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[blah123]

I know what bubble point and dew point are, but what I don't understand clearly is when to use which
This is what I understand, correct me if I'm wrong:
If a liquid product is leaving a distillation column, then you perform a bubble point calculation to find operating conditions?
If it is a liquid leaving the column, then you perform a dew point calculation to find operating conditions?
If there is a condenser at the top, you perform a bubble point calculation?

Also, there's a question I am trying to solve. And for it I need to find top and bottom temperatures.
In the solution it shows that a dew point calculation has been used to find top temperature, and a bubble point calculation to find bottom temperature. I don't understand why?

Thank you.

[Art Montemayor]

Blah:

We don’t do calculations related to a distillation Unit Operation simply to do them. We must have a solid reason for doing them. And there is where the dew point and bubble point calculations come in: you must have a reason for employing either one.

You say you know what the bubble point and dew point are – and I’m going to believe you, so I won’t define them here (although it would be the best way to make sure we are all talking about the same thing).

Firstly, if a liquid product is leaving a distillation column, you must presume that the liquid is SATURATED. This is a very, very important point (which you fail to even mention). Because it is saturated, it is on the verge of vaporizing and it should be at its bubble point. To find the column’s reboiler operating conditions, you would perform a bubble point calculation on the liquid composition desired (or expected) out of the column sump.

If you are running a distillation column, of course you have a condenser on the overheads product. How else would you generate the required Reflux to secure the desired overheads purity level? This is just common sense. And again, the overheads vapor exiting the top of the column is --- guess what? SATURATED! You can’t run a bubble point calculation on a saturated vapor (although you can try), so the obvious and common sense point of view is that you run a dew point on the vapor to find the operating conditions inside the condenser.

I haven’t tried to sound nonsensical in explaining what I consider to be very basic, straightforward, and important information. I’ve tried to show you how really simple and easy it is to know what these tools are used for. Common sense and intuition alone can tell you every time if you remember to apply the basic definition and the basic conditions under which both the bubble and dew point are defined: saturated conditions must exist. If you have already taken your first course in Thermodynamics, you will recall how the Mollier Diagram (or my favorite, the T-S Diagram) display the saturated vapor and saturated liquid curves and how they inter-act with the variations in temperature and pressure. This conceptualization of the saturated state of a fluid always helped me as a student to decide where I was in a process with respect to phase equilibria. I suspect that is what is hampering your ability to quickly grasp what you have to do to identify those points within the Mollier Diagram.

I hope I’ve helped you in better understanding when to use these tools and WHY.