Pulping of white birch: How to maximize yield
Transcription
Pulping of white birch: How to maximize yield
KRAFT PULPS Pulping of white birch: How to maximize yield By C. Luthe, R. Berry and K. Parsons Abstract: The individual contributions to yield gain provided by PS, AQ, and reduced AA charge were evaluated. At 20% AA, AQ (0.05% on wood), PS (1.5-1.8% on wood), and PSAQ increased the yields from white birch by 0.5%, 1.1%, and 1.6% respectively versus conventional kraft pulping. This yield gain was independent of the AA charge applied - 16, 17, and 20%. However, for kraft, kraft-AQ, PS, and PSAQ pulping processes, each 1% reduction in AA charge increased pulp yield by 0.5%. OLYSULPHIDE (PS) is recognized as an effective additive for increasing yield in kraft pulp mills [10,11,14,17,20, 22,27,30]. Its wider application may be encouraged by the invention of Paprican’s Paprilox® process [16] which produces polysulphide liquor in the recausticizing plant [13]. Paprilox® allows for the production of polysulphide liquor with less capital investment than the competing MOXY™ [23] and Chiyoda [18] processes. There have been several successful pilot plant and full scale batch trials involving the generation of polysulphide by the Paprilox® process at Canadian mills [4,8,9] and Paprilox® has undergone successful long term trials in the hardwood fibreline of a Canadian kraft mill [1,2]. While polysulphide is an effective additive for increasing yield, further yield gains in hardwood pulping can be provided by lowering the AA charge [5,29]. The reduced AA charge provides yield gain by minimizing the dissolution of alkalilabile xylan [28], the predominant hemicellulose in hardwoods. The disadvantage of this approach, however, is that the delignification rate is lowered. The original rate can be recovered by adding AQ which accelerates the cooking process [7,12,21] and further improves pulp yield [7]. This pulping strategy of lowering AA and adding AQ is the one that was adopted for the first millscale installation of Paprilox® [2]. Although the overall yield increase associated with the new cooking operations was measured with hanging baskets [17] and evaluation of longer term operating data [16], the individual contributions to the overall yield improvement provided by the PS, AQ, and reduced AA charge were not identified. As such information is valuable for optimizing PS performance, these individual contributions have now been evaluated in laboratory pulping experiments. P EXPERIMENTAL White birch chips were air-dried (90-95% solids content) and classified (2-6 mm) using a Domtar Classifier. The active alkali charges used ranged from 16 to 20% (expressed as Na2O on o.d. wood). For kraft cooking, the sulphidity (AA basis) was maintained at 34%; for the PS cooks, it was 17%. The PS charge on wood ranged from Pulp & Paper Canada T 277 1.5 to 1.8% (o.d. wood basis), depending on the AA charge. Anthraquinone, when used, was charged at the 0.05% level (o.d. wood basis). A liquor-to-wood ratio of 3.5:1 (including the moisture in the chips) was used unless otherwise specified. For all cooks (200 g chips, o.d. basis) the temperature was raised to 170 °C in 90 minutes and held at 170 °C for the time needed to attain the target H-factor. Carbohydrates in chloritedelignified pulp were measured using high performance anion-exchange chromatography coupled with amperometric detection [15]. To assess the error in yield and kappa measurements of kraft and polysulphide cooking of air-dried white birch, 6 replicate kraft and PS cooks were done at 1400 H-factor using a 17% AA charge. For the kraft cooks, the average yield was 50.2% with a standard deviation of 0.1 and the average kappa number was 16.7 with a standard deviation of 0.2. For the PS cooks, the average yield was 50.9% with a standard deviation of 0.2, and the average kappa number was 16.0 with a standard deviation of 0.3. C. LUTHE Paprican Pointe-Claire, QC RESULTS AND DISCUSSION Air-Dried Chips The initial work for this yield study was done using never-dried, white birch chips, but the non-uniform moisture distribution in the relatively small charges (200 g o.d.) led to poor reproducibility in the yield measurement of replicate cooks. This irreproducibility was overcome by using air-dried chips. However, because of a literature report that there is a 2% yield penalty associated with PSAQ cooking of air-dried spruce relative to its neverdried form [6], the effect of air-drying the birch on pulp yield was also evaluated. Triplicate PSAQ cooks were done using both air-dried and neverdried chips (68% solids). The data in Fig. 1 indicate that air-dried and never-dried birch chips provide comparable results but with a small advantage, rather than disadvantage, in yield being seen with air-dried chips. Chip Impregnation Good chip impregnation is critical for generating reliable delignification and yield data, so the impregnation methodology was validated. Both air-dried chips and re-wetted, air-dried chips were R. BERRY Paprican Pointe-Claire, QC K. PARSONS Paprican Pointe-Claire, QC 106:12 (2005) ❘❘❘ 97 KRAFT PULPS FIG. 1. Under PSAQ cooking conditions, air-dried and never-dried white birch gave comparable yields. FIG. 2. Both air-dried chips and air-dried chips subsequently saturated with water provided equivalent pulp yields in kraft and PS pulping. FIG. 3. At 20% AA, 1500, 800, 650, and 500 H were required to reach 15 kappa in the kraft, kraft-AQ, PS, and PSAQ cooking processes respectively. FIG. 4. A reduction in AA charge from 20 to 17% AA curtailed delignification to the extent that the kraft, kraft-AQ nor PS processes could decrease the kappa number to 15. exposed to liquor under vacuum in the cooking vessels. Six replicate cooks were done with both sets of chips, under kraft and PS cooking conditions. A constant liquor to wood ratio of 3.5:1 (including the moisture in the chips) was maintained in each case. The data in Fig. 2 clearly show that both air-dried chips and re-wetted, air-dried chips provide equivalent pulp yields. For the kraft cooks, however, delignification was faster for the air-dried chips which is likely caused by faster diffusion of cooking liquor into the void spaces of the dry chips. Delignification of White Birch The relative rates of delignification for kraft, kraft-AQ, PS, and PSAQ cooking of white birch were evaluated over a range of H-factors between 600 and 2000 H, the exact range used being dependent on the cooking process. As an active alkali charge of 20% AA was the upper limit at the kraft mill that was implementing Paprilox®, this AA level was selected as the upper limit for our laboratory studies. The kappa vs 98 ❘❘❘ 106:12 (2005) H-factor relationship at 20% AA is shown in Fig. 3. As expected, delignification was accelerated by AQ [7,12,21], particularly when AQ was added to the kraft process. While 1500 H was required to reach 15 kappa under kraft conditions, only 800 H was required when AQ was added. PS cooking was even faster than kraft-AQ cooking; the H-factor requirement for 15 kappa was only 650. With the combined addition of PS and AQ, the H-factor requirement dropped to 500. The enhanced delignification rates for the cooks containing AQ was expected, because AQ is a delignification catalyst. It is also recognised, however, that some of the increase in rate can be attributed to higher alkali levels during bulk delignification [24]. Reduced EA consumption has been related to the prevention of the peeling reaction and consequent minimising of the formation of sugar acids, which would otherwise consume alkali [12]. The peeling reaction is prevented by both AQ and PS and it should therefore be expected that reduced EA consump- tion and pulping acceleration should be seen with both. This expectation is borne out by the results obtained; the KAQ, PS and PSAQ cooks had residual effective alkali concentrations of 11 to 12 g/L compared to 7 g/L for the kraft cook. Pulping studies were also done at 17% AA, the lower AA target under which the first mill-scale installation of Paprilox® would operate. As shown in Fig. 4, the pulping rate was much lower at 17% AA than at 20% AA; a delignification plateau was reached at about 16.5 kappa and the kappa number target of 15 could not be achieved with either the kraft or PS process. Furthermore, in contrast to the results at 20% AA, AQ only marginally increased the pulping rate over that of kraft. The kappa numbers obtained were similar to those from the kraft cook, ranging from 16.1 to 16.9 (avg = 16.6; sd = 0.3); only the PSAQ cook gave kappa numbers close to 15. Three replicate PSAQ cooks at 1400 H gave kappa numbers ranging from 15.1 to 15.6. The slower pulping rate for cooking done at 17% T 278 Pulp & Paper Canada KRAFT PULPS FIG. 5. Pulp yields for white birch at 17% AA (data above dashed line) and 20% AA (data below dashed line). Lower AA charges gave higher yields. FIG. 6. Pulp yields for white birch at 16 kappa number for cooks done with 16, 17 and 20% AA. Lower AA charges gave higher yields. FIG. 7. At the three AA charges studied, AQ (0.05%, o.d. wood basis), PS (1.5-1.8%, o.d. wood basis), and PSAQ increased the yield of white birch at 16 kappa number by 0.5%, 1.1%, and 1.6% on wood respectively. FIG. 8. For kraft, kraft-AQ, PS, and PSAQ processes, a 1% reduction in AA charge increased pulp yield by ~0.5% on wood. AA, and the ineffectiveness of the additives to substantially accelerate it, are likely due to low EA concentrations during bulk delignification. This is reflected in low residual EA levels: as low as 3.6 g/L for the kraft process and only 5.3 g/L for the PSAQ process. The low EA levels during cooking may also have promoted lignin condensation reactions, resulting in hard-to-remove residual lignin [19] and the observed delignification plateau. These results show that any yield enhancement strategy based on AA reduction has to be applied carefully and that an acceptable residual alkali concentration must be maintained to ensure smooth pulping and recovery operations. The positive results obtained with the Paprilox® installation at Domtar Espanola [2], however, show that this strategy can be implemented without problems. Pulp Yields for White Birch Pulp yields were determined for the cook- Pulp & Paper Canada T 279 ing experiments at both 17 and 20% AA. The yield vs kappa data for all the individual pulps are shown in Fig. 5. As only limited kappa ranges were obtained for all but the kraft cooks at 17% AA, it was not possible to draw reliable linear regression lines through the various data sets. However, even without regression lines, a comparison of the data at 17 and 20% AA shows that a severe yield penalty is associated with pulping when using the higher AA charge; the PSAQ process at 20% AA gave a yield-kappa relationship very similar to that for the kraft process at 17% AA. Figure 6 offers an alternative view of the combined effects of pulping process and AA charge on pulp yield. The data for cooks at 17 and 20% AA were supplemented with data from cooks done at 16% AA. The pulp yields shown in Fig. 6 each represent an average of 3 to 6 replicate cooks. As most of the average kappa numbers for the pulps were within 1 unit of 16 kappa, all yield measurements reported in Fig. 6 were “normalized” to 16 kappa using the relationship 0.23% yield gain (or loss) per kappa unit cited for trembling aspen [25]. A similar relationship was found in our work for birch kraft cooks at 17% AA, Fig. 5, where the yield loss per kappa unit for white birch was found to be 0.22. The data in Fig. 6 once again underline the importance of the pulping process and alkali charge on pulp yield. As an extreme example, cooking with PSAQ at 16% AA provided a 3.8% yield advantage over the kraft process at 20% AA. The prohibitively large penalty of such a cooking strategy with regards to the delignification rate, however, would render it uneconomical for mill implementation. Clearly, a compromise must be struck between maximizing pulp yield and maintaining acceptable pulp production rates. Figure 7 shows the relative yield increases provided by the additives AQ, PS and PSAQ over the active alkali range of 106:12 (2005) ❘❘❘ 99 KRAFT PULPS FIG. 9. At 17% AA, PS preserves xylan, increasing xylan yield by 1.0 and 0.6% on wood when applied in the kraft and kraft-AQ processes respectively. In contrast, cellulose yield is unaffected by PS addition. 16 to 20%, compared to the control kraft cook. For all three AA charges, the average yield increase over the control kraft cook provided by AQ, PS, and the combined addition of PS and AQ was 0.5%, 1.1%, and 1.6% respectively, and appeared to be independent of AA charge. Furthermore, for white birch, no synergy from the combined addition of PS and AQ was observed for the three alkali charges evaluated. In view of the faster delignification provided by the additives AQ, PS, and PSAQ at 20% AA compared to 17% AA, the observed independence of yield gain from AA charge is unexpected. Under otherwise identical conditions, faster delignification should enhance yield. One explanation for this discrepancy may be that the dissolution of alkali-sensitive xylan towards the end of the 20% AA cook negates the yield gain provided by the faster pulping rate. The higher residual alkali levels found when cooking with additives, and the observed reduction in xylan for PS pulps (see carbohydrate section), support this hypothesis. Figure 8 offers another way to evaluate the effect of AA charge on pulp yield. For the 4 types of cooking processes examined, yield (at 16 kappa) was plotted against AA charge. Linear relationships, with slopes ranging from -0.49 to -0.58, were found for all 4 processes. Therefore, for K, KAQ, PS, and PSAQ cooking, a 1% change in AA charge translates into a ~0.5% change in pulp yield. Decreases in yield of 0.5% and 0.4% per one percent increase in AA charge have also been reported for birch [29] and hardwoods in general [26]. Carbohydrates in White Birch To determine which carbohydrates are responsible for the observed yield increases provided by AQ, PS, and PSAQ, and the decrease in AA charge, carbohydrate anal- 100 ❘❘❘ 106:12 (2005) FIG. 10. At 20% AA, xylan yield is decreased by 0.5% on wood during pulping with PS but cellulose yield is increased by between 1.5 and 1.9%. The cellulose yield increase is a consequence of accelerated cooking. ysis was done on chlorite delignified pulps from the 4 processes generated at 17% AA (kappa range: 15.6 to 16.6) and at 20% AA (kappa range: 14.2 to 15.4). The results are shown in Figs. 9 and 10. As the carbohydrate analysis only determines the percentage of each carbohydrate in a pulp sample, and not the carbohydrate yield, the carbohydrate yields presented in the figures are derived from corresponding pulp yields of the 8 distinct pulp samples analyzed for carbohydrates. The numbers above the bars represent the actual lignin-free pulp yields, always based on cooking data. Lignin-free yield was calculated using the relationship, percent lignin in brownstock = 0.15 x kappa. As shown in Figs. 9 and 10, the primary carbohydrate constituents in white birch are cellulose (glucose) and xylan (xylose). The data indicate that the protection provided by PS is dependent on AA charge. At 17% AA, PS preserves xylan, increasing xylan yield, shown by shaded bars in Fig. 9, by 1.0 and 0.6% when applied in the kraft and kraft-AQ processes respectively. In contrast, cellulose yield is unaffected by PS addition at this AA charge. Despite the lower H-factor requirements to reach the target kappa number, when PS is used in kraft or KAQ cooking at 20% AA, there is a 0.5% loss in xylan, relative to the kraft or KAQ process. This decrease, not observed at 17% AA, is probably the result of a higher alkali concentration during the cook caused by a lower rate of alkali consumption when PS is used at high rather than low AA levels. The difference in response, with AA charge, may also indicate different rates of hexenuronic acid formation and degradation at different AA levels [3]. While PS appears to be protecting cellulose at 20% AA, providing a 1.9 and 1.5% cellulose yield increase when applied in the kraft and kraft-AQ processes respectively, this yield increase is more likely a consequence of the lower H-factor requirements to reach the target kappa number. The observation that PS did not conserve cellulose at 17% AA supports this conclusion. Although the magnitude of the effects are smaller, AQ produces the same results as PS; at 17% AA, AQ provides a 0.1 to 0.5% yield increase in xylan, while at 20% AA, it provides a 0.3 to 0.7% yield increase in cellulose. The results also show that both xylan and cellulose are aggressively removed in the kraft process when applied under severe conditions of high AA charge. The application of AQ and/or PS at these higher AA charges improves cellulose retention by reducing H-factor requirements. SUMMARY AND CONCLUSIONS • AQ (0.05% on wood), PS (1.5-1.8% on wood), and PSAQ increased the yields from white birch by 0.5%, 1.1%, and 1.6% on wood respectively. • With white birch, no synergy was observed from the combined addition of PS and AQ. • For kraft, kraft-AQ, PS, and PSAQ pulping of white birch, a 1% reduction in AA charge increases pulp yield by ~0.5% on wood. • At low AA charges (17%), the yield increase from the application of PS in birch pulping is through protection of xylan. At high AA charges (20%), the yield increase is a consequence of retaining cellulose through accelerated cooking. • White birch pulp has a variable residual lignin content, depending on the pulping process, beyond which it cannot be delignified, even under high alkali charges and with prolonged cooking schedules. • In contrast to literature reports, PSAQ cooking of air-dried and never-dried white birch provided comparable yield benefits. T 280 Pulp & Paper Canada KRAFT PULPS ACKNOWLEGEMENTS The authors would like to thank Lisette Nadeau for the carbohydrate analyses, Jie Wang and Arnold Dort for their contributions to the pulping studies, and Vic Uloth for his careful review of the manuscript. LITERATURE 1. GRIFFIN, C.W., KUMAR, K.R., GRATZL, J., JAMEEL, H. Effects of adding anthraquinone and polysulfide to the modified continuous cooking (MCC) process. 1995 TAPPI Pulping Conference Proceedings, TAPPI Press, Atlanta, vol. 1, p. 19 (1995). 2. JIANG, J.E. Extended modified cooking with polysulfide for simultaneous pulp yield and strength improvement. 1992 TAPPI Pulping Conference Proceedings, TAPPI Press, Atlanta, vol. 2, p. 683 (1992). 3. JIANG, J.E. Extended modified cooking of southern pine with polysulfide: effects on pulp yield and physical properties. Tappi J. 77(2):120 (1994). 4. KLEPPE, P.J. Polysulfide pulping in a dual-vessel Kamyr digester. Tappi J. 58(8):172 (1975). 5. LANDMARK, P.A., KLEPPE, P.J., JOHNSEN, K. Cooking liquor oxidation and improved cooking technique in polysulfide pulping. Tappi J. 48(5):56 (1965). 6. SANYER, N., LAUNDRIE, J.F. Factors affecting yield increase and fiber quality in polysulfide pulping of loblolly pine, other softwoods, and red oak. Tappi J. 47(10):640 (1964). 7. NISHIJIMA, H., INABA, R., SMITH, M. Review of polysulfide/AQ pulping to date in Japanese kraft mills and the impact on productivity. 1995 TAPPI Pulping Conference Proceedings, TAPPI Press, Atlanta, vol. 1, p. 31 (1995). 8. YAMAGUCHI, A. Operating experiences with the MOXY Process and quinoid compounds. 1983 TAPPI Pulping Conference Proceedings, TAPPI Press, Atlanta, vol. 2, p. 544 (1983). 9. DORRIS, G.M. Process of producing kraft pulping liquor by the oxidation of white liquor in the presence of lime mud. US patent No. 5,082,526 (1992). 10. DORRIS, G.M. Oxidation of white liquor in the presence of lime mud. Pulp & Paper Can. 95(10):44 (1994). 11. SMITH, G.C., KNOWLES, S.E., GREEN, R.P. All it takes is MOXY: Mead oxidation system generates polysulfide liquor. Paper Trade J. 159:38 (1975). 12. NAKAMURA, M., ONO, T. Production of polysulfide using a new catalyst. 1988 TAPPI Pulping Conference Proceedings, TAPPI Press, Atlanta, vol. 2, p. 407 (1988). 13. ULOTH, V.C., DORRIS, G.M., THRING, R.W., HOGIKYAN, R.M., WEARING, J.T. In-situ production Pulp & Paper Canada T 281 of polysulfide liquor in a kraft mill’s causticizers. Part I. Pilot plant trials. Pulp & Paper Can. 97(4):38 (1996). 14. ULOTH, V., DORRIS, G., THRING, R., HOGIKYAN, R., WEARING, J., TENCH, L., AYTON, J. Production of polysulfide liquor in a kraft mill’s causticizers. Tappi J. 80(10):223 (1997). 15. TENCH, L., ULOTH, V., DORRIS, G., HORNSEY, D., MUNRO, F. Mill-scale implementation of Paprican’s process for polysulphide liquor production in kraft mill causticizers. Part I. Batch trials and process optimization. Tappi J. 87(10):120 (1999). 16. MUNRO, F., ULOTH, V., TENCH, L., MACLEOD, M., DORRIS, G. Mill scale implementation of Paprican’s process for polysulphide liquor production in kraft mill causticizers. Part II: Results of pulp mill production trials. Pulp & Paper Can. 103(1):57 (2002). 17. MACLEOD, M., RADIOTIS, T., ULOTH, V., MUNRO, F., TENCH, L. Basket cases IV: Higher yield with Paprilox polysulphide-AQ pulping of hardwoods. Tappi J. [New Series] 1(8):3 (2002). 18. AURELL, R. Kraft pulping of birch. Part 2. The influence of the charge of alkali on the yield, carbohydrate composition and properties of the pulp. Svensk Papperstid. 67(13):89 (1964). 19. ACHRÉN, S., HULTHOLM, T., LÖNNBERG, B., KETTUNEN, A., JIANG, J.E., HENRICSON, K. Improved pulp yield by optimized alkaline profiles in kraft delignification of birch wood. 1998 Breaking the Pulp Yield Barrier Symposium Proceedings, TAPPI Press, Atlanta, p. 91 (1998). 20. AURELL, R. Kraft pulping of birch. Part 1. The changes in the composition of the wood residue during the cooking process. Svensk Papperstidn. 67(2):43 (1964). 21. PARTHASARATHY, V.R., SMITH, G.C., RUDIE, G.F., DETTY, A.E., STEFFY, J.J. Application of anthraquinone in extending the delignification of kraft and polysulfide pulps. Part 1: Pulping and bleaching of mixed dense hardwoods. Tappi J. 78(2):113 (1995). 22. BLAIN, T.J. Low-sulphidity pulping with anthraquinone. Tappi J. 62(6):53 (1979). 23. LI, Z., MA, H., KUBES, G., LI, J. Synergistic effects of kraft pulping with polysulfide and anthraquinone on pulp yield improvement. J. Pulp Pap. Sci. 24(8):237 (1997). 24. SULLIVAN, J., DOUEK, M. Determination of carbohydrates in wood, pulp, and process liquor samples by high-performance anion-exchange chromatography with pulsed amperometric detection. J. Chromatogr. A 671:339 (1994). 25. KLEPPE, P.J., MINJA, R.J.A. The possibilities to apply polysulphide-AQ pulping in kraft mills. 1998 Breaking the Pulp Yield Barrier Symposium Proceedings, TAPPI Press, Atlanta, p. 113 (1998). 26. LÉMON, S., TEDER, A. Kinetics of the delignification in kraft pulping. Svensk Papperstidn. 76(11):407 (1973). 27. AXEGARD, P., WIKEN, J.E. Delignification studies - factors affecting the amount of “residual lignin”. Svensk Papperstidn. 86:178 (1983). 28. KEAYS, J.L., HATTON, J.V. Relationship of pulp yield with permaganate number and kappa number for kraft pulps: II Trembling aspen (populus tremuloides michx.). Pulp & Paper Mag. Can. 73(10):100 (1972). 29. KLEPPE, P.J. Kraft pulping. Tappi J. 53(1):35 (1970). 30. JIANG, Z.-H., LIEROP, B.V., BERRY, R. Hexenuronic acid groups in pulping and bleaching chemistry. Tappi J. 83(1):167 (2000). Résumé: Nous avons évalué la contribution individuelle au gain de rendement du PS, de l’AQ, et d’une charge réduite d’AA. À une teneur de 20% en AA, l’AQ (0,05 % sur le bois), le PS (1,5 à 1,8 % sur le bois), et le PSAQ ont accru le rendement du bouleau à papier de 0,5 %, 1,1 %, et 1,6 % respectivement, comparativement au procédé de mise en pâte kraft classique. Ce gain de rendement était indépendant de la charge en AA appliquée - 16, 17, et 20 %. Toutefois, pour les procédés de mise en pâte kraft, kraft-AQ, PS, et PSAQ, chaque 1 % de réduction de la teneur en AA a fait augmenter le rendement de la pâte de 0,5 %. Reference: LUTHE, C., BERRY, R., PARSONS, K. Pulping of white birch: How to maximize yield. Pulp & Paper Canada 106(12):T277-281 (December, 2005). Paper presented at the 90th Annual Meeting in Montreal, QC, Canada, January 26-29, 2004. Not to be reproduced without permission of PAPTAC. Manuscript received November 11, 2003. Revised manuscript approved for publication by the Review Panel on September 20, 2004. Keywords: BETULA PAPYRIFERA, YIELD, KRAFT PULPS, ANTHRAQUINONE, ACETIC ACID, POLYSULFIDE PULPING, POLYSULFIDES. 106:12 (2005) ❘❘❘ 101