CA1093538A - Process for preparing shaped particles from rehydratable alumina - Google Patents

Abstract

7,181 Title: PROCESS FOR PREPARING SHAPED PARTICLES FROM
REHYDRATABLE ALUMINA

ABSTRACT OF THE DISCLOSURE
Disclosed is a process for preparing shaped alumina particles for catalysts or catalyst supports by passing droplets of an aqueous slurry of a rehydratable alumina composition through a shaping medium, such as a column of water-immiscible liquid. The alumina composition undergoes rehydration while being shaped as it passes through the shaping medium, and accordingly, firm, discrete alumina bodies are produced.

Description

3:~53~
181/26,176 The invention relates to the preparation of shaped alumina particles. In particular, it relat~s to forming shaped particles through utilization of a heated shaping medium. More particularly, it relate~ to an oil drop method for producing spherical alumina bodies from an alumina composition containing a substantial portion of a rehydratable form of alumina~
Various processes for preparing spherical aluminas previously have been proposed. One such prior method for producing such spherical shapes incorporates an oil drop procedure whereby drops of an aqueous slurry of alumina are dispersed into a water-immiscible suspending medium. Though seemingly simplistic in theory, the oil drop method has presented considerable practical operational problems. The prior art methods, exemplified by U.S. Patent Nos. 2,620,314 and 3,558,508, primarily are directed to ~he use of an aqueou~ slurry of an alumina sol including various chemical agents, so that the sol will set to a gel within the time interval of spherical particle formation, while the alumina - 20 drops are passing through the column of water~immi~clble liquid. Hence~ the selection of the starting alumina compo-sition is critical in order to obtain firmly formed particles that, in addition, will not dissolve or crack during later processing or use.
The present invention is directed to the use of a novel alumina composition in an oil drop formation method.
Unlike the prior art advancements in this fieLd, the present invention does not incorporate the use of outside chemical reagents to induce rheological ch~ngeR; rather, the novelty rè3ts in the alumina composition itself which i8 capable of 3~38 forming firm spherical shapes independently through its own internal chemistry.
According to the present inven~ion there is provided a method for producing shaped alumina particles suitable for use as catalysts and catalyst supports comprising preparing an aqueous slurry of an alumina composition containing a substantial portion of a rehydratable alumina, shaping the `
alumina into desired form, rehydrating to harden the shaped alumina, and curing, drying, and calcining the shaped alumina particles to produce cata-lyst and catalyst support material, an improvement comprising introducing an aqueous slurry containing water, an alumina containing a substantial portion of a rehydratable alumina, and optionally a combustible filler to a shaping medium selected from a) a water immiscible phase into which droplets of said alumina slurry are introduced to be shaped by surface tension forces in-to a spherical beaded form, and b) tubing of desired cross sectional size an~ shape to shape said alumina into extrudate form, whereby the alumina is fashioned into a desired configuration, and applying heat to said shaping medium to rehydrate and harden the alumina while it i5 being subjected to ~he influence of the shaping medium.
The method for producing spherical alumina particles according to the invention uses an alumina composition including a substantial portion of a rehydratable alumina form. Such an alumina composition can be produced by flash calcining hydrated alumina, such as is generally made from bauxite ore using the Bayer process, to form a partially dehydrated, rehydratable product consisting of anydrous alumina, lower hydrate forms of alumina, alumina mono-hydrate, and unreacted trihydrate. The rehydratable alumina composition ; which can be used in the present process may vary in composition. The re-hydratable powder commonly can be characterized by its LOI ~loss on ignition) and its RI (rehydration index). The LOI is determined by measurement of the amount of weight loss on heating the alumina powder .7J - 2 -3~31 at 1800F. for 2 hours. The RI, which denot0~ the amount of rehydratable alumina present in the powder, is computed according to the formula:

RI= (LOI after rehvdration - LOI before rehydration) 3700 15 (100 - ~OI a~ter rehy~rat~
In general, the preparation method for the rehydratable alumina consists of partially dehydrating alumina trihydrate by passing it through a flow of high temperature ga~ for a fraction of a second to several qecondsD The composition of the resulting product varies according to the trihydrate feed rate, the particle size, the gaq temperature, and the residence time of the particle in the gas stream. This rehydratable powder composition can be milled or ground to reduce the particle size and then mixed with water and formed. In the rehydratable alumina compo3ition of the present invention LOI measuraments in the general range of 3-15 are considered preferable. The rehydration index of the powder should be between 15 and 80, with a preferred - range of 40 to 90. The formed alumlna may be hardened or cured to increase crush strength. Steam or hot water treat-ment curing has been found to be preferable. The alumina forms are then dried and calcined.
A key feature of the rehydratable alumina compo-- sition is that an aqueous slurry of the composition requires no additional chemical reagents in order to harden into an isotropic ~olid while passing through the immL~cible phase.
A slurry with a ~olids content of about 50-60~ has been found to be of a preferred con3istency. Mere application of heat to the alumina in the presence of water ~erve~ to effectively rehydrate the rehydratable slumina form and ~3.~3~1 accordingly harden the alumina sphere into a firm discrete particle Temperatures in the range of about 80-100C. have been found to be preferable. The surface tension forces in the immiscible phase form the slurry drops into spheres, while, at the same time, the alumina is firmed by internal rehydration. An important aspect of the process is to control the droplet buoyancy and oil viscosity so that critical hardness is achieved while the drops are in free fall. Any suitable water-immiscible liquid which does not vaporize at the rehydration temperatures may be employed. While it is possible to use immiscible liquid which has a higher density than that of the formed alumina, it is preferred to use a liquid of lower density so that the formed alumina will fr~e-fall to the bottom of the forming column rather than rise to the top thereof. A blend of polyterpene resin and mineral oil has been determined to be a suitable immisci-ble phase of acceptable viscosity and density.
Spherical shaped alumina offers certain advantages and hence is preferred in various catalyst applications. The lack of sharp edges on the spheres reduces wear and handling problems. Also the spherical shape often permits more uni~orm packing and thereby reduces channeling tendencies in a reaction zone.
By utilizing the ability of the rehydratable alumina slurry to harden directly into a formed shape, a distinct energy saving advantage i3 obtained over other practiced forming techniques. For example, the energy intensive ~teps of mulling and extruding, essential st~ps in the formation of extrudates~ can be totally obviated.
Further, the nature of the forming process of the present 3~

invention yield~ a more nearly isotropic solid alumina particle which minimizes planes of weaknes~, rendering catalyRt and catalyst support par~icles with higher cru~h strength and porosity equivalent to extrudate~.
Low density alumina catalyst substrates may be made using the present process by incorporating a combustible fil-ler in with the alumina plus water when preparing the aqueous slurry. The combustible filler is destroyed during the calcination subsequent to the shaping proces~ so as to yield a low density product. By low density is m~ant a compacted bulk density in the order of 12 to 32, preferably 20 ~ 30, and most preferably 26 - 30 lbs~/cû. ft.
Suitable combustible fillers include such as microcrystalline cellulose, starch, wood flour, fine particle carbon black, and saw dust. Preferably the filler is microcrystalline cellulose. The fillers are generally used in amounts of from about 2 to 25 percent by weight of the dry inyredients, preferably 5 to 20 percent. The solids content of the slurry should preferably be in the range of about 25 - 50~.
The microcrystalline cellulo~e usable herein is - purified, partially d~polymerized cellulo~e prepared by treating alpha cellulo~e, obtained as a pulp from fibrous plant material, with mineral acids. It may be prepared a~
disclo~ed in U S. Patent 2,978,446 and i3 availabIe under ~he tradename Avicel~ of FMC Corp~ Alternatively, dependlng upon the availability of m~terial~, it may be u~ed without the drying operation specified in the p~tent.
The following exampleq demonstrate preparation of catalysts and catalyst supports using the method of the presen~ invention. They are not intended to be limiting but merely illustrative.
Preparation of An Aqueous Alumina Slurry A suitable aqueous alumina slurry with a solids content of 5Q-60 can be prepared through a method including the utilization of an ion exchange resin for removing ionic impurities from the alumina prior to forming. Such a slurry ~as prepared by stirring 1262 g of a rehydra-table alumina powder composition (LOI 9.7 RI 41, 0.15% Na20) into 1000 ml of ice chips to which water was added to fill the interstices. This ;~ 10 resulted in a thick slurry at a temperature of 10C. To the chilled slurry was introduced 100 ml of an ion exchange resin ~Dowex S~ ~-X 8*, a sulfonated styrene divinyl ben~ene polymer) with a particle size of 20-50 mesh. An exotherm occurs as the resin removes ionic impurities and the slurry becomes less viscous. The pH of the slurry fell from 10.2 to 7.9 in 4 hours, accompanied by a temperature rise to about 20C. The slurry ~o~ quite fluid, was separated from the used ion exchange resin by screening the resin off on a 50 mesh screen. The ion exchanged alumina ~as analyzed at 0.03% Na20. The resultant aqueous alumlna slurry has a solids content of S0.3%.
EXAMPLE I
An aqueous alumina slurry prepared as described in the procedure set forth above (solids conten~ 50.3%~ was observed to undergo ra~id setting upon application of heat. In order to form the alumina into beaded shapes, droplets of ~he slurry formed from a small orifice 1.5 mm in diameter ~ere dropped into an oil bath. The surface tension forces in the immiscible oil phase forms the slurry drops into , * Trade Mark r~ -- 6 ~

3~i3~

~pheres as the alumina pa~es through the oil bath. In order to rehydrate and harden the alumina as the beaded shapes are passing through the oil, the oil was heated to 80-100C. Critical hardness was achieved while the drops are suspended in free fall through the oil, so that sufficient strength developed before the drops reached the bottom of the oil container where they would otherwi~e tend to flatten out on impact. A blend of 80% poly ~erpene resin and 20~
mineral oil was used as the immiscible phase~ ~roplet fall times in a ~-3" oil layer were in the 3-60 ~econd range;
this was quite ~ufficient for the development of firm formed beads capable of maintaining their ~hape. The bead~ then were cured in the oil for about another additional 3 hours.
A catalyst support was made from these beads by subjecting 15 them to drying and calcining treatments. The finished product exhibited the following properties:
Particle Diameter 0.19"
- Particle Shape generally l'tear drop"
Crush Strength 35 lbs.
Pore Structure*
Pore Volume 0.706 ml/g Micro Pore Volume (~105A radius) 0.378 ml/g Surface Area 210 m2/g Compacted Bulk Density 38.1 lbs./ft3 (* measured by mercury intrusion method) An alternative approach to utilizing the pre~ent inventive concept accommodates the formation of extrudates.

In~tead of ~haping bead~ through use of th~ 3urface tens10n force3 in an immi~cible phase, cylinder~ (or other preferred 53~

shapes~ can be formed by forcing an aqueous alumina slurry through tubes.
Application of heat to the tubes serves to rehydrate and harden the alumina as it is being formed. Accordingly, the shaped alumina is ejected from the tubes as rigid cylinders. The following example illustrates this embodi-ment of the invention.
EXAMPLE II
An aqueous alumina slurry was prepared according to the procedure set forth above. This slurry then was pumped by a Masterflex* Variable Speed Tubing Pump, equipped with 0.02 "ID binyl ~ubing. A tubing transition was made to thin wall polytetrafluoroethylene tubing of 1/16" ID. The polytetrafluoroethylene tubing was arranged to extend one foot into an enclosure heated by atmospheric pressure steam. As the alumina slurry ~as pumped through this heated extrusion die, the heat caused it to harden inside the die and be ejected as a rigid cylinder. The pump rate was adjusted so as to produce cylinders of desired firmness upon leaving the die. Upon emerging from the die, the extrudates fell into a pool of 100C.
steam condensate at the bottom of the steam enclosure, where they were allowed to cure.
EXAMPLE III
The basic procedure of Example I was repeated to make low densitr beads as below. The aqueous alumina slurry was diluted to 40%
~olids by the addition of 25.8 g of water to 100 g of the 50.3% solids ~lurry. As the dilution was made the alumina tended to settle into a dense cake with a supernatant water layer. At this point, 1 g of micro~
cryStalline cellulose ~Avicel* of F~C Corp.) was added with stirring. The settling ~as inhibited and beads were formed as in * Trade ~ark - ~ - 8 -,i 353~

Example I.
The beads w~re calcined, and pore volume measured by water titration was 1.07 ml/g.
The compacted bulk density at 0.38 void fraction was 28.1 lbs./cu. ft.
EXAMPLE IV
The procedure of Example III was repeated except the solids content was reduced to 30~ and the amount of microcrystalline cellulose was increased to 1.35 g.
The pore volume of the re~ultant beads was 1.65 ml~
g. The compacted bulk density at 0.38 void fraction was 20.0 lbs./cu. ft.
EXAMPLE V
Low density alumina beads were prepared by dispersing 20 g of microcrystal:line cellulose in 60 g of water with stirring. This mixture was then mixed with lll g ; . of a rehydratable alumina powde:r and 30 g of ice. The pH of the resultant slurry was 9.61. 20 drop~i of 70% HNO3 were : added and the pH was reduced to 7.48.
Then the beading procedure of Example I wa~ u ed on the above~prepared slurry.
The resultan~ beads had a compacted bulk density of 13.1 lbs./cu. ft.

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a method for producing shaped alumina particles suitable for use as catalysts and catalyst supports comprising preparing an aqueous slurry of an alumina composition containing a substantial portion of a rehydratable alumina, shaping the alumina into desired form, rehydrating to harden the shaped alumina, and curing, drying, and calcining the shaped alumina particles to produce catalyst and catalyst support material, an improvement comprising introducing an aqueous slurry containing water, an alumina containing a sub-stantial portion of a rehydratable alumina, and optionally a combustible filler to a shaping medium selected from a) a water immiscible phase into which droplets of said alumina slurry are introduced to be shaped by surface tension forces into a spherical beaded form, and b) tubing of desired cross sectional size and shape to shape said alumina into extrudate form, whereby the alumina is fashioned into a desired configuration, and applying heat to said shaping medium to rehydrate and harden the alumina while it is being subjected to the influence of the shaping medium.

2. The method of Claim 1 wherein the combustible filler is present and is used at about 2 to 25 percent by weight of the dry ingredients.

3. The method of Claim 2 wherein the combustible filler is selected from the group consisting essentially of microcrystalline cellulose, starch, wood flour, fine particle carbon black, and sawdust.

4. The method of Claim 2 wherein the combustible filler is micro-crystalline cellulose.

5. The method of Claim 1 wherein the solids content of said aqueous slurry is about 25 to 60 percent.

6. The method of Claim 1 wherein the alumina composition used to prepare said aqueous slurry is an alumina powder having a rehydration index of about 40 to 80.

7. The method of Claim 1 wherein said immiscible phase is a mineral oil-polyterpene resin mixture heated to a temperature of about 80 to 100°C.

8. The method of Claim 1 wherein steam heat is applied to the tubing shaping medium.