Fuel cell reactor and a process for direct conversion of a hydrocarbon-containing gas to a higher hydrocarbons product

Abstract

A fuel cell reactor, preferably a solid oxide fuel cell (SOFC) reactor, for performing direct conversion of a hydrocarbon-containing gas to a higher hydrocarbons product is confined by walls, where reactants are flown in the anode compartments and air is introduced to the cathode compartments, and where oxygen is transferred from one side of the walls to the other side to promote or inhibit a chemical reaction. The process for direct conversion of a hydrocarbon-containing gas to a higher hydrocarbons product takes place in the anode compartment of the reactor, in which produced hydrogen, limiting the conversion to the equilibrium, is reacted in situ with oxygen ions transferred from the cathode compartment to produce steam, thereby removing the equilibrium-limiting hydrogen from the reaction.

Claims

1. A process for direct conversion of a hydrocarbon-containing gas to a higher hydrocarbons product in an anode compartment of a fuel cell reactor, in which produced hydrogen, limiting the conversion to the equilibrium, is reacted in situ with oxygen ions transferred from a cathode compartment to produce steam, thereby removing the equilibrium-limiting hydrogen from the reaction.

2. The process according to claim 1, wherein the hydrocarbon-containing gas is any methane-containing gas, such as natural gas, biogas, synthetic natural gas or shale gas.

3. The process according to claim 1, wherein the higher hydrocarbons product is a gaseous or a liquid product.

4. The process according to claim 1, wherein methane is converted to aromatic products according to the equilibrium reaction
nCH.sub.4.Math.nC.sub.nH.sub.2n−6+(n+3)H.sub.2 with n=6,7,8 or 9 where hydrogen is reacted in situ with oxygen ions transferred from the cathode department to produce steam in the fuel cell reactor.

5. The process according to claim 4, wherein the fuel cell reactor is a solid oxide fuel cell reactor.

6. The process according to claim 1, wherein a conversion catalyst is coated on an anode surface, loaded in the anode compartment or a combination of both.

7. The process according to claim 2, wherein the methane-containing gas is cleaned of impurities.

8. The process according to claim 7, wherein the cleaned gas is further treated in a hydrocarbon treating unit to remove or convert any hydrocarbon species except methane in order to produce a methane-rich feed gas for the fuel cell reactor.

9. The process according to claim 8, wherein the hydrocarbon treating unit is a pre-reformer operating in an optimized condition to convert higher hydrocarbons with steam to hydrogen and carbon oxides.

10. The process according to claim 9, wherein unconverted feed gas in the fuel cell reactor is separated and recycled back to the reactor.

11. The process according to claim 10, wherein a fraction of gas from the fuel cell reactor effluent, which contains hydrogen, is used to clean and treat raw hydrocarbon feed gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a block diagram of a process for the preparation of aromatic hydrocarbons from methane.

(2) FIG. 2 is alternative to the method of FIG. 1 which includes the in situ removal of hydrogen from the reactor.

(3) FIG. 3 is a graph showing the equilibrium conversion of methane versus temperature.

(4) FIG. 4 shows how the conversion of methane to benzene can be enhanced beyond the thermodynamic equilibrium limit by employing a planar or tubular fuel cell reactor in which the produced hydrogen is consumed to generate steam and electricity.

(5) FIG. 5 is a block diagram showing the schematic layout of the process according to the present invention.

(6) FIG. 6a is a schematic drawing illustrating one possible SOFC reactor structure, and FIG. 6b shows a top view of a possible arrangement for one module of the SOFC reactor.

(7) FIG. 7 is a block diagram illustration of Example 1.

(8) FIG. 8 shows an alternative to the process layout shown in FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(9) More specifically, the present invention concerns a fuel cell reactor for performing direct conversion of a hydrocarbon-containing gas to a higher hydrocarbons product, the fuel cell reactor being confined by walls, where reactants are flown in the anode compartments and air is introduced to the cathode compartments, and where oxygen is transferred from one side of the walls to the other side to promote or inhibit a chemical reaction. Furthermore, the invention concerns a process for direct conversion of a hydrocarbon-containing gas to a higher hydrocarbons product in the anode compartment of a fuel cell reactor, in which produced hydrogen, limiting the conversion to the equilibrium, is either removed or reacted in situ with oxygen ions transferred from the cathode compartment to produce steam, thereby removing the equilibrium-limiting hydrogen from the reaction.

(10) Preferably, the fuel cell reactor is a solid oxide fuel cell (SOFC) reactor.

(11) The hydrocarbon-containing gas can be any gas containing methane, such as natural gas, biogas, synthetic natural gas or shale gas. Preferably, it is natural gas.

(12) The higher hydrocarbons product is either a gaseous or a liquid product.

(13) The methane-containing gas is preferably cleaned from impurities such as sulfur, nitrogen and any non-hydrocarbon species. The cleaned gas may be further treated to remove or convert any hydrocarbonic species except methane in order to produce a methane-rich feed gas for the solid oxide fuel cell reactor.

(14) The hydrocarbon treating unit is preferably a pre-reformer operating in an optimized condition to convert higher hydrocarbons, i.e. C2+, with steam to hydrogen and carbon oxides.

(15) Methane-rich gas is converted to aromatic products according to the equilibrium reaction
nCH.sub.4.Math.nC.sub.nH.sub.2n-6+(n+3)H.sub.2 with n=6,7,8 or 9
where hydrogen is reacted in situ with oxygen ions transferred from the cathode compartment to produce steam.

(16) Preferably the unconverted feed gas in the fuel cell reactor is separated and recycled back to the reactor.

(17) A conversion catalyst can be coated on the anode surface, loaded in the anode compartment or a combination of both.

(18) A fraction of the gas from the fuel cell reactor effluent, which contains hydrogen, may be used to clean and treat the raw hydrocarbon feed gas.

(19) The equilibrium conversion of methane (CH.sub.4) in mol % versus temperature is shown graphically in FIG. 3. From the graph it is seen that the conversion is limited to a maximum of about 20% in a practical operating temperature range below 1073 K (800° C.)

(20) It has surprisingly turned out that the conversion of methane to benzene can be enhanced beyond the thermodynamic equilibrium limit by employing a planar or tubular fuel cell reactor, in which the produced hydrogen is consumed to generate steam and electricity; see FIG. 4.

(21) The SOFC reactor, which is preferred for this purpose, is a special kind of SOFC in which anode activity is not needed. For this reason the anode active layer can be eliminated, leaving only the cathode, an electrolyte and possibly anode support layers.

(22) More specifically, the reactor for performing the process according to the invention is a reactor confined by fuel cell walls, where reactants are flown in the anode compartments and air is introduced to the cathode compartments, and wherein oxygen is transferred from one side of the walls to the other side to promote or inhibit a chemical reaction. Said chemical reaction is preferably a heterogeneous catalytic gas phase reaction.

(23) In the reactor according to the invention, the fuel cell walls are solid oxide fuel cell walls of any type operating at elevated temperatures, and where the catalytic chemical reaction is carried out on the anode side of the cell, resulting in oxygen ions being transferred from the cathode side (the air side) and reacted with hydrogen on the anode side (the fuel side).

(24) Oxygen from the cathode side is ionized and diffused through the electrolyte. On the anode side, where the conversion of methane to benzene takes place, hydrogen (formed as a by-product) reacts with oxygen to form steam. Electrons released from the anode surface are conveyed to the cathode surface via a closed circuit.

(25) When using SOFC reactors for the purpose of the invention, the reaction chamber can be designed in two possible ways:

(26) (1) When the SOFC is used as catalyst, the methane-to-aromatics catalyst is deposited on the anode surface. The reaction takes place on the anode surface, where hydrogen is continuously consumed.

(27) (2) When the SOFC is used as reactor wall, the methane-to-aromatics catalyst in pellet or monolith form is filled in a reactor with SOFC walls. Then the produced hydrogen will be converted to steam on the reactor walls.

(28) A combination of designs (1) and (2) is also possible.

(29) The schematic layout of the process according to the invention is shown in the block diagram of FIG. 5. The SOFC reactor produces aromatics and steam. Hydrogen is consumed to generate power. Air with a temperature of 700-800° C. is supplied to the cathode side of the SOFC wall. The separation unit can e.g. be a condensation unit. Water and aromatics are separated in a liquid settling vessel. Unconverted methane gas and hydrogen is compressed and recycled back to the reactor inlet.

(30) FIG. 6a is a schematic drawing illustrating one possible SOFC reactor structure. For the sake of simplicity a two-channel reactor is shown. Natural gas is passed through the channels 1. These channels are loaded with a suitable catalyst for converting methane to aromatics. The reaction channels are confined between the anode surfaces of the SOFC (4) and the metal interconnects (3). An intertwined metal network is inserted between two surfaces, allowing current flow across the channels.

(31) Air is blown to the channels (2). These channels are confined between the cathode surfaces of the SOFC (4) and the metal interconnects (3). A suitable interconnect network may also be provided here to pass the current flow. Alternatively, corrugated plates can be used as metal interconnects.

(32) FIG. 6b shows a top view of a possible arrangement for one module of the SOFC reactor. One reactor module is confined between two SOFCs each consisting of a cathode layer (1), an electrolyte layer (2) and an anode layer (3). Catalyst pellets (4) are loaded in the anode compartment (6). Air is flown through the cathode compartment (7). Two compartments are separated by an electrically conductive interconnect (5).

(33) Current is collected from both ends of the reactor.

(34) The process according to the invention presents a number of advantages over the known processes. These advantages are as follows: steam generation in the reactor will potentially suppress or inhibit carbon deposition; a high methane conversion is obtained by shifting the reaction towards aromatics production; a high hydrogen removal rate compared to high temperature membranes is obtained; a simple/cheap aromatics separation unit can be used; the hydrogen/oxygen reaction heat and the ohmic loss heat are supplied to the endothermic aromatics synthesis reaction, and electricity is generated.

(35) The invention is illustrated further by the following examples.

Example 1

(36) Referring to FIG. 7, natural gas (NG) is desulfurized in the NG HDS (hydrodesulfurization) unit A and fed to the pre-reforming unit B, which operates at a relatively low temperature, along with enough steam from the steam generation unit C to merely reform higher hydrocarbons. The pre-reformed natural gas is devoid of any higher hydrocarbons and almost devoid of carbon monoxide. The effluent from the pre-reformer is cooled in the CO.sub.2/water separation unit D to condense out the water. The content of carbon dioxide in the dry gas is also removed in this unit. Part of the hydrogen from the pre-reformed gas is separated in the membrane unit E and recycled back to the NG HDS unit A. Dry methane-rich gas from the membrane unit mixed with recycle gas from the SOFC reactor F is heated up to the reaction temperature by means of heat exchanger K and fed to the SOFC reactor, and hot air is blown through the cathode channels of the SOFC reactor. The product gas from the anode channels is cooled down using the reactor feed gas and then led to the phase separation unit G, where organic, aqueous and gaseous phases are separated. A small fraction of the gaseous phase is purged to prevent accumulation of inert gases in the synthesis loop, and the rest of the gas is cleaned up and recycled back to the SOFC reactor. The organic phase is sent to the distillation unit I for further purification of the final product. The aqueous phase is sent to the waste water treatment unit H.

(37) NG is supplied to the plant at a pressure of 30 barg.

(38) The NG HDS unit A is operated under the following operating conditions: Temperature 350° C.; pressure 30 barg; hydrogen-to-feed gas ratio 0.04 (mol/mol); NG composition (mol %):90% methane, 4% ethane, 2% propane; 4% inert; required heat 0.79 MW.

(39) The pre-reforming unit B is operated under the following operating conditions: Temperature 350° C.; pressure 29 barg; steam-to-carbon (S/C) ratio 0.6 (mol/mol); steam consumption 5.1 MTPD; heat removal 2.48 MW assuming superheated steam inlet at 350° C. The reactor is adiabatic.

(40) The CO.sub.2/water separation unit D is operated under the following operating conditions: Temperature 40° C.; pressure 27 barg; water condention temperature 60° C. (air cooler can be used); carbon dioxide removal yield over 99%; carbon dioxide inlet concentration 1.8 mol %.

(41) The membrane unit E, which is an optional unit, is operated under the following operating conditions: Temperature 40° C.; pressure 25 barg; hydrogen molar recovery 84%; methane molar slip 9%; recycle hydrogen gas compressor power 44 kW.

(42) The SOFC reactor F has the following technical data: Module (cell) dimension 1×2×0.05 m; number of cells 1080; catalyst W on HZSM-5 with a particle size of 1 mm; catalyst volume 10.8 m.sup.3; air blower power 786 kW; net electrical power output 3.3 MW (5% ohmic and DC/AC converter loss); pressure drop 0.9 bar.

(43) The reactor is operated under the following operating conditions: Temperature 800° C. (average); pressure 0.2 barg in the cathode side (air) and 1.1 barg in the anode side (process gas); air utilization 5.5%; current density 2000 A/m.sup.2; gas hourly space velocity (GHSV) 1000 h.sup.−1; process gas recycle ratio 9.6; recycle gas compressor power 1612 kW; air flow rate 10377 Nm.sup.3/h.

(44) The methane conversion is 5.75% compared to 0.57% conversion in a packed bed at the same process condition. The hydrogen consumption rate is 2379 Nm.sup.3/h equal to a water production rate of 1912 kg/h. Hot air exhaust from heat exchanger M is available at 95° C. for utility water heating. The generated heat amounts to 72.45 MW without considering ohmic loss.

(45) The phase separation unit is operated under the following operating conditions: Temperature 40° C.; atmospheric pressure; required heat removal 1.86 MW.

(46) The material balance values for the process layout of this example are given in Table 1.

Example 2

(47) An alternative to the process layout, which is shown in FIG. 7 and described in Example 1, is shown in FIG. 8. This alternative layout differs from the above layout in that the membrane unit is eliminated. In this case the hydrogen required for the NG HDS (hydrodesulfurization) unit is obtained from the effluent stream from the SOFC reactor.

Example 3

(48) An alternative to the process layout described in Example 2 and shown in FIG. 8 is a layout without water and carbon dioxide removal unit. In this layout, the SOFC feed gas is supplied directly from the pre-reforming unit at the pre-reformer outlet temperature.

(49) TABLE-US-00001 TABLE 1 Material balance values for Example 1 (see FIG. 7 for stream numbers) Stream No. 1 2 8 5 7 10 17 13 15 20 9 11 23 26 Name CO.sub.2- CH.sub.4-rich Rec. free Makeup Rec. Feed Effluent H2 Purge Air Exhaust NG MPS H.sub.2 Syngas syngas gas gas gas gas Product purge gas feed Air mol % N.sub.2 4.0 0.0 2.7 2.3 3.6 3.7 5.5 5.3 5.2 0.0 2.7 5.5 79.0 79.9 O.sub.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 21.0 20.1 H.sub.2 0.0 0.0 30.7 3.0 4.6 0.8 31.6 29.4 30.1 0.0 30.7 31.6 0.0 0.0 CH.sub.4 90.0 0.0 66.6 59.5 91.8 95.5 62.9 65.3 60.0 0.0 66.6 62.9 0.0 0.0 C.sub.2H.sub.6 4.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 C.sub.3H.sub.8 2.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 CO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 CO.sub.2 0.0 0.0 0.0 1.2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 C.sub.6H.sub.6 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.6 100.0 0.0 0.0 0.0 0.0 H.sub.2O 0.0 100.0 0.0 34.0 0.0 0.0 0.0 0.0 4.1 0.0 0.0 0.0 0.0 0.0 T (° C.) 21 350 90 60 60 40 100 800 113 40 40 40 25 95 P (barg) 30.0 30.0 30.0 27.5 25.5 3.5 1.3 1.1 0.1 0.1 5.0 0.1 0.0 0.1 Flow 172 118 26 324 210 183 2376 2559 2623 16 0.7 125 4686 4633 (kmol/h) MW 17.64 18.01 12.09 16.91 15.83 16.37 12.28 12.57 12.91 78.13 11.43 12.27 28.85 28.81 (kg/kmol) Density 23.45 11.05 12.42 15.50 2.85 0.914 0.30 856.23 2.79 0.52 1.18 1.05 (kg/m.sup.3)