WO2000056658A1

WO2000056658A1 - Method for selectively oxidizing hydrocarbons

Info

Publication number: WO2000056658A1
Application number: PCT/NL2000/000192
Authority: WO
Inventors: Renee Van Yperen; Johannes Maria Der Kinderen; Erwin Roderick Stobbe; John Wilhelm Geus
Original assignee: Plug Power Inc.
Priority date: 1999-03-22
Filing date: 2000-03-22
Publication date: 2000-09-28
Also published as: AU3575900A; NL1011627C2

Abstract

The invention relates to a method for oxidizing hydrocarbons which comprises contacting the hydrocarbon with a solid substance comprising at least one metal oxide reducible by the hydrocarbon, under conditions that the hydrocarbon is oxidized and the metal oxide is reduced, which oxidation takes place in the co-presence of nickel and/or at least one noble metal in the solid substance.

Description

WO 00/56658 PCTtNLOO/00192

Title: Method for selectively oxidizing hydrocarbons

The invention relates to a method for oxidizing hydrocarbons, for the purpose of producing, inter aha, hydrogen.

For preparing synthesis gas from hydrocarbons, more in particular from methane (natural gas), a number or processes are known. Methane-steam reforming is often used. According to this process methane reacts with steam to form a mixture of H₂ and CO, according to the reaction:

If only hydrogen is desired, the resulting carbon monoxide is reacted with water to form CO2 and hydrogen according to the reaction:

The CO2 is removed from the resulting mixture by means or known techniques, such as dissolving under pressure in aqueous solutions or by using regenerable solid sorbents.

Steam reforming may also be used for other hydrocarbons, or hydrocarbon mixtures, such as naphtha, provided that they can be brought into the gaseous phase. Steam reforming is an endothermic process, in which the required reaction heat must be supplied to the reaction mixture at high temperature. Conventional temperatures are of the order of 800 to 900 °C. In general, this heat is generated outside the reaction mixture by combustion of hydrocarbons, such as methane. Such reactions are mostly carried out in metal tubes, which are placed in an oven in which the combustion takes place. Owing to the high temperatures and the desired life of the materials, expensive, often nickel-containing, alloys must be used for the tubes. The generated reaction heat is transferred by radiation via the tube wall to the reaction mixture. Steam reforming is often carried out at elevated pressure, for instance 30 bar, which imposes stringent requirements on the oxidation resistance of the materials.

For methane steam reforming the work could theoretically be done at a molar ratio of methane/water of 1 : 1, as appears from the above reaction equation. In practice, however, it is not possible very well to work under such conditions, because carbon formation occurs on the employed reform catalyst. In practice, the work is therefore done with excess steam. A conventional molar ratio of water to methane ranges between 2 and 3.

Steam reforming has long since been used with excellent results, but the process also has a number of disadvantages. In the first place, much thermal energy is generated in the flue gas and in the process gas. From economical considerations, it must be recovered. This is done via the production of high-pressure steam which cannot always be used properly.

Related to this is the fact that for a good conversion of the hydrocarbons at elevated pressure a high temperature must be used. The temperatures are approximately between 800 and 1000 ° C. At temperatures above 850 ° C the Hfe of the reactor pipes which contain the catalyst is strongly declines.

There is also a need for great flexibility with respect to the carbon monoxide/hydrogen ratio in the produced gas mixture.

As already indicated above, the hydrogen content can be increased by carrying out the carbon monoxide shift conversion reaction. To obtain a good conversion, it is desirable, however, that this is done in two reactors, while rather much thermal energy must be discharged to allow this equilibrium reaction to shift to the hydrogen side.

For methanol synthesis of the Fisher-Tropfsch reaction the work is most preferably done with an H2/CO ratio < 3. To obtain ratios in this range, special measures must be taken.

It would further be very interesting if it is possible to efficiently carry out methane-steam reform processes on a small scale, since this enables production of a gas which could be efficiently supplied to a fuel cell. An alternative for steam reforming with heat generation outside the reaction mixture is a process in which the required reaction heat is generated in the reaction system. An advantage thereof is, inter alia, that a less complicated reactor construction is needed, while it is also possible to process base materials having a low reactivity, such as fuel oil. The temperatures required for these base materials are so high that these are not practicable via the conventional steam-reform methods.

In this type of processes a part of the feed is combusted to form the thermal energy required for the endothermic reaction. Combustion with deficient oxygen, as necessarily takes place in this type of systems, results in soot formation. This should then be separated from the system individually. It is also necessary in this type of systems to use pure oxygen, because the use of air would lead to an impermissible increase of the reactor volume. It has been proposed that the required heat is generated by an exothermic catalytic oxidation of a part of the hydrocarbon supplied to the process. See in this respect, for instance, EP-A 0 738 235.

It has also been proposed that hydrocarbons, in particular methane, are oxidized using copper oxide, which is simultaneously reduced to metallic copper. The reduced metallic copper may then be oxidized again in a reoxidation phase for reuse. Here the so-called oxygen storage properties of the metal (oxide) are therefore utilized.

By using this method, methane is completely combusted to carbon dioxide and water to form thermal energy. This thermal energy can then be used in a sequence step for the steam-reform process.

It would be attractive if these two steps could take place nearly simultaneously or in any case immediately after each other in one reactor.

Surprisingly, it has been found that it is possible to attain such an object by the use of a combination of a metal/metal oxide having oxygen storage properties and a reform catalyst (nickel and/or noble metal).

The invention therefore relates to a method for oxidizing hydrocarbons which comprises contacting the hydrocarbon with a solid substance comprising at least one metal oxide reducible by the hydrocarbon under conditions that the hydrocarbon is oxidized and the metal oxide is reduced, which oxidation takes place in the co-presence of nickel and/or at least one noble metal in the solid substance.

According to a preferred embodiment of the method, the thus obtained reduced metal oxide is reoxidized with oxygen and/or steam in absence of the hydrocarbon. In particular the reoxidation with steam is very interesting, since hydrogen is produced herein. The heat generated during the reoxidation is stored in the solid substance, the metal oxide, from which it can be transmitted to the steam- reform reaction. This leads to an efficient energy consumption, while, furthermore, a solution is given for the problems of the heat transfer to the steam reform reaction.

In the reactor, for one thing, hydrocarbon (methane) is oxidized:

CH + 4[O] -» CO2 + 2H2O + ΔH

In the presence of a reduced metal oxide and the second component, nickel and/or noble metal, according to the reaction:

WO 00/56658 4 PCTtNLOO/00192

and carbon dioxide react according to

The heat requirement of both reactions can be satisfied by the thermal energy stored in the solid substance.

In this connection, the water formed is of course a first source. If the conditions allow this, additional water (steam) may also be supplied from the outside. As metal oxide, the method according to the invention utilizes metal oxides reducible by hydrocarbons. Examples thereof are, inter alia, the metals of the groups IB, LIB, VB, VTB, VIIB and VIII of the Periodic Table, more in particular iron, manganese and copper.

It has been found that the application of the metal oxide to a carrier gives an evident improvement of the performance, more in particular with respect to the amount of oxygen which can be absorbed and emitted. For manganese, for instance, it appears that after a number of cycles unsupported oxide cannot be oxidized further than to Mn3θ₄, which is less favorable than to M θ3. As carriers, all the carriers can be used that are stable under the diverse conditions to be used in the reactions. Moreover, the carriers must not or only in a minor degree react with the metal or metal oxide to form inactive components. Examples of suitable carriers are alumina, titania, magnesia and zirconia. If desired, the carrier materials may be thermically and/or chemically stabilized.

The amount of metal oxide with respect to the carrier may vary within broad limits, in which connection, of course, efforts are directed to an optimum balance between the load (as high as possible) and the properties of the oxide (as good as possible). These criteria often lead to conflicting situations, so that an optimum compromise is found. In general, based on the oxide of the metal, 10 to 75 wt.% metal oxide is used with respect to the weight of the catalyst. The catalyst further contains an amount of finely divided nickel and/or noble metal. The nickel and/or the noble metal is preferably homogeneously distributed over the metal oxide, in an amount of from 0.01 to 40 wt.%, based on the weight of the catalyst. When using nickel, this amount will generally be at the top of this range, preferably from 3 to 40 wt.%, while in respect of noble metal this will rather be at the bottom, preferably from 0.01 to 5 wt.%. In the present case, the noble metal is preferably selected from platinum, palladium, rhodium and ruthenium.

The preparation of the catalyst can be carried out in a conventional manner, for instance by impregnation, deposition-precipitation and the like. The method according to the invention is suitable for oxidizing all kinds of hydrocarbons, from natural gas to heavy fuel oil fractions. The advantages of the invention, however, particularly manifest themselves in the lower hydrocarbons, such as natural gas, and also in naphtha.

The hydrocarbon is supplied to the oxidation step, in which the metal oxide is most preferably present in a highest possible degree of oxidation. During the reaction the temperature preferably ranges between 500 and 1100°C. The pressure may be rather freely selected, but it is preferred, from the viewpoint of reactor construction, to work at pressures from atmospheric to 50 bar (abs).

During the oxidation step the hydrocarbon is oxidized, under simultaneous reduction of the metal oxide. If this reaction is endothermic (for instance in the oxidation with steam), the reaction heat required for this is obtained by the heat stored in the solid substance, originating from the reoxidation of the reduced metal oxide with water or air. Subsequently, the reduced solid substance may be used as catalyst for the steam reforming. This is preferably done in a second reactor, in which the reaction mixture containing the oxidized hydrocarbon, is passed over the reduced solid substance. The reaction proceeds herein partly by the heat still present in the solid substance. If the heat content of the solid substance has decreased too strongly, it is reoxidized again and absorbs oxygen and thermal energy, so that the solid substance can be used again for the oxidation of the hydrocarbon. Thus the solid substance can be recycled over three phases of the process (see Fig. 2).

Besides, US-A 5,799,482 discloses a process for integrating a reformer and a combustion/regenerator unit which are operated as fluid beds. According to this known method, a metal oxide is used in combination with, for instance, a nickel catalyst. The nickel catalyst is used to catalyze the steam reforming. Furthermore, EP-A 0 016 648 discloses a process for oxidizing and/or cracking a heavy hydrocarbon fraction, also in fluid beds with an iron oxide catalyst. According to this known method, a nickel catalyst is used to crack hydrocarbons and to convert iron sulfide.

In the process of the present invention, however, the nickel and/or noble metal has a three-fold function. The metal accelerates both the oxidation step and the reduction step, while it also works as reform catalyst. WO 00/56658 6 PCTtNLOO/00192

Other variants, however, are also conceivable, provided that the starting- point of the invention, the storage of heat and oxygen in the solid substance, remains the binding criterion.

The reoxidation may be carried out in the same reactor as that for the oxidation step of hydrocarbon, after the hydrocarbon has been rinsed out, using an inert gas to prevent the formation of an explosion/mixture. It is also possible to remove the catalyst from the reactor, for instance when using a moving bed, and to reoxidize it in another reactor. It is further also possible to use a rotary reactor, for instance as described in WO-A 9801222. The reoxidation takes place with air or steam. As indicated, in the latter case there is further formed hydrogen which may be separated and used. The heat released during the oxidation reaction can then be simply stored in the solid substance, instead of in the gases, and subsequently, the heat is used in the endothermic steam reforming.

The invention is illustrated with the following examples:

Example 1

Preparation of a manganese oxide on aluminum oxide catalyst

The starting material was powdered α-aluminum oxide having a BET surface of 13 m² per gram and a pore volume of 0.08 ml per gram. After evacuation for 30 minutes manganese oxide was applied to the carrier by pore volume impregnation with a manganese(II)nitrate solution. After impregnation the carrier was dried for 16 hours at 80°C, after which the loaded carrier was calcined at 550°C. The temperature was increased by 300°C per minute, and the loaded carrier was kept at 550°C for 5 hours.

The loading with manganese oxide was 15 wt.%, based on M Oβ. After calcination the material was pulverized and pressed to tablets.

Example 2

Preparation of a manganese oxide on magnesium oxide catalyst The starting material was magnesium oxide extrudates supplied by

Engelhard, De Meern (Mg-0601, T 1/8"). The extrudates were kept at 900°C for 20 hours to remove sulfur and graphite. After this the material had a thermostable BET surface of 6 m² per gram.

After evacuation for 30 minutes manganese oxide was applied to the carrier by pore volume impregnation with a manganese(II)nitrate solution. After impregnation the carrier was dried for 16 hours at 80°C, after which the loaded carrier was calcined at 550°C. The temperature was increased by 300°C per minute, and the loaded carrier was kept at 550°C for 5 hours. The loading of the carrier was 12.2 wt.% Mn2O₃; the material showed the X-ray diffraction pattern of Mg₂MnO₄ and MgβMnOβ.

Example 3

Application of platinum to the catalysts of Examples 1 and 2

For applying platinum to the catalysts prepared according to Examples 1 and 2 the starting material was a solution of platinum hydroxide supplied by Alfaproducts. The platinum was apphed by pore volume impregnation with a solution of platinum hydroxide. After the impregnation the material was dried for 16 hours at 80°C and then calcined at 550°C for 5 hours.

In both catalysts the loading with platinum was 3 wt.% as metallic platinum.

Example 4

Reaction of the catalysts prepared according to Examples 1, 2 and 3 with methane. The catalysts were placed in a vertically disposed cylindrical reactor, through which a stream of methane of approximately 100 ml/min was passed. The temperature of the catalyst was increased by 5°C per minute to 900°C, after which it was cooled to room temperature and after changing the gas stream the catalyst was reoxidized. During the reoxidation the temperature was also increased by 5°C per minute

It was found that in contact with methane the catalyst of Example 1 reduced in two steps at 600 and 700°C. During the reduction the manganese reacted to MnO Reoxidation of the manganese led to M 0₄; although oxidation to M Oβ should proceed thermodynamically, the oxidation stuck at Mn₃Oι. When platinum was applied to the catalyst according to Example 3, the reduction took place at 310°C; the reduction profile showed a shoulder at 300°C. In this case the amount of reactive oxygen in the catalyst is substantially larger. Now Mn2θ₃ is always formed during reoxidation. The catalyst of Example 2 reduces at 630 °C and shows a shoulder at 730°C. In this case the Mg2MnO and MgβMnOβ reduce to MnO. Reoxidation proceeds with difficulty. After the application of platinum according to Example 3 the reduction proceeds at 420°C. Reoxidation now very smoothly leads to Mg₂MnO and MgβMnOβ. It is clear that through the application of platinum both the reduction and the reoxidation are highly accelerated and that the amount of oxygen available for reduction is larger.

In the catalysts according to Example 3 large amounts of hydrogen were formed at the moment that the reduction of the manganese oxide was nearly completed. Carbon deposition took place on the catalyst. With addition of steam to the methane stream, the deposition of carbon was fully suppressed. The gas mixture flowing out of the reactor contained large amounts of hydrogen and carbon monoxide.

Example 5

Recirculation of the catalysts according to Example 3

The catalysts according to Example 3 were placed in a reactor arrangement shown in Figure 1. Here it was found that a gas stream only consisting of carbon monoxide, hydrogen and steam flowed out of the right-hand reactor.

Claims

1 . A method for oxidizing hydrocarbons which comprises contacting said hydrocarbon with a solid substance comprising at least one metal oxide reducible by said hydrocarbon, under conditions that the hydrocarbon is oxidized and the metal oxide is reduced, which oxidation takes place in the co-presence of nickel and/or at least one noble metal in the solid substance.

2. A method according to claim 1, which comprises reacting hydrocarbon and steam in the presence of the reduced solid substance.

3. A method according to claim 2, wherein the reaction mixture containing the oxidized hydrocarbon, supplemented with hydrocarbon, is passed over said reduced solid substance.

4. A method according to claims 1-3, wherein the product of the reduction of the metal oxide is reoxidized by means of molecular oxygen or steam in the absence of hydrocarbon.

5. A method according to claims 1-4, wherein platinum, palladium, rhodium or ruthenium is used as noble metal.

6. A method according to claim 4 or 5, wherein said reoxidation takes place in the same reactor as that in which the reduction has taken place.

7. A method according to claim 4 or 5, wherein said reoxidation takes place in another reactor than in which the reduction has taken place.

8. A method according to claim 4 or 5, wherein reduction and reoxidation take place in a rotary reactor.

9. A method according to claims 1-8, wherein the metal of the metal oxide is selected from the metals of groups IB, IIB, VB, VIB, VIIB and VIII of the Periodic Table.

10. A method according to claim 9, wherein the metal is selected from the group consisting of manganese, iron and copper.

11. A method according to claims 1-10, wherein the reduction takes place at a temperature of 500 to 1100°C.

12. A method according to claims 1-11, wherein the reoxidation takes place at a temperature of 300 to 1100°C.

13. A method according to claims 1-12, wherein methane is used as hydrocarbon.

14. A method according to claims 1-13, wherein the metal oxide is applied to a carrier.

15. A method according to claims 1-14, wherein the nickel and/or noble metal is apphed to a carrier, preferably selected from the group of magnesia, alumina, zirconia and titania.

16. A method according to claims 1-15, wherein the nickel and/or noble metal is applied to the reducible metal oxide.