thank you sir

I don´t understand the question nor the given answer

in a given mass transfer operation involving two phases, which phase should be dispersed? are there any guidelines available for selecting the dispersed phase?

My point is that the extraction depends highly on the distribution coefficient, for me it does not matter who is dispersing in what but how to increase the value of the distribution coefficient, that will result in using less organic solvent (or aqueous) for the extraction

Omar is right. You don't really need to select the dispersed phase. What is important, is to select the best solvent for your system in a way that gives you the best separation efficiency.

There are a number of considerations and rules of thumb when deciding which phase to disperse, and some are in conflict:

• In general, the dispersed phase should be the one that does not wet the internals.  For example, if you're using an agitated column with PTFE rotors, disperse the aqueous phase because it will not wet the PTFE.  If the dispersed phase wets the internals, it will be difficult to keep it dispersed.
• Agitated columns: disperse the phase with the lower flow rate.  Sieve plate or packed columns: disperse the one with the higher flow rate.
• For higher extraction efficiency, disperse the less viscous phase, for more rapid diffusion inside the droplet.  For higher capacity, disperse the more viscous phase, to avoid droplets flowing through a more viscous continuous phase.
• Best approach: try both in the pilot plant.