Abstract:
An apparatus for processing hydrocarbon fuel (e.g., gasoline, kerosene, jet fuel, diesel and heating oil) to generate hydrogen (H 2 ), which can be used in fuel cells, includes a desulfurization reactor for removing sulfur from the fuel; a catalytic reactor for forming a reformate from the fuel; and, optionally, a separator for separating a light fraction of the fuel from a heavy fraction of the fuel. The fuel is first exposed to the desulfurization reactor and then, if present, to the separator. Finally, the fuel is exposed to the catalyst in the catalytic reactor; and the hydrogen gas generated there from is collected for use in the fuel cell.

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional application No. 60/500,134, filed Sep. 4, 2003, the entire teachings of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Hydrogen is playing an increasingly important role in clean combustion and zero-emission power generation. Fuel cells operating on pure hydrogen or hydrogen-rich gas (reformate) have the potential to revolutionize power generation for both stationary and transportation applications. Distribution of hydrogen to fuel-cell devices poses significant technical difficulties due to hydrogen&#39;s low energy density, poor distribution infrastructure, and high cost. Thus, a great and practical interest for fuel-cell power generation units is the conversion of liquid hydrocarbon fuels, such as gasoline, kerosene, jet fuel, diesel fuel and heating oil, to a gaseous stream rich in hydrogen. One common attribute of these fuels is that they all contain high concentrations of sulfur. Typical sulfur concentration in commercial grade gasoline, diesel and jet fuels currently range from 200 to 3000 parts per million by weight (ppmw). 
     Conversion of hydrocarbon fuels to hydrogen and carbon oxides is generally carried out in a reactor vessel via one of three processes: stream reforming, partial oxidation and autothermal reforming. To increase the process efficiency and reduce the operating costs, catalysts are normally used in all these reforming processes. Commercially available catalysts for these reforming processes include transition metals, such as nickel, and noble metals, such as platinum, supported on ceramic oxides. However, none of these catalysts can be used to directly reform high-sulfur fuels, such as diesel and gasoline, because the catalysts are extremely sensitive to poisoning by sulfur. Even a few ppmw sulfur may cause severe deactivation of these catalysts. 
     In addition to poisoning reforming catalysts, sulfur also has a detrimental effect on fuel cell performance and thus needs to be removed from the hydrogen or reformate. For the most-commonly used polymer electrolyte membrane (PEM) and solid oxide fuel cells (SOFC), the presence of less than one part per million by volume (ppmv) sulfur in the feed stream can result in an immediate drop in fuel-cell efficiency. Therefore, to protect reforming catalysts and fuel cells, sulfur must be removed from hydrocarbon fuels that are to be used as feeds for fuel-cell power generation systems. 
     Existing systems and apparatuses for converting hydrocarbon fuels to sulfur-free gas streams suitable for use in fuel cells are disclosed in U.S. Pat. No. 6,159,256; U.S. Pat No. 6,156,084; U.S. Pat. No. 6,210,821; U.S. Pat. No. 5,302,470; and U.S. Pat. No. 5,686,196. The first three of these patents disclose two complicated systems for reforming sulfur-containing hydrocarbon fuels. The first system is based upon stream reforming, and the second is based upon autothermal reforming. In both systems, a fuel desulfurizer is used to remove sulfur from raw fuels before reforming. The fuel desulfurization is based on the well-known mechanism of reactive adsorption of sulfur on transition metals, such a nickel (e.g., RSR′+Ni→NiS+R″H, where R, R′ and R″ are different hydrocarbon groups). Although the process is capable of treating low-sulfur fuels with very-high sulfur removal efficiencies (to &lt;1 ppmw), the process suffers from an inability to treat high-sulfur fuels, such as diesel and jet fuels, which typically contain hundreds of ppmw of sulfur, because of the formation of dense NiS shells on the outer surfaces of Ni particles; the NiS shells result in very-low sulfur uptake. As pointed out by Anumakoda, et al., in U.S. Pat. No. 6,221,280, a Ni:S weight ratio of at least 100:1 is needed for near complete removal of residual thiophenes from diesel or jet fuels. Thus, fuel desulfurization by this approach is costly and demanding in terms of metal weight and reactor volume. 
     The fuel processing systems disclosed in U.S. Pat. No. 5,302,470 and U.S. Pat. No. 5,686,196 employed another approach for desulfurization of raw fuels prior to stream reforming to convert desulfurized fuel into hydrogen-rich streams. The desulfurization process disclosed in these two patents is based on the traditional hydrodesulfurization (HDS) process widely used in the petroleum refining industry. In the HDS process, a raw fuel is mixed with gaseous hydrogen and fed into a reactor containing CoMo/Al 2 O 3  or NiMo/Al 2 O 3  catalyst at high temperature and pressure. Sulfur compounds in the fuel react with hydrogen (hydrogenation) over the catalysts to form hydrogen sulfide via the following reaction: RSR′+2H 2 →RH+R′H+H 2 S. The H 2 S is then removed with a regenerable metal or metal oxide absorbent, such as ZnO or CuO, via the following sulfiding reaction: H 2 S+MO→MS+H 2 O. The two reactions can be carried out in a single reactor by mixing the hydrogenation catalysts and the absorbent materials together. It is well known that in order to reach high efficiencies in desulfurizing heavy hydrocarbon fuels, such as diesel and jet fuels, the HDS process has to be operated at very-high pressure and with large amounts of excess hydrogen. While the process may be effective for use in larger, stationary hydrocarbon reforming systems, it is not attractive for smaller, mobile fuel reforming applications because of its dependence on an external hydrogen feed, large system size, complexity, and high operating pressure. 
     U.S. Pat. No. 6,296,814; U.S. Pat. No. 6,120,923; U.S. Pat. No. 5,931,658; U.S. Pat. No. 5,516,344; U.S. Pat. No. 6,254,839; U.S. Pat. No. 6,126,908; and U.S. Pat. No. 6,232,005 disclose a number of integrated apparatuses for reforming light hydrocarbon fuels. None of these patented designs contained a fuel desulfurizer, and thus these apparatuses are not employed to convert high-sulfur fuels into sulfur-free hydrogen or reformate for clean combustion or fuel cell power generation. 
     Consequently, there is a need for compact and efficient fuel processors for converting sulfur-containing hydrocarbon fuels into sulfur-free and hydrogen-rich gaseous streams. 
     SUMMARY 
     The following description relates to a fuel processing apparatus and method for hydrogen generation, and more particularly, to an integration of several components for processing hydrocarbon fuels having substantial sulfur contents into a hydrogen-rich fuel suitable for use as a feed for a fuel cell power generator. 
     The hydrocarbon fuel can be processed by passing the fuel through a desulfurization reactor. After desulfurization, the fuel can optionally be fed through a separator that separates a light fraction of the fuel from a heavy fraction, based on constituent boiling points. Where the separator is used, the light fraction is passed through a catalytic reactor that converts the light fraction into reformate containing hydrogen gas. Absent the use of a separator, the entire desulfurized fuel stream can be passed through the catalytic reactor. 
     An embodiment of a fuel-processing apparatus for hydrogen (H 2 ) generation is described, as follows. The fuel-processing apparatus includes a catalytic reactor, located in the center of the apparatus, in which fuel, air and water react to form reformate. Multiple tubular regenerable desulfurization reactors (desuflurizers) are located around the catalytic reactor. The catalytic reactor and the desulfurizers are enclosed in a chamber having thermal-insulation materials attached to its wall to prevent heat loss from the apparatus. The desulfurizers communicate thermally with the catalytic reactor via conduction, convection and radiation to permit transfer of heat between the desulfurizers and catalytic reactor. 
     There may be two or more tubular separators on the side of the chamber, and these separators are enclosed by separate cylindrical housings. A multi-port fuel-distribution valve is located on the top of the chamber. The valve has one fuel inlet, one air outlet and multiple fuel outlets and air inlets that are connected to each desulfurizer. By switching this fuel-distribution valve to different positions, at least one desulfurizer is operated in regeneration mode (in which sulfur species are emitted from the desulfurizer), and the rest of the desulfurizers are operated in desulfurization mode (in which sulfur species are absorbed in the desulfurizer). Two vessels beneath the reactor chamber are connected to the outlet of each desulfurizer. One vessel acts as a fuel collector to collect desulfurized fuels from the desulfurizers and to direct the fuel to the separators. The second vessel acts as air distributor to distribute air to desulfurizers that are in regeneration mode. On each of the tubes connecting the fuel collector and the desulfurizer outlets, there is a check valve that permits fuel to flow only from the desulfurizers to the fuel collector. On each of the tubes connecting the air distributors to the desulfurizer outlets, there is a check valve that permits air to flow only from the distributor to the desulfurizers. A burner is located beneath the catalytic reactor to provide heat for operation of the apparatus. 
     An exemplary method for operating the fuel-processing apparatus includes feeding a raw hydrocarbon fuel into the fuel inlet of the fuel distribution valve. The valve distributes the raw fuel evenly to desulfurizers that are in desulfurization mode. The desulfurizers contain metal- or metal-oxide-based absorbents and are operated at a pressure between about 790 kPa and 3.5 MPa and a temperature between about 300° C. and 600° C. In the desulfurizers, sulfur contained in the raw fuel is removed by reaction with the absorbent materials to form metal sulfides. Desulfurized fuel from the desulfurizers is collected in the fuel collector and flows to the separators; the separators separate the desulfurized fuel into a light fraction and a heavy fraction based upon boiling points. The light fraction of desulfurized fuel exits from the top of the separators and mixes with a controlled amount of preheated air and water. The light-fraction fuel/air/water mixture then enters into the catalytic reactor where the light-fraction mixture is converted over a catalyst to a reformate gas stream primarily comprising hydrogen, carbon monoxide, carbon dioxide, water, nitrogen and methane. The heavy fraction of the desulfurized fuel may flow to the burner in order to provide heat for operation of the apparatus, or the heavy fraction of the desulfurized fuel may return to the unprocessed diesel fuel tank in order to subsequently pass through the desulfurization apparatus an additional time. 
     Simultaneously, air is supplied to the air distributor, where the air is further directed to desulfurizers that are operating in regeneration mode. During regeneration, metal sulfides that are formed during the desulfurization mode react with oxygen contained in air to form sulfur oxides, thereby restoring the absorbent to tis active form. Sulfur oxides pass from the desulfurizers to the multi-port fuel-distribution valve. The sulfur oxide containing effluent exits from the fuel-processing apparatus at the air-outlet port on the multi-port valve. 
     At each state of the multi-port fuel distribution value, at least one desulfurizer is in regeneration mode, while the rest of the desulfurizer are in desulfurization mode. By switching the value to the next state, a regenerated desulfurizers is switched back to desulfurization mode, and another desulfurizer is switched to regeneration mode. By operating the apparatus in this manner, a continuous, stable and sulfur-free reformate is produced. 
     In another embodiment of the fuel processing apparatus and method for converting a high sulfur hydrocarbon fuel into a hydrogen-rich stream, the apparatus does not include separators that separate the desulfurized fuel into light fraction and a heavy fraction based upon their boiling points. In this embodiment, the entire desulfurized fuel stream is mixed with a controlled amount of preheated air and water and passed through the catalytic reactor where the fuel-containing stream is converted over a catalyst to a reformate gas stream primarily comprising nitrogen, hydrogen, carbon monoxide, carbon dioxide, water and methane. 
     In another embodiment of the fuel processing apparatus and method for converting a high-sulfur hydrocarbon fuel into a hydrogen-rich stream, water is not employed as a feed to the apparatus. In this embodiment, the desulfurized fuel that passes to the catalytic reactor is mixed only with preheated air. The fuel/air mixture is converted over a catalyst to a reformate gas stream primarily containing nitrogen, hydrogen, carbon monoxide, carbon dioxide, water and methane. 
     In another embodiment of the fuel processing apparatus and method for converting a high sulfur hydrocarbon fuel into a hydrogen-rich stream, the apparatus does not include a multi-port fuel distribution valve. In this embodiment, all of the desulfurizers are operated at the same time and all of the desulfurizers are regenerated at the same time. Desulfurizer operation and regeneration occur sequentially, with desulfurizer regeneration taking place when reformate production is not required. 
     The apparatus and methods described herein offer many advantages over existing approaches. The fuel processing apparatus and methods can operate on a variety of hydrocarbon fuels, such as gasoline, kerosene, jet fuel, diesel and heating oil, and convert them into sulfur-free reformate. Gaseous fuels that contain sulfur, such as natural gas, propane, LPG and naphtha can also be processed with the apparatus and methods. The use of regenerable desulfurizers can  reduce the volume of the fuel processor devoted to the task of desulfurization and can eliminate the need to periodically replace or otherwise maintain desulfurization system components. The regenerable desulfurizers can also mitigate problems associated with the vaporization of heavy distillate fuels because any carbonaceous residue that may form upon conversion of the liquid fuel into a gaseous state can be removed from the fuel-processing apparatus during regeneration. The multi-port valve of the apparatus allows saturated desulfurizers to be regenerated one at a time without disruption to the production of the sulfur-free reformate. The configuration of the catalytic reactor relative to the desulfurizers allows the heat generated by the chemical reaction within the catalytic reactor to be effectively utilized by the desulfurizers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, described below, like reference characters refer to the same or similar parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating particular principles of the methods and apparatus characterized in the Detailed Description. 
         FIG. 1  is a cross-sectional side view showing selected elements of one embodiment of the apparatus. 
         FIG. 2  is a more-detailed cross-sectional side view of the apparatus of  FIG. 1 . 
         FIG. 3  is a top view of the apparatus of  FIG. 2 . 
         FIG. 4  is a cross-sectional top view of the apparatus of  FIGS. 2 and 3 . 
     
    
    
     DETAILED DESCRIPTION  
     Now, referring to  FIGS. 1-4 , features and details of the fuel processing apparatus and method are described. The same numeral present in different figures represents the same item. Particular embodiments are detailed, below, for the purpose of illustration and not as limitations of the invention. 
     Selected components of the apparatus are illustrated and labeled in  FIG. 1 . This figure is a simplified view of the more-detailed view presented in  FIG. 2  and is provided for ease of illustrating broader aspects of the apparatus and method. The apparatus includes three principle components through which the hydrocarbon fuel is passed. Those components, through which the hydrocarbon fuel passes in sequence, are desulfurization reactors  107 , separators  116 , and a catalytic reactor  302 . 
     In the illustrated embodiment, the hydrocarbon fuel enters a distribution valve  105  through an inlet  103  at the top of the apparatus and is therein distributed to tubes  106 , which delivers the fuel to the desulfurizers  107 . After passing through the desulfurizers  107 , which remove sulfur from the fuel, the fuel is delivered via outlet tubes  109  to a fuel collector  112  at the bottom of the apparatus. From the fuel collector  112 , the fuel is sent through outlet  113  to a pressure regulator  114 , which distributes the fuel to tubes  115 . The fuel is directed around and up the outside of the apparatus through tubes  115  to separators  116 , which separate the fuel into light and heavy fractions. 
     The heavy fractions drop to the bottom of the separators  116  and are directed into an annular fuel tank  402 , which delivers the heavy-fraction fuel to a burner  406  via tube  403 . In the burner  406 , the heavy fraction is mixed with air from inlet  405  and burned to generate a hot combustion gas  407 , which heats the desulfurizers  107  and acts as a heat transfer medium between catalytic reactor  302  and desulfurizers  107 . 
     The light fractions rise through the separators  116 , are passed through the top of the separators  116 , mixed with air and water, which are introduced through inlets  203 , and passed to inlets  301 , which direct the light-fraction/air/water feed into the top of the catalytic reactor. The feed passes down through a catalyst bed  304  in the reactor  302 , which catalyzes a reaction of the fuel to produce reformate gas containing hydrogen gas. The reformate gas exits the bottom of the catalyst bed  304  and passes back up through a central tube  306  and out of the reactor  302 . The reformate gas can then be used in a fuel cell or can then be purified and used in a fuel cell. Exemplary reactions that may occur within the catalyst bed  304  are as follows:
         C n H m +n/2 O 2 →n CO+m/2 H 2 ;   C n H m +(m/4+n) O 2 →n CO 2 +m/2 H 2 O;   C n H m +n H 2 O→n CO+(m/2+n) H 2 ; and   C n H m +n CO 2 →2n CO+m/2 H 2 .       

     This embodiment of the apparatus is shown in greater detail in  FIGS. 2-4 . The fuel processor has a cylindrical shell  501  that has thermal insulation materials  502  attached to its surface to prevent heat loss. An annular plate  503  divides the space inside the shell  501  into an upper chamber  504  and a lower chamber  505 . In the upper chamber  504 , a cylindrical catalytic reactor  302  is located in the center and supported by the top plate of shell  501 . Two fuel/air/water inlets  301  are coupled with the upper section of reactor  302  to discharge a fuel/air/water mixture into the reactor  302 . On the bottom of the reactor  302 , there is a layer of insulation material  305 , on top of which a ceramic liner  303  is located. Liner  303  is loosely in contact with the inside wall of the reactor  302 . Liner  303  also holds the catalyst bed  304  in a way that there is a void space  307  beneath the catalyst bed  304 . A tube  306  is inserted from the top of the reactor  302  along the center line of the catalyst bed  304 , passes through the catalyst bed  304  and reaches space  307 . Over the tube  306 , reformed fuel  308  leaving the catalyst bed  304  is transported out of the reactor  302 . 
     Surrounding the catalytic reactor  302  is a series of desulfurization reactors (desulfurizers)  107 . In this particular embodiment, ten desulfurizers  107  are used. The desulfurizers  107  are evenly placed around the catalytic reactor  302  and supported by the top plate of shell  501  and the annular plate  503 . Desulfurizers  107  contain granular sulfur-absorption materials  108 , such as transition metal species supported on porous substrates, and are connected to a multi-port fuel distribution valve  105  through tubes  106 . The distribution valve  105  has one fuel inlet  103 , one outlet  104  and ten fuel outlet/air inlet tubes  106 . Beneath each desulfurizer  107 , there is a fuel outlet tube  109  that connects the desulfurizer  107  to a fuel collector  112  and to an air distributor  119 . 
     The fuel collector  112  is cylindrical in shape and is used to collect desulfurized fuel from all ten desulfurizers  107 . Fuel collector  112  has ten inlets  111  and one outlet  113 . The outlet  113  is connected to a pressure regulator  114 . 
     The desulfurizer outlets  109  are connected to the fuel collector inlet  111  via check valves  110  that allow the desulfurizer fuel to flow only in one direction from desulfurizers  107  to the fuel collector  112  when the pressure of the desulfurizer reaches a preset value. The air distributor  119  is a hollow ring and is used to direct air to the desulfurizer  107  that is being operated in regeneration mode. The air distributor has one inlet  118  and ten outlets  120 . The air distributor outlets  120  are connected to the desulfurizer outlets  109  via check valves  121 , which allow air to flow only in one direction from the air distributor  119  to the desulfurizers  107 . 
     By installation of the check valves  110 , the check valves  121  and the multi-port fuel-distribution valve  105 , nine of the ten desulfurizers  107  are operated in desulfurization mode, and one is operated in regeneration mode. For the desulfurizers  107  that are in desulfurization mode, raw fuel flows from the top of the desulfurizers  107  to the bottom, while for the desulfurizer  107  that is in regeneration mode, air flow upwards. In this particular embodiment, only one desulfurizer  107  is operated in regeneration mode at any given time although the apparatus can be easily configured so that multiple desulfurizers  107  can be operated in regeneration mode at the same time. 
     The upper chamber  504  has an inner vertical wall  506 . The void space between the inner wall  506  and the outer wall of partial oxidation reactor  302  forms an inner annular zone  408 , where desulfurization reactors  107  are located. In the inner annular space  408 , an inner set of fins  410  are attached to the outer walls of the partial oxidation reactor  302  and to desulfurizers  107 ; and an outer set of fins  409  are attached to desulfurizers  107  and to wall  506 . Fins  409  and  410  form a tortuous passageway for the combustion gas and increase the heat transfer efficiencies of the partial oxidation reactor  302  and desulfurizers  107 . A second annular zone  411  is formed between the wall  506  and the side wall of shell  501  in which coil  102  preheats raw fuel, coil  202  preheats air and coil  602  preheats water. There is a gap  507  along the upper edge of wall  506  that allows flue gas to leave space  408  and pass into space  411 . 
     Two cylindrical separators  116 , consisting of empty vessels or vessels filled with a porous packing, are symmetrically placed outside chamber  501 . Each of the two separators  116  has a cylindrical housing  413  enclosing it. The two separators  116  are connected to the pressure regulator  114  via tubes  115 . Each of the two separators  116  has two outlets  117  and  401 . Outlets  117  are located on the top of the separators and connected to the partial oxidation reactor inlets  301 . Outlets  401  are located on the bottom of the separators  116  and connected to an annular fuel storage tank  402 . Housings  413  have insulation layers  414  that prevent heat loss. Housings  413  are connected to the preheating zone  411  via four tubes  412  (see  FIGS. 3 and 4 ) that serve as the combustion gas inlets for the separator housings  413 . In the space between the separators  116  and their housings  413 , fins  415  form a tortuous passageway for the combustion gas and increase the heat transfer efficiencies of the separator  116 . Combustion gas exits from the separator housing  413  from outlets  417 . 
     In the lower chamber  505 , an annular fuel tank  402  is attached to the side wall of shell  501  and is used to store the heavy fraction of desulfurized fuel from the separator  116  for burner  406 . Burner  406  is located beneath the partial oxidation reactor  302  and is suspended from the annular dividing plate  503 . The fuel for the burner  406  is supplied from the fuel tank  402  via tube  403  and preheating element  404 . Air for the burner is provided from air inlet  405 . 
     During operation, a raw hydrocarbon fuel is fed into the apparatus at inlet  101  (see  FIGS. 3 and 4 ). The fuel is preheated in preheating coil  102  (see  FIGS. 2 and 4 ) and flows upwards into the fuel distribution valve  105  via inlet  103 . Valve  105  evenly distributes the preheated raw fuel into nine of the ten desulfurizers  107 . In the desulfurizer  107 , sulfur contained in the raw fuel is chemically absorbed onto the absorbent material  108 . After exiting from the desulfurizers  107 , desulfurized fuel flows into the fuel collector  112  via outlet  109  and check valve  110 . From collector  112 , the desulfurized fuel enters into separator  116  after being depressurized through pressure regulator  114 . 
     In the separator  116 , desulfurized fuel is separated into a light fraction and heavy fraction. The heavy fraction of the desulfurized fuel leaves the separator  116  via tubes  401  and enters into fuel tank  402  as fuel for burner  406 . The light fraction of the desulfurized fuel leaves the separator via tubes  117  and is mixed with preheated air from preheating coil  202  and preheated water from preheating coil  602  through inlet  203 . Air is supplied to coil  202  from inlet  201 ; water is supplied to coil  602  from inlet  601  (see  FIGS. 3 and 4 ). 
     After mixing in tube  301 , the fuel/air/water mixture enters into the catalytic reactor  302 . While passing through the catalyst bed  304  in the catalytic reactor  302 , the fuel/air/water mixture is reformed into a reformate stream comprising mainly N 2 , H 2 , CO, CO 2 , H 2 O and CH 4 . Exemplary catalyst beds  304  that may be used include noble metal or transition metal species supported on a monolithic or foamed ceramic substrate. The reformate gas  308  leaves the catalytic reactor  302  via tube  306  as the final product. 
     Simultaneously, air for desulfurizer  107  regeneration is fed into the air distributor  119  from inlet  118 . Through check valve  121 , the air flows upwards into one of the desulfurizers  107  that is operated in regeneration mode. During regeneration, sulfur is released from the absorbent material  108  as sulfur dioxide, which is carried out from the desulfurizer  107  through the outlet  104  on the fuel distribution valve  105 . 
     Fuel for burner  406  is supplied from fuel storage tank  402  via tube  403 . After being heated in coil  404 , the fuel mixes with the combustion air that is supplied from inlet  405 . The fuel/air mixture is combusted in burner  406  generating a hot combustion gas  407 . The hot flue gas first flows in a radial direction to space  408  containing desulfurizer  107 . In space  408 , the flue gas moves upwards in a tortuous pathway to heat desulfurizers  107 . Through gap  507  on wall  506 , the flue gas makes a U-turn and enters into space  411  in which air, water and fuel are preheated. After leaving space  411 , the flue gas enters into the two separator housings  414  via four tubes  412 . In housing  414 , the fuel gas moves upwards, also in a tortuous pattern, to heat separator  116 . The flue gas finally leaves the separator housings  414  from outlets  417 . 
     In describing embodiments of the invention, specific terminology is used for the sake of clarity. For purposes of description, each specific term is intended to at least include all technical and functional equivalents that operate in a similar manner to accomplish a similar purpose. Additionally, in some instances where a particular embodiment of the invention includes a plurality of system elements or method steps, those elements or steps may be replaced with a single element or step; likewise, a single element or step may be replaced with a plurality of elements or steps that serve the same purpose. Moreover, while this invention has been shown and described with references to particular embodiments thereof, those skilled in the art will understand that various other changes in form and details may be made therein without departing from the scope of the invention.