Hydrodynamic behavior analysis of a rotating disc contactor for aromatics extraction with 4-methyl-butyl-pyridinium·BF4 by CFD

An experimental and numerical simulation analysis of the hydrodynamic behavior for aromatics extraction with 4-methyl-N-butyl-pyridinium tetrafluoroborate is presented. Room temperature ionic liquids (RTILs) have proven to be promising solvents for the extraction of aromatic hydrocarbons, because of their non-volatile nature and their tailoring properties. The RTIL 4-methyl-N-butyl-pyridinium tetrafluoroborate ([4-mebupy]BF4) was therefore tested as a solvent for the extraction of toluene from toluene/n-heptane in a rotating disc contactor (RDC). Hydrodynamic characteristics, like Sauter mean diameter and hold-up, were measured for different total fluxes and stirrer speeds. Unexpected behavior for the hold-up was observed in experiments when the RTIL was applied as solvent. At lower fluxes, the hold-up decreases with increasing rotor speed, when an increase of hold-up was expected. This behavior, however, can very well be explained by the existence of three operating regimes in the used RDC.Computational fluid dynamics simulations of the two-phase flow in the RDC extractor have been performed to investigate the unexpected hold-up behavior. The numerical simulations were done using the commercial CFD software fluent, whereas an Euler–Euler model was applied together with the realizable k–ɛ turbulence model for the solution of the liquid–liquid problem. The numerical hold-up results are compared to the experimental profiles. Possible reasons for the hold-up anomalies, namely the path of the RTIL droplets as well as the velocity fields in both liquid phases, are presented and discussed. The work shows, that CFD can predict hydrodynamic characteristics even for extreme examples as in the present RTIL extraction.