Fischer-Tropsch wax and hydrocarbon mixtures for transport (law938)

The invention is a process for producing a mixture of Fischer-Tropsch product that is solid at ambient conditions (between 32.degree. F. and 95.degree. F.), such as wax, and hydrocarbon liquid, such as naphtha, that can be pumped at ambient temperature (between 32.degree. F. and 95.degree. F.). The temperature of the mixture is controlled below the melting point of the Fischer-Tropsch product. The present invention provides for the transport of Fischer-Tropsch product from a remote location in a readily available medium, such as naphtha, via pipeline, tanker or railcar. At the completion of the transport, the hydrocarbon liquid and Fischer-Tropsch product are separated by conventional methods such as flashing, distillation, or filtration with minimal contamination from the hydrocarbon liquid.

FIELD OF THE INVENTION
 The present invention pertains to a process for producing a mixture of a
 Fischer-Tropsch product that is solid at ambient conditions (between
 32.degree. F. and 95.degree. F.), such as Fischer-Tropsch wax, and a
 hydrocarbon liquid at ambient temperature, such as naphtha, that can be
 pumped from a remote location and subsequently separated by conventional
 methods such as flashing, distillation, or filtration with minimal
 contamination from the hydrocarbon liquid.
 BACKGROUND INFORMATION
 Oil fields typically have deposits of natural gas associated with them. In
 remote locations where transport of this gas may not be economically
 attractive, gas conversion technology can be used for chemically
 converting natural gas to higher molecular weight hydrocarbons. Current
 gas conversion technologies rely on the chemical conversion of natural gas
 to synthesis gas, which is a mixture of carbon monoxide and hydrogen.
 Synthesis gas is then reacted in a catalyzed hydrocarbon synthesis process
 commonly known as Fischer-Tropsch synthesis as described in U.S. Pat. No.
 5,348,982 to form higher molecular weight hydrocarbons.
 Waxes produced from the Fischer-Tropsch synthesis have many desirable
 properties. These waxes have very high purity since they are essentially
 free of any sulfur, nitrogen and aromatics. Additionally, Fischer-Tropsch
 waxes have high normal paraffin content.
 Generally, the transport of wax is not a problem because the wax, which is
 typically a solid below 100.degree. F., is produced at refineries or
 chemical plants with easy access to railcar or truck loading docks.
 However, most gas conversion plants are built in remote locations and
 hence, the above-mentioned conventional methods for shipping the wax are
 often unavailable.
 Some methods for transporting the wax from a remote location include
 shipping it in a cargo bay as a solid, in heated tanks and tankers, in a
 solvent, steam traced pipelines, or as a slurry. Solutions and slurries
 are attractive methods because they can be pumped at ambient conditions.
 However, the availability of solvents in remote locations can be a
 problem.
 Therefore, it is desirable to transport the Fischer-Tropsch product that is
 solid at ambient conditions in a medium that is readily available at a
 remote location and that is easily separated from the Fischer-Tropsch
 product upon completion of the transport with minimal contamination from
 the hydrocarbon liquid medium.
 SUMMARY OF THE INVENTION
 In accordance with the present invention, a Fischer-Tropsch product that is
 solid at ambient conditions (between 32.degree. F. and 95.degree. F.),
 such as a Fischer-Tropsch wax, is blended with hydrocarbon liquid at
 ambient temperature (between 32.degree. F. and 95.degree. F.), such as
 naphtha, to form a mixture that can be pumped at ambient temperature. The
 temperature of the mixture is controlled below the melting point of the
 Fischer-Tropsch product, thus producing a heterogeneous mixture.
 The Fischer-Tropsch product and hydrocarbon liquid mixture is transported
 via conventional methods for the movement of liquids such as via pipeline,
 tanker, or railcar.
 At the completion of the transport, the hydrocarbon liquid and
 Fischer-Tropsch product are separated by conventional methods such as
 flashing, distillation or filtration. The hydrocarbon liquid derived from
 the Fischer-Tropsch synthesis, which is available at a remote location,
 allows for the transport of the Fischer-Tropsch product with minimal
 contamination from the hydrocarbon liquid.

DETAILED DESCRIPTION OF THE INVENTION
 The present invention provides a process for producing a mixture of
 Fischer-Tropsch product that is solid at ambient temperature, such as a
 Fischer-Tropsch wax, and a hydrocarbon liquid at ambient temperature, such
 as naphtha. For illustrative purposes, the Fischer-Tropsch product is a
 Fischer-Tropsch wax and the hydrocarbon liquid is naphtha. However, those
 skilled in the art recognize that any Fischer-Tropsch product that is
 solid at ambient temperature and any hydrocarbon liquid at ambient
 temperature could be used.
 The mixture of Fischer-Tropsch wax and naphtha contains from about 1 to 22
 weight percent Fischer-Tropsch wax, preferably about 8 to 10 weight
 percent, that can be pumped at ambient temperature.
 As illustrated in FIG. 1, the Fischer-Tropsch product (1) from a
 Fischer-Tropsch reactor is fractionated into products such as light gases
 (2), naphtha (3), jet fuel (4), diesel fuel (5), and a heavy hydrocarbon
 stream (6). The Fischer-Tropsch product (1) may be hydrotreated,
 processed, or hydroisomerized before separation, or may be separated and
 the fractionated products processed individually. The products may vary
 with operational objectives and could be used as produced or with
 additional hydrotreating, upgrading, blending, or additives.
 The heavy hydrocarbon stream (6) could be the total wax from the
 Fischer-Tropsch synthesis, fractionated into specific boiling ranges,
 hydroisomerized to produce a lubricant basestock with solvent dewaxing to
 obtain the wax or any combination of these options. The wax from the heavy
 hydrocarbon stream (6) can be hydrotreated for sale of the wax as refined
 wax.
 The wax, refined or unrefined, is solidified, granulated, and blended with
 all or part of the naphtha (3) to produce a heterogeneous Fischer-Tropsch
 wax and naphtha mixture (8). As previously mentioned, the amount of
 Fischer-Tropsch wax that can be blended is about 1 to 22 weight percent
 Fischer-Tropsch wax, preferably about 8 to 10 weight percent. The pour
 point of the mixture should be below about 75.degree. F., more preferably
 below about 32.degree. F. These ranges and pour points are based on the
 tendency for naphtha to swell the wax to form a paste at amounts above
 these ranges.
 The viscosity of the mixture should be below about 1500 cP, preferably
 below about 500 cP. Otherwise, the increased viscosity will make the
 transport of the mixture more difficult.
 The temperature of the mixture is controlled below the melting point of the
 wax to limit the solubility of the wax. Additionally, the molecular weight
 difference between the wax and the naphtha also helps to limit the
 solubility of the wax. This objective is important because it is the
 soluble wax that becomes deposited on the walls of a pipeline or tanker.
 The deposited wax typically leads to an increase in the pressure drop in
 the pipeline due to a reduction in the cross-sectional area and hence, a
 reduced efficiency in the transport of the mixture.
 Although any Fischer-Tropsch derived wax may be used in this invention, the
 preferred boiling range of the wax to be blended is about 700+.degree. F.,
 more preferably about 725.degree. F. to 1025.degree. F.
 EXAMPLE
 A Fischer-Tropsch synthesis product was fractionated to obtain naphtha with
 a boiling range from about 95.degree. F. to about 320.degree. F. The
 quality of separation was measured by High Temperature Simulated
 Distillation Gas Chromatography (GCD) using a HP 6890 series gas
 chromatograph. The wax was the total solid product from the
 Fischer-Tropsch synthesis at ambient conditions with a boiling range of
 453.degree. F. to 1129.degree. F. based on 5 and 95 weight percent GCD,
 respectively. The GCD data are presented in Table 1 below.
 TABLE 1
 Naphtha and Wax GCD
 Boiling Range (.degree. F.) Naphtha (wt. %) Wax (wt. %)
 i/200 10.7 No detected
 200/320 51.6 0.7
 320/500 28.7 7.5
 500/700 8.4 32.0
 700/1000 0.6 45.9
 1000+ Not detected 13.9
 The mixtures were produced by granulating the wax into finely divided
 flakes and then mixing the wax with the naphtha in a colloid mill with
 varying rotor-stator gap widths and times. This blending process was
 repeated for a range of wax concentrations from about 7 to 30 weight
 percent.
 Pour points were measured by an ISL pour point analyzer and the Brookfield
 viscosity was measured using a viscometer from about 100.degree. F. to the
 pour point. The results are shown below in Table 2.
 TABLE 2
 Naphtha Wax Colloids Properties
 Total Wax (wt. %) Pout Point (.degree. F.)
 7 1
 10 41
 13 41
 19 50
 22 63
 25 86
 28 Paste
 30 Paste
 At total wax concentrations greater than about 28 weight percent, the
 mixture tended to form a paste due to the swelling of the wax caused by
 the naphtha. Total wax concentrations between about 7 and 22 weight
 percent wax yielded pour points below typical ambient conditions.
 The ability to pump the mixture, as measured by the Brookfield viscosity at
 32.degree. F., was obtained for the 7 and 13 weight percent wax. The
 resulting values were 372 cP and 1218 cP, respectively. As indicated by
 the data, an increase in the wax concentration caused a substantial
 increase in the low temperature viscosity.
 As previously mentioned, the dissolved wax deposits on the walls of the
 pipeline or tanker thereby decreasing the effectiveness of the transport
 operation. Plating on the walls occurs by deposition of dissolved wax on a
 cool surface and is proportional to the heat transfer at the interface. By
 limiting the amount of dissolved wax, surface coating can be reduced
 because the dissolved wax content is proportional to deposition. For the
 total wax having a boiling range of about 453.degree. F. to 1129.degree.
 F. only 5.5.+-.2.0 grams of wax per liter of mixture were dissolved.
 Increasing the wax concentration did not increase the dissolved wax thus
 indicating that the mixture was saturated. These experiments were done at
 room temperature. For heavier waxes such as those having a boiling range
 of about 725.degree. F. to 1025.degree. F. instead of the entire
 453.degree. F. to 1129.degree. F. fraction, the solubility of the wax in
 naphtha decreased and the separation became easier.
 Visual observations of the mixture after two weeks indicated that
 agglomerates did not form in the mixture. However, due to the density
 difference between the naphtha and wax, some settling of solid particles
 in the mixture occurred. These wax particles were easily suspended by mild
 agitation thus indicating that settling of the mixture in a tank or tanker
 could be addressed by circulation or agitation either during shipment or
 before unloading of the mixture.
 Separation of the wax and naphtha mixture was achieved by fractionating the
 mixture at 400.degree. F. for the 7, 13, and 19 weight percent wax with
 goodness of cut determined by GCD as shown in Table 3 below. Fractionation
 will be sharper for higher boiling range Fischer-Tropsch waxes.
 TABLE 3
 Distillation Products After Blending
 7 wt. % 13 wt. % 19 wt. %
 Boiling Naphtha Wax Naphtha Wax Naphtha Wax
 Range (.degree. F.) (wt. %) (wt. %) (wt. %) (wt. %) (wt. %) (wt. %)
 i/200 Not Not Not Not Not Not
 detected detected detected detected detected detected
 200/320 23.5 Not 37.3 Not 25.5 Not
 detected detected detected
 320/500 73.8 1.0 55.7 19.0 70.5 3.5
 500/700 2.7 66.4 2.0 49.0 1.2 59.6
 Numerous modifications and alternative embodiments of the invention will be
 apparent to those skilled in the art in view of the foregoing description.
 Accordingly, this description is to be construed as illustrative only and
 is for the purpose of teaching those skilled in the art the best mode of
 carrying out the invention. Details of the process may be varied
 substantially without departing from the spirit of the invention and the
 exclusive use of all modifications, which come within the scope of the
 appended claims, is reserved.