Process for converting alkanes to olefins
A process and apparatus for converting an alkane to an olefin. In one embodiment, the process involves oxidative coupling of an alkane, e.g., methane, with an oxidant, such as air, to produce an olefin having twice the number of carbon atoms as the alkane, e.g., ethylene. In another embodiment, the process involves oxidative dehydrogenation of an alkane, e.g., ethane, with an oxidant to form an olefin having the same number of carbon atoms as the alkane, e.g., ethylene. The process involves passing a flow of the oxidant from a first flow passage through a porous medium; diffusing a flow of the alkane from a second flow passage into the porous medium; and contacting the reactant alkane and the oxidant in the presence of a catalyst within the porous medium to produce the olefin.
1. A process of oxidative coupling for converting an alkane to an olefin having twice the number of carbon atoms as the alkane, comprising:
(a) passing a flow of an oxidant through a first inlet into a first chamber defining a first flow passage;
(b) passing a flow of an alkane through a second inlet into a second chamber defining a second flow passage, wherein a porous medium having an oxidative coupling catalyst supported therein separates the first flow passage from the second flow passage, wherein the oxidative coupling catalyst infills the porous medium at a depth from 40 to 60 percent of the thickness of the porous medium measuring from a side adjacent the second flow passage to a side adjacent the first flow passage;
(c) applying a pressure in the first flow passage sufficient to provide a bulk flux of the oxidant through the porous medium into the second chamber;
(d) adjusting the flow of the alkane such that a ratio of diffusive flux of the alkane into the porous medium to the bulk flux of the oxidant through the porous medium is greater than 1:1;
(e) contacting the oxidant and the alkane at the catalyst within the porous medium under reaction conditions sufficient to produce the olefin, which has twice the number of carbon atoms as the alkane, and which thereafter flows into the second flow passage and exits through an outlet in the second chamber.
2. The process of claim 1 wherein the alkane is a C1-4 alkane selected from methane, ethane, propane, n-butane, iso-butane, and mixtures thereof; and wherein the olefin is a C2-8 olefin having twice the number of carbon atoms as the C1-4 alkane.
3. The process of claim 1 wherein the oxidant comprises an oxidizer selected from oxygen, sulfur vapor, hydrogen sulfide and methylsulfide; and wherein the oxidant further comprises a diluent selected from nitrogen, carbon dioxide, inert gases, and mixtures thereof.
4. The process of claim 1 wherein the oxidant comprises an oxidizer and wherein a molar ratio of alkane to oxidizer fed to the process is greater than 1.9:1 and less than 5:1.
5. The process of claim 1 wherein the porous medium comprises a porous ceramic material selected from the group consisting of porous aluminas, silicas, aluminosilicates, titanium oxides, zirconium oxides, mullite, hexaluminates, and spinels, anyone of which is optionally modified with one or more alkali or alkaline elements; or wherein the porous medium comprises a porous metallic material selected from stainless steel, iron-chromium-aluminum alloys and nickel-chromium-aluminum alloys.
6. The process of claim 1 wherein the porous medium comprises a plurality of pores, channels, cavities, or void spaces, which have an average diameter or critical cross-sectional dimension ranging from 2 microns (2 μm) to 100 μm.
7. The process of claim 1 wherein the porous medium has a porosity of at least about 30 percent and less than about 90 percent.
8. The process of claim 1 wherein the oxidative coupling catalyst is selected from the group consisting of compounds of magnesium, manganese, sodium tungstate, and silica (Na/Mg/W/Mn/SiO 2 ), lithium magnesium oxide (Li/MgO), lanthanum oxide (La 2 O 3 ), strontium-magnesium-calcium-doped lanthanum oxide (Sr,Mg,Ca/La 2 O 3 ), acid or sodium modified ZSM-5 (H/Na-ZSM-5), and rhenium, gallium, tungsten, and/or molybdenum modified ZSM-5 and MCM-22.
9. The process of claim 1 wherein the ratio of diffusive flux of the alkane into the porous medium to the bulk flux of the oxidant through the porous medium (DF ALK :BF OX ) is greater 10:1 and less than 100:1.
10. The process of claim 1 wherein the process is conducted at a temperature greater than 500° C. and less than 900° C.
11. The process of claim 1 wherein the process is conducted at an overall process pressure of greater than 0.69 kPa and less than 3,447 kPa as measured at the outlet of the second chamber; while a pressure at the inlet to the first chamber is at least +13.8 kPa to +35 kPa greater than the overall process pressure.
12. The process of claim 1 wherein the process is conducted at an alkane flow rate greater than about 0.05 g/g-cat/hr and less than about 240 g/g-cat/hr, and at an oxidant flow rate greater than 0.2 g/g-cat/hr and less than about 40 g/g-cat/hr, wherein the oxidant flow rate is the total flow rate of an oxidizer and a diluent.
13. The process of claim 1 wherein a flow of air is fed through the first inlet into the first chamber; and wherein a flow of methane is fed into the second inlet into the second chamber; and wherein the oxidative coupling product produced is ethylene.