Closed cycle photocatalytic process for decomposition of...

Details Closed cycle photocatalytic process for decomposition of... Closed cycle photocatalytic process for decomposition of...

2001-02-15

2003-06-03

Mayekar, Kishor (Department: 1742)

Chemical apparatus and process disinfecting, deodorizing, preser

Chemical reactor

With means applying electromagnetic wave energy or...

Reexamination Certificate

active

06572829

ABSTRACT:

This invention relates to sulfur recovery from oil and gas refinery waste streams, and in particular to a method and apparatus for sulfur recovery from oil and gas refinery waste streams using semiconductor particulates, where hydrogen sulfide (H
2
S) having been absorbed into an alkaline solution is decomposed in a photochemical scheme to yield both sulfur and hydrogen which can be later sold or used internally at a refinery and is a continuation of Provisional Application No. 60/126,036 filed on Mar. 25, 1999, by the same inventors thereof.
BACKGROUND AND PRIOR ART
Oil refineries typically pump crude oil from the ground which contains unusable organic sulfur, the latter of which must be separated from the crude oil in order to allow the oil to become a useful product. Typically, the pumped crude oil is treated with hydrogen and a catalyst under a high temperature which decomposes the organic sulfur in the crude oil and releases the sulfur in the form of hydrogen sulfide (H
2
S) gas. The problem is what do the refineries do with the H
2
S gas, which is a noxious poisonous gas that cannot be released into the atmosphere.
Recent environmental regulations generally require oil and gas refineries to develop waste minimization technologies for the treatment of sulfur-containing compounds, such as hydrogen sulfide (H
2
S) within their facilities. Current technology typically requires a solvent absorption and stripping process to separate the acid gases, a Claus type reactor to perform a partial oxidation of the sulfide to make elemental sulfur and water and a tertiary gas treatment. The latter of which having specifics that can vary greatly but must effect at least another hundred-fold reduction in H
2
S concentration.
Currently refineries must now go out and buy or make hydrogen to perform hydrodesulfurization. Afterwards, the resultant H
2
S must then be decomposed at the plant.
As the world supply of petroleum decreases, the average sulfur content of crude oil has begun to rise. In recent years the average oil refinery has gone from being a net producer to a net consumer of H
2
. The oil producers would thus be interested in any process that increases the amount of H
2
available in-house.
SUMMARY OF THE INVENTION
A primary objective of the present invention is to provide a method of recovering sulfur from oil and gas refinery waste streams using semiconductor particulates, where hydrogen sulfide (H
2
S) having been absorbed into an alkaline solution is decomposed in a photochemical process to yield both sulfur and hydrogen.
The secondary objective of the present invention is to decompose noxious poisonous gas such as hydrogen sulfide into separate amounts of hydrogen and sulfur using light and a photocatalyst.
A third objective is to recoup the H
2
contained in H
2
S and return it to the hydrodesulfurization plant for reuse in oil refinery applications.
A preferred embodiment of the novel process decomposes H
2
S in a thermodynamically uphill process to its constituent elements:
H
2
S+light→S+H
2
The energy to drive the reaction can either come from the sun or an artificial light source. The decomposition process is photoelectrochemical in nature, in which the photovoltaic effect from semiconductor solid state physics is combined with conventional electrochemical principles to produce a light-driven electrochemical reactor using semiconductor electrodes. Thus, we call it the PEP (photoelectrochemical particle) process.
In the preferred embodiment, H
2
S from the stripper is passed into an alkaline solution to produce bisulfide ion, HS
−
(aq). The sulfide solution is then sent into a photoreactor, which consists of a bed of catalyst-activated semiconductor particles such as Cadmium Sulfide (CdS), distributed in a thin, planar or cylindrical array depending on the lamp configuration, and a light source containing a wavelength range of approximately 450 to approximately 500 nm. As the solution is passed over the surface of the photo-energized semiconductor particles, bisulfide ion is oxidized and water is reduced to sulfur and hydrogen, respectively. Other photo-atalysts useful can include as Platinized Cadmium Sulfide, Pt—CdS, Zinc Sulfide, ZnS, Zinc Ferrate, ZnFe
2
O
4
, and Indium Sulfide, In
2
S
3
.
The exit stream from the photoreactor will contain sulfur in the form of polysulfide ion, which results when sulfur is produced in the presence of excess bisulfide ion:
HS
−
+(
n
−1)S
0
+OH
−
=S
n
2−
+H
2
O
The polysulfide solution is returned to the scrubber tank, where pressurizing with H
2
S causes the sulfur to become separated from solution.
The following chemical process shows the effects of combining sulfur recovery and scrubber units, utilizing the acid properties of H
2
S itself to decompose polysulfide ions and free the elemental sulfur.
S
n
2−
+H
2
S=(
n−
1)S
0
+2HS
−
Further objects and advantages of this invention will be apparent from the following detailed description of a presently preferred embodiment which is illustrated schematically in the accompanying drawings.


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