Mineral oils: processes and products – Chemical conversion of hydrocarbons – With subsequent treatment of products
Reexamination Certificate
2002-02-08
2004-03-09
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
With subsequent treatment of products
C208S095000, C208S027000, C585S738000, C585S739000
Reexamination Certificate
active
06702937
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for upgrading the hydrocarbons having an initial boiling point above about 120 degrees C. recovered from a Fischer-Tropsch plant by bulk dewaxing a C
5
plus hydrocarbon Fischer-Tropsch syncrude followed by hydrofinishing and recovering diesel and lubricating base oil products having improved properties.
BACKGROUND OF THE INVENTION
The market for lubricating base oils of high paraffinicity is continuing to grow due to the high viscosity index, oxidation stability, and low volatility relative to viscosity of these oils. The products produced from the Fischer-Tropsch process (syncrude) contain a high proportion of wax which make them ideal candidates for processing into lubricating base stocks. Accordingly, the hydrocarbon products recovered from the Fischer-Tropsch process have been proposed as feedstocks for preparing high quality lubricating base oils. See, for example, U.S. Pat. No. 6,080,301 which describes a premium lubricating base oil having a high non-cyclic isoparaffin content prepared from Fischer-Tropsch waxes by hydroisomerization dewaxing and solvent dewaxing.
High quality diesel products also may be prepared from the syncrude recovered from the Fischer-Tropsch process. Fischer-Tropsch derived diesel typically has a very low sulfur content and an excellent cetane number. These qualities make Fischer-Tropsch derived diesel an excellent blending stock for upgrading lower quality petroleum-derived diesel.
In general, for the purposes of this disclosure, diesel is considered as having a 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.). Lubricating base oils typically will have an initial boiling point above about 600 degrees F. (about 315 degrees C.). The syncrude fractions boiling in the range of diesel and lubricating base oils as they are initially recovered from the Fischer-Tropsch plant will usually contain a high proportion of waxy normal paraffins and unsaturated hydrocarbons, and therefore, these fractions require upgrading to meet commercial specifications for the finished products prior to their use either alone or as blending stock. During upgrading the various fractions are separated according to their respective boiling ranges. In the case of those fractions boiling in the ranges of lubricating base oils and diesel, upgrading steps generally are intended to lower the pour point to an acceptable temperature and to improve the oxidation and UV stability of the products. In order to improve the properties of these fractions, they are separated from the syncrude and separately dewaxed and hydrofinished. Applicants have discovered that contrary to conventional teaching, it is advantageous to bulk dewax and hydrofinish the entire C
5
plus syncrude fraction prior to separation of the various fractions.
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” is contained in the higher boiling portion of the Fischer-Tropsch syncrude. Fischer-Tropsch wax contains at least 10 percent by weight of C
20
and higher hydrocarbonaceous compounds, preferably at least 40 percent by weight of C
20
and higher hydrocarbonaceous compounds, and most preferably at least 70 percent by weight of C
20
and higher hydrocarbonaceous compounds. Fischer-Tropsch wax is important for the present invention because this fraction will contain the heavier hydrocarbons which when sent to the catalytic dewaxing operation will be converted to high quality lubricating base oil and diesel.
C
5
plus syncrude refers to those fractions of the product from a Fischer-Tropsch plant which are normally a liquid or solid at ambient temperature. Usually hydrocarbons boiling above ambient temperature will include those hydrocarbons containing 5 or more carbon atoms in the molecule. However, one skilled in the art will recognize that other factors besides carbon number will affect the boiling ranges of the Fischer-Tropsch syncrude fractions, such as, for example, the presence of unsaturated bonds, branching, and heteroatoms in the molecule. Therefore, some hydrocarbons may be present in this fraction which contain less than 5 carbon atoms. 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.
Fractions boiling in the range of 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” which refers to a product comprising primarily hydrocarbons having more than 20 carbon atoms in the backbone of the molecule and having an initial boiling point at the upper end of the boiling range for diesel. 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 backbone 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. C
10
plus hydrocarbons generally boil 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. 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.
The dewaxing and hydrofinishing of C
20
plus hydrocarbons, including Fischer-Tropsch-derived syncrude is discussed in U.S. Pat. No. 5,135,638. The isomerization of lighter feeds boiling in the range of naphtha and diesel by contacting the feed with a catalyst containing a silicoaluminophosphate molecular sieve (SAPO) is discussed in U.S. Pat. No. 4,859,311.
As used in this disclosure the words “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 phrases “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
Broadly, the present invention is directed to a process for producing low pour point hydrocarbon products having an initial boiling point above about 150 degrees C. from a Fischer-Tropsch plant which comprises (a) recovering a feedstock comprising C
5
plus syncrude from a Fischer-Tropsch plant; (b) dewaxing the C
5
plus syncrude feedstock in a catalytic dewaxing zone by contacting the C
5
plus syncrude feedstock with a dewaxing catalyst under dewaxing conditions, whereby a C
5
plus intermediate is produced having a lowered pour point relative to the C
5
plus syncrude feedstock; (c) hydrofinishing the C
5
plus intermediate in a hydrofinishing zone under hydrofinishing conditions, whereby a UV stabilized C
5
plus product is produced; and (d) separately collecting from the UV stabilized C
5
plus product a low pour point product having an initial boiling point above about 150 degrees C. The low pour point product of step (d) will generally consist of diesel and lubricating base oil products, although depending on how the operation is run the proportional yields of the products may vary over a considerable range.
In one embodiment of the invention, the preferred dewaxing catalyst is a hydroisomerization catalyst, such as, for example, a catalyst comprising a silicoaluminophosphate molecular sieve, com
Johnson David R.
Lee Stephen K.
Miller Stephen J.
Mohr Donald H.
Simmons Christopher A.
Ambrosius James W.
Chevron U.S.A. Inc.
Griffin Walter D.
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