DE3382624T3 - Intermediate distillate preparations with improved flow behavior at low temperature. - Google Patents

Abstract

The flow properties of a distillate petroleum fuel oil whose 20% and 90% distillation points differ within the range of from 65 to 100 DEG C, and/or whose 90% to final boiling point is 10 to 20 DEG C is improved by the inclusion of a copolymer of ethylene and a vinyl ester of a carboxylic acid containing 1 to 4 carbon atoms containing 32 to 35 wt % of the vinyl ester and having a number average molecular weight of 1000 to 6000.

Description

Mineral oils that contain paraffin waxes have the property of becoming less fluid when the temperature of the oil drops. This loss of fluidity is due to the crystallization of the paraffin into plate-like crystals, which eventually form a spongy mass in which the oil is trapped.

It has long been known that various compositions act as paraffin crystal modifiers when blended with paraffin-containing mineral oils. These compositions change the size and shape of the paraffin crystals and reduce the adhesive forces between the paraffin and the oil in such a way that the oil remains fluid at a lower temperature.

Various pour point depressants have been described in the literature and several of them are used commercially. For example, US-A-3,048,479 teaches the use of copolymers of ethylene and C3-C5 vinyl esters, e.g. vinyl acetate, as pour point depressants for fuels, especially 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-3,961,916 teaches the use of a mixture of copolymers, one of which is a wax nucleating agent and the other a growth inhibitor, to control the size of the wax crystals.

Similarly, British Patent 1263152 suggests that the size of the wax crystals can be controlled by using a copolymer with a lower level of lateral branching.

With the increasing diversity of distillate fuels, fuel types have emerged that cannot be treated with existing additives or require uneconomically high levels of additives. A special group of fuels that exhibit such problems are those which have a relatively narrow boiling range. Fuels are often characterized by their initial boiling point (IBP), final boiling point (FBP) and the intermediate temperatures at which certain volume percentages of the initial fuel are distilled. Fuels whose 20% to 90% distillation points vary in the range 70 to 100ºC (ASTM D86) and whose 90% boiling temperature is generally 10 to 30ºC, particularly 10 to 25ºC, below the final boiling point have proven particularly difficult to treat, sometimes being virtually unaffected by additives or, on the other hand, requiring very high levels of additives. All distillations referred to here are to ASTM D86.

Furthermore, the increase in crude oil costs has made it important for the refiner to increase his production of distillate fuels and to optimize his processing operations by using what is known as sharp fractionation, which in turn results in distillate fuels that are difficult to treat with conventional additives or require a level of treatment that is economically prohibitive. Typical sharp fractionated fuels have a 90% to final boiling point of 10 to 20ºC, usually with a 20 to 90% boiling range of 90 to 110ºC. Both types of fuel have final boiling points above 350ºC, generally a final boiling point in the range of 350ºC to 375ºC and particularly 350ºC to 370ºC.

The copolymers of ethylene and vinyl acetate which found widespread use for flow improvement of the previously widely available distillate fuels generally contained up to 30 wt.% vinyl acetate when the additive was used to control the size of the wax crystals forming in the fuel, or around 36 wt.% or more vinyl acetate when their primary function was to lower the pour point of the distillate fuel. We have not found that one of these types of additives is effective in treating the narrow boiling and/or sharply fractionated fuels described above.

EP-A-61 894 describes, inter alia, the improvement of the flow properties of a distillate fuel whose 20% and 90% distillation points differ by 106ºC and whose 90% boiling point temperature is 31ºC below its final boiling point temperature of 375ºC; this fuel is described on page 14 as "Fuel 3". The document describes the improvement that results from adding "Polymer 15" to the fuel, the polymer being a mixture of about 75% by weight of a wax growth inhibitor consisting of a copolymer of ethylene and about 38% by weight of vinyl acetate and having a number average molecular weight of about 1800 and 25% by weight of a wax nucleating agent consisting of a copolymer of ethylene and about 16% by weight of vinyl acetate and having a number average molecular weight of about 3000.

Polymer 15 is described in the above-mentioned US-A-3,961,916, where the inhibitor is referred to as Copolymer B and the nucleating agent as Copolymer H. This polymer is considered to be one of the most preferred additives disclosed in this document.

We have found that the above-mentioned known additives are not sufficiently effective in improving the flow properties of the narrower boiling distillate fuel oils referred to above. However, we have found that certain blends of copolymers of ethylene and vinyl esters of carboxylic acids are particularly effective in treating these fuels.

The present invention therefore relates to the use as an additive for improving the flow properties of a distillate petroleum fuel oil, whose 20% and 90% distillation points differ in the range of 70 to 100°C, whose 90% boiling point is 10 to 30°C below the final boiling point and whose final boiling point is above 350°C, the additive comprising a mixture of two copolymers of ethylene and a vinyl ester of a carboxylic acid containing 1 to 4 carbon atoms, one copolymer of which is a growth inhibitor while the other is a wax nucleating agent, the mixture containing at least 10 parts by weight of the growth inhibitor for each part of the wax nucleating agent and an average of 32 to 35% by weight of the vinyl ester and having a number average molecular weight of 1000 to 6000.

The present invention further provides a distillate fuel having 20% and 90% distillation points differing by 70°C to 100°C, having a 90% boiling point 10°C to 30°C below the final boiling point and having a final boiling point above 350°C, comprising 50 to 500 ppm of a mixture of two copolymers of ethylene and a vinyl ester of a carboxylic acid containing 1 to 4 carbon atoms, one copolymer being a growth inhibitor and the other being a wax nucleating agent, the mixture containing at least 10 parts by weight of the growth inhibitor for each part of the wax nucleating agent and an average of 32 to 35% by weight of the vinyl ester, and having a number average molecular weight of 1000 to 6000. contains.

The additive is a mixture of two copolymers, which may or may not contain the same vinyl ester. Such an additive mixture is particularly suitable for treating the above-mentioned fuel type, as it allows additional flexibility.

In a preferred embodiment of the present invention, the additive comprises 10 to 15 parts by weight of a synthetic polymeric material having the properties of a paraffin growth inhibitor in the fuel for each part of a synthetic polymeric material having the properties of a paraffin growth stimulator, wherein the paraffin growth inhibitor and the paraffin growth stimulator are copolymers of ethylene and vinyl esters of carboxylic acids containing 1 to 4 carbon atoms, the average ester content of the copolymers being in the range of 23 to 35 wt.% and their average molecular weight (number average) being in the range of 1000 to 6000.

In a further embodiment of the present invention, the fuel contains 50 to 500 ppm of the additive mixture of the above-mentioned preferred embodiment.

The fuels used in the present invention may have final boiling points between 350°C and 375°C, usually between 350°C and 370°C.

The paraffin growth stimulator or nucleating agent is a synthetic polymeric material which is soluble in the distillate at temperatures substantially above the saturation temperature, but which progressively precipitates as small particles as the distillate cools as the temperature of the distillate approaches the saturation point, e.g. from a point slightly above (e.g. 10ºC above, preferably 5ºC above) the saturation temperature. The "saturation temperature" is defined as the lowest temperature at which the solute, e.g. paraffin, cannot be crystallized from solution even when using known crystallization-inducing methods. It is believed, although not certain, that as cooling continues, additional nucleating particles will precipitate in a more or less continuous manner. These additional particles will act as nucleating agents for continued paraffin crystallization which would in fact substantially prevent supercooling of the distillate. The advantages of the constantly formed fresh nucleating particles are that the Supersaturation of the distillate with n-paraffins is kept at the lowest possible level so that it is easier for a molecule of the growth inhibitor to incorporate itself into the growth center of growing crystals and thereby stop further growth.

It is believed that the inhibitory effect of a growth inhibitor is due to the presence of bulky groups in its molecule. Additional nucleating agent should separate out to replace the deactivated growth centers. The paraffin growth inhibitor is more soluble in the distillate than the nucleating agent and it acts as a growth inhibitor when the paraffin crystal forms.

The nucleating agent should not be insoluble in the distillate at elevated temperatures, nor should it begin to precipitate at a temperature substantially above that at which wax crystallization can occur. If nucleating agents precipitate at a temperature substantially above that at which crystallization can occur, they tend to settle to the bottom of the kettle containing the distillate rather than remaining dispersed in the distillate. This factor is particularly important when the distillate is subjected to repeated heating and cooling, such as during the warm and cool parts of the day, since there will not be a corresponding redistribution of the nucleating agent particles in the distillate. The synthetic polymeric materials used as wax growth stimulators and as wax growth inhibitors may contain the same or different vinyl esters.

For the purposes of this invention, paraffin crystal growth stimulators, paraffin nucleating agents and paraffin nucleating agents are used as equivalent terms and are used interchangeably.

Paraffin growth inhibitors (hereinafter sometimes referred to as paraffin inhibitors) generally include in their molecular structure paraffin-like polymethylene segments which are able to incorporate themselves into the lattice of the paraffin crystals at the point of lattice disorder and also contain bulky groups which prevent the incorporation of further n-paraffin molecules at the point of lattice disorder and thus stop further crystal growth.

A good synthetic wax nucleating agent can be selected by visually comparing a clear vessel containing a 0.1 to 3.0 wt.% solution of the potential nucleating agent in a distillate with an identical vessel containing the same distillate without any additive while lowering the temperature of the two materials. The onset of wax crystallization in the distillate containing a polymeric material having nucleating properties will occur at a higher temperature than crystallization will begin in the absence of the nucleating agent. Similarly, a wax inhibitor is usually characterized by the ability to delay the onset of crystallization.

The synthetic polymers used as nucleating agents or as wax growth inhibitors are copolymers of ethylene and vinyl ester and may contain the same or different ester monomers.

The vinyl ester content and molecular weight are the average of the blend.

Typical vinyl esters include vinyl acetate, vinyl propionate, and vinyl butyrate.

The flow improvers, when incorporated into the distillate fuels, are effective in

1. to keep the fuels liquid at operating temperatures,

2. To inhibit the growth of precipitating wax crystals when these oils are subjected to slow cooling, i.e. 0.2ºF to 2ºF/hour, which are typical of the rates attained when "bulk oils" are subjected to atmospheric cooling.

3. To inhibit the growth of precipitating wax crystals when the oils are subjected to rapid cooling, i.e. 10ºF to 100ºF/hour, which are typical of the rates occurring when relatively warm oil enters transport lines and is suddenly exposed to low temperatures.

All three criteria listed above are desirable to ensure that a fuel is pumpable and filterable under the conditions of its distribution and use.

As mentioned above, the molecular weight is the average of the two polymers and in general the preferred number average molecular weight (VPO) of the nucleating agent is within the range of 500 to 6000, particularly 1200 to 6000. For example, it has been found that particularly a relatively low molecular weight ethylene/vinyl ester copolymer with a relatively high vinyl ester content acts as a wax growth inhibitor. On the other hand, a copolymer of ethylene with a vinyl ester, wherein the copolymer has a relatively high molecular weight and a relatively low vinyl ester content, acts as a nucleating agent. More specifically, it has been found that blends containing ethylene/vinyl acetate copolymers having a number average molecular weight of 1200 to 6000 (VPO) with vinyl acetate contents of about 32 to 50 wt.% (e.g. 13.3 to 25 mol.% ester) as paraffin inhibitors and ethylene/vinyl acetate copolymers having a number average molecular weight of 500 to 10,000 (VPO) with vinyl acetate comonomer weight proportions of 1 to 30 wt.% (eg 0.3 to 12 mole % ester) are highly effective as paraffin growth stimulators. When the nucleating agent is an ethylene/vinyl acetate copolymer, its number average molecular weight is preferably at least 500, preferably higher than 1,000 and/or the ester content is at least 5% less than the corresponding proportion of the paraffin growth inhibitor.

All molecular weights mentioned here are number average molecular weights determined by gas phase osmometry (VPO), e.g. using Mechrolab Gas Phase Osmometer 301A. Vinyl acetate contents are determined by saponification. Thus, the nucleating agent may comprise a higher molecular weight ethylene/vinyl acetate copolymer compared to the growth inhibitor if the vinyl acetate content of the two polymeric materials is approximately equal. The two synthetic polymers may have been prepared separately or prepared sequentially in one batch by changing the reaction conditions. Thus, the reaction conditions may be chosen so that the initial polymerization reaction produces a polymer having primarily nucleating properties and the reaction conditions may be changed to produce a polymer having primarily paraffin growth inhibitory properties, or vice versa. In this way, a mixture of polymers can be created that possesses both types of functionality.

In the particular embodiment of the invention using two different copolymers of ethylene and vinyl acetate, the relationships between the vinyl acetate concentration in the copolymer and the molecular weight of the copolymers are important because they are the factors that determine the role of that particular copolymer in the fuel. That is, provided the properties of the other polymer are similar, determine whether or not the copolymer as a whole will behave as a wax inhibitor or as a wax nucleating agent in the composition. Accordingly, as a very general rule of thumb, the nucleating agents should have relatively long polymethylene segments and as these synthetic polymers approach low molecular weight regions, the proportion of vinyl acetate should also decrease. On the other hand, as the molecular weight is increased, the proportion of vinyl acetate should also be increased. Accordingly, the specific wax nucleating agents will comprise a copolymer of ethylene and a relatively low proportion of vinyl acetate with a relatively high molecular weight.

The paraffin inhibitor, on the other hand, will generally be a copolymer with a relatively low molecular weight and a relatively high vinyl acetate content, since the paraffin inhibition function depends more on the presence of bulky groups such as ester groups attached to the backbone of the copolymer molecule.

Although the separate copolymers can be blended directly into the fuel, it is normally considered desirable to prepare a concentrate. This can be accomplished by first adding each to a separate solvent, but most preferably each is dissolved in a common solvent. Accordingly, both the preferred (second) low molecular weight, high vinyl acetate copolymer and the preferred first relatively high molecular weight, low vinyl acetate copolymer can be dissolved in jet fuel (kerosene) or a heavy aromatic naphtha. Preferred concentrates contain from 5 to 60%, preferably from 10 to 50%, total copolymer, with the balance being hydrocarbon oil as solvent.

The inhibitor copolymers can be prepared by known methods using free radical initiators, preferably organic peroxide compounds. Suitable processes are high temperature and high pressure processes or the solution processes described in US specifications such as US-A-3,048,479 or US-A-3,093,623 or British Patent 1,263,152.

In one aspect, the fuels to which the present invention relates are difficult to treat with conventional additives because of the relatively narrow boiling range of the 20% to 90% fraction of the fuel, the 90% fraction boiling point being 70 to 100°C higher than that of the 20% fraction, and the relatively small gap between the 90% boiling point and the final boiling point of 10 to 30°C, such as 25°C and even 20°C in some cases.

Suitably a total amount of additive is present from 0.001 to 0.5% by weight, preferably from 0.005 to 0.1%, most preferably from 0.001 to 0.04%, based on the weight of the fuel, all percentages being by weight. The polymeric materials may be used in ratios of from 10 to 15 parts by weight of growth inhibitor per part of nucleating agent.

The present invention is illustrated by the following examples in which an additive according to the invention (Additive A) was an oil solution containing 63% by weight of a polymer combination comprising 13% by weight of a wax crystal growth inhibitor comprising an ethylene/vinyl acetate copolymer having a number average molecular weight of 2500 and a vinyl acetate content of 36% by weight and 1% by weight of a wax crystal stimulator having a number average molecular weight of 3500 and a vinyl acetate content of about 13% by weight. Additive B was an oil solution containing 45% by weight of an additive combination of three parts by weight of the above-mentioned wax crystal growth inhibitor and one part of the wax crystal stimulator according to US-A-3,961,916. Additive C was a 50% by weight solution of an ethylene/acetate copolymer having an average Molecular weight (number average) of 2000 and a vinyl ester content of 30 wt.% in oil.

The fuels used in the examples were as follows.

In these examples, wax crystal size is determined at rapid cooling rates by the Cold Filter Plugging Point (CFPP) test. This test is conducted according to the procedure described in the "Journal of the Institute of Petroleum", Vol 52, No. 510, June 1966, pp. 173 - 185. In brief, the CFPP test is conducted on a 45 ml sample of the oil to be tested. The oil, charged to the ASTM cloud point vessel, is cooled in a bath maintained at -30ºF. For each two degree decrease in temperature, beginning at 4ºF above the cloud point, the oil is drawn at a vacuum of 8 inches of water through a filter element fitted with a 350 mesh screen into a pipette to a mark indicating a volume of 20 ml, then the oil is allowed to flow back into the cooling chamber by gravity. The test is carried out at Tests are presented as CFPP (Cold Filter Plugging Point), which is the highest temperature at which the oil no longer fills the pipette.

The following amounts of Additive A, Additive B and Additive C were found to be required to achieve a reduction of 6ºC, 8ºC and 10ºC in the temperature at which these fuels pass the CFPP test.

* Comparison examples

In a further series of experiments, the amount of additive required to achieve a reduction in the CFPP value of various fuels by 6ºC, 8ºC and 10ºC and compared with the amounts used for additives outside the present invention The fuels used were

and the additives used were A, B and C as in the previous example together with the additives D to H as follows

The results are shown in the following table.

* Comparison examples

Claims (4)

1. Use as an additive for improving the flow properties of a distillate petroleum fuel oil, the 20% and 90% distillation points of which differ in the range of 70°C to 100°C, the 90% boiling point of which is 10 to 30°C below the final boiling point and the final boiling point of which is above 350°C, the additive comprising a mixture of two copolymers of ethylene and a vinyl ester of a carboxylic acid containing 1 to 4 carbon atoms, one copolymer being a growth inhibitor and the other being a wax nucleating agent, the mixture containing at least 10 parts by weight of the growth inhibitor for each part of the wax nucleating agent and an average of 32 to 35% by weight of the vinyl ester and having a number average molecular weight of 1000 up to 6000, includes. 2. Use according to claim 1, wherein the vinyl ester is vinyl acetate. 3C distillate fuel, the 20% and 90% distillation points of which differ by 70ºC to 100ºC, the 90% boiling point of which is 10ºC to 30ºC below the final boiling point and the final boiling point of which is above 350ºC, containing 50 to 500 ppm of a mixture of two copolymers of ethylene and a vinyl ester of a carboxylic acid containing 1 to 4 carbon atoms, one copolymer being a growth inhibitor and the other being a wax nucleating agent, the mixture containing at least 10 parts by weight of the growth inhibitor for each part of the wax nucleating agent and an average of 32 to 35% by weight of the vinyl ester and having a number average molecular weight of 1000 to 6000. 4. The distillate fuel of claim 3 wherein the vinyl ester is vinyl acetate.