MXPA00008105A - Hydrogen production via the direct cracking of hydrocarbons - Google Patents

Abstract

A process for producing substantially pure hydrogen by contacting a stream of a hydrocarbon gas with a nickel or nickel-copper containing catalyst at a temperature in the range of about 400 to 900°C. This results in the conversion of the hydrocarbon gas to substantially pure hydrogen, with said process being carried out until the catalyst is deactivated due to the deposition of carbon on the catalyst. The deactivated catalyst may be regenerated by oxidation in air or by steam gasification of the deposited carbon. The carbon deposited on the catalyst also has separate utility for electrochemical and fuel storage applications and may be recovered for further use.

Description

HYDROGEN PRODUCTION BY HYDROCARBON CATALYTIC PYROLYSIS FIELD OF INVENTION This invention is concerned generally with the production of hydrogen and more specifically with the production of hydrogen by direct catalytic pyrolysis of hydrocarbons, such as methane and natural gas.

BACKGROUND OF INVENTION The significant advance made in fuel cell technologies during the past decade has driven the exploration of a replacement of large traditional power plants with so-called distributed power generators consisting of a hydrogen generator and a plant. of membrane fuel cell power. This latest technology generates electricity in places where it is planned to use and, therefore, eliminates the loss of electricity during its transmission. In addition, a fuel cell process does not emit any environmental contaminants, such as NOx and SOx that are by-products or combustion byproducts. Such a process becomes attractive to the automotive industry, since vehicles can be propelled by electricity produced from an on-board fuel cell power plant instead of an internal combustion engine. The current proton exchange membrane (PEM) fuel cells use hydrogen as the energy source and require an essential removal (ideally less than 20 ppmv) of carbon monoxide from the hydrogen stream to prevent electrode poisoning. catalyst. Hydrogen is normally produced by means of steam reforming, partial oxidation or autothermal reforming of natural gas. However, in all these cases carbon monoxide is a co-product that has to be converted to carbon dioxide in subsequent stages, which adds to the cost of the hydrogen produced. An alternative route is to subject the hydrocarbon fuel to hydrogen and carbon directly to catalytic pyrolysis. In this case the formation of carbon oxides is avoided and the need for downstream reactions, such as water-gas displacement and selective oxidation for the conversion of carbon monoxide to carbon dioxide is eliminated. Surprisingly, this procedure has not been studied extensively. While there are commercial processes that use the thermal catalytic pyrolysis of methane at extremely high temperatures for the production of acetylene and carbon black, the production of hydrogen via methane catalytic pyrolysis has only been briefly considered in the past. In the North American patent 3,361,535 the catalytic pyrolysis at high temperature of methane is taught. The process taught by the 3,361,535 patent, however, results in the production of the undesirable carbon monoxide co-product which requires an additional elaborate process for its conversion to carbon dioxide and results in additional cost. Recently, Muradov Int. J. Hydrogen Energy 18, 211 (1993), studied the use of iron and nickel oxides supported on alumina as catalysts for catalytic methane pyrolysis and reports that equilibrium conversions are obtained at temperatures greater than 800 °. C. It is appreciated that the iron oxide maintains some of its activity for several hours, in contrast to a Pt / Al203 catalyst that was deactivated in minutes under similar conditions. Muradov in Energy & Fuels 12, 41 (1998) has also reported the use of carbon-based catalysts for the same reaction. Although they are more stable, these catalysts exhibit low activity. In addition, Ishihara et al, in Shokubai 35, 324 (1995) and Chem. Lett. , 93 (1995); reports that methane catalytic pyrolysis takes place at low temperatures over a 10% Ni / SiO2 catalyst that is not deactivated even after approximately 200 carbon atoms per nickel atoms have been deposited thereon. However, the results reported by Ishihara did not demonstrate the level of hydrogen production efficiency that would result in potential commercial use.

BRIEF DESCRIPTION OF THE INVENTION Accordingly it can be seen from the prior review of the prior art that an efficient method for the direct catalytic pyrolysis of hydrocarbons to produce hydrogen without the presence of undesirable co-products, such as carbon monoxide, has been a objective in the technique. Accordingly, it is an object of the present invention to provide a method for producing hydrogen by the direct catalytic pyrolysis of hydrocarbons. It is another object of the present invention to provide a method of producing pure hydrogen without the contamination of carbon monoxide by direct catalytic hydrolysis of hydrocarbons. It is still another object of the present invention to produce hydrogen of high purity and carbon by the catalytic pyrolysis of hydrocarbons. It is still a further object of the present invention to provide a method of hydrogen production by direct catalytic hydrolysis of methane by the use of a highly efficient catalyst. It is another object of the present invention to provide a method of hydrogen production by direct catalytic pyrolysis of methane or natural gas at low temperatures using a nickel-containing catalyst. It is still a further object of the present invention to provide a method of producing highly pure hydrogen by direct pyrolysis of methane at low temperatures using a nickel-copper catalyst supported on silica. It has been found that catalytic pyrolysis of methane or natural gas as a potential route for the efficient production of hydrogen can be carried out on silica-containing nickel-supported catalysts. In one embodiment, activity measurements were performed for the catalytic pyrolysis reaction of the methane with a 16.4% by weight Ni / SiO2 catalyst in a stream of 20% CH in He at 500 ° C and a space velocity per hour (GHSVE) of 30000h_1. Under these conditions the catalyst exhibits a high initial activity for the catalytic pyrolysis of methane (approximately 35% conversion of CH4). Hydrogen was the only gaseous product detected. In addition, it was found that the conversion ratios of methane and hydrogen formation are in a ratio of 1: 2, thus verifying the stoichiometry of the reaction for methane pyrolysis. The amounts of carbon deposited in the spent catalyst and the methane that reacted indicate a good approach to the rest of the carbon (100 + 5%). After deactivation of the catalyst due to carbon deposition; the activity of the catalyst can be fully restored by regeneration of the catalyst by means of oxidation in air or gasification by saturated steam. The process of the invention may be applicable to any other suitable hydrocarbon, such as ethane, ethylene, propane, propylene, butane, pentane, hexane and mixtures thereof and hydrocarbons with molecular weights in the range of gasoline and diesel. However, it is anticipated that the preferred hydrocarbons will be methane and natural gas. During the catalytic pyrolysis of higher molecular weight hydrocarbons, it is expected that several other undesirable products will be formed in addition to hydrogen. In a second embodiment, the activity measurements for the catalytic methane pyrolysis reaction were carried out in a set of 9 Ni-Cu / SiO2 catalysts in which the total amount of metal (on a molar basis) was kept constant to 2.6 millimoles of metal / g of support, while the ratio of Ni: Cu was varied from about 8: 1 to about 1: 8. The reaction was carried out in a stream of pure methane at 650 and 800 ° C and at a space velocity per hour of the gas of 6000 h "1. The results indicate that the presence of small amounts of Cu significantly improves the activity of Ni to 800 ° C. The initial conversion with respect to the composition of 2.3 millimoles of Ni / 0.3 millimoles of Cu / SiO2 for example, was measured at 63%, compared to 14.4% for the composition of 2.3 millimoles of Ni / SiO2. This is a surprising result, given that Cu alone is not active for the catalytic pyrolysis of methane under these conditions (0.3% initial methane conversion for the composition of 0.3 millimole Cu / SiO2). The promoter effect is also more pronounced when small amounts of Cu are added (that is, proportions of Ni: Cu greater than 1). The highest initial methane conversion observed with this set of catalysts is in the 8: 1 ratio of Ni: Cu. Even higher initial methane conversions are expected with the proportions of Ni: Cu even higher up to about 20: 1.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the nature and objects of the invention, reference should be made to the following detailed description of a preferred mode of implementing the invention, read in conjunction with the accompanying drawings in which : Figure 1 represents a graph of deactivation of a Ni / Si02 catalyst at 550 ° C and GHSVE = 30,000h_1 in a stream containing 20% CH in He; Figure 2 represents a graph of the • or p methane conversions obtained on new (•,) and regenerated NIE / S02 catalyst (O in air, or saturated steam) in air at 550 ° C at two different space velocities; Figure 3 represents a graph of Hydrogen selectivities waves (• to GHSV = 1500 h "1, to GHSV = 37500 h" 1) and carbon dioxide (O to GHSV = 15000 h "1, U to GHSV = 37500 h_1) during regeneration steam Ni / Si02 deactivated at 550 ° C and figure 4 shows a graph of the initial conversion of methane as a function of the composition of Eatalizador at two different temperatures over a series of catalysts of Ni-Cu / Si02 (O at 650 ° C and Q at 800 ° C).

DETAILED DESCRIPTION OF THE INVENTION The catalyst used in the first embodiment of this invention was prepared by impregnation of incipient moisture from an aqueous solution of nickel nitrate on the silica support, followed by calcination in ai e and reduction in situ in flowing hydrogen. This is a standard method of preparing metal-supported catalysts and several different nickel salts can be used in place of nickel nitrate as the nickel precursor. In addition, other standard methods for the preparation of supported metal catalysts could be used without having a detrimental effect on the properties of the catalyst. In addition to silica, other inorganic supports such as alumina and titania have been investigated. Although it was found that the nickel supported in these supports was effective for the catalytic pyrolysis of methane, the performance of the nickel supported on silica was superior to those of the other catalysts and therefore, this system was chosen to demonstrate the invention in this application. In addition, the performance of other transition metals such as Co and Fe, supported on silica for this reaction was examined. Although it was found that these catalysts are also effective for the reaction at 550 ° C, the performance of the nickel was again higher than the other catalysts. Finally, when examining several Ni / Si02 catalysts of variable Ni content, it was determined that the optimum performance for the catalytic pyrolysis of methane can be obtained with a nickel content of more than 5% by weight and in particular a content of about 16% by weight. As a result, a Ni / Si02 catalyst at 16% by weight was chosen to demonstrate the invention in this application. When the catalyst was placed in a reactor of conventional fixed bed and exposed to a stream containing 20% CH4 (by volume) in He at 550 ° C and under a GHSV of 30,000 h "1, a high initial activity was observed for the catalytic pyrolysis of methane (approximately 35% conversion of CH4) Hydrogen was the only gaseous product detected and it was found that the proportions of methane consumption and hydrogen production were in a ratio of 1: 2, thus verifying the The stoichiometry of the reaction for methane pyrolysis The catalyst used in the present invention will inevitably deactivate as a result of carbon deposition.Carbon can be deposited on the surface to cover the active sites (site blocking) or accumulate in the pore entry to additionally block the access of the reagents to the interior (pore mouth plugging). It has been estimated that in both cases the catalytic deactivation ica would be presented in a short period of time. Although if you need 10 carbon atoms to block each surface of Ni atom, for example, 11 mg of carbon deposition would be sufficient to completely deactivate one gram of the Ni / Si02 catalyst at 16.4%. In addition, if the plugging of the pore's mouth was the main deactivation mechanism, approximately 250 mg of carbon would be sufficient to plug the external 10% of the pores, in one gram of the Ni / SiO2 catalyst sample. It has been found that a significantly higher amount of carbon deposition on Ni / SiO2 catalysts occurs before deactivation occurs. For example, at a temperature of 550 ° C, a very slow deactivation of the Ni / SiO2 catalyst is observed for the first two hours (Fig. 1) followed by a more rapid loss of activity in the third hour. At the time when the catalyst was completely deactivated (200 minutes), approximately 0.59 g of carbon had accumulated on the 0.2 g of the Ni / Si02 catalytic sample. This amount is in good agreement with the amount of carbon calculated based on the integration of the methane conversion (0.61 g) and corresponds to approximately 2700 carbon atoms accumulated on the catalyst per nickel atom surface. It is therefore evident that the capacity of the nickel catalyst supported on silica to accommodate carbon is significantly higher than that predicted by either the site blocking or the pore mouth plugging models. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis of spent catalysts were used to better understand the deactivation mechanism. SEM micrographs indicate the formation of filamentous carbon on the catalytic surface. It is appreciated that these filaments grow out of the silica support surface, their length increases with time in the stream. Each filament had a bright tip, identified by the use of SEM / EDS (Energy Dispersion X-ray Spectroscopy) as a nickel particle. Exhausted catalytic samples were further studied by the use of X-ray diffraction (XDR). The XRD configurations suggest that graphite carbon constituents with different degrees of defect or distortion are present in the deactivated samples. The MET micrographs of the fully deactivated sample show that carbon growth is terminated as a result of spatial limitations. The modes of termination of the filament include the restriction of nickel particle by the silica surface, the arm and the tip of another carbon filament. The formation of carbon filaments as a result of catalytic hydrocarbon pyrolysis has been reported extensively in the literature with higher molecular weight hydrocarbons over supported nickel catalysts, iron, cobalt and various alloy catalysts. The carbon deposited on the catalyst in the embodiment of the present invention can be used in electrochemical applications such as superconductors, electrodes and fuel cells. The reversibility of filament growth and catalyst regeneration has previously been considered by others, but no results have been reported regarding whether the activity of the catalyst can be restored. Indeed, some workers have suggested that regeneration of the catalyst may be useless in view of the changes caused to the catalyst support structure as a result of the filament growth process. Conventional oxidation in air (C + 02 = C02) and saturated vapor gasification (C + xH20 = Cox + xH2) were considered as potential routes for the regeneration of the catalyst at 823 K. Surprisingly, it can be seen that both methods are suitable to fully restore catalyst activity, as shown in Figure 2, where the methane conversion is plotted against the current time for the new and regenerated catalyst. The oxidation process was faster than gasification with saturated steam, but it caused a high temperature front. This front moves gradually through the catalytic bed, causing the collapse of the sample to a fine powder. The XRD analysis suggests that the oxidation process completely eliminates the deposited carbon and converts the metallic nickel to nickel oxide that has to be reduced in fluid hydrogen before the next reaction cycle shown in Figure 2. On the contrary, the catalytic bed maintained a uniform temperature profile during the regeneration process with saturated steam and the catalyst retained its metallic nickel shape at the end of the process. Another difference between the two regeneration methods is that gasification with saturated steam leads to the production of additional hydrogen. This is at the expense of external thermal energy, since carbon oxidation releases a large amount of heat. However, the additional production of hydrogen may be of significant practical importance. The theoretical yields of four and three moles of hydrogen per mole of methane can be obtained, respectively, with steam reforming and partial oxidation of methane, provided that all carbon monoxide is converted to carbon dioxide by the displacement reaction water-gas. In comparison, the catalytic pyrolysis of methane produces less hydrogen (two moles) of each mole of methane. However, the hydrogen yield can be dramatically improved if the hydrogen produced during the regeneration step is also considered. In this case, two moles of hydrogen were obtained from each mole of methane during the pyrolysis step and an additional 1.4 moles were produced during the subsequent gasification with saturated steam from the deposited carbon, leading to an overall hydrogen yield of 3.4 moles per mole of methane. This overall hydrogen yield is slightly lower than that of steam reforming, but better than for partial oxidation.

The formation of carbon monoxide and methane is also detected during the initial stage of the regeneration with saturated steam, where, as shown in Figure 3, the selectivities of carbon dioxide and hydrogen are less than 100%. It is noted that the increased selectivities of carbon dioxide and hydrogen, obtained when regeneration is carried out at a higher space velocity, suggest that carbon monoxide and methane may be side products, formed due to water displacement reactions. -gas reversed (H2 + C02 = CO + H20) and methanation (CO + 3 H2 = CH + H20). These observations suggest that the ability to efficiently control the selectivity of hydrogen during the regeneration stage should be had by adjusting the space velocity. TEM micrographs of the sample regenerated with saturated steam show that some residues of large filaments are still present on this catalyst after the regeneration has been consummated. The presence of these residues, however, does not seem to have a negative effect on the catalytic activity that is fully restored immediately after the steam regeneration stage (Fig. 2) However, after some steam deactivation and regeneration cycles, an oxidation stage with air will probably be required to completely eliminate the accumulated debris. The set of Ni-Cu / SiO2 catalysts used in the second embodiment of this invention had the total amount of metal (in a molar basis) kept constant at 2.6 millimole metal / g support, while the Ni ratio: Cu was varied from about 8: 1 to about 1: 8. The catalysts were prepared by impregnation of incipient moisture impregnation of nickel and copper nitrate (Ni (N03) 2x6H20 and Cu (N03) 2x2.5H20) obtained from Aldrich (with a purity of 99.999%) on commercially available Si02 (Davison Syloid 74 ). Before impregnation, the silica support was pressed, pressed into pellets under a pressure of 1054 Kg / cm2 (15,000 psig), crushed and sieved to obtain a granulometric fraction of mesh size 20-35. The impregnated samples were dried in a vacuum oven at 120 ° C overnight and subsequently calcined in an oven at 700 ° C for 6 hours. The Ni and Cu loads were estimated by the weight difference between the blank support and the catalyst reduced overnight in a 1: 2 mixture of H2 / N2 (total flow rate of 120 ml / minute) at 650 ° C. . Following the reduction treatment, the samples were exposed to methane (GHSVE = 6000 h "1) at 650 and 800 ° C. Activity measurements were carried out at two different temperatures and the results are presented in figure 4. The results indicate that the presence of small amounts of Cu significantly improved the initial activity at 800 ° C, while the presence of Cu did not have any significant effect at 650 ° C. The initial conversion on the composition of 2.3 mmoles of Ni / 0.3 mmoles of Cu / Si02, for example, was measured at 63%, compared with 14.4% for the composition of 2.3 millimoles of Ni / SiO2 This is a surprising result since Cu alone is not active for methane pyrolysis under these conditions ( 0.3% initial methane conversion for the composition of 0.3 millimoles of Cu / SiO2.) The promoter effect is also more pronounced when small amounts of Cu are added (that is, Ni: Cu ratios greater than 1). 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 detail can be made therein without deviating from the spirit and scope of the invention. the invention as defined by the claims.

Claims (19)

1. CLAIMS 1. A process for producing hydrogen, characterized in that it comprises contacting a stream of hydrocarbon gas with a catalyst containing nickel at a temperature in the range of about 400 to 900 ° C which results in the conversion of such gas to hydrogen, such a process is carried out until the catalyst is deactivated due to the deposition of carbon on such a catalyst.
2. 2. The process in accordance with the claim 1, characterized in that the catalyst contains at least 5% by weight of nickel.
3. 3. The process according to claim 1, characterized in that the nickel-containing catalyst is supported on an inorganic support.
4. 4. The process according to claim 1, characterized in that the nickel-containing catalyst is supported on silica.
5. 5. The process according to claim 1, characterized in that the catalyst also contains copper.
6. 6. The process according to claim 5, characterized in that the nickel: copper ratio varies from 20: 1 to 1: 8.
7. 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, and mixtures thereof and hydrocarbons with molecular weights in the range of gasoline and diesel.
8. 8. The process in accordance with the claim 1, characterized in that the deactivated catalyst is regenerated by oxidation in air.
9. 9. The process according to claim 1, characterized in that the deactivated catalyst is regenerated by gasification with saturated steam of the deposited carbon.
10. 10. The process in accordance with the claim I, characterized in that the hydrocarbon gas is mixed with an inert gas carrier.
11. 11. A process for producing hydrogen, characterized in that it comprises contacting a stream of a hydrocarbon gas with a catalyst containing nickel at a temperature in the range of 400 to 900 ° C, which results in the conversion of such gas to hydrogen and wherein such hydrocarbon gas is selected from the group consisting of methane and natural gas.
12. 12. The process in accordance with the claim II, characterized in that the catalyst contains at least 5% by weight of nickel.
13. 13. The process according to claim 11, characterized in that the nickel-containing catalyst is supported on an inorganic support.
14. The process according to claim 11, characterized in that the nickel-containing catalyst is supported on silica.
15. 15. The process according to claim 11, characterized in that the catalyst also contains copper.
16. 16. The process according to claim 14, characterized in that the nickel: copper ratio varies from 20: 1 to 1: 8.
17. 17. The process according to claim 11, characterized in that the catalyst is regenerated by oxidation in air.
18. 18. The process in accordance with the claim 11, characterized in that the catalyst is regenerated by gasification with saturated steam of deposited carbon.
19. 19. The process according to claim 11, characterized in that the hydrocarbon gas is mixed with an inert carrier gas.