Chemistry: fischer-tropsch processes; or purification or recover – Plural zones each having a fischer-tropsch reaction
Reexamination Certificate
1999-12-21
2001-05-01
Richter, Johann (Department: 1621)
Chemistry: fischer-tropsch processes; or purification or recover
Plural zones each having a fischer-tropsch reaction
C518S702000, C518S703000, C518S704000, C518S706000, C208S028000, C208S049000, C585S001000, C585S643000, C585S700000, C585S708000
Reexamination Certificate
active
06225359
ABSTRACT:
The present invention relates to a process for conversion of natural gas and associated light hydrocarbons into salable hydrocarbon products, using molecular redistribution to reduce the amount of gas processed through the most expensive reforming and synthesis sections of the facility and to increase the yield of most valuable products.
BACKGROUND OF THE INVENTION
In a typical process for conversion of natural gas and associated light hydrocarbons into salable hydrocarbon products, methane, a gaseous oxidant, and C
2
to C
6
hydrocarbons from a gas well are fed to a reforming zone to produce a first product stream comprising carbon oxides (such as carbon monoxide and carbon dioxide), hydrogen, and some water. That first product stream is feed into a hydrocarbon synthesis zone with a hydrocarbon synthesis catalyst at elevated temperature and pressure to produce a second product stream having a wide range of products: from light hydrocarbons to heavy wax. The second product stream is separated in a distillation column into a light hydrocarbon fraction, at least one fraction of salable hydrocarbon products (such as a naphtha stream and a distillate fuels stream), and a waxy fraction. The waxy fraction is hydrotreated to remove heteroatoms and is hydrocracked, then the hydrotreated, hydrocracked waxy fraction is recycled to the distillation column. The light hydrocarbon fraction is recycled to the reforming zone.
A problem with such a process is that it requires that large volumes of light hydrocarbon fraction be fed to and recycled to the reforming zone, which means that both the reforming zone and the hydrocarbon synthesis zone have to be large to accommodate such large volumes of recycle gas. Therefore, it would be advantageous to develop processes that do not require such large volumes of recycle gas. In addition, processing C
3+
hydrocarbons in the reforming zone can cause coking and metal dusting problems which make operation difficult.
Another problem with such a process is that a mixture of products is made which typically include LPG, naphtha, distillate fuel, and hydrocarbons in the lube base oil range. The hydrocarbons in the lube base oil range and distillate fuel are the most valuable products, but with the existing technology it is not practical to increase the yield of the most valuable products beyond a certain limit. In particular the export of the least valuable LPG product requires the use of pressurized containers for storage and shipment, which represent a significant expense, operational difficulties, and safety hazards. The naphtha from such a process typically has a very low octane value and makes a relatively poor feedstock to a reformer, and is only of moderate value when used as a feed to an ethylene naphtha cracker. While it is desirable to reduce or eliminate the production of LPG and naphtha, with the current process it is not possible to do so. Similarly, the diesel and hydrocarbons in the lube base oil range fractions are the most valuable, but with the current technology the yield of these most valuable fractions cannot be increased beyond a given value.
The disproportionation of saturated hydrocarbons has been described in the patent literature in U.S. Pat. Nos. 3,484,499; 3,668,268; 3,856,876; 3,864,417; and 3,953,537. In the general literature see Hughes, T. R., et. al., Proc. Int. Congr. Catal., 5th (Paper 87) 1972 and Burnett R. L., et. al., Jour. of Cat. 31, pp 55-64, 1973. In the petroleum industry, disproportionation has been proposed for the conversion of refinery gases (see, for example, U.S. Pat. No. 3,773,845) and for the reforming of distillate transportation fuels (see, for example, U.S. Pat. No. 4,676,885).
SUMMARY OF THE INVENTION
The present invention provides processes for conversion of natural gas and associated light hydrocarbons into salable hydrocarbon products, wherein disproportionation of saturated hydrocarbons is used to reduce the amount of light gas recycle needed, and reduce the reactor size needed for the reforming zone and the hydrocarbon synthesis zone.
By “salable hydrocarbon products” we mean any hydrocarbon products having an identifiable market, including but not limited to naphtha, diesel, jet, distillate fuel, hydrocarbons in the lube base oil range, waxes, paraffinic solvents, feedstocks for making detergents, and alcohols.
In one embodiment, the present invention is an improvement on the process for the preparation of salable hydrocarbon products comprising:
(a) reacting a light hydrocarbon gaseous stream with a gaseous oxidant (preferably water, carbon dioxide, oxygen, air, or oxygen-enriched air, or mixtures thereof) in at least one reforming zone containing a reforming catalyst under conditions selected to produce a first product stream comprising carbon oxides (e.g., carbon monoxide and carbon dioxide) and hydrogen;
(b) contacting the first product stream in a hydrocarbon synthesis zone with a hydrocarbon synthesis catalyst under conditions selected to produce a second product stream comprising salable hydrocarbon products; and
(c) separating the second product stream into at least a gaseous fraction and at least one fraction of salable hydrocarbon products.
In that improvement, the light hydrocarbon gaseous stream comprised of predominately C
1
and C
2
alkanes is formed by separating a light hydrocarbon gaseous feed comprising alkanes into a lighter fraction (preferably comprised of predominately C
1
and C
2
alkanes) and a heavier fraction (preferably comprised of predominately C
3+
alkanes). At least a portion of this heavier fraction is contacted in a disproportionation zone with a disproportionation catalyst under conditions selected to convert a significant portion of the alkanes in the heavier fraction by disproportionation into both higher alkanes and lower alkanes. At least part of the lighter fraction and at least part of the lower alkanes from the disproportionation zone form the light hydrocarbon gaseous stream. The higher alkanes from the disproportionation zone constitute salable hydrocarbon products.
If the hydrocarbon synthesis zone is a methanol synthesis zone, then methanol is produced. If the hydrocarbon synthesis zone is a Fischer Tropsh zone then the salable hydrocarbon products are likely to be a naphtha fraction and a distillate fuels fraction.
Preferably, if the hydrocarbon synthesis zone is a Fischer Tropsh zone, then the second product stream is separated into at least a gaseous fraction, at least one fraction of salable hydrocarbon products (e.g., a naphtha fraction, a distillate fuels fraction, hydrocarbons in the lube base oil range), and a waxy products fraction. The waxy products fraction being hydrocracked and recycled to the second product stream.
In a second embodiment, the present invention is an improvement on the process for the preparation of salable hydrocarbon products comprising:
(a) reacting a light hydrocarbon gaseous stream (predominately C
1
and C
2
alkanes) with a gaseous oxidant (preferably water, carbon dioxide, oxygen, air, or oxygen-enriched air, or mixtures thereof) in at least one reforming zone containing a reforming catalyst under conditions selected to produce a first product stream comprising oxides (e.g., carbon monoxide and carbon dioxide) and hydrogen;
(b) contacting the first product stream in a hydrocarbon synthesis zone with a hydrocarbon synthesis catalyst under conditions selected to produce a second product comprising salable hydrocarbon products; and
(c) separating the second product stream into at least a gaseous fraction of predominately C
2
-alkanes, a gaseous fraction of predominately C
3
-C
6
alkanes, at least one fraction of salable hydrocarbon products (preferably a naphtha fraction and a distillate fuels fraction), and a waxy fraction
In that improvement, the light hydrocarbon gaseous stream is formed by separating a light hydrocarbon gaseous feed comprising alkanes into a lighter fraction and a heavier fraction, then at least part of the lighter fraction and at least part of the gaseous fraction form the light
Kibby Charles L.
Krug Russell R.
O'Rear Dennis John
Burns Doane , Swecker, Mathis LLP
Chevron U.S.A. Inc.
Parsa J.
Richter Johann
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