Chemistry of inorganic compounds – Sulfur or compound thereof – Elemental sulfur
Reexamination Certificate
2000-10-10
2003-01-14
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Sulfur or compound thereof
Elemental sulfur
C423S576800, C502S303000, C502S304000, C502S305000, C502S324000, C502S325000, C502S340000, C502S349000, C502S350000, C502S352000, C502S353000, C502S517000
Reexamination Certificate
active
06506356
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, in general, to a catalyst for oxidizing hydrogen sulfide gas to elemental sulfur and, more particularly, to a catalyst capable of selectively oxidizing hydrogen sulfide gas even when it contains moisture. Also, the present invention relates to a method for recovering elemental sulfur from hydrogen sulfide gas using the catalyst.
2. Description of the Prior Art
Representative of air pollutants, hydrogen sulfide is a colorless, toxic gas, giving out a bad smell. Hydrogen sulfide is produced in large quantity as a main or side product of biological metabolisms or industrial processes. Once hydrogen sulfide, whose sources are various at present, is released to the air, living organisms, including humans, absorb or adsorb the toxic gas directly or indirectly. In this course, the living organisms may suffer a fatal blow owing to the serious toxicity of hydrogen sulfide. Further, industrial facilities are increasingly deteriorated when coming into contact with hydrogen sulfide.
Since it is virtually impossible to effectively remove hydrogen sulfide from hydrogen sulfide-contaminated air, the best policy is to reduce the amount of the hydrogen sulfide gas released to the air. In this regard, there is a tendency toward the strengthening of laws regulating the hydrogen sulfide discharging industries. However, regulation by law is not preferable because the hydrogen sulfide discharging industries make a contribution to national economy. Rather than controlling hydrogen sulfide sources, appropriately processing inevitably generated hydrogen sulfide in advance of the discharge of hydrogen sulfide to the air is desired. That is, preferable is that an appropriate desulfurization process is adopted at the end of various processes in industrial facilities.
When fossil fuels themselves are directly processed or used as energy sources, a large quantity of hydrogen sulfide is produced. Relevant industrial facilities are representatively exemplified by oil refineries, iron manufacturing plants, and power plants. The effects of the quantity of the hydrogen sulfide generated from such large-scale plant facilities are not limited to local contamination, but may directly pollute other nations or the entire globe. Thus, the removal of hydrogen sulfide is of particular concern to all nations at present.
Claus reaction for converting toxic hydrogen sulfide into elemental sulfur, which is non-toxic to the body and the environment is one of the most well-known hydrogen sulfide removing methods. In a hydrogen sulfide-removing method using Claus reaction, toxic hydrogen sulfide is oxidized to elemental sulfur by passing through a reaction system comprising one high temperature furnace and two or three catalyst reactors, where thermal oxidation and catalytic reaction occur.
In detail, in the thermal oxidation process, one-third of the fed hydrogen sulfide(H
2
S) is oxidized to sulfur dioxide(SO
2
) in the high temperature furnace maintained at 1,100-1,200° C. This thermal oxidation is explained by the following reaction formula 1:
2
H
2
S+
3
O
2
→
2
SO
2
+
2
H
2
O
In the catalyst reaction process, which is subsequent to the thermal oxidation process, unconverted hydrogen sulfide and the sulfur dioxide produced in the high temperature furnace are mixed in a molar ratio of 2:1 and converted elemental sulfur through the condensation reaction, named Claus reaction, represented by the following reaction formula 2:
2
H
2
S+SO
2
←→
3
Sn+
2
H
2
O
The Claus process including the condensation of the reaction formula 2 suffers limited sulfur-recovery efficiency for the following reasons, making it difficult to increase hydrogen sulfide treating efficiency to a desirable level.
First, because the Claus process according to the reaction formula 2 is reversible chemically and thermodynamically, the equilibrium conversion rate is limited.
Next, the forward reaction, as shown in the reaction formula 2, is smoothly progressed with maintenance of the stoichiometric molar ratio of hydrogen sulfide to sulfide dioxide at 2:1. However, the stoichiometric molar ratio is not easy to control into the quantitatively accurate value. Thus, the forward reaction rate is lowered.
Finally, it is somewhat difficult to remove the water produced by the forward reaction, so the reverse reaction of the reaction formula 2 may predominate over the forward reaction. Accordingly, there may be caused a result contrary to the desulfurization purpose of converting hydrogen sulfide to elemental sulfur.
When the Claus process is used to treat a volume of hydrogen sulfide, 3-5% of the volume of the hydrogen sulfide typically remains unreacted owing to the above-mentioned problems. Usually, such tail gas is incinerated for discharge to the air. Incineration of hydrogen sulfide leads to sulfur dioxide as a main product. This incineration has been blamed for the emission of the serious air pollutant sulfur dioxide.
To avoid the pollution of the air, the tail gas must be further treated. In fact, a number of Claus tail-gas treatment processes have been developed to increase the total sulfur-recovery efficiency. Most of the conventional Claus tail-gas treatment processes, which take advantage of the adsorption or absorption of hydrogen sulfide, however, have the disadvantage of inhibiting the continuous operation of the facilities because of producing wastes after the treatment or requiring separate, periodic recycle processes.
In the most efficient Claus tail-gas treatment process, the removal of toxic hydrogen sulfide utilizes a catalyst. For example, a tail gas containing sulfur is hydrogenated to give hydrogen sulfide which is then oxidized on a catalyst to elemental sulfur. The following reaction formula 3 explains this reaction:
2
H
2
S+O
2
→
2
Sn+
2
H
2
O
The Claus tail-gas treatment processes following the reaction formula 3 are representative of a mobile direct oxidation process (MODOP), which is high in sulfur-recovery rate as disclosed in EP 0 078 690 A2, and a super-Claus process which is disclosed in U.S. Pat. No. 5,286,697.
In the MODOP, hydrogen sulfide is directly converted to elemental sulfur by being reacted with oxygen at the stoichiometric molar ratio (2:1) in the presence of a titanium dioxide (TiO
2
)-based catalyst. Through the three-step process including the Claus process, the conversion of hydrogen sulfide to elemental sulfur is achieved at a rate of as high as 90% or higher. The MODOP requires that the moisture level of the reaction gas be reduced to less than 4% prior to the catalytic reaction as the catalyst may be functionally deteriorated by water poisoning. Thus, the MODOP has the prerequisite condition of conducting a dehydration process in advance of the catalytic process, causing complexity in the conversion procedure.
The super Claus process is similar to the MODOP, but more useful in terms of requiring no separate dehydration processes. In other words, the iron- or chrome-based catalyst used in the super Claus process is not seriously vulnerable to moisture. Therefore, the super Claus process allows hydrogen sulfide gas containing excess moisture to be directly converted to elemental sulfur, showing as high a sulfur-recovery efficiency as that of the MODOP. Instead of the dehydration as in MODOP process, however, a prerequisite process is needed to prevent the catalyst from being poisoned by water and limiting the reverse super Claus reaction. The activity of the catalyst cannot be maintained high enough to drive the super Claus reaction without excessively using oxygen at an amount ten-fold larger than the stoichiometric equivalent required in the reaction formula 3. Indeed, the super Claus chemical reaction is smoothly conducted when the hydrogen sulfide gas is maintained at the level of less than 1 vol %. If the amount of the hydrogen sulfide gas exceeds 2 vol %, the hydrogen sulfide is difficult to treat by the super Claus process.
Therefore, there remains a need for a catalyst th
Chung Jong Shik
Shin Moon Young
Envichem Co., Ltd.
Silverman Stanley S.
Vanoy Timothy C.
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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