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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 g 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.

During the conversion of glycerol to propylene glycol, highly selective conversion is necessary for commercial viability. The greatest strides in achieving high selectivity are attained with catalyst and temperature. For the conversion of glycerol to propylene glycol, these parameters can be optimized to achieve selectivities of greater than 80%. This paper is on the optimization of more-subtle parameters such as concentration, water content, pressure, isothermal operation, and residence time to achieve selectivities in excess of 90%. Data reveal that low concentrations are important to reduce by-product whose formation relies on second-order reaction mechanisms. Water is important to reduce dehydration reactions and indirectly helps to maintain more-isothermal operation. An optimal hydrogen partial pressure between 5 and 15 bar minimizes the cumulative amount of by-product that results from hydrocracking versus dehydration side-reactions.