The solid-liquid extraction process of nylon 6 to eliminate small molecules

i.e.

caprolactam (CL)

cyclic dimers (CD) and cyclic trimers (CT)

is investigated in detail by both batch extraction experiments and numerical simulations. In the batch extraction experiments

due to the small molecules attaching to the polymeric surface

the basic physical mechanism shifts from surface diffusion to internal diffusion as the extraction proceeded. The experimental data are well reproduced by a diffusion model consisting of two distinct steps

characterized as surface diffusion and internal diffusion. Furthermore

based on the established mass transfer mechanism and diffusion model of the two distinct steps

the equilibrium constants and internal diffusion coefficients of CL

CD and CT are acquired. An industrial countercurrent extraction tower is further simulated. It is found that the extraction efficiency of CL can be significantly improved by increasing the temperature at the bottom portion of the tower. The elimination of CD

which can be greatly promoted by a high-concentration CL-water solution

is controlled by mass transfer resistance

whereas the removal of CL is mainly affected by the equilibrium.