2-Step Acid Digestion of Marine Fuels, Raw Oils and Distillation Residues as Rapid Alternative to Ashing/Fusion Procedures

Heavy petroleum products like marine fuels, crude oil and distillation residues are traditionally analyzed for trace metals after tedious ashing/fusion procedures (i.e. IP501 or ASTM D5708, D5863). This alternative microwave-assisted acid digestion method according to ASTM D7876 is faster and overcomes several serious drawbacks that the conventional methods suffer from.

Petroleum products are routinely analyzed for their elemental content. In crude, fuel oils, marine fuel-, and residual oils the concentrations of Si, Al, V, Ni, Fe and Na are used to define their quality and value. Ni and V in crude oil can deactivate catalysts during processing and initiate corrosion in motors and boilers during the combustion. Si and Al from catalyst break-down are abrasive in pumps and engines. Being present as particles requires very thorough high-shear mixing to ensure representative sampling.

Classical methods rely on dry ashing with/out fusion with subsequent acid digestion, where the sample is burnt off in a Pt-dish, then charred overnight in a muffle furnace. The inorganic residues (ash) are either taken up in a fluoride/borate flux or digested with acid in an open digestion system and finally brought in dilute acid to be analyzed as an aqueous solution. This method allows for processing of large sample quantities (> 10 g), but suffers from significant errors related to loss of volatile elements and process times of several hours up to two days. Common methods for dry ashing are IP 501, ASTM D5708 (Test Method B), or D5863 (Test Method A).

Due to the lower limits of determination, modern measuring techniques such as ICP-OES or ICP-MS do not require several grams of sample to obtain accurate analytical data.

In addition, appropriate sample preparation techniques like microwave-assisted acid digestion in closed-vessels are required. Multiwave’s Rotor 8NX systems provide the ability of high-speed pressure monitoring and control for all digestions of reactive petroleum products.

 

The release of the new standard practice ASTM D7876 (which covers sample digestion by using microwave heating) confirms the increased importance of this sample preparation method in the petrochemical industry.

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