3 replies to this topic

[Dizzy]

Hi, I'm looking for clarification with regard to the usage of dew point and bubble point calculations in finding condenser pressure or temperature. The following are the points I would like clarification on:

• For total condensers, all vapour is being condensed to liquid therefore should a dew point calculation be used to determine pressure and temperature? Or is it that the liquid will be saturated and therefore a bubble point calculation should be used? I think it should be dew point but I am seeing examples done in class where the bubble point was used.
• For partial condensers, only some vapour is condensed to liquid which is the reflux. I think here dew point calculations should be used since it's vapour going to liquid but I'm not sure.

Any help with this will be greatly appreciated. Thanks! 

[Lucian Gomoescu]

Hello Dizzy,

1. A total condenser at a pressure P operates at a temperature T <= T_bubble point, otherwise the outlet would include a vapor phase and, therefore, it would no longer be a TOTAL condenser.

2. A partial condenser at a pressure P operates at a temperature T that satisfies the inequalities T_bubble point < T <= T_dew point, otherwise it would be a total condenser (T<=T_bp) or only a cooler (no condensation because T>T_dew pt).

Observation: In a partial condenser, the condensed phase is not the reflux, it is reflux + liquid distilate.
Observation: In case of a vapor phase containing non-condensables (i.e. gas), it  is impossible to use a total condenser.

BR,
Lucian

[Francisco Angel]

Dear Dizzy:
I think Lucian mentioned the more important points. I would only like to point out that in the case of a partial condenser, the relative amount of liquid and vapour phase desired will determine the condensation temperature to use, based on the mass balance and the equilibrium compositions at that temperature.

Best regards.

[Dizzy]

Thanks to both of you A rough VLE diagram + Lucian's points made it crystal clear