ON-BOARD DESULFURIZATION SYSTEM

FIELD OF THE INVENTION

[0001] The present invention relates to a fuel desulfurization system that can be located on-board a transportation vehicle. The desulfurization system contains a unique sorption vessel having a vacuum shell design to ensure a prescribed axial and radial temperature profile under operating conditions.

BACKGROUND OF THE INVENTION

[0002] Growing environmental demands and the need for higher efficiency vehicles, dictated to a great extent by concerns relating to global warming, have triggered interest in fuel cells as power platforms for transportation vehicles. This has prompted an extensive search for technologies for delivering hydrogen to fuel cells. Despite much research and development on multiple fronts, commercial deployment of a pure hydrogen-based transportation sector has not been realized. One reason for this is that the production, transportation and retail distribution of hydrogen requires infusion of very large amounts of capital. Further, present on-board hydrogen storage technology does not allow fuel cell vehicles to meet a 300 mile driving range requirement. One of the most promising options for delivering hydrogen to a fuel cell is to provide on-board reforming of liquid hydrocarbon fuels coupled with hydrogen separation. As the prospect of vehicles employing fuel cells moves closer to being a reality, petroleum companies must consider how fuel specifications will change. For example, "octane" as used for gasoline, is a useless performance index for a fuel cell power platform and, in fact, typical octane enhancers will complicate matters. [0003] A host of chemical processes (reforming and water-gas shift reactions), physical processes (e.g., adsorption and membrane separation processes) and electrochemical processes (e.g., the Proton Exchange Membrane (PEM) Fuel Cell) are being pursued to convert liquid fuels to hydrogen. All of these processes are susceptible to sulfur and would be greatly improved if the level of sulfur could be reduced to less than about 1 wppm. Current refinery processes cannot provide sub-ppm sulfur fuels and the existing finished products distribution network would not be able to maintain sub-ppm sulfur concentration levels because of cross-contamination with higher sulfur-containing products. Desulfurization at retail sites is not feasible because of lack of expertise in running chemical processes, oversight of hazardous solids, requirement of substantial capital, and the like. A system that can produce sub-ppm sulfur fuel on-board a transportation vehicle would be advantageous over conventional systems.

[0004] Although several fuels have been suggested as possible candidates as fuels for fuelling fuel cell vehicles, naphtha-based fuels are the leading candidates. This is primarily because they have a relatively low degree of volatility, a high hydrogen content, are readily available, and are relatively low in cost. This option also provides a relatively safe fuel that is compatible with existing service stations, supply, distribution, and storage infrastructure, as well as being the most economical system of fueling. Further, low-volatility liquid hydrocarbon fuels can be stored in existing vehicular and service station tanks, pumped with existing equipment, and transported through existing pipelines and by truck, and marine and rail tankers.

[0005] Most petroleum and biomass derived fuels contain sulfur in excess of the level tolerable by fuel cell systems without significant loss in performance. This is particularly true of on-board fuel cell systems used as auxiliary power units. Most fuel cells give the best performance using pure hydrogen. Thus, sulfur-containing petroleum and biomass derived fuels must be desulfurized to extremely low sulfur levels. The desulfurization of petroleum based fuels is conventionally carried out using large-scale process units at a refinery. The most widely used desulfurization unit is one that treats a sulfur-containing petroleum derived feedstream with a desulfurization catalyst in the presence of hydrogen at elevated temperatures and pressures. Such processes, which are referred to as hydrodesulfurization processes, have been commercially used for many years and have met with great commercial success for meeting governmental sulfur regulations for transportation fuels in conventional combustion engine vehicles. Although conventional refinery desulfurization process units can produce fuels with low sulfur levels, the levels are still too high for the fuels needed to provide hydrogen to fuel cell power platforms for transportation vehicles.

[0006] Therefore, a need exists in the art for technology that is capable of providing a substantially sulfur-free fuel for use with a fuel cell.

SUMMARY OF THE INVENTION

[0007] In accordance with the present invention, there is provided an onboard desulfurization system for removing sulfur moieties from transportation fuels, which system comprises: a) a heat exchanger having a first passageway and a second passageway

contiguous to each other but not in fluid communication with each other,

wherein each of said passageways has an inlet and an outlet and wherein each

passageway is constructed to allow a fluid to pass from its inlet to its outlet and to allow heat to be transferred from a fluid of one passageway to a fluid in the other passageway; b) a first fuel pump having an inlet and an outlet wherein the outlet of said first fuel pump is in fluid communication with the inlet of said first passageway of said heat exchanger; c) a sulfur trap vessel having and inlet and an outlet and containing a bed of sorbent material having the capacity to sorb sulfur compounds from a transportation fuel stream and wherein said inlet of said sulfur trap vessel is in fluid communication with the outlet of said first passageway of said heat exchanger and said outlet of said sulfur trap vessel is in fluid communication with the inlet of said second passageway of said heat exchanger; d) an enclosed tank having an inlet and an outlet wherein said inlet is in fluid communication with the outlet of said second passageway of said heat exchanger; and e) a second fuel pump having an inlet and an outlet wherein said inlet is in fluid communication with the outlet of said enclosed tank.

[00081 In a preferred embodiment, the heat exchanger is a shell and tube type heat exchanger wherein said first passageway is a bundle of tubes sealingly passing through the shell of said heat exchanger.

[0009] In another preferred embodiment, the inlet of said first fuel pump is in fluid communication with a source of transportation fuel. [0010] In yet another preferred embodiment of the present invention the

sulfur trap vessel is insulated.

[0011] In another preferred embodiment, the sulfur trap vessel is insulated by

use of a surrounding containment vessel that is hermetically sealed to the outside

environment and is positioned about the sulfur trap vessel so as to provide a

substantially hollow gap between the two vessels.

[0012] In still another preferred embodiment, the outlet of said second fuel

pump is in fluid communication with a reforming unit capable of producing

hydrogen from the transportation fuel.

BRIEF DESCRIPTION OF THE FIGURES

[00131 Figure 1 hereof is a schematic representation of a preferred on-board desulfurization system of the present invention.

[0014] Figure 2 hereof is a representation of one preferred embodiment of a sulfur trap vessel of the present invention showing an internal baffle system.

[0015] Figure 3 hereof is side view of a preferred reactor of the present invention showing an internal baffle system.

[0016] Figure 4 hereof is a top view of the reactor of Figure 3 cut along plane A-A' and showing a preferred internal baffle arrangement. DETAILED DESCRIPTION QF THE INVENTION

[0017] Fuel processing technologies that can produce hydrogen from conventional liquid hydrocarbon fuels (such as diesel and gasoline) for on-board mobile platforms (vehicles, forklifts, etc.) offer advantages in reaching the goal of enabling fuel cell vehicles to meet a 300 mile driving range requirement. Such a platform, which has a higher efficiency than current internal combustion engines, will improve the tank-to- wheels efficiency. The production, transportation and distribution of hydrogen is presently significantly more energy intensive than systems based on conventional liquid fuels, such as gasoline. Thus, on-board propulsion platforms will also lead to higher tank-to- wheel efficiency. Further, on-board hydrogen generation can reduce the capital requirement by leveraging the existing liquid fuel distribution network for use with fuel cell power platforms.

[0018] As previously mentioned, various chemical processes (reforming and water-gas shift reactions), physical processes (e.g., adsorption and membrane separation processes) and electrochemical processes (e.g., the Proton Exchange Membrane (PEM) Fuel Cell) can be used to produce hydrogen from liquid transportation fuels. All these processes are susceptible to fuel sulfur and would be more effective with fuels having less than about lwppm sulfur.

[0019] The requirement of tailpipe emission control systems to comply with future regulations is spurring the development of catalytic systems for the control of such pollutants as nitrogen oxides, particulate matter, and chlorofluorocarbons etc. Sulfur in liquid fuel hampers performance of such catalytic systems because it leads to detrimental interaction with oxygen to form SO _x . Practice of the present invention can be an enabling technology for these systems because it ensures the delivery of sub-ppm sulfur level fuel, thereby minimizing SO _x in the exhaust. [0020] As previously mentioned, the present invention relates to an on-board desulfurization system for transportation fuels. The system is comprised of a high temperature sorption unit (sulfur trap) that desulfurizes the fuel without the use of hydrogen. The high temperature on-board desulfurizing system comprises a sulfur trap vessel that is a fixed bed reactor of a novel geometry; a bed of suitable sorbent material having a finite sulfur capacity; two fuel pumps; a heat exchanger; and a buffer tank. It is preferred that the fixed bed reactor be insulated, more preferably of a vacuum shell design that will ensure relatively low heat loss and require minimal heat input. The vacuum shell will also serve as a containment vessel in case of accidental rupture or other damage to the sorbent vessel. The terms "sorbent vessel", "reactor", and "sulfur trap vessel" are sometimes used interchangeably herein. The sorbent vessel will also preferably have an internal electrical heater that is used to provide heat to the bed of sorbent material. It is within the scope of this invention that heat can also be provided from a reducing gas, such as hot exhaust gas, either directly or via a heat pipe. The function of the buffer, or holding, tank is to provide substantially sulfur-free fuel without being limited by start-up time that the desulfurization system would need to produce on-spec substantially sulfur-free fuel. By on-spec substantially sulfur- free fuel we mean that the fuel will have less than 1 wppm, preferably less than about 0.75 wppm, more preferably less than about 0.5 wppm, and most preferably substantially zero sulfur.

[0021] This invention can be better understood with reference to the figures hereof. Figure 1 shows a preferred on-board desulfurization system comprised of a first fuel pump FPl, a heat exchanger HX capable of preheating fuel being conducted to sulfur trap vessel STV and cooling the desulfurized product stream exiting sulfur trap vessel STV. Sulfur trap vessel STV is preferably insulated. The preferred method of insulation is to have the sulfur trap vessel in a containment vessel (vacuum shell) CV. It is preferred that there by an effective void, or hollow, GAP, between the two vessels. The containment vessel CV will be hermetically sealed and the GAP will contain a vacuum to insulate sulfur trap vessel STV from the outside environment. Containment vessel CV also serves to protect against sorbent particles and fuel from being released to the environment in the event of a rupture. Both sulfur trap vessel STV and containment vessel CV are constructed of a suitable material that is capable of withstanding the physical and chemical environments that will be encountered during use. Non-limiting examples of such suitable materials include stainless steels and composites comprised of a polymer composited with a material selected from the group consisting of carbon particles, carbon fibers, carbon nanofibers, ceramic particles, and ceramic fibers. There is also provided a buffer tank BT and second fuel pump FP2. When the system is used on-board a transportation vehicle, first fuel pump FPl conducts fuel from a fuel storage tank (not shown), through heat exchanger HX where it is preheated by a hot desulfurized product fuel stream via line 14.

[0022] Any type of heat exchanger can be used in the practice of this invention as long as it is capable of indirectly transferring heat from the hot desulfurized product stream from sulfur trap vessel STV to the fuel being passed to the sulfur trap vessel STV to be processed. Non-limiting examples of heat exchangers that can be used in the practice of the present invention include shell and tube heat exchangers, plate heat exchangers, and plate and fin heat eachangers. Shell and tube heat exchangers are typically comprised of a series of tubes. One set of these tubes contains the fluid that must be either heated or cooled. The second fluid in introduced into the shell surrounding the tubes where it runs over the tubes and exchanges heat with the fluid in the tubes. In the embodiment shown in Figure 1 hereof, the fuel being conducted to the sulfur trap vessel STV is passed through the tubes and the hot product stream exiting the sulfur trap vessel is introduced into the shell. A set of tubes is typically called a tube bundle and can be made up of several types of tubes: plain, longitudinally finned, etc. A typical plate heat exchanger is a stacked-plate arrangement usually comprised of multiple, thin, slightly-separated plates that have very large surface areas and fluid flow passages for heat transfer. This stacked-plate arrangement can be more effective, in a given space, than the shell and tube heat exchanger. Plate and Fin heat exchangers are typically comprised of "sandwiched" passages containing fins to increase the effectiveness of the heat exchanger. Designs include cross-flow and counter-flow coupled with various fin configurations, such as straight fins, offset fins, and wavy fins. The heat exchanger used in the practice of the present invention will preferably be a "shell and tube" type of heat exchanger.

[0023] The preheated fuel stream is conducted from heat exchanger HX to sulfur trap vessel STV where it comes into contact with a bed of suitable sorbent material SB capable of sorbing sulfur moieties from the fuel stream. The orientation of sulfur trap vessel STV is not critical as long as the fuel contacts an effective amount of sorbent material for a given residence time, or flow rate, to provide the predetermined level of sulfur. The fuel may enter at the top of the bed or the bottom of the bed if the vessel is placed vertically. The vessel may also be operated horizontally. For simplicity, Figure 1 hereof shows a vertical up-flow configuration.

[0024] Any suitable sorbent can be used in the practice of the present invention. Preferred sorbents are those disclosed in co-pending United States Patent Applications 2005028077 and 20080099375, both of which are incorporated herein by reference.

[0025] Preferred sorbent materials include alumina-silica/nickel composite materials comprised of nickel particles distributed in a phase containing silica, alumina and an effective amount of promoter additives (copper and/or silver, lithium, or zinc, and mixtures thereof) that display high sulfur capacity for removing sulfur from hydrocarbon streams at relatively low temperatures, in the absence of additional liquids or gas additives. The alumina-silica/nickel composite is preferably obtained by the reduction of a composite material comprised of nickel oxide (NiO), and (CuO, Ag ₂ O, Li ₂ O, or ZnO). The composite is preferably formed by homogeneous deposition-precipitation of a nickel salt onto a mesoporous silica materials grafted with an effective amount of alumina species. An effective amount of copper, silver, lithium, or zinc or a mixture thereof as promoting additives are included by deposition onto the NiO- alumina-silica composite.

[0026] Also incorporated herein by reference is co-pending United States Patent Application No. 11/977,898 filed October 26, 2007. More preferred sorbent materials are transition metal phosphide materials having the formula MP _x , where M is selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Nb, Mo, Ta and W, x is between 0.1 and 10, and the material(s) are dispersed on a high surface area oxide support. Such materials are capable of converting the organo-sulfur compounds in the fuel to H ₂ S. Select materials belonging to this group adsorb substantial amounts of organic sulfur from hydrocarbon streams without added hydrogen. The more preferred sorbent is a nickel phosphide complex Ni _x P, with x = 2-3, stabilized in the form of 2-50 nm nanocrystals of Ni ₂ P, Ni] ₂ P ₅ , Ni ₃ P phases, or their mixtures, in mesoporous supports matrices. Sulfur adsorption with such a material is sufficient for ultra-deep desulfurization to the level of about 1 wppm residual sulfur or less of hydrotreated hydrocarbon fuels containing about 20 wppm sulfur, with sulfur capacity more than about 0.5 g, preferably more than about 0.75 g, and most preferably more than about 1 g per 100 g of sorbent. The present invention includes a process for ultra-deep desulfurization of hydrotreated hydrocarbon liquid feedstocks, especially of diesel fuels. [0027] In another more preferred embodiment of the present invention, the sorbent has high loading of disperse Ni _x P complexes, ranging from about 15 wt % to about 80 wt %, preferably 20 wt % to about 60 wt %. The dispersed Ni _x P complexes have crystal sizes ranging from about 2 nanometers to about 50 nanometers (preferably 2-30 nm), and are deposited on silica, mesostructured silica, silica-alumina, carbon or a combination thereof with surface area ranging from about 200 m ² /gm to about 800 m ² /gm, and pore diameter ranging from about 5 nanometers to about 30 nanometers. The material is prepared by reduction of nickel phosphate or nickel oxide (hydroxide) deposited on the mesoporous supports together with ammonium phosphate salt.

[0028] Liquid phase desulfurization is a process that attains sub-ppm sulfur concentration in gasoline and diesel fuels through the reaction between sulfur in the fuel with (supported) metals and metal phosphides. Such a technology is suited for on-board systems as it does not need hydrogen for the desulfurization and because of its relatively simple operation that involves contacting fuel, in the liquid state, with a solid sorbent material at moderate temperatures and pressures in a fixed bed mode. Two classes of sorbents have been tailored for gasoline and diesel desulfurization and are disclosed in U.S. Patent Application No. 11/977,898 which is incorporated herein by reference.

[0029] Formulations of nickel phosphide on silica have demonstrated a 3 wt.% sulfur uptake capacity while reducing the total sulfur content of full range diesel fuel containing from about 11 wppm sulfur to about 200 wppb at a temperature range from about 200° to about 375 ⁰ C at 1- 15/hr liquid hourly space velocity (LHSV). A Ni-based sorbent, a composite containing about 25-75 wt.% Ni with a metallic promoter additive dispersed on the silica-alumina support, has demonstrated a 3 wt.% sulfur capacity (measured at an exit sulfur concentration of <1 wppm) on a full range 22 wppm sulfur CA-2 gasoline fuel (California reformualed gasoline without add-packs and MTBE) containing 3 wt.% ethanol at 150°-3O0°C, 250 psi and LHSV of about 1-12/hr.

[0030] Returning to Figure 1 hereof, a heating element HE is preferably provided within the bed of sorbent material SB, which bed is preferably a fixed bed. Heating element HE can be any electrical heating element that is capable of heating the sorbent bed to a temperature of about 25O ⁰ C to about 35O ⁰ C. The heating element is preferably an electrical band heater, but it is within the scope of this invention that heat can be provided via a reducing gas, such as hot exhaust gases, either directly or via a heat pipe (not shown). Sulfur compounds are sorbed by the sorbent material and a stream of substantially sulfur-free fuel will exit sulfur trap vessel STV at an elevated temperature and pass through the heat exchanger HX wherein it will transfer heat to the fuel stream passing through the heat exchanger to the sulfur trap vessel. The cooled substantially sulfur free fuel stream is conducted from heat exchanger HX to a buffer tank BT, which can also be referred to as a holding tank. Buffer tank BT can provide substantially sulfur-free fuel without being limited by start-up time that the sulfur trap would need to produce on-spec substantially sulfur-free fuel. Second fuel pump FP2 conducts the on-spec substantially sulfur-free fuel to an engine or reformer, or other end use device that can use the product fuel.

[0031] Figure 2 hereof is another representation of a sulfur trap ST of the present invention, but showing an internal baffle system IB within sulfur trap vessel STV. Although a baffle system is optional, it is never-the-less preferred that the sulfur trap vessels of this invention contain a suitable baffle system. The use of a baffle system helps to ensure a substantially homogeneous axial and radial temperature profile of the sorbent bed. It will be understood that the number and arrangement of baffles is not critical as long as a predetermined flow of feed can flow through the sulfur trap vessel STV. Thus, any suitable connecting grid of baffles that runs the length of the vessel can be used in the practice of this invention. There is provided an inlet port for receiving fuel to be treated and an outlet port for conducting substantially sulfur-free fuel from sulfur trap vessel STV. It will be noted that inlet port IP in this Figure 2 is shown at a first end of sulfur trap vessel STV and outlet port OP at a second end of the sulfur trap vessel STV. It is within the scope of this invention that both the inlet port and the outlet port, particularly the inlet port can be at other locations on the vessel as long as the fuel enters and comes into contact with an effective volume of sorbent bed SB to remove enough sulfur to bring the sulfur level to about 1 wppm or less, preferably less than about 0.75 wppm, more preferably less than about 0.5 wppm, and most preferably substantially zero. This figure shows the fuel to be treated entering at the bottom of the vessel. It allows the fuel to enter the bed at the bottom and a collection mechanism is such that flow from the top can exit anywhere where the outlet port is located. There is also provided a gap, or hollow space, between the containment vessel CV and the sulfur trap vessel STV that is effectively hermetically sealed to allow vacuum to exist between the two vessels. It is preferred that sulfur trap vessel STV be held in place within the containment vessel CV by a support rings SR that are preferably porous enough to allow fluid communication throughout the gap.

[0032] Figure 3 and 4 are representations of a preferred internal baffle system for the sulfur trap vessel.

[0033] The embodiments set forth herein are presented in order to best explain the present invention and its practical application and to thereby enable those skilled in the art to make and use the invention. However, those skilled in the art will recognize that the foregoing description has been presented for the purposes of illustration of the best mode of the instant invention. Thus, the description as set forth is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teaching without departing from the spirit and scope of the following claims.