3 replies to this topic

[process.eng88]

The McCabe-Thiele construction should be based on the internal reflux ratio, which can be estimated by the following equation derived from an energy balance around the top tray:

R internal = R external *( 1+ ( CpL * DeltaTsubcooling/molar enthalpy of vap))

Ref: Separation Process Principles By Seader & Henley

CpL refers to Liquid Phase Heat Capacity.


Should I use CpL at the feed temperature or at the bubble point for calculating the internal Reflux Ratio ?

Please explain.

[process.eng88]

I have another query related to the internal reflux ratio:

If subcooled reflux is expected to occur in the batch rectification column do I need to use the external reflux ratio or the internal reflux ratio for determining the duty of the Total Condenser ?


I'm using the following energy balance for a total condenser

Qc = D(R+1) * Avg Heat of Vaporization

[USR]

Which feed temperature are you talking about? Where does that come into picture? You consider only the condenser-reflux drum system. It is when overhead vapours are cooled below the bubble point. External reflux will require heat to come back to the bubble point this comes from the condensation of overhead vapours only. This heat requirement is the expression you are inquiring about. The Cp is definitely at the bubble point.

Your terminology is confusing as well. What you state as your internal reflux has been taught to me as apparent reflux. Apparent reflux will consist of the external reflux plus the extra overhead vapours that has been condensed.

Now coming to internal and external reflux ratio, for V mole/hr overhead vapours, L mole/hr reflux and D mole/hr distillate product. External reflux ratio is L/D; internal reflux ratio is L/V.

[Root]

I have another query related to the internal reflux ratio:

If subcooled reflux is expected to occur in the batch rectification column do I need to use the external reflux ratio or the internal reflux ratio for determining the duty of the Total Condenser ?


I'm using the following energy balance for a total condenser

Qc = D(R+1) * Avg Heat of Vaporization

1st of do overall material balance and then calculate heat duty for required D (distillate) and after that if liquid is sub-cooled then you can very condenser duty of you can add more heat to produce more internal and external reflux.
Your calculation all depend on mass balance.
Cheers