Simulation of a slurry airlift using a two-fluid model

A two-fluid approach is used to simulate the flow of a three-phase mixture in an internal-loop airlift reactor. The loop consists of a riser, in which gas is injected and two downcomers on both sides. The airlift loop is at first treated as a two-phase system (gas-liquid). A full two-fluid formulation is used to describe the dynamics of the two-phase how. Subsequently, the distribution of the third, solid phase is considered by solving a mass balance in which the solids velocity is the superposition of the liquid velocity, a gravitational settling and turbulent dispersion. An extended version of Tchen's theory is applied for the dispersed phases; turbulence in the carrier-phase is modelled by a modified k-epsilon equation. The model is implemented in the in-house finite-volume code DISSIM (Lathouwers, D. (1999). Modelling and simulation of turbulent bubbly how. Ph.D. Thesis), a 2-D pressure-based code. Variation of the gas flow rate revealed the existence of different how regime with respect to the gas fraction in the downcomer, in agreement with literature. In case the gas separation at the top is complete, the circulation velocity simulated agreed well with a simple mechanical energy balance. The solid phase is found to accumulate in the dead corners if the turbulent dispersion is ignored. The settling process is very slow and part of the solids are trapped in the circulation of the liquid. When the turbulent dispersion is taken into account a smooth solids distribution is found with a higher volume fraction of solids in the lower part of the downcomers, where the liquid velocity is minimal. (C) 2001 Elsevier Science Ltd. All rights reserved.