WO2001034295A1

WO2001034295A1 - Sorbent, method of making the sorbent, and method of using the sorbent in fixed bed applications

Info

Publication number: WO2001034295A1
Application number: PCT/US2000/030539
Authority: WO
Inventors: Angela Breithor; John Michael Macaoay; Michael Hoffmeister
Original assignee: Engelhard Corporation
Priority date: 1999-11-12
Filing date: 2000-11-06
Publication date: 2001-05-17
Also published as: AU1468601A

Abstract

A sorbent contains an acid-leached clay and a binder. The acid-leached clay may be attapulgite, kaolin, bentonite, montmorillonite, or a combination thereof. The binder may be colloidal silica sol, sodium silicate, sodium di-silicate, hydrated alumina, starch, sucrose, or a combination thereof. Sorbent particles may contain 40 to 95 wt. % of the acid-leached clay and 5 to 30 wt. % of the binder. The sorbent has high dry and wet crush strengths and good water stability.

Description

SORBENT, METHOD OF MAKING THE SORBENT, AND METHOD OF USING THE SORBENT IN FIXED BED APPLICATIONS

BACKGROUND OF THE INVENTION The present invention is directed to a sorbent, a method of making the sorbent, and a method of using the sorbent in fixed bed applications.

Calcium bentonite clays are also referred to as sub- bentonites, calcium montmorillonites, or calcium smectites. Generally, these clays are hydrated aluminosilicate crystalline minerals. For many years, selected bentonite source clays have been treated with acids to leach alumina from the clay structure. The starting clays which are used to produce known forms of acid-leached bentonite typically contain approximately 20% alumina (based on the dry weight) in octahedral and tetrahedral bonding structures . The clays are usually leached to a residual aluminum content in the range of about 10-15 wt.%, depending upon the intended use of the acid- leached clay.

Acid dosages of about 40-50g of 96% H₂SO₄/100g clay are typically used for the leaching. While sulfuric acid has preferably been used as the leaching agent, other acids such as phosphoric acid and citric acids have been proposed. Generally, both octahedral and tetrahedral aluminum remain in the solid residue which, when studied by x-ray diffraction, exhibits lines characteristic of the clay crystals. The acid- treated clay is invariably washed to remove soluble salts and entrained acid. Acid-leached clays are used to produce bleaching earths for the removal of pigments from oils; as petroleum cracking catalysts; as reactive pigments for carbonless copying paper; and for other commercial applications .

U.S. Patent No. 5,883,035, incorporated by reference herein in its entirety, discloses a method for manufacturing mesoporous particulate silicoaluminate from calcium bentonite by the extraction of octahedral aluminum by acid. The process comprises mixing calcium bentonite with an amount of acid, such as phosphoric acid, sufficient to leach substantially all of the octahedral alumina while leaving at least a predominant amount of the tetrahedral alumina. The mesoporous particles have a pore diameter (volume average) of about 20-100 Angstroms (i.e., 2-10 nanometers) as measured by N₂ adsorption. The mesoporous silicoaluminates are used as a cracking catalyst and as a matrix for a zeolitic cracking catalyst.

U.S. Patent No. 5,008,226, incorporated by reference herein in its entirety, discloses a process for making acid- activated bleaching earth products from naturally-occurring mixtures of calcium bentonite and attapulgite clay. The process involves treating such clay with low levels of activating acid, which are mixed with the dried and ground clay, or spray dried from slurries containing the clay-acid mixture. The acid-activated clays are used to adsorb colored pigments and colorless pigments from edible and inedible oils.

U.S. Patent No. 5,008,227, incorporated by reference herein in its entirety, discloses a process for making acid- activated bleaching earth from a crude attapulgite clay that is mildly acidic and contains at least about 90% attapulgite.

WO 92/16291 discloses a filter for taking up gaseous substances. The filter includes a porous, self-regenerating material containing two components. The first component consists of one or more porous materials such as perlite, zeolite, bentonite, ground silicon dioxide powder, or christobalite . The second component consists of one or more materials with the capability to take up water molecules directly from the surrounding air, such as silicon dioxide gel, starch or derivatives of cellulose, gypsum, silicates, aluminum silicate, or fuller's earth. The components are joined into a lattice by a binder. - 3 -

WO 98/47614 discloses compositions suitable as bleaching earths in the purification of edible or inedible oils. The compositions may comprise agglomerated particles, or microspheres, of a bleached clay and a binder material. The microspheres have an average diameter of at least about 10 microns with 90% of the microspheres having a diameter of 10- 100 microns. The microspheres have a pore diameter of 600- 10,000 Angstroms. The compositions filter colored and colorless impurities of oils.

U.S. Patent No. 3,526,322 discloses absorbent filter bodies containing molecular sieves and two compounds from the group consisting of silica gel, activated alumina, and attapulgite clay. The body is bonded by means of an inert bonding material, preferably a fired glass frit. The molecular sieves are alkali metal and alkaline earth metal zeolites or aluminosilicates .

Known filters or sorbents, however, have many disadvantages. Silica gels are synthetic materials manufactured according to the sol-gel process, which is expensive in comparison to manufacturing sorbents comprising natural minerals such as clay. Activated aluminas, as basic oxides, deactivate quickly due to acidic attack from carbon dioxide and moisture in air streams. Zeolites bond water strongly and therefore require a high desorption energy at temperatures greater than about 200°C to regenerate the filter material. In addition, conventional acid-leached clays cannot be used in fixed bed applications because they do not have sufficient mechanical and water stability.

Thus, there remains a need for sorbents containing natural clay materials that are inexpensive to manufacture and that can be easily regenerated at lower temperatures than known filter materials. There also remains a need for sorbents containing natural clay materials that have sufficient dry and wet crush strengths and good water stability for use in fixed bed applications. SUMMARY OF THE INVENTION

The present invention is directed to a sorbent comprising an acid-leached clay and a binder and to methods of making the sorbent and using the sorbent in fixed bed applications.

Preferably, the acid-leached clay is attapulgite, kaolin, bentonite, montmorillonite, smectite, or a combination of at least two of these clays. The binder is, for example, sodium silicate, sodium di-silicate, hydrated alumina, or a combination thereof. The sorbent may contain about 40 to 95 wt.% of the clay and about 5 to 30 wt . % of the binder. The sorbent has acceptable dry and wet crush strengths . In addition, the sorbent does not fall apart after being contacted with liquid water.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The sorbent of the present invention comprises an acid- leached clay and a binder. The clay may be either an acid- leached mono- or double- layered clay. Mono-layered clays include attapulgite and kaolin. Double-layered clays include bentonite, montmorillonite, and smectite. The acid-leached clay is present in an amount of about 40 to 100 wt.%, preferably 40 to 95 wt.%, of the sorbent. Good results may be achieved when the clay is, for example, (1) F-20^® acid-leached calcium montmorillonite obtained by acid leaching raw Cheto clay, available from Engelhard Corporation; or (2) F-160^® acid-leached Fowlke clay, available from Engelhard Corporation. Both F-20 and F-160 are powdered aluminosilicates . Typical particle sizes for F-20 and F-160 are as follows:

Average Particle Size Range 10% less than 2.6 μm 1-3 μm

50% less than 19.6 μm 10-25 μm 90% less than 84 μm 75-95 μm.

The binder may be, but is not limited to, sodium silicate (Na₂Si0₃) , sodium di-silicate (SDS) , bentonite, hydrated alumina, starch, colloidal silica sol, sucrose (e.g., molasses), or combinations thereof. The binder allows the acid-leached clay to be formed into spherical particles or into other discrete shapes. The binder is present in an amount of up to about 30 wt.%, preferably about 5 wt.% of the sorbent. Sodium silicate having a Si0₂:Na₂0 ratio of 3:22 is preferably used as the binder. Other useful commercially available binders include NALCO 1034A^® (colloidal silica sol) ; VERSAL^® (alumina) ; ATTAGEL^® (clay) ; METHOCEL^® (methyl cellulose), and BAUXIT^® (i.e., Al₂0₃«H₂0, 20-25 wt.% Fe₂0₃ and 1-5 wt.% Si0₂) .

Other additives, can optionally be added to the sorbent, including lime, molecular sieves, silica gels, aluminas, Y- zeolites, and mixtures thereof. An example of a Y-zeolite additive is NaY, which may be present in an amount of 10-20 wt.%.

The sorbent preferably has a BET surface area of greater than about 150 m²/g and an attrition of less than 0.1 wt.%. The sorbent may be in the form of pellets comprising a plurality of particles and having a pellet size of about 1-10 millimeters. The pellets have a mean pore diameter of about 3-4 nanometers; negligible macropores; and a total pore volume of about 0.3 mL/g. The pore volume is calculated according to ASTM D 4641-87: Standard Practice for Calculation of Pore Size Distributions of Catalysts from Nitrogen Desorption Isotherms. The pellet size is calculated by conventional screening using a RETSCH AS 200 Control G instrument and sieves according to DIN 4188.

The sorbent of the present invention preferably has a dry crush strength of greater than about 60 Newtons (N) , and a wet crush strength of greater than about 45 Newtons (N) . A Newton is 1 x 10⁵ dynes. In addition, the sorbent according to the present invention is easily regenerated at temperatures from about 120-150°C. In order to form the sorbent, and to achieve sufficient mechanical and water stability, the acid-leached clay and binder are mixed together at ambient temperature for at least 30 minutes and up to about 2 hours. Mixing for less than 30 minutes does not result in sorbent pellets that are mechanically strong because a sufficient amount of energy or work has not been added to the mixture . The clay and binder mixture is then subjected to a process such as extrusion, agglomeration, and the like. The sorbents of the present invention cannot be prepared by spray drying or flash drying. The sorbent may be extruded one or more times . A second extrusion may be through die openings that are smaller than those used for the first extrusion. For example, a first extrusion may be through die openings having a one inch or greater diameter; whereas, the second extrusion may be through die openings having 1/16 inch to 1/4 inch openings. After particles are formed (e.g., by extrusion), they are calcined at a temperature of about 150°C to about 540°C, preferably at about 250-500°C, in order to make the sorbent.

The sorbent of the present invention is suitable for gas- solid phase separations including, but not limited to, dynamic fixed bed applications, such as temperature swing adsorption, pressure swing adsorption, and removing moisture from air. The pressure swing adsorption corresponds to heaterless dryer operation.

EXAMPLES

I . Method of Making

F-20^® acid-activated calcium montmorillonite powder and sodium silicate were added to a Littleford mixer. Water was then added to make a mixture .

The resulting mixture was extruded through a 1/8 -inch (3.3 mm) die to form pellets, which were dried in an oven. The resulting pellets were calcined in air at temperatures of 300°C, 500°C, or 600°C with a 4-hour ramp up from room temperature and a 3 -hour hold time, followed by cooling to room temperature . II . Examples 1-2

Examples 1-2 were prepared by mixing acid-leached clay and a binder in the proportions shown in Table 1 below in an Eirich R02 mixer to form granules. The largest agglomerated granules were separated. The remaining granules were extruded through a 1/8-inch (i.e., 3.3 mm) die and dried. The resulting granules were then calcined in air for 4 hours at the temperature listed in Table 3 (T_calc is the calcination temperature applied for 4 hours in a lab muffle oven) .

TABLE 1

III. Examples 3-9: Examples 3-9 were prepared by mixing an acid-leached clay and a binder in the proportions shown in Table 2 below in an Littleford mixer. Water and nitric acid were added to make a mixture. The nitric acid (HN0₃) was added to peptize the binder. The resulting mixture was extruded through a 1/8-inch (i.e., 3.3 mm) die to form pellets and then dried in an oven. No granules were formed. The resulting pellets were then calcined in air for 4 hours at the temperatures listed in Table 3.

TABLE 2

TABLE

The dry crush strength is measured after Examples 1-9 were calcined at 160°C for 4 hours and then cooled to ambient temperature in an exsiccator. According to German Industrial Standard DIN 8948, 50 pellets were randomly selected. The dry crush strength of 25 of those pellets was measured with an ERWEKA TBH 20 apparatus, and the average dry crush strength was calculated.

The wet crush strength was determined after the examples were calcined at 160°C for 4 hours and then cooled to ambient temperature in an exsiccator. About 50 randomly selected pellets were immersed in a beaker full of room temperature water for about 30 minutes. The pellets without cracks, splits or other visible damage were separated and used to determine the water stability (% of pellets without cracks, splits, etc.) . The wet crush strength of the undamaged pellets (a minimum of 25 pellets) was then measured with an ERWEKA TBH 20 apparatus.

The moisture uptake in weight percent from air at 25°C is given at 10-80% relative humidity according to German Industrial Standard DIN 55473.

IV. Examples 10-12:

Examples 10-12 were prepared by mixing acid-leached clay and a binder in the proportions shown in Table 4 below in an Eirich R02 mixer. First, the clay and any other dry ingredients were mixed for about 10 minutes. Second, water and binder were added and the resulting mixture was mixed for about 20 minutes to 2 hours. Third, the mixture was then extruded one or more times through a Bonner extruder having a triangular die with 8 holes. The extrudates were then collected on Pyrex pans and dried in a forced air oven at 120°C for about 2 hours. The extrudates were hand-sized and calcined at 250°C for about 240 minutes in a box furnace. The extrudates were then resized and a minimum of 25 extrudates were crushed with an ERWEKA TBH 20 apparatus to determine the average dry crush strength. Several extrudates were allowed to sit in room temperature water for 30 minutes. The crush strength of a minimum of 25 extrudates that did not deform while being handled was measured with an ERWEKA TBH 20 apparatus to determine the average wet crush strength. The dry and wet crush strengths for Examples 10-12 are shown in Table 4.

Table 4

V. Comparative Examples

Table 5

A comparison of the sorbents of Table 3 with those of Table 5 shows that the sorbents of the present invention have wet and dry crush strengths and water stability that are superior to conventional acid-leached clay sorbents, such as Desiccite 25 and F24, and which are comparable to known silicagels such as TrockenperlenWS and alumina products such as F200.

Tests were also conducted using the above conventional acid-leached sorbents in pressure swing adsorption using a heaterless dryer, PALL XP26HAP. Unlike the sorbents of the present invention, Desiccite 25 and F24 had high attrition. Due to their poor crush strength, breakage of the pellets caused blocking of the dryer valves, an increased pressure drop, and inoperability of the dryer.

The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed broadly to include everything within the scope of the appended claims and equivalents thereof.

Claims

WHAT IS CLAIMED IS:

1. A sorbent comprising a plurality of pellets, said pellets comprising an acid-leached clay and a binder and having a dry crush strength of greater than about 60 N.

2. A sorbent according to Claim 1, wherein said pellets have a wet crush strength of greater than about 45 N and a water stability greater than 95%.

3. A sorbent according to Claim 1, wherein said acid- leached clay is selected from the group consisting of acid- leached attapulgite, kaolin, montmorillonite, bentonite, smectite, and a combination of two or more of these clays.

4. A sorbent according to Claim 1, wherein said binder is selected from the group consisting of colloidal silica sol, sodium silicate, sodium di-silicate, hydrated alumina, starch, sucrose, and a combination of two or more of these binders.

5. A sorbent according to Claim 1, wherein said acid- leached clay is present in an amount of about 40 to 95 wt.% of the sorbent .

6. A sorbent according to Claim 1, wherein said binder is present in an amount of about 5 to 30 wt.% of the sorbent.

7. A sorbent according to Claim 1, wherein said pellets have a pellet size of about 1-10 millimeters and a mean pore diameter of about 3-4 nanometers.

8. A method of making a sorbent, comprising: treating a clay with an acid to leach aluminum; mixing the acid-leached clay and a binder for at least 30 minutes to form a mixture; extruding the mixture one or more times to form particles; and calcining the extruded particles.

9. A method according to Claim 8, wherein said mixing is from about 30 minutes to about 2 hours.

10. A method according to Claim 8, where the calcining is at a temperature of from about 150°C to about 540^CC.

11. A method according to Claim 8, wherein said calcining comprises increasing the temperature of the extruded particles over a period of about 4 hours from room temperature to a temperature of from about 150°C to about 540°C and maintaining said temperature for about 3 hours .

12. A method of adsorbing a gas comprising contacting the gas with a fixed bed of a sorbent according to Claim 1.

13. A method according to Claim 12, wherein said contacting comprises temperature swing adsorption and pressure swing adsorption.

14. A method for maintaining a dry atmosphere in a container, comprising: placing a package comprising a sorbent according to Claim 1 in the container; and adsorbing water vapor in the container with said sorbent.