Question

How to calculate the pressure inside the distillation column?

Answer 1

There are many parameters need to be considered while selecting operating pressure of distillation column. But primarily it is decided to a value that can enable the use of cheapest cooling media in condensor and/or heating media in reboiler (trade off between two is considered to achieve minimum utility cost.).

For example; if your vapour temperature is supposed to be “x” and we want it to be at less than its bubble point, to condensed it down in condensor using cheapest cooling media, which has temperature “y”. Now using VLE data we can find out operating pressure of column, that would enable us to achieve desired bubble point(ideally which would be greater than operating temperature of cheapest cooling media). So, by this way we can manipulate operating pressure to achieve desirable bubble point for vapours, wchich can be condensed using cheapest cooling media. (Calculate operating pressure such that your x>y.) Same procedure can be used for deciding cheapest heating media. By considering both, total utility cost maintained at minimum.

However for heat sensitive products change in operating pressure would be secondary parameter to be considered.