Mineral oils: processes and products – Chemical conversion of hydrocarbons – Plural serial stages of chemical conversion
Reexamination Certificate
1997-05-08
2001-02-20
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
Plural serial stages of chemical conversion
C208S061000, C208S089000, C585S251000, C585S648000, C585S649000, C585S650000
Reexamination Certificate
active
06190533
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for upgrading hydrocarbon feedstocks for subsequent use in steam cracking. In particular, this invention describes a process for upgrading hydrocarbon feedstocks for use in steam cracking by the application of hydrotreating and concomitant partial hydrogenation of the unsaturated and/or aromatic species found therein, and the resultant yield increase of hydrogen, C
1
-C
4
hydrocarbons, steam cracked naphtha and steam cracked gas oil, and the concomitant decrease in the yield of steam cracked gas tar, upon steam cracking of the hydrotreated hydrocarbon feedstocks.
BACKGROUND OF THE INVENTION
Steam cracking is a process widely known in the petrochemical art. The primary intent of the process is the production of C
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-C
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hydrocarbons, particularly ethylene, propylene, and butadiene, by thermal cracking of hydrocarbon feedstocks in the presence of steam at elevated temperatures. The steam cracking process in general has been well described in the publication entitled “Manufacturing Ethylene” by S. B. Zdonik et. al, Oil and Gas Journal Reprints 1966-1970. Typical liquid feedstocks for conventional steam crackers are straight run (virgin) and hydrotreated straight run (virgin) feedstocks ranging from light naphthas to vacuum gas oils. Gaseous feedstocks such as ethane, propane and butane are also commonly processed in the steam cracker.
The selection of a feedstock for processing in the steam cracker is a function of several criteria including: (i) availability of the feedstock, (ii) cost of the feedstock and (iii) the yield slate derived by steam cracking of that feedstock. Feedstock availability and cost are predominantly a function of global supply and demand issues. On the other hand, the yield slate derived by steam cracking of a given feedstock is a function of the chemical characteristics of that feedstock. In general, the yield of high value C
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hydrocarbons, particularly ethylene, propylene and butadiene, is greatest when the steam cracker feedstocks are gaseous feedstocks such as ethane, propane and butane. The yield of high value steam cracked naphtha and low value steam cracked gas oil (SCGO) and particularly low value steam cracked tar (SCT) upon steam cracking of a straight run (virgin) or hydrotreated straight run (virgin) feedstocks increases as the boiling range of the feedstock increases. Thus, the steam cracking of liquid feedstocks such as naphthas, gas oils and vacuum gas oils generally results in a greater proportion of particularly low value steam cracked products, i.e., steam cracked tar. In addition, steam cracking facilities where naphthas and gas oils are processed require additional capital infrastructure in order to process the large volume of liquid co-products resulting from steam cracking of those feedstocks.
What is more, the yield of the least desirable products of steam cracking, steam cracked tar, is generally even higher when low quality hydrogen deficient cracked feedstocks such as thermally cracked naphtha, thermally cracked gas oil, catalytically cracked naphtha, catalytically cracked gas oil, coker naphthas and coker gas oil are processed. The significantly increased yield of the low value steam cracked tar product relative to production of high value C
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hydrocarbon products obtained when processing the low quality hydrogen deficient cracked feedstocks is such that these feedstocks are rarely processed in steam crackers.
Catalytic hydrodesulfurization (sulfur removal), hydrodenitrification (nitrogen removal) and hydrogenation (olefins, diolefins and aromatics saturation) are well known in the petroleum refining art. Hydrodesulfurization, hydrodenitrification and partial hydrogenation have been applied to upgrading feedstocks for steam cracking as described by Zimmermann in U.S. Pat. No. 4,619,757. This two stage approach employed base metal, bi-metallic catalysts on both non-acidic (alumina) and acidic (zeolite) supports.
Minderhoud et. al., U.S. Pat. No. 4,960,505, described an approach for upgrading of kerosene and fuel oil feedstocks by first pre-treating the feedstock to effect hydrodesulfurization and hydrodenitrification to yield a liquid product with sulfur and nitrogen contaminants at levels of less than 1,000 and 50 ppm wt., respectively. Thereafter, the low impurity hydrocarbon stream was subjected to hydrogenation to yield a high cetane number fuel oil product.
Winquist et. al., U.S. Pat. No. 5,391,291, described an approach for upgrading of kerosene, fuel oil, and vacuum gas oil feedstocks by first pre-treating the feedstock to effect hydrodesulfurization and hydrodenitrification, and thereafter hydrogenation of the resultant liquid hydrocarbon fraction to yield a high cetane number fuel oil product.
It has been found that the present invention which comprises hydrotreating followed by steam cracking results in significant yield improvements for hydrogen, C
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hydrocarbons and steam cracked naphtha when applied to straight run (virgin) feedstocks; and results in high yields of hydrogen, C
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hydrocarbons and steam cracked naphtha and reduced yields of steam cracked tar when applied to low quality, hydrogen deficient, cracked feedstocks such as thermally cracked naphtha, thermally cracked kerosene, thermally cracked gas oil, catalytically cracked naphtha, catalytically cracked kerosene, catalytically cracked gas oil, coker naphthas, coker kerosene, coker gas oil, steam cracked naphthas and steam cracked gas oils. The ability of this process to treat low quality hydrogen deficient cracked feedstocks, such as steam cracked gas oil, permits these heretofore undesirable feedstocks to be recycled to extinction through the combined feedstock upgrading and steam cracking system.
It has further been found that hydrogen, C
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hydrocarbons and steam cracked naphtha can be produced in higher quantities in a process in which the effluent from at least one hydrotreating zone containing at least two hydrotreating catalysts is passed to a steam cracking zone. The effluents from the steam cracking zone are then passed to one or more fractionating zones in which the effluents are separated into a fraction comprising hydrogen and C
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hydrocarbons, a steam cracked naphtha fraction, a steam cracked gas oil fraction and a steam cracked tar fraction. The process of the present invention results in improved yields of the high value steam cracked products, i.e., C
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hydrocarbons, particularly ethylene, propylene, and butadiene, and steam cracked naphtha, particularly isoprene, cis-pentadiene, trans-pentadiene, cyclopentadiene, and benzene, and reduced yields of steam cracked tar.
SUMMARY OF THE INVENTION
This invention provides an integrated process for converting a hydrocarbon feedstock having components boiling above 100° C. into steam cracked products comprising hydrogen, C
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hydrocarbons, steam cracked naphtha (boiling from C
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to 220° C.), steam cracked gas oil (boiling from 220° C. to 275° C.) and steam cracked tar (boiling above 275° C.).
The process of the present invention therefore comprises: (i) passing the hydrocarbon feedstock through at least one hydrotreating zone wherein said feedstock is contacted at an elevated temperature and at a pressure in the range of from about 400 psig to about 1,250 psig with a hydrogen source and at least two hydrotreating catalysts to effect substantially complete conversion of organic sulfur and/or nitrogen compounds contained therein to H
2
S and NH
3
, respectively; (ii) passing the product from said hydrotreating zone to a product separation zone to remove gases and, if desired, light hydrocarbon fractions; (iii) passing the product from said separation zone to a steam cracking zone and thereafter; (iv) passing the product from said steam cracking zone to one or more product separation zones to separate the product into a fraction comprising hydrogen and C
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hydrocarbons, a steam cracked naphtha fraction, a steam cracked gas oil fraction and a steam cracked tar fraction, wherein the yields of
Bradow Carl W.
Foley Richard M.
Grenoble Dane Clark
Milam Stanley N.
Murray Brendan D.
Exxon Chemical Patents Inc.
Griffin Walter D.
Russell Linda K.
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