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Bubbling gas through a liquid is common in many processes like air sparging to remove contaminants from water, or froth flotation to recover minerals. Performance of these processes typically depends on the amount of bubble surface area available for the process to occur. This can be described in terms of the area holdup in a vessel – the amount of bubble surface area per volume of liquid.

Describing aeration with area holdup is useful for comparisons between different scales, e.g., scaling a commercial process to laboratory scale for testing, or sizing commercial equipment based on pilot tests. To calculate area holdup, gas holdup and bubble size must be known. Gas holdup is the fraction of gas in an aerated liquid and can be measured with a variety of techniques. Bubble size, however, can be tricky.

Bubble size can be measured by imaging, although this is often only works in laboratory systems with optical access and clear liquids. Many bubbles throughout the vessel must be visualized to get an appropriate average size. Isokinetic sampling is also used sometimes. In this method, bubbles are captured from an aerated vessel and visualized. As for imaging, this method is limited to suitable liquids and vessel geometries.

It is often more practical to estimate bubble size. This can be done using the drift flux method. Given fluid properties, gas flow rate and gas holdup in a vessel, bubble size can be determined based on the flux of gas relative to the liquid phase. This allows bubble size to be determined from measurable quantities. The underlying principle is that the superficial velocity of the gas through a vessel depends on the balance of buoyancy driving the gas flow and viscous drag resisting it. Using appropriate mathematical expressions for the buoyancy and drag forces, the bubble size can be calculated.

The bubble size determined from the drift flux method is an average value over the entire vessel. Based on experimental studies in the literature, it corresponds well to the relevant bubble surface area for various area-dependent processes.