Effect of ultrasound irradiation on asphaltene aggregation and implications to rheological behavior of bitumen

Abstract

The present work investigates the contribution of asphaltene aggregation to bitumen viscosity subject to ultrasound irradiation. A West-African bitumen with a viscosity of 12043cP at room temperature was sonicated at low (38 kHz) and mild frequency (200 kHz) under controlled gas environment including air, nitrogen (N₂) and carbon dioxide (CO₂). The rheology of the bitumen, asphaltene content analyses as well as spectral studies were conducted. Herein was found that sonicating the bitumen at 200 kHz under air-environment reduces the initial viscosity up to 2079cP, which was twice larger than that obtained when a low frequency was used. In respect of the gas environment, it was shown that ultrasound irradiation under N₂ environment could lower the bitumen viscosity up to 3274cP. A positive correlation between the asphaltene content and the viscosity reduction was established. The results from the spectral analyses including Fast Fourier Infrared and the observations from Scanned Electron Microscope were consistent with the rheological studies and led to the argument that the viscosity reduction results from either the scission of long chain molecules attached to the aromatic rings (when the applied frequency was altered under fixed gas environment) or the self-aggregation of asphaltene monomers (when gas environment was changed at fixed frequency).

Keywords: Asphaltene; Bitumen; Bitumen viscosity; Sonication; Ultrasound.

Fig. 1

Schematic representation of ultra-sonication equipment used for bitumen irradiation; (1) submersible traducer; (2) bitumen sample; (3) thermostatic water bath; (4) thermocouple; (5) gas flowmeter; (6) gas holding tank.

Fig. 2

Photographs of (a) fresh bitumen and sonicated bitumen after (b) 2 h and (c) 3hr; no efforts were performed to trap the light ends of the oil.

Fig. 3

Sample photographs of BT-1 exposed for 3 h at different temperatures.

Fig. 4

Apparent viscosity of the bitumen before and after sonication under at 70 °C under air environment.

Fig. 5

(a) Influence of applied ultra-sound frequency conducted under air environment and (b) influence of gas environment on viscosity alteration after 3 h.

Fig. 6

Viscosity alteration as function of temperature of (a) applied ultra-sound frequency and (b) gas environment.

Fig. 7

Influence of (a) applied frequency and (b) gas sonication environment on activation energy estimated from Arrhenius equation (Eq. (4)).

Fig. 8

Infrared spectra of fresh and sonicated bitumen under different (a) applied frequency and (b) different gas-environment; the spectra were acquired after 1 h of sonication.

Fig. 9

SEM photographs of asphaltene precipitation at different ultrasound frequencies.

Fig. 10

SEM images of the surface morphology of asphaltenes under different gas environment.

Fig. 11

Effect of applied frequency (b, c) and gas enviroment (d, e, f)on viscosity enhancement and asphaltene yield.