The impact of methanol mass transport on its conversion for the production of hydrogen and oxygenated reactive species in sono-irradiated aqueous solution

Abstract

This study aims principally to assess numerically the impact of methanol mass transport (i.e., evaporation/condensation across the acoustic bubble wall) on the thermodynamics and chemical effects (methanol conversion, hydrogen and oxygenated reactive species production) of acoustic cavitation in sono-irradiated aqueous solution. This effect was revealed at various ultrasound frequencies (from 213 to 1000 kHz) and acoustic intensities (1 and 2 W/cm²) over a range of methanol concentrations (from 0 to 100%, v/v). It was found that the impact of methanol concentration on the expansion and compression ratios, bubble temperature, CH₃OH conversion and the molar productions inside the bubble is frequency dependent (either with or without consideration of methanol mass transport), where this effect is more pronounced when the ultrasound frequency is decreased. Alternatively, the decrease in acoustic intensity decreases clearly the effect of methanol mass transport on the bubble sono-activity. When methanol mass transfer is eliminated, the decrease of the bubble temperature, CH₃OH conversion and the molar yield of the bubble with the rise of methanol concentration was found to be more amortized as the wave frequency is reduced from 1 MHz to 213 kHz, compared to the case when the mass transport of methanol is taken into account. Our findings indicate clearly the importance of incorporating the evaporation and condensation mechanisms of methanol throughout the numerical simulations of a single bubble dynamics and chemical activity.

Keywords: Acoustic bubble; Bubble sono-activity; Bubble sonochemistry; Methanol evaporation and condensation; Ultrasound.

Fig. 1

Temporal evolution of bubble radius (a), temperature (b), number of moles of methanol (c) and its mole fraction (d) within the oscillating Ar-CH₃OH [0, 10, and 80 % (v/v)] bubble. Conditions: frequency = 355 kHz, Intensity = 1 W/cm², Liquid temperature = 20 °C, Static pressure = 1 atm. It should be noted that (b)-(d) are revealed at around the bubble collapse.

Fig. 1

Temporal evolution of bubble radius (a), temperature (b), number of moles of methanol (c) and its mole fraction (d) within the oscillating Ar-CH₃OH [0, 10, and 80 % (v/v)] bubble. Conditions: frequency = 355 kHz, Intensity = 1 W/cm², Liquid temperature = 20 °C, Static pressure = 1 atm. It should be noted that (b)-(d) are revealed at around the bubble collapse.

Fig. 2

Expansion ratios (R_max/R₀) as function of methanol concentration (from 0 to 100 %, v/v) over a range of ultrasound frequency (from 213 to 1000 kHz) either with (a) or without (b) consideration of methanol evaporation and condensation.

Fig. 3

Compression ratios (R_max/R_min) as function of methanol concentration (from 0 to 100%, v/v) over a range of ultrasound frequency (from 213 to 1000 kHz) either with (a) or without (b) consideration of methanol evaporation and condensation.

Fig. 4

Maximal bubble temperature as function of methanol concentration (from 0 to 100%, v/v) over a range of ultrasound frequency (from 213 to 1000 kHz) either with (lines) or without [dashed lines) consideration of methanol evaporation and condensation.

Fig. 5

Total production of the bubble (a) and methanol conversion (b) as function of methanol concentration (from 0 to 100%, v/v) over a range of ultrasound frequency (from 213 to 1000 kHz) either with (lines) or without (dashed lines) consideration of methanol evaporation and condensation.

Fig. 5

Total production of the bubble (a) and methanol conversion (b) as function of methanol concentration (from 0 to 100%, v/v) over a range of ultrasound frequency (from 213 to 1000 kHz) either with (lines) or without (dashed lines) consideration of methanol evaporation and condensation.

Fig. 6

Molar production of hydrogen (a) and reactive oxygen species (^•OH, HO₂^• and H₂O₂) (b) as function of methanol concentration (from 0 to 100 %, v/v) over a range of ultrasound frequency (from 213 to 1000 kHz) either with (lines) or without (dash lines) consideration of methanol evaporation and condensation.

Fig. 6

Molar production of hydrogen (a) and reactive oxygen species (^•OH, HO₂^• and H₂O₂) (b) as function of methanol concentration (from 0 to 100 %, v/v) over a range of ultrasound frequency (from 213 to 1000 kHz) either with (lines) or without (dash lines) consideration of methanol evaporation and condensation.

Fig. 7

Maximum bubble temperature as function of methanol concentration (from 0 to 100%, v/v) and acoustic intensity (1 and 2 W/cm²) either with (lines) or without (dashed lines) consideration of methanol evaporation and condensation.

Fig. 8

Total molar production of bubble as function of methanol concentration (from 0 to 100 %, v/v) and acoustic intensity (1 and 2 W/cm²) either with (lines) or without (dashed lines) consideration of methanol evaporation and condensation.

Fig. 9

Molar conversion of methanol as function of its concentration (from 0 to 100 %, v/v) and acoustic intensity (1 and 2 W/cm²) either with (lines) or without (dashed lines) consideration of methanol evaporation and condensation.