EP1246893A2

EP1246893A2 - Diesel fuel composition

Info

Publication number: EP1246893A2
Application number: EP00990759A
Authority: EP
Inventors: Robert Howie Barbour; Paul J. Berlowitz; David John Patents & Licences RICKEARD; Alan Mark Patents and Licences SCHILOWITZ
Original assignee: ExxonMobil Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1999-12-16
Filing date: 2000-12-14
Publication date: 2002-10-09
Also published as: GB2357297A; WO2001044411A2; CA2393280A1; WO2001044411A3; JP2003517090A; GB9929805D0

Abstract

This invention relates to a diesel fuel composition having enhanced lubricity, said composition having a sulphur content of <25ppm and an aromatic content of not less than about 12%, characterised in that the kinematic viscosity measured at 40 °C (KV40) of the fuel composition is greater than 3.0 cSt. Such compositions have the desired lubricity without having to use or by using in relatively lower amounts of conventional lubricity enhancing additives.

Description

FUEL COMPOSITION

This invention relates to fuel compositions of low sulphur content and high viscosity which have improved lubπcity performance and thus a reduced dependency on lubπcity additive

Fuels such as diesel are widely used in automotive transport due to their low cost However, one of the problems with such fuels is the presence of relatively high concentrations of sulphur compounds Excessive sulphur contributes to exhaust particulate emissions and can also degrade the effectiveness of some exhaust after- treatment technology which is being introduced in response to regulated limits on exhaust emissions As a result, the permitted level of sulphur in diesel fuel has been progressively reduced over the years and further reductions are planned for the future Whilst a reduction in sulphur content can be readily achieved by well known processes such as hydrodesulphuπsation which is generally earned out in the presence of a catalyst, such process also adversely affect the lubπcity of the resultant desulphuπsed product The hydrotreatment process can also reduce the level of aromatic compounds in fuel compositions This reduction is also considered to have a beneficial effect on emission levels and as such the California Air Resources Board has set a limit of 10% aromatics but will allow some dispensation where fuels with higher aromatics levels have been shown to have comparable emissions levels A reduction in aromatic levels has been shown by Nikanjam and Henderson (SAE 920825) to have a detπmental effect on lubπcity These workers showed that the deteπoration in lubπcity can be reversed by back-blendmg low aromatic fuels with commercial blends having typical aromatics levels

Consequently, it is necessary to formulate compositions which are low m sulphur content but are also of the desired lubπcity in order to minimise wear and friction when used m automotive engines and to minimise the damage to the injection system of a diesel engine It has hitherto been the practice to add anti-wear agents to such formulations including fatty acid, fatty acid esters, lactones, polyoxyalkylene ethers, ammo compounds and the like for this purpose However, compositions containing compounds such as esters are expensive in terms of both mateπal costs and the cost of additive storage facilities A publication by Wei and Spikes entitled "The lubricity of diesel fuels" in Wear, 111, (1986), page 217 discloses that heterocyclic nitrogen compounds, like quinoline and indole, also have a beneficial effect on the antiwear performance of base fuels. These compounds were investigated because they fall within the same general structure as the natural compounds that are destroyed during hydrotreatment.

A further article by D. Wei et al in Lubrication Science, 1989, 2(1), pp 63-67 entitled "The Influence of Chemical Structure of Certain Nitrogen-Containing Organic Compounds on Their Antiwear Effectiveness: The Critical Role of Hydroxy Group" shows that hydroxy groups involved in some nitrogen-containing compounds have been found to improve their antiwear performance significantly and states that hydroxy substituted benzothiazoles are most effective in wear reduction and anti-scuffmg. With this in view the author reports the results of the tests carried out on films formed on rubbing surfaces by the benzo-derivatives of pyridine and thiazole, with or without hydroxy groups on the rings. The article concludes that protective films formed on rubbing surfaces by the above heterocyclic compounds bearing a hydroxy group are significantly different from those produced by their analogues with similar chemical composition and physical properties.

In JP-A-1 10001692 a specific mixture of C₈-C₃o fatty acid esters are used to improve the lubricity of low sulphur (<0.2 wt%) middle distillate fuel oils having an aromatic content of < 40 wt% suitable for use as diesel fuels. There is no mention of the viscosity of the final fuel oil.

In each of these instances, the lubricity enhancing component generally has to be synthesised separately and introduced into the fuel from an external additive. This is not only wasteful of resources but also causes proliferation of chemicals into this industry. Moreover, extensive testing is needed to ensure that such externally sourced additives do not have any undesirable side-effects.

Prior published RU-A-2079542 relates to a highly viscous fuel for ship's diesel engines comprising a narrow fraction from the atmospheric distillation of oil (b.p. range 350-500°C) and a depressor additive in the form of a thermal cracking residue (density 1040-1095 kg/m³). There is no mention of the sulphur content or the viscosity of the final product. It has now been found that an increase in fuel viscosity has a beneficial effect on the lubricity performance of fuel compositions with ultra low S levels. Fuels formulated to have a higher viscosity also have enhanced lubricity performance without excessive recourse to additives from an external source.

Accordingly, the present invention therefore provides diesel fuel compositions having enhanced lubricity, said compositions having a sulphur content of <25ppm and an aromatic content of not less than about 12%, characterised in that the kinematic viscosity measured at 40°C (KV o) of the fuel composition is greater than 3.0 cSt.

Such fuel compositions can be prepared by blending at least two components one of which has a relatively higher viscosity than the final fuel composition and the other of which has a lower viscosity than the final fuel composition. When such blending is used, the relative ratios by volume of the relatively higher viscosity component to the relatively lower viscosity component may vary over a very wide range depending the viscosity of each and the amount blended such as eg from 10:90 to 80:20 respectively. Such a volume ratio would suitably be in the range from 70:30 to 80:20 respectively. Thus, a conventional hydrotreated fuel can be used as the relatively higher viscosity component which can be obtained either from a pipestill (eg heavy gas oil) of a refining process, or, from secondary processing of a refinery product stream such as eg hydrocracking (ie hydrocrakate). Such components may have been severely hydrotreated to reduce the sulphur content thereof but will still retain the aromatics content therein. Such aromatic content may typically be in the range of 12 to 35%. In one embodiment, the fuel composition of the present invention can be prepared by blending a relatively higher viscosity refinery stream (which may in itself be a blend) obtained from a hydrocracker with a typical viscosity automotive diesel oil (ADO) or even a low viscosity component like kerosene. By blending these components in appropriate proportions, a fuel composition can be formulated which has a KV ₀ viscosity of more than 3.0 cSt. For example, a hydrocracked component with a KV₄₀ of 6.7 cSt can be blended with a lower viscosity component with a KV₄₀ of 1.1 cSt in a volume ratio of 74:26 respectively to give a blend with a KV₄₀ of 3.6 cSt. Alternatively, if an ultra-low sulphur automotive diesel oil ("ULSADO") with a sulphur content of 10 ppm and a KV₄₀ of 2.6 cSt KV₄₀ was blended with the aforementioned hydrocracked component in a ratio of 80:20 respectively, the resultant blend would have a viscosity of 3.0 cSt. The fuel compositions of the present invention provide an acceptable lubricity performance when used in diesel injection equipment that is less susceptible to wear problems. In the relatively more susceptible rotary distribution systems such as eg pumps, which are solely lubricated by the fuel itself the enhanced lubricity performance will enable a reduction in the conventional additive treat rate required for acceptable performance. These pumps contain precisely engineered components to maintain the consistency and precision of the injected fuel volume and to ensure a long service life. If the pump components become worn, irregular fuel injection may occur thereby leading to poor drivability, and increased emissions and may eventually lead to pump seizure.

The diesel fuel composition has a sulphur content of less than 25 ppm, suitably less than 10 ppm and is preferably a zero sulphur fuel. The low sulphur levels can be achieved in a number of ways. For instance, this may be achieved by well known methods such as catalytic hydrodesulphurisation. Furthermore, the fuel composition has a KV₄₀ for >3.0 cSt, preferably >3.5 cSt.

The base fuels of the present invention may comprise mixtures of saturated, olefinic and aromatic hydrocarbons and these can be derived from straight run streams, thermally or catalytically cracked hydrocarbon feedstocks, hydrocracked petroleum fractions, catalytically reformed hydrocarbons, or synthetically produced hydrocarbon mixtures. The present invention is particularly applicable to fuels with less than 25 ppm sulphur where the natural lubricity polar compounds have been reduced during processing to an ineffective level.

Fuel compositions of the present invention will contain more than normal levels of the higher viscosity components from the pipestill or from secondary processing such as hydrocracking. Methods of processing petroleum crude to obtain various process streams are well known in the art and are described in detail for instance by Keith Owen and Trevor Colley in "Automotive Fuels Reference Book", Second Edition, published by the Society of Automotive Engineers, Inc, Warrendale, PA, USA (1995). Specifically Chapter 3 of this text-book at pages 29-49, Chapter 15 on Diesel Fuel Characteristic Influencing Combustion at pages 385-418, Chapter 18 at pages 519-522 relating to lubricity additives for diesel fuels, and Appendix 12 at pp 865-890 which is a 'Glossary of Terms' give all the information that is necessary to make and characterise such streams. The antiwear and lubricity performances of the fuel compositions of the present invention were measured according to the so-called high frequency reciprocating rig test (hereafter referred to as "HFRR"). The HFRR test consists of a loaded upper ball 6mm in diameter, which oscillates against a static lower plate. Both friction and contact resistance are monitored throughout the test. The tests are conducted according to the standard procedure published as CEC F-06-A-96 in which a load of 2N (200g) was applied, the stroke length was 1mm, the reciprocating frequency was 50 Hz and sample temperature of 60°C. The ambient temperature and humidity were controlled within the specified limits and the calculated value of wear scar diameter was corrected to the standardized water vapour pressure of 1.4 kPa. The specimen ball was of a grade 28 (ANSIB3.12), AISI E-52100 steel with a Rockwell hardness "C" scale (HRC) number of 58-66 (ISO 6508) having a surface finish of less than 0.05μm R_a and the lower plate was of AISI E-52000 steel machined from an anealed rod, with a Vickers hardness "HV30" scale number of 190-210 (ISO 6507/1). It is turned, lapped and polished to a surface finish of 0.02μm R_a.

TABLE 1 Summary of HFRR test conditions

A series of test samples were prepared by blending a refinery component from the hydrocracker with a low viscosity kerosene with an S content of 1 lppm. Details of these blend components are shown in Table 2 below. Table 3 shows details of the blend ratios as well as KV₄₀ (ASTM D445-97/446-97) and aromatics composition by IP391-95. TABLE 2 Composition of blend components

RA - ring aromatics

TABLE 3 Details of base fuel blends

RA - ring aromatics

These data show that the blends prepared and having a span of KV o's ranging from 2.0 to 4.0 cSt and have an aromatics content which is consistent with typical diesel fuels. Blends 1 to 3 (according to the invention) are compared with Blends 4 and 5 (comparative tests not according to the invention) having a KV₄n below 3.0 cSt.

Table 4 below shows the HFRR results for the base blends with no additives and for low and high viscosity blends treated with ester lubricity additive. TABLE 4 HFRR Lubricity performance of test blends

The results in the base blend column show that with the decrease in viscosity from Blend 1 to blend 5 there is a corresponding detrimental effect on lubricity. The HFRR results for Blends 1 and 5 treated with ester lubricity additive show that this benefit translates through the treat curve and provides a means to meet the claimed performance specification (460μm) at a lower additive treat rate.

Claims

1 A diesel fuel composition having enhanced lubπcity, said compositions having a sulphur content of <25ppm and an aromatic content of not less than about 12%, characteπsed m that the kinematic viscosity measured at 40°C (KV₄₀) of the fuel composition is greater than 3 0 cSt

2 A composition according to Claim 1 wherein said composition has a KV o of more than 3 5 cSt

3 A composition according to any one of the preceding Claims wherein the fuel composition is prepared by blending a component of relatively higher viscosity than the final fuel composition with a component of relatively lower viscosity than the final fuel composition

4 A composition according to any one of the preceding Claims wherein the weight ratio of the component of relatively higher viscosity to that of relatively lower viscosity is in the range from 10 90 to 80 20

5 A composition according to Claim 3 or 4 wherein the weight ratio of the component of relatively higher viscosity to that of relatively lower viscosity is in the range from 70 30 to 80 20

6 A composition according to any one of the Claims 2-5 wherein the component of relatively higher viscosity is a hydrocrackate from a refinery process stream which may itself be a blend and the component of relatively lower viscosity is selected from ADO and kerosene

7 A composition according to any one of the preceding Claims wherein the aromatic content of the composition is in the range of about 12 to 35%

8 A composition according to any one of the preceding Claims wherein the sulphur content thereof is less than 10 ppm