Distillation - Downcomer Flood % Calculation

[BBMM]

Can anyone tell me how "Downcomer Flood" can be manually easily calculated (for valve trays) based on both "backup flooding" approach and also "choke flooding" approach. I need to calculate it in percent form (however that is done) so I can compare it to a percent magnitude given by a supplier's rating sheet result. The supplier is unwilling to discuss how it is calcuated. Can anyone help.

 

[pmureiko]

In your case the vendor blindly uses a computer program to calculate pressure drop and is embarrassed to admit they don't know how to do it manually. Even worse is that they won't help fix the problem, whose solution could be selling you more trays. I suggest you get a better vendor that wants your business. If so this problem is also a poor reflection on the state of chemical engineering in industry.

Downcomer flooding is when liquid from the tray below backs up to the tray above. For backup flooding this is because the pressure drop across the tray is greater than the liquid head available from the tray spacing. The formulas to calculate pressure drop area readily available in Distillation Design by Henry Kister, Perry ChemE Handbook, and Unit Operation of ChemE by McCabe, Smith, & Harriott among others. You shouldn't have any problems calculating this on your own and it will be an empowering experience.

Choke flood occurs when there is insufficient time for the liquid to dearate. Downcomer residence time preferrably should be at least 5 s, more for foaming systems. This is any easy calc. and you should be able to determine if this is your problem.

Good luck.

 

[BBMM]

Thanks for the tips and I agree with all you said about vendors. In the mean time I'll be using Kister and Perry's for the calculations. Thanks

 

[STI666]

This does not involve the calculations but to actually see the extent of flooding on the trays or liquid in the downcomers you could use a service called a Gamma Scan. It is non-invasive and is done while on-line. It can tell you the height of liquid on trays, flooding, foaming, entrainment, weeping, etc. Check out some of the good web sites by Scanning Technologies or Tower Scan.