EP1899439A2

EP1899439A2 - Low foaming distillate fuel blend

Info

Publication number: EP1899439A2
Application number: EP06784692A
Authority: EP
Inventors: Jeffrey J. Toman; Dennis J. O'rear; Graham Nancekievill; Rony H. Danschutter
Original assignee: Chevron USA Inc
Current assignee: Chevron USA Inc
Priority date: 2005-06-10
Filing date: 2006-06-07
Publication date: 2008-03-19
Also published as: US20060278565A1; AU2006257955A1; NL2000093C2; WO2006135731A2; NL2000093A1; JP2008545872A; CN101278035A; ZA200800264B; BRPI0611727A2; WO2006135731A3

Abstract

A distillate fuel composition having low foaming characteristics and process for making the composition is described. The composition and process uses Fischer Tropsch distillates to reduce the foaming in distillate fuels. The use of Fischer Tropsch distillates to control foaming in distillate fuels reduces or eliminates the need to use silicon anti-foam agents. A preferred composition comprises at least 20 vol. % of a petroleum derived distillate having a foam vanishing time greater than 20 seconds; and at least 5 vol. % of a Fischer-Tropsch derived distillate having a foam vanishing time of less than 15 seconds; where the resulting distillate fuel blend has a foam vanishing time of 15 seconds or less in the absence of an antifoam additive.

Description

Low Foaming Distillate Fuel Blend

BACKGROUND OF THE INVENTION

Foam can form in distillate fuels during movement or agitation of the fuels. Foam formation in distillate fuels can be a particular problem during transfer of the fuel such as when filling of tanks in vehicles, service stations, terminals and other operations. When foam is generated during fueling of a vehicle, it can cause the fill sensors on the fuel nozzle to shut off the flow of fuel. When the foam breaks, fueling of the vehicle can continue. However, this can be a very frustrating experience for the customer, can cause delays in fueling, and can result in only partial filling of the vehicle fuel tank. Foaming can also lead to spilled fuel which creates a potential safety concern from potential fires, slipping on a fuel-wetted surface, as well as an environmental hazard. In order to minimize foaming, silicon antifoam agents are often used in distillate fuels, especially in Europe where the problem of foaming is more common. Silicon antifoam agents are typically used in the range of 0.1 - 20 ppm (weight), most commonly 1 - 10 ppm. Despite their low concentration, silicon antifoam agents can be one of the most expensive additives in distillate fuels. Silicon antifoam agents have also been associated with deposits on injector nozzles and potentially are a concern for other engine problems.

The World Wide Fuel Charter of December 2002 describes an emerging specification for diesel fuels. It defines various categories with Number Three being for "Markets with advanced requirements for emission controls or other market demands" and Number four being for "Markets with further advanced requirements for emission control, to enable sophisticated NOx and PM after- treatment technologies." See pages 15 and 16. The proposed requirements for foaming for Category 3 and 4 diesel fuel - a maximum of 100 ml of foam and a foam vanishing time of 15 seconds or less as determined by NF M 07- 075 - see page 9. Of the two measurements of foam, the vanishing time is felt to be the most appropriate indicator of performance. Further discussion of diesel foaming is on pages 45-46. Page 46 states the goal of selection of additives to control foaming: "It is important that the eventual additive chosen should not pose any problems for the long-term durability of the emission post- treatment control systems."

GE Silicons is a supplier of silicon antifoam additives. In their brochure on their product SAS®TP-325 they state that this additive is "... a major step towards minimizing the potential risks of silica deposits in engine car injectors."

Thus while silicon antifoam agents are effective in controlling foaming, they are expensive and have been indicated in problems associated with both emission control systems and injectors. Approaches to minimize or eliminate their use are desirable.

Art that relates to the control of foaming in hydrocarbon fuels includes USP 4,690,688 to Adams et al. which relates to the use of certain siloxane polyoxyalkylene copolymers as antifoaming agents in diesel and jet fuel. Another patent that relates to silicone foam control agents is USP 5,620,485 (Fey). A patent that relates to Diesel Fuel and Lubricating oil antifoams and methods of their use is USP 6,221 ,815 (Grabowski et al.). A patent that provides a method for reducing foaming of lubricating oils is USP 6,090,758 (Pillion et al.). A patent that provides antifoaming agents of lubricating oils is USP 5,766,513 to Pillion et al. Another patent that describes silicone antifoam compositions is USP 5,531 ,929 (Kobayashi). A patent that describes a device for the controlled release of antifoaming agents in the diesel fuel tank filling nozzle is USP 4,687,034, Graiff et al. The patents listed above all provide background and some theory on the problem of hydrocarbon foam formation and provide various solutions to the problem. They all also use silicon based antifoams (in various forms) and highlight some of the problems associated with the use of silicon based antifoams. The present invention provides an alternative to the use of high levels of silicon based antifoams or the elimination of the use of silicon based antifoams entirely hence minimizing the problems/costs associated with silicon based antifoam use. Art that relates to blends of Fischer-Tropsch and petroleum distillate for fuels includes USP 6,663,767 and USP 6,822,131 (Berlowitz et al.). A European patent that relates to diesel or turbine engine fuels consisting of a mixture of petroleum refinery hydrocarbons and Fischer-Tropsch hydrocarbons is EP 1 ,365,007 (Pavoni) which is incorporated by reference herein in its entirety. Another patent that provides teachings about distillate fuel blends from Fischer Tropsch products is USP 6,890,423 (O'Rear) which is also incorporated herein by reference in its entirety. As mentioned above silicone-containing anti-foams ("silicon based anti- foams") are an integral part of diesel packages in Europe. However, there is a great demand for a non-silicon based anti-foam product, or a reduced need to use silicon based anti-foams since several problems are associated with silicon based anti-foams. For example, silicone anti-foams can separate from the diesel fuel package due to poor solubility. This causes inconsistent fuel and anti-foam performance. Excess silicon anti-foam may need to be added to consistently achieve the desired low level of foaming. Furthermore, silicone anti-foams can contribute to the dispersion of sediments (rust, water, etc.) into the diesel fuel. This may increase emissions and cause damage to the engine. Additionally, there is some concern that the silicone anti-foams themselves contributes to engine deposits and emissions. Lastly, silicones can lose their effectiveness as an anti-foam after the treated diesel fuel package has been stored for just a few days unless a high dosage is charged.

Thus there is a need to develop a fuel that either does not require anti- foam additives (particularly silicon based anti-foams) or requires only reduced amounts of antifoam additives. The present invention provides such a fuel.

SUMMARY OF THE INVENTION

The present invention provides a distillate fuel composition and a process for making a distillate fuel having improved foaming properties. As discussed above in the Background foaming can be a serious problem in distillate fuels particularly during transfer of fuel such as but not limited to fueling of a vehicle. The present invention seeks to provide a distillate fuel having low foaming characteristics and a process for reliably and consistently making low foaming distillate fuels with reduced need for anti-foaming additives or preferably without any anti-foaming additives.

In a preferred embodiment of the present invention a distillate fuel blend is provided, comprising:

at least 20 vol. % of a petroleum derived distillate having a foam vanishing time greater than 20 seconds; and at least 5 vol. % of a Fischer-Tropsch derived distillate having a foam vanishing time of less than 15 seconds; wherein the resultant distillate fuel blend has a foam vanishing time of 15 seconds or less in the absence of an antifoam additive.

The present invention also provides a process for making distillate fuel having improved foaming characteristics, comprising: combining a petroleum derived distillate fuel having a foam vanishing time greater than 20 seconds with a Fischer-Tropsch derived distillate having a foam vanishing time of less than 15 seconds to form a distillate fuel blend having a distillate fuel blend having a foam vanishing time of 15 seconds or less in the absence of an antifoam additive.

As mentioned above an antifoam may be used in the present invention however following the teaching of the present invention will allow the use of antifoam additives to be reduced if not eliminated entirely. It is advantageous to reduce the amounts of antifoam additives used in a distillate to avoid some or all of the negative effects of antifoam additives (particularly silicon based antifoams) that are discussed in the Background section of this application. Another embodiment of the, present invention comprises an improved distillate fuel composition for the prevention of foaming during fuel transfer, wherein the improvement comprises blending an effective amount of a FT derived distillate, with a petroleum derived distillate having a foam vanishing time of above 20 seconds, sufficient to achieve a blended distillate fuel having a foam vanishing time of 15 seconds or less in the absence of an antifoam additive.

We have discovered that Fischer Tropsch distillate can be used to control foaming when used with petroleum distillate fuel components which exhibit foam formation in excess of 15 seconds of vanishing time. This technology is useful for petroleum derived distillate fuels that have a foam vanishing time in excess of 15 seconds, preferably in excess of 20 seconds, more preferably in excess of 25 seconds, still more preferably in excess of 30 seconds, and most preferably in excess of 50 seconds.

Optionally a silicon based antifoam agent can be added to further reduce foam formation or provide a safety margin. Preferably less silicon based antifoam agent can be used when employing the teachings of the present invention.

In the present invention the volume fraction of Fischer Tropsch distillate in the blend (x) is less than or equal to 0.7 (70 vol. %), preferably less than or equal to 0.5 (50 vol. %), more preferably greater than or equal to 0.05 (5 vol. %) and less than or equal to 0.4 (40 vol. %), still more preferably between about 0.05 (5 vol. %) and 0.3 (30 vol. %), even more preferably between about 0.05 (5 vol. %) and 0.25 (25 vol. %),still more preferably between about 0.05 (5 vol. %) and 0.20 (20 vol. %), and most preferably between about 0.10 (10 vol. %) and 0.20 (20 vol. %).

Not to be limited by theory the improvement (decrease) of foam vanishing time by blending of Fischer-Tropsch derived distillate with a petroleum derived distillate appears to be non-linear and not simply a dilution effect. Thus significant improvements in foaming in a blend can be achieved by use of less Fischer-Tropsch derived distillate than might be expected.

As mentioned above the response of foam vanishing time to addition of Fischer Tropsch distillate has been found to be highly non-linear. The reduction in vanishing time when adding a Fischer-Tropsch distillate is greater than what one would calculate from a linear blend.

With a target foam vanishing time (Y)₁ a measured vanishing time of the petroleum-derived distillate (A) and a measured vanishing time of the Fischer Tropsch derived distillate of B, the volume fraction of Fisher Tropsch derived distillate fuel required to at least reach the target vanishing time is

x < (Y - A)/(B - A)

According to an embodiment of the present invention a method to decrease the foam vanishing time of a petroleum-derived distillate product to a target vanishing time Y is by adding to the petroleum-derived distillate product an amount of a Fischer-Tropsch derived distillate product having a lower vanishing time, B, than the vanishing time of the petroleum-derived distillate product, A, wherein the amount of added Fischer-Tropsch derived distillate product is less than the amount which would be added if linear blending is assumed.

In this embodiment of the present invention one can determine the maximum amount of Fischer-Tropsch distillate that is required to be added to a petroleum distillate to at least meet a desired foam vanishing time. The volume fraction of Fischer-Tropsch distillate product that is required, to at least achieve the foam vanishing time of the blend, is less than x', wherein x' is the target volume fraction that would be added if linear blending assumptions would have been made according to the following equation: Y = A + x'(B-A).

Among other factors the present invention is based on the surprising finding that a relatively small amount of Fischer-Tropsch derived distillate added to a petroleum derived distillate can have a substantial effect on the foam vanishing time of the blend. The teachings of the present invention can be used to make distillate fuel blend compositions having desired low foam vanishing times. Surprisingly low foam distillate fuel can be made by blending a Fischer-Tropsch distillate with a petroleum derived distillate without the use of an antifoam additive or with minimal use of an antifoam additive while still meeting foam vanishing time requirements for a finished fuel.

DETAILED DESCRIPTION OF THE INVENTION

In a preferred embodiment of the present invention a process is provided for making a distillate fuel blend having improved foaming characteristics, comprising:

selecting a petroleum derived distillate having a foam vanishing time greater than 20 seconds; and

blending an amount of a Fischer-Tropsch derived distillate having a foam vanishing time of less than 15 seconds sufficient to achieve a distillate fuel blend having a foam vanishing time of 15 second or less.

Using the teachings of the present invention it is possible to decrease or eliminate entirely the use of anti foam additives such as silicon based antifoams. Elimination or reduction of antifoam additives is quite desirable for economic as well as performance reasons. These reasons are discussed in detail in the background section of this application. Surprisingly, the use of Fischer-Tropsch derived distillate having a foam vanishing time of less than 15 seconds, blended with petroleum derived distillate having a high foam vanishing time, can result in a distillate fuel blend having greatly reduced foaming characteristics as expressed in foam vanishing time. In the present invention the Fischer-Tropsch derived distillate should have a low foam vanishing time, preferably less than 15 seconds, more preferably less than 12 seconds, still more preferably less than 10 seconds, most preferably less than 8 seconds. The terms "distillate fuel, distillate fuel fraction, petroleum derived distillate, Fischer-Tropsch derived distillate " means a hydrocarbon with boiling points between about 250 degrees F and 1100 degrees F, preferably 300 degrees F and 700 degrees F. The preferred method to measure boiling ranges is with ASTM D2887 or for materials with Final Boiling Points greater than 1000⁰F ASTM D6352. The lower value of the boiling range is the Initial Boiling Point (IBP) and the higher value of the boiling range is the Final Boiling Point (FBP). While not preferred, ASTM D-86 and ASTM D1160 can be used, but their results must be converted to True Boiling Points (TBP) for comparison. The term "distillate" means that typical conventional fuels of this type can be generated from vapor overhead streams of petroleum crude distillation or Fischer-Tropsch derived hydrocarbons. In contrast, residual fuels cannot be generated from vapor overhead streams of petroleum crude distillation, and are a non-vaporizable remaining portion. Within the broad category of distillate fuels are specific fuels that include: naphtha, jet fuel, diesel fuel, kerosene, aviation gasoline, fuel oil, and blends thereof. Distillate fuel as used herein may mean distillate fuels prepared by Fischer Tropsch processes as well as distillate fuels generated from conventional petroleum crude distillation as appropriate in the context. A salable distillate fuel is a distillate fuel meeting the specifications for one or more of naphtha, jet fuel, diesel fuel, kerosene, aviation gas, fuel oil, and blends thereof.

The terms "Fischer-Tropsch derived distillate, petroleum derived distillate and distillate fuel blend component" are components which can be used with other components, to form a salable distillate fuel meeting at least one of the specifications for naphtha, jet fuel, diesel fuel, kerosene, aviation gas, fuel oil, and blends thereof, especially diesel fuel or jet fuel, and most especially diesel fuel. The component by itself does not need to meet all specifications for the distillate fuel, only the salable distillate fuel needs to meet the specifications. In a preferred embodiment of the present invention the Fischer Tropsch distillate is made in a Low Temperature Fischer Tropsch (LTFT) process. Most preferably the Fischer Tropsch distillate is made using a cobalt catalyst and operated in the slurry bed mode. For the purposes of the present invention the terms "foam vanishing time, foam release time, and time of disappearance of foam" are equivalent and are measured using the protocol described in AFNOR NFM07-075. The test method of the AFNOR French Standards Organization can be obtained from AFNOR, 11 avenue Francis de Pressense, 93571 Saint-Denis La Plaine Cedex (France). Their web site is http://www.afnor.fr.

A diesel fuel is a material suitable for use in diesel engines and conforming to at least one of the following specifications:

. ASTM D 975— "Standard Specification for Diesel Fuel Oils" • European Norm EN590.

• Japanese Fuel Standards JIS K 2204.

• The United States National Conference on Weights and Measures (NCWM) 1997 guidelines for premium diesel fuel.

• The United States Engine Manufacturers Association recommended guidelines for premium diesel fuel (FQP-1A).

A jet fuel is a material suitable for use in turbine engines for aircraft or other uses meeting at least one of the following specifications:

• ASTM D1655. • DEF STAN 91-91/3 (DERD 2494), TURBINE FUEL, AVIATION,

KEROSENE TYPE, JET A-1 , NATO CODE: F-35.

• International Air Transportation Association (IATA) Guidance Materials for Aviation, 4^th edition, March 2000.

The term "petroleum-derived diesel components", "petroleum derived distillate" or "petroleum-derived distillate" means the vapor overhead streams from distilling petroleum crude directly or with intermediate refinery processing steps. A source of the petroleum-derived crude can also be from a gas field condensate. Other processing steps may also be employed in the refining of petroleum crude such as but not limited to hydroprocessing, hydrocracking, hydrotreating, alkylation, oligomerization, catalytic reforming resulting in "petroleum-derived diesel components", "petroleum derived distillate" or "petroleum-derived distillate".

A highly paraffinic distillate fuel component is a distillate fuel component that contains more than 70 wt. % paraffins, preferably more than 80 wt. % paraffins, and most preferably more than 90 wt % paraffins.

A distillate-boiling Fischer Tropsch product is a product derived from a Fischer Tropsch process that boils within 60° F. and 1100° F., preferably boiling between 250 and 700° F. This stream is typically converted to a highly paraffinic distillate fuel component by processes that may include one or more additional step selected from the group consisting of isomerization, hydroprocessing, hydrocracking, and hydrotreating.

A heavy Fischer Tropsch product is a product derived from a Fischer Tropsch process that can boil above the range of commonly sold distillate fuels: naphtha, jet or diesel fuel. This means greater than 400° F., preferably greater than 550° F., and most preferably greater than 700° F. This stream is typically converted to a highly paraffinic distillate fuel component by processes that include a hydrocracking step.

Syngas is a mixture that includes both hydrogen and carbon monoxide. In addition to these species, water, carbon dioxide, unconverted light hydrocarbon feedstock and various impurities may also be present.

According to the present invention, a portion of the fuel blend components of the present invention may be obtained from Fischer Tropsch processes. In Fischer-Tropsch chemistry, syngas is converted to liquid hydrocarbons by contact with a Fischer-Tropsch catalyst under reactive conditions. Typically, methane and optionally heavier hydrocarbons (ethane and heavier) can be sent through a conventional syngas generator to provide synthesis gas. Generally, synthesis gas contains hydrogen and carbon monoxide, and may include minor amounts of carbon dioxide and/or water. The presence of sulfur, nitrogen, halogen, selenium, phosphorus and arsenic contaminants in the syngas is undesirable. For this reason and depending on the quality of the syngas, it is preferred to remove sulfur and other contaminants from the feed before performing the Fischer Tropsch chemistry. Means for removing these contaminants are well known to those of skill in the art. For example, ZnO guardbeds are preferred for removing sulfur impurities. Means for removing other contaminants are well known to those of skill in the art. It also may be desirable to purify the syngas prior to the Fischer Tropsch reactor to remove carbon dioxide produced during the syngas reaction and any additional sulfur compounds not already removed. This can be accomplished, for example, by contacting the syngas with a mildly alkaline solution (e.g., aqueous potassium carbonate) in a packed column.

In the Fischer Tropsch process, liquid and gaseous hydrocarbons are formed by contacting a synthesis gas comprising a mixture of H2 and CO with a Fischer Tropsch catalyst under suitable temperature and pressure reactive conditions. The Fischer Tropsch reaction is typically conducted at temperatures of about 300 to 700° F. (149 to 371° C), preferably about from 400 to 550° F. (204 to 228° C); pressures of about from 10 to 600 psia, (0.7 to 41 bars), preferably 30 to 300 psia, (2 to 21 bars) and catalyst space velocities of from about 100 to about 10,000 cc/g/hr, preferably 300 to 3,000 cc/g/hr.

Examples of conditions for performing Fischer-Tropsch type reactions are well known to those of skill in the art. Suitable conditions are described, for example, in U.S. Pat. Nos. 4,704,487, 4,507,517, 4,599,474, 4,704,493, 4,709,108, 4,734,537, 4,814,533, 4,814,534 and 4,814,538, the contents of each of which are hereby incorporated by reference in their entirety.

The products of the Fischer Tropsch synthesis process may range from Ci to C200+ with a majority in the C5-C100+ range. The reaction can be conducted in a variety of reactor types; for example, fixed bed reactors containing one or more catalyst beds, slurry reactors, fluidized bed reactors, or a combination of different type reactors. Such reaction processes and reactors are well known and documented in the literature. Slurry Fischer Tropsch process are preferred for the process of the invention.

In general, Fischer-Tropsch catalysts contain a Group VIII transition metal on a metal oxide support. The catalysts may also contain a noble metal promoter(s) and/or crystalline molecular sieves. Certain catalysts are known to provide chain growth probabilities that are relatively low to moderate, and the reaction products include a relatively high proportion of low molecular (C2-e) weight olefins and a relatively low proportion of high molecular weight (C30+) waxes. Certain other catalysts are known to provide relatively high chain growth probabilities, and the reaction products include a relatively low proportion of low molecular (C2-8) weight olefins and a relatively high proportion of high molecular weight (C3o+) waxes. Such catalysts are well known to those of skill in the art and can be readily obtained and/or prepared. The preferred catalysts of this invention contain either Fe or Co, with Co especially preferred. The teachings of this invention may also be useful for making a dual use or multi use fuel. An example of a dual use fuel is one that can be used as both in Diesel and Jet engines. Jet fuel specifications can have strict limitations on the use of additives. The present invention provides a fuel that has desirable low foaming characteristics without the use of additives. Such a fuel can be used in both diesel and jet fuel applications. Dual use fuels may become increasingly useful in the future. Dual use fuels would be desirable to minimize infrastructure (such as tankage and dedicated pipelines) or where infrastructure is limited. An example of this is military uses where it would be very useful to have a "flexible" fuel or "unifuel" that could be used as the fuel for two or even several end uses. Alternatively a flexible fuel that requires only a minimal addition of an additive package for specific sues (such as diesel) would be useful.

Foam inhibitors

The present invention allows for the use of supplemental amounts of an antifoam additive. Addition of a small amount of antifoam additive can be used to guarantee low foaming properties or to ensure very rapid foam vanishing times. Use of the teachings of the present invention allows for significantly less antifoam additives to be used to achieve a desired foam vanishing time. Preferably a foam inhibitor, if used at all, is a silicone-based foam inhibitor. Examples of silicone-based foam inhibitors include siloxane-polyoxyalkylene copolymers, for example those described in U.S. Patent No. 3,233,986, the disclosure of which is incorporated by reference herein, which comprise at least one siloxane block containing at least two siloxane groups of the formula R2SiOO 5(4-b) wherein R represents a halogen atom or an optionally halogenated hydrocarbon group and b represents from 1 to 3, and at least one polyoxylalkylene block containing at least two oxyalkylene groups.

Generally, the alkylene groups have 2 or 3 carbon atoms, and usually both ethylenoxy and propyleneoxy groups are present. Advantageously, the copolymer is a polymethylsiloxane-polyoxylalkylene copolymer, preferably of the general formula

(CH₃)3SiO[CH₃(A)SiO]_m[(CH3)2SiO]nSi(CH₃)3 in which A represents -(CH₂)pO(C2H₄O)χ(C₃H₆O)yZ in which Z represents hydrocarbyl, OC(hydrocarbyl) or, preferably, hydrogen, and in which the absolute values of m and n, and their ratios, and the values of p, x, and y, and their ratios, may vary widely but total values advantageously give a weight average molecular weight in the range of from 600 to 25000. The ratio of m:n is advantageously in the range of from 10:1 to 1 :20, or the value of n may be zero, and the ratio of x:y is advantageously in the range of from 1 :100 to 100:1 , preferably 20:80 to 100:1 , or one of x or y, but not both, may be zero.

Other anti-foams also useful in the present invention may be non-silicon containing such as those made by acylating polyamines as described in WO r 94/06894.

This invention allows one to reduce the amount of foam inhibitor used. Advantageously, the foam inhibitor is present in the fuel at concentrations of less than 10 ppm, more preferably less than 8 ppm, even more preferably less than 5 ppm by weight relative to the total amount of fuel. Most preferably, the foam inhibitor is not present at all.

Examples

The Examples that follow are intended to help illustrate aspects of the present invention and are not meant to limit the scope of the invention.

Example 1

Foam Measurements of Commercial Distillate Fuel Samples.

Fourteen commercial diesel fuel samples not containing antifoam additives were obtained from throughout Europe and measured to determine foaming properties. The foam height and foam vanishing time for the samples were determined using the AFNOR (Association Francaise de Normalisation) NF M 07-075 test (dated June 1997) which is incorporated by reference herein in its entirety. The samples were also measured for other characteristics. Results are shown in Table 1.

Example 2 Preparation of a Fischer Tropsch Distillate

The preparation of the Fischer-Tropsch distillate sample is described in

Example 1 of US Published Application 20040152930 which is incorporated by reference herein in its entirety and is Fuel A of Table Il of said reference. The Fischer Tropsch process used to make this sample is a Low Temperature Fischer Tropsch (LTFT) process using a cobalt catalyst and operated in the slurry bed mode. It is important for this invention that the Fischer Tropsch distillate not contain components which induce foaming or give long foam retention times. The foam release time (foam vanishing time) of the Fischer Tropsch distillate should be 15 seconds or less. Components to be minimized include heteroatoms, such as sulfur, nitrogen and oxygen. Preferably the Fischer Tropsch distillate will contain less than 1 ppm of sulfur and less than 1 ppm of nitrogen. Distillates directly from a Fischer Tropsch process can contain oxygenates, such as primary linear alcohols. These compounds are well known surfactants, and their composition should be minimized by hydroprocessing (hydrocracking, hydrotreating, hydroisomerization and combinations). The oxygenate content of the Fischer Tropsch distillate should be less than 100 ppm of oxygen, preferably less than 25 ppm of oxygen, more preferably less than 10 ppm oxygen, and very most preferably not detectable. US20040152930 describes methods for measuring the oxygen content of Fischer Tropsch distillates. The oxygenate content of the Fischer Tropsch distillate in US20040152930 was below the limit of detection, that is, less than 6 ppm. The oxygen content is expressed on both a water-free and air-free basis.

Example 3

Properties of a Petroleum Derived Distillate with High Form Formation

A petroleum-derived distillate fuel was obtained and tested. It was a non- additized (without additives) diesel fuel from the Belgium market and had the following properties shown in Table 2: Table 2

Property Value

Density, ASTM D4052 kg/I 0.8238

Sulfur, ISO 20884, ppm m/m 38.5

Kinematic Viscosity at 4O⁰C, ASTM D445-Aut, cSt 2.40

Lubricity at 6O⁰C, CEC F06A96microns 275

Electrical Conductivity, ASTM D2624 pS/m 169

Distillation, ASTM D-86 by LV%, ⁰CV⁰F

IBP 160.9/322

5 LV% 181.3/358

10 LV% 190.2/374

30 LV% 219.6/427

50 LV% 256.3/493

70 LV% 295.2/563

90 LV% 334.4/634

95 LV% 347.7/658

FBP 357.2/675

Evaporated LV% 98.4

Residue LV% 1.6

Loss LV% 0

This material is used in the later examples. Its foaming properties are described in Table 3 (Test A).

Example 4 Foam Formation Measurements

The foam height and vanishing time for various samples were determined by the AFNOR (Association Frangaise de Normalisation) NF M 07-075 test (dated June 1997). To determine the effect of blending of Fischer Tropsch derived distillate fuels, a blend was made of 70% volume petroleum derived distillate fuel (from Example 3) and 30% volume Fischer Tropsch derived distillate fuel (from Example 2). Duplicate measurements on the Fischer Tropsch derived distillate were obtained.

To determine the comparative effects of a commercial silicon antifoam agent, blends were prepared with 250 volume ppm of a multi-functional package containing a mixture of detergent, demulsifier, corrosion inhibitor, solvents and 1.24 wt% silicon antifoam additive. This is equivalent to 3 ppm weight silicon antifoam additive in the diesel fuel. The silicon antifoam additive is a commercial product supplied by Wacker. These results are shown in tests H, I₁ and J shown below.

Table 3

I est A snows tnat tne petroleum derived distillate fuel has foam properties that do not comply with Category three or four diesel fuels in the World Wide Fuel Charter. Both the vanishing time and the amount of foam exceed the maximum values. In comparison the Fischer Tropsch derived fuel meets both limits as shown by test B.

Tests C to G show that blending a Fischer Tropsch derived distillate fuel with a petroleum-derived distillate fuel significantly improves the foaming tendency, especially the most important foam vanishing time. A 20% blend of Fischer Tropsch derived distillate fuel in 80% petroleum-derived distillate fuel meets the foam vanishing time requirement of the Category three and four fuels in the World Wide Charter, and just meets the maximum amount of foam. The impact of blending the Fischer Tropsch distillate fuel on foam is dramatic. Addition of Fischer Tropsch distillate fuel reduces the vanishing time far more than what would be expected from a linear blend or even from the drop in the viscosity. Not wishing to be limited by theory it is speculated that the polar functions in the petroleum derived distillate contribute to foaming, and highly paraffinic nature of the Fischer Tropsch product disrupts them. However, the reduction in foam is highly non-linear and more than can be expected by a simple dilution of the polar species in the conventional diesel fuel.

Blends with the antifoam additive show it to be highly effective in reducing foaming, both in the vanishing time and the amount of foam. But even here the blending of a Fischer Tropsch distillate component leads to a lowering in the vanishing time for the petroleum derived fuel indicating that less of the antifoam agent would be needed to obtain a given foam value.

Example 5 - Foam Formation with Low Sulfur Diesel A diesel fuel containing less than 10 ppm by weight sulfur and conforming to emerging diesel fuel specifications was obtained. Properties are shown in Table 4. Table 4

Blends of this diesel fuel with the Fischer Tropsch distillate of Example 2 were made and evaluated for foam formation by the AFNOR NF M 07-075 test as shown in Table 5. Table 5

The results on blend of the low sulfur petroleum derived diesel with the Fischer Tropsch derived distillate fuel are very similar to those obtained from Example 4. Specifically, the petroleum derived distillate fuel has foam properties that do not comply with Category three or four diesel fuels in the World Wide Fuel Charter. Both the vanishing time and the amount of foam exceed the maximum values. Blends that contain a Fischer Tropsch derived distillate fuel show an improvement in the foam vanishing time, with blends that contain 30% Fischer Tropsch derived distillate having a vanishing time of less than 15 seconds.

Claims

What is claimed is:

1. A distillate fuel blend, comprising: at least 20 vol. % of a petroleum derived distillate having a foam vanishing time greater than 20 seconds; and

; at least 5 vol. % of a Fischer-Tropsch derived distillate having a foam vanishing time of less than 15 seconds; wherein the resultant distillate fuel blend has a foam vanishing time of 15 seconds or less in the absence of an antifoam additive.

2. A process for making a distillate fuel blend, comprising:

3. A distillate fuel blend according to Claim 1 wherein the petroleum derived distillate has a foam vanishing time in excess of 25 seconds.

4. A distillate fuel blend according to Claim 1 wherein the petroleum derived distillate has a foam vanishing time in excess of 30 seconds.

5. A distillate fuel blend according to Claim 1 wherein the petroleum derived distillate has a foam vanishing time in excess of 40 seconds.

6. A distillate fuel blend according to Claim 1 wherein the Fischer-Tropsch derived distillate has a foam vanishing time of less than 12 seconds.

7. A distillate fuel blend according to Claim 1 wherein the Fischer-Tropsch derived distillate has a foam vanishing time of less than 10 seconds.

8. A distillate fuel blend according to Claim 1 wherein the Fischer-Tropsch derived distillate has a foam vanishing time of less than 8 seconds.

9. A distillate fuel blend according to Claim 1 wherein the petroleum derived distillate has a foam vanishing time in excess of 50 seconds.

10. A distillate fuel blend according to Claim 1 wherein the resultant distillate fuel blend is diesel fuel.

11.A distillate fuel blend according to Claim 1 wherein the resultant distillate fuel blend is jet fuel.

12. A distillate fuel blend according to Claim 1 wherein the Fischer Tropsch derived distillate contains less than 1 ppm nitrogen, less than 1 ppm sulfur and less than 100 ppm oxygen as oxygenates.

13.A distillate fuel blend according to Claim 1 wherein the Fischer Tropsch derived distillate fuel contains less than 25 ppm oxygen as oxygenates.

14. A distillate fuel blend according to claim 1 comprising between 5 vol. % and 30 vol. % of a Fischer-Tropsch derived distillate.

15.A distillate fuel blend according to claim 1 comprising between 5 vol. % and 20 vol. % of a Fischer-Tropsch derived distillate.

16. The process of claim 2 wherein an antifoam additive is added to the distillate fuel blend having a foam vanishing time of 15 second or less.

17. A distillate fuel blend according to claim 1 , further comprising addition of an antifoam additive to the resultant distillate fuel blend.

18. The process of claim 2 wherein the Fischer-Tropsch derived distillate has a foam vanishing time of less than 12 seconds.

19. The process of claim 2 wherein the petroleum derived distillate has a foam vanishing time in excess of 25 seconds.

20. A distillate fuel blend according to Claim 1 wherein the resultant distillate fuel blend is a dual use fuel.