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Design Of Multi-Pass Distillation Trays

multi-pass distillation trays single-pass stripper amine

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#1 rajasingh

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Posted 23 March 2017 - 08:15 AM

Dear all

 

I am in the process of designing amine regeneration stripper column using tray internals.

 

Flow into column is 42763.21 kmol/hr, of which:

  • CO2 = 1811.7 kmol/hr
  • H2O = 38301.8 kmol/hr
  • MDEA = 2075.46 kmol/hr
  • PZ = 574.25 kmol/hr

Loading of rich amine is 0.45, loading of lean amine output is assumed 0 (MDEA is readily stripped)

Stripper column operates at 185 kpa, 120C in reboiler.

Reboiler produce 2052 kmol/hr H2O for stripping.

20 stages are present with extra 4 for top water wash.

 

I have determined that 4-pass trays are required due to extremely high input flow rate.

I wish to know:

 

What is design procedure for 4 pass trays?

I use Chemical Engineering Design by Coulson, Richardson and Sinnott however procedure is specific for single pass trays.

 

4 pass trays will have different dimensions, such as downcomer area is different and weir length/height is different.

 

What relations must I use to design such a tray system?

 

Many thanks



#2 Bobby Strain

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Posted 23 March 2017 - 08:54 AM

Tray design is best left to the tray vendor. So you only need to provide loads and properties.

 

Bobby



#3 PingPong

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Posted 23 March 2017 - 11:21 AM

Bobby is right, but probably did not realise that this is a student exercise.

 

I have determined that 4-pass trays are required
I wonder how you determined that.

There is no law that demands that above a certain diameter you have to use 4-pass trays.

 

4-pass trays are difficult to design, so I suggest you first try 2-pass trays.

Read also the good old Glitsch Ballast Tray Design Manual:

http://www.koch-glit...lletin-4900.pdf

 

But why do you want to use trays? I think all newly designed amine regenerators and amine absorbers are nowadays equipped with random (dumped) packing or structured packing. And most old existing ones have been revamped for packing to increase capacity and/or number of theoretical stages, and lower pressure drop resulting in lower regenerator bottoms pressure and consequently lower bottoms temperature.



#4 Bobby Strain

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Posted 23 March 2017 - 01:30 PM

PingPong, you are right. And, I still fail to understand why engineering students must waste time with exercises such as this. Instead, they should be developing skills for the fundamentals. Or, having a beer at the pub.

 

Bobby



#5 Saml

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Posted 23 March 2017 - 03:39 PM

What is the amine unit for? For large units I've seen a double concentration absorber design. But it is difficult to know without details.

Why do you regenerate all the circulating amine?



#6 rajasingh

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Posted 24 March 2017 - 03:43 AM

PingPong,

 

Trays chosen due to low corrosivity of amine solution, large column diameter required, large flowrate of amine provided, low foaming of amine solution and recommendation of Kohl and Nielsen (Gas Purification). For conclusion of report, suggestion will be made to implement packing however proprietary vendor software must be used for accurate design. 

 

4 pass trays determined due to calculation of pressure drop on tray and downcomer backup. Single pass produced back up in excess 1 meter however with 4 pass weir length is sufficient to reduce backup.

 

Saml,

 

Amine unit is simple gas sweetening however removal of CO2 is only concern as H2S not present in system. Absorber design was performed by fellow student who determined 42763 kmol/hr CO2-containing rich amine is produced. Rich amine must pass through stripper to remove CO2 and produce lean amine, which is recycled back to absorber unit for re-use.



#7 PingPong

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Posted 24 March 2017 - 08:00 AM

What Kohl&Nielsen (or any other book) states is not necessarily true.

 

Almost everybody uses packing for amine regenerator and absorber.

I strongly suggest you do the same.


Edited by PingPong, 24 March 2017 - 08:04 AM.


#8 breizh

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Posted 25 March 2017 - 02:26 AM

hi ,

Take a look at the resource attached to support your work.

Good luck

Breizh



#9 Napo

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Posted 06 April 2017 - 09:07 AM

Rajasingh,

 

The following article: "Hydrodynamic calculation of distillation and absorption columns with multipass perforated trays" by Bakhshiyan D., et. al., in the Chemistry and Technology of Fuels and Oils, December 1978, maybe can help you. The link is:

https://link.springer.com/article/10.1007/BF00724041

 

I employed this information when I was student (class exercises), but now in the web there are more information (you need to pay!!).

 

Also I send you related information.

 

Napo.

 

Attached Files



#10 Art Montemayor

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Posted 06 April 2017 - 01:46 PM

rajasingh:

 

The subject of amine CO2 strippers is one that has been dealt with on our forums in many, many threads ad nauseam.  This application is a basic and simple one that is older than I am!  Mr. R. R. Bottoms patented it in 1930 and described it in detail.  The Girdler Corporation built many plants based on it and I personally designed, built, and operated several units in the field with success.  As I have stated many times before: the amine CO2 stripper is a simple one requiring no more than 10 simple trays.  The calculated theoretical stages for stripping amine are approximately 3-4.  So why are you complicating your life and such a simple unit operation?

 

Read and ponder carefully what PingPong and Bobby Strain are telling you:

  • Kohl & Nielsen, to my knowledge, never designed or built amine strippers.  I have, and thoroughly agree with PingPong on his comment;
  • Bobby is right on; you can best spend your time drinking some good draft beer rather than trying to apply multiple pass trays on such a simple operation.

How did you - or anyone else - arrive at 20 actual trays needed to strip out the CO2?  You yourself admit that it is “readily” stripped out!

 

Structured packing is easily applied and why do you expect the MDEA to “foam”?  What evidence or experience are you basing yourself on?  Kohl & Nielsen mention that some amine units have been reported as foaming and suggest some causes - but I don’t believe they recommend that you design for this to happen to you.  Read their report on page 10:

 

“Packed columns are gaining favor for a wide range of applications because of the development of packings that offer superior performance, as well as the emergence of more reliable design techniques.  The most commonly used packing elements are packed randomly in the column.  Non-random ordered (or structured) packings were originally developed for small scale distillation columns to handle difficult separations.  Their use has recently expanded, however, and ordered packings are now offered by several companies for large scale commercial applications.  The current availability of performance data and rational design procedures makes the use of ordered packing worth considering for cases requiring high mass transfer efficiency and low pressure drop.”

 

Also read what they state in “Tray Versus Packed Columns”, page 111:

 

“The choice between trays and packing is somewhat arbitrary because either can usually be designed to do an adequate job, and the overall economics are seldom decisively in favor of one or the other.  At this time, sieve tray columns are probably the most popular for both absorbers and strippers in conventional, large commercial amine plants; while packed columns are often used for revamps to increase capacity or efficiency and for special applications.”

 

Kohl & Nielsen are NOT EXPERTS in designing or building amine plants.  They have never claimed to be that.  They are expert, knowledgeable reporters who have documented a lot of information and data on the subject and put it into an organized book form.

 

In my opinion, you are trying to use an Alfa Romeo as city taxi when you should be applying the K.I.S.S. rule of engineering.






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