Basic modelling of extraction process

The concentration of the species is supposed to be sufficiently low, so that the interfacial flux density (the number of moles of S crossing a surface of unit area) may be written 

This equation shows that k has the dimension ofm s . Moreover, k must be a constant as long as C is not too high. Above some critical concentration eq 3.1 can be expected not too hold any longer because of nonlinearities due to interactions between solute partieles S at, or near, the interface. 

As mentioned above, the global extraction process consists of a series of steps involving transport and chemical reactions. This type of process is sequential by nature. 

Very often, transport in a liquid near the interface is modelled as a purely diffusive process in a stagnant layer of some thickness S. Generally this thickness has different values S/i and 6b in phases A and B. The concept of stagnant layer was introduced by Nemst at the beginning of this century and it constituted the basis of the two-film theory proposed by Whitman [9]. The parameter 6 represents the 

Application of first Pick s law gives the corresponding flux density 

---