27.8 How can I convert cooking oil into diesel fuel?
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This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
27.8 How can I convert cooking oil into diesel fuel?
Diesel engines can run on plant and animal triglycerides such as tallow
and seed oils, however most trials have resulted in reduced engine life, or
increased service costs. The solution is to transesterify the triglycerides
into esters, taking care to avoid the formation of monoacylglycerides
that will precipitate out at low temperatures or when diesel is encountered.
There are several plants in Austria that produce Rapeseed Oil Methyl Esters
as fuels for diesel engines. The economics of the process are very
dependant on the price of diesel and the market for the glycerol byproduct.
The common catalysts used to transesterify triglycerides are sodium
hydroxide, sodium methoxide and potassium carbonate. If the esters are to
be blended with diesel fuel, then a two stage reaction is usually required
to ensure that monoacylglycerides are kept below 0.05%. Usually this
involves using 22g of methanol ( containing 0.6g of sodium hydroxide ) and
100g of tallow refluxed for 30 minutes. The mixture is cooled, the glycerol
layer removed, and a further 0.2g of sodium hydroxide is reacted for 5
minutes at 35C in a stirred reactor. The glycerol phase is allowed to
separate, and the ester phase is washed with water to remove residual
catalyst, glycerol and methanol. Note that sodium hydroxide is the most
cost-effective catalyst, but also has the worst tendency to form soaps.
The catalyst and methanol can be of industrial grade without further
purification required, however care should be taken to prevent additional
water entering the reaction [10].
The fuel can be converted into other esters, such as ethyl and butyl, but
it really depends on the availability of cheap alcohol along with the
desired properties of the fuels. The effect of catalysts, reagent ratio,
temperature, and moisture on the production of methyl, ethyl, and butyl
esters from some common oils has been investigated [11]. The New Zealand
government investigated a wide range of techniques for turning various
vegetable and animal triglycerides into esters for diesel, and the reports
cover many aspects of the kinetics and efficiencies [12]. There is a general
overview of the current processes and technology available in Inform [13].
A specific technique for analysing the monoglycerides has been published
[14], however I have found that acetylation followed by narrow bore
( 0.1mm ID ) capillary chromatography is faster and cheaper.
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