Hydrodynamics And Acoustic Emission Detection Of Particle Loss And Wall Sticking In Multizone Circulating Reactor

Polypropylene(PP)is widely used in the industrial area and our daily life because of its high performance properties.According to the rapidly development of our society,the innovation polymerization process which can improve the performance of PP and broaden its application is the focus of industrial and academic attention.At the early stage of the technique development,the properties of PP have been expanded by conducting polymerization in two successive steps,generating in each step a different polymer in terms of molecular weight,crystallinity.The two step technology has the typical limitation related to the polymer unhomogeneity because of the resistance time distribution.Multizone circulating reactor(MZCR)has been put forward to manufacture PP with higher homogeneity.In MZCR,PP particles are kept continuously circulating between riser and downer,where two distinct and different fluid regimes are realized.In the riser,the polymer is kept in a“fast fluidization”and the polymer in the downer act as“moving bed”.A complete and massive circulating is obtained between the two zones,managed by pressure balance.With a proper dimensioning of the two zones,it is thus possible to get a homogeneous broad polymer molecular weight distribution.However,the operation of MZCR exhibits pressure drop fluctuations in the riser and the downer and significant entrainment.Moreover,significant entrainment leads the blockage of recycle gas heat exchanger and further leading to the risk of emergency shutdown.In addition,the agglomeration and wall sticking are easily formed in the downer of MZCR because of the poor heat remove ability.Therefore,it deserves further investigation of the detection of particle loss,agglomeration and wall sticking.In this thesis,the particle loss in MZCR is studied respectively with acoustic emission(AE)detection technology,combined with FFT analysis and wavelet analysis.A model of particle loss mass flowrate is proposed to explore the quantitative relationship between AE energy,superficial gas velocity and particle loss mass flowrate.What's more,the effect of gas flowrate,valve opening and cyclone structure parameters on the particle loss rate are investigated in the cold model.On the other hand,the detection of agglomeration and wall sticking in the cold model is established based on the AE technique.The results had important guiding significance for safe and stable production,optimizing operating conditions in the MZCR.This thesis mainly focused on the following three aspects:(1)the hydrodynamic characteristic of MZCRAccording to the basic hydrodynamic principles in circulating fluidized bed,the macro and local hydrodynamic characteristics on MZCR are calculated.The results show that the gas-solids flow pattern in riser is belong to the fast fluidization,while the gas-solids flow pattern in the downer act as moving bed.The axial solids distribution in the riser is shown as exponential function,while an typical core-annulus structure is observed in the radial solids distribution.For the local hydrodynamic characteristics,large cluster structure is shown in the riser bottom.In addition,the cyclone separation efficiency is calculated,the results show that the central tube is too longer which decrease the separation efficiency.(2)AE detection of particle loss and the effect of operation on the particle lossIn the 3-dimensional MZCR cold model,acoustic emission(AE)technique is used to analyze the particle loss during the solids circulation processes.A model of particle loss mass flowrate is proposed to explore the quantitative relationship between acoustic emission(AE)energy,superficial gas velocity,and particle loss mass flowrate,by using wavelet transform and based on the mechanism that AE signals with different characteristic frequency band energies are emitted when the particles with different Velocities impact with the wall.The effect of operation condition and cyclone structure parameters are studied.The particle loss rate increases with higher gas flowrate,smaller valve opening.Besides,the particle loss rate increases and then decreases with longer central tube.(3)AE detection of agglomeration and wall stickingWe put some agglomerations and wall sticking in the cold model to simulate the condition in industrial reactor.At first,the flow pattern of the agglomeration in the MZCR cold is studied and determine the location of AE sensor.The AE energy characteristic frequency range is calculated which is regarded as the criteria of agglomeration.For detection of the wall sticking,the frequency distribution is also proposed,when the magnitude of AE frequency decrease sharply,the wall sticking is formed.