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Liquid Levels In Distillation Columns

distillation column level hll nll lll

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#1 miguel29chem


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Posted 04 June 2017 - 10:45 AM



I am trying to set the operating levels for a distillation tower to design the control loops. Which criteria should I adopt to define the HLL, NLL and LLL of the tower?


Thank you.


#2 Bobby Strain

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Posted 04 June 2017 - 10:53 AM

Whatever your client requires.



#3 gegio1960


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Posted 04 June 2017 - 10:55 AM

which kind of distillation tower?

which kind of reboiler?

have you developed a preliminary sketch?

#4 miguel29chem


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Posted 04 June 2017 - 11:00 AM



There's no client, this is a project for college. We have already designed the tower with its diameter, height, amount of trays and type, etc.   


The proyect we are working on is Air Distillation, production of Oxygen. We have two integrated towers in a cryogenic temperature. There is a high pressure tower with a condenser, and a low pressure tower with reboiller which is integrated to the condenser of the first one. 


We have every single aspect of the towers designed, but we are having trouble in defyning the HLL, NLL and LLL since we don´t know the residence time in the bottom of the column.


I don´t know if that is enough data for you to help us.


Thank you.

#5 Art Montemayor

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Posted 04 June 2017 - 02:40 PM



Use your common sense.  You are dealing with a student design project - not a real-world project.


You may think you know the required physical dimensions of the Linde double column you have designed, but you really don't.  What is expected of you is to give the relative heights of the liquids in the sumps - enough to cover the reboilers.  You may think you have covered every aspect - but you really haven't.  Believe me, I've worked too many years working with air separation columns and know the internals well.


Make sure you know the type of reboiler you have designed and how it is expected to work.  You're doing submerged boiling, so common sense tells you you should have the bundle totally submerged.


Where you set the Low-Low level depends on how you design and pipe up the tower sumps where the reboilers are located.  You should have already prepared a detailed sketch of the towers - complete with dimensions.  Refer to my simplified profile of how a typical Linde double column is built and operates.  You should have generated and submitted this to our members if you are seeking help and advice.



Attached File  Air Separation - Lindes Double Column.xlsm   37.38KB   87 downloads

#6 breizh


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Posted 04 June 2017 - 09:38 PM

Hi ,

To add to the previous post , let you consider this resource .

Good luck


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#7 Mariano Aiub

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Posted 01 May 2018 - 03:49 PM


Hi Art,


I would like to ask you this question: I need to have a liquid level in a column to provide natural circulation to a kettle reboiler.  What is the best choice?



Have a higher skirt, and less level in the column?   Or have a shorter skirt, and more level in the column?


There is a surge time in the column, when I only need level to provide natural circulation?


Thank you,



#8 Art Montemayor

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Posted 01 May 2018 - 08:09 PM



I would like for you to get a copy of:

“Process Design for Reliable Operations” by Norman Lieberman
Gulf Publishing Co.


You can buy it from Amazon Books on the internet - or, if you are lucky, you may find it in your technical library at your school in Argentina.  Either way, you will find an excellent discussion of real-life problems and design answers on the application of a kettle reboiler to a distillation column and some problems encountered in the design.  The example Norm gives is 100% accurate of a similar problem I resolved in 1967 in a plant located in Alcala de Henares, Spain.


The basic design points involved in the configuration found in a kettle reboiler servicing a distillation column, side-by-side are:

  • The column sump acts as a reservoir to collect and feed the bottom liquids to the body of the kettle reboiler, located below it.
  • The sump also has to function as a potential reservoir to store the system liquid in the event of a shut down.  You have to have the ability to catch and store the liquids from all the trays, some reflux, and residual feed stock - in the reboiler and the sump.
  • The hydraulic head between the normal operating sump level and the reboiler liquid level should be such that all the column’s bottom liquid can easily flow at the rated maximum flow rate into the reboiler - over coming all the piping and system resistance.
  • You must allow for the flow resistance of the reboiler’s tube bundle as well as the height of the liquid that flows over the internal weir.

In order to describe the details, you should supply a detailed sketch of the system - much as Lieberman does.  It would take several pages of written explanation.

So, the answer to your question is: It depends on how your system is configured and what the hydraulics required turn out to be.

#9 Bobby Strain

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Posted 01 May 2018 - 08:45 PM


Art gives a good description of one piping arrangement between the column and the kettle reboiler. The other, and the one I prefer, is to draw liquid from a chimney tray and direct it to the kettle. Liquid from the kettle overflow is then discharged to the column sump. This puts the surge in the sump for reboiler outlet liquid. Elevations and pipes sizes must be set accordingly.



Edited by Bobby Strain, 01 May 2018 - 08:47 PM.

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