Mixing Flashcards
Baffles
Number required for mixing and suspension
- At least 3 required (better 4)
- B = T/10
- Suspended from vessel lid or integrated into cylindrical vessel shell
Ideal height of a tank in terms of its diameter for mixing
H = T
Ideal Clearance of the stirrer/impeller
C = T/3
Ideal shape of the base of a vessel
Dish based for optimum mixing
Axial Flow
- Thrust in direction of axial (vertical in tank)
- Low shear
- Good for solid suspension
Radial Flow
- Thrust Perpendicular to axial direction
- High shear
- Good for dispersions
Static Volume
Liquid Volume beneath the impeller
~1% V
Minimum stirred Volume
- Volume that upon impeller motion induces liquid motion
- Impeller is partially submerged
(~ 5% V)
Minimum mixed volume
- Impeller + baffle are both submerged
- Must operate above this
~30-40% V
Conical Tanks
- Good Separators
- Bad Mixers
- High Vmin,mix
- Low Vmin,stir
Mixed flow impeller
Flow predominantly in axial direction with also a radial component
Close clearance impeller
Ensures good motion near vessel walls
Good for high viscous mixtures to facilitate heat transfer near the walls
Application of radial flow impellers
- Turbulent & transitional regime
- Gas-Liquid (Provided gas introduced below impeller)
- Liquid-Liquid dispersions (provided density difference is not too great and the impeller is relatively close to the liquid-liquid interface)
Applications of axial and mixed flow
- Turbulent and transitional regime
- Blending
- Solid Suspension
- Liquid-Liquid dispersions
Applications of close clearance impellers
- Laminar regime
- Blending
Baffles
- Promote flow pattern characteristic of the impeller types
- Tangential flow prevails in unbaffled tank (air entrainment, vortex formation and poor top-to-bottom mixing)
Controlling duty of processes
- Liquid blending
- Solid-liquid mixing
- Gas-liquid mixing
- Dispersing immiscible liquid
- Heat Transfer
Importance of power dissipation
Aids for predicting:
mixing time, mass transfer coefficient, droplet size
Power Consumption Equation
P = 2piN*Lambda
N = Stirrer Rate Lambda = Torque
Impeller Power equation
P = Po roN^3 * D^5
Po = Power Number
Factors affecting Impeller power Number (Po)
- Impeller type
- Impeller/vessel dimensions
- Properties of the phases present
Impeller Re Number
Re = roND^2/mu
N = impeller rotation rate
Power Curve Details
- Laminar (Re < 10)
(Po proportional to 1/Re) - Transitional (10 1000)
Po = Constant
Blend Time Definition
Time to 95% homogeneity (Theta)
Blend Time Equation
N*Theta = 6/(Po)^1/3 * (T/D)^2 = Constant
For turbulent regime
Po = Power Number Theta = Blend Time T = Tank Diameter D = Impeller Diameter
Why can’t the average slurry density be used at low impeller speeds
A proportion of slurry solids will not be suspended
Zwietering Correlation
Suspension speed equation that ensures no particle remains stationary at the bottom of the vessel
Relationship between Zwietering impeller speed and homogeneous impeller speed
H_homogeneous = 1.25*N_js
Flow regimes in gas-liquid mixed tank
Increasing shaft speed (i.e. power) at constant gassing rate:
- Impeller flooded
- Column of bubbles
- Start of bubble distribution below impeller
- Fully developed bubble distribution
- Recirculation of gas bubbles
Russian Disc Turbine Po
5
Flat Blade Turbine Po
3
45 degree Pitch Blade Turbine Po
1.25
Wide Blade Hydrofoil
0.6
Retrieved Curved Impeller
0.5
Narrow Blade Hydrofoil
0.25
Define the pitch of the turbine blade
Angle between the blade and the horizontal. The higher the pitch the more radial the flow
Zwiettering Correlation equation
N_js = S*v^0.1 *dp^0.2 (g(ro_s - ro_l)/ro_l)^0.45 *X^0.13 *D^-0.85
S = Vessel related constant v = kinematic viscosity dp = particle diameter X = solid to liquid mass ratio D = impeller diameter
