Part 33: Other Things - Mass Contact by Feinne

Let's talk about simple mass contact for a bit. So, mass contact is a basic separation process driven by concentration gradients. So, the idea is that we bring two immiscible phases into contact (for example, a liquid and a vapor phase) that are differentially concentrated in some component. Where the two phases contact, it is possible for components to be transferred between them (assuming the component in question is both soluble and physically able to enter the phase in question). I think it's best to talk about some simple mass contact 'experiments' you can do yourself.

So, say you pour a glass of water. It's at rough equilibrium with the air, so it's got a minimal concentration of carbon dioxide. Now, take a straw and push it down to the bottom of the glass. If you exhale through it, you'll see bubbles rise up through the water. These bubbles are, compared to the water, enriched in carbon dioxide. Since the carbon dioxide is soluble in water, it'll enter the liquid phase at the interface. It's actually very easy to see that this is happening if you've got a pH indicator, because carbon dioxide will form carbonic acid in water. This may seem like a silly and simple thing, but it's a great example of very basic mass contact.

So, a basic mass contactor is actually sort of problematical because a lot of them use two immiscible liquid phases brought into physical contact and then agitated (think oil and water). In order to actually get your two phases back out, you then need to settle the solution. That's not really something we want to do if we can help it, so next time I'll talk about some more complex strategies for mass contact.