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PPD > Mixing > Flashcards

Mixing Flashcards

1
Q

Baffles

Number required for mixing and suspension

A
  • At least 3 required (better 4)
  • B = T/10
  • Suspended from vessel lid or integrated into cylindrical vessel shell
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2
Q

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

A

H = T

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3
Q

Ideal Clearance of the stirrer/impeller

A

C = T/3

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4
Q

Ideal shape of the base of a vessel

A

Dish based for optimum mixing

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5
Q

Axial Flow

A
  • Thrust in direction of axial (vertical in tank)
  • Low shear
  • Good for solid suspension
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6
Q

Radial Flow

A
  • Thrust Perpendicular to axial direction
  • High shear
  • Good for dispersions
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7
Q

Static Volume

A

Liquid Volume beneath the impeller

~1% V

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8
Q

Minimum stirred Volume

A
  • Volume that upon impeller motion induces liquid motion
  • Impeller is partially submerged
    (~ 5% V)
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9
Q

Minimum mixed volume

A
  • Impeller + baffle are both submerged
  • Must operate above this
    ~30-40% V
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10
Q

Conical Tanks

A
  • Good Separators
  • Bad Mixers
  • High Vmin,mix
  • Low Vmin,stir
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11
Q

Mixed flow impeller

A

Flow predominantly in axial direction with also a radial component

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12
Q

Close clearance impeller

A

Ensures good motion near vessel walls

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

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13
Q

Application of radial flow impellers

A
  • 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)
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14
Q

Applications of axial and mixed flow

A
  • Turbulent and transitional regime
  • Blending
  • Solid Suspension
  • Liquid-Liquid dispersions
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15
Q

Applications of close clearance impellers

A
  • Laminar regime

- Blending

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16
Q

Baffles

A
  • Promote flow pattern characteristic of the impeller types

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

17
Q

Controlling duty of processes

A
  • Liquid blending
  • Solid-liquid mixing
  • Gas-liquid mixing
  • Dispersing immiscible liquid
  • Heat Transfer
18
Q

Importance of power dissipation

A

Aids for predicting:

mixing time, mass transfer coefficient, droplet size

19
Q

Power Consumption Equation

A

P = 2piN*Lambda

N = Stirrer Rate
Lambda = Torque
20
Q

Impeller Power equation

A

P = Po roN^3 * D^5

Po = Power Number

21
Q

Factors affecting Impeller power Number (Po)

A
  • Impeller type
  • Impeller/vessel dimensions
  • Properties of the phases present
22
Q

Impeller Re Number

A

Re = roND^2/mu

N = impeller rotation rate

23
Q

Power Curve Details

A
  • Laminar (Re < 10)
    (Po proportional to 1/Re)
  • Transitional (10 1000)
    Po = Constant
24
Q

Blend Time Definition

A

Time to 95% homogeneity (Theta)

25
Q

Blend Time Equation

A

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
26
Q

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

A

A proportion of slurry solids will not be suspended

27
Q

Zwietering Correlation

A

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

28
Q

Relationship between Zwietering impeller speed and homogeneous impeller speed

A

H_homogeneous = 1.25*N_js

29
Q

Flow regimes in gas-liquid mixed tank

A

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
30
Q

Russian Disc Turbine Po

A

5

31
Q

Flat Blade Turbine Po

A

3

32
Q

45 degree Pitch Blade Turbine Po

A

1.25

33
Q

Wide Blade Hydrofoil

A

0.6

34
Q

Retrieved Curved Impeller

A

0.5

35
Q

Narrow Blade Hydrofoil

A

0.25

36
Q

Define the pitch of the turbine blade

A

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

37
Q

Zwiettering Correlation equation

A

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

PPD (7 decks)

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