Fast and Efficient Numerical-Simulation Method for Supersonic-Gas Processing

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 131239, ’A Fast and Efficient Numerical-Simulation Method for Supersonic-Gas Processing,’ by Dengyu Jiang, Qitai Eri, Changliang Wang, Huoxing Liu, and Yao Yuan, Beijing University of Aeronautics and Astronautics, prepared for the 2010 CPS/SPE International Oil & Gas Conference and Exhibition in China, Beijing, 8-10 June. The paper has not been peer reviewed. Supersonic-swirling-separation technology separates heavy hydrocarbons and water vapor from natural gas. The de Laval nozzle, where condensation occurs, is used to generate supersonic flow and achieve a high degree of supersaturation in a natural-gas dehydration unit. To optimize the structure of the nozzle and achieve higher separation efficiency, numerical simulation was used to accelerate development cycles and reduce the cost of experiment. Rather than use multiphase models and a real-gas model, a quick and efficient method was validated and used to determine the location of the nucleation zone and the droplet-growth zone. Introduction Spontaneous condensation occurs when a supersaturated gas passes through a strong expansion process. As part of the process, latent heat is released to the environment and the latent heat makes the state of the parameters of the flow field (e.g., temperature, pressure, and density) change suddenly, the so-called condensation-jump phenomenon. If the amount of heat released exceeds some critical value, the flow becomes thermally choked and the structure of the flow field changes, even forming a condensation shock. Condensation can be divided into two physical processes: nucleation and droplet growth. Homogeneous nucleation occurs when the condensation nuclei are gas molecules that collect spontaneously rather than because of contact with dust particles. Then, the nuclei grow into droplets because of gas molecules colliding with and adhering to the surface of other gas molecules under chemical reaction. Nonequilibrium condensation occurs if the scale of the change of gas-state parameters is much less than the time scales of gas molecules adhering to and merging with other particles. At that moment, the gas is supersaturated, indicative of a metastable non-equilibrium state of the vapor phase. As these metastable clusters exceed some critical size, the process of droplet growth sets in, leading to the formation of a stable liquid phase. The full-length paper details the numerical-simulation method of condensation. Adapting This Method for a de Laval Nozzle The engineering objective of super-sonic-gas processing is superior dew-point depression and a high-pressure recovery; therefore, research on the interaction between the condensation and the flow condition provides some directive standards. To determine the interaction between condensation and the flow conditions, this method was adapted to model the nonequilibrium-condensation flow field in a de Laval nozzle. Fig. 1 shows the nozzle and structure mesh. By use of precision and efficiency, dramatic-change regions of density in the flow field and the near-wall area were refined, and wall functions were chosen for solving the boundary layer.