Hydrodynamic Characteristics Of Inverse Fluidized Bed

Beside the advantages of traditional fluidized bed with an upward cocurrent flow of the gas and liquid,inverse fluidized bed has such many remarkable benefits as low liquid flowrate,low energy consumption,efficient heat and mass transfers,easy re-fluidization and minimum solids attrition,etc.It thus has a great prospect in various applications in the fields of biochemical,food,environmental,petrochemical and other industries.A pilot-scale inverse fluidized bed(0.33 m ID×3 m Height)coupling with the measurement system was custom designed and constructed to investigate flow regimes and inter transitions,local phase holdup distributions and bubble dynamic characteristics of bubble-induced inverse fluidized bed(BIFB),on the basis of the study of liquid-solid inverse fluidized bed and three-phase inverse fluidized bed.The results could provide basic data and theoretical supports for the design and industrial application of inverse fluidized bed.In the liquid-solid inverse fluidized bed,the minimum fluidization velocity and the bed expansion were investigated.Both experimental data from this project and reported in the literatures were collected and fitted in the form of prediction equations reported in the literatures.It has proven that the Wen and Yu equation is valid to predict the minimum fluidization velocity of liquid-solid inverse fluidized bed.An empirical equation has been developed to determine the bed expansion ratio as a function of Ar_mand Re_l.In three-phase inverse fluidized bed system,the hydrodynamic characteristics including flow regimes,transition velocities between neighboring flow regimes,bed expansion,and phase holdups were studied in detail.A flow regime map(including fixed bed regime,transition regime and fluidization regime separately induced by gas,liquid and combination of gas and liquid)was proposed and the variations of the bed expansion and the phase holdups were obtained under different operating gas velocities and liquid velocities.In bubble-induced inverse fluidized bed(BIFB),some basic hydrodynamic characteristics including flow regimes,transition velocities between neighboring flow regimes,fluidization ratio,and phase holdups were comprehensively investigated.A flow regime map was also proposed and a new concept named“fluidization ratio”was defined to describe the fluidization quality in BIFB.A new telecentric photographic probe was applied at the first time to obtain the detailed phase holdup distributions,the bubble size distributions and the bubble rising velocity in the bubble-induced inverse fluidized bed.By analyzing all collected pictures,the trends of the axial and radial phase holdup distribution were obtained in the complete fluidization regime.The bubble size is logarithmic normal distribution in different positions.The average bubble size increases with the axial height and is mostly uniform in the radial direction except a decreasing trend near the zone of the wall.The bubble rising velocity increases with the bubble size and the gas velocity.An equation was proposed to predict the rising velocities of the bubble in BIFB.In addition,a gas-induced wake model of the BIFB is modified by combining the generalized wake model and the gas disturbed fluid model.Two equations were proposed to predict separately the complete fluidization gas velocity and the solid holdup(liquid holdup)in BIFB system.