Gas hold-up, the amount of gas retained in the column at a given time, is one of the key variables that determine how intense the contact between gas and liquid is. It can reach a stationary value or can vary periodically. Together with size and form factors, the actual throughputs, feed distribution, and whether or not there is a packing or additional mixing, hold-up values determine the extent of the interfacial area, as well as the flow regimes; these in turn determine transport efficiency (heat & mass).
Gas holdup is the averaged void fraction over the entire domain of the bubble column.
In many processes such as cultivation, slurry phase reactions, mixing and extractions in bubble columns, the gas is injected in or contact with liquid phase. The volume fraction of the dispersed gas phase in liquid is referred to as the gas hold‐up.
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