DE3788585T2 - Liquid fuel compositions. - Google Patents

Description

This invention relates to distillate fuel compositions containing a flow improver.

Heating oils and other distillate petroleum fuels, e.g. diesel fuels, contain n-alkane waxes which at low temperatures tend to precipitate as large crystals in such a way that they form a gel structure which causes the fuel to lose its ability to flow. The lowest temperature at which the fuel will still flow is generally known as the pour point.

When the fuel temperature reaches or falls below the pour point and the fuel no longer flows freely, difficulties arise in transporting the fuel through flow lines and pumps, such as when attempting to transfer the fuel from one storage container to another by gravity or under pump pressure, or when attempting to feed the fuel into a burner.

The crystals falling out of solution also tend to clog fuel lines, screens and filters at temperatures above the pour point. These problems have been well recognized in the past and various additives have been proposed to lower the pour point of the fuel oil and reduce the size of the wax crystals. One function of such additives has been to change the nature of the crystals precipitating from the fuel oil and thereby reduce the tendency of the wax crystals to form a gel. Small size crystals are desirable so that the precipitated wax does not clog the fine mesh screens present in fuel transport, storage and distribution equipment. It is therefore desirable to obtain not only fuels with low pour points, but also oils that form small paraffin crystals so that clogging of the filters does not affect the flow of the fuel at low operating temperatures.

Effective wax crystal modification (determined by CFPP and other operability tests, as well as simulated performance and field performance) can be achieved by flow improvers, most commonly based on ethylene vinyl acetate copolymer (EVA), in distillates containing up to 4 wt.% n-alkanes at 10ºC below the cloud point, determined by gravimetric or DSC (differential scanning calorimetry) methods. Additional response in these distillates is normally stimulated by the refiner adjusting the ASTM-D86 distillation characteristics of the distillates to increase the final range from 90% to the final boiling point to differences between 20ºC and 25ºC.

It has been suggested in US-A-3,620,696 that the response of the low paraffin content middle distillate fuels available in the United States in 1968 to copolymers of ethylene and vinyl esters prepared according to FR-A-1,461,008 can be improved by incorporating a small amount of a paraffin wax to provide 0.03 to 2.0 wt.% paraffin having an average molecular weight within the range of 300 to 650. Similarly, US-A-3,640,691 suggests that the response of the same types of middle distillates to similar additives can be improved by adding a paraffinic distillate fraction containing normal alkanes higher than n-hexacosane and as high as n-tetracontane to provide 0.1 to 2 wt.% normal alkanes of C24 and higher. In these patents it is shown that as little as 0.1 wt.% and a maximum of 2.0 wt.% of the C24 and higher added paraffin improves the response.

These practices are not effective, however, when treating narrow-boiling distillates with high paraffin content, similar to those encountered in the Far East and Australia, which, although they exhibit similar distillation characteristics, have a much higher paraffin content (between 5 and 10% at 10ºC below the cloud point, determined by DSC or gravimetric analyses) and a different carbon number distribution particularly in the C₂₂ to C₂₈ range. Fuels which are particularly difficult to treat are those with a high paraffin content and a relatively low final boiling point, ie not higher than 370ºC, sometimes below 360ºC, which have a high paraffin content with a narrow carbon number distribution. The most difficult to treat are the fuels obtained from crude oils with a high paraffin content, such as those from Australian and Far Eastern crude oils, in which the total n-alkane content of the distillate can be greater than 20%, the total content being C₁₂ and higher n-alkanes, determined by gas-liquid chromatography.

More recently, it has been suggested in JP-A-615 811 586 that a middle distillate receptive to flow improvers can be obtained by adjusting the total paraffin content of the fuel to between 5.5 and 12 wt%, preferably by blending low and high paraffin content fuels, the paraffin content being the amount precipitated with methyl ethyl ketone from one gram of the fuel at -20°C. This technique is not a satisfactory indication of the paraffin content of the fuel treated with the additives, since it is the paraffin precipitated between the cloud point of the fuel and its serviceability point that is treated with the additives and that is important for the low temperature characteristics of the fuel. We have found that the ability of these fuels to respond to flow improvers is not dependent on the total paraffin content of the fuel.

A typical difficult-to-treat distillate fuel containing 5 to 10 wt.% paraffin at 10ºC below the cloud point and/or containing more than 20 wt.% n-alkanes C₁₂₋ has the following ASTM D-86 characteristics:

Initial boiling point (IBP) 212ºC

5% 234ºC

10% 243ºC

20% 25SºC

30% 263ºC

40% 279ºC

50% 288ºC

60% 298ºC

70% 303ºC

80% 321ºC

90% 334ºC

95% 343ºC

Final boiling point (FBP) 361ºC

We have discovered that, contrary to the recommendations of JP-A-615 811 586, the response of such distillate fuels to flow improvers can be improved by adding materials to broaden the carbon number distribution of the n-alkane content within a defined range.

According to the invention, a liquid fuel composition comprises a major weight proportion of a distillate fuel containing between 4 and 10 weight percent paraffin at 10°C below its cloud point and having a narrow n-alkane distribution, i.e., containing essentially no paraffins longer than n-triacontane (C30), from 0.001 to 2.0 weight percent, based on the weight of the distillate fuel, of a low temperature flow improver, and added n-alkanes providing C24 and higher alkanes in an amount of more than 0.35 weight percent of the fuel.

The invention also provides the use as a cold flow improver for a distillate fuel containing between 4 and 10% by weight of paraffin at 10°C below the cloud point and having a narrow carbon distribution, ie containing essentially no paraffins longer than n-triacontane (C₃₀), of a mixture of a distillate fuel-low temperature flow improver and added n-alkanes whose C₂₄ and higher amount to more than 0.35 wt.% of the weight of the fuel.

The flow improvers used in the present invention can be any of those commonly available, although we prefer to use the type comprising copolymers of ethylene and at least one second unsaturated monomer. The second unsaturated monomer can be another monoolefin, e.g. a C3 to C18 alpha-monoolefin, or it can be an unsaturated ester such as vinyl acetate, vinyl butyrate, vinyl propionate, lauryl methacrylate, ethyl acrylate or the like. The second monomer can also be a mixture of an unsaturated mono- or diester and a branched or straight chain alpha-monoolefin. Mixtures of copolymers can also be used, such as mixtures of a copolymer of ethylene and vinyl acetate with an alkylated polystyrene or with an acylated polystyrene. Alternative materials are aminosuccinic acid derivatives, esters such as polyacrylates or esterified maleic anhydride copolymers, poly-α-olefins, etc.

The preferred distillate fuel flow improver useful in this invention is a copolymer consisting of 1 to 40, and preferably 1 to 20, especially 3 to 20 molar parts of ethylene per molar part of the ethylenically unsaturated monomer, the latter monomer being a single monomer or a mixture of such monomers in any proportions. The polymer is oil soluble and has a number average molecular weight in the range of about 1,000 to 50,000, and preferably about 1,000 to about 5,000. Molecular weights can be determined by cryoscopic methods or by vapor phase osmometry, for example, using a Mechrolab vapor phase osmometer, Model 310A.

The unsaturated monomers, which may be homopolymerized or copolymerized with ethylene or with each other, include unsaturated acids, acid anhydrides and mono- and diesters of the general formula:

wherein R₁ is hydrogen or methyl; R₃ is a -OOCR₄ or -COOR₄ group wherein R₄ is hydrogen or a straight chain or branched C₁ to C₁₆, preferably C₁ to C₄ alkyl group and R₃ is hydrogen or -COOR₄. The monomer includes, when R₁ to R₃ are hydrogen and R₂ is -OOCR₄, vinyl alcohol esters of C₂ to C₁₇ monocarboxylic acids. Examples of such esters include vinyl acetate, vinyl isobutyrate, vinyl laurate, vinyl myristate, vinyl palmitate, etc. When R₂ is -COOR₄ such esters include C8 oxoalcohol acrylate, methyl acrylate, methyl methacrylate, lauryl acrylate, isobutyl methacrylate, palmityl alcohol ester of alpha-methacrylic acid, C13 oxoalcohol ester of methacrylic acid, etc. Examples of monomers in which R1 is hydrogen and R2 and R3 are -OOCR4 groups include mono-C12 oxoalcohol fumarate, diisopropyl maleate, dilauryl fumarate, ethyl methyl fumarate, fumaric acid, maleic acid, etc. When R2 is hydrogen, R1 is COOR4 and R3 is CH2COOR4 they include the itaconates.

Other unsaturated monomers copolymerizable with ethylene for preparing pour point depressants or flow improvers useful in the invention include branched or straight chain C3 to C16 alpha-monoolefins such as propylene, n-octene-1, 2-ethyldecene-1, n-decene-1, etc.

Small amounts, e.g. about 0 to 20 mole percent, of a third monomer or even a fourth monomer may also be included in the copolymers, such as a branched or straight chain C₂ to C₁₆ α-monoolefin, e.g. propylene, n-octene-1, n-decene-1, etc. Accordingly, for example, copolymers of 3 to 40 moles of ethylene with one mole of a A mixture of 30 to 99 mol.% unsaturated ester and 70 to 1 mol.% olefin can be used.

The copolymers formed are random copolymers consisting mainly of an ethylene polymer backbone over which hydrocarbon or oxygen-substituted hydrocarbon side chains are distributed.

The above-mentioned alcohols used to prepare the above-mentioned esters are isomeric mixtures of branched chain aliphatic primary alcohols prepared from olefins such as polymers and copolymers of C3 to C4 monoolefins which have been reacted with carbon monoxide and hydrogen in the presence of a cobalt-containing catalyst such as cobalt carbonyl at temperatures of about 150 to 205°C (300°F to 400°F) under pressures of about 70 to 210 bar (1000 to 3000 psi) to form aldehydes. The resulting aldehyde product is then hydrogenated to form the alcohol, the latter being recovered by distillation from the hydrogenated product.

It is also preferred that the copolymers have a low degree of side chain branching, in particular they contain less than 10 and preferably less than 8 methyl-terminated side groups (other than the ester groups) per 100 methylene groups, determined by NMR, in particular 500 MHz H¹ NMR analysis.

The flow improver is used in a concentration in the range of about 0.001 to about 2.0 weight percent, and preferably about 0.005 to about 0.2 weight percent, based on the weight of the distillate fuel being treated.

The second additive provides the n-alkanes higher than C₂₄ and preferably has a carbon number distribution of from about 20 to about 40. It is also preferable that the additive be predominantly linear alkanes, although it may contain a small amount of branched hydrocarbons. The additive may be in the form of a refinery stream, such as a heavy atmospheric gas oil, vacuum gas oil or heavy cracked gas oil, containing alkanes having carbon numbers in the required range. It is believed that the additive stimulates crystallization of the n-alkanes in the fuel and also co-crystallizes with the first n-alkanes to precipitate from the fuel. The preferred n-alkane distribution of the additive will therefore depend on the particular fuel. Although the C24 and higher n-alkane component should be greater than 0.35 wt.% based on the fuel, we prefer that it be greater than 0.5 wt.%.

Further additives may also be used to provide further improvements in low temperature properties, for example a diamide or preferably a half amide/half salt of a dicarboxylic acid or dicarboxylic anhydride such as phthalic anhydride and a secondary amine, wherein the alkyl groups preferably contain from 12 to 20 carbon atoms, may be added. A particularly preferred compound is the half amide/half amine salt of phthalic acid and di(hydrogenated tallow)amine - Armeen 2HT (approximately 4 wt% n-C14, 30 wt% n-C16 alkyl, 60 wt% n-C18 alkyl, the balance being unsaturated). The amount of diamide or half amide/half amine salt added is typically 0.001 to 2 wt.%, preferably 0.005 to 0.2 wt.%, based on the weight of the distillate fuel.

Examples of other additives are the glycol esters such as those defined in our EP-B-0 061 895, the esters and amines of maleic anhydride copolymers such as those defined in EP-A-0 214 786, polyolefins and chlorinated polyolefins and the amines or amides of alkyl succinic anhydride.

The cold flow properties of the distillate fuel can be further improved by adding a paraffin-naphthalene condensate. A typical condensate is prepared by chlorinating a paraffin containing n- and branched C₁₈ to C₃₉ (average C₂₆) paraffins to obtain a chlorinated paraffin containing about 15 wt.% chlorine. The chlorinated paraffin thus obtained is converted to chlorine via an alkylation reaction polymerized with naphthalene to give a condensate containing alternating paraffin and naphthalene units.

The amount of condensate added is typically 0.00005 to 0.1 wt.% based on the weight of the distillate fuel.

The additives of the invention can be sold as concentrates for incorporation into the fuel as such, such as a concentrate comprising a solution containing 30 to 70% by weight and preferably 40 to 60% by weight of a mixture of the copolymer of ethylene and another ethylenically unsaturated monomer and the n-alkane composition.

The following additives were used in the examples:

Additive 1 63 wt% solution of a blend of two ethylene-vinyl acetate copolymers marketed by Exxon Chemicals as ECA 8400. (This additive was not used alone.)

Additive 2 Additive 1 plus 10 wt.% of a paraffin-naphthalene condensate

Additive 3 Blend of (1) an ethylene-vinyl acetate copolymer (2 parts by weight), (2) a paraffin-naphthalene condensate and (3) one part by weight of a half amide/half amine salt of phthalic acid and di(hydrogenated tallow)amine (Armeen 2HT). The amount of paraffin-naphthalene condensate was 5% by weight of the total weight of copolymer (1) and half amide/half amine salt (2).

Additive 4 45% by weight solution of a highly branched ethylene-vinyl acetate copolymer with a molecular weight of about 2,000 and a vinyl acetate content of about 30% by weight, prepared according to FR-A-1 461 008.

Additive 5 Blend of ethylene-vinyl acetate copolymers and fumarate-vinyl acetate copolymers, marketed by Exxon as Paraflow 206.

Additive 6 Concentrate in an aromatic diluent of about 50% by weight of a mixture of two ethylene-vinyl acetate copolymers in a ratio of about 75% by weight wax growth inhibitor and about 25% by weight nucleating agent. The wax growth inhibitor consists of ethylene and about 38% by weight vinyl acetate and has a number average molecular weight of about 2500 to 3500. The nucleating agent consists of ethylene and about 16% by weight vinyl acetate and has a number average molecular weight of about 3000 (VPO). It is identified in GB-A-1 374 051 as Copolymer H.

example 1

To a high paraffin content distillate fuel with a cloud point of +6ºC, obtained from a Chinese crude oil with the following ASTM D-86 characteristics:

Initial boiling point (IBP) 212.8ºC

5% 234.8ºC

10% 243.8ºC

20% 255.8ºC

30% 263.4ºC

40% 279.1ºC

50% 288.8ºC

60% 298.6ºC

70% 303.3ºC

80% 321.0ºC

90% 334.8ºC

95% 343.8ºC

Final boiling point 361.0ºC

and a paraffin content at 10ºC below the cloud point of about 8 wt.% was added to 7 vol.% of a vacuum gas oil (VGO) having n- alkanes essentially in the C₂₅ to C₃₅ range and the following ASTM D-86 distillation characteristics:

Initial boiling point 252.0ºC

10% 301.5ºC

50% 358.0ºC

90% 435.0ºC

Final boiling point 480.0ºC

The n-alkane distributions of the fuel and vacuum gas oil were: n-Alkanes Fuel VGO

The base fuel contained 23.40 wt.% n-alkanes of C₁₂ and higher. The following blends were prepared using the mentioned distillates as base fuel: Base fuel, wt.% Vacuum gas oil D86 Distillation of blends Initial boiling point Final boiling point Added C₂₄ paraffins

The ability of the fuel to pass through filters was evaluated by the Cold Filter Plugging Point Test (CFPPT) which is carried out by the procedure detailed in the "Journal of the Institute of Petroleum", Vol. 52, No. 510, June 1966, pp. 173-185. Briefly, a 40 ml sample of the oil to be tested is cooled in a bath maintained at about -34ºC. Periodically (at each one degree Celsius drop in temperature, starting from not less than 5ºC above the cloud point) the cooled oil is tested for its ability to pass through a fine sieve in a specified time. This cold property is tested in an apparatus consisting of a pipette with an inverted funnel attached to the lower end positioned below the surface of the oil to be tested. A 350 mesh screen is stretched over the mouth of the funnel, having an area of approximately 2.9 cm² (0.45 square inch). Periodic tests are initiated by applying a vacuum to the top of the pipette, drawing oil through the screen up the pipette to a mark indicating 20 ml of oil. The test is repeated for each one-degree decrease in temperature until the oil can no longer fill the pipette to the mark indicating 20 ml of oil.

The test is repeated for each drop in temperature of one degree until the oil no longer leaves the pipette within 60 seconds. The temperature at which the last filtration started was recorded is noted as the CFPP (Cold Filter Plugging Point). CFPP response to Additive 2 (ºC)

With 1,250 ppm Additive 3 the CFPP point was -1ºC.

Example 2

In this example, the addition of a vacuum gas oil (VGO) with a cloud point of +12ºC having maximum n-alkane C₃₂ was compared with the addition of a heavy cracked distillate with a cloud point of +35ºC (maximum n-alkane C₃₃) to a base fuel with a cloud point of +3ºC made from Australian Bass Strait crude oil and having a wax content of 8.8 wt% wax measured by wax precipitation at a temperature 10ºC below the cloud point. The composition of the components was: VGO Heavy Cracked Base Fuel Distillation Type VGO heavy cracked base fuel Paraffin content, wt.% n-Alkane distribution (wt.% in fuel)

The base fuel contained 27.3 wt% alkanes of C₁₂ and higher. The following blends were prepared: Kerosene, wt% base fuel, VGO heavy cracked The D-86 distillation of the mixtures was as follows: Mixture Initial boiling point Final boiling point added C₂₄+n-alkanes The CFPP response to the following additives was as follows: Amount of Additive Mixture 1 Additive 4

Example 3

In this example, the addition of a component containing C32 to C33 n-alkanes to a base fuel with cloud point of +7°C was compared with the addition of heavy gas oil streams (HGO-1 and HGO-2) containing higher n-alkanes obtained from a paraffinic Chinese crude oil from Daqing. The components were as follows: Base fuel Distillation type Initial boiling point Final boiling point Base fuel n-alkane distribution

The base fuel contained 32 wt.% alkanes of C₁₂ and higher. The following blends were prepared: Base fuel, wt% D-86 distillation added C₂₄+n-alkanes CFPP value when treated with Additive 2 was (ºC)

Example 4

This example illustrates the improvement in response to flow improvers by replacing heavy atmospheric gas oil (HGO) with heavy cracked gas oil (HCO), base fuel 1 had a cloud point of -1ºC and base fuel 2 of -2ºC. The components used were as follows: Base Fuel 1 HGO Heavy Cycle HCO Base Fuel 2 Distillation Type Initial Boiling Point Final Boiling Point Paraffin Content at 10ºC below Cloud Point Base fuel 1 HGO Heavy cycle material n-alkane distribution

Base fuel 1 contained 22.8 wt% n-alkanes of C₁₂ and higher and base fuel 2 contained 27.6 wt%. The following blends were prepared: Base Fuel 1, wt.% Distillation D-86 Final Boiling Point Added C₂₄+n-Alkanes CFPP Response to Additive 5

Example 5

Using the base fuel 2 mentioned in Example 4, the following mixtures were prepared: Blends Kerosene, wt% Base Fuel 2, CFPP Response 200 ppm Additive 4 added C₂₄+n-alkanes

In this example, the additive amounts given are the actual polymer amounts.

The effect of using several other commercially available low temperature flow improvers was also evaluated and found to be as follows: ppm Active ingredient used Additive used Additive 3 Amoco 2042E Keroflux H BASF CE 5323 Bayer FI 1814 Hoechst Dodiflow 3592 Sumitomo FI 20 Elf 8320

Claims (16)

1. A liquid fuel composition comprising a major weight proportion of a distillate fuel containing at 10°C below the cloud point between 4 and 10 wt.% paraffin and substantially no paraffins longer than n-triacontane, 0.001 to 2.0 wt.% based on the weight of the distillate fuel of a low temperature flow improver and added n-alkanes providing added C₂₄ and higher alkanes in an amount greater than 0.35 wt.% based on the weight of the fuel. 2. A composition according to claim 1, wherein the distillate fuel contains about 8% paraffin by weight at 10°C below the cloud point. 3. Composition according to claim 1 or 2, wherein the amount of flow improver is 0.005 to 0.2 wt.% based on the weight of the distillate fuel. 4. Composition according to one of the preceding claims, in which the added n-alkanes have a carbon number distribution of 20 to 40. 5. A composition according to any preceding claim, wherein the amount of C₂₄ and higher n-alkanes added to the distillate fuel is greater than 0.5% by weight of the distillate fuel. 6. A composition according to any preceding claim, wherein the n-alkanes are added by mixing the distillate with a vacuum gas oil, a heavy cracked gas oil or a heavy atmospheric gas oil. 7. A composition according to claim 6, wherein 5 to 10 wt.% vacuum gas oil or heavy atmospheric gas oil is used. 8. A composition according to claim 6 or 7, wherein vacuum gas oil is used which comprises n-alkanes substantially in the C₂₅ to C₃₅ range. 9. A composition according to claim 6 or 7, wherein heavy atmospheric gas oil is used which comprises n-alkanes substantially in the C₁₄ to C₃₇ range. 10. A composition according to any preceding claim, wherein the fuel has a final boiling point below 370°C. 11. Use as a cold flow improver for distillate fuel containing between 4 and 10% by weight of paraffin at 10ºC below the cloud point and essentially no paraffins longer than n-triacontane, of (a) a mixture of n-alkanes providing at least 0.35 wt%, based on the weight of the fuel, of C₂₄ and higher n-alkanes and (b) 0.001 to 2% by weight, based on the weight of the fuel, of a low temperature flow improver. 12. Use according to claim 11, which provides at least 0.5 wt.% added C₂₄ and higher alkanes. 13. Use according to claim 11 or 12, wherein the n-alkanes are added as a component in vacuum gas oil or heavy gas oil. 14. Use according to any one of claims 11 to 13, wherein the n-alkanes contain 20 to about 40 carbon atoms. 15. A liquid fuel composition containing a major proportion by weight of a distillate fuel which at 10ºC is below the cloud point between 4 and 10 weight percent paraffin and essentially no paraffins longer than n-triacontane, 0.001 to 2.0 weight percent, based on the weight of the distillate fuel, of a copolymer of ethylene and an ethylenically unsaturated monomer and 5 to 10 volume percent, based on the volume of distillate fuel and copolymer, of a vacuum gas oil, heavy cracked gas oil or heavy atmospheric gas oil. 16. A process for improving the cold flow properties of a distillate fuel containing between 4 and 10 weight percent paraffin and substantially no paraffins longer than n-triacontane at 10°C below the cloud point by adding thereto from 0.001 to 2 weight percent, based on the weight of the fuel, of a low temperature flow improver and n-alkanes providing C24 and higher alkanes in an amount of more than 0.35 weight percent, based on the weight of the fuel.