ULSD could trigger other problems besides lubricity

Diesel Fuel News,  Feb 4, 2002  by Jack Peckham

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Severe hydrotreating of diesel fuel to achieve the <15 ppm sulfur limits for up coming ultra-low-sulfur diesel (ULSD) standards will have varying impacts on lubricity of diesel fuel, hence the intense interest in a new lubricity standard at American Society for Testing & Materials (ASTM).

But meantime, evidence is emerging that other fuel properties also could be affected, including cold-flow, corrosion resistance, oxidation tendency and conductivity, according to some leading diesel additives makers.

A potential corrosion problem with ULSD may seem paradoxical, given that hydro-treating diesel fuel tends to destroy organic acids that could -- if present in high-enough amounts -- cause corrosion in fuel delivery systems, as Chevron pointed out in its 1998 manual, "Diesel Fuels Technical Review."

Example: Virgin distillate streams from high-naphthene crudes can be caustic-treated to avoid excessive corrosion. On the other hand, hydrotreating to meet ULSD specs could offset the need for steps such as caustic treating.

However, it's not necessarily true that all crudes and all hydrotreating processes always will have the same impact on the anticorrosive properties of the resulting fuel.

Ironically, anti-corrosive agents added to diesel fuel are themselves organic acids, but due to their unique properties - and since they're in such small quantities, as additives - they improve the corrosion resistance of fuel, rather than degrade it, Infineum fuel additives expert Chip Millard explains.

However, the destruction of corrosive organic acids via hydrotreating indicates that "ULSD should not be more corrosive," as a Chevron technical source told us.

"All corrosive components are removed and [ULSD] dissolves less water than normal diesel fuel. It is possible that people realize that in some cases hydrotreating to reduce sulfur can reduce lubricity. Some add corrosion inhibitors to restore fuel lubricity; maybe they feel that if they are adding corrosion inhibitor to the ULSD, then it is more corrosive.

Another viewpoint on emerging ULSD quality issues appears in a recent booklet, "New Diesel Fuels: They Are In Your Future For Nonroad Equipment," produced for Equipment Manufacturers Institute (EMI), makers of agricultural and other diesel equipment.

The EMI booklet (not original research, but rather a compilation of some commercial web site information) points out that "ultra-low sulfur fuels can lose natural antioxidants that help prevent the fuel from forming gums and sludges. A fuel's antioxidation properties are particularly important in modern fuel systems, where the fuel is exposed to higher operating temperatures. Ultra-low sulfur fuels can also be more corrosive than conventional diesel, requiring corrosion-inhibiting additives."

It turns out that this information comes from Lubrizol's "Diesel Trends" series on its web site, as EMI booklet author Glenn Keller (formerly the executive director of Engine Manufacturers Association) told us.

Besides the anti-oxidant and anticorrosion property changes, electrical conductivity of diesel fuel also could be degraded by the loss of polar compounds during hydrotreating, Lubrizol contends. That could be an issue because some high-velocity pumps at fuel terminals can expose fuel to "significant static electrical charges," Lubrizol says.

As for the Chevron view on ULSD likely to be less corrosive, here's what Lubrizol's Keith Corkwell told us when we asked about that:

"The true effect of hydrotreating on the anti-corrosive properties of the fuel will depend on the composition of the fuel before and after hydrotreating . . . Hydrotreating to produce ULSD can certainly affect the fuel's lubricity, anti-corrosive, anti-oxidant and conductivity properties.

"While the magnitude of the effect (or possibly the direction of the effect in the anticorrosive properties) will depend on the fuel and the severity of the hydrotreating, all of these properties should be monitored by the refiner.

"Keep in mind that fuels are very complex mixtures and these properties can also be affected by refinery processes other than hydrotreating. The final effect on the fuel will depend greatly on how the refinery is rebalanced or reconfigured to produce ULSD."

Still another potentially major issue with ULSD is cold-flow, according to Infineum.

Reason: Some refiners will make changes to the way they make diesel fuel in order to accommodate limited hydrodesulfurization (HDS) capacity, Infineum points out.

"Changes in refinery operations are often needed, such as reducing [distillation] cut points to accommodate limited HDS capacity or altering fuel blending patterns to change the use of components between diesel fuel and heating oil," Infineum says.

"As with lubricity, the impact [of ULSD production] is refinery-specific: MDFI [middle distillate flow improver] response can get better or worse depending on several factors, including the crude oil source and the boiling range of the final blend. Maintaining low temperature performance will involve a change in the additive treatment - perhaps moving to a different treat rate, or changing to a new additive that is best suited to the type of fuel being produced.