Chemistry of inorganic compounds – Hydrogen or compound thereof – Elemental hydrogen
Reexamination Certificate
2000-05-22
2002-12-24
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Hydrogen or compound thereof
Elemental hydrogen
C423S220000, C423S242100, C423S244090, C423S244100, C423S246000, C423S247000, C423S418200, C423S512100, C423S539000, C423S542000
Reexamination Certificate
active
06497855
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a catalytic process for selectively oxidizing carbonyl sulfide (COS) and carbon disulfide (CS
2
) to carbon monoxide (CO) and sulfur dioxide (SO
2
). The process of the invention is particularly useful in combination with the known equilibrium reaction for producing hydrogen from hydrogen sulfide. Such combination provides a cost effective way of producing hydrogen from what are often considered waste sulfur streams. The so-produced hydrogen can be used for a variety of purposes as is well known in the art.
BACKGROUND OF THE INVENTION
Carbonyl sulfide (COS) is found in many industrial process streams such as those associated with natural gas production, petroleum refineries and coal gasification plants. COS often is catalytically converted by hydrogenation to methane (CH
4
), hydrogen sulfide (H
2
S) and water; by hydrolysis to carbon dioxide (CO
2
) and H
2
S, or by oxidation to CO
2
and sulfur dioxide (SO
2
). Hydrogenation is an expensive waste of valuable hydrogen (H
2
), while hydrolysis and total oxidation produces CO
2
, considered by many to be an environmental threat due to its alleged impact on global warming.
Carbon disulfide (CS
2
) is another organic sulfur pollutant. CS
2
is commonly encountered in paper manufacturing waste streams and in petroleum refineries. It can be treated much like COS to yield the same end products. Even though these methods provide the important removal of environmentally unsafe reduced sulfur compounds, they do not produce valuable chemicals from the original sulfur pollutants.
H
2
S is commonly encountered in petroleum refineries where hydrodesulfurization is widely used to remove sulfur from gasoline. A significant amount of H
2
S (oftentimes as much as 10-30%) also is found in natural gas recovered from geological formations. The H
2
S is typically separated from the methane and the resulting de-sulfurized methane typically is distributed for industrial and personal uses, such as for home heating. The isolated H
2
S often is converted to elemental sulfur via the Claus Process. In the Claus Process, a first portion of the separated hydrogen sulfide is converted (oxidized) to sulfur dioxide (SO
2
) and water and the remaining portion of the hydrogen sulfide is reacted with the water-laden sulfur dioxide in the presence of a suitable catalyst to produce additional water and elemental sulfur. The so-produced sulfur represents a low value-added, commodity product, while essentially all of the potentially valuable hydrogen resident in the H
2
S is converted into water. The development of a cost effective alternative for using the hydrogen sulfide, and especially for recovering at least a portion of its hydrogen content, would significantly improve the economies of sour natural gas recovery and processing and would greatly benefit the operation of petroleum refinery operations.
Herrington et al., U.S. Pat. No. 4,618,723 describes processes for using H
2
S as a reducing agent to reduce carbon oxides and generate hydrogen which is then used, as described, to make such organic compounds as alkanes, alcohols, alkenes and the like. The focus of such processes is to upgrade carbon dioxide and carbon monoxide to valuable organic compounds using H
2
S as a reducing agent. The patent presents the equilibrium reaction in which H
2
S is used to produce H
2
from carbon monoxide, with by-product COS. The patent also shows the oxidation reaction involving COS to produce CO and SO
2
. This reaction, however, is conducted either in the absence of a catalyst or in the presence of quartz chips (SiO
2
) as a catalyst. Unfortunately, the yield (conversion) and selectivity of this step, as obtained under these conditions, has simply not been sufficiently high to make hydrogen production from H
2
S a cost effective alternative.
Thus, it would be advantageous if a process were available for selectively converting organic sulfur pollutants, such as COS and CS
2
, at high yield (conversion) and selectivity to CO and SO
2
. Such a process would make practicably feasible the conversion of H
2
S to the valuable end product, H
2
.
U.S. Pat. No. 4,427,576 generally describes catalytically oxidizing H
2
S and/or organosulfur compounds to SO
2
using titanium dioxide (titania) with zirconia or silica (SiO
2
), an alkaline earth metal sulfate and one of the elements Cu, Ag, Zn, Cd, Y, lanthanides, V, Cr, Mo, W, Mn, Fe, Co, Rh, Ir, Ni, Pd, Pt, Sn and Bi.
SUMMARY OF THE INVENTION
The present invention is directed to a process for selectively oxidizing carbonyl sulfide (COS) and carbon disulfide (CS
2
) to carbon monoxide (CO) and sulfur dioxide (SO
2
):
COS+O
2
→CO+SO
2
2CS
2
+5O
2
→2CO+4SO
2
The CO can be used to produce valuable hydrogen, such as via the water gas shift reaction;
CO+H
2
O⇄CO
2
+H
2
or more preferably from H
2
S via the known equilibrium reaction:
H
2
S+CO⇄COS+H
2
In this later reaction, the COS, in turn, is used to generate additional CO for the above reaction with H
2
S. The by-product SO
2
also can be converted, if desired, to elemental sulfur, or to sulfuric acid.
In particular, the present invention is based on the discovery that by contacting COS, and/or CS
2
, in the presence of excess oxygen, with a supported metal oxide catalyst, one can selectively oxidize these organic sulfur compounds to CO and SO
2
. In particular, applicant has discovered that a supported metal oxide catalyst using the oxide of one or more of a variety of different catalytic metal oxides, i.e., comprising a metal oxide of a metal selected from the group consisting of vanadium (V), niobium (Nb), molybdenum (Mo), chromium (Cr), rhenium (Re), titanium (Ti), tungsten (W), manganese (Mn), tantalum (Ta) and mixtures thereof, supported on a metal oxide support selected from the group consisting of titania, antimony oxides, tantala, tin oxide, lanthana, indium oxides, iron oxides, nickel oxide, cobalt oxide, gallium oxides, manganese oxides, chromia, tungsten oxide, hafnia, zirconia, ceria, niobia, silica and alumina, such as vanadia on a titania support, is able to promote the selective oxidation of COS and CS
2
to CO and SO
2
. As a general rule, the metal oxide support and the supported metal oxide surface layer should not be the same.
On the basis of cost and availability the preferred metal oxide supports are selected from the group consisting of titania (TiO
2
), zirconia (ZrO
2
), ceria (CeO
2
), niobia (Nb
2
O
5
), silica (SiO
2
) and alumina (Al
2
O
3
). Vanadia is often preferred as the metal oxide surface coating. Especially preferred catalysts are a 5% V
2
O
5
surface coating on a Nb
2
O
5
support (5% V
2
O
5
on Nb
2
O
5
catalyst), which for COS oxidation at 290° C. provides a selectivity to CO of 98% and for CS
2
oxidation provides an 80% selectivity to CO and a coating of V
2
O
5
on a SiO
2
support, which for CS
2
oxidation provides a selectivity towards CO higher than 90% at 270° C.
By coupling these oxidation reactions with the known equilibrium reaction involving the conversion of H
2
S and CO to H
2
and COS (and often some CS
2
and CH
3
SH as byproducts), one is able to convert what are otherwise environmentally undesirable sulfur pollutants to a very valuable material, H
2
and by-product SO
2
. The SO
2
is not laden with as much moisture as when produced directly from H
2
S oxidation and thus can be sent to a Claus plant for more efficient conversion to elemental sulfur, or alternately it can be further oxidized to sulfuric acid (H
2
SO
4
).
REFERENCES:
patent: 4277458 (1981-07-01), Sugier et al.
patent: 4427576 (1984-01-01), Dupin
patent: 4432960 (1984-02-01), Herrington et al.
patent: 4489048 (1984-12-01), Kuch
patent: 4500505 (1985-02-01), Jevnikar et al.
patent: 4618723 (1986-10-01), Herrington et al.
patent: 4668825 (1987-05-01), Ratcliffe et al.
Sun et al. “Partial Oxidation of Methane by Molecular Oxygen Over Supported V2O5Catalysts: ACatalytic and in situ Raman Spectroscopy Study”, Methane and Alkane Conversion Chemistry
Lehigh University
Vanoy Timothy C.
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