Mineral oils: processes and products – Refining – Sulfur removal
Reexamination Certificate
1999-08-27
2002-12-17
Preisch, Nadine (Department: 1764)
Mineral oils: processes and products
Refining
Sulfur removal
C208S212000, C208S217000, C208S226000, C208S211000
Reexamination Certificate
active
06495029
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the hydrodesulfurization (HDS) of the multiple condensed ring heterocyclic organosulfur compounds present in petroleum and chemical streams. The stream is passed through at least one reaction zone countercurrent to the flow of a hydrogen-containing treat gas, and through at least one sorbent zone. The reaction zone contains a bed of Group VIII metal-containing hydrodesulfurization catalyst and the sorbent zone contains a bed of hydrogen sulfide sorbent material.
BACKGROUND OF THE INVENTION
Hydrodesulfurization is one of the fundamental processes of the refining and chemical industries. The removal of feed sulfur by conversion to hydrogen sulfide is typically achieved by reaction with hydrogen over non-noble metal sulfides, especially those of Co/Mo and Ni/Mo, at fairly severe temperatures and pressures to meet product quality specifications, or to supply a desulfurized stream to a subsequent sulfur sensitive process. The latter is a particularly important objective because some processes are carried out over catalysts which are extremely sensitive to poisoning by sulfur. This sulfur sensitivity is sometimes sufficiently acute as to require a substantially sulfur free feed. In other cases environmental considerations and mandates drive product quality specifications to even lower sulfur levels.
There is a well established hierarchy in the ease of sulfur removal from the various organosulfur compounds common to refinery and chemical streams. Simple aliphatic, naphthenic, and aromatic mercaptans, sulfides, di- and polysulfides and the like surrender their sulfur more readily than the class of heterocyclic sulfur compounds comprised of thiophene and its higher homologs and analogs. Within the generic thiophenic class, desulfurization reactivity decreases with increasing molecular structure and complexity. While simple thiophenes represent the more labile sulfur types, the other extreme, which is sometimes referred to as “hard sulfur” or “refractory sulfur” is represented by the derivatives of dibenzothiophene, especially those mono- and di-substituted and condensed ring dibenzothiophenes bearing substituents on the carbons beta to the sulfur atom. These highly refractory sulfur heterocycles resist desulfurization as a consequence of steric inhibition precluding the requisite catalyst-substrate interaction. For this reason, these materials survive traditional desulfurization and they poison subsequent processes whose operability is dependent upon a sulfur sensitive catalyst. Destruction of these “hard sulfur” types can be accomplished under relatively severe process conditions, but this may prove to be economically undesirable owing to the onset of harmful side reactions leading to feed and/or product degradation. Also, the level of investment and operating costs required to drive the severe process conditions may be too great for the required sulfur specification.
Typically, catalytic hydrodesulfurization of liquid-phase petroleum feedstocks is carried out in co-current, downflow, “trickle bed” reactors in which both the preheated liquid feedstock and a hydrogen-containing treat gas are introduced to the reactor at a point, or points, above one or more fixed beds of hydrodesulfurization catalyst. The liquid feedstock, any vaporized hydrocarbons, and hydrogen-containing treat gas all flow in a downward direction through the catalyst bed(s). The resulting combined vapor phase and liquid phase effluents are normally separated in a series of one or more separator vessels, or drums, downstream of the reactor.
Conventional co-current catalytic hydrodesulfurization has met with a great deal of commercial success, however, it has limitations. For example, because of hydrogen consumption and treat gas dilution by light reaction products, hydrogen partial pressure decreases between the reactor inlet and outlet. At the same time, any hydrodesulfurization reactions that take place results in increased concentrations of H
2
S which strongly inhibits the catalytic activity and performance of most hydroprocessing catalysts through competitive adsorption onto the catalyst. Thus, the downstream portion of catalyst in a trickle bed reactor are often limited in reactivity because of the simultaneous occurrence of multiple negative effects, such as low H
2
partial pressure and the presence of the high concentrations of H
2
S. Further, if a bed of is present downstream of a co-current hydrodesulfurization zone its effect is quickly diminished if substantial sulfur breakthrough occurs. Further, liquid phase concentrations of the targeted hydrocarbon reactants are also the lowest at the downstream part of the catalyst bed.
Another type of hydroprocessing is countercurrent hydroprocessing which has the potential of overcoming many of these limitations, but is presently of very limited commercial use. U.S. Pat. No. 3,147,210 discloses a two stage process for the hydrofining-hydrogenation of high-boiling aromatic hydrocarbons. The feedstock is first subjected to catalytic hydrofining, preferably in co-current flow with hydrogen, then subjected to hydrogenation over a sulfur-sensitive noble metal hydrogenation catalyst countercurrent to the flow of a hydrogen-containing treat gas. U.S. Pat. Nos. 3,767,562 and 3,775,291 disclose a countercurrent process for producing jet fuels, whereas the jet fuel is first hydrodesulfurized in a co-current mode prior to two stage countercurrent hydrogenation. U.S. Pat. No. 5,183,556 also discloses a two stage co-current/countercurrent process for hydrofining and hydrogenating aromatics in a diesel fuel stream.
In light of the above, there is still a need for a desulfurization process that can convert feeds bearing the refractory, condensed ring sulfur heterocycles at relatively mild process conditions to products containing substantially no sulfur.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a process for the desulfurization of a stream selected from petroleum and chemical streams containing condensed ring sulfur heterocyclic compounds in a process unit comprised of at least one reaction zone having a non-reaction zone upstream and downstream thereof and at least one sorbent zone containing a bed of hydrogen sulfide sorbent material downstream of the first of said at least one reaction zones, which reaction zone(s) contain hydrodesulfurization catalyst, which process comprises:
(a) feeding said stream to one or more hydrodesulfurization reaction zones counter-current to upflowing hydrogen-containing treat gas;
(b) passing the resulting liquid phase effluent from at least one of said reaction zones through a zone containing a bed of hydrogen sulfide sorbent material;
(c) recovering a vapor phase effluent from said reaction zone in an upstream non-reaction zone, which vapor phase effluent is comprised of hydrogen-containing treat gas and vaporized sulfur reaction products from said reaction zone:
(d) recovering downstream from said sorbent zone a liquid phase effluent characterized as having substantially no sulfur.
In a preferred embodiment of the present invention, the Group VIII metal is a noble metal selected from Pt, Pd, Ir, and mixtures thereof.
In another preferred embodiment of the present invention, the hydrogen sulfide sorbent is selected from supported and unsupported metal oxides, spinels, zeolitic based materials, and layered double hydroxides.
In yet another preferred embodiment of the present invention, the sorbent zone contains a mixed bed of hydrodesulfurization catalyst and hydrogen sulfide sorbent.
DETAILED DESCRIPTION OF THE INVENTION
It is well known that so-called “easy” sulfur compounds, such as non-thiophenic sulfur compounds, thiophenes, benzothiophenes, and non-beta dibenzothiophenes can be removed without using severe process conditions. The prior art teaches that substantially more severe conditions are needed to remove the so-called “hard” sulfur compounds, such as condensed ring sulfur heterocyclic compounds which are typically present
Ellis Edward S.
Gupta Ramesh
Schorfheide James J.
Touvelle Michele S.
Exxon Research and Engineering Company
Hughes Gerard J.
Nguyen Tam M.
Preisch Nadine
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