20-10-2012, 05:36 PM
Agitation
Agitation.pdf (Size: 1.49 MB / Downloads: 40)
Devices to Produce Agitation
• For overview of equipment see section 13.3 in
course notes
• Impellers
– Propellers (high‐speed, low viscosity)
– Turbines (Moderate viscosity)
– High‐efficiency impellers
• Shafts
– Europeans prefer bottom entering shafts
• Balance between bending and sealing
Without Baffles a Centre Vortex Forms
1. Poor mixing between adjacent layers
2. If P0 is too great, air can be be entrained into liquid
3. Vortex causes upward displacement
To Prevent Centre Vortex
1. Baffles
2. Operate in viscous region
3. Off‐centre location or side entering impeller
4. Closed tank operating full with no gas‐liquid
surface
1. Good for continuous flow
2. Impractical for gas‐liquid contacting
Blending and Mixing
• Blend number, Nb
• Nb =N.θ
• Mixing time, Θ, depends on method of
measurement
• Mixing times in vessels
– Solid with baffles
– Dashed no baffles