Guilhem Caumette, University of Pau - IFP, France, outlines the techniques used to find metal contaminants in petroleum and how they will lead to superior fuels

Petroleum is, and always has been, the primary source of energy on our planet. The shortage of oil reserves, combined with increasing energy demands has brought a surge of interest in revisiting petroleum processing technology in the quest for better performing and cheaper fuels. Oil companies are searching for alternative sources of carbonaceous fuels, such as biofuels and gas condensates, or are trying to improve the efficiency of heavy crude oil (and heavy distillation fractions') conversion to transportation fuels. 

Metals and sulfur found in these heavy petroleum fractions can poison the catalysts used during refining processes. They also corrode equipment and contaminate the environment. So investigations focus on detecting metal content in petroleum and petroleum products. The metals' behaviour during the refining processes depends on their speciation - their chemical form. Knowing the size and structure of metal complexes is crucial in choosing catalysts suitable for removing them. Their identity can also give information about the geological origin and migration of oils, and can be used to find new oil fields.

 

Relatively little is known about the metal species in crude oils

 

 

Yet, despite progress in analytical methodology, relatively little is known about the metal species in crude oils. Metalloporphyrin complexes with nickel and vanadium are often cited, but non-porphyrin complexes with molecular weights reaching several thousand Daltons should also be present. More information is available about the forms of sulfur in petroleum or mercury and arsenic in gas condensates but there is no definite agreement on their exact nature. 

Analysing the elements in petroleum is a nightmare for analysts as the fuel contains thousands of molecules of different sizes and polarities, which interact with different forces. Direct  analysis is barely possible and multistep analytical techniques are required to separate and properly identify target metallomolecules. The interest in speciation of metals and other heteroelements in petroleum-related products has led analysts to develop dedicated analytical techniques and methods. They include direct analytical techniques such as x-ray absorption spectroscopy, electron paramagnetic resonance and molecular mass spectrometry (using laser desorption, electrospray and chemical ionisation) as well as hyphenated techniques that combine chromatography's high separation potential (e.g. high performance liquid chromatography or gas chromatography) with an element specific detector's sensitivity (e.g. atomic absorption spectrometry). 

As these dedicated techniques become more widely available, identifying metal species in complex organic mixtures such as petroleum becomes easier and meeting the challenge of removing them for cleaner fuels becomes a step away.

Read more in 'Element speciation analysis of petroleum and related materials' in issue 3, 2009 of Journal of Analytical Atomic Spectrometry.

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