Biodiesel
consists of
the
monoesters
formed when
the
triglycerides
react with
an alcohol
such as
methanol.
What are
esters and
monoesters?
Esters
are a type
of chemical
compound
that
contains the
following
grouping of
carbon and
oxygen:
|
3333333333333O
||
-
C
-
O - |
One example
is methyl
palmitate:
erereere O
||
CH3(CH2)14
- C
- O
- CH3
|
dO
||
Here the
CH3(CH2)14
- C - O -
part of the
molecule
represents
the palmitic
acid and the
CH3
designates
this as a
methyl
ester. Other
examples of
ester
molecules
are:
Note that
each of
these
example
contains
only one
occurrence
of the ester
functional
group
All
of these
examples are
known as
monoesters.
Other
organic
molecules
can contain
more than
one
occurrence
of the ester
group, such
as the
triglyceride
shown below
Note that
this
molecule
contains the
ester group
three
times.
Thus, it is
called a
triester.
Sometimes
people refer
to the
reaction
that
converts oil
or fat
(triglyceride)
to methyl
esters
(biodiesel)
as
"esterification"
as if the
reaction
were
converting
something
that is not
an ester
into an
ester. In
reality, the
reaction
converts one
type of
ester into
another type
of ester.
This is why
the reaction
is more
properly
known as
transesterification.
Transesterification
Transesterification
is the
process of
reacting a
triglyceride
molecule
with an
excess of
alcohol in
the presence
of a
catalyst (KOH,
NaOH, NaOCN3,
etc.) to
produce
glycerin and
fatty
esters. The
chemical
reaction
with
methanol is
shown
schematically
below.
Triglyceride
+
methanol
mixture
of fatty
esters +
glycerol
In this
transesterification
reaction,
one mole of
triglyceride
reacts with
3 moles of
methanol to
produce
three moles
of fatty
esters
(biodiesel)
and one mole
of
glycerol.
On a weight
basis, the
mole of
triglyceride
will weigh
about 884
grams, the
three moles
of methanol
about 96
grams, the
resulting
three moles
of biodiesel
will weight
about 888
grams and
the mole of
glycerol
will weigh
92 grams.
Most
biodiesel
producers
will add
from 1.6 to
2.0 times
the
chemically
correct
amount of
methanol to
ensure that
the reaction
is driven to
completion.
This means
that a
substantial
amount of
methanol
will be left
in the
products
that should
be removed
and
recovered.
Expressed on
the basis of
100 grams of
oil, the
mass balance
for the case
of 100%
excess
methanol
becomes:
100 g
soybean oil
+ 21.7 g
methanol
100.4 g
biodiesel +
10.4 g
glycerol +
10.9 g
methanol
|