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Lime Treatment of Prehydrolysis Liquor from the Kraft-Based Dissolving Pulp Production Process Jing Shen,†,‡ Pedram Fatehi,*,‡,§ Pendar Soleimani,‡ and Yonghao Ni*,‡ †
Key Laboratory of Bio-based Material Science and Technology of Ministry of Education, Material Science and Engineering College, Northeast Forestry University, Harbin 150040, China ‡ Department of Chemical Engineering & Limerick Pulp and Paper Centre, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada § Department of Chemical Engineering, Faculty of Engineering, Lakehead University, Thunder Bay, Ontario P7B 5E1, Canada ABSTRACT: In this study, the concept of using lime for the treatment of industrially produced prehydrolysis liquor (PHL) of kraftbased dissolving pulp production process was tested, and its effect on the concentrations of acetic acid, furfural, UV lignin, and hemicelluloses was explored. Lime treatment resulted in a significant increase in the acetic acid concentration due to alkaline hydrolysis of the acetyl groups bound to the dissolved hemicelluloses in PHL, and the degradation of sugars. At relatively high lime dosages (e.g., above 1.7%), furfural was completely eliminated, while the lignin concentration decreased by 20 25%. The hemicelluloses concentration can be significantly decreased due to the alkaline oxidative degradation. However, when performed in the absence of oxygen/air by bubbling carbon dioxide to the lime/PHL system, the degradation of hemicelluloses was effectively minimized.
1. INTRODUCTION The manufacturing concept of converting renewable biomass for the production of valuable biofuels/biochemicals, sometimes referred to as the biorefinery, would be of strategic significance because such processes are renewable and sustainable.1 3 The prehydrolysis kraft-based dissolving pulp production process may be a demonstrable example of an integrated forest biorefinery. In this process, the majority of hemicelluloses or hemicellulosic sugars in the wood is depolymerized and solubilized,4 and then removed during the high-temperature prehydrolysis stage. The prehydrolysis liquor (PHL) also contains other bioproducts, including lignin, acetic acid, and furfural.5 11 Once economically and effectively recovered, the utilization of these dissolved organics in PHL would provide interesting integrated forest biorefinery possibilities. Hemicelluloses can be further treated and processed to produce many value-added products, such as bioethanol, furfural, and hydroxymethylfurfural, or used as functional papermaking chemicals/additives.7,12 14 Lignin and its derivatives have many applications, such as fuel for energy production, and starting material for producing polyurethane and carbon fibers.15 17 Acetic acid and furfural, if recovered in a reasonable purity, can be directly utilized to meet various industrial needs. In the literature, the treatment of hemicelluloses-rich biomass hydrolyzates using lime has been widely addressed,18 27 and such a treatment has been found to be effective in the detoxification of the hydrolyzates to make them more suitable for the subsequent fermentation process to produce such products as ethanol. Generally speaking, the relevant available research reports on treatment of biomass hydrolyzates with lime have been focused on the hydrolyzates from acid pretreatments, although lime treatment of lignocellulosic hydrolyzates produced from steam explosion is also available in the literature.28 However, there have been no reports regarding the application of the r 2011 American Chemical Society
lime treatment concept to industrially produced hydrolyzates from steam or hot water hydrolysis at high temperature (e.g., 170 °C), a critical stage of the commercial prehydrolysis kraftbased dissolving pulp production process.5 For the steam/hot water prehydrolysis or autohydrolysis29 process, it is worth noting that the release of sticky lignin-derived compounds can hinder the recovery of value-added products and their commercial practices.30 32 These compounds need to be removed to facilitate the downstream processing and utilization of the dissolved organics. Also, the removal of inhibitors (e.g., dissolved lignin and furfural) from the pre-extracts is a necessary step to utilize the sugars to produce such products as bioethanol via a fermentation process. In this study, we applied the concept of lime treatment to an industrially produced PHL of kraft-based dissolving pulp production process, which contains unique compositions as compared to other hydrolyzates.5 It was assumed that treating PHL with lime may result in the reduction of the amount of some fermentation inhibitors (lignin and furfural), while the acetyl groups bound to the dissolved hemicelluloses might be liberated5 under the alkaline condition. On the basis of such an assumption, the effect of lime treatment on the concentrations of acetic acid, furfural, UV lignin, and hemicelluloses was explored.
2. EXPERIMENTAL SECTION 2.1. Materials. Calcium oxide was purchased from SigmaAldrich Co. The prehydrolysis liquor (PHL) produced from a mixture of 70% maple, 20% poplar, and 10% birch10,33 was Received: August 27, 2011 Accepted: November 17, 2011 Revised: November 4, 2011 Published: November 17, 2011 662
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collected from a mill located in Eastern Canada. The prehydrolysis was conducted by using the VisCBC technology, and the PHL was produced via steaming at 170 °C for 30 min.10,33 To remove the large particles/impurities, the PHL was first thoroughly filtered using two filter papers and then a Nylon 66 membrane filter (47 mm diameter membrane with pores of 0.45 μm) obtained from Supelco Analytical Group, U.S. By using the membrane filtration, some lignin fragments/precipitates30 32 with relatively large particle sizes can be removed from the PHL. 2.2. PHL Treatment. In one set of experiments, the lime was mixed with PHL in an Erlenmeyer flask and shaken at 150 rpm and 78 °C for 1 h. In another set of experiments, to conduct the lime treatment in the absence of oxygen/air, calcium oxide was mixed with PHL in a Pyrex gas-washing bottle (Sigma-Aldrich) at 78 °C for 1 h, while carbon dioxide was slowly bubbled into the aqueous mixture to exclude oxygen/air from the system. During this process, the mixing was achieved by carbon dioxide bubbling only, and no extra mixing (e.g., shaking or stirring) was applied. After being cooled to room temperature using running tap water, the mixtures were filtered through the Nylon 66 membrane filter mentioned earlier, and the filtrates, that is, treated PHLs, were then collected. The lime dosages (wt %) reported throughout this Article were all based on the weight of the original PHL (untreated PHL). 2.3. Dissolved Lignocelluloses Analysis. The lignin (UV lignin) concentrations of the lime-treated PHLs were determined on the basis of a UV/vis spectrometric method at a wavelength of 205 nm according to Tappi UM 250.6,8,10,28 The hemicelluloses concentrations of the lime-treated PHLs were measured using an ion chromatography unit equipped with a CarboPac PA1 column (Dionex-300, Dionex Corp., U.S.) and a pulsed amperometric detector (PAD), by following the previously applied procedures.6,8 10,29 For some lime-treated PHLs, to measure the amount of hemicelluloses adsorbed on calcium hydroxide particles (the reaction products of lime with the water in PHL), the filter cakes were collected (during the lime treatment process) and dissolved for subsequent sugar analysis. The acetic acid and furfural concentrations of the lime-treated PHLs were determined by using a Varian 300 1H NMR spectrometer. The principles and procedures relevant to this method have been published elsewhere.5,8,9,33 35 To measure the amount of acetyl groups in the PHL, the PHL was treated with sulfuric acid to totally hydrolyze the acetyl groups by following the same procedure for the measurement of total hemicelluloses concentration;6,8 10,33 the difference between the acetic acid concentration of the treated PHL and that of the untreated PHL represents the amount of acetyl groups.
Figure 1. The pH of lime treated PHL as a function of lime dosage (based on the weight of PHL).
pH (see Figure 1) with lime addition is due to the solubility limit of the formed calcium hydroxide from the slaked lime. 3.2. Effect of Lime Treatment on Acetic Acid Concentration. For the PHL of kraft-based dissolving pulp production process produced from steam (or hot water) hydrolysis at high temperature, the dissolved hemicellulosic components can be still highly acetylated.5,37 These acetyl groups in hemicelluloses can potentially be utilized as acetic acid when further hydrolyzed. In this study, the acetic acid concentration of the PHL was 10.11 g/L, and the amount of acetyl groups bound to the dissolved hemicelluloses of the PHL was 7.34 g/L (calculated on the basis of acetic acid). When lime was used for the treatment of PHL, its effect on the acetic acid concentration in the treated PHL is shown in Figure 2. It should be noted that the quantitative proton NMR concept5,8,9,33 35 determines the acetic acid concentration of the PHL based on the methyl group that is next to the carboxyl group, and the sum of dissociated and undissociated acetic acid was measured. As seen from Figure 2, under the experimental conditions studied, the use of lime for the treatment of PHL generally increased the acetic acid concentration of PHL. When the lime dosage was relatively high, for example, above 1.7% (i.e., above pH 11.5), the acetic acid concentration in the PHL was more than doubled. Also, as the lime dosage increased from 2% to 4.5%, the acetic acid concentration of the treated PHL had a small decrease, presumably due to the adsorption of acetic acid onto calcium hydroxide particles, similar to the lime mud/PHL system, as reported earlier.33 As discussed earlier, the maximum acetic acid concentration of the PHL can be 17.45 g/L (10.11 g/L + 7.34 g/L) provided that acetyl groups bound to the dissolved hemicelluloses are completely hydrolyzed to form acetic acid. Obviously, this maximum number was substantially lower than that of lime-treated PHL at a relatively high lime dosage (i.e., above 1.7%), which can be explained by the reported research findings that sugars can be degraded to form acetic acid under strong alkaline conditions.27,38 As discussed earlier, the results available in the literature on the use of lime for treatment of lignocellulosic hydrolyzates have been mainly focused on the hydrolyzates from acid pretreatment.
3. RESULTS AND DISCUSSION 3.1. Effect of Lime Addition on pH. As the PHL of the kraftbased dissolving pulp production process contains acetic acid that is released due to the cleavage of acetyl groups in the lignocellulosic material,4,36 it has an acidic nature, and the PHL used in this study had a pH of 4.0. The effect of lime addition on the pH of PHL is shown in Figure 1. Various amounts of lime were mixed with PHL at room temperature before the pH measurements. It can be seen from Figure 1 that when lime dosage was below a certain level, for example, around 1.7%, the pH increased very quickly with the increasing amount of lime; a further increase in the lime charge had only a small effect on the pH. The asymptotic upper limit on 663
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Figure 3. Effect of lime dosage on the furfural content of the treated PHL (furfural concentration of the untreated PHL was 1.43 g/L). Figure 2. Effect of lime dosage on the acetic acid concentration of the treated PHL (acetic acid concentration of the untreated PHL was 10.1 g/L).
It has been reported that lime treatment of such hydrolyzates generally did not produce more acetic acid at relatively low temperature (e.g., 25 °C), even at a very high pH (e.g., pH 12).21 This is due to the fact that the cleavage of acetyl groups is completed during the process of dilute-acid hydrolysis, and the lime treatment can not degrade the sugars to form acetic acid at low temperature. However, when the temperature is relatively high (e.g., 80 °C), the lime treatment at a high pH (e.g., pH 11 12) can result in an obvious increase in acetic acid concentration due to the degradation of sugars,27 which is consistent with our results on the PHL of kraft-based dissolving production process. 3.3. Effect of Lime Treatment on Furfural and UV Lignin Concentrations. The effect of lime treatment on the amounts of furfural and UV lignin of the PHL is shown in Figures 3 and 4. The lime treatment was very effective in decreasing the amount of furfural. When the lime dosage was higher than 1.7% (around pH 11.5), furfural in the PHL was completely removed. These results are generally in agreement with those reported in the literature on treatment of dilute-acid hydrolyzates with lime.21,26,27 It was proposed that furfural was further oxidized under the alkaline conditions.39 UV lignin was also influenced by the lime treatment, which decreased the amount of UV lignin in the PHL. In the lime dosage range of 1.3 4.5%, UV lignin content was generally reduced by 25 30%. Similarly, Agblevor et al. (2004) reported that for the corn stover hydrolyzates from dilute-acid pretreatment, the lime treatment resulted in significant removal of ligninderived compounds.18 The decrease in UV lignin concentration as a result of lime treatment can be due to the fact that free calcium ions originating from lime can complex with the ionic groups of the dissolved lignin, resulting in the precipitation of lignin-based complexes.40 Furthermore, the adsorption of dissolved lignin onto the calcium hydroxide particles may also result in the decreased UV lignin concentration.
Figure 4. Effect of lime dosage on the UV lignin content of the treated PHL (UV lignin concentration of the untreated PHL was 9.22 g/L).
3.4. Effect of Lime Treatment on Hemicelluloses Concentration. The PHL of kraft-based dissolving pulp production
process contains both monomeric and oligomeric sugars.5 8,33 Various sugars, including xylose, mannose, glucose, galactose, arabinose, and rhamnose, are present. The effect of lime treatment on the hemicelluloses concentration of PHL is shown in Figure 5. When the lime dosage was below 1% (i. e., pH less than 10), the lime treatment had a negligible effect on the contents of monomeric and oligomeric sugars; however, a further increase in the lime dosage resulted in a striking decrease in the hemicelluloses concentration. At a lime dosage of about 2.5%, mono sugars in the PHL were essentially consumed, while the total hemicelluloses content decreased by about 70%. At a lime dosage 664
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Figure 5. Effect of quick lime dosage on the hemicelluloses contents of the treated PHL (concentrations of total sugars, oligomeric sugars, and monomeric sugars of the untreated PHL were 50.33, 41.93, and 8.40 g/L, respectively).
Figure 6. Hemicelluloses degradation versus adsorption during the lime treatment of PHL (lime dosage: 3%).
of 3%, the total hemicelluloses content of the PHL decreased by about 80%, and a further increase in the lime dosage did not significantly change the hemicelluloses content. In the literature, the use of lime for the treatment of dilute-acid hydrolyzates can lower the amount of dissolved sugar components especially at a pH of 11 and higher,21,22,24,27 and the mechanism was proposed to be hydroxide-catalyzed degradation reactions.22 For example, Millati et al. reported that for diluteacid hydrolyzates of forest residues (mainly from spruce) from acid pretreatment, the lime treatment at pH 12 resulted in up to 70% loss of sugars at 60 °C.21 Our results are to some degree similar to these research findings. As shown in Figure 5, at a high lime dosage, not only the mono sugars in the PHL are degraded, but the oligomeric sugars can also be degraded. Those that were difficult to be degraded even at lime dosage of as high as 4.5% (see Figure 5) might possibly be considered as alkali-tolerant components of the dissolved hemicelluloses (the high molecular weight fraction).41 Another factor that decreased the total hemicelluloses concentration of the PHL from the lime treatment might be the adsorption of hemicelluloses (mono sugars and/or oligomeric sugars) onto the calcium hydroxide particles, particularly at a high lime dosage. Further experiments were carried out to verify this hypothesis. When the lime dosage was 3%, the filter cake (containing calcium hydroxide particles) was collected to measure the total sugar concentration, and the results regarding the percentage of sugars adsorbed on the calcium hydroxide particles and those degraded sugars are shown in Figure 6. It can be seen that the decrease in total hemicelluloses concentration was predominantly due to sugar degradation, and their adsorption on calcium hydroxide particles was generally very limited. 3.5. Lime Treatment in the Absence of Oxygen. The above results showed that a significant decrease in the hemicelluloses concentration of the PHL occurred during the lime treatment, mostly due to sugar degradation. For value-added utilization of
Figure 7. Hemicelluloses degradation versus adsorption for PHL treatment with lime (lime dosage: 3%) in the absence of oxygen/air by bubbling carbon dioxide.
these sugars in the PHL, it would be desirable to minimize the oxidative degradation. For this purpose, we designed another set of experiments so that the lime treatment was carried out in the absence of dissolved oxygen/air by bubbling carbon dioxide. Interestingly, it was found that during the PHL treatment process using lime with the dosage of 3%, continuously bubbling carbon dioxide gas into aqueous mixture can effectively decrease the sugar degradation (see Figure 7). Also, the carbon dioxide bubbling did 665
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Table 1. Effect of Lime Treatment in the Absence of Oxygen/Air by Bubbling Carbon Dioxidea hemicelluloses concentration (g/L) furfural PHL sample
a
acetic acid
lignin concentration (g/L)
mono-
oligo-
total
concentration (g/L) concentration (g/L)
treated PHL with carbon dioxide bubbling
6.60
4.71
37.33
42.04
0
treated PHL without carbon dioxide bubbling
6.85
0.09
10.12
10.21
0
22.23
untreated PHL
9.22
8.40
41.93
50.33
1.43
10.11
20.23
Lime dosage, 3%; temperature, 78 °C; treatment time, 1 h.
not significantly change the concentrations of UV lignin and furfural of the lime-treated PHL (see Table 1). It is noted that the PHL lime treatment resulted in a significant increase in the acetic acid concentration (Table 1), which may be explained by (1) the alkaline hydrolysis of acetyl groups bound to dissolved hemicelluloses, together with sugar degradation; and (2) sugar degradation.39 On the other hand, the carbon dioxide bubbling during the lime treatment only slightly decreased the acetic acid concentration of the lime-treated PHL (trial 1 vs trial, Table 1), presumably due to the fact the amount of acetic acid generated from the sugar is less. Furthermore, Table 1 showed that the carbon dioxide bubbling did not significantly change the concentrations of UV lignin and furfural of the lime-treated PHL (see Table 1). Rowell et al. reported that the alkaline oxidative degradation of cellubiose could be effectively eliminated when the lime treatment was carried out in the nitrogen medium.42 However, on the basis of the availability of industrial gases at the mill, it was decided that carbon dioxide was used. On the basis of the results shown in Figure 7 and Table 1, it can be concluded that under the conditions studied, the lime treatment in the absence of oxygen/ air would significantly decrease the alkaline oxidative degradation of sugars. The reduction of system pH as a result of the introduction of acidic carbon dioxide can also possibly contribute to the protection of sugars from alkaline degradation. Further work on a more comprehensive understanding of lime treatment of prehydrolysis liquor of dissolving pulp production process, for example, sugar degradation, precipitate formation, and recovery of other products, such as acetic acid, furfural, may still be needed in the future.
’ ACKNOWLEDGMENT This project was funded by an NSERC CRD grant and Canada Research Chairs program of the Government of Canada. We wish to thank the reviewers for their valuable comments and suggestions, which helped in improving the quality of this Article. ’ REFERENCES (1) Liu, W.; Yuan, Z.; Mao, C.; Hou, Q.; Li, K. Removal of hemicelluloses by NaOH pre-extraction from aspen chips prior to mechanical pulping. BioResources 2011, 6, 3469–3480. (2) Jahan, M. S.; Saeed, A.; Ni, Y.; He, Z. Pre-extraction and its impact on the alkaline pulping of bagasse. J. Biobased Mater. Bioenergy 2009, 3, 380–385. (3) Zhu, J. Y.; Pan, X. J. Woody biomass pretreatment for cellulosic ethanol production: Technology and energy consumption evaluation. Bioresour. Technol. 2010, 101, 4992–5002. (4) Alfaro, A.; Rivera, A.; Perez, A.; Ya~ nez, R.; García, J. C.; Lopez, F. Integral vaporization of two legumes by autohydrolysis and organosolv delignification. Bioresour. Technol. 2009, 100, 440–445. (5) Li, H.; Saeed, A.; Ni, Y.; van Heiningen, A. R. P. Hemicellulose removal from hardwood chips in the pre-hydrolysis step of the kraftbased dissolving pulp production process. J. Wood Chem. Technol. 2010, 30, 48–60. (6) Liu, Z.; Fatehi, P.; Jahan, M. S.; Ni, Y. Separation of lignocellulosic materials by combined processes of prehydrolysis and ethanol extraction. Bioresour. Technol. 2011, 102, 1264–1269. (7) Liu, Z.; Ni, Y.; Fatehi, P.; Saeed, A. Isolation and cationization of hemicelluloses from pre-hydrolysis liquor of kraft-based dissolving pulp production process. Biomass Bioenergy 2011, 35, 1789–1796. (8) Saeed, A.; Jahan, M. S.; Li, H.; Liu, Z.; Ni, Y.; van Heiningen, A. R. P. Mass balance of hemicelluloses and other components in the pre-hydrolysis kraft-based dissolving pulp production process. Biomass Bioenergy 2010, doi: 10.1016/j.biombioe.2010.08.039. (9) Saeed, A.; Fatehi, P.; Ni, Y. Chitosan as a flocculant for prehydrolysis liquor of kraft-based dissolving pulp production process. Carbohydr. Polym. 2011, 86 (4), 1630 1636. (10) Shi, H.; Fatehi, P.; Xiao, H.; Ni, Y. A combined acidification/ PEO flocculation process to improve the lignin removal from the prehydrolysis liquor of kraft-based dissolving pulp production process. Bioresour. Technol. 2011, 102, 5177–5182. (11) Tunc, M. S.; van Heiningen, A. R. P. Hemicellulose extraction of mixed southern hardwood with water at 150 °C: Effect of time. Ind. Eng. Chem. Res. 2008, 47, 7031–7037. (12) Ren, J. L.; Peng, F.; Sun, R. C. The effect of hemicellulosic derivatives on the strength properties of old corrugated container pulp fibres. J. Biobased Mater. Bioenergy 2009, 3, 62–68. (13) Shen, J.; Song, Z.; Qian, X.; Ni, Y. Carbohydrate-based fillers and pigments for papermaking: A review. Carbohydr. Polym. 2011, 85, 17–22. (14) Liu, Z.; Fatehi, P.; Sadeghi, S.; Ni, Y. Application of hemicelluloses precipitated via ethanol treatment of pre-hydrolysis liquor in high-yield pulp. Bioresour. Technol. 2011, 102 (20), 9613 9618.
4. CONCLUSIONS The treatment of industrially produced PHL of kraft-based dissolving pulp production process with lime resulted in increased acetic acid concentration due to the hydrolysis of bound acetyl groups of the dissolved hemicelluloses as well as the degradation of sugars under the alkaline condition. When the lime dosage was relatively high, for example, above 1.7% (i.e., above pH 11.5), the acetic acid concentration was more than doubled. In the same process, the concentrations of furfural and UV lignin can be decreased by 100% and 25 30%, respectively. Hemicelluloses concentration could be significantly reduced at high lime dosage due to the alkali oxidative degradation of sugars; however, the application of lime treatment concept in the absence of oxygen/air can essentially eliminate the sugar degradation. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: (506) 451-6857. Fax: (506) 453-4767. E-mail: pfatehi@ lakeheadu.ca (P.F.);
[email protected] (Y.N.). 666
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