept lab-scale ultrafiltration set-up, maximum obtainable volume reduction (VR) was mainly determined by the cut-off. While using a membrane with 5 kDa cut-off, the transmembrane flux at a given operating pressure of 5 bar was too low to reach over 70% VR within reasonable time frame. Even moderate increase in the ambient filtration temperature would have positively affected the transmembrane flux (Keyoumu et al., 2004), but this was not possible with the given set-up. Therefore, in diafiltration experiments (BL sample withdrawn from the experiment at 60% EA and 130 °C), membrane with 10 kDa cut-off was used. Owing to the low-molar-mass, cooking chemicals are not fractionated in a membrane separation process, regardless of the cut-off (Wallberg and Jönsson, 2003). Therefore, diafiltration was used to retain dissolved polysaccharides and lignin while decreasing the BL alkalinity to a level tolerable by the recovery boiler. Accordingly, four liquor fractions were prepared, namely primary and secondary retentate (R1 and R2), as well as primary and secondary permeate (P1 and P2) (Supplementary material). With a 10 kDa membrane and VR of 70%, R1 contained 68% and 91% of the initial lignin and carbohydrates, respectively (Table 2). According to the discussion, practically no difference between P1 and R1 was observed in respect to alkali concentration.
101
M. Paananen, H. Sixta / Bioresource Technology 193 (2015) 97–102 Table 2 Lignin, carbohydrate, and residual alkali mass balance during diafiltration with a 10 kDa membrane. Black liquor extracted from HALT cooking with 60% EA at 130 °C. Vol-frac
Initial black liquor Primary retentate (R1) Primary permeate (P1) Diafiltration Secondary retentate (R2) Secondary permeate (P2) Total identification* (%) *
Lignin
1.00 0.24 0.70 1.80 0.36 1.44
Carbohydrates
Residual EA
g/L
% of all
g/L
% of all
g/L
% of all
39.8 112.3 19.7
100.0 67.8 34.5
5.6 21.1 0.4
100.0 90.6 5.1
52.1 53.1 55.7
100 24.5 74.5
50.4 6.7
45.6 24.2 104
12.5 0.2
80.6 6.3 92
7.7 7.0
5.3 19.4 99
Total identification from P1, R2 and P2.
Table 3 Pulp characterisation before and after oxygen delignification sequence. HALT pulp cooked at 60% EA and 130 °C.
REF pulp REF-O HALT pulp HALT-O HALT-OPx HALT-OPxO
Kappa number
Viscosity
Yield (% odw)
CELL (% odw)
GGM (% odw)
AX (% odw)
26.2 13.5 54.7 37.3 21.3 18.7
1145 1106 1250 1260 985 945
46.6 45.5 56.9 54.9 53.5 52.7
36.1 36.1 38.6 38.7 38.8 38.6
4.2 4.2 8.8 8.7 8.5 8.5
4.1 4.0 4.2 4.0 3.8 3.5
Therefore, based on the VR, P1 contained roughly 75% of the overall residual alkali. Dilution of R1 and subsequent filtration resulted in a secondary retentate R2, considered suitable for the recovery boiler owing to the low residual alkali content. Volume of R2 was decreased to 36% of the initial volume, while it contained approximately 45% of the lignin, 81% of the polysaccharides and only 5% of the residual alkali amount in respect to initial BL. Due to the small obtainable sample size, additional processing of R2, P1 and P2 could not be performed. However, subsequent ultra- and nanofiltration of the primary and secondary permeates would even further purify the liquor fractions especially from lignin; consequently enabling the recirculation of P1 and P2 back into alkali intensive process steps, e.g. pulping or oxygen delignification stage, respectively. 3.3. Oxygen delignification of high-kappa pulp In order to remove residual lignin before bleaching, oxygen delignification (OD) is preferred over extended pulping. Transformation from pulping to OD is optimally defined by the distinctive decrease in pulping stage selectivity. In HALT conditions, delignification selectivity decreased at around K-60, far from the conventional practice of approximately K-25. Conventionally used two-stage oxygen delignification processes are suitable for kappa reductions from 25 to 10, though they generally lack the performance to obtain over 70% kappa reduction from a high-kappa pulp. To promote delignification during extended two-stage oxygen delignification processes, a wide range of activators or catalysts was listed by Suchy and Argyropoulos (2002). To produce a starting pulp for oxygen delignification study according to REF and HALT concepts, similar cooking times could be used (REF: K-26, 440 min; HALT: EA 60%, K-55, 450 min). In this study, a two-stage OD process with inter-stage peroxymonosulphuric acid (Px) treatment was utilised for a HALT pulp. In a recent study with high-kappa number softwood pulp (Jafari et al., 2014), a 70% kappa reduction was obtained in a two-stage oxygen delignification sequence, including an inter-stage Px treatment (OPxO). Both REF and HALT pulps were chelated prior to the first O-stage. With the high-yield HALT pulp used in this study (K-55), a kappa reduction of 66% was obtained with a given OPxO sequence. Pulp properties before and after oxygen delignification are presented in Table 3.
Oxygen bleaching was found to be selective toward lignin for high-yield HALT pulps. Minor polysaccharide losses occurred, mainly attributed to AX and, to a lesser extent, to GGM. The cellulose yield was practically unaffected. Surprisingly, extensive kappa reduction was obtained in a 10 min Px treatment, almost equal to the first oxygen stage. As a downside, pulp viscosity decreased greatly during Px, indicating insufficient chelation prior to the OPxO sequence. To prevent certain cellulose depolymerisation and pulp viscosity losses during Px treatment, as was shown previously by Allison and McGrouther (1995), the effective removal of metal ions is critical. The main motivation of the presented oxygen delignification study was not to optimise the oxygen bleaching sequence, but to demonstrate the preservation of pulp yield throughout extended oxygen delignification. HALT pulp maintained its yield advantage over REF pulp even after OD. Lignin-free pulp yield was found to be 6.3%/unit higher; approximately two-thirds attributed to the improved GGM yield and the rest to cellulose (Table 3). 3.4. The HALT process as a base of a biorefinery: possibilities and challenges The economic benefit that reflects from the significant increase in pulp yield is obvious. Especially in Nordic countries, wood raw material comprises up to 90% of the variable costs per produced air dry tonne of pulp and over half of the total production costs in a modern pulp mill (Kangas et al., 2014). Thus, savings from improved raw material utilisation and additional revenue from increased pulp production would be expected. Nevertheless, before commercial realisation of the proposed HALT concept, improvements and modifications in respect to prevailing oxygen delignification practice and BL handling are required. Even though it is acknowledged that delignification selectivity can be maintained in extended oxygen delignification stages and not in extended pulping, industrial solutions for high-kappa oxygen delignification processes are missing. Also, owing to high residual alkalinity, additional BL processing and certain liquor circulation outside the recovery boiler is mandatory. Described liquor circulation would enable accumulative enrichment of other low-molar-mass compounds, such as hydroxy acids, in the liquor. However, a recent study successfully utilised
102
M. Paananen, H. Sixta / Bioresource Technology 193 (2015) 97–102
size-exclusion chromatography to separate hydroxy acids from alkaline pulping liquor (Hellstén et al., 2013). Therefore, in order to recover the unused alkali by removing the dissolved lignin and hemicelluloses, ultrafiltration seems viable. Separation of lignin and hemicelluloses as well as hydroxy acids could create side streams with added value, e.g., via (bio)chemical conversion to biofuels or other chemicals (Mesfun et al., 2014). 4. Conclusions High-alkali low-temperature polysulfide (HALT) pulping was used to obtain maximal polysaccharide stability and effective delignification; at around K-60, the pulp yield increased up to 6.7 wt% on wood compared to conventional practice. At best, the proposed pulping concept significantly improves mill economy through decreased energy consumption owing to lower pulping temperature and enhanced pulp production in respect to raw material utilisation. The novel concept of HALT pulping reveals a substantial potential for yield increase and the recovery of high value-added side products, which, however, cannot be exploited without modifications to the conventional kraft pulping practice. Acknowledgements For their financial support, Andritz, Kemira, Metsä Fibre, Stora Enso, and UPM are highly acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech.2015.06. 075. References Allison, R., McGrouther, K., 1995. Improved oxygen delignification with interstage peroxymonosulfuric acid treatment. Tappi J. 78, 134–142. Brännvall, E., Gustafsson, R., Teder, A., 2003. Properties of hyperalkaline polysulphide pulps. Nordic Pulp Pap. Res. J. 18, 436–440. da Silva Perez, D., van Heiningen, A., 2002. Determination of cellulose degree of polymerization in chemical pulps by viscometry. In: Proceedings in 7th European Workshop on Lignocellulosics and Pulp, Turku, Finland. pp. 393–396. DuPont™ OxoneÒ Monopersulfate compound – Technical attributes. (accessed March 30, 2015). FitzPatrick, M., Champagne, P., Cunningham, M., Whitney, R., 2010. A biorefinery processing perspective: treatment of lignocellulosic materials for the production of value-added products. Bioresour. Technol. 101, 8915–8922. Gustafsson, R., Ek, M., Teder, A., 2004a. Polysulfide pretreatment of softwood for increased delignification and higher pulp viscosity. J. Pulp Pap. Sci. 30, 129–135. Gustafsson, R., Freysoldt, J., Teder, A., 2004b. Influence of the alkalinity in polysulfide pretreatment on results of cooking with normal liquor-to-wood ratios. Pap. Puu. 86, 169–173.
Hellstén, S., Heinonen, J., Sainio, T., 2013. Size-exclusion chromatographic separation of hydroxy acids and sodium hydroxide in spent pulping liquor. Sep. Purif. Technol. 118, 234–241. Jafari, V., Sixta, H., van Heiningen, A., 2014. Multistage oxygen delignification of high-kappa pine kraft pulp with peroxymonosulfuric acid (Px). Holzforschung 68, 497–504. Janson, J., 1970. Calculation of polysaccharide composition of wood and pulp. Pap. Puu. 52, 323–329. Johansson, B., Mjöberg, J., Sandström, P., Teder, A., 1984. Modified continuous kraft pulping: now a reality. Svensk Papperstidn. 87, 30–35. Kangas, P., Kaijaluoto, S., Määttänen, M., 2014. Evaluation of future pulp mill concepts: reference model of a modern Nordic kraft pulp mill. Nordic Pulp Pap. Res. J. 29, 620–634. Keyoumu, A., Sjödahl, R., Henriksson, G., Ek, M., Gellerstedt, G., Lindström, M., 2004. Continuous nano- and ultrafiltration of kraft pulping black liquor with ceramic filters: a method for lowering the load on the recovery boiler while generating valuable side-products. Ind. Crops Prod. 20, 143–150. Li Jansson, Z., Brännvall, E., 2014. Effect of kraft cooking conditions on the chemical composition of the surface and bulk of spruce fibers. J. Wood Chem. Technol. 34, 291–300. Lindgren, C., Lindström, M., 1995. Thermal decomposition of inorganic polysulfides at kraft cooking conditions. Nordic Pulp Pap. Res. J. 10, 41–45. Mesfun, S., Lundgren, J., Grip, C.-E., Toffolo, A., Nilsson, R., Rova, U., 2014. Black liquor fractionation for biofuels production: a techno-economic assessment. Bioresour. Technol. 166, 508–517. Paananen, M., Tamminen, T., Nieminen, K., Sixta, H., 2010. Galactoglucomannan stabilization during the initial kraft cooking of Scots pine. Holzforschung 64, 683–692. Paananen, M., Liitiä, T., Sixta, H., 2013. Further insight into carbohydrate degradation and dissolution behavior during kraft cooking under elevated alkalinity without and in the presence of anthraquinone. Ind. Eng. Chem. Res. J. 52, 12777–12784. Paananen, M., Rovio, S., Liitiä, T., Sixta, H., 2015. Stabilization, degradation, and dissolution behavior of Scots pine polysaccharides during polysulfide (K-PS) and polysulfide anthraquinone (K-PSAQ) pulping. Holzforschung. http:// dx.doi.org/10.1515/hf-2014-0220. Sanyer, N., Laundrie, J.F., 1964. Factors affecting yield increase and fiber quality in polysulfide pulping of loblolly pine, other softwoods, and red oak. Tappi 47, 640–652. Schlesinger, R., Götzinger, G., Sixta, H., Friedl, A., Harasek, M., 2006. Evaluation of alkali resistant nanofiltration membranes for the separation of hemicellulose from concentrated alkaline process liquors. Desalination 192, 303–314. Suchy, M., Argyropoulos, D., 2002. Catalysis and activation of oxygen and peroxide delignification of chemical pulps: a review. Tappi J. 1, 9–26. Swan, B., 1965. Isolation of acid-soluble lignin from the Klason lignin determination. Svensk Papperstidn. 68, 791–795. Tasman, J.E., 1959. The permanganate consumption of pulp materials. IV. The KAPPA correction coefficient. Pulp Pap. Can. 60, 231–235. Teder, A., 1969. Some aspects of the chemistry of polysulfide pulping. Svensk Papperstidn. 72, 294–303. Vaaler, D., 2008. Yield-Increasing Additives in Kraft Pulping: Effect on Carbohydrate Retention, Composition and Handsheet Properties (Ph.D. thesis). Norwegian University of Science and Technology. Wallberg, O., Jönsson, A.-S., 2003. Influence of the membrane cut-off during ultrafiltration of kraft black liquor with ceramic membranes. Chem. Eng. Res. Des. 81, 1379–1384. Wallberg, O., Jönsson, A.-S., 2006. Separation of lignin in kraft cooking liquor from a continuous digester by ultrafiltration at temperatures above 100 °C. Desalination 195, 187–200. Young, R.A., Liss, L., 1978. A kinetic study of the alkaline endwise degradation of gluco- and galactomannans. Cellul. Chem. Technol. 12, 399–411.
