Gas-Liquid Separation Characteristics Through Holes At The Wall Of A Swirling-Pipe Separator

Through the efficient separation and controllable injection of condensed liquid,the gas-liquid polyethylene process enables the coexistence of different temperature zones and reaction environment,based on which high-performance polyethylene resin is produced in a single reactor.The progress of polyolefin industry is hence greatly promoted.In the gas-liquid polyethylene process,the rapid and efficient separation of circulating gas and condensed liquid is the basis of the controllable injection of liquid,and the precise adjustment of temperature and component concentration distributions.Therefore,it is of significant importance to develop a high-performance gas-liquid separator for the operation optimization and product improvement of the gas-liquid polyethylene process.The swirling-pipe separator is a centrifugal gas-liquid separator,and consists of a rotating stream tray(RST),a porous inner pipe and outer pipe.At the guidance of RST,gas-liquid two phase flow swirls and redistributes between the holes and inner pipe.The regulation of the two phase flow and the gas-liquid separation characteristics are the key factors to the performance of the separator.Employed computational fluid dynamics(CFD)method and combined with cold model experiment,three-dimensional model of gas-liquid separator is established to quantificationally study the gas-liquid separation characteristics and the influencing factors through introducing the liquid separation ratio KL(defined as the ratio of liquid mass flow rate in the hole and total liquid mass flow rate)and Φ(defined as the ratio of KL and KG).The main conclusions of this thesis are as followed:(1)The steady state model of gas-liquid separator in experimental scale with the industrial physical parameters(the same with the following chapters)is established to validate the model and to study the effect of the axial position,diameter and the pressure difference of the four holes on the separation characteristics.And the four holes are uniformly arranged at circumferential direction The results indicate the flow pattern transfers from a mist flow to an annular flow along the axial direction.Under the condition investigated,in the zone with L/D<1.0(L is axial location of the holes,D is pipe diameter),no liquid film forms on the wall and the liquid separation ratio increases with the axial distance of the holes.While in the zone with L/D>1.0,liquid film forms on the wall and the liquid separation ratio increases and keeps basically constant around 0.007.KL raises as the hole diameter increases while Φ decreases,which means the gas is more sensitive to the resistance.Besides,increasing pressure difference,the area of influenced zone is larger,hence KL increases and Φ decreases,which means the gas is more sensitive to the pressure.(2)The effect of the operating parameters of gas-liquid separator(including superficial gas velocity,vs,g and superficial liquid velocity,vs,l)and structural parameters of RST(including the angle of vanes,numbers of vanes and height of RST)on the gas-liquid separation characteristics are studied.KL decreases with vs,l,increases with the liquid superficial velocity.When vs,g is lower than a certain value,KL increases linearly with vs,g.When vs,g is higher than the certain value,the growth of KL decreases.Φdecreases with vs,g.As the angle of vanes of RST increases,KL decreases while Φincreases.KL and 0 increases and then keep steady as numbers of vanes increases,the effect of height of RST on the gas-liquid characteristics is not remarkable,and KL keeps as a constant.(3)The mutual effect of the seven holes(six holes are uniform arranged around the central hole,and the seven holes are arranged in equilateral triangle)is studied under different gas superficial velocity(vs,g)and liquid superficial velocity(vs,l).The simulated results indicated that KL decreases as vs,l increases or vs,g decreases,which the regulation is the same with that of the single hole,while KL and Φ in the center hole are relatively low compared with the single hole.The gas-liquid separation characteristics among the seven holes are quite different,and the holes upstream perform great influence on the holes downstream.Besides,KL increases as v,l increases or vs,g decreases.It is not the change of distribution but the change of liquid fraction that leads to the difference.Liquid rotates at the pipe wall,so along the moving direction,part of liquid film has been removed at the holes upstream.And the liquid fraction at the holes downstream are relatively low.