Hydrotalcite Contact Materials

The present invention relates to new hydrotalcite contact materials, including sorbents useful in sulfur abatement processes.

Industry is lacking a composition such as fluidized bed catalyst or additive contact solids for economically removing sulfur oxides from emissions from a variety of industrial processes. Further, the industry lacks effective and economical methods for using a fluidized bed of particles to remove sulfur oxides from emissions.

Synthetic hydrotalcites, known as layered double hydroxides, are useful as contact solids in numerous industrial processes. In view of a limited supply of naturally-occurring hydrotalcite, various efforts have been made to achieve a reasonably economic alternative material in the form of synthetic Layered Double Hydroxides [`LDH`]. Production of analogous crystalline materials has been fruitful, and it may be expected that important new uses for the class of hydrotalcite crystalline solids will increase industrial demand.

The `hydrotalcite-like` materials of Shutz et al (U.S. Pat. Nos. 5,399,329 and 5,507,980) are characterized by crystals having very high broadness to thickness ratios in the 50:1 to 5000:1 range. These interesting materials have desirable mechanical and chemical properties, which will render them useful as a solids binders or as active chemical sites. Recent efforts in this aspect of layer propagation have identified certain anionic species which contribute to forming the large sheets of synthetic hydrotalcites. The mechanism whereby these lower alkanoic acid anions direct the important plate shape is not understood; however, large plate crystals having a maximum dimension of about 5 to 500 microns (.mu.) impart excellent mechanical properties.

A structural description of the class of "brucite" crystals is found in the work of Pinnavaia et al (U.S. Pat. No 5,358,701, incorporated by reference). Briefly, the metal oxide layers consist essentially of magnesium oxide (MgO) configured structurally with octahedral hydroxy groups. A trivalent metal oxide, such as alumina (Al.sub.2 O.sub.3), can be inserted into the brucite crystalline lattice in the octahedrally-coordinated metal oxide layer. In the case of natural hydrotalcite, the carbonate layer or gallery may be replaced by substitution of various acidic anions, sulfate, sulfite, nitric, etc.

It has been discovered that incorporation of certain organic acid anionic species can be effective in modifying the hydrotalcite/brucite structure to result in large crystalline sheet contact materials having increased sorption of SO.sub.x. Since hydrotalcite-like materials are stable in the crystalline oxide structure and essentially reversible in anion exchange up to about 400.degree. C., stable sorbent may be employed in moderately elevated industrial processes, such as cold side combustion effluent treatment of fossil fuel burning power plants.

In a preferred embodiment of the invention, an improved contact material comprises at least one layered hydrotalcite sheet having a brucite crystalline structure containing divalent and trivalent metal oxides comprising predominantly magnesia and alumina present in the brucite structure in molar ratio of 3:1 to 8:1 magnesia:alumina, together with a sulfur oxide replaceable anion, the hydrotalcite sheet having a breadth to thickness ration of 50:1 to 5000:1. The amount of hydrotalcite in the solid sorbent can vary from about 10 to 100 wt %.

Sulfur oxide sorption may be enhanced by depositing on the hydrotalcite-containing solid sorbent material an effective amount (eg--50 to 500 ppm) of oxidative metal catalyst, such as platinum. In one embodiment the contact material comprises at least 10 wt % hydrotalcite and 10 to 50 wt % of a ZnO:TiO.sub.2 mixture.

In the following description, units are given as parts by weight and metric units unless otherwise indicated.

The sulfur-laden hydrotalcite crystalline sheet material is a novel composition containing a sulfate anionic crystal moiety having at least 5 weight percent sulfur oxide based on sulfur-free hydrotalcite. The capture of SO.sub.3 in the fluid particle bed regenerator as an inorganic sulfate on the SOx-capturing oxide is usually followed by reducing the inorganic sulfate to a sulfide in a fluid particle bed reactor/stripper. Then, the sulfur can be released as H.sub.2 S upon hydrogenolysis or reduction in the stripper. Incorporation of a minor amount of a vanadium oxide component in the contact solids promotes reduction of sulfate during regeneration of the hydrotalcite.

The catalyst composition is desirably formed into a fluidizable particle having an average particle size in the range of about 20-300.mu. that is useful in both conventional fluidized bed and ebullating bed processes. The SOx-capturing oxide of the invention is desirably a sheet-like hydrotalcite and/or mixed inorganic oxide additive for catalyzing the capture and release of sulfur-containing compounds in the fluid cracking catalyst provides refiners with an effective means for reducing fluid cracking catalyst SOx emissions. The invention provides a contact solids or catalyst composition employed as a formed spheroidal catalyst that reduces the emissions of sulfur containing compounds from industrial processes.

For example, in the preferred embodiment of the invention, the hydrotalcite captures and thereby removes the sulfur containing species, such as H.sub.2 S or SOx that is present in the gas stream being treated.

The hydrotalcite composition may include a combination of inorganic oxides with an inorganic binder, an organic binder, or a mixtures of an inorganic and organic binder. Desirable inorganic oxides include a member selected from the group consisting of oxides or hydroxides of aluminum, calcium, cobalt, copper, iron, magnesium, molybdenum, silicon, titanium, vanadium, zinc, tungsten, strontium, nickel, manganese, zirconium, barium, members of the lanthanide series and mixtures thereof.

The contact solid composition may be self-bound or may include a binder component to "glue" the inorganic oxides together into the desired shape, such as spheroidal particles. Depending on the application, different binding systems are used. For example, in more severe high temperature applications such as fluid cracking catalyst (eg--700-820.degree. C.), a hydrothermally stable inorganic binder such as aluminum chlorohydrol or peptized alumina is used.

Effective inorganic binders include sols of aluminum such as aluminum chlorohydrol, peptized aluminas, sols of silica, colloidal silicas, sols of titanium, sols of zirconium clays such as bentonite, calcined kaolinite, kaolinite, metakaolin, montmorillonite, chlorite, talc, and mixtures of these. Desirable inorganic binders include a sol of aluminum, peptized alumina, a sol of silica, colloidal silica, a sol of titanium, a sol of zirconium, a clay, and mixtures thereof.

The smectite clays are especially desirable alone or in combination with zinc oxide/titania mixtures, various inorganic binders and smectite. In some applications, such as a fluidized or ebullating bed, found in many flue gas desulfurization units, an organic binder such as hydroxypropyl methyl cellulose or polyvinyl alcohol is used.

Other components can be added to the catalyst composition to enhance the performance of conversion catalyst in specific applications or for a specific set of operating conditions. An oxidation promoter such as platinum or cerium can be added to the catalyst solids to facilitate the oxidation of SO.sub.2 to SO.sub.3, for instance in the fluid cracking catalyst regenerator, thus improving the overall performance of the catalyst. Desirable additional components can include any number of activators, promoters oxidizers, reducers, scavengers, and mixtures of these components. These materials may enhance performance. The solid composition preferably is prepared in the form of a spheroidal particle of about 1 to 10,000 microns diameter.

Advantageously, this invention is employed to reduce emissions of sulfur compounds from a fluid cracking catalyst (FCC) process. Particulate solids are added to the FCC unit in the same manner as the conventional cracking catalyst (eg--REY zeolite). The hydtrotalcite additive can be introduced separately or together in a m accomplished by a pneumatic conveying system to blow the material directly into the FCC catalyst regenerator. The catalyst additive then circulates through the fluid cracking catalyst unit in direct contact and along with the fluid cracking catalyst. The quantity of fluid cracking catalyst additive that is added and which then circulates in the unit is sufficient to effectively reduce the emissions of sulfur containing compounds from the FCC unit, yet it need not be present in a proportion so large that it deleteriously affects the operation of the cracking process reactions. Fluid cracking catalyst additive level of between about 0.5 and about 10 weight percent of the circulating fluid cracking catalyst inventory is desirable.

In the coal gasification process, the flue gas or hot gas is passed through a fluidized or ebullating bed of catalyst to remove sulfur compounds.

Industrial processes for sulfur oxide abatement may be conducted at various temperatures. Whereas cold side flue gas abatement may be carried out advantageously at about 200 to 400.degree. C., more elevated temperatures in the 400 to 700.degree. C. range may be employed. FCC processes may employ the hydrotalcite sorbents at 700 to 820.degree. C. or higher temperature, as will be understood by the skilled chemical engineer.

Synthesis Methods:

A preferred method of manufacturing hydrotalcite-like materials having a sheet morphology is disclosed by Shutz et al in U.S. Pat. Nos. 5,399,329 and 5,507,980, incorporated herein by reference. These materials may be synthesized from an aqueous slurry of pseudoboehmite and magnesia in the presence of C1-C6 lower monocarboxylic acid, such as acetic acid. The coprecipitated crystalline sheet materials have a very high breadth:thickness ratio, eg--50:1 to 5000:1.

The process of sulfur oxide sorption is preferably carried out by a hydrotalcite having the following formula, as synthesized:

where A is a lower monocarboxylic anion, x is 0.2 to 0.4, and m is 0 to 4.

Divalent Mg may be substituted in the brucite crystalline structure up to 50 mol % by other divalent metal cation, such as Ni, Cu, Zn, Co and/or Mn; and Al ion may be substituted in the brucite crystalline structure up to 50 mol % by trivalent metal cation, such as Cr and/or Fe.

Sulfur oxide sorption may be enhanced by depositing on the hydrotalcite-containing solid sorbent material an effective amount (eg--50 to 500 ppm. Pt) of oxidative metal catalyst, such as platinum, palladium, cerium, etc. This is believed to permit sulfur dioxide to be oxidized to sulfur trioxide to provide a sulfate ion in the sorbent lattice as a `gallery layer` anion.

Sulfur Oxide Sorption

It is understood that hydrotalcite crystalline material may have up to four moles of water bound to the ideal formula unit moiety of the crystals. Accordingly, there is a significant weight decrease when water of hydration is lost during heating to about 200-400.degree. C.

It is known that natural hydrotalcite and other LDH materials are sorbents for sulfur dioxide (SO.sub.2) and sulfur trioxide (SO.sub.3), collectively--"SO.sub.x ". According to Pinnavaia et al (U.S. Pat. No. 5,358,701), these sulfur oxides can replace the carbonate anion moiety or other anions in the brucite lattice in the temperature range of 100.degree. C. to 400.degree. C. A standard test method for measuring such sorption characteristics is thermogravimetric analysis (TgA), wherein a test sample of the crystalline solid material is heated at predetermined rate to 700.degree. C., releasing water of hydration. A gaseous stream containing the sulfur oxide is then contacted with the sample and weight of sorbed material is measured. Regeneration of the sorbent (ie--by reduction of sulfate to sulfite) can be effected by passing a hot gas, such as hydrogen over the sample.

In the following examples, various contact solids test by TgA for comparing SO.sub.x sorption in a controlled heating, sorption and regeneration cycle. A standard duPont TgA unit is employed to test samples in the TgA ramp method. The sample is heated at uniform rate to 700.degree. C. with a flowing stream of nitrogen through the sample. The sorption cycle employs 3000 ppm. SO.sub.2 in a carrier gas stream containing 3 wt % oxygen and balance nitrogen and maintains the sample at predetermined temperature of 700.degree. C. for at least 30 minutes during sulfur oxide sorption. Regeneration is performed by hot hydrogen gas flowing over the sample at 650.degree. C.

A mixed hydrotalcite-Pt contact solid is tested by standard TgA method. The sheet-type hydrotalcite component (invention) comprises LaRoche HTC powder having a particle size range of about 4 to 90 microns (predominantly 15-40 microns), produced by the method of U.S. Pat. No. 5,399,329. The hydrotalcite powder is blended with equal parts of spheroidal oxidation catalyst component containing 550 ppm Pt on alumina substrate to provide a 50:50 admixture having average 225 ppm. Pt, based on total sample. SO.sub.x sorption peaks at 17 weight percent (based on HTC) in 35 minutes and is released slowly during regeneration.

A comparative sample material is employed containing `DESOX` spinel (MgAl2O4), a standard commercial desulfurization solid having 225 ppm. Pt deposited thereon. The procedure of Example 1 is repeated employing the standard sample.

An unmodified hydrotalcite powder sample as in Example 1 is tested for SOx sorption without Pt catalyst.

A zinc oxide/titanium dioxide mixture having a 1.5:1 ZnO:TiO.sub.2 wt.ratio (35 wt %) is blended with 10 wt % hydrotalcite powder as in Example 1, 15 wt % talc and 40 wt % fine kaolin clay.

Improved oxidation/sorption performance may be obtained by incorporating the Pt component directly onto the hydrotalcite-containing particles. Cerium and vanadium oxide are also suitable oxidization/reduction components.

In one aspect, this invention includes processes for producing the hydrotalcite solids. Typical methods include spray drying to make small (eg--20-300.mu.) microspheroidal particles and beading or tumbling processes to make larger spheroidal particles.

While the invention has been shown by particular examples, there is no intent to limit the inventive concept, except as set forth in the following claims.