NO172650B - APPLICATION OF AN ADDITIVE TO IMPROVE LOW TEMPERATURE FLOW CHARACTERISTICS OF A DISTILLATE PETROLEUM FUEL OIL - 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

The present invention relates to the use of an additive to improve the low-temperature flow properties of a distillate petroleum fuel oil.

Mineral oils containing paraffin wax have the property that they become less fluid as the oil's temperature drops. This loss of fluidity is due to the crystallization of the wax into plate-like crystals which will eventually form a sponge-like mass that traps the oil in it.

It has long been known that various compositions act as crystal modifiers when mixed with waxy mineral oils. These compositions modify the size and shape of wax crystals and reduce the adhesive forces between the wax and the oil in such a way that the oil is allowed to remain liquid at a lower temperature. Various pour point depressants have been described in the literature, and several of these are in commercial use. E.g. US patent no. 3,048,479 mentions 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, is also known. US patent 3,961,916 discloses the use of a mixture of copolymers, one of which is a wax crystal nucleator and the other a growth arrester to control the size of the wax crystals. Likewise, GB patent 1,263,152 suggests that the size of the wax crystals can be regulated by using a copolymer which has a low degree of side chain branching.

With the increasing diversity of distillate fuels, types of fuel have emerged which cannot be treated with the existing additives or which require an uneconomically high level of additive. A particular group of fuels associated with such problems are those with a relatively narrow boiling range. Fuels are often characterized by their initial boiling point, final boiling point and the interim temperatures at which certain volume percentages of the initial fuel are distilled. Fuels whose 20-90% distillation point deviates in the range from 65 to 100°C, especially from 70 to 100°C (ASTM D86) and the 90% boiling point is generally from 10 to 30°C, especially from 10 to 25°C of it initial boiling point, have been found to be particularly difficult to treat, being sometimes practically unaffected by additives or requiring very high additive levels. All distillations referred to here are according to ASTM D86. Furthermore, with the increase in the price of crude oil, it has also become important for a refiner to increase their distillate fuels production and to optimize their operations using what is known as sharp fractionation, which in turn results in distillate fuels that are difficult to process with conventional additives or that require a level of processing that is unacceptably high from an economic point of view. Typical sharply fractionated fuels have a 90% to final boiling point of 10-20°C, usually in a 20-90% boiling range of 90-110°C. Both types of fuels have final boiling points above 350°C, usually a final boiling point in the range 350-375°C, especially 350-370°C.

The copolymers of ethylene and vinyl acetate which have found widespread use for improving the flow properties of the previously widely available distillate fuels usually contained up to about 30# of vinyl acetate, where the additive was used to regulate the size of wax crystals that formed in the fuel or they contained about 36 wt-# or more of vinyl acetate where their main function was to lower the pour point of the distillate fuel. It has not been found that any of these types of additive are effective for treating the narrow-boiling and/or sharply fractionated fuels described above.

However, it has been found that copolymers of ethylene and vinyl esters of carboxylic acids containing 1-4 carbon atoms, containing 32-35 weight-# of the vinyl ester and with a number average molecular weight of 1000-6000, are particularly effective for treating these fuels.

According to the present invention, an additive comprising a copolymer of ethylene and a vinyl ester of a carboxylic acid with 1-4 carbon atoms containing 32-35 weight-# of the vinyl ester and with a number-average molecular weight of 1000-6000 is used to improve the low-temperature flow properties of a distillate petroleum fuel oil whose 20% and 90% distillation points differ by from 65 to 100°C and whose 90% boiling point is from 10 to 30°C from the final boiling point which is higher than 350°C.

Said 90% boiling temperature is preferably from 10 to 20 °C from the final boiling point, and the additive preferably constitutes 10-15 parts by weight of said copolymer.

The copolymer of ethylene and the vinyl ester of the carboxylic acid may be a mixture of two copolymers such as those generally described in US Patent 3,961,916 which may or may not contain the same vinyl ester. It has been found in particular that an additive combination containing at least 10 parts by weight of the growth arrester for each part by weight of the wax-crystal nucleator is suitable for treating this type of fuel. The blend is particularly useful because it allows for additional flexibility.

A distillate fuel that is used according to the present invention preferably contains 50-500 ppm of an additive mixture of 10-15 parts by weight of a synthetic polymer material, which has the property of a wax growth arrester in said fuel, for each part of a synthetic polymer material, which has the properties of a wax-growth stimulator, the growth stopper and the growth stimulator being said copolymers of ethylene and vinyl esters of carboxylic acids containing 1-4 carbon atoms, where the average ester content in the copolymers is in the range 32-35 wt-#, and the number average molecular weight thereof is in the range 1000-6000.

The fuels whose 20-90 H> distillation points differ within the range 65-100'C and their 90# boiling point is usually from 10 to 30"C from the final boiling point, have a final boiling point above 350°C, usually between 350 and 37S<P >C, more commonly between 350 and 370° C. The fuels whose 90% final boiling point is 10-20°C usually have a 20-90% destination range from 90 to 110°C and also usually have final boiling points above 350°C, usually between 350 and 375°C, more commonly between 350 and 370°C.

When the additive is a mixture, the wax-growth stimulator or nucleating agent is a synthetic polymer material which is soluble in the distillate at temperatures substantially above the saturation temperature, but will, on cooling the distillate, progressively separate in the form of small particles as the temperature of the distillate approaches the saturation point, f .ex. is cooled from a point slightly above (e.g. 10°C above, preferably approx. 5°C above) said saturation temperature. The term "saturation temperature" is defined as the lowest temperature at which dissolved material, e.g. wax, cannot be crystallized from the solution, even if known crystallization-induced methods are used. While it is not known with certainty, it is believed that as cooling continues, additional nucleating particles will be precipitated in a more or less continuous manner. These additional particles act as nucleators for continued wax crystallization, which will prevent significant subcooling of the distillate. The advantages of having fresh nucleating particles formed continuously is that the over-saturation of the distillate with n-paraffins is kept at the lowest possible level and thus makes it easier for a molecule of growth arrester to build up in the growth center of mature crystals and thereby stop further growth. The inhibitory effect of a growth arrester is believed to result from the presence of bulky groups in its molecule. Additional nucleators should be segregated to replace the deactivated growth centers. The wax growth arrester is more soluble in said distillate than said nucleating agent, and it acts as a growth arrester as the wax crystal forms. The nucleating agent should not be insoluble in the distillate at elevated temperatures nor should it begin to separate at a temperature substantially above that at which wax crystallization can occur. If nucleators are separated at a temperature substantially above the temperature at which crystallization can occur, they tend to settle at the bottom of the vessel containing the distillate instead of remaining dispersed in the distillate. This factor is particularly important when the distillate is subjected to repeated heating and cooling such as during the hot and cold parts of a day, because this does not result in sufficient redispersion of the nucleating particles in the distillate. The synthetic polymer materials used as wax-growth stimulators and wax-growth stoppers may contain the same or different vinyl esters.

For the purposes of the present invention, wax crystal growth stimulators, wax nucleators and wax nucleating agents are all considered equivalent terms, and are used interchangeably.

Wax growth stoppers (in the following sometimes referred to as wax stoppers) probably include in their molecular structure waxy polymethylene segments that can build themselves up in the lattice of the wax crystals at the point of lattice dislocation, and also contain bulky groups that prevent the incorporation of further molecules of n-paraffins at the point of lattice dislocation and thereby stops further crystal growth.

A good synthetic polymer wax nucleator can be selected by visually comparing a transparent container containing 0.1.3.0 wt-# solution of the potential nucleator in a distillate with an identical container of the same distillate without any additive, while the temperature of the two materials lowered. The beginning of the crystallization of wax from the distillate containing a polymer material having nucleating properties will occur at a higher temperature than that at which crystallization will start in the absence of said nucleating agent. Likewise, a wax stopper is usually characterized by its ability to delay the onset of crystallization.

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

With the preferred additive being a mixture of polymers as described above, the vinyl ester content and molecular weight are the average of the mixture. However, the additive can also be a single polymer, and by this is meant material produced in a single polymerisation. In this case, the materials can be obtained by means of the well-known high pressure or solution polymerization techniques previously proposed for the production of ethylene vinyl ester, especially vinyl acetate copolymers for fuel additives. Typical vinyl esters for both blends and single polymers include vinyl acetate, vinyl propionate and vinyl butyrate.

The flow-improving agents, when incorporated into distillate fuels, are preferably effective with respect to: 1. maintaining these fuels in a liquid state at operating temperatures; 2. stop the growth of separating wax crystals when the oils are subjected to slow cooling, i.e. 0.11-1.11°C/hour, which is typical of the rates applicable when "bulk oil" is subjected to atmospheric cooling; 3. stop the growth of separating wax crystals when the oils are subjected to rapid cooling, i.e. 5.56 to 55.56°C/hour, which is typical of the rates applicable when relatively hot oil enters transmission lines and the sudden exposed to low temperatures.

All of the above three criteria are desired to ensure that a fuel is pumpable and filterable under the conditions of its distribution and use. When mixtures of polymers are used, as mentioned, the molecular weight is the average of the two polymers and usually the preferred number average molecular weight (VPO) for the nucleator will be in the range 500-6000, more preferably 1200-6000. In particular, e.g. an ethylene-vinyl ester copolymer of relatively low molecular weight and with a relatively high vinyl ester content, has been found to act as a wax growth inhibitor. On the other hand, a copolymer of ethylene with a vinyl ester and of relatively high molecular weight and with a relatively low content of vinyl ester acts as a nucleating agent. Even more particularly, it has been found that mixtures containing ethylene/vinyl acetate copolymers with number average molecular weights of 1200-6000 (VPO) with vinyl acetate content of 32-50 wt-#

(e.g. 11-25 mol% ester (as wax stoppers and ethylene/vinyl acetate copolymers with a number average molecular weight of 500-10,000 (VPO), and with vinyl acetate comonomer weight fractions of 1-30 wt-# (e.g. 0.3-12 mol% ester) as wax growth stimulators, are very effective. When the nucleating agent is an ethylene/vinyl acetate copolymer, its number average molecular weight is preferably at least 500, preferably 1000, higher and/ or the ester content at least 5 # lower than the equivalent property growth-growth arrester.

All molecular weights specified herein are "number average molecular weights", which are molecular weights as measured by vapor phase osmometry (VPO), e.g. using the "Mecrolab vapor Phase Osmometer 301A". The vinyl acetate contents are determined by saponification. Thus, in relation to the growth stopper, the nucleator may comprise an ethylene-vinyl acetate copolymer with a higher molecular weight if the vinyl acetate content in both polymer materials is approximately the same. When two synthetic polymers are used, they can be prepared separately, or they can be prepared one after the other in a batch by varying the reaction conditions. Thus, the reaction conditions can be chosen so that the initial polymerization reaction gives a polymer that has mainly nucleating properties, and the reaction conditions can be changed to form a polymer that has mainly wax-growth-stopping properties or vice versa. In this way, a mixture of polymers can be produced that have both functional types.

In an additive using two different copolymers of ethylene and vinyl acetate, the relationship between the concentration of vinyl acetate in the copolymer and the molecular weight of the copolymers is important, because they are the factors that determine the role of the particular copolymer in the fuel. That is, given that the other polymer properties are equal, they determine whether the copolymer as a whole will behave in the composition as a wax stopper or as a wax nucleating agent. Thus, and very generally as a 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 will also decrease. On the other hand, as the molecular weight increases, the proportion of vinyl acetate should also increase. The specific wax-core forming agents will thus comprise a copolymer of ethylene and a relatively small proportion of vinyl acetate with a relatively high molecular weight.

The wax stopper, on the other hand, will generally be a relatively low molecular weight copolymer with a relatively high vinyl acetate content, because the wax stopper function depends more on the presence of bulky groups, such as ester groups, which are attached to the main chain of the copolymer molecule.

Although the separate copolymers can be mixed directly into the fuel, it will normally be desirable to produce a concentrate. This can be effected by first associating each of them with a separate solvent, but preferably by dissolving each of them in a common solvent. Thus, both the preferred copolymer (second) of low molecular weight and high vinyl acetate content and the preferred first copolymer of relatively high molecular weight and low vinyl acetate content can be dissolved in a kerosene or a highly aromatic naphtha. Preferred concentrates will contain 5-60%, preferably 10-50%, of total copolymer, the remainder being a hydrocarbon oil solvent.

The stop copolymers can be prepared by known methods that use free radical initiators, preferably organic peroxide compounds. Suitable methods are high temperature and high pressure processes or dissolution processes as described in US patents 3,048,479 and 3,093,623 as well as GB patent no. 1,263,152.

In one respect, the fuels to which the present invention relates are difficult to treat with conventional additives due to the fuel's relatively narrow boiling range for the 20-90 56 fraction, as the 90 Æ fraction boils from 65 to 100"C above what is the case for the 20% fraction and/or due to the relatively small distance between the 90% boiling point and the final boiling point of less than 25 °C, and even in certain cases less than 20°C.

A total of 0.001-0.5 wt-# of additive is suitably present, based on the weight of fuel, preferably 0.005-0.196, preferably 0.01-0.04%, all percentages being wt-#. When a mixture is used, the polymer materials are used in a ratio of 10-15 parts by weight of growth stops per share nucleus.

The present invention is illustrated by the following examples in which an additive that is used according to the present invention (additive A) was an oil solution containing 63 parts by weight of a combination of a polymer including 13 parts by weight of a wax crystal growth inhibitor comprising an ethylene vinyl acetate copolymer with a number average molecular weight of 2500 and vinyl acetate content of 36 wt-# and one part by weight of a wax crystal simulator with a number average molecular weight of 3500 and a vinyl acetate content of approx. 13 weight-96. Additive B was an oil solution containing 45% by weight of an additive combination of 3 parts by weight of the above wax crystal growth arrester and one part of the wax crystal simulator according to US patent 3,961,916. Additive C is a 50% by weight solution in oil of an ethylene acetate copolymer with a number average molecular weight of 2000 and a vinyl acetate content of 30% by weight.

The fuels used in the examples were as follows:

In the examples, wax crystal size is measured at rapid cooling rates using the "Cold Filter Plug Point" (CFPP) test. This test is carried out according to the method described in the "Journal of the Institute of Petroleum", Volume 52, No. 510, June 1966, pages 163-185. Briefly, the CFPP test is carried out with a 45 ml sample of the oil to be examined. The oil placed in the ASTM cloud point container is cooled in a bath maintained at approx. -34.4°C. Every 2" drop in temperature, starting from 2.22°C above the cloud point, the oil is forced by a suction of 20.3 cm of water through a filter element fitted with a 350 mesh screen and into a pipette to a mark indicating a volume of 20 ml, at which point the oil is allowed to flow back to the cooling chamber under the influence of gravity. The test is repeated with every 2°C drop in oil temperature until the oil no longer fills the pipette within a period of 60 seconds to the previously mentioned mark. The results of the experiment are reported as the "Cold Filter Plugging Point" which is the highest temperature at which the oil will not fill the pipette."

The amounts of Additive A, Additive B and Additive C required to achieve a 6°C, 8°C and 10°C reduction in the temperature at which these fuels would pass the CFPP test were determined to be as follows: In a further series of experiments, the amount of additive needed to produce a reduction of 6, 8 and 10° C in the CFPP value for different fuels was tested and compared with the amounts needed for additives outside the scope of the invention. The fuels used were:

Claims (7)

1. Use of an additive comprising a copolymer of ethylene and a vinyl ester of a carboxylic acid having 1-4 carbon atoms containing 32-35 wt-# of the vinyl ether and having a number average molecular weight of 1000-6000, for improving the low temperature flow properties of a distillate petroleum oil if 20 1o and 90 Sé distillation points differ by from 65 to 100°C and if 90% boiling point is from 10 to 30°C from the final boiling point higher than 350°C 2. Application according to claim 1, where said 20% to 90% destination range for the fuel is 70-100°C, and where 90% to the final boiling point is from 10 to 25°C. 3. Use according to claim 2, where 90% of the final boiling point is from 10 to 20°C. 4. Use according to any of the preceding claims where the fuel has a final boiling point in the range 350-370°C. 5. Use according to any of the preceding claims, of an additive which is a mixture of two copolymers, one of which acts as a nucleator for the wax, and the other acts as a growth arrester for the wax crystals. 6. Use according to claim 5, of a mixture containing at least 10 parts by weight of the growth arrester for each part by weight of the nucleating agent. 7. Use according to any one of the preceding claims wherein the vinyl ester is vinyl acetate.