6 replies to this topic

[Human6]

Hello Folks, 

 

I am designing a CO2 absorption tower. The primary objective is to determine the height and diameter of the column. 

 

The feed gas composition is specified.

The exit gas composition is specified.

The solvent in is assumed to contain no co2.

The Temperature and pressure of the inlet feed gas and column is specified. 

 

Design selection:

Counter-current, differential operation

15.2% MEA solution 

 

To calculate the solvent flow, Kohl, 1997 suggests using 0.5 mol CO₂/ mol amine rich gas loading, However  I used values suggested by P. Chattopadhyay, 2007 based on empirical data given as 0.35 mol CO₂/ mol amine rich gas loading .  I then used modified Sherwoods correlation to find the diameter.

 

 

The problem I have is that the calculated diameter is too large (approx 7m). I have use various packings, increased the strength of the solution, used a higher pressure loading % but still the lowest value i could possibly get was ~4.5m. These diameters fall within the allowable loading % limit of 50-85%. 

 

The problem I figured is that the gas flow rate is too high. So the question is, is having a large diameter good justification for having 2 towers with split flow? I found this to decrease the diameter to within ~2-3m which is reasonable. I know that large diameters cause channelling which is why they are best avoided. Or is having a tower of that diameter ok?. 

 

This is not an industrial design, so please understand that I am not trying to complicate the design but trying to simplify it as much as possible in order to move forward.

 

If further data is required to answer those questions  please let me know.

 

Regards 

Edited by Human6, 02 April 2013 - 08:30 AM.

[shan]

Higher gas pressure to reduce gas velocity in the tower.

[Art Montemayor]

It is obvious you are doing something(s) wrong - both in your basic data and in your calculations.  But, since we have not been furnished any basic data or calculations from you, we have no way of even starting to analyze what you have done - much less finding out what is wrong.

 

What Shan has suggested is not what I see as a positive answer.  You don't simply increase the pressure in a CO₂ absorber to solve the problem.  You have to resolve the problem with the basic data you were given - and presumably this is a low pressure (< 1psig), CO₂ capture problem.

 

Your description does not make sense: "To calculate the solvent flow, Kohl, 1997 suggests using 0.5 (co2/amine) when designing, However I used empirical data from P. Chattopadhyay, 2007 given as 0.35 (co2/amine)."   What do you mean by "0.5 (co2/amine)" and "0.35 (co2/amine)"???  Do you mean solution gas loadings in mol CO₂/mol MEA???   If so, then STATE IT.  This makes for sloppy and misunderstood communications.

[Pilesar]

If higher MEA concentration would help you, make it a bit stronger. Your prof would probably not look twice at 20% MEA. Your calc'd diameter is too large. If you sized the column using your inlet gas at standard conditions...use the actual flowing conditions to get a smaller column diameter. 0.35 mol CO2/mol MEA sounds reasonable for your rich amine loading. Your lean amine will have non-zero CO2. I would assume lean amine loading of about 0.1 mol CO2/mol MEA. This will make a difference when you calculate the total solvent flow needed. Your syngas effluent will have CO2 ... probably at least 100 ppm CO2 concentration. Your syngas effluent will also be saturated with water.

 

As for tower height, use whatever calculation procedure you like as long as you end up with the equivalent of about 20-25 trays for your absorber.

Edited by Pilesar, 01 April 2013 - 01:25 PM.

[Human6]

I made a few changes, This case is unrealistic but I wish to show how the column diameter changes with these parameters; 

 

Increased the solution to 30wt% (which is unrealistic), assumed 0 mol CO₂/ mol MEA loading in the solvent inlet, and a loading of 0.5 mol CO₂/ mol MEA in rich solvent.

The biggest change I made is by using an average gas flow rate to calculate the diameter after finding G'.( I took the average of the dirty gas flow & clean gas flow - to give an average mass flow of gas in the tower.) The resultant diameter is 2.47 m .

 

Is taking the average gas flow viable? 

Edited by Human6, 02 April 2013 - 06:20 AM.

[Pilesar]

I like your spreadsheet layout. You give a "Fractional Recovery of CO2" which probably should be recalculated to reflect the CO2 fraction recovered as CO2 product. I did not check your calcs in much detail. My CO2 removal experience (primarily with MEA or Ryan-Holmes in trayed columns) is with higher pressure applications so it is difficult to point to any specific flaw in your column sizing. For MEA, you can boost your concentration to 30% since you have no H2S -- see your Kohl reference page 50. How is your feed made? I am surprised the feed has no water in it. When I consider possible alternatives to using MEA, the low pressure still greatly affects the design. I think in any case you will want to use one absorber and not parallel columns for cost efficiency. Columns do come large and yours may be one of them so check your calcs and justify your sizing then defend it to your prof if necessary. You should plan to use a sparger pipe for gas distribution. You may want to look harder at your diameter as it approaches 4.5 meters -- there are transportation limits (roads and bridges) to consider unless your plant has a useful water dock.

[Art Montemayor]

 Human:

 

Thank you for the submitted calculations.  With this information now in hand, we can ask more specific and detailed questions.  Some comments are:

• With a removal of 4 tons CO₂/hr, you are dealing with a relatively large stream – not a humongous one, but still a large stream.  I’ve operated units producing 3 tons/hr – but with the MEA absorber (contactor) at < 1psig.  In other words, this was essentially CO₂ capture.  The absorber that I remember was approx. 3 meters diameter (9 feet).  Since my absorber was handling 15% CO₂ inlet at 1 psig, using a combination of soda ash and MEA (1961), it was less efficient than your proposal.  Therefore, I know you are doing something wrong to come up with your results.
• If you have an acid gas stream at 12 barg, you should be looking at using trays in your absorber rather than packing – due to the larger, expected diameter.  If that were the case, then the diameter should be calculated using the Brown-Souders relationship (as was always done with trayed columns).  That should give you a realistic estimate on what size your absorber should be.
• Pilesar is correct in stating that you are probably looking at 20 trays for a conventional MEA absorber.  Kohl and Neilsen even mention that.  What they don’t mention (they lack field operating experience) is that 20 trays is a very conservative number of trays – for the usual performance liability put on contractors.  I have seen absorbers working with as little as 15 trays.
• The usual (and practical) CO₂ gas loadings for a 20% MEA solution are: 0.1 for the Lean and 0.35 for the Rich solution.  You can take that to the bank.  I have designed and built units under those data and they performed exactly as calculated.
• You never mentioned in your original post that you have 25% of Carbon Monoxide (CO) in your feed stream.  Your feed stream, as revealed now, shows that it is nothing more than a “Syn Gas” stream, probably the product of a steam reformer and prior to Shift Conversion.  What you propose is (and never was) not done in the field.  CO will oxidize and degrade your MEA since the monoxide is reactive and forms carboxylic acids.  Consequently, your process, in my opinion, will not work as shown.  The normal way to remove the acid gas is to convert the CO to hydrogen and CO₂ in a Shift Converter prior to introducing it in an acid gas removal system.
• Please show your reference when using equations in your calculations.  Your calculations cannot be “checked” without proper reference as to source and reasoning for applying the equation(s).
• You do not have to increase the MEA solution beyond 20% to have a viable process.  In fact, I have always designed for much lower MEA solution concentrations – 12-15% - in order to alleviate the MEA degradation and corrosion problems.