4 replies to this topic

Hi everyone,

I am currently carrying out a hydraulic design on a distillation column and am following the example in 'Distillation Design', H Kister, Chapter 6. Does this example apply for Valve trays and are any alterations required?

Help Much Appreciated!

Henry

[Zauberberg]

The answer is - yes. Equations, nomograms and correlations presented in Kister's book are there for the purpose of preliminary design of tower internals. Make sure that you always follow the correct equations which are describing your system and are related to your particular application. For example, there is a huge difference between liquid- and vapor-driven hydraulic designs of fractionation equipment.

One additional and very important note: regardless of your design efforts, these calaculations will always have to be confirmed from tray manufacturing vendors. In other words, if you are specifying tower diameter, height and the type of internals, and at the same time you are requesting from tray vendor to guarantee the performance of YOUR design - that is something that no one will accept. So keep in mind that you should be able at least to deliver accurate vapor and liquid profiles across the tower (inlets/outlets and transition zones), physical properties and product quality parameters. Vendor will usually give you the optimum design, and your calculations are there to prove to yourself how accurate this is, or not.

Best of luck,

Brilliant, very useful. One point on the vapour/liquid driven aspect. My column is an H2S stripper (not steam!) with very little vapour flow. Will the column be therefor Liquid driven hydraulically, and how would this change the calculations? At the moment I am taking the maximum vapour load as the design limit...

Thanks again

Henry

[Zauberberg]

I don't know the flowrates in your case (liquid feed, stripping medium, and bottom product specs), neither the operating conditions of the tower. However, here are some rules of thumb - and you'll find them in Kister's book, and in many other chemical engineering textbooks as well.

1. If you set the tower diameter based on maximum allowable vapor velocity (including system factor), then perform a cross-check of maximum weir loading, downcomer velocity and downcomer pressure drop - for a given liquid flowrate you have. If you exceed maximum values for any of these critical hydraulic parameters, then you need to increase tower diameter, use multiple tray passes, or switch to structured/random packing.

2. For a tower diameter which is determined based on liquid loadings, vapor flowrate is not a big concern. Just make sure that you properly adjust tray open area and valve characteristics (valve lift and weight), which will make dry tray pressure drop and hydraulic tray pressure drop approx. equal. This gives very good turndown characteristics and minimum weeping at low vapor rates, so the tray efficiency will not be adversely affected and you will not loose separation sharpness.

[djack77494]

Henry,
I think Zauberberg has about nailed it. If you use any of the tray vendors' commercial software to check your results (which I heartily recommend), you will usually find out how close you are to the various flooding limits. They will often output factors like %Flooding due to multiple factors such as downcommer backup, jet flooding, etc,. I think it useful and educational to review and understand the entire output.

A further point of interest is that the vendors will often provide their proprietary software FREE of charge. After all, their hope is that you will eventually purchase their product, and the software is a tool that you may use to (preliminarily) select the product that you may ultimately purchase.
Doug