Chemistry of hydrocarbon compounds – Plural serial diverse syntheses – To produce unsaturate
Reexamination Certificate
2002-08-07
2004-03-02
Parsa, J. (Department: 1621)
Chemistry of hydrocarbon compounds
Plural serial diverse syntheses
To produce unsaturate
C585S316000, C585S510000, C208S028000, C208S136000
Reexamination Certificate
active
06700027
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for upgrading Fischer-Tropsch products by increasing the yield of diesel and lubricating base oils through the oligomerization of olefins in the condensate using a chromium catalyst and high temperature.
BACKGROUND OF THE INVENTION
Products prepared from the Fischer-Tropsch process comprise a mixture of various solid, liquid, and gaseous hydrocarbons. Those Fischer-Tropsch products which boil within the range of lubricating base oil and diesel are usually of higher value than lower boiling products, such as naphtha, or normally gaseous products, such as LPG. Obviously, it is advantageous to capture the carbon value of the lower boiling and normally gaseous products by upgrading them to higher molecular weight and higher value products.
High quality lubricating base oils may be prepared from the Fischer-Tropsch wax recovered as one of the products of the Fischer-Tropsch synthesis. A process for upgrading the other hydrocarbon products recovered from the Fischer-Tropsch process into suitable feedstocks for preparing high quality lubricating base oils is highly desirable. Lubricating base oils typically will have an initial boiling point above about 315 degrees C. (600 degrees F.). High quality diesel products also may be prepared from the syncrude recovered from the Fischer-Tropsch process. Fischer-Tropsch derived diesel typically has very low sulfur and aromatics content and an excellent cetane number. These qualities make Fischer-Tropsch derived diesel an excellent blending stock for upgrading lower quality petroleum-derived diesel. Accordingly, it is desirable to be able to maximize the yields of such higher value hydrocarbon products which boil within the range of lubricating base oils and diesel. At the same time, it is desirable to minimize the yields of lower value products, such as naphtha, and C
4
minus products.
All syncrude Fischer-Tropsch products as they are initially recovered from the Fischer-Tropsch reactor contain varying amounts of olefins depending upon the type of Fischer-Tropsch operation employed. In addition, the crude Fischer-Tropsch product also contains a certain amount of oxygenated hydrocarbons, especially alcohols, which are reported to act as poisons to most oligomerization catalysts. In conventional processes, these oxygenates may be removed through a hydrotreating step, or, alternatively, they may be readily converted to olefins by a dehydration step. The olefins originally present in the Fischer-Tropsch products or derived from converted oxygenates may be oligomerized to yield hydrocarbons having a higher molecular weight than the original feed. The oligomerization of olefins has been well reported in the literature and a number of commercial processes are available. See, for example, U.S. Pat. Nos. 4,417,088; 4,827,064; 4,827,073, and 4,990,709. Oligomerization also introduces desirable branching into the hydrocarbon molecule which lowers the pour point of the diesel and lubricating base oil products, thereby improving the cold flow properties of the product. See for example U.S. Pat. No. 4,417,088. The use of chromium as the active metal in the oligomerization catalyst has been described in the literature. See, for example U.S. Pat. Nos. 4,827,073; 4,990,709, and 5,264,642. However, the maximum temperature under which the oligomerization reaction will proceed without promoting undesired side reactions has been reported to be about 480 degrees F. (250 degrees C.).
Fischer-Tropsch wax refers to a high boiling fraction from the Fischer-Tropsch derived syncrude and is most often a solid at room temperature. For the purpose of this disclosure “Fischer-Tropsch wax” will be contained in the higher boiling portion of the Fischer-Tropsch syncrude and will have an initial boiling point approximately the same as that for lubricating base oil, i.e., above about 315 degrees C. (600 degrees F.). The term “Fischer-Tropsch condensate” refers in this disclosure to that fraction of the Fischer-Tropsch syncrude which is normally liquid at ambient temperature. Accordingly, the Fischer-Tropsch condensate fraction is that fraction which includes those hydrocarbons generally having five or more carbon atoms in their molecular structure but having a lower boiling range than Fischer-Tropsch wax.
As used in this disclosure, the term “C
19
minus Fischer-Tropsch product” refers to a product recovered from a Fischer-Tropsch reaction zone which is predominantly comprised of hydrocarbons having 19 carbon atoms or less in the molecular structure. One skilled in the art will recognize that such products may actually contain a significant amount of hydrocarbons containing greater than 19 carbon atoms. In general, what is referred to are those hydrocarbons having a boiling range of diesel and below. In general, for the purposes of this disclosure, diesel is considered as having an upper boiling point of about 700 degrees F. (370 degrees C.) and an initial boiling point of about 300 degrees F. (about 150 degrees C.). Diesel may also be referred to as C
10
to C
19
hydrocarbons. Likewise, Fischer-Tropsch wax preferably is comprised predominantly of “C
20
plus product” and refers to a product comprising primarily hydrocarbons having 20 or more carbon atoms in the structure of the molecule. C
20
plus product will have an initial boiling point at the upper end of the boiling range for diesel, i.e., above about 600 degrees F. (315 degrees C.). It should be noted that the upper end of the boiling range for diesel and the lower end of the boiling range for Fischer-Tropsch wax have considerable overlap. The term “naphtha” when used in this disclosure refers to a liquid product having between about C
5
to about C
9
carbon atoms in the molecular structure and will have a boiling range generally below that of diesel but wherein the upper end of the boiling range will overlap that of the initial boiling point of diesel. The term C
10
plus hydrocarbons refers to those hydrocarbons generally boiling above the range of naphtha, i.e., the fractions boiling within the range of diesel and lubricating base oils or above about 150 degrees C. Products recovered from the Fischer-Tropsch synthesis which are normally in the gaseous phase at ambient temperature are referred to as C
4
minus product in this disclosure. LPG which is primarily a mixture of propane and butane is an example of a C
4
minus product. The precise cut-point selected for each of the products in carrying out the distillation operation will be determined by the product specifications and yields desired.
As used in this disclosure the word “comprises” or “comprising” is intended as an open-ended transition meaning the inclusion of the named elements, but not necessarily excluding other unnamed elements. The phrase “consists essentially of” or “consisting essentially of” is intended to mean the exclusion of other elements of any essential significance to the composition. The phrase “consisting of” or “consists of” are intended as a transition meaning the exclusion of all but the recited elements with the exception of only minor traces of impurities.
SUMMARY OF THE INVENTION
The present invention is directed to a process for increasing the yield of C
10
plus hydrocarbon products from a Fischer-Tropsch plant which comprises (a) recovering a Fischer-Tropsch condensate fraction boiling below about 700 degrees F. from the Fischer-Tropsch plant, wherein said fraction contains at least 10 weight percent olefins; (b) contacting the olefins in the Fischer-Tropsch condensate fraction under oligomerization conditions, at a reaction temperature between about 650 degrees F. and 800 degrees F. with an oligomerization catalyst comprising active chromium on an inert support; and (c) recovering a C
10
plus hydrocarbon product having a higher average molecular weight than the Fischer-Tropsch condensate. The amount of olefins present in the Fischer-Tropsch condensate fraction will vary depending on the way in which the Fischer-Tropsch synthesis is carried out. From an economic perspect
Johnson David R.
Simmons Christopher A.
Ambrosius James W.
Chevron U.S.A. Inc.
Parsa J.
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