3 replies to this topic

[jatinder]

Dear all,

I'm 4th year chemical process Engineering student from The Netherlands. Currently, I am working on gas sweetening and my main point of focus is only the contactor stage. I would like to know the procedure of calculating number of trays and height of amine contactor column.
I have succesfully calcuated lean amine loading, rich amine loading, amine circulation rates, diameter of the column.

Given:

gas inlet pressure = 6200kPa
gas inlet temperature = 32 degree C
H2S feed in gas stream = 0.50%
CO2 feed in gas stream = 2%
inlet gas stream = 1410000 cubic meter/da

The calculated data is as follow:

residual concentraion of CO2 in lean amine = 0.32 Kmol CO2/cubic meter
residual concentraion of H2S in lean amine = 0.0059 Kmol H2S/cubic meter
lean amine CO2 loading = 0.127 kmol CO2/kmol MEA
lean amine H2S loading = 0.00234 kmol H2S/kmol MEA
H2S partial pressure = 236 mm Hg
CO2 partial pressure = 945 mm Hg
rich amine CO2 loading = 0.3955 kmol CO2/kmol MEA
rich amine H2S loading = 0.0672 kmol H2S/kmol MEA
diameter of amine contactor = 2.35 meter
amine circulation rate for CO2 = 3.09 Kmol MEA/min
amine circulation rate for H2S = 3.19 Kmol MEA/min

So, now I would like to know a proper formula to calcuate number of trays and height of column.

Thank you,

Kind regards,

Jatinder

[Art Montemayor]

Jatinder:

As a 4^th year student, you obviously have taken Unit Operations and Plant Design courses during your university training. You show some basic knowledge of the MEA acid gas removal process. Have you read and studied the classic book on this process – “Gas Purification” by Arthur Kohl and Fred Riesenfeld? This text specifically addresses each of the design steps in this process. My latest copy is the 3^rd Edition. I would ask you for the following basic data:

• Why have you selected a plate type of MEA absorber?
• What strength of MEA solution are you proposing to use?
• What method have you used to determine the absorber diameter at 2.35 meters?
• Why have you selected MEA as the amine solution for this mutual absorption of CO2 and H2S?
• What is the base composition of your process gas entering the absorber? Is it a natural gas?

The absorption of acid gases by an amine solution is a complex process. This is specifically addressed by Kohl and Riesenfeld:

“The problem of estimating the column-height requirement is considerably more difficult than determining that of the diameter as the mathematical formulation of absorption with chemical reaction is exceedingly complex. It has therefore been found necessary to correlate absorption coefficients and tray-efficiency data on an empirical basis for application to practical problems in plant design.”

The above should alert you to the fact that you are not dealing with a “cookbook”, “special” equation that you can simply plug in your known data and immediately resolve the height and number of trays required. I am afraid it is far more complex and rigorous than that. I know, I did it for many years.

On page 69 of the above referenced text you will find the section “Absorption of CO2 in Plate Columns”. There, the authors present a methodology for manually calculating the number of absorption stages for such a separation. Additionally, in the subsequent section, the authors present “Absorption of H2S and H2S – CO2 Mixtures”.

This complex subject is also treated and discussed by John Campbell in his books on gas processing as well as by Bryan Research and Engineering at their website, http://www.bre.com/ . I highly recommend you visit and download all the material BRE offers for free on the absorption of H2S – CO2 Mixtures using MDEA (not MEA). I have made the above recommendations on countless threads in our Forums in the past. You will discover that ProMax, BRE's acid gas design program is widely used for the design of this application in industry.

Await your response.

[jatinder]

Dear Admin,

Thanks for your consideration. My univeristy, do not provide course on subjects like gas sweetening or other gas related treatment. This one is my thesis project work. As per asked by you, I already read books like Gas Purification and Campbell. In gas purification things are not that clear for the height and number of trays and in Cample height is not mentioned at all. The answers of other question that you have asked me are as follow:

1 - I have selected plate type for MEA not on base of any strong reasons. I just selected MEA and plate type only to understand the process.
2 - I am using 20 wt% of MEA
3 - To determine absorber diameter I have used few equation. Please have a look in the ttached excel file. Iteration method is used to calculate the value of drag coefficient. (If required I can send you my real excel sheet)
4 - it is a natural gas entering the absorber. I am provided with the flow rate that is 1410000 cubic meter per day and it is only mentioned the amount of H2s and CO2 present. H2S = 0.5%, CO2 is 2%, therefore 2.5% total acid gas. Thats all, no more composition is provided.

Thank you

Attached Files

•  Book1.xlsx   63.25KB   275 downloads

[Art Montemayor]

Jatinder:

I am not familiar with the equations you employ in calculating the diameter of a tray tower. I recognize that the equations relate to the drag coefficient and, as such, are also probably related to what I have used in the past: the Brown-Souders Equation. The Brown-Souders equation has been around a very long time (it is as old as me) and has traditionally been used to determine the diameter of tray towers. It is also employed in designing the diameter of a vapor-liquid separator. You do not furnish any reference as to where you obtained the equations and why they are applicable to the application. Therefore, I can’t comment on your calculated diameter’s accuracy.

If you have read Kohl and Reisenfeld (or Kohl and Nielsen, the latest edition), then you have no basis for saying “things are not that clear for the height and number of trays”. This textbook – and its latest version – are well recognized as the authoritative text on this subject and the book clearly explains the manual procedure for calculating the height of an MEA absorber – in detailed, step-by-step instructions. Therefore, I can’t make any further comment to try to help you on that point.

By selecting a 20% wt. MEA solution, you have picked a solution strength that has caused a lot of grief and problems in actual practice. You will inherit a lot of serious corrosion problems in the MEA side. That is why people who have years of experience with MEA will employ a 12 to 15% wt. solution instead. Of course, this means a higher reboiler heat duty, but that is the price to pay for selecting MEA. Practical field experience has proven that point many years ago.

If you have a need to also mutually remove H2S, then you have selected the wrong amine solution. MDEA is the best acceptable amine for that type of sour + acid gas removal. This has also been verified with empirical field experience.

You haven’t asked any direct or specific questions nor have you asked for additional help, so I have not further comments to add.

I hope, nevertheless, that my comments and advice help you out.