Recovery of toluene from high temperature boiling absorbents by pervaporation

Abstract

The treatment of gaseous effluents containing hydrophobic volatile organic compounds (VOCS) can be carried out by absorption with the use of a heavy hydrophobic solvent. These solvents must be regenerated in order to be reused in the absorption process. A possible solution to continuously regenerate the absorbent is a hybrid absorption–pervaporation process, with the pervaporation step serving to regenerate the solvent. This piece of research examines the feasibility of that regeneration step. The VOC used was toluene and three high temperature boiling absorbents were considered: di(2-ethylhexyl) adipate (DEHA), diisobutyl phthalate (DIBP) and polyethylene glycol 400 (PEG 400). Bibliographical research and a preliminary theoretical evaluation led to the choice of PDMS for separating the toluene/absorbent mixture, whatever the absorbent. Experiments using various absorbents showed that toluene passed through the membrane. The extracted toluene flows from DEHA were considerably higher that those measured using diisobutyl phthalate or polyethylene glycol. Therefore, di(2-ethylhexyl) adipate would be the most easily regenerable absorbent. No absorbent was found in the permeate, but a slow accumulation of DEHA was observed in the porous support. This point is of interest and could decrease the separation efficiency. The predominant effect of the liquid boundary layer was highlighted. The resistance-in-series theory allowed the impact of the boundary layer to be quantified. The flow rates of toluene extraction from a DEHA solution were low and require improving the pervaporation regeneration performance to use this kind of separation in an industrial hybrid process.