Ind. Eng. Chem. Process Des. Dev. 1982, 21, 457-465
days this operating procedure has wide perspectives in industrial adsorption separation. If a complete mathematical model whose parameter values are obtained by a few experimental runs is available, it is possible to explore the whole range of operating conditions without restrictions and therefore to obtain a proper design. Acknowledgment
The authors gratefully acknowledge financial support from SISAS S.p.A. Nomenclature a = external particle surface/particle volume, l/cm c, = concentration in external liquid phase, mol/L
ci = concentration in liquid phase into macroporosity (internal
phase), mol/L Deff= effective diffusion coefficient (eq E),cm2/s DL = axial diffusion coefficient, cm2/s fq = mathematical expression of adsorption isotherm k , = film mass transfer coefficient, cm/s ki = intraparticle mass transfer coefficient (into macroporosity), cm/s K = equilibrium constant of adsorption, L/mol K, K* = pseudolinear equilibrium constant of adsorption, defined by eq 10, 12, L/mol KL = global mass transfer coefficient, cm/s L = length of the column, cm N.C. = number of the components Pe = Peclet number (= 2uRp/D~.e,) R, = radius of spherical particle equivalent to the solid pellet, cm t = time, s tR = retention time of a component t (= L / u ) , s u = fluid velocity, cm/s u = interstitial fluid velocity, cm/s u j = effective velocity of component j (eq l l ) , cm/s z = axial length coordinate, cm Greek Symbols a,p = coefficients introduced in eq 21
457
r = concentration in solid phase, mol/g r- = loading capacity of adsorbent, mol/g t,
= external void fraction (external liquid phase volume/total
bed volume)
s, = total bed void fraction (eq 13) ti
= intraparticle void fraction (internal liquid phase vol-
ume/solid phase volume) B = coverage degree (= I'/Fm) p = molar density of a pure compound, mol/L ps = bulk density of the solid phase, g/L u = coefficient defined by eq 12
Superscripts F = feed variable 0 = initial value Subscripts e = external phase variable i = internal phase variable j = component j , j = 1, N.C. L i t e r a t u r e Cited Broughton, D. B.; Neuzll, R. W.; Pharis, J. M.; Brearley, C. S. Chem. Eng. Prog. 1970. 66, 70. Butt, J. B. "Reaction Kinetics and Reactor Design"; Rentice-Hall: Engelwood Cllffs, NJ, 1960 p 274. De Rosset, A. J.; NeuzU, R. W.; Korous, D. J. Ind. Eng . Chem Process Des. Dev. 1978, 15, 261. Garg, D. R.; Ruthven, D. M. Chem. Eng. Sci. 1974. 29, 571. Keulemans, A. I.M. "Qas Chromatography", 2nd ed.; Relnhold New York, 1959. Liapis, A. I.; Rlppin, D. W. T. Chem. €ng. Sci. 1977, 33, 593. Morbldelll. M.; Servida, A.; Can& S. submltted to Chem. Eng. Comp. 1982. Santacesarla, E.; MorbMelll, M.; Denise, P.; Mercenari, M.; C a d , S. Ind. Eng. Chem. Process Des. Dev. 1982a. Part 1 of thls series, published in this issue. Santacesaria, E.; Morbldelli, M.; Servlda, A.; Storti, G.; Car& S. Ind. Eng. Chem. Process Des. Dev. 1882b. Part 2 of this series, publlshed in thls Issue. Seko, M.; Mlyake, T.; Inada, K. Hydocarbon Process 1980, 59, 133. Spencer, F. S. Anal. Chem. 1989, 35, 592. Villadsen, J.; Micheisen, M. L. "Solution of Differential Equatlon Modeis by Polynomials Approximation"; Prentice-Hall: Engelwood Cliffs, NJ, 1978.
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Receiued for reuieur May 13, 1981 Accepted January 20, 1982
Product Compositions and Kinetics for Rapid Pyrolysis of Cellulose Mohammed I?. Hajaligol, Jack B. Howard, John P. Longwell, and Willlam A. Peters" Energy Laboratory and Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Systematic studies of the independent effects of temperature (300-1100 "C), solids residence time (0-30 s), and 100-15 I 000 "CIS)on the yields, compositions, and rates of formation of products from the rapid heating rate ( pyrolysis of 0.0101 cm thick sheets of cellulose under 5 psig pressure of helium have been performed. The experiments mainly probe the primary decomposition of the cellulose, wtth contributions from post-pyrdysis reactions being confined to those occurring within and closely proximate to the sample. Temperature and sample residence time are the most important reaction conditions in determining the pyrolysls behavior, while heating rate effects are explicable in terms of their influence on these two parameters. A heavy liquid product of complex molecular composition accounted for 40 to 83 w-t % of the volatiles above 400 "C. Secondary cracking of this material increased with increasing residence time or temperature and was a signiflcant pathway for producing several light gases. At a heating rate of 1000 ' C I S and temperatures above 750 "C, CO dominated the product gases for all sample residence times and attained a yield above 23 wt % at 1000 "C.
Introduction
Previous research at M.I.T. and elsewhere (Lewellen et al., 1977; Peters, 1978) indicates that biomass pyrolysis offers promise for producing commercially interesting quantities of high heating value gases, and liquids suitable for replacing petroleum-based products such as distillate 0198-4305/82/1121-0457$01.25/0
fuels, high octane motor fuel additives, and olefinic feedstocks. Realization of this potential, however, is critically dependent on identification of reaction conditions achievable in practical scale equipment. There have been many previous investigations of the pyrolysis of wood, cellulose, and other forms of renewable materials. How@ 1982 American Chemical Society
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Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 3, 1982
100- and 50-A variable transhrmers connected to two 100-Arelays which are in turn respectively activated by 0-1 s and 0-60 s timer switches. This system allows in-
IiWII
Figure 1. Captive sample apparatus.
ever, there still remains a great need for reliable systematic studies of the independent effects of the type of material and commercially interesting reaction conditions such as temperature, heating rate, solids residence time, volatiles residence time, pressure, gaseous atmosphere, and sample dimension, on the yields, compositions, and rates of production of pyrolysis gases, liquids, and cham Theae details are needed to gain better fundamental understanding of biomass thermal conversion pathways and to provide predictive modeling capability for existing and future processes for converting renewable reeources to clean fuels and chemical feedstocks. Further, since pyrolytic decomposition supplies the volatiles that allow ignition and support flaming combustion of most materials, such information is important in studies on materials flammability, flame spread and stability, and related ieeuea in fire research. This paper presents recent results from one facet of an ongoing M.I.T. research program aimed at providing better quantitative understanding of the rapid thermal decomposition behavior of biomass, including data and interpretive models responsive to the above needs. Effects of temperature, solids residence time, and heating rate on the yields, compositions, and rates of formation of products from the rapid pyrolysis of cellulose under a pressure of 5 psig of helium are treated here. Subsequent communications will furnish similar information on wood, lignin, and hemicellulose. Experimental Section Reactor Description. Laboratory scale batch reactors were employed in this work. These devices are designed: (a) to allow total product collection for direct measurement of material and elemental balances; (b) to permit inversion of product yields from rapid thermal processes to kinetic parameters via nonisothermal models; (c) to allow independent determination of collaborative effects of heating rate, sample residence time, sample size, extent of sample dispersion, and sample temperature; and (d) to minimize secondary (post-pyrolysis)reactions of volatiles evolved from the decomposing sample. A schematic of one such reactor and the product collection equipment is shown in Figure 1. This reactor vessel is designed for atmospheric pressure and vacuum pyrolysis work. It is a Corning Pyrex cylindrical pipe, 9 in. in diameter and 9 in. long, closed at each end with stainleas steel flanges,with electrical feedthroughs, and gas inlet and outlet ports. Another version of this reador capable of operating at pressures up to 1500 psig is also available for studies of elevated pressures. The sample is heated inside a folded strip of 325 mesh stainless steel screen held between two massive brass electrodes mounted within the above vessel. The heating circuit consists of
I
dependent variation of the following reaction conditions over the indicated ranges: heating rates (100-100OOO OC/s), final temperaturea (200-1100"C), sample reaidence time at final temperature (0- 8). During operation most of the gas within the reactor remains close to room temperature so that upon exiting the immediate neighborhood of the hot stage, volatilea are rapidly diluted and quenched. The data therefore mainly reflect the primary decomposition behavior of the cellulose with contributions from post-pyrolysis reactions being limited to those occurring within and closely proximate to the sample. The time-temperature history of the sample over the entire run is measured in each experiment using a rapid response (time constant = 0.003 s) type K (chromel-alumel) thermocouple fabricated by joining 25.4pm diameter bare wires to give an approximately 76 pm diameter bead. The thermocouple is placed within the folded screen and its output (millivolt range) is monitored by a fast response strip chart recorder. Experimental Procedure. The cellulose samples used in this work were a&roximately 100 mg,thin strips of low ash (
