(133e) Catalytic Transesterification of Waste Cooking Oil to Biofuel and Conversion of Glycerin with Supercritical Methanol

In the conventional transesterification of fats/vegetable oils for biodiesel production, free fatty acids and water always present a problem and result in a low conversion.  The object of this study was to investigate the yield of methyl esters treated by acid-free and alkaline-free with and without a heterogeneous catalyst in supercritical methanol (SCM) condition.  Glyceride tributyrate was used as a model compound for waste cooking oil to study transesterification with methanol at supercritical condition (239.4^oC and 1140 psi).  The transesterification reaction was studied with an automatic controlled fixed bed tubular reactor in a continuous mode as shown in Figure 1.   The reactor was constructed from a 3/8” OD x 4’ long stainless tube with wall thickness of 0.049” (reactor volume = 47.4 mL). This system was continuously used for the biodiesel synthesis from and waste cooking oil and animal fats.

Figure 1.  Continuous Process Flow Diagram for biodiesel production with supercritical methanol.

For glyceride tributyrate, the conversion was 100% under SCM condition. For waste cooking oil, the fatty acid methyl esters (FAMEs) yield was  >90% at the following reaction conditions: reaction temperature = 300^oC and 250°C, reaction pressures = 2000 psi, 2500 psi, and 3000 psi, residence time = 18 minutes, and molar ratio of methanol to oil = 40:1, flow rate = 2.5 mL/min.  However, when feed flowrate increased to 7 mL/min. 250 ^oC and 2500 psi, the transesterifcation conversion dropped to <86 %.  Heterogeneous catalyst, ETS-10, lowered the transestrification reaction to sub-supercritical conditions, the conversion was consistently achievable over 99% for a 24 hours study at a flow rate of 6mL/min (LHSV 4.47 g. min./mL).  Other nanoporous solid catalysts were also studied in similar SCM conditions.  The catalysts include hydrotalcite, Zeolite-X, Zeolite-Y, sulfate-ZrSO₂, ML-3, and ZSM-5.  The findings demonstrated that, by a supercritical methanol approach with heterogeneous nanoporous catalysts, vegetable oil and its wastes could be readily used for biodiesel fuel production in a simple, continuous, and automatic process.

Glycerin is a main byproduct in triglyceride transesterification for biodiesel production as shown in Equation 1.

CH₂-OCO-R                                                                                                          CH₂-OH                                     

l                                                                                                                               l              

CH-OCO-R           +       3R’OH        →        3R-COOR’’                 +             CH-OH                                  Eq (1)

l                                       Alcohol                        Alkyl Ester                             l

CH₂-OCO-R                                                            Biodiesel                          CH₂-OH 

Triglyceride                                                             Product                                Glycerol (byproduct)          

R = fatty acid chain              R’ = CH₃ : fatty acid methyl esters                     R’’ = CH₂CH₃ : fatty acid ethyl esters

Continued expansion of biodiesel capacity has led to a rapid increase in glycerin supply, the supply of glycerol to the market could affect the need for chemical derivatives.  When the transesterification reaction was conducted at 350ºC with SCM, there was no phase separation of the byproduct glycerol and biodiesel; both components were dissolved in the methanol solvent.  GC-MS analysis showed that there were some oxygenated low molecular weight compounds, which are miscible with biodiesel.  After carefully analyzing these compounds, glycerol had reacted with methanol and produced isobutanol and 3-methoxy-1,2-propandiol at the higher temperature. 

At lower temperature (300 ^oC) in SCM, spiked glycrol remained unconverted.  Heterogeneous catalysts, ETS-10 and Zeolite-X, contain basic surface states had catalyzed the complete conversion of the glycerol bio-based byproduct to useful fuel additives with higher carbon/oxygen ratio (C/O) at sub-supercritical temperature and pressure.