hydraulic limit of a distillation column

[sb00]

Hi folks,
the limiting conditions in a distillation column are because of limitations on Vapor/Liq ratio- if V/L is very high, the column will go under flooding, if V/L is very low, it will exhibit weeping and so on...
Now, I have a serious difference of opinion from two of my colleagues (who have considerable experience of refineries).
Let us say that the hydraulic limit is 20% of naptha cut. What does this exactly mean? This 20% is that of the designed flowrate or is it 20% of the operating flowrate?
let us say that the designed throughput is 100m3/hr and the turndown is 60m3/hr.
These guys are saying that even if the column is operated at 60m3/hr, the hydraulic limit will be 20m3/hr (20% of design)!
but, as far as I can see, at turndown of 60m3/hr, the V/L will be too high if we try to draw a naptha of 20m3/hr.
What I believe is that the hydraulic limit should be on the operating throughput rather than the designed. That is, if I am operating the column at 60m3/hr, the maximum naptha I can draw is 12m3/hr (20% of 60m3/hr).
Please enlighten as to what is correct.
Much thanx in advance.......this forum really provides us a breather on things that are difficult to find in books!
-SB

 

[pmureiko]

If your tower is capable of handling 20 m3/hr of naptha at the high feed rate, it should certainly be capable hydraulicaly and mass transfer wise at the lower feedrate.

From a mass balance point of view, the composition of the feed would have to change to continue to yield 20 m3/hr of on spec. naptha product.