Separation of chlorobenzoic acids by dissociation extractive crystallization

Separation of o- and p-chlorobenzoic acids (CBAs) has been investigated by a two-phase process of dissociation extractive crystallization. On the basis of a screening procedure developed from thermodynamic considerations, methanol and piperazine were selected as a solvent and reagent, respectively. The ternary phase equilibrium diagram depicting several isotherms has been constructed for the o-CBA–p-CBA–methanol system. Also included are the corresponding calculated values based on a modified UNIFAC group contribution method and these show good agreement for such a complex system. A series of batch equilibrium precipitation experiments was performed and the proposed thermodynamic model predicted comparable concentrations of all species in the solution phase at equilibrium. Good recovery of o-CBA (in excess of 90%) was achieved in a single stage and very high separation factors (usually approaching infinity) were possible indicating the potential efficacy of the separation technique for this system.