Initial studies using ultrasonic spectroscopy for monitoring changes in residua with pyrolysis

Ultrasonic spectroscopy was explored for monitoring microstructure changes during pyrolysis of petroleum residua. Attenuation and sound velocity measurements were obtained for original and pyrolyzed residua using ultrasonic transducers with frequencies ranging from 1 to 10 MHz. Transducers at 1.0, 2.2, and 3.5 MHz provide signals that can be measured at room temperature. Little useful signal passes the solid samples at room temperature above 4 MHz. Symmetrical amplitude vs. time pulse signals are observed through water, which exhibits essentially no attenuation of ultrasound. When the ultrasound signal passes through a solid residuum sample, however, the raw pulse signal in the time domain shows significant asymmetry. By obtaining fast Fourier transform (FFT) of the time domain waveforms, amplitude vs. frequency spectra are obtained. The FFT spectrum for water is symmetrical. For FFT spectra of solid residua, the higher frequencies are attenuated more than the lower frequencies. Maximum intensities are near 1.1 MHz, regardless of the frequency of the transducer. For the 2.2 and 3.5 MHz transducers, the FFT spectra for all of the residua studied exhibit a shoulder above 1 MHz. The shoulder diminishes early during pyrolysis as the solvation shell structures associated with the suspended particles are destroyed, and the nature of the particles changes. The FFT shoulder begins to grow again as pre-coke particles form.