Experimental investigation of the surfactant effect on liquid removal in vertical pipes

Abstract

In order to investigate the effect of surfactant on liquid removal in vertical pipes, an experimental study has been conducted in a 50 mm inner diameter vertical pipe. The superficial gas velocities ranged from 0.06 m/s to 27 m/s to include bubble, slug, churn, and annular flow. The superficial liquid velocities were 0.01 m/s and 0.1 m/s. The surfactant concentration was 2000 ppm. In the presence of surfactant, foaming postpones film reversal dramatically and makes the flow much more regular at low superficial gas velocities, due to the decrease of film density and increase of interfacial-friction factor. However, the slug-churn and bubble-slug transitions are barely influenced with surfactant injecting. At high superficial gas velocities, adding surfactant in the air-water flow creates an increased frictional pressure gradient, leading to an increase in the total pressure gradient. When the superficial gas velocity decreases to some point, there is a limit where the pressure gradients in the flow with and without surfactant are equal. A higher superficial liquid velocity leads to a smaller value for this equality. Thus, proper timing for surfactant injection is of significant importance in gas wells. As the superficial gas velocity decreases, the lifting efficiency of the surfactant increases at first and subsequently decreases dramatically, due to the interaction effect of liquid holdup and hydrodynamic agitation. An optimal area in lifting efficient is in the churn flow near the slug-churn transition. Furthermore, a decrease in lifting efficiency is observed with the increase of superficial liquid velocity and thus we infer that the transition to dispersed bubble flow is the limit for superficial liquid velocity to prevent foaming.