How to size a Knock out drum?

[Imran.Idris]

Can anybody help me in sizing a knock out drum?

[tosif]

Imran, its a three phase process. First you will have to freeze the design basis, including gas and liquid flowrates, densities at FTP, the allowed droplet size and its usage (KO for compressor or for pipeline) . In second phase you will evaluate whether you have to go with vertical or horizontal orientation, size of vessel, usage of demister, nozzles and dimensions. Third phase will be liquid level controller, including LLL, NLL, HLL defining along nozzle size.

hope this will help you in your sizing.

[Imran.Idris]

Thanks.
Its good for understanding but I need help in detailed designing of a knock out drum for carbon dioxide service.

[Ali]

is it a 2-phase separation or a 3-phase one.

[Imran.Idris]

Its a two phase separator; mixture of carbon dioxide and moisture.

[Ali]

One thing is sure from your system description, that you will be going for vertical separator due to low Water-gas ratio. You can get 2-phase vertical separtor sizing easily in books like Ludwig, Evans, GPSA engg. data Handbook etc. The major steps are:
1.Determination of K factor (based on operating pressure, service and provision of mist eliminator)
2.Evaluation of Terminal velocity. Use 0.75 of the terminal velocity for your further calc.
3.Determination of Vessel dia based on the vol. flowrate and the abovementioned calculated velocity. (add 3 to 6 in for mist eliminator installation and if required,then increase the dia so that it could be in a multiple of 6-in)
4.Calculate Liq. Holdup volume (based on liquid flow rate and the residence time. May be taken 10 min.).
5. Low liquid level (HLL) setting (based on vessel dia and operating pressure). Take it 200 mm from T.L
6.Height from HLL to Normal Liquid Level. (min. 1 ft)
7.Height from N.L to HHL. (Min. 6 in)
8. Height from HHL to Inlet nozzle (12 + inlet nozzle dia, in). Inlet diverter is installed.
9. Height from inlet nozzle upto the demister pad: 0.5 Vessel Dia
10. 6 in. for demister
11. 1 ft above demister upto the vessel T.L.
12. Add all of them and adjust the height if required to get L/D in the range of 1.5 to 3.0.
13. Inlet / Outlet Nozzle sizing has its own criteria based on recommended momentum (Rho-v2) thru them.
14.Level connection may be taken as min. 2".

I have tried to give a precise overlook of 2-phase vertical separator sizing. Rest you can get from the literature.

[Imran.Idris]

Thanks Ali for a very comprehensive and informative reply.
What you have suggested is quite good if I am going to size a vertical KOD with no internal like additional fin etc. What if I provide vertical fins inside the KOD, for explanation please see the attached drawing, for moisture to condense and settle on it. In that case how much size reduction is possible for me? In other words to what fraction of terminal velocity can I reduce my vessel dia? Is there any correlation available for this??

[Black Onyx]

The sizing steps have been adequately mentioned, just as a reference, following link gives step by step demostration of how to proceed.

http://www.red-bag.com/jcms/index.php/e ... sel-sizing

Regards

[Imran.Idris]

Thanks Balck Onyx

[Imran.Idris]

Finally we have developed a software that can help you to design a two phase (liquid-vapor) separator. It is free to download and results are as reliable as per GPSA Sec: 7. It is beta version so far and is limited to size KO drum without demister pad.
Following is the link where you can download it. Download it, use it and enjoy
http://www.thepetrostreet.com/forums/vi ... f=23&t=193
For any query you can respond to the forum.