Acetosolv delignification of depithed cardoon (Cynara cardunculus) stalks

Industrial Crops and Products 25 (2007) 294–300

Acetosolv delignification of depithed cardoon (Cynara cardunculus) stalks P. Ligero a , J.J. Villaverde a , A. Vega a,∗ , M. Bao b a

Departamento de Qu´ımica F´ısica e Enxe˜ner´ıa Qu´ımica I, Universidade de A Coru˜na, Campus da Zapateira s/n, 15071 A Coru˜na, Spain b Escola T´ ecnica Superior de Enxe˜ner´ıa, Universidade de Santiago de Compostela, R´ua Lope G´omez de Marzoa s/n, 15782 Santiago de Compostela, Spain

Received 27 July 2006; received in revised form 19 December 2006; accepted 19 December 2006

Abstract Modelling of the Acetosolv treatment of the cardoon bark (Cynara cardunculus) was accomplished using a second-order face-centred factorial design. We considered as independent (experimental) variables: cooking time (60–180 min), acetic acid concentration in the cooking liquor (60–90%) and hydrochloric acid concentration in the cooking liquor (0.20–0.80%); as well as dependent variables: pulp yield, kappa number and viscosity. Empirical models were deduced to satisfactorily fit experimental data with the values of the independent variables and allow quantifying the effects of each variable. An optimisation with constraints led to the calculation of the region of the experimental domain (time = 180 min, acetic acid concentration ≥ 71.3% and HCl concentration > 0.41%) leading to pulps with kappa numbers < 25 at a maximal pulp yield and viscosity, giving us maximum possible values for pulp yield (46.3%) and viscosity (557 mL/g). © 2007 Elsevier B.V. All rights reserved. Keywords: Cynara cardunculus; Acetic acid pulping; Delignification; Modelling

1. Introduction Along with other treatments for lignocellulosic materials used to produce cellulose pulp, in recent decades organosolv methods have attracted growing interest. Organosolv gets its name from its use of organic solvents in the processing, a method designed with a view to taking fuller advantage of the primary material. An important aspect of all organosolv processes is the investigation of quantitative fractionation or the breaking down of the main components of the plant tissue. This

∗

Corresponding author. Tel.: +34 81167000; fax: +34 81167065. E-mail address: [email protected] (A. Vega).

0926-6690/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.indcrop.2006.12.009

part of the process should leave the material as unaltered as possible so that it can be incorporated into further processes of transformation and revalorization. The number of organosolv systems which has been tested is very large, owing to the fact that each different organic solvent capable of dissolving lignin-type compounds may be considered determinative in a procedure that leads to a more or less efficient delignification. However, only a small number of organosolv processes show high selectivity and efficiency, among other properties. This has led to their becoming the object of intensive investigation on the part of many research groups. Processes based on the use of acetic acid (Nimz and Casten, 1986) as an organic solvent have been applied with success to hard and softwoods, and even

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to non-woody materials, in catalysed systems using HCl (Acetosolv method) and non-catalysed systems (Acetocell method) (Abad et al., 2003; Davis et al., 1986; Kin, 1990; Ligero et al., 2005; Neto and Robert, 1992; Pan et al., 1999; Paraj´o et al., 1993; V´azquez et al., 1995). The EU has expressed great interest in the exploitation of certain high-yield non-food crops. A broad spectrum of species has been and continues to be studied for this purpose. For rocky soils, characterised by drought conditions and a large temperature variations (as in many Mediterranean ecosystems), cardoon contains the properties necessary for it to be among the first on the list of candidates (Abbate and Patane, 1996; Curt et al., 2002). In our study we have applied the Acetosolv method to examine the delignification of the cardoon stem and to establish models to ensure the efficient fractionation of the raw material and to establish operational conditions leading to optimal results. 2. Materials and methods 2.1. Raw material For this study we have used cardoon (Cynara cardunculus) provided by the Escuela Superior de Ingenieros Agr´onomos, Universidad Polit´ecnica de Madrid, obtained from an experimental plantation (European Union AIR Programme) located at the School of Agricultural Engineering. The air-dried cardoon stems we received were cut longitudinally into strips of 20–30 cm and then treated with an abrasive rotating element, applying it to the inner section of the stem to eliminate the pith. The remaining bark was submitted to a further grinding and sieving. The material thus obtained, below 1 mm, was spread over sheets of paper to allow equilibration of moisture (8.8%), and finally stored in polypropylene hermetic containers. 2.2. Pulping The Acetosolv treatments were carried out in glass flasks with refluxing liquors at atmospheric pressure (boiling point approximately 110 ◦ C). For each cooking, the predetermined quantities of cardoon, water and acetic acid were systematically added to the flask, heated the system to the point of boiling and then added the hydrochloric acid, attached the system agitator and began timing the process. After the reaction, the pulps were filtered into medium-porosity crucibles and washed four times with acetic acid solutions (85%) in proportions of 2.5, 2.5, 5 and 5 (v/p), respectively, in relation to the initial dry

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weight of the cardoon. Finally, the pulps were washed repeatedly in distilled water until neutrality. 2.3. Analytical methods Pulp yields (PY) were determined gravimetrically after oven drying until constant weight was achieved. TAPPI standards were applied to determine the kappa number of pulps (KI) (T 236) and the pulp viscosity (T 230) in copper–ethylenediamine solution. Residual lignin (RL) of pulps and original material was determined using the Klason method of two-stage acid digestion (T222 om-88): the first with H2 SO4 (72%) at 15 ◦ C for 2 h, and the second, lowering the acid concentration to 3% at 120 ◦ C for 1 h. Acid soluble lignin was quantified by measuring the absorbance of the hydrolysed solution at 205 nm. Other TAPPI standards used were: ash (T211 om-02) and extracts (T204 cm-97). Monosaccharides were analysed by liquid chromatography of the Klason hydrolysate, lyophilised and extracted with methanol. The chromatographic conditions were as follows: column, Biorad Aminex HPX-87P; mobile phase, water; flow, 0.6 cm3 /min; oven temp., 85 ◦ C; index of refraction detected at 50 ◦ C. 2.4. Experimental design and statistical analysis To examine and quantify the effect of the cooking variables on the parameters defining the progress of the processes of delignification and hydrolysis, we opted for a factorial experimental design. Factorial designs have been used extensively to examine the behaviour of organosolv systems (Jim´enez et al., 2002; Tjeerdsma et al., 1994). We constructed our design with a face-centred second-order structure (Dean and Voss, 1999). According to standard procedures in experimental design construction and analysis, the independent variables (cooking time, concentration in weight of acetic acid in cooking liquor and concentration in weight of hydrochloric acid in cooking liquor) were standardised so that their respective ranges of variation were into the interval (−1,1). The following experimental results were analysed as dependent variables: pulp yield, kappa number and pulp viscosity; and submitted to multivariate regression against the independent variables using polynomic fit functions such as: DV = b0 +

3  i=1

bi X i +

3  i=1

bii Xi2 +

 i
bij Xi Xj

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where DV represents each of the dependent variables examined in this study; and Xi and Xj are the normalised independent variables explained above. The different values of b0 , bi and bij represent the fitting parameters by which, due to normalisation, we can determine and compare the effects of each of the independent variables on the dependent variables (experimental results). All calculations were performed with a statistics module of Excel. 3. Results and discussion The chemical analysis of the depithed cardoon stem is presented in Table 1. One remarkable result is the relatively high solubility in ethanol (typical of polar compounds) which, together with the content in cold and hot water extractives, is in accordance with previously published data (Gominho et al., 2001) for the same material. In our case the cardoon was probably obtained from an earlier crop which may explain the significant difference between the levels of acid insoluble lignin in each of the samples: our sample represents a more ligneous material, confirming results obtained by other authors for cardoon specimens from plantations in different phases of development (Antunes et al., 2000). In general, it is the levels of acid insoluble lignin which present the greatest variation from author to author. A whole range of values have been presented: from 11.3 to 26.4% (Gominho et al., 2001; Antunes et al., 2000; Oliet et al., 2005). The holocellulose content of our sample lied within the range presented by different authors (Oliet et al., 2005; Villar et al., 1999), although it is the lowest among Table 1 Chemical analysis of the cardoon bark Component Ash

Content (% over d.m.) 7.9

Extracts

Hexane 96% ethanol Hot water

0.6 7.3 10.8

Lignin

Acid insoluble (Klason) Acid soluble

17.1 3.8

Holocellulose Monosaccharides (HPLC)

70.8 Glucose Xylose Galactose Arabinose Mannose

35.9 14.5 1.6 0.9 3.2

all of them. The level and distribution of monosaccharides in our sample showed values similar to those published elsewhere (Gominho et al., 2001), even taking into account the different procedures used to determine them. Ash levels are high, as is to be expected in a nonwoody material. To establish the variable value ranges to be considered in experimental designs having into account those applied in other studies using different primary materials (Ligero et al., 2005; Pan et al., 1999; Jim´enez et al., 2002; Pan and Sano, 2000; Shukry et al., 1992) a series of preliminary experiments were carried out, the results of which are summarised in Table 2 (experiments 1–8). The importance of the HCl concentration in the reaction is clear (see sequence of experiments 1–3 and 6–8), obscuring the influence of cooking time (experiments 1 and 4) and acetic concentration (experiments 1, 6 and 3, 8). Furthermore, only pulps obtained using an HCl concentration of more than 0.3% (numbers 3, 7 and 8) were clearly defibrated; when the 0.3% concentration was used (no. 2), defibration was only incipient (maybe as a result of the short reaction time). In view of all this, we decided to maintain the ranges for cooking time (60–180 min) and acetic acid concentration (60–90%) from the earlier experiments, and increase the levels of HCl concentration from 0.2 to 1.0%. A second-order factorial design was constructed and implemented, the results of which appear in Table 2 (experiments 9–25). Table 3 contains the regression coefficients obtained for the models used to describe the pulp according to the principal variables, together with their level of importance as determined by a Student t-test. The table also includes parameters to measure the correlation and statistical signification of the mathematical model (Dean and Voss, 1999). Pulp yield reveals considerable dependence on the three experimental variables (its values decrease accordingly as the value of each of the variables increase), especially in both acid concentrations, as observed in values b2 and b3 (Table 3) and likewise in Fig. 1. Fig. 1 represents the behaviour of pulp yield according to variations in the concentrations of acetic and hydrochloric acid, at a constant cooking time of 169 min, for which the mathematical model offers a minimum yield value of 35.2%. Under these conditions, the model predicts a kappa number and viscosity index of 6 and 523 mL/g, respectively. In addition to the main effects of the process, the quadratic effect caused by the HCl (represented by the parameter b33 ) is also significant: the influence of the acid solution is indicated by large curves on the response surface of the graph corresponding to variations

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Table 2 Experimental design results for the pulping of cardoon bark Experiment number

Cooking time (min)

AcOH concentration (% by wt.)

HCl concentration (% by wt.)

Yield (%)

Kappa index

Viscosity (mL/g)

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

60 60 60 180 180 60 60 60 60 180 60 180 60 180 60 180 60 180 120 120 120 120 120 120 120

60 60 60 60 60 90 90 90 60 60 90 90 60 60 90 90 75 75 60 90 75 75 75 75 75

0.05 0.3 1 0.05 0.15 0.05 0.5 1 0.2 0.2 0.2 0.2 0.8 0.8 0.8 0.8 0.5 0.5 0.5 0.5 0.2 0.8 0.5 0.5 0.5

84.1 70.6 51.1 79.4 73.4 82.2 45.2 41.7 75.7 63.5 58.6 42.4 55.9 44.5 39.6 37.3 51.7 41.3 51.4 38.8 58.7 38.8 47.4 48.4 43.9

38 43 38 17 36 25 7 11 32 17 32 8 40 11 17 21 19

315 395 440 535 423 525 504 467 497 553 443 494 415 484 534 581 571

in the concentration of HCl (Fig. 1). In other words, the system’s ability to dissolve substances is much greater when the HCl concentration is between approximately 0.2 and 0.6% (steeper slope in relation to the other

constant variables), than in the second section of the experimental range. If we look at the model for kappa number, the results are qualitatively similar because, here too, the acid concentrations yield the greatest influence. There are some

Table 3 Regression parameter for each and goodness of fit Factor

Yield

Klason lignin

Kappa index

Viscosity

b0 b1 b2 b3 b12 b13 b23 b123 b11 b22 b33

45.45* −5.25* −7.43* −8.28* 0.64 1.84** 1.84** 1.64** 1.89 0.49 4.14*

5.65 −2.46 −3.89 −3.81 −0.09 0.49 −0.16 2.39 0.93 0.68 2.48

20.15 −3.80 −9.30 −8.60 −1.37 1.12 −2.12 5.12 3.48 −1.02 4.48

532.4* 29.6* 33.9* 30.3* −15.5 −13.7 −30.2* −19.2 14.9 −41.6** −60.6*

R2 R2 aj. F Sig. F (%) * **

0.9831 0.9549 34.85 >99.9

0.9598 0.8927 14.32 >99.7

0.9333 0.8222 8.40 >99.1

0.9063 0.7502 5.80 >97.7

Indicates significant parameters at a confidence level of 95%. Indicates significant parameters at a confidence level of 90%.

Fig. 1. Influences of acetic and hydrochloric acid concentrations on the pulp yield for a cooking time of 169 min.

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Fig. 2. Influences of acetic and hydrochloric acid concentrations on the kappa index for a cooking time of 111 min.

interesting differences, however: the relative importance of the cooking time decreases and there is no especially significant quadratic term. All this translates in the response graph into surfaces with large negative slopes (for variables representing the acid concentrations) and very moderate curvatures (Fig. 2). The predicted minimum for this variable lies right at the upper end of the scale of values, obtaining at 111 min, 90% acetic and 0.8% HCl; yielding the following values: a very low pulp yield 36.4%, 3.5 kappa number and 467 mL/g viscosity. The results for viscosity, apart from being the least satisfactory in terms of goodness of fit, are also the most difficult to explain. The influence of the variables can be observed from the parameter values presented in Table 3. In summary, once again, the time and concentrations are very important; however, now the results for two quadratic terms (b22 and b33 ) are also significant, causing the appearance of maximums in the graph representations for acetic, HCl concentration and viscosity values (Fig. 3). The fall in viscosity caused by increasing HCl concentration probably is due to the degradative (hydrolytic) action of acid, when added in higher proportions, on cellulose. It is less easy to explain why viscosity decreases at low acid concentrations, though we believe this may be the result of a lack of specificity in the analytical procedure when applied to insufficiently delignified samples with values greater than 40 (Table 2). Anyway, viscosity values show little variation around the maximum values stated and indicate a wide operational area with predicted values of over 500 mL/g. Maximum viscosity predicted by the

regression equation is 579 mL/g, for 180 min, 78% acetic acid and 0.2% HCl. Under these conditions: PY = 50.6% and kappa = 29.0. If we optimise the process under the joint conditions of maximum yield and viscosity, with a minimum kappa number, we obtain the following results: PY = 37.7%, kappa = 11.8 and viscosity = 569 mL/g; for variable values: time = 180 min, acetic concentration = 86% and HCl concentration = 0.47%. These results offer very good delignification with good viscosity values, but with considerable mass losses. Then, an alternative optimisation was attempted, therefore, using the following, less restrictive conditions: obtain maximum pulp yield and viscosity, and a kappa number lower than or equal to 25. Results offered three definite conclusions: (1) cooking time must always be the maximum (180 min); (2) acetic acid concentration must be equal to or greater than 71.3% and (3) HCl concentration must be greater than 0.41%. One particularly striking feature detected during the experimental phase of this study was the noticeably less bright and whiteness obtained for Acetosolv cardoon pulp than that of other primary materials with the same kappa number and system of pulping. To establish the ratio between kappa number and the percentage of residual lignin in the pulps, all of the pulps were submitted to a residual Klason lignin analysis (RKL), the results of which appear in Fig. 4 along with the results from an earlier study carried out on Miscanthus sinensis (Ligero et al., 2005). We find that all the cardoon pulps contain higher levels of residual lignin than the Miscanthus pulps with the same kappa number, even at the lower range of

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Fig. 3. Influences of acetic and hydrochloric acid concentrations on the viscosity of pulps for a cooking time of 180 min.

Fig. 4. Relationships between kappa index and residual Klason lignin of pulps for two different annual plants (cardoon and Miscanthus) delignified by the Acetosolv process.

kappa values where the values for both series are similar. This would seem to indicate that the early stages of delignification are different for each plant. Summarising, pulps with kappa numbers lower than 30 were all obtained using HCl concentrations equal to or greater than 0.5%, with the exception of one obtained using a 0.2% concentration, but with maximum cooking time and acetic concentration.

cooking liquor and hydrochloric acid concentration in the cooking liquor) in an Acetosolv treatment at normal atmospheric pressure of a cardoon (C. cardunculus) bark, and quantified these effects using empirical polynomial models. By optimising our models mathematically, we were able to calculate the maximum conditions of the experimental variables under which bleachable pulps (kappa number ≤ 25) might be obtained. Using these maximum values, we then modified the experimental conditions to achieve a greater degree of delignification (lower kappa), or cellulose preservation (greater viscosity). To obtain maximum values, samples must be cooked for the maximum time (180 min) using intermediate values for the other variables (71.3% AcOH and 0.41% HCl), with the following results: yield = 46.3%, kappa number = 25 and viscosity = 557 mL/g. The ratio between the kappa number of the pulps and the Klason levels of residual lignin is very different from that obtained for another herbaceous species (M. sinensis): at equivalent kappa values, cardoon retains a higher proportion of lignin, which has a negative effect on the appearance of the pulps. Acknowledgements

4. Conclusions We have studied the effects of three operational variables (cooking time, acetic acid concentration in the

The authors wish to express their gratitude to the Comisi´on Interministerial de Ciencia y Tecnolog´ıa of the Ministry for Education for granting this work (Proyecto

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de investigaci´on PPQ2001-1215-C03-02). We would also like to thank Dr. D. Jes´us Fern´andez Gonz´alez of the ETSI Agr´onomos of the Universidad Polit´ecnica de Madrid, for supplying the cardoon samples. References Abad, S., Santos, V., Paraj´o, J.C., 2003. Two-stage acetosolv pulping of Eucalyptus wood. Cellul. Chem. Technol. 35, 333–343. Abbate, V., Patane, C., 1996. Biomass energy crops for possible introduction to Sicily. In: Chartier, Ph. (Ed.), Biomass for Energy and the Environment. Proceedings of the Ninth European Bioenergy Conference, Copenhagen, June 24–27, pp. 622–627. Antunes, A., Amaral, E., Belgacem, M.N., 2000. Cynara cardunculus L.: chemical composition and soda-anthraquinone cooking. Ind. Crops Prod. 12, 85–91. Curt, M.D., S´anchez, G., Fern´andez, J., 2002. The potential of Cynara cardunculus L. for seed oil production in a perennial cultivation system. Biomass Bioenergy 23, 33–46. Dean, A., Voss, D., 1999. Design and Analysis of Experiments. Springer-Verlag, New York. Davis, J.L., Young, R.A., Deodhar, S.S., 1986. Organic acid pulping of wood III. Acetic acid pulping of spruce. Mokuzai Gakkaishi 32, 905–914. Gominho, J., Fern´andez, J., Pereira, H., 2001. Cynara cardunculus L.—a new fibre crop for pulp and paper production. Ind. Crops Prod. 13, 1–10. Jim´enez, J., P´erez, I., L´opez, F., Ariza, J., Rodr´ıguez, A., 2002. Ethanol–acetone pulping of wheat-straw. Influence of the cooking and the beating of the pulps on the properties of the resulting paper sheets. Bioresour. Technol. 83, 139–143. Kin, Z., 1990. The acetolysis of beech wood. Tappi J. 11, 237–238.

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