Natural Gas Sweetening Using Supersonic Swirling Separation Technology

Supersonic swirling separation is an emerging natural gas treatment technology in recent years,which has been successfully used to separate water and heavy hydrocarbons from natural gas.A new type of natural gas sweetening method,in which the sour gas is removed by the combination of condensation due to expansion refrigeration and centrifugal separation,is proposed for the treatment of high sulfur natural gas.Based on the theories of gas dynamics,phase transformation kinetics,thermodynamics and fluid dynamics,the condensation characteristics of acid gas and the separation of condensed droplets under supersonic swirling flow conditions are systematically studied using theoretical analysis,numerical simulation and experimental investigation.The main studies and results are as follows:1)The numerical simulation on the expansion cooling process of sour gas within the Laval nozzle and the swirling characteristics under supersonic flow is carried out and the influencing factors of cooling effect and swirling strength are analyzed.The refrigeration effect caused by high speed expansion in Laval nozzle can provide a low temperature environment for the condensation of acidic components in natural gas.Better refrigeration performance can by achieved in nozzles with larger outlet Mach number,but at the same time the pressure recovery ability of the device is restrained.When the swirling device is installed at the entrance of the nozzle,the tangential velocity in the nozzle increases first and then decreases,which reaches a peak value at the throat.The swirl intensity increases with the increase of the swirl angle of the spiral blade,but the flow capacity of the separator is restricted.2)An Eulerian-Eulerian two-phase model containing non-equilibrium condensation behavior is developed to predict the condensation characteristics of H₂S and CO₂ in CH₄-H₂S and CH₄-CO₂ binary gas during supersonic expansion.The simulation results show that H₂S and CO₂ vapor nucleate spontaneously and the condensed droplets grow to sub-micron scale during the rapid expansion in supersonic nozzles,while the H₂S and CO₂ concentration remaining in the gas phase decrease.The onset of the condensation occurs earlier with bigger droplets and more acid component can be condensed out of the gas mixture at lower inlet temperature,higher inlet pressure and bigger acid gas feed concentration.Shock wave moves towards to the nozzle throat with the increase of the back pressure,which will destroy the supercooled state required by the condensation process and cause the re-evaporation of the condensed droplets.A mathematical model containing the spontaneous condensation process of two components which are immiscible is developed to investigate the nucleation and droplet growth process when H₂S and CO₂ condense together.The results show that CO₂nucleation dominates particle formation while the presence of H₂S reduces the nucleation rate.H₂S vapor can condense on these nuclei which inhibits its spontaneous nucleation process.H₂S condenses faster at the initial droplet growth process while CO₂ mainly condenses at the later stage.The latent heat released by the condensation of one component leads to the mild evaporation of the other.The degree of supersaturation ratio during nucleation process is lower in nozzles with smaller maximum expansion angle,which will lead to condensation of lower droplet number density and greater droplet size.The increment of outlet Mach number can improve the condensation performance of supersonic nozzle at the expense of bigger pressure energy loss.The condensation process of acid gas will continue in the downstream constant cross area conduit if phase equilibrium is not yet established at nozzle exit,but with the extension of the conduit,the combination effect of phase transition,compression wave and friction will lead to the dynamic equilibrium of condensation and re-evaporation.3)An experimental system is set up and supersonic nozzles with different maximum expansion angle are designed to test the condensation characteristics under supersonic flow condition for further investigation and validation.The size and concentration of the droplets are measured by an optical particle testing system based on the principle of multi-wave-length extinction detection method.The measured droplet number density distribution is in good agreement with the numerical results and the number of the droplets remains constant after the clusters are formed,which strongly verifies the reasonability of neglecting the coagulation effect.The onset position measured in the experiment agrees well with the simulation result,which further proves the accuracy of the established condensation model under supersonic flow condition.In the experimental nozzle with smaller maximum expansion angle,lower number of droplet is nucleated and larger droplets are formed,which is consistent with the law obtained in the simulation.In order to get condensed droplets with larger size,nozzle with smaller maximum expansion angle is recommended in the supersonic separator.4)The condensation characteristic of sour gas and the cyclonic separation of condensed droplets under supersonic swirling flow conditions are numerically calculated and the removal efficiency of acid gas by supersonic swirling separation is obtained.The condensation process of acid gas in supersonic swirling flow is similar to that in the axial flow.With the increase of the swirling strength,the onset of condensation moves forward and the condensation efficiency increases,but the change is not significant.Most of the condensed droplets are thrown into the liquid-collection space by the centrifugal force,achieving the gas-liquid separation.The separation efficiency increases with the enlargement of the droplet size and the advancing of the condensation onset.The removal efficiency of the acid component is determined by the condensation efficiency and the separation efficiency.Both the condensation efficiency and separation efficiency are enhanced by increasing the inlet pressure and decreasing the inlet temperature,resulting in bigger removal efficiency.A cyclonic conduit with proper length can be added to make full utilization of the supercooling degree and to achieve better gas-liquid swirling separation.A novel natural gas sweetening technology based on supersonic swirling separation mechanism is proposed for the preliminary deacidification of high sulfur natural gas.The corresponding technological process is designed in which two-stage or multi-stage supersonic swirling separation,inlet cooling and interstage supercharging can be used to improve the removal efficiency.