Droplet Deposition in High- Pressure Natural-Gas Streams

Abstract

The Norwegian University of Science and Technology has been leading a project that investigates the phenomena related to high pressure gas liquid separation (HiPGLS) in natural-gas streams. At high pressures, liquid from gas cannot be separated with the known techniques.

The present is a study of the deposition phenomena at high pressures. Under these regimes, a dispersed phase composed of small droplets (from 10 to 100 microns) interacts with surfaces giving as outcome a complete deposition, a total rebound of the liquid droplet into the stream or some stage in between.

A novel experimental technique based on stereoscopic particle tracking is proposed. Experiments were run up to a pressure of 100 bar with three different systems. The interfacial tension values and liquid to gas density ratios achieved were low. The most direct observations are stochastic regions better described by a deposition efficiency. Most droplets interacting with a wetted film will not deposit. The increase of the deposition efficiency for the dry wall cases compared with wet walls was observed.

The basic governing equations for multiphase high pressure modelling are derived. The limitations of traditional techniques are exposed. Some solutions includes macroscopic results on simplified separator efficiency calculations and numerical simulations of microscopic droplet collisions. Furthermore, a theory where an additional variable is incorporated into the analysis, namely the diffusion coefficient, is presented with direct implications for the efficiency. A reduced diffusion, present in high pressure systems, was found to be an important coalescence inhibitor. Numerical simulations also showed that low droplet velocities benefits the deposition efficiency but decrease the deposition rate.

General conclusions are drawn from both the experiments and simulations regarding some system parameters. Higher gas densities, small droplet sizes, the presence of liquid films and other thermodynamic parameters where found to be responsible for low separation efficiencies.

The implications of the results on separator unit modelling and design are discussed. A section with suggested design criteria is provided.