MXPA00008106A - Use of a membrane reactor for hydrogen production via the direct cracking of hydrocarbons - Google Patents

Abstract

A process for producting substantially pure hydrogen by contacting a stream of a hydrocarbon gas with a nickel containing catalyst in a membrane reactor. The membrane reactor combines a hydrogen permeable membrane and a catalyst capable of producing hydrogen via the direct cracking of hydrocarbons. The stream of a hydrocarbon gas is contacted with the catalyst at a temperature in the range of about 400 to 900°C which results in the conversion of the gas to substantially pure hydrogen, which selectively permeates through the membrane wall.

Description

USE OF A MEMBRANE REACTOR FOR THE PRODUCTION OF HYDROGEN VIA DIRECT PIROLYSIS OF HYDROCARBONS FIELD OF THE INVENTION The present invention is generally concerned with the production of hydrogen, and more specifically the use of a membrane reactor for the production of hydrogen by the direct pyrolysis of a hydrocarbon.

BACKGROUND OF THE INVENTION 7__ Inorganic membranes such as palladium (Pd), palladium-silver (Pd-Ag) and various other alloys have been used in the past to separate hydrogen from other reagents and products in various reactions including hydrogenations and dehydrogenations. Due to the high cost of these membranes, a great effort has also been directed during the last years to the development of composite and alloy membranes. Membranes of this type consist of a thin film of palladium (which provides permselectivity) deposited on a porous or non-porous support that provides the required mechanical strength. A special type of membrane has been developed by Buxbaum et al. (J. Membr. Sci., 85.29 (1993) and patents • North American No. 5,149,420 and 5,215,729. This membrane takes advantage of the fact that several refractory metals such as niobium (Nb), zirconium (Zr) and vanadium (V) are an order of magnitude more permeable to hydrogen than palladium and have an acceptable mechanical strength. A non-electrode deposition technique was used to deposit a thin palladium film (1-2 microns thick) on the surface of the refractory metals. The membranes prepared in this way and particularly Pd-Nb and Pd-Ta ensure a high purity of the extracted hydrogen and are capable of permeating larger amounts of hydrogen than the pure palladium membranes. In addition, they are more resistant and more durable, and can be used at higher temperatures. The ability to produce hydrogen via direct pyrolysis of methane and other suitable hydrocarbons has been previously demonstrated. The details of this process are described in a separate invention disclosure entitled "Hydrogen Production via the Direct Cracking of Hydrocarbons". The methane pyrolysis reaction, however, is limited by thermodynamic equilibrium. In addition, kinetic experiments suggest that the rate of reaction inhibited by the -hydrogen product. -For both purposes, it would be beneficial to eliminate the hydrogen produced during the reaction of the reaction zone.

BRIEF DESCRIPTION OF THE INVENTION In a separate invention disclosure it has been demonstrated the feasibility of producing hydrogen via direct pyrolysis of methane on a Ni nickel containing catalyst in a conventional fixed bed reactor. The efficiency of such design, however, is adversely affected by the presence of hydrogen in the reaction zone. The aforementioned disclosure of the prior art suggests that the use of a membrane reactor could improve the efficiency of the system for catalytic methane pyrolysis by effectively removing the hydrogen from the reaction zone. It is therefore an object of the present invention to efficiently produce high purity hydrogen by the catalytic pyrolysis of hydrocarbons. It is another object of the present invention to provide a method for producing hydrogen by direct pyrolysis of hydrocarbons using a membrane reactor. It is another object of the present invention to provide a method for producing substantially pure hydrogen without carbon monoxide contamination by the direct pyrolysis of hydrocarbons in a membrane reactor using a nickel-containing catalyst. It is still yet another object of the present invention to provide a method for producing high purity hydrogen by direct pyrolysis of methane at low temperatures using a nickel-containing catalyst supported on silica in a membrane reactor.

The present invention overcomes the problems described above and demonstrates the feasibility of producing substantially pure hydrogen by the direct pyrolysis of hydrocarbons by the use of a membrane reactor. A membrane reactor can remove hydrogen from the reaction zone and therefore eliminate its negative effects on both, the reaction equilibrium and the reaction rate. As a result, the use of a membrane reactor can significantly increase the efficiency of the hydrogen production process. The membrane can be of any type of material that is selectively permeable only to hydrogen, and can thus effectively separate hydrogen from carbon monoxide and other components of the reaction mixture (e.g., unreacted hydrocarbons, carbon dioxide, steam, etc.). water, etc). The invention has been demonstrated with a type of Pd-Nb membrane, which is believed to have certain advantages discussed in the background section of this application. This, however, in no case excludes the use of other membranes either of the compound ceramic or mixed ion type. In one embodiment, the membrane preferably comprises Pd-Nb. The invention may be applicable to pyrolysing any suitable hydrocarbon such as methane, natural gas, ethane, ethylene, propane, propylene, butane, pentane, hexane or mixtures thereof, and hydrocarbons with molecular weights in the range of gasoline and diesel . The membrane reactor uses a catalytic bed which preferably comprises a nickel-containing catalyst supported on a silica support. The hydrogen produced in the reaction zone permeates selectively through the wall of the membrane and is carried out by a sweeping gas. In operation, the reactor typically operates at a temperature in the range of about 400 to 900 ° C.

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the nature and objects of the invention, reference should be made to the following detailed description of a preferred mode of practicing the invention, read in connection with the accompanying drawings, in which: 1 is an enlarged side sectional view of the catalytic reaction zone of the double tubular catalytic membrane reactor. Figure 2 depicts a graph of a comparison of the methane conversion on 0.2 grams of a Ni / SiO2 catalyst of 16% by weight at 550 ° C in a conventional fixed bed reactor and the reactor of Figure 1. Figure 3 represents a graph of the comparison of methane conversion on 0.2 grams of a Ni / S02 catalyst at 16% by weight at 7600 h "1 in a conventional fixed-bed reactor and the reactor of Figure 1. The figure 4 is an enlarged side sectional view of the catalytic reaction zone of an alternative design of a fixed bed catalytic reactor having a membrane separator.

DETAILED DESCRIPTION OF THE INVENTION The invention was demonstrated with the double tubular catalytic membrane reactor (10) shown in Figure 1. The Pd-Nb membrane tube used has an outside diameter of 9.525 (3/8 inch) and a thickness 0.25 mm wall and was prepared according to the procedures described in the corresponding patents (US 5,149,420 and 5,215,729) covering its manufacture and use, which are incorporated herein by reference. The reactor consists of an inner membrane tube (12) and a stainless steel or outer quartz tube (14) defining a flow passage (16). A catalytic bed (18) is located inside the inner tube (12). An electric heater (20) controls the reaction temperature. The hydrogen produced in the reactor zone permeates selectively through the wall of the membrane and is carried out by the sweep gas indicated by the dashed arrows. The outer tube (SS, 1 inch OD, 0.028 inches thick) is directly connected to a sweep gas supply (not shown) the membrane occupies the center section of the inner tube and was connected to the inlet and outlet of the reactor with appropriate unions. The catalyst (16% by weight of Ni / S02) was packed into the membrane tube and the produced hydrogen was purged with an inert scavenging gas such as argon on the side of the shell. The additional hydrogen that is also leaving the reactor in the lower part of the catalyst bed, as indicated by the arrows of solid lines. In a typical experiment, 0.2 grams of the catalyst (25-35 mesh) were uniformly diluted in 0.3 grams of inert silica (25-35 mesh) and subsequently packed in the middle section of the membrane tube. The reactor was purged with inert gases and heated to the reaction temperature. The flow rate of the scanning argon was kept constant at 150 cm3 / minute. A feed indicated by solid arrows from the top that consisted of 20% CH4 in He was introduced into the reactor to initiate the reaction. The exit currents of both the scanning and reaction sides were analyzed by gas chromatography. It should be understood that the current invention is not limited to the specific configuration presented in Figure 1. Indeed, any other configuration that effectively contains a catalyst capable of producing hydrogen via direct pyrolysis of hydrocarbons and a membrane that is selectively permeable only Hydrogen can be incorporated in the current invention. The configuration of Figure 1 was selected for the demonstration of the invention due to its simplicity. Another example of a membrane reactor configuration suitable for use in the present invention is shown in Figure 4, wherein a fixed bed catalytic reactor or fuel processor is equipped with a membrane separator. The fuel processor, which uses direct pyrolysis, converts the hydrocarbon feed to hydrogen and carbon products and, with the membrane separator, selectively extracts hydrogen to produce an essentially pure hydrogen product. The membrane separator reactor 30 illustrated in Figure 4 includes a bundle of tubes 32 of metal or metal alloy membrane, up to 50 or more small hollow tubes or tubes sealed at one end 34 and open at the opposite end 36. The tubes are surrounded by a bed of catalytic material 38 and all together contained within the outer shell 40 of the reactor. In operation, the hydrocarbon gas is fed through the inlet port 32 and is pyrolyzed in the inner chamber 44. The hydrogen formed from the pyrolysis permeates the porous membrane tubes 32 selectively and travels to the outlet port 38 (see small arrows), while carbon monoxide, other reaction products and unreacted hydrocarbons exit through port 46. As can be seen from the previous description, the hydrocarbon gas feed enters one end of the reactor, passes through the catalytic bed and the reaction products and the unreacted hydrocarbons exit at the other end. In the membrane separator, the flow of hydrogen goes from the outside to the inside of the membrane tubes. The essentially pure hydrogen flux of all the membrane tubes are combined in a common collector 50, and collected in the output port 48. Other alternative reactor configurations for this type of reactor in commercial operations may include moving bed or fluidized bed reactors. The catalyst used in the present invention will eventually be deactivated as a result of the deposition of carbon. The carbon deposited on the catalyst can be recovered and used in electrochemical applications (superconductors, electrodes and fuel cells) or fuel storage applications. Alternatively, the deactivated catalyst can be completely regenerated by oxidation in air or vapor gasification of the deposited carbon. Figure 2 compares the methane conversions obtained from a conventional fixed bed reactor and the reactor of Figure 1 at 550 ° C at different space velocities. The CH conversion in the conventional fixed-bed reactor ranged from 31.7% at a space velocity of 60,000 h-1 to 42.2% at 7,500 h "1. In the membrane reactor the CH4 conversion increased from 37.2% at 60,000 h "1 to 70.8% at 7,500 h" 1. The difference (and hence the benefits of using the membrane reactor) was more significant at low space velocities, because the negative effects of hydrogen are more pronounced under These are conditions.The hydrogen permeability of the Pd-Nb membrane used in this demonstration was measured at different temperatures and found to increase with temperature.Thus, the hypothesis was made that the observed methane conversions may benefit the most. By using the membrane reactor at higher temperatures, this was actually demonstrated experimentally as shown in Figure 3, for temperatures up to 550 ° C. At even higher temperatures, the difference in the efficiency of the two types of reactor probably decreased due to the deterioration of the membrane under these conditions. While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawings, it will be understood by one skilled in the art that various changes in details may be made therein without departing from the spirit and scope of the invention. the invention defined by the claims.

Claims (12)

1. CLAIMS 1. A process for producing substantially pure hydrogen, characterized in that it comprises: (a) providing a membrane reactor that includes a hydrogen permeable membrane and a catalytic bed; (b) contacting a stream of a hydrocarbon with the catalyst at a temperature in the range of about 400 to 900 ° C, which results in the conversion of the gas to substantially pure hydrogen, which selectively permeates through the membrane wall. 2.' The process in accordance with the claim 1, characterized in that the membrane is selectively permeable only to hydrogen and effectively separates the hydrogen from the carbon monoxide and other components of the reaction mixture. 3. The process in accordance with the claim 1, characterized in that the membrane comprises a metal or metal alloy. 4. The process according to claim 1, characterized in that the membrane comprises Pd or a Pd alloy. 5. The process in accordance with the claim 1, characterized in that the membrane comprises Pd-Nb. The process according to claim 1, characterized in that the membrane comprises a composite or ceramic-type membrane. 7. The process according to claim 1, characterized in that the catalyst has nickel and is supported on a silica support. 8. The process according to claim 1, characterized in that the catalyst contains at least 5% by weight of nickel. The process according to claim 1, characterized in that the hydrocarbon gas is one selected from the group consisting of methane, natural gas, ethane, ethylene, propane, propylene, butane, pentane, hexane or mixtures thereof and hydrocarbons with molecular weights in the range of gasoline and diesel. 10. The process in accordance with the claim 1, characterized in that the hydrocarbon gas is mixed with an inert carrier gas. 11. The process according to claim 1, characterized in that the catalyst is regenerated, by oxidation of the carbon deposited in air. 12. The process according to claim 1, characterized in that the catalyst is regenerated by vapor gasification of the deposited carbon.