Chemistry of inorganic compounds – Hydrogen or compound thereof – Elemental hydrogen
Reexamination Certificate
2000-02-08
2001-11-13
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Hydrogen or compound thereof
Elemental hydrogen
C208S074000, C208S121000, C422S198000, C422S198000, C422S211000, C423S44500R, C423S447300
Reexamination Certificate
active
06315977
ABSTRACT:
BACKGROUND OF THE INVENTION
(i) Field of the Invention
The present invention relates to the field of the production of hydrogen by the thermocatalytic decomposition (cracking) of hydrocarbons.
(ii) Description of the Related Art
It is known that hydrogen production, in particular on-site, is a major field, especially because of a growing demand in the industrial market for hydrogen, for example for heat-treatment applications.
Many studies have been published in recent years on this subject, particularly studies using catalysts consisting of nickel powder supported on silica (SiO
2
).
In particular, the following documents may therefore be referred to:
the article by Zhang et al., published in “Applied Catalysis A,” 1998, vol. 167, pp. 161-172;
the article by Muradov, published in “energy and Fuels,” 1998, vol. 12, pp. 41-48;
the article by Chen et al., published in the journal “carbon,” 1997, vol. 35, pp. 1495-1501;
the article by Poirier et al., published in “International Journal of Hydrogen Energy,” 1997, vol. 22. pp. 429-433; or
the article by Steinberg, published in “International Journal of Hydrogen Energy,” 1998, vol. 23, pages 419-425.
The mechanism for hydrogen production under such conditions, which is most commonly accepted in the literature, thus seems to be the adsorption of the hydrocarbon molecule on the surface of a catalyst particle (for example a nickel particle supported on porous silica) followed by the successive dehydrogenation of the hydrocarbon (for example going from CH
4
to CH
3
, then CH
2
, then CH), in order to end in a carbon atom adsorbed on the surface of the catalyst. This carbon then travels, by thermal diffusion, through the catalyst particle in order to form what are called carbon “nanotubes” or “nanofilaments,” a phenomenon allowing the catalyst to be active for a much longer time (the metal surface remains free, accessible and active for a longer time).
It is known that the phenomenon of nanotube formation depends especially on the size of the catalyst particles, on the metal content of the catalyst and on the porosity of the material serving as support for the metal.
This literature can therefore be rapidly summarized by the fact that it has demonstrated the feasibility of the cracking reaction on such catalysts and the activation of the reaction between approximately 550 and 800° C., the fact that maintaining the activity of the catalyst depends essentially on forming these carbon nonofilaments, and that specifically this reaction becomes deactivated when there is so not enough space for these filaments to grow, therefore resulting in the need to regenerate the catalyst, for example by an air flush.
Thus, although all this literature presents the direct catalytic cracking of hydrocarbons as a very promising avenue to explore for the purpose of producing hydrogen (especially on-site), developments undeniably remain to be carried out in order to provide a really industrial process based on this concept, especially when considering the fact that the prior art has obtained all these feasibility results with low gas flow rates and small amounts of catalysts (typically a few milliliters).
Extensive studies by the Applicant have also demonstrated that certain key technical questions still need to be addressed:
the problem of carbon deposition after cracking: occurring immediately from the start of cracking, and entailing a significant risk of the reactor becoming blocked (only the first portion of the catalyst is then used);
the difficulties encountered during regeneration of the catalyst: which regeneration is a source of CO and CO
2
, therefore of soot deposition;
the regeneration is, moreover, an exothermic process: the increase in temperature and thus the thermocycling resulting therefrom may throw doubt on the integrity of the material;
short regeneration times have to be obtained, especially when it is hoped to achieve truly industrial conditions.
SUMMARY AND OBJECTS OF THE INVENTION
One of the objectives of the present invention is therefore to propose a response to the technical questions mentioned above.
The process according to the invention, for producing a production gas mixture comprising hydrogen, by thermocatalytic decomposition of an initial mixture which comprises a hydrocarbon or a mixture of hydrocarbons, the decomposition taking place over a catalyst which is capable, on contact with the hydrocarbon, of forming carbon nanotubes, is characterized in that:
at least one succession, which comprises at least one first and one second catalytic reaction zone, these zones being separate within at least two different consecutive reactors or else being consecutive reactors or else being consecutive within the same reactor, is used;
the at least one first and one second consecutive catalytic reaction zone are subjected to an increasing temperature gradient and/or have an increasing metal concentration gradient in the catalyst;
the initial mixture is made to flow into the first catalytic reaction zone so as to form therein a first intermediate mixture which is directed toward the second catalytic reaction zone of the succession, in order to form the required production mixture.
As will have been understood on reading the foregoing, the notion according to the invention of “first zone” and “second zone” of the succession should be understood by considering the direction of flow of gas to be cracked in the succession.
The process according to the invention may also adopt one or more of the following technical characteristics:
the succession comprises at least two catalytic reaction zones, each zone being located in a separate reactor;
the succession comprises at least two catalytic reaction zones positioned consecutively in the same reactor;
after a phase in which the production mixture is produced, the process continues with a phase in which the catalytic reaction zones of the succession are regenerated in the following manner: each of the catalytic reaction zones of the succession are independently and simultaneously flushed with the aid of a regeneration gas (for example an oxidizing gas);
after a phase in which the production mixture is produced, the process continues with a phase in which the catalytic reaction zones of the succession are regenerated in the following manner: each of the catalytic reaction zones of the succession are independently and simultaneously flushed with the aid of an oxidizing regeneration gas, the regeneration gas used differing from one zone to another by the fact that it has a different residual oxygen concentration;
after a phase in which the production mixture is produced, the process continues with a regeneration of the catalytic reaction zones of the succession by independently and simultaneously flushing each of the catalytic reaction zones with the aid of a regeneration gas, each of the reaction zones following the first zone of the succession being regenerated in the following manner: a pipe for feeding a regeneration gas is used for each of the catalytic reaction zones which follow the first zone of the succession, each feed pipe being connected to the line used for directing, toward the zone to be regenerated in question, the intermediate mixture produced by the zone preceding it in the succession (the “first zone” and “the following zones” of the succession will be defined by considering the direction of flow of the gas to be cracked in the succession—the regeneration configuration thus described therefore corresponds to concurrent regeneration with respect to the direction of flow of the gas to be cracked in the succession);
after a phase in which the production mixture is produced, the process continues with a regeneration of the catalytic reaction zones of the succession by independently and simultaneously flushing each of the catalytic reaction zones with the aid of a regeneration gas, each of the reaction zones which precede the last zone of the succession being regenerated in the following manner, a pipe for feeding a regeneration gas is used for each of the catalytic reaction zones whic
Burns Doane Swecker & Mathis L.L.P.
Griffin Steven P.
L'Air Liquide, Societe Anonyme pour l'Etude et l'
Medina Sanabria Maribel
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