Ind. Eng. Chem. Res. 2001, 40, 6201-6206
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Acetone Pulping of Wheat Straw. Influence of the Cooking and Beating Conditions on the Resulting Paper Sheets L. Jime´ nez,*,† J. C. Garcı´a,† I. Pe´ rez,† J. Ariza,‡ and F. Lo´ pez‡ Departamento de Ingenierı´a Quı´mica, Campus de Rabanales, Edificio C-3, Universidad de Co´ rdoba, 14071 Co´ rdoba, Spain, and Departamento de Ingenierı´a Quı´mica, Campus de la Rabida, Universidad de Huelva, Carretera de Palos s/n, 21919 Huelva, Spain
This paper reports on the influence of independent variables in the pulping of wheat straw [viz., cooking temperature (140-180 °C), time (60-120 min), and acetone concentration (40-80%)] and of the number of PFI beating revolutions (0-1750) to which the pulp is subjected, on the yield and Shopper-Riegler index of the pulp, and on the breaking length, stretch, burst index, and tear index of the resulting paper sheets. By using a central composite factorial design, equations that relate each dependent variable to the different independent variables were obtained that reproduced the experimental results for the dependent variables with errors of less than 25%. Ensuring the optimal Shopper-Riegler index for the pulp (29.53 °SR), and also the optimal stretch (2.37%) and burst index (2.63 kN/g) for the paper sheets, entails using a 180 °C temperature and 1750 number of PFI beating revolutions. The use of these conditions in conjunction with a 40% acetone concentration and a 60 min cooking time allows one to save solvent in the recovering and recycling steps and also to increase production. The values thus obtained for the breaking length and tear index differ by less than 12% and 29% from their optimum levels (5781 and 4.72 mN‚m2/g, respectively). Introduction The process by which raw materials are pulped produces large amounts of wastewater of a highly polluting power, especially in sulfite- and sulfate-based processes. One way of circumventing this shortcoming is by removing fiber and lignin using suitable organic solvents. Although such solvents have long been known to be effective, they have not yet been used on a industrial scale.1-4 Organosolv pulping procedures provide a number of advantages including the following: (a) They can be used with any type of woody and nonwoody raw material. (b) The properties of the resulting pulp are similar to those of kraft pulp, but the yield obtained is higher and the lignin content lower. (c) The pulp is whiter and more readily bleached than kraft pulp, which saves bleaching reagents. (d) The pulp is easier to beat. (e) The process uses no sulfur-containing reagents, so it is less contaminant than traditional alternatives. (f) The process is more economical than kraft pulping at small-to-medium scale plants. (g) The solvents can be efficiently recovered. (h) The byproducts possess commercial value. (i) The procedure has a much weaker environmental impact than conventional ones. (j) The process uses less water, energy, and reagents than traditional alternatives. (k) The bleaching effluents are more readily degraded. (l) Raw materials are more efficiently used (virtually the whole mass can be used for some purpose). * To whom correspondence should be addressed. Phone: +34 957 218658. Fax: +34 957 218625. E-mail:
[email protected]. † Universidad de Co ´ rdoba. ‡ Universidad de Huelva.
Organosolv processes have been applied with varying success to hardwood and softwood and also, to a lesser extent, to nonwood materials. One such material, wheat straw, has been studied in this respect by authors such as Papatheophanus et al.,5 Lawther et al.,6 Elsakhawy et al.,7,8 and Jime´nez et al.9-15 Zarubin et al.16 studied delignification in hardwood and softwood chips using oxygen in an acetone/water medium containing no acid or base. They obtained good results at an acetone concentration of 40-60% and found the cooking temperature and time to be the two most influential variables. Jime´nez et al.12 examined the influence of the variables in the acetone pulping of wheat straw (viz., temperature, time, and acetone concentration) on the holocellulose, R-cellulose, and lignin contents in the resulting pulp and found temperature to exert the strongest influence and time and the acetone concentration the weakest. The current pulp output is inadequate to meet the increasing demand, particularly in developing countries; this is leading to an increasing shortage of wood raw materials and to gradual deforestation of some areas in the world. This makes nonwood materials such as wheat straw and various other agricultural residues abundant in our country especially attractive choices for pulp making. Pulping processes have been studied in light of a variety of models with a view to deriving equations for predicting the quality of the resulting pulp in terms of the process variables and for establishing the best operating conditions. Most such models are mathematical designs based on the kinetics of delignification and intended to predict the extent to which it is bound to occur. So far, few authors have used a factorial design to develop empirical models involving several independent variables to identify patterns of variation in the de-
10.1021/ie0010161 CCC: $20.00 © 2001 American Chemical Society Published on Web 12/04/2001
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pendent variables (viz., chemical composition, yield, κ index, and degree of polymerization) of various pulping processes as applied to diverse vegetable materials. Organosolv pulping has been studied in this respect by Parajo´ et al.,17 Tjeerdsma et al.,18 Va´zquez et al.,19 Vega et al.,20 Gilarranz et al.,21,22 and Jime´nez et al.;9-15 none, however, considered the effect of the operating variables on the physical properties of the paper sheets obtained from the pulp. These empirical models are preferred to theoretical ones because the latter are rather complex when more than two independent variables are involved. For this reason, in this work we used a central composite design to examine the influence of the independent cooking variables (viz., temperature, time, and acetone concentration) in the organosolv pulping of wheat straw with acetone/water mixtures and of the number of PFI beating revolutions to which the pulp is subjected, on the yield and Shopper-Riegler index of the pulp, and on various properties of paper sheets obtained from it (viz., breaking length, stretch, burst index, and tear index) in order to establish the optimum operating conditions for the process. Experimental Section Raw Material. Sun-dried wheat straw (Triticum vulgaris), the composition of which was determined to be 70.30% holocellulose, 17.73% lignin, 6.84% ash, 4.23% ethanol/benzene extractables, and 0.90% others, was used throughout. Analysis of the Raw Material, Pulp, and Paper Sheets. The starting materials and the products obtained from them were characterized according to the following standard methods: holocellulose (Wise et al. method23), lignin (TAPPI 222), ash (TAPPI 211), ethanol/benzene extractables (TAPPI 204), Shopper-Riegler index (TAPPI 227), breaking length (TAPPI 494), stretch (TAPPI 404), burst index (TAPPI 494), and tear index (TAPPI 414). Pulping and Sheetmaking. Pulp was obtained by using a 15 L batch cylindrical reactor that was heated by means of electrical wires and was linked through a rotary axle (to ensure proper agitation) to a control unit including a motor actuating the reactor and the required instruments for measurement and control of pressure and temperature. The wheat straw was cooked in the reactor. Next, the cooked material was fiberized in a wet desintegrator at 1200 rpm for 30 min, and the screenings were separated by sieving through a screen of 1 mm mesh size. The pulp obtained was beat in a PFI refiner. Paper sheets were prepared on an ENJO F-39.71 sheet machine according to the TAPPI 220 standard method. Experimental Design. The tested model uses a series of points (experiments) around a central one (central experiment) and several additional points (additional experiments) to estimate the first- and secondorder interaction terms of a polynomial. This design meets the general requirement that every parameter in the mathematical model can be estimated from a fairly small number of experiments.24 The total number of observations (experiments) required for our four independent variables [viz., temperature (T), cooking time (t), acetone concentration (A), and number of PFI beating revolutions (R)] was found to be 25.
Table 1. Processing Conditions Used in the Acetone Pulping of Wheat Straw and the Beating of the Pulp and Experimental Results of the Characterization of Paper Sheets Obtained from the Beaten Pulpa XT
Xt
XA
XR
YI, %
SR, °SR
BL, m
ST, %
BI, TI, kN/g mN‚m2/g
0 +1 +1 +1 +1 -1 -1 -1 -1 +1 +1 +1 +1 -1 -1 -1 -1 0 0 0 0 +1 -1 0 0
0 +1 +1 -1 -1 +1 +1 -1 -1 +1 +1 -1 -1 +1 +1 -1 -1 0 0 +1 -1 0 0 0 0
0 +1 +1 +1 +1 +1 +1 +1 +1 -1 -1 -1 -1 -1 -1 -1 -1 0 0 0 0 0 0 +1 -1
0 -1 +1 -1 +1 -1 +1 -1 +1 -1 +1 -1 +1 -1 +1 -1 +1 -1 +1 0 0 0 0 0 0
64.36 50.48 50.48 57.63 57.63 76.53 76.53 79.62 79.62 40.34 40.34 46.42 46.42 73.40 73.40 74.86 74.86 64.36 64.36 60.14 68.72 47.35 75.47 69.16 57.27
15.00 12.50 30.00 13.00 27.50 8.50 28.00 9.50 23.00 12.50 35.00 12.00 30.00 9.50 24.00 10.00 25.50 9.00 20.00 16.00 17.50 20.00 17.00 15.00 15.00
3244 3610 4962 2954 4891 608 1197 601 967 2638 4125 3697 5789 641 1474 622 1094 2129 3885 3726 2017 5749 1199 2541 3803
1.36 1.29 2.07 1.11 1.54 0.94 0.79 0.54 0.56 1.03 1.55 1.46 2.93 0.59 0.65 0.72 0.79 0.94 1.82 1.60 1.04 2.24 0.63 1.03 1.51
1.23 1.37 2.45 1.14 2.59 0.13 0.32 0.15 0.34 1.26 2.04 1.60 3.02 0.17 0.52 0.14 0.35 0.69 1.66 1.31 1.00 2.52 0.23 0.93 1.49
4.14 3.53 3.36 3.72 3.93 1.80 2.72 1.36 2.03 2.82 2.18 4.54 3.62 2.60 4.31 1.84 2.61 4.06 4.04 4.28 3.66 4.00 2.00 4.31 4.03
a X ) normalized temperature. X ) normalized time. X ) T t A normalized acetone concentration. XR ) normalized number of PFI beating revolutions. YI ) yield. SR ) Shopper-Riegler index. BL ) breaking length. ST ) stretch. BI ) burst index. TI ) tear index.
The values of the independent variables were normalized from -1 to +1 by using eq 1 to facilitate direct comparison of the coefficients and visualization of the effects of the individual independent variables on the response variable.
X-X h Xn ) 2 Xmax - Xmin
(1)
where Xn is the normalized value of T, t, A, or R; X is the absolute experimental value of the variable concerned; X h is the mean of all of the experimental values for the variable in question; and Xmax and Xmin are the maximum and minimum values, respectively, of such a variable. Experimental data was fitted to the following secondorder polynomial:
Z ) a + bXT + cXt + dXA + eXR + fXT2 + gXt2 + hXA2 + iXR2 + jXTXt + kXTXA + lXTXR + mXtXA + nXtXR + oXAXR (2) where Z denotes the response variables [yield (YI), Shopper-Riegler index (SR), breaking length (BL), stretch (ST), burst index (BI), and tear index (TI)], XT, Xt, XA, and XR are the normalized values of T, t, A, and R; and letters a-o denote constants. The 25 experiments conducted, together with the corresponding normalized values for the independent variables, are given in Table 1. Results and Discussion A set of four preliminary experiments were conducted under the central operating conditions, namely, 60%
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acetone, 160 °C, 90 min, and 875 PFI beating revolutions. The experimental results obtained in the determinations of the dependent variables differed from the mean values, shown in the first row of Table 1, by less than 5-10%. Subsequent tests, corresponding to the experimental design adopted, provided the results shown in the other rows. The operating variables were varied over the following ranges: 60-120 min, 140-180 °C, 40-80% acetone concentration, and 0-1750 PFI beating revolutions. A constant liquid/solid ratio of 40 was used in all tests to ensure thorough mixing in the reactor. The time needed for the operating temperature to be reached was 2-4 min and was excluded from the pulping time; such a short time was the result of the wheat straw and the water/acetone mixture being heated prior to meeting in the reactor. The short time (in the region of 2-4 min) required to attain the operating temperature (140-180 °C) was found to have no substantial influence on the final properties of the pulp, first, because such a time was only a small fraction of the overall time and, second, because, owing to the combined effects of temperature and time, the severity of the treatment was negligible during the time (2-4 min) taken to raise the temperature from about 100 to 180 °C relative to the treatment involving the operating temperature (140-180 °C) and cooking times of 60120 min.25 The BMDP software suite26 was used to conduct a multiple linear regression analysis involving all terms in eq 2 except those with Snedecor’s F values of less than 4, which were left out using the stepwise method.27 The following equations (accompanied by their corresponding F and r2 values and the highest p and lowest Student’s t values for their terms at a confidence limit of 95%) were obtained:
Figure 1. Variation of the pulp yield with the temperature and acetone concentration at 60 min and constant cooking time.
YI ) 64.05 - 13.73XT - 2.45Xt + 3.91XA 1.75XT2 - 1.09XTXt + 1.68XTXA (F ) 507.3; r2 ) 0.99; p < 0.003; t > 3.5)
(3)
SR ) 15.36 + 2.08XT + 8.14XR + 3.95XT2 (F ) 124.3; r2 ) 0.95; p < 0.0001; t > 4.7) (4) BL ) 3321 + 1667XT + 605XR - 687Xt2 + 288XTXR (F ) 49.8; r2 ) 0.91; p < 0.05; t > 2.1) (5) ST ) 1.23 + 0.50XT + 0.23XR + 0.20XTXR + 0.21XtXA (F ) 20.9; r2 ) 0.81; p < 0.01; t > 2.8)
(6)
BI ) 1.15 + 0.87XT + 0.37XR + 0.24XTXR (F ) 99.2; r2 ) 0.93; p < 0.0006; t > 4.0) (7) TI ) 4.08 + 0.58XT - 1.13XT2 - 0.47XTXt + 0.30XTXA - 0.35XTXR (F ) 24.53; r2 ) 0.87; p < 0.0007; t > 3.1)
(8)
The yield values calculated from eq 3 differ from their experimental counterparts by less than 5%. Nonlinear programing28 (More and Toraldo’s method29) was applied to eq 3 in order to determine the highest yield over the ranges studied for the process variables (normalized values from -1 to +1 for all); the maximum yield thus calculated for a low temperature and a short
Figure 2. Variation of the pulp yield with the cooking time and acetone concentration at 140 °C and constant temperature.
time (a normalized value of -1 for both variables), in addition to a high acetone concentration (normalized value +1), was 79.62%, typical of semichemical pulp. Equation 3 allows one to estimate the variation of the yield with changes in each independent variable over the range considered on constancy of the other variables. With constant temperature, time, and ethanol concentration at their normalized values -1, -1, and +1, respectively, the greatest changes in yield resulted from variation of the temperature (21.92 units or 27.53% with respect to the maximum value) and the smallest ones from the processing time (2.72 units or 3.42%); the effect of the acetone concentration (4.46 units or 5.60%) was between the previous two extremes. The yield was thus much more sensitive to changes in temperature than to variations in time or the acetone concentration. Figures 1 and 2 confirm these results. From the foregoing it follows that, if low values of the three independent variables (temperature, time, and acetone
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Table 2. Optimum Values of the Dependent Variables and Variations with Changes in the Independent Variables for the Acetone Pulping of Wheat Straw and the Subsequent Beating of the Pulp
dependent variable yield, %
Shopper-Riegler index, °SR breaking length, m stretch, % burst index, kN/g tear index, mN‚m2/g
percent errors made in estimating the dependent variables with respect to optimum value the experimental values (max) 5
20 25
normalized values of the independent variables leading to optimum values of the dependent variables XT
Xt
XA
79.62
-1
-1
+1
39.44 (min)
+1
+1
-1
29.53
+1
5781
0 -1
2.37
+1
25
2.63
+1
20
4.72
+0.75
Figure 3. Variation of the Shopper-Riegler index with the temperature and number of PFI beating revolutions at all values and constant cooking time and acetone concentration.
concentration) are used, the yield decreases to 75.66% (i.e., by only 5.60% with respect to its highest level, 79.62%) but with substantial savings of cooking solvent. There is always some loss through recovery and recycling. To produce chemical-grade pulps, it is advisable to use a high temperature and a long cooking time (normalized values of +1 for both variables), as well as a low acetone concentration (normalized value -1); the lowest yield thus obtained as calculated using the nonlinear programing is 39.44%. Following a reasoning similar to that in the previous paragraph, from the foregoing it follows that the yield obtained by using a short time, a high temperature, and a low acetone concentration, 46.52%, is 17.95% greater than the minimum value. The results of Table 2 were obtained by using a similar procedure with the other dependent variables. As can be seen, all of the equations relating the dependent variables to the independent ones repro-
+1
+1
25
XR
+1 -1
+1 +1
-1
+1
-1
maximum changes in the dependent variables (percentages with respect to the optimum values are shown in brackets) with changes in the independent variables (from -1 to +1) T 21.92 (27.53) 33.00 (83.60) 2.14 (7.25) 3910 (67.64) 1.40 (59.07) 2.22 (84.41) 3.47 (73.52)
t
A
R
2.72 4.46 (3.42) (5.60) 7.08 11.18 (17.95) (28.35) 16.28 (55.13) 687 1786 (11.88) (30.89) 0.42 0.42 0.86 (17.72) (17.72) (36.29) 1.22 (46.39) 0.71 0.45 0.53 (15.04) (9.35) (11.23)
Figure 4. Variation of the breaking length with the temperature and cooking time at 1750 and constant number of PFI beating revolutions.
duce the experimental results with errors of less than 25%. As can be seen from Table 2, a high temperature is required to ensure a high Shopper-Riegler index, breaking length, stretch, and burst index. However, the best values for the tear index are obtained with lower temperatures. Also, with the exception of the tear index, it is advisable to use a large number of PFI beating revolutions. The cooking time should be short; by exception, maximizing the breaking length entails using a medium time. Also, the acetone concentration should be low except if the tear index is to be optimized. As can also be seen from Table 2, the temperature is the individual independent variable most strongly influencing the yield, breaking length, stretch, burst index, and tear index. The number of PFI beating revolutions is the individual independent variable most strongly influencing the Shopper-Riegler index; on the other hand, the processing time and the acetone concentration are the least influential variables. Figures 3-7 (which correspond to the Shopper-Riegler index, breaking length, stretch, burst index, and tear index, respectively) and similar others confirm the above-described results.
Ind. Eng. Chem. Res., Vol. 40, No. 26, 2001 6205 Table 3. Operating Conditions Required To Ensure Near-Optimal Values of the Dependent Variables in the Acetone Pulping of Wheat Straw and the Subsequent Beating of the Pulp
dependent variable
optimum value
near-optimal value
variation, %
yield, % Shopper-Riegler index, °SR breaking length, m stretch, % burst index, kN/g tear index, mN‚m2/g
79.62 29.53 5781 2.37 2.63 4.72
46.52 29.53 5094 2.37 2.63 4.27 4.19 3.35
41.57 11.88 9.35 11.23 29.03
Figure 5. Variation of the stretch with the temperature and cooking time at 1750 and 40% and constant number of PFI beating revolutions and acetone concentration.
Figure 6. Variation of the burst index with the temperature and number of PFI beating revolutions at all values and constant cooking time and acetone concentration.
Table 3, which was constructed in light of the previous considerations, compares the optimum values of the dependent variables with others obtained with more
normalized values of the independent variables required to obtain near-optimal values of the dependent variables XT Xt XA XR +1 +1 +1 +1 +1 +0.75 +0.75 +1
-1 -1 -1 -1 -1 -1
-1 -1 -1 +1 -1
+1 +1 +1 +1 -1 +1 +1
Figure 7. Variation of the tear index with the temperature and cooking time at 1750 and 40% and constant number of PFI beating revolutions and acetone concentration.
favorable values (more desirable conditions in terms of process economy) of the independent variables less markedly influencing the dependent ones; the values of the independent variables considered are also shown, as are the variations (as percentages) of the most suitable values of the dependent variables relative to their optimum levels. As can be seen from Table 3, using a short processing time, a low acetone concentration, a high temperature, and a large number of PFI beating revolutions results in Shopper-Riegler index, stretch, and burst index values that differ by less than 12% from their optimum levels and in tear index values within 29% of the optimum level. In view of these results, using these milder conditions is advisable as they save solventsalthough this is recovered and recycled, some loss is always inevitablesand boost yield, at the expense of a slightly lower tear index. A comparison of the results for the properties of paper sheets made under the conditions of Table 3 from acetone pulp and those for sheets produced from wheat straw pulp obtained by soda pulping12 (viz., 7677 m breaking length, 2.06% stretch, 3.44 kN/g burst index, and 4.33 mN‚m2/g tear index with a yield of 46.62% and a Shopper-Riegler index of 27.0 °SR) reveals that the stretch, burst index, and tear index for the sheets from acetone pulp exceed those for sheets from soda pulp and also that the breaking length is lower for the sheets from acetone pulp.
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Conclusions As shown above, temperature and the number of PFI beating revolutions are the variables most strongly influencing the properties of paper sheets obtained by acetone pulping of wheat straw (viz., breaking length, stretch, burst index, and tear index); the opposite is true of the acetone concentration and cooking time. Using a temperature of 180 °C, an acetone concentration of 40%, a cooking time of 60 min, and 1750 beating revolutions provides pulp of quite acceptable properties (viz., 5094 m breaking length, 2.37% stretch, 2.63 kN/g burst index, and 3.55 mN‚m2/g tear index). On the other hand, the properties of paper sheets made from acetone pulp are similar to or even better than those for soda pulp, with the advantages that the acetone pulping process is less contaminating (the acetone can be recycled by distillation) and that the residual aqueous solution can be concentrated and dried to obtain lignin of use in the formulation of novel materials. Acknowledgment The authors are grateful to Ecopapel, S.L. (E Ä cija, Seville, Spain), and ENCE (Huelva, Spain) for their support and to Spain’s DGICyT for funding of this research within the framework of Project PPQ20001068-C02-01. Literature Cited (1) Asiz, S.; Sarkanen, K. Organosolv pulping. A review. TAPPI J. 1989, 72 (3), 169-175. (2) Stockburger, P. An overview of near-commercial and commercial solvent-based pulping processes. TAPPI J. 1993, 76 (6), 71-74. (3) Jime´nez, L.; Maestre, F.; Pe´rez, I. Organic solvents for cellulose pulp. Bibliographic review. Afinidad 1997, 44 (467), 4550. (4) Hergert, H. L. Developments in organosolv pulping. An overview. In Environmental friendly technologies for the pulp and paper industry; Young, R. A., Akhtar, M., Eds.; John Wiley and Sons Inc.: New York, 1998. (5) Papatheophanus, M. G.; Koullas, D. P.; Koukios, E. G. Alkaline pulping of prehydrolyzed wheat-straw in aqueousorganic solvent systems at low-temperatures. Cellulose Chem. Technol. 1995, 29 (1), 29-40. (6) Lawther, J. M.; Sun, R. C.; Banks, W. B. Characterization of dissolved lignins in two-stage organosolv delignification of wheat straw. J. Wood Chem. Technol. 1996, 16 (4), 439-457. (7) Elsakhawy, M.; Fahmy, Y.; Ibrahim, A. A.; Lonnberg, B. Organosolv pulping. 3. Ethanol pulping of wheat straw. Cellulose Chem. Technol. 1996, 30 (1-2), 161-174. (8) Elsakhawy, M.; Fahmy, Y.; Ibrahim, A. A.; Lonnberg, B. Organosolv pulping. 4. Kinetics of alkaline ethanol pulping of wheat straw. Cellulose Chem. Technol. 1996, 30 (3-4), 281-296. (9) Jime´nez, L.; De la Torre, M. J.; Maestre, F.; Ferrer, J. L.; Pe´rez, I. Organosolv pulping of wheat straw by use of phenol. Bioresour. Technol. 1997, 60 (3), 199-205. (10) Jime´nez, L.; Maestre, F.; De la Tore, M. J. Organosolv pulping of wheat straw by use of methanol-water. TAPPI J. 1997, 80 (12), 248-154.
(11) Jime´nez, L.; Bonilla, J. L.; Ferrer, J. L.; Pe´rez, I. Obtention of cellulose pulps with mixtures of ethanol-water. Afinidad 1998, 55 (474), 133-138. (12) Jime´nez, L.; De la Torre, M. J.; Bonilla, J. L.; Ferrer, J. L. Organosolv pulping of wheat straw by use of acetone-water mixtures. Process Biochem. 1998, 33 (1), 229-238. (13) Jime´nez, L.; De la Torre, M. J.; Maestre, F.; Ferrer, J. L.; Pe´rez, I. Delignification of wheat straw by use of low-molecularweight organic acids. Holzforschung 1998, 52 (2), 191-196. (14) Jime´nez, L.; Maestre, F.; Pe´rez, I. Use of butanol-water mixtures for making wheat straw pulp. Wood Sci. Technol. 1999, 33, 97-109. (15) Jime´nez, L.; Pe´rez, I.; De la Torre, M. J.; Lo´pez, F.; Ariza, J. Use of formaldehyde for making wheat straw cellulose pulp. Bioresour. Technol. 2000, 72, 283-288. (16) Zarubin, M. Y.; Dejneko, I. P.; Evtuguine, D. V.; Robert, A. Delignification by oxygen in acetone-water media. TAPPI J. 1989, 72 (11), 163-168. (17) Parajo´, J. C.; Alonso, J. L.; Va´zquez, D.; Santos, V. Optimization of catalysed acetosolv fractionation of pine wood. Holzforschung 1993, 47 (3), 188-196. (18) Tjeerdsma, B. F.; Zomers, F. H. A.; Wilkison, E. C.; SierraAlvarez, R. Modelling organosolv pulping of hemp. Holzforschung 1994, 48 (5), 415-422. (19) Va´zquez, G.; Antorrena, G.; Gonza´lez, J. Acetosolv pulping of Eucalyptus globulus wood. 1. The effect of operational variables on pulp yield, pulp lignin content and pulp potential glucose content. Holzforschung 1995, 49 (1), 69-74. (20) Vega, A.; Bao, M.; Lamas, J. Application of factorial design to the modeling of organosolv delignification of Miscanthus sinensis (Elephant grass) with phenol and dilute-acid solutions. Bioresour. Technol. 1997, 61 (1), 1-7. (21) Gilarranz, M. A.; Oliet, M.; Rodrı´guez, F.; Tijero, J. Ethanol-water pulping. Cooking variables optimization. Can. J. Chem. Eng. 1998, 76 (2), 253-260. (22) Gilarranz, M. A.; Oliet, M.; Rodrı´guez, F.; Tijero, J. Methanol-based pulping of Eucalytus globulus. Can. J. Chem. Eng. 1999, 77 (3), 515-521. (23) Wise, L. E.; Marphy, M.; D’Adieco, A. Chlorite holocellulose, its fractionation and bearing on summative wood analysis and studies on the hemicelluloses. Pap. Trade J. 1946, 122 (2), 35-43. (24) Montgomery, D. C. Disen˜ o y ana´ lisis de experimentos; Grupo Editorial Iberoamericana: Mexico, 991. (25) Jime´nez, L.; De la Torre, M. J.; Ferrer, J. L.; Garcı´a, J. C. Influence of process variables on the roperties of pulp obtained by ethanol pulping of wheat straw. Process Biochem. 1999, 35, 143-148. (26) Dixon, J. P. BMDP statical software manual; University of California Press: Berkeley, CA, 1988. (27) Draper, N.; Smith, H. Applied regression analysis; Wiley: New York, 1981. (28) Press, W. H.; Teukolsky, S. A.; Vetterling, W. T.; Flannery, B. B. Numerical recipes in C: The art of scientific computing; Cambridge University Press: Cambridge, U.K., 1992. (29) More, A.; Toraldo, A. Algorithms for bound constrained quadratic programing problems. Numer. Math. 1989, 55, 377400.
Received for review November 29, 2000 Revised manuscript received May 11, 2001 Accepted July 27, 2001 IE0010161
