Mineral oils: processes and products – Refining – Sulfur removal
Reexamination Certificate
2001-04-03
2002-11-05
Norton, Nadine (Department: 1764)
Mineral oils: processes and products
Refining
Sulfur removal
C208S217000, C208S212000, C208S209000, C208S21600R
Reexamination Certificate
active
06475376
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a process for reducing the sulfur content in a fuel, such as gasoline, to a very low level in order to make it suitable for use in a fuel cell.
BACKGROUND OF THE INVENTION
Fuel cells offer certain advantages over conventional internal combustion engines in certain applications. Fuel cells are usually more efficient and emit less pollutants into the environment when compared to heat engines. Accordingly, fuel cell systems are being proposed for both stationary and mobile applications which have traditionally been occupied by internal combustion engines. Fuel cells with which the present invention is concerned are referred to as proton exchange membrane (PEM) fuel cells. PEM fuel cells require hydrogen as a fuel and a source of oxygen for their operation. During operation the hydrogen is contacted with a catalyst, usually a platinum based catalyst, where it is ionized and gives up an electron. The resulting electrical charge provides a source of electricity which may be used to power an electric motor.
Demonstration PEM fuel cell systems typically have used pure hydrogen as a fuel. However, hydrogen has a number of significant disadvantages as a practical fuel for commercial applications. Therefore, other fuels such as natural gas and methanol which are easily converted to hydrogen at the site of the fuel cell have been proposed, but these fuels also have serious drawbacks. For example, methanol is expensive as a fuel, lacks an extensive distribution network, and presents groundwater pollution problems. Natural gas while useful for stationary applications is less practical for wide spread use as a transportation fuel due to its storage and handling problems. Gasoline has been suggested as a suitable alternative fuel for use in fuel cells and has the advantages over other fuels of being relatively inexpensive, of already being widely available through a commercial distribution network, and of lacking the storage problems associated with gases such as hydrogen and natural gas. However, a principal disadvantage of gasoline as a fuel for use in fuel cells is its sulfur content. The catalysts used to convert the hydrogen and oxygen in the fuel cell to electricity are very sensitive to even very low levels of sulfur and are rapidly deactivated at the sulfur levels normally present in conventional gasoline which typically falls within the range of from about 10 to 50 ppm. Gasoline intended for fuel cells would require a significantly lower sulfur content than 10 ppm, usually 5ppm or less, preferably less than 1 ppm and even more preferably below 0.5 ppm.
Gasoline is generally prepared from a number of blend streams. Typical examples include butanes, light straight run, isomerate, FCC cracked products, hydrocracked naphtha, coker gasoline, alkylate, reformate, added ethers, etc. Of these, gasoline blend stocks from the FCC, the reformer and the alkylation unit account for a major portion of the gasoline pool. FCC gasoline, and if present, coker naphtha and pyrolysis gasoline, generally contribute a substantial portion of the pool sulfur.
Sulfur present in the gasoline pool may be in one of several molecular forms, including thiophenes, mercaptans and disulfides. Typical thiophenes include thiophene (<(CH:CH)
2
>S) and its alkylated derivatives, and benzothiophene (alternatively thianaphthene). Typical mercaptans occurring in the sulfur-containing gasoline streams include thiophenol (C
6
H
5
SH), and the alkylthiols from ethanethiol to nonanethiol, with potentially smaller amounts of the higher alkylthiols.
A number of methods have been proposed for removing sulfur from gasoline. In general, hydrotreating is the method of choice, on account of the cost and ease of processing using the catalytic method. However, sulfur removal by hydrotreating has several disadvantages for preparing fuels suitable for use in a fuel cell. Hydrotreating requires relatively severe operating conditions which make it impractical in most instances to include the hydrotreating unit as part of a fuel processor in association with the fuel cell itself. In addition, hydrotreating converts olefins present in the gasoline which leads to octane loss. A special advantage of the process of the present invention is that the process is capable of reducing the sulfur content of the gasoline to a level suitable for use in fuel cells while minimizing octane loss. Therefore, fuels prepared according to the present invention may serve either conventional internal combustion engines or fuel cells.
According to U.S. Pat. No. 3,957,625, the sulfur impurities tend to concentrate in the heavy fraction of the gasoline and a method for removing the sulfur includes hydrodesulfurization of the heavy fraction of the catalytically cracked gasoline so as to retain the octane contribution from the olefins which are found mainly in the lighter fraction. U.S. Pat. No. 5,290,427 teaches fractionating a sulfur containing gasoline feed, and introducing each fraction in turn into a hydrodesulfurization reactor at spaced locations along the length of the reactor according to boiling point. By this method, low boiling, olefin containing fractions are treated for a relatively shorter time, and higher boiling fractions, with lesser amounts of olefins and higher amounts of sulfur containing molecules, are treated for a relatively longer time. U.S. Pat. No. 5,290,427 further teaches contacting the intermediate product from the hydrodesulfurization reaction zone with an acidic catalyst, reportedly to produce a product having a higher octane number than that of the intermediate product.
In U.S. Pat. No. 4,049,542, Gibson et al. discloses a process in which a copper catalyst is used to desulfurize an olefinic hydrocarbon feed such as catalytically cracked light naphtha. This catalyst is stated to promote desulfurization while retaining the olefins and their contribution to product octane.
In U.S. Pat. 5,059,304 a desulfurization process is described in which a naphtha feed is subjected to a mild reforming step followed by treatment with sulfur sorbent to remove the hydrogen sulfide. The reforming step is conducted at a temperature above 575° F., and preferably above 660° F.
The process of the present invention is particularly advantageous because, due to the very mild process conditions employed, it may be used in direct association with the fuel cell, that is, the desulfurization process may be carried out as part of an integrated fuel cell system. This is of major importance since the presence of sulfur at the levels with which the present invention is concerned may readily result from contamination during handling, such as during tankage or passage through contaminated fuel lines. Therefore, fuels which as manufactured may meet the stringent sulfur standard required for use in fuel cells, may easily become contaminated during delivery to the site of the fuel cell. With the present invention it is possible to desulfurize the fuel at the site of the fuel cell, if stationary, or in the case of a vehicle by use of an on-board fuel processor. Thus the desulfurized fuel may be fed directly into the fuel cell with minimal opportunity for contamination.
SUMMARY OF THE INVENTION
The present invention is directed to a process for desulfurizing a hydrocarbonaceous fuel intended for use in a fuel cell which comprises (a) contacting a hydrocarbonaceous fuel containing sulfur with hydrogen in the presence of a hydrotreating catalyst at a temperature of between 300° F. and 500° F., a total pressure between 50 psig and 500 psig, and a feed rate between 0.5 hr
−1
and 10 hr
−1
for a time sufficient to convert up to 95% of the thiophenes present in the fuel, to produce a hydrotreated hydrocarbonaceous fuel; (b) contacting the hydrotreated hydrocarbonaceous fuel with a solid adsorbent or a liquid extractant selected to rereduce the sulfur compounds remaining in the hydrotreated hydrocarbonaceous to about 5.0 ppm or less; (c) recovering a hydrocarbonaceous fuel having a reduced sulfur content
Dieckmann Gunther H.
Jossens Lawrence W.
Munson Curtis L.
Ambrosius James W.
Chevron U.S.A. Inc.
Norton Nadine
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