Electrorheological effects of waxy crude oils under high-voltage AC electric field

Electrical treatment is an emerging method for improving the cold flowability of waxy crude oil by taking advantage of the electrorheological effect of the oil. Previous investigations have been focused on the effect of DC electric fields, with the effect of AC electric fields unknown. In this paper, experiments were made by exposing a waxy crude oil to AC electric fields with different frequencies, field amplitudes, and waveforms (square wave, sine wave, and triangle wave), and the viscosity and yield stress of the crude oil before and after the treatment were compared. It was found that the AC electric field could also improve crude oil cold flowability, but its electrorheological effect is somewhat weaker than that of a DC field under the same test conditions of treatment time and temperature. The lower the frequency, the stronger the effect. The essential reason could be the reduced force of charged particle migration under an AC field with a higher frequency. At the same frequency, the vis-cosity reduction is significantly improved with increased field strength amplitude. At the same field strength amplitude, the electrorheological effect of the three studied waveforms is ranked, from strong to weak, as square one, sinusoidal one, and triangle one, which is essentially attributed to the difference in the Root Mean Square (RMS) values of the field strength of each waveform. The higher the RMS value, the better the electrorheological effect. The main conclusion was confirmed by additional experiments using another crude oil.