L5/6- Agitation & mixing

Question 1

Agitation

Answer 1

Induced motion of a fluid in a specified way (homogeneous material can be agitated)

Question 2

Why Agitate?

Answer 2

Ensures contents are uniform in temp & composition;
Improves heat transfer between contents and wall/coil;
Speed up dissolving solid
Improve mass transfer
Suspend solids in solution

Question 3

Mixing

Answer 3

Distribution of one initially separate phase into another (initially inhomogeneous phase only mixed)

Question 4

Impeller Choices

Answer 4

Axial flow impellers or radial flow impellers

Propeller- (high speed axial flow) produces highly turbulent liquid & effective in bulk flow scenarios

Paddle- (simple flat paddle) no axial motion - radial and tangental, with up/downward motion at wall

Turbine- high speed rotation induces strong current in low viscosity systems

Question 5

High speed Impellers for bulk fluid mixing

Answer 5

Propeller and pitched-bladed turbines

Question 6

Low Speed agitators for viscous fluids

Answer 6

Paddle, anchor agitator, helical ribbon agitator

Prevents deposits on HT surface

Question 7

Rushton turbine

Answer 7

Radial flow mixing- suitable for turblent flow also

Question 8

Agitation Selection

Answer 8

Typically turbines except for high viscosity

Propeller for low viscosity

Angle blade turbine or axial flow propeller for keeping solids in suspension

Question 9

Flow patters in mixing

Answer 9

Radial and longitudinal - provide flow for mixing

Tangential flow - suppresses flow by creating vortexes

Question 10

Preventing swirl

Answer 10

Baffles to impede tangential motion without interfering with radial/longitudinal flows

Mounting impeller off centre

Question 11

What happens if no baffles are present?

Answer 11

Overtopping as fluid rotates as solid body

Question 12

Standard turbine - define variables

Answer 12

Da - agitator diameter
Dt - tank diameter
H - height of liquid
J - depth of baffle
E = height of centre-line of agitator
W = width/depth of agitator
L = length of agitator blade

Question 13

Power number

Answer 13

Po = P/[(Da^5)(N^3)p]

Analagous to grag coefficient.

Proportional to ratio of drag force acting in unit area of impeller and inertial stress (flow of momentum of bulk motion)

Constant for high Re

Question 14

Reynolds Number

Answer 14

Re = [(Da^2)Np]/mu

Proportional to diameter and peripheral speed of impeller

Question 15

Froude Number

Answer 15

Fr = [(N^2)*Da]/g

Ratio of inertial stress to gravitational force per unit area acting on fluid

Question 16

Rotator tip speed

Answer 16

Vtip = PIND

Question 17

Turbine impellers See lecture figure of 6 impellers

Answer 17

1) Flat six-blade disk turbine
2) Flat six blade open turbine
3) Six blade open tubrine at 450
4) Six-blade open turbine
5) Pitched curve blade turbine
6) Pitched blade turbine

2 and 4 only differ by blade depth- use for flat blade

Question 18

Critical Re number

Answer 18

Re c = [6370/Po^(1/3)]

For mixing to take place, flow must be fully turbulent, i.e. Re > Re c

Question 19

t95 parameter

Answer 19

Extent of mixing - time in seconds required for deviations from mean temp, conc. etc to be no more than 5% of the mean, starting with a fully stratified situation

Fo = [mut95]/[p(Dt^2)]

Question 20

Fourier Number

Answer 20

(Po^(1/3))ReFo = 5.2

Question 21

Mixing regimes

Answer 21

Different length and time scales of dominating mechanisms:

Macro, micro, meso

Question 22

Macromixing

Answer 22

Relates to scale of equipment (mm to m)

Question 23

Micromixing

Answer 23

Relates to size of smallest turblent eddies (micro m)

Length scale- smallest which flow has turbulent characteristics

Question 24

Mesomixing

Answer 24

In between macro and micro mixing

Question 25

Problems in micromixing

Answer 25

Local inhomogeneity on length-scale

Question 26

Kolmogoroff Microscale

Answer 26

/\k = [(v^3)/E]^(1/4)

Question 27

Lifetime of eddy, tow k

Answer 27

tow k = 12*[v/E]^(1/2)

Note E here is epsilon

Question 28

Engulfment rate, E

Answer 28

E = ln2/tow k

Question 29

Rate of engulfment, tow E

Answer 29

tow E = 1/E

Question 30

What ratio is wanted to avoid problems of selectivity in micromixing?

Answer 30

tow R > tow E

Avoid problem of selectivity determined by micromixing rather than intrinsic kinetics

Question 31

Kinematic viscosity

Answer 31

v = (mu/p)