Mineral oils: processes and products – Refining – Sulfur removal
Reexamination Certificate
2000-07-21
2004-04-20
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Refining
Sulfur removal
C208S211000, C208S212000, C208S217000, C208S226000, C208S299000, C502S020000, C502S034000, C502S053000
Reexamination Certificate
active
06723230
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for regenerating iron-based hydrogen sulfide sorbents comprising treating a spent iron-based hydrogen sulfide sorbent with steam. In a preferred embodiment, the iron-based sorbent is first contacted with steam, preferably mixed with at least one of a hydrogen gas and an inert gas, such as nitrogen, followed by contacting with hydrogen under regeneration conditions.
BACKGROUND OF THE INVENTION
The removal of sulfur from feedstocks is a fundamental process of the refining and petrochemical industries. One process for removing sulfur from a feedstock is hydrodesulfurization. Hydrodesulfurization involves the reaction of sulfur in the feedstock with hydrogen over supported noble metals, such as Pt, Pd, or supported non-noble metal catalysts, especially Co/Mo and Ni/Mo catalysts, at fairly severe temperatures and pressures thereby forming hydrogen sulfide.
The performance of the hydrodesulfurization catalysts, especially those containing a noble metal, can be inhibited by the presence of hydrogen sulfide. The use of sorbents to remove hydrogen sulfide produced during desulfurization improves the effectiveness of the overall hydrodesulfurization process.
The performance of a hydrogen sulfide sorbent depends on a variety of properties. Thermodynamics and kinetics of sulfidation clearly are important, because they determine the overall sulfur capacity before breakthrough at some predetermined level of hydrogen sulfide. Other important sorbent properties include stability and regenerability in extended use, the operating conditions required for regeneration, and the composition of the regeneration off-gas, which largely determines the choice of a downstream sulfur recovery process. A practical limitation on the use of any hydrogen sulfide sorbent is the ability to regenerate the sorbent. Zinc oxide, one of the most promising and widely studied sorbents, has a very high equilibrium constant for sulfidation, but it is difficult to regenerate. The use of zinc oxide may, therefore, be limited by economic constraints relating to the level of sulfur being processed, the reactor volumetrics required, and issues pertaining to removal and disposal of the spent sorbent. These limitations are relieved if the sorbent is capable of multicycle operation made possible by a means for regenerating the sorbent.
Regenerable solid sorbents currently used for treating hot gaseous streams are typically based on metal oxides and are regenerated under oxidizing conditions at temperatures frequently greater than about 600° C. The regeneration of these sorbents using an oxidizing atmosphere requires an initial displacement of combustible organics, which not only is costly, but can also be hazardous.
Regeneration using hydrogen gas has been proposed as an alternative to oxidizing conditions for sorbents containing one or more of iron, cobalt, nickel, and/or copper. The use of hydrogen gas is effective for cobalt, nickel, and copper containing sorbents, but it is difficult to achieve substantially complete regeneration of an iron-containing sorbent using hydrogen alone. Therefore, methods are needed for the substantially complete regeneration of iron-containing sorbents using a non-oxidizing atmosphere.
SUMMARY OF THE INVENTION
The present invention provides a process for regenerating a spent iron-based hydrogen sulfide sorbent, comprising: exposing the spent iron-based hydrogen sulfide sorbent to a sufficient concentration of steam under conditions effective for the steam to regenerate the spent iron-based hydrogen sulfide sorbent.
In a preferred embodiment the spent iron-based sorbent is treated with steam in a first step, followed by being treated with hydrogen in a second step.
In another preferred embodiment of the present invention a mixture of steam and hydrogen are used in the first treatment step.
In still another preferred embodiment of the present invention a mixture of steam and hydrogen and an inert gas are used in the first treatment step.
DETAILED DESCRIPTION OF THE INVENTION
The present invention uses steam, preferably in combination with at least one of hydrogen and an inert gas, to regenerate an iron-based spent hydrogen sulfide sorbent. In a more preferred embodiment, the spent iron-based sorbent is contacted with a mixture of steam and at least one of hydrogen and nitrogen gas in a first treatment step, followed by being treated with hydrogen in a second treatment step, both steps being conducted under sorbent regeneration conditions.
It is within the scope of this invention that the iron-based sorbents be either bulk iron materials, or iron on a suitable support, such as an inorganic oxide. Non-limiting examples of suitable iron-based supported regenerable sorbents include, but are not necessarily limited to: 5 Fe/Al
2
O
3
, 10 Fe/SiO
2
, and 20 Fe/ZrO
2
, wherein the numbers 5, 10, and 20 refer to the wt. % Fe based on the total weight of the sorbent. As previously mentioned, the hydrogen sulfide sorbent may be employed as a metal oxide or as bulk iron. If bulk iron is used as the sorbent in may be used as one or more type of finely divided skeleton metal, including Raney metals, ponderous metals, Rieke metals, and metal sponges.
If a support material is used, it is preferably one that will increase at least one of the surface area, pore volume, and pore diameter of the overall sorbent. Suitable support materials include, but are not limited to alumina, silica, zirconia, carbon, silicon carbide, kieselguhr, amorphous and crystalline silica-aluminas, silica-magnesias, aluminophosphates, boria, titania, and combinations thereof. Preferred support materials include alumina, silica, and zirconia. The iron or iron oxide may be loaded onto these support materials by conventional techniques known in the art. Such techniques include impregnation by incipient wetness, adsorption from an excess impregnating medium, and ion exchange. In a preferred embodiment, the regenerable sorbents are prepared by conventional impregnation techniques using aqueous solutions of iron halides, oxides, hydroxides, carbonates, nitrates, nitrites, sulfates, sulfites, carboxylates and the like. The iron or iron oxide loadings may vary with the quantity of sulfur to be adsorbed per cycle, the cycle frequency, and the regeneration process conditions and hardware. Iron loadings will range from about 2 wt. % to about 80 wt. %, preferably from about 3 wt. % to about 60 wt. %, and more preferably from about 5 wt. % to about 50 wt. %, based on the total weight of the regenerable sorbent. After impregnation onto a support, the sorbent typically is dried, calcined, and reduced; the latter may either be conducted ex situ or in situ, as preferred. The regenerable sorbent may comprise iron only, or iron with one or more additional metals.
In addition to its activity as a hydrogen sulfide sorbent, Fe is also a hydrocracking metal. Unless its hydrocracking activity is suppressed, Fe may cause hydrocracking of the other hydrocarbon stream being treated, leading to the production of low value light gas. The hydrocracking activity of the sorbent metal can be suppressed by incorporating from about 1 wt. % to about 10 wt. %, preferably from about 1.5 wt. % to about 7 wt. %, and most preferably from about 2 wt. % to about 6 wt. %, of a metal selected from Group IB or Group IVA of the Periodic Table of the Elements, such as Cu, Ag, Au, Sn, or Pb, preferably Cu.
Hydrogenolysis also can be suppressed by incorporating a small amount, preferably from about 0.01 wt. % to about 1 wt. %, of one or more of the elements selected from Group VIA of the Periodic Table of the Elements. The Periodic Table of the Elements referred to herein appears on the inside cover of the Merck Index, Twelfth Edition, Merck & Co., 1996.
Accordingly, the sorbent may be presulfided conventionally, for example, by exposing the virgin sorbent to dilute hydrogen sulfide in hydrogen at a temperature from about 200° C. to about 400° C. for about 15 minutes to about 15 hours, or until sulfur breakthrough is detect
Baird Jr. William C.
Brown Leo D.
Chen Jingguang G.
Ellis Edward S.
Klein Darryl P.
ExxonMobil Research & Engineering Company
Griffin Walter D.
Hughes Gerard J.
Nguyen Tam M.
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