Ind. Eng. Chem. Res. 2001, 40, 413-419
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Evaluation of Eucalyptus globulus Wood Processing in Media Made up of Formic Acid, Water, and Hydrogen Peroxide for Dissolving Pulp Production S. Abad, V. Santos, and J. C. Parajo´ * Department of Chemical Engineering, University of Vigo (Campus Ourense), Polytechnical Building, Faculty of Science, Campus Universitario, As Lagoas, 32204 Ourense, Spain
Eucalyptus globulus wood samples were reacted with formic acid, water, and hydrogen peroxide (first stage of treatment). After the desired reaction times, the temperature was raised, and isothermal treatments (second stage) were carried out in the same media. This operational procedure (which followed the philosophy of the two-stage Milox process) was assessed by means of second-order models describing the interrelationships among the main operational variables and both the composition and technical properties of the pulps. The concentrations of xylose and furfural in the pulping liquors were also considered as experimental variables, since these compounds have commercial value. Optimized pulping conditions were selected from the model predictions. Bleacheable pulps (containing 89% cellulose and 2.2% xylan) were obtained under these conditions, at good pulp yield (51.2%) and with a favorable kappa number (23.9). The results achieved for SCAN viscosity (1045 mL/g) and alkaline resistance (R-10 test ) 90.4%; R-18 test ) 93.9%) confirmed the quality of the pulps. Introduction Environmentally friendly pulping process allowing the recovery of valuable hemicellulose- and lignindegradation products can be based on the utilization of organic solvents (“organosolv pulping”). Among the variety of organosolvents proposed for this purpose,1 carboxylic acids (especially formic acid and acetic acid) present excellent characteristics in terms of delignification degree, hemicellulose removal, and selectivity toward cellulose degradation. In this field, processes such as Acetosolv, Acetocell, Formacell, and Milox,2 based on the utilization of acetic acid and/or formic acid, allow a “fractionation” of the lignocellulosic raw materials into separate streams containing hemicellulosedegradation products, lignin-degradation products, and cellulose, all of them being useful for different endproduct applications. The Milox process (nomenclature derived from “Milieu pure oxidative pulping”) is the most well-known technology dealing with concentrated solutions of formic acid. This process (developed at the Finnish Pulp and Paper Research Institute) is based on the simultaneous action of formic acid and peroxyformic acid, which is formed in situ by reacting formic acid with hydrogen peroxide. This mixture provides a reaction medium suitable for delignification, causing little alteration in cellulose and giving pulps easily bleachable by totally chlorine free (TCF) technologies, leading to a final product suitable for paper or viscose manufacture.2,3 The advantages of pulping in formic acid media can be summarized as follows: (i) the chemicals used are environmentally friendly (the fully bleached pulp is obtained using chemicals made up of carbon, hydrogen, and oxygen, the only inorganic materials being sodium hydroxide and magnesium salts used in bleaching);3 (ii) * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +34988387033. Fax: +34988387001.
the yield of the process is comparable to that of bleached kraft pulp; (iii) pulps exhibit high levels of brightness and viscosity; (iv) pulping byproducts are suitable for further utilization; (v) the equipment is simple, and no special materials are required;2,4 and (vi) this technology can be applied to a wide range of raw material such as hardwoods,5-7 softwoods,2,8-10 and nonwood materials.7,11,12 Typically, the Milox process is made up of three sequential stages.2,5 In the first stage, the lignocellulosic raw material is reacted in a medium containing water, formic acid and hydrogen peroxide. Peroxyformic acid is generated in situ through an equilibrium reaction between formic acid and hydrogen peroxide, and the electrophilic HO+ ions formed react with lignin.13 The second stage is performed in the same reaction media, but the temperature is raised and kept constant during a given time. In this stage, the main delignification takes place, the hydrolysis of β-aryl ethers being the major reaction involved. In the third stage, the delignified solid phase from the previous treatments is reacted again with formic acid and hydrogen peroxide, leading to peroxyformic acid generation and further demethylation with formation of additional phenolic hydroxyl groups. The general procedure described above has been modified in some cases according to the different susceptibilities of the raw material.2 For example, nonwood materials have been successfully pulped using a first stage without peroxide addition,11 Obrocea and Cimpoesu9 proposed a single peroxyformic acid stage for sprucewood pulping, and da Silva et al.7 replaced the third stage by an alkaline extraction in the processing of Eucalyptus grandis wood and sugarcane bagasse. This work deals with the optimization of Eucalyptus globulus wood pulping in a two-stage treatment, according to the Milox philosophy. Based on the surfaceresponse methodology, the effects of the operational conditions on the major process variables have been
10.1021/ie000347a CCC: $20.00 © 2001 American Chemical Society Published on Web 11/21/2000
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Table 1. Fixed and Operational Variables Considered in the Experimental Design type of variable fixed independent
dependent
a
variable formic acid concentration temperature of the first stage temperature of the second stage concentration of solids hydrogen peroxide concentration duration of the 1st stage duration of the 2nd stage pulp yield cellulose content of the pulps xylan content of the pulps kappa number of the pulps acid-soluble lignin content content in saponifiable groups viscosity alkaline resistance R-10 alkaline resistance R-18 % xylan converted into xylose % xylan converted into furfural
nomenclature SC HPC t1 t2 y1 y2 y3 y4 y5 y6 y7 y8 y9 y10 y11
value
units
80 70 NBTa 8-16 1-5 30-90 60-180
weight percent of liquors °C °C g of woodb/100 g of liquid g/100 g of woodb min min g of pulpb/100 g of woodb g of cellulose/100 g of pulpb g of xylan/100 g of pulpb g of a-s lig./100 g of pulpb g of NaOH/100 g of pulpb mL/g % % % %
Nomal boiling temperature. b Oven-dry basis.
quantitatively assessed using empirical models that were employed for optimization purposes to obtain bleachable pulps with improved composition and properties. Materials and Methods Raw Material. Eucalyptus globulus wood samples were collected in a local pulp mill (ENCE, Pontevedra, Spain), passed through a two-roll mill fitted with a 2-mm screen, homogenized in a single lot to avoid compositional differences among aliquots, and stored. Analysis of Wood. DCM extracts were assayed by the SCAN C7:62 method. The composition of the raw material in the structural fractions was established by quantitative acid hydrolysis according to the TAPPI T13m method. Monosaccharides and acetic acid in the hydrolyzates were determined by HPLC.15 From the analysis of the monosaccharides, the cellulose, xylan, and araban contents of the wood were calculated by material balances and corrected for sugar decomposition, whereas the acetyl group content of wood was estimated from the concentration of acetic acid in the hydrolyzates. The solid residues from the acid hydrolyzates were considered as Klason lignin, and the percentages of acid-soluble lignin were determined spectrophotometrically by the method of Maekawa et al.16 The results regarding the composition of the wood (obtained as the average of five replicate determinations and expressed as weight percentages on an oven-dry basis) are as follows: DCM extractives, 0.44%; cellulose, 45.5%; xylan, 15.5%; araban, 0.10%; acetyl groups, 4.46%; Klason lignin, 24.3%; and acid-soluble lignin, 4.14%. Processing of Wood Samples. Pulping assays were carried out in stirred glass reactors with temperature control under a variety of operational conditions (see below). Pulps from treatments were sequentially washed with warm 80% formic acid and water, and treated for 30 s at 5000 rpm in an UltraTurrax T-50 defibrator (IKA Labortechnik, Germany) before analytical processing. Analysis of the Pulps. Pulps were subjected to the same analysis described for raw wood as well as to additional determinations of the kappa number, viscosity, and alkaline resistance by the standard methods ISO 302:1981, SCAN C15:88, and ISO 699:1982, respectively. Saponifiable (ester) groups in the pulps were
determined by titration after saponification with KOH/ EtOH (0.5 M) solution. Fitting of Data. The experimental data were fitted to the proposed models by nonlinear regression using commercial software with a built-in optimization routine dealing with the Newton’s method. Results and Discussion Wood pulping is a complex problem from the point of view of both chemistry and engineering. Two approaches can be used for evaluating a given technology: a kinetic study (based on the utilization of pseudohomogeneous or heterogeneous models) or phenomenological modeling, based on the response-surface methodology. Even if this second possibility is not valid for predicting reaction rates or conversions out of the range of conditions tested, it presents advantages in terms of simplicity and allows for the development of equations suitable for design and economic evaluation with a reasonable amount of experimental work. This is important in a preliminary study of a problem like organosolv pulping, in which many operational variables affect a set of dependent effects that must be measured (see below). For this reason, the surface-response methodology has been used in studies dealing with related subjects.17,18 The first and second stages of the Milox process are affected by a number of operational variables. To limit the experimental work to a reasonable extent, some of these variables were fixed according to literature data and our own experimental results (data not shown). Seisto et al.,11 da Silva et al.,7 and Obrocea and Cimpoesu9 suggested isothermal operation in the first stage instead of a linear temperature profile from 60 to 80 °C. The validity of this idea was confirmed in preliminary experiments, and the temperature was kept at 70 °C during the fist stage of all of the experiments performed in this work. Technical-grade formic acid (80 wt % solution) was utilized for reasons of economy, even if better results could be achieved with higher concentrations.3 With these simplifications, a set of experiments (data not shown) was carried out in order to investigate the limits to be considered for the major operational variables. Based on this information, Table 1 shows the fixed and experimental variables considered in the experimental design, as well as their values or variation ranges. The same table includes the definition
Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 415 Table 2. Operational Conditions Assayed in Experiments 1-31 Expressed in Terms of the Independent Variables Defined in Table 1a and in Terms of the Dimensionless, Normalized, Independent Variables Defined in Eqs 1-4b dimensional, independent variables exp 1 2 3 4 5 6 7 8 9 10 11 12 13-19 20 21 22 23 24 25 26 27 28 29 30 31
SC HPC(%, t1 t2 (g/100 g) wood basis) (min) (min) 8 10 10 10 10 10 10 10 10 12 12 12 12 12 12 12 14 14 14 14 14 14 14 14 16
3 2 2 2 2 4 4 4 4 1 3 3 3 3 3 5 2 2 2 2 4 4 4 4 3
60 45 45 75 75 45 45 75 75 60 30 60 60 60 90 60 45 45 75 75 45 45 75 75 60
120 90 150 90 150 90 150 90 150 120 120 60 120 180 120 120 90 150 90 150 90 150 90 150 120
dimensionless, normalized, independent variables x1
x2
x3
x4
-2 -1 -1 -1 -1 -1 -1 -1 -1 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 2
0 -1 -1 -1 -1 1 1 1 1 -2 0 0 0 0 0 2 -1 -1 -1 -1 1 1 1 1 0
0 -1 -1 1 1 -1 -1 1 1 0 -2 0 0 0 2 0 -1 -1 1 1 -1 -1 1 1 0
0 -1 1 -1 1 -1 1 -1 1 0 0 -2 0 2 0 0 -1 1 -1 1 -1 1 -1 1 0
a Solid concentation, SC; hydrogen peroxide concentration, HPC; duration of the first stage, t1; and duration of the second stage, t2. b x , x , x , and x . 1 2 3 4
of the dependent variables selected to measure the composition of the pulps, their technical characteristics, and the content of marketable byproducts in the liquors. The interrelationships between operational and experimental variables were investigated by means of a central composite design in which four independent variables were evaluated at five different levels. This design is made up of a 24 factorial design completed with a star and seven replications in the central point.19,20 Table 2 lists the operational conditions assayed in the experiments. The operational conditions are also expressed in terms of the dimensionless, normalized experimental variables x1, x2, x3, and x4, which were defined for calculation purposes and are calculated from the corresponding dimensional, independent variables listed in Table 1 by means of the equations
x1 ) dimensionless concentration of solids ) 2 (SCi - SCmed)/(SCmax - SCmin) (1) x2 ) dimensionless hydrogen peroxide concentration ) 2(HPCi - HPCmed)/(HPCmax - HPCmin) (2) x3 ) dimensionless duration of the first stage) 2 (t1i - t1med)/(t1max - t1min) (3) x4 ) dimensionless duration of the second stage) 2 (t2i - t2med)/(t2max - t2min) (4) where SC, HPC, t1, and t2 refer to the operational variables defined in Table 1; the subscript i refers to the number of the experiment considered; and the subscripts med, max, and min refer to the central point,
Table 3. Experimental Results Obtained for Variables y1-y11 exp
y1a
y2b
y3c
y4d
y5e
y6f
y7g
y8h
y9i
y10j
y11k
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
53.1 55.8 53.9 53.9 52.5 55.2 52.2 54.3 53.2 54.7 53.0 57.7 53.4 53.6 53.4 53.5 53.8 53.4 53.6 53.3 53.9 52.4 54.6 53.1 55.4 54.4 54.2 53.0 54.8 53.7 54.8
82.9 75.5 78.4 77.7 82.8 76.4 79.8 82.4 85.0 74.0 78.9 75.6 81.2 81.3 81.4 81.5 80.9 81.1 81.4 83.8 80.6 77.4 74.1 78.4 74.0 83.4 80.3 81.8 75.4 83.2 77.8
2.77 3.49 3.13 2.70 2.84 3.36 2.8 3.09 2.64 2.83 3.60 3.35 3.13 3.06 3.07 2.97 3.02 3.03 3.12 2.41 2.80 3.31 3.41 2.99 3.33 2.94 3.46 2.68 3.52 2.73 3.06
30.2 31.6 28.3 35.7 33.2 33.1 27.7 35.1 28.6 36.0 34.7 39.0 35.0 35.0 34.8 34.6 34.6 34.5 34.6 31.3 35.4 33.5 35.5 35.6 37.0 36.8 39.1 34.8 35.6 35.5 39.9
0.70 0.95 0.77 1.06 0.80 0.88 0.80 0.86 0.68 1.01 0.76 1.12 0.90 0.88 0.85 0.91 0.89 0.91 0.88 0.69 1.01 0.91 0.93 0.85 1.10 0.78 0.98 0.92 1.13 0.85 1.04
6.75 7.00 7.19 6.86 4.65 6.70 6.97 6.96 5.20 6.77 6.78 6.55 6.23 6.36 6.40 6.38 6.36 6.32 6.39 6.54 6.07 6.16 6.62 7.91 6.35 6.43 5.55 6.39 7.10 6.16 6.58
1037 979 1026 1069 1025 947 938 959 925 1030 968 817 924 925 941 944 925 927 947 900 973 953 923 923 858 947 894 918 884 975 930
89.0 87.6 87.7 88.3 90.4 87.2 88.7 89.3 88.9 86.9 87.4 86.2 88.6 88.4 88.4 88.4 88.5 88.4 88.3 87.9 87.8 86.9 87.0 87.7 88.9 88.1 87.2 87.0 87.2 87.5 89.0
91.0 90.1 89.7 90.2 93.7 89.3 91.0 90.4 91.6 90.0 88.7 87.2 90.1 90.6 90.2 90.3 90.8 89.7 90.4 90.5 90.4 89.6 89.3 90.3 90.9 91.8 90.8 89.7 90.3 91.4 89.5
88.0 77.7 80.9 77.7 88.1 78.1 85.8 75.9 91.2 78.1 84.9 67.5 84.1 84.0 82.6 80.9 82.6 84.0 80.5 82.8 76.7 86.3 80.2 80.9 70.7 80.2 78.0 73.5 73.8 74.1 80.9
3.35 4.62 4.00 2.09 5.33 2.32 4.39 2.16 4.97 4.00 3.24 1.12 4.13 4.25 4.54 4.57 4.83 4.42 4.28 4.86 3.60 3.43 4.05 3.72 1.87 5.07 2.05 3.40 1.92 4.79 2.99
a y ) pulp yield, %. b y ) % cellulose. c y ) % xylan. d y ) 1 2 3 4 kappa number. e y5 ) % acid-soluble lignin. f y6 ) saponifiable g h groups, g of NaOH/100 g y7 ) viscosity, mL/g. y8 ) R-10 test for alkaline resistance, %. i y9 ) R-18 test for alkaline resistance, %. j y10 ) % xylan converted into xylose. k y11 ) % xylan converted into furfural.
upper limit, and lower limit, respectively, of the variable variation ranges. Table 3 lists the results determined for the pulp yield (y1), for the composition of the pulps (measured by the cellulose content of the pulps, y2; the xylan content of the pulps, y3; the kappa number, y4, which was found to be proportional to the Klason lignin content; the acidsoluble lignin content of the pulps, y5; and the content in saponifiable groups, y6), for the technical characteristics of the pulps (including viscosity, y7; R-10 test for alkaline resistance, y8; and R-18 test for alkaline resistance, y9), and for the composition of the liquors (measured by the percentage of xylan converted into xylose, y10, and by the percentage of xylan converted into furfural, y11). The experimental results allowed for the development of empirical models, in which the dependent variables were evaluated as the sum of linear, interaction, and second-order terms involving the independent, dimensionless variables, according to the generalized expression
yj ) b0j +
∑i bijxi + ∑i ∑k bikjxixk
(5)
where yj is the dependent variable under consideration; xi and xk (i, k ) 1-4, k g i) are the dimensionless, independent variables defined in eqs 1-4; and b0j, ..., bikj are the regression coefficients, calculated from the experimental data by multiple regression using the least-squares method. Table 4 presents the regression coefficients obtained for variables y1-y7, y10, and y11, which led to statistically
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Table 4. Regression Coefficients and Significance (Based on a t-Test) and Statistical Parameters Measuring the Correlation and Significance of Models parameter b0j b1j b2j b3j b4j b12j b13j b14j b23j b24j b34j b11j b22j b33j b44j r2 F test signif. levell
y1a
y2b
53.5j
81.3j
0.23j -0.32j 0.07 -0.88j -0.05 0.40j 0.15 0.17 -0.03 0.18 0.10 -0.01 -0.03 0.49j 0.8941 9.64 >99%
-0.73j 1.11j 0.95j 2.23j 0.10 -1.03j 0.55 -0.24 -0.40 0.81j -0.16 -1.32 -0.31 -0.32 0.9516 11.0 >99%
y3c 3.06j 0.07k 0.02 -0.13j -0.23j 0.00 0.09j -0.07j 0.06 -0.10j 0.04 -0.03 0.01 0.04k -0.04 0.8964 9.89 >99%
y4d 34.7j 2.33j -0.38k 0.55j -1.57j 0.28 -0.75j 0.82j -0.71j -0.66j 0.22 -0.09 -0.16 -0.10 -0.06 0.9424 18.7 >99%
y5e 0.89j 0.06j -0.01 0.03j -0.10j 0.04j 0.01 0.00 -0.02 0.01 -0.04j -0.01 0.02 0.00 0.00 0.909 11.4 >99%
y6f 6.35j 0.03 -0.13j -0.25j -0.09k -0.14j 0.24j 0.30j 0.26j -0.06 -0.46j 0.07 0.02 0.01 0.04 0.9133 12.0 >99%
y7g 933j
-31.7j -19.3j 4.36 13.7j 21.9j -4.95 15.2j -0.15 -1.14 2.44 12.3j 14.3j 9.03k -19.0j 0.8906 9.30 >99%
y10h 82.7j
-2.43j 0.44 -0.82 3.06j -1.20 -1.51k -1.92j 0.17 -0.32 1.78j 0.20 -0.36 -0.71 -2.12j 0.8423 6.10 >99%
y11i 4.43j -0.16k -0.25j 0.02 0.92j -0.02 0.08 -0.02 0.23j 0.23j 0.60j -0.28j -0.14k -0.22j -0.32j 0.9304 15.3 >99%
a y ) pulp yield, %. b y ) % cellulose. c y ) % xylan. d y ) kappa number. e y ) % acid-soluble lignin. f y ) saponifiable groups, g 1 2 3 4 5 6 of NaOH/100 g g y7 ) viscosity, mL/g. h y10 ) % xylan converted into xylose. i y11 ) % xylan converted into furfural. j Coefficients significant k at the 95% confidence level (based on the t-test). Coefficients significant at the 90% confidence level (based on the t-test). l Based on the F test.
significant models. The significance of the coefficients was measured on the basis of a t-test. The same table includes the statistical parameters measuring the correlation (r2) and the statistical significance (F test) of the models.19-20 The validity of the models for quantitative predictions is also confirmed by the comparison between experimental and calculated results: the deviations determined for the most important variables of this study (y1, y2, y3, y4, and y7) were in the range of (10% in all cases. Dependence of the Pulp Yield on the Operational Conditions. Pulp yield (variable y1) was mostly affected by the hydrogen peroxide concentration (variable HPC) and the reaction time of the second stage (t2). Figure 1a shows the predicted dependence of the pulp yield on both variables, for media with a concentration of solids SC ) 12 g/100 g of liquors in experiments whose first stage lasted t1 ) 60 min. As expected, the pulp yield decreased with the severity of treatments, the main effects being caused by t2 (particularly in assays carried out with low hydrogen peroxide concentration). The lowest experimental pulp yield (52.2%, experiment 7) was in good agreement with the result predicted in Figure 1a for the severest conditions (t2 ) 150 min, HPC ) 4%). Effects of Treatments on the Pulp Composition. The values of the regression coefficients shown in Table 4 demonstrate that the cellulose content of the pulps (variable y2) was mostly affected by the hydrogen peroxide concentration (HPC) and the duration of the second stage (t2). Figure 1b shows the isoresponse curves calculated for y2 when SC and t1 were fixed at the central point of their variation ranges. The effects caused by t2 were more important when HPC was higher than 3%. Pulps containing more than 84% cellulose can be reached using 4% hydrogen peroxide in treatments with a second stage longer than 140 min. This prediction is in good agreement with the experimental results achieved in experiment 9 (y2 ) 85.0%), which was carried out under operational conditions closely related to those cited above. Considering the experimental data determined for the wood composition, pulp yield, and cellulose content of
the pulps, the selectivity of delignification toward cellulose decomposition can be evaluated on the basis of the percent of cellulose recovered in the pulps with respect to the cellulose contained in the wood. The average value obtained for this parameter in the whole set of experiments was 94.3%, a result confirming the excellent selectivity of the studied process. The xylan content of the pulps (variable y3) was mainly affected by the duration of the process (measured by t1 and t2), the relative influence of t1 being of less importance in the experimental domain defined by t1 > 60 and t2 > 130 min (see Figure 1c). Under these conditions, pulps with a remarkably low xylan content ( 800 mL/g are desirable for reaching the desired final quality. The experimental results listed in Table 3 show that this condition is satisfied by all of the pulps obtained in this study (variation range of y7 ) 817-1067 mL/g). A significant dependence of viscosity on the operational conditions was found (see Table 3). Based on the empirical model, Figure 2c shows the results calculated for experiments carried out at fixed values of t1 (60 min) and t2 (120 min). Even if good pulps were obtained in the whole experimental domain, viscosity was improved by low solid concentrations and limited hydrogen peroxide concentrations. Variables y8 and y9, measuring the alkaline resistance of the pulps led to non-significant models, because of the limited variation range and the incidence of experimental error. Even if a quantitative interpretation of the data is not feasible, the ranges of values determined for R-10 (86.2 e y8 e 92.4) and for R-18 (87.2 e y8 e 93.7) are considered very promising, because they show a favorable distribution of molecular weights and because the target values for dissolving-grade pulps (R10 g 91, R-18 g 94) seem easy to reach in further TCF bleaching stages without causing a significant reduction in viscosity. Effects of Treatments on the Xylan-Degradation Products. To assess the generation of valuable byproducts from xylan degradation, the conversions of xylan to xylose (variable y10) and furfural (variable y11) were included in this study. The experimental results of Table 3 and the model predictions confirmed that xylose accounted for a significant portion of the initial xylan and suggested that this potentially marketable byproduct can contribute to the overall economy of the process.
The results derived from the model show that the conversion of xylan into xylose was improved by low concentrations of solids and by long durations of the second stage. Comparatively, a reduced proportion of xylose was dehydrated to furfural: the variation range of variable y11 (which was expressed as percent of equivalent xylan in order to allow a direct comparison with y10) was only 1.12-5.33% xylan converted into furfural. The model predictions for variable y11 suggest that longer durations of the first and second stages resulted in increased conversions of xylan into furfural to reach a maximum value of 5.4%. Pulping under Optimized Conditions. The empirical models developed for the variables were optimized (maximized in the case of y2 and y7 and minimized in the case of y3, y4, and y6) by derivatization of eq 5 respect xj (j ) 1-4) with the restriction |xj| e 1 (j ) 1-4), and the corresponding sets of optimum values x10, ..., x40 obtained for each dependent variable cited above were calculated. On the basis of this information and in light of the relative importance of the dependent variables considered in this step, the following conditions were considered to correspond to a technical optimum: SC, 10.8 g/100 g of liquor; HPC, 4% (wood basis); t1 ) 75 min; and t2 ) 150 min. For these conditions, the models predicted y1 ) 52.7% yield, y2 ) 84.4% cellulose, y3 ) 2.67% xylan, y4 ) 30.2 kappa number units; y7 ) 942 viscosity units (mL/g), and y10 ) 88.0% xylan converted into xylose. To confirm the validity of the above calculations, triplicate pulping assays were conducted under the same pulping conditions, and the following average values were determined for the major experimental variables: y1 ) 51.2% yield, y2 ) 86.5% cellulose, y3 ) 2.55% xylan, y4 ) 23.9 kappa number units; y7 ) 1045 viscosity units (mL/g), and y10 ) 88.2% xylan converted into xylose. Additional experimental data were obtained for the alkaline resistances (y8 ) 90.4% and y9 ) 93.9%). The concordance between experimental and calculated results confirmed the validity of the models for quantitative predictions, and the composition and quality parameters of the pulp showed that the requeriments for dissolving-grade pulps are likely to be fulfilled by TCF bleaching. Acknowledgment The authors are grateful to Dr. Ju¨rgen Puls and Dr. Bodo Saake (Institut fu¨r Holzchemie und Chemische Technologie des Holzes, Hamburg, Germany) for their valuable help, to the Commission of the European Community for the financial support of this work (in the framework of the European Project “New Dissolving Pulps”, reference FAIR-CT98-3855-DG12-SSMI), to the Ministry of Education and Culture of Spain for the financial support of the Project “Development of processes with low environmental impact for production of high quality cellulose pulp” (reference QUI1999-0346), and to Ms. Aida Ramos Nespereira for her excellent technical assistance. Literature Cited (1) Johansson, A.; Aaltonen, O.; Ylinen, P. Organosolv pulping. Methods and pulp properties. Biomass 1987, 13, 45-65. (2) Sundquist, J.; Poppius-Levlin, K. Milox pulping and bleaching. In Environmentally friendly technologies for the pulp and paper industry; Young, R. A., Akhtar, M., Eds.; John Wiley & Sons: New York, 1998.
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Received for review March 20, 2000 Revised manuscript received September 14, 2000 Accepted September 21, 2000 IE000347A
