Study On The Condensation Characteristics Of CO₂ And The Structure Optimization Of Supersonic Separators

Supersonic swirling separation technology is a new approach of natural gas treatment in recent years,which has been widely used in the fields of natural gas dehydration,hydrocarbon removal and liquefaction.Compared with the traditional natural gas dehydration technology,the supersonic swirling separation technology has the advantages of simple structure,small footprint,light weight,no moving parts,no external power consumption and other advantages.A great deal of recent scientific research has focused on the basic theory,numerical simulation and experimental research,and the gas flow characteristics and spontaneous condensation performance of the nozzle.However,up to now,there is no related research on the supersonic separation of CO₂ from natural gas.Therefore,the study on the removal mechanism of CO₂ in natural gas based on supersonic expansion characteristic is not only helpful to the research and development of the new technology of natural gas processing,but also helpful to deepen the understanding of the phase transition mechanism of supersonic condensation process under high pressure and low temperature conditions.In this paper,the main studies and results are as follows:Firstly,the supersonic condensation flow characteristics of CH₄-CO₂ mixture gas were studied.The mathematical model of CH₄-CO₂ two-component mixture of supersonic condensation process in the nozzle was established based on the droplet nucleation,growth model and the surface tension of CO₂ prediction model,the numerical simulation of two-component mixed gas supersonic condensation and flow process was carried out and flow and condensation parameters were investigated.The influences of inlet parameters on the supersonic flow and condensation characteristics of the mixture gas were studied.The results show that with the increase of the inlet pressure,the condensation position moves forward,the maximum nucleation rate and the droplets number decrease,while the droplet radius and the liquid fraction of CO₂ increase;with the increase of the inlet temperature,the condensation position moves away from the inlet,the maximum nucleation rate and the droplets number increase,while the droplet radius and the liquid fraction of CO₂ decrease;With the increase of the inlet CO₂ fraction,the condensation position moves forward,the maximum nucleation rate and the droplets number decrease,while the droplet radius and the liquid fraction of CO₂ increase,and the liquefaction efficiency remained at 57%⁵8%.The simulation results confirm the feasibility of the supersonic separation technology on the removal of CO₂ from natural gas.Secondly,the structure improvement of the supersonic separator and the distribution law of flow field in the supersonic separator were studied.The structure of the supersonic separator was improved on the basis of shock wave control mechanism,and the numerical simulation and analysis of flow field was carried out.The effect of inlet pressure on shock position and the distribution law of flow field in the separator under different inlet pressure were investigated,and the effect law of pressure loss ratio on shock location was obtained.The results show that the shockwave occurs in the back of the cyclone and the improved supersonic separator can achieve a wider range of deep cooling.The shockwave moves back with the increase of the pressure loss ratio,when the pressure loss ratio increases to 70%,the position of shockwave moves back to the diffuser.In order to avoid the secondary evaporation of condensed droplets before the cyclone and reduce the pressure loss of the gas as far as possible,the pressure loss ratio should be controlled in the range of 35%to 70%.Thirdly,the structure design and research of the separation performance of the cyclone were carried out.The flow characteristics of natural gas in the cyclone back-placed supersonic separator were obtained by numerical simulation using FLUENT software by changing the parameters of the cyclone.The influence of cyclone structure on the flow field was studied,especially the change law of the shock-wave position in the separator.The results show that with the increase of the rotation angle,the influence on the pressure is more obvious,the shock-wave moves forward,and the tangential velocity increases;With the increase of the rotation angle and thickness,the influence on the pressure is more obvious,the shock-wave moves forward,and the tangential velocity decreases;With the decrease of the vane length,the influence on the pressure is more obvious,but the change of shock-wave position is not obvious,and the tangential velocity is increases,however,the swirling flow distance is shorter,which is not conducive to the gas-liquid sufficiently separated.Lastly,the motion law of droplets in the supersonic separator was studied.The mathematical model of the continuous phase?methane gas?and the dispersed phase?CO₂droplets?was established,and the droplets motion was numerically calculated.The distribution of the gas flow field and the droplets path in the supersonic separator were obtained,the droplet separation efficiency and gas removal efficiency were defined,and the effect of the droplet sizes and speed on the separation efficiency was analyzed.The results show that when the droplet diameter is 0.25?m,the separation efficiency of supersonic separator was only about 50.76%,and when the droplet size increases to 1.25?m,the separation efficiency reached 96.97%,the CO₂ droplets can be separated completely when the droplet size is greater than 1.5 m;The axial velocity has no obvious effect on the separation efficiency of the supersonic separator;The inlet pressure has an obvious effect on the gas removal efficiency of the supersonic separator,when the inlet pressure is 6 MPa,the removal efficiency of CO₂ gas was only about 31.25%,and when the inlet pressure increases to 9 MPa,the removal efficiency of CO₂ gas was reached 54.52%.