Numerical Simulation on Hybrid Lifting Operation of Polymetallic Nodules and Rare-Earth Elements-Rich Mud by Air-Lift Pump in Deep Sea around Minamitorishima Island

Polymetallic nodules and REE-rich mud under the seabed of 5500–5700 m water depth around Minamitorishima island are promising and attractive for exploration and development. Following our previous research, numerical analysis was used to investigate the unsteady flow characteristics and the lifting performance of a commercial production system using an air-lift pump for hybrid lifting, lifting both polymetallic nodules and REE-rich mud. Gas–liquid–solid three-phase flow and gas–liquid two-phase flow in the system were analyzed using the one-dimensional drift–flux model. First, the reliability of the schemes and program was verified by comparing the numerical results with the experimental ones. Next, numerical simulations were conducted, in which the model’s dimensions were related to a commercial production system operated in the deep sea around Minamitorishima island, and the conditions fit the expected production rate. The results revealed the unsteady flow characteristics under the operations, such as start-up, shut-down, feed of polymetallic nodules and REE-rich mud, and those associated with disturbances, such as feed rate fluctuations. We demonstrate that the program and the schemes can simulate the unsteady flow characteristics and the lifting performance of a commercial production system with an air-lift pump well, and they can derive useful information and know-how in advance for the safe and continuous operation of the system.