10 replies to this topic

[Steve90]

Hi everybody ;

 

Is there any analytical method / equations that can be used to calculate The Sweetening gas unit regenerator number of trays !

 

Thanks

[Zauberberg]

For design procedures you can refer to Kohl & Nielsen "Gas purification", available online at: https://books.google...ication&f=false

[Steve90]

Mr Dejan thank you very much you were always helpful but i looked in the book and since i'm not too familiar with number of trays calculation

(we didn't do this in university) i want if possible a simplified procedure to calculate number of trays in the amine regenerator ! thanks

[Zauberberg]

Number of trays/stages largely depends on the contaminants absorbed by amine. It doesn't work the same way for H2S- or CO2-governing cases. I'm afraid that only unit suppliers can provide shortcut and detailed methods.

[Steve90]

I don't have H2S i only have CO2 in my feed stream

[RockDock]

Most amine regenerators have 20-22 trays. I design it by modeling the column in ProMax and making sure I have an appropriate temperature profile, as well as CO2 stripping in the column, not the reboiler.

[Art Montemayor]

All simulated CO₂ removal systems I've seen have been totally WRONG in predicting excessive number of trays required to regenerate amine solutions - especially MEA solutions.  I know this to be factually true because I've had the advantage of designing, building, installing, and operating these units out in the field.  I have reported these factual, field experiences many times in many prior threads on the same subject here in our Forums in the past and these can be found in our SEARCH engine.

 

I normally have used 10 to 15 theoretical separation stages for an MEA stripper working with 12-15% MEA and a rich MEA loading of approximately 0.3 mol CO₂/mol MEA.  In fact, I have used as little as six actual trays - without any bubble caps, sieve openings, valves, or anything else and successfully run these units for years producing 0.1mol loadings in the lean solution.  I have proven - in the field, with actual operating units - that you don't need any more than this to effectively regenerate amine solutions.  I can assure you that designers and fabricators of amine units in the 1950's and '60's knew this and used it in their design.

 

The last article I read on this (I can't remember when) came up with a theoretical tray requirement of 2-3 stages for an amine stripper.  I firmly believe that is true from a theoretical view because my field experience shows this.  Another issue I have with some designs of amine strippers is the use of so-called "reflux".  There is no such thing in a stripper design that releases a non-condensable gas.  NO REFLUX IS REQUIRED, AS SUCH, IN AN MEA STRIPPER.   I also have proven this in the field with units that have produced for years - longer than most of the operators that ran them.  To put condensate back into the top of an MEA stripper is to increase the amount of reboiler duty and that, in my opinion, is nothing short of being stupid if it isn't required for proper stripping action - which it isn't.  Yet, simulation programs insist on using the so-called "reflux".  And they do this without any mathematical or process calculation method.  What they are doing is nothing more than "scrubbing" the exit sour gas and this is not needed in a properly designed stripper tower.

 

What Rockdock reports is also true.  Fluor, Bechtel, and almost all the big world-wide processors have always insisted on using 20-25 trays in absorbers and an equal amount in strippers.  The reason(s) I found out in the field for their exaggerated size of towers (by taking their field engineers out and buying them a couple of drinks) is because of license requirements that insist on conservative design in order to ensure that no negative results will occur in the field.  In other words, they are protecting their warranty on operational results and playing it safe.  Also, a lot of these projects were cost-plus contracts with process guarantees.  The corporation that invented and patented the amine system - the Girdler Corporation - built a lot of these units, especially during World War II when they were needed as skid-mounted units in large air craft carriers in order to have a continuous supply of CO₂ available for fire fighting.  Girdler never used more than 12 to 14 trays in an MEA stripper to my knowledge - and they should have known, since they invented the process. 

 

My advice to you is to follow Zauberberg's experienced recommendations.  He is guiding you to the best compiled information on amine unit design that is currently available.  Even Kohl & Neilsen admit to the facts that I have stated - although they don't have the field and hands-on experience that I accumulated.  Read the book carefully and you will find that it admits to the difficulty in simulating and predicting the correct, accurate design of an amine unit.  See the attached condensed material from their book.

 

Engineering is not all about just relying on a given math equation or a computer simulation program to give you the correct answer.  Thank God for that!  Otherwise there would be no need for any university training or preparation.  The vast amount of professional engineering is about using practical, experienced knowledge based on common sense and past results.  That's why the experienced engineers found in our Forums are so valuable and useful for learning and self improvement.  In my younger years we had engineering mentors that guided us through the learning process.  Today, these are no longer available to you and that is why our Forums exist as one way to obtain the necessary help and counsel.

 Acid Gas Removal Design Criteria.docx   111.38KB   232 downloads

[gegio1960]

Dear Art,

Thank you for sharing, once more, your valuable and "independent" experience.

Could you better explain the item relevant to the type of trays "without any bubble caps, sieve openings, valves, or anything else" ?

My experience is mostly in H2S removal where I've been used to specify sieve trays for this service (and never received negative comments from final users).

Kind regards,

gegio

[Art Montemayor]

Gegio:

 

What I briefly described as some internals in an MEA stripper that I have used is nothing more than 3/16” thick plates without any holes or other liquid-vapor contact mechanisms other than the flat horizontal surface offered.  These plates were cut just as bubble cap plates would be - but without perforations - with a “cut” of approximately 15-20% of the diameter, giving the ascending vapor enough area to rise with a minimum of pressure drop.  These simple devices were used for many years and for many projects, by such companies as IPEC in Dallas, TX, who specialized in acid and sour gas removal.  IPEC and Girdler units never used any so-called “reflux” to the amine stripper.

 

Look at the attached workbook I put together some years back when I had time to consolidate my thoughts and experiences in this area.  I never augmented this workbook with the rest of acid and sour gas removal operations that I had experiences in because I ran out of time and other priorities started to come up.  I hope this explains what I’ve preached.

 

Above everything I have written and said, do not take my comments as criticism of using sophisticated and improved unit operations and tower internals as employed today.  There are invaluable advantages to using such items as valve trays, structured packings, improved vapor and liquid distributors, etc., etc.  My main purpose in pointing to simpler techniques used in the past is to emphasize - especially to students and young engineers - that the basic unit operation is a very simple and direct one that can be confidently relied upon when using design methods and steps.  It is not nuclear science.  The fact that CO₂ can be removed so easily and reliably is a fact and should give all gas engineers confidence in using even better techniques and separation internals than were available in the past.  You don’t need a computer or an expensive and elaborate simulation program to successfully - and profitably - resolve the problem.  Stripping out H₂S is, I believe, easier than stripping out CO₂.  Using Sieve or Valve trays when justifiable should, of course, be the action to take as a reasonable step.  And, based on my past experience, you should certainly never obtain bad results when using them.  On the contrary, you should feel super-safe in achieving the desired separation results.

 

Another point I keep on harping upon every time this topic comes up is that there are far more important things to bear in mind and take into design consideration than those involving whether or not to use 15 or 20 trays in an amine stripper or absorber.  Things such as amine carry-over from the absorber, proper control of amine solution strength, proper control of raw gas treatment and separation from liquids prior to introduction into the absorber, proper and timely analysis and controls on amine solution gas loads - both rich and lean solutions - and even such items as efficient steam condensate removal from the reboiler tube bundle are items that can be much more important to consider and design for than the optimum number of trays in the stripper or absorber.  One will always install the correct number of stages in either tower with an additional “contingent” amount to ensure proper performance for a determined period of operation.

 

I don’t want to make my response lengthy, but feel a lot of related details are needed for explaining to students and young engineers exactly what ALL is involved in designing and ultimately operating an amine unit.  It’s isn’t just calculating the exact number of trays required; it involves a variety of other, important and related process variables.  I hope this helps out.

 

 Amine Acid & Sour Gas Plant Experiences.xlsx   96.33KB   211 downloads

[Zauberberg]

Art, great post - as usual!

[gegio1960]

Thank you Art.

I could add that, in my knowledge of the amine systems, sieve trays are better than valve trays because of the absence of moving and little parts, more subject to troubles originated by fouling and corrosion.... a little point :-)

Edited by gegio1960, 02 April 2015 - 03:00 AM.