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Distillation: Multipass Trays


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#1

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Posted 04 April 2010 - 09:37 PM

I have been designing a single pass sieve tray. Whilst the calculations I have for a 4 meter diameter tray is theoretically sound, reading around literature made me think that since it is a large diamter column I should be employing two-pass trays.

However my search for books which discusses multipass trays in detail has been to no avail. I would like to have a solid basis to justify a two-pass design, however it seems that most books quote it as "according to previous experiences". Should I just stick with a one-pass design for academic purposes?

Thanks!

#2 Zauberberg

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Posted 05 April 2010 - 12:07 PM

Multi-pass trays do work in reality. I have seen those in conventional distillation towers (e.g. Atmospheric Distillation Unit in refinery, 2- to 4-pass trays) and also in some amine absorbers (although they call for special care during design because of amine residence time issues), and also in gas condensate strippers (the one here has 6-pass trays).

Normally, we employ multi-pass trays when handling extremely large liquid flows. Having single-pass trays would result in somewhat unacceptable big tower diameter, and opting for multi-pass trays is one fancy way of solving the problem.

You can try to compare two different designs (i.e. single- and multi-pass trays) for the same system by using vendor software such is KG Tower from Koch-Glitsch. It is available at: http://www.koch-glit...re/default.aspx

Cheers,

#3 katmar

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Posted 06 April 2010 - 07:23 AM

In order to decide between single pass cross flow plates and multi-pass plates you need to consider the hydraulic gradient. There is a resistance to the liquid flowing across the plate due to the rising vapor bubbles, and the valves (if a valve tray). This effect was far worse in the days of bubble cap trays as the liquid had to flow around and between the caps.

The hydraulic gradient means that the liquid is deeper on the inlet side of the tray than near the overflow weir. Because the vapor pressure drop over the tray has to be the same on the side with deeper liquid as it is where the liquid is shallow, you get less vapor flow through the deep liquid. This results in maldistribution of the vapor, causing decreases in efficiency and capacity.

The answer is to use shorter flow paths, as with multi-pass trays. For more detail see Perry or Kister.

#4 Zauberberg

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Posted 06 April 2010 - 12:21 PM

Katmar is right. By using multi-pass trays you'll definitely loose some efficiency as compared to a single-pass tower. I remember when I was doing efficiency calculations for condensate stripper (dryer) where two-pass design was calling for approx. 15% more theoretical stages required for the same product specs. Or, in other way, by saving money on tower diameter you'll have to spend additional resources on a little bit extra height.

#5 abhi_agrawa

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Posted 06 April 2010 - 07:34 PM

LeeFauxe,

The number of passes in a distillation column is really a function of vapor-liquid loadings only. I have seen one pass, two pass as well as four pass trays being used in same service in different plant. Usually if the weir loading (volumetric flow of liquid/length of outlet weir) is more than 6 GPM/in, the number of passes is increased. Really speaking there is no basis of this 6 GPM/in. I know of a column with as high as ~9 GPM/in weir loading, but still operates well.

As Zauberberg has rightly pointed out, be careful about the efficiency. As you increase the number of passes, the tray efficiency comes down.

Hope this helps,
Abhishek

#6 katmar

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Posted 07 April 2010 - 02:16 AM

There is an excellent discussion on the design and operation of multi-pass trays the the following article :-

http://kolmetz.com/p...gn AIChE R5.pdf

This highlights the fact that it is very difficult to design a multi-pass tray with all the V/L ratios the same in the different sections on the tray. If the weir overflow rates are not all the same (i.e. GPM/inch) there will be varying liquid depths, and this results in the same maldistribution that I mentioned earlier as being caused by the liquid gradient in large single pass trays. I guess the simulator that Zauberberg used made an allowance for this balancing difficulty and lowered the estimated tray efficiency, resulting in the additional 15% trays.

#7 sheiko

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Posted 08 April 2010 - 01:51 AM

Other articles of interest FYI.

Attached Files


Edited by sheiko, 27 February 2011 - 12:24 PM.


#8

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Posted 08 April 2010 - 09:04 PM

thank you so much guys. Ive had a look at my stripper section and the liquid loading is at the limit. This would be very helpful in sorting out a discussion and possibly an alternative tray design for my project. Thank you so much!




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