Effects of Operating Parameters on Flow Characteristics of Natural Gas in Supersonic Separators
Authors: Wen Chuang , Feng Yuqing , Cao Xuewen , Yang Yan , Witt Peter 
Published: May 2013

The supersonic separator is introduced to condense and separate water and heavy hydrocarbons from natural gas. The dynamic parameters of natural gas in a supersonic separator were simulated using the standard k-e and Redlich-Kwong real gas model. The pressure based implicit solver was used to solve the governing equations. The SIMPLE algorithm was applied to couple the velocity field and pressure. The results show that natural gas expands in the supersonic separator to supersonic velocities with resulting in low pressure and temperatures. In this calculation, the lowest pressure and temperature are about 20 atm and -82 °C, respectively, which creates good conditions for the nucleation and condensation of the water vapor and some higher hydrocarbons. The higher back pressure induces the shock position shift forward. If the back pressure is more than 75% of the inlet pressure, the shock moves into the nozzle, resulting in the re-evaporation of the condensed components. The Mach number will be less than unity at the nozzle throat and the flow will never be choked if the flow rate is lower than the designed.

Introduction

The natural gas usually contains some water vapour and free water in its exploitation. The presence of water vapour and free water causes a series of damages in the transportation and storage. For example, it increases the risk of the formation of gas hydrates with line plugging due to hydrate deposition on the pipe walls; it can result in corrosion combined with acid gases including carbon dioxide and hydrogen sulfide; also, it will reduce the delivery capacity of the pipelines as a result of a collection of free water. Therefore, the water vapour and free water have to be removed from natural gas.

At present, a lot of conventional techniques have been employed for the natural gas separation, such as absorption, adsorption, refrigeration, membranes and so on. The supersonic separation as a novel technique has been introduced to the natural gas processing from the the beginning of this century [1–5]. In essence, the supersonic separation technique is one of the refrigeration like the Joule-Thompson effect and Turbine expansion, both of which induce a low temperature for the condensation of water vapour.

The supersonic separator mainly consists of a Laval nozzle, a swirl device and a diffuser [6–8]. The Laval nozzle is used to expand the saturated gas to supersonic velocity, resulting in a low temperature and pressure. In these conditions, the nucleation of the water vapour and hydrocarbon may occur, followed by the droplet growth. The swirl device generates a high vorticity swirl to centrifuge the droplets to the wall and liquids are separated from the gas mixtures. And then the diffuser will recover 60–80% of the initial pressure of the dry gas, which will stay in the center area of the tube. There are a lot of advantages of this novel technique. Firstly, it is an enviorental friendly apparatus because it does not require chemicals and have no emissions to the environment. Secondly, it is a static device as it does not use rotating equipment. Also it is small size and light weight and enables unmanned operation for personnel safety. Therefore, it is suitable for platforms and subsea gas processing.