Natural Gas Condensation Characteristics In Supersonic Flows

The supersonic swirling separation technique is a new technique in gas gathering and transportation field,which is compact in structure,small in size,without moving part inside and without consuming power.The process of the new technique is expanding cooling,swirling separation and recompression.At present,many researches focus on the condensate of vapor and alcohol,while the research on the condensate of hydrocarbon is little.This paper describes the gas condensate characteristics in supersonic condition by studying the hydrocarbon condensate flow process in Laval nozzle based on the supersonic swirling separation technique.(1)Due to the deviation of nucleation rate calculation based on classic nuclear theory(CNT),this paper improves the ICCT nuclear theory which is more accurate than CNT,considering the effect of drop diameter on surface tension and establishes hydrocarbon condensate model.The calculation results of nucleation rate of vapor and nonane show that the error based on improved ICCT between theory calculation results and experiment results is smaller than that based CNT.This paper calculates the nucleation rates of five components,methane,ethane,propane,butane and pentane respectively.The results show that for a component,the increase of pressure promotes the degree of super saturation and nucleation rate with same temperature,and the increase of the temperature decreases the degree of super saturation and nucleation rate with same pressure.For different components with same temperature and pressure,larger the molecule weight is,faster the nucleation rate gets.(3)The condensate characteristics with different components are compared in the condition of same nucleation rate.For a component,the required degree of super saturation is low when the temperature is high.And For different components,the required degree of super saturation is high with large molecule weight in the same temperature.(2)The supersonic Laval nozzle model is established.The pressure and temperature in the nozzle of methane is simulated using ANSYS FLUENT,without considering the gas condensate.The simulation results show that the pressure and temperature decrease along the axis,and there is no expanding wave in nozzle expanding section and no unreasonable phenomenon,proving the rationality of the model.(3)The hydrocarbon condensate mathematical model is established.The governing equations consider the change of mass,momentum and energy due to the condensate drop by adding 4 sources and 2 User-defined scalar in UDF.(4)The methane condensate flow process is studied.Similar to the condensate process of water,the methane doesn't condensate before reaching the nozzle throat.After methane enters the nozzle throat,as the increase of the degree of supercooling and supersatuation the nucleation rate increases rapidly in a small region.When it reaches hyper saturated state,the condensate begins.Condensate occurs with the release of latent heat,heating the surroundings,resulting in the increase of temperature and pressure transiently.On account of the collapse of gas molecule,there are large quantities of critical condensate core.When the number of condensate core reaches a certain number,abundant drop is generated.And the condensate core ceases to produce after the decrease of the degree of supercooling and supersatuation.Without considering the coalescence of the drop,the droplet number will reach a maximum and then keep constant.The change rules of nucleation rate and droplet number show that when they reach a limit,a large quantity of gas molecule gathers in drop surface due to the large magnitude,resulting in the drop diameter and liquid mass fraction increase rapidly.The difference between methane condensate and water condensate is that the condensate wave is not obvious in methane condensate process.(5)The effects of inlet pressure,temperature and outlet backpressure on methane gas condensate process in Laval nozzle are analyzed.When the inlet pressure increases,the condensate position moves forward,the nucleation rate,outlet liquid mass fraction and droplet number increase.And as the temperature increases,the condensate position moves backward,the nucleation rate,outlet liquid mass fraction and droplet number decrease.Overall,the outlet backpressure is suggested 30%~40% of the inlet pressure.(6)The propane condensate process in Laval nozzle is simulated.The physical model,inlet temperature and inlet degree of supersatuation are the same with that of methane.The result indicates that(1)propane gas doesn't condensate until the degree of supersatuation reaches a certain degree;(2)The maximum nucleation rate in the nozzle outlet is smaller than that of methane;(3)The limit value of nucleation rate,droplet number,degree of supercooling and supersatuation are not indicated,which is different from methane.