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Study of formic acid as an agent for biomass fractionation
Biomass and Bioenergy 22 (2002) 213 – 221
Study of formic acid as an agent for biomass fractionation S. Dap a, V. Santos, J.C. Paraj o ∗ Department of Chemical Engineering, University of Vigo (Campus Ourense), Potytechnical Building, Campus Universitario, As Lagoas, 32004 Ourense, Spain Received 25 October 2001
Abstract Beech, hardwood samples were treated in 80% formic acid solutions under a variety of experimental organosolv pulping conditions. The e0ects of the main operational variables on the composition and yield of cellulose pulps obtained in treatments have been assessed. The SCAN viscosity of cellulose pulps was measured in order to determine the damage of the cellulose produced during the treatments, and additional data on the composition of liquors from treatments were determined to assess the type and amount of valuable byproducts. ? 2002 Elsevier Science Ltd. All rights reserved. Keywords: Beech wood; Fractionation; Formic acid; Kinetics; Fagus sylvatica; Organosolv pulping
1. Introduction In order to achieve a complete and pro:table utilisation of lignocellulosic biomass, many e0orts have been made all over the world to develop processes based on the utilisation of organic solvents [1]. Following the general idea of the “biomass fractionation” [2], these processes should improve the weak points of the kraft pulping technology, including: (i) mitigation of the environmental impact, (ii) generation of valuable byproducts from hemicelluloses, (iii) production of soluble, sulphur-free lignin fragments useful for further processing, and (iv) reduction of the investment needed for pro:table operation. Among organosolvents, formic acid shows interesting features as an agent for fractionation, including its ability to reach an extensive deligni:cation with simultaneous hemicellulose degradation at good ∗
Corresponding author. Tel.: +34-9-8838-7047; fax: +34-98838-7001. E-mail address: [email protected] (J.C. Paraj o).
pulp yield. Formic acid was proposed as an agent for deligni:cation as early as 1917 [3], but no systematic studies were made until the 1980s. Several approaches dealing with the fractionation in formic acid media have been reported, including operation in aqueous formic acid, acid-catalysed aqueous formic acid and formic acid-peroxyformic acid mixtures in aqueous media [4 –7]. Among them, the latter technology (known as Milox process) is the most studied since it was developed at the “Finnish Pulp and Paper Research Institute” (KCL) [4 – 6]. Some studies are available on the utilisation of formic acid–water media of susceptible lignocellulosic materials, including Phragmites comunis [8] and wheat straw [9]. Beech (Fagus sylvatica) wood shows good characteristics as a raw material for the chemical industry, including high polysaccharide content, reduced proportion of lignin and hemicelluloses (having xylan as the main component), which can be converted into marketable chemicals such as xylose and furfural.
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This work deals with the kinetics of beech wood fractionation in 80% formic acid media at a :xed liquid=solid ratio of 10:5 g g−1 . Fractionation exper◦ ◦ ◦ iments were carried out at 110 C; 120 C or 130 C considering the reaction time as an operational variable. The e0ects of the operational conditions on solid residue yield, solid residue composition (including the contents in lignin, cellulose and hemicellulose) and reaction products from hemicelluloses (including sugars and furfural) were assessed. Moreover, the SCAN viscosity of pulps was measured in order to measure the depolymerisation of cellulose produced during the treatments. Kinetics equations were derived to describe the time course of selected variables. 2. Materials and methods Beech (Fagus sylvatica) wood chips were kindly provided by Lenzing A.G. (Lenzing, Austria). Wood chips were screened to select a fraction of particles with size below 2 mm, homogenised in a single lot to avoid compositional di0erences and stored. The composition of wood and pulps was determined using the following methods: moisture, ISO 638:1978; cellulose (as glucan), xylan, other polysaccharides and acetyl groups, by HPLC determination of the glucose, xylose, other sugars and acetic acid contained in hydrolysates from a quantitative acid hydrolysis carried out according to the TAPPI T13m assay; Klason lignin, TAPPI T13m assay; kappa number, ISO 302:1981 method; SCAN viscosity, SCAN C15:62 method; saponi:able groups (as NaOH equivalent), by saponi:cation with KOH-ethanol; ethanol and ethanol–benzene extractives, by Soxhlet extraction according to Browning [10] and acid-soluble lignin as per Maekawa [11]. The composition of the wood lot used, expressed as the average of three replicate determinations and expressed in terms of weight percent (o.d. basis), was as follows: cellulose, 38.6%; xylan, 17.7%; other polysaccharides, 0.21%; acetyl groups, 5.6%; Klason lignin, 23.7%; acid-soluble lignin, 2.7%; ethanol extractives, 0.57% and ethanol–benzene extractives, 0.49%. In material balance calculations, the moisture of wood was considered as water. Wood chips and formic acid solutions were mixed at the desired liquid-to-solid ratio and treated in autoclave under various oper-
ational conditions in 80% formic acid media (see below). Pulps were successively washed with 300 ml of warm 80% formic acid for 15 min and :nally with additional formic acid and warm water until pH = 7, and then treated in an UltraTurrax T-50 de:brator (IKA Labortechnik, Germany) for 30 s at 5000 rpm and air-dried before analysis.
3. Results and discussion Table 1 shows the set of :xed and experimental variables considered in this study, as well as their values or variation ranges. The same table includes the de:nitions of the dependent variables selected to characterise both pulps and pulping liquors. ◦ Sets of experiments were performed at 110 C, ◦ ◦ 120 C or 130 C using technical-grade formic acid (80 wt% solution) at a liquor-to-solid ratio of 10:5 g g−1 . Time zero for reaction was considered to correspond to the beginning of the isothermal operation stage. In order to take into account the e0ect of the heating-up and cooling periods, experiments were run at each temperature with no isothermal reaction stage (time zero). The average times to reach the preset temperatures were 5, 8.5 and 13 min for operation ◦ ◦ ◦ at 110 C, 120 C and 130 C, respectively. 3.1. Pulp yield During the fractionation process, several phenomena are responsible for the weight loss of the solid phase, including extractives removal, deligni:cation and hemicellulose hydrolysis. It can be observed from Fig. 1 that the experimental dependence of the pulp yield on the reaction time at di0erent temperatures showed a similar variation pattern, with higher initial solubilisation for the assays carried out under harsher conditions. On the basis of the above considerations, and taking into account reported results on deligni:cation [12,13], it was assumed that the solid was composed of a soluble fraction (P g g−1 wood) and a non-soluble fraction (1 − P g g−1 wood). The P parameter was found to be dependent on temperature. Assuming that the soluble fraction is degraded with a :rst-order kinetics with partial conversion at time zero, it can be
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Table 1 Fixed, operational and dependent variables considered in this study
(a) Fixed variables Formic acid concentration: 80 wt% of liquors Liquid=solid ratio: 10:5 g solution=g wood (o.d. basis) (b) Operational variables Duration of the treatment (t): 0 –180 min ◦ ◦ ◦ Temperature (T ): 110 C, 120 C or 130 C (c) Dependent (experimental) variables Pulp yield (PY), g pulp=100 g wood, o.d. basis Variables used to measure the composition of pulps: Per cent of cellulose recovery (%CR), g cellulose contained in pulp=100 g cellulose present in the raw material Per cent of residual hemicelluloses (%RH), g hemicelluloses contained in pulp=100 g hemicelluloses present in the raw material Per cent of residual lignin (%RL), g lignin contained in pulp=100 g lignin present in the raw material Acetyl group content, g acetyl groups content=100 g treated sample, o.d. basis Acid-soluble lignin content, g acid-soluble lignin=100 g treated sample, o.d. basis Viscosity (ml g−1 ) Saponi:able group content, g saponi:able groups=100 g treated sample, o.d. basis Variables used to measure the composition of liquors: Concentration of glucose in pulping liquors (g l−1 ) Concentration of xylose in pulping liquors (g l−1 ) Concentration of other sugars in pulping liquors (g l−1 ) Concentration of furfural in pulping liquors (g l−1 )
where kP is a kinetic coeRcient and PY0 is the pulp yield at the beginning of the isothermal reaction stage (t = 0). Table 2 shows the results achieved when the experimental series of data pulp yield=time were :tted to Eq. (1). The soluble fraction P increased with temperature from 0.50 up to 0:55 g g−1 , whereas the exponential decrease of the remaining “soluble fraction” gave a close interpretation of results. The major effects in pulp yield were caused during the :rst 30 min of the reaction. 3.2. Deligni5cation
Fig. 1. Experimental and predicted time courses of pulp yield (PY).
inferred that the dependence of pulp yield PY with the reaction time t is given by the equation PY = 100(1 − P ) + (100P + PY0 − 100) × exp(−kP t);
(1)
In order to allow an easier calculation scheme based on pseudohomogenous kinetics, the variables measuring the composition of pulps are expressed as per cent of the amount contained in the raw material (per cent of residual Klason lignin, %RL; per cent of residual hemicelluloses, %RH and per cent of cellulose recovery, %CR). Several studies [14 –16] reported on increased lignin contents when organosolv-deligni:ed samples were
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Table 2 Results of data analysis for variable PY (pulp yield) ◦
Temperature ( C)
PY0
P (×10)
kP (×102 )
r2
Fstat (d.f.)
110 120 130
98.0 89.7 71.9
5.10 5.34 5.50
3.77 7.79 7.97
0.9995 0.9960 0.9906
13203 (5) 1998 (7) 738 (6)
Note: PY0 = pulp yield obtained at the beginning of the isothermal reaction stage, g=100 g untreated solid, P = soluble fraction of the raw material, g (g raw material)−1 , kP = :rst-order kinetic coeRcient for substrate solubilisation, min−1 , r 2 and Fstat = statistical parameters measuring the correlation and signi:cance of the equations. d.f.: degrees of freedom in the determination of the Fstat coeRcient.
Table 3 Results of data analysis for variable %RL (percent of residual lignin) ◦
Temperature ( C)
%RL0
L (×10)
kL (×102 )
r2
Fstat (d:f :)
110 120 130
89.6 80.2 58.5
8.32 8.74 9.14
3.33 4.93 6.20
0.9994 0.9931 0.9923
8777 (5) 1008 (7) 774 (6)
Note: RL0 = per cent of residual lignin corresponding to the beginning of the isothermal reaction stage, g=100 g lignin contained in the raw material, L = soluble fraction of lignin, g (g initial lignin)−1 , kL = :rst-order kinetic coeRcient for native lignin solubilisation, min−1 , r 2 , Fstat and d.f. as in Table 1.
subjected to long treatments under harsh conditions, suggesting the occurrence of recondensation reactions. In our case, the experimental data concerning deligni:cation were adequately interpreted by a :rst-order decrease of the lignin fraction susceptible to degradation (“soluble lignin fraction”, with a weight fraction measured by the parameter L g soluble lignin=g lignin contained in the raw material). Under these hypotheses, the kinetic equation describing the time course of residual lignin was as follows: %RL = %RL0 − [%RL0 − 100(1 − L )] × [1 − exp(kL t)];
(2)
where %RL0 is the per cent of residual lignin at time zero, L the soluble fraction of lignin, and kL is the kinetic coeRcient (Table 3). Fig. 2 shows the agreement between experimental and predicted values at the various temperatures assayed, as well as a variation pattern closely related to the one determined for pulp yield. The minimum ◦ ◦ ◦ lignin contents achieved at 110 C, 120 C and 130 C (7.8%, 6.1% and 4.4% Klason lignin) corresponded to kappa numbers of 27.4, 26.8 and 23.0, respectively,
Fig. 2. Experimental and predicted time courses of the per cent of residual lignin (%RL).
con:rming that bleachable pulps can be obtained in a single fractionation step carried out under the best conditions determined in this work. In comparison with other deligni:cation technologies, reported data for concentrated, HCl-catalysed
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3.3. Hemicellulose degradation The time course of the per cent of residual hemicelluloses (%HR) on the reaction time showed a typical exponential decrease (see Fig. 3). Assuming a :rst-order decrease of the “soluble fraction” of hemicelluloses (weight fraction, H g soluble hemicelluloses=g hemicelluloses contained in the raw material), the kinetic equation describing the time course of residual hemicelluloses (%RH) was as follows: %RH = %RH0 − [%RH0 − 100(1 − H )] × [1 − exp(−kH t)];
Fig. 3. Experimental and predicted time courses of the per cent of residual hemicelluloses (%RH).
media (Acetosolv pulping) of beech wood [17] con:rmed the comparative advantage of the approach considered in this work in terms of deligni:cation degree, even if no catalyst was added to the reaction media in our case. Though the Acetosolv process is known to be very e0ective for deligni:cation, the comparatively high kappa numbers reported for beech wood deligni:cation seem to be in relation with an increased participation of condensation reactions. Finally, the minimal lignin contents reached in this work are slightly lower than the results reported for the Milox processing of beech wood, a technology involving two sequential stages and the utilisation of hydrogen peroxide as an additional chemical [18].
(3)
where RH0 is the per cent of residual hemicelluloses at the beginning of the isothermal reaction stage (time zero) and kH is the :rst-order kinetic coeRcient. The regression analysis of experimental data (performed considering both H and kH as optimisation parameters) led to the results listed in Table 4. Similarly to the behaviour observed for the per cent of residual lignin, the main decrease in residual hemicelluloses was reached during the :rst 30 min of reaction. A comparison with literature data shows that a single stage of formic acid fractionation of beech wood reached higher degree of hemicellulose removal than Acetosolv or Milox treatments of the same raw material [17,18] and that the formic acid pulping of other raw materials [9]. 3.4. Cellulose recovery A preliminary analysis of experimental data showed that this process was very selective towards cellulose solubilisation. The per cents of cellulose recovery in the whole set of experiments, calculated from the
Table 4 Results of data analysis for variable %RH (per cent of residual hemicelluloses) ◦
Temperature ( C)
%RH0
H (×10)
kH (×102 )
r2
Fstat (d:f :)
110 120 130
99.4 80.8 58.8
8.98 9.17 9.51
4.59 7.91 9.25
0.9971 0.9955 0.9983
2044 (5) 1773 (7) 4057 (6)
Note: RH0 = per cent of residual hemicelluloses corresponding to the beginning of the isothermal reaction stage, g=100 g hemicellulose contained in the raw material, H = soluble fraction of hemicelluloses, g (g initial hemicelluloses)−1 , kH = :rst-order kinetic coeRcient for hemicellulose solubilisation, min−1 , r 2 , Fstat and d.f. as in Table 1.
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Fig. 4. Comparison of experimental and calculated data of the cellulose content of pulps (variable CC). Solid line: experimental = calculated, dashed lines = 10% deviation.
experimental data of pulp yield and cellulose content of samples, were in the range 95 –103%, with an average value of 98%. This :nding is in agreement with literature data reported for formic acid pulping of wheat straw [9]. Considering 98% cellulose recovery for all the experiments, a good interpretation of the experimental data was obtained (see Fig. 4). So, the data concerning the cellulose content of the solid phase in oven-dry basis (variable CC) can be interpreted simply by the equation: CC = 0:98CC0 =PY;
(4)
where CC0 is the cellulose content of untreated wood (38:6 wt%) and PY denotes the pulp yield. 3.5. Results concerning other fractions in solid phases after treatments In order to allow a better characterisation of the solid phases from treatments, two additional properties of pulps were measured: the content in saponi:able groups (which are responsible for increased alkali consumption in further bleaching stages owing to saponi:cation reactions) and the content in acid-soluble lignin, which is not included in Klason lignin and is a term to be considered in material balances.
Fig. 5. Time course of pulp viscosity in fractionation treatments ◦ ◦ ◦ carried out at 110 C, 120 C or 130 C.
All the samples analysed were characterised for comparatively-high per cents of saponi:able groups (in the range 8.0 –14:6 wt% of pulps). Comparing these results with the composition of the raw material, it is evident that the saponi:able groups came from esteri:cation reactions between the organic acid present in the media and polysaccharides. The limited content of samples in acetyl groups (0.5 –2.5%) also supported this idea. The results determined for the acid-soluble lignin content of treated solids (within the range 0.4 –1.7%) showed that this fraction was of importance for the purposes of this work. 3.6. Physico-chemical properties of solid phases after treatments The SCAN viscosity of solid phases from treatments is a variable of primary importance to quantify the depolymerisation of cellulose caused by treatments, since the molecular weight distribution is a key factor for many :nal applications (for example, in the manufacture of dissolving pulps). As the samples obtained at short reaction times (with comparatively high lignin contents) were diRcult to be de:bred and dissolved in the assay to measure viscosity, the corresponding data are not reported. Fig. 5 shows the time courses
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Table 5 Composition of liquors from fractionation treatments
Operational conditions ◦
Composition of fractionation media
Temperature ( C)
Time (min)
Glucose conc. (g l−1 )
Xylose conc. (g l−1 )
Conc. of other pentoses (g l−1 )
Furfural conc. (g l−1 )
110
0 30 60 90 120 135 180
0.0 0.4 0.3 0.8 0.5 0.8 0.8
0.1 4.3 10.6 13.9 14.2 15.2 14.9
0.0 0.6 0.1 1.7 1.1 1.0 0.5
0.0 0.0 0.5 0.7 0.6 0.6 0.7
120
0 15 30 60 90 120 150 165 180
0.9 0.9 1.4 1.3 2.7 2.8 3.7 3.0 2.4
3.3 6.9 12.3 14.8 13.7 12.6 11.2 11.4 11.2
0.0 1.7 1.3 0.1 0.8 0.5 0.7 0.7 0.1
0.0 0.0 0.3 0.5 0.9 1.4 1.7 1.8 1.9
130
0 15 30 60 120 150 165 180
0.2 1.1 2.0 3.3 3.6 4.4 5.0 5.6
4.5 10.1 12.5 13.1 12.5 9.1 8.6 7.8
0.0 0.0 0.0 0.1 0.2 0.1 0.1 0.0
0.0 0.0 1.1 1.6 2.4 2.7 3.1 3.0
of viscosity at various temperatures considered. As expected, the experimental results (in the range 992– 609 ml g−1 ) decreased with time in all the cases as◦ sayed, this tendency being more marked at 130 C. −1 Assuming that a viscosity of 800 ml g is a threshold under which the unbleached pulps are not easily utilisable for dissolving pulp production (owing to the limited viscosity loss allowable during the bleaching ◦ stages) [18], the samples obtained at 130 C are out of the desired range. However, it must be taken into account that the viscosity data are underestimated, since they were not corrected for the presence of saponi:able groups in substrates. 3.7. Composition of liquors In order to assess the generation of valuable byproducts from cellulose and hemicelluloses, the concentra-
tions of glucose, xylose, other pentoses and furfural were considered in the experimental plan as dependent variables. Table 5 shows the experimental results obtained for all the conditions assayed. The glucose concentrations determined in all the experiments (in the range, 0.0 –5:6 g l−1 ) were coherent with the data concerning cellulose recovery, and con:rmed the selectivity of treatments towards cellulose solubilisation. Xylose was the main degradation product from hemicelluloses, whereas other pentoses (mainly arabinose) appeared in low proportions. The composition of valuable byproducts in fractionation media increased with the temperature. The experiments at ◦ 110 C were characterised by low furfural concentrations with xylose concentrations (up to 15:2 g l−1 ) which increased with temperature. In experiments ◦ ◦ carried out at 120 C or 130 C, the xylose concentration achieved maximal values at intermediate
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reaction times, owing to the increased participation of sugar-degrading reactions. Under the conditions leading to minimal lignin contents, the per cent of xylan converted into xylose decreased with tempera◦ ◦ ture (70.5%, 58.6% and 52.2%, at 110 C, 120 C and ◦ 130 C, respectively). On the other hand, the furfural concentrations increased steadily with time to reach maximal concentrations in the range 1.9 –3:0 g l−1 . Under the conditions leading to maximum deligni:cation of pulp :bres, the xylan converted into furfural ◦ increased with temperature (from 8.3% at 110 C up ◦ to 19.6% at 130 C), but xylose remained as the main byproduct of hemicellulose decomposition in all the cases considered. 4. Conclusions Kinetic equations describing the behaviour of the several reactions involved in wood fractionation are useful tools for the technical and economic evaluation of processes. The development of mathematical equations describing the dependence of the most important factors involved in treatments on the operational conditions allow the formulation of generalised material and energy balances, providing a sound basis for the understanding of the system’s behaviour. The pulping of beech wood in uncatalysed formic acid media allowed both extensive deligni:cation and excellent cellulose recovery. The mathematical interpretation of the e0ects caused by the reaction time on pulp yield and pulp composition at three di0erent temperatures were interpreted by means of equations providing a close reproduction of the experimental data. Four experimental variables (pulp yield, percent of residual lignin of pulps, per cent of residual hemicelluloses of pulps and cellulose content of pulps) were calculated either by :rst-order kinetic equations or by material balances based on the assumption of 98% cellulose recovery. Additional data (including xylose and furfural concentrations in liquors from fractionation treatments, and pulp content in saponi:able groups and acid-soluble lignin) provided the basic information needed for process evaluation. Acknowledgements The authors are grateful to the “Comisi on Interministerial de Ciencia y Tecnologia” of the Spanish
Ministry of Education for the :nancial support of this work (in the scope of the Research Project “Desarrollo de Procesos con Bajo Impacto Ambiental para la Producci on de Pastas de Celulosa de Alta Calidad”, reference QUI99-0346), as well as to Ms. Aida Ramos Nespereira and Ms. Antonia Rodriguez Jard on for their excellent technical assistance. References [1] Johansson A, Aaltonen O, Ylinen P. Organosolv pulping-methods and pulp properties. Biomass 1987;13:45– 65. [2] Myerly RC, Nicholson MD, Katzen R, Taylor JM. The forestry re:nery. Chemical Technology 1981;11:186–92. [3] Hergert HL. In: Young RA, Akhtar M, editors. Environmentally friendly technologies for the pulp and paper industry. New York: Wiley, 1998. p. 5–67. [4] Poppius K, Sundquist J, Wartiovaara I. In: Kennedy JF, Phillips GO, Williams PA, editors. Wood processing and utilization. New York: Ellis Horwood, 1989. p. 87–92. [5] Poppius K, Laamanen L, Sundquist J, Wartiovaara I, Kauliomaki S. Bleached pulp by peroxyacid=alkaline peroxide deligni:cation. Pap Puu 1986;68:87–92. [6] Sundquist J. Summary of Milox research. Pap Puu 1996;78:92–5. [7] Ruggiero R, Machado EH, da Silva D, Greler S, Nourmamode A, Castellan A. Bleached chemical pulp from Eucalyptus grandis wood produced by peroxyformic acid pulping and photochemical bleaching. Holzforsch 1998;52:325–32. [8] Seisto A, Poppius-Levlin K. Peroxyformic acid pulping of nonwood plants by the Milox method. Part I: Pulping and bleaching. TAPPI Journal 1997;80:215–21. [9] Jim enez L, De la Torre MJ, Maestre F, Ferrer JL, P erez I. Deligni:cation of wheat straw by use of low molecular weight organic acids. Holzforsch 1998;52:191–6. [10] Browning BL. Methods of wood chemistry. New York: Wiley, 1967. [11] Maekawa E, Ichizawa T, Koshijima TJ. An evaluation of the acid-soluble lignin determination in analyses of lignin by the sulfuric acid method. Wood Chemistry and Technology 1989;9:549–67. [12] Tirtowidjojo S, Sarkanen KV, Pla F, McCarthy JL. Kinetic of organosolv deligni:cation in batch and Vow-thorough reactors. Holzforsch 1988;42:177–83. [13] Paraj o JC, Alonso JL, Santos V. Kinetics of Eucalyptus wood fractionation in acetic acid–HCl–water media. Bioresource Technology 1995;51:153–62. [14] Abad S, Alonso JL, Santos V, Paraj o JC. Furfural from wood in catalyzed acetic acid media: a mathematical assessment. Bioresource Technology 1997;62:115–22. [15] Paraj o JC, Alonso JL, V azquez D. On the behaviour of lignin and hemicelluloses during the Acetosolv pulping of wood. Bioresource Technology 1993;46:233–40.
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Study of formic acid as an agent for biomass fractionation S. Dap a, V. Santos, J.C. Paraj o ∗ Department of Chemical Engineering, University of Vigo (Campus Ourense), Potytechnical Building, Campus Universitario, As Lagoas, 32004 Ourense, Spain Received 25 October 2001
Abstract Beech, hardwood samples were treated in 80% formic acid solutions under a variety of experimental organosolv pulping conditions. The e0ects of the main operational variables on the composition and yield of cellulose pulps obtained in treatments have been assessed. The SCAN viscosity of cellulose pulps was measured in order to determine the damage of the cellulose produced during the treatments, and additional data on the composition of liquors from treatments were determined to assess the type and amount of valuable byproducts. ? 2002 Elsevier Science Ltd. All rights reserved. Keywords: Beech wood; Fractionation; Formic acid; Kinetics; Fagus sylvatica; Organosolv pulping
1. Introduction In order to achieve a complete and pro:table utilisation of lignocellulosic biomass, many e0orts have been made all over the world to develop processes based on the utilisation of organic solvents [1]. Following the general idea of the “biomass fractionation” [2], these processes should improve the weak points of the kraft pulping technology, including: (i) mitigation of the environmental impact, (ii) generation of valuable byproducts from hemicelluloses, (iii) production of soluble, sulphur-free lignin fragments useful for further processing, and (iv) reduction of the investment needed for pro:table operation. Among organosolvents, formic acid shows interesting features as an agent for fractionation, including its ability to reach an extensive deligni:cation with simultaneous hemicellulose degradation at good ∗
Corresponding author. Tel.: +34-9-8838-7047; fax: +34-98838-7001. E-mail address: [email protected] (J.C. Paraj o).
pulp yield. Formic acid was proposed as an agent for deligni:cation as early as 1917 [3], but no systematic studies were made until the 1980s. Several approaches dealing with the fractionation in formic acid media have been reported, including operation in aqueous formic acid, acid-catalysed aqueous formic acid and formic acid-peroxyformic acid mixtures in aqueous media [4 –7]. Among them, the latter technology (known as Milox process) is the most studied since it was developed at the “Finnish Pulp and Paper Research Institute” (KCL) [4 – 6]. Some studies are available on the utilisation of formic acid–water media of susceptible lignocellulosic materials, including Phragmites comunis [8] and wheat straw [9]. Beech (Fagus sylvatica) wood shows good characteristics as a raw material for the chemical industry, including high polysaccharide content, reduced proportion of lignin and hemicelluloses (having xylan as the main component), which can be converted into marketable chemicals such as xylose and furfural.
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This work deals with the kinetics of beech wood fractionation in 80% formic acid media at a :xed liquid=solid ratio of 10:5 g g−1 . Fractionation exper◦ ◦ ◦ iments were carried out at 110 C; 120 C or 130 C considering the reaction time as an operational variable. The e0ects of the operational conditions on solid residue yield, solid residue composition (including the contents in lignin, cellulose and hemicellulose) and reaction products from hemicelluloses (including sugars and furfural) were assessed. Moreover, the SCAN viscosity of pulps was measured in order to measure the depolymerisation of cellulose produced during the treatments. Kinetics equations were derived to describe the time course of selected variables. 2. Materials and methods Beech (Fagus sylvatica) wood chips were kindly provided by Lenzing A.G. (Lenzing, Austria). Wood chips were screened to select a fraction of particles with size below 2 mm, homogenised in a single lot to avoid compositional di0erences and stored. The composition of wood and pulps was determined using the following methods: moisture, ISO 638:1978; cellulose (as glucan), xylan, other polysaccharides and acetyl groups, by HPLC determination of the glucose, xylose, other sugars and acetic acid contained in hydrolysates from a quantitative acid hydrolysis carried out according to the TAPPI T13m assay; Klason lignin, TAPPI T13m assay; kappa number, ISO 302:1981 method; SCAN viscosity, SCAN C15:62 method; saponi:able groups (as NaOH equivalent), by saponi:cation with KOH-ethanol; ethanol and ethanol–benzene extractives, by Soxhlet extraction according to Browning [10] and acid-soluble lignin as per Maekawa [11]. The composition of the wood lot used, expressed as the average of three replicate determinations and expressed in terms of weight percent (o.d. basis), was as follows: cellulose, 38.6%; xylan, 17.7%; other polysaccharides, 0.21%; acetyl groups, 5.6%; Klason lignin, 23.7%; acid-soluble lignin, 2.7%; ethanol extractives, 0.57% and ethanol–benzene extractives, 0.49%. In material balance calculations, the moisture of wood was considered as water. Wood chips and formic acid solutions were mixed at the desired liquid-to-solid ratio and treated in autoclave under various oper-
ational conditions in 80% formic acid media (see below). Pulps were successively washed with 300 ml of warm 80% formic acid for 15 min and :nally with additional formic acid and warm water until pH = 7, and then treated in an UltraTurrax T-50 de:brator (IKA Labortechnik, Germany) for 30 s at 5000 rpm and air-dried before analysis.
3. Results and discussion Table 1 shows the set of :xed and experimental variables considered in this study, as well as their values or variation ranges. The same table includes the de:nitions of the dependent variables selected to characterise both pulps and pulping liquors. ◦ Sets of experiments were performed at 110 C, ◦ ◦ 120 C or 130 C using technical-grade formic acid (80 wt% solution) at a liquor-to-solid ratio of 10:5 g g−1 . Time zero for reaction was considered to correspond to the beginning of the isothermal operation stage. In order to take into account the e0ect of the heating-up and cooling periods, experiments were run at each temperature with no isothermal reaction stage (time zero). The average times to reach the preset temperatures were 5, 8.5 and 13 min for operation ◦ ◦ ◦ at 110 C, 120 C and 130 C, respectively. 3.1. Pulp yield During the fractionation process, several phenomena are responsible for the weight loss of the solid phase, including extractives removal, deligni:cation and hemicellulose hydrolysis. It can be observed from Fig. 1 that the experimental dependence of the pulp yield on the reaction time at di0erent temperatures showed a similar variation pattern, with higher initial solubilisation for the assays carried out under harsher conditions. On the basis of the above considerations, and taking into account reported results on deligni:cation [12,13], it was assumed that the solid was composed of a soluble fraction (P g g−1 wood) and a non-soluble fraction (1 − P g g−1 wood). The P parameter was found to be dependent on temperature. Assuming that the soluble fraction is degraded with a :rst-order kinetics with partial conversion at time zero, it can be
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215
Table 1 Fixed, operational and dependent variables considered in this study
(a) Fixed variables Formic acid concentration: 80 wt% of liquors Liquid=solid ratio: 10:5 g solution=g wood (o.d. basis) (b) Operational variables Duration of the treatment (t): 0 –180 min ◦ ◦ ◦ Temperature (T ): 110 C, 120 C or 130 C (c) Dependent (experimental) variables Pulp yield (PY), g pulp=100 g wood, o.d. basis Variables used to measure the composition of pulps: Per cent of cellulose recovery (%CR), g cellulose contained in pulp=100 g cellulose present in the raw material Per cent of residual hemicelluloses (%RH), g hemicelluloses contained in pulp=100 g hemicelluloses present in the raw material Per cent of residual lignin (%RL), g lignin contained in pulp=100 g lignin present in the raw material Acetyl group content, g acetyl groups content=100 g treated sample, o.d. basis Acid-soluble lignin content, g acid-soluble lignin=100 g treated sample, o.d. basis Viscosity (ml g−1 ) Saponi:able group content, g saponi:able groups=100 g treated sample, o.d. basis Variables used to measure the composition of liquors: Concentration of glucose in pulping liquors (g l−1 ) Concentration of xylose in pulping liquors (g l−1 ) Concentration of other sugars in pulping liquors (g l−1 ) Concentration of furfural in pulping liquors (g l−1 )
where kP is a kinetic coeRcient and PY0 is the pulp yield at the beginning of the isothermal reaction stage (t = 0). Table 2 shows the results achieved when the experimental series of data pulp yield=time were :tted to Eq. (1). The soluble fraction P increased with temperature from 0.50 up to 0:55 g g−1 , whereas the exponential decrease of the remaining “soluble fraction” gave a close interpretation of results. The major effects in pulp yield were caused during the :rst 30 min of the reaction. 3.2. Deligni5cation
Fig. 1. Experimental and predicted time courses of pulp yield (PY).
inferred that the dependence of pulp yield PY with the reaction time t is given by the equation PY = 100(1 − P ) + (100P + PY0 − 100) × exp(−kP t);
(1)
In order to allow an easier calculation scheme based on pseudohomogenous kinetics, the variables measuring the composition of pulps are expressed as per cent of the amount contained in the raw material (per cent of residual Klason lignin, %RL; per cent of residual hemicelluloses, %RH and per cent of cellulose recovery, %CR). Several studies [14 –16] reported on increased lignin contents when organosolv-deligni:ed samples were
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Table 2 Results of data analysis for variable PY (pulp yield) ◦
Temperature ( C)
PY0
P (×10)
kP (×102 )
r2
Fstat (d.f.)
110 120 130
98.0 89.7 71.9
5.10 5.34 5.50
3.77 7.79 7.97
0.9995 0.9960 0.9906
13203 (5) 1998 (7) 738 (6)
Note: PY0 = pulp yield obtained at the beginning of the isothermal reaction stage, g=100 g untreated solid, P = soluble fraction of the raw material, g (g raw material)−1 , kP = :rst-order kinetic coeRcient for substrate solubilisation, min−1 , r 2 and Fstat = statistical parameters measuring the correlation and signi:cance of the equations. d.f.: degrees of freedom in the determination of the Fstat coeRcient.
Table 3 Results of data analysis for variable %RL (percent of residual lignin) ◦
Temperature ( C)
%RL0
L (×10)
kL (×102 )
r2
Fstat (d:f :)
110 120 130
89.6 80.2 58.5
8.32 8.74 9.14
3.33 4.93 6.20
0.9994 0.9931 0.9923
8777 (5) 1008 (7) 774 (6)
Note: RL0 = per cent of residual lignin corresponding to the beginning of the isothermal reaction stage, g=100 g lignin contained in the raw material, L = soluble fraction of lignin, g (g initial lignin)−1 , kL = :rst-order kinetic coeRcient for native lignin solubilisation, min−1 , r 2 , Fstat and d.f. as in Table 1.
subjected to long treatments under harsh conditions, suggesting the occurrence of recondensation reactions. In our case, the experimental data concerning deligni:cation were adequately interpreted by a :rst-order decrease of the lignin fraction susceptible to degradation (“soluble lignin fraction”, with a weight fraction measured by the parameter L g soluble lignin=g lignin contained in the raw material). Under these hypotheses, the kinetic equation describing the time course of residual lignin was as follows: %RL = %RL0 − [%RL0 − 100(1 − L )] × [1 − exp(kL t)];
(2)
where %RL0 is the per cent of residual lignin at time zero, L the soluble fraction of lignin, and kL is the kinetic coeRcient (Table 3). Fig. 2 shows the agreement between experimental and predicted values at the various temperatures assayed, as well as a variation pattern closely related to the one determined for pulp yield. The minimum ◦ ◦ ◦ lignin contents achieved at 110 C, 120 C and 130 C (7.8%, 6.1% and 4.4% Klason lignin) corresponded to kappa numbers of 27.4, 26.8 and 23.0, respectively,
Fig. 2. Experimental and predicted time courses of the per cent of residual lignin (%RL).
con:rming that bleachable pulps can be obtained in a single fractionation step carried out under the best conditions determined in this work. In comparison with other deligni:cation technologies, reported data for concentrated, HCl-catalysed
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3.3. Hemicellulose degradation The time course of the per cent of residual hemicelluloses (%HR) on the reaction time showed a typical exponential decrease (see Fig. 3). Assuming a :rst-order decrease of the “soluble fraction” of hemicelluloses (weight fraction, H g soluble hemicelluloses=g hemicelluloses contained in the raw material), the kinetic equation describing the time course of residual hemicelluloses (%RH) was as follows: %RH = %RH0 − [%RH0 − 100(1 − H )] × [1 − exp(−kH t)];
Fig. 3. Experimental and predicted time courses of the per cent of residual hemicelluloses (%RH).
media (Acetosolv pulping) of beech wood [17] con:rmed the comparative advantage of the approach considered in this work in terms of deligni:cation degree, even if no catalyst was added to the reaction media in our case. Though the Acetosolv process is known to be very e0ective for deligni:cation, the comparatively high kappa numbers reported for beech wood deligni:cation seem to be in relation with an increased participation of condensation reactions. Finally, the minimal lignin contents reached in this work are slightly lower than the results reported for the Milox processing of beech wood, a technology involving two sequential stages and the utilisation of hydrogen peroxide as an additional chemical [18].
(3)
where RH0 is the per cent of residual hemicelluloses at the beginning of the isothermal reaction stage (time zero) and kH is the :rst-order kinetic coeRcient. The regression analysis of experimental data (performed considering both H and kH as optimisation parameters) led to the results listed in Table 4. Similarly to the behaviour observed for the per cent of residual lignin, the main decrease in residual hemicelluloses was reached during the :rst 30 min of reaction. A comparison with literature data shows that a single stage of formic acid fractionation of beech wood reached higher degree of hemicellulose removal than Acetosolv or Milox treatments of the same raw material [17,18] and that the formic acid pulping of other raw materials [9]. 3.4. Cellulose recovery A preliminary analysis of experimental data showed that this process was very selective towards cellulose solubilisation. The per cents of cellulose recovery in the whole set of experiments, calculated from the
Table 4 Results of data analysis for variable %RH (per cent of residual hemicelluloses) ◦
Temperature ( C)
%RH0
H (×10)
kH (×102 )
r2
Fstat (d:f :)
110 120 130
99.4 80.8 58.8
8.98 9.17 9.51
4.59 7.91 9.25
0.9971 0.9955 0.9983
2044 (5) 1773 (7) 4057 (6)
Note: RH0 = per cent of residual hemicelluloses corresponding to the beginning of the isothermal reaction stage, g=100 g hemicellulose contained in the raw material, H = soluble fraction of hemicelluloses, g (g initial hemicelluloses)−1 , kH = :rst-order kinetic coeRcient for hemicellulose solubilisation, min−1 , r 2 , Fstat and d.f. as in Table 1.
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Fig. 4. Comparison of experimental and calculated data of the cellulose content of pulps (variable CC). Solid line: experimental = calculated, dashed lines = 10% deviation.
experimental data of pulp yield and cellulose content of samples, were in the range 95 –103%, with an average value of 98%. This :nding is in agreement with literature data reported for formic acid pulping of wheat straw [9]. Considering 98% cellulose recovery for all the experiments, a good interpretation of the experimental data was obtained (see Fig. 4). So, the data concerning the cellulose content of the solid phase in oven-dry basis (variable CC) can be interpreted simply by the equation: CC = 0:98CC0 =PY;
(4)
where CC0 is the cellulose content of untreated wood (38:6 wt%) and PY denotes the pulp yield. 3.5. Results concerning other fractions in solid phases after treatments In order to allow a better characterisation of the solid phases from treatments, two additional properties of pulps were measured: the content in saponi:able groups (which are responsible for increased alkali consumption in further bleaching stages owing to saponi:cation reactions) and the content in acid-soluble lignin, which is not included in Klason lignin and is a term to be considered in material balances.
Fig. 5. Time course of pulp viscosity in fractionation treatments ◦ ◦ ◦ carried out at 110 C, 120 C or 130 C.
All the samples analysed were characterised for comparatively-high per cents of saponi:able groups (in the range 8.0 –14:6 wt% of pulps). Comparing these results with the composition of the raw material, it is evident that the saponi:able groups came from esteri:cation reactions between the organic acid present in the media and polysaccharides. The limited content of samples in acetyl groups (0.5 –2.5%) also supported this idea. The results determined for the acid-soluble lignin content of treated solids (within the range 0.4 –1.7%) showed that this fraction was of importance for the purposes of this work. 3.6. Physico-chemical properties of solid phases after treatments The SCAN viscosity of solid phases from treatments is a variable of primary importance to quantify the depolymerisation of cellulose caused by treatments, since the molecular weight distribution is a key factor for many :nal applications (for example, in the manufacture of dissolving pulps). As the samples obtained at short reaction times (with comparatively high lignin contents) were diRcult to be de:bred and dissolved in the assay to measure viscosity, the corresponding data are not reported. Fig. 5 shows the time courses
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Table 5 Composition of liquors from fractionation treatments
Operational conditions ◦
Composition of fractionation media
Temperature ( C)
Time (min)
Glucose conc. (g l−1 )
Xylose conc. (g l−1 )
Conc. of other pentoses (g l−1 )
Furfural conc. (g l−1 )
110
0 30 60 90 120 135 180
0.0 0.4 0.3 0.8 0.5 0.8 0.8
0.1 4.3 10.6 13.9 14.2 15.2 14.9
0.0 0.6 0.1 1.7 1.1 1.0 0.5
0.0 0.0 0.5 0.7 0.6 0.6 0.7
120
0 15 30 60 90 120 150 165 180
0.9 0.9 1.4 1.3 2.7 2.8 3.7 3.0 2.4
3.3 6.9 12.3 14.8 13.7 12.6 11.2 11.4 11.2
0.0 1.7 1.3 0.1 0.8 0.5 0.7 0.7 0.1
0.0 0.0 0.3 0.5 0.9 1.4 1.7 1.8 1.9
130
0 15 30 60 120 150 165 180
0.2 1.1 2.0 3.3 3.6 4.4 5.0 5.6
4.5 10.1 12.5 13.1 12.5 9.1 8.6 7.8
0.0 0.0 0.0 0.1 0.2 0.1 0.1 0.0
0.0 0.0 1.1 1.6 2.4 2.7 3.1 3.0
of viscosity at various temperatures considered. As expected, the experimental results (in the range 992– 609 ml g−1 ) decreased with time in all the cases as◦ sayed, this tendency being more marked at 130 C. −1 Assuming that a viscosity of 800 ml g is a threshold under which the unbleached pulps are not easily utilisable for dissolving pulp production (owing to the limited viscosity loss allowable during the bleaching ◦ stages) [18], the samples obtained at 130 C are out of the desired range. However, it must be taken into account that the viscosity data are underestimated, since they were not corrected for the presence of saponi:able groups in substrates. 3.7. Composition of liquors In order to assess the generation of valuable byproducts from cellulose and hemicelluloses, the concentra-
tions of glucose, xylose, other pentoses and furfural were considered in the experimental plan as dependent variables. Table 5 shows the experimental results obtained for all the conditions assayed. The glucose concentrations determined in all the experiments (in the range, 0.0 –5:6 g l−1 ) were coherent with the data concerning cellulose recovery, and con:rmed the selectivity of treatments towards cellulose solubilisation. Xylose was the main degradation product from hemicelluloses, whereas other pentoses (mainly arabinose) appeared in low proportions. The composition of valuable byproducts in fractionation media increased with the temperature. The experiments at ◦ 110 C were characterised by low furfural concentrations with xylose concentrations (up to 15:2 g l−1 ) which increased with temperature. In experiments ◦ ◦ carried out at 120 C or 130 C, the xylose concentration achieved maximal values at intermediate
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reaction times, owing to the increased participation of sugar-degrading reactions. Under the conditions leading to minimal lignin contents, the per cent of xylan converted into xylose decreased with tempera◦ ◦ ture (70.5%, 58.6% and 52.2%, at 110 C, 120 C and ◦ 130 C, respectively). On the other hand, the furfural concentrations increased steadily with time to reach maximal concentrations in the range 1.9 –3:0 g l−1 . Under the conditions leading to maximum deligni:cation of pulp :bres, the xylan converted into furfural ◦ increased with temperature (from 8.3% at 110 C up ◦ to 19.6% at 130 C), but xylose remained as the main byproduct of hemicellulose decomposition in all the cases considered. 4. Conclusions Kinetic equations describing the behaviour of the several reactions involved in wood fractionation are useful tools for the technical and economic evaluation of processes. The development of mathematical equations describing the dependence of the most important factors involved in treatments on the operational conditions allow the formulation of generalised material and energy balances, providing a sound basis for the understanding of the system’s behaviour. The pulping of beech wood in uncatalysed formic acid media allowed both extensive deligni:cation and excellent cellulose recovery. The mathematical interpretation of the e0ects caused by the reaction time on pulp yield and pulp composition at three di0erent temperatures were interpreted by means of equations providing a close reproduction of the experimental data. Four experimental variables (pulp yield, percent of residual lignin of pulps, per cent of residual hemicelluloses of pulps and cellulose content of pulps) were calculated either by :rst-order kinetic equations or by material balances based on the assumption of 98% cellulose recovery. Additional data (including xylose and furfural concentrations in liquors from fractionation treatments, and pulp content in saponi:able groups and acid-soluble lignin) provided the basic information needed for process evaluation. Acknowledgements The authors are grateful to the “Comisi on Interministerial de Ciencia y Tecnologia” of the Spanish
Ministry of Education for the :nancial support of this work (in the scope of the Research Project “Desarrollo de Procesos con Bajo Impacto Ambiental para la Producci on de Pastas de Celulosa de Alta Calidad”, reference QUI99-0346), as well as to Ms. Aida Ramos Nespereira and Ms. Antonia Rodriguez Jard on for their excellent technical assistance. References [1] Johansson A, Aaltonen O, Ylinen P. Organosolv pulping-methods and pulp properties. Biomass 1987;13:45– 65. [2] Myerly RC, Nicholson MD, Katzen R, Taylor JM. The forestry re:nery. Chemical Technology 1981;11:186–92. [3] Hergert HL. In: Young RA, Akhtar M, editors. Environmentally friendly technologies for the pulp and paper industry. New York: Wiley, 1998. p. 5–67. [4] Poppius K, Sundquist J, Wartiovaara I. In: Kennedy JF, Phillips GO, Williams PA, editors. Wood processing and utilization. New York: Ellis Horwood, 1989. p. 87–92. [5] Poppius K, Laamanen L, Sundquist J, Wartiovaara I, Kauliomaki S. Bleached pulp by peroxyacid=alkaline peroxide deligni:cation. Pap Puu 1986;68:87–92. [6] Sundquist J. Summary of Milox research. Pap Puu 1996;78:92–5. [7] Ruggiero R, Machado EH, da Silva D, Greler S, Nourmamode A, Castellan A. Bleached chemical pulp from Eucalyptus grandis wood produced by peroxyformic acid pulping and photochemical bleaching. Holzforsch 1998;52:325–32. [8] Seisto A, Poppius-Levlin K. Peroxyformic acid pulping of nonwood plants by the Milox method. Part I: Pulping and bleaching. TAPPI Journal 1997;80:215–21. [9] Jim enez L, De la Torre MJ, Maestre F, Ferrer JL, P erez I. Deligni:cation of wheat straw by use of low molecular weight organic acids. Holzforsch 1998;52:191–6. [10] Browning BL. Methods of wood chemistry. New York: Wiley, 1967. [11] Maekawa E, Ichizawa T, Koshijima TJ. An evaluation of the acid-soluble lignin determination in analyses of lignin by the sulfuric acid method. Wood Chemistry and Technology 1989;9:549–67. [12] Tirtowidjojo S, Sarkanen KV, Pla F, McCarthy JL. Kinetic of organosolv deligni:cation in batch and Vow-thorough reactors. Holzforsch 1988;42:177–83. [13] Paraj o JC, Alonso JL, Santos V. Kinetics of Eucalyptus wood fractionation in acetic acid–HCl–water media. Bioresource Technology 1995;51:153–62. [14] Abad S, Alonso JL, Santos V, Paraj o JC. Furfural from wood in catalyzed acetic acid media: a mathematical assessment. Bioresource Technology 1997;62:115–22. [15] Paraj o JC, Alonso JL, V azquez D. On the behaviour of lignin and hemicelluloses during the Acetosolv pulping of wood. Bioresource Technology 1993;46:233–40.
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