Characterization of naphthenic acids in crude oils and refined petroleum products

Abstract

Naphthenic acids (NAs) or naphthenic acid fraction compounds (NAFCs) are generally recognized as a family of cycloaliphatic carboxylic acids naturally occurring in petroleum. These acid extractable organics (AEOs) from petroleum industry activities including oil sands produced waters (OSPW) have led to increasing environmental concern in recent years due to their potential release into the environment. This study presents a characterization of naphthenic acids in a number of crude oils and refined petroleum products from various sources. NAs with unsaturated degree of z −2 to z −24 and carbon number ranging from 6 to 60 were determined by liquid chromatography-high resolution Orbitrap mass spectrometry (LC-HRMS). NA profiles generally vary from oil to oil. Conventional light crude oils generally contain low concentrations of NAs, while heavier crudes and oil sands bitumen contain significant levels of NAs. NAs in Federated and Alaska North Slope crude oils are relatively low with O2-NA concentrations of 139 µg/g and 419 µg/g, respectively, while their abundances are as high as 7994 µg/g in Venezuelan Orinoco bitumen. The ratio of even to odd (E/O) carbon number NAs in all petroleum oils studied is close to 1.0 for z −2 to z −24 NAs. NA series z 0 to z −12 in bitumen account for about 90% of the total determined NAs, and z −14 to z −24 NAs make up the remaining ∼10%. Moreover, z −2∼−6 (1- to 3-ring) NAs are the most predominant in all oil samples. In terms of distribution according to carbon number, C6 to C21 NAs (α-group) make up about 44.2% of the total NAs in an Alberta oil sands bitumen. Caustic extraction of oil sands bitumen mostly transports the more toxic low-molecular weight portion of NAs into OSPW, in which α-group NAs constitute about 90% of the total NAs. Evaporation (up to 23.6% by weight) weathering slightly enriched the NAs but likely did not affect the distribution profile of NAs in a diluted bitumen (dilbit).