1. INTRODUCTION:
Chemical transesterification or
alcoholysis of triglycerides or the esterification of free fatty acids using
short-chain alcohols in the production of alkyl esters or biodiesel was first
reported1 on August 31st 1937 in a Belgian Patent by
Chavanne of the University of Brussels2. In the early 1940s
researches that did not actually anticipate the production of alkyl esters as
fuel were patented3-8. The original objective of the work was to
develop a simplified method for extracting glycerol during soap production9.
Glycerol was needed for wartime explosives production. Glycerol could be
separated since it is insoluble in the esters and has a much higher density
that makes settling or centrifugation a choice process in its removal.
Transesterification reaction of vegetable oils can be represented as in Scheme
1.
Scheme 1: A typical Transesterification
reaction
The reaction does not proceed to
any appreciable extent in the absence of catalysts or supercritical condition.
Various homogeneous and heterogeneous catalysts, ranging from base, acid to
enzyme10, 11 as well as carbon catalysts produced from sugar starch
and cellulose have been developed for use in biodiesel production. It is an
alternative to petroleum diesel for reducing emissions of gaseous pollutants
such as COx, SOx, particulate matter and organic
compounds12,13. Considering that it is made from renewable resources,
its production provides a mean to recycle CO2 14, 15. Its
properties are similar to petroleum-based diesel, allowing its use either as a
substitute for diesel fuel or more commonly in fuel blends13, 16, 17.
2. HOMOGENEOUS CATALYSIS:
There are several routes to
obtain biodiesel using oilseeds, edible oils, animal fats and waste oils and
greases as feedstock. Transesterification of triglycerides with low molecular
weight alcohols catalyzed by homogeneous catalysts is the most used one18–22.
This route has some advantages, like low cost and mild reaction conditions.
Since, alkali-catalyzed transesterification is much faster i.e, about 4,000
times than the acid-catalyzed transesterification12,18,19. Basic
catalysts such as alkaline hydroxides, methoxides and carbonates are more often
used to promote the reaction. However, the overall base-catalyzed process
suffers from serious limitations concerning strict feedstock specifications12,
especially the content of free fatty acids (FFA) and water. Saponification
reaction is an undesired reaction which may be promoted, depending on the
reaction conditions and the free acid content of the vegetable oil used. The
soap decreases selectivity toward biodiesel, inhibits separation of the alkyl
esters and glycerol and contributes to emulsion formation during the water wash
10, 23, 24. Esterification of FFA with low molecular weight alcohols
is another route to produce biodiesel and can be used as a pretreatment for
basic transesterification reaction to convert the FFA into methyl esters and
avoid saponification, especially when FFA content is higher than 1% w/w12,
18, 19,22 . Some authors have shown that alkyl esterification of fatty
acids is faster than transesterification of triglycerides 25, 26.
This observation was assigned to the fact that alkyl esterification is a kind
of one step reaction, while transesterification of triglycerides consists of
three stepwise reactions, with diglycerides and monoglycerides as
intermediates, and the presence of glycerol. The much higher solubility of
fatty acids in low chain alcohols may also be related to this observation. In
some cases, fatty acids are by-products of the food processing, like in the
edible Palm Oil-based oleochemical industry, which produces 4–8% of FFA of total
crude Palm Oil in the physical refining. The recovery of fatty acids residue is
difficult and not economically feasible27. Therefore, its
alternative usage as feedstock for biodiesel production looks promising.
2.1Base-catalyzed process:
Several conventional and
non-conventional base-catalyzed transesterification processes have been
reported in a review by Knothe et al28. Boiler ashes, potassium
hydroxide (KOH) amongst other catalysts were successfully used in the
ethanolysis and methanolysis of palm and coconut oils with yields as high as
90%29-33. It has also been reported that methyl and ethyl esters
with 90% yield can be obtained from palm and coconut oil from the press cake
and oil mill and refinery waste with the ashes of the wastes (fibers, shell, and
husks) of these two oil seeds, and with lime, clay, zeolites, etc34-37.
Methanolysis has been reported to yield 96-98% esters when palm oil is refluxed
2hrs. Using coconut-shell ash and other ashes from the combustion of plant
wastes such as fibers of palm tree that contain potassium and sodium carbonate38.
Calcium oxide on magnesium oxide has been shown, at 600C-63 0C,
to be the best catalyst system amongst potassium carbonate, sodium carbonate,
iron (III) oxide, sodium methoxide, sodium aluminate, zinc, copper, tin, lead
and zinc oxide in the methanol transesterification of low-erucic rapeseed oil39.
Generally, the mechanism of the
base-catalysed transesterification of vegetable oils involves four steps. The
first step is the reaction of the base with the alcohol, producing an alkoxide
and the protonated catalyst. The second step is the nucleophilic attack of the
alkoxide at the carbonyl group of the triglyceride generating a tetrahedral
intermediate40-42. The third step involves the formation of the alkyl
ester and the corresponding anion of diglyceride. The final step involves
deprotonating the catalyst, thus regenerating the active species, which is now
able to react with a second molecule of the alcohol, starting another catalytic
cycle. Diglycerides and monoglycerides are converted by the same mechanism to a
mixture of alkyl esters and glycerol. The above steps are summarized in
Scheme 2. The base-catalyzed transesterification of vegetable oils are reported
to proceed faster than the acid- catalyzed reactions43Because of
this and the fact that the alkaline catalysts are less corrosive than acidic
compounds, industrial processes usually favour base catalysts, such as alkaline
metal alkoxides44 and hydroxides45-47 as well as sodium
or potassium carbonates34,48.
Scheme 2: Mechanism for Base
catalysed transesterification process
Alkaline metal alkoxides (as CH3ONa
for the methanolysis) are the most active catalysts. They give yields greater
98% in a relatively short reaction time of 30 min. even at low molar
concentrations of about 0.5 mol%, but their requirement of the absence of water
makes them inappropriate for typical industrial processes in which water cannot
be avoided completely49. Alkaline metal hydroxides (e.g KOH and
NaOH) are cheaper than metal alkoxides, but less active. Nevertheless, they are
good alternatives since they can give the same high conversions of vegetable
oils just by increasing the catalyst concentration to 1 or 2 mol%. However,
even if water-free alcohol/oil mixture is used, some water is produced in the
system by the reaction of the hydroxide and the alcohol. The presence of water
gives rise to hydrolysis of some of the produced ester (Scheme 3), with
consequent soap formation49.
Scheme 3: Hydrolysis of ester
and formation of soap by the presence of water
The typical process for
continuous production of biodiesel is shown in the scheme-4 using homogeneous
catalysis50. Especially, in case of waste vegetable oil, when using
alkali catalyst, the first step involves removal of water. Strong alkali is
added to methanol solution in a closed reaction vessel, vegetable oil is added
and the vessel is then sealed to prevent alcohol losses. The temperature is
raised to about 550 C – 700 C to promote the reaction.
Competing reactions can be minimized by adding excess alcohol, beyond the
stoichiometric amounts needed to react with the triglycerides50. As
shown, the glycerol is separated from the mixture after the transesterification
stage. The higher density of glycerol allows gravity separation, though some
operations include centrifugal separation to speed up the process. Residual
alcohol in the glycerol is distilled and reused. The separated oil phase is
washed with water to remove any undesired soap which may have formed during the
reaction. Fatty acids have become the by-product of the continuous alkali
catalyzed process, rather than being converted to biodiesel.
Scheme 4: Typical process for
continuous production of biodiesel using alkali catalyst
Shortcomings of homogeneous alkali catalyzed
processes:
Reports already reviewed showed
that base-catalyzed transesterification of vegetable oils results in good
yields of the esters. Nevertheless, there are obvious problems encountered by
their use. Some of these problems have been identified to include:
·
High energy
demand.
·
Post-reaction
treatment to remove the catalyst from the product-biodiesel.
·
Interferences
occasioned by the presence of free fatty acid and water during the reaction.
·
Difficulty
in the recovery of glycerol after the reaction and
·
Post-reaction
treatment of the alkaline waste-water to obviate the environmental effects of
its disposal.
The development of acid and
heterogeneous catalyst systems, some of which run in continuous reactors, have
addressed many of these problems which ordinarily meant higher production costs
and less economic viability relative to petroleum-based diesel.
2.2Acid-catalyzed process:
The mechanism of the
acid-catalyzed transesterification of vegetable oils is as shown in Scheme 5
for a monoglyceride. The protonation of the carbonyl group of the ester leads
to the carbocation which after a nucleophilic attack of the alcohol produces
the tetrahedral intermediate. This in turn eliminates glycerol to form the new
ester, and to regenerate the catalyst. The mechanism can be extended to di and
triglycerides51. Carboxylic acids can be formed by reaction of the
carbocation with water present in the reaction mixture. This suggests that an
acid-catalyzed transesterification should be carried out in the absence of
water, in order to avoid the competitive formation of carboxylic acids, which
reduce the yield of alkyl esters.
Scheme 5: A typical mechanism of
acid catalysed transesterification of vegetable oils
The transesterification process
in biodiesel production is catalyzed by Bronsted acids like HCl, BF3,
H3PO4, H2SO4 and sulphonic acids52,53.
Preferably, sulphonic and sulphuric acids are mostly used. These catalysts give
very high yields in alkyl esters, but the reactions are slow, requiring
typically, temperatures above 100 oC and from 3-48 h reaction time
to reach complete conversion54-59. It was shown that the
methanolysis of soybean oil, in the presence of 1 mol% of H2SO4,
with an alcohol/oil molar ratio of 30:1 at 65 oC, takes 50h to reach
complete conversion of the vegetable oil (>99%), while the butanolysis (at
117o C) and ethanolysis (at 78oC) using the same
quantities of catalyst and alcohol take 3 h and 18 h, respectively43.
Reaction rates in acid-catalyzed
processes may be increased by the use of larger amounts of catalyst. Typically,
catalyst concentrations in the reaction mixture have ranged between 1 and 5 wt
% in most academic studies using sulphuric acid43. Different amounts
of sulphuric acid were used (1, 3 and 5 wt %) in the transesterification of
grease with methanol and reported60. In these studies, a rate
enhancement was observed with the increased amounts of catalyst and ester yield
went from 72.7% to 95.0% as the catalyst concentration was increased from 1% to
5 wt%. The dependence of reaction rate on catalyst concentration has been
further verified by same authors and other groups61,62. A further
complication of working with high acid catalyst concentration becomes apparent
during the catalyst neutralization process, which precedes product separation.
The liquid acid-catalyzed transesterification process does not enjoy the same
popularity in commercial applications as its counterpart, the base-catalyzed
process. The fact that the homogeneous acid-catalyzed reaction is about 4000
times slower than the homogeneous base-catalyzed reaction has been one of the
main reasons68. However, acid-catalyzed transesterification holds an
important advantage with respect to base–catalyzed ones; the performance of the
acid catalyst is not strongly affected by the presence of free fatty acids in
the feedstock. Thus, a great advantage with acid catalysts is that they can
directly produce biodiesel from low-cost feedstocks, generally associated with
high free fatty acid concentrations. A two step esterification process in which
the free fatty acid is converted to fatty acid methyl esters in an
acid-catalyzed treatment followed by base-catalyzed process has been proposed61.
In the first stage, the free fatty acids are converted into esters. In the
second stage, alkali-catalyzed transesterification of triglycerides present in
the mixture is employed as shown in the scheme 6. The latter reaction can be
completed in a much less time than would be possible with acid-catalyzed
transesterification alone. Though acid catalysis of transesterification
provides a way to avoid undesired saponification reactions, the reaction rates
tend to be much lower, in comparison to the alkaline system. The ester
conversion is strongly inhibited by the presence of water in the oil. If the
water content is greater than 0.5%, the ester conversion drops below 90%
efficiency61.
The acid-catalyzed process is
thought to be more suitable for the production of biodieselfrom low feedstocks
(used frying oil, waste animal fat), mainly because of the fact that these
feedstocks contain greater amounts of free fatty acids (FFAs)63. The
base- and acidcatalyzed transesterification processes were compared with respect
to the FFAs content of the feedstock. The greater tolerance of an acid catalyst
to the FFA content compared to an alkaline catalyst was confirmed in a report
by Canakci and Van Gerpen66. They also showed that acid catalyzed
reactions are more susceptible to water content of the feedstock than the
base-catalyzed process and that the presence of more than 0.5% water in the oil
will decrease the ester conversion to below 90%61,64,65. The fact
that the water content is more crucial in acid catalysis than in alkaline
catalysis is mainly caused, according to Siakpas et al63, by the
greater affinity of water by sulphuric acid, which will lead to the acid
catalyst preferentially interacting with water rather than alcohol with the
consequent deactivation of the catalyst. Also, there is evidence that large
quantities of acid catalyst in biodiesel production may lead to ether formation
by alcohol dehydration60 and the consequent high use of calcium
oxide in the acid neutralization after production with its attendant high
production cost and waste generation. It has been suggested that acid-catalyzed
transesterification achieves greater and faster conversions at high alcohol
concentrations67.
3. SCOPE FOR FUTURE WORK:
Future work can be focused using
combination of bases, acid and alcohols.
4. ACKNOWLEDGEMENT:
The authors would like to thank
the Principal Dr. K.V.L.Raju and the management of MVGR College for their
constant support and encouragement. The authors would also like to thank Prof
Ch.Durga Prasada Rao, Retired Professor, IIT-Chennai for useful discussions.
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