Viscosity of Prehydrolysis Liquor of a Hardwood Kraft-Based

In this study, experimental viscosity results of hardwood prehydrolysis liquor (PHL) from the kraft-based dissolving pulp production process were obta...
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Viscosity of Prehydrolysis Liquor of a Hardwood Kraft-Based Dissolving Pulp Production Process Haitang Liu,*,†,‡ Huiren Hu,† Ashwini Nairy,‡ M. Sarwar Jahan,‡,§ Guihua Yang,‡,⊥ and Yonghao Ni*,†,‡ †

Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science and Technology, Tianjin 300457, China Limerick Pulp and Paper Centre, Department of Chemical Engineering, University of New Brunswick, NB, Canada E3B 5A3 § Pulp and Paper Research Division, BCSIR Laboratories, Dhaka, Dhaka-1205, Bangladesh ⊥ Key Laboratory of Pulp & Paper Science and Technology, Shandong Polytechnic University, Ministry of Education, Jinan, Shandong 250353, P.R. China ‡

ABSTRACT: In this study, experimental viscosity results of hardwood prehydrolysis liquor (PHL) from the kraft-based dissolving pulp production process were obtained and used to develop empirical models describing the effect of temperature, solid content, the interaction between solute and solvent, and the concentration of lignin and hemicelluloses. The concentration, molar mass, and molecular weight of lignin and polysaccharide of PHL, all are the factors that affect the rheological behavior of the PHL. The results showed that, on the one side, Zaman and Fricke’s model gave better fitting when the viscosity is lower, on the other hand, a much better fitting of the Moosavifar’s model could be obtained by taking interaction between solute and solvent, solids content, concentration of lignin and hemicelluloses into account when the viscosity is higher, because of the interaction between solute lignin/hemicelluloses and solvent water, and the aggregation of lignin with itself, hemicelluloses with itself, and lignin with hemicelluloses. Consequently, using two different correlations in covering different viscosity value regions (η > 2 mPa s or η < 2 mPa s) of PHL led to better fitting of the data than those using just one single correlation. a continuous need to fire black liquor at higher solids concentration in the recovery furnace.16−18 While the viscosity of black liquor increases exponentially with solids content,19 the viscosity affects the heat transfer in the evaporation units, the capacity of the pumps and the size of the droplets of black liquor being fed into the recovery boiler,20−23 thus impacting the process operation and economics. These cases of black liquor are also applicable and analogous to prehydrolysis liquor, especially when high concentration liquor is demanded. There are many reports on utilization of prehydrolysis liquor or hydrolyste by evaporation or nanofiltration to concentrate different sugars in order to ferment for ethanol and xylitol production or dehydration for furfural formation. For economic recovery of ethanol by distillation, it should be 4−5% w/w in the feed to the distillation column.24 According to the stoichiometry of ethanol fermentation, this requires at least 80−100 g/L of monosaccharides in the feed to fermentation. Stoutenburg et al.25 employed a membrane which purified and concentrated hydrolysate derived from sugar maple to produce ethanol. Canilha et al.26 increased the Eucalyptus hemicellulosic hydrolysate concentrations in a evaporator operating at 75 ◦C by either three- or six- fold, in order to increase the xylose content used for xylitol production by Candida guilliermondii FTI20037. Eventually, the xylose concentration could be increased to 70.91 g/L after evaporation and detoxification with ion-exchange resin. Xing et al.12 obtained the hemicellulose solution containing xylose content

1. INTRODUCTION An integrated forest biorefinery (IFBR) allows the production of other coproducts such as ethanol and polymers, besides pulp, has attracted more and more researchers and manufacturers to devote efforts to it.1 The pre-extraction of lignocellulosic materials prior to pulping has been considered an important aspect for the implementation of an integrated forest biorefinery.1−3 In the dissolving pulp production process, prehydrolysis is carried out prior to kraft pulping to remove hemicellulose from the lignocelluloses.4 In the prehydrolysis liquor (PHL), xylose/ xylan, acetic acid, lignin, and furfural are present.5,6 In this context, the commercial prehydrolysis kraft-based dissolving pulp production process fits well into the biorefinery concept.5−9 Therefore, a successful exploitation of these components from the PHL in an economic way makes the biorefinery concept a reality. All of these can be recovered or converted to potential products in industry, e.g., lignin can be adsorbed by activated carbon10 then recovered for fuel production, hemicelluloses can be used for ethanol, furfural, or xylitol production.11−13 The utilization and study of black liquor have been developed for many years, while prehydrolysis liquor (PHL) is a new and emerging field currently. Black liquor contains various organic materials (low molecular weight acids and degradation products of the complex polyphenolic compound, lignin) and inorganic pulping chemicals in an aqueous medium.14 Recovery of pulping chemicals from the black liquor of the kraft process is necessary to maintain cost effectiveness. Most current papermaking operations typically evaporate black liquor to 60−70% solids and then combust it in the recovery furnace.15 In order to improve the process, there is © 2013 American Chemical Society

Received: Revised: Accepted: Published: 3974

January January January January

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the correlation, since the content of these organic compounds was believed to have a major influence on the viscosity:

of around 10.7 wt %, which was concentrated by the evaporation of unconcentrated extracts (about 2 wt %) at the University of Maine, to produce furfural and carboxylic acids. The prehydrolysis liquor of a hardwood kraft-based dissolving pulp production process contains 50.0 g/L hemicelluloses, 9.22 g/L lignin, 10.1 g/L acetic acid, and 1.43 g/L furfural,6,9 the dominant high molecular weight compounds that contribute to solid content and in turn to the viscosity are lignin and polysaccharides present in the PHL. There are no reports on the viscosity of prehydrolysis liquor so far; therefore, the objective of this paper is to study the viscosity of prehydrolysis liquor with different solid content, lignin concentration, and sugar concentration, so that some conducive guidance to the evaporation and heat transfer coefficient design and power usage with recirculation pumps can be given. The models for the viscosity of black liquor were adapted for developing new models with the experimental viscosity data of PHL and are briefly discussed here. Generally, the logarithm of viscosity of liquids is inversely proportional to the temperature of the solution as log10 η = A +

E RT

⎛η⎞ ⎞ 1010 ⎞ S ⎛⎜⎛ S C HC KL⎟ 4 ⎟ − K3 loge⎜⎜ ⎟⎟ = ⎜ K1 + K 2 ⎠T ⎠ (1 − S) ⎝⎝ 1−S ⎝ η0 ⎠ (6)

Where η0 is set to 1 mPa s, CH and CKL are the concentrations of hemicelluloses and Klason lignin (g/kg dry solids), respectively, and K1, K2, and K3 are constants. In the present work, eqs 2, 4, 5, and 6 were used to model the viscosity data of PHL.

2. EXPERIMENTAL WORK 2.1. Materials and Methods. The industrially produced PHL of the kraft-based dissolving pulp production process was collected from a mill located in Eastern Canada. To remove large particles and impurities, the PHLs were filtered using Whatman qualitative filter papers (GE Healthcare UK Limited, UK) and Nylon 66 membrane with a pore size of 0.45 μm and diameter of 0.45 mm (Supelco Analytical Group, USA), and then concentrated by distillation using a rotary evaporator to obtain mass reduction factors of 2−5 resultant liquors approximately, or by nanofiltration using NF HL membranes. The solid content of these samples and the original PHL sample were then determined. 2.2. Sugar Analysis. The monomeric sugar contents were determined by using an Ion Chromatography5,31 with a Pulse Amperometric Detector and CarboPacTM PA1 column (Dionex-300, Dionex Corporation, Canada). Deionized water was used as eluant with a flow rate of 1 mL/min. The regeneration agent used was 0.2 N NaOH with 1 mL/min flow rate and 0.5 N NaOH was used as the supporting electrolyte with 1 mL/min flow rate. The samples were filtered and diluted prior to analysis. The experimental error for the sugar analysis was within 6%. In order to measure the oligomeric sugar of original PHL or concentrated PHL, the oligomers in the PHL need to be hydrolyzed into monomeric sugars. To hydrolyze the oligomeric sugars in the prehydrolysis liquor into monomeric sugars, a vial containing 1 mL of the prehydrolysis liquor and 5 mL of 4% sulfuric acid was sealed in an autoclave,32 which was then put in an oil bath (Neslab Instruments, Inc., Portsmouth, N.H., USA). The autoclave was kept in the oil bath at 121 °C for 1 h. Total saccharides in the PHL were calculated from the monomeric sugar content in the post acid hydrolysate. The sugar contents in oligomeric form in the prehydrolysis liquor were calculated from the difference of the monomeric sugar contents with and without the post acid hydrolysis. 2.3. Lignin Analysis. The lignin content of original and treated PHLs was measured based on the UV−vis spectrometric method at a wavelength of 205 nm (TAPPI UM 250). 2.4. Viscosity. Viscosity of the liquor samples was measured at temperatures 25, 50, 75, and 90 °C with an Ostwald’s Viscometer. 2.5. Data Analysis. Nonlinear least-squares fitting method was used to fit equations with the viscosity data. The term residual (R), defined as given below, was minimized by Solver in Excel software to find out the values of constants that appear in the correlations.

(1)

Where, A is a constant and E is the activation energy. Moosavifar et al.21 developed an expression (eq 2) for the viscosity of black liquor based on a combination of absolute reaction rate theory and free volume theory, so that the influence of solids concentration can be considered as ⎛η⎞ K2 loge⎜⎜ ⎟⎟ = K1S1.5 + η − 250 T ⎝ w⎠

(2)

Where, S is the solids content, and K1 and K2 are constants Zaman and Fricke27 suggested that, at low solids contents, black liquor could be treated as a polymer solution. Thus, the concept of the reduced variables method for dilute polymer solutions to the viscosity of black liquors: n

loge ηR =

⎛ S ⎞i ⎜ ⎟ T⎠

∑ ai⎝ i=1

(3)

Where, ηR is defined as ηR = η/ηw, η and ηw are the kinematic viscosities of liquor and water at temperature T, respectively, and ai is a constant that varies with the liquor type. Zaman and Fricke28 further modified eq 4 to a two-constant equation as ⎛η⎞ ⎛ S ⎞2 S loge⎜⎜ ⎟⎟ = K1 + K 2⎜ ⎟ ⎝T ⎠ T ⎝ ηw ⎠

(4)

Where, K1 and K2 are constants The model gave a good fit for the data they obtained from black liquor samples, including thin black liquor after the extraction of lignin. Also the model can predict their results well, especially at low solid content and lignin concentration. Wennberg29 considered the interaction between solute and solvent, and gave the equation as ⎛η⎞ A loge⎜⎜ ⎟⎟ = 4 + K 2 , η T ⎝ 0⎠

⎛ S ⎞ 10 ⎟10 A = K1⎜ ⎝1 − S ⎠

(5)

Where, η0 is set to 1 mPa s, and K1 and K2 are constants. Moosavifar et al.30 developed eq 6 by adding the product of concentrations of lignin and hemicelluloses as another term in 3975

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Table 1. Composition of PHL Samples Used in the Studya

a

no.

1

2

3

4

5

6

7

solid content (%) solid fraction lignin content (g/kg dry solid) lignin fraction hemicellulose content (g/kg dry solid) hemicellulose fraction

7.81 0.08 107.97 0.0087 628.43 0.0491

15.92 0.16 107.97 0.0198 628.43 0.0965

25.12 0.25 107.97 0.0238 628.43 0.1508

32.47 0.32 107.97 0.0362 628.43 0.1879

41.34 0.41 107.97 0.0439 628.43 0.2465

18.71 0.19 106.23 0.0216 674.84 0.1262

28.70 0.29 103.42 0.0332 646.34 0.1855

1−5 obtained from rotary evaporation; 6 and 7 from nanofiltration. 2 ⎛ ηfitted ⎞ 1⎜ ⎟ R = ⎜1 − N⎝ ηexp ⎟⎠

temperature. These results are in good agreement with other studies.21,27 As can be seen from the Figure 1a, the viscosity of PHL also increases exponentially with solids content like black liquor.19 Figure 1b shows the relationship between the logarithm of viscosity data and temperature, an Arrhenius type equation is extensively used in the description of the viscosity dependence on temperature for Newtonian fluids, it is an empirical model developed from the theory of the liquid state, based on the movement of molecules through the formation of “holes” in the system.36 The logarithm of viscosity is proportional to the inverse of absolute temperature. 3.2. Modeling and Fitting. The modeling results from using eqs 2, 4, 5, and 6 were listed in Table 3. The plots of viscosity predicted using different correlations against viscosity obtained experimentally were shown in Figure 2. Equation 2 does not show less residual than eqs 4 and 6 (Table 3), its predicted viscosity data does not correspond to the experimental data at higher solid content (Figure 2). Since the viscosity is exponentially proportional to solid content in Figure 1a, the power exponent is 1.5 of eq 2, it is lower than the power exponent 2 of eqs 4 and 6. Consequently, eq 2 underestimated the viscosity when the solid content is higher. Equation 5 takes consideration of the non-solid fraction, namely the solvent that dissolves the solute, because viscosity is generally considered to be a case of sliding friction between molecules, and in any simple solution, there are three possible kinds of such friction, such as friction between solute molecules, solvent molecules, and solute and solvent.37 Equation 5 shows good fitting for black liquor which is thin black liquor and filtrate after the extraction of lignin;21 nevertheless, it does not give better fitting for the PHL in our work. The reason is that the viscosity of a solution or suspension varies directly with the size and asymmetry of the solutes or suspended particles. A large or elongated solute molecule imparts higher viscosity to its solution than a smaller or spherical molecule, because a nonspherical (or large) particle presents a larger surface toward the direction of flow than a spherical (or small) particle and consequently has a higher frictional coefficient and a proportionately higher contribution to the viscosity of the solution.38 For PHL, the reactions of lignin during prehydrolysis includes cleavage of aryl ether bonds, lower β-O-4 bonds, generation of new phenolic units, partially by a homolytic cleavage mechanism, and condensation reactions, so the degradation products dissolved in PHL to a limited extent attributed to their limited solubility in water and only low molecular weight products remain soluble in water.39 The molecular weight of lignin was 6400−9400 for black liquor, while it was 2975 for PHL.40,41 The molecular weights of xylan of black liquor was also higher, than the hemicelluloses molecular weight in the PHL (17000−19000 vs 5800).21,41

(7)

Where N is the number of experimental data. ηfitted is predicted viscosity, ηexp is experimental viscosity data.

3. RESULTS AND DISCUSSION 3.1. Composition and Rheological Behavior of Prehydrolysis Liquor (PHL). The black liquor can be considered as a complex aqueous solution, comprising organic materials from wood or fibrous plants (lignin, polysaccharides, and resinous compounds of a low molar mass) and inorganic compounds (mainly soluble salt ions). The solid content of black liquor prior to combustion is typically 60−70%,15 while it is much lower in the original PHL.5,31 The composition of the samples in our study with their solid content ranging from 7.81% to 41.34% are given in Table 1. The lignin content of original or concentrated PHL is 103.42−107.97 g/kg dry solid, which are much less than that of 350−435 g/kg dry solid in black liquor from the Slash Pine in Table 2.33 Table 2. Lignin Concentration and Molecular Weight of Black Liquor at Different Time and Temperaturea,33 no.

t, min

T, K

CL, g/kg solids

Mw

1 2 3 4

40 80 80 40

438.8 449.9 438.8 449.9

350 435 408 427

6411 6618 7960 9358

a The term t is cooking time; T is cooking temperature; effective alkali is 13%; sulfidity is 20%; CL is lignin concentration; Mw is lignin molecular weight.

Lignin makes the largest contribution to the viscosity of softwood kraft black liquor.34 The increase of polysaccharides concentration also caused the increase of viscosity of black liquor.35 For the PHL, the sugars content are 628.43−674.84 g/ kg dry solid, which are about 6−7 times of that of lignin.21 The hemicelluloses present in the PHL have very low molecular weights, in fact the monomeric sugars accounted for 18% of the total sugar while the oligomeric sugars were 82% of the total sugar. Owing to the different solid content and composition of the PHL compared to black liquor, the highest viscosity of the PHL was not more than 18 mPas in our study at which the solid content of PHL was 41.34% at 298 K (Figure 1a). Figure 1a shows the experimentally measured viscosity of the liquor as a function of solid fraction at different temperatures. The viscosity increases with the increase of solid fraction and 3976

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Figure 1. Plots of (a) viscosity of PHL vs its solid fraction at different temperature and (b) log(viscosity of PHL) vs inverse of temperature at different solid fraction.

4 fits well for their results of low solid content black liquor. And also it is obvious in Figure 2 that, at low solid content and viscosity region, the experimental data fitted better to the eq 4 model than those at high solid content and viscosity region. Because lignin is believed to be able to aggregate with polysaccharides to form a cluster and have larger size than individual lignin or polysaccharides molecule.42 And the cohesion, adhesion, molecule interaction, and molecular volume should also be considered, since the chances of collision between different molecules will depend on the ratio of the volumes of the molecules to the volume of the space between them.43 Eventually, the most comprehensive eq 6 which includes the interaction between the solute and the solvent, the solid fraction, and concentration of lignin as well as sugars concentration was evaluated. Equation 6 shows better fitting than the other three equations and a residual of 0.0375, which was smaller than the values obtained from eqs 2 and 5 (Table 3). Equation 6 gives good fit at high solid content and viscosity region (Figure 2), it is different and opposite to eq 4 that gives a good fit at low solid content and viscosity region. From Figure 3a, liquors with high lignin and polysaccharides concentration are seen; these two compounds can cluster into amorphous and voluminous molecules of high molar mass, so they tend to have a high viscosity. Conversely, as schematized in Figure 3b, liquors with low lignin and polysaccharides concentration have a smaller more compact

Table 3. Constants and Residuals Calculated for Equations 2, 4, 5, and 6 equation

K1

K2

2 4 5 6

6.7782 445.2001 4.6300 7.6681

17.1098 1040808.1472 1.0623 −0.0001

K3

R

1.3757

0.0498 0.0202 0.0620 0.0375

Figure 2. Plots of predicted viscosity versus experimental viscosity of PHL using eqs 2, 4, 5, and 6. Figure 3. Schematic representation of lignin and polysaccharide conglomerates presented in black liquor: (a) voluminous and shapeless and (b) compact and spherical.42

It can be found that eq 4 gives the best fit owing to the lowest residuals (Table 3). Mossavifar et al.21 concluded that eq 3977

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and spherical molecular structure, and so they tend to present a lower viscosity.44 It has been shown that when black liquor contains a large fraction of low molecular weight lignin, the impact of lignin concentration on viscosity decreases.28 Thus, residuals are evaluated separately for viscosity data which is higher than 2 mPa s and lower than 2 mPa s. Table 4 shows the values of residuals and constants obtained. Table 4. Constants and Residuals Calculated Separately in Two Regions of Viscosity for Equations 4 and 6 equation

viscosity (η)

K1

K2

4

low (η < 2 mPa s) high (η > 2 mPa s) low (η < 2 mPa s) high (η > 2 mPa s)

384.4210

999999.76

0.0082

558.9662

1030908.2650

0.0248

7.5871

−0.0001

1.3247

0.0443

6.3167

−0.0000

0.9195

0.0074

6

K3

R

Figure 4. Plots of predicted viscosity vs experimental viscosity, where estimation was done by separate evaluation for two regions of solid content according to eqs 4 and 6.

Separating lower and higher regions of viscosity of PHL from eqs 4 and 6, respectively, to evaluate the constants and residuals produced a better fitting of predicted values with experimental viscosity. The residual values of eqs 4 (lower viscosity region) and 6 (higher viscosity region) calculated separately were lower than those obtained without the separation (Table 3). At lower values of viscosity (η < 2 mPa s), eq 4 gave the best fitting and less residual, whereas at higher values of viscosity (η > 2 mPa s), eq 6 produced the best results based on the residual values. For low viscosity PHL, the interaction of lignin and hemicelluloses are not significant, while for high viscosity PHL, the solid content is relatively high, the lignin and hemicelluloses play an important role to the viscosity of PHL, their interaction, concentration, molar mass, and molecular conformation affect strongly its rheological behavior.42 For example, hemicellulose can cluster with lignin,44 there are also gel formed from lignin that it is believed to be able to aggregate with polyphenols.45 Overall, a combination of eq 4 for lower region viscosity and eq 6 for higher region viscosity was recommended for estimation of viscosity of PHL (Figure 4). It shows the combined model has extremely good fitting and prediction on the viscosity of PHL.

the viscosity of different viscosity PHLs than any single correlation. This combined model supplies a good description for the rheology of PHL, and also can be a good instruction to the industrial design related to prehydrolysis liquor process.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.L.); [email protected] (Y.N.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by NSERC CRD and Canada Research Chairs programs of the Government of Canada and Tianjin Municipal Science and Technology Commission (Grant No. 12ZCZDGX01100), China.

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REFERENCES

(1) van Heiningen, A. Converting a Kraft pulp mill into an integrated forest biorefinery. Pulp Paper Canada 2006, 107, 38−43. (2) Amidon, T. E.; Liu, S. Water- based woody biorefinery. Biotechnol. Adv. 2009, 27, 542−550. (3) Jahan, M. S.; Saeed, A.; Ni, Y.; He, Z. Pre- Extraction and its Impact on the Alkaline Pulping of Bagasse. J. Biobased Mater. Bio. 2009, 3, 380−385. (4) Sixta, H. HandBook of Pulp; WILEY-VCH Verlag GmbH & Co. KGaA: Weinheim, 2006. (5) Saeed, A.; Jahan, M. S.; Li, H.; Liu, Z.; Ni, Y.; van Heiningen, A. Mass balances of components dissolved in the pre-hydrolysis liquor of kraft-based dissolving pulp production process from Canadian hardwoods. Biomass Bioenergy 2012, 39, 14−19. (6) Shen, J.; Fatehi, P.; Soleimani, P.; Ni, Y. Recovery of lignocelluloses from pre-hydrolysis liquor in the lime kiln of kraftbased dissolving pulp production process by adsorption to lime mud. Bioresour. Technol. 2011, 102, 10035−10039. (7) 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, 9613−9618. (8) Liu, X.; Fatehi, P.; Ni, Y. Removing the inhibitors of prehydrolysis liquor of kraft- based dissolving pulp production process using adsorption and flocculation processes. Bioresour. Technol. 2012, 116, 492−496.

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CONCLUSION The lignin or the polysaccharides in PHL have lower molecular weight than those in black liquor, namely smaller size, thus there are less collisions between the molecules in PHL when the solid content and viscosity is low. When the solid content and viscosity are high, the interaction between lignin and polysaccharides influences the viscosity a lot; these two compounds can agglomerate to form larger size molecules and then change the rheological behavior of liquors. Different expressions were studied to correlate viscosity of PHL with temperature, solid fraction, interaction between solute and solvent, lignin concentration, and hemicelluloses concentration. Using different equations in two different regions of viscosity of PHL (>2 and