8 replies to this topic

[carib1]

I've never designed a packed column before using procedures in textbook, and since I'm doing chemical absorption process I'm very confused.

Basically I'm asking advice on how to use VLE data correctly.

The inlet gas has 19% carbon dioxide, it isn't dilute, but the approach in coulson and richardson using henry's law, can I use it?

Does anyone know where I can find the solubility data for carbon dioxide in 30% MEA?

I'm designing a packed bed column. I have managed to find the column diameter and height utilising PRO/II. My supervisor suggested following the procedure in Coulson and Richardson and comparing it with the PRO/II design. The example in coulson and richardson uses partial pressure and percent w/w solution, but I cannot find this data for the system I am designing or perhaps I am no interpreting the data correctly.

I have found data from jout 1995 that has graphical data for partial pressure against loading at various temperatures for 30% MEA with carbon dioxide, but I don't know how to utilise it in this design. Is the loading the same as percent w/w solution in coulson and richardson? I would like to find the partial pressure that 1% w/w solution of carbon dioxide has in the vapour.

Also the example in Coulson and Richardson is at 1 bar and my design is at 19 bar.

If I were to find how much 1% mole fraction generates as partial pressure then divide that by 19 bar and then find the mole fraction in the liquid. Could I then generate the Henry's law constant or equilibrium line slope.

[carib1]

So jou et al gives a graph of partial pressure in kilopascals against mole of carbon dioxide per mole of amine. If my outlet concentration of carbon dioxide is 1% w/w solution, how does that correlate to mole of carbon dioxide per mole of amine, ie. how do I use it to determine the partial pressure?

[breizh]

http://www.google.co...iw=1106&bih=592

carib 1
Let you try Google !
Breizh

[carib1]

Thank you Breizh

[kkala]

1. A source of needed equilibrium data (diagrams) can be found in "Gas Purification" by Arthur Khol and Fred Riesenfeld (2nd edition, Gulf, 1974), Chapter 2 (Ethanolamines for H2S and CO2 removal, 62 pages), also giving a good theoretical background on the design. Following Chapter 3 deals with "Mechanical Design and Operation of Ethanolamine Plants", if required later.

Searching Web can be also fruitful, e.g. <http://www.ieaghg.or...stract00040.pdf>

It is noted  30% MEA is not recommended, due to risk for corrosion; see <http://www.cheresour...fired-reboiler/> , post no 8.

2.  Consider 100 kg of spent solution, containing 1 kg CO2 and 99 kg solution of 30% MEA.

It contains 1000/44 gmol CO2 and 99x30% kg MEA, that is 99x0.30x1000/61.09 gmol MEA
So molar ratio CO2/MEA = (1000/44)/(99x0.30x1000/61.09)=61.09/(44x99x0.30)=0.04675;

mentioned book indicates  CO2 partial pressure =0.0017 atm a at 75 oC (from diagram 2-18, not precisely seen)

3. For small molar ratios of CO2/MEA, one can use Henry constant H, plus equilibrium constant K of RNHCOO^--+RNH3⁺<=>2RNH2+CO2  (R is HOCH2CH2).

E.g. for molar ratio=0.15, 2.5 gmol/l MEA, 37 oC :  K=5.5E-5 gmol/l, (RNHCOO^--)=(RNH3⁺) = 0.375 gmol/l, (RNH2) = 2.5-2*0.375 = 1.75 gmol/l, (CO2) in solution =5.5E-5x(0.375x0.375)/(1.75)²=2.53E-6 gmol/l.

MEA solution density = 1030 g/l, H2O contained=1030-2.5x61.08=877 g, gmoles contained=2.5+877/18=51. Mol fraction CO2 in solution=2.53E-6/51=4.96E-8.

Henry constant for CO2=2350 atma, equilibrium CO2 pressure in gas 2350x4.96E-8 atma = 1.17E-4 atma.

Reference is P.V. Danckwerts "Gas-Liquid Reactions", McGraw-Hill (1970), reported in the local university book, from where this procedure has been copied.  It may be useful to somebody in the future.

Comments on the above welcomed.

Edited by kkala, 23 February 2013 - 02:03 PM.

[Art Montemayor]

carib1:

 

If you are trying to design a CO₂ capture process using MEA, then Coulson and Richardson is not the best text to find information on this process.

 

"Gas Purification” by Kohl and Reisenfeld (now Kohl & Neilsen) as well as the GPSA are the two best recognized gas purification texts in the English language.  You will find all the directions and examples in these two texts for removing acid gas (CO₂) with MEA.

 

The solubility of CO₂ in MEA solution is usually not important unless your absorber is under high pressure – a piece of important information you fail to mention.

Vapor-liquid equilibria is not of much importance for process design work when dealing with the amines because empirical gas loadings are usually applied to the Rich and the Lean amine solution and these set the absorption duty as well as the stripping duty in the CO₂ stripper.  HySys is not going to tell you anything about the recommended MEA strength of solution used and it seems your supervisor doesn’t know much about the process either – otherwise, you would have been warned about the grave and serious corrosion problems facing you with the 30% wt. MEA solution.  I speak with many years of hands-on experience and the literature bears me out: 12 to 15% MEA is the recommended strength that can be employed with controllable corrosion.  And to control corrosion, you require a Reclaimer still and activated carbon adsorbers.
 

Amine gas loadings are set according to empirical, field experience (even Kohl & Neilsen don't admit this) and are set as:

Lean MEA gas loading = 0.1 to 0.15 mols CO₂/ mol MEA (or lower, if you can get it)

Rich MEA gas loading = 0.35 mols CO₂/ mol MEA

 

I have written extensive responses on many threads in our Forums dealing with acid gas removal using amine solutions.  One example is found at:  http://www.cheresour...ng-calculation/

 

Use our SEARCH engine with key words like CO₂ and MEA and you will find many of these responses.

[kkala]

It may not be important now, however absorber pressure = 19 bar (abs, gauge ?), post no 1 by carib.

[mohammad uzair]

sir i want some help, kindly tell me how can i start topic here? as i am brand new member

[Art Montemayor]

Mohammed:

 

I've already given you the way to find your answer - use our SEARCH ENGINE.   If you cannot find sufficient information there to help you, then START a new thread over your specific topic.   Use the "start a new tab: button on the right-hand side, top of the page to follow instruction on how to do it.   You have to put your effort into it.