When production of biodiesel is by
transesterification, the raw material may need to
be blended prior to process. This is dictated by the specific
properties of the various vegetable oils, which may also be
affected by temperature and seasonal variation.
Although the raw material is not liable to excise duty at
this stage, the trader would still be expected to exercise
effective stock control and maintain accurate production records to
demonstrate that the materials have been put to biodiesel
production.
(This text has been withheld because of exemptions in the
Freedom of Information Act 2000)
(This text has been withheld because of exemptions in the
Freedom of Information Act 2000).
Transesterification involves the identification of
the quantity of the catalyst (caustic soda) required to convert a
given batch of vegetable oil (often delivered to a user
specification concerning the Free Fatty Acid (FFA) content). If the
FFA factor is high, the oil will not readily convert. The free
fatty acids in vegetable oil change the more it is heated, e.g. for
frying foods.
The quantification is established via ‘titration’
using a small known volume of oil dissolved in a known volume of
alcohol (e.g. IPA). The caustic is dissolved in water and added in
millilitre amounts until the pH reaches around 8/9.
The proportion of caustic soda is determined by adding
millilitres to a given factor to compute the grams of catalyst
required per litre of oil (specification).
The caustic is usually pre-mixed with methanol (10/15% by
weight in relation to the mass of oil to be converted). The mixing
of caustic soda generates considerable heat. The sodium methoxide
produced is added to the pre-heated oil and vigorously mixed. As
the reaction progresses, conversion and separation take place and
crude glycerol sinks to the bottom of the reaction vessel. The
biodiesel (methyl ester) rests on top.
The glycerol is drained off and the biodiesel may be washed
with water to remove any soaps or salts and to neutralise the
effect of any excess alkali present. The solution is allowed to
separate before the biodiesel is filtered/centrifuged, ready for
use. The purpose of the reaction is to separate the fatty acid
chains (typically palmatic, stearic oleic and linolic acids).
