910
I n d . Eng. Chem. Res. 1989, 28, 910-919
Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Catalytic Processes; McGraw Hill: New York, 1978; Chapter 3. Guisnet, M.; Perot, G. Zeolite Bifunctional Catalysis. In Zeolites: Science and Technology; Ribeiro, F. R., et al., Eds.; NATO AS1 Series E; Martinus Nijhoff: The Hague, 1984; Vol. 80, pp 397-420. Himmelblau, D. M.; Jones, C. R.; Bischoff, K. B. Determination of Rate Constants for Complex Kinetic Models. Ind. Eng. Chem. Fundam. 1967, 6, 539. Hogeveen, H. The Reactivity of Carbonium Ions Towards Carbon Monoxide. In Aduances in Physical Organic Chemistry; Gold, V., Ed.; Academic Press: London, 1973; Vol. 10, pp 29-52. Jacobs, P. A.; Martens, J. A.; Weitkamp, J.; Beyer, H. K. Shape Selectivity Changes in High-Silica Zeolites. Faraday Discuss. Chem. Soc. 1981, 72, 123. Kramer, G. M.; McVicker, G. B. Hydride Transfer and Olefin Isomerization as Tools To Characterize Liquid and Solid Acids. Ace. Chem. Res. 1986, 19, 78. Kramer, G . M.; McVicker, G. B.; Ziemak, J. J. On the Question of Carbonium Ions as Intermediates over Silica-Alumina and Acidic Zeolites. J . Catal. 1985, 92, 355. Martens, J. A.; Jacobs, P. A. Attempts To Rationalize the Distribution of Hydrocracked Products. I. Qualitative Description of the Primary Hydrocracking Modes of Long Chain Paraffins in Open Zeolites. Appl. Catal. 1986a, 20, 239. Martens, J. A.; Jacobs, P. A. Attempts To Rationalize the Distribution of Hydrocracked Products. 11. Relative Rates of Primary Hydrocracking Modes of Long Chain Paraffins in Open Zeolites. Appl. Catal. 1986b, 20, 283. Martens, J. A.; Weitkamp, J.; Jacobs, P. A. Primary Cracking Modes of Long Chain Paraffinic Hydrocarbons in Open Acid Zeolites. In Catalysis by Acids and Bases; Imelik, B., et al., Eds.; Studies in Surface Science and Catalysis; Elsevier: Amsterdam, 1985; Vol. 20, p 247.
Pollak, E.; Pechukas, P. Symmetry Numbers, Not Statistical Factors, Should Be Used in Absolute Rate Theory and in Bronsted Rela1978, 100, 2984. tions. J . Am. Chem. SOC. Poutsma, M. L. Mechanistic Considerations of Hydrocarbon Transformations Catalyzed by Zeolites. In Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; ACS Monograph 171; American Chemical Society: Washington, DC, 1976; p 437. Rabo, J. A. Unifying Principles in Zeolite Chemistry and Catalysis. Catal. Reo. Sci. Eng. 1981, 23, 293. Steijns, M.; Froment, G. F. Hydroisomerization and Hydrocracking. 3. Kinetic Analysis of Rate Data for n-Decane and n-Dodecane. Ind. Eng. Chem. Prod. Res. Deu. 1981, 20, 660. Steijns, M.; Froment, G. F.; Jacobs, P. A.; Uytterhoeven, J. B.; Weitkamp, J. Hydroisomerization and Hydrocracking. 2. Product Distributions from n-Decane and n-Dodecane. Ind. Eng. Chem. Prod. Res. Deu. 1981, 20, 654. Vansina, H.; Baltanas, M. A.; Froment, G. F. Hydroisomerization and Hydrocracking. 4. Product Distribution from n-Octane and 2,2,4-Trimethylpentane. Ind. Eng. Chem. Prod. Res. Deu. 1983. 22, 526. Weitkamp, J. Hydrocracken, Cracken und Isomerisieren von Kohlenwasserstoffen. Erdol, Kohle, Erdgas, Petrochem. 1978,31, 13. Weitkamp, J. Isomerization of Long-chain n-Alkanes on a Pt/CaY Zeolite Catalyst. Ind. Eng. Chem. Prod. Res. Deu. 1982,21, 550. Weitkamp, J.; Jacobs, P. A,; Martens, J. A. Isomerization and Hydrocracking of C9 through C16 n-Alkanes on Pt/HZSM-5 Zeolite. A p p l . Catal. 1983, 8, 123. Willems, P.; Froment, G. F. Kinetic Modeling of the Thermal Cracking of Hydrocarbons. 1. Calculation of Frequency Factors. Znd. Eng. Chem. Res. 1988, 27, 1959. Received f o r review September 3, 1988 Accepted March 14, 1989
Granulation and Rehydration of Rehydratable Alumina Powders Ching-Chung Huang* and Hisashi 0. Kono Department of Chemical Engineering, West Virginia University, Morgantown, West Virginia 26506-6101
T h e spouted fluidized bed granulation of fine rehydratable alumina powders and the strength development of alumina granules in an aging process were investigated. A rehydration mechanism for rehydratable alumina was also proposed and verified by experiment. First-order kinetics were found to be applicable to the formation of metastable transition aluminas (i.e., aquohydroxo complex gel and pseudoboehmite). Pretreating the rehydratable alumina powders with cold water to form aquohydroxo complex gels is necessary to stabilize the powder for granulation. T h e prewetted granulation method (the powder being premixed with water before granulation) combined with heterogeneous moisture distribution in the feed has proven the ability to produce spherical granules of 1-3-mm diameter in a spouted fluidized bed. Optimal pretreating and aging conditions were determined by solution calorimetry and granule compressive strength experiments. T h e granule growth rate and the physical properties of alumina granules can be controlled by the fluidization condition and water-powder ratio in the premixed stage. Granulation of rehydratable alumina powders is one of many important tasks in preparing aluminas for various types of process applications. The objective of granulation is to improve the bulk properties of particulates. Among the many beneficial effects, substitution of granular aluminas for fine powders can create the desired pore structures, improve the flow properties and product appearance, reduce the pressure drop in packed bed reactors, and reduce the dusting loss. Techniques to accomplish this beneficiation are classified according to the principal methods used, i.e., agitation, pressure, thermal, multiphase spray, and agglomeration from liquids. Since each method is different in its character, the resulting granules can be characteristically different in their properties.
* Author t o whom correspondence should be addressed. 08S8-5SS5/89/2628-0910$01.50/0
The existence of a liquid bridge is crucial in powder granulation mechanisms that use liquid capillary energy. Because of its strong water adsorption characteristics, rehydratable alumina has to be partially rehydrated to ensure the existence of a liquid bridge during granulation. Furthermore, granule strength will be insufficient if the degree of rehydration of the rehydratable alumina is inadequate. The understanding of the rehydration mechanism and kinetics of the rehydratable alumina is necessary to control the pretreatment, granulation, and aging conditions to ensure the granule strength. Although some fundamental studies in the rehydration of active alumina were reported (Wefers and Bell, 1972; Yamada et al., 1981), no comprehensive kinetics information has been presented. Owing to its peculiar chemical characteristics, the prewetted granulation method is feasible for producing C 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 7, 1989 911 spherical granules from rehydratable alumina powder. Many applications of granulation technology, including granulation of pharmaceuticals (David and Gloor, 1971; Holm et al., 1985; Kristensen et al., 1985), granulation of iron ores (Meyer, 1980; Abouzeid, 1985), granulation of inorganic salts (Mortensen and Hovmand, 1975; Uemaki and Mathur, 1976), granulation of radioactive waste (Bjorklund and Offutt, 1973), and the coating of granules (Kono, 1981b; Weiss and Meisen, 1983), have been reported. The first systematic investigations of prewetted granulation were carried out by Rumpf (1958) and Newitt and Conway-Jones (1958). They postulated that the pellets grow by direct coalescence with appropriate water content and the tumbling action kneads such a pair into a nearly spherical shape. In reference to this coalescence mechanism model, various studies have been reported by Capes and Danckwerts (1965), Kapur and Fuerstenau (1964), Duffy et al. (1972), and Sastry (1975). However, the small granules were assumed to be in a condition of homogeneous moisture content. This condition is difficult to obtain for fine-powder granulation (Kapur, 1967; Herd, 1979). Transition alumina powders are particularly difficult, since they adsorb up to 100% of their mass of water with chemical change of the surface properties. The granulation mechanism model proposed by Huang and Kono (1988) was employed in this study. The effect of a heterogeneous moisture distribution in the granulation feed on the granule growth was considered in terms of local deformability and moisture content. On the basis of this granulation concept, the fine rehydratable alumina powder can be granulated efficiently. Fluidization techniques have been known and used for many decades. Although the particle-forming aspect of fluidization has been somewhat secondary to other process objectives such as chemical reaction, it is becoming increasingly important in applications such as the production of granulated materials. Because of the inherent nature of fluidized beds (i.e., higher degree of solid-particle momentum and mixing), granules of l-3-mm diameter can be efficiently produced; the temperature of granulation charge can easily be controlled. The ease of temperature control may be critical to chemical reactions in the rehydration of rehydratable alumina powder. Fluidized bed granulation, however, is a coarse-particle fluidization system. The conventional fluidized bed cannot stably operate in this case due to segregation and stagnant zones in the fluidized bed. Spouted fluidized bed is one in which gas is blown through a conical-shaped gas distributor and also through the gas jet opening at the bottom of the conical distributor. The gas velocity in the bed is controlled by adjusting the gas flow through the gas distributor and the jet opening. This bed eliminates the above fluidization difficulties and is feasible for granulation. In this study, the rehydration mechanism of rehydratable alumina was investigated to study the optimal pretreatment, granulation, and aging conditions. The prewetted granulation method of heterogeneous moisture distribution feed was also demonstrated to produce small spherical granules in a spouted fluidized bed granulator.
Mechanism of Rehydration of the Rehydratable Alumina This research used rehydratable alumina, which is a highly active, porous technical aluminum oxide prepared by ALCOA. Its composition is shown in Table I, equivalent to about A1203.0.5Hz0. A rehydratable alumina particle can be considered as a broken up piece of a calcined gibbsite crystal. It is a highly porous particle with about 25% of the pore volume in the meso- and macropore
Table I. Properties of Rehydratable Alumina Powder (ALCOA CP) Physical Properties median particle size 7m particle size range, 5-95 wt % 0.5-20 Hm packed bulk density 0.72 g/cm3 particle (Hg) density 1.6 g/cm3 true (He) density 2.8 g/cm3 loss on ignition, 250-1100 OC 7% surface area (BET) 240 m2/g Pore Volume Analysis (Mercury Porosimetry) total pore volume 0.28 cm3/g 0.21 cm3/g micropore, 30 A, pore vol, strength, area, W l P , time, min MPa % m2/g cm3/g cm3/g 15 0.722 0.826 1.17 38 377 42 15 1.99 0.818 360 0.665 2.02 46 15 0.769 0.626 391 15 1.86 48 415 0.785 0.614 15 1.96 50 396 0.739 0.654 0.77 5 46 0.817 0.469 399 1.19 10 0.760 46 400 0.450 1.71 20 46 0.723 0.494 343 2.52 30 0.629 46 0.461 323 2.73 0.592 50 46 0.436 346
UO= 1.1 m/e
3 (Yo) 0
34
0
38
A
42
/
2.0
-
Dp(mm)
Table IV. Physical Properties of Semicontinuous Granulation Granules residence surface pore vol, total av >30 A, pore vol, strength, area, W/P, time, % min m2/g cm3/g MPa cm3/g 0.726 1.92 0.659 38 40 403 40 40 0.625 0.721 2.35 430 42 40 0.613 0.701 2.43 400 0.682 2.35 44 40 420 0.587 0.657 3.14 46 40 421 0.545 0.737 1.66 42 20 0.649 422 0.729 2.40 42 30 0.639 415 2.72 42 60 392 0.617 0.739
1.5
I. I
200
400
600
800
1000
1200
t (s) Figure 13. Effect of water-powder ratio on the granule growth rate of CP-7 powder.
to stimulate the formation of pseudoboehmite (the transformation of liquid bridges to solid bridges) in the
granules. The aged granules were further calcined to reach the final phase of granule product. The physical properties of the activated alumina granules were measured. BET method and mercury porosimetry (Satterfield, 1980) were used to measure the surface area, pore volume, and pore size distribution of the granules. Some measurement results are given in Tables I11 and IV. The pore volumes are closely related to granulation time and water-powder
918 Ind. Eng. Chem. Res., Vol. 28, No. 7, 1989 W/P RATIO 46% I
2.5
//i
UO( m h )
0
1.4
a
1.1
~
0.8
a
0.6
.. I
Q
%
0.4
E
2 .o
7j
0.2
-
D p (mm)
0
M E A N P O R E DIAMETER
- ANGSTROMS
Figure 16. Differential pore distribution of activated alumina granules from batch granulation. 1.5
RESIDENCE TIME 12
1.
,
-
40 MIN
1 I
................
.
ILEGEND
I .c
400
200
600
000
-
e
1000
1
06
t (s) Figure 14. Granule growth rate of CP-7 powder as a function of superficial gas velocity a t W / P = 42%.
W
p ' 4 2 96 R.T.(s)
0
1.0'
0
Figure 17. Differential pore distribution of activated alumina granules from semicontinuous granulation.
1200
0
1800
A
2400
0
3600
"
'
I
"
1.0
"
"
'
2 .o
U O (m h )
Figure 15. Mean product size of CP-7 granules as a function of superficial gas velocity a t various residence times in semicontinuous granulation.
ratio. In general, increasing the granulation time and water-powder ratio causes the total pore volume and volume of pores with diameters above 30 A to decrease. Decreases in the pore volume result in stronger granules. Granules generated by semicontinuous granulation have a larger surface area than those generated by batch granulation. This may be because of different granulation mechanisms involved in batch and semicontinuous granulation. In addition, pore size distributions, as measured by mercury porosimetry, show that a bimodal pore size distribution can be obtained for both batch and semicontinuous granulations at some granulation conditions (Figures 16 and 17). This special pore size distribution characteristic may provide additional application for activated alumina granules. Fluidized bed granulation can produce spherical alumina granules with satisfactory mechanical strength and high surface area from rehydratable alumina powder. Various grades of activated alumina granules (Le., granule size, granule strength, surface area, and bimodal pore size distribution) can be obtained by appropriately controlling the fluidization condition and water-powder ratio. Conclusion The spouted fluidized bed granulation and granule strength development of rehydratable alumina have been investigated. Prewetted granulation combined with heterogeneous pretreatment was proven able to produce
Ind. Eng. Chem. Res., Vol. 28, No. 7 , 1989 919 spherical granules of 1-3-mm diameter with a spouted fluidized bed. A rehydration mechanism of rehydratable alumina and the solid bridge formation in the alumina granule were proposed and experimentally verified. The formation of metastable transition aluminas in the rehydration reaction of rehydratable alumina powders is a function of aging temperature and aging time. The pretreatment of rehydratable alumina powders at low temperatures to suppress the rehydratioi: reaction and to ensure stable granulation is confirmed by the investigation of the rehydration mechanism. The transformation of liquid bridges to solid bridges is very important for the development of granule strength which depends on the formation of pseudoboehmite. The kinetics of the formation of solid bridges in the green alumina granule is a first-order reaction with an activation energy of 1.185 kcal/mol. The optimal aging time and temperature to develop solid bridges in the green alumina granule are determined. The granule growth rate and the physical properties of the final activated alumina granules can be controlled by the fluidization condition and water-powder ratio in the premixed stage. A bimodal pore size distribution can be obtained at some operation conditions. The possibility of the granulation of chemically reactive and porous powders is demonstrated under pretreated conditions. This study provides a granulation methodology for powders that have properties similar to rehydratable alumina powders.
Acknowledgment The authors express their appreciation to ALCOA for financial and technical support in this study. Special thanks to Dr. W. C. Sleppy, Dr. H. Fleming, and Dr. C. L. Chou for their valuable assistance and suggestion.
Nomenclature = weight mean diameter of granules E, = activation energy k = rate constant P = weight of powder PD1 = pore diameter 1 PD2 = pore diameter 2 R = gas constant RT = residence time t = operation time T = temperature U, = superficial gas velocity V = voltage V * = cumulative pore volume X = conversion W = weight of water
D,
Greek Symbol up = compressive strength of granules Registry No. A1,0,, 1344-28-1.
Literature Cited Abouzeid, A. Z. M.; Kotb, I. M.; Negm, A. A. Iron Ore Fluxed Pellets and Their Physical Properties. Powder Technol. 1985, 42, 225-230.
Barrett, C. S.; Massalski, T. B. The Structure of Metals, 3d ed.; McGraw Hill: New York, 1966. Bjorklund, W. J.; Offutt, G. F. Fluidized Bed Denitration of Uranyl Nitrate. AIChE Symp. Ser. 1973, 128, 123-129. Capes, C. E.; Danckwerts, G. C. Granule Formation by the Agglomeration of Damp Powder. Trans. Inst. Chem. Eng. 1965, 43, T116-Tl30. David, W. L.; Gloor, W. T. Batch Production of Pharmaceutical Granulations in a Fluidized Bed. J. Pharm. Sci. 1971, 60, 1869-1874. Duffy, G.; Linksoon, P. B.; Glastonbury, J. R. A Clarification of the Mechanism of Granule Growth. Pre. PACHEC Jpn. 1972, Section 4.1, 1-10. Gitzen, W. H. Alumina as a Ceramic Material; The American Ceramic Society: 1970. Hartley, P. A.; Parfitt, G. D. An Improved Split-Cell Apparatus for the Measurement of Tensile Strength of Powders. J . Phys. E Sci., Instrum. 1984, 17, 347-349. Herd, A. C. Induction Periods in the Granulation Kinetics of Very Fine Powders. Powder Technol. 1979, 24, 103-104. Holm, P.; Schaefer, T.; Kristensen, H. G. Granulation in High Speed Mixer. Powder Technol. 1985,43, 225-233. Huang, C. C.; Kono, H. 0. The Granulation of Partially Prewetted Alumina Powders-A New Concept in Coalescence Mechanism". Powder Technol. 1988, 55, 19-34. Kapur, P. C. Ph.D. Thesis, University of California, Berkeley, 1967. Kapur, P. C.; Fuerstenau, D. W. Kinetics of Green Pelletization. Trans. Am. Inst. Min. Metall. Pet. Eng. 1964, 229, 348-355. Kono, H. 0. Attrition Rates of Relatively Coarse Solid Particles in Various Types of Fluidized Beds. AIChE Symp. Ser. 1981a, 77, 96-106. Kono, H. 0. Granulation of Urea in a Fluidized Bed Granulator-an Application of Three Phase Fluidized Bed. In 3rd International Symposium of Agglomeration; Druckerei Heinrich Schuster: Nuremberg, West Germany, May, 1981b; Vol. 2, pp G16-G27. Kristensen, H. G.; Holm, P.; Schaefer, T. Mechanical Properties of Moist Agglomerates in Relation to Granulation Mechanism. Powder Technol. 1985, 44, 227-237. Marboe, E. C.; Bentur, S.Silicates Ind. 1961, 26, 389-399. Meyer, K. Pelletization of Iron Ores; Springer Verlag, Berlin, 1980. Mortensen, S.; Hovmand, S. Particle Formation and Agglomeration in a Spray Granulator. First Engineering Foundation Conference on Fluidization, Pacific Grove, CA, 1975; pp 519-544. Newitt, D. M.; Conway-Jones, J. M. A Contribution to the Theory and Practice of Granulation. Trans. Inst. Chem. Eng. 1958, 36, 422-442. Rumpf, H. Grundlagen und Methoden des Granulierens. Chem. Ing. Tech. 1958, 30, 144-158. Sastry, K. V. S.Similarity Size Distribution of Agglomerates during Their Growth by Coalescence in Granulation or Green Pelletization. Int. J . Mineral Process. 1975, 2, 187-203. Satterfield, C. N. Physical Characterization and Examination. Heterogeneous Catalysis I n Practice; McGraw-Hill: New York, 1980. Uemaki, 0.;Mathur, K. B. Granulation of Ammonium Sulfate Fertilizer in a Spouted Bed. Ind. Eng. Chem. Process Des. Dev. 1976, 15, 504-508. Wefers, K.; Bell, G. M. Oxides and Hydroxides of Aluminum. Technical Paper 19, ALCOA Research Laboratories, 1972. Weiss, P. J.; Meisen, A. Laboratory Studies on Sulfur-Coating Urea by the Spouted Bed Process. Can. J . Chem. Eng. 1983, 61, 440-447. Yamada, K.; Hamano, S.; Horinouchi, K. The Strength and Catalytic Activity of Active Alumina Body. Chem. SOC.Jpn. 1981, 9, 1486-1491. Received for review August 30, 1988 Revised manuscript received March 17, 1989 Accepted April 24, 1989
