3 replies to this topic

[kougon]

Hi, 

 

I'm designing an adsorbent system for removing Carbon Dioxide from a Carbon Monoxide gas stream. I need to ensure that the adsorbent system will not add any impurities to the gas stream.I tried looking up literature but could not find a design guideline for an NaOH based unit. The system specifications are as follows: 

CO Gas Flow Rate: 200SLPM

CO Gas Pressure: 1-36 Bar (Variable)

Temperature of Gas Stream: 300Kelvin

 

CO2 Concentration (Inlet Stream):   200PPM

CO2 Concentration (Outlet Stream): <1PPM    (After passing through adsorbent system)

Breakthrough Concentration (CO2) : 1PPM

 

Designed Changout time: 30 Days

Adsorbent System is operated 24x7

 

-------------------------------------------------------DESIGN-------------------------------------------------------

Contaminant Information:

Max CO2 concentration is 200PPM,

 

Therefore, 1L of gas has 1L*200*10^-6 = 0.2ml of CO2.

From Ideal Gas Law, 22.4L ---> 44g of CO2  ====> 1ml of CO2 --> 0.4*10^-3g 

 

Total CO2 contaminant generated by flowing 200SLPM of gas in 1 day 

= 200 * (0.4*10^-3) * (60 *24) g/day 

= 115 g/day

 

Total Contaminant to be removed in a Month = 115g/day * 30day = 3,450g = 3.45Kg

 

 

Adsorbent Information:

The Adsorbent being used are pure dry NaOH Pellets. 

 

https://www.merckmil...MDA_CHEM-106462

 

Assume NaOH pellets can adsorb 10% or (0.1) by weight of Carbon Dioxide. (I'm making a wild assumption without any basis. I need help on this.)

 

Kg of NaOH needed per month = (Amount of Contaminant to be removed) / (Amount of CO2 adsorbed by NaOH)

= 3.45Kg/ 0.1

= 34.5Kg

 

Safety Factor of 1.75 

 

==> ~60Kg of NaOH/month is needed. 

 

 

Vessel Sizing:

Adsorbent used in industries have a superficial velocity between 5 to 50cm/s (Using lower value. I'm not sure what is perfect for NaOH pellets)

 

Q = Gas Flow Rate = 200SLPM

Qa= Actual Gas Flow rate = Q/ sqrt(System Pressure)            //Pressure of the system varies between 1-36Bar 

Qa = 200LPM to 33LPM. 

 

Use the higher flow. 

 

Superficial Velocity = Actual Flow/Cross-section Area

 

==> Diameter = Sqrt(4*Qa/ pi*velocity) = 29cm 

 

For simplicity, Choose Vessel Diameter = 30cm 

 

Density = Mass/Volume     [Density of Sodium Hydroxde Pellets = 1g/ml  ; Density of NaOH  = 2.13g/ml]

==> Vessel Height = 4*M/(pi*D^2 * density)

      Vessel Height = 85cm

 

Contact time with adsorbent = Volume of Vessel/Superficial Flow = 18s (Is this sufficient?)

 

This unit is followed by a Mol-Sieve 13x unit which has a diameter of 8cm and height of 100cm. This is to remove any trace water that is picked up by the gas streams. 

 

-------------------------------EXPERIMENTS TILL NOW-------------------------------------------

I tried building smaller prototypes to test if this system would work. The first time around, I built a unit with a Diameter of 2.5cm and Height of 100cm. It was loaded with ~0.75Kg of NaOH. This was expected to work for an hour or so. But the system did not work. CO2 levels in the system would saturate at ~1000PPM and stay there. i.e It was incapable of bringing CO2 to below 1PPM but was good enough to adsorb excess CO2 being generated by the system. This system used a gas pressure of 20Bar. 

 

I built a larger unit, with a diameter of 9cm and a height of 100cm. It held about 5Kg of NaOH pellets. But this could only bring down CO2  levels to ~250PPM. It would adsorb the additional CO2 that was being added but could not bring the levels to required. The system pressure for this was again 20Bar. 

----------------------------------------------------------------------------------------------------------------

My doubts for the new system are: 

 

I think the very low CO2 removal I'm getting is because of using dry pellets as is. Am I using NaOH pellets in an industrially acceptable way? I've read often that wetted NaOH pellets are excellent CO2 adsorbents. But how do I wet them? Do I pour a few drops of water. When I tried pouring water, I always found that the wetted NaOH pellets would melt, reform and impede gas flow substantially. Is there a thumb rule I can use? 

 

Are my assumptions for contact time proper? Is there an "acceptable" contact time for NaOH pellets to remove CO2? 

 

Is the vessel sizing I've used something that will work for a month? 

 

Looking forward to tap into the forum's experience to design this system. 

 

 

Thanks

 

 

[Art Montemayor]

Kougon:

 

Where have you “read often that wetted NaOH pellets are excellent CO₂ adsorbents”?  The action between NaOH and CO₂ is not aDsorption.  It is a chemical reaction and mistakenly called aBsorption.  I know because I’ve done it for years in the past.  Your knowledge of the two processes in question may be what is holding you back in this application.

You can quickly gather information in the internet by googling: "sodium hydroxide absorption of CO₂".  You should get:

 

https://www.google.c...sorption of co2

 

...with about 291,000 results in 0.32 seconds.  There you will find information that tells you that what happens when you combine CO₂ with a solution of NaOH, you get the following:

 

2NaOH(aq) + CO₂(g) → Na₂CO₃(aq) + H₂O(l)

 

You will also find information on how a sodium hydroxide scrubber is designed and used to remove CO₂ from a gas mixture (air, for example).

[kougon]

Dear Art, 

 

Thank you very much for your reply. I went through the links. They were similar to results I'd gotten earlier. I thought that it was possible to use solid pellets. Looks like the liquid is way better that anything I can ever get with the pellets. 

In the new searches, I found a thesis with detailed design of a Bubble packed CO2 scrubber. It talk of using MEA but i'll stick with a 1M NaOH and use design data from the thesis to build a scrubber. 

 

http://ir.canterbury...is_fulltext.pdf

 

Will post results of the system. 

 

 

Edit: rephrased text for clarity. 

Edited by kougon, 13 September 2015 - 12:17 PM.

[Art Montemayor]

Kougon:

 

What and how to design a CO₂ removal process depends a lot on the concentrations of the CO2 in the feed gas and in the product.  It also depends on other factors, such as flow rates and pressures.  If you still want to use caustic pellets in a fixed bed (and thereby avoid a pump-around solution and a packed tower), then read the attached document (that was included in the Google Search results).

 

 Decarbite for CO2 Removal.docx   130.14KB   10 downloads