Abstract:
In general, the invention provides methods and associated apparatuses for removing odorant and sulfur compounds from a gas stream such as natural gas. As an example, such systems are typically required by fuel processor systems adapted to convert natural gas into reformate for use in fuel cell systems, where the odorant and sulfur compounds might otherwise poison the fuel processor and fuel cell catalysts Systems under the present invention are based on the use of at least two filtration stages such that the odorant removal function is segregated from the general removal of H 2 S.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority under 35 USC 119(e) from U.S. Provisional Application No. 60/287,046, filed Apr. 27, 2001, naming Lieftink, et al. as inventors, and titled “GAS PURIFICATION SYSTEM.” That application is incorporated herein by reference in its entirety and for all purposes. 
     
    
     
       BACKGROUND  
         [0002]    The invention generally relates to a technique and apparatus for desulfurizing a hydrocarbon stream.  
           [0003]    A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a polymer electrolyte membrane (PEM), often called a proton exchange membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the protons to form water. The anodic and cathodic reactions are described by the following equations:  
           [0004]    H 2 →2H + +2e −  at the anode of the cell, and  
           [0005]    O 2 +4H + +4e − →2H 2 O at the cathode of the cell.  
           [0006]    A typical fuel cell has a terminal voltage of up to one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.  
           [0007]    The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to act as a gas diffusion media and in some cases to provide a support for the fuel cell catalysts. In this manner, reactant gases from each side of the PEM may pass along the flow channels and diffuse through the GDLs to reach the PEM. The PEM and its adjacent pair are often assembled together in an arrangement called a membrane electrode assembly (MEA).  
           [0008]    A fuel cell system may include a fuel processor that converts a hydrocarbon (natural gas or propane, as examples) into a fuel flow for the fuel cell stack. For a given output power of the fuel cell stack, the fuel flow to the stack must satisfy the appropriate stoichiometric ratios governed by the equations listed above. Thus, a controller of the fuel cell system may monitor the output power of the stack and based on the monitored output power, estimate the fuel flow to satisfy the appropriate stoichiometric ratios. In this manner, the controller regulates the fuel processor to produce this flow, and in response to controller detecting a change in the output power, the controller estimates a new rate of fuel flow and controls the fuel processor accordingly.  
           [0009]    The fuel cell system may provide power to a load, such as a load that is formed from residential appliances and electrical devices that may be selectively turned on and off to vary the power that is demanded by the load. Thus, the load may not be constant, but rather the power that is consumed by the load may vary over time and abruptly change in steps. For example, if the fuel cell system provides power to a house, different appliances/electrical devices of the house may be turned on and off at different times to cause the load to vary in a stepwise fashion over time.  
           [0010]    Fuel cells generally utilize hydrogen as a fuel, and fuel cell systems are known that process natural gas or propane into a hydrogen rich stream (often referred to as reformate) for use as the fuel. Natural gas is a loose term that can describe a variety of hydrocarbon gas compositions which may vary widely, for example, according to geography, temperature, time of year, etc. For example, some sources of natural gas may typically contain about 75% CH 4 , 15% ethane (C 2 H 6 ), and 5% other hydrocarbons, such as propane (C 3 H 8 ) and butane (C 4 H 10 ). In the U.S., natural gas from municipal utilities may contain the following general composition: 94% methane (CH 4 ); 3.2% ethane (C 2 H 6 ); 0.7% propane (C 3 H 8 ); 2.6% CO 2 +N 2 ; 0.25˜1 grains/100 ft 3  hydrogen sulfide (H 2 S); 1.0-10 grains/100 ft 3  Mercaptans; 10-20 grains/100 ft 3  Total sulfur.  
           [0011]    Methane is a colorless, odorless gas with a wide variation in distribution in nature. Methane is not toxic when inhaled, but it can produce suffocation by reducing the concentration of oxygen inhaled. For this reason, trace amounts of odorous organic sulfur compounds are usually added to natural gas in order to provide a detectable odor (e.g., to make gas leaks readily detectible). Such compounds are generally referred to as “odorants”. It will be appreciated that an odorant refers to any compound which is added to a gas to make the gas detectable to humans by smell. Typical odorants mixtures include tertiary butyl mercaptan, (CH 3 ) 3 CSH and dimethyl sulfide, CH 3 —S—CH 3 . Additional compounds may also be used: tetrahydrothiophene (THT, or “thiophanes”); isopropyl mercaptan; propyl mercaptan; and methyl ethyl sulfide. Additional components may also be present in natural gas, such as carbonyl sulfide (COS), carbon disulfide (CS 2 ), etc. Similarly, propane fuels may also contain a mixture of components.  
           [0012]    The sulfur compounds associated with natural gas, propane, and other fuels (e.g., H 2 S, COS) can degrade fuel cell catalysts, as well as the fuel processor catalysts. For this reason, fuel cell systems utilizing such streams generally include a desulfurization process to remove sulfur compounds from such streams. Many other processes also require desulfurization of gas or liquid streams, either to protect catalysts or for other reasons. For example, sulfur may be removed from combustion fuels to prevent formation of sulfur dioxide and other exhaust components that cause acid rain.  
           [0013]    There is a continuing need for an arrangement and/or technique to desulfurize gas streams, and to address one or more of the issues discussed above.  
         SUMMARY  
         [0014]    In general, the invention provides methods and associated apparatuses for removing odorant and sulfur compounds from a gas stream such as natural gas (e.g., removing such compounds to less than 50 parts per billion). As an example, such systems are typically required by fuel processor systems adapted to convert natural gas into reformate for use in fuel cell systems, where the odorant and sulfur compounds might otherwise poison the fuel processor and fuel cell catalysts. Systems under the present invention are based on the use of at least two filtration stages such that the odorant removal function is segregated from the general removal of H 2 S. This advantageously enables the size and make-up of each stage to be tailored to a specific application. Some embodiments also provide modular systems allowing individual stages to be replaced independently as they become saturated with odorants and sulfur compounds. Other features and advantages are described herein.  
           [0015]    As an example, whereas zeolite and activated carbon materials are often used to filter odorant and sulfur compounds from natural gas, the natural gas (sometimes referred to as “utility gas”) in some areas of Europe, as an example, tends to contain levels of carbonyl sulfide (COS) and other components that are not absorbed by zeolites or activated carbon. In such systems, a second material such as nickel oxide may be required to remove materials such as COS. Such materials are typically expensive relative to more conventional absorbents such as zeolites and activated carbon. However, even in areas where difficult-to-absorb components like COS are present, levels are still generally low enough that only small amounts of the more expensive absorbents are required (as an example, to achieve a one year life as a filter for a given fuel processing application). Since COS absorbents also tend to be suitable for absorbing odorants and H 2 S, it is advantageous to use a less expensive material in a primary absorption stage to remove odorants and H 2 S in order to conserve the amount of secondary absorbents required for the other sulfur compounds present (e.g., COS).  
           [0016]    In the context of this invention, the terms absorption and adsorption may be used interchangeably. In some embodiments, odorants and H 2 S can be removed from a gas stream in a first stage, and then the remaining carbonyl sulfide and carbon disulfide is reacted with a catalyst suitable to hydrolyze these components into H 2 S. The H 2 S is then absorbed into an additional material such as a zeolite or active carbon in a subsequent stage. In one aspect, the invention provides a method of removing sulfur compounds from a gas stream, comprising the following steps: removing an odorant component of a gas in a first step by contacting the gas with a first material; and removing H 2 S from the gas in a second step by contacting the gas with a second material different from the first material.  
           [0017]    Embodiments of this method can further any of the following details and additional steps, either alone or in combination. The first material can comprise a zeolite (e.g., a type X zeolite). The odorant compound can be at least one of: tetrahydrothiophene, tertiary butyl mercaptan, isopropyl mercaptan, propyl mercaptan, dimethyl sulfide, and methyl ethyl sulfide. The second material can comprise an H 2 S absorbent. A further step may include: reacting the gas with a COS hydrolysis catalyst to convert a COS sulfide component of the gas to H 2 S after removing the odorant and prior to removing the H 2 S in the second step. The COS hydrolysis catalyst can comprise a material selected from the group comprising titania, zirconia, thoria, lanthanide oxide, alumina, ceria, molybdenum oxide, vanadium oxide, manganese oxide, cobalt oxide, iron oxide and nickel oxide. Further steps may also include: heating a third material to a temperature greater than 10° C., wherein the third material is adapted to adsorb H 2 S (e.g., a zeolite having a mean pore size less than 10 angstroms); and flowing the gas through the third material prior to contacting the gas with the first material.  
           [0018]    Another step may include flowing the gas from the second material through a fourth material adapted to provide a visual indication of H 2 S detection. Methods under the invention may also include: maintaining the first material at a first temperature; and maintaining the second material at a second temperature, wherein the first temperature is different from the second temperature. Other methods under the invention may also include: absorbing the odorant component into the first material; and replacing the first material with fresh first material while not replacing the second material. Still other methods under the invention may include: absorbing H 2 S into the second material; and replacing the second material with fresh second material while not replacing the first material.  
           [0019]    In another aspect, the invention provides an apparatus for removing sulfur compounds from a gas stream. The system includes a first material and a second material, and a conduit having an inlet and an outlet. The conduit provides fluid communication from the inlet to the first material, from the first material to the second material, and from the second material to the outlet (i.e., the gas flows along the conduit through each of the two stages). The first material is suitable for absorbing an odorant compound, and the second material is suitable for absorbing H 2 S. The first material can comprise a zeolite, and the first material and second material can be different substances.  
           [0020]    Advantages and other features of the invention will become apparent from the following description, drawing and claims. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 shows a flow diagram of a method of removing sulfur compounds from a gas stream.  
         [0022]    [0022]FIG. 2 shows a flow diagram of a method of removing sulfur compounds from a gas stream.  
         [0023]    [0023]FIG. 3 shows a schematic diagram of an apparatus for removing sulfur compounds from a gas stream.  
         [0024]    [0024]FIG. 4 shows a schematic diagram of an apparatus for removing sulfur compounds from a gas stream.  
         [0025]    [0025]FIG. 5 shows a schematic diagram of an apparatus for removing sulfur compounds from a gas stream. 
     
    
     DETAILED DESCRIPTION  
       [0026]    Referring to FIG. 1, a flow diagram is shown of a method of removing sulfur compounds from a gas stream, including the following steps: ( 100 ) Flowing gas through a first material to remove odorant components from the gas; and ( 102 ) Flowing the gas through a second material to remove H 2 S from the gas. Alternatively, referring to FIG. 2, a flow diagram is shown of another method of removing sulfur compounds from a gas stream according to the present invention, including the following steps: ( 200 ) Flowing gas through a,first material to remove odorant components from the gas; ( 202 ) Flowing the gas through a COS hydrolysis catalyst to convert COS in the gas into H 2 S; and ( 204 ) Flowing the gas through a third material to remove H 2 S from the gas.  
         [0027]    Referring to FIG. 3, a schematic diagram is shown of an apparatus  300  for removing sulfur compounds from a gas stream. The system  300  includes a vessel  301  having an inlet  302  and an outlet  304 . The vessel  301  includes a first compartment  306  containing a first material, and a second compartment  308  containing a second material. A gas such as natural gas from a utility line is fed through inlet  302 . The gas flows up the vessel  301  and exits outlet  304 . As an example, outlet  304  may be connected with the inlet of a fuel processor adapted to convert the natural gas into reformate for use by a fuel cell. However, it will be appreciated that the invention is not necessarily limited to this application (e.g., it can also be used to de-sulfurize other types of gasses and for other applications).  
         [0028]    The first material in compartment  302  is a material suitable for absorbing odorant compounds from the gas. As an example, where the gas is methane and contains sulfur-based odorants, the first material can be a type X zeolite or activated carbon, either in monolith or pellet form, as examples. Since it may also be necessary to remove COS and other compounds not absorbed by the first material, the second material is provided in the second compartment  308  as a second filtration stage. As an example, nickel materials (e.g., NiO) are suitable materials. The second material can also be another zeolite. For example, where CO 2  is present in the gas stream and a type X zeolite is used as the first material, the CO 2  can react in the pores of the zeolite with H 2 S to form COS. To prevent such COS formation, a zeolite may be selected as the second material that has a pore size small enough (e.g., smaller than 10 angstroms) to prevent CO 2  from being absorbed.  
         [0029]    As previously mentioned, one aspect of the invention is that with multiple filtration stages, it may be possible to replace individual vessel compartments or materials (e.g., at service intervals) without having to replace the entire de-sulfurization vessel. The individual compartments and/or the capacity of the material quantities provided in the vessel can also be tailored to a given application. Another feature of the invention may include an electric heater associated with the first material to heat the first material up during a cold start (e.g., below 20° C. or 0° C., as examples). It will be appreciated that the effectiveness of various materials to absorb gas components may be diminished at relatively low temperatures. Without such an arrangement, the start-up time of a system may be prolonged while the de-sulfurization bed heats up with the rest of the system.  
         [0030]    As another example, a cold-start module (not shown) may be associated with any of the systems described herein (e.g.,  300 ,  400 ,  500 ). In such a cold start module, an electric heater can be activated to provide heat to an adsorption material that is only used during start-up. For example, a small quantity of a highly active, less-temperature dependent material such as nickel can be used in the cold-start module. The quantity need only be relatively small since the cold-start module is only used during start-up. When the system reaches a desired temperature, the gas flow can be by-passed from the cold-start module. In some embodiments, the material used in the cold start module is active enough that the electric heater is not necessary.  
         [0031]    Referring to FIG. 4, a schematic diagram is shown of another apparatus  400  for removing sulfur compounds from a gas stream. The system  400  includes a vessel  401  having an inlet  402  and an outlet  404 . The vessel  401  includes a first compartment  406  containing a first material, a second compartment  408  containing a second material, and a third compartment  410  containing a third material. In this embodiment, the gas (methane in this example) flows through inlet  402  and through material  406 , which is a bed of zeolite pellets that remove the odorant components of the gas (mercaptans, THT, etc.). The gas, which contains water vapor, flows into material  408 , which is a COS hydrolysis catalyst (The COS is reacted with water to produce H 2 S: COS+H 2 O→CO 2 +H 2 S). One suitable COS hydrolysis catalysts is the SCOS catalyst available from Elf Atofina. Other suitable materials include titania, zirconia, thoria, lanthanide oxide, alumina, ceria, molybdenum oxide, vanadium oxide, manganese oxide, cobalt oxide, iron oxide and nickel oxide.  
         [0032]    In some embodiments, the water for this reaction may be supplied by injecting water into the vessel at a location associated with the second material  408 . The gas then flows to a third material  410  that absorbs the H 2 S produced in the second material. Since CO 2  is produced by the hydrolysis reaction of COS, and as previously discussed, since CO 2  can react with H2S in the pores of a zeolite to form COS, where the third material is a zeolite, it may be desirable the third material have a pore size small enough (e.g., &lt;10 angstroms) such that the CO 2  is not absorbed.  
         [0033]    Referring to FIG. 5, a schematic diagram is shown of an apparatus  500  for removing sulfur compounds from a gas stream. The system  500  includes a vessel  501  having an inlet  502  and an outlet  504 . The vessel  501  includes a first compartment  506  containing a first material, and a second compartment  508  containing a second material. A third compartment  510  is positioned between the second compartment  508  and the outlet  504 . The third compartment contains a third material and includes a window  512 .  
         [0034]    The third material is selected to provide a visual indication when contacted with sulfur compounds. As an example, the gas may be passed through a material containing lead acetate as it exits the vessel  501  through outlet  504 . As known in the art, lead acetate, which is normally white, will turn black when contacted with sulfur compounds as it is converted to lead sulfide. Other suitable materials are known in the art. This visual indication can be observed through window  512 . In this way, the system  500  provides a visual indication of when sulfur begins “breaking through”. In other words, a visual indication is provided that the absorbent materials in the system need to be refreshed or replaced (e.g., a service call).  
         [0035]    While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the invention covers all such modifications and variations as fall within the true spirit and scope of the invention.