A supercritical (SC) fluid technology coupled with power cogeneration is proposed to produce biodiesel fuels without the conventional complex separation/purification steps. The core of the integrated system consists of the transesterification (TE) of various triglyceride sources (i.e., vegetable oils and animal fats) with SC methanol/ethanol. Part of the reaction products can be combusted by a diesel engine integrated in the system, which, in turn, provides the power needed to pressurize the system and the heat of the exhaust gases for the TE process. This article reports laboratory-scale investigations directed to system optimum performance (i.e., near complete conversion in short processing time) in connection with the effects of process operating conditions. TE experiments have been conducted at 100-300 bar, 250-425 °C, and 0.73-8.2 min residence time with soybean/sunflower oils as triglycerides and SC methanol/ethanol at ratios of alcohol to oil from 3 (stoichiometric) to 24. Special emphasis was on reactant phase transitions from liquid to SC states. These transitions were monitored with a high-pressure, high-temperature view cell connected to the reactor outlet for the continuous TE experiments and also serving as a batch reactor. Under selected parameters, near complete oil conversion to biodiesel has been achieved with the glycerol decomposition products included in the fuel. Commercial biodiesel production by this method estimated processing costs as low as $0.26/gal for a plant capacity of 5 million gal/year, significantly lower than the current processing costs of ∼$0.51/gal for biodiesel produced by conventional catalytic methods. The retail cost of biodiesel produced by the proposed method is likely to be competitive with diesel fuel prices.
Supercritical transesterification of chicken fat with methanol was investigated at various temperatures (350, 375, and 400 °C), pressures (100, 200, and 300 bar), methanol-to-chicken-fat molar ratios (from stoichiometric 3:1 to 12:1), and residence times (3-10 min). The best experimental results for the conversion of triglycerides and the decomposition of glycerol to fuel components were obtained under the following conditions: 400 °C, 300 bar, methanol-to-triglycerides molar ratio = 9:1, and residence time = 6 min. The thermal decomposition products of long-chain methyl esters and glycerol were identified in the biodiesel samples and their potential influence on fuel characteristics such as viscosity, cold flow, cetane number, and flash point is discussed. Because of the low excess of methanol used in this study in comparison with similar supercritical transesterification processes (methanol-to-feedstock molar ratios of 3-12 vs 40þ), costs associated with the pumping, preheating, and recovery of the excess methanol will be greatly reduced in commercial applications. Use of low-cost feedstocks and a moderate excess of methanol, coupled with glycerol decomposition to valuable fuel components, certainly will increase the profitability of a much simplified method, in comparison with competitive technologies.
Supercritical water oxidation of extracted contaminants is the second step of a two-stage
supercritical fluid technology proposed to remediate soils and sediments contaminated with
polychlorinated biphenyls (PCBs) and/or polyaromatic hydrocarbons. In connection with the
second step, the supercritical water oxidation rate of Aroclor 1248 (A1248), a mixture of ∼76
PCB congeners, is investigated at 25.3 MPa and temperatures of 723, 748, 773, and 823 K. The
reactions are conducted in an isothermal, isobaric plug-flow tubular reactor, and GC/ECD, GC/FID, GC/TCD, and GC/MS chromatographic methods are employed for product analysis.
Experiments are conducted at a nominal A1248 feed concentration of 5.75 × 10-5 mol/L (reaction
conditions) using a methanol solution of 5.245 g/L (5245 ppm) and H2O2 as an initial oxidant
(providing ∼20 mol % excess of O2). Molar global conversion of A1248 varies from 36.06% (for
residence time equal to 6.29 s at 723 K) to 99.95% (54.4 s at 823 K). The overall conversion
follows apparent second order, and the rate constant calculated from the data leads to Arrhenius
parameters of frequency factor A = 1017.0±0.1 s-1 (mol/L)-1 and energy of activation E
a = 186 ±
2 kJ/mol (44.43 ± 0.51 kcal/mol). The congener specific analysis indicates a buildup of
intermediate congener byproducts, which also undergo oxidation decomposition. The identified
reaction products are mainly biphenyl, low-chlorinated PCB congeners such as 2-chlorobiphenyl
and 2,2‘-dichlorobiphenyl, CO, and CO2.
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