DE3751401T2 - Fuel additives. - Google Patents

Abstract

The use as an additive for wax containing distillate fuel of a compound containing at least 2 substituent groups and having a spacing and configuration such that they may occupy the positions of wax molecules in the intersection of the (001) plane and the (110) and/or (111) planes in crystals of the wax, the substituent groups being alkyl, alkoxy alkyl or polyalkoxy alkyl having at least 10 atoms in the main chain.

Description

Mineral oils containing paraffin wax, such as the distillate fuels used as diesel fuel and heating oil, have the property of becoming less fluid as the temperature of the oil drops. This loss of fluidity is due to the crystallization of the paraffins into platelet-shaped crystals which form a spongy mass and enclose the oil in it, the temperature at which the paraffin crystals begin to form being known as the cloud point and the temperature at which the paraffins prevent the oil from pouring being known as the pour point.

It has long been known that numerous additives act as pour point depressants when mixed with paraffinic mineral oils. These compositions modify the size and shape of the paraffin crystals and reduce the cohesive forces between the crystals and between the paraffin and the oil so that the oil remains fluid at lower temperatures and passes through coarse filters.

Various pour point depressants have been described in the literature and several are commercially available. For example, US-A-3,048,479 teaches the use of copolymers of ethylene and C1-C5 vinyl esters, e.g. vinyl acetate, as pour point depressants for fuels and in particular for heating oils, diesel and jet fuels. Polymeric hydrocarbon pour point depressants based on ethylene and higher alpha-olefins, e.g. propylene, are also known. US-A-3,252,771 relates to the use of polymers of C16-C18 alpha-olefins with aluminum trichloride/alkyl halide catalysts as pour point depressants in "broad boiling range" distillate fuels, which are easy to handle types and were available in the USA in the early 1960's. In the late 1960s and early 1970s, more emphasis was placed on improving the filterability of oils at temperatures between the cloud point and the pour point, as determined by the Cold Filter Plugging Point (CFPP) test (IP 309/80), and many patents have since been granted relating to additives for improving fuel performance in this test. US-A-3 961 916 teaches the use of a mixture of copolymers to control the size of the wax crystals. According to GB-A-1 263 152 the size of the wax crystals can be controlled by using a copolymer with a lower degree of side chain branching.

Furthermore, it has been proposed, for example in GB-A-1 469 016, that copolymers of di-n-alkyl fumarates and vinyl acetate, which have previously been used as pour point depressants for lubricating oils, can be used as co-additives together with ethylene/vinyl acetate copolymers in the treatment of distillate fuels with high final boiling points to improve their low temperature flow properties.

It has also been proposed to use additives based on olefin/maleic anhydride copolymers. For example, according to US-A-2 542 542, copolymers of olefins such as octadecene and maleic anhydride esterified with an alcohol such as lauryl alcohol are used as pour point depressants; furthermore, according to GB-A-1 468 588, copolymers of C22-C28 olefins and maleic anhydride esterified with behenyl alcohol are used as co-additives for distillate fuels. Similarly, according to JP-A-5 654 037, olefin/maleic anhydride copolymers reacted with amines are used as pour point depressants.

According to JP-A-5 654 038, derivatives of olefin/maleic anhydride copolymers are used together with conventional middle distillate flow improvers such as ethylene/vinyl acetate copolymers. JP-A-5 540 640 discloses the use of olefin/maleic anhydride copolymers (non-esterified) and states that the olefins used should contain more than 20 carbon atoms in order to obtain CFPP effectiveness. According to GB-A-2 192 015, mixtures of certain esterified olefin/maleic anhydride copolymers and low molecular weight polyethylene are used, whereby the esterified copolymers are ineffective when used as sole additives.

The improvement in CFPP effectiveness achieved by introducing the additives of these patents is due to a modification of the size and shape of the paraffin crystals formed, which are most often formed as needle-like crystals with a general particle size of 10 u or larger, typically 30 to 100 u. When diesel engines are operated at low temperatures, these crystals do not pass through the vehicle paper fuel filters, but rather form a permeable filter cake on the filter that allows the liquid fuel to pass through. The paraffin crystals subsequently dissolve when the engine and fuel are warmed up, which can occur due to the majority of the fuel being warmed by recirculated fuel. However, a buildup of paraffin can block the filters, causing problems starting diesel vehicles and problems starting in cold weather or when heating systems fail.

With the aid of a computer, it is possible to accurately calculate the geometry of n-alkane paraffin crystal lattices and the energetically preferred geometry of a potential additive molecule in a simple and convenient manner. The results become particularly clear when they are subsequently illustrated graphically using a plotter or drawing device. This procedure is known as "molecular modeling".

Computer programs suitable for this purpose are commercially available and a particularly useful set of programs is sold by Chemical Design Ltd., Oxford, England (Technical Director: E.K. Davies) under the name "Chem-X".

The growth of paraffin crystals occurs by association of individual paraffin molecules with their long sides to the edge of an existing paraffin platelet, as shown schematically in Figure 1. Association with the top or bottom of the platelet is energetically disadvantageous because in this case only one end of the paraffin chain molecule interacts with the existing crystal. Association occurs mainly in the crystallographic (001) plane (see Figure 2). The (001) plane contains the largest number of the strongest intermolecular bonds and therefore causes the crystal to form large, flat, rhombohetric plates (Figure 3). The next most stable side of the crystal is the (11x) plane (e.g. the (110) plane). The intermolecular association is strongest where the n-alkane molecules are closest together and thus the (110) disk is stronger than the (100) disk.

The crystal grows by extension of the (001) plane. It is therefore important to note that the edges of this plane are the fast-advancing sides, and they are the (11x) side (e.g. the (110) side) and the (100) side to a lesser extent. Consequently, growth is most controlled by the advance of the (11x) plane.

Since the "head-to-head" bonds are relatively weak, only one molecular (001) plane was considered for the purposes of the invention. Therefore, it can be assumed that the (110), (111) planes, etc. are equivalent for the present purposes, since the lattice spacings and relative orientations of neighboring molecules stacked side by side in the (001) plane are the same (Figure 4).

Figure 3, which is a photograph of a paraffin crystal plate, shows that the macroscopic appearance of the plate is dominated by the (001) plane. It is these platelets that Filters in fuel lines, such as those in vehicles, can become blocked.

According to the present invention, this problem is overcome by inhibiting or at least greatly reducing the crystal growth in the (001) plane and the (11x) plane, e.g. in the (110) and (111) directions.

This makes it possible to produce paraffinic fuels that have paraffin crystals of a sufficiently small size at low temperatures to pass through filters commonly used in diesel engines and fuel oil systems, and this is achieved by the addition of certain additives.

The present invention therefore provides the use of a compound as an additive for paraffin-containing distillate fuel, the compound containing at least two substituent groups which have a spacing and a configuration in the energetically preferred geometry of that compound such that they are capable of occupying the positions of paraffin molecules in the intersection line of the (001) plane and the (11x) planes, e.g. the (110) and/or (111) planes, and of inhibiting or at least greatly reducing crystal growth in the (001) plane and in the (110) and (111) directions in crystals of the paraffin, the substituent groups being at least two n-alkyl groups having more than 10 carbon atoms, the spacings of which are in the order of 0.45 to 0.55 nm (4.5 to 5.5 Å) and the dihedral angle between the local symmetry planes of the two n-alkyl groups is 75º to 90º.

The occupation of the planes of the crystal can be explained by referring to Figure 4, which shows an enlarged view of the crystal lattice of a paraffin plate. The viewing direction corresponds to the direction of the long axis of the paraffin chains. From each paraffin molecule, only two carbon atoms and four Hydrogen atoms (the latter shown only once) are visible because the other atoms lie beneath the atoms shown, since all carbon atoms of a molecule lie in a molecular plane of symmetry.

From Figure 4 it can be seen that the distances between two molecules in the (100) plane and the (110) plane are almost the same.

These distances are labeled "b" and "d" in Figure 4 and b = 0.49 nm (4.9 Å) and d = 0.45 nm (4.5 Å). The main difference between the (100) and (110) planes (or more rigorously the (11x) planes, where x is zero or an integer) is the relative orientation of the adjacent paraffin molecules. Within the (100) plane, the molecular symmetry planes of individual molecules are parallel to each other, i.e. the dihedral angle is 0º. In the crystallographic (110) plane, the dihedral angle between the molecular symmetry planes of adjacent molecules is about 82º.

According to the invention, the additives are intended to replace the positions of paraffin molecules designated by "C" and "D" in Figure 4. This is only possible if the distance between the substituents is about 0.45 to 0.50 nm (4.5 to 5.0 Å) in an energetically available conformation and the dihedral angle between the local symmetry planes of the two substituents is about 82º and preferably 75º to 90º.

A molecule can be modeled either by entering its known atomic coordinates into a computer or by building the structure according to the rules of chemical physics using the computer. The structure can then be improved, for example by:

(a) Calculation of partial charges on each atom from electronegativity differences (force field method) or quantum mechanical Procedures (e.g. CNDO, QCPE 141, Indiana University);

(b) optimization of the structure using molecular mechanics (cf. "Molecular Mechanics", U. Burkert and N.L. Allinger, ACS, 1982);

(c) minimizing the total energy by optimizing the conformation, i.e. by rotation about rotatable bonds. Care must be taken here because the environment on the paraffin crystal surface is different from that in the gas phase or in solution and it is possible that the additive molecules co-crystallize with the paraffin in a conformation that is not the most energetically favorable conformation in the gas phase. The fact that the additive cannot adopt a conformation that is hindered by steric or electronic factors must also be taken into account.

Using the following criteria, it can be checked whether an additive molecule fits into the (001) plane of the paraffin crystal lattice in the desired positions C and D:

(1) The distance between the substituents in the molecule must be approximately equal to the distance between two adjacent paraffin molecules in the (001) plane intersection with the (11x) plane, i.e. about 4.5 Å. The relative orientation of the substituents should have the orientation of the n-alkanes in the (110) direction, i.e. the dihedral angle between the local symmetry planes of the substituents should be approximately 82º. The distance and angle can be easily measured using the computer programs.

(2) The additive molecule should fit into the paraffin crystal lattice structure and "dock" to two free lattice sites.

Compounds that have previously been suggested as additives are certain derivatives of phthalic acid, maleic acid, succinic acid and vinylidene olefins. None of the compounds mentioned fulfill the criterion (1) given above. This is illustrated by the following selected examples.

(a) Phthalic acid diamide adopts, in the best case, the conformation shown in Figure 5, in which the distance between the substituents and the dihedral angle are too small. The conformation is energetically unfavorable due to steric hindrance. Further miniation leads to the twisted conformation shown in Figure 6.

(b) The situation is similar for maleic acid diamide, as can be seen in Figure 7. Due to steric hindrance, further minimization leads to the twisted conformation of Figure 8.

(c) Due to steric hindrance, succinic acid derivatives cannot adopt a conformation that comes close to the special arrangement according to the invention (see Figure 9). By rotating around the C-C single bond, the succinic acid snaps into the conformation shown in Figure 10.

The improvement in CFPP activity by introducing these previously known additives of the cited patents is achieved by modifying the size and shape of the wax crystals, mostly forming needle-like crystals with a general particle size of 10,000 nm or larger, typically 30,000 to 100,000 nm. When diesel engines are operated at low temperatures, these crystals do not pass through the vehicle paper fuel filters, but form a permeable cake on the filter that allows the liquid fuel to pass through. The wax crystals subsequently dissolve when the engine and fuel are warmed, which may be followed by the bulk of the fuel passing through recirculated fuel. However, a buildup of paraffin can block the filters, causing diesel vehicle starting problems and problems starting in cold weather. Similarly, it can cause failure of fuel heating systems.

The known additives acting as paraffin growth inhibitors are able to interact to some extent with the surface of a growing paraffin crystal. However, the energetically preferred geometries of these previously known additives do not match the positions of at least two neighboring paraffin molecules at the intersection of the (001) plane and the (11x) planes in paraffin crystals.

When an additive according to the invention approaches a growing paraffin crystal, the probability that it actually "docks" onto two free lattice sites at the intersection of the (001) plane and the (11x) plane is high because its substituent groups are energetically fixed in the required geometry. For a known additive, this probability is comparatively lower and in many cases it is zero.

Some of the known additives described here before and those described in EP-A-0 061 894 and EP-A-0 061 895 also have two substituent groups with more than 10 carbon atoms. However, in the energetically preferred geometry of these additive molecules, the substituent groups do not have the required arrangement, i.e. a distance of the order of 0.45 to 0.55 nm (4.5 to 5.5 Å) and a dihedral angle between the local symmetry planes of the two n-alkyl groups of 75 to 90º. For many known additives, it is therefore impossible to occupy the positions of two adjacent paraffin molecules at the intersection of the (001) plane and the (11x) planes of the paraffin crystals. In some cases, it may be possible to twist the geometry of the additive molecule so that the substituent groups have the required configuration. can assume. The higher the energy required for such a twist, the lower the probability that it will occur. In other cases, a large number of different geometries are equally energetically accessible. In such cases, it is also unlikely that the additive molecule will have the required geometry when it approaches a growing wax crystal. Because they do not, or usually do not, have the required geometry that allows them to occupy the positions of neighboring wax molecules at the intersection of the (001) plane and the (11x) planes, the known additives are considerably less effective in reducing or inhibiting the growth of wax crystals at low temperatures than the additives of the invention.

The following test procedures are used in this application.

The wax appearance temperature (WAT) of the fuel is determined using differential scanning calorimetry (DSC). In this test, a small sample of fuel (25 microlitres) is cooled at 2ºC per minute together with a reference sample of the same thermal capacity but which shows wax precipitation (such as kerosene) in the temperature range of interest. An exothermic phenomenon is observed as the sample begins to crystallise. For example, the WAT of the fuel can be measured using an extrapolation technique on a Mettler TA 2000B instrument.

The paraffin content of the fuel is derived from the DSC trace by integrating the area enclosed by the baseline and the exotherm curve up to the specified temperature. Calibration is previously performed with a known amount of crystallizing paraffin.

The average particle size of the wax crystal is determined by analyzing a scanning electron image of a fuel sample at a magnification of between 4,000 and 8,000 and measuring the longest dimension of at least 40 of 88 points on a given grid. It has been found that, provided the average size is < 4000 nm, the wax will begin to pass through typical paper filters, such as those used in diesel engines, together with the fuel, it being preferred that the size be below 3000 nm, and more preferably below 2500 and most preferably below 2000 nm, especially below 1000 nm, thereby achieving the actual benefits of passing the crystals through the paper fuel filters. The actual size achieved will depend on the original nature of the fuel and the type and amount of additive used, but it has been found that these sizes and even smaller are achievable.

The use of the additives of the invention allows such small wax crystals to be obtained in the fuel, which lead to a significant advantage in diesel engine operability. This can be demonstrated by pumping stirred fuel through a diesel filter paper such as that used in a VW Golf or a Cummins diesel engine at 8 to 15 ml/s and 1.0 to 2.4 l/min/m² filter surface at a temperature of at least 5°C below the wax appearance temperature, with at least 0.5% by weight of the fuel being in the form of solid wax. Both the fuel and the wax are considered to have passed successfully through the filter if one or more of the following criteria are met:

(i) When 18 to 20 litres of fuel have passed through the filter, the pressure drop across the filter is not more than 50 kilopascals (kPa), preferably 25 kPa, more preferably 10 kPa and most preferably 5 kPa.

(ii) When at least 60%, preferably at least 80%, and more preferably at least 90% of the paraffin present in the original fuel, as determined by the DSC test described above, is found in the fuel exiting the filter.

(iii) When pumping 18 to 20 litres of fuel through the filter, the flow rate always remains above 60% of the original flow rate and preferably above 80%.

The proportion of crystals that pass through the vehicle filter and the operability advantage resulting from the small crystals are largely dependent on crystal length, although crystal shape is also important. It has been found that cube-shaped crystals pass through filters somewhat more easily than flat crystals and that, if they do not pass through, they offer less resistance to fuel flow. Nevertheless, the preferred crystal shape is a flat shape which in principle allows more wax to be deposited as the temperature drops and more wax to precipitate before the critical crystal length is reached than would be the case with crystals of the same length in cubic form.

The fuels obtained by the methods of the present invention have outstanding advantages over previous distillate fuels which have been improved in their cold flow properties by the addition of conventional additives. For example, the fuels are operable at temperatures closer to the pour point and are not limited by the inability to meet the CFPP test because they either meet the CFPP test at significantly lower temperatures or eliminate the need to meet this test. The fuels also have improved cold start performance at low temperatures while not relying on the circulation of warm fuel to remove undesirable wax deposits. Furthermore, the wax crystals tend to tends to remain in suspension rather than settling and forming waxy layers in storage tanks, as happens with fuels treated with conventional additives.

Distillate fuels from the general class of those boiling from 120ºC to 500ºC vary considerably in their boiling characteristics, n-alkane distributions and paraffin contents. Fuels from northern Europe generally have lower final boiling points and cloud points than those from southern Europe. Paraffin contents are generally greater than 1.5% (at 10º below WAT). Similar fuels from other countries around the world vary in the same way with respect to climate, but paraffin content also depends on the source of the crude oil. A fuel derived from a Middle Eastern crude is likely to have a lower paraffin content than one derived from paraffinic feedstocks such as those from China and Australia.

The extent to which the very small crystals can be obtained depends on the nature of the fuel itself and with some fuels it may not be possible to produce extremely small crystals. If this situation exists, the fuel properties can be modified to enable such small crystals, for example by adjusting the refinery conditions and blending to allow the use of suitable additives.

It has been found that the paraffin precipitating from distillate fuels consists mainly of normal alkanes crystallizing in an orthorhombic unit cell via a rotator or hexagonal form, as described, for example, by A. Muller in Proc. Roy. Soc. A. 114, 542 (1927), ibid. 120, 437, 1928), ibid 127, 417, (1930), ibid. 138, 514, (1932), AE Smith in J. Chem. Phys., 21, 2229, (1953) and PW Teare in Acta. Cryst., 12, 294, (1959). As mentioned, the two factors important in fitting the additive molecules to the paraffin crystal structure are primarily the distance between the n-alkane chains in selected crystal planes.

The additive chains must occupy the lattice sites (more than one) at the intersection of the (001) plane and the (11x) planes of the crystal. The distances are on the order of 0.45 to 0.55 nm (4.5 to 5.5 Å) and the closer the additive chains are to these distances, the more effective the additive is. Secondly, the relative orientations of the additive molecular chains should preferably match those of the n-alkanes in the crystal. The n-alkyl chains of the additive must be able to closely match the intermolecular distances of the n-alkanes in the paraffin crystal along the (11x), e.g. the (110) and (111) planes, where they intersect with the (001) plane, and be able to adopt conformations that allow the chains to orient themselves at angles similar to those of the n-alkanes of the crystal planes mentioned above. It has been found that these distances and orientations of the chains on the additive molecule can best be achieved by arranging them to adjacent carbon atoms in a cyclic or ethylenically unsaturated compound, provided that they are in a cis orientation.

This adjustment is preferably also achieved along the length of the n-alkane chains of the paraffin and the total chain length of the additive is preferably of the same order of magnitude as the average chain length in the paraffin. The paraffin precipitate from the fuel or oil generally consists of a range of n-alkanes, hence the reference to the average dimensions.

The additives therefore contain n-alkyl chains that can co-crystallize (crystallize together) with the paraffin. It is It has been found that there should be two or more such chains per molecule and that they should be located on the same side of the additive molecule. It is desirable that the additive molecule have two distinct "sides". One "side" contains the co-crystallizable n-alkyl chains and the other "side" contains the smallest number of hydrocarbon groups possible to block or prevent further crystallization after the additive molecule has co-crystallized into the n-alkane lattice sites of the wax crystal. It is further desirable that this "blocking" group be located halfway or approximately halfway down the co-crystallizable n-alkyl chains of the additive.

Preferred additives correspond to the formula:

in the

-Y-R²

-SO₃(-)(+)NR³₃R², -SO₃(-)(+)HNR³₂R₂),

-SO₃(-)(+)H₂NR³R², -SO₃(-)(+)H₃NR²,

-SO₂NR³R² or -SO₃R²;

-X-R¹

-Y-R² or -CONR³R¹,

-CO₂(-)(+)NR³₃R¹, -CO₂(-)(+)HNR³₂R¹,

-CO₂(-)(+)H₂NR³R¹,-CO₂(-)(+)H₃NR¹,

-R⁴-COOR⁴, -NR³COR¹,

-R⁴OR¹, -R⁴OCOR¹, -R⁴R¹,

-N(COR³)R¹ or Z(-)(+)NR³₃R¹;

where -Z(1) is SO₃(-) or CO₂(-);

R¹ and R² are alkyl, alkoxyalkyl or polyalkoxyalkyl groups with at least 10 carbon atoms in the main chain,

R¹ and R² are alkyl, alkoxyalkyl or polyalkoxyalkyl groups with at least 10 carbon atoms in the main chain,

R³ is a hydrocarbon group, where each R³ may be the same or a different group, and R⁴ is either meaningless or is a C₁ to C₅ alkylene group, and

is such that the additive of the above formula is a cyclic compound, the ring atoms of such a cyclic compound preferably being carbon atoms, but can also contain an N, S or O atom in the ring to form a heterocyclic compound.

Examples of aromatic-based compounds from which the additives can be made are

in which the aromatic group may be substituted.

Alternatively, they can be obtained from polycyclic compounds, i.e. those with two or more ring structures that can take various forms. They can be (a) fused benzene structures, (b) fused ring structures in which none or not all of the rings are benzene rings, (c) rings connected at their ends, (d) heterocyclic compounds, (e) non-aromatic or partially saturated ring systems, or (f) three-dimensional structures.

Condensed benzene structures from which the compounds can be derived include, for example, naphthalene, anthracene, phenathrene and pyrene.

Fused ring structures in which no ring or not all rings are benzene rings include, for example, azulene, indene, hydroindene, fluorene and diphenylene. Compounds in which all rings are connected at their ends include, for example, diphenyl. Suitable heterocyclic compounds from which they can be derived include, for example, quinoline, pyridine, indole, 2,3-dihydroindole, benzofuran, coumarin, isocoumarin, benzothiophene, carbazole and thiodiphenylamine. Suitable non-aromatic or partially saturated ring systems include decalin (decahydronaphthalene), α-pinene, cadinene and bornylene. Suitable three-dimensional compounds include, for example, norbornene, bicycloheptane (norbornane), bicyclooctane and bicyclooctene.

The two substituents X and Y must be bonded to adjacent ring atoms in the ring if there is only one ring, or to adjacent ring atoms in one of the rings if the compound is polycyclic. In the latter case, this means that when using naphthalene, the substituents are not bonded to the 1,8 or 4,5 positions, but to the 1,2, 2,3, 3,4, 5,6, 6,7 or 7,8 positions.

These compounds are reacted to give the esters, amines, amides, half-esters/half-amides, half-ethers or salts used as the additives. Preferred additives are the salts of a secondary amine having a hydrogen and carbon-containing group or hydrogen and carbon-containing groups having at least 10, preferably at least 12 carbon atoms. Such amines or salts can be prepared by reacting the acid or anhydride described above with an amine or by reacting a secondary amine derivative with carboxylic acids or anhydrides. Removal of water and heating are generally necessary to prepare the amides from the acids. Alternatively, the carboxylic acid can be reacted with an alcohol containing at least 10 carbon atoms or with a mixture of an alcohol and an amine.

The hydrogen and carbon containing groups in the substituents are preferably hydrocarbon groups, although halogenated hydrocarbon groups can be used, preferably those containing only a small proportion of halogen atoms (e.g. chlorine atoms) such as less than 20 wt.%. The hydrocarbon groups are preferably aliphatic groups, e.g. alkylene groups. They are preferably straight chain. Unsaturated hydrocarbon groups, e.g. alkenyl groups, can be used but they are not preferred.

The alkyl groups preferably have at least 10 carbon atoms, preferably 10 to 22 carbon atoms, e.g. 14 to 20 carbon atoms, and are preferably straight chained or branched at the 1 or 2 positions. If branching exists in over 20% of the alkyl chains, the branches must be methyl groups. The other hydrogen and carbon containing groups may be shorter, e.g. contain less than 6 carbon atoms, or may, if desired, contain at least 10 carbon atoms. Suitable alkyl groups include methyl, ethyl, propyl, hexyl, decyl, dodecyl, tetradecyl, eicosyl and docosyl (behenyl). Suitable alkylene groups include hexylene, octylene, dodecylene and hexadecylene, but these are not preferred.

In the preferred embodiment where the intermediate is reacted with the secondary amine, one of the substituents is preferably an amide and the other is an amine or a dialkylammonium salt of the secondary amine.

The particularly preferred additives are the amides and amine salts of secondary amines.

These additives are generally used in combination with other additives and examples of the other additives include those referred to as "comb" polymers having the general formula

in which D = R, CO.OR, OCO.R, R'CO.OR or OR,

E = H or CH₃ or D or R',

G = H or D,

m = 1.0 (homopolymer) to 0.4 (molar ratio) is

J = H, R', aryl or a heterocyclic group or R'CO.OR,

K = H, CO.OR', OCO.R', OR or CO₂H,

L = H, R', CO.OR', OCO.R', aryl or CO₂H ,

n = 0.0 to 0.6 (molar ratio),

R ≥ C₁₀₀ and

R' ≥ C₁

Another monomer may be terpolymerized if required.

When these other additives are copolymers of α-olefins and maleic anhydride, they may conveniently be prepared by reacting the monomers solvent-free or in a solution of a hydrocarbon solvent such as heptane, benzene, cyclohexane or white oil at a temperature generally in the range of 20°C to 150°C and usually using a peroxide or azo catalyst such as benzoyl peroxide or Azodiisobutyronitrile can be polymerized under an inert gas atmosphere of, for example, nitrogen or carbon dioxide to exclude oxygen. It is preferred, but not necessary, that equimolar amounts of the olefin and maleic anhydride are used, although molar proportions in the range of 2 to 1 and 1 to 2 are suitable. Examples of olefins which can be copolymerized with maleic anhydride are 1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene and 1-octadecene.

The copolymers of olefins and maleic anhydride can be esterified by any conventional procedure and, although preferred, it is not essential that the maleic anhydride be at least 50% esterified. Examples of alcohols that can be used include n-decan-1-ol, n-dodecan-1-ol, n-tetradecan-1-ol, n-hexadecan-1-ol and n-octadecan-1-ol. The alcohols can also contain up to one methyl branch per chain, e.g., 1-methylpentadecan-1-ol and 2-methyltridecan-1-ol. The alcohol can be a mixture of normal and singly methyl branched alcohols. Any alcohol can be used to esterify copolymers of maleic anhydride with one of the olefins. It is preferred to use pure alcohols rather than the commercially available alcohol mixtures, but if mixtures are used, R¹ refers to the average number of carbon atoms in the alkyl group and if alcohols having branching at the 1- or 2-position are used, R¹ refers to the straight chain backbone segment of the alcohol. The choice of alcohol will of course depend on the choice of olefin copolymerized with maleic anhydride, with R + R¹ ranging from 18 to 38. The preferred value of R + R¹ may depend on the boiling properties of the fuel in which the additive is to be used.

These comb polymers may also be fumarate polymers and copolymers such as those described in European patent applications with publication numbers 0 153 176 and 0 153 177 Other suitable comb polymers are the polymers and copolymers of α-olefins and the esterified copolymers of styrene and maleic anhydride.

Examples of other additives which can be used together with the cyclic compound are the polyoxyalkylene esters, ethers, ester/ethers and mixtures thereof, and especially those containing at least one, preferably at least two linear, saturated C10 to C30 alkyl groups and a polyoxyalkylene glycol group having a molecular weight of 100 to 5,000, preferably 200 to 5,000, wherein the alkyl group in the polyoxyalkylene glycol contains 1 to 4 carbon atoms. These materials are the subject of European Patent 0 061 895 B. Other such additives are described in US-A-4 491 455.

The preferred esters, ethers or ester/ethers useful in the present invention can be structurally represented by the following formula:

R-O(A)-O-R",

in which R and R" are the same or different and

i) n-alkyl

ii) n-alkyl -

iii) n-alkyl - O - - (CH₂)n -

iv) n-alkyl - O - (CH₂)n - -

wherein the alkyl group is linear and saturated and contains 10 to 30 carbon atoms and A represents the polyoxyalkylene segment of the glycol in which the alkylene group has 1 to 4 carbon atoms such as a polyoxymethylene, polyoxyethylene or polyoxytrimethylene group which is substantially linear; some degree of branching with lower alkyl sides (such as in polyoxypropylene glycol) can be tolerated but it is preferred that the glycol be substantially linear. A may also contain nitrogen.

Suitable glycols are generally the substantially linear polyethylene glycols (PEG) and polypropylene glycols (PPG) having a molecular weight of about 100 to 5,000, preferably about 200 to 2,000. Esters are preferred and fatty acids containing 10 to 30 carbon atoms are suitable for reaction with the glycols to form the ester additives and it is preferred to use a C18 to C24 fatty acid, preferably behenic acids. The esters can also be prepared by esterifying polyethoxylated fatty acids or polyethoxylated alcohols.

Polyoxyalkylene diesters, diethers, ethers/esters and mixtures of these are suitable as additives, with diesters being preferred for use in narrow-boiling distillates, while small amounts of monoethers and monoesters may also be present and are often formed during manufacture. It is important for the additive behavior that a larger amount of the dialkyl compounds is present. In particular, stearic or behenic acid diesters or polyethylene glycol, polypropylene glycol or polyethylene/polypropylene glycol mixtures are preferred.

The additives may also contain ethylenically unsaturated ester copolymer flow improvers. The unsaturated monomers that may be copolymerized with ethylene include unsaturated mono- and diesters of the general formula:

in which R6 is hydrogen or a methyl group, R5 is a -OOCR8 group, where R8 is hydrogen or a C1 to C28, more usually C1 to C17, and preferably C1 to C8 straight or branched chain alkyl group, or R3 is a -COOR8 group, where R8 is as previously described but not hydrogen, and R7 is hydrogen or -COOR8 as previously defined. The monomer comprises, when R6 and R7 are hydrogen and R5 is -OOCR8, vinyl alcohol esters of C1 to C28, to C₂₉, more usually C₁ to C₁₈ monocarboxylic acid and preferably C₂ to C₂₉, more preferably C₁ to C₁₈ monocarboxylic acid and preferably C₂ to C₅ monocarboxylic acid. Examples of vinyl esters which may be copolymerized with ethylene include vinyl acetate, vinyl propionate and vinyl butyrate or isobutyrate, with vinyl acetate being preferred. When used, it is preferred that the copolymers contain from 5 to 40% by weight of the vinyl ester, more preferably from 10 to 35% by weight of the vinyl ester. They may also be blends of two copolymers such as those described in US-A-3,961,916. It is preferred that these copolymers have a number average molecular weight, as measured by vapor phase osmometry, of 1,000 to 10,000, preferably 1,000 to 5,000.

The additives may also contain other polar compounds, either ionic or non-ionic, which have the ability to act as wax crystal growth inhibitors in the fuels. Polar nitrogen-containing compounds have been found to be particularly effective when used in combination with the glycol esters, ethers or esters/ethers. These polar compounds are generally amine salts and/or amides formed by reacting at least one molar proportion of hydrocarbon-substituted amines with a molar proportion of hydrocarbon acid having 1 to 4 carboxylic acid groups or their anhydrides; they may also contain Esters/amides containing a total of 30 to 300, preferably 50 to 150 carbon atoms may be used. These nitrogen compounds are described in US-A-4 211 534. Suitable amines are usually long chain, primary, secondary, tertiary or quaternary C₁₂ to C₄₀ amines or mixtures thereof, but shorter chain amines may be used provided the resulting nitrogen compound is insoluble and it therefore normally contains a total of about 30 to about 300 carbon atoms. The nitrogen compound preferably contains at least one straight chain C₈ to C₂₄ alkyl segment.

Suitable amines include primary, secondary, tertiary or quaternary amines, but secondary amines are preferred. Tertiary and quaternary amines can only form amine salts. Examples of amines include tetradecylamine, cocoamine, hydrogenated tallow amine and the like. Examples of secondary amines include dioctadecylamine, methylbehenylamine and the like. Amine mixtures are also suitable and many amines derived from natural materials are mixtures. The preferred amine is a secondary hydrogenated tallow amine of the formula HNR1R2, in which R1 and R2 are alkyl groups derived from hydrogenated tallow fat and are composed of about 4% C14, 31% C16 and 59% C18.

Examples of suitable carboxylic acids (and their anhydrides) for preparing these nitrogen compounds include cyclohexane-1,2-dicarboxylic acid, cyclohexene-1,2-dicarboxylic acid, cyclopentane-1,2-dicarboxylic acid, nphthalenedicarboxylic acid, and the like. Generally, these acids have about 5 to 13 carbon atoms in the cyclic portion. Preferred acids useful in the present invention are benzenedicarboxylic acids such as phthalic acid, isophthalic acid, and terephthalic acid. Phthalic acid or its anhydride is particularly preferred. The most preferred compound is the amide-amine salt formed by reacting one molar portion of phthalic anhydride with two molar portions of dihydrogenated tallow amine. Another preferred compound is the diamide formed by dehydration of this amine-amine salt.

Hydrocarbon polymers can also be used as part of the additive combination to produce the fuels. These can be represented by the following general formula:

where T = H or R¹,

U = H, T or aryl,

v = 1.0 to 0.0 (molar ratio),

w = 0.0 to 1.0 (molar ratio) and

R¹ is alkyl.

These polymers can be prepared directly from ethylenically unsaturated monomers or indirectly, for example by hydrogenating the polymer prepared from other monomers such as isoprene and butadiene.

A particularly preferred hydrocarbon polymer is a copolymer of ethylene and propylene with an ethylene content between 50 and 60% (w/w).

The amount of additive required to prepare the distillate fuel oil will vary depending on the fuel, but will generally be from 0.001 to 0.5 wt.%, e.g. 0.01 to 0.1 wt.% (active ingredient), based on the weight of the fuel. The additive may conveniently be dissolved in a suitable solvent to give a concentrate of 20 to 90 wt.%, eg 30 to 80 wt.% in the solvent. Suitable solvents include kerosene, aromatic naphthas, mineral lubricating oils, etc.

The present invention is illustrated by the following examples, where the size of the wax crystals in the fuel was determined by placing samples of the fuel in 2 oz. bottles (about 5.9 x 10-4 m3) in cooling boxes which were maintained at about 8°C above the fuel cloud point for one hour while the fuel temperature stabilized. The box was then cooled to the test temperature at 1°C per hour and then held there.

A previously prepared filter support consisting of a 10 mm diameter sintered ring surrounded by a 1 mm wide circular metal ring holding a 200 nm silver membrane filter held in position by two vertical pins was placed in a vacuum unit. A vacuum of at least 80 kPa was applied and the cooled fuel dripped onto the membrane from a clean dropper until the membrane was covered by a small pool. The fuel dripped slowly to maintain the pool for a few minutes. After about 10 to 20 drops of fuel had been applied, the pool was allowed to pass through, leaving a very thin, dull, matte layer of fuel-wet paraffin cake on the membrane. A thick layer of paraffin cannot be washed acceptably and a very thin layer can be washed away. The optimum layer thickness is a function of crystal shape, with sheet-like crystals requiring thinner layers than nodular crystals. It is important that the final cake has a matte appearance. A shiny cake indicates excessive fuel residue and smearing of the crystals and should be discarded.

The cake was then washed with a few drops of methyl ethyl ketones, which were allowed to drain completely. The procedure was repeated several times. After washing was complete, the methyl ethyl ketone disappeared very quickly, leaving a "brilliant matte white" surface, which turned grey when another drop of methyl ethyl ketone was added. The washed sample was then placed in a cold desiccator and left until coating could be done in the SEM. It may be necessary to keep the sample under refrigeration to preserve the paraffin, in which case it should be stored in a cold box (in a suitable sample transport container) before transfer to the SEM to avoid ice crystal formation on the sample surface.

During coating, the sample must be kept as cool as possible to minimize damage to the crystals. Electrical contact with the stage is best achieved with a locking screw that presses the circular ring against the side of a recess in the stage designed to place the sample surface on the focal plane of the instrument. An electrically conductive lacquer can also be used.

After coating, the micrographs are obtained in the conventional manner by scanning electron microscopy. The micrographs are analyzed to determine the average crystal size by attaching a transparent sheet with 88 marked locations (like dots) at the intersections of a regular, evenly spaced grid of 8 rows and 11 lines in size to a suitable micrograph. The magnification should be such that only a few of the largest crystals are touched by more than one dot and 4,000 to 8,000 times has been found to be suitable. At any grid point, if the dot touches a crystal dimension whose shape can be clearly defined, the crystal can be measured. The scatter in the form of Gaussian Standard deviation of the crystal length determined with a Bessel correction.

The wax content upstream and downstream of the filter is measured using a differential scanning calorimeter (DSC) (such as du Pont's 9900 series) capable of producing a plot of approximately 100 cm2 per 1% of fuel in the form of wax with an instrument having a noise induced output variation with a standard deviation of less than 2% of the main output signal.

The DSC is calibrated using an additive that produces large crystals that are safely removed by the filter, this calibration fuel is run through the apparatus at a test temperature and the WAT of the thus dewaxed fuel is measured on the DSC. The samples of the tank fuel and the fuel behind the filter to be tested are then analyzed on the DSC and for each fuel the area above the baseline down to the WAT of the calibration fuel is determined.

The ratio of

DSC area for the sample behind the filter/DSC area for the tank sample x 100%

is the percentage of paraffin that is still present behind the filter.

The cloud point of the distillate fuel was determined using the standard cloud point test (IP-219 or ASTM-D 2500). Other measurements/metrics for the onset of crystallization are the paraffin appearance point (WAP) test (ASTM D.3117-72) and paraffin appearance temperature (WAT). The WAT is measured by differential scanning calorimeter using, for example, a Mettler TA 2000 differential scanning calorimeter.

The ability of the fuel to pass through a diesel engine main filter was determined in an apparatus consisting of a typical diesel engine main filter fitted into a standard housing in a fuel feed line; the Bosch type as used in a 1980 VW Golf diesel passenger car and a Cummins FF105 used in the range of Cummins NTC engines were suitable. A reservoir and feed system capable of supplying half the fuel of a normal fuel tank and connected to a fuel injection pump as used in the VW Golf were used to draw fuel from the tank through the filter at a constant flow rate as in the vehicle. Instrumentation was provided to measure the pressure drop across the filter, the flow rate from the injection pump and the temperatures of the unit. There were receiving tanks to hold the pumped fuel, both the injected fuel and the excess fuel.

In the test the tank was filled with 19 kg of fuel and checked for leaks. Once this was satisfactory the temperature was stabilised at an air temperature of 8ºC, the fuel cloud point. The unit was then cooled to the desired test temperature at 3ºC per hour and held for a minimum of three hours to stabilise the fuel temperature. The tank was vigorously shaken to completely disperse any paraffin present, a sample was taken from the tank and 1 litre of fuel was passed through a sampling point on the discharge line immediately behind the tank and returned to the tank. The pump was then started with the pump speed set to correspond to a road speed of 110 km per hour in terms of pump revolutions per minute. In the case of the VW Golf this is 1900 rpm, which corresponds to an engine speed of 3800 rpm. The pressure drop across the filter and the flow rate of fuel from the injection pump were recorded. until the fuel was used up, typically 30 to 35 minutes.

Provided that the fuel flow to the injectors could be maintained at 2 ml/s (with excess fuel, about 6.5 to 7 ml/s), the result was a "pass." A drop in the fuel flow to the injectors indicated a "borderline" result; no flow indicated a "fail."

Typically a "pass" result may be associated with an increasing pressure drop across the filter which may be as high as 60 kPa. Generally, significant amounts of wax must pass through the filter for such a result to be achieved. A "good pass" is a run in which the pressure drop across the filter does not exceed 10 kPa and is the first indication that most of the wax has passed through the filter, with an excellent result being a pressure drop below 5 kPa.

In addition, fuel samples were taken from the "excess" fuel and "injector feed" fuel, ideally every four minutes during the test. These samples were compared with pre-test samples from the tank using DSC to determine the proportion of feed paraffin that passed through the filter. Pre-test fuel samples were also used and SEM samples were made from the post-test fuel to determine paraffin crystal size and type under actual conditions.

The additives used were:

Additive 1

The N,N-dialkylammonium salt of 2-dialkylamidobenzenesulfonate, in which the alkyl groups were nC₁₆₋₁₈H₃₃₋₃₇, was prepared by reacting one mole of the cyclic anhydride of orthosulfobenzoic acid with two moles of di(hydrogenated)tallow amine in a xylene solvent at a 50% concentration (w/w). The reaction mixture was stirred between 100°C and the reflux temperature. The solvent and chemical compounds were kept as dry as possible to avoid hydrolysis of the anhydride.

The reaction product was analyzed by 500 MHz nuclear magnetic resonance spectroscopy, which revealed the structure as

was confirmed.

The molecular model of this compound is shown in Figure 11.

Additive 2

A copolymer of ethylene and vinyl acetate having a vinyl acetate content of 17 wt.%, a molecular weight of 3,500 and a degree of side chain branching of 8 methyl groups per 100 methylene groups as measured by 500 MHz NMR.

Additive 3

A styrene/dialkyl maleate copolymer prepared by esterifying a 1:1 molar styrene/maleic anhydride copolymer with two moles of a 1:1 molar mixture of C12H25OH and C14H29OH per mole of anhydride groups. A slight excess of alcohol (5%) was used in the esterification and the catalyst was p-toluenesulfonic acid (1/10 mole) in xylene solvent. The copolymer thus produced had a molecular weight (Mn) of 50,000 and contained 3% (w/w) of virgin alcohol.

Additive 4

The dialkylammonium salts of 2-N,N-dialkylamidobenzoate obtained by mixing one molar part of phthalic anhydride with two molar parts of dihydrogenated tallow amine at 60°C.

The results were as follows:

example 1

Fuel properties Cloud point -14ºC

Paraffin appearance temperature -18.6ºC

Initial boiling point 178ºC

20% 230ºC

90% 318ºC

Final boiling point 355ºC

Paraffin content at -25ºC 1.1 wt.%.

250 ppm of each of Additives 1, 2 and 3 were introduced into the fuel and the test temperature was -25ºC. The wax crystal size was measured to be 1200 nm long and more than 90 wt% of the wax passed through the Cummins FF105 filter.

During the test, the passage of paraffin was further demonstrated by observing the pressure drop across the filter, which increased by only 2.2 kPa.

Example 2

Example 1 was repeated, whereby the paraffin crystal size was determined to be 1300 nm and the maximum final pressure drop across the filter to be 3.4 kPa.

Example 3

Fuel properties

Cloud point 0ºC

Paraffin appearance temperature -2.5ºC

Initial boiling point 182ºC

20% 220ºC

90% 354ºC

Final boiling point 385ºC

Paraffin content at test temperature 1.6 wt.%.

250 ppm of each of Additives 1, 2 and 3 were used and the wax crystal size was determined to be 1500 nm with approximately 75% of the wax passing through the Bosch filter 145434106 at the test temperature of -8.5ºC. The maximum pressure drop across the filter was 6.5 kPa.

Example 4

Example 3 was repeated and it was found that the wax crystal size was 2000 nm long with about 50 wt.% passing through the filter giving a maximum pressure drop of 35.3 kPa.

Example 5

The fuel used in Example 3 was added with 400 ppm of Additive 1 and 100 ppm of a mixture of Additive 2 and tested as in Example 3 at -8ºC, at which temperature the wax content was 1.4 wt.%. The wax crystal size was 2500 nm and 50 wt.% of the wax passed through the filter at a maximum final pressure drop of 67.1 kPa.

When this fuel was used in the test apparatus, the pressure drop increased relatively quickly and the test was "failed". It is believed that this is because, as shown in the photograph, the crystals are flat and flat crystals do not pass through the filter and tend to cover the filter with a thin, impermeable layer. "Cuboid" (or "nodular") crystals, on the other hand, if they do not pass through the filters, collect in a relatively loose "cake" through which the fuel can still pass until the mass becomes so large that the filter is filled and the total thickness of the wax cake is so great that the pressure drop is again excessive.

Example 6 (comparison)

The fuel used in Example 3 was treated with 500 ppm of a mixture of four parts Additive 4 and one part Additive 2 and tested at -8ºC. The wax crystal size was 6300 nm and 13 wt.% of the wax passed through the filter.

This example is one of the best examples of the state of the art, with excellent results being achieved, apart from the passage of the crystals.

Scanning electron micrographs of the paraffin crystals formed in the fuel of Examples 1 to 6 are shown in Figures 1 to 6.

Examples 1 to 4 demonstrate that if the crystals can reliably pass through the filter, the excellent cold temperature performance can be extended to much higher fuel paraffin contents than has been possible heretofore and also to temperatures further below the paraffin appearance temperature than has been possible heretofore. This is possible without having to take into account the fuel system, such as the ability to recirculate fuel from the engine to warm up the stock fuel taken from the fuel tank, the ratio of stock fuel flow to recirculated fuel, the ratio of main filter surface area to the flow of fuel used, and the size and location of prefilters and screens.

These examples show that for the filters tested, crystal lengths below about 1800 nm lead to dramatically better fuel behavior.

Example 7

In this example, Additive 1 was added to a distillate fuel with the following properties:

Initial boiling point 180ºC

20% 223ºC

90% 336ºC

Final boiling point 365ºC

Paraffin appearance temperature 5.5 ºC

Cloud point -3.5ºC

For comparison purposes, the following additives were also added to the distillate fuel:

Additive A: A blend of ethylene/vinyl acetate copolymers, one of which was Additive 2 (1 part by weight) and the other (3 parts by weight) having a vinyl acetate content of 36 wt.%, a molecular weight (Mn) of 2000 and a degree of side chain branching between two and three methyl groups per 100 methylene groups as measured by 500 MHz NMR.

Additive B: A mixture of Additives 4 and 2, where the molar ratio was 4:1.

Additive C: The dibehenate of a polyethylene glycol blend with an average molecular weight of 600.

Additive D: An ethylene/propylene copolymer in which the ethylene content was 56 wt.% and the number average molecular weight was approximately 60,000.

The additives were added in the amounts given in the table below and the tests were carried out in accordance with the PCT, with details given below:

Programmed cooling test (PCT)

This is a slow cooling test to correlate with pumping of stored fuel oil. The cold flow properties of the fuel containing the additives were determined by the PTC as follows. 300 ml of fuel was cooled linearly at 1ºC per hour to the test temperature and then the test temperature was held constant. After two hours at the test temperature, approximately 20 ml of the surface layer was removed by suction to prevent the test from being affected by abnormally large wax crystals that had formed at the oil/air interface during cooling.

Paraffin which had settled in the bottle was dispersed by gentle stirring and then a CFPPT(1) filter assembly was inserted. The tap was opened to apply a vacuum of 500 mm of mercury and closed when 200 ml of the fuel had passed through the filter into a graduated receiving vessel. A "pass" was recorded if the 200 ml were collected within 10 seconds through a sieve of a specified size, or a "fail" if the flow rate was too slow, indicating that the filter had become blocked.

The sieve number passed through at the test temperature was recorded.

(1) Cold Filter Plugging Point Test (CFPPT) described in detail in "Journal of the Institute of Petroleum", Volume 52, No. 510, June 1966, pages 173-185.

Example 8

The fuel used in this example had the following properties:

ASTM-D86) Initial boiling point 190ºC

20% 246ºC

90% 346ºC

Final boiling point 374ºC

Paraffin appearance temperature -1.5 ºC

Cloud point +2.0ºC

It was treated with 1000 ppm active ingredient of the following additives:

(E) A mixture of Additive 2 (1 part by weight) and Additive 4 (9 parts by weight)

(F) The commercial ethylene/vinyl acetate copolymer additive sold by Exxon Chemicals as ECA 5920.

(G) A mixture of:

1 part Additive 1

1 part Additive 3

1 part Additive D

1 part Additive K.

(H) The commercial ethylene vinyl acetate copolymer additive sold by Amoco as 2042E.

(I) The commercial ethylene vinyl propionate copolymer additive sold by BASF as Keroflux 5486.

(J) No additive

(K) The reaction product of four moles of dihydrogenated tallow amine and one mole of pyromelitic anhydride. The reaction was carried out solventless at 150°C with stirring under nitrogen for six hours.

The following behavioral properties of these fuels were subsequently determined.

(i) The ability of the fuel to pass through the diesel fuel main filter at -9ºC and the percentage of wax that passed through the filter, giving the following results: Additive Time to Failure % of Paraffin Passed Minutes No Failure

(ii) The pressure drop across the main filter versus time, the results being shown graphically in Figure 12.

(iii) The wax deposition in the fuels was measured by cooling 100 ml of fuel in a graduated cylinder. The cylinder was cooled at 1ºC per hour from a temperature preferably 10ºC above the cloud point of the fuel but not less than 5ºC above the cloud point of the fuel to the test temperature which was then held for the specified time. The test temperature and exposure time depend on the application, i.e. diesel fuel and heating oil. It is preferred that the test temperature be at least 5ºC below the cloud point and the minimum cold exposure time to the test temperature be at least 4 hours. Preferably the test temperature should be 10ºC or more below the cloud point of the fuel and the exposure time should be 24 hours or more.

After the end of the exposure time, the graduated cylinder was examined and the extent of wax crystal deposition was determined visually as the height of a layer of wax above the bottom of the cylinder (0 ml) and expressed as a percentage of the total volume (100 ml). A clear fuel can be seen above the settled wax crystals and this form of measurement is often used to give an assessment of wax deposition.

Sometimes the fuel above the layer of settled paraffin crystals is cloudy or the paraffin crystals can be considered denser towards the bottom of the cylinder. In this case, a A more quantitative method of analysis is used. Here the top 5% (5 ml) of the fuel is carefully aspirated and stored, the next 45% is aspirated and discarded, the next 5% is aspirated and stored, the next 35% is aspirated and stored and finally the bottom 10% is collected at the bottom after heating to dissolve the paraffin crystals. These stored samples are referred to below as top, middle and bottom samples. It is important that the vacuum applied to remove the samples is quite low, ie 200 mm water pressure, and that the head of the pipette is placed just on the surface of the fuel to avoid currents in the liquid disturbing the concentration of paraffin in the various layers within the cylinder. The samples were then warmed to 60ºC for 15 minutes and analyzed for paraffin content by differential scanning calorimetry (DSC) as previously described.

In this case a Mettler TA 2000B DSC apparatus was used. A 25 µl sample was placed in the sample cell and regular kerosene in the reference cell. They were then cooled at 22ºC per minute from 60ºC to at least 10ºC, but preferably 20ºC, above the wax appearance temperature (WAT). They were then cooled at 2ºC per minute to approximately 20ºC below the WAT. A reference must be made with the unsettled, uncooled treated fuel. The extent of wax settling then correlates with the WAT (or ΔWAT = WAT of the settled sample - WAT of the original sample). Negative values indicate dewaxing of the fuel. Positive values indicate wax enrichment by settling. The wax content can can also be used as a measure of the settling from these samples. This is illustrated by % paraffin or Δ% paraffin (Δ% paraffin = % paraffin of the settled sample - % paraffin of the original sample) and again a negative value indicates dewaxing of the fuel and a positive value indicates paraffin enrichment by settling.

In this example, the fuel was cooled from +10ºC to -9ºC at 1ºC per hour and cold treated for 48 hours prior to testing. The results were as follows: WAT ºC Settled Sample Data Additive Visual Settling of Paraffin Top 5% Middle 5% Bottom 10% Total Cloudy Closer to Bottom 50% Clear Clear Above 100% Semi-Gel

(The results are also shown graphically in Figure 13). Δ WAT (ºC) (settled samples) WAT original (non-settled fuel) top 5% middle 5% bottom 10%

It should be noted that a significant reduction in WAT can be achieved by the most effective additive (G). Δ % WAT (settled samples) top 5% middle 5% bottom 10%

These results show that when the crystal size is reduced by the presence of additives, the wax crystals settle relatively quickly. For example, untreated fuels, when cooled below their cloud points, tend to show little wax crystal settling because the platelet-shaped crystals interlock and cannot sink freely in the liquid and a gel-like structure is built up, but when a flow improver has been added, the crystals can be modified to become less platelet-like in behavior and result in the formation of needles of a size in the range of tenths of a micrometer which can move freely in the liquid and settle relatively quickly. This wax crystal settling can cause problems in storage tanks and vehicle systems. Concentrated wax layers can be drawn off unexpectedly, particularly when the fuel level is low or the tank is agitated (eg when a vehicle turns corners), and filter blockage can occur.

If the paraffin crystal size can be further reduced to below 10 nanometers, the crystals will settle relatively slowly and paraffin settling can be avoided, which has advantages in terms of fuel performance compared to a fuel with settled paraffin crystals. If the paraffin crystal size can be reduced to below about 4 nanometers, the tendency of the crystals to settle within the storage tank period is virtually eliminated. If the crystal sizes are reduced to the preferred size of below 2 nanometers, the paraffin crystals will remain suspended in the fuel for the many weeks required in some storage tanks, with the problems of settling essentially eliminated.

(iv) The CFPP behaviour was as follows: Additive CFPP temperature (ºC) CFPP reduction

The average crystal size was: Additive Size (nanometer) thin platelets over

Claims (1)

1. Use of a compound which contains at least two substituent groups and a distance which have a configuration in the energetically preferred geometry of the compound such that they are able to occupy the positions of paraffin molecules at the intersection boundary of the (001) plane and the (11x) planes, e.g. the (110) and/or (111) planes, and to inhibit or at least greatly reduce the crystal growth in the (001) plane and the (110) and (111) directions in the crystals of the paraffin, wherein the substituent groups are at least two n-alkyl groups with more than 10 carbon atoms, the distances of which are in the size range of 0.45 to 0.55 nm (4.5 to 5.5 Å) and wherein the dihedral angle between the local symmetry planes of the both n-alkyl groups is 75º to 90º, as an additive for paraffin-containing distillate fuel.