(431f) Hydrothermal Production of Methanol from Biodiesel By-Products

HYDROTHERMAL PRODUCTION OF METHANOL FROM BIODIESEL BY-PRODUCTS

Adam G. Carr^a,b, Lawrence Lai^b, Tamba Monrose^b, William H. Green^b

a – Present address: Aerodyne Research Inc., 45 Manning Rd. Billerica MA, 01821

b – Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge MA 02139

Glycerol was hydrothermally treated in supercritical water for longer residence times than reported previously. Methanol was produced as a major product, along with the expected acetic acid and acetaldehyde co-products. Selectivity of methanol changed over a period of 40 mins, indicating there is an optimum residence time for the production of methanol from glycerol. Methanol is a feedstock of the biodiesel process for the methanation reactions of trialkylglycerides, the major byproduct of which is crude glycerol. The successive production of methanol from glycerol demonstrates potential for the recycling of a waste feedstock back into the biodiesel process.

We studied the effects of sulfur as a homogeneous catalyst in the system. Sulfur was added in the form of diethylsulfide, which is known to produce hydrogen sulfide rapidly at elevated temperatures in supercritical water. The presence of hydrogen sulfide can facilitate the transfer of hydrogen from water, effectively hydrotreating the glycerol. Our studies showed that the maximum concentration of methanol in the system was reached faster with sulfide present, though the conversion of glycerol was consistent between runs.  This may represent a slight change in selectivity at lower residence times when sulfur is present.

The reaction mechanism was investigated using an ab-initio model: Reaction Mechanism Generator (RMG), an open source modeling tool developed at MIT. Methanol, acetaldehyde and acetic acid were all produced through a similar pathway, going through a hydroxyacetone intermediate. Predicted major products at the timescales used in the experiments were acrolein, hydroxyacetone, acetic acid, acetaldehyde and methanol. Acrolein and hydroxyacetone concentrations were overpredicted, whereas methanol and acetaldehyde were underpredicted. The conversion of glycerol over time was well represented, as was the predicted concentrations of acetone, methane and acetic acid. Improvements in the predictions can be made by adding ionic reaction kinetics to the mechanism, and will be the focus of a future study.