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Temperature Gradient In Distillation Column


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#1 ChemRXN

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Posted 21 December 2016 - 10:50 PM

Hello everyone,

 

As a chemical engineering student learning about separations I understand that in an ideal distillation column (Constant Molal Overflow assumption, adiabatic) there is a temperature and pressure gradient across stages. I do not entirely understand these concepts or how one would control/ensure that a gradient is maintained so I would appreciate if you can clear my doubts. 

 

First and foremost, I understand a pressure gradient is needed otherwise the vapor will not flow upwards but how does one set the pressure for each stage and what causes this pressure drop? Is it the overall gas pressure or the partial pressure of the most volatile component that drops as the vapor rises?

 

Furthermore, I understand that based on the Phase rule, fixing the pressure (or temperature) fixes the other state variable (if P, then T is fixed or if T, then P is fixed). So with a  pressure drop, temperature will drop as well.

 

But from a conceptual standpoint any heat added by the reboiler is ultimately removed by the condenser from an energy balance standpoint because the column has no heat lost or gained to the surroundings. But then, why would there be a temperature gradient in the column? Any heat transfer used to vaporize the falling liquid in the rectifying or stripping sections as the vapor from the bottom of the column rises is not removed from the system so why would the temperature drop? Furthermore, some of the more volatile component also condenses which is removing heat but i don't see where it goes. 

 

Thanks for the help

 

 



#2 Bobby Strain

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Posted 21 December 2016 - 11:47 PM

Well, you need to start with elementary fundamentals of distillation. Then return and ask what you don't understand.

 

Bobby



#3 breizh

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Posted 22 December 2016 - 02:36 AM

https://escholarship...96n0xv#page-489

 

Consider this resource to support your study . All you need is there.

 Breizh



#4 colt16

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Posted 22 December 2016 - 10:11 AM   Best Answer

Hello everyone,

 

As a chemical engineering student learning about separations I understand that in an ideal distillation column (Constant Molal Overflow assumption, adiabatic) there is a temperature and pressure gradient across stages. I do not entirely understand these concepts or how one would control/ensure that a gradient is maintained so I would appreciate if you can clear my doubts. 

 

First and foremost, I understand a pressure gradient is needed otherwise the vapor will not flow upwards but how does one set the pressure for each stage and what causes this pressure drop? Is it the overall gas pressure or the partial pressure of the most volatile component that drops as the vapor rises?

 

Furthermore, I understand that based on the Phase rule, fixing the pressure (or temperature) fixes the other state variable (if P, then T is fixed or if T, then P is fixed). So with a  pressure drop, temperature will drop as well.

 

But from a conceptual standpoint any heat added by the reboiler is ultimately removed by the condenser from an energy balance standpoint because the column has no heat lost or gained to the surroundings. But then, why would there be a temperature gradient in the column? Any heat transfer used to vaporize the falling liquid in the rectifying or stripping sections as the vapor from the bottom of the column rises is not removed from the system so why would the temperature drop? Furthermore, some of the more volatile component also condenses which is removing heat but i don't see where it goes. 

 

Thanks for the help

 

You do not control the pressure of the individual stages, you control the pressure at the top of the tower. 

As this occurs, there is a pressure buildup in order for the liquid at the bottom to boil. As each stage has a liquid holdup, there will be some pressure for the vapour to overcome resulting in the pressure at the tower bottoms being higher than the one at the top. The reboiler which consistently supplies heat at the bottoms is responsible for generating the vapour that causes the pressure build-up.

 

The temperature gradient is not so surprising if you think of the compositions. The top of the tower consists almost entirely of the purer lighter material which boils at a lower temperature than the bottom which consists almost entirely of the purer heavy material. In a benzene-toluene tower for example, you expect the temperature at the top to be closer to benzene dew point, and the temperature at the bottom to be toluene bubbling point. In between the top and bottom are varying compositions of benzene-toluene with decreasing benzene composition until the bottom of the tower. That is why a temperature gradient also exists. 



#5 rdcrags

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Posted 22 December 2016 - 11:02 PM

Exactly. as answered. Consider that there is also a composition gradient as well, and each composition has its own boiling point. Why the composition gradient? At each stage, there occurs heat and mass transfers.






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