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The air-lift method—in which compressed air is used to lift liquids or liquids and solid particles—has been known for a long time. The principles of the method are fairly simple: air is injected into a vertical pipe and the rising bubbles carry the liquid along due to interfacial friction and the change of hydrostatic pressure. If solid particles are present near the bottom of the pipe, they will be sucked up as well due to the drag exerted on them by the liquid.

Even though the principle of air-lift is straightforward, its operation is not. Lack of direct control of the liquid and solid outputs (when compared to submersible liquid-solid pumps) makes the stable continuous operation a non-trivial task. To make it possible, a greater understanding of the process is required, which can be achieved through extensive experimental and modeling work.

We applied three-phase numerical modeling to investigate air-lift for various depths and pipe diameters. After model validation based on published data for shallower depths, the simulations were carried out for more relevant operating conditions. The graph illustrates the variation of the flow rates for all phases at the start of the air-lift operation; the initial flow rate oscillations die down to produce almost steady state operation. The time shift between top and bottom peaks and troughs is also worthy of notice.

As presented in the contour plots, prior to air injection, the radial particle distribution in the pipe is relatively uniform with a slightly higher concentration towards the centre. Once air is added, the distribution changes and keeps developing due to the bubble expansion with elevation change.

The demonstrated capability of air-lift modeling, which can provide substantial spatial and temporal details, enables the design of deep-sea lifting systems based on knowledge of the complex multi-phase flow inside the pipe.