The Physics of Interdigitated Dielectrometry Sensors and Application as In-Situ Oil Oxidation Monitoring

Abstract

Monitoring of hydrocarbon oxidation is of great importance in many industry applications and reliable in-situ measurements are a challenge. In literature, it was shown that new versus degraded hydrocarbons show a change in their dielectric properties. In this thesis, the degradation of the oil was investigated by means of two thermal oil degradation experiments and the Fourier transform infrared spectroscopy. In addition, the impact. on the dielectric properties were determined using a novel type of a dielectric test cell that is temperature compensated. It was found that ketones, acids and moisture were generated through a thermal oil aging process. These products have been found to change in the dielectric properties of the liquid which are reflected through the complex permittivity. Ketones increased largely the real part of the permittivity and organic acids affected predominantly the imaginary part of the complex permittivity in a nonlinear fashion, which could be described using a modified polaron theory model. These measurements served as the base for the development of a novel kind of interdigitated sensor that can measure the dielectric properties such as the relative permittivity and the intrinsic conductivity with high accuracy and precision, without being affected by temperature. This is a crucial step in the development of a suitable in-situ sensor, as it does not need to undergo a complicated temperature curve compensation or calibration using calibration-liquids. The interdigitated sensor, made using cost efficient printed circuit board technology, exhibited an accuracy in measuring the complex permittivity of about 99%. The sensing precision was practically limited by the measurement instrumentation using a developed Faraday shield for the sensor. The sensor was used in an oil degradation experiment. to verify the in-situ capability. These measurements of the relative permittivity and conductivity yielded values such as a degree of oxidation and acidity number. For the first time: it was possible to measure in-situ the complex dielectric properties of liquids at temperatures between 20 °C to 140 °C using interdigitated sensors.