Stabilized Ceramic Composition, Apparatus and Methods of Using the Same (Exxonmobil)

CATEGORY: HIGH SEVERITY HYDROCARBON CRACKING
PATENT
Stabilized Ceramic Composition, Apparatus and Methods of Using the Same (Exxonmobil)
Publication numberUS20130231238 A1
Publication type Application
Application number US 13/769,620
Publication date Sep 5, 2013
Also published as US8399372, US20100292522, WO2010135045A1
Inventors
ChangMin Chun, Frank Hershkowitz
Original Assignee
Exxonmobil Chemical Patents Inc.
Abstract
In one aspect, the invention includes a refractory material, said material comprising: (i) at least 20 wt. % of a first grain mode stabilized zirconia based upon the total weight of said material, said first grain mode having a D50 grain size in the range of from 5 to 2000 μm, said stabilized zirconia including a matrix oxide stabilizer; (ii) at least 1 wt. % of a second grain mode having a D50 grain size in the range of from 0.01 μm up to not greater than one-fourth the D50 grain size of said first grain mode zirconia, based upon the total weight of said material; and (iii) at least 1 wt. % of a preservative component within at least one of said first grain mode stabilized zirconia, said second grain mode stabilized zirconia, and an optional another grain mode; wherein after sintering, said material has porosity at 20° C. in the range of from 5 to 45 vol %.
FIELD OF THE INVENTION
This invention pertains to ceramic materials, components, methods, thermal reactor apparatus and processes using the same, demonstrating improved physical and chemical stability in refractory applications. In some embodiments, the components may have particular utility in pyrolysis reactors such as may be used for thermally cracking, converting hydrocarbons, or for performing other high temperature chemistry. The invention includes refractory grade ceramic components that are resistive to progressive compositional degradation, thereby demonstrating improved retention of certain performance properties, such as strength, toughness, chemical stability, and thermal stability at high temperatures such as above 1500° C., as compared to prior art refractory grade ceramics.
BACKGROUND OF THE INVENTION
Economical operation of high severity hydrocarbon cracking processes and equipment requires overcoming numerous competing operational and engineering challenges. The high temperatures and process stresses can exceed the long term viability of most conventional materials, including ceramics. In addition to component physical and thermal performance considerations, component chemical inertness and crystalline stability also become significant impediments requiring consideration. Component degradation and corrosion during long-term use present still further obstacles requiring address, particularly with regard to severe hydrocarbon processing.
One problem in the art pertains to ceramic stabilizer volatility and progressive loss of such stabilizer from the ceramic due to the severe pyrolysis temperatures and cyclic temperature swings. This stabilizer loss results in progressive reduction in crystalline stability and component degradation, eventually leading to premature component failure.
Conventional steam crackers are a common tool for cracking volatile hydrocarbons, such as ethane, propane, naphtha, and gas oil. Other higher severity thermal or pyrolysis reactors are also known to be useful for cracking hydrocarbons and/or executing thermal processes, including some processes that are performed at temperatures higher than can suitably be performed in conventional steam crackers. As compared to conventional cracking equipment and processes, higher temperature reactions and processes typically require more complex, costly, and specialized equipment to tolerate the intense heat and physical stress conditions. Properties such as temperature, reaction environment, component strength, and toughness limitations commonly defining upper limits for many of the processes and facilities.
In addition to processes utilizing high temperatures (e.g., >1500° C.), processes involving high temperatures plus large cyclic temperature swings and process fluid directional changes, such as regenerative or reverse flow reactor processes, pose even greater challenges. For example, the art discloses that to efficiently obtain relatively high yields of acetylene from pyrolyzing methane feed, such as in excess of 75 wt. % yield, reactor temperatures in excess of 1500° C. are required and preferably in excess of 1600° C., with relatively short contact times (generally <0.1 seconds). Due to the high temperatures involved, such processes are generally limited to relatively small amounts or batches using cyclical processes yielding a mixture of acetylene, CO, and H2. Due to the high severity, such methane cracking processes, however, have been relatively inefficient, impractical, and of very limited commercial value as compared to other more economical processes for generation of acetylene. Acetylene is typically generated commercially by cracking feeds is other than methane, which may be done at lower temperatures.
The high temperature processes (e.g., >1500° C.) have previously not scaled well and are generally only useful for relatively high-cost, specialty applications. Processes such as thermally cracking methane to acetylene have largely been commercially unattractive due in large part to thermal, chemical, and mechanical degradation of the reactor equipment, including ceramic materials used therein. Cyclic temperature changes and product flow direction changes impose severe physical strength and toughness demands upon the refractory materials at high temperature. Such stresses and performance demands have also typically limited manufacturing and use of the refractory materials to relatively simple shapes and components, such as bricks, tiles, spheres, and similar simple monoliths. Reactor component functions and shapes have been limited for high severity services.
In addition to physical temperature limitations for reactor materials, many prior art ceramic reactor materials that are relatively inert at lower temperatures become susceptible to chemical degradation, ceramic corrosion, and/or crystalline alteration at higher temperatures, leading to premature equipment degradation and/or process interference, such as by generation of unacceptable levels of contaminants in the process. Although high temperature regenerative pyrolysis reactors are generally known in the art as capable of converting or cracking hydrocarbons, they have not achieved widespread commercial use, due significantly to the fact that they have not been successfully scaled to a commercially economical size or useful life span as compared to less severe alternatives, such as steam cracking.
The identified prior art pertaining to refractory materials for high-severity hydrocarbon pyrolysis dates primarily to the 1960's and earlier. However, that art merely occasionally provides generalized lists of some exemplary materials such as ceramics, alumina, silicon carbide, and zircon as reactor materials. These sparse, non-specific disclosures left the art largely incapable of providing a large-scale, commercially useful reactor or reactor process. The teachings of the art was only effective for enabling relatively small scale specialty applications that see vastly inferior use as compared to large scale processes such as hydrocarbon steam cracking. The identified art is void of teaching or providing a refractory ceramic material that is capable of sustaining the complex set of properties that are required for extended use in the reactive or other most-demanding regions of a high-severity (≧1500° C.) pyrolysis reactor, such as for the commercial production of acetylene and/or olefins from methane or other hydrocarbon feed. The studied art does not teach preferred crystalline structures or compositions for particular reactor furnace uses, or for complex reactor component shapes and/or functions. Multimodal ceramics are also is known in the ceramics art, as are ceramic compositions utilizing nanoparticles. However, the art remains void of teaching a ceramic or other composition or method of preparing the same that meets the rigorous performance properties needed for commercial application and long-term stability in high temperature cyclic pyrolysis processes. Further, the desired materials must maintain their formulations, crystalline structure, and corresponding physical and chemical properties for prolonged periods of time, at commercial scale and within the confines of an economic requirement. The studied art is believed to be similarly deficient at teaching materials suitable for complex, irregular, relatively fragile, or functionally-shaped reactor components.
For further example, the “Wulff” process represents one of the more preferred commercial processes for generation of acetylene. Wulff discloses a cyclic, regenerative furnace, preferably including stacks of Hasche tiles (see U.S. Pat. No. 2,319,679) as the heat exchange medium. However, such materials have demonstrated insufficient strength, toughness, and/or chemical inertness, and are not amenable to use as certain desirable reactor components, such as for use as reactor fluid conduits, to facilitate large-scale commercialization. Although some of the “Wulff” art disclose use of various refractory materials, a commercially useful process for methane cracking or other extreme high-temperature processes (e.g., >1500° C., >1600° C., and even >1700° C.) has not previously been achieved utilizing such materials. The aforementioned practical obstacles have impeded large scale implementation of the technologies. Materials availability for high temperature, high-stress applications is one of the most critical issues in design and operation of large-scale, commercial, high-productivity, thermal reactors. Due to high temperatures involved in cyclic pyrolysis reactors, generally only ceramic components have the potential to meet the materials characteristics needed in such aggressive applications.
One attempt to overcome the above listed problems involved use of a “deferred combustion” process that delayed combustion and heat generation until the reaction components were positioned into the core of the reactor, thermally isolated from flow control equipment that was subject to premature degradation. The deferred combustion, regenerative reactor process and equipment was disclosed in a U.S. patent application filed Dec. 21, 2006, Ser. No. 11/643,541, entitled “Methane Conversion to Higher Hydrocarbons,” related primarily to methane feedstocks for pyrolysis systems. Although the disclosed process of the '541 application effectively controls the location of combustion within the reactor, the internal reactor components must still contend with the severely high temperatures, temperature changes, and physical stresses incurred during methane pyrolysis, is particularly for a commercially desirable reactor life term. The refractory material comprising the reactive regions may typically be a ceramic or related refractory material. In some embodiments, however, the disclosed processes and apparatus may utilize relatively complex shaped refractory components, such as a thin-walled honeycomb monolith used to conduct process fluids through the reactor. Such reactors and reactor component geometries may demand materials that have strength, toughness, chemical inertness, and other required properties that exceed the capabilities of previously identified or known refractory materials under such temperature and stress conditions.
Ceramics components generally can be categorized in three material categories: engineering grade, insulation grade, and refractory grade. The term “engineering grade” has been applied to ceramic materials which typically have very low porosity, high density, relatively high thermal conductivity, and comprise a complete component or a lining. Examples include dense forms of aluminum oxide (Al2O3), silicon nitride (Si3N4), silicon carbide (SiC), silicon aluminum oxynitride (SIALON), zirconium oxide (ZrO2), transformation-toughened zirconia (TTZ), transformation-toughened alumina (TTA), and aluminum nitride (AlN). These materials usually possess high strength and toughness, which have been dramatically improved to the degree that ceramics are now available that can compete with metals in applications previously thought impossible for ceramics. Strength is a measurement of the resistance to formation of a crack or structural damage in the material when a load is applied. Toughness is a measurement of the resistance of the material to propagation of a crack or extension of damage to the point of failure. Even though engineering grade ceramics have superior strength and toughness at relatively low temperatures, they are relatively poor in thermal shock resistance (both strength and toughness) and many grades, such as but not limited to borides, carbides, and nitrides are not chemically stable at high temperature. Many are also not suitable for use at the high temperatures encountered with some pyrolysis reactions.
The second category of ceramic materials is insulation grade ceramics, which are typified by relatively high porosity. Many may have fibrous crystalline grain structures and are more porous than engineering grade ceramics, have lower density, and have lower thermal conductivity than engineering grade ceramics. Insulating monolithic ceramics and composite ceramics are often fabricated into various forms such as rigid boards, cylinders, papers, felts, textiles, blankets, and moldables. Many are primarily used for thermal insulation at elevated temperatures, such as up to 1700° C. A broad range of porosities and pore sizes can be produced, depending on the intended application, but in general, insulation is grade ceramics tend to be relatively porous as compared to engineering grade ceramics. Porous ceramics have many open or closed internal pores that provide the thermal barrier properties. Often, quite porous ceramics, such as those having porosity of greater than 50 vol. % and commonly even in excess of 90 vol. %, are used for thermal insulation where extremely low thermal conductivity (<0.08 W/m·K) is required. However, insulation grade ceramics typically lack the structural strength and functional toughness needed for the internal components of many pyrolysis reactors and processes. Insulation grade ceramics typically are recognized as having a flexural strength or toughness of less than about 4 Kpsi (27.6 MPa) and often of less than even 1 Kpsi (6.9 MPa). Also, the insulation properties of porous ceramics may tend to degrade as the pores may fill with coke accumulation.
The third generally recognized category of ceramic materials is refractory grade ceramics. Many refractory grade ceramics typically have porosity, strength, and toughness properties intermediate to such properties in engineering grade and insulation grade. Refractory grade ceramics typically have thermal shock resistance properties similar to some insulation grade ceramics but higher than engineering grade ceramics. Conversely, refractory grade ceramics typically lack the strength and toughness of engineering grades ceramics, but which properties exceed those of insulation grade ceramics. However, typically as strength increases, thermal shock resistance and related properties are compromised. All relevant properties must be considered when selecting a ceramic for a particular application.
As compared to insulation grade ceramics, refractory grade ceramics tend to be stronger across broader temperature ranges. Refractory grade ceramics also generally tend to be more resistant to thermal shock than engineering grade ceramics. However, while some ceramics tend to be somewhat inert or chemically stable at moderately elevated temperatures, many ceramics become chemically and/or structurally unstable at severely elevated temperatures, tending to degrade and corrode within undesirably short periods of time, rendering them unsuitable for some applications. Exemplary chemically and/or thermally unstable ceramics include certain silicas, aluminas, borides, carbides, and nitrides. Also, some refractory grade ceramics are known to possess lower thermal conductivities and coefficients of expansion than certain other refractory or engineering grade ceramics. Refractory grade ceramics are also known to undergo alterations in crystalline structure at elevated temperatures. Such alterations can result in changes in bulk volume which can result in creation of stress fractures and/or cleavage planes which can reduce the material's strength or performance properties.
Some advanced engineering ceramics, such as aluminas, zirconias, and silica, such as SiC and Si3N4, also provide superior strength, but their thermal shock resistance in grossly inadequate. Moreover, these silicon based ceramics can not be used at high temperatures (i.e. >1500° C.) due to high temperature oxidation issues. On the other end of the spectrum lie the insulation grade ceramics. These ceramics offer excellent thermal shock resistance, but they fall quite short of the required strength performance.
Zirconia is a crystalline material that is commonly used in certain ceramics, also having thermal application. However, zirconia undergoes a crystalline change at different temperatures in the way its atoms are stacked (polymorphic transformation). Zirconia has a monoclinic crystal structure between room temperature and about 1200° C. Above about 1200° C., zirconia converts to a tetragonal crystal structure. At a still higher temperature, such as above 2370° C., zirconia changes from tetragonal to cubic structure and melts at 2715° C. These transformations are accompanied by greater than one percent volumetric shrinkage during heating and equivalent expansion during cooling. The volumetric changes associated with alterations in crystalline structure can produce crystalline fractures or cleavages along grain boundaries. In polycrystalline zirconia, this tetragonal-monoclinic transition results in a reduction in strength and potential catastrophic failure of the component. Stabilizers, such as yttria and some metal oxides are can be into the crystal structure to arrest or prevent the crystalline shifts, rending the crystal structure across a more broad temperature spectrum.
However, it has recently been learned that extended exposure of stabilized ceramic components, such as but not limited to stabilized zirconias, to high temperature processes and severe environments can result in gradual evacuation or loss of the stabilizer component from the ceramic crystals. This loss undesirably results in progressing temperature-related re-alteration of the crystal structure over time, further leading to onset of the aforementioned cleaving and fracturing problems. Such stabilizer material loss and crystal alteration result in a corresponding degradation and reduction in life expectancy of the component, due to compromised performance properties.
The pyrolysis art needs a stabilized ceramic composition or material that provides the desirable set of performance properties and that can sustain those properties for a commercially meaningful period of use, by resisting loss of stabilizer, maintaining crystalline stability, and enduring prolonged exposure to high severity temperatures, substantial temperature swing cycles, cyclic flows of combustion and reaction materials. The desired materials must concurrently provide the needed structural integrity, crystalline stability, is relatively high heat transfer capability, and chemical inertness required for large scale, commercial, high productivity applications. Unavailability of such materials, components, and associated processes has been one of the most critical impediments against large scale, commercial adoption and application of many high temperature pyrolysis and chemistry processes and apparatus.
SUMMARY OF THE INVENTION
The present invention provides materials, compositions, components, reactors, processes, and methods that overcome the aforementioned deficiencies. Particularly, the present inventions include materials, components, and methods that among other attributes rectify the loss of stabilizer and corresponding crystalline instability problems. Inventive embodiments and aspects may have broad application to stabilizing ceramic materials for use in high temperature pyrolysis and/or highly active environments, resulting in extended component and equipment life expectancy and predictability. For example, the present inventions may have particular application for use in high temperature pyrolysis reactors and furnaces, and more particularly with such apparatus and processes involving high temperatures, aggressive environments, or other thermodynamically active reactions.
In one aspect, the invention includes but is not limited to ceramic components and pyrolysis reactors utilizing such components, including but not limited to manufacturing processes and uses related thereto. According to the present invention, the unique combination of stabilized zirconia, distribution of multiple grain sizes, stabilization of the grains, presence of a preservative component, and prescribed porosity is believed to provide inventive materials and components differing from anything previously known in pyrolysis and refractory arts.
In one aspect, the invention includes a refractory material comprising: (i) at least 20 wt. % of a first grain mode based upon the total weight of the refractory material, the first grain mode comprising stabilized zirconia, the first grain mode stabilized zirconia having a D50 grain size in the range of from 5 to 2000 μm (some embodiments in the range of from 5 to 800 μm), the stabilized zirconia including a matrix oxide stabilizer; (ii) at least 1 wt. % of a second grain mode having a D50 grain size in the range of from 0.01 μm up to not greater than one-fourth the D50 grain size of the first grain mode stabilized zirconia (the second grain mode is herein selected as the “fine” grit size mode, relative to the size of the larger or more “coarse” grit first grain mode), based upon the total weight of the refractory material; and (iii) at least 1 wt. % of a preservative component; wherein after sintering the material has porosity at 20° C. in the range of from 5 to 45 vol %. The preservative component may be is provided within one or more of (a) the first grain mode, (b) the second grain mode, and/or (c) an optional another grain mode, and the amount of the at least 1 wt. % of the preservative component is determined based upon the aggregate of preservative component within the refractory material. That is, the at least 1 wt. % minimum is not limited to a minimum amount in each mode, but instead to an aggregate amount within the entirety of the refractory material. The term “zirconia” is defined broadly to include not only zirconium oxide, as in many embodiments the crystals or grains comprising the refractory material are primarily made up on zirconium oxide crystals, but also the term zirconia as used herein also includes the element zirconium and other compounds including zirconium, as appropriate.
In many embodiments, the first or coarse grain mode stabilized zirconia is a partially stabilized zirconium oxide, a.k.a. herein as partially stabilized zirconia. The first grain mode zirconia stabilizer may include a matrix oxide stabilizer, such as a metal oxide. In many embodiments, the fine or second grain mode includes a stabilized zirconia. In many other embodiments, the second grain mode includes or even consists essentially of the preservative component. In some embodiments, the preservative component is provided primarily within the second grain mode. In still other embodiments, the preservative component is provided within the first grain mode, as a separate grain mode, or in a combination of both the first grain mode, second grain mode, and/or a distinct grain mode, including but not limited to embodiments where the another or distinct grain mode consists essentially of the preservative component. As used herein, the terms “grit,” “particle” and “grain” generally may be used interchangeably, except as otherwise indicated.
In other embodiments, the second grain mode comprises a fully stabilized zirconia stabilized by a second grain mode zirconia stabilizer, wherein at least a portion of the second grain mode zirconia stabilizer also functions as the preservative component. In such embodiments, preferably the second grain zirconia stabilizer is present in an amount sufficient to at least partially stabilize and in some embodiments more preferably to fully stabilize, the second grain mode ceramic and also, surprisingly, with the remainder or excess amount of stabilizer in the second mode functioning as the preservative component to help the matrix oxide stabilizer to stabilize the first grain mode matrix without rendering the first grain matrix to be a fully stabilized. Sometimes, having a partially stabilized first grain mode may be preferable due to the superior physical and thermal performance properties offered by some of such embodiments. Therefore, it may be preferred sometimes to provide the preservative component primarily within the second grain mode, instead of within the first is grain mode, such that the first grain mode can remain substantially partially stabilized. In still other embodiments, however, the first grain mode may also be fully stabilized and still provide the desired physical and thermal performance properties.
Surprisingly, providing a fully stabilized zirconia within the material, such as for example within the second grain mode, has been found to provide a stabilizing function within the first grain mode for prolonged periods of exposure or use, particularly when the second grain mode stabilizer is present in an amount in excess of the amount required to merely fully stabilize the second grain mode. In some embodiments, the second grain mode zirconia stabilizer and second grain mode preservative component each comprise substantially the same materials or compounds as comprise the first grain matrix oxide stabilizer.
According to some embodiments, the second grain mode comprises a fully stabilized zirconia, the second grain fully stabilized zirconia stabilized by at least 14 wt. % of a second grain mode zirconia stabilizer based upon the weight of the second grain stabilized zirconia, wherein at least a portion of the second grain zirconia stabilizer within the fully stabilized second grain zirconia functions as the preservative component. In other of such embodiments, the second grain mode also further comprises a preservative component that also functions as the preservative component. Often, both excess mode stabilizer and preservative component comprise the same material and perform the same function of stabilizing the first grain mode of the refractory material so as to overcome the progressive loss of stabilizer therefrom due to the high temperature pyrolysis.
In still other embodiments, the second grain mode comprises at least 1 wt. % of preservative component, or for example at least 5 wt. %, 10 wt. %, 14 wt. %, 15 wt. %, 20 wt. %, 50 wt. %, 80 wt. %, 90 wt. %, or substantially 100 wt. % preservative component, based upon the weight of the second grain mode. The amount of preservative component is defined as that amount of stabilizer material present in the ceramic material or component that is in addition to the amount of such stabilizer material that is actively, initially engaged or employed in stabilizing the structure of the respective zirconia-containing mode or modes in which it is provided. For example, when a second grain mode stabilized zirconia is present, typically the second grain zirconia is a fully stabilized zirconia, stabilized by at least 14 wt. % of second grain matrix stabilizer. As only a fraction of the 14 wt. % is needed to at least partially stabilize the second grain mode stabilized zirconia, the extra amount of stabilizer is expendable for use or function as the preservative component without compromising the stability of the second grain mode crystal structure. It is not prohibitive or detrimental to practicing or making the invention that determining exactly how much of the fully stabilizing material or stabilizer is ambiguous or difficult to determine, so long as the total amount present in the second mode renders the second mode fully stabilized. It is sufficient to have learned that a fully stabilized second mode can offset the progressive loss of stabilizer from the first grain mode structure during prolonged use. The preservative component portion thus includes that portion of the stabilizer that is inn excess of the amount of stabilizer actually utilized in stabilizing the second grain zirconia matrix. It has been learned that surprisingly, a portion of the 14 wt. % minimum in a fully stabilized zirconia is useful to function as the preservative component. In many embodiments, the preservative component includes or is substantially the same compounds as comprise the first grain matrix oxide stabilizer such that the stabilizers are fully mutually compatible. (Weight percents are the amount of a component after initial sintering for at least ten minutes at a temperature of at least 1500° C., as appropriate, unless otherwise specified.)
In some embodiments, the second grain mode may comprise only a fraction by weight of fully stabilized zirconia or may not even comprise any stabilized zirconia at all, such that the second grain mode is comprised of at least 50 wt. % or substantially completely (at least 90 wt. %) or consists essentially of (at least 99 wt. % or even substantially 100 wt. %) the preservative oxide, based upon the weight of the second grain mode.
In still other embodiments, the inventive formed ceramic components include a flexural strength (modulus of rupture, MOR) of at least 6 kpsi and a normalized thermal shock resistance rating of at least four (4), as described herein. In other embodiments, the inventive formed ceramic components provide an MOR of at least 6 kpsi, while still other embodiments include MOR of at least 10 kpsi. The inventive components may also include a normalized thermal shock resistance rating of at least four (4) and preferably at least five (5).
In yet other aspects, the invention includes process for the manufacture of a hydrocarbon product from a hydrocarbon feed using a pyrolysis reactor, the process comprising the steps of: (a) providing a pyrolysis reactor with a reactive region comprising a refractory material that includes: (i) at least 20 wt. % of a first grain mode based upon the total weight of the refractory material, the first grain mode comprising stabilized zirconia having a D50 grain size in the range of from 5 to 800 μm, the stabilized zirconia including a matrix oxide stabilizer; (ii) at least 1 wt. % of a second grain mode having a D50 grain size in the range of from 0.01 μm up to not greater than one-fourth the D50 grain size of the first grain mode stabilized zirconia, based upon the total weight of the refractory material; and (iii) at least 1 wt. % of a preservative component within the aggregate of at least one of (a) the first grain mode, (b) the second grain mode, and (c) an optional another grain mode; wherein after sintering, the material has porosity at 20° C. in the range of from 5 to 45 vol %; (b) heating the reactive region to a temperature of at least 1500° C. to create a heated region; and (c) feeding a hydrocarbon feed into the heated region to pyrolyze the hydrocarbon feed and create a pyrolyzed hydrocarbon product.
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