Biodiesel production from vegetable oils by supercritical methanol

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					Journal of Scientific & Industrial Research
Vol. 64, November 2005, pp. 858-865




             Biodiesel production from vegetable oils by supercritical methanol
                                                            Ayhan Demirbas*
                            Department of Chemical Engineering, Selcuk University, Campus, Konya, Turkey

           Transesterification of vegetable oils in supercritical methanol are carried out without using any catalyst. Methyl esters
      of vegetable oils or biodiesels have several outstanding advantages among other new-renewable and clean engine fuel
      alternatives and can be used in any diesel engine without modification. The most important variables affecting the methyl
      ester yield during the transesterification reaction are molar ratio of alcohol to vegetable oil and reaction temperature.
      Compared to no. 2 Diesel fuel, all vegetable oils are more viscous, while the methyl esters of vegetable oils are slightly more
      viscous. Biodiesel has become more attractive because of its environmental benefits. The cost of biodiesel, however, is the
      main obstacle to commercialization. With cooking oils as raw material, viability of a continuous transesterification process
      and recovery of high quality glycerol as a biodiesel by-product are primary options to be considered to lower the cost of
      biodiesel. Supercritical methanol has a high potential for both transesterification of triglycerides and methyl esterification of
      free fatty acids to methyl esters for diesel fuel substitute. In supercritical methanol transesterification method, yield of
      conversion rises 95% in 10 min. Viscosity of vegetable oils (27.2-53.6 mm2/s) get reduced in vegetable oil methyl esters
      (3.59-4.63 mm2/s). 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 increases the viscosity from 3.59 to 4.63 mm2/s.

      Keywords: Alternative fuel, Biodiesel, Methanol, Transesterification, Vegetable oil
      IPC Code: C10L 1/10

Introduction                                                             (respectively): methanol, 512.2, 8.1; ethanol, 516.2,
   Petroleum based fuels are limited reserves                            6.4; 1-propanol, 537.2, 5.1; and 1-butanol, 560.2 K,
concentrated in certain regions of the world.                            4.9 Mpa. Biodiesel is made from vegetable oils and
Therefore, countries lacking such resources are facing                   animal fats by catalytic and non-catalytic supercritical
foreign exchange crisis, mainly due to the import of                     alcohol transesterification methods5-7. A non-catalytic
crude oil. Alternative fuels and propulsion systems                      biodiesel production route with supercritical methanol
have potential to solve such problems and concerns                       allows a simple process and high yield because of
from air pollution and global warming to other                           simultaneous transesterification of triglycerides and
environmental and sustainability issues1,2.                              methyl esterification of fatty acids4. Methanol is a
   Biodiesel, an alternative diesel fuel, is made from                   relatively inexpensive alcohol. A reaction mechanism
renewable biological sources such as vegetable oil                       of vegetable oil in supercritical methanol was
and animal fats. It is biodegradable, non-toxic and                      proposed based on the mechanism developed by
possesses low emission profiles. It has become more                      Krammer & Vogel8 for the hydrolysis of esters in
attractive because of its environmental benefits and                     sub/supercritical water. The basic idea of supercritical
renewable resource3. Chemically, bio-diesel is mono-                     treatment is a relationship between pressure and
alkyl-ester especially mono ethyl ester of long-chain-                   temperature upon thermo physical properties of the
fatty acids derived from renewable lipid sources.                        solvent such as dielectric constant, viscosity, specific
Biodiesel is ester based oxygenated fuel from                            weight, and polarity9.
renewable biological sources. It can be used in
compression ignition engines with little or no                              In conventional transesterification of animal fats
modifications4.                                                          and vegetable oils for biodiesel production, presence
   Transesterfication of triglycerides by lower                          of free fatty acids (FFAs) and water causes soap
aliphatic alcohols (1-8 carbon atoms) has proved to be                   formation, consumes catalyst and reduces catalyst
the most promising process. Critical temperature and                     effectiveness, all of which result in a low
critical pressure of such alcohols are as follows                        conversion10. Transesterification of sunflower oil11 has
_______________                                                          been investigated in supercritical methanol and
*E-mail: ayhandemirbas@hotmail.com                                       ethanol at various temperatures (475-675 K).
                  DEMIRBAS: BIODIESEL FROM VEGETABLE OILS BY SUPERCRITICAL METHANOL                                       859

Effect of Reaction Parameters on Conversion
Yield of Transesterification
   Methyl esters (MEs) formation is affected by reaction
temperature, pressure, molar ratio, water content and
free fatty acid content. At subcritical state of alcohol,
reaction rate is so low and gradually increased as either
pressure or temperature rises. Increasing the reaction
temperature, especially to supercritical conditions, had a
favorable influence on the yield of ester conversion. The
yield of alkyl ester increased with increasing the molar
ratio of oil to alcohol5.
Biodiesel via Transesterification from Vegetable
Oil                                                            Fig. 1Simple autoclave for supercritical transesterification
   Transesterified vegetable oils are a viable
alternative diesel engine fuel with characteristics          After settling is complete, water is added (5.5 % by
similar to those of diesel. Transesterification means        vol of methyl ester of oil) and then stirred for 5 min
taking a triglyceride molecule or a complex fatty acid,      and glycerin is allowed to settle again. Washing ester
neutralizing the free fatty acids, removing the              is a two-step process and carried out with extreme
glycerin, and creating an alcohol ester (Eq. 1).             care. A water wash solution (28 % by vol of oil) and
Triglycerides + Monohydric alcohol            Glycerin +     1 g of tannic acid per liter of water is added to the
Mono-alkyl esters                                …(1)        ester and gently agitated. Air is carefully introduced
                                                             into the aqueous layer while simultaneously stirring
   Transesterification reaction (TR) is an equilibrium       very gently. This process is continued until ester layer
reaction, where more amount of methanol is used to           becomes clear. After settling, aqueous solution is
shift reaction equilibrium to right side and produce         drained and water alone is added (28 % by vol of oil)
more MEs as proposed product. Ethanol is a preferred         for the final washing3,5,6.
alcohol in TR compared to methanol because it is
derived from agricultural products and is renewable          Supercritical Methanol Transesterification Method
and biologically less objectionable in the                      In a typical run, the autoclave (Fig. 1) for
environment, however, methanol is preferred because          supercritical transesterification (SCTE) is charged
of its low cost and its physical and chemical                with a given amount of vegetable oil and liquid
advantages (polar and shortest chain alcohol). TR can        methanol with changed molar ratios. After each run,
be catalyzed by alkalis7, acids12, or enzymes13-17. A        the gas is vented, and the autoclave is poured into a
catalyst is usually used to improve reaction rate and        collecting vessel. Rest of the contents is removed
yield. Satisfactory performance for several                  from autoclave by washing with methanol. The most
alcoholysis reactions was achieved with calcium              important variables5 affecting MEs yield during TR
carbonate catalysts even though at higher                    are molar ratio of alcohol to vegetable oil and reaction
temperatures, typically greater than 475 K18.                temperature (Fig. 2). Increasing reaction temperature,
Catalytic Transesterification Method                         especially to supercritical temperatures, had a
   The catalyst is dissolved into methanol by vigorous       favorable influence on ester conversion5. Viscosities
stirring in a small reactor. The oil is transferred into     of MEs from the vegetable oils were slightly higher
the biodiesel reactor and then the catalyst/alcohol          than that of no. 2 diesel fuel.
mixture is pumped into the oil. The final mixture is            In TR, vegetable oil should have an acid value less
stirred vigorously for 2 h at 340 K in ambient               than 1 and all materials should be substantially
pressure. A successful TR produces two liquid                anhydrous. At acid value greater than 1, more NaOH
phases: ester and crude glycerin. Crude glycerin, the        or KOH is spent to neutralize the FFAs. Water also
heavier liquid, will collect at the bottom after several     caused soap formation and frothing. The resulting
hours of settling. Phase separation can be observed          soaps caused an increase in viscosity, formation of
within 10 min and can be complete within 2 h of              gels and foams, and made separation of glycerol
settling. Complete settling can take as long as 20 h.        difficult3, 19. The stoichiometric ratio for TR requires
860                                           J SCI IND RES VOL 64 NOVEMBER 2005




Fig. 2Changes in yield percentage of methyl esters as treated with subcritical and supercritical methanol at different temperatures as a
       function of reaction time6. Molar ratio of vegetable oil to methyl alcohol: 1:41; Sample: Hazelnut kernel oil

three moles of alcohol and one mole of triglyceride to                  esters. However, the presence of water affected
yield three moles of fatty acid ester and one mole of                   positively the formation of methyl esters in present
glycerol. Higher molar ratios result in greater ester                   supercritical methanol method.
production in a shorter time. The vegetable oils were                   Non-catalytic Supercritical Alcohol Transesterification
transesterified 1:6-1:40 vegetable oil-alcohol molar                       Viscosity of biodiesel affects the operation of fuel
ratios in catalytic and supercritical alcohol                           injection equipment, particularly at low temperatures
conditions5.                                                            when the increase in viscosity affects the fluidity of the
   Catalysts are classified as alkali, acid, or                         fuel. Biodiesel has viscosity close to diesel fuels. High
enzyme7,12-17. TR was performed using triglycerides                     viscosity leads to poorer atomization of the fuel spray
and short-chain alcohol by immobilized lipase in non-                   and less accurate operation of the fuel injectors. A
aqueous conditions. TR of rapeseed oil in                               novel process of biodiesel fuel production has been
supercritical methanol required the shorter reaction                    developed by a non-catalytic supercritical methanol
time and simpler purification procedure because of                      method. The supercritical methanol process, which is
the unused catalyst5,6. TR can occur at different                       non-catalytic, simpler in purification, takes lower
temperatures, which influence the reaction rate and                     reaction time and lower energy use (Table 1) than the
yield of esters, depending on the oil used. In the                      common commercial process9,20. The conversion of
supercritical alcohol transesterification method at 575                 triglycerides into MEs or ethyl esters (EEs) through TR
K, yield20 of conversion rises from 50 to 95 percent                    reduces the molecular weight to one-third that of the
for first 10 min (Fig. 3).                                              triglyceride reduces the viscosity by a factor of about
   Water content is an important factor in the                          eight. Viscosities show the same trends as
conventional catalytic transesterification of vegetable                 temperatures, with the lard and tallow biodiesels higher
oil. In the conventional transesterification of fats and                than the soybean and rapeseed biodiesels. Density is
vegetable oils for biodiesel production, FFAs and                       another important property of biodiesel. It is the weight
water always produce negative effects since the                         of a unit volume of fluid. Specific gravity of biodiesels
presence of FFAs and water causes soap formation,                       ranges between 0.87 and 0.89. Fuel injection equipment
consumes catalyst and reduces catalyst effectiveness                    operates on a volume metering system, hence a higher
(Fig. 4 & 5). In catalyzed methods, the presence of                     density for biodiesel results in the delivery of a slightly
water has negative effects on the yields of methyl                      greater mass of fuel.
               DEMIRBAS: BIODIESEL FROM VEGETABLE OILS BY SUPERCRITICAL METHANOL                                                    861




Fig. 3Plots for changes in fatty acids alkyl esters conversion from triglycerides as treated in supercritical alcohol at 575 K77




         Fig. 4Plots for yields of methyl esters as a function of water content in transesterification of triglycerides




                        Fig. 5Plots for yields of methyl esters as a function of free fatty acid content
862                                            J SCI IND RES VOL 64 NOVEMBER 2005


   Table 1Comparisons between catalytic methanol (MeOH) process and supercritical methanol (SCM) method for biodiesel from
                                             vegetable oils by transesterification
                                                         Catalytic MeOH process                              SCM method
Methylating agent                                               Methanol                                     Methanol
Catalyst                                                     Acid or alkali                                    None
Reaction temperature, K                                         303-338                                      523–573
Reaction pressure, MPa                                             0.1                                         10–25
Reaction time, min                                               60–360                                         6–15
Methyl ester yield, wt%                                             97                                           98
Removal for purification                         Methanol, catalyst, glycerol, saponifieds                   Methanol
Free fatty acids                                          Saponified products                            Methyl esters, water
Process                                                       Complicated                                     Simple
                           Table 2Comparisons of properties of vegetable oils and their esters with diesel fuel
Fuel type                         Caloricfic value               Density                Viscosity at 300 K          Cetane number
                                      MJ/kg                      kg/m3                       mm2/s
Diesel fuel                            43.4                        815                         4.3                        47.0
Sunflower oil                          39.5                        918                        58.5                        37.1
Sunflower methyl ester                 40.6                        878                        10.3                        45.5
Cotton seed oil                        39.6                        912                        50.1                        48.1
Cotton seed methyl ester               40.6                        874                        11.1                        45.5
Soybean oil                            39.6                        914                        65.4                        38.0
Soybean methyl ester                   39.8                        872                        11.1                        37.0
Corn oil                               37.8                        915                        46.3                        37.6
Opium poppy oil                        38.9                        921                        56.1                         –
Rapeseed oil                           37.6                        914                        39.2                        37.6

   Cetane number (CN), prime indicator of the quality                    components in the fossil diesel fuel–ethanol–rapeseed
of diesel fuel, is based on hexadecane (cetane, 100)                     oil EE system was slightly lower than in the fossil
and heptamethylnonane (cetane, 15). The straight                         diesel fuel–ethanol–rapeseed oil ME mixture.
chain, saturated hydrocarbons have higher CN                             Moisture content of ethanol had a great influence on
compared to branched-chain or aromatic compounds                         mixture solubility. With decrease of temperature,
of similar molecular weight and number of carbon                         solubility of components in the fossil diesel fuel–
atoms. The CN of biodiesel is generally higher than                      ethanol–rapeseed oil ME system decreased21.
conventional diesel. It relates to the ignition delay                    Previous studies on the effects of vegetable oil MEs
time of a fuel upon injection into the combustion                        on diesel fuel lubricity have shown an increase in
chamber. The CN is a measure of ignition quality of                      lubricity associated with the addition of these
diesel fuels and high CN implies short ignition delay.                   esters22,23.
The longer the fatty acid carbon chains and the more                        Biodiesel fuels are generally nontoxic and
saturated the molecules, the higher the CN. The CN of                    biodegradable, which may promote their use in
biodiesel from animal fats is higher than those of                       applications where biodegradability is desired. Neat
vegetable oils. Two important parameters for low                         biodiesel and biodiesel blends reduce particulate
temperature applications of a fuel are Cloud Point                       matter (PM), hydrocarbons (HC) and carbon
(CP) and Pour Point (PP). The CP is the temperature                      monoxide (CO) emissions and increase nitrogen
at which wax first becomes visible when the fuel is                      oxides (NOx) emissions compared with petroleum-
cooled. The PP is the temperature at which the                           based diesel fuel used in an unmodified diesel
amount of wax out of solution is sufficient to gel the                   engine24.
fuel, thus it is the lowest temperature at which the fuel                   Results indicate that the transformities of biofuels
can flow. Biodiesel has higher CP and PP compared                        are greater than those of fossil fuels (Table 2), thus
to conventional diesel.                                                  showing that a larger amount of resources is required
   Solubility of components in the fossil diesel fuel–                   to get the environmental friendly product. This can be
ethanol–rapeseed oil ME system at 293 K was                              explained by the fact that natural processes are more
substantially higher than in the fossil diesel fuel–                     efficient than industrial ones. On the other hand, the
methanol–rapeseed oil ME system. Solubility of                           time involved in the formation of the fossil fuels is
                   DEMIRBAS: BIODIESEL FROM VEGETABLE OILS BY SUPERCRITICAL METHANOL                                863

considerably different from that required for the              volumetric efficiency and reduced compression
production of the biomass25. Coconut biodiesel can             temperatures. The oxygen contents of alcohols
yield reductions in net CO2 emissions (80.8– 109.3%)           depress the heating value of fuel in comparison with
relative to petroleum diesel26.                                HC fuels. The heat of combustion per unit volume of
                                                               alcohol is approx half that of isooctane.
Comparison of Fuel Properties and Combustion Characteristics      Methanol is not miscible with hydrocarbons and
of Methyl and Ethyl Alcohols and their Esters                  separation ensues readily in the presence of small
   In general, physical and chemical properties and            quantities of water, particularly with reduction in
performance of EEs are comparable to those of MEs.             temperature. On the other hand, anhydrous ethanol is
EEs and MEs have almost the same heat content.                 completely miscible in all proportions with gasoline,
Viscosities of EEs are slightly higher and CP and PP           although separation may be effected by water addition
are slightly lower than those of MEs. MEs produce              or by cooling. If water is already present, the water
slightly higher power and torque than EEs27. Some              tolerance is higher for ethanol than for methanol, and
desirable attributes of EEs over MEs are significantly         can be improved by the addition of higher alcohols,
lower smoke opacity, lower exhaust temperatures,               such as butanol. Also benzene or acetone can be used.
more injector coking and lower PP.                             The wear problem is believed to be caused by formic
   Ethanol is a more environmentally benign fuel. It is        acid attack, when methanol is used or acetic acid
rapidly oxidized in the body to CO2 and water, and in          attack when ethanol is used. Dry methanol is very
contrast to methyl alcohol no cumulative effect                corrosive to some aluminum alloys, but additional
occurs. Ethanol is also a preferred alcohol in the             water (1%) almost completely inhibits corrosion.
transesterification process compared to methanol               Methanol with additional water (> 2%) becomes
because it is derived from agricultural products and is        corrosive again. Ethanol always contains some acetic
renewable and biologically less objectionable in the           acid and is particularly corrosive to aluminum alloys.
environment. Methanol use in current-technology                   Since alcohols, especially methanol, can be readily
vehicles has some distinct advantages and                      ignited by hot surfaces, pre-ignition can occur. It must
disadvantages. Methanol has a higher octane rating             be emphasized here that pre-ignition and knocking in
than gasoline. It has high heat of vaporization,               alcohol engine is more dangerous condition than
resulting in lower peak flame temperatures than                gasoline engines. Other properties, favorable to the
gasoline and lower nitrogen oxide emissions. Its               increase of power and reduction of fuel consumption
greater tolerance to lean combustion higher air-to-fuel        are as follows: (1) Number of molecules or products is
equivalence ratio results in generally lower overall           more than that of reactants; (2) Extended limits of
emissions and higher energy efficiency. However,               flammability; (3) High octane number; 4) High latent
energy density of methanol is about half that of               heat of vaporization; (5) Constant boiling temperature;
gasoline, reducing the range a vehicle can travel on an        and (6) High density.
equivalent tank of fuel.
   There are some important differences in the                 Biodiesel Economy
combustion characteristics of alcohols and HC.                    Cost of biodiesel fuels varies depending on the
Alcohols have higher flame speeds and extended                 base stock, geographic area, variability in crop
flammability limits. Pure methanol is very flammable           production from season to season, and the price of the
and its flame is colorless when ignited. Alcohols mix          crude petroleum. Biodiesel costs over double the price
in all proportions with water due to polar nature of           of diesel. High price of biodiesel is due to the high
OH group. Low volatility is indicated by high boiling          price of feedstock. However, biodiesel can be made
point and high flash point. Combustion of alcohol in           from other feedstocks, including beef tallow, pork
presence of air can be initiated by an intensive source        lard, and yellow grease. Fatty acid methyl ester could
of localized energy, such as a flame or a spark and            be produced from tall oil, a by-product in the
also, mixture can be ignited by application of energy          manufacture of pulp by Kraft process. Tall oil consists
by means of heat and pressure, such as happens in              of free C18 unsaturated fatty acids (oleic acid, linoleic
compression stroke of a piston engine. High latent             acid and its isomers), resin acids and relatively small
heat of vaporization of alcohols cools air entering the        amounts of unsaponifiables. Tall oil fatty acids are
combustion chamber of engine, thereby increasing the           easily converted into their MEs by reaction with
air density and mass flow. This leads to increased             methanol, whereas resin acids are virtually
864                                   J SCI IND RES VOL 64 NOVEMBER 2005


unesterified due to hindered effect28. With cooking       the presence of water has negative effects on the
oils used as raw material, viability of a continuous      yields of methyl esters. However, presence of water
transesterification process and recovery of high          affected positively the formation of methyl esters in
quality glycerol as a biodiesel by-product are primary    present supercritical methanol method. Supercritical
options to be considered to lower the cost of             methanol has a high potential for both
biodiesel3. With recent increases in petroleum prices     transesterification of triglycerides and methyl
and uncertainties concerning petroleum availability,      esterification of FFAs to methyl esters for diesel fuel
there is renewed interest in vegetable oil fuels for      substitute.
diesel engines.                                              Vegetable oils can be used as fuel for combustion
   Most biodiesels uses soybean oil, methanol, and an     engines, but its viscosity is higher than usual diesel
alkaline catalyst. Soybean oil as a food product cannot   fuel and requires modifications of the engines.
produce a cost-effective fuel. However, large amounts     Therefore, vegetable oils are converted into their
of low-cost oils (restaurant waste) and fats (animal      methyl esers (biodiesel) by transesterification. The
fats) could be converted to biodiesel. But these low      viscosity values of vegetable oils are between 27.2
cost oils and fats often contain large amounts of FFA     and 53.6 mm2/s, whereas those of vegetable oil
that cannot be converted to biodiesel using an alkaline   methyl esters are between 3.59 and 4.63 mm2/s. The
catalyst29.                                               viscosity values of vegetable oil methyl esters highly
   A review of 12 economic feasibility studies shows      decrease after transesterification process. The flash
that the projected costs for biodiesel from oilseed or    point values of vegetable oil methyl esters are highly
animal fats have a range US$0.30-0.69/l, including        lower than those of vegetable oils. An increase in
meal and glycerin credits and the assumption of           density from 860 to 885 kg/m3 for biodiesels increases
reduced capital investment costs by having crushing       viscosity from 3.59 to 4.63 mm2/s.
and/or esterification facility added onto an existing
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