1888
Energy & Fuels 2004, 18, 1888-1895
Optimization of Base-Catalyzed Transesterification Reaction of Used Cooking Oil Merve C¸ etinkaya and Filiz Karaosmanogˇlu Chemical Engineering Department, Istanbul Technical University, Maslak, 34469 Istanbul, Turkey Received April 29, 2004. Revised Manuscript Received September 10, 2004
Biodiesel is one of the environmentally friendly alternative liquid biofuels that has proven itself commercially, with international standards all around the world. Industrial and scientific studies on reducing biodiesel production costs are one of the major contributions that have strengthened the position of biodiesel commercially. The type of vegetable oil used for biodiesel production is the parameter that has the greatest effect on biodiesel production cost. For this reason, investigations on the types of no-to-low-cost vegetable oils become crucial. In this study, the optimum conditions for biodiesel production from restaurant-originated used cooking oil (which is composed primarily of oleic and linoleic acids) and the refining procedure were investigated. A refining method of “washing with hot water” was used for biodiesel refinement. One of the properties of biodiesel that has an influence on biodiesel purity is glycerin content. In the refining studies, the effects of glycerin amount, used washing water amount, and the number of washing steps were discussed and biodiesel that meets EN 14214 standards was produced.
Introduction Diesel fuel and diesel fuel emissions have serious environmental effects. Biodiesel obtained from vegetable oils has an important role among biomass products. Biodiesel has proven itself as a technically sufficient alternative diesel fuel in the fuel market since the beginning of the 1990s in several commercial applications. The most important advantages of this fuel include the following: its reducing characteristic on greenhouse gas emissions; its help in regard to reducing a country’s reliance on crude oil imports and its supportive effect on agriculture by providing a new market for domestic crops; its effective lubricating property, which eliminates the need of a lubricating additive; and its wide acceptance by vehicle manufacturers. Biodiesel is an oxygenated fuel that is produced by transesterifying triglycerides such as animal fats or vegetable oils with alcohol in the presence of a homogeneous or heterogeneous catalyst. Any source of complex fatty acid can be used to create biodiesel and glycerin. Transesterification of vegetable oils has been used since the mid-1800s. This method was originally used to distill out the glycerin used to make soap. The “byproducts” of this process are monoalkyl esters, depending on the type of alcohol used. Biodiesel is composed of these esters. The general transesterification process combines animal fats and vegetable oils with an alcohol in the presence of a catalyst to produce glycerin and monoalkyl esters. There are several options for the choice of alcohol and catalyst in the transesterification process. Generally, methanol and ethanol are * Author to whom correspondence should be addressed. Telephone: +90 212 285 68 37. Fax: +90 212 285 29 25. E-mail address:
[email protected].
the most common alcohol types used in transesterification reactions. The process can be conducted with acidic, basic, or enzymatic catalysts. Basic catalysts can be potassium hydroxide (KOH), sodium hydroxide (NaOH), sodium methoxide (NaOCH3), or sodium ethoxide (NaOCH2CH3); the most common acidic catalyst is sulfuric acid (H2SO4). Biodiesel can be produced from several sources, including the following: vegetable oils, such as soybean oil, canola oil, sunflower oil, corn oil, cottonseed oil, mustard oil, palm oil; animal fats, such as beet tallow or pork lard; restaurant waste oils, such as frying oils/used cooking oils; trap grease (from restaurant grease traps); float grease (from wastewater treatment plants); and edible oil technology byproducts, such as acid oil and soapstock. Biodiesel also can be obtained from bleaching earth, which is used in the bleaching unit of vegetable oil refining.1-3 Biodiesel production costs are highly dependent on feedstock costs. The cost of the fat or oil used to produce biodiesel affects the cost of the finished product, up to 60%-75%; therefore, less-expensive raw materials are preferred. To produce biodiesel economically, a production facility must have access to low-value feedstock, develop quality, high-value co-products, and enjoy a cost-effective and high-yielding process. In this study, used cooking oil, which is composed primarily of oleic and linoleic acids, is chosen as a feedstock for biodiesel production. All related studies in the literature and industrial applications were taken (1) Schuchardt, U.; Sercheli, R.; Vorgas, R. M. J. Braz. Chem. Soc. 1998, 9 (1), 1999-1210. (2) Hui, Y. H. Bailey’s Industrial Oil and Fat Products, Vol. 5; Wiley: New York, 1996. (3) Gunstone, F. D.; Hamilton, R. J. Oleochemical Manufacture and Applications; Sheffield Academic Press: Sheffield, U.K., 2001; pp 106164.
10.1021/ef049891c CCC: $27.50 © 2004 American Chemical Society Published on Web 10/08/2004
Optimized Transesterification of Used Cooking Oil
into consideration to set optimum reaction condition alternatives for pilot and industrial-scale production purposes. For this reason, the effects of varying oil: alcohol molar ratios of 1:3, 1:4, 1:5, and 1:6 and catalyst (NaOH) amounts of 1% and 2% of the weight of the oil on the transesterification reaction yield were investigated. In the second part of the laboratory studies, the “washing with hot water” method was performed to investigate the effect of the number of washing steps on the total glycerin content of the biodiesel. Literature Review The production of biodiesel that originates from used cooking oil has been the subject of several research and development (R&D) studies and commercial applications throughout the world. Tomasevic and SilerMarinkovic4 investigated the transesterification reaction conditions for heated refined sunflower oil and used frying oil. At constant temperature (25 °C), molar ratios of 4.5:1, 6:1, and 9:1 and catalyst percentages of 0.5%1.5% were studied. It was determined that the quality of used frying oil does not have an essential effect on the quality of produced methyl esters, if optimal conditions of methanolysis are chosen. As a result, with 1% KOH, a temperature of 25 °C, a molar ratio of 6:1, and 30 min of reaction, heated refined sunflower oil and used frying oil were sufficiently transesterified and suggested to be useable as an alternative fuel in diesel engines.4 Lee et al.5 studied the base-catalyzed reactions of restaurant grease. Raw material candidates were manipulated through acetone fractionation or on a chromatography column packed with an absorbent to obtain maximum reaction rates. Regenerated restaurant grease was converted to ester via base-catalyzed methanolysis. After 24 h of reaction, a reaction rate of 96% was obtained, whereas only 25% conversion was observed from crude grease.5 Alcantara et al.6 studied two different biodiesel production methods on three different types of feedstocks. The methods were transesterification and amidation reactions, with methanol and diethylamine, respectively. The transesterification reaction of used frying oil was conducted under the following conditions: molar ratio of 6:0.8, catalyst ratio of 1%, a temperature of 60 °C, and 600 rpm. The ignition properties of the fuels produced were compared by calculating the cetane indexes of the products, according to ASTM Standard D- 4737, which was based on a fourvariable equation, and by blending the amide biodiesel with No. 2 diesel fuel. As a result, it was determined that amide biodiesel enhances the ignition properties of No. 2 diesel fuel and can be one of the substitutes for No. 2 diesel fuel.6 Al-Widyan and Al-Shyoukh7 worked on the transesterification of used palm oil under various conditions. Different concentrations of hydrochloric acid (HCl), H2SO4, and ethanol at different excess levels were used. From the obtained results, it was investigated that, at higher catalyst concentrations (1.5-2.25 M), (4) Tomasevic, A. V.; Siler-Marinkovic, S. S. Fuel Process. Technol. 2003, 81, 1-6. (5) Lee, K.; Foglia, T. A.; Chang, K. J. Am. Oil Chem. Soc. 2002, 79 (2), 191-195. (6) Alcantara, R.; Amores, J.; Canoira, L.; Fidalgo, E.; Franco, M. J.; Navarro, A. Biomass Bioenergy 2000, 18, 515-527. (7) Al-Widyan, M. I..; Al-Shyoukh, A. O. Bioresource Technol. 2002, 85, 253-256.
Energy & Fuels, Vol. 18, No. 6, 2004 1889
biodiesel with a lower specific gravity was produced in a shorter reaction time. H2SO4 performed better than HCl at a catalyst concentration of 2.25 M. The best reaction condition was 2.25 M H2SO4 with 100% excess ethanol, which reduced the specific gravity of the used palm oil from 0.916 to a final value of 0.8737 in 3 h of reaction time.7 Dorado et al.8 studied the two-step transesterification reaction of waste oils from Brazil, Spain, and Germany. Stoichiometric amounts of methanol and the necessary amounts of KOH, supplemented with the exact amount of KOH to neutralize acidity, were used during the experiments. Both reactions were completed in 30 min. The temperature range was 4060 °C. Parameters such as density, viscosity, water content, and energy content were investigated. It was concluded that a two-step, alkaline-catalyzed transesterification reaction is an economic method for biodiesel production from used vegetable oil.8 C¸ anakc¸ ı and Van Gerpen9 studied a technique to reduce the free fatty acid (FFA) content of restaurant wastes and animal fats, using an acid-catalyzed pretreatment to esterify the FFA before transesterifying the triglycerides with an alkaline catalyst to complete the reaction. Initial process development was performed with synthetic mixtures that contained 20% and 40% FFA, prepared using palmitic acid. The research showed that the acid level of the high-FFA feedstocks could be reduced to
