Bubble characteristics measured using a monofibre optical probe in a bubble column and freeboard region under high gas holdup conditions

Local bubble characteristics, including gas holdups, bubble rise velocities, and chord lengths, were measured using a monofibre optical probe manufactured to withstand elevated pressures. Previous studies have validated the use of single tip probes for simultaneous measurement of local bubble properties at atmospheric conditions; however no study has been currently reported for these probes at elevated pressures. Experiments were conducted in a 101.6mm diameter column operating at pressures up to 9.0MPa. Surfactant addition and operating pressure were studied to simulate high gas holdups observed in many industrial reactors containing liquid mixtures with surface-active compounds. Experiments were hence completed using two liquid phases: tap water and a 0.5wt% aqueous ethanol solution. Liquid and gas superficial velocities were varied between 0–90mm/s and 0–150mm/s, respectively. Radial profiles at atmospheric conditions validated the probe measurements in water. Local holdups, rise velocities and chord lengths were adequately measured in water up to 9.0MPa. The probe struggled in the aqueous ethanol solution due to its physical constraints (i.e., tip diameter and sensing length) when compared to the significant bubble size reduction (chord lengths below 0.5mm). Comparisons with fluidized bed freeboard measurements demonstrated that flow through the bed enhanced bubble breakup for a coalescing system, but had a negligible impact with the added surfactant. [Display omitted] •Local gas holdups, rise velocities and chord lengths measured using 1C optical probe.•High gas holdup conditions achieved with elevated pressures and surfactant addition.•Probe measurements were comparable to global values in water up to 9.0MPa.•Significant bubble size reduction with surfactant diminished probe׳s effectiveness.•Freeboard and bubble column global hydrodynamics comparable at high gas holdups.