The present invention is directed to a catalytic process for selectively oxidizing carbonyl sulfide (COS) and carbon disulfide (CS2) to carbon monoxide (CO) and sulfur dioxide (SO2). 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.
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 (CH4), hydrogen sulfide (H2S) and water; by hydrolysis to carbon dioxide (CO2) and H2S, or by oxidation to CO2 and sulfur dioxide (SO2). Hydrogenation is an expensive waste of valuable hydrogen (H2), while hydrolysis and total oxidation produces CO2, considered by many to be an environmental threat due to its alleged impact on global warming.
Carbon disulfide (CS2) is another organic sulfur pollutant. CS2 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.
H2S is commonly encountered in petroleum refineries where hydrodesulfurization is widely used to remove sulfur from gasoline. A significant amount of H2S (oftentimes as much as 10-30%) also is found in natural gas recovered from geological formations. The H2S 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 H2S 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 (SO2) 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 H2S 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 H2S 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 H2S as a reducing agent. The patent presents the equilibrium reaction in which H2S is used to produce H2 from carbon monoxide, with by-product COS. The patent also shows the oxidation reaction involving COS to produce CO and SO2. This reaction, however, is conducted either in the absence of a catalyst or in the presence of quartz chips (SiO2) 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 H2S a cost effective alternative.
Thus, it would be advantageous if a process were available for selectively converting organic sulfur pollutants, such as COS and CS2, at high yield (conversion) and selectivity to CO and SO2. Such a process would make practicably feasible the conversion of H2S to the valuable end product, H2.
U.S. Pat. No. 4,427,576 generally describes catalytically oxidizing H2S and/or organosulfur compounds to SO2using titanium dioxide (titania) with zirconia or silica (SiO2), 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.
The present invention is directed to a process for selectively oxidizing carbonyl sulfide (COS) and carbon disulfide (CS2) to carbon monoxide (CO) and sulfur dioxide (SO2):
COS+O2xe2x86x92CO+SO2
2CS2+5O2xe2x86x922CO+4SO2
The CO can be used to produce valuable hydrogen, such as via the water gas shift reaction;
CO+H2O⇄CO2+H2
or more preferably from H2S via the known equilibrium reaction:
H2S+CO⇄COS+H2
In this later reaction, the COS, in turn, is used to generate additional CO for the above reaction with H2S. The by-product SO2 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 CS2, in the presence of excess oxygen, with a supported metal oxide catalyst, one can selectively oxidize these organic sulfur compounds to CO and SO2. 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 CS2 to CO and SO2. 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 (TiO2), zirconia (ZrO2), ceria (CeO2), niobia (Nb2O5), silica (SiO2) and alumina (Al2O3). Vanadia is often preferred as the metal oxide surface coating. Especially preferred catalysts are a 5% V2O5 surface coating on a Nb2O5 support (5% V2O5 on Nb2O5 catalyst), which for COS oxidation at 290xc2x0 C. provides a selectivity to CO of 98% and for CS2 oxidation provides an 80% selectivity to CO and a coating of V2O5 on a SiO2 support, which for CS2 oxidation provides a selectivity towards CO higher than 90% at 270xc2x0 C.
By coupling these oxidation reactions with the known equilibrium reaction involving the conversion of H2S and CO to H2 and COS (and often some CS2 and CH3SH as byproducts), one is able to convert what are otherwise environmentally undesirable sulfur pollutants to a very valuable material, H2 and by-product SO2. The SO2 is not laden with as much moisture as when produced directly from H2S 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 (H2SO4).