Acid gas extraction of pyridine from water

Pyridine was extracted from aqueous solutions initially containing 5 or 15 wt % pyridine by using liquid or supercritical carbon dioxide at 10 MPa as a solvent in a mechanically agitated countercurrent extraction column. The lowest pyridine concentration in the raffinate was 0.06 wt %, whereas the pyridine concentration in the extract was 86−94 wt %. From the initial amount of pyridine, 96−99% was transferred from the feed stream to the extract by using relatively small solvent-to-feed ratios of 2.8−4.6 (kg of solvent/kg of feed). The measured distribution coefficients for the water/pyridine/carbon dioxide system ranged from 0.3 to 1 (weight units), depending on the initial pyridine concentration in water. Carbon dioxide is a particularly suitable solvent for the extraction of pyridine from concentrated aqueous solutions. The efficiency may be the result of an acid−base interaction between weakly basic pyridine solute and weakly acidic carbon dioxide solvent in an aqueous environment.