Influence of small amounts of additives on gas hold-up, bubble size, and interfacial area

The gas-liquid interfacial area, which is determined by the gas hold-up and the Sauter mean bubble diameter, determines the production rate in many industrial processes. The effect of additives on this interfacial area is, especially in multiphase systems (gas-liquid-solid, gas-liquid-liquid), often not understood. The addition of a third phase can cause the gas-liquid system to become completely opaque, which means that conventional techniques to study the interfacial area cannot be used. For this reason ultrasonic spectroscopy was used in this work to study the interfacial area and the bubble size distribution in these systems. The influence of different additives on the interfacial area was studied in a stirred vessel and in a bubble column under coalescing and noncoalescing conditions. It was found that the addition of toluene to a noncoalescing electrolyte system decreased the interfacial area to a large extent by turning it into a coalescing system, due to the interaction between gas bubbles and liquid organic droplets. Furthermore, around the toluene solubility concentration, both the gas hold-up (measured using an electric conductivity technique) and the interfacial area increased to values similar to those observed in noncoalescing systems. The cause of this remarkable phenomenon lies probably in the presence of a small toluene layer around the gas bubbles, which can be formed beyond the solubility point. This layer is absent at concentrations below the solubility limit and a large surface tension gradient exists between those two situations, which can be responsible for the sharp change in coalescence behavior.