# Understanding dissolution of CO₂ in Selexol and Rectisol

I'm trying to understand pre-combusiton carbon capture processes as described in this presentation.

I'm told that solvents such as chilled methanol ($\ce{CH3OH}$) and dimethyl ethers of polyethylene glycol ($\ce{C_{2n}H_{4n + 2}O_{n + 1}}$) can dissolve $\ce{CO2}$ from a gasified coal fuel (before combustion) under the action of increased pressure. I'm struggling in understanding how the increased pressure induces the dissolution of $\ce{CO2}$ from the gasified coal. I'm also a bit rusty with my chemistry, by saying $\ce{CO2}$ is dissolved by the solvents, what is really going on, chemically?

• dissolving something is really something simple that you can find on wikipedia. When water dissolves a salt the ions are seperated and surrounded by water molecules. Anything can dissolved with a proper solvent. The molecules in the air we are breathing is dissolved in nitrogen if I recall correctly, although I do not think bonds other than ionic bonds are broken in dissolutions. Think of the solvents you mention as the policemen tackle the bad guy, more pressure means more cops nearby I suppose. – Leonardo Sep 17 '12 at 4:18
• @Leonardo: is it proper to say that the CO2 chemically bonds to the solvent? – Paul Sep 17 '12 at 4:57
• Honestly I can not be sure and say that no chemical reaction happens. For the most part, when a solvent dissolves a solute, ionic bonds tend to be broken (CO2 is covalent not ionic) and what is usually left are the intermolecular attractions, but in the case of the CO2 I would not like to mislead you by saying that no chemical bonds are formed with the CO2, although that is my guess due to pressure being one of the ingredients. – Leonardo Sep 17 '12 at 14:54

Simply, $\ce{CO_{2}}$ is dissolved in those solvents in a similar way to gases dissolved in water. The molecules do not react with the solvent but mix. The higher the pressure of the gas you pump into the liquid, the more that dissolves in it (rather than turning into bubbles and escaping). In fact, amount of gases dissolved in liquids can be measured both as a concentration (mol/L) or as a partial pressure (Pa).

Henry's law states that:

"At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid."

The formula is:

$p = k_{H}c$

where $p$ is partial pressure of the gas in solution, $c$ is its concentration and $k_{H}$ is a constant based on what the solute is (in this case $\ce{CO_{2}}$), what the solvent is (eg methanol) and the temperature.

The harder you push, the tighter you can pack it in.

Here is a picture:

In contrast, with the amine-based solvents, the $\ce{CO_{2}}$ actually reacts, which is why a degasification process needs to happen to release it.