CFD Simulation Of Non-uniform Flow Characteristics In The Loop Reactor

The loop reactor has been widely used in the polypropylene industry,owing to its characteristics of the simple structure and the fast heat transfer.However,in the loop reactor,the uneven particle structure will increase the unit operation energy consumption,even hinder the normal and efficient operation of the device.Studying the formation mechanism of the heterogeneous flow structure is of great significance to the normal,stable and efficient operation of industrial equipment.In this paper,the steady state and dynamic three-dimensional model of industrial and pilot scale loop reactor are established by using the computational fluid dynamics method,to study the heterogeneous flow structure and its influencing factors in the loop reactor.Based on this,we offer the proposed measures to reduce the non-uniform flow.The main research contents and achievements of this thesis are as follows:(1)The steady state model of industrial scale loop reactor was established by using the computational fluid dynamics method,to study the effect of industrial loop reactor structure(the height and the number of straight section)and operating parameters(speed,particle size,concentration)on the reactor of non-uniform flow structure influence.Simulated results indicate that the non-uniformity of the 2-leg loop reactor's outlet does not change with the variation of height/diameter ratio when it exceeds 43.However,the flow uniform of the 4-leg loop reactor's outlet keeps increasing with higher height/diameter ratio.When the height/diameter ratio of the loop reactor's straight tube is fixed with 65,the outlet flow uniform is improved with more straight tubes.But,the influence of the height/diameter ratio on the outlet's non-uniformity is more significant than that of straight pipe number.When flow rate is in the range of 7-13 m/s,flow rate has little effect on non uniformity.If reducing the flow rate to 4 m/s,although the non uniformity of export can be reduced,but it is not conducive to the stable operation of the device.The bigger the particle size,the more violently the solid volume fraction fluctuates,the more non-uniform the outlet.The higher the inlet concentration,the smaller the non-uniformity of the whole loop reactor,the better the uniformity of the outlet.(2)Taking the 4-leg industrial loop reactor as the research object,the CFD simulation was used to study the flow field of the non-uniform distribution,concentration and velocity in the loop reactor under single and double dispersed particles.Based on this,the action mechanism of double disperse particles is put forward.The results showed that the higher the content of large particles,the greater the non-uniformity of the axial direction,more uneven the distribution of the particles,and more easy to form a larger particle band and do the spiral movement in the straight pipe;forming the "ring core structure" concentration distribution.Particle band continuously develop to uniform when doing the spiral movement,and the greater the particle size,the greater the speed difference between the inner and outer sides of the elbow,the weaker the circumferential rotation ability,the more difficult to develop even.Based on the above,the mechanism of the action of the two particles is proposed that large particle has inhibitory effect on uniformity,which shows that the large particles promote the formation of particles on the curved pipe and inhibit the dissipation of particles band in the straight pipe.(3)Three kinds of loop reactors,without a belt connection,with a belt connection and with imported belt connection,were studied by using the computational fluid dynamics method.The influence of the belt connection on the local uniformity in the reactor was studied,and based on this,the influence mechanism of connection is proposed.The results showed that the joining of the belt connection will cause the non-uniform fluctuation of the loop reactor,which will let the non-uniformity of the inlet section increase dramatically;but little influence on the non-uniformity of the outlet section,causing fluctuation of the non-uniformity in the near area.The effect of the connection on the inlet pipe is greater than the outlet pipe.The mechanism of belt caoonction in the loop is the diverging effect of the inlet pipe and the collecting current at the outlet pipe.The slurry in the inlet pipe wall swarm into the belt connection continuously,changing the distribution of liquid-solid two-phase in the vicinity of the inlet pipe from the uniform distribution to the distribution of thin near wall and thick in the center;the low concentration slurry in the belt connection swarm into the outlet pipe of loop reactor along the wall,mainly flow down,changing the liquid-solid two-phase distribution in the vicinity of the outlet pipe near belt from the structure of thick in the wall and thin in the center into the structure of thin in the wall and thick in the center.(4)The unsteady state model of small size loop reactor was established by using the computational fluid dynamics method.The variation of export solid volume fraction and non-uniformity with time presented.Taking the solid phase volume fraction at different positions of the loop reactor as an example,we analyse the spectrum and the energy spectrum to further understand the characteristics of the non-uniform flow in the loop reactor.The results showed both the nonuniformity and the particle concentration of the loop reactor fluctuate around a certain stable value with time at different particle diameters.The bigger the particle sizes,the bigger the export fluctuation of solid volume fraction,and more easily to cause the fluctuation of the axial flow pump.By analyzing the spectrum and energy spectrum,we find that the formation of the cohesive structure of particle agglomerates after the bend,will result in a new periodic behavior(about 10 Hz)at the solid fraction volume fraction in the reactor.And the turbulence of the flow field is strengthened after the bend.The friction between the particles and the wall of the granule is increased,and the energy dissipation is accelerated.