Ind. Eng. Chem. Res. 1990, 29, 1776-1785
1776
= KCOCCO,B 4 = Thiele modulus defined by eq 6 fJ
Subscripts
0 = argon; circumstance 1 = carbon monoxide 2 = hydrogen m = methane s = external surface Registry No. C, 7440-44-0; Ni, 7440-02-0; CO, 630-08-0.
Literature Cited Anderson, J. R. Methane to Hydrocarbons. A p p l . Catal. 1989,47. 177-196. Bajpai, P. K.; Bakhshi, N. N.; Mathews, J. F. Deactivation Studies of Nickel Catalysts Used in the Methanation of Carbon Monoxide. Can. J . Chem. Eng. 1982,60,4-10. Carberry, J. J. Chemical and Catalytic Reaction Engineering; McGraw-Hill: New York, 1976. Chang, C. D. Hydrocarbons from Methanol. Catal. Reu.-Sci. Eng. 1983, 25, 1-118. Chemburkar, R. M.; Varma, A. Optimal Catalyst Activity Profiles in Pellets-VIII. Experimental Results under CSTR Conditions. Chem. Eng. Sci. 1990, in press. Chemburkar, R. M.; Morbidelli, M.; Varma, A. Optimal Catalyst Activity Profiles in Pellets-VII. The Case of Arbitrary Reaction Kinetics with Finite External Heat and Mass Transport Resistances. Chem. Eng. Sci. 1987, 42, 2621-2632. Dougherty, R. C.; Verykios, X. E. Nonuniformly Activated Catalysts. Catal. Reo.-Sci. Eng. 1987, 29, 101-150. Hegedus, L. L.; Summers, J. C.; Schlatter, J. C.; Baron, K. Poisonresistant Catalysts for the Simultaneous Control of Hydrocarbon, Carbon Monoxide and Nitrogen Oxide Emissions. J. Catal. 1979, 56, 321-335. Lee, C. K.; Varma, A. An Isothermal Fixed-bed Reactor with Nonuniformly Active Catalysts: Experimental and Theory. Chem. Eng. Sci. 1988, 43. 1995-2000.
Masi, M.; Sangalli, M.; Carra, S.; Cao, G.; Morbidelli, M. Kinetics of Ethylene Hydrogenation on Supported Platinum; Analysis of Multiplicity and Nonuniformly Active Catalyst Particle Behavior. Chem. Eng. Sei. 1988, 43, 1849-1854. Morbidelli, M.; Servida, A.; Varma, A. Optimal Catalyst Activity Profiles-1. The Case of Negeligible External Mass Transfer Resistance. Ind. Eng. Chem. Fundam. 1982,21, 278-284. Morbidelli, M.; Servida, A.; Paludetto, R.; Carra, S. Optimal Catalyst Design for Ethylene Oxide Synthesis. J.Catal. 1984,87, 116-125. Morbidelli, M.; Servida, A.; Carra, S.; Varma, A. Optimal Catalyst Activity Profiles in Pellets-3. The Nonisothermal Case with Negligible External Transport Limitations. Ind. Eng. Chem. &;dam. 1985, 24, 116-119. Rice. R. W.: Hahn. R. S. Deactivation of Nonuniform Nickelialum'ina Methanation Catalysts. Ind. Eng. Chem. Prod. Res.'Dev. 1984, 23, 208-214. Tronconi, E.; Ferlazzo, N.; Forzatti, P.; Pasquon, I. Synthesis of .4lcohols from Carbon Monoxide and Hydrogenation. 4. Lumped Kinetics for the Higher Alcohol Synthesis over Zn-Cr-K Oxide Catalyst. Ind. Eng. Chem. Res. 1987, 26, 2122-2129. Vayenas, C. G.; Pavlou, S. Optimal Catalyst Distribution for Selectivity Maximization in Pellets: Parallel andd Consecutive Reactions. Chem. Engl. Sci. 1987a, 42, 1655-1666. Vayenas, C. G.; Pavlou, S. Optimal Catalyst Activity Distribution and Generalized Effectiveness Factors in Pellets: Single Reactions with Arbitrary Kinetics. Chem. Eng. Sci. 1987b, 42,2633-2645. Vayenas, C. G.; Pavlou, S. Optimal Catalyst Distribution for Selectivity Maximization in Nonisothermal Pellets: the Case of Parallel Reactions. Chem. Eng. Sci. 1988, 43, 2729-2740. Wu, H.; Yuan, Q.; Zhu, B. An Experimental Investigation of Optimal Active Catalyst Distribution in Nonisothermal Pellets. Ind. Eng. Chem. Res. 1988,27, 1169-1174. Wu, H.; Brunovska, A.; Morbidelli, M.; Varma, A. Optimal Catalyst Activity Profiles in Pellets-IX. General Nonisothermal Reacting System with Arbitrary Kinetics. Chem. Eng. Sei. 1990, 45, 1856-1862. Received f o r reuiew December 26, 1989 Accepted May 8, 1990
Kinetics of COz Gasification of Fast Pyrolysis Black Liquor Char J i a n Li a n d A d r i a a n
R.P. van
Heiningen*
Department of Chemical Engineering, McGill University, Pulp and Paper Research Institute of Canada, Montreal, PQ, Canada H 3 A 2A7
The C 0 2 gasification rate of black liquor char (BLC) was studied in a thermogravimetric analysis setup a t temperatures between 600 and 800 "C. The BLC was prepared via fast pyrolysis of dry solids of spent liquor of the kraft wood pulping process. BLC gasification by COPis well described by Langmuir-Hinshelwood type kinetics. The gasification rate of BLC is 1 order of magnitude larger than that of a high surface area activated carbon impregnated with 12% Na2C03. Also, the gasification rate of BLC remains high a t sodium/carbon ratios where the rate of Na2C03impregnated chars would normally be strongly reduced. The rate of BLC gasification was also influenced by the drying method and pyrolysis heating rate. Using a scanning electron microscope and energy dispersive spectroscopy (SEM-EDS)mapping and line scan techniques, it is shown that the unique gasification properties of BLC are caused by a very fine distribution of sodium in the carbon matrix. The present data suggest that internal surface area measurements rather than swelling tests might be more appropriate to characterize combustion properties of black liquors. Introduction Black liquor char (BLC) is the pyrolysis product of kraft black liquor solids (BLS). Kraft black liquors are dark viscous liquids resulting from digestion of wood with an aqueous solution of sodium hydroxide and sodium sulfide. In general, during kraft pulping, about 50% of the wood enters the solution. This dissolved wood substance originates primarily from hemicellulose and lignin and is present in the liquor as, respectively, hydroxy acids and alkali lignin. A black liquor char bed is formed at the bottom of a chemical recovery furnace after drying, py0888-5885/90/2629-1776$02.50/0
rolysis, and partial gasification of a spray of concentrated black liquor droplets. The porous char has a very high sodium content and also contains inorganic sulfur compounds like Na2S03,Na2S203,and Na2S04. Gasification of the char provides the reducing atmosphere required to convert the inorganic sulfur compounds to Na2S, one of the active pulping chemicals. Fundamental knowledge of the processes occurring in a kraft recovery furnace is still incomplete because many interrelated reactions proceed simultaneously. Also the corrosive nature and complex composition of the mixture of char and inorganic smelt 1990 American Chemical Society
Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990 1777 Table I. Chemical and Elemental Analysis of Black Liquor Solids and Chars chemical analvsis. wt % [SO?-] P"1 iSOS2-I [W-I [S2032-1 solids 8.13 "--
L U U U
-
u
I
Figure 14. SEM picture of black liquor char obtained by drying in a dish.
sustained high rates at almost complete carbon conversion when catalytic gasification of coal chars is substantially reduced due to pore plugging and catalyst sintering (Hamilton et al., 1984). The importance of the uniformity of dispersion of sodium is also confirmed by the effect of the drying method on char reactivity. Further evidence of separation of inorganics and organics during dish drying was obtained by analyzing the fast pyrolysis char prepared from a wellmixed sample of the dish-dried solids. Shown in Figure 14 is a SEM picture of dish-dried char particles. The sodium content of the dendritic structures in Figure 14 measured by EDS was much higher than that of surrounding particles, indicating that these are sodium salt crystals. These dendritic crystals were never seen in film-dried chars. When comparing Figure 10a and Figure 14, one can see that the morphological character of the two types of BLC chars is quite different. Based on the above results and discussion, it can be concluded that the loss of uniformity of sodium dispersion during dish drying is responsible for the lower gasification rate. The results in Figure 9 show that the gasification rate of slow pyrolysis char is a few time smaller than that of fast pyrolysis char. The only difference between the two chars is that the latter was obtained by fast prepyrolysis of dry solids a t 580 "C before being heated in the TGA from room temperature to 775 "C a t 25 "C/min, while slow pyrolysis char was obtained without the fast prepyrolysis step. Both chars were kept a t 775 "C for the same time
1784 Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990
before introduction of C02. Since the sodium dispersion in the bulk of the black liquor char is mostly determined by the drying technique, and the distribution of reduced catalyst sites on the internal surface is mostly determined by the severity of the pyrolysis conditions (i.e., time at the highest temperature), it is unlikely that the differences in gasification rate in Figure 9 are caused by differences in sodium dispersion. No SEM-EDS pictures were available of the earlier study of slow pyrolysis char to confirm this. The fast pyrolysis char has, however, a significantly larger porosity (degree of swelling) and internal surface area than the slow pyrolysis char. Specifically the surface areas of the fast and slow pyrolysis chars are, respectively, 160 and 30 m2/g. The ratio of these surface areas compares favorably with the ratio of the gasification rates in Figure 9. Assuming that the number of catalytic sites per unit internal surface area is comparable for these two cham, this suggests that the gasification rate of black liquor char is also proportional to internal surface area. It has been shown that the carbon combustion rate of black liquor is related to the degree of swelling during pyrolysis (Milanova and Kubes, 1986). Since the present setup is free of significant gas-phase diffusional resistances (except Knudsen diffusion (Li, 1989)),it is unlikely that the increased porosity of the fast pyrolysis char might explain its higher reactivity. Indeed, a recent study (Noopila et al., 1989) has shown that swelling of black liquor in air with or without ignition (respectively, 700 and 420 "C) does not always give a good indication of the combustion rate for laboratory liquors. Also, unreported work by Hupa and co-workers (Hupa, 1989) showed that there was no correlation between swelling and combustion properties when industrial liquors were analyzed. The present data suggest, then, that it might be more relevant to correlate the combustion properties of black liquors with the internal surface of their respective chars obtained by pyrolysis under inert conditions at 700-800 "C rather than their degree of swelling. Conclusions Black liquor char is a unique carbonaceous material because of its fine dispersion of sodium throughout the bulk of the carbon matrix. The fine dispersion of sodium in the carbon structure is confirmed by SEM-EDS techniques. The high loading and fine distribution of sodium are responsible for the gasification rate, at least 1 order of magnitude higher than that obtained for activated carbon or coal chars impregnated with an optimal loading of Na2C03. Fine dispersion of sodium in the bulk of the carbon structure of BLC rather than just on the internal surface is probably responsible for the sustained high gasification rates a t Na/C ratios, where the rate of Na2C03-impregnatedchars is normally greatly reduced due to pore plugging and sintering of the catalyst. The method of drying of BLC has an influence on the gasification rate because it influences the sodium dispersion. The influence of gas composition on BLC gasification rate can be described by Langmuir-Hinshelwood type kinetics. This behavior, as well as the activation energy, is the same as for alkali-metal-catalyzed carbon gasification. This indicates that despite the much higher reactivity, the mechanism of carbon gasification by COBfor BLC is the same as for other alkali-metal-impregnated chars. The internal surface area rather than porosity of BLC also appears to have an influence on the gasification rate. This suggests that internal surface area measurements rather than swelling tests might be more appropriate to characterize
combustion properties of black liquors. Registry No. CO, 630-08-0; C02, 124-38-9; Na, 7440-23-5.
Literature Cited Austin, L. G.; Walker, P. L., Jr., Effect of Carbon Monoxide in Causing Nonuniform Gasification of Graphite by Carbon Dioxide. AIChE J. 1963,9 (3), 303-306. Cerfontain, M. B. Alkali Catalyzed Carbon Gasification. Ph.D. Dissertation, University of Amsterdam, The Netherlands, 1987. Cerfontain, M. B.; Meijer, R.; Kapteijn, F.; Moulijn, J. A. AlkaliCatalyzed Carbon Gasification in CO/C02 Mixtures: An Extended Model for the Oxygen Exchange and Gasification Reaction. J. Catal. 19878, 107, 173-180. Cerfontain, M. B.; Algalianos, D.; Moulijn, J. A. C02 Step-Response Experiments during Alkali Catalyzed Carbon Gasification; Evaluation of the So-called CO Overshoot. Carbon 1987b, 25 (3), 351-359. Freriks, I. L. C.; van Wechem, H. M. H.; Stuiver, J. C. M.; Bouman, R. Potassium-catalyzed Gasification Carbon with Steam: A Temperature-Programmed Desorption and Fourier Transform Infrared Study. Fuel 1981,60, 463. Grace, T. M.; Walsh, A.; Jones, A.; Sumnicht, D.; Farrington, T. A Three-Dimensional Mathematical Model of the Kraft Recovery Furnace. 1989 Intern. Chem. Recov. Conf., Ottawa, Canada, April 1989, 1-8. Hamilton, R. T.; Sams,D. A.; Shadman, F. Variation of Rate during Potassium-Catalyzed C02 Gasification of Coal Char. Fuel 1984, 63 (7), 1008-1012. Hupa, M. Private communication, Department of Chemical Engineering, Abo Akademi, Finland, 1989. Kapteijn, F.; Moulijn, J. A. Kinetics of the Potassium CarbonateCatalyzed COPGasification of Activated Carbon. Fuel 1983, 62 (2), 221-225. Kapteijn, F.; Moulijn, J. A. Kinetics of Catalyzed and Uncatalyzed Coal Gasification. In Carbon and Coal Gasification;Figueiredo, J. L., Moulijn, J. A., Eds.; NATO AS1 Series E, No. 105; Martinus Nijhoff Publishers; Boston, 1986; pp 291-360. Kapteijn, F.; Abbel, G.; Moulijn, J. A. COz Gasification of Carbon Catalyzed by Alkali Metals: Reactivity and Mechanism. Fuel 1984,63 (S),1036-1042. Li, J. Rate Processes during Gasification and Reduction of Black Liquor Char. Ph.D. Dissertation, McGill University, Montreal, Canada, 1989. Li, J.; van Heiningen, A. R. P. Mass Transfer Limitations in the Gasification of Black Liquor Char by C02. J. Pulp Paper Sci. 1986,12 (5), 5146-5151. Li, J.; van Heiningen, A. R. P. Reaction Kinetics of Gasification of Black Liquor Char. Can. J. Chem. Eng. 1989,67,693-697. McKee, D. W. The Catalyzed Gasification Reactions of Carbon. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Thrower, P. A., Us.Marcel ; Dekker, Inc.: New York, 1981; Vol. 16; pp 1-118. McKee, D. W. Gasification of Graphite in Carbon Dioxide and Water Vapol-The Catalytic Effects of Alkali Metal Salts. Carbon 1982, 20 ( l ) , 59-66. McKee, D. W.; Chatterji, D. The Catalytic Behavior of Alkali Metal Carbonate and Oxides in Graphite Oxidation Reactions. Carbon 1975, 13, 381. Milanova, E.; Kubes, G. L. The Combustion of Kraft Liquor Chars. J. Pulp Paper Sci. 1986, 12 (6), 5187-5192. Mims, C. A.; Pabst, J. K. Role of Surface Salt Complexes in Alkali-Catalyzed Carbon Gasification. Fuel 1983, 62 (2), 176-179. Moulijn, J. A.; Kapteijn, F. The Mechanism of the Alkali Metal Catalyzed Gasification Carbon. Erdl Kohle (Sci. Technol.) 1987, 40, 15-21. Nahas, N. C. The EXXON Catalytic Coal Gasification ProcessFundamentals to Flowsheets. Int. Symp. on Fund. of Cat. Coal of Carbon Gasification, 1982, Amsterdam, The Netherlands; pp 1-9. Noopila, T.; Alen, R.; Hupa, M. Combustion Properties of Laboratory Made Cooking Liquors. 1989 Intern. Chem. Recov. Conf., Ottawa, Canada, April 1989; pp 75-80. Oren, M. J.; Nassar, M. M.; MacKay, G. D. M. Infrared Study of Inert Carbonization of Spruce Wood Lignin Under Helium Atmosphere. Can. J. Spectrosc. 1984, 29 (I), 10-12. Sams, D. A. The Kinetics and Mechanism of the Potassium-Catalyzed Carbon/Carbon Dioxide Gasification Reaction. Ph.D. Dissertation, University of Arizona, Phoenix, 1985. Sams, D. A.; Shadman, F. Mechanism of Potassium-Catalyzed Carbon/C02 Reaction. AIChE J . 1986,32 (7), 1132-1137.
I n d . Eng. Chem. Res. 1990,29, 1785-1792
1785
Ph.D. Dissertation, University of Amsterdam, The Netherlands, 1982. Wigmans, T.; Goebel, J. C.; Moulijn, J. A. The Influence of Pretreatment conditions on the Activity and Stability of Sodium and Potassium Catalysts in Carbon-steam Reactions. Carbon 1983, 21 (3), 295-301. Wood, B. J.; Sancier, K. M. The Mechanism of the Catalytic Gasification of Coal Char, A Critical Review. Final Report, SRI International, DOE Contract DE-AC21-80MC14953, 1984. Yuh, S. L.; Wolf, E. E. Kinetics and FTIR Studies of the Sodium Catalyzed Steam Gasification. Fuel 1984, 63, 1604.
Shadman, F.; Sams, D. A.; Punjak, W. A. Significance of the Reduction of Alkali Carbonate in Catalytic Carbon Gasification. Fuel 1987,66 (12), 1658-1663. Spiro, C. L.; McKee, D. W.; Kosky, P. G.; Lamby, E. J.; Maylotte, D. H. Significant Parameters in the Catalyzed C 0 2 Gasification of Coal Chars. Fuel 1983,62 (3), 323-330. Van Heiningen, A. R. P.; Li, J.; Fallafollita, J. Canadian Patent Applicaton 583,409, Nov 17, 1988. Walker, P. L., Jr.; Rusinko, F., Jr.; Austin, L. G. Gas Reactions of Carbon. Advances in Catalysis; Academic: New York, 1959; Vol. 11, pp 133-221. Wen, W.-Y. Mechanisms of Alkali Metal Catalysis in the Gasification of Coal, Char, or Graphite. Catal. Rev.-Sci. Eng. 1980,22 (l), 1-30. Wigmans, T. Catalytic Gasification of Carbon; A Mechanistic Study.
Received for review December 12, 1989 Revised manuscript received April 18, 1990 Accepted May 5, 1990
Hydrodynamics and Mixing of Solids in a Recirculating Fluidized Bed D. Corleen Chesonis and George E. Klinzing Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
Yatish T. Shah* and Carlos G. Dassori College of Engineering and Applied Sciences, The University of Tulsa, Tulsa, Oklahoma 74104
Voidage and mixing of solids were measured in a 10-cm-diameter, 6.2-m-tall recirculating fluidized bed column. The solids were alumina with an average particle size of 120 pm, a wide particle size distribution, and a density of 3460 kg/m3. For mixing of solids, CaC12 impregnated on alumina as a tracer and the carbon-alumina system were examined. The experimental measurements were carried out in the ranges of gas velocities from 3.5 to 4.5 m/s, solid recycle rates from 7 to 11kg/(m2.s), column inventories from 19 to 24 kg, and average riser voidages from 0.879 to 0.938. The experimental data for the solid fraction were correlated well (mean deviation 26.9%) with an expression similar to Kwauk et a1.k model. Mixing of solids in the riser was well correlated assuming a dilute core region, a wall region with a high concentration of solids, and interchange of solids between these two regions. The mixing model contained two parameters; the mass-exchange coefficient It, between the core and wall regions and the effective average residence time for the solids in the return leg, tPp The experimental data obtained in this study were well correlated by the mass-exchange coefficient, It,, whose value varied between 0.02 and 0.04 s-l. In general, It, decreases with the average residence time of solids in the riser and t increases with the solid recirculation rate. In the range of experimental measurements examinei in this study, a high degree of backmixing of solids was observed.
Introduction In recent years, a recirculating fluidized bed in which gas circulates an appropriate amount of solid particles at a velocity much higher than their natural settling velocity has found an increasing number of practical applications. This type of fluidized bed is quite different from the conventionalbubbling fluidized bed because of the absence of bubbles in the gas phase. The first detailed experimental work on this subject was published by Lewis et al. (1949). More recently, various attempts have been made to correlate the average voidage as a function of gas and solid flow rates (Yerushalmi et al., 1978; Li et al., 1982; Avidan and Yerushalmi, 1982; Malladi et al., 1982). This voidage is averaged over the entire volume of the bed. However, over the last few years, several investigators (Kwauk et al., 1986; Yang, 1988; Schnitzlein and Weinskin, 1988; Yoshida and Mineo, 1989) have shown that existence of a voidage distribution along the bed axis. In general, the recirculating fluidized bed can be divided into two main regions, a dense region at the bottom and a dilute region at the top, with a transition region in between. Yang (1988) also considers an acceleration region
* Author t o whom correspondence should be addressed. 0888-5885/90/2629-1785~02.50/0
at the entrance to the bed. Fluidization properties of the two main regions are functions of gas velocity, solid rate, solid inventory, bed diameter, and properties of the solid and the gas. The dense phase shows a large degree of backmixing in the solid phase (Yang, 1988; Schnitzlein and Weinstein, 1988). The dilute phase, leaner in content of solids, approaches plug flow in the solid phase. The objective of the present work is to investigate the hydrodynamics and mixing of solids in a recirculating fluidized bed. Experimental data are obtained for the solid content along th8 bed axis as functions of gas and solid flow rates and particle size. The degree of mixing of solids in a recirculating fluidized bed is evaluated by using the transient response analysis. The transient response data are correlated with a model for the mixing of solids for the recirculating fluidized bed.
Experimental Apparatus and Procedure Voidage measurements and circulation tests of solids were conducted in a 10-cm-diameter and 6.2-m-tall recirculating fluidized bed. Figure 1 shows a sketch of the system. The column was made of Plexiglas and had eight ports, 19 mm in diameter, at intervals of about 0.8 m. Five of these ports could be used for sampling, while the other three and any of the sample ports that were not used in 0 1990 American Chemical Society
