Note: Descriptions are shown in the official language in which they were submitted.
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"METHOD FOR PRODUCTION OF ALKYL ESTERS "
FIELD OF THE INVENTION
The present invention relates to a method for production of alkyl esters from
mixtures
of glycerides and free fatty acids, in particular mixtures of glycerides and
free fatty
acids from grease trap waste, tallows, animal fats, vegetable lipids and other
high
fatty acid content lipids. In addition to these lipids it has been found to be
very
effective in efficiently producing alkyl esters from lower fatty acid, higher
glyceride
content lipids.
BACKGROUND OF THE INVENTION
Alkyl esters of fatty acids found in natural lipids, commonly known as
biodiesel, are
becoming increasingly recognised as viable fuel alternatives for diesel
engines. They
are known to reduce the output of particulate, hydrocarbon pollutants and
improve the
lubrication of diesel engines. Biodiesel may also be successfully used as a
lubricant,
a hydraulic fluid and as a mufti-purpose solvent.
Furthermore, biodiesel is attractive from a commercial and environmental
viewpoint
because the raw materials applicable to this invention may be sourced from
renewable
oilseed feedstocks, used vegetable oils, palm oil, coconut oil, soapstock
materials as a
lipid-rich byproduct of vegetable oil refining, tallows, animal fats, or from
grease trap
waste which would normally be disposed of in land fill and other waste
disposal
facilities.
The widespread adoption of biodiesel as an alternative to petroleum-based
diesel fuel
is hampered by inefficient methods for the production of biodiesel. Known
methods
for the production of biodiesel are limited by incomplete esterification of
all fatty
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acids in the starting material, lengthy purification methods such as water
washing,
relatively long reaction times, contamination and separation difficulties
associated
with co-production of glycerol, and saponification of the starting material
under
certain reaction conditions.
The Boocock method for formation of methyl esters from vegetable oils has
sought to
accelerate reaction times for a two-step acid then base-catalysed methanolysis
of
vegetable oils by forming a single phase reaction mixture using a cosolvent
such as
tetrahydrofuran (THF) or methyltetrabutylether (MTBE). Boocock reports that a
99%
recovery of methyl ester is achieved with this method. Glycerol is a byproduct
and
the resultant alkyl ester normally requires extensive purification including
water
washing of the alkyl ester.
The Haas et al method for synthesis of fatty acid methyl esters from soapstock
involves another two-step process wherein the soapstock is first saponified
and then
esterified in an acidic alcohol solution. Haas et al reports that a 60%
recovery of fatty
acid methyl ester is achieved with this method. The method is characterised,
however, by high consumption of the reagents.
The present invention attempts to overcome at least in part some of the
aforementioned disadvantages and to provide new advantages including low or
nil
glycerol production and the method is amenable to low capital plant costs and
plant
mobility.
SUMMARY OF THE INVENTION
In accordance with a first aspect of the present invention there is provided a
method
for production of alkyl esters from a mixture of glycerides and free fatty
acids,
comprising:
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a) addition of an acid to an alcoholic solution of the mixture to decrease the
pH of
the solution to about 1 to 2 to effect acid-catalysed alkyl esterification;
b) treatment of the solution from step a) with a concentrated alkoxide
solution to
raise the pH of the solution to about 12 to effect base-catalysed
transesterification
of glycerides contained in the mixture;
c) treatment of the solution from step b) with acid to decrease the pH of the
solution
to about 2 to effect acid-catalysed esterification of residual saponified by-
products
from step b);
d) removal of alcohol from the solution from step c); and
e) separation of resulting alkyl esters.
DESCRIPTION OF THE INVENTION
The present invention describes a method for reacting a mixture of free fatty
acids and
glycerides, commonly found in grease trap waste, tallows, animal fats, palm
oil,
coconut oil, and other naturally occurring lipids, including waste or fresh
unused
vegetable oils and tallows with short-chain alcohols such as methanol,
ethanol,
propanol, or butanol to produce corresponding alkyl esters.
It will be understood that grease trap waste encompasses a highly variable
mixture of
organic fats, greases and vegetable oils, comprising a high free fatty acid
and mono-,
di-, and triglyceride content, contaminated by detergents, water, soaps,
inorganic and
particulate matter. Throughout this specification the term free fatty acid
(FFA) and
glycerideswill be taken to mean the total content of free fatty acids and mono-
, di-,
and triglycerides comprised in grease trap waste, tallows, animal fats, and
other
naturally occurring lipids, and available for alkyl esterification. Typically,
the grease
trap waste is between 80 to 100% free fatty acid and glycerides.
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The alcohol includes low-boiling point alcohols, preferably methanol, ethanol,
propanol, and butanol in their single-chain and isomerised forms. However, it
is
envisaged that longer-chain alcohols can be utilised in a similar fashion in
accordance
with the present invention.
A mixture of free fatty acids and glycerides, in the form of grease extracted
from
grease trap waste, tallows, animal fats, and other naturally occurring animal
or
vegetable lipids, is mixed and agitated in the alcohol to form a miscible
single-phase
alcoholic solution of the mixture.
Depending on the composition of the mixture, an immiscible suspension of
lipids may
form in the alcohol. In this case, the suspension is continually agitated to
prevent
separation of the lipid and alcohol into two distinct phases. For the purposes
of the
invention, the agitated suspension is to be regarded and treated as an
alcoholic
solution of the mixture of lipids and glycerides.
Neither type of mixture is dependent on, or requires a co-solvent in the
process.
However, use of such co-solvents will not impair the reaction or its final
alkyl ester
product. Co-solvent usage may marginally increase the reaction rate but the
reaction
is quite rapid and goes to completion in the absence of co-solvent.
Preferably, the mixture of free fatty acids and glycerides is dissolved in a
similar
volume of alcohol to afford about 50% v/v alcoholic solution of the mixture.
In this
way, the alcohol is in molar excess of the total free fatty acid content,
preferably in a
range of 1:10 to 1:25.
The reaction proceeds satisfactorily with a lesser or greater proportion of
alcohol to
lipids, however the reaction proceeds optimally at about 50% v/v.
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Certain mixtures of free fatty acids and glycerides, such as tallow and
vegetable oils,
may not form a single-phase alcoholic solution. However, vigorous agitation of
the
two-phase homogeneous solution throughout the reaction procedure effects
similar
results with regard to alkyl ester production.
The alcoholic solution of the mixture is first treated with an acid catalyst
to effect
acid-catalysed alkyl esterification, then treated with a base catalyst to
effect base
catalysed transesterification of glycerides contained in the mixture. The
resulting
reaction mixture is finally treated with an acid catalyst to effect acid-
catalysed alkyl
esterification of residual saponified by-products resulting from the previous
treatment
steps.
Preferably, the acid-catalysed esterification and base-catalysed
transesterification
steps are performed under reflux conditions, although said steps may also be
performed at ambient temperatures, albeit with longer reaction times unless
high
pressures are used.
The method of the present invention is preferably performed at ambient
atmospheric
pressure. However, it is envisaged that said method can be successfully
performed
under higher pressures with use of a pressure'vessel, or under autoclave
conditions.
Acid catalysts include, but are not limited to, concentrated strong inorganic
acids such
as sulphuric acid, hydrochloric acid, phosphoric acid, and perchloric acid.
Preferably,
the ratio of added acid-catalyst to total fatty acid content is about 1:20 in
the first
acid-catalysed alkyl esterification reaction step, and about 1:5 in the second
acid- _.
catalysed esterification reaction step.
Base catalysts include, but are not limited to, alcoholic solutions of sodium
hydroxide
and/or potassium hydroxide, or sodium or potassium dissolved in the short-
chain
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alcohol of choice. Preferably, a 10% w/w alkali metal alkoxide solution is
used as the
base catalyst, such that the ratio of added base-catalyst to total fatty acid
content is
about 1:5.
The alcohol is removed from the resulting reaction mixture by distillation or
by
separation with microfiltration membranes, with or without vacuum assistance,
leaving a two-phase heterogeneous mixture of alkyl esters and a solid phase.
The
alkyl esters are separated from the solid phase by decantation, and further
purified by
filtration or separated by filtration alone.
Once the alcohol has been removed from the reaction mixture by distillation or
microfiltration, and the alkyl ester has been separated from the solid phase
by
decantation or filtration, the alkyl ester can be treated with a weak base to
effectively
neutralise residual acid remaining in the alky ester phase, or alternatively
by ion
exchange or microfiltration. It is envisaged that the pH of the alkyl ester
phase will
be neutralised to about pH 6-7.
The neutralisation step is achieved by addition of small amounts of sodium
bicarbonate or calcium carbonate or magnesium carbonate to the alkyl ester
phase. It
is envisaged that other well known means for neutralising pH such as elution
of the
liquid phase through an ion exchange resin or membrane separation, can be
successfully employed to effect neutralisation of residual acid within the
alkyl ester
phase.
Residual water can then be removed by conventional means such as passing the
alkyl
ester phase over a hygrosopic dessicant or by heating the alkyl ester
sufficiently to
remove steam at atmospheric pressures or under vacuum or by microfiltration.
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Depending on the final intended use of the alkyl ester, a small amount of
entrained
water may be acceptable and its removal may not be necessary.
Surprisingly, the inventors have found that the alkyl esters produced by the
method of
the present invention remain uncontaminated by glycerol byproducts from the
acid
s catalysed esterification and base-catalysed transesterification reactions of
this
invention. An exception was found from the treatment of fresh unused vegetable
oil
whereby a small quantity of good quality glycerol was readily separated. The
amount
of glycerol byproduct was at less than 8% of the starting oil and noticeably
less than
the amount generated by other esterification methods. No noticeable glycerol
production occurred when treating other lipids such as palm oil, coconut oil,
animal
tallow or grease trap waste. The residual product after alkyl ester separation
showed
no signs of glycerol by gas chromatography testing.
The present invention will now be illustrated according to the following
example.
Example
A single phase 1:1 v/v methanolic solution of grease trap waste (800 ml) in
methanol
(800 ml) was prepared. A GC/MS analysis of the grease trap waste indicated
that the
major components of the grease trap waste were octadecanoic acid and
hexadecanoic acid. The pH of the methanolic solution of grease trap waste was
4,
reflecting a high fatty acid content.
Concentrated sulphuric acid (98%) (8.5 g) was added with stirring to the
methanolic
solution resulting in a solution pH of between l and 2. The methanolic
solution was
refluxed for 30 minutes.
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A sodium methoxide solution of 10% w/w NaOH in methanol (160 ml) was then
added dropwise to the stirred.refluxing methanolic solution resulting in a
final pH of
12.
Concentrated sulphuric acid (98%) (21 ml) was added dropwise to the stirred
refluxing methanolic solution resulting in a final pH of 2.
Excess methanol was removed by distillation. The resulting methyl esters (825
ml)
were separated from a semi solid basal residue (64.5 g) by decantation, and
finally
filtered to remove fine sediment. It was estimated that there was a better
than 91
conversion of waste trap grease to methyl esters.
A similar amount of grease trap waste was treated according to the present
invention
as described in the abovementioned Example wherein ethanol, rather than
methanol,
was used as the solvent. It was found that a similar degree of conversion of
the grease
trap waste to corresponding ethyl esters was achieved.
Furthermore, similar experiments using a potassium alkoxide solution rather
than a
sodium hydroxide solution produced similar results as those exemplified in the
abovementioned Example.
Although the residual sediment formed from the reaction mixture using ethanol
and
potassium hydroxide was less crustiform than the sediment described in the
Example,
it was readily separated from the ethyl esters by filtration.
On testing, the crustiform residue was found to comprise mostly sodium
sulphate with
minor entrained methyl ester that combined with the sodium sulphate totalled
9% by
weight of the starting grease trap waste. The conversion of grease trap waste
to
methyl ester therefore exceeded 91%. The entire processing time to final
product
alkyl ester is less than 1 hour. Furthermore, by increasing the amount of acid
in the
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first step it was found that the reaction time was accelerated thereby
reducing the
overall reactions processing time.
The process is not sensitive to or damaged by residual water contaminating the
starting lipid.
It is a rapid and efficient process and method that can be made even faster by
increasing reaction kinetics by increasing the catalytic dosage amounts and/or
many
well known methods including increased heat, pressure and agitation.
The final product alkyl ester does not require water washing thereby
eliminating a
major processing step common to other methods that is both time consuming and
polluting. Glycerol byproduction is eliminated in most lipid feedstocks
tested. The
method and process permits design of commercial plants that should be low in
capital
cost, small in size, with high production rates and be portable as truck
mounted,
trailer-coupled units, or as a static plant.
Modifications and variations as would be apparent to a skilled addressee are
deemed
to be within the scope of the present invention. The process and method allows
the
alkylesterification of various stated lipids.
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