Effect of Hemicellulose Pre-extraction on the Properties and

Jul 27, 2012 - with dilute sulfuric acid on the properties and bleachability of aspen (Populus tremuloides) chemithermomechanical pulp (CTMP)...
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Effect of Hemicellulose Pre-extraction on the Properties and Bleachability of Aspen (Populus tremuloides) Chemithermomechanical Pulp Wei Liu,†,‡,§ Qingxi Hou,*,† Changbin Mao,‡ Zhirun Yuan,*,‡ and Kecheng Li§ †

Tianjin Key Laboratory of Pulp & Paper, Tianjin University of Science & Technology, Tianjin, China 300457 FPInnovations, Pointe-Claire, Quebec, Canada H9R 3J9 § Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3 ‡

ABSTRACT: Pre-extraction of hemicelluloses prior to pulping and conversion of the extracted hemicelluloses to other byproducts will provide additional revenue to the traditional pulp and paper industry. The effect of hemicelluloses pre-extraction with dilute sulfuric acid on the properties and bleachability of aspen (Populus tremuloides) chemithermomechanical pulp (CTMP) was investigated. The acid pre-extraction significantly reduced refining energy consumption to a given freeness. However, at the same freeness, the acid pre-extraction resulted in the reduction of bulk, tensile index, and zero-span breaking length, fewer long fibers and more fines, but an increase of the Scott bond strength. Alkaline peroxide bleaching can compensate the strength loss, but the bleachability of pulps with acid pre-extraction was lower than that of the control.

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INTRODUCTION Chemithermomechanical pulp (CTMP)/bleached chemithermomechanical pulp (BCTMP) has been widely used in the pulp and paper industry with increasing applications in many paper products such as printing and writing paper, paperboard, and hygiene products.1−5 Hardwood is one typical wood species for making CTMP/BCTMP and contains 22% hemicelluloses on average.6 Most hardwood hemicelluloses are xylans that are estimated to account for one-third of all renewable organic carbon available in nature.7−9 Part of the hemicelluloses in wood can be potentially extracted from wood prior to making pulp. The extracted hemicelluloses could be used to produce value-added bioproducts (e.g., furfural, xylitol,10 sorbitol, bifunctional organic molecules, barrier films,11,12 and hydrogels13). This is the so-called “value prior to pulping (VPP)” concept.14,15 VPP aims to partially or completely pre-extract hemicelluloses for producing high-value-added byproducts or for biofuel production while using the remains (mainly cellulose and lignin) for wood pulp production.16 Therefore, a delicate balance is required between the pre-extraction and the resultant pulp properties. So far VPP has been studied mainly for Kraft pulping processes.17−20 Few attempts have been made from traditional mechanical pulping processes such as CTMP. Preextraction of hemicelluloses from wood chips prior to pulping can not only offer new feedstocks for the production of chemicals and fuels, but also remedy some of the operational problems in pulp and paper mills6 such as the reduction of dissolved and colloidal substances (DCS) from dissolved hemicelluloses in a typical CTMP process. Also, the hemicellulose pre-extraction with sulfuric acid can help reduce the refining energy, which has been confirmed by our previous work.21 In the previous work,21 up to 11% of xylan could be possibly extracted from wood chips in the CTMP process, and the chemical compositions of the pulp fibers were analyzed. © 2012 American Chemical Society

The carbohydrate streams obtained from acid pre-extraction could be a potential hemicellulose source for producing bioproducts due to higher concentrations of hemicelluloses (arabinose, galactose, xylose, and mannose) in comparison with the control; the pulping yield loss was limited since the preextraction yields were about 95% for both CTMP2 and CTMP3. The effects of acid pre-extraction on aspen CTMP properties were also examined at the same refining energy to understand the sole effect of acid pre-extraction on the pulp properties. In this paper, the effect of acid pre-extraction on pulp properties at the same freeness was reported because freeness is one of the important customer specifications for market CTMP/BCTMP. Since the acid pre-extraction can significantly reduce the refining energy consumption to a given freeness and refining energy is known to have a significant effect on fiber development, it is expected that the difference in pulp properties at the same freeness with and without acid preextraction would be even larger than that at the same refining energy. Also, the effect of acid pre-extraction on the bleachability of the CTMP has not been known. Alkaline peroxide bleaching can have a significant effect on properties of BCTMP including brightness, strength, and optical properties.22−24 Therefore, the focus of this paper is the effect of acid pre-extraction on properties and bleachability of aspen CTMP at the same freeness. Received: Revised: Accepted: Published: 11122

January 30, 2012 July 27, 2012 July 27, 2012 July 27, 2012 dx.doi.org/10.1021/ie300265s | Ind. Eng. Chem. Res. 2012, 51, 11122−11127

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Experimental Procedure. The experimental procedure is shown in Figure 1. The screened aspen chips were press impregnated with dilute sulfuric acid and cooked under various temperatures and times to pre-extract hemicelluloses. The cooked chips were pressed and impregnated with sodium sulfite and then refined to produce CTMPs. The resultant CTMPs were bleached with alkaline peroxide for the production of BCTMPs. Hemicellulose Pre-extraction with Dilute Sulfuric Acid. The screened aspen chips were presteamed at 100 °C for 15 min. Then, an Andritz-Bauer 6 in. MSD press impregnator was used for the impregnation of screened chips with 0.5% (w/v) sulfuric acid at 60 °C. The resulted sulfuric acid charges for CTMP2 and CTMP3 were both 1.0% (w/w) based on the oven-dried wood chips. The impregnated chips were then cooked in the vapor phase of a digester under different conditions to pre-extract part of the hemicelluloses from wood chips, as shown in Table 1. After cooking, the cooked chips were pressed again to collect the pressates. CTMP Pulping Process. The CTMPs were produced in the pilot plant of FPInnovations, Canada. The screened aspen chips were impregnated with 1.5% (w/v) sodium sulfite (Na2SO3) in the press impregnator. After the impregnated chips were retained at 70 °C for 30 min and preheated at 100 °C for 3 min, the chips were then refined in two stages with a 22 in. Andritz pressurized TMP refiner and a Bauer 400 36 in. atmospheric refiner. Table 1 lists the conditions of these trials: CTMP1 (control without acid pre-extraction), CTMP2 (with acid pre-extraction at 110 °C for 30 min), and CTMP3 (with

Figure 1. Block diagram of the experimental procedure.

Table 1. Pulping Conditions for CTMP Trials CTMP1 CTMP2 H2SO4 on oven-dried wood chips, % pre-extraction temp, °C pre-extraction time, min concn of Na2SO3, % (w/v) ratio of oven-dried wood chips wt (kg) to Na2SO3 soln vol (L) preheating time, s refining press. in TMP refiner, kPa

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CTMP3

− − − 1.5 1:5

1.0 110 30 1.5 1:5

1.0 140 10 1.5 1:5

180 250

180 250

180 250

EXPERIMENTAL SECTION Materials. Aspen chips from an eastern Canadian mill were screened before the process. The chemicals used in this study were of analytical grade.

Figure 2. Total specific refining energy versus freeness and contents of shives, long fibers, and fines of CTMPs. 11123

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Figure 3. CTMP properties versus freeness.

acid pre-extraction at 140 °C for 10 min). The sulfonation levels of these CTMPs were the same. The contents of sulfonic groups for CTMP1, CTMP2, and CTMP3 were 17.2, 18.2, and 17.9 mmol/kg oven-dried pulp, respectively, which was determined according to the conductometric titration method proposed by Katz et al.25 As a result, the differences in pulp properties were mainly attributed to the acid pre-extraction. Bleaching of the Resultant CTMP. The bleaching was performed in a polyethylene bag according to the following three conditions: NaOH charge at 1.0, 2.0, and 3.0% at a fixed ratio of H2O2/NaOH of 2.0. The other fixed conditions were 2.0% Na2SiO3, 0.5% diethylene triamine pentacetate acid (DTPA), 20% bleaching consistency, 70 °C, and 120 min. All chemical dosages were based on the weight of the oven-dried CTMP. The bleaching liquor was prepared following the order of deionized water, Na2SiO3, NaOH, and H2O2. After the prepared bleaching liquor was thoroughly mixed with 40 g (oven dry) of CTMP with a Hobart mixer, the pulp suspension was put into a polyethylene bag. The bag was then sealed and placed into a water bath at 70 °C, and it was kneaded every 10 min in order to make the bleaching reaction uniform. Once the bleaching duration time was reached, the polyethylene bag was promptly taken from the water bath and placed into a cold water bath to cool the pulp to room temperature. After bleaching, the bleached CTMP was washed thoroughly with deionized water, and the pH value of the pulp suspensions was adjusted to 5−6 with sodium metabisulfite. Pulp Handling and Testing. The refined pulps were hotdisintegrated and screened with a four-cut plate. The pulp properties were determined according to PAPTAC standard

methods: C.8P (latency removal by hot disintegration with Domtar method); C.1 (Canadian standard freeness, CSF); C.5U (fiber classification by Bauer-McNett); C.4 (handsheets forming for physical tests of pulp); D.12 (physical and optical testing of pulp handsheets); D.27U (zero-span breaking length of pulp).

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RESULTS AND DISCUSSION

Effect of Hemicellulose Pre-extraction on Refining of CTMPs. Figure 2a shows that the acid pre-extraction

Figure 4. Bauer-McNett fiber fractions of CTMPs at a freeness of about 380 mL. 11124

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Figure 5. Optical properties of CTMPs before and after peroxide bleaching.

Table 2. Properties of CTMPs and Bleached Ones at a Freeness of about 380 mL

a

bulk (cm3/g)

tensile index (N·m/g)

CTMP1 CTMP2 CTMP3

3.47 3.26 2.90

10.33 8.67 9.85

BCTMP1 BCTMP2 BCTMP3

3.39 2.73 2.53

11.66 13.59 15.41

zero-span breaking length (km) CTMPs 10.34 7.39 7.26 Bleached CTMPsa 11.56 8.47 8.15

burst index (kPa·m2/g)

tear index (mN·m2/g)

Scott bond (J/m2)

0.43 0.37 0.40

1.59 1.31 1.44

32.40 37.00 41.60

0.59 0.61 0.73

1.92 1.84 1.86

45.20 60.20 73.00

Bleaching conditions: 3.0% NaOH, 6.0% H2O2, 2.0% Na2SiO3, 0.5% DTPA, 20% bleaching consistency, and 70 °C for 120 min.

60.08%. The results showed that, compared with the CTMP without acid pre-extraction (i.e., CTMP1), the CTMPs with acid pre-extraction (i.e., CTMP2 and CTMP3) had a somewhat lower bulk, tensile index, and zero-span breaking length at the same freeness, but a higher Scott bond. The lower bulk could be mainly attributed to the lower long fiber content and higher fines content at a similar freeness, as shown in Figure 4. The slight reduction of tensile index mainly resulted from the loss of a single fiber strength (as indicated by the zero-span breaking length) caused by the acid pre-extraction. The above results suggest that the acid pre-extraction could make the aspen fibers more fragile and easier to break down in the refining process, and as a result, the refining energy was reduced, as shown in Figure 2a. The higher Scott bond was mainly caused by the increased fines content after the acid pre-extraction. Bleachability of CTMPs. Compared to the CTMP without acid pre-extraction, the brightness of CTMPs with acid preextraction decreased 5−7 points. The strong cooking

significantly reduced the refining energy to a given freeness. Compared to the specific refining energy for CTMP1, the specific refining energy was accordingly reduced by 54 and 86% for CTMP2 and CTMP3 to reach a freeness of 400 mL, respectively. At the same refining energy, the shive content of the CTMP with acid pre-extraction was lower than that of the CTMP without acid pre-extraction (Figure 2b), which implies that the cost of screening and refining energy in rejects refining will be reduced for CTMPs with acid pre-extraction. Meanwhile, the long fiber content decreased and the fines content increased after the acid pre-extraction with dilute sulfuric acid, as shown in Figure 2c and Figure 2d, respectively. Properties of CTMPs at the Same Freeness. The properties of resultant CTMPs were compared at the same freeness, as shown in Figure 3. The control sample (CTMP1) can represent the properties of a typical CTMP pulp: Canadian standard freeness (CSF) 380 mL, bulk 3.47 cm3/g, tensile index 10.33 N·m/g, tear index 1.59 mN·m2/g, and ISO brightness 11125

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conditions (110−140 °C and 10−30 min in the pre-extraction) were probably the main reasons for the brightness loss. The question arose of whether the brightness loss could be compensated by the following alkaline peroxide bleaching. The effect of hemicellulose pre-extraction on bleachability of CTMP was also unclear. Therefore, three CTMPs with a freeness of about 380 mL were chosen for the bleaching investigation which was carried out at three different hydrogen peroxide charges of 2.0, 4.0, and 6.0%. The ratio of H2O2/ NaOH was kept at 2.0. The other fixed conditions were specified in the Experimental Section. It was found that the magnitude order of bleachability for these three CTMPs is CTMP1 > CTMP2 > CTMP3, as shown in Figure 5a. The brightness gain increased with increasing charges of sodium hydroxide and hydrogen peroxide. However, the brightness gain of CTMPs with acid pre-extraction was still lower than that of the CTMP without acid pre-extraction. Mainly due to the lower initial ISO brightness (60.08, 53.20, and 49.85% for CTMP1, CTMP2, and CTMP3, respectively) and lower bleachability, under the same charges of 3.0% H2O2 and 6.0% NaOH, the ISO brightnesses of the bleached CTMP2 and CTMP3 were lower than that of the control CTMP1 (72.70, 67.43, and 81.89%, respectively). More hydrogen peroxide is required for CTMP2 and CTMP3 to reach the same brightness as the bleached CTMP1. The reduction of pulp brightness can be mainly explained by (1) decreased light scattering coefficient and increased light absorption coefficient, as shown in Figure 5b and Figure 5c, respectively, which would decrease the brightness based on the theory of Kubelka−Munk,26 and (2) higher fines content in the CTMPs with acid pre-extraction (Figure 4). The contents of metal ions (e.g., Fe3+) in short fibers (fiber fraction of 100/200) and fines were higher than those in other pulp fiber fractions.26 This could lead to lower brightness and decrease the peroxide bleaching efficiency. In addition, the reduction of pulp brightness may also result from the lignin condensation and chromophore formation during the acid pre-extraction. Also, the light scattering coefficient of CTMPs with acid preextraction was lower than that of the CTMP without acid preextraction, and would decrease with increasing charges of NaOH and H2O2. It was reported that the fines in pulp were the major contributor to the high scattering capacity.27 By contrast, the light absorption coefficients of CTMPs with acid pre-extraction were much higher than that of the CTMP without acid pre-extraction, and would decrease due to increased charges of NaOH and H2O2. Physical Properties of Bleached CTMPs. The physical properties of resultant CTMPs and the bleached ones are listed in Table 2. As expected, alkaline peroxide improved pulp strength properties but reduced bulk. However, after bleaching, the tensile, burst, and Scott bond of BCTMP2 and BCTMP3 were higher than those of the BCTMP1, reversing the trend for unbleached pulps. This indicates that alkaline peroxide led to a larger improvement in strength for the pulps with acid preextraction. Before bleaching, the strength properties of CTMP2 and CTMP3 with acid pre-extraction were much lower than those of CTMP1. This could be mainly explained by the finding that alkaline peroxide bleaching tends to improve the swelling and bonding ability of fines. Figure 4 shows that pulps with acid pre-extraction had more fines than the control CTMP1. The large improvement in Scott bond by bleaching supported the above explanation. The lower zero-span breaking length for bleached pulps with acid pre-extraction further supported that

the increase in strength was mainly due to the improvement in fiber−fiber bonding. Therefore, alkaline peroxide can compensate the strength loss caused by the acid pre-extraction. Caution is required to preserve the bulk of BCTMP since this is also a very important property of market BCTMP.

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CONCLUSIONS The acid pre-extraction significantly reduced refining energy consumption to a given freeness. Compared with the CTMP without acid pre-extraction at the same freeness, the acid preextraction resulted in the reduction of bulk, tensile index, and zero-span breaking length, fewer long fibers, and more fines. Alkaline peroxide bleaching can compensate the strength loss (e.g., tensile index, burst index, and Scott bond), which indicates that alkaline peroxide led to a larger improvement in strength for the pulps with acid pre-extraction. This is explained by the fact that more fines are present in the pre-extracted pulps than in the control. The bleachability of acid preextracted pulp is lower than that of the control, and more peroxide is required for pre-extracted pulps to reach the same brightness as the control.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Q.H.); zhirun.yuan@ fpinnovations.ca (Z.Y.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge the financial support from National Science and Engineering Research Council of Canada (NSERC) Discovery Grant and the in-kind contribution from FPInnovations, Canada. The authors would also like to thank Michael Hellstern, Daniel Gilbert, and David Giampaolo for their help with the chip impregnation.

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REFERENCES

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