Modelling gas–liquid flows in degassing risers used in extracting gases from gas-bearing volcanic lakes

Abstract

The paper presents a one-dimensional two-fluid model for the simulation of gas–liquid flow in vertical degassing devises used in extracting dissolved gases for energy production or for protection against lake overturns. The inter-phase mass transfer is determined assuming gas–liquid phase equilibrium with a gas mixture comprising two gases (CO2, CH4). The resolution of the conservation equations of the multiphase system makes it possible to describe the two-phase (gas–liquid) flow parameters along the degassing riser. The one-dimensional two-fluid model is applied to the simulation of the degassing devises installed in Lakes Nyos and Monoun respectively in 2001 and 2003. The results of the model at the outlet (gas volume fraction and mixture velocity) are compared with reported data and good concordance has been obtained. Number of important insights on the physical mechanisms of the degassing process have emerged from the numerical simulations the results of which have been commented with regard to the sensitivity of the model to the various closures and hypothesis.