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[KennethNoisewater]

I think this should be a fairly quick question to answer, although it may take a bit to explain.

Our senior unit-ops lab consists of six pilot scale experiments which we must take data on over two weeks, then use that data to scale-up for a design problem. The experiment my group is starting next is a 12 tray column distillation separating . Week 1 the column is operating at total reflux and we use a density meter to find out the concentration of the liquid and vapor at each tray, and the concentration in the reboiler and condenser.

Using the information from week 1 we determine the tray efficiency and use that to choose which tray we will feed to at week 2. During week 2 we operating the column and try to reach steady state using our calculated information.

Throughout the course other groups have done this experiment, and the one question no one has answered to a satisfactory degree is this: Why are we able to use the efficiency we calculated at total reflux for week 2 and during scale-up since the conditions change?

From the discussions my friends and I have had we have decided that it is because the reboiler is at equilibrium. This means that as long as we are feeding the same material as was originally put into the column during week 1 and will be fed to the scaled-up column the vapor coming up from the reboiler is the same concentration throughout all the tests. I think this is the beginning of the reason, but I can't really explain it well past there, and I was curious what else I could add or maybe what else I haven't thought of.

TIA for any help.