VAPOR PHASE ASSOCIATION

[thubelihle]

WHAT IS VAPOR PHASE ASSOCIATION AND HOW IS IT PREDICTED GIVEN COMPOUNDS BEFORE SIMULATION. WHAT ARE HEXAMERS AND DIMERS WITH REGARDS TO VAPOR PHASE ASSOCIATION.

[jasper]

Vapor phase association is where multiple molecules form a conglomerate in the vapor phase, and effectively occupy the space of a single molecule (in an ideal gas) rather than two molecules. An example is carbonic acids, where the OH group of one molecule forms a H-bond with the =O group of the other molecule and at the same time the =O of the first molecule forms a H-bond with the -OH of the other molecule. These two molecules are then a dimer in the vapor phase (di = 2, hexa = 6, also think of oligomers (few molecules) and polymers (many molecules)).

For the special case of carbonic acids, Prausnitz' Chemical Theory can be used. Implemented by TEA (http://cocosimulator.org/index_help.php ... virial.htm). In general packages that model association need to model the equilibrium reaction between the monomer and oligomers.

[thubelihle]

Wow. Is there any way to if given components will form vapor phase association, like a rule of the thumb. Thanks for your help.

[jasper]

Components that form multiple hydrogen bonds, such as water and carbonic acids, are good candidates. Or very strong hydrogen bonds, such as HF.

If not hydrogen bonds, then they must be chemical bonds. So now you are looking for systems with chemical reactions.

[thubelihle]

Just wondering, lets say we have water and some other component that is not involved in vapor phase association, can we still say the system has vapor phase association, since the water molecules in the vapor phase will associate with one another? Thank you.

[Usseglio]

Hello, I work in the field of carboxylic acids stripping from vegetable oils.
Vapor association, as it was already said, it is one of the chemical explanations regarding why some compounds departure from vapor phase ideality, in case of carboxylic acids, can form dimers. But this is only significant under high pressure conditions. In the case of carboxylic acids stripping, that is carried out a very low absolute pressure, I found that against actual plant results, there is no difference in distillate composition considering ideal phase on one hand and non-ideal phase including vapor association.

So if the pressure of your process works under low pressure, I would say you can treat the system as vapor phase as ideal, since results will be in accordance with actual results.

[jasper]

Water will not behave ideal in the vapor phase, due to several effects, including vapor phase association. In my experience, vapor phase association of just water in the vapor phase is not typically explicitly account for though.

[jasper]

And as Marcello indicated, if the pressure is low, all vapor will be close enough to ideal.