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Estimating Efficiencies - Coggle Diagram
Estimating Efficiencies
100% = Perfect equilibrium
0% = Vapour compositions are the same so no separation
The Scale of Efficiency
E(OC)=Nt/Na
Nt=total number of ideal stages for the separation
Na=number of actual stages needed to achieve separation
Good for designing a column that already exists
Drawbacks
Too simplistic
Efficiency is affected by composition
With lots of stages, there are large changes in composition
Good method if a relatively small column
Physical factors affect efficiency
Composition causes changes in other properties
Viscosity
Surface tension
Geometry
Empircal methods
Drickamer and Braford Correlation
Mainly hydrocarbon mixtures
Limited range of data
O'Neills
Good reliability
Reasonably accurate
Simpler
Not good for nitriles
THE OVERALL COLUMN EFFICIENCY
Stage efficiency
Factors affecting stage efficiency :
Vapour and liquid flowrates
Different efficiencies in the rectifying and stripping section
5 Hydraulic regimes
Bubble
Cellular foam
Froth
Spray
Emulsion
Low vapour flowrate ::Star::
Weeping
High Vapour flow rate :star:
Flooding and entrainment
Entrainment
Vapour pulls liquid from the previous tray up
Stage geometry
Weir height
Height of end that liquid flows over
Hole size
Smaller hole = higher surface area/volume ratio for the bubbles
Liquid and vapour flow paths
Sieve, Valve tray or bubble cap
Valve tray
Increases the efficiency
Bubble cap
Increases the pressure drop - better than valve tray as more efficient
Sieve = least efficient
Physical properties
Viscosity
Bubble size and surface area
Higher surface area contact is wanted by CE
The bubble is trying to reduce it
Diffusivity - hence mass transfer
Flow regime
Surface tension
Rising and settling velocity - stokes law
Vapour and liquid contact time
Higher liquid viscosity --> Higher efficiency
Drag force is high with high viscosity (stokes law)
Slower the rise through the liquid, more contact time, better mass transfer
Liquid composition
Assume liquid is moving across the tray like a PFR or pipe
Point Efficiency
Always less than 1
Oldershaw Column