Mineral oils: processes and products – Refining – Sulfur removal
Reexamination Certificate
2001-01-11
2003-10-21
Parsa, J. (Department: 1621)
Mineral oils: processes and products
Refining
Sulfur removal
C208S031000, C208S20800M, C208S25400R, C208S244000, C208S245000, C208S25100H, C208S25400R, C585S820000, C585S821000
Reexamination Certificate
active
06635171
ABSTRACT:
The present invention relates to a process for upgrading nitrogen-containing Fischer-Tropsch products.
BACKGROUND OF THE INVENTION
Fischer-Tropsch products can be upgraded before being sold as products. The usual processes that are used are hydrocracking to make distillate fuels such as diesel and jet fuel, naphtha, and feeds for lube processing. These products can also be upgraded by wax isomerization to make lube base oils. Finally, the light naphtha can be reformed to make aromatics for use in gasoline or petrochemicals.
Several catalysts used in these upgrading processes require low levels of heteroatoms for efficient operation. The key heteroatoms that must be controlled to low levels are nitrogen and oxygen. Nitrogen is the most serious catalyst poison, while oxygen is a lesser concern. The presence of nitrogen-containing impurities in a feedstock causes the reactions to be performed at higher than desired hydroprocessing reactor temperatures, with a serious reduction in the yield of valuable products.
Products from Fischer-Tropsch synthesis are well known to contain oxygen impurities and have little or no sulfur. It was unknown that Fischer-Tropsch products can contain nitrogen. If oxygen is the only impurity to be considered, the upgrading can be quite mild, as described in EP 583 836 B1 and EP 668 342 A1. These patents speak of “mild hydrogenation, under conditions such that substantially no isomerization or hydrocracking of hydrocarbons occurs” (lines 50-55 in 583 836 B1). These mild conditions typically remove sulfur and oxygen impurities, but not nitrogen impurities. Likewise, U.S. Pat. No. 4,943,672 describes severe hydrotreating to improve the processing of Fischer-Tropsch wax, but states that Fischer-Tropsch wax contains essentially no nitrogen or sulfur.
SUMMARY OF THE INVENTION
We have discovered that some Fischer-Tropsch waxes and products can contain nitrogen, and it is advantageous to reduce the nitrogen content of these products below a threshold value. When Fischer-Tropsch products contain nitrogen, the mild hydrotreating conditions used in the prior art can be insufficient to reduce the nitrogen to an effective low level. For best (or at least, satisfactory) operation in Fischer-Tropsch wax and condensate upgrading processes, the nitrogen content must be below 15 ppm, preferably below 5 ppm, and most preferably below 1 ppm.
The present invention provides a process for treating nitrogen-containing, substantially paraffinic products derived from a Fischer-Tropsch process. That process comprises three steps: a purification step, a monitoring step, and an adjustment step. In the purification step, the substantially paraffinic product,is purified in a purification process to remove oxygen, nitrogen, and other impurities. Then, in the monitoring step, the nitrogen content (and other impurities) of the reduced product is monitored. Finally, in the adjustment step, the conditions of the purification step are adjusted to increase nitrogen removal if the nitrogen content of the reduced product exceeds a preselected value. If the nitrogen content of the reduced product does not exceed the preselected value, no adjustment is necessary.
The nitrogen reduction can be achieved by a number of methods, including hydrotreating, adsorption and extraction.
DETAILED DESCRIPTION OF THE INVENTION
In its broadest aspect, the present invention involves the discovery that some Fischer-Tropsch waxes and other products can contain nitrogen, and it is advantageous to reduce the nitrogen content of these products below a preselected threshold value.
As used herein, the following terms have the following meanings unless expressly stated to the contrary:
The term “substantially paraffinic product” refers to a product comprising at least 50% paraffins.
The term “nitrogen-containing, substantially paraffinic product” refers to a product comprising at least 50% paraffins and at least 1 ppm nitrogen.
The term “nitrogen-containing, substantially paraffinic product of a Fischer-Tropsch process” refers to a product of a Fischer-Tropsch process, wherein that product comprises at least 50% paraffins and at least 1 ppm nitrogen.
Unless otherwise specified, all percentages are in weight percent and all parts per million (ppm) are by weight.
As defined above, the “nitrogen-containing, substantially paraffinic product of a Fischer-Tropsch process” refers to a product produced by a Fischer-Tropsch process comprising at least 50% paraffins and at least 1 ppm nitrogen.
Our invention is based on the surprising discovery that some Fischer-Tropsch products have too much nitrogen for use in some catalystic processes. It is not known exactly why some Fischer-Tropsch products have too much nitrogen and other Fischer-Tropsch products do not. It may be related to a combination of catalyst and reactor system.
In Fischer-Tropsch chemistry, syngas is converted to liquid hydrocarbons by contact with a Fischer-Tropsch catalyst under reactive conditions. Fischer-Tropsch synthesis may be effected in a fixed bed, in a slurry bed, or in a fluidized bed reactor. The Fischer-Tropsch reaction conditions may include using a reaction temperature of between 190 C. and 340 C., with the actual reaction temperature being largely determined by the reactor configuration. Thus, when a fluidized bed reactor is used, the reaction temperature is preferably between 300 C. and 340 C.; when a fixed bed reactor is used, the reaction temperature is preferably between 200 C. and 250 C.; and when a slurry bed reactor is used, the reaction temperature is preferably between 190 C. and 270 C.
An inlet synthesis gas pressure to the Fischer-Tropsch reactor of between 1 and 50 bar, preferably between 15 and 50 bar, may be used. The synthesis gas may have a H
2
:CO molar ratio, in the fresh feed, of 1.5:1 to 2.5:1, preferably 1.8:1 to 2.2:1. The synthesis gas typically includes 0.1 wppm of sulfur or less. A gas recycle may optionally be employed to the reaction stage, and the ratio of the gas recycle rate to the fresh synthesis gas feed rate, on a molar basis, may then be between 1:1 and 3:1, preferably between 1.5:1 and 2.5:1. A space velocity, in m
3
(kg catalyst)
−1
hour
−1
, of from 1 to 20, preferably from 8 to 12, may be used in the reaction stage.
In principle, an iron-based, a cobalt-based or an iron/cobalt-based Fischer-Tropsch catalyst can be used in the Fischer-Tropsch reaction stage. The iron-based Fischer-Tropsch catalyst may include iron and/or iron oxides which have been precipitated or fused. However, iron and/or iron oxides which have been sintered, cemented, or impregnated onto a suitable support can also be used. The iron should be reduced to metallic Fe before the Fischer-Tropsch synthesis. The iron-based catalyst may contain various levels of promoters, the role of which may be to alter one or more of the activity, the stability, and the selectivity of the final catalyst.
Preferred promoters are those influencing the surface area of the reduced iron (“structural promoters”), and these include oxides or metals of Mn, Ti, Mg, Cr, Ca, Si, Al, or Cu or combinations thereof.
The nitrogen-containing, substantially paraffinic product of a Fischer-Tropsch process is purified in a purification zone (e.g., hydrotreated in a hydrotreating zone) to remove nitrogen, oxygen and other impurities to form a treated waxy heavy fraction. Such hydrotreating zones are well known in the industry. Other treatments useful for removing nitrogen, oxygen and other impurities include, but are not limited to, adsorption (e.g., with an acid clay) and extraction.
Hydrogenation catalysts can be used for the purification. For example, a noble metal from Group VIIIA according to the 1975, rules of the International Union of Pure and Applied Chemistry, such as platinum or palladium on an alumina or siliceous matrix, or unsulfided Group VIIIA and Group VIB, such as nickel-molybdenum or nickel-tin on an alumina or siliceous matrix, is a suitable catalyst. U.S. Pat. No. 3,852,207 to Stangeland et al. (“Production of Stable Lubricating Oils By Seque
O'Rear Dennis J.
Rosenbaum John M.
Simmons Christopher A.
Burns Doane Swecker & Mathis L.L.P.
Chevron U.S.A. Inc.
Parsa J.
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