Lubricity improver chemistry matters

Low sulphur diesel fuels need to be treated with lubricity improver (LI) additives to protect the high pressure components of the diesel fuel system. Their extensive use in the field over the past 20 years might suggest that new technology development work in this area would be limited. However, over that period, significant changes have taken place in the refining and automotive markets. This means that continuing research to understand the additive appetite in the latest fuel types and combinations is still needed today.

With so much change in the market, Infineum has conducted a series of studies to help identify the best LI chemistry to ensure safe application from the refinery through the transportation infrastructure and into the vehicle. The fuels studied were representative of typical diesel grades found in the marketplace and the lubricity and cetane improver additives represented commercially available components.

HFRR performance

HFRR performance of LI additives was compared over a range of treat rates

The ester LI provided a much deeper response, with wear scar diameters (WSD) below 350 µm achieved at realistic market treat rates. For mono-acid LI, the HFRR response was good but tended to plateau as the treat rate was increased.

It was possible to relate the difference in performance at higher treat rate to the average film coverage, with the acid LI having much lower average film coverage, which resulted in higher wear scar diameters.

The deeper HFRR response provided by ester LIs can be beneficial in real world applications.

At the refinery, for example, it can provide additional security where there are diesel fuel pools that contain intermittently severe fuels. It may also help to ensure internal specifications are consistently met. In the vehicle, it can be useful in providing increased protection for high pressure common rail systems, where injection pressures already exceed 2,000 bar and are expected to increase to 3,000 bar by 2025.

Impact of cetane number improver

The production of automotive diesel fuel uses a range of refinery streams. So called ‘cracked components’, when used in diesel blending, can result in fuels with low cetane values. The addition of cetane number improvers (CNI) is the most common way to restore the fuels’ ignition properties.

To test the impact of CNI on lubricity, test fuels were treated with either ester or acid LIs and CNI was added. HFRR results indicated that, in the presence of CNI, acid LIs have a large negative effect on HFRR performance compared with ester LIs.Acid lubricity improvers, in the presence of CNI, have a negative effect on HFRR performance

The findings were further substantiated by using film coverage analysis. This showed that the acid LI films were relatively unstable compared to the film coverage for the ester LI, which appeared to be more robust with less variability over the duration of the test.

As the export and global movement of diesel fuels increases, additive selection is becoming increasingly complex. The use of ester LIs, which demonstrate the most robust HFRR performance, can ensure specification limits are met regardless of the source refinery and end market use.

Pipeline complications

Clearly, to service broader markets, diesel fuel must be moved from the refinery to the user. In some cases this involves transportation via multi-product pipelines (MPPs).

Recently the Energy Institute has updated EI1535, which sets out the minimum criteria to determine the acceptability of additives for use in MPPs co-transporting jet fuel. These criteria now include a new water-mapping test to help ensure jet fuel is manufactured and distributed water free from the refinery to the aircraft.

Since MPPs transport both diesel and jet fuel, there is potential for residual diesel additives to be transferred into the jet fuel.

Systems are in place using coalescers with polar elements to attract water, which then forms into large droplets that can be removed. Here, any surfactant-type additive could cause a reduction in the performance of the coalescers, which means the water content of the jet fuel could increase. The use of diesel lubricity improvers in MPPs is therefore of concern due to their surface-active chemistry.

Regionally, this risk is handled in various ways. In North America, for example, LIs are added at distribution terminals rather than passing through the MPPs. In Europe, pipeline operators have lists of approved additives that are considered safe to use up to a specified maximum treat rate.

In this study, Infineum tested two different ester LIs (both considered suitable for use in low temperature regions), which had high solubility but that were synthesised in different ways. For one of the LIs, the water content exceeded the maximum level of 15 ppm in effluent at a lower flow rate than a reference additive, suggesting it may not be suitable for multi-product pipeline use. The second LI had no impact on the water content of the jet fuel, even at high flow rates and had no negative impact on the jet fuel coalescer used in the test and means that the additive could be further assessed for first use monitoring in a MPP.

Advanced fuel injectors

The potential for diesel fuel lubricity additives to solubilise metals has been extensively studied. However, in field tests using fuels treated with mono-acid and ester LIs, there have been mixed reports of the build up of zinc and calcium. It appears that, as well as additive type, fatty acid methyl ester (FAME) in the fuel may also impact metal pick up.

There is no published data on whether there is the potential for metal contamination to occur prior to the vehicle fuelling. For several years Infineum has monitored diesel fuels in a market where the fuels contained a maximum of 100 ppm sulphur and no FAME for the entire period. Samples were collected from 26 service stations at the same time each year. For each fuel, it was possible to ascertain whether it was treated with an ester or an acid lubricity additive. On analysis, zinc pick up was predominantly found in fuels that had been treated with acid LIs, while for fuels containing ester LIs average zinc values were close to zero.

Zinc pick up was predominantly found in fuels that had been treated with acid lubricity improvers

When fuels are contaminated with zinc at levels as low as 1 ppm, large levels of spray-hole and internal injector deposits can be generated. In addition, other metals such as calcium and sodium can be associated with the formation of deposits.

Conclusions

Infineum has explored new areas associated with lubricity additive use and examined historical trends with a view to ensuring robust performance and trouble free use in the field.

The LI response observed in a large number of fuels confirms that acid and ester LIs behave differently.

Ester LIs provide more robust HFRR performance than acid LIs, even in fuels treated with CNI.

This is a key factor in the current market environment where fuel exchanges are now the norm.

En route to the consumer, fuels may be transported in MPPs. This work has shown that the use of an approved LI is critical to the safe transportation of jet fuel in these pipelines.

In the vehicle, soluble metals have been shown to contribute to the formation of injector deposits. Here again, LI chemistry has been shown to have a significant effect: with ester LIs there is essentially no zinc pick up and less potential for injector deposit formation than for acid LIs. 

Ester and acid LIs have been deployed in the field for many years. In our view, carefully selected ester LIs are the most robust choice to consistently meet diesel fuel lubricity specifications in an increasingly complex market.

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