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I know that flooding will ruin the performance of a distillation column. There are many correlations (Eckert, kister, billet, etc.) that will tell you when a column will flood based on your fluid conditions. Most design guides suggest choosing a diameter that will lead 70-80% of the flooding condition. This makes sense as the size of the column (IE, the cost) is as small as possible while still giving you a 20-30% margin of error.

However, what if due to other constraints you want the column to be large so that it is only operating at 10% of the flooding conditions (IE, a much larger diameter than normal). Would this column still operate? Is this type of operation just avoided because it is not economically practical and a smaller column would do the same thing operating closer to flooding?

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Using a much too large column means you loose a lot more heat through its walls, meaning it will take longer for the equilibrium temperature/concentration gradient in it to form, and the destillate comes not only later but also considerably slower.

And you'll have to put more power into heating the still, with following danger of decomposition, unwanted side-products, etc.

If you use an expensive column with silvered vacuum insulation, this effect might not be too large even at only 10% of the maximum capacity. I wouldn't want to judge without trying, though.

Education and experience taught me to always use the proper type and size of equipment. It's faster, cheaper, and gives higher yields. I you have a large column that costs thousands, you can sure afford an additional smaller one at a tenth of the price, can't you?

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  • $\begingroup$ Good answer. The issue is I am using a diabatic column so the vapor flow rate changes significantly over the height of the column. The average flooding condition is 30%, the max is 90%, and the min is 10%. Its pretty linear over the entire height of the column. $\endgroup$ – Murenrb Dec 9 '15 at 21:27
  • $\begingroup$ So you can imagine the issue. Either half the column is overdesigned or we have a shrinking diameter over the column. Our cost is pretty low in either case so it seems simpler to just have it be overdesigned for half. But I want to make sure it will still be functioning in that realm. $\endgroup$ – Murenrb Dec 9 '15 at 21:33
  • $\begingroup$ Help me out, what is a "diabatic" column? $\endgroup$ – Karl Dec 9 '15 at 21:39
  • $\begingroup$ diabatic just means that the heat is added at each tray instead of all at the bottom. Here is a nice link: sci.sdsu.edu/~salamon/OptDiabDist.pdf $\endgroup$ – Murenrb Dec 9 '15 at 21:44
  • $\begingroup$ Ah. Well, the bad disadvantages mentioned above don't apply then, but i guess a sort of "conical" column would still deliver your destillate initially faster , and at generally lower energy consumption. Should be easy to calculate the flux here, with constant temperatures everywhere. If you custom build it anyway, why no go for the optimum? $\endgroup$ – Karl Dec 9 '15 at 22:00

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