WO2017093859A1

WO2017093859A1 - A two-step process of co2 assisted oxidative conversion of methane to syngas and methane assisted conversion of syngas to hydrocarbons

Info

Publication number: WO2017093859A1
Application number: PCT/IB2016/057075
Authority: WO
Inventors: Aghaddin Mamedov
Original assignee: Sabic Global Technologies B.V.
Priority date: 2015-12-02
Filing date: 2016-11-23
Publication date: 2017-06-08

Abstract

Processes for converting methane into olefins are presented. Certain exemplary processes can involve use of both oxidative dry reforming of methane to syngas and conversion of the syngas to olefins via Fischer-Tropsch synthesis.

Description

A TWO-STEP PROCESS OF C0₂ ASSISTED OXIDATIVE CONVERSION OF METHANE TO SYNGAS AND METHANE ASSISTED CONVERSION OF SYNGAS

TO HYDROCARBONS CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to and the benefit of U.S. Provisional Application No. 62/262, 116, filed December 2, 2015. The contents of the referenced application are incorporated into the present application by reference.

FIELD

[0002] The presently disclosed subject matter relates to processes and systems for converting methane to olefins.

BACKGROUND

[0003] Synthesis gas, also known as syngas, is a gas mixture containing hydrogen (H₂) and carbon monoxide (CO). Syngas can also include carbon dioxide (C0₂). Syngas is a chemical feedstock that can be used in numerous applications. For example, syngas can be used to prepare liquid hydrocarbons, including olefins (e.g., ethylene (C₂H₄)), via Fischer-Tropsch synthesis.

[0004] Conversion of syngas to olefins, for example ethylene, under Fischer-Tropsch conditions can proceed according to the following chemical equation:

2CO + 4H₂→ C₂H₄ + 2H₂0

[0005] Syngas is commonly generated on a large scale from methane (CH ), e.g., through steam reforming processes or through oxidative reforming with oxygen (in the absence of carbon dioxide). Existing processes suffer from drawbacks. For example, steam reforming processes can be affected by harmful coke formation. Steam reforming processes can also be highly endothermic and energy intensive. Oxidative reforming with oxygen can be highly exothermic. [0006] An additional drawback with preparation of syngas via steam reforming and oxidative reforming with oxygen can be that certain reactions provide syngas with a molar ratio of hydrogen to carbon monoxide of approximately 3 : 1 or higher, greater than the 2: 1 ratio ideal for formation of ethylene. Thus, there remains a need for improved processes for preparation of syngas from methane and improved processes for preparation of olefins (e.g., ethylene) from syngas.

SUMMARY OF THE DISCLOSED SUBJECT MATTER

[0007] The presently disclosed subject matter provides processes for preparing olefins from methane. The processes can include contacting methane, oxygen gas, and carbon dioxide with a first catalyst in a first reactor zone to form a mixture of syngas, water, and unreacted methane at a temperature of from about 650 to about 710°C. The process can further include separating the water produced to produce a stream comprising unreacted methane and syngas. The process can also include contacting the unreacted methane and syngas stream with a second catalyst in a second reactor zone to undergo Fischer- Tropsch synthesis at a temperature of from about 420 to about 460°C to form a second stream comprising olefins, carbon dioxide, and methane. The process can also include separating the olefins from the second stream to form a third stream comprising carbon dioxide and methane and recycling the third stream by feeding the third stream into the first reactor zone.

[0008] In certain embodiments, the first catalyst is 3% Ni/La₂0₃.

[0009] In certain embodiments, the temperature of a first reactor zone is about 710°C.

[0010] In certain embodiments, the temperature of the Fischer-Tropsch synthesis is about 460°C.

[0011] In certain embodiments, methane and carbon dioxide are present in a ratio of about 1.7.

[0012] In certain embodiments, the olefins can include C₂-C₃ and/or C₂-C₄ olefins. [0013] In certain embodiments, carbon monoxide and hydrogen are present in a ratio of from about 1 :2 to about 2: 1.

[0014] In certain embodiments, the unreacted methanol and syngas are present in amounts of 20% CH₄, 40% H₂, and 40% CO.

[0015] In certain embodiments, the Fischer-Tropsch synthesis reaction is performed from about 20%) to about 90% conversion of CO. In other embodiments, the Fischer-Tropsch synthesis reaction is performed to about 40% conversion of CO.

[0016] In certain embodiments, the Fischer-Tropsch synthesis reaction is performed from about 5% to about 50% conversion of methane. In other embodiments, the Fischer-Tropsch synthesis reaction is performed to about 15% conversion of methane.

[0017] In certain embodiments, the process can be performed from about 65% to about 90% conversion of C0₂. In certain embodiments, the process can be performed from about 50%) to about 90%) conversion of methane.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 is a schematic diagram showing an exemplary system that can be used in conjunction with processes for converting methane into olefins in accordance with the presently disclosed subject matter.

[0019] FIG. 2 is a schematic diagram depicting an exemplary method in accordance with one non-limiting embodiment of the disclosed subject matter.

DETAILED DESCRIPTION

[0020] The presently disclosed subject matter provides processes for converting methane into olefins (e.g., ethylene and propylene). The presently disclosed processes can involve use of both oxidative dry reforming of methane to syngas and conversion of the syngas to olefins via Fischer-Tropsch synthesis. The presently disclosed processes have advantages over existing processes, as described below, including improved efficiency, reduced energy consumption, and reduced cost.

[0021] As used herein, the term "about" or "approximately" means within an acceptable error range for the particular value as determined by one of ordinary skill in the art, which will depend in part on how the value is measured or determined, i.e., the limitations of the measurement system. For example, "about" can mean a range of up to 20%, up to 10%, up to 5%), and or up to 1% of a given value.

Reactors and Reaction Chambers

[0022] The methods of the present disclosure can involve fixed bed isothermal or adiabatic reactors suitable for reactions of gaseous reactants and reagents catalyzed by solid catalysts. The reactors can be constructed of any suitable materials capable of holding temperatures, for example from about 200°C to about 900°C. Non-limiting examples of such materials can include metals, alloys (including steel), glass, ceramics or glass lined metals, and coated metals. The reactor can also include a reaction vessel enclosing a reaction chamber. In certain embodiments, the reactor is capable of holding temperatures of from about 600 to about 800°C, e.g., the temperature required for conversion of methane. In certain embodiments, the reactor is capable of holding temperatures of from about 400 to about 500°C, e.g., the temperature required for olefin synthesis.

[0023] The dimensions of the reaction vessel and reaction chamber are variable and can depend on the production capacity, feed volume, and catalyst. The geometries of the reactor can be adjustable in various ways known to one of ordinary skill in the art.

[0024] The pressure within the reaction chamber can be varied, as is known in the art. In certain embodiments, the pressure within the reaction chamber can be atmospheric pressure, i.e., about 1 bar. Catalysts

[0025] Catalysts suitable for use in conjunction with the presently disclosed subject matter can be catalysts capable of catalyzing conversion of methane and/or olefin synthesis. The catalyst can be a metal oxide or mixed metal oxide. In certain embodiments, the catalyst can be located in a fixed packed bed, i.e., a catalyst fixed bed. In certain embodiments, the catalyst can include solid pellets, granules, plates, tablets, or rings.

[0026] In certain embodiments, the catalyst can include one or more transition metals. The catalyst can include nickel (Ni).

[0027] In certain embodiments, the catalyst can include a solid support. That is, the catalyst can be solid-supported. In certain embodiments, the solid support can include various metal salts, metalloid oxides, and/or metal oxides, e.g., titania (titanium oxide), zirconia (zirconium oxide), silica (silicon oxide), alumina (aluminum oxide), magnesia (magnesium oxide), and magnesium chloride. In certain embodiments, the solid support can include alumina (A1₂0₃), lanthanum (La₂0₃), silica (Si0₂), magnesia (MgO), titania (Ti0₂), zirconia (Zr0₂), cerium(IV) oxide (Ce0₂), or a combination thereof. The amount of the solid support present in the catalyst can be between about 40% and about 95%, by weight, relative to the total weight of the catalyst. By way of non-limiting example, the solid support can constitute about 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95% of the total weight of the catalyst.

[0028] In certain embodiments, the catalyst can include one or more additional metals in addition to Ni. In certain embodiments, the additional metal(s) can be present in an amount between about 1% and 25%, relative to the total weight of the catalyst. For example, the catalyst can include about 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 18%, 20%, 22%, or 25% of the additional metal(s), by weight. [0029] The catalysts of the presently disclosed subject matter can be prepared according to various techniques known in the art. For example, metal oxide catalysts suitable for use in reactions can be prepared from various metal nitrates, metal halides, metal salts of organic acids, metal hydroxides, metal carbonates, metal oxyhalides, metal sulfates, and the like. In certain embodiments, a transition metal oxide (e.g., a Ni oxide) can be precipitated along with a solid support (e.g., La₂0₃). In certain embodiments, catalysts can be prepared by precipitation of metal nitrates by ammonium hydroxide (NH₄OH) at pH equal to 8.5. The suspension can be washed from H₄NO3 and dried at 120°C for 8 hours. The product can then be calcined at 400°C for 12 hours. The product catalyst can be size 20-50 mesh.

[0030] In certain non-limiting embodiments, the catalysts used for Fischer-Tropsch synthesis can be catalysts with a composition known in art. In certain non-limiting embodiments, the catalyst is a mixture of Fe and Mn oxides. A catalyst comprising a mixture of Fe and Mn oxides can lead to the formation of C2-C3 olefins and methane. Alkali metals, e.g., potassium (K), can be incorporated into the mixture of Fe and Mn oxides to increase selectivity for C₂+ hydrocarbons. In certain non-limiting embodiments, the catalysts used for Fischer-Tropsch synthesis can be a mixture of Fe, Mn, and K oxides. In certain embodiments, the catalyst composition is 55%Fe₂O₃-40%MnO-5%K₂O.

Methods of Preparing Syngas and Olefins

[0031] Oxidative dry reforming of methane is a process in which methane is reacted with carbon dioxide and oxygen to provide carbon monoxide, hydrogen, and water. Oxidative dry reforming can be summarized by the following chemical equation:

2CH₄ + C0₂ + 0₂→ 3 CO + 3H₂ + H₂0

Oxidative dry reforming can accordingly generate syngas with a hydrogenxarbon monoxide ratio of approximately 1 : 1.

[0032] Processes for converting methane into olefins of the presently disclosed subject matter can generally include contacting methane, carbon dioxide, and oxygen with an oxidative dry reforming catalyst to provide an oxidative dry reforming product mixture that includes carbon monoxide, hydrogen, and water. The processes can additionally include contacting the oxidative dry reforming product mixture with an olefin preparation catalyst to provide an olefin product mixture that includes an olefin and carbon monoxide.

[0033] For the purpose of illustration and not limitation, FIG. 2 is a schematic representation of methods according to non-limiting embodiments of the disclosed subject matter. In one embodiment, an exemplary method 200 can include providing a system 100 for converting mixtures of methane, H₂, and C0₂ into syngas and olefins.

[0034] The method 200 can include contacting methane, oxygen gas, and carbon dioxide with a first catalyst in a first reactor zone to form a mixture of syngas, water, and unreacted methane at a temperature of from about 650 to about 710°C 201. The method can further comprise separating the water produced to produce a stream comprising unreacted methane and syngas 202. The method can further include contacting the unreacted methane and syngas stream with a second catalyst in a second reactor zone to undergo Fischer- Tropsch synthesis at a temperature of from about 420 to about 460°C to form a second stream comprising olefins, carbon dioxide, and methane 203. The method can include separating the olefins from the second stream to form a third stream comprising carbon dioxide and methane 204. The method can further include recycling the third stream by feeding the third stream into the first reactor zone 205.

[0035] Methane, carbon dioxide, and oxygen can be fed into a reaction chamber at various flow rates. The flow rate and gas hourly space velocity (GHSV) can be varied, as is known in the art. In certain non-limiting embodiments, GHSV can be from about 500 to about 3600h-l . In certain embodiments, the flow rate can be from about 5 to about 50 cc/min. In certain embodiments, the flow rate can be from about 10 to about 30 cc/min. In certain embodiments, the flow rate can be about 15 cc/min. In certain embodiments, the flow rate can be about 30 cc/min. In certain non-limiting embodiments, catalyst loading can be from about 0.5 to about 3ml and have contact time from about 1.5 to about 2 seconds.

[0036] The reaction temperature can be understood to be the temperature within a reaction chamber. The reaction temperature can influence the oxidative dry reforming reaction, including conversion of C0₂ and H₂, the ratio of H₂:CO in the product mixture, and the overall yield. In certain embodiments, the reaction temperature for an oxidative dry reforming reaction can be from about 550 °C to about 950 °C. In certain embodiments, the reaction temperature can be from about 650 °C to about 720 °C. In certain embodiments, the reaction temperature can be about 710 °C.

[0037] In certain embodiments, the reaction temperature for an olefin synthesis reaction can be from about 400 °C to about 500 °C. In certain embodiments, the reaction temperature can be from about 420 °C to about 460 °C. In certain embodiments, the reaction temperature can be about 460 °C.

[0038] The oxidative dry reforming reaction can proceed with partial conversion of methane, C0₂, and H₂, thus providing a product mixture that includes methane, CO, H₂0, C0₂, and H₂. In certain embodiments, the oxidative dry reforming reaction can be performed from about 65% to about 90% conversion of C0₂. In certain embodiments, the oxidative dry reforming reaction can be performed to about 86.4% conversion of C0₂. In certain embodiments, the oxidative dry reforming reaction can be performed from about 50% to about 90% conversion of methane. In certain embodiments, the oxidative dry reforming reaction can be performed to about 72.7% conversion of methane.

[0039] The oxidative dry reforming reaction products can be dried and produce a gas mixture for the Fischer- Tropsch synthesis reaction. The gas mixture can comprise 20% CH₄, 40% H₂, and 40% CO or 20% CH₄, 50% H₂, and 30% CO. [0040] The Fischer- Tropsch synthesis reaction can proceed with partial conversion of methane and CO, thus providing a product mixture that includes methane, CO, C0₂, olefins, and paraffins. In certain embodiments, the Fischer-Tropsch synthesis reaction can be performed from about 20% to about 90% conversion of CO. In certain embodiments, the Fischer-Tropsch synthesis reaction can be performed to about 40% conversion of CO. In certain embodiments, the Fischer-Tropsch synthesis reaction can be performed from about 5% to about 50% conversion of methane. In certain embodiments, the Fischer-Tropsch synthesis reaction can be performed to about 15% conversion of methane.

[0041] For the purpose of illustration and not limitation, FIG. 1 is a schematic representation of an exemplary system that can be used in conjunction with the processes of the presently disclosed subject matter. The system 100 can include an oxidative dry reforming reactor 102. The oxidative dry reforming reactor 102 can include an oxidative dry reforming catalyst. A stream 101 that contains methane, carbon dioxide, and oxygen can be fed into the reactor 102 and can be contacted with the oxidative dry reforming catalyst to provide an oxidative dry reforming product mixture that contains carbon monoxide, hydrogen, and water. The proportions of methane, carbon dioxide, and oxygen in the stream 101 can be varied. The oxidative dry reforming product mixture can be removed as a stream from the reactor 102. In certain embodiments, the oxidative dry reforming product mixture can also include unreacted methane and/or carbon dioxide.

[0042] In certain embodiments, water can be removed from the dry reforming product mixture in a separations unit 103. Alternatively, the oxidative dry reforming product mixture can be dried by distillation and/or by passage through a drying agent (e.g., calcium chloride). The product mixture can then be transferred to a reactor 104 to undergo olefin synthesis, e.g., Fischer-Tropsch synthesis. The reactor 104 can include an Fischer-Tropsch synthesis catalyst. In certain embodiments, additional methane can be introduced into reactor 104. [0043] The products of the Fischer-Tropsch synthesis reaction can be separated in a separations unit 105. In certain embodiments, C2-C3 olefins can be separated and transferred via a transfer line 106. Unreacted C0₂, methane, paraffins, and hydrocarbons >C₃ can be separated into a stream 107. Stream 107 can be reintroduced to the system by merging with reactant stream 101. Alternatively, stream 107 can be undergo further processing, e.g., thermal cracking.

[0044] An advantage of the presently disclosed subject matter can be the use of oxidative dry reforming for conversion of methane to syngas, rather than exclusive use of steam reforming. Whereas steam reforming is highly endothermic (and consequently highly energy intensive), oxidative dry reforming is only mildly exothermic, which can reduce energy consumption and facilitate control of heat released by the reaction. Conversion of methane at low temperatures controls the products of the reaction so at to produce unreacted methane for syngas conversion to hydrocarbons. The methane partially replaces H₂ as in the Fischer- Tropsch reaction, reducing production of high molecular weight hydrocarbons.

EXAMPLES

[0045] The following examples are provided by way of illustration and not by way of limitation.

Example 1 - Generation of syngas

[0046] This Example describes generation of syngas by methane oxidative dry conversion.

[0047] A gas composition as described in Table 1 was converted at 710°C in the presence of a 3% Ni/La₂0₃ catalyst. The catalyst loading was 0.75 grams. The flow rate of the gases was 30 cc/minute. Table 1. Gas composition

The conversion of methane was 72.7%. The conversion of carbon dioxide was 86.1%>. The conversion of oxygen was 96%.

Example 2 - Conversion of syngas

[0048] This Example describes conversion of syngas to olefins.

[0049] A gas composition comprising 20% CH₄, 40% H₂, and 40% CO, was reacted at 460°C in the presence of a 55% Fe₂O₃-40% MnO-5% K₂0 catalyst. The catalyst loading was 3.0 grams. The flow rate of the gases was 30 cc/minute.

[0050] Carbon monoxide conversion was 40%. Methane conversion was 15%. The selectivity for synthesis of C₂-C₄ olefins was 40.0 %. * * *

[0051] Although the presently disclosed subject matter and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made herein without departing from the spirit and scope of the disclosed subject matter as defined by the appended claims. Moreover, the scope of the disclosed subject matter is not intended to be limited to the particular embodiments described in the specification. Accordingly, the appended claims are intended to include within their scope such alternatives.

Claims

1. A process for preparing olefins from methane, the process comprising:

a) contacting methane, oxygen gas, and carbon dioxide with a first catalyst in a first reactor zone to form a mixture of syngas, water, and unreacted methane at a temperature of from about 650 to about 710°C;

b) separating the water produced in step a) to produce a stream comprising

unreacted methane and syngas;

c) contacting the unreacted methane and syngas stream with a second catalyst in a second reactor zone to undergo Fischer-Tropsch synthesis at a temperature of from about 420 to about 460°C to form a second stream comprising olefins, carbon dioxide, and methane;

d) separating the olefins from the second stream to form a third stream

comprising carbon dioxide and methane; and

e) recycling the third stream by feeding the third stream into the first reactor zone.

2. The process of claim 1, wherein the first catalyst is 3% Ni/La₂0₃.

3. The process of claim 1, wherein the temperature of step a) is about 710°C.

4. The process of claim 1, wherein the temperature of step c) is about 460°C.

5. The process of claim 1, wherein methane and carbon dioxide are present in a ratio of about 1.7 in step a).

6. The process of claim 1, wherein the olefins comprise C₂-C₃ and/or C₂-C₄ olefins.

7. The process of claim 1, wherein carbon monoxide and hydrogen are present in a ratio of from about 1 :2 to about 2: 1 in step c).

8. The process of claim 1, wherein the unreacted methanol and syngas are present in

amounts of 20% CH₄, 40% H₂, and 40% CO.

9. The process of claim 1, wherein the Fischer-Tropsch synthesis reaction is performed from about 20% to about 90% conversion of CO.

10. The process of claim 1, wherein the Fischer-Tropsch synthesis reaction is performed to about 40% conversion of CO.

11. The process of claim 1, wherein the Fischer-Tropsch synthesis reaction is performed from about 5% to about 50% conversion of methane.

12. The process of claim 1, wherein the Fischer-Tropsch synthesis reaction is performed to about 15%) conversion of methane.

13. The process of claim 1, wherein step a) can be performed to from about 65% to about

90%) conversion of C0₂.

14. The process of claim 1, wherein step a) can be performed to from about 50% to about

90% conversion of methane.