Chemistry of hydrocarbon compounds – Unsaturated compound synthesis – By dehydrogenation
Reexamination Certificate
2002-10-11
2004-12-14
Wood, Elizabeth D. (Department: 1755)
Chemistry of hydrocarbon compounds
Unsaturated compound synthesis
By dehydrogenation
C585S658000, C585S660000, C585S661000, C585S662000, C585S663000
Reexamination Certificate
active
06831204
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to high temperature, oxidation-resistant, aluminum containing oxide-dispersion-strengthened (ODS) alloy-supported catalyst compositions for oxidative dehydrogenation processes and a method of using such catalysts in the presence of hydrocarbons. More particularly, this invention relates to compositions of MCrAlY-supported catalysts for the production of olefins by oxidative dehydrogenation of hydrocarbons in short-contact time reactors (SCTRs).
BACKGROUND OF THE INVENTION
Dehydrogenation of hydrocarbons is an important commercial process. Dehydrogenation is the process used to convert aliphatics to olefins, mono-olefins to di-olefins, cycloalkanes to aromatics, alcohols to aldehydes and ketones, aliphatics and olefins to oxygenates, etc., by chemically removing hydrogen from the starting molecule(s). In more practical terms, this process has been used to produce commercially many of the precursors of products such as detergents, gasolines, pharmaceuticals, plastics, polymers, synthetic rubbers and many others. For example, polyethylene is made from ethylene, which is made from the dehydrogenation of ethane (i.e. aliphatic to olefin). More ethylene is produced in the U.S. than any other organic chemical. Thus, it is easy to appreciate the significance of the dehydrogenation process to industry.
Traditionally, the dehydrogenation of hydrocarbons has been carried out using steam cracking or non-oxidative dehydrogenation processes. Thermal or steam cracking is an extremely energy intensive process that requires temperatures in excess of 800° C. About 1.4×10
15
BTU's (equivalent to burning 1.6 trillion ft
3
of natural gas) are consumed annually to produce ethylene. In addition, much of the reactant (ethane) is lost as coke deposition. Non-oxidative dehydrogenation is dehydrogenation whereby no molecular oxygen is added.
Oxidative dehydrogenation of hydrocarbons (ODH) with short contact time reactors is an alternative to traditional steam cracking and non-oxidative dehydrogenation processes. During an ODH reaction, an oxidant is co-fed with saturated hydrocarbons. Typically the oxidant is a gas containing oxygen. The oxygen-containing gas may be pure molecular oxygen, air, oxygen-enriched air, oxygen mixed with a diluent, and so forth. However the presence of a diluent such as an inert gas in the oxidant increases the reactor and equipment size. The oxidant in the desired amount may be added in the feed to the dehydrogenation zone and the oxidant may also be added in increments to the dehydrogenation zone. Gas hourly space velocity (GHSV) is typically from 20,000 to 10,000,000 hr
−1
. For the present process, GHSV is defined by the ratio of the volumetric flow rate (m
3
/hr) of gaseous feed at normal pressure and temperature over the catalyst bed volume (m
3
). The contact time of the reactants with the catalyst is typically in the 10-200 ms range. The reaction pressure is typically between 1 and 50 bars.
The capital costs for olefin production via ODH are significantly less than with the traditional processes, because ODH uses simple fixed bed reactor designs and high volume throughput. In addition, ODH is an autothermal process, which requires no or very little energy to sustain the reaction. Energy savings over traditional, endothermal processes can be significant if the heat produced with ODH is recaptured and recycled. Often, the trade-off for saving money in commercial processes is loss of yield or selectivity; however, the ODH reactions are comparable to steam cracking in olefin selectivity and alkane conversion.
As mentioned above, ODH is an exothermic reaction, and temperatures at typical reaction conditions in excess of 1,000° C. may be required for successful operation. It is known that ceramic monolith catalyst supports are susceptible to thermal shock; that is, either rapid changes in temperature with time or substantial thermal gradients across the catalyst structure. Catalysts and catalyst supports for use in such a process must therefore be very robust, and avoid structural and chemical breakdown under the relatively extreme conditions prevailing in the reaction zone.
U.S. Pat. No. 5,639,401 discloses a porous monolithic foam catalyst support of relatively high tortuosity and porosity, preferably comprising at least 90 wt % zirconia for thermal shock resistance.
Complete oxidation of hydrocarbons, such as occur in automobile catalytic converters, also require catalysts, which function at high space velocities and also are stable at elevated temperatures of greater than about 700° C. U.S. Pat. No. 5,511,972 discloses a catalyst structure that is effective under the severe conditions encountered in automobile catalytic converters. The catalyst structure comprises a ferrous alloy as the catalyst support. The ferrous alloy contains aluminum, which forms micro-crystals or whiskers of alpha-alumina on the alloy surface when heated in air. A washcoat of gamma-alumina is added to the alpha-alumina surface followed by the deposition of palladium.
Materials such as oxide-dispersion-strengthened (ODS) alloys can withstand high temperatures (>700° C.) similar to those used in the ODH reaction system. As an example of ODS alloys, MCrAlY alloys have been used as a thermal coating or thermal barrier in high-temperature or corrosive environments such as diesel exhaust systems or gas turbine engines. As disclosed by Czech, et al., in
Surface and Coatings Technology
, 108-109 (1998) p. 36-42, stationary gas turbine engines for electric power generation operate at gas inlet temperatures that are as high as those in the ODH reaction zone. The turbine blades are subjected to very high thermal and mechanical loads and are additionally attacked by oxidation. To deal with the mechanical loads, the base material of the turbine blades is metallic in composition. To deal with the thermal and chemical stresses, the turbine blades have a coating with an MCrAlY composition, where M comprises Ni and/or Co, as a protective overlay coating against oxidation. Additional coatings may be added as thermal barriers. The overlay coatings are typically applied by either Low Pressure Plasma Spray or Vacuum Plasma Spray. The base material is protected in operation by an alumina scale, which forms from the overlay coating.
A MCrAlY alloy is comprised of chromium, alumina, yttrium and another metal or metal alloy M, with the metal preferably selected from the group of Ni, Co, Fe. The aluminum in MCrAlY forms the oxide scale. As a major constituent of the alloy, it provides a reservoir from which the alumina scale is constantly replenished. Replenishment, or film growth, is controlled by oxygen diffusing inwardly along alumina grain boundaries. The oxidation rate of MCrAlY is directly proportional to the formation rate of the alumina scale on its surface. Scale formation is attributed to the aluminum's activity and its diffusivity in the alloy. This activity is increased by the presence of chromium, which also enhances diffusion rate of the metal M. Adding chromium lowers the amount of aluminum needed to form and maintain the protective oxide film. If the aluminum content were increased instead of adding chromium, the MCrAlY alloy would show signs of brittleness.
FeCrAlY alloys exhibit high corrosion resistance and high sulfadation resistance. For optimum protection from hot corrosion, CoCrAlY alloys are preferred. “Hot corrosion” or high temperature corrosion is a form of corrosion that does not require the presence of a liquid electrolyte. An example of hot corrosion is oxidation. Nonetheless, CoCrAlYs are preferably limited to applications operating at temperatures below 927° C. NiCrAlYs can be used in a slightly higher temperature range than CoCrAlYs (up to 982° C.) and offer better oxidation protection than CoCrAlYs. The shortfalls of these two alloys can be overcome by substituting a small percentage of one for the other, or making either a NiCoCrAlY or CoNiCrAlY alloy.
In addition to combining M-base alloys, other metals known to
Allison Joe D.
Carmichael Lisa M.
Chen Zhen
Ramani Sriram
Conley & Rose, P.C.
ConocoPhillips Company
Wood Elizabeth D.
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