PhD Thesis - Energy Systems Research Unit - University of Strathclyde

2.5.3 Transesterification
The use of vegetable oils (straight or blended) in a diesel engine gives higher
particulate emissions, reduced efficiency, and greater maintenance requirements
compared to the use of conventional diesel. However, if these oils are
transesterified to make methyl or ethyl esters (biodiesel), increased relative
efficiency, and lower particulate emissions can be achieved, with little or no
engine modifications necessary [68]. The transesterification process can use
waste vegetable or animal oils, or specially grown energy crops with high oil
yields (including soybeans, oilseed rape, olives, or sunflowers) as a feedstock.
Where energy crops are used, energy in the forms of heat and/or electricity is
required for oil extraction from the crop. The residue left after oil extraction can
be used as cattle feed, put into an anaerobic digester or gasifier, or burned
directly to produce heat and/or electricity, depending on the residue type, and
energy requirements [69].
Small-scale production of biodiesel is batch-wise, where batches of up to 2000
litres can be processed, each batch taking around 10 hours to complete. Figure
2.6 shows the stages of a typical batch process for producing ‘backyard
biodiesel’. If necessary, the oil is first filtered to remove particles, and boiled
for between 30 to 60 minutes to remove any water, which is often required when
waste oil is being used. Ethanol or methanol (between 10 to 15% of the amount
of oil by volume) and a catalyst (sodium hydroxide or potassium hydroxide,
around 1% by weight) are mixed together and added to the oil. This is mixed
mechanically for around one hour, and then allowed to settle for about eight
hours, to allow separation of the biodiesel and glycerine. The glycerine is
drained off, and any soap in the mixture is removed, if necessary, by adding
water and mixing, or bubbling air through the biodiesel. This, again, is allowed
to settle, and the biodiesel is drained off, leaving the soapy water. This process
yields around the same volume of biodiesel as oil that was input, and around the
same volume of glycerine as alcohol that was input [24,70,71]. Glycerine is a
valuable by-product that can be sold to the pharmaceuticals or explosives
industry or used to make soap or cosmetics.
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