Biodiesel Production Using Supercritical Methanol with Carbon Dioxide and Acetic Acid

Biodiesel Production Using Supercritical Methanol with Carbon Dioxide and Acetic Acid Chao-Yi Wei, Tzou-Chi Huang, and Ho-Hsien Chen Department of Food Science, National Pingtung University of Science and Technology, Neipu, Pingtung 91201, Taiwan Received 29 September 2012; Revised 11 December 2012; Accepted 17 December 2012 Academic Editor: Ahmed A. Mohamed Copyright © 2013 Chao-Yi Wei et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract Transesterification of oils and lipids in supercritical methanol is commonly carried out in the absence of a catalyst. In this work, supercritical methanol, carbon dioxide, and acetic acid were used to produce biodiesel from soybean oil. Supercritical carbon dioxide was added to reduce the reaction temperature and increase the fats dissolved in the reaction medium. Acetic acid was added to reduce the glycerol byproduct and increase the hydrolysis of fatty acids. The Taguchi method was used to identify optimal conditions in the biodiesel production process. With an optimal reaction temperature of 280°C, a methanol-to-oil ratio of 60, and an acetic acid-to-oil ratio of 3, a 97.83% yield of fatty acid methyl esters (FAMEs) was observed after 90 min at a reaction pressure of 20 MPa. While the common approach to biodiesel production results in a glycerol byproduct of about 10% of the yield, the practices reported in this research can reduce the glycerol byproduct by 30.2% and thereby meet international standards requiring a FAME content of >96%. 1. Introduction Due to higher energy demands, more problems are associated with the widespread use of fossil fuels, recent rises in petroleum prices, and other energy concerns; it is increasingly necessary to develop renewable energy sources with smaller environmental impacts. In recent years, there have been increased demands for biodiesel, which is used as fuel in diesel engine systems. Generally, biodiesel is a mixture of fatty acid methyl esters (FAMEs), which may be derived from a variety of oils, fats, and waste oils and has similar physicochemical properties to conventional diesel [1–3]. Hence, biodiesel is compatible with existing diesel engines and can be utilized without major engine modifications. In addition, biodiesel combustion decreases emissions of CO2, , and unburned hydrocarbons. Biodiesel obtained from energy crops favorably affects the environment and can help develop new industries, such as the agroenergy industry, which create employment and boost regional development. For these reasons, this renewable and environmentally friendly biofuel has the potential to ensure the sustainability of energy sources in the future by replacing exhaustible fossil fuels as the main energy supply. Despite the enormous benefits of biodiesel, high processing costs and expensive feedstock have acted as barriers to its development [4]. Transesterification of oils and lipids into biodiesel consists of replacing the glycerol of triglycerides with a short-chain alcohol, which can be achieved using various processes. Transesterification reactions can be catalyzed using alkaline, acidic, enzymatic, or other kinds of catalysts [5]. Most commonly, biodiesel production utilizes an alkaline catalyst, but it is difficult to adapt this process for use with some waste oils and fats. Enzymatic catalysis takes a long time to completely convert oils and fats into FAMEs. Biodiesel fuels can also be processed from oils via noncatalytic transesterification with supercritical alcohol [6], a process developed to resolve various problems. Such supercritical treatment can significantly reduce the reaction time, and the properties of the product mixture were found to fulfill international standards as well [7]. The raw material of oils or fats with high contents of free fatty acids can also be converted to FAMEs by an esterification reaction in supercritical methanol [7, 8]. With this process, a high yield of FAMEs can be obtained, and there are no alkaline soaps generated. Moreover, separation and purification of the products are easy [9, 10]. Unlike the alkali-catalyzed method, this method can also be applied to relatively long-chain alcohols. The production of glycerol as a byproduct, however, has not been avoided (1), and with the increased production of biodiesel in years to come, a glut of glycerol may result. A method for producing biodiesel without producing glycerol, therefore, may prove efficacious. Ilham and Saka [11] proposed that dimethyl carbonate be used for noncatalytic, supercritical treatment in biodiesel production. Subsequently, Saka and Isayama [12] carried out a process utilizing noncatalytic, supercritical methyl acetate to produce a mixture of FAMEs and triacetylglycerol, commonly known as triacetin, without producing glycerol [12, 13]. Hence, produc (...truncated)