L5/6- Agitation & mixing Flashcards
Agitation
Induced motion of a fluid in a specified way (homogeneous material can be agitated)
Why Agitate?
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
Mixing
Distribution of one initially separate phase into another (initially inhomogeneous phase only mixed)
Impeller Choices
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
High speed Impellers for bulk fluid mixing
Propeller and pitched-bladed turbines
Low Speed agitators for viscous fluids
Paddle, anchor agitator, helical ribbon agitator
Prevents deposits on HT surface
Rushton turbine
Radial flow mixing- suitable for turblent flow also
Agitation Selection
Typically turbines except for high viscosity
Propeller for low viscosity
Angle blade turbine or axial flow propeller for keeping solids in suspension
Flow patters in mixing
Radial and longitudinal - provide flow for mixing
Tangential flow - suppresses flow by creating vortexes
Preventing swirl
Baffles to impede tangential motion without interfering with radial/longitudinal flows
Mounting impeller off centre
What happens if no baffles are present?
Overtopping as fluid rotates as solid body
Standard turbine - define variables
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
Power number
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
Reynolds Number
Re = [(Da^2)Np]/mu
Proportional to diameter and peripheral speed of impeller
Froude Number
Fr = [(N^2)*Da]/g
Ratio of inertial stress to gravitational force per unit area acting on fluid
Rotator tip speed
Vtip = PIND
Turbine impellers See lecture figure of 6 impellers
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
Critical Re number
Re c = [6370/Po^(1/3)]
For mixing to take place, flow must be fully turbulent, i.e. Re > Re c
t95 parameter
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)]
Fourier Number
(Po^(1/3))ReFo = 5.2
Mixing regimes
Different length and time scales of dominating mechanisms:
Macro, micro, meso
Macromixing
Relates to scale of equipment (mm to m)
Micromixing
Relates to size of smallest turblent eddies (micro m)
Length scale- smallest which flow has turbulent characteristics
Mesomixing
In between macro and micro mixing
Problems in micromixing
Local inhomogeneity on length-scale
Kolmogoroff Microscale
/\k = [(v^3)/E]^(1/4)
Lifetime of eddy, tow k
tow k = 12*[v/E]^(1/2)
Note E here is epsilon
Engulfment rate, E
E = ln2/tow k
Rate of engulfment, tow E
tow E = 1/E
What ratio is wanted to avoid problems of selectivity in micromixing?
tow R > tow E
Avoid problem of selectivity determined by micromixing rather than intrinsic kinetics
Kinematic viscosity
v = (mu/p)
