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Process simulation, economic analysis and synthesis of biodiesel from waste vegetable oil using supercritical methanol Lee, Soo Jin


Biodiesel production using supercritical methanol received attention as an alternative method to replace the conventional alkali-catalyzed method being practiced in industry. Due to its flexibility to feedstock compared to the conventional method, the supercritical method for waste vegetable oil conversion appears to be promising in environmental and economical points of views. Four industrial-scale biodiesel production processes were simulated using Hysys. Each process used either the conventional or the supercritical method. The first model simulated the alkali-catalyzed process using fresh vegetable oil. The second process model resembled the first one as it uses sodium hydroxide catalyst in transesterification, but pre-treatment process of waste vegetable oil was included. The third and fourth models were supercritical biodiesel production processes using waste vegetable oil. Fourth model had differences from the third one in terms of the amount of methanol being introduced to a plug flow reactor and the way of recovering methanol from reaction products, both of which can reduce energy consumption of the process. To improve the accuracy of the process simulations, properties of a model compound (triolein) of the vegetable oils were examined via thermogravimetric analysis, and the experimental data were incorporated into the simulation models. Economical aspects of the developed simulation models were then assessed using Aspen Icarus Process Evaluator. The economic assessment revealed that supercritical processes using waste vegetable oil were competitive to the conventional process based on their better profitability indicators such as discounted payback period and net present value. The net present value prediction formulas were derived for the four processes via statistical analysis of the vegetable oil price, biodiesel selling price, by-product selling price and interest rate that were found to most strongly affect the profitability of the biodiesel production processes by sensitive analysis. Experiments of biodiesel synthesis from waste canola oil were conducted using supercritical methanol. High methyl ester yields over 96% were achieved after 45 min of reaction time at 270°C/10 MPa with methanol to oil ratios of 1:1 and 2:1. Side reactions such as glycerol decomposition and glycerol methanolysis were confirmed by water content measurement using Karl-Fischer titration and Gas chromatography-mass spectrometry (GC-MS) analysis.

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