WO2002002208A1

WO2002002208A1 - Regenerable gas desulfurizer

Info

Publication number: WO2002002208A1
Application number: PCT/US2001/020648
Authority: WO
Inventors: Mark A. Perna; Andrew F. Kisik; Robert M. Privette
Original assignee: Mcdermott Technology, Inc.
Priority date: 2000-06-29
Filing date: 2001-06-28
Publication date: 2002-01-10
Also published as: AU2001273058A1

Abstract

A continuous desulfurization system (10) for removing sulfur fromfuel cell fuels comprises at least a pair of beds (12, 14) conta ining a regeneratable sorbent and switches (28, 30, 32, 36, 42, 44, 50, 56, 58) for sequentially connecting the beds to either thefuel (18) or regenerating air (20) (46) whenever the sorbent is depleted in one and requires regeneration.

Description

REGENERABLE GAS DESULFURIZER

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention generally relates to systems which remove hydrogen sulfide from fuel cell fuels and more particularly to such systems where the sulfide removing elements are sequentially regenerated on a continuous basis to provide continuous hydrogen sulfide removal by the system.

Description of the Prior Art

The preferable fuels for many military and nonrnilitary fuel cell markets contain sulfur compounds. For example, natural gas has sulfur-containing odorants (mercaptans, disulfides, or commercial odorants). Sulfur is a poison to fuel cells and also to the low temperature, water-gas shift catalyst used with proton exchange membrane (PEM) fuel cell fuel-processing systems.

When fuel cell fuels such as synthesis gas are generated from hydrocarbonaceous fuels, the sulfur present in the fuel is converted to mostly hydrogen sulfide. The removal of hydrogen sulfide from the gas stream using zinc oxide sorbents at temperatures between 600°F and 800°F is well known.

Suppliers of zinc oxide for use in chemical processing plants include catalyst vendors such as Haldor Topsoe and United Catalysts, Inc. However, systems used to achieve such low levels of hydrogen sulfide are very large and heavy since they require a large inventory of zinc oxide. When the bed used to remove the hydrogen sulfide in the desulfurizing system is saturated, the zinc oxide pellets must be replaced. This causes an expensive and time consuming shutdown of the desulfurizing system in order to permit the zinc oxide pellet replacement therein. A regenerable sorbent was developed by the U.S. Department of Energy for removal of hydrogen sulfide from gas generated by coal gasification. This regenerable sorbent is described in US Patents 5,494,880; 5,866,503; and 5,703,003 all of which are incorporated herein by reference. These sorbent pellets contain metal oxides for removing hydrogen sulfide from a gas stream at a temperature of 800°F to 1,200°F and comprise a mixture of the following components, in weight percent; zinc oxide - 40% to 60%, calcium sulfate - 15% to.25%, calcium oxide - 5% to 10%, silica- less than 5%, nickel oxide - 5% to 15%, Bentonite - 5% to 15%. The mixture is moistened with water, compressed into pellets, dried, and calcined.

Thus the use of these regenerable sorbent pellets in the prior art desulfurization systems can provide a system where the desulfurization pellets would not have to be replaced. Nevertheless, the pellets would still have to be regenerated, thereby still requiring the system to shut down to permit regeneration.

Thus it is seen that what the prior art lacked was a compact desulfurizer system capable of removing sulfur on a continuous basis with no need to shut down the system for recharging or pellet removal.

SUMMARY OF THE INVENTION

The present invention solves the problems associated with prior art desulfurization systems as well as other problems by providing an improved desulfurization system comprising an apparatus and method for continuously removing hydrogen sulfide from synthesis gas generated from liquid hydrocarbon fuels by using on-line regeneration of the sorbent in the system. The system of the present invention removes substantial quantities of H₂S from hot synthesis gas (syngas) to provide a continuous stream of nearly sulfur-free gas to downstream equipment. The desulfurizer consists of two (or more) fixed beds of sulfur sorbent material installed in parallel housings which are used alternately. The sorbent may be a zinc oxide, a suitable derivative thereof, and or any sorbent capable of regeneration to provide good sulfur adsorption capacity and also able to withstand multiple high temperature (approximately 1100°F) regeneration cycles. Notably, this sorbent may also be the regenerable sorbent described earlier in the prior art section of this disclosure as the sorbent developed by the Department of Energy. In operation, one bed of the system removes sulfur from the syngas stream while the other bed regenerates. The sulfided bed is "regenerated" in-situ by admitting a flow of oxygen-depleted air to the inlet side of the bed to bum off or oxidize the accumulated sulfur. After a predetermined period of time, high-temperature inlet valves redirect the gas flow from the sulfided bed to the clean bed. The cycle time can be adjusted based on the sulfur sorbent and regeneration gas oxygen content and flow rate. The valve timing may preferably be controlled by a programmable logic controller (PLC). Heating elements or other means may also be used to help maintain temperatures necessary for complete combustion of the sulfur. The flue gas produced by burning the sulfur is vented to the atmosphere.

In view of the foregoing it will be seen that one aspect of the present invention is to provide a continuously operating desulfurization system for fuel cell fuels containing sulfur. Another aspect is to provide a desulfurization system having on-line regeneration of the sorbent in the system.

These and other aspects of the present invention will be more fully understood after careful review of the following description of the preferred embodiment when considered with the accompanying drawings.

BRIEF DESCRIPπON OF THE DRAWINGS

In the drawings:

Fig. 1 is a perspective view of the desulfurization system of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing, a continuous regeneration desulfurizing system (10) is shown having the unique features of a multiple bed (12, 14) arrangement with an associated bed regeneration capability, which provides the ability to deliver a continuous stream of desulfurized gas from an outlet (16). When compared to a nonregenerable prior art desulfurizer, the present system substantially reduces the size of the desulfurization equipment, requires less maintenance, and is capable of continuous operation.

Significantly, the system (10) is not restricted to having any particular sulfur sorbent in the beds (12, 14) but rather will operate with any known sorbent with certain inherent sulfur adsorption and regeneration characteristics. At least two prior art sorbents are known to have characteristics that allow this system (10) to work. Both have been described in the present disclosure in the "Prior Art" section, and are incorporated herein by reference for the purpose of exemplifying some of the types of sorbents contemplated by the present invention. During the operation of the system (10), one bed (12) removes sulfur from a syngas stream supplied by line (18) while the other bed (14) regenerates. The sulfided bed is "regenerated" in-situ by admitting a flow of oxygen-depleted air to the inlet side of the bed along line (20) to bum off or oxidize the accumulated sulfur therein. While bed (14) is being regenerated, fuel cell fuel gas is supplied to the active bed (12) by open valve (44) along line (24). The desulfurized fuel is exhausted from the bed (12) along outlet line (26) through open valve (28) to that outlet (16).

While the first bed (12) is in the desulfurizing mode (described above), the supply of fuel from line (18) is shut-off to the bed (14) by closed valve (30) and only the regenerative gas from line 20 through open valve (32) is allowed to flow through regenerating bed (14) and out therefrom along line (34) to open valve (36) to be exhausted along line (38).

After a predetermined period of time, the open valves (O) in the drawing are closed and the closed valves (C) in the drawing are opened to redirect the regenerative gas flow from the sulfided bed (14) to the clean bed (12). The cycle time can be adjusted based on the sulfur sorbent and regeneration gas oxygen content and flow rate. The valve timing is controlled by a known programmable logic controller (PLC). Heating elements or other means are used to help maintain temperature necessary for complete combustion of the sulfur. The flue gas produced by bivning the sulfur can be vented to the atmosphere. In this switched operation, the fuel from line (18) passes through now open valve (30) to the bed (14) along line (40) and is exhausted therefrom through now open valve (42) to the exhaust line (16) for desulfurized fuel. The now closed valves (36, 44, 32, 28) prevent flow to any of the other branches. Bed (12) is now desulfurized by passing regenerative air thereto along line (46) and out therefrom along line (48) through now open valve (50). The open valves (O) on the drawing are now closed to prevent flow through other branches.

Controlling the rate of combustion in the system (10) is a key element of the system. The combustion rate must be slow enough that the sorbent is not overheated and damaged by high temperature. Sorbent developers DOE/FETC and Phillips Petroleum have recommended minimal regeneration temperatures of approximately 1050°F with regeneration gas containing approximately 2% O₂. Oxygen-depleted regeneration air may be introduced across the sulfided bed during the entire regeneration period, so that the regeneration time matches the sulfidation time. The oxygen-depleted airflow rate is quite low relative to the syngas flow, and is set to just match the quantity of air supplied in the regeneration period to the quantity of sulfur to be removed by oxidation. There is no penalty for supplying excess air if the flue gas is vented.

Certain sorbents may require a subsequent reductive regeneration where the bed being regenerated is exposed to a hydrogen atmosphere before being put back on-line for desulfurization. In such a case, the system (10) uses lines (52, 54) and valves (56, 58) in a known manner to allow flowing the raw syngas to the bed undergoing reductive regeneration followed by the desulfurizing bed providing to ensure capture of any sulfur released during reductive regeneration. This series arrangement of the beds would be maintained until the reductive regeneration is complete.

The system described above reduces the sulfur content of the hydrogen-rich gas stream down to about 10 ppm. A non-regenerable, polishing sulfur sorbent bed, using conventional zinc oxide sorbent, can be added to the system (10) in a known manner to further reduce the sulfur content down to less than 1 ppm. Certain modification and additions have been deleted herein for the sake of conciseness and readability but are fully intended to fall within the scope of the following claims.

Claims

CLAIMSWe claim:

1. A fuel desulfurization system comprising: 5 a source of fuel containing sulfur; a source of regenerative air for regenerating said desulfurizing pellets; a first and second bed containing regenerable sorbent for removing sulfur from the fuel; a series of valves selectively connecting said first and second bed to said 10 source of fuel and to said source of regenerative air; and a controller for switchably connecting only said fuel to one of said first and second beds and said regenerative air to the other of said first and second beds.

2. A system as set forth in claim 1 wherein said controller is a programmable logic 15 controller connected to said series of valves to selectively open and close said valves according to a programmed sequence and time.

3. A system as set forth in claim 2 wherein said sorbent in said first and second bed is a zinc oxide derivative able to withstand multiple high temperature regeneration cycles.

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4. A system as set forth in claim 3 wherein said beds are heated to approximately 1100°F during regeneration of the sorbent therein.

5. A system as set forth in Claim 4 wherein said regeneration gas comprises 25 approximately 2% oxygen.

6. A system as set forth in Claim 5 wherein said fuel is synthesis gas.

7. A method of continuously desulfurizing a fuel containing sulfur comprising the steps 30 of: providing a first and second bed containing a regeneratable sorbent for desulfurizing the fuel; providing a source of regenerating air for regenerating a depleted sorbent; and alternately connecting the first and second bed only to either the regenerating air or to the fuel.

8. A method as set forth in claim 7 wherein the first bed is connected to the regenerative air when the sorbent therein is spent and the second bed is connected to the fuel whenever the first bed is connected to the regenerative air.

9. A method as set forth in claim 8 including the step of heating the sorbent in said first or second bed to approximately 1100°F whenever the bed is connected to the regenerative air.

10. A method as set forth in claim 9 including the step of supplying at least 2% oxygen to the regenerative air.

11. A method as set forth in claim 7 including the step of exposing the first or second bed to a hydrogen atmosphere subsequent to supplying the bed with regenerative air.

12. A method as set forth in claim 7 including the steps of providing a third bed containing a non-regeneratable polishing sulfur sorbent and passing exhaust from the first or second bed to the third bed.