Characterization of the wax precipitation in Iranian crude oil based on wax appearance temperature (WAT): The influence of ultrasonic waves

Abstract

Abstract In this study, the effect of ultrasonic waves on the wax appearance temperature (WAT) of a crude oil sample was investigated. For this purpose, the crude oil was exposed to ultrasonic radiation at 2, 4, 6, 8, and 10 min. Then, the WAT of each sample was measured using the cross polarized microscope (CPM) method. The WAT was reduced to 8 °C and 12 °C at 4–8 min and 10 min, respectively. The origin of this reduction by ultrasonic radiation were studied by Fourier-transform infrared (FTIR) spectroscopy analysis. Because of the ultrasonic radiation, the first heavy crude compounds are cracked to lighter compounds, which reduces the side alkyl chain index in the FTIR spectra. Depending on the temperature of the crude oil after the irradiation, some light petroleum components, such as aliphatic components evaporated, resulting in the reduction of the aliphatic index calculated from the FTIR spectra. The wave radiation causes the wax crystals to become smaller, and their diameter dispersion reduces. The crystals were seen as rod-type before the irradiation, but the ultrasonic radiation caused the wax crystals to be rounded. Moreover, the wax crystals became morphologically more uniform. These waves, because of their high energy levels, cause changes in the percentage of saturate, aromatic, resin, and asphaltene (SARA) components of the crude oil, that are effective in altering the WAT. The aromatic compounds of the crude oil are pyrolyzed in two ways. First, in the presence of the saturate compounds, they are converted to resin. The resin, as a surfactant, reduces crystallization seeds in the oil and reduces the WAT point. Secondly, the aromatic compounds are first converted into the liquid saturate, and they evaporate if energy is supplied. The asphaltenes are also sensitive to pyrolysis by transferring energy into the oil and converting into the saturate compounds, especially methyl. Increasing the resin and light components within the structure of oil, such as paraffin, and reducing the aromatic and asphaltene compounds eventually reduced WAT. Finally, the shape of wax particles, isolated by IP143 method, was analyzed by scanning electron microscopy (SEM) imaging. The high-energy ultrasonic waves caused the average size of wax grains to reduce, the space between them to be greater, and the form of seeds changed from rounded to snow-like and irregular. Also, after the irradiation, bright spots were observed at the surface of the wax grains, which is related to the presence of heavy metals.