Chemistry of hydrocarbon compounds – Plural serial diverse syntheses – To produce unsaturate
Reexamination Certificate
2000-06-23
2003-05-20
Dang, Thuan (Department: 1764)
Chemistry of hydrocarbon compounds
Plural serial diverse syntheses
To produce unsaturate
C585S708000, C585S253000, C585S275000, C585S331000, C585S656000, C585S664000, C585S734000, C208S057000, C208S049000
Reexamination Certificate
active
06566569
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the production of hexanes and butanes from a predominantly C
5-
paraffinic feedstock.
BACKGROUND OF THE INVENTION
Regulations for gasoline are changing and MTBE may be phased out and replaced by more volatile ethanol. If this happens, other volatile components in gasoline, for example pentanes, will have to be removed in order to keep the volatility of the blended gasoline constant. It would be advantageous to provide a commercially attractive use for the displaced pentanes.
Pentane itself has a relatively low octane, and its use as a refinery fuel is therefore of relatively low value. Pentanes can be used as a feedstock to flexi-crackers to produce ethylene and propylene. However, pentanes tend to give excessive yields of heavier, lower-valued products. Yet another potential use is to dehydrogenate the pentanes to form pentenes, and oligomerize the pentanes to form decenes, which can be hydrogenated to form jet fuel. However, this process is relatively expensive.
It would be desirable to provide a process for producing useful products from pentanes that do not require using a Flexi-cracker, or require using capital intensive equipment. The present invention provides such a process.
SUMMARY OF THE INVENTION
In its broadest aspect, the present invention is directed to an integrated process for producing butanes and hexanes from a feedstock that includes pentanes. The process involves obtaining an appropriate pentane-containing feedstock and subjecting the pentanes to conditions of alkane dehydrogenation to olefins, olefin metathesis, and olefin hydrogenation to alkanes, which collectively are referred to as molecular redistribution. The molecular redistribution reaction provides a product stream that includes C
2-4
alkanes, predominantly butane, and C
6
+ alkanes, predominantly hexane, in addition to un-converted pentanes.
The product stream can be distilled to provide a first fraction rich in butanes, a second fraction rich in pentanes and a third fraction containing predominantly C
6
+ alkanes.
The majority of the ethane and propane in the C
2-4
fraction can be removed using deethanization and/or depropanization columns, using technology well known to those of skill in the art. N-butane can be isomerized to form isobutane and then used to form a high-octane alkylate. Alternatively, the butane stream can be subjected to a separate molecular redistribution reaction to form ethane and propane, along with additional C
5
+ products. N-butane can also be dehydrogenated to form butenes, which can be alkylated with iso-butane to form high octane gasoline, or used to form other compounds, such as butanol and butadiene.
The C
6
+ fraction tends to be highly paraffinic and have low ppb sulfur and can be used, for example, as a solvent or as a feedstock for reforming reactions, for example using the AROMAX™ process or conventional platforming or rheniforming processes, to produce aromatic compounds. Alternatively, the C
6
+ fraction can be isomerized to improve the octane value and used in gasoline compositions.
When n-pentane is used as a feed, the products tend to be highly linear. An isomerization catalyst can be added to the catalyst bed, which produces intermediate isoalkanes during the molecular redistribution reaction. This is advantageous, since the reaction of n-butane with isopentane produces a mixture of isomeric products, including n-hexane, 2-methylpentane, and 3-methylpentane. The n-pentane can also be isomerized in a separate upstream unit using known dual function catalysts which include a hydrogenation/dehydrogenation component (typically Pd or Pt catalysts) and an acidic component (typically a zeolite catalyst).
If an isomerization catalyst is not added to the catalyst bed, the products of the molecular redistribution reaction can optionally be subjected to catalytic isomerization using the dual function catalysts described above. Useful isomers of hexane include 2-methylpentane, with an octane value of about 73 and 3-methyl pentane, with an octane value of about 74, where normal hexane has an octane value of only about 25.
Depending on the nature of the molecular redistribution chemistry, it may be desirable that the feedstock not include appreciable amounts (i.e., amounts that would adversely affect the catalyst used for molecular redistribution) of hydrogen, alkenes, alkynes, thiols, amines, water, air, oxygenates or cycloparaffins such as cyclohexane and methylcyclopentane which might convert to aromatics in the presence of the catalysts.
Methods for removing sulfur and nitrogen compounds are well known, and generally involve hydrotreating the feedstock. Methods for removing cyclic compounds are also known in the art and generally involve adsorption and separation by molecular sieves. Methods for hydogenating alkenes and alkynes are also well known in the art.
DETAILED DESCRIPTION OF THE INVENTION
In its broadest aspect, the present invention is directed to an integrated process for producing C
2-4
and C
6
+ paraffinic product streams from a feedstock that includes pentanes. The process involves obtaining an appropriate pentane feedstock, dehydrogenating the pentanes to form pentenes, methathesizing the resulting pentenes, and rehydrogenating the resulting metathesized olefins to form alkanes. The three steps preferably occur in the same reactor. At least a portion of the initial pentanes is converted to C
2-4
and C
6
+ alkanes. Unconverted pentanes can be recycled if desired and converted to additional C
2-4
and C
6
+ alkanes.
The process described herein is an integrated process. As used herein the term “integrated process” refers to a process which involves a sequence of steps, some of which may be parallel to other steps in the process, but which are interrelated or somehow dependent upon either earlier or later steps in the total process.
An advantage of the present process is the effectiveness and relatively inexpensive processing costs with which the present process may be used to prepare high quality components for incorporation into gasoline compositions. In particular, an advantage is that feedstocks that are not conventionally recognized as suitable sources for such product streams can be used.
I. PREPARATION OF FEEDSTOCKS FOR THE MOLECULAR REDISTRIBUTION REACTION
Feedstocks for the Molecular Redistribution Reaction
The feedstocks for the molecular redistribution reaction include predominantly pentanes. Primarily, the feedstocks are derived from crude oil and/or natural gas. Any feedstock that includes predominantly pentanes and which does not include an appreciable amount of hydrogen, water, air, olefins, alkynes, cycloparaffins, and heteroatom-containing compounds can be used. Pentane-containing feedstocks can be derived from natural gas, cracked gas feed streams, LPG and refinery waste gas by removing the bulk of the above-listed compounds and C
1-4
alkanes from the feedstock.
Hydrotreating Chemistry
As noted above, it may be preferable that the feedstocks not include appreciable amounts of heteroatoms or saturated cyclic compounds. If any heteroatoms or saturated cyclic compounds are present in the feedstock, they are preferably removed before the molecular redistribution reaction. Alternatively, if the molecular redistribution occurs at conditions which do not dehydrogenate the cycloparaffins, they can be tolerated.
Certain saturated and partially saturated cyclic hydrocarbons (cycloalkanes which can form aromatics in the presence of the catalysts, such as cyclohexane and methylcyclopentane, aromatic-cycloalkanes, and alkyl derivatives of these species) can form hydrogen during the molecular redistribution reaction. The hydrogen produced from the formation of aromatics can inhibit the reaction, and should therefore be substantially excluded from the feed. The term “substantially excluded,” as used herein, means less than about 5 wt. % of the feed, and more preferably less than about 2 wt. % of the feed.
The desired paraffins can be separated from the satu
Brundage Scott R.
Chen Cong-Yan
O'Rear Dennis J.
Chevron U.S.A. Inc.
Dang Thuan
Roth Steven H.
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