Matrix Effects Affecting the Indirect Calibration of the Static Headspace-Gas Chromatographic Method Used for Dissolved Gas Analysis in Dielectric Liquids

Abstract

To evaluate the effects of the density and polarity of the matrix on the dissolution of H2, O2, N2, CH4, CO, CO2, C2H2, C2H4, C2H6, C3H6, and C3H8 in various dielectric liquids, the gas−liquid partitioning coefficients (Ostwald coefficients) have been measured under the conditions of method C of ASTM D 3612-01 using a vapor−liquid-phase equilibrium technique. Altogether, 286 gas−liquid systems distributed throughout samples of mineral oils, vegetable oils, and synthetic oils were investigated with the aim of extending the scope of applicability of method C for dissolved gas analysis. The possibility of applying the method to a range of high-viscosity liquids (up to 120 cSt at 40 °C) was first demonstrated by measuring the equilibration of the species in the coexisting vapor phase of the samples. The effect of oil aging upon gas solubility was then addressed by assessing samples of a given mineral oil collected at different ages of in-service equipment. The oxidation of a weak polar dielectric was found to exert a negligible influence on the solubility of the nonpolar gas solutes, while for the only species with a permanent dipole moment (CO), an important reduction is seen in the solubility with the building up of oxidation products in the matrix. The Ostwald coefficients determined with samples of mineral oils (N = 13) obtained from naphthenic and paraffinic petroleum crudes showed that in the absence of strong intermolecular interactions, the solubility of the individual gases decreased with the matrix density as expected from the literature. Other measurements carried out with samples of vegetable oils (N = 7) revealed that solutes with highly polarizable π electrons (e.g., C2H2 and CO2) are susceptible to strong intermolecular interactions with some matrix polar components, which increase their ability to dissolve in this type of dielectrics. Last, the results collected with samples of synthetic oils (N = 6) were conclusive regarding the role played by some functional groups of the matrix components (carbonyl groups, conjugated double bonds, SiO bonds) in the intermolecular forces acting on the gas solutes. In light of these direct measurements of solubility data, the possibility of using a typical set of Ostwald coefficients for assessing mineral oils of various origins and as they age in the electrical apparatus was confirmed.