Hi Iason,

As you say, literature is quite flexibel. In my work I used 50 g/L of KOH solution and it worked fine. If you have access to a GC you can analyze the same sample with both methods and see if the solution retains all the produced CO2. 

Cheers

The process that you describe will react CO2 with NaOH to produce NaHCO3 or sodium bicarbonate.  The reaction is quite fast once the CO2 contacts the NaOH solution.  The limiting factor will be the size of bubbles that the gas sparger can produce.  Specifically, the smaller the bubbles, the faster the reaction because the volume to surface area ratio will be lower.  If your sparger only produces large bubbles, then CO2 will always break through because the CO2 in the middle of the bubble will never contact the NaOH solution. 

The other concern to be aware of is that the NaOH is used up in the process (converted to NaHCO3) and so the concentration that you end with will not be the same that you begin with.  If you are running a short experiment, you may be able to use a very low concentration.  If you are running a longer experiment, then you will need to use a higher concentration or change solutions from time to time.  If you have a pH probe or an indicator dye to tell you when the solution is spent, then you can change the solution when needed.  Otherwise, you will need to practice excess chemistry by guessing how much you will need and then adding extra to account for potential error. 

Personally, I use finely powdered Ca(OH)2 in CO2 scrubbers.  The solution is cloudy, but when it is close to being used up, it turns clear and I change it.  It is much easier to handle and dispose of.  If it goes too long, then it turns cloudy again due to a further chemical reaction, so it does need to be carefully watched.  Alternatively, if I don't want to add humidity, I use Ca(OH)2 granules.  There is enough humidity in the air to dissolve the CO2 and the Ca(OH)2.  Eventually, it just becomes a caustic liquid in the bottom of the container.  This only works when there is more moisture in the air than there is CO2.

As said above, the size of the bubbles are important. Smaller there are, better it is. On an other the contact time of the bubbles in the NaOH solution is also important. Longer it is, better it is. So, for your example, you can place your solution in a bottle with a smaller diameter. The height of the solution will be higher and so the contact time will increase. To know the concentrration of your solution you can calculate it by using the stoecheometry of the reaction.

Thanks for the anwsers!

The liquid used is a solution of NaOH or KOH at a concentration between 15 to 20g / L (1.5% to 2% m/m). Among this range excess equivalent of NaOH to make sure that all the CO2 is absorbed by the liquid is guaranteed.

Hi Iason,

In addition to the answer of T. Salmon, you can place the gas sparger under a bed of inert material to increase the residence time of biogas bubbles.

Best regards

Hi Iason,

In addition to the answer of T. Salmon, you can place the gas sparger under a bed of inert material to increase the residence time of biogas bubbles.

Best regards

Ray H. Kamps thank you for your detailed explanation. Can you please help me out with a design of sparger. I want to use this for biogas purification.

Bidya Dhar Biswas , Your question is very open-ended. If you are interested in industrial-scale biogas purification, then you need the services of an engineer, which I am not. If you are interested in laboratory-scale biogas purification so that you can run gas samples through a gas analyzer, then what I have used in the past was to run the gas through a packed column of Calcium Hydroxide (to remove CO2) and then through a packed column of desiccant (to remove H2O). It was then ready to run into the Gas Chromatograph. Both the Calcium Hydroxide and desiccant will be used up in the process. On an industrial scale, they can be regenerated with heat. On a laboratory scale, some desiccants can be regenerated with heat effectively, but others are discarded when spent. Spent Calcium Hydroxide is typically discarded due to the high heat needed to convert Calcium Carbonate back into Calcium Oxide. Calcium Oxide can be used directly in the packed column since it naturally turns back into Calcium Hydroxide due to humidity.

Ray H. Kamps ,Thanks for the information.I am performing experiments in laboratory. Please help me by giving a detailed explanation about the design of those packed columns. I am stuck at implementation.

You can also mail me at [email protected]

Thanks in advance

Bidya Dhar Biswas , Packed columns are fairly standard items in chemistry, especially for desiccants. The ones that I used were supplied by the manufacturer of the gas analyzer, LiCor Inc. In practice, they are simply a tube filled with a reactant, in this case, Calcium Hydroxide. Generic packed columns are typically made of glass, but the ones supplied by LiCor were made of plastic. A sample is drawn in to the packed tube at one end, passes over the reactant in packed in the tube, and is drawn out the other end. There is some method of retaining the reactant in the tube. In the case of the LiCor packed columns, it was a circular piece of filter paper at each end. The gas analyzer that I was using required both CO2 and H2O to be removed, so I used two packed columns, one for CO2 and one for H2O. If you are using a gas analyzer, you will need to follow the manufacturer's instructions for scrubbing CO2. Otherwise, I would suggest that you discuss this with a chemist in your own institution.

All good answers above. You may also find the following helpful.

It is likely that your biogas will be 30-50% CO2, that 1 mole of gas is about 22L, and that the stoichiometey is 1:1. So 40 g of NaOH will scrub up to 70 L of biogas.

This rule of thumb helped me achieve peace of mind in the past when thinking about preparing solutions and planning when to replenish them.

Happy experimenting.