The vertical flow centrifugal gas-liquid separator and its application to the removal of gases from geothermal wells

Abstract

Two vertical flow centrifugal gas-liquid separators were designed, built and tested using steam-water and air-water mixtures as the two-phase fluids. The theoretical analysis of the vertical flow centrifugal gas-liquid separator and the experimental data from the model separators is given in this report. It has been shown that the performance of the vertical flow centrifugal separator is determined by the inlet mass flow rate, vapour-phase mass fraction and the dimensions of the separator components. The important dimensions influencing the performance of the separator have been shown to be the gastube diameter, the gastube inlet diameter and the design of the vortex generator. Large gastube diameters have been shown to give high separation efficiencies. Sharp vortex generator blade exit angles have been shown to give the best performance especially with slugging inlet flow which occurs at relatively low vapour mass fractions. The distance between the vortex generator and the gastube inlet has been shown to have little effect on the separation efficiency. Improvements in the design of the vortex generator, especially the curvature of the blades and the diameter of the hub have been shown to increase both the mass flow capacity and separation efficiency, and to reduce the overall pressure drop which mainly occurs across the vortex generator. The amount of flashing caused by the vortex generator has been shown to be small. The effects of the various parameters on the separation efficiency are discussed in this report. The vertical flow centrifugal separator has been shown to be capable of liquid separation efficiencies close to one over a wide range of inlet flow conditions. Vapour separation efficiencies in excess of 90% have been shown to be possible and higher efficiencies may be obtained when the dimensions of the separator are optimised. The application of the vertical flow centrifugal separator to the removal of gases from geothermal wells has been investigated. A method of estimating the onset of two-phase flow in a geothermal well and hence the location of a separator has been presented. Methods for calculating the pressure drops between the separator and the wellhead have been presented. Sample calculations illustrating the effect of reservoir properties, fluid chemistry and well casing program on pressure drops have also been presented. A general discussion on vertical two-phase flow is given as a background for these calculations. The theory of vapour-liquid equilibria, and the properties of the main components of geothermal fluids have been included as appendices. The available literature on other methods of gas removal is reviewed and a preliminary comparison of some of these other methods and the downhole separator is also presented. Recommendations for future work and other uses of the vertical flow centrifugal separator are given at the end of this report.