Conversion of Glycerol to Lactic Acid under Low Corrosive Conditions with Homogeneous and Heterogeneous Catalysts

With the increasing demand for biodiesel, the accumulation of byproduct, crude glycerol has become a problem which needs to be solved. Lactic acid is one of the value-added chemical which can be produced from glycerol that has wide uses in food and chemical industry. Although glycerol can be converted to lactic acid with an alkali as the catalyst at high glycerol conversion (100 mol%) and lactic acid yield (around 90 mol%), the high alkalinity would cause severe corrosiveness to a stainless steel reactor. In this study two tasks were performed to convert glycerol to lactic acid with satisfactory conversion and selectivity, and to reduce the corrosiveness of reaction medium.

First, CaO was used as solid base catalyst. The highest lactic acid yield achieved was 40.8 mol% with a 97.8 mol% glycerol conversion, when operating at 290°C after 150 min reaction with molar ratio of CaO: glycerol=0.3. Also CaO has advantages such as high lactic acid productivity (3.35 g/(min·L)) and reusability. Meanwhile, CaO can be used as the catalyst for both biodiesel production and the following crude glycerol conversion to lactic acid. Second, for glycerol conversion with NaOH as catalyst, a fed-batch reactor was applied to continuously supply NaOH during reaction process, compensating the OH^- neutralized by newly formed lactic acid. The optimal lactic acid yield of 80.5 mol%, with 92.8% glycerol conversion was obtained at 300 °C for 220min, with 1.1 M glycerol initial concentration. A first-order kinetic model for glycerol concentration versus time was developed and verified experimentally under conditions with different initial glycerol concentration and reaction temperature. Although crude glycerol samples contained large amount of impurities, both methods, conversion with solid base catalyst and with fed-batch reactor, were applied successfully to three crude glycerol samples provided by biodiesel manufacturers, and the lactic acid yield reached 52.3 mol% and 72.7 mol% respectively.

Finally, the corrosion issue of different methods was compared based on the Fe³⁺ concentration (analyzed with atomic-absorption spectroscopy) in the products. Both methods of glycerol conversions, with solid base catalyst and fed-batch reactor, can reduce the corrosiveness of glycerol conversion with an alkali as the catalyst.