Chemistry of hydrocarbon compounds – Unsaturated compound synthesis – By dehydrogenation
Reexamination Certificate
2000-09-18
2003-02-11
Dang, Thuan D. (Department: 1764)
Chemistry of hydrocarbon compounds
Unsaturated compound synthesis
By dehydrogenation
C585S658000, C585S661000, C585S662000, C585S663000, C585S943000
Reexamination Certificate
active
06518476
ABSTRACT:
FIELD OF THE INVENTION
This invention relates, in general, to manufacturing olefins from lower alkanes. As used herein, the term “olefins” means ethylene, propylene, butenes, pentenes, hexenes and higher olefins. The term “lower alkanes” means methane, ethane and/or propane. More particularly, the present invention relates to methods for manufacturing olefins such as ethylene and propylene from methane, ethane, and/or propane by oxidative dehydrogenation at elevated pressure, wherein the olefins are recovered from unconverted methane, ethane, and/or propane and reaction byproducts by using a complexation separation. In one embodiment of the invention, recycle of reaction byproducts is reduced or eliminated by adding an effluent containing unconverted methane, ethane, and/or propane and reaction byproducts to a methane gas transport system, such as a natural gas pipeline.
BACKGROUND OF THE INVENTION
Methane is an attractive raw material because it is widely available and inexpensive; however, it is used mainly as a fuel. Natural gas liquids, such as ethane and propane, are the major raw materials for the production of ethylene and propylene, from which many petrochemicals are produced. But the supply of natural gas liquids has not kept pace with increasing demand for olefins, so more costly cracking processes that use naphtha from petroleum are being commercialized. Therefore, the development of economical processes for manufacturing olefins from methane and other lower alkanes is highly desirable.
Methane has low chemical reactivity, so severe conditions are required to convert it to higher hydrocarbons such as olefins. Oxidative dehydrogenation is favored because conversion is not thermodynamically limited and reactions are exothermic. But selectively producing ethylene by partial oxidation, while avoiding over-oxidation to carbon oxides, has been elusive and is difficult to achieve. Therefore, since the first screening of oxidative dehydrogenation coupling catalysts was reported by G. E. Keller and M. M. Bhasin in “Synthesis of Ethylene via Oxidative Coupling of Methane. I. Determination of Active Catalysts”,
Journal of Catalysis
73: 9-19 (1982), great effort has been made to develop selective catalysts and processes for methane coupling.
Catalyst studies have nearly all been at atmospheric pressure, with only a few studies conducted at elevated pressure. This is the case because it has been reported that increasing pressure reduces coupling selectivity, primarily due to increased homogeneous or heterogeneously catalyzed combustion. The oxidative dehydrogenation coupling reaction is highly exothermic, and a high reaction temperature is usually generated within a hot spot after the reactants are heated to the initiation temperature. The temperatures employed generally exceed 650° C. and are typically 800 to 900° C. An important catalyst characteristic is lifetime, especially under such high temperature conditions. Sustained operation at excessively high temperatures usually causes significant to substantial decay in selectivity and may also result in the loss of catalytic and promoter components through slow-to-rapid vaporization.
Process studies have developed cofeed (continuous) processes and sequential (pulsed) processes. The cofeed processes pass methane and oxygen simultaneously over a catalyst in a fixed-bed or fluidized-bed reactor. They typically use low methane conversion for safety and because olefin selectivity decreases as conversion increases. The reactions are operated under oxygen-limited conditions, i.e., very high or total oxygen conversion. The sequential processes alternately contact the catalyst with oxygen (oxidation) and then methane (reduction), either in cyclic pulses or in separate reactors. Because methane does not contact gaseous oxygen in sequential processes, homogeneous oxidation is suppressed, and conversion can be higher.
Sequential catalysts are typically reducible metal oxides that function as oxygen transfer agents. Materials that have been used as sequential catalysts include a wide variety of reducible metal oxides, mixed metal oxides and other reducible compounds of the following metals: Sn, Pb, Bi, Tl, Cd, Mn, Sb, Ge, In, Ru, Pr, Ce, Fe, Re, Tb, Cr, Mo, W, V or mixtures thereof. Promoters include oxides or compounds containing alkali metals, alkaline earth metals, boron, halogens, Cu, Zr, or Rh. Processes which utilize a reducible metal oxide catalyst are disclosed, for example, in the following references: U.S. Pat. No. 4,547,607 discloses methane coupling wherein a portion of the C
2
+ alkanes recovered are subsequently recycled to the reactor. No examples under pressure are given. U. S. Pat. No. 4,554,395 discloses methane coupling at elevated pressure (100 psig and 700° C.) to promote formation of C
3
+ hydrocarbons, but does not disclose the effect on C
2
hydrocarbons. The higher C
3
+ selectivity decreases considerably after just a few minutes. U.S. Pat. No. 4,754,093 discloses reacting methane and air, adsorbing higher hydrocarbons on activated carbon at atmospheric pressure, selectively desorbing olefins under vacuum, and recycling higher alkanes with the uncoverted methane.
Many metal oxides, carbonates, and promoted mixtures, often supported on substrates such as alumina, silica, and titania, have been used as cofeed catalysts for oxidative dehydrogenation coupling. These include alkaline earth metal oxides, alkali metal oxides or halides, and oxides of Mn, Co, Ni, Zn, Bi, Pb, Sb, Sn, Tl, In, Cd, Ge, Be, Ca, Sr, Ba, Sc, Y, Zr, La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, or Lu, either individually or as mixtures thereof. Other metal-containing materials such as various zeolites have also been used. The metal oxides are often promoted with alkali metals and/or alkaline earth metals or their oxides, halides, or carbonates. Basic oxides promoted with alkali metal carbonates are important catalysts, as well as transition metal compounds.
Cofeed catalysts and oxidative dehydrogenation coupling processes utilizing such catalysts are disclosed, for example, in the following references: U.S. Pat. Nos. 4,695,668 and 4,808,563 disclose catalysts containing Mo-W compounds, which gave C
2
and oxygenated hydrocarbons, and much CO, at 520 to 800 psig. U.S. Pat. Nos. 5,066,629 and 5,118,898 disclose separating natural gas into methane and higher alkanes, oxidatively coupling the methane, pyrolyzing the higher alkanes by using the heat released, cryogenically separating the combined products, and recycling recovered ethane to the pyrolysis reaction. Integrated processes for converting natural gas into higher hydrocarbons are further disclosed in U.S. Pat. Nos. 5,025,108; 5,254,781; 5,736,107; and 5,336,825. The latter discloses recycling, to the coupling reaction, which is preferably done at 1-2 atmospheres pressure, the methane and C
2
hydrocarbons left over from subsequently converting the olefins to liquid hydrocarbons. Note also, J. L. Matherne and G. L. Culp, “Direct Conversion of Methane to C
2
's and Liquid Fuels: Process Economics”, pages 463-482, in E. E. Wolf,
Methane Conversion by Oxidative Processes, Fundamental and Engineering Aspects,
Van Nostrand Reinhold (1992).
A number of these prior art references disclose recycling unconverted methane containing byproduct alkanes to the oxidative dehydrogenation coupling reaction. These references suggest that the reaction may be done under elevated pressure, but they do not demonstrate that recycling such a composition is feasible or beneficial when the reaction is done under elevated pressure. Furthermore, the aforementioned processes that demonstrate conducting the oxidative dehydrogenation coupling reaction under elevated pressure do not suggest or demonstrate recycling unconverted methane containing byproduct alkanes to the reaction. The aforementioned processes also disclose cryogenic distillation separation, adsorption/desorption separation using activated carbon or charcoal, and separation by subsequent olefin reaction as methods by which olefins may be separa
Bhasin Madan Mohan
Culp Gary Lynn
Nelson James Russell
Nielsen Kenneth Andrew
Stricker Vincent Joseph
Dang Thuan D.
Union Carbide Chemicals & Plastics Technology Corporation
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