Research On Phase Holdup Measurement In Stirred Tank Reactor

Stirred tank reactors are widely applied in many process industries.According to statistics,there are 70%percent of three principal synthetic materials are fabricated in stirred reactor.The mixing characteristics in reactors have remarkable effect on the quality of products,production cost and energy cost.However,research on mixing process becomes rather complicated and difficult when concerning problems of gas dispersion,liquid mixing and solid suspension.During the plants amplification design, some parameters are not only important basis for reactor design and optimization but also crucial for industrial process control,such as gas holdup,the minimum speed to prevent flooding,solid holdup,critical suspension impeller speed etc.Although some researchers have already studied on these aspects,the results are controversial even contrary due to the complexity in stirred reactor and the restrictions of detection methods.Therefore,it is essential to adopt multiple detection means to thoroughly and systematically inspect the characteristics of stirred reactor.Take polyethylene slurry reactor for example,this thesis proposes using acoustic emission(AE) and hydrophone as main measuring methods to study stirred tank systematically,meanwhile adopting conductivity probe and sampling method to assist in detection.First,the characteristic information of gas and solid are extracted from the original signals by studying the underlying structure characteristics of AE signals.Basing on the analysis,a new criterion for determining the flooding-loading transition by AE method and a new method for measuring the critical suspension speed on account of algorithm complexity is presented respectively.And the characteristic impeller speed are investigated and analyzed based on predetermined criteria in three phase system;Then,the gas and solid profile are investigated and PLS(partial least squares regression) prediction model for gas and solid layout are established separately;Finally,phase holdup in gas-liquid,liquid-solid and gas-liquid-solid systems are examined,the performance of stirred device is evaluated and hydrophone-PLS prediction models for phase holdup are obtained. The innovative results can be summarized as follows:1.The underlying structure characteristics of AE signals measured in gas-liquid system,liquid-solid system and gas-liquid-solid system are researched and compared by resorting to rescaled range analysis and wavelet transform.And the research on the minimum speed to prevent flooding,solid suspension,critical suspension impeller speed and gas dispersion is conducted on the basis of extracted characteristic frequency band of gas and solid. Multi-scale resolution of AE signals in gas-liquid system,liquid-solid system and gas-liquid-solid system implied that AE signals in macro-scale represent totally the stability of stirred reactor and environment,while AE signals in meso-scale represent the interaction between different phases.But micro-scale of AE signals represent the irregular motion of liquid in gas-liquid system and the motion of particles in liquid-solid and gas-liquid-solid system.For gas-liquid system,a characteristic frequency band of acoustic emission(AE) signals G₁(d₄,d₅,d₆ frequency band) which represents the interactions between bubbles and liquid is obtained.Basing on the observation that the energy fraction of AE signals on characteristic frequency changes regularly with the increasing impeller speed,a new method for determining the flooding-loading transition is presented.When the AE energy fraction in the characteristic frequency band increases rapidly and begin to level off,the corresponding impeller speed is the minimum speed to prevent flooding,N_f.Based on this new method,the aeration rate and total liquid depth is investigated by the air-water experiment and it is found that Nf increases with aeration rate and decreases with total liquid depth.Compared to visual observation,the AARE of AE method is 2.62%,better than power drawn method.For liquid-solid system,apply algorithm complexity(AC) to AE signals processing,and it is observed that the relationship between AC and AE changes regularly with the increasing impeller speed in a stirred tank.Then a new method for measuring the critical suspension speed is proposed.That is,when AC decreases rapidly and begins to level off,the corresponding impeller speed is the critical suspension speed.This new method is verified by the water-glass bean system which shows that the RD is 5.90%compared with the results calculated by Zweitering formula,and it is found that N_jc increases with solid volume fraction.2.The minimum speed to prevent flooding and critical suspension impeller speed are investigated and analyzed based on predetermined criteria in gas-liquid-solid system.The gas profile in gas-liquid system and solid profile in liquid-solid system are investigated and PLS(partial least squares regression) prediction model for gas and solid layout are established respectivelyFor gas-liquid-solid system,the critical suspension impeller speed decreases due to the positive effect of gas on solid suspension.However,the minimum speed to prevent flooding increases because solid impedes the dispersion of gas.Besides, the minimum speed to prevent flooding is larger than critical suspension impeller speed in the same operating condition,so the dispersion of gas requires more energy than solid dispersion in this system.The layout of gas dispersion and solid suspension along axial direction are examined separately,and AE prediction models are established.The model shows for gas dispersion a correlation coefficient(R) larger than 0.90,an average relative deviation(ARD) less than 1.78%and a root mean square error for prediction (RMSEP) less than 2.25%.For the solid suspension,a prediction model with an R larger than 0.93,an ARD less than 0.87%and a RMSEP less than 1.03%is obtained.3.Research on gas holdup in gas-liquid system and gas-liquid-solid system by conductivity probe proves that gas disperses regularly in the stirred tank,and the hydrophone prediction models for gas holdup are obtained.Gas holdup in gas-liquid and gas-liquid-solid stirred tank are detected by conductivity probe.It is found that gas disperses regularly in the stirred tank,that is, concentration is relatively low around middle impeller and in the bottom zone. Besides,gas holdup decreases with the solid volume fraction dramatically and increases with aeration rate.In addition,the uniformity of gas dispersion is improved with increasing impeller speed.A series of experiments were carried out to investigate the gas holdup in gas-liquid system and gas-liquid-solid system by hydrophone.Then apply partial least squares(PLS) regression to establish prediction models based on energy calculated by wavelet analysis and gas holdup measured by conductivity probe.The model shows for gas-liquid system an R larger than 0.91,an ARD less than 1.12% and a RMSEP less than 1.36%.For the gas-liquid-solid system,a prediction model with an R larger than 0.91,an ARD less than 0.95%and a RMSEP less than 1.23% is obtained.4.It is found that gas dispersed regularly in stirred tank through the investigation on gas-liquid system and gas-liquid-solid system by sampling,and the hydrophone prediction models for gas holdup are obtained.It turns out that gas and solid holdup can be simultaneously detected by hydrophone.Solid holdup in liquid-solid and gas-liquid-solid stirred tank are measured by sampling method.The result shows that solid disperses regularly in the stirred tank, that is,concentration is relatively low around middle impeller and relatively high in the bottom zone.Besides,solid holdup increases with solid volume fraction,and the uniformity of solid dispersion is improved with increasing impeller speed. Hydrophone was applied to investigate the solid holdup in gas-liquid system and gas-liquid-solid system.And solid holdup prediction models are established by applying partial least squares(PLS) regression to the energy calculated by wavelet analysis and solid holdup measured by sampling method.The model shows for liquid-solid system an R larger than 0.9,an ARD less than 2.09%and a RMSEP less than 1.52%.For the gas-liquid-solid system,a prediction model with an R larger than 0.89,an ARD less than 1.02%and a RMSEP less than 1.73%is obtained.5.It is concluded that gas and solid cannot fully dispersed after the phase profile is carefully analyzed in gas-liquid system,liquid-solid system and gas-liquid-solid system.So a new stirred device is recommended based on the experiment results and recent research results on three-phase stirred reactor,that is, using half elliptical blades disk turbine(HEDT) as lower impeller,4-wide-blade hydrofoil impellers as middle and upper impellers,finger-type baffle as gas inlet pipe.