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  • Biodiesel production from vegetable oils via catalytic and

    non-catalytic supercritical methanol transesterification methods

    Ayhan Demirbas *

    Department of Chemical Engineering, Selcuk University, Campus, 42031 Konya, Turkey

    Received 18 April 2005; accepted 23 September 2005

    Abstract

    This paper reviews the production and characterization of biodiesel (BD or B) as well as the experimental work carried out by

    many researchers in this field. BD fuel is a renewable substitute fuel for petroleum diesel or petrodiesel (PD) fuel made from

    vegetable or animal fats. BD fuel can be used in any mixture with PD fuel as it has very similar characteristics but it has lower

    exhaust emissions. BD fuel has better properties than that of PD fuel such as renewable, biodegradable, non-toxic, and essentially

    free of sulfur and aromatics. There are more than 350 oil bearing crops identified, among which only sunflower, safflower, soybean,

    cottonseed, rapeseed and peanut oils are considered as potential alternative fuels for diesel engines. The major problem associated

    with the use of pure vegetable oils as fuels, for Diesel engines are caused by high fuel viscosity in compression ignition. Dilution,

    micro-emulsification, pyrolysis and transesterification are the four techniques applied to solve the problems encountered with the

    high fuel viscosity. Dilution of oils with solvents and microemulsions of vegetable oils lowers the viscosity, some engine

    performance problems still exist. The viscosity values of vegetable oils vary between 27.2 and 53.6 mm2/s whereas those of

    vegetable oil methyl esters between 3.59 and 4.63 mm2/s. The viscosity values of vegetable oil methyl esters highly decreases after

    transesterification process. Compared to no. 2 diesel fuel, all of the vegetable oil methyl esters were slightly viscous. The flash

    point values of vegetable oil methyl esters are highly lower than those of vegetable oils. An increase in density from 860 to

    885 kg/m3 for vegetable oil methyl esters or biodiesels increases the viscosity from 3.59 to 4.63 mm2/s and the increases are highly

    regular. The purpose of the transesterification process is to lower the viscosity of the oil. The transesterfication of triglycerides by

    methanol, ethanol, propanol and butanol, has proved to be the most promising process. Methanol is the commonly used alcohol in

    this process, due in part to its low cost. Methyl esters of vegetable oils have several outstanding advantages among other new-

    renewable and clean engine fuel alternatives. The most important variables affecting the methyl ester yield during the

    transesterification reaction are molar ratio of alcohol to vegetable oil and reaction temperature. Biodiesel has become more

    attractive recently because of its environmental benefits. Biodiesel is an environmentally friendly fuel that can be used in any diesel

    engine without modification.

    q 2005 Elsevier Ltd. All rights reserved.

    Keywords: Alternative fuel; Vegetable oil; Biodiesel; Viscosity; Transesterification; Methanol

    Contents

    1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467

    1.1. History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 468

    Progress in Energy and Combustion Science 31 (2005) 466–487

    www.elsevier.com/locate/pecs

    0360-1285/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

    doi:10.1016/j.pecs.2005.09.001

    * Tel.: C90 462 230 7831; fax: C90 462 248 8508. E-mail address: [email protected]

    http://www.elsevier.com/locate/pecs

  • A. Demirbas / Progress in Energy and Combustion Science 31 (2005) 466–487 467

    1.2. The use of vegetable oils and their derivatives as alternative diesel fuels . . . . . . . . . . . . . . . . . . . . . . . . . 468

    1.3. Global vegetable oil resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 469

    2. Biodiesel (BD) as an alternative fuel for diesel engine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 470

    2.1. The importance of alcohols for diesel engines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471

    2.1.1. Methanol production methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 471

    2.1.2. Ethanol production methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472

    2.2. Hydrogen production methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 473

    3. Transesterification of vegetable oils and fats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474

    3.1. Catalytic transesterification method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 474

    3.2. Supercritical methanol transesterification method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475

    3.3. Recovery of glycerol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477

    3.4. Reaction mechanism of transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477

    3.4.1. Acid-catalyzed processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478

    3.4.2. Alkali-catalyzed processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478

    3.4.3. Enzyme-catalyzed processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479

    3.4.4. Non-catalytic supercritical alcohol transesterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479

    4. Fuel properties of vegetable oils and biodiesels (BDs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479

    4.1. Emissions from biodiesel combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 482

    4.2. Comparison of fuel properties and combustion characteristics of methyl and ethyl alcohols and their esters 482

    5. Engine performance tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483

    6. BD economy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 483

    7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484

    References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484

    1. Introduction

    The major part of all energy consumed worldwide

    comes from fossil sources (petroleum, coal and natural

    gas). However, these sources are limited, and will be

    exhausted by the near future. Thus, looking for

    alternative sources of new and renewable energy

    such as hydro, biomass, wind, solar, geothermal,

    hydrogen and nuclear is of vital importance. Alterna-

    tive new and renewable fuels have the potential to

    solve many of the current social problems and

    concerns, from air pollution and global warming to

    other environmental improvements and sustainability

    issues [1].

    Vegetable oil is one of the renewable fuels.

    Vegetable oils have become more attractive recently

    because of its environmental benefits and the fact

    that it is made from renewable resources. Vegetable

    oils are a renewable and potentially inexhaustible

    source of energy with an energetic content close to

    diesel fuel. The vegetable oil fuels were not

    acceptable because they were more expensive than

    petroleum fuels. However, with recent increases in

    petroleum prices and uncertainties concerning pet-

    roleum availability, there is renewed interest in

    vegetable oil fuels for diesel engines [2]. Diesel

    boiling range material is of particular interest

    because it has been shown to significantly reduce

    particulate emissions relative to petroleum diesel

    [3]. There are more than 350 oil-bearing crops

    identified, among which only sunflower, safflower,

    soybean, cottonseed, rapeseed, and peanut oils are

    considered as potential alternative fuels for diesel

    engines [4,5]. The major problem associated with

    the use of pure vegetable oils as fuels, for diesel

    engines are caused by high fuel viscosity in

    compression ignition.

    The use of vegetable oils as alternative renewable

    fuel competing with petroleum was proposed in the

    beginning of 1980s. The advantages of vegetable oils as

    diesel fuel are [2]:

    † Liquid nature-portability † Ready availability † Renewability † Higher heat content (about 88% of no. 2 diese

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