I
WM. D. STILLWELL The Harshaw Chemical Co., Cleveland, Ohio
Preformed Catalysts and Techniques of Tableting
Tableted catalysts pack uniformly for smooth liquid and gas flow and may have high resistance against crushing and abrasion. They are easily handled and catalytic material secured by impregnation has greater surface area.
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ATALYSTS in industrial use today may be solids, liquids, gases, chemical compounds, solutions, or mixtures. Also they may be solids with absorbed films or activated areas of catalytic promoters often appearing as powders, pellets, or pressed tablets. This general group of solid forms, usually with some added catalytic component, is considered here under the generic term, preformed catalysts. The simplest type of preformed catalyst is one wherein the pellet is made directly from a catalytic material such as alumina tablets for alcohol dehydration (2), or the fixed-bed forms of reduced nickel on kieselguhr (3) or of copper chromite (7). Mixtures of oxides such as the iron oxidechromia-potash combination of butylenebutadiene dehydro catalysts (9),and the polymerization catalyst mixture of kieselguhr-phosphoric acid (23), may be formed directly and develop strength on subsequent calcination. Logical tablet variants are those forms produced on a carrier such as alumina, kieselguhr, or carbon, and subsequently impregnated with catalytic components. Tableted type of catalysts are but a single form of a series of preformedcatalyst shapes briefly mentioned as follows.
1. Ground powders often sized to specifications for fluid-type or slurry-type operations. Reduced nickel on kieselguhr (3), and copper chromite (7) are typical formulations. 2. Microspheroidal powders, spray
dried to predetermined particle-size distributions, make up the gel-type synthetic silica-alumina cracking catalysts (22) and molybdena-on-alumina hydroforming catalysts (27). 3. Granules of dried press cake in irregular broken shapes or the more uniform product of preform dryers. 4. Flakes of hardened vegetable oil containing reduced nickel (7) are an unusual type of preformed catalyst used in the vegetable shortening and related industry. 5. “Shotted” pellets of gel-type carriers, made by forming droplets in a medium immiscible with the medium of which the droplets are composed (76), or by “casing” small particles with gel under analogous conditions (20), are used in moving-bed units. 6. Molded pellets, tablets, rings, and spheres in which a plasticlike mass is shaped by a mold or similar forming device, without the application of external pressure, are widely used in many industrial applications. 7. Simple extrusions are made by wiping or roller devices which force spaghettilike strings of plasticized catalyst through perforations. 8. Pellets can be formed by matching half molds in revolving heavy steel compression drums. 9. Extrusions may be formed by means of an auger-type feed or hydraulic piston forcing a plasticized catalyst through an orifice. I t is difficult and often impossible to simulate these operations on a laboratory scale, particularly where compaction is accomplished by high pressures with rugged plant equipment. Laboratory spray dryers will produce beautiful microspheres, but generally not typical in size to those from plant units. Plastic catalyst mixes with starch or other binders can be “buttered” into perforated plates, dried therein, punched out, and subsequently calcined. Laboratory ex-
trusions can be made with a grease gun fitted with an orifice and mounted in a vise; a spatula can serve as a cutter blade. On a more ambitious scale, an old-fashioned meat grinder with orifices can produce adequate extrudates. While each catalyst shape produced by the many possible techniques has its particular field of application-and most of them are adequate for laboratory work-many lack compaction or cohesiveness necessary to withstand the pressures and abrasion of commercial installations. Compensating factors such as drying, or calcining soluble salts, to a bonding cement, compound formation, solid solution, or shrinkage caused by calcination can produce added strength. Gellike structures often shrink to exceptionally strong forms on calcination.
Ta bleting Many solid catalysts are used in packed towers or reactors wherein a uniform feed flow is desirable, or maximum strength is needed to resist the crushing load of the catalyst charge. Tablets pack uniformly to give smooth liquid and gas flow, while superior crushing strengths are often possible from the great compaction of the tableting machines. All tablet compressing operations, no matter what the material, are essentially the same. A regulated amount of feed is placed in the die cavity and pressure is applied to form a tablet of the desired form and density. Speed of operation and shape and size of tablets which can be made on a given machine, vary widely, depending on type of material being compressed, its granular condition, or temperature and humidity. I n commercial production, rotary tableting machines are used which can turn out an amazing number of tablets per hour; VOL. 49, NO. 2
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the pound rate usually increases with the larger tablet sizes. Some high-speed machines can produce nearly 2000 I/*inch tablets per minute. The usual commercial-catalyst tableting machine consists of a rotary horizontal plate holding a series of dies, while cam-actuated punches work in pairs from both above and below the die plate. The lower punches thus serve as the bottom of the die. but are adjusted in such a way that they are dropped down during the feed cycle to allow a gravityflow feed frame to fill the die. The lower punches are then pushed up by the cam to expel excess feed; thus a measured amount of material is always filled into the die. As the cycle continues, both punches, brought slowly toward each other, exert uniform pressure on both top and bottom of the tablet and thus make for uniform strength in the product. .4s the cycle continues the upper punch is lifted and the lower punch knocks the tablet out of the die. Experience seems to be the main guide in catalyst-tableting problems; with standard materials. conditions may be well established and tableting machine settings can be fairly reproducible. Considerable experimentation is usually necessary when tablets of strange and novel materials are required. The machine operator may vary the moisture content of his feed, particle size, or particle-size distribution, and experiment with several lubricants. He may adjust the length of the stroke, amount of feed to the machine, pressure, and other factors to obtain the desired length and strength of the tablet. Normally, tablets are made with slightly rounded ends although flat-end tablets also can be made. Usually, the smallest are slightly longer than their diameter (about inch), but with increasing diameter the length-diameter ratio decreases. Thus, while tablet manufacturers have some control over their product, nature of the material rather than desires of users, often dictates both length and strength of the tablet. While many installations call for tablets of maximum strength. a number of specifications actually put limits on both maximum and minimum average crushing strengths, and on percentage distribution of several strength brackets within the acceptable spread. .4 considerable number of tablets must be crushed to give a fair determination of value for a large shipment. Normally, punches and dies for tableting machines are made of hardened steel, but special steel alloys and tungsten carbide are also used. When caught in time, worn punches can be dressed down, perhaps three times, and still produce tablets within size tolerances. Even with tungsten carbide, a particular catalyst mix may be so abrasive that several thousand dollars’ worth of
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punches and dies can be worn out during the tableting of but a few hundred pounds of catalyst. Unfortunately, certain catalyst formulations cannot be tableted at all to produce products of desired strengths. Tablets of 100% chrome oxide, 100% silica, and 1 0 0 ~ carbon o are particularly difficult to form. A few tablets can be made for experimental purposes with a laboratory hydraulic press, but these seldom show the physical characteristics of plant-produced catalysts. Satisfactory tablets can be made in the laboratory with single punch machines or, better still, with a small rotary plate unit on which punches and dies are installed at but one or two of the numerous stations. Such a unit is absolutely necessary in working out the permutations of a new production job; unfortunately, tableting a few pounds of catalyst does not always anticipate plant problems of tonnage production, continuous operation, and equipment wear. A commercial pilled catalyst, for example, may have to meet specifications of hardness, diameter, length, curvature, uniformity, chemical analysis, surface area, and perhaps other criteria. Only an extended run can predict accurately its ability to meet stringent specifications of this nature. The best feed for a tableting machine is a fine, free-flowing powder; sometimes, however, the powder can be too fine to feed properly and often causes excessive abrasion and wear of the tableting equipment. Materials are sometimes densified by mixing with water, starch, or sugar solutions, drying, and repowdering in a granulator. This is a power-driven device which minimizes the production of fines by rubbing the aggregates through a screen. O n other occasions, satisfactory tablets can be accomplished only by a “slugging” operation wherein large tablets are knocked out in a rough way and then broken up in the granulator to provide the desired feed. Materials high in volatile breakdown products, such as carbonates or hydrates, can be tableted only with reservations as to ultimate strength; such tablets may, in fact, disintegrate during final calcination. The obvious remedy for this is to precalcine the carbonates or hydrates prior to tableting, but only to the extent proved effective by experience. Gel-type materials, despite high content of bound water, prove to be exceptions; they exhibit shrinkage which tends to impart tablet strength. Large quantities of calcined aluminas have been tableted and used as mounts for important catalysts. High-quality aluminum trihydrate produced by the Bayer process is available in powders with some degree of size classification and excellent chemical control. Blending of various lots in the calciner will tend to average the variations in particle-size distribution. As this material on ignition shows a loss of about 35%, the hydrate
INDUSTRIAL AND ENGINEERING CHEMISTRY
must be broken down by calcination to a loss of about 10% for best pilling characteristics and formation of socalled active alumina (79). Such dehydrations can be readily carried out in indirect rotary calciners equipped with cyclones and operating at a temperature of about 350’ C. Because of their greater surface area and often greater activity, gel aluminas are finding wider acceptance as catalyst bases or mounts for promoter metals, although in many formulations they cannot replace acdve Bayer-process aluminas. Methods of preparing alumina gels are beyond the scope of this article; dozens of patents have been issued, covering many methods of manufacture and applications in industry. Lubricants are usually incorporated into tableting mixes to minimize die-wall drag and to help flow of the mix in the die. Graphite of various types and hydrogenated vegetable oils, either alone or in various combinations of 1 to 4%, may be used. At times, incorporating a small amount of water into the mix may be necessary. For laboratory batches, catalyst powder surfaces are adequately coated with lubricants by shaking the components together in a jar and for plant installations by means of a pony mixer or tumbling drums.
Calcination Tablets must be calcined at several stages of production. Calcination prior to impregnation with catalytic components develops strength, increases surface area, and effects burn-off of pilling lubricants, which is important for complete and uniform impregnation. Alumina tablets without calcination, for example, will fall apart in hot impregnating solutions or even react chemically with certain impregnants. Burn-off of pilling lubricants can be accomplished in simple vertical canisters, wherein the burning organic material provides most of the heat after ignition is started. Tunnel kilns provide more accurate control and a more uniform product. Calcination between impregnations given to a catalyst, and again after final impregnation, is usually necessary to fix the carrier in the proper form, convert the active component to the desired compound, or both. Normally, conversion of active alumina mounts, for example, to the most active form will take place in the temperature range 600’ to 700”C., which thus limits all subqequent heat treatment. Calcination of higher temperatures may deactivate carriers by lowering surface area, converting the oxides to other crystalline forms, or inducing solid solution or compound formation. At times, some of these effects are deliberately sought; precious metal catalysts are often most effective on mildly active supports, and certain
P R E P A R I N G CATALYSTS IN T H E L A B O R A T O R Y iron catalysts are purposely converted to solid solutions. Catalyst calcinations are carried out in equipment more or less standard for the nature of the material and the temperatures desired. Rotary calciners, more commonly the indirect fired types, are widely used but may be too rough on tablets, especially “green” tablets fresh from the tableting machines. Double shells are effective. Large tonnage calcinations are often made in tunnel kilns wherein 10 to 20 pounds of tablets are placed in saggers (ceramic boxes) which are slowly carried through a predetermined drying, calcining, and cooling cycle. At times the atmosphere over catalysts must be controlled; such catalysts as nickel on kieselguhr can be reduced by hydrogen at elevated temperatures, the furnace design depending on whether the catalyst is in powdered or tabular form. Rotary and fixed-bed reactors are used. As such reductions can easily get out of hand, the rate must be controlled by carefully valving the inlet hydrogen in accordance with thermocouple data. Laboratory calcinations of preformed catalysts may be carried out in standard equipment. A passable rotary furnace can be devised from a ball-mill drive and rack, and reduction ovens from tube furnaces, while program-control systems can materially aid in laboratory-to-plant conversions. The latter is critical because time-temperature relations are most important for the oxide components of many formulations, and the usual laboratory calcination differs markedly from plant techniques.
Impregnation A great number of solid catalysts are made by extending the desired catalytic material through the pores or over the surface of a carrier by means of impregnation. This not only secures the catalytic material but puts it in a form readily handled, or exposes a greater surface for maximum utilization and efficiency. All types of preformed carriers are used as mounts for catalyst impregnations. Such diverse shapes as powders, granules, or spheres of Alundum, clay, and porcelain, and tablets of carbon, alumina, and silica are used in quantity. Perhaps the most simple catalyst of this type is that formed by dipping pieces of firebrick in nickel nitrate solution and subsequently reducing in situ (4). A catalyst of aluminaplatinum mounted on porcelain rods (74) is an interesting entry in the field of air pollution control. Typical of preformed impregnated catalysts prominent in the petroleum field today are cobalt-molybdenurn on alumina, desulfurization catalyst ( 5 ) , platinum on alumina re-
forming catalysts ( 7 7 ) , and chrome on alumina dehydro catalysts (8). Practically any element of the periodic table, except the noble gases, is a potential catalytic additive, particularly for research laboratories. These most widely used are metals of Groups VI, VII, and VIII, particularly chromium, molybdenum, nickel, iron, cobalt, platinum, tungsten, and palladium; also, portions of Groups I and I1 are used, such as copper, silver, zinc, and mercury. Coupled with these elements, such components as stabilizers, promoters, or activators, may be added in small amounts. Alumina stabilizes silica; silica heat stabilizes alumina; barium is said to stabilize copper chromite; zinc, cerium, and potassium compounds, fluorides, and chlorides find use in specific applications. I n this article, no attempt is made to differentiate among these functions. Impregnations can be made from molten salts or by sublimation; both vacuum and pressure methods have been described, but the most widely used system is impregnation from aqueous media at normal atmospheric pressure. Effective impregnating salts may be nitrates, amines, or any water-soluble form which readily decomposes to the desired oxide without by-product impurities; therefore, sulfates are not popular. Less common salts such as formates, tartrates, and citrates supplement such better known impregnants as ammonium molybdate for molybdenum, and chromic acid (CrOs) for chromium. Because of volatilization or decomposition, influence of the anion involved in an impregnation is generally negligible, but patent claims are made for anion effects in important catalysts-eg., the influence of halogens on platinum (70) and chrome-alumina catalysts (72). Impregnation of a catalyst carrier will depend on the amount of impregnating material desired and its solubility, porosity of the carrier, and degree of penetration to be effected. Many catalyst formulations contain some generally established and accepted metal contenteg., most platinum hydroforming catalysts contain 0.1 to 1.0% platinum, and most cobalt-molybdenum desulfurization catalysts contain 3% cobalt oxide and 9% molybdenum trioxide. Although catalyst producers offer a line ofstandard products, a considerable number of impregnated catalysts are made to customer specifications. Laboratory work, however, often reaches into the unknown, and optimum impregnating characteristics can be learned only from experience. Differences in feedstock, operating conditions, reactor designs, and end-product requirements may vary metal content somewhat or degree of reduction of two nickel catalysts designed for the same
reaction. Probably, active catalyst sites on the carrier, as determined by experience, limit many metal impregnations to the 5 to 10% range. Some chromiaimpregnated aluminas, however, have been prepared with as much as 30 to 4070 chromic oxide. For high metal assays, the impregnating solution is generally used at a convenient elevated temperature so as to hold a maximum of metal and avoid immediate crystallization on the tablet. Obviously, the key to uniform impregnations and consistent results over a period of time, is to absorb out a solution which is not being depleted of any one component during the impregnation. A quick way to determine metal pickup for laboratory purposes is to impregnate a weighed amount of tablets with a solution having a known metal content, dry off the surface solution by rolling the tablets in a towel, reweigh, and after adjusting for calcination characteristics of the mount, calculate the pickup of metal. I n meeting specifications, the only safe method is to determine the solution strengthpickup factor through chemical analysis, and then maintain the solution at constant assay and temperature. The pickup of metal salt solutions will vary considerably with the nature of the mount; it is not always possible to obtain a desired maximum metal content with one impregnation. Then, when all likely impregnating salts have been considered, it is necessary to employ multiple impregnations. This introduces added calcinations between each impregnation, and substantially increases cost of the finished catalyst. Occasions such as one which required 14 separate impregnations, may be necessary, but are poor plant practice. While no hard and fast rules can be drawn, it appears that impregnations on porous supports tend to approach a maximum pickup after a number of cycles, whereas pickup on nonabsorptive mounts appears to increase continuously at a low rate. Thus, a point of no return can be encountered not only in the amount of metal picked up by multiple impregnations, but in the economics of such treatment also (Tables I and 11). When two or more metals are to be impregnated in a single carrier, a joint impregnation can be made, or a compound of the two metals can be precipitated in the presence of the carrier, or in its pores. Also, the carrier can be impregnated with each metal separately. In the latter method, the first impregnated material should be insolubilized by precipitation or heat treatment, so that solvent action of the second impregnant does not “lift” the first metal. Choice of a given technique will depend on convenience and cost of prepVOL. 49, NO. 2
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Figure 1,
Continuous apparatus for plant impregnation of tablets from aqueous solutions
aration, but most important, 011 actual performance of the resulting products. Excellent examples of these techniques are shown by a series of patents of cobalt-molybdena on alumina:
1. Cobalt molybdate precipitated on a n alumina gel (5). 2. Cobalt salt dissolved in water containing slurried alumina gel. 4mmonium molybdate solution wirh an excess of ammonia is added to precipitate the combined oxides of cobalt and molybdenum ( 7 5 ) . 3. Cobalt salt and molybdenum salt dissolvcd in an ammonia solution and used as a single impregnant (77). 4. A carrier impregnated with cobalt salt solution, dried, and calcined to decompose the salt. Treatment is then repeated with molybdenum salt (13).
Figure 2. solutions
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Laboratory technique for
impregnating tablets from
INDUSTRIAL A N D ENGINEERING CHEMISTRY
aqueous
Specifications for impregnated catalysts usually call for uniform impregnation-i.e., an even distribution of the impregnating metal throughout the tablet. While this is impossible for highly impervious carriers, careful control of impregnating time and adjustment of concentration of impregnating solutions can usually give satisfactory results. There are patent claims to the use of acyclic polyhydric alcohol ( 6 ) , for example, to effect better metal penetration. Often a dipping period of but 2 to 3 minutes will prove adequate, although 30-minute dips are sometimes required. Longer immersion times are usually not economically feasible in commercial installations.
P R E P A R I N G CATALYSTS IN THE L A B O R A T O R Y Table I.
Pickup from Multiple Impregnations o f Low Absorption Supports (From analysis) Alundum Pellets Mullite Bricks Spent Cat., 5% N I Support c o o , 11% Ni, 10% Ni, 22% Concn. of metal wanted Ni(NOs)z, 18% Ni NiNOs, 18% Ni Impregnating sol. Co(NOa)z, 11% Co Cumulative Metal Pickup, %
Dips, N o . 1 2 3 4
...
2.4 4.8 6.8 8.5
6.7 9.6 12.2
5
+ 1.1
= 6.1 6.9 8.4 9.3
e.g., carbon mounts cannot be exposed to high oxidizing temperatures, and noxious fumes or dusts must be removed. Acknowledgment
The author expresses appreciation to the Harshaw Chemical Co. for permission to publish this paper, and to the personnel of the Catalyst Laboratory who have been most helpful with advice and criticism. literature Cited
Table II.
Pickup from Multiple Impregnations of High Absorption Supports (From analysis) Gel Alumina Act. Alumina Kieselguhr Support Tab. Gran. Pellets Ni, max. Cr~03,30% Ni, 3040% Concn. of metal wanted CrO3, 50% Ni(NO&, 17% Ni Ni(N03)2, 17% Ni Impregnating Sol. Cumulative Metal Pickup, %
Dips, N o . 1 2 3 4 5
13 23 32
... ...
One trouble often encountered in aqueous impregnations is migration, or crawling of the impregnant. This often can be eliminated or minimized by changing the impregnating compound, impregnating with more dilute solutions (and thus possibly requiring a greater number of impregnations), very slow drying of the wet tablets, controlled humidity in the dryer, or freguent mixing or tumbling of the wet tablets during the drying stage. There is patent evidence to indicate that sometimes high surface concentration of impregnated metals may be desired. An alumina tablet with a thin skin or shell of impregnated chromia (78) is claimed to possess superior catalytic properties with less chromia content, and thus lower cost. This skin structure is obtained by burning out only the outer portion of the binder or lubricant in the original tablet, thus leaving an inner impervious core and an outer absorbing layer in which chromic acid is impregnated; subsequent calcination burns out the core, leaving a white-centered alumina tablet with a chromia-alumina shell. Impregnation of preformed shapes in a catalyst plant is usually accomplished in dipping baskets made of stainless steel mesh or suitable material. Tough granular mounts and coarse powders can be handled by a dragline or spiral conveyor moving through a dip tank. Each dipping is usually followed by
11
... ... 27 29
10 18 24 28
...
draining, drying, and calcining to complete the cycle. Production line techniques can be applied to largescale operations when a mechanized unit can be reserved for one particular metal. A continuous unit, as shown in Figure 1, may hold 5 to 10 pounds of tablets per each stainless steel basket, pass the load through a controlled impregnating liquor, drain, and discharge a uniformly dried product from a wire belt conveyor. O n a more modest scale, larger baskets holding 15 to 20 pounds of tablets may be dipped in racks by chain hoist; or, in preparation of special formulations, single baskets may be hand dipped. Laboratory impregnations can be made with small stainless steel baskets (Figure 2). Monel burner gauzes can be easily formed into small handy baskets. Full and adequate draining of free impregnating solution prior to drying is always important for uniform results. Because heat shock often causes rupture or spalling, it is desirable to dry freshly impregnated tablets slowly before calcination. This can be accomplished with normal chemical-plant type of equipment and usual laboratory means. Continuous screen dryers, rotary units, and tray dryers are used, depending on the nature of formulations and the strength or abrasion resistance of particles. Direct, indirect, and controlled atmosphere variations may be required-
(1) Adkins, H., “Reactions of Hydrogen with Organic Compounds over Copper-Chromium Oxide and Nickel Catalysts,” Univ. of Wisconsin Press, Madison, Wis., 1944. (2) Badische Anilin- und Soda Fabric Process, p. 736, Reinhold, New York, 1940. ( 3 ) Bailey, A. E., “Industrial Oil and Fat Products,” 1st ed., p. 589, Interscience, New York, 1946. (4) Ibid., p. 605. ( 5 ) Byrns, A. C. (to Union Oil Co. of Calif.), U. S. Patent 2,325,033 (July 27,1943). ( 6 ) Engel, W. F., Hoog, H., Ibid., 2,690,433 (Sept. 28, 1954). ( 7 ) Freed, M. L.‘(to Seymour Mfg. Co.), Zbid., 2,424,811 (July 29, 1947). ( 8 ) Groll, H. P. A., Burgin, J (to Shell Development Co.), Zbid., 2,184,234 (Dec. 19, 1939). ( 9 ) Gutzeit, C. L. (to Shell Development Co.), Zbid., 2,408,140 (Sept. 24, 1946). (10) Haensel, V. (to Universal Oil Products Co.), Zbid., 2,479,109 (Aug. 16. 1949).
(11) Ibid., 2,623,861 (Dec. 30, 1953). (12 ) Hansford, .R. (to Socony-Vacuum Oil Go., Inc.), Zbid., 2,678,923 (May 18,1954). (13) Hendricks,.G. W. (to Union Oil Co. of Calif.), Ibid., 2,687,381 (Aug. 24, 1954). (14) Houdry, E. J., Zbid., 2,664,340 (Dec. 29, 1953). (15) Huffman, H. C. (to Union Oil Co. of Calif.), Zbid., 2,437,533 (March 9, 1948). (16) Marisic, M. M. (to Socony-Vacuum Oil Co., Inc.), Zbid., 2,384,946 (Sept. 18, 1945); 2,418,232 (April 1, 1947). (17) Nahin, P. G., Huffman, H. C. (to Union Oil Co. of California), Zbid., 2,486,361 (Oct. 25, 1949). (18) Owen, J. R. (to Phillips Petroleum Co.),Ibid., 2,606,159 (Aug. 5, 1952). (19) Ibid., 2,664,451 (Dec. 29, 1953). (20) Pierce, J. A., Kimberlin, C. N. (to Standard Oil Development Co.), Zbid., 2,454,942 (Nov. 30, 1948). (21) Proc. Am. Petroleum Znst. 32M, Sect. 111. DD. 224. ff. (22) Sitig, M. J., Petroleum Rejner 31, 263 (1952). (23) Watson, K. M. (to Universal Oil Products Co.), U. S. Patent 2,120,723 (June 14, 1938). 1.1
RECEIVED for review May 18, 1956 ACCEPTED December I , 1956 VOL. 49, NO. 2
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