Gas dispersion tubes and gas washing bottles are typically used to saturate a liquid with a gas or a gas with another vapor. For example, one might want to saturate a carbon dioxide gas stream with water vapor. The devices can also be used to dry gas streams. For example, one might pass a gas stream through sulfuric acid to remove base impurities or water.
The principle of these is the same. A gas stream enters a tube that ends in with a porous fritted glass disc which provides a high surface/contact area at the gas/liquid interface. The dispersed gas bubbles through the absorbent material (which is usually a liquid), before exiting to the atmosphere (for simple tubes) or the outlet (for gas washing bottles).
Some people also use bubblers for these purposes as well. Bubblers usually don't give as good drying or saturationg, but can be good enough in some cases.
Select your tube or fritted disc based on the size/shape of your reaction vessel and its openings. Select the porosity of the frit based on the flow rate and volume of liquid. Finer porosities produce smaller bubbles but have slower flow rates than coarse porosities. Coarse or extra-coarse porosity is usually the best choice for large volumes and high flow rates. For more information on glass frit porosities, consult the Fritted Funnels Page of our Glassware Gallery.