Method for dry reforming of at least one alkane

Abstract

The present invention relates to a method for dry reforming of at least one alkane carried out in at least one reaction chamber, preferably with a catalytic bed, having a stream of gas passing through same. According to the invention, said at least one reaction chamber comprises a catalytic solid which is cyclically and alternatively exposed to a stream of at least one alkane and a stream containing carbon dioxide, such that said catalytic solid is used as an oxidation vector.

Claims

1. Method for dry reforming of at least one alkane, wherein said dry reforming of at least one alkane is carried out in at least one reaction chamber exposed to a stream of gas, wherein said at least one reaction chamber comprises a catalytic solid, wherein said method comprises cyclically and alternatively exposing said catalytic solid (i) to a stream containing an alkane and (ii) to a stream containing carbon dioxide, wherein said catalytic solid is an oxidation vector and consists of Me1-Ox1-Ox2 where: Me1 is an element selected from the group consisting of Ag, Au, Co, Cr, Ir, La, Mn, Ni, Os, Pd, Pt, Re, Rh, Ru, Sc, W, Mo and any combination thereof and, wherein Me1 cannot be oxidised in carbon dioxide; Ox1 is a reducible oxide in alkane and can be re-oxidised in carbon dioxide and wherein Ox1 does not contain Fe; Ox2 is an inert oxide with respect to said alkane and carbon.

2. The method according to claim 1, wherein said Ox2 is chosen from among Al.sub.2O.sub.3, MgO, Ta.sub.2O.sub.5, Y.sub.2O.sub.3, ZrO.sub.2 and in that Ox2/(Me1+Ox1) mass ratio is between 0 and 100.

3. The method according to claim 1, wherein said at least one reaction chamber is a catalytic bed reaction chamber.

4. The method according to claim 1, wherein said at least one reaction chamber is a fixed catalytic bed reaction chamber.

5. The method according to claim 1, wherein the alkane is methane.

6. The method according to claim 1 wherein Me1/Ox1 mass ratio is approximately between 0 and 0.9.

7. The method according to claim 1, wherein the element Me1 is initially used in oxide or hydroxide form.

8. The method according to claim 1, wherein Ox1 is chosen from among an oxide of the following elements taken alone or in combination: Ce, Nb, Ti, W, Mo.

9. The method according to claim 1, wherein Ox1 may contain V and/or Zr.

10. The method according to claim 1, wherein said catalytic solid comprises a CeO.sub.2 support including Ni and/or Co in a ratio of 8.7% in weight.

11. The method according to claim 1, wherein up to 80% of injected carbon dioxide is part of the stream of alkane.

Description

DETAILED DESCRIPTION OF A MODE TO CARRY OUT THE INVENTION

(1) As already mentioned, the present invention, concerns, according to one of its aspects, a method for dry reforming in which the catalytic solid is exposed in an alternate and repeated manner to each of the dry reforming reagents (methane and carbon dioxide) and where it is used as an oxygen vector. Initially, the solid reacts with the methane to form carbon monoxide and dihydrogen. Then, it reacts with the carbon dioxide to recharge in oxygen. During this second phase, any possible carbonated residues are re-oxidised by the carbon dioxide. Since the solid is regenerated during each cycle, de-activation by coking is avoided. The produced hydrogen is never in contact with the carbon dioxide (reagent), thereby avoiding the reverse water gas-shift reaction.

(2) The activity and selectivity of the solid are assured by the presence of a reducible oxide (CeO2 in the example) serving as the oxygen vector, and a metal (Ni, Co in the examples) that is not oxidised in the conditions used in the method, assuring the activation of the methane.

(3) Such a reaction thereby makes a catalytic solid undergo repeated cycles of oxidation-reduction by alternatively exposing it to methane and carbon dioxide. In practice, this is obtained by supplying the fixed-bed reactor in a periodic manner. For this type of reaction chamber, the reaction temperature should necessarily be identical in the two stages of the periodic method.

(4) It consists of a simple method to carry out this type of reaction on both the laboratory and industrial scales. In an interesting manner, the method may be used with a circulating fluidised bed. In this case, the solid is carried between two distinct reactors where it is exposed to each of the reagents independently. Moreover, this method has the advantage that is can most effectively optimise each of the reaction phases and thereby a still more effective use of the invention.

(5) The type of analytic tool used (on-line mass spectrometry) requires working with reduced concentration of reagents (typically 5 to 25 in a quantitative manner, up to 50% in a less quantitative manner). Nevertheless, the defended concept should be perfectly possible with streams of pure gases.

(6) The principle of the method according to the invention consists of using a solid especially with the following specific properties:

(7) 1. It should present a capacity for storage and oxygen transfer;

(8) 2. It should enable the activation of the methane;

(9) 3. It should, in the conditions used, selectively lead to the syngas (CH4+Sol-O.fwdarw.CO+2H2+Sol-R) and not total oxidation (CH4+4Sol-O.fwdarw.CO2+2H2O+4Sol-R) (Sol-O and Sol-R represent the oxidised and reduced solid, respectively). The formation of derivatives should also be limited (for example, alkenes). The formation of solid carbon (CH4.fwdarw.C+2H2 or 2CO.fwdarw.CO2+C) should be limited as far as possible but is not prohibitive as long as the solid is constantly regenerated by the CO2+C.fwdarw.2CO reaction;

(10) 4. The oxygen storage capacity should be replenished by re-oxidation by CO2.

(11) In an interesting manner, the composition of a catalytic solid may be generalised as follows: Me1-Ox1-Ox2; where:

(12) Me1 is an element that cannot be re-oxidised under carbon dioxide. Me1 may be one of the following elements or a combination of the following elements: Ag, Au, Co, Cr, Ir, La, Mn, Ni, Os, Pd, Pt, Re, Rh, Ru, Sc, W, Mo. Initially, Me1 may be in a reduced form or in any form of oxide or hydroxide, or a mixture of both, reducible under methane. The selection of elements proposed for Me1 is based on the thermodynamics of the reducibility of the oxide under methane and that of the re-oxidation of the metal reduced by the carbon dioxide. The elements in question should also show a certain ability to activate methane, for any type of reaction.

(13) Ox1 is an oxide that is reducible in methane and can be re-oxidised in carbon dioxide. The Ox1 elements are chosen so that the thermodynamics (in the temperature conditions proposed) of the partial oxidation of the methane into carbon monoxide and dihydrogen is more favourable than the total oxidation into carbon dioxide and water. Ox1 may therefore be an oxide of the following elements or an oxide from a combination of the following elements: Ce, Fe, Nb, Ti, W, Mo. The oxide may also contain less than 30% V and/or Zr associated with the previous elements. Although these two elements don't directly comply with the thermodynamic criteria or reducibility and selectivity, they are known to affect the properties of solids based on the oxides proposed for Ox1.

(14) The oxides Al2O3, MgO, Ta2O5, Y2O3, ZrO2 may be used as supports of solid reagents in order to, for example, improve: the dispersion, the reactivity or the chemical and mechanical stability. Ox2 may thereby be chosen from among Al2O3, MgO, Ta2O5, Y2O3, ZrO2.

(15) The mass proportion between Me1/Ox1 may vary from 0 to 0.9; that of Ox2/(Me1+Ox1) from 0 to 100.

(16) In general, conditions 1 and 4 should be provided by a reducible oxide. Condition 2 may be provided by this same oxide or by another solid phase associated with the former, for example, a metal supported on the reducible oxide. Condition 3 is determined by the type of oxide or all of the present phases as well as the operating conditions.

(17) Successful trials have been obtained using a cerium oxide (CeO) solid associated with metal particles (Ni, Co) supported on this oxide. The support acts as an oxygen vector by oxidation and reduction in CO2 and CH4, respectively. The metal enables the good activation of the methane and thereby high conversions. It should be noted that the metals mentioned do not re-oxidise in the presence of CO2 in the reaction conditions as it has been possible to check by thermodynamic calculations. In these conditions, the selectivity during the oxidation of the methane is excellent (refer to examples) while in oxidised form (for example by dioxygen) these same elements lead to the total oxidation of the methane in the same operating conditions. Moreover, the supported Ni oxide catalysts are well known for the total oxidation of methane in a chemical loop for the production of heat.

(18) The proposed method has been tested with laboratory-produced solids. The behaviour of the Ni catalyst supported on CeO2 has been verified using a laboratory-prepared support as well as a commercial support (Aldrich). The performance in terms of activity (conversion of the methane and carbon dioxide) are lower but this may be attributed to the reduced specific area of the commercial support. However, in terms of selectivity, the performance has validated the principle of the periodic method independently of the origin of the support. The same is true for the performance of the supported cobalt solid, an element known for its ability to activate methane.

(19) The following tables present several representative results. The first table provides: the type of solid used (the type of Ox1 support used and its source; the type of Me1 metal, the mass ratio Me1/Ox1 as well as the reaction conditions (the amount of solid used, the reaction temperature, the methane and carbon dioxide concentrations used, the total gas inflow, the periodic operating conditions (length of the cycles)). The second table provides: during the exposure of the methane, the conversion of the methane, the measured H2/CO ratio, the percentage of converted methane leading to the formation of solid carbon; during the exposure of the carbon dioxide, the conversion of the carbon dioxide, the CO/(converted CO2) ratio.

(20) Ideally, the H2/CO ratio should be equal to 2. A higher value indicates the formation of solid carbon, a lower value indicates the presence of a reverse water-gas shift reaction. The percentage of carbon formed should be as low as possible. Ideally, the CO/CO2 ratio should be equal to 1. A higher value indicates the oxidation of the carbon deposited on the support. The conversion of methane and carbon dioxide should be equivalent.

(21) Each experiment consists of 12 full cycles. The values indicated are integrated averages for the last 6 cycles carried out. Experiments with 60 cycles have also been carried out and demonstrate the excellent stability of the behaviour of the solids (examples 1, 2, 3).

(22) The results show that, in very extensive operating conditions, both in the composition of the reaction streams, temperature, quantities and the type of solid, the performance of the system is close to that of the ideal in terms of selectivity (CO/H2). The results differ only in terms of reactivity (conversion) and more or less high percentage of methane transformed from solid carbon.

(23) The results obtained show the robustness of this method when the above conditions are respected, as well as its adaptability to vast operating conditions. They also demonstrate great flexibility for optimisation, in particular by:

(24) The control of the quantity and type of metallic phase, or even multi-metallic phase, by associating several elements in order to optimise the activation of the methane and CO2,

(25) The control of the oxygen supply from the support by modifying its type (oxides, mixed oxides, doped oxides),

(26) Control of the activation of the CO2 by modifications in the support (impregnation, doping).

(27) In principle, the method according to the invention is well adapted for the reforming of methane without excluding other alkanes (for example ethane, propane, etc.).

(28) The dry reforming of methane involves the consumption of an equivalent of CH4 for an equivalent of CO2. The invention foresees a separate supply of these two reagents in order to optimise the performance of the catalysts used. Nevertheless, without going beyond the invention, it is possible to supply part of the CO2 simultaneously with the methane (co-supply) while maintaining part of the advantages of this method (solid carbon eliminated during each cycle, lower contribution of the reverse water-gas shift reaction). The portion CO2 supplied with the alkane should in any case not exceed 80% of the total CO2 required for dry reforming.

(29) TABLE-US-00001 TABLE 1 Experimental details of the examples Metal m.sub.sol T Reagent Cycles Inflow Example CeO2 Metal (% weight) (g) ( C.) concentration (number) (cc/min) Cycle 1 produced Ni 8.7 0.40 800 25% CH.sub.4 60 100 1 min. CH.sub.4 25% CO.sub.2 1 min. CO.sub.2 2 produced Ni 8.7 0.20 800 5% CH.sub.4 60 100 1 min. CH.sub.4 5% CO.sub.2 1 min. CO.sub.2 3 produced Co 8.7 0.20 800 5% CH.sub.4 60 100 1 min. CH.sub.4 5% CO.sub.2 1 min. CO.sub.2 4 produced Ni 2.0 0.20 700 5% CH.sub.4 12 100 1 min. CH.sub.4 5% CO.sub.2 1 min. CO.sub.2 5 produced Ni 8.7 0.20 700 5% CH.sub.4 12 100 1 min. CH.sub.4 5% CO.sub.2 1 min. CO.sub.2 6 produced Ni 8.7 0.20 800 5% CH.sub.4 12 100 1 min. CH.sub.4 5% CO.sub.2 1 min. CO.sub.2 7 produced Ni 8.7 0.20 750 5% CH.sub.4 12 100 1 min. CH.sub.4 5% CO.sub.2 1 min. CO.sub.2 8 produced Ni 8.7 0.20 700 5% CH.sub.4 12 100 1 min. CH.sub.4 5% CO.sub.2 1 min. CO.sub.2 9 produced Ni 8.7 0.20 650 5% CH.sub.4 12 50 1 min. CH.sub.4 5% CO.sub.2 1 min. CO.sub.2 10 Commercial Ni 8.7 0.20 800 5% CH.sub.4 12 100 1 min. CH.sub.4 5% CO.sub.2 1 min. CO.sub.2 11 produced Co 8.7 0.20 800 5% CH.sub.4 12 100 1 min. CH.sub.4 5% CO.sub.2 1 min. CO.sub.2 12 produced Co 8.7 0.20 750 5% CH.sub.4 12 100 1 min. CH.sub.4 5% CO.sub.2 1 min. CO.sub.2 13 produced Co 8.7 0.20 700 5% CH.sub.4 12 100 1 min. CH.sub.4 5% CO.sub.2 1 min. CO.sub.2

(30) TABLE-US-00002 TABLE 2 Experimental results of the examples Conversion C.sub.deposited/ Conversion CO/ CH4 H2/C0 C.sub.conv. CO.sub.2 CO.sub.2conv. Example (%) 1 0.2 (%) 2 (%) 1 0.1 1 54 2.2 3 60 1.0 2 84 2.0 3 83 1.0 3 69 2.0 <2 74 1.0 4 38 1.9 <2 37 1.2 5 68 1.9 <2 71 1.1 6 88 2.1 5 87 1.0 7 80 2.0 1 79 1.0 8 65 2.1 5 65 1.0 9 41 2.1 1 43 1.1 10 13 1.8 <2 22 0.9 11 77 2.1 <2 84 1.0 12 68 2.1 2 71 1.0 13 51 2.1 <2 52 1.0

(31) It is therefore interesting to note that the method according to the invention avoids the reverse water gas-shift reaction between carbon dioxide (dry reforming reagent) and dihydrogen (dry reforming product), a reaction that affects the selectivity of the method when the two reagents are simultaneously supplied. The coking of the catalyst in contact with the methane may also be considerably reduced or even cancelled. The periodic recycling of the catalytic solid enables the re-oxidation of the material and therefore maintains performance.

(32) In an especially interesting manner, the method according invention allows for the valorisation of the biogas. Other more complex sources, such as, for example, mixtures with other alkanes or alkenes from natural gas, may also be valorised by the method according to the invention.