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Note for Fluidization Engineering - FE by Janani Balasubramanian

  • Fluidization Engineering - FE
  • Note
  • KARPAGAM UNIVERSITY - KAHE
  • Chemical Engineering
  • B.Tech
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Objectives_template Module 9: Packed beds Lecture 33: Minimum fluidization velocity Minimum fluidization velocity file:///D|/Web%20Course/Dr.%20Nishith%20Verma/local%20server/fluid_mechanics/lecture33/33_1.htm[5/9/2012 3:40:34 PM]

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Objectives_template Module 9: Packed beds Lecture 33: Minimum fluidization velocity Minimum fluidization velocity The minimum fluidization velocity can be calculated by equating the pressure–drop across the fixed packed–bed, calculated from Ergun's equation to that from the expression for fluidized bed under particulate (smooth) conditions. Let us calculate the pressure-drop from the 2 nd expression: Under fluidization conditions, pressure–drop equals effective weight of solid, as intraparticle forces disappear and solids float in the bed exhibiting ‘liquid–like ‘behavior. For a fluidized bed of length of L and bed-porosity of , Weight of solid-particles–buoyancy Or , etc. where R-call: (Fig. 33a) file:///D|/Web%20Course/Dr.%20Nishith%20Verma/local%20server/fluid_mechanics/lecture33/33_2.htm[5/9/2012 3:40:34 PM]

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Objectives_template Module 9: Packed beds Lecture 33: Minimum fluidization velocity At the minimum fluidization condition: Apply Ergun's equation for ‘fixed–bed' at minimum fluidization condition or at the incipience of fluidization: , where superficial average velocity at minimum fluidization state Equate: The above-equation is quadratic on (minimum fluidization velocity) and may be written in the following form: , where For small particles For large particles file:///D|/Web%20Course/Dr.%20Nishith%20Verma/local%20server/fluid_mechanics/lecture33/33_3.htm[5/9/2012 3:40:34 PM]

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Objectives_template Module 9: Packed beds Lecture 33: Minimum fluidization velocity To avoid or reduce carryover of particles form the fluidized bed, keep the gas velocity between . Recall for low Reynolds number and, Terminal velocity, for high Reynolds number With the expressions for and known for small (viscous–flow) and large (inertial flow) particles or Reynolds number, one can take the ratio of and : For small For spherical particles, and assuming Therefore, a bed that fluidizes at 1cm/s could preferably be operated with velocities < 50 cm/s, with few particles carried out or entrained with the exit gas. For large Or, Therefore, operating safety margin in a bed of coarse particles is smaller and there is a disadvantage for the use of coarse particles in a fluidized bed. However, make a note that the operating particle size is also decided by the other factors such as grinding cost, pressure-drop, heat and mass-transfer aspects. file:///D|/Web%20Course/Dr.%20Nishith%20Verma/local%20server/fluid_mechanics/lecture33/33_4.htm[5/9/2012 3:40:35 PM]

Lecture Notes