Removal of Residual Catalyst from Simulated Biodiesels Crude Glycerol for Glycerol Hydrogenolysis to Propylene Glycol

The removal of sodium from glycerol solutions by crystallization/precipitation of hydroxyapatite (HAP) through the co-addition of lime [Ca(OH)2] and phosphoric acid was evaluated as a means to remove soluble catalyst from the glycerol byproduct of biodiesel production. Phosphate ions precipitated as hydroxyapatite upon reacting with calcium and hydroxide ions. Seed crystals and pH impacted crystallization. The yield decreased due to the polymerization of glycerol at high pH values (pH >= 11). The continuous removal of phosphate by a lime packed column method was also evaluated for process scale-up considerations. Higher temperatures favored the phosphate removal efficiency with higher temperatures raising the pH and the supersaturation region of the respective effluents to the desired level for HAP crystallization/precipitation. The suitability of the resulting product was evaluated as a hydrogenolysis feedstock for producing propylene glycol. The yield of propylene glycol increased with increasing filtrate pH.