Temperature difference along a distillation column 1

[Mike4chemic]

Hello,
We try to model the separation of n-pentane from the syntetic seal oil/n-pentane miscible mixture by a simple batch distillation process (reboiler + riser+condenser) at the atmosheric pressure.
The n-pentane boiling temperature is 38 deg.C. and oil boiling temperature is about 270 deg.C.
According to the oil/n-pentane vapor-liquid equilibrium data (which were created based on the Raoult's law),for example at the reboiler's temperature of 60 deg.C, the equilibrium n-pentane (Y) concentration in vapor is 0.9998 and oil is 0.0002.Is it correct to assume that at the simple batch distillation the vapor (top) temperature when n-pentane (Y) 0.9998 is similar to the botomm temperature of 60 deg.C ?
On the other hand,when the vapor phase enriched with low boiling n-pentane, I would expect to see the vapor temperature closely to 38 deg.C.
Is there a temperatute difference between a reboiler and condenser for a simple batch distllation process ?

Thanks, Mike

 

[jari001]

You can expect a temperature and pressure gradient, the magnitude of which should be easily measurable based of the wide difference in boiling points. The pressure gradient will be a sum of the drop through the column internals as well as the vapor pressure changes through the different stages.

I could be remembering my gen chem incorrectly, but miscible solutions are non-ideal and therefore Raoult's law wouldn't be a good method to generate VLE data. Feel free to correct me if I'm wrong.

 

[Mike4chemic]

jari001,
Thank you.
In our case the pressure gradient along the column is neglegible,since the column is a simple riser without any internals.
I am aware that the miscible n-pentane/oil mixture is non-ideal and deviates from the Raoult's law.
My question is whether based on the ideal VLE data (temperature vs composition)the bubble point of the liquid phase is always similar to the dew point of the vapor phase?
As I wrote, at the reboiler (liquid phase) temperature of 60 deg.C (bubble temperature),the dew point of the equilibrium vapor phase is 60 deg. C too. At the reboiler temperature of 60 deg.C the vapor phase is enriched with n-pentane vapor(Y-0.9998).Is it feasible that the dew point of the equilibrium vapor (Y-0.9998) is 60 deg.C.? Or it should be closely to 38 deg.C ? Where is a catch ?

Thanks. Mike

 

[jari001]

The dew point and bubble point temperatures will be different for a standard ideal solution, you can make a P-xy diagram and confirm. Assuming that the vapor is not subject to good heat transfer, and that the pressure at that point of the column is close to 1 atm, then I would expect the vapor to be close to it's normal boiling point since it is basically a single phase, pure component.

 

[Compositepro]

You will not have equillibrium conditions in such a column. Vapor will be rising in the column and some degree of refluxing liquid will run down the walls. There will be very poor contact between the vapor and the reflux. Amount of insulation on the column will be a big factor.

 

[georgeverghese]

The dewpoint of the vapor phase would be expected to be close to 60degC since you have a binary mix for the vapor phase, one of the components being this high bp oil. However, the liquid phase fraction of this vapor at this vapor dewpoint would be very low, and this liquid would be almost all oil.
What you are expecting to be 38degC is actually approx the bubble point temp of the vapor phase.

 

[Mike4chemic]

Thank you Gentlemen.

georgeverghese
I try to summarise my understanding.For the simple batch distillation (with an empty column and without reflux),the vapor temperature at the top of column always is close to the reboiler's(liquid ) temperature, assuming the heat-losses and pressure drop over column are neglegible. Is it correct?
The condensation of vapor phase will start at the dew-point of 60 deg.C,and in order to condense all vapor phase the vapor's should be cooled below of the bubble temperature of 38degC. Is it correct?
For a sake of curiosity,I tried to model a simple batch distillation (with one stage)with the Winsim process simulator.
At the reboiler's temperature of 60 deg.C, the vapor(top) and the dew-point temperatures are the same of 38 deg.C
How it can be explained?

Thank you, Mike

 

[georgeverghese]

Q1. Yes Q2. Yes.

The vapor top temp and its corresponding dewpoint will obviously be the same. Remember you are dealing with a wide boiling binary mix here ( not a single component). And it is at equilibrium at the column top also. But this vapor's bubble point will be much less than its dewpoint.