Hydrodynamics And Mass Transfer Characteristics In A Gas-liquid Stirred Tank With Draft Tube

Research on the gas-liquid flow,hydrodynamics and mass transfer characteristics are important for the design,scale-up and optimization of the gas-liquid stirred tank with draft tube.In the present work,a stirred tank with inner diameter of 300mm and height of 650mm was designed.Two different configurations with draft tube were adopted.One is a single impeller configuration,the other is a dual impeller configuration.The gas-liquid flow,dispersion,power consumption,gas hold-up and gas liquid mass transfer were investigated experimentally.The main conclusions are listed as below:（1）In the single impeller configuration of stirred tank with draft tube,the power number is obviously enhanced by the draft tube.The power consumption of PBTD impeller is increased with agitation speed rising,while the power number decreases.However,when the agitation speed is more than 600 r·min^-1,the power number is approaching the same under different gas velocity.The power consumption of Rushton impeller is increased with the increase of agitation speed,but decreased with the increasing gas velocity.The style and location of the gas sparger has little influence on PBTD,but the location has some influence on Rushton.With the increase of Reynolds number Re,the relative power consumption P_g/P₀ for PBTD decreases sharply,while Re is higher than2.5×10⁵,the relative power consumption P_g/P₀ tends to be stable.For Rushton case,when the Re is higher than 3.5×10⁵,the power consumption and power number become constanting.Compared with that of ungased condition,the gassed power consumption of Rushton is reduced to 40%.（2）For the dual impeller configuration of stirred tank with draft tube,the power consumption increases as the increasing of agitation speed and the impeller spacing,but decreases with the gas inlet volume rising.Compared to the single impeller configuration,the dual impeller configuration can disperse gas more efficiently,accelerate the gas-liquid flow circulation velocity in the tank,and show more stable performance under higher gas and stirred speed.At low agitation speed,the power consumption for the S-sparger is nearly 20%higher than that of the C-sparger,but this gap is declined with the increasing agitation speed.When the agitation speed is higher than 700r·min^-1,the power consumption becomes nearly the same for both spargers.（3）For the dual impeller configuration of stirred tank with draft tube,the gas hold-up increases with the increasing agitation speed.The gas hold-up also rises as the superficial gas velocity increases.Higher gas hold-up is observed with higher volume power consumption P_V.Both show a positive correlation.（4）In the dual impeller stirred reactor with draft tube,the gas-liquid mass transfer coefficient k_La increases with the increasing volume power consumption P_V.The gas-liquid mass transfer coefficient k_La shows higher at higher u_G,which means a higher gas-liquid interfacial area.At a fixed volume power consumption,the increase of u_G can only enhance the gas-liquid mass transfer coefficient k_La in a limited range.