Chemistry of inorganic compounds – Sulfur or compound thereof – Elemental sulfur
Reexamination Certificate
2000-06-09
2003-07-08
Silverman, Stanley S. (Department: 1759)
Chemistry of inorganic compounds
Sulfur or compound thereof
Elemental sulfur
C423S576200, C423S576400, C423S576500, C423S576600
Reexamination Certificate
active
06589498
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved process intended for desulfurization of a gaseous feed containing hydrogen sulfide in order to eliminate more readily the elementary sulfur contained in a catalytic desulfurization solution and formed by chemical oxidation reduction reaction after a contacting stage. At least part of the catalytic solution from the contacting stage, containing elemental sulfur, is therefore recycled to the absorption stage under selected conditions allowing to increase the average size of the sulfur grains and notably to reduce the number of sulfur grains of very small size.
The process according to the invention is notably applied in a <<Redox>> type process for desulfurizing a gas containing at least hydrogen sulfide, where the gaseous feed is contacted with a catalytic solution. The catalytic solution comprises for example a polyvalent metal chelated by at least one chelating agent under suitable conditions for oxidation of the hydrogen sulfide to elemental sulfur and simultaneous reduction of the chelated polyvalent metal from a higher oxidation level to a lower oxidation level. After the absorption stage, a gaseous effluent practically free of hydrogen sulfide is recovered on the one hand and the catalytic solution at least partly reduced and containing elemental sulfur as a suspended solid is recovered on the other hand. At least part of the catalytic solution is recycled for example to the absorption stage so as to increase the size of the sulfur particles. The non-recycled part of the catalytic solution containing the sulfur is freed from the most part of the sulfur present as a suspension during a separation stage, then it is sent to a regeneration stage, air regeneration for example, prior to being used in the absorption stage.
BACKGROUND OF THE INVENTION
The prior art describes many redox processes and associated devices allowing to eliminate hydrogen sulfide and to recover the elemental sulfur formed during the process.
The desulfurization process comprises for example the following two oxidation reduction stages:
during a first stage (absorption stage, oxidation reduction reaction), the hydrogen sulfide present in the gas to be processed reacts with chelated ferric ions according to the reaction:
H
2
S+2 Fe
3+
(chel)→S+2H
+
+2 Fe
2+
(chel) (1)
The sulfur resulting from this chemical reaction has the form of grains of very small size. This sulfur is referred to as <<native sulfur>> in the description hereafter.
during a second stage (regeneration stage), the ferrous ions are reoxidized by the oxygen of air according to the reaction:
2 Fe
2+
(chel)+2 H
+
+½O
2
→2 Fe
3+
(chel)+H
2
O (2)
These stages are notably described in patents U.S. Pat. No. 5,753,189, U.S. Pat. No. 4,859,436 and U.S. Pat. No. 4,532,118, and in patent applications WO-92/17,401, DE-34 44,252, but none of these documents describes or suggests recycling part of the catalytic solution at least partly reduced and containing elemental sulfur to the absorber in order to decrease the quantity of (small-size) native sulfur in the catalytic solution to be purified and regenerated. In fact, patent U.S. Pat. No. 5,753,189 describes recycling of part of the reduced catalytic solution freed from sulfur in order to control the sulfur concentration in the absorption zone.
The aqueous catalytic solutions used are generally very poor sulfur solvents. The presence of native sulfur grains of very small size can therefore disturb the course of these processes.
The devices generally comprise sulfur separation equipments using mechanical processes such as filtration, decantation or centrifligation. Sulfur grains of very small size can disturb this separation, for example by generating frequent filter clogging.
Furthermore, when the gas to be desulfurized is under high pressure, the process generally comprises a stage of preliminary expansion of the catalytic solution containing the sulfur prior to air regeneration. This expansion leads to degassing of the solution, which causes foaming problems that can be amplified by the presence of sulfur grains of very small size.
On the other hand, when the stage of regeneration of the catalytic solution at least partly reduced is carried out by contacting with air at atmospheric pressure, and the sulfur is most often recovered by flotation and/or decantation, the presence of sulfur grains of very small size can reduce the efficiency of this recovery.
SUMMARY OF THE INVENTION
The object of the invention is a new approach consisting in reducing the number of sulfur grains of very small size at least after the absorption stage in order to overcome notably the aforementioned drawbacks of the prior art. In order to obtain this result, at least part of the catalytic solution at least partly reduced and containing elemental sulfur is used and recycled to the absorption stage.
The invention relates to an improved process intended for desulfurization of a gaseous feed containing hydrogen sulfide, comprising at least the following stages:
a) contacting the gaseous feed with a catalytic solution comprising at least one polyvalent metal chelated by at least one chelating agent, under suitable conditions for oxidation of the hydrogen sulfide to elemental sulfur and concomitant reduction of the polyvalent metal from a higher oxidation level to a lower oxidation level (absorption stage),
b) recovering on the one hand a gaseous effluent substantially freed from hydrogen sulfide and, on the other hand, said catalytic solution at least reduced and containing elemental sulfur.
The process is characterized in that at least a fraction F
1
of said catalytic solution at least reduced and containing solid elementary sulfur is recycled to absorption stage a) in order to reduce the number of sulfur grains of very small size.
Fraction F
1
proper cannot correspond to the whole of the reduced catalytic solution coming from the contacting zone. This fraction F
1
contains sulfur whose grain size can be selected.
According to an embodiment, the grain size of the sulfur grains suspended in the catalytic solution at least reduced after stage a) is for example determined and the quantity of the recycled fraction F
1
of said solution is adjusted so as to maintain a particle grain size distribution within a given range.
The recycled quantity of the reduced catalytic solution containing the suspended sulfur can also be adjusted according to the quantity of sulfur in the feed. Generally, the larger the quantity of sulfur in the feed, the lower the recycle ratio. Advantageously, this recycled quantity containing the sulfur of selected grain size can represent between 1 and 95% by weight of the reduced solution, for example between 20 and 90%, preferably between 30 and 85% and more preferably between 50 and 80% by weight.
According to a feature of the process, the solid elemental sulfur is separated from the non-recycled fraction F
2
of the catalytic solution (stage c)).
According to another feature of the process:
c) said reduced catalytic solution depleted in sulfur and preferably substantially freed from all of the sulfur can be at least partly regenerated,
d) at least part of the regenerated catalytic solution is recycled to a stage of contacting the regenerated solution with a gaseous feed consisting at least of hydrogen sulfide.
According to another feature, part of the reduced catalytic solution substantially freed from all of the sulfur can be recycled to contacting stage a).
According to an embodiment, the sulfur produced during separation stage c) can be washed with water in order to recover the catalytic solution imbibing it and the catalytic solution is processed by reverse osmosis in order to control the water content of said catalytic solution.
The sulfur produced during separation stage c) can be washed with water in order to recover the catalytic solution imbibing it and the catalytic solution can be processed by nofiltration i
Barrere-Tricca Cecile
Huard Thierry
Streicher Christian
Antonelli Terry Stout & Kraus LLP
Institut Francais du Pe'trole
Silverman Stanley S.
Vanoy Timothy C.
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