Research On Designing Of Laval Nozzle And Characteristics Of Phase Transition For Supersonic Swirling Separator

Gas dehydration and heavy hydrocarbon separation play an important role in field operation of oilfield. Traditional low-temperature separation has proven inefficient and costly, requiring high capital and running cost. To that question, a new gas conditioning process—supersonic gas/liquid separator has been developed. It is a revolutionary gas conditioning process.The supersonic swirling separator is composed of nozzle, rectification tubular section, supersonic wing section and diffuser. Natural gas is induced to efficiently expand and flow at supersonic velocity through Laval nozzle so as to decrease the temperature below which heavy hydrocarbon and water begin to condense and form separate droplets. Then the droplets are separated from the natural gas through swirling. The separator has thermodynamics similar to a turbo-expander, combining thermal insulation expansion, low temperature condensation, swirling separation and re-compression in a compact tubular device. It has no moving parts, and the need for associated regeneration systems is eliminated. The new technology has the potential to lower capital and operating expenditures by 10 to 25%.The pressure of natural gas in the supersonic swirling separator is as high as to several MPa and the temperature is as low as to minus decades degrees, this is a kind of high pressure and low temperature gas, so it should be computed as the real gas.Based on the hypothesis of the flow in the Laval nozzle is a one dimension and reversible flow, the paper make use of the equation of BWRS, computed the different critical parameters under the different inlet conditions and the relevant area of the throat. Based on the theory of gas dynamics, thermodynamics and fluid dynamics, the paper presents designing methods for the Laval nozzle of supersonic swirling separator. Subsonic convergent section of Laval nozzle is designed according to BumowuHCKuu method, throat is designed as a smooth circular arc, and supersonic divergent section is designed according to Foelsch method. The stream from this type of nozzle has uniform velocity with designed Mach number.Based on the theory of phase transition, gas dynamics and the equation of state of real gas, the paper has set up the mechanical model of high speed natural gas flow with phase transition in Laval nozzle and discussed the flow characteristics of natural gas with phase transition in Laval nozzle. The phase transition can take place only when the vapor is supersaturated. As it takes place, the gas will be heated by the latent heat which is released by the vapors, then the"condensation shocks"phenomenon of the gas will appear. Under different inlet conditions, the starting points of the phase transition and the mass proportion between condensed water and vapors in the outlet of the nozzle are calculated, and when the inlet temperature of the system is constant, the relations between those and inlet pressure are analyzed. The results indicate that, with the increase of the inlet pressure, the starting points of the phase transition will move along, the mass proportion between condensed water and vapors will increase. The paper discusses the factors which effect the normal operation of the separator and establishes a method which can be used to predict the range of the normal operating pressure.