Patent Publication Number: US-2007119751-A1

Title: Combination of zeolite and alumina impregnated with a noble metal(s) for COS and THT removal at low temperatures (&lt;40 degree C) in fuel cell processor applications

Description:
BACKGROUND  
      The present development is a method for removing sulfur-containing compounds from fluids, such as gaseous fuels and hydrocarbons. More specifically, the method comprises placing a feedstream in contact with a catalyst-sorbent which comprises a noble metal, a zeolite, and an alumina. The method is suitable for removing carbonyl sulfide and saturated heterocyclic sulfur compounds such as tetrahydrothiophene as well as thiols and hydrogen sulfide.  
      The presence of sulfur compounds in fuels is problematic for a number of reasons. Burning such fuels produces sulfur oxides which are a form of pollution. Sulfur-laden exhaust also poisons the catalysts used to remove other harmful substances from the exhaust, resulting in additional pollution. Many of the catalysts used in refining and processing fuels are also subject to sulfur poisoning. The electrocatalysts used in fuel cells to extract energy from the fuel are highly sensitive to sulfur. Consequently, sulfur content in fuels used to power fuel cells must be rigorously restricted.  
      It may also be necessary, for one reason or another, to remove sulfur from fluids other than fuels. Ethylene used for polymerization, for example, must be of high purity and have a low sulfur content. The same can be said for the hydrogen used in ammonia synthesis.  
      The removal of carbonyl sulfide (COS) is particularly troublesome because it is resistant to many desulfurization processes. Thus, the ability to remove COS is a desirable feature in a desulfurization process. Typically, COS is removed by adsorption, hydrolysis, or a combination of the two.  
      Other sulfur compounds that may need to be removed include hydrogen sulfide, thiols, and organic compounds containing a C—S—C or C—S—S—C linkage, such as diethyl sulfide, diethyl disulfide, and tetrahydrothiophene (THT). These compounds may be naturally co-occurring with the combustible hydrocarbons in the fuel, or may be added as odorants to allow humans to detect dangerous leaks of otherwise odorless gases. In either case, these sulfur compounds must be removed when the fuel is used certain applications, such as in a fuel cell. It is known in the art that hydrogen sulfide is relatively easy to remove by adsorption relative to organic sulfur compounds. As a result, a process that converts carbonyl sulfide and organic sulfur compounds to hydrogen sulfide is useful, even if the overall sulfur content does not appreciably change.  
      It is known in the art to remove sulfur compounds from hydrocarbons at elevated temperature and high partial pressures of hydrogen, a process referred to as hydrotreating or hydrodesulfurization. Typically temperatures of 150° C.-400° C. and hydrogen partial pressures in excess of 1 MPa are used in hydrodesulfurization processes. For example, U.S. Pat. No. 6,855,653 teaches a hydrodesulfurization process that uses a catalyst which is similar in composition to the catalyst-sorbent of the present development. However, the process of the &#39;653 patent relies on relatively high temperature and pressure, and requires hydrogen to be present in the process stream in order to effect sulfur compound removal.  
      Methods for removing noxious impurities, such as sulfur compounds, from a fluid by contacting the fluid with a solid material are well known in the art. Whether the solid material acts by adsorption, catalysis, or a combination of the two, it is necessary to ensure efficient and effective contact between the catalyst-sorbent and the fluid, and to make sure that substantially all of the fluid has been brought into effective contact with the catalyst-sorbent. Examples of some methods to achieve this can be found in the literature of chemical engineering and petroleum engineering, including in the three articles “Adsorption”, “Adsorption, Gas Separation”, and “Adsorption, Liquid Separation” in the  Kirk - Othmer Encyclopedia of Chemical Technology,  5 th  Edition, and in the three chapters “Adsorption and Ion Exchange,” “Liquid-Gas Systems,” and “Solids Drying and Gas-Solid Systems” in  Perry&#39;s Chemical Engineers&#39; Handbook,  6 th  Edition. These three articles from Kirk-Othmer and three chapters from Perry&#39;s Handbook are hereby incorporated by reference.  
      Sulfur compounds can be absorbed metals or metal oxides, such as metallic nickel, nickel oxide, and zinc oxide. This type of desulfurization process does not require high partial pressures of hydrogen, but does require elevated temperatures. Nickel-based sorbents, for example, typically work at a temperature range of 200° C.-400° C. In some situations, such as during startup of a fuel cell system designed for intermittent use, removal of sulfur compounds at about room temperature is needed.  
      U.S. Pat. No. 4,455,446 teaches a method for removing carbonyl sulfide from propylene which comprises passing the propylene stream through a bed of platinum sulfide supported on alumina. Unlike the present development, the catalyst-sorbent used in the process claimed in the &#39;446 patent does not include a zeolite. The method described in the &#39;446 patent can be performed at temperatures only slightly above ambient (35° C. to 65° C.), but requires rather high pressures (200 psia to 450 psia). Alternatively, the process can be carried out at near-atmospheric pressure but at elevated temperatures (135° C. to 260° C.).  
      U.S. Patent Application 2002-0121093 teaches a process using a Pt/Al 2 O 3  catalyst to hydrolyze COS present in synthesis gas. The temperature 300° F. (150° C.) is stated to be a typical process temperature. As with the &#39;446 patent, the catalyst-sorbent in the &#39;093 application includes a noble metal and alumina, but does not include a zeolite. In addition, an elevated temperature is required for the hydrolysis of COS.  
      U.S. Pat. No. 6,843,907 teaches a process for removing carbonyl sulfide at near-ambient temperatures (15° C. to 100° C.) from a hydrocarbon stream which uses a generic, regenerable sorbent. The sorbent is preferably an alkali-impregnated alumina, a zeolite, or a combination thereof. Thus the process taught in the &#39;907 patent uses zeolite and an alumina, but lacks a noble metal.  
      U.S. Patent Application 2004-0007506 teaches a process for deep desulfurization of hydrocarbon fuels, primarily liquid fuels. The fuel stream is contacted with any of a number of sulfur sorbents which work at temperatures ranging from 10° C. to 340° C. In claims  3 ,  4 , and  5  of the &#39;506 application, adsorbents containing mixed base-metal noble-metal chlorides supported on MCM-41, silica gel, activated carbon, or zeolites are mentioned. But the desulfurization process taught in the &#39;506 application requires a narrow class of noble metal compounds supported on alumina or zeolite.  
      Japanese Patent JP H04-106194 (to Kawasaki Steel Corp., 1992) teaches a process for removing organic sulfur compounds from coke oven gas. The gas to be treated is passed first over a bed of Pd/Al 2 O 3 , then through a bed of zeolite. The process temperature in the JP&#39;194 patent is 250° C.-350° C.  
     SUMMARY OF THE INVENTION  
      The present invention is a process for removing sulfur-containing compounds, such as carbonyl sulfide, thiols, disulfides, and saturated heterocyclic sulfur compounds, including tetrahydrothiophene, from gaseous fuels and hydrocarbons, such as natural gas. The process includes at least a step in which a gaseous sulfur-contaminated feedstream is placed in contact with a catalyst-sorbent which comprises a noble metal, a zeolite, and an alumina in a single reactor bed. Use of a single bed reactor is advantageous when conserving space or weight is desirable. Further, the process is intended to operate at near-ambient temperature.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present development is a method for removing sulfur-containing compounds from gaseous feedstreams. The sulfur compounds are removed from the feedstream by allowing the feedstream to have intimate contact with a catalyst-sorbent comprising a noble metal, a zeolite and an oxide compound. During the period of feedstream contact with the catalyst-sorbent, the temperature of the catalyst-sorbent is maintained at from about 0° C. to about 100° C. and the feedstream is fed to the catalyst-sorbent at a gas hourly space velocity of less than about 3000 h −1 .  
      The term catalyst-sorbent, as used herein, denotes a solid substance which by catalysis, absorption, adsorption, or any combination thereof changes the composition of the stream passed over it. For purposes of discussion herein, the term may be used interchangeably with “catalyst.” 
      The term noble metal, as used herein, denotes any one or more of the elements ruthenium, rhodium, palladium, osmium, iridium, and platinum, or any alloys or compounds containing at least one of these elements.  
      The term oxide comprises compounds of oxygen and at least one other element. In particular the term as used herein includes hydroxides and hydrated oxides. Similarly, the term alumina includes aluminum oxides, hydrated aluminum oxides, aluminum hydroxides, and aluminum oxide hydroxides.  
      The term desulfurization refers to any process designed to lower the total sulfur content or the concentration of particular sulfur compounds or classes of sulfur compounds. When used with the preposition “of”, the noun phrase after the “of” indicates the substance or mixture from which sulfur or its compounds are to be removed.  
      The term gaseous when applied to fuels, chemical compounds, or mixtures indicates that said fuel, compound, or mixture has a vapor pressure of greater than or equal to about 100 kPa at 25° C. The physical state under process conditions is irrelevant.  
      The process of the invention may be used wherever a fluid in the gaseous state contains excessive amounts of total sulfur, or of particular sulfur compounds, or of classes of sulfur compounds, including without limitation, carbonyl sulfide, thiols, disulfides, and saturated heterocyclic sulfur compounds, including tetrahydrothiophene, which the catalyst-sorbent is able to remove. In fuel cell processor train applications, the sulfur levels must be low enough to prevent premature degradation of the catalysts used to convert the fuel into a pure hydrogen stream and of the electrocatalyst in the fuel cell itself. Other situations may arise in which the reduction in total sulfur or of particular sulfur compounds or classes of sulfur compounds is desirable for one reason or another.  
      The process of the invention is particularly well suited for removing sulfur compounds from gaseous fuels such as natural gas, methane, propane, and liquefied petroleum gas (LPG). These gases often contain sulfur compounds as impurities, either naturally occurring or deliberately added as odorants. A sulfur concentration of 20 ppm to 500 ppm is typical for pipeline natural gas. When sulfur-containing gases are fed into a fuel cell processor train, the sulfur can poison the reforming catalyst(s), water-gas-shift catalyst(s), and other catalysts required to convert the fuel into a pure hydrogen stream. Sulfur poisoning of the fuel cell electrocatalyst itself can also occur.  
      The catalyst-sorbent used in the process of the invention has three necessary components: at least one noble metal, at least one zeolite, and at least one oxide support. In an exemplary embodiment of the catalyst, the noble metal comprises from about 0.1 wt % to about 5 wt % of the total catalyst weight, the zeolite comprises from about 50 wt % to about 90 wt % of the total catalyst weight, and the oxide support comprises from about 10 wt % to about 50 wt % of the total catalyst weight. In a more preferred embodiment of the catalyst, the noble metal comprises from about 0.5 wt % to about 2 wt % of the total catalyst weight, the zeolite comprises from about 75 wt % to about 85 wt % of the total catalyst weight, and the oxide support comprises from about 25 wt % to about 35 wt % of the total catalyst weight. The pore volume of the catalyst-sorbent is preferably at least about 0.3 cm 3 /g, and the specific surface area, measured by the BET procedure, is preferably at least 300 m 2 /g.  
      The noble metal may be selected from the group consisting of ruthenium, rhodium, palladium, osmium, iridium, platinum, alloys of these elements, other compounds containing at least one of these elements, and combinations thereof. The amount of noble metal to be added can vary and may, for example, be chosen to balance the need for a high activity with the cost of the metal(s). If noble metals are combined, for example if the catalyst-sorbent comprises platinum and palladium, the concentration of the combined noble metals will be from about 0.1 wt % to about 5 wt % of the total catalyst weight. The relative ratio of the noble metals may also vary as necessary to obtain a balance of performance versus cost. In an exemplary embodiment, palladium and platinum having a Pd:Pt ratio of from about 1:1 to about 6:1 is used. The noble metal or noble metals may be added to the zeolite and/or the support by standard methods of catalyst preparation which are known in the art, such as impregnation, ion exchange, chemisorption, or vapor deposition.  
      Zeolites are microporous materials having a framework comprising oxygen, silicon, and aluminum. The framework has a formal negative charge, which is balanced by adsorbed cations. One property of zeolites is that these cations can be exchanged for other cations, for example, Na +  ions can be replaced by NH 4   +  ions. It is found that aluminosilicate zeolites of the BEA and FAU structures (including zeolites β, X, and Y) are suitable zeolite components for the catalyst-sorbent. Preparative procedures for several zeolites can be found in the monograph  Verified Syntheses of Zeolitic Materials,  edited by H. Robson (Amsterdam; N.Y.: Elsevier, 2001), which is hereby incorporated by reference. Zeolite Y containing Na +  exchangeable cations (commonly known as “NaY”) is preferred.  
      The oxide support is typically alumina, silica, titania, zirconia, or mixtures thereof. Alumina is preferred. If alumina is used, it can be obtained in various forms with differing surface areas (high- or low-surface-area), differing levels of hydration, and different crystallographic phases (alpha, gamma, kappa, etc.) Any one or more of these forms may be used to prepare the catalyst-sorbent, and any one or several of these forms may be present in the final product. As is known in the art, some forms of alumina are more suitable for use in catalysts and sorbents than other forms. In general, high surface areas and crystallographic phases other than the alpha phase are more catalytically active and have higher sorbent capacity. This is especially important in fuel cell processor trains, where weight or space is often at a premium.  
      The process is suitable for use at near-ambient conditions. That is, a process wherein the catalyst-sorbent bed temperature is held below about 100° C. is preferred; a catalyst-sorbent bed temperature below about 60° C. is more preferred; and a catalyst-sorbent bed temperature below 40° C. is most preferred. A catalyst-sorbent bed temperature above 0° C. is preferred. No hydrogen is required for the process. If hydrogen is present, it is preferable that the partial pressure of hydrogen at the catalyst-sorbent bed be less than about 500 kPa, and more preferably less than about 50 kPa. The gas hourly space velocity of the feedstream as it is fed into the catalyst-sorbent bed is preferably less than about 3000 h −1 , more preferably less than about 1500 h −1 .  
      The following example illustrates and explains the present invention, but is not to be taken as limiting the present invention in any regard:  
      In an exemplary process, a catalyst-sorbent is prepared by physically mixing gamma-alumina and zeolite NaY such that the ratio of alumina to zeolite is approximately 1:4, and the alumina-zeolite mixture is then extruded. Palladium and platinum are impregnated onto the extruded alumina-zeolite pieces to produce a catalyst precursor having a Pd to Pt ratio of about 3:1 and a combined noble metal loading of between 1.0% and 1.5%. The precursor is dried at about 120° C. for about 12 hours, then calcined at about 550° C. for about 4 hours. The resulting catalyst-sorbent comprises about 37.49 wt % A; 2 O 3 , 57.08 wt % SiO 2 , 4.49 wt % Na 2 O, 0.83 wt % Pd, 0.30 wt % Pt, 0.029 wt % C and 0.003 wt % S, and has a pore volume of 0.48 cm 3 /g, a BET specific surface area of 481 m 2 /g, and a loss of ignition at 1000° F. (540° C.) of 23.07%. The catalyst-sorbent has about an 86% NaY content according to X-ray diffraction. The catalyst-sorbent is packed into a tubular reactor having a diameter of about 1.9 cm and a catalyst bed volume of about 10 cm 3 . The catalyst bed is heated to a temperature of about 38° C. and this temperature is maintained as a hydrocarbon feedstream comprising about 93 vol % methane, 3 vol % ethane, 2 vol % propane, 0.25 vol % butane, 1 vol % nitrogen, 1 vol % carbon dioxide, 5 ppm carbonyl sulfide, 5 ppm mercaptan, 5 ppm tetrahydrothiophene, and 5 ppm hydrogen sulfide is fed to the reactor at a gas hourly space velocity (GHSV) of about 3000 −hr , and at a gas pressure of about 15 psig. After about 50 hours on stream, at the reactor outlet, the gaseous feedstream comprises by volume less than about 0.1 ppm mercaptan, less than about 0.1 ppm tetrahydrothiophene, about 3 ppm carbonyl sulfide, and about 10 ppm hydrogen sulfide.  
      The present invention has the advantage of being able to remove mercaptans, tetrahydrothiophene, and carbonyl sulfide in a single step. However, the catalyst-sorbent used in the invention is not designed to absorb hydrogen sulfide; in fact the hydrogen sulfide level of the process stream may increase due to hydrolysis of other sulfur compounds, such as carbonyl sulfide. Because of this, it is almost always preferable to include a subsequent step in which hydrogen sulfide is removed. For example, a bed of H 2 S-specific sorbent may be used downstream from the claimed catalyst-sorbent.  
      It is understood that the composition of the catalyst-sorbent and the specific processing conditions may be varied within limits known in the art without exceeding the scope of this development.