Petroleum Mineral Oil Refining and Evaluation of Cancer Hazard

Abstract

Petroleum base oils (petroleum mineral oils) are manufactured from crude oils by vacuum distillation to produce several distillates and a residual oil that are then further refined. Aromatics including alkylated polycyclic aromatic compounds (PAC) are undesirable constituents of base oils because they are deleterious to product performance and are potentially carcinogenic. In modern base oil refining, aromatics are reduced by solvent extraction, catalytic hydrotreating, or hydrocracking. Chronic exposure to poorly refined base oils has the potential to cause skin cancer. A chronic mouse dermal bioassay has been the standard test for estimating carcinogenic potential of mineral oils. The level of alkylated 3-7-ring PAC in raw streams from the vacuum tower must be greatly reduced to render the base oil noncarcinogenic. The processes that can reduce PAC levels are known, but the operating conditions for the processing units (e.g., temperature, pressure, catalyst type, residence time in the unit, unit engineering design, etc.) needed to achieve adequate PAC reduction are refinery specific. Chronic dermal bioassays provide information about whether conditions applied can make a noncarcinogenic oil, but cannot be used to monitor current production for quality control or for conducting research or developing new processes since this test takes at least 78 weeks to conduct. Three short-term, non-animal assays all involving extraction of oil with dimethylsulfoxide (DMSO) have been validated for predicting potential carcinogenic activity of petroleum base oils: a modified Ames assay of a DMSO extract, a gravimetric assay (IP 346) for wt. percent of oil extracted into DMSO, and a GC-FID assay measuring 3-7-ring PAC content in a DMSO extract of oil, expressed as percent of the oil. Extraction with DMSO concentrates PAC in a manner that mimics the extraction method used in the solvent refining of noncarcinogenic oils. The three assays are described, data demonstrating the validation of the assays are shown, and test results of currently manufactured base oils are summarized to illustrate the general lack of cancer hazard for the base oils now being manufactured.

Scientific Opinion on Mineral Oil Hydrocarbons in Food

“…BaP may be inhibited by less active MOAH) (Brooks et al, 1995;Booth et al, 1998;Mackerer et al, 2003). Hence, it is not possible to sum up the activity of a number of single fractions.…”

“…With the exception of highly purified oil varieties consisting of alkanes and naphthenes, all mineral oils are mutagenic in the Salmonella typhimurium assay ( IARC, 1984;Roy et al, 1988;Mackerer et al, 2003). It is the aromatics, including alkylated PAHs, that cause the mutagenicity of mineral oils and this fraction is also responsible for the formation of DNA adduct in mice skin following skin painting (Granella and Clonfero, 1991;Ingram et al, 2000).…”

“…Most studies on the genotoxicity of mineral oil and mineral oil fractions have been performed in the Salmonella typhimurium mutagenicity assay (Blackburn et al, 1984(Blackburn et al, , 1986IARC, 1984IARC, /1987Roy et al, 1988;Granella and Clonfero, 1991;Brooks et al, 1995;Mackerer et al, 2003). With the exception of highly purified oil varieties consisting of alkanes and naphthenes, all mineral oils are mutagenic in the Salmonella typhimurium assay ( IARC, 1984;Roy et al, 1988;Mackerer et al, 2003).…”

“…These modified methods include prior fractionation by solvent extraction (with DMSO alone or in combination with cyclohexane) in order to concentrate the total polycyclic aromatic hydrocarbon fraction. Also chromatographic separation has been employed for testing of individual components (Epler et al, 1979;Brooks et al, 1995;Mackerer et al, 2003). Since the mineral hydrocarbons are not soluble in water, procedures involving emulsification of oils have also been used (Brooks et al, 1995).…”

“…It is mainly the alkylated and non-alkylated (usually low amounts from oil, but they may accumulate at high temperatures) aromatics of 3-7 fused rings including sulphur containing aromatics that causes genotoxicity (mutagenicity and DNA-adduct formation) (and carcinogenicity) of mineral oils (Roy et al, 1988;Mackerer et al, 2003). Because of this also two other tests in addition to the modified Ames assay have been used for classification of mineral oils; the measurement of 3-7-ring aromatics by extraction and chromatography (Mobil method) and the gravimetric IP 346 assay based on extractions and the weight of the MOAH fraction after removal of solvent (Mackerer et al, 2003). The IP 346 test has been approved for printing ink mineral oils by the Institute of Petroleum and is accepted by the European Chemical Agency (ECHA) for carcinogenicity classification and labelling of mineral oils.…”