Ind. Eng. Chem. Res. 1991,30, 2416-2425
2416
temperature. This can be attributed to the loss of volatile with increasing temperature. 4. Effect of Impregnation Ratio and Reagent on Yield. From Figure 12,one can observe that yield increases with impregnation ratio. However, this increase in yield is marginal beyond an impregnation ratio of 25%. This is true for both reagents studied and holds good for both fluidized bed and static bed processes. 5. Effect of Gas Medium on Yield. The influence of the gas medium (nitrogen, air,and carbon dioxide) on yield is shown in Figure 13. It can be clearly seen that the static bed process gives a higher yield. Use of nitrogen and carbon dioxide results in a better yield compared to air as the medium. Phosphoric acid gives a slightly better yield than ZnC12, when nitrogen and carbon dioxide are employed. When air is used as the medium, H3P04gives a low yield compared to ZnC1,. This may be due to the combustion reactions, which are more pronounced with H3P04because of its lower temperature stability (boiling point 213 OC). From the experimental data it may be concluded that air is unsuitable as the medium of activation since it gives low activation and also low yield compared to nitrogen and carbon dioxide.
Conclusions On the basis of the experimental study carried out, it can be concluded that the fluidized bed process gives better
activation (high adsorption capacity of carbon) in less time and at a lower temperature compared to the static bed process. High activation and greater yield are both possible with nitrogen and carbon dioxide compared to air as the medium of activation. Under the experimental conditions investigated for the chemical activation of coconut shells in the fluidized bed, a process time of 2 h, a temperature of around 500 "C with zinc chloride as the activating agent, and an impregnation ratio of 25% in the presence of nitrogen or carbon dioxide may be taken as optimal parameters.
Nomenclature = average particle diameter, mm = temperature, O C t = time, min Abbreviations I.R. = impregnation ratio MED = medium of activation R = reagent for activation Registry No. ZnC12,7646-85-7;H3PO4,7664-38-2;Nz,772737-9;COz, 124-38-9;carbon, 7440-44-0.
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Literature Cited Edwards, G. D.; Robert, Y.; Joseph, T.; Loveday, P. E.; Williams, F. D. Steam Activation of Charcoal: A systematic study of weight and volume losses. Proceedings of the Fifth Carbon Conference; Pergamon Press: New York, 1963;Vol. 2, pp 265-285. Hassler, J. W. Activated Carbon; Chemical: New York, 1977;pp 171-177. Kirubakaran, C. J. Studies on Activated Carbon Production from Coconutshells Using a Fluidized Bed Reactor. M.S. Thesis, Indian Institute of Technology, Madras, 1990. Kunii, D.; Levenspiel, 0. Fluidization Engineering; Wiley: New York, 1969 pp 8-11. Mantell, C. L. Carbon and Graphite Handbook; Interscience: New York, 1977;pp 181-184. Ruiz, Bevia, F.; Prats Rico, D.; Marcilla Gomis, A. F. Activated Carbon from Almond Shells. Chemical Activation. 1. Activating Reagent Selection and Variables Influence. Znd. Eng. Chem. Prod. Res. Dev. 1984a, 23,266-269. Ruiz Bevia, F.; Prats Rico, D.; Marcilla Gomis, A. F. Activated Carbon from Almond Shells. Chemical Activation. 2. ZnC12 Activation Temperature Influence. Znd. Eng. Chem. Prod. Res. Dev. 1984b, 23, 269-271. Snell, F. D., Ettre, L. S., Eds. Encyclopedia ofzndustrial Chemical Analysis; Interscience: New York 1973;Vol. 17,pp 28-32. Vogel, A. I. Text Book of Quantitative Inorganic Analysis, 4th ed.; ELBS and Longman: London, 1978; pp 370-379. Wagner, N. J.; Jula, R. J. Activated Carbon Adsorption. In Activated Carbon Adsorption for Waste Water Treatment; Perrich, J. R., Ed.; CRC Press: London, 1981;Chapter 3.
Received for review December 6,1990 Revised manuscript received May 29,1991 Accepted June 20,1991
Phenomenological Kinetics of Complex Systems: Mechanistic Considerations in the Solubilization of Hemicelluloses following Aqueous/Steam Treatments K. Belkacemi,?N. Abatzoglou,t R. P. Overend,+?$ and E. Chornet*t+ Departement de genie chimique, UnioersitB de Sherbrooke, Sherbrooke, Quebec, J l K 2R1, Canada, and Division of Biosciences, National Research Council of Canada, Ottawa, Ontario, K1A OR6,Canada The solubilization of hemicelluloses from a lignocellulosic matrix is generally described by two parallel fint-order reactions. In this paper we develop a phenomenological description that relates the kinetic parameters associated with the hydrolysis of the glucosidic bond to linear free energy correlations of the Leffler type. Such a description has resulted in the correct prediction of the acid hydrolysis kinetics of a number of substrates including the published data on Populus tremuloides and Betula papyrifera and our own work on corn stalks and Stipa tenacissima. The key advantage of our approach is that it facilitates the obtention of kinetic data for a given substrate with fewer experimental points than frequently used in isothermal experiments.
Introduction In a previous work, Abatzoglou et al. (1991),we have shown that the kinetics of hemicellulose solubilization can
* Author to whom correspondence should be addressed. t
Universit6 de Sherbrooke. National Research Council of Canada.
be modeled by combining the three main operational variables (i.e., residence time, temperature, and acid concentration) within a single reaction coordinate,named ROH, which describes the severity of the thermochemical treatment. The use of ROHleads to the derivation of three parameters: the energy-related parameters w' and y have been interpreted as indicators of the nonhomogeneity of
0888-5885/91/2630-2416$02.50/00 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 11, 1991 2417 the biomass substrates studied, while the parameter X is linked to the catalytic role of the acid. In the present paper we develop a link between the reaction coordinate ROHand the thermodynamic parameters (i.e., activation energy and entropy) that characterize the hydrolytically induced glucosidic bond splitting. The so-called prehydrolysis of lignocellulosic materials leading to the hemicellulose solubilizationwill be the focus of our analysis. In this type of treatment the biomass undergoes significant structural and chemical changes, under appropriate combinations of temperature, acid concentration, and reaction time, as described by Roudier and Eberhard (1967) and Casebier et al. (1969). The polymeric carbohydrates treated with acids in aqueous media participate at least in two distinct reactions: hydrolysis of the glucosidic bonds; dehydrations, as reported by Feather and Harris (1973). The mechanism of the hydrolysis and the reaction rates of both polysaccharides and oligosaccharides have been extensively studied (Capon, 1967; Casebier et al., 1969; Defaye, 1981; Harris, 1975). The concept of the conformational mechanism is generally accepted. Although there is some uncertainty on whether the proton H+ attacks the oxygen in the ring or the glucosidic bond preferentially, the conformational mechanism remains the most accepted explanation of the homogeneous hydrolysis of glucosides in the aqueous phase. Wayman (1980,1983) reviewed the main reactions taking place during the hemicellulose hydrolysis and studied their kinetics. Since then a limited number of works have been published, proposing kinetic models for the hydrolysis of hemicelluloses. Maloney et al. (1984), Conner et al. (19851, and Harris et al. (1985) showed that the hemicelluloses are solubilized following first-order kinetics and claimed the existence of two distinct profiles of solubilization due to the coexistence of reactively different hemicellulosic components. On the other hand, Carrasco et al. (1987) claimed that a single kinetic law is sufficient to describe the same reaction system. None of these kinetic models has, however, been linked to the basic conformationalmechanism. The reason is that the mathematical calculations of the kinetic parameters of the overall reaction via optimization methods lead to results lacking precise physical meaning. The term pseudokinetics is then used as in the case of the cellulose hydrolysis to signify a phenomenological approach. The obvious lack of a model linking classical thermodynamic concepts, reaction mechanisms, and the kinetics of the complex carbohydrate solubilization led us to the present work, which is based on the severity factor concepts developed by our group (Abatzoglou et al., 1991).
Hydrolysis of Lignocellulosic Materials: Rsactions and Mechanisms The solubilizationof hemicellulose is dependent on both the acid concentration of the aqueous medium and the nature of the hemicellulosic material. The salient controlling parameters according to Harris (1975), Defaye (19811, and Fengel and Wegener (1984) are the acid type, the concentrations, the acid strength, the temperature and pressure, the phase in which the reaction takes place, the physicochemical structure and the accessibility of the reactants, if the reaction is heterogeneous, the conformation effects, and the electronic effects due to the chains structure and their substitutes. The process of fragmentation (a partial depolymerization) to yield soluble glucosides takes place by means of scission of the glucosidic bonds (C-0-C). Two mechanisms are possible: (a) protonation of the C-0-C
a . Hydrolysisvis the cyclic arbonium-oxoniumion
HP H OH
CH,OH
H OH
&H!
b
. Hydrolysisvia the acyclic carboniumion
Figure 1. Mechanisms involved in scission of glucosidic C-O-C bonds.
glucosidic bond followed by formation of a cyclic carbanion and the elimination of the aglycon part through scission of the corresponding bond; (b) protonation of the anomeric oxygen taking part in the glucosidic ring followed by an opening of the latter and formation of an acyclic carbanion complex. These two mechanisms are described in Figure 1. The most probable case is that both oxygen atoms are protonated with the position of the scission depending upon the distribution of the electronic density on the intermediate carbanion (Harris, 1975). It is generally admitted that in homogeneous reactions the mechanism through the cyclic carbanion is the most probable (Capon, 1967). However, the step involving the cyclic complex formation requires an important modification of the conformation of the molecule whose activation energy depends on the steric and electronic hindrances encountered (Fengel and Wegener, 1984); in fact the mandatory "half-chair" conformation of the cyclic carbanion is obtained through a slight rotation of the ring constitutive (pendant) groups; thus the energy required for this step depends on the steric and electronic intereffects of the latter and is likely to be difficult in the solid state. In the case of an heterogeneous reaction (i.e., the acidcatalyzed depolymerization of the hemicellulosic polysaccharides) the reaction mechanisms described above are still likely to be valid but the measured rate of the (243-42 bonds cleavage is generally lower. Ross and Jurasek (1978) and Fengel and Wegener (1984) have proposed that limited accessibility of the oxygen atoms of the C-0-C bonds is the main reason; however, no means have been so far proposed to satisfactorily quantify these effects. Given the complexity of the overall reaction and the unknown nature of some of the elementary steps taking place in the reaction, the only positive way to monitor the phenomenon is to follow the rate of the polymer disappearance from the
2418 Ind. Eng. Chem. Res., Vol. 30, No. 11, 1991
solid phase (its solubilization in the aqueous phase).
Kinetics of the Solubilization of the Hemicelluloses Veeraraghavan et al. (1982), Maloney et al. (19841, Conner et al. (1985), and Harris et al. (1985) have shown that the quantity of xylans remaining in the fibers following the prehydrolysis of lignocellulosic material as function of the treatment time follows two distinct solubilization profiles: an initial rapid solubilization process and a subsequent slow step. Both profiles are well fitted by first-order kinetics. Simmonds et al. (1955), Kobayashi and Sakai (1956), Springer et al. (1963), Brasch and Free (19651, Springer (1966), Springer and Zoch (1968), Planes et al. (1981), Harris et al. (1985), Veeraraghavan (1982), Trickett and Neytzell de Wilde (1982a), Conner (1984), Maloney et al. (1984), and Conner et al. (1985) have estimated through their experimental results the pseudo-first-order reaction rate constants k related to both rapid and slow solubilization steps for a variety of biomasses including wood, bagasse, and annual plants (Trickett and Neytzell de Wilde, 1982b; Dutoit et al., 1983). The experimental data generated by our work (to be described later) show that the hydrolysis of corn stalks and Stipa tenacissima follows the same pattern (two distinct solubilization profiles). On the basis of structural considerations as well as the experimental observations we can formulate the following set of assumptions which are the basis for the modeling approach: 1. The mechanisms reported in the literature for the homogeneous acid-catalyzed hydrolysis of the glucosidic bonds are valid for the heterogeneous depolymerization of the labile hemicellulosic polysaccharides present in lignocellulosics; however, we must consider that the accessibility of the oxygen atoms of the different C-O-C bonds present in the macromolecules is not uniform, thus leading to reaction rates that may vary with the extent of the overall solubilization. It is necessary to point out that when introducing the hypothesis of varying accessibilities, we do not consider the proton diffusion rates in the polymeric matrix as being a limiting step; the small size of the hydrogen proton, the high activation energies for acid hydrolysis reported in the literature (higher than 126 kJ/mol), as well as the reduced particle size of the biomasses used (
