Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2000-05-08
2002-10-08
Bell, Mark L. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S159000, C502S313000, C502S321000, C502S328000, C502S330000, C502S338000, C502S527110, C502S527190, C502S527240, C428S116000
Reexamination Certificate
active
06461995
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to extruded honeycomb catalysts, and more particularly to a method for making improved catalysts for the catalytic dehydrogenation of hydrocarbons.
One dehydrogenation reaction of particular commercial interest is the dehydrogenation of ethylbenzene to styrene:
Ethylbenzene←→H
2
+Styrene
Dehydrogenation of ethylbenzene is a major petrochemical process for styrene monomer production. In this process, deep conversion to styrene is limited by thermodynamic equilibrium:
K
P
=
x
H2
·
x
ST
x
EB
·
P
wherein x
i
are the molar fractions of the reactants and product and P corresponds to the total pressure within the reactor. This process is typically carried out in a packed bed reactor in the gas phase, with high conversion to styrene being favored by low reactor pressures and high temperatures.
When ethylbenzene dehydrogenation is run using a conventional catalyst pellet bed (e.g., beds made up of ⅛″ cylinders of the catalyst), large pressure drops across the reactor significantly limit reactor utilization efficiency. Increasing pellet size to reduce pressure drop is not a viable solution in this case since efficient mass and heat transfer within the catalyst bed require small catalyst pellet sizes. Thus conventional pellet beds cannot meet both low reactor pressure drop and high mass/heat transfer requirements.
Attempts to solve this problem have included both different pellet configurations and the substitution of honeycomb catalysts for the pellets. U.S. Pat. No. 5,097,091 describes toothed-wheel shaped catalyst pellets designed to improve reactor efficiency, but such pellets do not fundamentally change the nature of the back-pressure problem.
The potential advantages that could be realized through the use of an effective honeycomb catalyst for styrene processing are several. First, a catalyst offering large geometric surface areas, short diffusion paths within the thin channels walls, and straight thin channels of low back-pressure could in principle lead to increases in both catalytic activity and styrene selectivity. In addition, the development of a durable honeycomb catalyst could permit easy installation and removal of the catalyst in either retrofitted radial reactors or straight-flow integral reactors, and could offer reduced catalyst attrition due to abrasion than conventional pellets.
Finally, thin catalyst walls could limit the migration and loss of the potassium component of the catalyst, an effect which has been identified as a major cause of catalyst deactivation in catalyst beads. Ethylbenzene dehydrogenation catalysts typically comprise potassium salts, usually as carbonates, with the carbonate residue remaining after calcination and activation acting as an additional catalyst for the oxidation of coke in the presence of steam during reactor operation.
U.S. Pat. No. 4,711,930 discloses the use of honeycomb catalysts to reduce pressure drop in the ethylbenzene dehydrogenation process, but the catalyst mixtures provided in accordance with that patent are difficult to shape into honeycombs of the required low flow impedance, high catalyst activity, and chemical and mechanical durability (strength and attrition resistance) needed for conducting a stable commercial dehydrogenation process. One specific source of difficulty with these mixtures relates to a fundamental incompatibility between the chemical constituents required for catalyst formation and the vehicle systems required for shaping the catalysts into active yet durable honeycomb shapes.
SUMMARY OF THE INVENTION
This invention provides extruded honeycomb catalysts of high quality together with a method for extruding such catalysts in a thin-wall, high cell density configuration that preserves catalytic activity while enhancing the manufacturability and physical durability of the catalyst. The method is applicable to ethylbenzene dehydrogenation catalysts as well as a variety of other industrial-grade catalysts made from catalyst formulations that necessarily comprise relatively high concentrations of salts or other water-soluble catalyst constituents.
In a first aspect, then, the invention may be characterized as an improvement in the method of making a honeycomb catalyst by the extrusion of a plasticized catalyst precursor mixture including at least one water-soluble metallic salt catalyst precursor and a temporary organic binder through a honeycomb extrusion die. The improvements of the invention are secured through the use of a temporary organic binder that includes at least one highly-dispersed water-insoluble organic polymer in place of the water-soluble temporary binders more typically employed for the extrusion of inorganic honeycomb bodies.
The method of the invention comprises the initial step of preparing a honeycomb extrusion batch having a plasticity, lubricity, and viscosity suitable for forming a strong, green honeycomb. The batch is formulated from a catalyst precursor mixture that includes a substantial proportion of one or more water-soluble metallic catalyst precursors. This plasticized catalyst precursor mixture will comprise a water vehicle, at least one water-insoluble catalyst precursor, at least one water-soluble metallic salt catalyst precursor, and at least one temporary extrusion binder consisting essentially of a finely-divided water-insoluble organic polymer. Typically a plasticizer and a lubricant will also be included.
After combining and mixing these constituents for a time sufficient to achieve suitable plasticization of the catalyst precursor mixture, the plasticized mixture is extruded through a honeycomb die to form a green honeycomb. Providing appropriate proportions of water and temporary binders insures that green honeycomb bodies can be formed to closely prescribed honeycomb shapes, cell configurations, cell sizes and wall thicknesses at relatively low extrusion pressures and with excellent body integrity. Furthermore, the wet green honeycombs are sufficiently durable to resist deformation during handling and subsequent processing.
Following forming, the green honeycombs are dried and heat-treated to convert the catalyst precursors in the green honeycombs to the final catalyst. The characteristics of the temporary binder are such that drying and heat treatment can be carried out efficiently and with minimal processing losses.
The product of the above-described process is an improved honeycomb dehydrogenation catalyst of high activity and strength. For example, a typical ethylbenzene dehydrogenation catalyst is a durable honeycomb composed of an active potassium oxide (carbonate)-promoted, oxide-stabilized iron oxide catalyst with an axial crushing strength (parallel with the honeycomb channel axis) of at least 100 psi. Honeycomb geometries particularly appropriate for this particular catalyst, which may include honeycomb cell densities in the range of 15-400 cells/in 2 or more and channel wall thicknesses in the range of 0.2-3 mm, are readily obtained by this process.
DETAILED DESCRIPTION
A wide variety of different catalyst formulations for the dehydrogenation of ethylbenzene and similar feed stocks are known. Among the families of catalysts particularly suitable for ethylbenzene dehydrogenation are iron oxide catalysts containing ferric oxides, other transition metal oxides, potassium oxide, vanadium oxide, molybdenum and tungsten oxides, chromium oxide, aluminum oxide, cerium oxide, in addition to other rare earth oxides, and alkaline earth oxides. Known catalyst compositions for this dehydrogenation process include those comprising, when calculated as oxides, about 20-95 wt % of total of iron oxide, magnetite, or potassium ferrite yielding compounds, 0.1-40 wt % of potassium compounds, 0.1-30 wt % of cerium compounds, 0.1-30 wt % of molybdenum compounds, 0-25 wt % of Ca compounds, and 0-25 wt % of Mg compounds. One specific catalyst family of interest includes compositions comprising, in weight percent on an oxide basis, about 50-80% Fe
2
O
3
, 10-27% K
2
O, 0-5% Ce
2
O
Addiego William P.
Liu Wei
Bell Mark L.
Corning Incorporated
Hailey Patricia L.
Sterre Kees van der
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