GB2197878A - Middle distillate compositions with reduced wax crystal size - Google Patents

Abstract

Distillate fuel oil boiling in the range 120 DEG C to 500 DEG C which has a wax content of at least 0.3 wt.% at a temperature of 10 DEG C below the Wax Appearance Temperature, the wax crystals at that temperature having an average particle size less than 4000 nanometres.

Description

Middle Distillate Compositions with Reduced Wax Crystal Size This invention relates to improved distillate fuel oil.

Mineral oils containing paraffin wax such as the distillate fuels used as diesel fuel and heating oil have the characteristic of becoming less fluid as the temperature of the oil decreases. This loss of fluidity is due to the crystallisation of the wax into plate-like crystals which eventually form a spongy mass entrapping the oil therein, the temperature at which the wax crystals begin to form being known as the Cloud Point, the temperature at which the wax prevent the oil pouring is known as the Pour Point.

It has long been known that various additives act as Pour Point depressants when blended with waxy mineral oils.

These compositions modify the size and shape of wax crystals and reduce the cohesive forces between the crystals and between the wax and the oil in such a manner as to permit the oil to remain fluid at a lower temperature so being pourable and able to pass through coarse filters.

Various Pour Point depressants have been described in the literature and several of these are in commercial use.

For example, US Patent No. 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, specifically heating oils, diesel and jet fuels.

Hydrocarbon polymeric pour depressants based on ethylene and higher alpha-olefins, e.g. propylene, are also known. US Patent 3,252,771 relates to the use of polymers of C16 to C18 alpha-olefins with aluminium trichloride/alkyl halide catalysts as pour depressants in distillate fuels of the "broad boiling", easy-to-treat types available in the United States in the early 1960's.

In the late 1960's, early 1970's, greater emphasis was placed upon improving the filterability of oils at temperatures between the Cloud Point and the Pour Point as determined by the more severe Cold Filter Plugging Point (CFPP) Test (IP 309/80) and many patents have since been issued relating to additives for improving fuel performance in this test. US Patent 3,961,916 teaches the use of a mixture of copolymers, to control the size of the wax crystals. United Kingdom Patent 1,263,152 suggests that the size of the wax crystals may be controlled by using a copolymer having a lower degree of side chain branching.

It has also been proposed in, for example, United Kingdom Patent 1,469,016, that the copolymers of di-n-alkyl fumarates and vinyl acetate previously used as pour depressants for lubricating oils may be used as co-additives 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/naleic anhydride copolymers, For example, U.S.

Patent 2,542,542 uses copolymers of olefins such as octadecene with maleic anhydride esterified with an alcohol such as lauryl alcohol as pour depressants and United Kingdom Patent 1,468,588 uses copolymers of C22-C28 olefins with maleic anhydride esterified with behenyl alcohol as co-additives for distillate fuels.

Similarly, Japanese Patent Publication 5,654,037 uses olefin/maleic anhydride copolymers which have been reacted with amines as Pour Point depressants.

Japanese Patent Publication 5,654,038 uses the derivatives of the olefin/maleic anhydride copolymers together with conventional middle distillate flow improvers such as ethylene vinyl acetate copolymers.

Japanese Patent Publication 5,540,640 discloses the use of olefin/maleic anhydride copolymers (not esterified) and states that the olefins used should contain more than 20 carbon atoms to obtain CFPP activity. United Kingdom Patent 2,192,012 uses mixtures of certain esterified olefin/maleic anhydride copolymers and low molecular weight polythene, the esterified copolymers being ineffective when used as sole additives.

The improvement in CFPP activity from incorporation of the additives of these Patents is achieved by modifying the size and shape of the wax crystals forming to produce mostly needle like crystals generally of particle size 10 microns or bigger typically 30 to 100 microns. In operation of diesel engines at low temperatures, these crystals do not pass through the vehicle paper fuel filters but form a permeable cake on the filter allowing the liquid fuel to pass, the wax crystals will subsequently dissolve as the engine and the fuel heats up, which can be by the bulk fuel being heated by recycled fuel. A build up of wax can, however, block the filters, leading to diesel vehicle starting problems and problems at the start of driving in cold weather similarly it can lead to failure of fuel heating systems.

We have now surprisingly found that waxy fuels having wax crystals of sufficiently small size at low tempertures to pass through paper main filters typically used in diesel engines may be obtained by the addition of certain additives.

The present invention therefore provides distillate fuel oil boiling in the range 1200C to 5000C which has a wax content of at least 0.5 wt.% at a temperature of 100C below the Wax Appearance Temperature, the wax crystals at that temperature having an average particle size less than 4000 nanometres.

The Wax Appearance Temperature (WAT) of the fuel is measured by differential scanning calorimetry (DSC). In this test a small sample of fuel (25 microlitres) is cooled at 20C/minute together with a reference sample of similar thermal capacity but which will not precipitate wax in the temperature range of interest (such as kerosene). An exotberm is observed when crystallisation commences in the sample. For example, the WAT of the fuel may be measured by the extrapolation technique on a Mettler TA 20003 Differential Scanning Calorimeter.

The wax content of the fuel is derived from the DSC trace by integrating the area enclosed by the base line and the exotherm down to the specified temperature. The calibration having been previously performed on a known amount of crystallising wax.

The wax crystal average particle size is measured by analysing a Scanning Electron Micrograph of a fuel sample at a magnification of between 4000 and 8000 and measuring the longest dimension of at least 40 out of 88 points on a predetermined grid. We find that providing the average size is less than 4000 nanometres the wax will begin to pass through the typical paper filters used in diesel engines together with the fuel although we prefer that the size be below 3000 nanometres, more preferably below 2000, even more preferably below 1500 nanometres most preferably below 1000 nanometres where the real benefits of passage of the crystals through the paper fuel filters is achieved. The actual size attainable depends upon the original nature of the fuel and the nature and amount of additive used but we have found that these sizes and smaller are attainable.

The ability to obtain such small wax crystals in the fuel results in significant benefit in diesel engine operability. This may be demonstrated by pumping stirred fuel through a diesel filter paper as used in a V.W. Golf or Cummins diesel engine at from 8 to 15 ml/second and 1.0 to 2.4 litres per minute per square metre of filter surface area at a temperature at least 50C below the wax appearance temperature with at least 0.5 wt% of the fuel being present in the form of solid wax. Both fuel and wax are considered to successfully pass through the filter if one or more of the following criteria are satisfied.

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

(ii) At least 60%, preferably at least 80%, more preferably at least 90 wt.% of the wax present in the original fuel, as determined by the DSC test previously described is found to be present in the fuel leaving the filter.

(iii) Whilst pumping 18-20 litres of fuel through the filter, the flow rate always remains at above 60% of the initial flow rate and preferably above 80%.

The portion of crystals passing through the vehicle filter and the operability benefit arising fron small crystals are highly dependant on crystal length although crystal shape is also significant. We find that cube-like crystals tend to pass through filters slightly more easily than do flat crystals and, when they do not pass, offer less resistance to fuel flow. Nonetheless the preferred crystal form is a flat form, which in principle allows more wax to be laid down as the temperature falls, and therefore more wax precipitates before the critical crystal length is reached than would a crystal of the same length in cube-like form.

The fuels of the present invention have outstanding benefits compared to previous distillate fuels improved in their cold flow properties by the addition of conventional additives. The fuels also have improved cold start performance at low temperatures not relying on recirculation of warm fuel to remove undesirable wax deposits. Furthermore the wax crystals tend to remain in suspension rather than settle and form waxy layers in storage tanks as occurs with fuels treated with conventional additives giving benefits in distribution.

In addition the fuels frequently have improved operability in the Cold Climate Chassis Dynamometer Test as compared with fuel containing conventional additives.

Distillate fuels within the general class of those boiling from 1200C to 5000C vary significantly in their boiling characteristics and wax contents and the extent to which the very small crystals can be obtained depends upon the nature of the fuel and especially the wax therein, and in some fuels it may not be possible to produce the extremely small crystals required by the present invention. Fuel characteristics may however be modified to enable such small crystals to be obtained by for example adjustment of refinery conditions to enable the use of suitable additives.

The additives we prefer to use are or salts or compounds of the general formula

in which Y is SO3(-) (+)NR33, SO2NR3- or SO3-; X is Y or -CON(R3)-, -C02(-) (+)NR3- -R4-Co.o- , -NR3CO-I -R40- , -R40 C0- , -N(COR3)- or (+)NR3 Z(-)-; 3 z(-) is - So3(-) or - co2(-); R1 and R2 are alkyl, alkoxy alkyl or polyalkoxy alkyl containing at least 10 carbon atoms in the main chain;; R3 is hydrocarbyl and may be the same or different and R4 nothing or is C1 to C5 alkylene and in

the carbon-carbon (C-C) bond is either a) ethylenically unsaturated when A and B may be alkyl, alkenyl or substituted hydrocarbyl groups or b) part of a cyclic structure which may be aromatic, polynuclear aromatic or cyclo-aliphatic.

The ring atoms in such cyclic compound are preferably carbon atoms, but could, however, include a ring N, S or 0 atom to give a heterocyclic compound.

Examples of aromatic based compound from which the additives may be prepared are

In which the aromatic group may be substituted; Alternatively they may be obtained from polycyclic compounds, that is those having two or more ring structures which can take various forms. They can be (a) condensed benzene structures, (b) condensed ring structures where none or not all rings are benzene, (c) rings joined "end-on", (d) heterocyclic compounds (e) non-aromatic or partially saturated ring systens or (f) three-dimensional structures.

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

The condensed ring structures where none or not all rings are benzene include for example Azulene, Indene, Hydroindene, Fluorene, Diphenylene. Compounds where rings are joined end-on include for example diphenyl.

Suitable heterocyclic compounds from which they may be derived include for example Quinoline, Pyrindine, Indole, 2:3 dihydroindole, benzofuran, coumarin, isocoumarin, benzothiophen, carbazole and thiodiphenylamine. Suitable non-aromatic or partially saturated ring systems include decalin (decahydronaphthalene),d -Pinene, cadinene, bornylene.

Suitable 3-dimensional compounds include for example norbornene, bicycloheptane (norbornane), bicyclo octane and bicyclo octene.

The two substituents X and Y must be attached to adjoining ring atoms in the ring when there is only one ring or to adjoining ring atoms in one of the rings where the compound is polycyclic. In the latter case this means that if one were to use naphthalene these substituents could not be attached to the 1,8- or 4,5positions, but would have to be attached 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 which has a hydrogen- and carbon-containing group or groups containing at least 10 preferably at least 12 carbon atoms. Such amines or salts may be prepared by reacting the acid or anhydride previously described 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 may be reacted with an alcohol containing at least 10 carbon atoms or a mixture of an alcohol and an amine.

The hydrogen- and carbon-containing groups in the substituents are preferably hydrocarbyl groups, although halogenated hydrocarbyl groups could be used, preferably only containing a small proportion of halogen atoms (e.g.

chlorine atoms), for example less than 20 weight per cent. The hydrocarbyl groups are preferably aliphatic, e.g. alkylene. They are preferably straight chain.

Unsaturated hydrocarbyl groups, e.g. alkenyl, could be used but they are not preferred.

The alkyl groups preferably have at least 10 carbon atoms, preferably 12 to 22 carbon atoms, for example 14 to 20 carbon atoms and are preferably straight chain or branched at the 1 or 2 positions. If branching exists in over 20% of the alkyl chains then the branches must be methyl. The other hydrogen- and carbon-containing groups can be shorter e.g. less than 6 carbon atoms or may if desired have 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 will preferably be an amide and the other will be an amine or dialkylammonium salts of the secondary amine.

The especially preferred additives are the amides or amine salts of secondary amines.

In order to obtain the fuels of this invention these additives will generally be used in combination with other additives and Examples of the other additives include those termed as "comb" polymers which have the general formula:

where 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 (mole ratio) J = H, R', Aryl or Heterocyclic group, R'CO.OR K = H, CO.OR', OCO.R', OR', CO2H L = H, R', CO.OR', OCO.R', Aryl, CO2H n = 0.0 to 0.6 (mole ratio) R ;C10 R' > C1 Another monomer may be terpolymerized if necessary Where these other additives are copolymers of alpha olefins and maleic anhydride they may conveniently be prepared by polymerising the monomers solventless or in s solution of a hydrocarbon solvent such as heptane, benzene, cyclohexane, or white oil, at a temperature generally in the range of from 200C to 1500C and usually promoted with a peroxide or azo type catalyst, such as benzoyl peroxide or azo-di-isobutyro-nitrile, under a blanket of an inert gas such as nitrogen or carbon dioxide, in order to exclude oxygen. It is preferred but not essential that equimolar anounts of the olefin and maleic anhydride be used although molar proportions in the range of 2 to 1 and 1 to 2 are suitable.Examples of olefins that may be copolymerised with maleic anhydride are l-decene, l-dodecene, l-tetradecene, l-hexadecene, 1-octodecene.

The copolymer of the olefin and maleic anhydride may be esterified by any suitble technique and although preferred it is not essential that the maleic anhydride be at least 50% esterified. Examples of alcohols which may be used include n-decan-l-ol, n-dodecan-l-ol, n-tetradecan-l-ol, n-hexadecan-l-ol, n-octadecan-l-ol.

The alcohols may also include up to one methyl branch per chain, for example, 1-methyl, pentadecan-l-ol, 2-methyl, tridecan-l-ol. The alcohol may be a mixture of normal and single methyl branched alcohols. Each alcohol may be used to esterify copolymers of maleic anhydride with any of the olefins. It is preferred to use pure alcohols rather than the commercially available alcohol mixtures but if mixtures are used then R1 refers to the average number of carbon atoms in the alkyl group, if alcohols that contain a branch at the 1 or 2 positions are used R1 refers to the straight chain backbone segment of the alcohol. When mixtures are used, it is important that no more than 15% of the R1 groups have the value R1+2.

The choice of the alcohol will, of course, depend upon the choice of the olefin copolymerised with maleic anhydride so that R + R1 is within the range 18 to 38.

The preferred value of R + R1 may depend upon the boiling characteristics 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 our European Patent Applications 0153176, 0153177, 85301047 and 85301048.

Other suitable comb polymers are the polymers and copolymers of alpha olefins and the esterified copolymers of styrene and maleic anhydride.

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

The preferred esters, ethers or ester/ethers useful in the present invention may be structurally depicted by the formula: R-0(A)-0-RB where R and RB are the same or different and may be i) n-alkyl ii) n-alkyl

iii) n-alkyl

iv) n-alkyl

the alkyl group being linear and saturated and containing 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 polyoxymethylene, polyoxyethylene or polyoxytrimethylene moiety which is substantially linear; some degree of branching with lower alkyl side chains (such as in polyoxypropylene glycol) may be tolerated but it is preferred the glycol should be substantially linear, A may also contain nitrogen.

Suitable glycols generally are 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 from 10-30 carbon atoms are useful for reacting with the glycols to form the ester additives and it is preferred to use a C18-C24 fatty acid, especially behenic acids. The esters may also be prepared by esterifying polyethoxylated fatty acids or polyethoxylated alcohols.

Polyoxyalkylene diesters, diethers, ether/esters and mixtures thereof are suitable as additives with esters preferred for use in narrow boiling distillates whilst minor amounts of nonoethers and monoesters may also be present and are often formed in the manufacturing process. It is important for additive performance that a major amount of the dialkyl compound is present. In particu'ar, stearic or behenic diesters or polyethylene glycol, polypropylene glycol or polyethylene/polypropylene glycol mixtures are preferred.

The additives used may also contain ethylene unsaturated ester copolymer flow improvers. The unsaturated monomers which may be copolymerised with ethylene include unsaturated mono and diesters of the general formula:

wherein R6 is hydrogen or methyl, R5 is a -00CR8 group wherein R8 is hydrogen or a C1 to C28, more usually C1 to C17, and preferably a C1 to Cg, straight or branched chain alkyl group; or R5 is a -COORg group wherein R8 is as previously described but is not hydrogen and R7 is hydrogen or -COORg as previously defined.The monomer, when R6 and R7 are hydrogen and R5 is -00CR8, includes vinyl alcohol esters of C1 to C29, more usually C1 to Cl8, monocarboxylic acid, and preferably C2 to C29, more usually C1 to C18, monocarboxylic acid, and preferably C2 to C5 monocarboxylic acid. Examples of vinyl esters which may be copolymerised with ethylene include vinyl acetate, vinyl propionate and vinyl butyrate or isobutyrate, vinyl acetate being preferred. When these are used, we prefer that the copolymers contain fron 5 to 40 wt.% of the vinyl ester, more preferably from 10 to 35 wt.% vinyl ester. They may also be mixtures of two copolymers such as those described in US Patent 3,961,916. It is preferred that these copolymers have a number average molecular weight as measured by vapour phase osmometry of 1,000 to 10,000, preferably 1,000 to 5,000.

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

Suitable anines include primary, secondary, tertiary or quaternary, but preferably are secondary. Tertiary and quaternary amines can only form amine salts. Examples of amines include tetradecyl amine, cocoamine, hydrogenated tallow amine and the like. Examples of secondary amines include dioctacedyl amine, methyl-behenyl amine and the like Arnine mixtures are also suitable and many amines derived fron natural materials are mixtures.The preferred amine is a secondary hydrogenated tallow amine of the formula HNR1R2 where in R1 and R2 are alkyl groups derived from hydrogenated tallow fat composed of approximately 4% C14, 31% C16 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, naphthalene dicarboxylic acid and the like.

Generally, these acids will have about 5-13 cabon atoms in the cyclic moiety. Preferred acids useful in the present invention are benzene dicarboxylic acids such as phthalic acid, isophthalic acid, and terephthalic acid.

Phthalic acid or its anhydride is particularly preferred. The particularly preferred compound is the amide-amine salt formed by reacting 1 molar portion of phthalic anhydride with 2 molar portions of di-hydrogenated tallow amine. Another preferred compound is the diamide formed by dehydrating this amine-amine salt.

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

where T = H or R' U = H, T or Aryl v = 1.0 to 0.0 (mole ratio) w = 0.0 to 1.0 (mole ratio) R1 is a normal alkyl group containing more than 10 carbon atoms.

These polymers may be made directly from ethyenically unsaturated monomers or indirectly by for example hydrogenating the polymer made from other monomers such as isoprene and butadiene.

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

The amount of additive required to produce the distillate fuel oil of this invention will vary according to the fuel but is generally 0.001 to 0.5 wt.%, for example 0.01 to 0.1 wt.% (active matter) based on the weight of fuel.

The additive may conveniently be dissolved in a suitable solvent to form a concentrate of from 20 to 90, e.g. 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 in which the size of the wax crystals in the fuel was measured by placing samples of fuel in 2 oz.

bottles in cold boxes held about 80C above fuel cloud point for 1 hour while fuel temperature stabilises. The box is then cooled at 10C an hour down to the test temperature, which is then held.

A pre-prepared filter carrier, consisting of a 10 mm diameter sintered ring, surrounded with a 1 mm wide annular metal ring, supporting a 200 nanometres rated silver membrane filter which is held in position by two vertical pins, is then placed on a vacuum unit. A vacuum of at least 80 kPa is applied, and the cooled fuel dripped onto the membrane from a clean dropping pipette until a small domed puddle just covers the membrane. The fuel is dropped slowly to sustain the puddle; after about 10-20 drops of fuel have been applied the puddle is allowed to draindown leaving a thin dull matt layer of fuel wet wax cake on the membrane. A thick layer of wax will not wash acceptably, and a thin one may be washed away. The optimal layer thickness is a function of crystal shape, with "leafy" crystals needing thinner layers than "nodular" crystals.It is important that the final cake have a matt appearance. A "shiny" cake indicates excessive residual fuel and crystal "smearing" and should be discarded.

The cake is then washed with a few drops of methyl ethyl ketone which are allowed to completely drain away. The process is repeated a number of times. When washing is complete the methyl ethyl ketone will disappear very quickly, leaving a "brilliant matt white" surface which will turn grey on application of another drop of methyl ethyl ketone.

The washed sample is then placed in a cold dessicator, and kept until ready for coating in the SEM. It may be necessary to keep the sample refrigerated to preserve the wax, in which case it should be stored in a cold box prior to transfer (in a suitable sample transfer container) to the SEM to avoid ice crystal formation on the sample surface.

During coating, the sample must be kept as cold as possible to minimise damage to the crystals. Electrical contact with the stage is best provided for by a retaining screw pressing the annular ring against the side of a well in the stage designed to permit the sample surface to lie on the instrument focal plane.

Electrically conductive paint can also be used.

Once coated, the micrographs are obtained in a conventional way on the Scanning Electron Microscope.

The photomicrographs are analysed to determine the average crystal size by fastening a transparent sheet with 88 points marked (as dots) at the intersections of a regular, evenly spaced grid 8 rows and 11 columns in size, to a suitable micrograph. The magnification should be such that only a few of the largest crystal are touched by more than one dot and a nagnification of 4000 to 8000 times has proved suitable. At each grid point, if the dot touches a crystal dimension whose shape can be clearly defined, the crystal may be measured. A measure of "scatter", in the form of the Gaussian standard deviation of crystal length with Bessel correction applied is also taken.

The wax content before and after the filter is measured using a Differential Scanning Calorimeter DSC (such as the du Pont 9900 series) capable of generating a plot with an area of about 100 cm2/1% of fuel in the form of wax with an instrument noise-induced output variation with a standard deviation less than 2% of the mean output signal.

The DSC is callibrated by using an additive to produce large crystals certain to be removed by the filter, running this calibration fuel at test temperature in the rig and measuring the WAT of the thus dewaxed fuel on the DSC. The samples of tank fuel and post-filter fuel to be tested are then analysed on the DSC, and for each fuel the area above base line down to the calibration fuel WAT determined.

The ratio of DSC area for post filter sample X 100% DSC area for tank sample is the % wax left after the filter.

The cloud point of distillate fuels was determined by the standard Cloud Point Test (IP-219 or ASTM-D 2500) other measures of the onset of crystallisation are the Wax Appearance Point (WAP) Test (ASTM D.3117-72) and the Wax Appearance Temperature (WAT) are measured by different scanning calorimetry using a Mettler TA 2000B differential scanning calorimeter.

The ability of the fuel to pass through a diesel vehicle main filter was determined in an apparatus consisting of a typical diesel vehicle main filter mounted in a standard housing in a fuel line; the Bosch Type as used in a 1980 v Golf diesel passanger car, and a Cummins FF105 as used in the Cummins NTC engine series are appropriate. A reservoir and feed system capable of supplying half a normal fuel tank of fuel linked to a fuel injection pump as used in the VFJ Golf is used to draw fuel through the filter from the tank at constant flowrate, as in the vehicle. Instruments are provided to measure pressure drop across the filter, the flow rate from the injection pump and the unit temeratures.

Receptables are provided to receive the pumped fuel, both "injected" fuel and the surplus fuel.

In the test the tank is filled with 19 kilogrammes of fuel and leak tested. When satisfactory, the temperature is stabilised at an air temperature 80C above fuel cloud point. The unit is then cooled at 30C/hour to the desired test temperature, and held for at least 3 hours for fuel temperature to stabilise. The tank is vigorously shaken to fully disperse the wax present; a sample is taken from the tank and 1 litre of fuel removed through a sample point on the discharge line immediately after the tank and returned to the tank. The pump is then started, with pump rpm set to equate to pump rpm at a 110 kph road speed. In the case of the VW Golf this is 1900 rpm, corresponding to an engine speed of 3800 rpm.

Pressure drop across the filter and flow rate of fuel from the injection pump are monitored until fuel is exhausted, typically 30 to 35 minutes.

If fuel feed to the injectors can be held at 2 ml/sec (surplus fuel will be about 6.5 - 7 ml/sec) the result is a "PASS". A drop in feed fuel flow to the injectors signifies a "BORDERLINE" result; zero flow a "FAIL".

Typically, a "PASS" result may be associated with an increasing pressure drop across the filter, which may rise as high as 60 kPA. Generally considerable proportions of wax must pass the filter for such a result to be achieved. A "GOOD PASS" is characterised by a run where the pressure drop across the filter does not rise above 10 kPa, and is the first indication that most of the wax has passed through the filter, an excellent result has a pressure drop below 5kPa.

Additionally, fuel samples are taken from "surplus" fuel and "injector feed" fuel, ideally every four minutes throughout the test. These samples, together with the pre-test tank samples, are compared by DSC to establish the proportion of feed wax that has passed through the filter. Samples of the pre-test fuel are also taken and SEM samples prepared from them after the test to compare wax crystal size and type with actual performance.

The additives used were Additive 1 The N,N-dialkyl ammonium salt of 2-daialkylamido benzene sulphonate where the alkyl groups are nC16-18 H33-37 prepared by reacting 1 mole of ortho-sulphobenzoic acid cyclic anhydride with 2 moles of di-(hydrogenated) tallow amine in a xylene solvent at 50% (w/w) concentration.

The reaction mixture was stirred at between 1000C and the refluxing temperature. The solvent and chemicals should be kept as dry as possible so as not to enable hydrolysis of the anhydride.

The produce was analysed by 500 MgHz Nulcear Magnetic Resonance Spectroscopy which confirmed the structure to be

Additive 2 A copolymer of ethylene and vinyl acetate content 17 wt.% molecular weight 3500 and a degree of side chain branching of 8 methyls per 100 methylene groups as measured by 500 MHz NMR.

Additive 3 A styrene-dialkyl maleate copolymer made by esterifying a 1:1 molar styrene-maleic anhydride copolymer with 2 moles of a 1:1 molar mixture of C12H250H and C14H290H per mole of anhydride groups were used in the esterification (slight excess, 5% alcohol used) step using p-toluene sulphonic acid as the catalyst (1/10 mole) in xylene solvent. which gave a molecular weight (Mn) of 50,000 and contained 3% (w/w) untreated alochol.

Additive 4 The dialkyl-ammonium salts of 2-N,N dialkylamido benzoate formed by mixing one molar proportion of phthalic anhydride with two molar proportions of di-hydrogenated tallow amine at 600C.

The results were as follows.

Example 1 Fuel Characteristics Cloud point -140C Wax Appearance Point -18.60C Initial Boiling Point 1780C 20% 236oC 90% 3180C Final Boiling Point 3550C Wax content at -250C 1.1 wt.% 250 p.p.m. of each of Additives 1, 2 and 3 were incorporated in the fuel and the test temperature was -250C. The wax crystal size was found to be 1200 nanometres long and above 90 wt.% of the wax passed through the Cummins FF105 filter.

During the test, passage of wax was further evidenced by observing the pressure drop over the filter, which only increased by 2.2 kPa.

Example 2 Example 1 was repeated and the wax crystal size was found to be 1300 nanometres and the maximum final pressure drop across the filter was 3.4 kPa.

Example 3 Fuel characteristics Cloud point OOC Wax Appearance Point -2.50C Initial Boiling Point 1820C 20% 2200C 90% 3540C Final Boiling Point 3850C Wax content at test temperature 1.6 wt%.

250 p.p.m. of each of Additives 1, 2 and 3 were used and the wax crystal size was found to be 1500 nanometres and about 75 wt% of the wax passed through the Bosch 145434106 filter at the test temperature of -8.50C. The maximum pressure drop across the filter was 6.5 kPa.

Example 4 Example 3 was repeated and found to give wax crystal size 2000 nanometres long of which about 50 wt.% passed through filter giving a maximum pressure drop of 35.3 k?a.

Example 5 The fuel used in Example 3 was treated with 400 ppm of Additive 1 and 100 ppm of a mixture of Additive 2 and tested as in Example 3 at -80C at which temperature the wax context 1.4 wt.%. The wax crystal size was found to be 2500 nanometres and 50 wt.% of the wax passed through the filter with a maximum final pressure drop of 67.1 kPa.

Comparative Example 6 The fuel used in Example 3 was treated with 500 ppm of a mixture of 4 parts of Additive 4 and 1 part of Additive 2 and tested at -80C, the wax crystal size was found to be 6300 nanometres and 13 wt.% of the wax passed through the filter.

This example is among the very best examples of the prior art, where excellent results are achieved without crystal passage.

A scanning electron micrograph of the wax crystals forming in the fuel of Examples 1 to 6 are Figures 1 to 6 hereof.

Examples 1-4, therefore show that if crystals can pass through the filter reliably and the excellent cold temperature performance can be extended to higher fuel wax contents than heretofore practicable and also at temperatures further below fuel Wax Appearance Point than heretofore practicable. This is without regard to fuel system considerations such as the ability of recycle fuel from the engine to warm the feed fuel being drawn from the fuel tank, the ratio of feed fuel flow to recycle fuel, the ratio of main filter surface area to feed fuel flow and the size and position of prefilters and screens.

Examples 1 to 3 show that for the filters tested crystal lengths below about 1800 nanometers result in dramatically better fuel performance.

Claims (5)

1 Distillate fuel oil boiling in the range 1200C to 5000C which has a wax content of at least 0.3 wt.% at a temperature of 100C below the Wax Appearance Temperature, the wax crystals at that temperature having an average particle size less than 4000 nanometres.

2 Distillate fuel according to Claim 1 in which the wax crystals have an average particle size less than

3000 nanometres.

3 Distillate fuel according to Claim 1 i-n which the wax crystals have an average particle size less than

2000 nanometres.

4 Distillate fuel according to Claim 1 in which the wax crystals have an average particle size less than

1500 nanometres.

5 Distillate fuel according to Claim 1 in which the wax crystals have an average particle size less than

1000 nanometres.