Thread: Question about CO2 scrubbing with soda lime

Hi all,

This question goes out to those more learned than I in chemistry.

Is there any reason why a soda lime scrubber could not be fitted to a car's emissions system in the same way that a catalytic converter is fitted - in order to reduce CO2 emissions? any draw backs etc ?

A soda lime scrubber doesn't act in the same way as a catalytic converter. A catalytic converter acts catalytically, whereas the soda lime in a scrubber would be very quickly consumed.

Another point is how the soda-lime is produced and what you do with the spent soda lime. I suspect soda lime is made by heating limestone (CaCO3) and, er, driving off CO2!

If so, all you do when using it is to recapture the same amount of CO2 as you originally generated, and turn it back to CaCO3 again - zero net benefit.

Soda lime is usually made by treating calcium hydroxide with a concentrated sodium hydroxide solution...

The calcium hydroxide probably would be made from heating calcium carbonate to get the oxide and adding water.

Originally Posted by PhDemon

Soda lime is usually made by treating calcium hydroxide with a concentrated sodium hydroxide solution...

The calcium hydroxide probably would be made from heating calcium carbonate to get the oxide and adding water.

Thanks for the clarification. But the calcium carbonate would release CO2 when heated - the same amount, I presume, as a the amount the soda lime would be able to absorb.

The sodium hydroxide present would also react with CO2 so the soda lime would absorb more CO2 than was released making the calcium hydroxide, but as the NaOH is produced by electrolysis the CO2 produced making the soda lime would be less than absorbed by it unless the electrolysis was powered with a non-fossil fuel energy source...

Originally Posted by PhDemon

The sodium hydroxide present would also react with CO2 so the soda lime would absorb more CO2 than was released making the calcium hydroxide, but as the NaOH is produced by electrolysis the CO2 produced making the soda lime would be less than absorbed by it unless the electrolysis was powered with a non-fossil fuel energy source...

That's interesting. One doesn't normally think of NaOH absorbing CO2 in the way Ca(OH)2 does. I had wondered if it did, but decided it probably didn't. Perhaps I'm just rusty......

Yep, it does. This is why sodium hydroxide solutions used in analytical chemistry or ion testing practicals must be freshly made. Once they been left for a while you have some sodium carbonate in there... In fact solid sodium hydroxide usually has some carbonate impurity because of this reaction.

Originally Posted by PhDemon

Yep, it does. This is why sodium hydroxide solutions used in analytical chemistry or ion testing practicals must be freshly made. Once they been left for a while you have some sodium carbonate in there... In fact solid sodium hydroxide usually has some carbonate impurity because of this reaction.

OK, thanks. Maybe I knew this, once upon a time, but I haven't been in a lab since I moved out of quality assurance work in about 1990 - and even then there was precious little wet chemistry!