Tray column feed inlet pipe guidelines 1

[koshyeng]

Could someone please tell me the guidelines for feed inlet pipe velocity or mometum flux required.

100% vapor enters vacuum distillation column through a 6" pipe (I.D 6.065 inch). Earlier we ran at lower rates but with increased capacity, feed flow rate is increased to 90,000 ACFH. Vapor density is 0.0817 lb/ft3.

hydraulic rating shows 75% flooding at maximum load on top tray.

Another interesting thing to note is this feed pipe is directly welded to the column without any internal projection ( like bends) or distribution nozzles. Is this acceptable? Vapor feed enters between two trays and liquid feed which has a T pipe enters two tray above vapor feed line at 7 gpm.

Thank You

Koshy

 

[sshep]

koshyeng,

Kister's Distillation Design chapter 2 discusses the subject with some different arrangements. The text generally gives citations of guidence that for vapor and mixed phase, the velocity head of the feed should not exceed 10% of the pressure drop through the bed (or accross tray) above. Kisters considers this as the ideal, but by experience conservative in that a velocity head of up to 100% (upper limit of design) of the bed pressure drop is not uncommon.

Andrew Sloley has a paper (045) that can be ordered from his website:

http://www.distillationgroup.com.

This site is the best on-line distillation resource that I have ever found.

From my own experience it seems that many designs are encountered- though radically different, most work fine and so we can conclude that Kister's comment that simplest is cheapest (best) is good advice.

For liquids the advice is easy in that the velocity should be about 1m/s or less- unfortunately most of my own distributor retrofit designs have been for liquid without problem. For a retrofit, I once asked for design of a distributor for a mixed phase, and got a bottom slotted style sold as a "flashing feed gallery". The tower had a few spec problems and a later vendor claimed that the style of distributor that was designed for me created a shadow effect for the vapor on the bed above (poor vapor distribution)- we admittedly tried to cram alot into a small space so maybe he was right, but we just lived with it. Anyway, spacing is possibly another consideration.

Good luck and best wishes,
sshep

 

[FandF]

For the inlet nozzle I think the following is also to be considered: d * V² < 1000 where d is the vapour density in kg/m³ and V is the vapour velocity in m/s inside the inlet nozzle.

 

[koshyeng]

I heard so much about Rho* Square(V) but I dunno what is the physical meaning of this value.

This value for my vapor feed nozzle is 2500. what does this mean?

 

[sshep]

koshyeng,

I have always rationalized rho*v^2 to be proportional to a velocity component (kinetic energy) of flow expressed as pressure instead of head. This reasoning comes straight from the Bernoulli equation, and is in any event the route to take to calculate the inlet velocity head as 100% of pressure drop described above.

I think many engineers could agree with this thinking, but some scientists might think it too simplistic.

best wishes,
sshep

 

[FandF]

The meaning of rho*v^2 is that with too high values of this term you could have mechanical and hydraulic problems like noise, vibration of piping and liquid dragging inside the vessel