Some Basic Questions about
Biodiesel Production
Jon Van Gerpen
Department of Biological and Agricultural
Engineering
University of Idaho
2012 Collective Biofuels Conference
Temecula, CA
August 17-19, 2012
1
Views of Biodiesel Production
• How do we usually look at biodiesel
production?
• Can we learn something from looking at it
in a new way?
o From a microscopic perspective.
2
How do we usually talk about biodiesel
production? We always start with the chemical
equation.
O
||
CH2 - O - C - R1
|
|
O
|
||
CH - O - C - R2 + 3 CH3OH ----------->
|
(KOH)
|
O
|
||
CH2 - O - C - R3
Triglyceride
methanol
O
||
CH3 - O - C - R1
O
||
CH3 - O - C - R2
+
O
||
CH3 - O - C - R3
mixture of fatty esters
CH2 - OH
|
CH - OH
|
CH2 - OH
glycerin
Transesterification
3
Biodiesel Production – 3 basic
steps
Oil
Methanol
Catalyst
Reactor
Sometimes we
recognize that there are
other steps in the
process.
Settling
tank
Clean
up
Glycerin
Finished
biodiesel
4
Oil
Biodiesel Production Schematic
Finished
biodiesel
Methanol
Catalyst
Reactor
Separator
Methyl
esters
Neutralization
and methanol
removal
Dryer
Water
washing
Acid
Acid
Free
fatty
acids
Glycerin
(50%)
Acidulation
and
separation
Methanol
removal
Crude glycerin
(85%)
Wet
methanol
Water
Gray
water
Methanol/water
Sometimes
rectification
Wet methanol
Methanol
storage
we want to
look at
Water
individual
processes.
5
Microscopic perspective
• To look at biodiesel production in a
different way, let’s look at what is
happening on a small scale.
• Questions we should ask:
o What affects the speed of reaction and why?
o How can we guide the reaction to give higher
yield and purity?
6
During the Reaction
Methanol solubility in triglyceride
Most biodiesel
producers
operate at
atmospheric
pressure close to
the boiling point
of methanol
(65°C). Solubility
is 10% but we
add about 20%.
7
Binary phase diagram
Concentration
of oil in
methanol
phase
Concentration of
methanol in oil phase
Cerce, T., S. Peter and E. Weidner. 2005.
8
Initial Reaction Conditions
Due to its low solubility, 50-70%
of the methanol is distributed
throughout the oil as small
droplets. The catalyst tends to
be more attracted to the
methanol.
Methanol
+
Catalyst
Oil
Oil
Methanol
Methanol
+
+
Catalyst
Catalyst
Methanol in the
oil phase is
about 8-10%. Oil
in the methanol
droplets is <1%.
Oil
9
Later stages of the reaction
Glycerol
The glycerol has low
solubility in the oil and
methyl esters. As more
glycerol is produced
during the reaction, it
will attract the catalyst
and form small droplets.
Methyl
esters
+
catalyst
Glycerol
Methyl
esters
+
catalyst
Glycerol
+
catalyst
Methyl
esters
10
We have always referred to biodiesel
production as a “mass transfer limited
reaction.” But is this right?
Oil
Methanol
Methyl ester
+
catalyst
Reaction is limited by the rate at
which oil can get to “reaction zone”
in the methanol/catalyst droplets
and the rate at which the methyl
esters can get away.
11
Faster stirring
If we stir
faster, the
reaction will
occur faster.
This is
what we
would
expect of
a masstransfer
limited
reaction.
Noureddini, H. and D. Zhu. 1997. Kinetics of
Transesterification of Soybean Oil. JAOCS 74(11):1457-1463.
12
Reaction time
• Transesterification reaction will proceed at
ambient (25°C) temperatures but needs 48 hours to reach completion.
• Reaction time can be shortened to 2-4
hours at 40°C and 0.5-1 hours at 60°C.
• This generally follows the old “rule-ofthumb” that reaction rates double for
every 10°C rise in temperature.
13
Higher temperature
Higher temperature
makes the reaction go
faster.
This
makes it
seem as
if the
reaction
rate is
limiting.
Noureddini, H. and D. Zhu. 1997
.
Balance between the relative significance
of diffusion and reaction
Reaction is slow and
occurs throughout
the droplet.
These generic
curves show the
concentration of
oil as the
primary reactant.
1.2
1
CA/CAR
0.8
Beta=1
0.6
Beta=3
0.4
Beta=5
Beta=10
0.2
0
0
0.2
0.4
0.6
r/R
0.8
1
Reaction consumes
oil as fast as it enters
droplet.
15
Reaction zone for
slow reaction/fast
diffusion. Entire
droplet is reaction
zone.
Reaction zone
1.2
1
CA/CAR
0.8
Beta=1
0.6
Beta=3
0.4
Beta=5
Beta=10
0.2
0
0
0.2
0.4
0.6
r/R
0.8
1
Reaction zone for fast
reaction/slow
diffusion. Outside
surface of droplet
only. Mass transfer
limited!
16
Which case is appropriate for
biodiesel transesterification?
• The only way we can sort this out is with
modeling the diffusion and reaction
processes.
droplet
Diffusion flux of species in
and out of droplet.
Internal diffusion flux within the
droplet.
+ chemical reaction within the droplet
depending on catalyst
Need this
piece!
17
Where can we get the chemical reaction rates?
Boocock
• David Boocock, a chemical engineering
professor at the University of Toronto,
recognized the limitations of the twophase reaction.
• He proposed, and patented, the concept of
adding a co-solvent, which makes the
reaction mixture a single phase.
Boocock, D.G.B , “Single phase process for
production of fatty acid methyl esters from
mixtures of triglycerides and fatty acids,”
US Pat#6,642,399
18
Co-solvents
• Patented process
now marketed by
Biox corp.
• Cosolvent makes
methanol, oil,
methyl ester, and
glycerol soluble.
• Complete reaction
in minutes.
Recent news
20
Single phase reaction
• There should be no mass transfer
limitation.
• Measured reaction rates should be “true”
chemical reaction rate.
• These kinetics have been measured:
R. Doell, S.K. Konar, and D.G.B. Boocock, J. Am.
Oil Chem. Soc., 2008, 85, 271.
21
Single phase reaction rate
Time to reach BG=0.24%
T=20 C 13.5 min
T=30 C 5.78 min
T=40 C 2.59 min
T=50 C 1.21 min
T=60 C 0.59 min
Van Gerpen, J. and B. He. Biodiesel Production and Properties, Chapter 15 in
Thermochemical Conversion of Biomass to Liquid Fuels and Chemicals, 2010.
22
Modeling results
Concentration, mole/L
120
100
80
TG
60
DG
MG
40
20
0
0
0.2
0.4
0.6
0.8
1
The distribution of
the main reactant,
triglycerides,
indicates a mixed
reaction where both
diffusion and
chemical kinetics
are important.
23
Modeling results
• Results are only preliminary!
• Important factors are not yet included.
o We don’t have good data for the solubility
limits of mono- and diglycerides in methanol
and oil.
o Surface tension effects are not included.
(Important!)
24
To speed up the reaction
• Need to break the methanol/catalyst
droplets into smaller droplets to increase
their surface area.
25
Droplet size
Interfacial tension still
draws droplets within
another liquid into spheres.
Droplets in air have
surface tension that
acts like a flexible
membrane that pulls
the droplet into a
sphere.
Liquid droplets within a
liquid have the same
force – called interfacial
tension – that resists
deformation.
26
Internal pressure
The tension in the
outer layer of the
droplet from the
interfacial tension
causes an internal
pressure in the
droplet.
27
To break up droplets
• You need to have a high shear velocity
field to distort the droplet against the
pressure caused by the interfacial tension.
28
Effect of surfactants
• Surfactants collect at
surfaces and reduce
surface tension.
• Lower surface (and
interface) tension
makes it easier to
produce smaller
droplets.
Schramm, L.L., Editor. 1992. Emulsions. ACS
29
Surfactants
Many biodieselrelated
compounds are
strongly surfaceactive. Especially
monoglycerides.
Kuhrt, N.H., E.A. Welch and F.J. Kovarik. 1950. JAOCS 27:310-313.
30
Monoglycerides are natural products of
transesterification.
• The reaction proceeds in three steps:
TG + MeOH → DG + methyl ester
DG + MeOH → MG + methyl ester
MG + MeOH → Glycerin + methyl ester
• The products of complete reaction are methyl esters
and glycerin.
• The products of incomplete reaction include TG, DG,
and MG. Monoglycerides, and to some extent,
diglycerides, are surface active.
31
Effect of
Stoichiometry on
Monoglycerides
(alcohol to oil ratio)
• For a single step
reaction, at least 100%
excess alcohol is required
to get a complete
reaction (based on Le
Chatelier’s principle).
• This corresponds to
about 22% methanol, by
weight.
32
Modeling can tell us
• Where the mono- and di-glycerides reside
– in the droplets or the continuous phase.
• How the methanol droplets get smaller as
the reaction proceeds and then transition
to glycerin droplets in the methyl ester
continuous phase.
• Can we optimize the timing of agitation
and settling to get the fastest overall
process?
Still a lot of work to do…
33
Summary
• Biodiesel production is a deceptively
simple process involving mass-transfer
limited reactions and complex separations.
• There are many technology options for
biodiesel production.
• The technology choice is a function of
desired capacity, feedstock type and
quality, alcohol recovery, and catalyst
recovery.
34