Mixing

Question 1

Baffles

Number required for mixing and suspension

Answer 1

• At least 3 required (better 4)
• B = T/10
• Suspended from vessel lid or integrated into cylindrical vessel shell

Question 2

Ideal height of a tank in terms of its diameter for mixing

Answer 2

H = T

Question 3

Ideal Clearance of the stirrer/impeller

Answer 3

C = T/3

Question 4

Ideal shape of the base of a vessel

Answer 4

Dish based for optimum mixing

Question 5

Axial Flow

Answer 5

• Thrust in direction of axial (vertical in tank)
• Low shear
• Good for solid suspension

Question 6

Radial Flow

Answer 6

• Thrust Perpendicular to axial direction
• High shear
• Good for dispersions

Question 7

Static Volume

Answer 7

Liquid Volume beneath the impeller

~1% V

Question 8

Minimum stirred Volume

Answer 8

• Volume that upon impeller motion induces liquid motion
• Impeller is partially submerged
  (~ 5% V)

Question 9

Minimum mixed volume

Answer 9

• Impeller + baffle are both submerged
• Must operate above this
  ~30-40% V

Question 10

Conical Tanks

Answer 10

• Good Separators
• Bad Mixers
• High Vmin,mix
• Low Vmin,stir

Question 11

Mixed flow impeller

Answer 11

Flow predominantly in axial direction with also a radial component

Question 12

Close clearance impeller

Answer 12

Ensures good motion near vessel walls

Good for high viscous mixtures to facilitate heat transfer near the walls

Question 13

Application of radial flow impellers

Answer 13

• 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)

Question 14

Applications of axial and mixed flow

Answer 14

• Turbulent and transitional regime
• Blending
• Solid Suspension
• Liquid-Liquid dispersions

Question 15

Applications of close clearance impellers

Answer 15

• Laminar regime

- Blending

Question 16

Baffles

Answer 16

• Promote flow pattern characteristic of the impeller types

- Tangential flow prevails in unbaffled tank (air entrainment, vortex formation and poor top-to-bottom mixing)

Question 17

Controlling duty of processes

Answer 17

• Liquid blending
• Solid-liquid mixing
• Gas-liquid mixing
• Dispersing immiscible liquid
• Heat Transfer

Question 18

Importance of power dissipation

Answer 18

Aids for predicting:

mixing time, mass transfer coefficient, droplet size

Question 19

Power Consumption Equation

Answer 19

P = 2piN*Lambda

N = Stirrer Rate
Lambda = Torque

Question 20

Impeller Power equation

Answer 20

P = Po roN^3 * D^5

Po = Power Number

Question 21

Factors affecting Impeller power Number (Po)

Answer 21

• Impeller type
• Impeller/vessel dimensions
• Properties of the phases present

Question 22

Impeller Re Number

Answer 22

Re = roND^2/mu

N = impeller rotation rate

Question 23

Power Curve Details

Answer 23

• Laminar (Re < 10)
  (Po proportional to 1/Re)
• Transitional (10 1000)
  Po = Constant

Question 24

Blend Time Definition

Answer 24

Time to 95% homogeneity (Theta)

Question 25

Blend Time Equation

Answer 25

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 ```

Question 26

Why can't the average slurry density be used at low impeller speeds

Answer 26

A proportion of slurry solids will not be suspended

Question 27

Zwietering Correlation

Answer 27

Suspension speed equation that ensures no particle remains stationary at the bottom of the vessel

Question 28

Relationship between Zwietering impeller speed and homogeneous impeller speed

Answer 28

H_homogeneous = 1.25*N_js

Question 29

Flow regimes in gas-liquid mixed tank

Answer 29

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

Question 30

Russian Disc Turbine Po

Answer 30

5

Question 31

Flat Blade Turbine Po

Answer 31

3

Question 32

45 degree Pitch Blade Turbine Po

Answer 32

1.25

Question 33

Wide Blade Hydrofoil

Answer 33

0.6

Question 34

Retrieved Curved Impeller

Answer 34

0.5

Question 35

Narrow Blade Hydrofoil

Answer 35

0.25

Question 36

Define the pitch of the turbine blade

Answer 36

Angle between the blade and the horizontal. The higher the pitch the more radial the flow

Question 37

Zwiettering Correlation equation

Answer 37

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 ```