The inside of the column is designed to have a large contact area between the liquid and gas to allow the liquid to absorb the undesired gases. The clean gas stream exits the top of the column. The contaminated amine solution leaves the bottom and is then cleaned and can be recycled into the column again. What makes this different than regular diffusion is that there are many reactions going on. This makes modeling much more complicated.
Diffusion alone is relatively simple.
What we do is measure the actual absorption and then calculate the diffusion coefficient of the species under consideration by accounting for the reactions. It's a bit complicated. We measure the uptake of the gas using an inverted tube apparatus. The liquid is held on top by capillary action to avoid free convection from denser products formed in the reactions. The pressure drop over time for a known volume of gas is measured and recorded.