Process for the treatment of a tail gas stream

Abstract

A process for the treatment of a tail gas stream comprising one or more sulfur compounds and / or carbon monoxide in the presence of water and hydrogen, under hydrolysis conditions, in a reactor system comprising one or more reactors, wherein said process takes place in the presence of a catalyst system comprising a first catalyst composition and a second catalyst composition, wherein the first catalyst composition and the second catalyst composition each comprise a cobalt compound and / or a nickel compound in combination with a molybdenum compound and an inorganic oxide selected from alumina and / or titania and the first catalyst composition has a higher activity for the removal of carbon monoxide and / or a higher activity for the removal of carbonyl sulfide (COS) and / or a higher activity for the removal of carbon disulfide (CS2) from a tail gas stream relative to the second catalyst composition under the same conditions, and wherein the first and second catalyst compositions are arranged such that at least some of the first catalyst composition is situated upstream of the second catalyst composition in the reactor system.

Description

[0001] SP 2823 PD PROCESS FOR THE TREATMENT OF A TAIL GAS STREAM Field of the Invention 5 The present invention relates to a process for the treatment of a tail gas stream comprising one or more sulfur compounds and / or carbon monoxide. Background of the Invention In the well-known Claus process, an acid gas that 10 contains a significant percentage of hydrogen sulfide (H2S) is combusted in a thermal stage in order to oxidize a portion of the H2S to sulfur dioxide (SO2). This combustion is controlled to provide a process gas stream containing H2S and SO2that are present in an 15 approximate molar ratio of 2 moles of H2S per mole of SO2(2:1). This process gas stream is passed to a catalytic stage providing for the reaction of H2S and SO2in the presence of an alumina catalyst in accordance with the Claus reaction to yield elemental sulfur and 20 water. The sulfur is then condensed from the Claus reaction gas to yield a Claus tail gas stream. The Claus tail gas stream typically contains small concentrations of H2S and other sulfur compounds, such as, SO2, carbon disulfide (CS2), carbonyl sulfide (COS), 25 methyl mercaptan (CH3SH), and elemental sulfur (Sx). In order for this tail gas stream to be combusted, or otherwise disposed of, it must further be processed to remove most of the sulfur. This provides a treated gas stream having a sufficiently low sulfur content that 30 allows its combustion or release into the atmosphere. One method of treating the tail gas is to pass it to a reduction reactor. The reduction reactor provides for catalytic reduction of sulfur compounds (i.e., SO2, SP 2823 PD - 2 – CS2, COS, and S) contained in the tail gas to H2S to yield a treated gas stream having a reduced concentration of the sulfur compounds. This treated gas stream may further be processed to remove H2S, for 5 example, by passing it to an absorption unit that removes the H2S. This is typically done by contacting the treated gas stream with an H2S removal absorbent. Shell’s SCOT process (Shell Claus Off-gas Treating Process) was developed to treat tail gas streams by a 10 combination of catalytic hydrogenation and hydrolysis reactions. Accordingly, U.S. Patent No. 3554689 (Bloembergen et al.) describes for the removal of carbonyl sulfide, i.e., COS, from a gas stream by catalytic hydrolysis into H2S. Disclosed in this patent 15 is a process by which COS is removed from combustion gases that also contain oxygen by first contacting the gases with an active hydrogenation catalyst for converting the oxygen and, thereafter, contacting the resulting substantially oxygen-free gases with a COS 20 conversion catalyst for converting the COS to H2S. The H2S can then be removed by absorption. The conversion of COS may be affected at temperatures below 150oC. The COS conversion catalyst includes alumina, having a specific surface area of more than 50 m2 / g and can 25 contain one or more Group VI and / or Group VIII metal oxides. Further embodiments of the COS conversion catalyst include the presence therein of an amount of alkali metal phosphate. One requirement of the process of the ‘689 patent is for the combustion gases to first 30 undergo a catalytic oxygen removal step so that the gas that is treated to remove the COS by catalytic hydrolysis is substantially oxygen free. SP 2823 PD - 3 – U.S. Patent No. 4668491 (Wimmer et al.) discloses a process and catalyst for selective catalytic hydrolysis of the sulfur compounds COS and / or CS2that are present in a carbon monoxide containing process gas. 5 The hydrolysis catalyst disclosed by the ‘491 patent is an alkalized chromium oxide-aluminum oxide catalyst that includes chromium oxide and an alkali metal compound supported on an aluminum oxide carrier with gamma alumina being the preferred form of aluminum oxide. The 10 carbon monoxide content of the process gas is significant and is passed over the hydrolysis catalyst at temperatures in the range of from 100oC to 350oC. The alkalized chromium oxide-aluminum oxide catalyst is produced by immersing an aluminum oxide carrier in a 15 chromium salt solution followed by drying and calcining the impregnated carrier. The resulting chromium- impregnated and calcined support is then immersed in a potassium salt and dried. U.S. Patent No. 5132098 (Kvasnikoff et al.) 20 discloses a process in which the sulfur compounds of SO2, CS2, COS and elemental sulfur contained in a Claus unit tail gas (residual gas) are catalytically converted by either hydrogenation or hydrolysis to H2S. This hydrogenation or hydrolysis treatment is carried out at 25 a temperature in the range of from 140oC to 550oC using a catalyst that contains a compound of a metal selected from the metals of groups Va, VIa and VIII of the periodic table which is deposited on a silica or silica / alumina support. A more specific catalyst 30 disclosed in U.S. Patent No. 5132098 is an impregnated bead that includes cobalt oxide and molybdenum oxide deposited on alumina, so as to contain 1.75 wt.% cobalt SP 2823 PD - 4 – and 8 wt.% molybdenum, expressed as weight of the catalyst. U. S. Patent No. 6080379 (Nedez et al.) discloses an alumina catalyst used for the treatment of sulfur- 5 containing gases either by carrying out the Claus reaction or by hydrolysis. The catalyst has an optimized macroporosity wherein its porosity is such that the volume in the pores of diameter greater than 0.1 μm (1,000 Å) is greater than 12 ml / 100g of catalyst and 10 that the ratio of the volume in the pores of diameter greater than 1 μm (10,000 Å) to the volume in the pores of diameter greater than 0.1 μm (1,000 Å) is greater than or equal to 0.65. The alumina may possibly be a transition alumina selected from the group consisting of 15 rho (ρ), chi (χ), eta (η), gamma (γ), kappa (κ), theta (θ), delta (δ), and alpha (α). The catalyst may additionally contain a metal oxide. The use of the catalyst in the hydrolysis of CS2appears to require a significantly high reactor temperature but still without 20 providing for a high CS2 conversion. WO 98 / 07502 A1 (van den Brink et al.) discloses a process for reducing the total sulfur content of a gas stream such as a Claus off-gas, wherein the hydrogenation / hydrolysis step takes place by contacting 25 the gas stream, in the presence of a reducing gas, first with a hydrogenation catalyst comprising hydrogenating metal on a carrier of amorphous silica-alumina and then over a hydrolysis catalyst. It is mentioned in WO 98 / 07502 A1 that as a result of using a hydrogenation 30 catalyst based on an amorphous silica-alumina carrier, the hydrogenation can be carried out at a lower temperature than in conventional hydrogenation processes for Claus off-gas. SP 2823 PD - 5 – U.S. Patent No. 8142748 (Massie) discloses a hydrolysis catalyst that for the low- temperature reduction of sulfur compounds contained in a gas stream. The catalyst is an impregnated catalyst. One 5 significant characteristic of the catalyst is its macroporosity, which provides a pore structure such that a large percentage, in particular, greater than 30%, of its total pore volume is contained within the pores of a pore diameter greater than 10,000 Å. The catalyst 10 comprises alumina and relatively high loadings of Group VI and Group VIII metals. A preferred embodiment of the catalyst has at least 50 percent alumina in the form of eta-alumina. EP 2944367 A1 (Ito et al.) discloses a process for 15 reducing the total sulfur content of a gas stream comprising hydrogen sulfide and sulfur dioxide, which process comprises: (a) contacting the gas stream comprising hydrogen sulfide and sulfur dioxide first with a hydrogenation 20 catalyst and then with a hydrolysis catalyst, in the presence of a reducing gas and under hydrogenation conditions, to obtain a hydrogenated gas stream enriched in hydrogen sulfide; and (b) removing hydrogen sulfide from the gaseous stream 25 enriched in hydrogen sulfide, wherein the hydrogenation catalyst comprises nickel and molybdenum on an alumina carrier and the hydrolysis catalyst is a catalyst capable of catalyzing hydrolysis of carbonyl sulfide to form hydrogen sulfide. Suitable 30 hydrolysis catalysts are said to be a catalyst comprising alumina and CeO2and a catalyst comprising, more preferably consisting essentially of titania. Preferably, the hydrogenation catalyst and the hydrolysis catalyst are contained in a single reactor in SP 2823 PD - 6 – a stacked bed configuration. It is indicated that in said configuration, the two beds are arranged in such manner that the feed gas comprising hydrogen sulfide and sulfur dioxide is first passed through the bed of 5 hydrogenation catalyst and subsequently through the bed of hydrolysis catalyst. Specific stacked bed catalyst combinations exemplified in EP 2944367 A1 are hydrogenation catalysts comprising nickel and molybdenum on alumina or amorphous silica-alumina and either 10 titania (with no metals added) or cerium on alumina as the hydrolysis catalyst. U.S. Patent No. 10562014 (Krueger et al.) discloses a high metals content catalyst useful in low-temperature conversion of sulfur compounds, such as sulfur dioxide, 15 carbonyl sulfide, carbon disulfide and sulfur, that may be contained in a gas stream. The catalyst is prepared by co-mulling its components that include cobalt and molybdenum metals. The co-mulled mixture is dried and calcined. The catalyst contains at least 7.5 wt.% up to 20 15 wt.% molybdenum and at least 2.75 wt.% up to 6 wt.% cobalt. The pore structure is an important property of the catalyst. It is bimodal with a first major portion of the total pore volume in pores of diameter in the range of from 50 Å to 150 Å and a minor portion of the 25 total pore volume in pores of diameter in the range of from 1,000 Å to 10,000 Å; and wherein pore diameter is determined by surface area from mercury porosimetry (with a contact angle of 140 degrees). Less than 6 % of the total pore volume of the catalyst is contained 30 within its pores having a pore diameter greater than 10,000 Å, wherein pore diameter is determined by surface area from mercury porosimetry (with a contact angle of 140 degrees). SP 2823 PD - 7 – US 2022 / 0016603 A1 (Krueger) describes a catalyst composition useful in the processes for the hydrolysis of sulfur compounds and carbon monoxide contained in gas streams. The catalyst composition comprises an 5 inorganic oxide, underbedded molybdenum, underbedded cobalt, overlaid molybdenum, and overlaid cobalt. In particular, said catalyst composition comprises a calcined co-mulled mixture of an inorganic oxide, a first molybdenum compound, and a first cobalt compound. 10 The calcined co-mulled mixture is impregnated with a second molybdenum compound and a second cobalt compound. The impregnated metals are overlaid to provide the underbedded molybdenum, underbedded cobalt, overlaid molybdenum, and overlaid cobalt. 15 Whilst catalysts such as those described in US 2022 / 0016603 A1 and U.S. Patent No. 10562014 have tremendous advantages in the treatment of gas streams comprising sulfur compounds and carbon monoxide, there are ongoing efforts to develop improved processes for 20 the hydrolysis of sulfur compounds contained in gas streams which achieve overall advantageous performance in the presence of reduced amounts of such catalysts and / or in the presence of catalyst compositions having lower activities for the treatment of such gas streams. 25 It is further desired to develop advantageous catalyst systems that provide for high percentage conversion under low-temperature reaction conditions of sulfur compounds contained in gas streams such as Claus tail gas streams to hydrogen sulfide. 30 Summary of the Invention The present invention provides a process for the treatment of a tail gas stream comprising one or more sulfur compounds and / or carbon monoxide in the presence SP 2823 PD - 8 – of water and hydrogen, under hydrolysis conditions, in a reactor system one or more reactors, wherein said process takes place in the presence of a catalyst system comprising a first catalyst 5 composition and a second catalyst composition, wherein the first catalyst composition and the second catalyst composition each comprise a cobalt compound and / or a nickel compound in combination with a molybdenum compound and an inorganic oxide selected from 10 alumina and / or titania and the first catalyst composition has a higher activity for the removal of carbon monoxide and / or a higher activity for the removal of carbonyl sulfide (COS) and / or a higher activity for the removal of carbon disulfide (CS2) from a tail gas 15 stream relative to the second catalyst composition under the same conditions, and wherein the first and second catalyst compositions are arranged such that at least some of the first catalyst composition is situated upstream of the second 20 catalyst composition in the reactor system. Detailed Description of the Invention The catalyst system described in the present invention has properties that make it particularly useful in the low-temperature hydrolysis of carbonyl 25 sulfide (COS) and carbon disulfide (CS2) that may be contained in tail gas streams such as Claus tail gases having concentrations of carbonyl sulfide and carbon disulfide that need to be removed or reduced to acceptable levels. 30 COS + H2O ^ CO2+ H2S (1) specification to mean the reaction of carbonyl sulfide SP 2823 PD - 9 – with water to yield hydrogen sulfide and carbon dioxide and the reaction of carbon disulfide with water to yield hydrogen sulfide and carbon dioxide. The catalyst system described in the present 5 invention also has application in low-temperature conversion of carbon monoxide present in such Claus tail gases by the water-gas shift reaction. The references herein to the “water-gas shift reaction” are to the equilibrium reaction of carbon monoxide and water to 10 carbon dioxide and hydrogen. H2O + CO ^ CO2+ H2(3) The catalyst system described in the present invention also promotes low temperature hydrogenation of SO2to yield water and sulfur and the hydrogenation of 15 Sxto H2S. The process of the present invention provides for the treatment of a tail gas stream comprising one or 20 more sulfur compounds and / or carbon monoxide in the presence of water and, optionally, hydrogen in a reactor system. Said process takes place in the presence of a catalyst system comprising a first catalyst composition and a second catalyst composition, wherein the first and 25 second catalyst compositions are arranged such that at least some of the first catalyst composition is situated upstream of the second catalyst composition in the reactor system. Preferably, all of the first catalyst composition in the catalyst system is situated upstream 30 in the reactor system of the second catalyst composition. SP 2823 PD - 10 – In a preferred embodiment of the process of the present invention, the first catalyst composition and the second catalyst composition are present in the same reactor in the reactor system in a stacked catalyst bed 5 arrangement. However, in another embodiment of the process of the present invention, the first catalyst composition and the second catalyst composition may be present in catalyst beds in separate reactors in the reactor system in a sequential arrangement. 10 In a preferred embodiment of the present invention, the catalyst system consists of a first catalyst composition and a second catalyst composition, wherein the first catalyst composition and the second catalyst composition each comprise a cobalt compound and / or a 15 nickel compound in combination with a molybdenum compound and an inorganic oxide selected from alumina and / or titania and the first catalyst composition has a higher activity for the removal of carbon monoxide and / or a higher activity for the removal of carbonyl 20 sulfide (COS) and / or a higher activity for the removal of carbon disulfide (CS2) from a tail gas stream relative to the second catalyst composition under the same conditions. In the present invention, preferably, the catalyst 25 volume ratio of the first catalyst composition to the second catalyst composition in the catalyst system is in the range of from 5:95 to 95:5 and more preferably in the range of from 10:90 to 90:10. The catalyst system used in the process of the 30 present invention may be prepared using conventional methods and conventional means. Any type of reactor may be loaded in such a way that the tail gas stream will initially come into contact with the first catalyst composition and subsequently with the second catalyst SP 2823 PD - 11 – composition. When, for example, an industrial reactor is operated in a top-down flow, the preparation of the catalyst system generally will comprise loading the reactor with the first and second catalyst compositions 5 in such a way that the second catalyst composition is positioned in the lower sections of the reactor (when operated top-down flow) and loading the first catalyst composition above that. For commercial units, various loading techniques are available which are often 10 executed by specialized companies. Whilst the first and second catalyst compositions present in the catalyst system used in the process of the present invention each comprise a cobalt compound and / or a nickel compound in combination with a 15 molybdenum compound and an inorganic oxide selected from alumina and / or titania, said compositions are different from each other in terms of their overall compositions, that is to say, said compositions are distinct either with regard to the overall combination of cobalt 20 compound, nickel compound, molybdenum compound and / or inorganic oxide present therein and / or they are distinct with regard to the amounts of said compounds and / or inorganic oxide present therein and / or they are distinct with regard to the arrangement of said compounds therein 25 as a result of the respective methods of manufacturing said catalyst compositions. In a preferred embodiment of the present invention, the first catalyst composition has a higher total wt. % content of cobalt, nickel, and molybdenum than the 30 second catalyst composition, wherein said wt.% are based on the total weight of each catalyst composition and the metals as an oxide regardless of their actual form. As a result of the afore-mentioned differences in the overall compositions of the first and second SP 2823 PD - 12 – catalyst compositions, the first and second catalyst compositions exhibit differences in their activities for the removal of carbon monoxide and / or carbonyl sulfide (COS) and / or carbon disulfide (CS2) 5 from a tail gas stream under the same operating conditions. Said activities refer to the so-called “individual” activities of said catalyst compositions for the removal of carbon monoxide and / or carbonyl sulfide (COS) and / or carbon disulfide (CS2) from a tail 10 gas stream under the same operating conditions. By “individual” activity in the present specification, is meant the activity of a catalyst composition for the removal of carbon monoxide and / or carbonyl sulfide (COS) and / or carbon disulfide (CS2) from a tail gas stream 15 when operating alone as the single catalyst composition under a given set of operating conditions for the treatment of a tail gas stream. The set of operating conditions under which the individual activities of the first and second catalyst 20 compositions are determined is not limited, provided that each catalyst composition is evaluated for its activity under the same set of operating conditions. In a preferred embodiment, the operating conditions selected for evaluating the individual activities of the 25 first and second catalyst compositions are identical to the operating conditions under which the process of the present invention (i.e. in the presence of the catalyst system consisting of the first and second catalyst compositions) is to be conducted. For example, in a 30 more preferred embodiment, the operating conditions selected for evaluating the individual activities of the first and second catalyst compositions are identical to the operating conditions under which the process of the SP 2823 PD - 13 – present invention (i.e. in the presence of the catalyst system consisting of the and second catalyst compositions) is to be operated in order to achieve the target sulfur concentration, preferably as described 5 hereinafter, in the reactor outlet. A suitable set of operating conditions for evaluating the individual activities of the first and second catalyst compositions is described in Example 2 herein and conveniently comprises employing a reactor 10 temperature of 220 C and feeding the tail gas stream at a rate so as to provide a 2052 h-1nGHSV (normal gas hourly space velocity) and 0.02068 MPa (3 psi, (0.2068 bar g)) unit pressure). The tail gas stream used for evaluating the 15 individual activities of the first and second catalyst compositions is not limited, provided that each catalyst composition is evaluated for its activity using the same tail gas stream. In a preferred embodiment, the tail gas stream selected for evaluating the individual 20 activities of the first and second catalyst compositions is identical to the tail gas stream to be used in the process of the present invention (i.e. in the presence of the catalyst system comprising or consisting of the first and second catalyst compositions). However, in 25 another preferred embodiment, the tail gas stream selected for evaluating the individual activities of the first and second catalyst compositions is a synthetic tail gas composition used for testing purposes only. Suitable synthetic tail gas compositions for such 30 purposes are not limited. Such synthetic tail gas compositions may conveniently comprise H2S, SO2, COS, CS2, S, H2, CO, N2, and steam. An example of a SP 2823 PD - 14 – synthetic tail gas composition that may be conveniently employed is shown in Table 2 herein. The first catalyst composition has a higher activity for the removal of carbon monoxide and / or a 5 higher activity for the removal of carbonyl sulfide (COS) and / or a higher activity for the removal of carbon disulfide (CS2) from the tail gas stream relative to the second catalyst composition under the same operating conditions. Preferably, the first catalyst composition 10 has a higher activity for the removal of carbon monoxide and also a higher activity for the removal of carbonyl sulfide (COS) from the tail gas stream relative to the second catalyst composition under the same operating conditions. More preferably, the first catalyst 15 composition has a higher activity for the removal of carbon monoxide and also higher activities for the removal of carbonyl sulfide (COS) and carbon disulfide (CS2) from the tail gas stream relative to the second catalyst composition under the same operating 20 conditions. Where the relative activity performance of individual catalyst compositions is less differentiated with regard to the removal of one or more of the afore- mentioned compounds (i.e. carbon monoxide, carbonyl 25 sulfide and / or carbon disulfide), particularly at higher temperatures, then testing may be focused on those activity performances which display the most significant differences. For example, at higher temperatures, there may be less differentiation between the activity of 30 different catalyst compositions for the removal of carbon disulfide, with all catalysts being tested demonstrated high efficacy. Thus, in a particularly preferred embodiment of the present invention, the relative activities of the first and second catalysts SP 2823 PD - 15 – are determined only with regard to the removal of carbon monoxide and / or carbonyl sulfide. In a preferred embodiment, the activity of the first catalyst composition for the removal of carbon 5 monoxide is at least 10% higher on a relative volumetric activity basis than the activity of the second catalyst composition for the removal of carbon monoxide from the same tail gas stream and under the same operating conditions. 10 In another preferred embodiment, the activity of the first catalyst composition for the removal of carbonyl sulfide (COS) is at least 10% higher on a relative volumetric activity basis than the activity of the second catalyst composition for the removal of carbonyl 15 sulfide (COS) from the same tail gas stream and under the same operating conditions. In another preferred embodiment, the activity of the first catalyst composition for the removal of carbon disulfide (CS2) is at least 10% higher on a 20 relative volumetric activity basis than the activity of the second catalyst composition for the removal of carbon disulfide (CS2) from the same tail gas stream and under the same operating conditions. In a preferred embodiment, the first catalyst 25 composition has an individual activity level, X1, for the removal of carbon monoxide, an individual activity level, Y1, for the removal of carbonyl sulfide, and an individual activity level, Z1, for the removal of carbon disulfide (CS2), when operating under a given 30 set of operating conditions and the second catalyst composition has an individual activity level, X2, for the removal of carbon monoxide, an individual activity level, Y2, for the removal of carbonyl sulfide, and an SP 2823 PD - 16 – individual activity level, Z2, for the removal of carbon disulfide (CS2), under the same set of operating conditions and wherein X1>X2 and / or Y1>Y2 and / or Z1>Z2. In a preferred embodiment, X1 is at least 10% > 5 than X2 on a relative volumetric basis and / or Y1 is at least 10% > Y2 on a relative volumetric basis and / or Z1 is at least 10% > than Z2 on a relative volumetric basis, under the same set of operating conditions. More preferably, X1 is at least 20% > than X2 on a 10 relative volumetric basis and / or Y1 is at least 20% > Y2 on a relative volumetric basis and / or Z1 is at least 20% > than Z2 on a relative volumetric basis, under the same set of operating conditions. As noted hereinbefore, the first and second 15 catalyst compositions each comprise a cobalt compound and / or a nickel compound in combination with a molybdenum compound and an inorganic oxide selected from alumina and / or titania. Preferably, the first catalyst composition comprises a cobalt compound in combination 20 with a molybdenum compound and an inorganic oxide selected from alumina and / or titania and the second catalyst composition comprises a cobalt compound in combination with a molybdenum compound and an inorganic oxide selected from alumina and / or titania. Generally, 25 the first and second catalyst compositions are not limited, provided that they are compositionally different from each other in one or more aspects, as previously described, and the first catalyst composition has a higher activity for the removal of carbon monoxide 30 and / or a higher activity for the removal of carbonyl sulfide (COS) and / or a higher activity for the removal of carbon disulfide (CS2) from the tail gas stream relative to the second catalyst composition under the same operating conditions. Said first and second SP 2823 PD - 17 – catalyst compositions may conveniently comprise a cobalt compound and / or nickel compound in an amount of at least 0.5 wt. %, preferably in the range of from 0.5 to 8 wt. %, and a molybdenum compound in an amount of at least 3 5 wt. %, preferably in the range of from 3 to 22 wt. %, based on the total weight of said catalyst composition and the metal (i.e. cobalt, nickel and / or molybdenum) as an oxide regardless of its actual form. In particular, the first catalyst composition may have a high metals 10 content of cobalt and / or nickel, and molybdenum. Preferably, said first catalyst composition contains at least 2.0 wt. %, more preferably at least 2.7 wt.%, even more preferably at least 2.75 wt. % and, in particular, an amount in the range of from 2.75 to 8 wt.%, 15 preferably in the range of from 3 to 8 wt.%, more preferably in the range of from 3 to 6 wt.%, even more preferably in the range of from 3.3 to 6 wt.%. and most preferably in the range of from 3.3 to 5 wt. % cobalt and / or nickel. Preferably, said first catalyst 20 composition also contains at least 7.4 wt.%, more preferably at least 7.5 wt. % and, in particular, an amount in the range of from 7.6 to 22 wt.%, preferably in the range of from 7.8 to 20 wt.%, more preferably in the range of from 7.8 to 18 wt.% and most preferably in 25 the range of from 7.8 to 15 wt.% molybdenum. Said wt.% are based on the total weight of the catalyst composition and the metal as an oxide regardless of its actual form. As noted hereinbefore, one way in which the second catalyst composition can have a lower 30 activity for the removal of carbon monoxide and / or a lower activity for the removal of carbonyl sulfide (COS) and / or a lower activity for the removal of carbon disulfide (CS2), from the tail gas stream can be by having a different metals content to the first catalyst SP 2823 PD - 18 – composition. Preferably, said second catalyst composition contains an amount in the range of from 1 to 3.75 wt.%, more preferably in the range of from 1.5 to 3.6 wt.%, and most preferably in the range of from 1.75 5 to 3.6 wt. % cobalt and / or nickel. Preferably, said second catalyst composition also contains an amount in the range of from 3 to 9.5 wt.%, more preferably in the range of from 5 to 9.5 wt.%, and most preferably in the range of from 5.75 to 9.5 wt.% molybdenum. Said wt.% 10 are based on the total weight of the catalyst composition and the metal as an oxide regardless of its actual form. The inorganic oxide component in each of the first and second catalyst compositions is selected from 15 alumina and / or titania. Preferably, said inorganic oxide is alumina in both of the first and second catalyst compositions. The inorganic oxide is preferably present in each of the first and second catalyst compositions in an amount in the range of from 20 60 to 95 wt. %, based on the total weight of the catalyst composition. It may also be present in each catalyst composition in an amount in the range of from 70 to 90 wt.% or 75 to 89 wt.%, based on the total weight of the catalyst composition. 25 The first and second catalyst compositions may be conveniently prepared by known methods, for example, as described in U.S. Patent No. 8142748, U.S. Patent No. 10562014 and US 2022 / 0016603 A1. General methods for preparing catalyst compositions 30 for use in the catalyst system include methods comprising the co-mulling of cobalt compounds and / or nickel compounds, molybdenum compounds with the inorganic oxide, followed by drying and calcining the resulting formed agglomerate. SP 2823 PD - 19 – What is meant by the use of the term “co-mulled” is that the starting of the catalyst are combined and mixed together to form a mixture of the individual components that is preferably or substantially uniform 5 or homogeneous. This term is intended to be broad enough in scope to include mixing of the starting materials that include alumina, a cobalt and / or nickel compound, and a molybdenum compound to yield a co-mulled mixture that can be formed into agglomerate particles. The co- 10 mulled mixture can be a paste or a plastic mixture that is capable of being formed into agglomerate particles by any of the known agglomeration methods or extruded into extrudate particles by any of the known extrusion methods. 15 The preferred method of agglomerating the co-mulled mixture is by extrusion to form extrudate particles having overall diameters in the range of from 0.5 mm to 10 mm or from 0.75 mm to 8 mm and length to diameter ratios of from 1:1 to 10:1 or even higher. The 20 extrudates can be any of the typical shapes such as cylinders and multi-lobal shapes. Thus, the formation of the co-mulled mixture may be done by any method or means known to those skilled in the art, including, but not limited to, the use of such 25 suitable types of solids-mixing machines as tumblers, stationary shells or troughs, muller mixers, which are either batch type or continuous type, and impact mixers, and the use of such suitable types of either batch-wise or continuous mixers for mixing solids and liquids or 30 for the formation of paste-like mixtures that are extrudable. Suitable types of batch mixers include, but are not limited to, change-can mixers, stationary-tank mixers, SP 2823 PD - 20 – double-arm kneading mixers that are equipped with any suitable type of Suitable types of continuous mixers include, but are not limited to, single or double screw extruders, 5 trough-and-screw mixers and pug mills. The mixing of the starting materials used in the preparation of the co-mulled mixture can include water and appropriate amounts of a mineral acid, such as nitric acid, as is necessary to provide the 10 aforementioned paste-like mixtures having a loss on ignition (LOI) in the range of from 40% to 80% as determined by the standard test method ASTM D7348. It has been found that the co-mulled mixture with an LOI in this range provides for a paste having desirable 15 extrusion properties, and may contribute to a finished catalyst product having the certain pore structure characteristics as described in detail throughout this specification. The mixing of the starting materials used in the 20 preparation of the co-mulled mixture is conducted for a time period necessary to properly homogenize the co- mulled mixture. Generally, the blending time is in the range of upwardly to 12 or more hours. Typically, the blending time is in the range of from 0.1 hours to 1 25 hour. The cobalt compound that may be present in the co- mulled mixture is a cobalt compound that is convertible to an oxide upon calcination within the presence of oxygen. The cobalt compound can be selected from 30 suitable cobalt salt compounds. Such compounds may include a cobalt compound selected from ammonium cobalt compounds, and phosphates, nitrates, oxalates, sulfates, and halides of cobalt. A particularly favorable cobalt salt that is found to be a useful cobalt compound for SP 2823 PD - 21 – the co-mulled mixture is cobalt nitrate. It is preferable to combine cobalt compound with the other components of the co-mulled mixture in the form of a first aqueous solution that comprises cobalt. The first 5 aqueous solution may be formed by dissolving the cobalt salt in water. The most preferred cobalt salt is cobalt nitrate. The nickel compound that may be present in the co- mulled mixture is a nickel compound that is convertible 10 to an oxide upon calcination within the presence of oxygen. The nickel compound can be selected from suitable nickel salt compounds. Such compounds may include a nickel compound selected from ammonium nickel compounds, and phosphates, nitrates, oxalates, sulfates, 15 and halides of nickel. A particularly favorable nickel salt that is found to be a useful nickel compound for the co-mulled mixture is nickel nitrate. It is preferable to combine the nickel compound with the other components of the co-mulled mixture in the form of a 20 first aqueous solution that comprises nickel. The first aqueous solution may be formed by dissolving the nickel salt in water. The most preferred nickel salt is nickel nitrate. The molybdenum compound of the co-mulled mixture is 25 a molybdenum compound that is convertible to an oxide upon calcination within the presence of oxygen. The molybdenum compound can be selected from suitable molybdenum salt compounds. Such compounds may include a molybdenum compound selected from such compounds as 30 ammonium molybdate, potassium molybdate, sodium molybdate, phosphomolybdic acid, molybdenum disulfide, molybdenum trioxide, and molybdic acid. It is preferable to combine the molybdenum compound with the other components of the co-mulled mixture in the form of a SP 2823 PD - 22 – second aqueous solution that comprises molybdenum. The second aqueous be formed by dissolving the molybdenum salt in water. The most preferred molybdenum salt is an ammonium molybdate such as ammonium 5 heptamolybdate and ammonium dimolybdate. The formed agglomerate of the co-mulled mixture is dried and then calcined. The drying of the co-mulled mixture is not a critical step and is generally performed in air and at a drying temperature in the 10 range of from 20oC to 125oC. The time period for drying is any suitable time period that can provide the desired amount of drying. Calcination of the co-mulled mixture is a necessary step. It provides for the conversion of the metal 15 compounds to their oxide forms and yields calcined formed agglomerates or particles. The calcination of the formed aggregates or agglomerates of the co-mulled mixture is conducted in the presence of an oxygen-containing gas, such as air, 20 at a temperature and for a time period that are suitable for achieving the desired degree of calcination to provide the final catalyst composition of the invention. Generally, the calcination temperature is in the range of from 300oC to 800oC, preferably, from 350oC to 700 25oC, and more preferably, from 400oC to 600oC. The calcination time period can be in the range of from 0.1 hour to 96 hours. In some embodiments, such dried and calcined formed agglomerates may undergo further modification, for 30 example, by impregnation of additional compounds, including additional quantities of cobalt compounds and / or nickel compounds and molybdenum compounds. The resulting impregnated formed agglomerate or impregnated particle is dried and then calcined. The SP 2823 PD - 23 – drying of the impregnated particle is not a critical step and is generally performed in air and at a drying temperature in the range of from 20oC to 125oC. The time period for drying is any suitable time period that 5 can provide the desired amount of drying. The calcination of the impregnated particle is conducted in the presence of an oxygen-containing gas, such as air, at a temperature and for a time period that are suitable for achieving the desired degree of 10 calcination to provide the final catalyst composition. Generally, the calcination temperature is in the range of from 300oC to 800oC, preferably, from 350oC to 700 oC, and more preferably, from 400oC to 600oC. The calcination time period can be in the range of from 0.1 15 hour to 96 hours. In other embodiments, rather than employing co- mulling, the catalyst compositions may be prepared by conventional methods including impregnation of the inorganic oxide with one or more solutions of cobalt 20 compounds and / or nickel compounds and / or molybdenum compounds and subsequent drying and calcination steps. As discussed hereinbefore, U.S. Patent No. 10562014 and US 2022 / 0016603 A1 each disclose catalyst compositions having advantageous activity for the 25 removal of carbon monoxide and sulfur compounds from the tail gas stream. Examples of suitable catalyst compositions that may be conveniently employed in the catalyst system used in the process of the present invention include those described in U.S. Patent No. 30 10562014 and US 2022 / 0016603 A1. The subject matter of said disclosures is incorporated herein by reference. Specifically, U.S. Patent No. 10562014 discloses a high metals content catalyst prepared by co-mulling its components that include cobalt and molybdenum metals and SP 2823 PD - 24 – an alumina precursor and drying and calcining the co- mulled mixture. Said high metals content catalyst contains at least 7.5 wt.% up to 15 wt.% molybdenum and at least 2.75 wt.% up to 6 wt.% cobalt. Preferably, 5 said catalyst is bimodal with a first major portion of the total pore volume in pores of diameter in the range of from 50 Å to 150 Å and a minor portion of the total pore volume in pores of diameter in the range of from 1,000 Å to 10,000 Å and, optionally, may have less than 10 6 % of the total pore volume of the catalyst is contained within its pores having a pore diameter greater than 10,000 Å, wherein pore diameter is determined by surface area from mercury porosimetry (with a contact angle of 140 degrees). 15 The catalyst composition described in U.S. Patent No. 10562014 is conveniently made by a co-mulling procedure as is hereinbefore described. US 2022 / 0016603 A1 describes a catalyst composition comprising both molybdenum and cobalt metals in a so- 20 called underbedded form and also both molybdenum and cobalt as overlaid metals. When referring herein to a metal that is in an “underbedded” form, what is meant is a catalytic metal or metal precursor or compound that is uniformly 25 dispersed in an inorganic oxide support material, such as alumina, to form a mixture that is thereafter calcined and subsequently contacted with a second catalytic metal or metal precursor or compound followed by a second calcination. The subsequent contacting of 30 the calcined mixture with the second metal is preferably done by metals impregnation. The metal that is first mixed with the inorganic oxide and followed by calcination is referred to as “underbedded” metal; because the subsequent contacting of this calcined SP 2823 PD - 25 – mixture with the second metal is thought of as “overlaying” the metal the calcined mixture with an overlayer of the second metal. Thus, the term “overlaid” or “overlayer” or similar terminology refers herein to 5 the second catalytic metal or metal precursor or compound that is placed on top or overlaid onto the metal of the calcined mixture that has become underbedded as a result. It will be appreciated that in such catalyst compositions, one or more metals may be in 10 underbedded form and one or more metals may be overlaid. The catalyst composition described in US 2022 / 0016603 A1 may be made by conducting two metals incorporation steps and two calcination steps, as is hereinbefore described. Each of these steps is believed 15 to provide for different active catalytic phases in the final catalyst. The first metals incorporation step is performed by co-mulling the starting metal components with the inorganic oxide to give a co-mulled mixture having the metal components uniformly or homogenously20 dispersed within the inorganic oxide material. This co- mulled mixture is preferably formed into agglomerates or particles, such as extrudates, that are dried and calcined. The second metals incorporation step is performed by contacting the calcined particles with an 25 overlayer of metal components, preferably by applying one or more aqueous metal impregnation solutions, to give metals-impregnated formed agglomerates that are dried and calcined. Accordingly, the first catalyst composition 30 employed herein may conveniently be a catalyst composition in accordance with the general teaching of U.S. Patent No. 10562014 and / or US 2022 / 0016603 A1. Hence, in such scenarios, the second catalyst composition will be a catalyst composition having a SP 2823 PD - 26 – lower activity for the removal of carbon monoxide and / or carbonyl sulfide (COS) carbon disulfide (CS2) from the tail gas stream relative to said first catalyst composition. Said second catalyst composition 5 conveniently comprises a formed agglomerate of a co- mulled mixture, comprising pseudoboehmite, a cobalt and / or nickel compound and a molybdenum compound, wherein said formed agglomerate has been calcined to provide said catalyst composition. 10 Thus, in one preferred embodiment of the present invention, the catalyst system consists of a catalyst composition in accordance with the disclosure of U.S. Patent No. 10562014 as the first catalyst composition, which composition comprises a cobalt compound in 15 combination with a molybdenum compound and an inorganic oxide selected from alumina and / or titania and, as the second catalyst composition, a catalyst composition which comprises a cobalt compound and / or a nickel compound in combination with a molybdenum compound and 20 an inorganic oxide selected from alumina and / or titania and which has a lower activity for the removal of carbon monoxide and / or a lower activity for the removal of carbonyl sulfide (COS) and / or carbon disulfide (CS2) from a tail gas stream than the first catalyst 25 composition. Thus, in said embodiment, the first catalyst composition is a high metals content catalyst prepared by a process comprising co-mulling components that include cobalt and molybdenum metals and an alumina and / or titania precursor and drying and calcining the 30 co-mulled mixture. For example, the first catalyst composition may conveniently comprise a formed agglomerate of a comulled mixture, comprising pseudoboehmite, a cobalt compound and a molybdenum compound. Said formed agglomerate may be calcined to SP 2823 PD - 27 – provide the catalyst composition comprising gamma- alumina. Said first catalyst composition preferably contains at least 7.4 wt.%, and preferably up to 15 wt.% molybdenum and preferably at least 2.7 wt.%, and 5 preferably up to 6 wt.% cobalt, wherein each wt.% is based on the total weight of the catalyst composition and the metal as an oxide regardless of its actual form. Preferably, the first catalyst composition has a bimodal structure with a first major portion of the total pore 10 volume in pores of diameter in the range of from 50 Å to 150 Å and a minor portion of the total pore volume in pores of diameter in the range of from 1,000 Å to 10,000 Å and, optionally, may have less than 6 % of the total pore volume of the catalyst is contained within its 15 pores having a pore diameter greater than 10,000 Å, wherein pore diameter is determined by surface area from mercury porosimetry (with a contact angle of 140 degrees). More preferably, the first catalyst composition has a bimodal pore structure such that: 20 less than 6 % of the total pore volume of said catalyst composition is contained within pores having a pore diameter greater than 10,000 Å; a first major portion of greater than 25 % and less than 60 % of the total pore volume of said catalyst 25 composition is contained within pores having a pore diameter in the range of from 50 Å to 150 Å; a second major portion of greater than 15 % and less than 50 % of the total pore volume of said catalyst composition is contained within pores having a pore 30 diameter in the range of from 1000 Å to 10,000 Å; and a minor portion of less than 15 % of the total pore volume of said catalyst composition is contained within pores having a pore diameter in the range of from 150 Å to 1000 Å; and wherein pore diameter is determined by SP 2823 PD - 28 – surface area from mercury porosimetry, with a contact angle of 140 degrees. In another particularly preferred embodiment of the present invention, the catalyst system consists of a 5 catalyst composition in accordance with the disclosure of US 2022 / 0016603 A1 as the first catalyst composition, which composition comprises a cobalt compound in combination with a molybdenum compound and an inorganic oxide selected from alumina and / or titania and, as the 10 second catalyst composition, a catalyst composition which comprises a cobalt compound and / or a nickel compound in combination with a molybdenum compound and an inorganic oxide selected from alumina and / or titania and which has a lower activity for the removal of carbon 15 monoxide and / or a lower activity for the removal of carbonyl sulfide (COS) and / or carbon disulfide (CS2) from a tail gas stream than the first catalyst composition. Thus, in said embodiment, the first catalyst composition comprises both molybdenum and 20 cobalt metals in a so-called underbedded form and both molybdenum and cobalt as overlaid metals. Preferably, said first catalyst composition comprises a calcined co- mulled mixture of an inorganic oxide selected from alumina and / or titania, a first molybdenum compound, and 25 a first cobalt compound, wherein said calcined co-mulled mixture is impregnated with a second molybdenum compound and a second cobalt compound, thereby providing underbedded molybdenum, underbedded cobalt, overlaid molybdenum, and overlaid cobalt. The metals in said 30 first catalyst composition are preferably present in amounts of a total molybdenum content of at least 7.4 wt. %, more preferably in the range of from 7.6 wt.% to 22 wt.%, even more preferably in the range of from 7.8 wt.% to 20 wt.% and most preferably in the range of from SP 2823 PD - 29 – 7.8 wt.% to 18 wt.% and a total cobalt content preferably of at least %, more preferably in the range of from 3 wt.% to 8 wt.%, even more preferably in the range of from 3.3 wt.% to 6 wt.% and most preferably 5 in the range of from 3.6 wt.% to 5 wt.%, based on the total weight of the catalyst composition and the respective metal components as an oxide regardless of actual form. When referring to the “total molybdenum content” or “total cobalt content” of the catalyst, what 10 is meant by these terms is the total weight % of the indicated metal contained in the entire composition including the sum of both the underbedded metal content and the overlaid metal content. In such embodiments for the first catalyst composition, the total metals wt.% 15 ratio of the total amount of molybdenum content of the catalyst divided by the total amount of cobalt content of the catalyst is preferably in the range of from 1.5:1 to 8:1, more preferably in the range of from 1.7:1 to 6:1 and, most preferably in the range of from 2:1 to 20 5:1. Preferably, the molybdenum ratio in said catalyst composition is in the range of from 1:5 to 4:5 and the cobalt ratio in said catalyst composition is in the range of from 1:5 to 4:5. It is preferred for these ratios to both be within the range of from 1:4 to 3:4, 25 and, more preferred, in the range of from 1:3 to 2:3. When referring to the “molybdenum ratio”, what is meant is the weight percent of the molybdenum of the catalyst that is in the underbedded form divided by the total molybdenum content (in weight percent) of the catalyst 30 composition. The ratio is computed on the basis of the molybdenum as an oxide. When referring to the “cobalt ratio”, what is meant is the weight percent of the cobalt of the catalyst that is in the underbedded form divided by the total cobalt content (in weight percent) SP 2823 PD - 30 – of the catalyst. The ratio is computed on the basis of the cobalt as an oxide. The total amount of underbedded molybdenum in the first catalyst composition in this embodiment may be within the range of from 1.2 to 12.5 5 wt.%. Preferably, the underbedded molybdenum is present in said catalyst composition in an amount within the range of from 1.15 to 11 wt.%, and more preferably, within the range of from 1.2 to 10 wt.%. The total amount of underbedded cobalt in the first catalyst 10 composition in this embodiment may be within the range of from 0.5 to 5 wt.%. Preferably, the underbedded cobalt is present in said catalyst composition in an amount within the range of from 0.55 to 4 wt.%, and, more preferably, within the range of from 0.6 to 3 wt.%. 15 These wt.% values are based on the total weight of the first catalyst composition and each respective metal component as an oxide regardless of actual form. In a most preferred embodiment of the present invention, the catalyst system consists of a catalyst 20 composition in accordance with the disclosure of US 2022 / 0016603 A1 as the first catalyst composition, which composition comprises a cobalt compound in combination with a molybdenum compound and an inorganic oxide selected from alumina and / or titania and, a catalyst 25 composition in accordance with the disclosure of U.S. Patent No. 10562014 as the second catalyst composition, which composition comprises a cobalt compound in combination with a molybdenum compound and an inorganic oxide selected from alumina and has a lower activity for 30 the removal of carbon monoxide and / or a lower activity for the removal of carbonyl sulfide (COS) from a tail gas stream than the first catalyst composition. Thus, in said embodiment, the first catalyst composition comprises both molybdenum and cobalt metals in a so- SP 2823 PD - 31 – called underbedded form and both molybdenum and cobalt as overlaid metals, the metals in said first catalyst composition are preferably present in amounts of a total molybdenum content of at least 7.4 wt. %, 5 more preferably in the range of from 7.6 wt.% to 22 wt.%, even more preferably in the range of from 7.8 wt.% to 20 wt.% and most preferably in the range of from 7.8 wt.% to 18 wt.% and a total cobalt content preferably of at least 2.7 wt. %, more preferably in the range of from 10 3 wt.% to 8 wt.%, even more preferably in the range of from 3.3 wt.% to 6 wt.% and most preferably in the range of from 3.6 wt.% to 5 wt.%, based on the total weight of the catalyst composition and the respective metal components as an oxide regardless of actual form. When 15 referring to the “total molybdenum content” or “total cobalt content” of the catalyst, what is meant by these terms is the total weight of the indicated metal contained in the entire composition including the sum of both the underbedded metal content and the overlaid 20 metal content. In such embodiments for the first catalyst composition, the total metals wt.% ratio of the total amount of molybdenum content of the catalyst divided by the total amount of cobalt content of the catalyst is preferably in the range of from 1.5:1 to 25 8:1, more preferably in the range of from 1.7:1 to 6:1 and, most preferably in the range of from 2:1 to 5:1. Preferably, the molybdenum ratio in said catalyst composition is in the range of from 1:5 to 4:5 and the cobalt ratio in said catalyst composition is in the 30 range of from 1:5 to 4:5. It is preferred for these ratios to both be within the range of from 1:4 to 3:4, and, more preferred, in the range of from 1:3 to 2:3. When referring to the “molybdenum ratio”, what is meant is the weight percent of the molybdenum of the catalyst SP 2823 PD - 32 – that is in the underbedded form divided by the total molybdenum content of catalyst. The ratio is computed on the basis of the molybdenum as an oxide. When referring to the “cobalt ratio”, what is meant is 5 the weight percent of the cobalt of the catalyst that is in the underbedded form divided by the total cobalt content of the catalyst. The ratio is computed on the basis of the cobalt as an oxide. The total amount of underbedded molybdenum in the first catalyst composition 10 in this embodiment may be within the range of from 1.2 to 12.5 wt.%. Preferably, the underbedded molybdenum is present in said catalyst composition in an amount within the range of from 1.15 to 11 wt.%, and more preferably, within the range of from 1.2 to 10 wt.%. The total 15 amount of underbedded cobalt in the in the first catalyst composition in this embodiment may be within the range of from 0.5 to 5 wt.%. Preferably, the underbedded cobalt is present in said catalyst composition in an amount within the range of from 0.55 20 to 4 wt.%, and, more preferably, within the range of from 0.6 to 3 wt.%. These wt.% values are based on the total weight of the first catalyst composition and each respective metal component as an oxide. Furthermore, in said embodiment, the second catalyst composition is a 25 high metals content catalyst prepared by co-mulling components that include cobalt and molybdenum metals and an alumina precursor and drying and calcining the co- mulled mixture. For example, the second catalyst composition may conveniently comprise a formed 30 agglomerate of a calcined, comulled mixture of pseudoboehmite, a cobalt compound and a molybdenum compound. Said formed agglomerate may be calcined to provide the catalyst composition comprising gamma- alumina. Said second catalyst composition contains at SP 2823 PD - 33 – least 7.5 wt.%, and preferably up to 15 wt.% molybdenum and at least 2.75 wt.%, and preferably up to 6 wt.% cobalt, wherein each wt.% is based on the total weight of the catalyst composition and the metal as an oxide 5 regardless of its actual form. Preferably, said second catalyst composition is bimodal with a first major portion of the total pore volume in pores of diameter in the range of from 50 Å to 150 Å and a minor portion of the total pore volume in pores of diameter in the range 10 of from 1,000 Å to 10,000 Å and, optionally, may have less than 6 % of the total pore volume of the catalyst is contained within its pores having a pore diameter greater than 10,000 Å; and wherein pore diameter is determined by surface area from mercury porosimetry 15 (with a contact angle of 140 degrees). More preferably, said second catalyst composition has a bimodal pore structure such that: less than 6 % of the total pore volume of said catalyst composition is contained within pores having a pore diameter greater than 10,000 Å; a 20 first major portion of greater than 25 % and less than 60 % of the total pore volume of said catalyst composition is contained within pores having a pore diameter in the range of from 50 Å to 150 Å; a second major portion of greater than 15 % and less than 50 % of 25 the total pore volume of said catalyst composition is contained within pores having a pore diameter in the range of from 1,000 Å to 10,000 Å; and a minor portion of less than 15 % of the total pore volume of said catalyst composition is contained within pores having a 30 pore diameter in the range of from 150 Å to 1,000 Å; and wherein pore diameter is determined by surface area from mercury porosimetry (with a contact angle of 140 degrees). SP 2823 PD - 34 – The catalyst system described herein is useful in a process for the treatment of a tail gas stream comprising one or more sulfur compounds and / or carbon monoxide in the presence of water and hydrogen, under 5 hydrolysis conditions, in a reactor system comprising one or more reactors. The catalyst system has certain unique catalytic properties when used in the treatment of Claus unit tail gas streams that allows for the operation of a reactor 10 at lower temperature conditions and with overall reduced amounts of catalyst having high individual activity for the removal of carbon monoxide and / or activity for the removal of carbonyl sulfide (COS) and / or activity for the removal of carbon disulfide (CS2) from a tail gas 15 stream. In the operation of a typical tail gas reactor system, which includes a reactor loaded with a catalyst, the tail gas stream is required to be heated up significantly prior to its introduction into the 20 reactor. This is due to the tail gas stream that is discharged from a Claus unit passing from the sulfur condenser that operates close to the condensation temperature of elemental sulfur. The temperature of a typical Claus unit tail gas stream is in the range of 25 from 110oC to 125oC. For conventional hydrolysis units, the tail gas stream typically must be heated up so that the introduction temperature, or reactor inlet temperature, of the tail gas stream to the reactor is in the range of from 250oC to 350oC. Any reduction of this 30 required tail gas stream inlet temperature to the reactor will provide significant energy savings in its operation. The use of the catalyst system described herein in the treatment of a Claus tail gas stream can, thus, SP 2823 PD - 35 – provide significant savings by reducing the temperature required a Claus tail gas stream. The tail gas stream that can be treated using the catalyst system described herein comprises one or more 5 sulfur compounds and / or carbon monoxide. As the term is used herein, a sulfur compound is a molecular or elemental compound selected from the group of compounds consisting of carbonyl sulfide (COS), carbon disulfide (CS2), sulfur dioxide (SO2), sulfur compounds of general 10 formula R-SH, wherein R is a hydrocarbon group, for example, methyl mercaptan (CH3SH), and elemental sulfur (Sx). Hydrogen sulfide is omitted from this definition of a sulfur compound; because, the catalyst system is not intended to provide for the conversion of H2S, but, 15 rather, the sulfur compounds are intended to be reduced by a reduction reaction to hydrogen sulfide. The hydrogen sulfide may afterwards be removed from the treated gas stream. The tail gas stream, thus, includes a compound that 20 is normally gaseous or is in the gas phase at the temperature and pressure conditions of the reactor operation. Examples of gaseous compounds, other than carbon monoxide and the aforementioned sulfur compounds, include nitrogen, oxygen, carbon dioxide, hydrogen, 25 water, and lower hydrocarbons such as methane, ethane and ethylene. The total concentration of sulfur compounds contained in the tail gas stream that is charged to or introduced into the reactor(s) containing the catalyst 30 system described herein can be in the range of from 0.01 volume % (100 ppmv) to 5 volume % of the total gas stream. More typically, the sulfur compound SP 2823 PD - 36 – concentration is in the of from 0.02 vol. % (200 ppmv) to 3 vol. %. As earlier noted, the catalyst system described herein is particularly suited for the treatment of a 5 Claus tail gas stream in order to convert the sulfur compounds contained therein to hydrogen sulfide so as to provide a treated gas stream having a reduced concentration of sulfur compounds below the concentration of sulfur compounds in the tail gas stream 10 to be treated. The following Table 1 presents typical ranges for the more common components that make up a Claus tail gas stream. Table 1 – Claus Tail Gas Composition Component Broad Intermediate Narrow Range Range (vol. Range (vol. %) %) (vol. %) H2S 0.2 – 2 0.4 – 1.5 0.6 – 1.2 SO20.1 – 1 0.2 – 0.75 0.3 – 0.6 Sx0 – 0.2 0.005 – 0.15 0.01 – 0.1 CO21 – 25 2 - 22 3 - 20 H2O 20 – 50 25 - 40 30 –35 N240 – 80 45- 70 50 –60 H20.5 – 4 1 – 3 1.5 – 2.5 CO 0.01 – 2 0.1 – 1 0.2 – 0.8 COS 0.005 – 1 0.015 – 0.5 0.01 – 0.1 CS20.005 – 1 0.015 – 0.5 0.01 – 0.1 Total Sulfur 0.11 – 3.2 0.23 – 1.9 0.33 – 0.9 Comp. 15 In the process of the present invention, a tail gas stream, having a concentration of one or more sulfur compounds, is introduced into a reactor system comprising one or more reactors, wherein said process 20 takes place in the presence of a catalyst system consisting of a first catalyst composition and a second catalyst composition as hereinbefore described and SP 2823 PD - 37 – wherein the first and second catalyst compositions are arranged such that the catalyst composition is situated upstream in the reactor system of the second catalyst composition. In one embodiment of the present invention, the first catalyst composition and the second catalyst composition are present in catalyst beds in separate reactors in the reactor system in a sequential arrangement. However, in a preferred embodiment of the process of the present invention, the first catalyst composition and the second catalyst composition are present in the same reactor in the reactor system in a stacked catalyst bed arrangement. The reactor system is operated at suitable reaction conditions for hydrolysis and / or reduction reactions to occur. As previously noted, one advantage from the use of the catalyst system described herein in the treatment of a Claus tail gas stream is that it allows for the operation of the reactor system at a relatively low inlet temperature, for example, of less than 250oC. The process of the present invention is operated under so-called “hydrolysis conditions”. That is to say, the inlet temperature at which the tail gas stream is charged to or introduced into one or more reactors of the reactor system may be generally in the range of from 115 to 400 C, particularly in the range of from 140oC to 400oC. It is preferred for the inlet temperature to be in the range of from 140oC to 320oC, more preferably in the range of from 150oC to 240oC, and, even more preferably the inlet temperature is in the range of from 160oC to 230oC. It is most preferred for the inlet temperature of the tail gas stream into the reactor system to be in the range of from 170oC to 220oC. SP 2823 PD - 38 – The operating of the one or more reactors of reactor system hydrolysis conditions may be generally in the range of from 0.006 to 10 MPa (0.06 to 100 bar g), preferably, in the range of from 0.006 to 5 7 MPa (0.06 to 70 bar g), and, more preferably, in the range from 0.006 to 1 MPa (0.06 to 10 bar g). The flow rate at which the tail gas stream and, if any, the added reducing gas, are introduced into the one or more reactors of reactor system may generally be such 10 as to provide a gaseous hourly space velocity (GHSV) that is in the range of from 10 hr-1to 10,000 hr-1. The term “gaseous hourly space velocity” refers to the numerical ratio of the rate at which the hydrocarbon feedstock, that is to say, the combined tail gas stream 15 and reducing gas (if any), is charged to the reactor in volume per hour divided by the volume of catalyst contained in the reactor to which the feedstock is charged. The preferred GHSV is in the range of from 10 hr-1to 8,000 hr-1, more preferably, from 500 hr-1to 20 5,000 hr-1, and, most preferably, from 1000 hr-1to 4,000 hr-1.In a preferred embodiment, the process of the present invention is operated under hydrolysis conditions comprising an inlet temperature in the range 25 of from 140-400 °C, a pressure in the range of from 0.006 to 10 MPa (0.06 bar g to 100 bar g) and a gaseous hourly space velocity (GHSV) in the range of from 10 hr-1to 10,000 hr-1. In the processing of a Claus tail gas stream, in 30 most instances, said gas stream will inherently contain concentrations of water and hydrogen, which can be the source of the reducing gas required for the hydrolysis reaction of the process. But, in the event that the gas SP 2823 PD - 39 – stream does not contain a sufficient concentration of reducing gas components, reducing gas may be added as needed to the tail gas stream. It is generally desirable to have amounts of the reducing gases in the tail gas 5 stream that are stoichiometrically required to allow for the hydrolysis reactions to proceed to close to completion. A treated gas stream, having a reduced concentration of the sulfur compound, is yielded from 10 the reactor system. While the treated gas stream will have an increase in the concentration of H2S over that of the tail gas stream, the treated gas stream will have a reduced concentration of sulfur compounds over that of the tail gas stream. The total concentration of sulfur 15 compounds in the treated gas stream should, generally, be less than 500 ppmv, preferably less than 100 ppmv, more preferably less than 75 ppmv, even more preferably less than 50 ppmv, and, most preferably, less than 30 ppmv. 20 The following examples illustrate certain aspects of the invention, but they are not to be construed as unduly limiting the scope of the invention. EXAMPLES EXAMPLE 1 – Preparation of catalyst compositions 25 This Example describes methods of preparing catalyst compositions for use in catalyst systems described and tested herein. (i) Catalyst composition A Catalyst composition A was prepared by mulling an 30 alumina powder, which comprised primarily pseudoboehmite, with nitric acid, and water in such proportions as to provide a plastic mixture, e.g., an extrudable mixture, having a water content such that its loss on ignition is around 62%. SP 2823 PD - 40 – A cobalt-containing solution including cobalt was prepared by cobalt nitrate in water, and a molybdenum-containing aqueous solution including molybdenum was prepared by dissolving ammonium 5 dimolybdate in water with 30% hydrogen peroxide. The two metal solutions were added to the mulling mixture and, after mixing for a period of time, a small percentage of ammonium hydroxide was mixed with the mulling mixture. The resulting mixture was then extruded 10 through 3.2 mm trilobe extrusion dies, and the extrudates were dried and calcined above 482 ° C. for > 1 hour. Catalyst composition A included alumina that was predominately in the gamma form, 8.2 wt.% molybdenum, 15 and 2.5 wt.% cobalt. The wt.% of the metals is based on the total weight of the catalyst with the metals in the oxide form. (ii) Catalyst composition B A sample of catalyst composition A prepared 20 hereinabove was further modified by an impregnation process as described below to prepare catalyst composition B. An impregnation solution was prepared by mixing aqueous ammonia, ammonium di-molybdate and cobalt 25 carbonate in amounts such as to target in the finished catalyst 14 wt. % molybdenum and 3.4 wt. % cobalt based on the total weight of the finished catalyst with the metals in the oxide form. This mixture was heated to 45° C. and an amount of 30 monoethanolamine (MEA) of from 0.5 to 0.9 moles MEA per mole cobalt was added to the mixture. The mixture was stirred while maintaining the temperature until the metal salts were digested. The solution was then cooled to approximately 30° C. and topped-off with water so as SP 2823 PD - 41 – to provide a total volume of solution that approximated the pore volume of composition A which was to be impregnated with the solution. A sample of the above prepared catalyst composition 5 A was impregnated with the impregnation solution and aged for two hours with occasional mixing to prevent agglomeration. The impregnated extrudates were dried in a convection oven at a temperature of 80° C. for two hours followed by further drying at 125° C for two 10 hours. The dried extrudates were calcined above 482 ° C. for > 1 hour in a muffle furnace. Catalyst composition B included alumina that was predominately in the gamma form, 14.0 wt.% molybdenum, and 3.4 wt.% cobalt. The wt.% of the metals is based on 15 the total weight of the catalyst with the metals in the oxide form. EXAMPLE 2 - Testing of catalyst compositions A and B This Example illustrates the use of catalyst compositions A and B described in Example 1 in the 20 hydrolysis of a gas stream containing a concentration of at least one sulfur compound and carbon monoxide and presents performance data for the catalysts. Catalyst compositions A and B were performance tested for their individual activity for the removal of 25 carbon monoxide, carbonyl sulfide (COS) and carbon disulfide (CS2) from a tail gas stream using a tail gas pilot unit reactor equipped with a tube furnace used to control the reactor temperature. In preparation for the activity testing, each 30 respective catalyst composition was sulfided by introducing into the reactor 3 hours at 300oC and a 467 h-1nGHSV a feed comprising H2S and H2. SP 2823 PD - 42 – A synthetic tail that included H2S, SO2, COS, CS2, S, H2, CO, N2, and and having the typical composition as shown in Table 2, was then charged to the tail gas reactor, operated at various reactor 5 temperatures, at a rate so as to provide a 2052 h-1nGHSV (normal gas hourly space velocity), 0.02068 MPa (3 psi, 0.2068 bar g) unit pressure. Table 2. Typical Feed Composition Component Mole % H22CO27H2S 0.8 CO 1 COS 0.025 SO20.4 CH3SH 0 CS20.025 CH40H2O 26 S 0 N262.75 10 The composition of the reactor effluent for each of the reactor temperature conditions was analyzed using gas chromatography. The results from the testing each of catalyst 15 compositions A and B are presented in Table 3. Table 3 shows the results of testing each of catalysts compositions A and B as the single catalyst in the presence of the synthetic tail gas composition of Table 2. 20 It is apparent from Table 3 that catalyst composition B has a higher overall individual activity of than catalyst composition A with regard to the SP 2823 PD - 43 – removal of CO, COS and CS2from the tail gas composition in testing at a of 220 C. Table 3. Testing of Single Catalyst Compositions 220°C Cat. CO Outlet COS Outlet CS Outlet Comp.2mole% ppm ppm A 0.1249 90 28 B 0.0129 2 nd 5 nd= not detected, below detection limit. EXAMPLE 3 - Testing of catalyst systems comprising combinations of compositions A and B This Example illustrates the use of catalyst 10 systems consisting of combinations of catalyst compositions A and B, as described in Example 1, in various stacked bed arrangements in the hydrolysis of a gas stream containing a concentration of at least one sulfur compound and carbon monoxide and presents 15 performance data for the catalysts. As described in Table 4, various catalyst systems were prepared having stacked bed arrangements of catalysts compositions A and B. Said stacked catalyst bed arrangements were 20 prepared by loading catalysts into a long tubular reactor. The reactor was oriented vertically with the feed gas entering at the top. The catalyst to be at the bottom of the reactor was loaded first followed by the catalyst to be at the top of the reactor. The reactor 25 was then filled to the top with inert glass beads and closed. Effluent from the reactor exited at the bottom of the reactor. SP 2823 PD - 44 – The catalyst were performance tested using a tail gas pilot unit equipped with a tube furnace used to control the reactor temperature. The feed gas (Table 2) and testing conditions used in this 5 Example were identical to those employed in Example 2. In preparation for the activity testing, each loaded catalyst system was sulfided by introducing into the reactor for 3 hours at 300oC and a 467 h-1nGHSV, a feed comprising H2S and H2. 10 In Table 4, the catalyst composition indicated to be the “1st catalyst composition” represents the catalyst composition that first comes into contact with the tail gas composition to be treated. Said 1st catalyst composition is upstream of the catalyst 15 composition indicated as being the “2nd catalyst composition”. Table 4 also indicates the volume ratio of the 1st catalyst composition to the 2nd catalyst composition in the stacked bed arrangement. 20 Table 4. Testing of Catalyst Systems Experiment Catalyst System Catalyst Volume Composition Ratio of 1st:2nd Catalyst 1st 2nd Composition 1 Single CatalystAn / an / a2 Single CatalystBn / an / a3 (comp.) Stacked BedAB75:254 (comp.) Stacked BedAB50:505 (comp.) Stacked Bed A B 25:75 6 (inv.) Stacked Bed BA 75:257 (inv.) Stacked Bed B A 50:50 8 (inv.) Stacked Bed BA 25:75Experiments 1 and 2 in Table 4 relate to the testing of single catalysts indicated in Table 3. SP 2823 PD - 45 – Experiments 3-5 relate to the testing of catalyst systems wherein a composition having lower independent activity for removal of CO, COS and CS2is arranged upstream of a catalyst composition having 5 higher independent activity for removal of CO, COS and CS2(as shown in Table 3). Hence, said experiments are comparative in nature. Experiments 6-8 relate to the testing of catalyst systems wherein a catalyst composition having higher 10 independent activity for removal of CO, COS and CS2is arranged upstream of a catalyst composition having a lower independent activity for removal of CO, COS and CS2(as shown in Table 3). Hence, said experiments are in accordance with the present invention. 15 RESULTS Table 5 shows the results of testing the various stacked bed combinations of Catalysts A and B using the afore-mentioned methodology and the tail gas composition described in Table 2. 20 As shown in Table 2, the tail gas composition used in testing had inlet concentrations of 1 mole % CO, 0.025 mole % (250 ppm) COS and 0.025 mole % (250 ppm) CS2.The outlet concentrations of CO, COS and CS2after 25 testing using stacked bed combinations of Catalysts A and B are given in said Tables at an operating temperature of 220 C.

[0002] SP 2823 PD - 46 – Table 5. Testing of Catalysts A and B Expt. Cat. Vol. 220°C System Ratio CO COSCS21st:2nd1st:2ndCat. Outlet Outlet Outlet Cat. Comp. mole % ppm ppm Comp. 1 A:B 100:0 0.1249 90 28 3 (comp.) A:B 75:25 0.0607 33 6 4 (comp.) A:B 50:50 0.0355 21 0 5 (comp.) A:B 25:75 0.0140 5 0 6 (inv.) B:A 75:25 0.0120 0 0 7 (inv.) B:A 50:50 0.0267 9 0 8 (inv.) B:A 25:75 0.0507 19 0 2 B:A 100:0 0.0129 2 0 DISCUSSION 5 (i) Catalyst systems consisting of Catalyst compositions A and B As shown in Table 5, for a given percentage loading of each of catalyst compositions A and B in a stacked bed arrangement, the order of arranging said 10 compositions surprisingly influences the overall performance of the catalyst system. It is apparent that catalyst systems wherein the higher activity catalyst, i.e. catalyst composition B, is placed upstream of the lower activity catalyst, i.e. 15 catalyst composition A results in improved conversion of the one or more sulfur compounds and carbon monoxide in the tail gas feed. The effect of the catalyst bed arrangement in improving conversion is particularly noted at lower temperatures. SP 2823 PD - 47 – Tables 6 and 7 indicate the improvements in conversion of carbon monoxide and carbonyl sulfide (COS) using catalyst systems in accordance with the present invention. 5 In the tables hereinbelow, improvements in conversion are determined by calculating the % conversion of CO and COS for each catalyst bed arrangement and then comparing conversion of alternate arrangements. 10 Table 6. Conversion of CO for Catalyst Systems consisting of Catalyst A and B Reactor Isothe ormal d Expt. 1st:2nTemp. (220 C) Catalyst BedImprovement in conversion of CO for B:A vs. A:B (%) 75% A / 25% B 3 (comp.) A:B 1.06 8 (inv.) B:A 50% A / 50% B 4 (comp.) A:B 0.91 7 (inv.) B:A 25% A / 5% B 5 (comp.) A:B 0.19 6 (inv.) B:A A 1% improvement in conversion can result in 100 ppm 15 less CO at the reactor outlet which is significant for many tail gas operations.

[0003] SP 2823 PD - 48 – Table 7. Conversion COS for Catalyst Systems consisting of Catalyst A and B Reactor Isothe ormal pt. 1sTemp. (220 C) Catalyst Bed Ext:2ndImprovement in conversion of COS for B:A vs. A:B (%) 75% A / 25% B 3 (comp.) A:B 6.48 8 (inv.) B:A 50% A / 50% B 4 (comp.) A:B 5.01 7 (inv.) B:A 25% A / 75% B 5 (comp.) A:B 2.23 7 (inv.) B:A 5 A 6.5% improvement in conversion can result in 14 ppm less COS at the reactor outlet which is significant for many tail gas operations. The data presented in the above Tables shows that in catalyst systems consisting of catalysts comprising 10 cobalt, molybdenum and alumina, by placing the catalyst having higher activity for removal of carbon monoxide and / or carbonyl sulfide (COS) upstream, there are surprising improvements in overall activity of the catalyst system vis-à-vis placing the catalyst having 15 lower activity upstream. Furthermore, as shown in Table 8, such arrangements unexpectedly allow for catalyst systems having advantageous overall activities for the removal of carbon monoxide and carbonyl sulfide, whilst also 20 employing lower amounts of the higher activity catalyst, thereby reducing costs. SP 2823 PD - 49 – Table 8. Systems Catalyst 220°C or Expt. Catalyst Vol. Outlet System Ratio 1st:2nd CO COSCS2mole% ppm ppm 2 B only 100 0.0129 2 0 5 (comp.) A:B 25:75 0.0140 5 0 6 (inv.) B:A 75:25 0.0120 0 0 5 Indeed, per Table 8, the performance of a catalyst system consisting of catalyst composition B upstream of catalyst composition A (as shown in Experiment 6) at 220 C actually exceeds the performance of catalyst composition B alone at 220 C for the removal of carbon 10 monoxide and carbonyl sulfide. It will be apparent to one of ordinary skill in the art that many changes and modifications may be made to the described invention without departing from its spirit and scope as set forth in this specification. 15

Claims

SP 2823 PD - 50 – C L A I M S 1. A process for the treatment of a tail gas stream comprising one or more sulfur compounds and / or carbon monoxide in the presence of water and hydrogen, under 5 hydrolysis conditions, in a reactor system comprising one or more reactors, wherein said process takes place in the presence of a catalyst system comprising a first catalyst composition and a second catalyst composition, 10 wherein the first catalyst composition and the second catalyst composition each comprise a cobalt compound and / or a nickel compound in combination with a molybdenum compound and an inorganic oxide selected from alumina and / or titania and the first catalyst 15 composition has a higher activity for the removal of carbon monoxide and / or a higher activity for the removal of carbonyl sulfide (COS) and / or a higher activity for the removal of carbon disulfide (CS2) from a tail gas stream relative to the second catalyst composition under 20 the same conditions, and wherein the first and second catalyst compositions are arranged such that at least some of the first catalyst composition is situated upstream in the reactor system of the second catalyst composition. 25 2. Process according to Claim 1, wherein the catalyst system consists of the first catalyst composition and the second catalyst composition.

3. Process according to Claim 1 or 2, wherein all of the first catalyst composition is situated upstream in 30 the reactor system of the second catalyst composition.SP 2823 PD - 51 – 4. Process according to any of Claims 1 to 3, wherein the first catalyst has an individual activity level, X1, for the removal of carbon monoxide, an individual activity level, Y1, for the removal of 5 carbonyl sulfide (COS), and an individual activity level, Z1, for the removal of carbon disulfide (CS2), when operating under a given set of operating conditions and the second catalyst composition has an individual activity level, X2, for the removal of carbon monoxide, 10 an individual activity level, Y2, for the removal of carbonyl sulfide, and an individual activity level, Z2, for the removal of carbon disulfide, under the same set of operating conditions and wherein X1>X2 and / or Y1>Y2 and / or Z1>Z2. 15 5. Process according to any of Claims 1 to 4, wherein the first catalyst composition and the second catalyst composition are present in the same reactor in the reactor system in a stacked catalyst bed arrangement.

6. Process according to any of Claims 1 to 5, wherein 20 the first catalyst composition and the second catalyst composition are present in catalyst beds in separate reactors in the reactor system in a sequential arrangement.

7. Process according to any of Claims 1 to 6, wherein 25 the catalyst volume ratio of the first catalyst composition to the second catalyst composition in the catalyst system is in the range of from 5:95 to 95:

5.

8. Process according to any of Claims 1 to 7, wherein the first and second compositions comprise a cobalt 30 compound and / or nickel compound in an amount of at least 0.5 wt. %, preferably in the range of from 0.5 to 8 wt. %, and a molybdenum compound in an amount of at least 3 wt. %, preferably in the range of from 3 to 22 wt. %,SP 2823 PD - 52 – based on the total of said catalyst composition and the metal as an regardless of its actual form.

9. Process according to any of Claims 1 to 8, wherein the first catalyst composition contains at least 2.0 5 wt.%, preferably at least 2.7 wt. %, cobalt and / or nickel and at least 7.4 wt.% molybdenum, based on the total weight of the catalyst composition and the metal as an oxide regardless of its actual form.

10. Process according to any of Claims 1 to 9, wherein 10 the second catalyst composition contains an amount in the range of from 1 to 3.75 wt.% cobalt and / or nickel and an amount in the range of from 3 to 9.5 wt.% molybdenum, based on the total weight of the catalyst composition and the metal as an oxide regardless of its 15 actual form.

11. Process according to any of Claims 1 to 10, wherein the inorganic oxide is preferably present in each of the first and second catalyst compositions in an amount in the range of from 60 to 95 wt. %, based on the total 20 weight of the catalyst composition.

12. Process according to any of Claims 1 to 11, wherein the first catalyst composition comprises a cobalt compound in combination with a molybdenum compound and an inorganic oxide selected from alumina and / or titania 25 and the second catalyst composition comprises a cobalt compound in combination with a molybdenum compound and an inorganic oxide selected from alumina and / or titania.

13. Process according to any of Claims 1 to 12, wherein the first catalyst composition comprises a 30 cobalt compound in combination with a molybdenum compound and an inorganic oxide selected from alumina and / or titania, and wherein the first catalyst composition is prepared by a process comprising co- mulling components that include cobalt and molybdenumSP 2823 PD - 53 – metals and an alumina and / or titania precursor and drying and calcining the co-mulled mixture.

14. Process according to Claim 13, wherein the first catalyst composition contains at least 2.0 wt.%, 5 preferably at least 2.7 wt. %, cobalt and at least 7.4 wt% molybdenum, wherein each wt.% is based on the total weight of the catalyst composition and the metal as an oxide regardless of its actual form.

15. Process according to any of Claims 1 to 14, wherein 10 the first catalyst composition comprises a cobalt compound in combination with a molybdenum compound and an inorganic oxide selected from alumina and / or titania, and wherein the first catalyst composition comprises both molybdenum and cobalt metals in a so-called 15 underbedded form and also both molybdenum and cobalt as overlaid metals.

16. Process according to Claim 15, wherein said first catalyst composition comprises a calcined co-mulled mixture of an inorganic oxide selected from alumina 20 and / or titania, a first molybdenum compound, and a first cobalt compound, wherein said calcined co-mulled mixture is impregnated with a second molybdenum compound and a second cobalt compound, thereby providing underbedded molybdenum, underbedded cobalt, overlaid molybdenum, and 25 overlaid cobalt.

17. Process according to Claim 15 or 16, wherein said first catalyst composition has a molybdenum ratio of underbedded molybdenum content-to-total molybdenum content in the range of from 1:5 to 4:5, and a cobalt 30 ratio of underbedded cobalt content-to-total cobalt content in the range of from 1:5 to 4:

5.

18. Process according to any of Claims 15 to 17, wherein said first catalyst composition has a totalSP 2823 PD - 54 – metals wt.% ratio of total molybdenum content-to-total cobalt content in the of from 1.5:1 to 8:

1.

19. Process according to Claim 13 or 14, wherein the first catalyst composition comprises a formed 5 agglomerate of a co-mulled mixture, comprising pseudoboehmite, a cobalt compound and a molybdenum compound, wherein said formed agglomerate has been calcined to provide said catalyst composition, comprising gamma-alumina. 10 20. Process according to Claim 19, wherein the first catalyst composition has a bimodal pore structure with a first major portion of the total pore volume in pores of diameter in the range of from 50 Å to 150 Å and a minor portion of the total pore volume in pores of diameter in 15 the range of from 1,000 Å to 10,000 Å and, optionally, may have less than 6 % of the total pore volume of the catalyst is contained within its pores having a pore diameter greater than 10,000 Å, wherein pore diameter is determined by surface area from mercury porosimetry, 20 with a contact angle of 140 degrees.

21. Process according to any of Claims 1 to 20, wherein the second catalyst composition comprises a formed agglomerate of a co-mulled mixture, comprising pseudoboehmite, a cobalt compound and a molybdenum 25 compound, wherein said formed agglomerate has been calcined to provide said catalyst composition.

22. Process according to any of Claims 1 to 21, wherein said one or more sulfur compounds are present in the tail gas stream at a sulfur compound concentration in 30 the range of from 0.002 volume % to 5 volume %, and wherein said one or more sulfur compounds are selected from the group of compounds consisting of carbonyl sulfide (COS), carbon disulfide (CS2), sulfur dioxideSP 2823 PD - 55 – (SO2), methyl mercaptan (CH3SH), and elemental sulfur (Sx).

23. Process according to any of Claims 1 to 22, wherein said process is operated under hydrolysis conditions 5 comprising an inlet temperature to said one or more reactors in the reactor system that is in the range of from 115 to 400oC, preferably in the range of from 140 to 400oC.

24. Process according to any of Claims 1 to 23, wherein 10 said process is operated under hydrolysis conditions comprising an inlet temperature in the range of from 140-400 °C, a pressure of in the range of from 0.006 to 10 MPa (0.06 bar g to 100 bar g) and a gaseous hourly space velocity (GHSV) in the range of from 10 hr-1to 15 10,000 hr-1.

25. Process according to any of Claims 1 to 24, wherein the total concentration of sulfur compounds in the treated gas stream yielded from the reactor system is less than 500 ppmv. 20

Images