Study On Mechanism Of CO₂ Removal From Natural Gas With Supersonic Swirling Separation Technology

In this paper,the application of supersonic swirling separation technology to natural gas deacidification is proposed.Based on the thoery of supersonic condensation and swirling,the research on mechanism of CO₂ removal from natural gas with supersonic swirling separation technology is carried out by combining theoretical analysis,numerical simulation and experimental testing.Main works and results are as follows:?1?The mathematical model of supersonic condensation flow in nozzle is established,the calculation methods are studied.Ignoring the velocity slip between gas and liquid,the flow control equations were built.Using the internal consistent classical theory to calculate nucleation rate,and the Gyarmathy theory was used to calculate droplet growth rate.All eight thermophysical parameters were clarified according to research object,with choosing appropriate calculation formulas as well as modification methods for each of them.After that,the reliability of these calculation methods was verified with NIST data.UDF and UDS were both used to develop the fluent software.?2?Using experimental data in literature,the process of CO₂ condensation is simulated and analyzed for the first time,then the characteristics of CO₂-CH₄ gas condensation flow in self-designed Laval nozzle is studied.The results show that:As the pressure and temperature decreased,spontaneous condensation of CO₂ compoenent occurred just behind the nozzle throat.The process of nucleation showed a sharp both in time and space,while the process of droplet growth could continue until nozzle outlet.Obvious condensation wave occurred in nozzle due to the latent heat released in the process of condensation.Increasing inlet pressure,reducing inlet temperature,as well as increasing inlet CO₂ gas mole fraction,could all contribute a promotion to gas condensation in nozzle.Shock wave appeared as pressure ratio increased,gradually affecting the droplet growth process and then the nucleation process,until the nozzle losed its liquefaction ability.By selecting reasonable back pressure,the shock wave could be controlled.Under a given nozzle throat size,increasing the expansion ratio could enhance the nozzle outlet humidity,while increasing the contraction ratio had no effect on the nozzle's cooling performance.?3?With process arranged and equipments established,the experiments of supersonic gas condensation flow in self-designed nozzles of different expansion angles are carried out,using dry air with water vapor as the test medium.The results show that:Nozzles were well sealed and parameters in the system could mantian on a satable and accurate level while the compressor outlet pressure was set to 6 bar.The pressure distribution of water vapor in experimental data were in good agreement with simulation results,condensation wave was weak due to small condensation of H₂O.The experimental data of droplet number and droplet radius were both consistent with numerical simulations too,with the diviation level of droplet number was in the range of-1.78¹.01 and the droplet radius was between145.88%²38.61%,both macthing the conclusions in worldwide tests.The difference between Wilson point in experiments and simulations were within+2.65⁺2.99mm,showing little diviation.Effect of different expansion angles on Wilson point was not obvious,but its effect on condensation parameters distribution of water vapor was significant.As the nozzle expansion angle increased,the droplet number increased while the droplet radius decreased.Numerical simulation of CO₂ showed the same results,except its Wilson point moving forward under bigger expansion angle.?4?Designing swirling nozzle with centerbody and swirl vanes,the flow and condensation process in swirling flow field of CO₂-CH₄ gas are studied.The results show that:As the swirl intensity increased,the velocity,pressure as well as temperature line all moved towards upstream of the nozzle throat,causing increasement of radial gradient.Swirling flow had a great iMPact on the central flow area,then the area near the inner wall,the area near outer wall showed the minimal effect.As the gas flowed in the nozzle,the tangential velocity increased first and then decreased,reaching its peak value at nozzle throat,and the maximum was promoted as swirl intensity enhanced.High swirl intensity increased the disorder of flow field,causing energy dissipation exacerbated,which would be bad for the separation process of gas and liquid.As the swirl angle and vane number increased,the tangential velocity also raise,while the nozzle flow capacity was limited.By optimizing swirl angle to 67.9?and vane number to 4,the nozzle's flow capacity and swirl performance could both be satisfied.Under swirling flow field,the pressure and temperature of gas was lower,thus the supercooling degree was higher at the same axial position of nozzle,the Wilson point of gas condensation was advanced,and the maximum of nucleation rate as well as droplet number were both improved,but swirling flow showed no promotion on the overall liquefication performance of nozzle.