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