Shortcut Modeling of Natural Gas Supersonic Separation

Supersonic separation is a novel technology for natural gas separation. The theoretical design uniquely combines concepts from aerodynamics, thermodynamics, physical separation and fluid-dynamics resulting in an innovative gas conditioning process. It is used to condition the gas by removing condensable vapors and natural gas liquids. The supersonic separator is composed of a converging section, a Laval nozzle and a diverging section. Natural gas flows from reservoirs with low velocity and high pressure. In the supersonic separation process, the temperature drops below the dew point of the natural gas. A multiphase flow is formed. Undesired components form liquid condensates that are centrifugally removed through side collection streams.

In this work, a one-dimensional numerical model was developed. The model presents a great potential as a fast and accurate tool that enables the simulation of supersonic separators with significant details. The model is comprised of rigorous thermodynamic and phase equilibrium calculations as well as multi-phase sound speed calculations. Although the Peng-Robinson equation of state was used, the approach is general and compatible with other equations of state. The model fills certain gaps found in literature with a shortcut modeling technique. It would best fit the category of preliminary design tools with significantly lower computational loads. Significance associated with the shortcut model pertains to its ability to simulate supersonic separators at a fraction of the time taken by other existing numerical models. Although the results will not detail the profiles of the thermodynamic conditions along the nozzle, six fundamental points are accurately predicted. Those points represent the inlet, nozzle throat, side stream location, upstream of the shockwave, downstream of the shockwave and the nozzle exit. These fundamental points serve as the necessary pivotal information in the conceptual design of supersonic separators.

Validation tests included different working fluids, feed conditions, nozzle geometries, specifications for side streams and liquid phase recovery specifications. Reduction in simulation run time and computational load represented by number of locations tested in the diverging section was demonstrated. A decrease of 75%-97%, depending on the specific case, in either the simulation time, number of locations tested or both has been achieved.