CELIA M. HENRY

As recently as a few years ago, who would have thought that we could have a detailed picture of the components of petroleum?

At one time, scientists thought that the average molecular weight of crude oil constituents was on the order of 10,000 daltons, with some components as large as 1 million Da. Now, they are finding that crude oil contains few components larger than 1,000 Da. At a symposium at Pittcon earlier this month, scientists presented research that is increasing the understanding of the composition of this complicated mixture.

Playing off the “omics” terminology of the biological field, symposium organizer Alan G. Marshall, chemistry professor at Florida State University and director of the ICR program at the National High Magnetic Field Laboratory (NHMFL), has dubbed this new focus “petroleomics” because it involves the simultaneous analysis of many components of petroleum products. “I felt if there are as many components in one sample of crude oil as there are genes in the genome, then it’s not crazy to use the word petroleomics,” Marshall told C&EN.

At the symposium, Oliver C. Mullins, a chemist at Schlumberger-Doll Research in Ridgefield, Conn., described research aimed at unraveling the composition of asphaltenes, the most aromatic and least soluble portion of crude oil. The classification of asphaltenes is based on solubility rather than chemical composition. Over the past 20 years, debate has raged in the petroleum field about the molecular weight of the compounds that make up the asphaltenes, Mullins said.

“Who cares about the molecular weight of the asphaltenes?” Mullins asked at the symposium. “Is this just for scientists or does it have an impact on operations?” As recently as 10 years ago, he might have said that it was just a question for scientists, but that changed with the development of deepwater facilities. “We don’t want to put in 50 miles of pipeline and then have it clog up.”

Mullins uses fluorescence depolarization, which measures the rotational diffusion constant of molecules, to determine the size of the components. The fluorescence depolarization data indicate that the asphaltenes are relatively lowmolecular-weight species, Mullins said.

Randall E. Winans, senior chemist and group leader in carbon chemistry at Argonne National Laboratory, described research to identify the classes of compounds responsible for corrosion in refineries. The acidic compounds, which are usually measured as a quantity called total acid number, are thought to be the culprits. Total acid number is measured by titrating how much base is required to neutralize crude oil. Using more sophisticated techniques such as high-resolution mass spectrometry, infrared spectroscopy, and nuclear magnetic resonance spectroscopy, Winans, working with Argonne’s Nancy Tomczyk and Chevron’s John Shinn, found that the acids in crude oil are not just aliphatic acids, but contain heteroatoms such as sulfur and nitrogen in addition to oxygen.

Winans has found that there is no straightforward correlation of total acid number with corrosiveness. For example, he described corrosive petroleum samples from the San Joaquin Valley and the North Sea that had high total acid numbers. In contrast, a Venezuelan oil had the lowest total acid number of the samples tested but was also very corrosive. “You need to be careful about using total acid number to predict corrosiveness,” he said.

Ryan P. Rodgers, an assistant scholar/scientist who works with Marshall at NHMFL, described the technique known as Fourier transform ion cyclotron resonance mass spectrometry. FT-ICR MS can distinguish between ions that differ in mass by less than the mass of an electron. Therefore, it can be used to separate compounds that have the same nominal mass.

So far, Rodgers and his coworkers have been able to resolve more than 11,000 compounds using positive-ion mass spectrometry and more than 6,000 compounds using negative-ion mass spectrometry. They can resolve even more compounds, including low-abundance species, by only looking at ions within a certain mass-to-charge ratio segment. They then combine the segments to stitch together the complete mass spectrum. By summing the segments, they can double the number of compounds they can identify.

One application of FT-ICR MS is differentiating crude oil samples from different parts of the world. At the symposium, Rodgers compared North American and Chinese crude oil. The North American crude oil was dirtier and was dominated by nitrogen-containing compounds. In contrast, the cleaner Chinese oil was dominated by oxygen-containing compounds.

The next step, Marshall says, is for the petroleum industry to start using the information that’s being gathered. “We can get the information that they didn’t have before. Now what they do with it will depend on them, because they’re the ones that know the other half, like which batches cause corrosion,” Marshall said.

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