The Formation of Bubbles and Drops

This chapter describes the single bubble formation at isolated nozzles both under constant flow and constant pressure conditions, though considerable work still needs to be done in the intermediate region where bubble size is highly influenced by resonance effects. The method of dispersion through submerged nozzles, slots, or holes is the simplest and hence the most common. It permits equipment of extremely simple design and leads to reasonably large interfacial areas. Some industrial operations involving bubble and drop formation are extraction, direct-contact heat exchange, distillation, absorption, sparger reactors, spray drying and atomization, fluidization, nucleate boiling, air lifts, and flotation. In all these operations involving bubbles and drops, three stages have to be studied: the formation of bubbles or drops, the movement of bubbles or drops through the continuous phase and possible coalescence therein, and the formation of the interface. Various factors that influence bubble size are experimental set-up, effect of orifice characteristics, chamber volume, submergence, surface tension of the liquid and the wetting properties of the orifice, liquid viscosity, liquid density, gas properties, effect of gas-flow rates, and effect of continuous-phase velocity. The two-phase theory of aggregative fluidization considers the bed to be made up of two parts: a particulate phase wherein the gas flow rate corresponds to that required for incipient fluidization and a bubble phase, which conveys the extra gas through the bed in the form of bubbles.