Visualization Measurement of Gas Absorption Process of Single Droplets in CO₂ Atmosphere Using Acoustic Levitation Method

Abstract

On-board CO₂ capture technology has attracted attention as a method for reducing GHG (greenhouse gas) from ships by scrubbing the ship’s exhaust gas with wash water to absorb CO₂. In this study, an acoustic levitation method was incorporated to visually observe and detail the CO₂ absorption process by droplets sprayed from a scrubber. First, an acoustic levitation device with enhanced structural strength and self-supporting capabilities was constructed, and droplets were levitated in an air atmosphere using the modified device. Subsequently, the ambient gas was replaced with CO₂, and the behavior of the levitated droplets was directly observed. Three types of aqueous solutions with varying pH values (water and two types of NaOH solutions) were used and a pH indicator (litmus reagent) was added beforehand at a concentration of 0.15wt%. As a result, it was calculated that pH of the droplet of NaOH aqueous solution with an initial pH of around 9 decreased by approximately 2.9 based on the hue change of the droplet. Under all CO₂ flow conditions, the pH was lower than that of the water droplets. On the other hand, the droplet in pH was slight, ranging from 0.1 to 0.3, for water droplets and the droplets of NaOH aqueous solution at pH over 13. The droplet shape started to flatten as CO₂ was supplied, eventually leading to a rapid decrease in the aspect ratio, and finally, a nearly spherical shape was obtained for all experimental conditions before the droplet fell. Finally, the CO₂ absorption amount and absorption rate were quantitatively evaluated from the pH changes of the levitated droplets measured through visualization.