Catalyst for catalytically reducing sulfur contained in a gas stream, and method for preparing and using such a catalyst

Abstract

The present invention provides a catalyst composition having excellent properties for converting sulfur, sulfur compounds, and carbon monoxide contained in a gas stream by catalytic hydrolysis, hydrogenation, and water gas shift reactions. The catalyst comprises a base layer of molybdenum and cobalt and an overlay of molybdenum and cobalt. These metals are present in the catalyst in a certain concentration range and relative weight ratio. The base layer metals are present in the catalyst in a specified range relative to the overlay and total metals. The base layer metals are formed by co-grinding an inorganic oxide with the catalytically active metals molybdenum and cobalt. The co-ground mixture is calcined and then impregnated with overlay molybdenum and overlay cobalt.

Description

Technical Field

[0001] The present invention relates to a catalyst composition for catalytically reducing sulfur compounds contained in a gas stream, a method for preparing such a catalyst composition, and a hydrolysis process for reducing and converting sulfur compounds contained in a gas stream. Background Art

[0002] In the well-known Claus process, an acid gas containing a relatively large percentage of hydrogen sulfide (H2S) is burned in a thermal stage to oxidize a portion of the H2S to sulfur dioxide (SO2). This combustion is controlled to provide a process gas stream containing H2S and SO2 in an approximate molar ratio of 2 moles of H2S / mole of SO2 (2:1). The process gas stream is conveyed to a catalytic stage, which provides for the reaction of H2S and SO2 in the presence of an alumina catalyst according to the Claus reaction to produce elemental sulfur and water. The sulfur is then condensed from the Claus reaction gas to produce a Claus tail gas stream.

[0003] The Claus tail gas stream typically contains low concentrations of H2S and other sulfur compounds, such as SO2, carbon disulfide (CS2), carbonyl sulfide (COS), and elemental sulfur (S). In order to combust or otherwise dispose of this tail gas stream, it must be further treated to remove most of the sulfur. This provides a treated gas having a sulfur content low enough to permit its combustion or release to the atmosphere.

[0004] One method of treating the tail gas is to convey it to a reduction reactor. The reduction reactor provides for the catalytic reduction of the sulfur compounds contained in the tail gas (i.e., SO2, CS2, COS, and S) to H2S to produce a treated gas stream having a reduced concentration of sulfur compounds. The treated gas stream can be further treated to remove H2S, for example, by conveying it to an absorption unit for removing H2S. This is typically accomplished by contacting the treated gas stream with an H2S removal absorbent.

[0005] U.S. Patent No. 5,132,098 (Kvasnikoff et al.) discloses a method in which sulfur compounds such as SO2, CS2, COS, and elemental sulfur contained in the Claus unit tail gas (residual gas) are catalytically converted to H2S by hydrogenation or hydrolysis. The hydrogenation treatment or hydrolysis treatment is carried out at a temperature in the range of 140°C to 550°C using a catalyst containing a metal compound, the metal being selected from metals in Groups Va, VIa, and VIII of the Periodic Table of the Elements, deposited on a silica or silica / alumina support. A more specific catalyst disclosed in the '098 patent is an impregnated bead containing cobalt oxide and molybdenum oxide deposited on alumina. Although the '098 patent discloses a catalyst comprising alumina impregnated with 1.75 wt% cobalt and 8 wt% molybdenum, there is no teaching regarding the range of these components or regarding the form of the alumina of the catalyst. In addition, the application of a bottom metal and a cladding metal in a hydrolysis catalyst has not been recognized, nor has the importance of the catalyst preparation method in providing low-temperature hydrogenation and hydrolysis reactions or in providing a high conversion of sulfur compounds to hydrogen sulfide been recognized.

[0006] U.S. Patent No. 10,562,014 (Krueger et al.) discloses a high-metal-content catalyst that can be used for the low-temperature conversion of sulfur compounds (such as sulfur dioxide, carbonyl sulfide, carbon disulfide, and sulfur) that may be contained in a gas stream. The catalyst is prepared by co-grinding its components containing cobalt metal and molybdenum metal. The co-ground mixture is dried and calcined. The catalyst contains at least 7.5 wt% to at most 15 wt% molybdenum and at least 2.75 wt% to at most 6 wt% cobalt. The pore structure is an important property of the catalyst. It is bimodal, where the first major portion of the total pore volume is in pores with diameters in the to range, and a small portion of the total pore volume is in pores with diameters in the to range. Less than 6% of the total pore volume of the catalyst is contained in pores with a diameter greater than No mention is made of a bottom metal component or a cladding metal component in the catalyst.

[0007] U.S. Patent No. 5,686,375 (Iyer et al.) discloses a hydrotreating catalyst for treating hydrocarbon feeds (such as gas oils) to reduce the concentrations of nitrogen and sulfur compounds by converting the nitrogen compounds and sulfur compounds into ammonium sulfide and hydrogen sulfide, respectively. The catalyst contains a Group VIII hydrogenation metal component in the bottom layer, preferably nickel. The catalyst also has a coating of catalytic promoter metals, which may include a Group VIB metal (preferably molybdenum) and optionally a Group VIII metal. The catalyst contains more than 3.0 wt% of the Group VIII metal component and more than 10 wt% of the Group VIB metal component. The catalyst has a narrow pore size distribution, approximately medium pore diameter, with at least 50% of the total pore volume distributed within 20 angstroms of the median pore diameter. The '375 patent does not distinguish the amounts of metals located in the bottom layer or the coating. Only the Group VIII metal is in the bottom layer. There is no bottom layer Group VIB metal in the catalyst. There is no teaching in the '375 patent regarding the ratio of the bottom layer metal to the coating metal, especially the ratio necessary to promote the hydrolysis reaction. The catalyst is used for hydrodenitrogenation and hydrodesulfurization and does not promote hydrolysis.

[0008] U.S. Patent No. 10,220,379 (Bhan et al.) discloses a catalyst for selectively desulfurizing sulfur compounds in an olefin-containing hydrocarbon feed. The catalyst has a bimodal pore structure and contains nickel, molybdenum, and optionally phosphorus in the bottom layer, and a coating of molybdenum and cobalt. The nickel content is substantially or completely in the form of bottom layer nickel. To prepare the catalyst, an inorganic oxide, molybdenum, and nickel are co-ground, shaped, and calcined to provide shaped and calcined particles impregnated with a coating of cobalt, molybdenum, and phosphorus. The amount of molybdenum in the calcined and shaped particles is 2 wt% to 7 wt%, and the amount of nickel is 0.5 wt% to 2 wt%. The molybdenum content in the final catalyst is in the range of 9 wt% to 23 wt%, and the cobalt content is 2 wt% to 8 wt%. SUMMARY OF THE INVENTION

[0009] Efforts are currently underway to develop improved catalyst compositions for hydrolyzing sulfur compounds contained in a gas stream. In addition, there is a desire to develop improved catalysts that provide a high percentage conversion of sulfur compounds contained in a gas stream (such as a Claus tail gas stream) to hydrogen sulfide under low temperature reaction conditions.

[0010] Accordingly, there is provided a catalyst composition for catalytically reducing sulfur compounds contained in a gas stream. The catalyst composition comprises an inorganic oxide, bottom layer molybdenum, bottom layer cobalt, coating molybdenum, and coating cobalt.

[0011] Another embodiment of the catalyst composition comprises a calcined co-ground mixture of an inorganic oxide, a first molybdenum compound, and a first cobalt compound. The calcined co-ground mixture is impregnated with a second molybdenum compound and a second compound. The impregnated metals are covered to provide a bottom layer of molybdenum, a bottom layer of cobalt, a cladding layer of molybdenum, and a cladding layer of cobalt.

[0012] The catalyst composition of the present invention is prepared by co-grinding an inorganic oxide, a first molybdenum compound, and a first cobalt compound to provide a co-ground mixture. The co-ground mixture is formed into shaped agglomerates. The shaped agglomerates are dried and then calcined under first calcination conditions to provide a calcined shaped agglomerate. A second molybdenum compound and a second cobalt compound are incorporated into the calcined shaped agglomerate to provide an impregnated shaped agglomerate. The impregnated shaped agglomerate is dried and then calcined under second calcination conditions to provide the catalyst composition.

[0013] The catalyst composition can be applied to the hydrolysis of sulfur compounds and carbon monoxide contained in a gas stream. The method includes introducing a gas stream containing a sulfur compound or carbon monoxide or both into a reactor defining a reaction zone containing the catalyst composition of the present invention. The gas stream is contacted with the catalyst composition under hydrolysis reaction conditions. Description of the Drawings

[0014] Figure 1 is a bar graph comparing the performance of the catalyst of the present invention and a comparative catalyst in hydrolytically converting carbonyl sulfide (COS) contained in a synthesis tail gas feed by showing unreacted COS in the effluent of a reactor operated at various reactor temperatures.

[0015] Figure 2 is a bar graph comparing the performance of the catalyst of the present invention and a comparative catalyst in hydrolytically converting carbon disulfide (CS2) contained in a synthesis tail gas feed by showing unreacted CS2 in the effluent of a reactor operated at a given reactor temperature.

[0016] Figure 3 is a bar graph comparing the performance of the catalyst of the present invention and a comparative catalyst in converting carbon monoxide (CO) contained in a synthesis tail gas feed by showing unreacted CO in the effluent of a reactor operated at various reactor temperatures.

[0017] Figure 4 is a bar graph comparing the performance of the catalyst of the present invention and a comparative catalyst in converting methanethiol (CH3SH) contained in a synthesis tail gas feed by showing unreacted CH3SH in the effluent of a reactor operated at various reactor temperatures. Detailed Description

[0018] The catalyst of the present invention has properties that make it particularly useful for the low-temperature hydrolysis of carbonyl sulfide (COS) and carbon disulfide (CS2) contained in a gas stream, such as a Claus tail gas or other gas streams, in which the concentrations of carbonyl sulfide and carbon disulfide need to be removed or reduced to acceptable levels. The catalyst can also be applied to the low-temperature conversion of carbon monoxide in the water-gas shift reaction.

[0019] As used herein, the term "hydrolysis reaction" refers to the reaction of carbonyl sulfide with water to produce hydrogen sulfide and carbon dioxide and the reaction of carbon disulfide with water to produce hydrogen sulfide and carbon dioxide.

[0020] The catalyst of the present invention also promotes the low-temperature hydrogenation of SO2 to produce water and sulfur and the hydrogenation of S x to H2S. An undesired reaction that the treatment catalyst may promote is the hydroconversion of carbon disulfide to methanethiol and hydrogen sulfide.

[0021] The water-gas shift reaction referred to herein is the equilibrium reaction of carbon monoxide and water to carbon dioxide and hydrogen.

[0022] A novel catalyst composition has been invented that has excellent properties for converting sulfur, sulfur compounds, and carbon monoxide contained in a gas stream by catalytic hydrolysis, hydrogenation, and water-gas shift reactions. It is the unique combination of the various features of the catalyst of the present invention that provides its enhanced catalytic properties. One of these features that is different from prior art compositions is that the catalyst of the present invention contains both molybdenum metal and cobalt metal in an underlying form and both molybdenum and cobalt as overlying metals. These metals should be present in the composition of the present invention in certain defined concentration and relative weight ratio ranges. Other embodiments of the catalyst of the present invention require a specific weight ratio of underlying metal to overlying metal within a specified range.

[0023] When referring to the "underlying" metal herein, it means a catalytic metal or metal precursor or compound that is uniformly dispersed in an inorganic oxide support material (such as alumina) to form a mixture, which is then calcined and subsequently contacted with a second catalytic metal or metal precursor or compound, followed by a second calcination. The contact of the calcined mixture with the second metal is preferably carried out by metal impregnation. The metal that is first mixed with the inorganic oxide and then calcined is called the "underlying" metal; because the subsequent contact of the calcined mixture with the second metal is considered to "cover" the metal of the calcined mixture with a coating of the second metal. Thus, the term "overlying" or "overlay" or similar terms are used herein to mean that the second catalytic metal or metal precursor or compound is placed on top of or covers the metal of the calcined mixture, and the metal of the calcined mixture thus becomes the underlying one.

[0024] The catalyst of the present invention comprises both a bottom layer of molybdenum and a bottom layer of cobalt, both of which are covered with molybdenum and cobalt. In fact, the basic feature of the catalyst of the present invention is that it comprises a bottom layer metal and a coating metal. Not only is it important for the catalyst of the present invention to include both a bottom layer metal and a coating metal, but these metals should be present in appropriate relative amounts in the composition to obtain a catalyst having the enhanced catalytic properties. The total amount of metal contained in the catalyst is also important for providing a catalyst having the performance characteristics of the catalyst of the present invention.

[0025] The catalyst of the present invention should have a total molybdenum content of at least 7.4% by weight and a total cobalt content of at least 2.7% by weight. This % by weight is based on the total weight of the catalyst composition and the corresponding metal component as an oxide. When referring to the total molybdenum content or the total cobalt content of the catalyst, these terms mean the total weight of the said metal contained in the whole composition, including the sum of both the bottom layer metal content and the coating metal content.

[0026] The total molybdenum content of the catalyst is preferably in the range of 7.6% to 22% by weight. More preferably, the total molybdenum content of the catalyst is in the range of 7.8% to 20% by weight, and most preferably in the range of 7.8% to 18% by weight.

[0027] The total cobalt content of the catalyst is preferably in the range of 3% to 8% by weight. More preferably, the total cobalt content of the catalyst is preferably in the range of 3.3% to 6% by weight, and most preferably in the range of 3.6% to 5% by weight.

[0028] Thus, generally, the total metal ratio of the total amount of the molybdenum content of the catalyst divided by the total amount of the cobalt content of the catalyst is in the range of 1.5:1 to 8:1. Preferably, the total metal ratio is in the range of 1.7:1 to 6:1, and more preferably in the range of 2:1 to 5:1. This total metal ratio is calculated on an elemental basis and based on the total weight of the catalyst.

[0029] As mentioned above, the basic feature of the catalyst of the present invention is that it contains both a bottom layer metal and a coating metal. It has been found that the ratio of the amount of the bottom layer metal to the amount of the coating metal is important for providing the catalyst of the present invention with the enhanced catalytic properties described herein. If the relative ratio is too low, the catalytic benefit provided by the combination of the bottom layer and the coating of the metal is small, but if the relative ratio is too high, the catalytic benefit is also small. Although it is not yet fully understood why the bottom layer metal and the coating metal are combined in the desired relative weight ratio, it is believed that several different active catalyst phases can be formed by co-grinding the metal with an inorganic support and by metal impregnation. Theoretically, one catalyst phase is more effective in promoting one type of reaction, while another catalyst phase is more effective in promoting another type of reaction.

[0030] Thus, in the embodiments of the catalyst of the present invention, its molybdenum ratio is in the range of 1:5 to 4:5, and the cobalt ratio is in the range of 1:5 to 4:5. Preferably, both of these ratios are in the range of 1:4 to 3:4, and more preferably in the range of 1:3 to 2:3.

[0031] When referring to the molybdenum ratio, it means the weight percentage of molybdenum in the underlying form in the catalyst divided by the total molybdenum content of the catalyst. This ratio is calculated based on molybdenum as the oxide.

[0032] When referring to the cobalt ratio, it means the weight percentage of cobalt in the underlying form in the catalyst divided by the total cobalt content of the catalyst. This ratio is calculated based on cobalt as the oxide.

[0033] The total amount of underlying molybdenum in the catalyst of the present invention should generally be in the range of 1.2 wt% to 12.5 wt%. Preferably, the amount of underlying molybdenum present in the catalyst is in the range of 1.15 wt% to 11 wt%, and more preferably in the range of 1.2 wt% to 10 wt%. The total amount of underlying cobalt in the catalyst of the present invention should be in the range of 0.5 wt% to 5 wt%. Preferably, the amount of underlying cobalt present in the catalyst is in the range of 0.55 wt% to 4 wt%, and more preferably in the range of 0.6 wt% to 3 wt%. These wt% values are based on the total weight of the catalyst and the respective metal components as the oxides.

[0034] The inorganic oxide component of the catalyst is selected from any refractory oxide material having properties suitable for use as a support component in the catalyst composition of the present invention. Examples of possible suitable porous refractory oxide materials include silica, magnesia, silica - titania, zirconia, silica - zirconia, titania, silica - titania, alumina, silica - alumina, and aluminosilicate. Preferred porous refractory oxides are selected from the group consisting of alumina, silica, and alumina - silica. A more preferred refractory oxide for the catalyst of the present invention is alumina. The amount of the inorganic oxide component of the catalyst is present in the range of 65 wt% to 90 wt% of the total weight of the composition. The amount of it present in the catalyst can also be in the range of 70 wt% to 90 wt% or 75 wt% to 89 wt%.

[0035] As described above, theoretically, the manner of preparing the catalyst of the present invention and its physical properties are related to enhancing its catalytic properties and forming various active catalytic phases. Co - grinding the metal with the inorganic oxide to provide a homogeneous mixture, which is calcined before being impregnated with the metal, is believed to produce a specific catalytic phase different from the catalytic phase formed by calcining after metal impregnation.

[0036] In the method for preparing the catalyst of the present invention, two metal incorporation steps and two calcination steps are used. Each of these steps is believed to provide a different active catalytic phase in the final catalyst. The first metal incorporation step is carried out by co-grinding a starting metal component with an inorganic oxide to obtain a co-ground mixture having the metal component uniformly or homogeneously dispersed within the inorganic oxide material. The co-ground mixture is preferably formed into an agglomerate or granule, such as an extrudate, which is dried and calcined. The second metal incorporation step is carried out by contacting the calcined granules with an overlay of a metal component, preferably by applying one or more aqueous metal impregnation solutions, to obtain a metal-impregnated shaped agglomerate, which is dried and calcined.

[0037] The use of the term "co-grinding" refers to combining and mixing the starting materials of the catalyst to form a mixture of the various components, which are preferably or substantially uniform or homogeneous. The scope of the term is broad enough to include mixing starting materials including alumina, cobalt compounds, and molybdenum compounds to produce a co-ground mixture capable of forming agglomerate granules. The co-ground mixture can be a paste or plastic mixture capable of forming agglomerate granules by any known agglomeration method or being extruded into extrudate granules by any known extrusion method.

[0038] A preferred method of agglomerating the co-ground mixture is by extrusion to form extrudate granules having a total diameter in the range of 0.5 mm to 10 mm or 0.75 mm to 8 mm and an aspect ratio of 1:1 to 10:1 or even higher. The extrudate can be of any typical shape, such as a cylinder and a multi-lobe shape.

[0039] Thus, the formation of the co-ground mixture is carried out by any method or means known to those skilled in the art, including but not limited to using such suitable types of solid mixers (such as drums, stationary shells or troughs, batch or continuous pan mixers, and impact mixers), and using such suitable types of batch or continuous mixers to mix solids and liquids or to form an extrudable paste mixture.

[0040] Suitable types of batch mixers include but are not limited to change-can mixers equipped with any suitable type of mixing blades, stationary trough mixers, and double-arm kneading mixers.

[0041] Suitable types of continuous mixers include but are not limited to single-screw or twin-screw extruders, trough-screw mixers, and agitators.

[0042] The mixing of the starting materials for preparing the co-ground mixture may include water and a suitable amount of an inorganic acid, such as nitric acid, which is necessary to provide the above-mentioned paste mixture having a loss on ignition (LOI) in the range of 40% to 80% as determined by the standard test method ASTM D7348. It has been found that the co-ground mixture with an LOI in this range will provide a paste having desirable extrusion properties and which contributes to giving the finished catalyst product the desired pore structure characteristics of the catalyst of the present invention as described in detail throughout the specification.

[0043] The mixing of the starting materials for preparing the co-ground mixture is carried out for a necessary period of time to properly homogenize the co-ground mixture. Generally, the blending time is in the range up to 12 hours or longer. Usually, the blending time is in the range of 0.1 hour to 1 hour.

[0044] The cobalt compound of the co-ground mixture is a cobalt compound that can be converted into an oxide when calcined in the presence of oxygen. The cobalt compound can be selected from suitable cobalt salt compounds. Such compounds can include cobalt compounds selected from ammonium cobalt compounds and phosphates, nitrates, oxalates, sulfates, and halides of cobalt. A particularly advantageous cobalt salt found to be useful for the co-ground mixture is cobalt nitrate. Preferably, the cobalt compound is combined with the other components of the co-ground mixture in the form of a first aqueous solution containing cobalt. This first aqueous solution can be formed by dissolving a cobalt salt in water. The most preferred cobalt salt is cobalt nitrate.

[0045] The molybdenum compound of the co-ground mixture is a molybdenum compound that can be converted into an oxide when calcined in the presence of oxygen. The molybdenum compound can be selected from suitable molybdenum salt compounds. Such compounds can include molybdenum compounds selected from compounds such as ammonium molybdate, potassium molybdate, sodium molybdate, phosphomolybdic acid, molybdenum disulfide, molybdenum trioxide, and molybdic acid. Preferably, the molybdenum compound is combined with the other components of the co-ground mixture in the form of a second aqueous solution containing molybdenum. This second aqueous solution can be formed by dissolving a molybdenum salt in water. The most preferred molybdenum salt is ammonium molybdate, such as ammonium heptamolybdate and ammonium dimolybdate.

[0046] The shaped agglomerates of the co-ground mixture are dried and then calcined. The drying of the co-ground mixture is not a critical step and is generally carried out in air and at a drying temperature in the range of 20 °C to 125 °C. The period of drying is any suitable period that provides the desired amount of drying.

[0047] The calcination of the co-ground mixture is a necessary step in the method of the present invention. This step provides the conversion of the metal compounds into their oxide forms and produces an intermediate calcined shaped agglomerate or particle.

[0048] Calcination of the shaped aggregates or agglomerates of the co-ground mixture is carried out in the presence of an oxygen-containing fluid such as air, at a temperature and for a period of time suitable to achieve the desired degree of calcination to provide the final catalyst composition of the present invention. Generally, the calcination temperature ranges from 300 °C to 800 °C, preferably from 350 °C to 700 °C, and more preferably from 400 °C to 600 °C. The calcination period can range from 0.1 hour to 96 hours.

[0049] The concentration levels of the metal components of the calcined co-ground mixture and their importance are as described above.

[0050] The resulting impregnated shaped agglomerates or impregnated particles are dried and then calcined. Drying of the impregnated particles is not a critical step and is generally carried out in air and at a drying temperature in the range of 20 °C to 125 °C. The period of drying is any suitable period that provides the desired amount of drying.

[0051] Calcination of the impregnated particles is carried out in the presence of an oxygen-containing fluid such as air, at a temperature and for a period of time suitable to achieve the desired degree of calcination to provide the final catalyst composition of the present invention.

[0052] Generally, the calcination temperature ranges from 300 °C to 800 °C, preferably from 350 °C to 700 °C, and more preferably from 400 °C to 600 °C. The calcination period can range from 0.1 hour to 96 hours.

[0053] The concentration levels of the metal components of the final catalyst composition and their importance are as described above.

[0054] The catalyst composition of the present invention can be used to hydrolyze sulfur compounds contained in a gas stream, and more particularly, the catalyst composition can be used in particular to treat the tail gas stream generated by a Claus process unit in order to convert the sulfur compounds contained in the tail gas stream into H2S, which can then be removed by any one of a number of suitable means or methods known to those skilled in the art for removing H2S from a gas stream.

[0055] When used to treat a Claus unit tail gas stream, the catalyst composition has certain unique catalytic properties that allow the hydrolysis reactor to operate at a lower temperature than that required for a hydrolysis reactor using a conventional catalyst, and the catalyst composition provides a high conversion of sulfur compounds even at lower reactor temperature conditions.

[0056] The catalyst composition also allows a gas stream to pass through the hydrolysis reactor at a much higher flow rate and thus a much higher space velocity than is allowed for a hydrolysis reactor loaded with a conventional catalyst, but still provides a high conversion of sulfur compounds at reduced reactor temperature conditions.

[0057] In the operation of a typical conventional hydrolysis reactor system, which includes a reactor loaded with a conventional hydrolysis catalyst, it is necessary to significantly heat the tail gas before introducing it into the hydrolysis reactor. This is because the tail gas discharged from the Claus unit passes through a sulfur condenser operating at a temperature close to the condensation temperature of elemental sulfur. The temperature of a typical Claus unit tail gas stream is in the range of 110 °C to 125 °C. For a conventional hydrolysis unit, the tail gas usually has to be heated so that the introduction temperature of the tail gas feed entering the hydrolysis reactor or the reactor inlet temperature is in the range of 250 °C to 350 °C. Any reduction in this tail gas feed inlet temperature required for the hydrolysis reactor will provide significant energy savings in its operation.

[0058] Therefore, using the catalyst composition of the present invention in treating a Claus tail gas stream can provide significant energy savings by reducing the temperature required to treat the Claus tail gas stream.

[0059] The gas stream that can be treated using the catalyst composition of the present invention includes one or more gaseous compounds, and further, it includes at least one sulfur compound. As used herein, the term sulfur compound is a molecular or elemental compound selected from the group consisting of: carbonyl sulfide (COS), carbon disulfide (CS2), sulfur dioxide (SO2), and elemental sulfur (S x ). Hydrogen sulfide is omitted from this definition of sulfur compounds; this is because the catalyst composition of the present invention is not intended to provide the conversion of H2S, but rather is intended to reduce sulfur compounds by reduction to hydrogen sulfide.

[0060] The hydrogen sulfide can then be removed from the treated gas stream. Thus, the gas stream includes compounds that are typically gaseous or in the gas phase under the temperature and pressure conditions of the hydrolysis reactor operation. Examples of gaseous compounds other than the above sulfur compounds include nitrogen, oxygen, carbon dioxide, carbon monoxide, hydrogen, water, and lower hydrocarbons such as methane, ethane, and ethylene.

[0061] The total concentration of sulfur compounds contained in the gas stream charged or introduced into the hydrolysis reactor containing the catalyst composition of the present invention can be in the range of 0.01 volume % (100 ppmv) to 5 volume % of the total gas stream. More typically, the sulfur compound concentration is in the range of 0.02 volume % (200 ppmv) to 3 volume %.

[0062] As described above, the catalyst composition is particularly suitable for treating a Claus tail gas stream to convert sulfur compounds contained therein into hydrogen sulfide, thereby providing a treated gas stream having a reduced sulfur compound concentration to below the sulfur compound concentration in the tail gas stream to be treated. Table 1 below gives typical ranges of the more common components making up the Claus tail gas stream.

[0063] Table 1 - Claus tail gas composition

[0064]

[0065]

[0066] In the hydrolysis process of the present invention, a gas stream having a certain concentration of sulfur compounds is introduced into a hydrolysis reactor containing the catalyst composition and operating under suitable hydrolysis or reduction reaction conditions. Inside the hydrolysis reactor, the gas stream contacts the catalyst composition contained therein. A treated gas stream with a reduced sulfur compound concentration is produced from the hydrolysis reactor. Although the H2S concentration of the treated gas stream increases compared to the gas stream, the sulfur compound concentration of the treated gas stream decreases compared to the gas stream. The reduced concentration of sulfur compounds should generally be less than 100 ppmv, preferably less than 50 ppmv, and most preferably less than 30 ppmv.

[0067] As described above, one advantage of using the catalyst composition of the present invention in the hydrolysis of a Claus tail gas stream is that it allows the hydrolysis reactor to operate at a relatively low inlet temperature (e.g., less than 250 °C). There is a minimum temperature at which the gas stream should be introduced into the hydrolysis reactor, and thus, the inlet temperature at which the gas stream is charged or introduced into the hydrolysis reactor is generally in the range of 140 °C to 250 °C. It is preferred that the introduction temperature is in the range of 150 °C to 240 °C, and more preferably, the introduction temperature is in the range of 160 °C to 230 °C. Most preferably, the temperature at which the gas stream is introduced into the hydrolysis reactor is in the range of 170 °C to 220 °C.

[0068] The operating pressure of the hydrolysis reactor is generally in the range of 1 bar (14.5 psi) to 100 bar (1450.3 psi), preferably in the range of 2 bar (29.0 psi) to 70 bar (1015.3 psi), and more preferably in the range of 3 bar (43.5 psi) to 50 bar (725.2 psi).

[0069] The flow rate at which the gas stream and the added reducing gas (if any) are introduced into the hydrolysis reactor is generally such that, for example, it provides in 10 hours -1 to 10,000 hours -1The gas hourly space velocity (GHSV) within a range. The term "gas hourly space velocity" refers to the numerical ratio of the rate of loading the hydrocarbon feedstock into the hydrolysis reactor (in volume / hour) divided by the volume of the catalyst contained in the hydrolysis reactor into which the gas stream is to be loaded. The preferred GHSV is in the range of 10 hours -1 to 8,000 hours -1 and more preferably in the range of 500 hours -1 to 5,000 hours -1 and most preferably in the range of 1,000 hours -1 to 4,000 hours -1 and within this range.

[0070] When treating a Claus tail gas stream, in most cases, it will contain a certain concentration of water and hydrogen, which can be a source of reducing gas required for the hydrolysis reaction of the hydrolysis process. However, in the case where the gas stream does not contain a sufficient concentration of reducing gas components, reducing gas can be added to the gas stream as needed. It is generally desirable to have a stoichiometric amount of reducing gas in the gas stream to allow the hydrolysis reaction to proceed close to completion.

[0071] The following examples illustrate certain aspects of the present invention, but they should not be construed as unduly limiting the scope of the present invention.

[0072] Example

[0073] Example I

[0074] Example I illustrates the preparation methods and catalyst compositions of the present invention and comparative catalysts.

[0075] Catalyst composition A of the present invention

[0076] An embodiment of the catalyst composition of the present invention is prepared by grinding wide-pore alumina powder mainly containing pseudoboehmite with nitric acid and water in a certain ratio such that a plastic mixture (e.g., an extrudable mixture) with a certain water content is provided, and its loss on ignition is about 62%. An aqueous cobalt-containing solution containing cobalt is prepared by dissolving cobalt nitrate in water, and an aqueous molybdenum-containing solution containing molybdenum is prepared by dissolving ammonium dimolybdate in water containing 30% hydrogen peroxide. These two metal solutions are added to the grinding mixture, and after mixing for a period of time, a small percentage of ammonium hydroxide is mixed with the grinding mixture. Then the resulting mixture is extruded through a 3.2 mm three-lobed extrusion die, and the extrudate is dried and calcined. This intermediate catalyst composition contains alumina mainly in the γ form, 8.2 wt% of molybdenum, and 2.5 wt% of cobalt. The wt% of the metal is based on the total weight of the intermediate catalyst and the metal in oxide form.

[0077] An impregnation solution is prepared by mixing ammonia water, ammonium dimolybdate and cobalt carbonate in certain amounts such that, for example, in the finished catalyst there is 14 wt% molybdenum (elemental) and 3.4 wt% cobalt (elemental). The mixture is heated to 45 °C and monoethanolamine (MEA) in an amount of 0.5 mol MEA / mol cobalt to 0.9 mol MEA / mol cobalt is added to the mixture. The mixture is stirred while maintaining the temperature until the metal salts are digested. The solution is then cooled to about 30 °C and made up with water to provide a total volume of solution approaching the pore volume of the intermediate to be impregnated with the solution. The co-ground intermediate prepared above is impregnated with the solution and aged for two hours, mixing from time to time to prevent agglomeration. The impregnated extrudate is dried in a convection oven at 80 °C for two hours and then further dried at 125 °C for two hours. The dried extrudate is calcined in a muffle furnace at 552 °C for 75 minutes.

[0078] Comparative catalyst composition B

[0079] A comparative catalyst composition is prepared by grinding broad pore alumina powder mainly containing pseudoboehmite with nitric acid and water in a certain ratio such that a plastic mixture (e.g., an extrudable mixture) with a certain water content is provided, such that its loss on ignition is about 62%. An aqueous cobalt-containing solution containing cobalt is prepared by dissolving cobalt nitrate in water, and an aqueous molybdenum-containing solution containing molybdenum is prepared by dissolving ammonium dimolybdate in water containing 30% hydrogen peroxide. These two metal solutions are added to the grinding mixture, and after mixing for a period of time, a small percentage of ammonium hydroxide is mixed with the grinding mixture. The resulting mixture is then extruded through a 3.2 mm trilobal extrusion die, and the extrudate is dried and calcined. The intermediate catalyst composition contains alumina mainly in the γ form, 9.4 wt% molybdenum and 3.6 wt% cobalt. The wt% of the metals is based on the total weight of the finished catalyst and the metals in oxide form.

[0080] Example II

[0081] This Example II illustrates the use of the catalyst described in Example I in the hydrolysis of a gas stream containing a certain concentration of at least one sulfur compound and gives the performance data of the catalyst.

[0082] The performance of the catalyst of Example I was tested using a tail gas pilot unit reactor equipped with a tube furnace for controlling the reactor temperature. When preparing for the activity test, each catalyst was sulfided by introducing a feed containing H2S and H2 into the reactor at 300 °C and 467 GHSV for 3 hours. Then, a synthetic tail gas containing H2S, SO2, COS, CS2, S, H2, CO, N2, and steam and having a typical composition as shown in Table 2 was charged into the tail gas reactor operating at various reactor temperatures at a rate such that 2052 nGHSV (standard gas hourly space velocity, unit pressure of 3 psi) was provided.

[0083] Table 2. Typical feed composition

[0084]

[0085]

[0086] The composition of the reactor effluent was analyzed by gas chromatography for various reactor temperature conditions. The test results are shown in the following Tables 3 to 5 and are further illustrated by Figures 1 to 4 bar graphs.

[0087] Table 3. Unconverted COS in reactor effluent

[0088]

[0089] Table 4. Unconverted CS2 in reactor effluent

[0090]

[0091] Table 5. Unconverted CO in reactor effluent

[0092]

[0093] The data given in the above tables show that the catalysts of the present invention, including both the base metal and the cladding metal, exhibit significantly better catalytic performance than the comparative catalysts without the base metal.

[0094] Figure 1 The data presented in Table 3 are shown in the form of bar graphs, and they help to illustrate the enhanced performance characteristics of the catalysts of the present invention (Catalyst A of the present invention) when compared with the catalysts without the base metal (Comparative Catalyst B). These experimental results show that the catalysts of the present invention can be operated at a temperature approximately 20 °C lower than the comparative catalysts while still maintaining substantially the same COS in the effluent. This is a significant improvement in catalyst activity of approximately 144%. For any given reactor temperature, the catalysts of the present invention provide significantly improved COS removal compared with the comparative catalysts.

[0095] Figure 2 The data presented in Table 4 are shown in the form of a bar graph and serve to illustrate the enhanced performance characteristics of the catalyst A of the present invention when compared with the comparative catalyst B. These experimental results show that the catalyst of the present invention can be operated at a temperature more than 20 °C lower than the comparative catalyst while still maintaining substantially the same CS2 in the effluent. This is a significant improvement with a catalyst activity of approximately 72%. For lower reactor temperatures, the catalyst of the present invention provides a significantly improved CS2 removal compared to the comparative catalyst.

[0096] Figure 3 The data presented in Table 5 are shown in the form of a bar graph, illustrating the enhanced performance characteristics of the catalyst A of the present invention when compared with the comparative catalyst B. These experimental results show that the catalyst of the present invention can be operated at a temperature more than 20 °C lower than the comparative catalyst while still maintaining substantially the same CO in the effluent. This is a significant improvement with a catalyst activity of approximately 72%. When compared with the reduction provided by the comparative catalyst at the same reactor temperature, the catalyst of the present invention provides a significantly reduced CO in the reactor effluent.

[0097] Figure 4 The enhanced performance characteristics of the catalyst A of the present invention when compared with the comparative catalyst B are shown. These experimental results show that the catalyst of the present invention can be operated at a temperature approximately 20 °C lower than the comparative catalyst while maintaining substantially the same CH3SH in the effluent. This is a significant improvement with a catalyst activity of approximately 72%.

[0098] It will be apparent to those of ordinary skill in the art that many changes and modifications can be made to the described invention without departing from the spirit and scope of the invention as set forth in this specification.

Claims

1. A catalyst composition for catalytically reducing sulfur compounds contained in a Claus tail gas, wherein the catalyst composition comprises: an inorganic oxide, a bottom layer of molybdenum, a bottom layer of cobalt, a coating layer of molybdenum, and a coating layer of cobalt, and wherein the catalyst composition has a total molybdenum content of at least 7.4 wt% and a total cobalt content of at least 2.7 wt%, where both wt% are based on the total weight of the catalyst composition and the corresponding metal components as oxides, wherein the molybdenum ratio of the bottom layer molybdenum content to the total molybdenum content of the catalyst composition is in the range of 1:3 to 4:5, and its cobalt ratio of the bottom layer cobalt content to the total cobalt content is in the range of 1:3 to 4:

5.

2. The catalyst composition according to claim 1, wherein the molybdenum ratio of the bottom layer molybdenum content to the total molybdenum content of the catalyst composition is in the range of 1:3 to 3:4, and its cobalt ratio of the bottom layer cobalt content to the total cobalt content is in the range of 1:3 to 3:

4.

3. The catalyst composition according to claim 2, wherein the total metal ratio of the total molybdenum content to the total cobalt content of the catalyst composition is in the range of 1.5:1 to 6:

1.

4. The catalyst composition according to claim 3, wherein the total inorganic oxide content of the catalyst composition is in the range of 65 wt% to 90 wt% based on the total weight of the catalyst composition.

5. The catalyst composition according to claim 4, wherein the bottom layer molybdenum content is in the range of 1.1 wt% to 12.5 wt%, and the bottom layer cobalt content is in the range of 0.5 wt% to 5 wt%, the wt% being based on the total weight of the catalyst composition and the corresponding metal components as oxides.

6. The catalyst composition according to any one of claims 1 - 5, wherein the molybdenum ratio of the bottom layer molybdenum content to the total molybdenum content of the catalyst composition is in the range of 1:3 to 2:3, and its cobalt ratio of the bottom layer cobalt content to the total cobalt content is in the range of 1:3 to 2:

3.

7. A method for preparing the catalyst composition according to any one of claims 1 - 6, wherein the method comprises: a) co - grinding an inorganic oxide, a first molybdenum compound, and a first cobalt compound to provide a co - ground mixture; b) forming the co - ground mixture into a shaped agglomerate; c) drying and calcining the shaped agglomerate under first calcination conditions to provide a calcined shaped agglomerate; d) incorporating a second molybdenum compound and a second cobalt compound into the calcined shaped agglomerate to provide an impregnated shaped agglomerate; and e) drying and calcining the impregnated shaped agglomerate under second calcination conditions to provide the catalyst composition.

8. The method according to claim 7, wherein the first calcination conditions include calcining the shaped agglomerate in the presence of oxygen and at a first calcination temperature in the range of 300°C to 800°C, and wherein the second calcination conditions include calcining the impregnated shaped agglomerate in the presence of oxygen at a second calcination temperature in the range of 300°C to 800°C.

9. A method for catalytically reducing sulfur compounds contained in a Claus tail gas stream, the method comprising: introducing a Claus tail gas stream containing sulfur compounds or carbon monoxide or both into a reactor, the reactor defining a reaction zone containing a catalyst composition according to any one of claims 1 to 6, the catalyst composition comprising an inorganic oxide, an underlying molybdenum, an underlying cobalt, an overlying molybdenum, and an overlying cobalt; and contacting the gas stream with the catalyst composition under hydrolysis reaction conditions.

10. The method according to claim 9, wherein the sulfur compounds are present in the gas stream at a sulfur compound concentration in the range of 0.01 vol% to 2 vol%, and wherein the sulfur compounds are selected from the group consisting of: Carbonyl sulfide (COS), carbon disulfide (CS2), sulfur dioxide (SO2), and elemental sulfur (S x ).

11. The method according to claim 10, wherein the hydrolysis reaction conditions include an inlet temperature of the reactor in the range of 115 °C to 300 °C.

12. The method according to claim 10, wherein, After treatment, the concentration of sulfur compounds in the gas stream is reduced to less than 75 ppmv.

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