METHODS OF PRODUCING ETHYLENE AND SYNTHESIS GAS BY COMBINING THE OXIDATIVE COUPLING OF METHANE AND DRY REFORMING OF METHANE REACTIONS

Abstract

Disclosed is a method for production of synthesis gas and ethylene by a combined oxidative coupling and dry reforming of methane process. Heat generated from the oxidative coupling of methane can be used to drive the endothermic dry reforming of methane reaction.

Claims

1. A method of producing ethylene and synthesis gas from a reactant mixture comprising methane (CH.sub.4), oxygen (O.sub.2) and carbon dioxide (CO.sub.2), the method comprising: contacting the reactant mixture with a catalytic material to produce a product stream comprising ethylene and synthesis gas, wherein the ethylene is obtained from oxidative coupling of CH.sub.4 and the synthesis gas is obtained from CO.sub.2 reforming of CH.sub.4, wherein heat produced by the oxidative coupling of CH.sub.4 is used in the CO.sub.2 reforming of CH.sub.4.

2. The method of claim 1, wherein the catalytic material comprises a catalyst, or a mixture of catalysts, that catalyze the oxidative coupling of CH.sub.4 and the CO.sub.2 reforming of CH.sub.4.

3. The method of claim 2, wherein the mixture of catalysts includes a first catalyst that catalyzes the oxidative coupling of CH.sub.4 and a second catalyst that catalyzes the CO.sub.2 reforming of CH.sub.4.

4. The method of claim 3, wherein the mixture of catalysts includes Na.sub.2O, Mn.sub.2O.sub.3, WO.sub.3, and La.sub.2O.sub.3, Na.sub.2O, Mn.sub.2O.sub.3, WO.sub.3 and SiO.sub.2 or both.

5. The method of claim 1, wherein the ratio of CH.sub.4:O.sub.2:CO.sub.2 in the reactant mixture is 1:0.5:1.

6. The method of claim 5, wherein the reaction temperature is 750° C. to 900° C.

7. The method of claim 4, wherein 20% to 60% methane was converted, and the selectivity to ethylene is 30% to 35% and the selectivity to carbon monoxide is 15% to 70%, or 65% to 70%.

8. The method of claim 1, wherein the method occurs in a continuous flow reactor.

9. The method of claim 8, wherein the continuous flow reactor is a fixed-bed reactor or a fluidized reactor.

10. The method of claim 1, wherein heat produced by the oxidative coupling of CH.sub.4 is (1) used in the CO.sub.2 reforming of CH.sub.4 and (2) transferred to an inert material in an amount sufficient to reduce thermal deactivation of the catalytic material.

11. The method of claim 1, wherein the catalytic material and the inert material are configured in multiple alternating layers, and wherein the total number of layers of the catalytic material is equal to x, and the total number of layers of the inert material is equal to x−1, x+1, or x.

12. The method of claim 11, wherein the total number of layers of the catalytic material ranges from 3 to 50, 3 to 25, or 3 to 5.

13. The method of claim 12, wherein the inert layer has a thickness that is greater than the thickness of the catalytic material layer.

14. The method of claim 10, wherein the catalytic material is dispersed in the inert material.

15. The method of claim 15, wherein the ratio, by wt. %, of the catalytic material to the inert material is 5 to 30, 5 to 20, or 7 to 15.

16. The method of claim 10, wherein the inert material is magnesium oxide, silicon dioxide, quartz, or any combination thereof.

17. The method of claim 10, wherein the temperature of the catalytic material does not exceed its deactivation temperature of 800° C. to 900to ° C.

18. The method of claim 1, wherein the catalytic material comprises a catalyst, or a mixture of catalysts, that catalyzes the oxidative coupling of CH.sub.4 and the CO.sub.2 reforming of CH.sub.4.

19. The method of claim 1, wherein the catalyst comprises manganese or a compound thereof, lanthanum or a compound thereof, sodium or a compound thereof, cesium or a compound thereof, calcium or a compound thereof, and any combination thereof.

20. The method of claim 19, wherein the catalyst comprises La/MgO, Na—Mn—La.sub.2O.sub.3/Al.sub.2O.sub.3, Na—Mn—O/SiO.sub.2, Na.sub.2WO.sub.4—Mn/SiO.sub.2, or any combination thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 depicts a schematic of a system of the present invention for the production of ethylene and synthesis gas.

[0023] FIG. 2 depicts a schematic of a second system of the present invention for the production of ethylene and synthesis gas.

[0024] FIG. 3 is a graphical depiction of temperature versus length of reactor for the system depicted in FIG. 2.

[0025] FIG. 4 depicts a schematic of a third system of the present invention for the production of ethylene and synthesis gas.

[0026] FIG. 5 is a graphical depiction of temperature versus length of reactor for the system depicted in FIG. 4.

[0027] FIG. 6 depicts a schematic of a fourth system of the present invention for the production of ethylene and synthesis gas.

DETAILED DESCRIPTION OF THE INVENTION

[0028] The currently available processes to produce ethylene often result in catalyst deactivation through agglomeration of material on the catalyst surface (coking) and runaway heat due to the heat generated from the highly exothermic reaction between oxygen and methane. This can lead to inefficient ethylene production as well as increased costs associated with its production.

[0029] A discovery has been made that controls the generated heat and avoids the catalyst deactivation described above. The discovery is based on a method to produce ethylene and synthesis gas from a reactant mixture containing methane, oxygen and carbon dioxide. The method includes contacting the reactant mixture with a catalytic material to produce a product stream containing ethylene and synthesis gas, where the ethylene is obtained from oxidative coupling of CH.sub.4 and the synthesis gas is obtained from CO.sub.2 reforming of CH.sub.4. The heat produced by the oxidative coupling of CH.sub.4 is (1) transferred to an inert material in an amount sufficient to reduce thermal deactivation of the catalytic material and (2) used in the CO.sub.2 reforming of CH.sub.4.

[0030] These and other non-limiting aspects of the present invention are discussed in further detail in the following sections.

A. Reactants

[0031] The reactant mixture in the context of the present invention is a gaseous mixture that includes, but is not limited to, a hydrocarbon or mixtures of hydrocarbons, carbon dioxide and oxygen. The hydrocarbon or mixtures of hydrocarbons can include natural gas, liquefied petroleum gas containing of C.sub.2-C.sub.5 hydrocarbons, C.sub.6+heavy hydrocarbons (e.g., C.sub.6 to C.sub.24 hydrocarbons such as diesel fuel, jet fuel, gasoline, tars, kerosene, etc.), oxygenated hydrocarbons, and/or biodiesel, alcohols, or dimethyl ether. In a preferred aspect, the hydrocarbon is methane. Oxygen used in the present invention can be air, oxygen enriched air, oxygen gas, and can be obtained from various sources. Carbon dioxide used in the present invention can be obtained from various sources. In one non-limiting instance, the carbon dioxide can be obtained from a waste or recycle gas stream (e.g. from a plant on the same site, like for example from ammonia synthesis) or after recovering the carbon dioxide from a gas stream. A benefit of recycling such carbon dioxide as a starting material in the process of the invention is that it can reduce the amount of carbon dioxide emitted to the atmosphere (e.g., from a chemical production site). The reactant mixture may further contain other gases, provided that these do not negatively affect the reaction. Examples of such other gases include nitrogen and hydrogen. The hydrogen may be from various sources, including streams coming from other chemical processes, like ethane cracking, methanol synthesis, or conversion of methane to aromatics. The reactant mixture is substantially devoid of water or steam. In a particular aspect of the invention the gaseous feed contains 0.1 wt. % or less of water, or 0.0001 wt. % to 0.1 wt. % water. In the reactant mixture a molecular ratio of CH.sub.4 to O.sub.2 ranges from 0.3 to 1, 0.5 to 0.8, or 0.6 to 0.7, a molecular ratio of CH.sub.4 to CO.sub.2 from 1 to 2, and/or a molecular ratio of O.sub.2 to CO.sub.2 ranges from 0.5 to 2, 0.75 to 1.5, or 1 to 1.25.

B. Catalytic Material and Inert Material

[0032] Catalytic material used in the context of this invention may be the same catalysts, different catalysts, or a mixture of catalysts. The catalysts may be supported or unsupported catalysts. The support may be active or inactive. The catalyst support may include MgO, Al.sub.2O.sub.3, SiO.sub.2, or the like. All of the support materials can be purchased or be made by processes known to those of ordinary skill in the art (e.g., precipitation/co-precipitation, sol-gel, templates/surface derivatized metal oxides synthesis, solid-state synthesis, of mixed metal oxides, microemulsion technique, solvothermal, sonochemical, combustion synthesis, etc.). One or more of the catalysts can include one or more metals or metal compounds thereof. Catalytic metals include Li, Na, Ca, Cs, Mg, La, Ce, W, Mn, Ru, Rh, Ni, Pt. Non-limiting examples of catalysts of the invention include La on a MgO support, Na, Mn, and La.sub.2O.sub.3 on an aluminum support, Na and Mn oxides on a silicon dioxide support, Na.sub.2WO.sub.4 and Mn on a silicon dioxide support, or any combination thereof. Non-limiting examples of catalysts that promote oxidative coupling of methane to produce ethylene are Li.sub.2O, Na.sub.2O, Cs.sub.2O, MgO, WO.sub.3, Mn.sub.3O.sub.4, or any combination thereof. Non-limiting examples of catalysts that promote dry reforming of methane to produce synthesis gas include Ni on a support, Ni in combination with noble metals (for example, Ru, Rh, Pt, or any combination thereof) on a support, Ni and Ce on a support, or any combination thereof. A non-limiting example of a catalyst that promotes oxidative coupling of methane and CO.sub.2 reforming of methane is a catalyst that includes metals of Ni, Ce, La, Mn, W, Na, or any combination thereof. A non-limiting example of a mixture of catalysts is a catalyst mixture that include a supported catalyst containing Ni, Ce and La, and another supported catalyst containing Mn, W, and Na. The catalysts of the present invention may be layered to promote oxidative coupling in one portion of a reactor system and dry reforming of methane in another portion of the reactor. In some instances, the catalysts that promote oxidative coupling and dry reforming of methane are mixed in a desired ratio to obtain a selected amount of heat for the endothermic dry reforming reaction.

[0033] The inert material may be one or more chemically inert compounds and/or non-catalytic compounds. Non-limiting examples, of the inert material include, for example, MgO, SiO.sub.2, quartz, graphite, or any combination thereof. The inert material can have any size or shape (for example, spheres, tubes, conical, planar, and the like) that is suitable for layering between the catalytic material. The inert material can have the same or different particle size and/or surface area as the catalytic material. The inert material does not include inert gases (for example, argon, nitrogen or both) used as in the process. In one aspect, the inert material has substantially little to no catalytic activity for oxidative coupling of methane and/or the oxidative reforming of methane. Heat generated from the oxidative coupling of methane transferred away from the catalytic material by the inert material. The heat may be removed through heat transfer from the inert material to the walls of a vessel. The inert material can be layered between catalytic material layers, mixed with the catalytic material and/or dispersed in the catalytic material. A portion of the heat generated from the oxidative coupling reaction can be removed by the inert material in amount to reduce thermal deactivation of the catalytic material.

C. Process

[0034] Continuous flow reactors can be used in the context of the present invention to treat methane with carbon dioxide and oxygen to produce ethylene and synthesis gas. Generally, the ethylene is obtained for oxidative coupling of methane and the synthesis gas is obtained from reforming of methane. Sufficient heat is generated to drive the endothermic dry reforming methane reaction. Non-limiting examples of the configuration of the catalytic material and the inert material in a continuous flow reactor are provided below and throughout this specification. The continuous flow reactor can be a fixed bed reactor, a stacked bed reactor, a fluidized bed reactor, or an ebullating bed reactor. In a preferred aspect of the invention, the reactor is a fixed bed reactor. The catalytic material and the inert material can be arranged in the continuous flow reactor either as separate layers in the reactor or mixed together (i.e., the catalytic material is dispersed in the inert material). Non-limiting examples of the configuration of the layers in the continuous reactor (FIGS. 1, 2 and 4) are provided below. A non-limiting example of the catalytic material dispersed in the inert material (FIG. 6) is also provided. Non-limiting examples of catalytic material and inert material that can be used in the context of the present invention are provided throughout this specification.

[0035] FIG. 1 is a schematic of system 100 for the production of ethylene and synthesis gas. System 100 may include a continuous flow reactor 102, a catalytic material 104, and an inert material 106. A reactant stream comprising methane enters the continuous flow reactor 102 via the feed inlet 108. An oxygen source and carbon dioxide are provided in via oxidant source inlet 110. In some aspects of the invention, the three reactants are fed to the reactor via separate inlets. Methane, carbon dioxide and oxygen can be provided to the continuous flow reactor 102 such that the reactants mix in the reactor to form a reactant mixture prior to contacting the first catalytic layer. The catalytic material 104 and the inert material 106 may be layered in the continuous flow reactor 102. As shown in FIG. 1, a first layer 112 of the catalytic material 104 is thin, for example, about 2-5 catalyst pellets in thickness. A first layer 114 of the inert material 106 that is thicker than the first catalytic material layer 112, for example, about 5 times thicker is positioned downstream of the catalytic material layer. A second catalytic material layer 116 is positioned downstream of the first inert material layer 114. The second inert material layer 114 is about twice the thickness of the first catalytic material layer 112, for example, 6, 7, 8 or 10 catalyst pellets in thickness. A second inert material layer 118 is about 2 times thicker than the second catalytic material layer 116, for example about 30, 40, or 50 pellets thick, and is placed downstream of the second catalytic material layer 116. A third catalytic material layer 120 fills the remainder of the continuous flow reactor 102. Contact of the reactant mixture with the first layer catalytic material 112 produces a product stream (for example, ethylene and synthesis gas (carbon monoxide and hydrogen) and generates heat (i.e., an exotherm or rise in temperature is observed). Wishing not to be bound by theory, it is believed that the product stream from contact of the feed stream with the catalytic material in the presence of oxygen generates only a small amount of carbon dioxide, due to the presence of excess carbon dioxide in the reactor. The generation of heat after contact with the catalytic layers drives the carbon dioxide reforming of methane to synthesis gas as the feed stream flows through the continuous flow reactor. A portion of the generated heat after contact with the catalytic layers is transferred to the inert layer 114, which can then transfer the heat to the walls of the reactor and/or to cooling jacket 122. The cooling jacket 122 can include one or more heat transfer fluids (for example, water, air, hydrocarbons or synthetic fluid) that can facilitate removal of heat in a controlled manner. In some instances of the invention, the continuous flow reactor 102 can include internal cooling coils, a heat exchange system or other types of heat removal components. The product stream containing ethylene and synthesis gas can exit continuous flow reactor 102 via product outlet 124.

[0036] Referring to FIG. 2, FIG. 2 is a schematic of system 200 for the production of ethylene and synthesis gas that can include the continuous flow reaction 102, the catalytic material 104, the inert material 106, and the cooling jacket 122 (such as those used in system 100 for the production of ethylene and synthesis gas). Similar to system 100, the catalytic material 104 and the inert material 106 of system 200 are layered, however, the thickness of the layers are different than those shown for system 100. As shown in system 200, a first catalytic material layer 202 and a second catalytic material layer 204 are about the same thickness (for example, about two catalyst pellets thickness) and a third catalytic material layer 206 fills the remainder of continuous flow reactor 102. The catalytic layers 202, 204 and 206 are separated by inert layers 208 and 210 that are thicker than the first catalytic material layer 202 and the second catalytic material layer 204, but thinner than the third catalytic material layer 206. As shown in FIG. 2, P is less than 0.1 (P<0.1) in the inert layers 208 and 210, and P is greater than 0.1 (P>0.1) in the catalytic material layers 202 and 204. P is much less than 0.1 (P<<0.1) in catalytic layer 206. Catalytic layer 206 is used to convert the last small increment of reactants. When P is greater than 0.1 (P>0.1), the transport rate between the fluid and the catalyst limits the temperature rise in the catalyst phase, which decreases coking (or other deactivation) of the catalyst and produces more ethylene and synthesis gas instead of carbon dioxide. FIG. 3 is a graphical depiction of reaction temperature versus length of the continuous flow reactor for contact of the reactant mixture having the configuration of catalytic material layers and inert material layers described for system 200. As shown in FIG. 3, the temperature profile increases rapidly (data 302) when the feed contacts the catalytic material (P>0.1), and the temperature decreases rapidly (data 304) when the mixture of reactant mixture and product stream contact the inert material 106 (P<0.1) and heat is removed from the system. As the mixture of feed stream and product stream flow through the catalytic material layers 202, 204 and 206 along the length of the continuous flow reactor 102, the temperature profile becomes more constant as the mixture of product stream and feed stream becomes enriched in product (e.g., enriched in ethylene, carbon monoxide and hydrogen). The product stream composed of ethylene and synthesis gas can exit continuous flow reactor 102 via product outlet 124.

[0037] Referring to FIG. 4, a schematic of system 400 for the production of ethylene and synthesis gas that can include the continuous flow reaction 102, the catalytic material 104, and the inert material 106 (such as those used in systems 100 and 200 for the production of ethylene and synthesis gas) is described. Similar to systems 100 and 200, the catalytic material 104 and the inert material 106 of system 400 are layered, however, the thickness of the layers are different than those shown for systems 100 and 200. As shown in system 400, the first catalytic material layer 402, the second catalytic material layer 404, and the third catalytic layer 406 are about the same thickness (for example, about two catalyst pellets thickness). The catalytic material layers 402, 404, and 406 are separated by the inert material layers 408 and 410 that are substantially thicker than the catalytic material layers, for example about 10 times as thick. FIG. 5 is a graphical depiction of reaction temperature versus length of the continuous flow reactor for system 400. As shown in FIG. 5, the temperature profile small increases in temperature (data 502) occurs when the feed contacts the catalytic material (P>0.1), and a less rapid decrease in temperature is observed (data 504) as the inert material removes heat (P<0.1) from the system in a controlled manner as the feed stream and product stream flow through continuous flow reactor 102. The product stream composed of ethylene and synthesis gas can exit continuous flow reactor 102 via outlet 124.

[0038] In some aspects of the present invention, the catalytic material is dispersed in or mixed with the inert material. FIG. 6 depicts system 600 for the production of ethylene and synthesis gas that has the catalytic material 104 mixed with the inert material 106.

[0039] The resulting syngas, water, and ethylene produced from the systems of the invention (for example, systems 100, 200, 300 and 400) are separated using gas/liquid separation techniques, for example, distillation, absorption, membrane technology to produce a gaseous stream that includes carbon monoxide, hydrogen, ethylene product, and a water stream. The ethylene is separated from the hydrogen and carbon monoxide using gas/gas separation techniques, for example, a hydrogen selective membrane, a carbon monoxide selective membrane, or cryogenic distillation to produce, ethylene, carbon monoxide, hydrogen or mixtures thereof. The separated or mixture of products can be used in additional downstream reaction schemes to create additional products or for energy production. Examples of other products include chemical products such as methanol production, olefin synthesis (e.g., via Fischer-Tropsch reaction), aromatics production, carbonylation of methanol, carbonylation of olefins, the reduction of iron oxide in steel production, etc. The method can further include isolating and/or storing the produced gaseous mixture or the separated products.

D. Conditions

[0040] The reaction processing conditions in the continuous flow reactor 102 can be varied to achieve a desired result (e.g., ethylene product and/or synthesis gas production). The method includes contacting a feed stream of hydrocarbon and oxidant (oxygen and carbon dioxide) with any of the catalysts described throughout the specification under sufficient conditions to produce hydrogen and carbon monoxide at a ratio of 0.35 or greater, from 0.35 to 0.95, or from 0.6 to 0.9 and ethylene. Such conditions can include a temperature range of 700 to 900° C. or a range from 725, 750, 775, 800, to 900° C., or from 700 to 900° C. or from 750 to 850° C., a pressure of about 1 bara, and/or a gas hourly space velocity (GHSV) from 1800 to 80,000 h.sup.−1, preferably from 1800 to 50,000 h.sup.−1, or more preferably from 1,800 to 20,000 h.sup.−1. Severity of the process conditions may be manipulated by changing, the hydrocarbon source, oxygen source, carbon dioxide source, pressure, flow rates, the temperature of the process, the catalyst type, and/or catalyst to feed ratio. A process in accordance with the present invention is carried out at atmospheric pressure but using pressures more than atmospheric should not have negative effect to the conversion of methane because the reaction at the above mentioned conditions is not regulated by thermodynamic equilibrium where pressure may have significant effect.

EXAMPLES

[0041] The present invention will be described in greater detail by way of specific examples. The following examples are offered for illustrative purposes only, and are not intended to limit the invention in any manner. Those of skill in the art will readily recognize a variety of noncritical parameters which can be changed or modified to yield essentially the same results.

Example 1

Production of Ethylene and Synthesis Gas from Methane, Oxygen and Carbon Dioxide Using Random Dilution

[0042] A fixed bed catalyst reactor was filled with a catalyst that was a mixture of Na.sub.2O, Mn.sub.2O.sub.3, WO.sub.3, and La.sub.2O.sub.3. The catalyst bed was diluted with inert quartz particles having the same particles size of the catalyst (about 20-50 mesh) at an inert material to catalyst ratio of 4. The reactor was heated to about 870° C. and a mixture of methane (CH.sub.4), oxygen (O.sub.2) and carbon dioxide (CO.sub.2) in a CH.sub.4:O.sub.2:CO.sub.2 ratio of 1:0.5:1 was fed to the reactor at a gas hourly space velocity of 3600 h.sup.−1. The methane conversion was 50% with selectivity to ethylene at 33% and selectivity to carbon monoxide at 67%. Methane conversion was calculated using internal standard (argon) on the basis of difference of inlet and outlet concentrations of methane. Selectivity was calculated also using internal standard on the basis of concentrations of C2 products in comparison all the converted amount of methane.

Example 2

Production of Ethylene from Methane and Oxygen

[0043] The experiments in this example were carried out at conditions of Example 1, except that the feed was a mixture of CH.sub.4:O.sub.2 in a 4:1 ratio. Conversion of methane was 35% with the selectivity to ethylene at 65%, the selectivity to CO at 5%, and the selectivity to CO.sub.2 at 30%.

[0044] When comparing Examples 1 and 3, the selectivity of ethylene was higher in Example 6 while and the selectivity to CO was higher in Example 1. It is believed that the excess CO.sub.2 used in Example 1 reacted with methane to produce the reformation product of CO.

Example 3

Production of Ethylene and Synthesis Gas from Methane, Oxygen and Carbon Dioxide Using Random Dilution

[0045] A fixed bed catalyst reactor was filled with a catalyst that was a mixture of Na.sub.2O, Mn.sub.2O.sub.3, WO.sub.3, and SiO.sub.2. The catalyst bed was diluted with inert quartz particles having the same particles size of the catalyst (about 20-50 mesh) at an inert material to catalyst ratio of 4. The reactor was heated to about 775° C. and a mixture of methane (CH.sub.4), oxygen (O.sub.2) and carbon dioxide (CO.sub.2) in a CH.sub.4:O.sub.2:CO.sub.2 ratio of 1:0.5:1 was fed to the reactor at a gas hourly space velocity of 2168 h.sup.−1. The methane conversion was 30.0% with selectivity to C.sub.2+at 80.3% and selectivity to carbon monoxide at 15.2% and selectivity to carbon dioxide at 4.5%. Methane conversion was calculated using internal standard (neon) on the basis of difference of inlet and outlet concentrations of methane. Selectivity was calculated also using an internal standard on the basis of concentrations of C.sub.2+ products in comparison all the converted amount of methane.

Example 4

Production of Ethylene from Methane and Oxygen

[0046] The experiments in this example were carried out at conditions of Example 3, except that the feed was a mixture of CH.sub.4:O.sub.2 in a 4:1 ratio. Conversion of methane was 32.2% with the selectivity to C.sub.2+ at 76.2%, the selectivity to CO at 10.9%, and the selectivity to CO.sub.2 at 12.9%.

[0047] When comparing Examples 3 and 4, the selectivity of C.sub.2+ was higher in Example 3 and the selectivity to CO was higher in Example 3 as well and the selectivity to CO.sub.2 is lower in Example 3. It is believed that the excess CO.sub.2 used in Example 3 reacted with methane to produce the reformation product of CO and the coupling of endothermic reaction and exothermic reaction reduces the hot spot temperature in the catalyst bed and lowers the CO.sub.2 production.