Module 3 – Biodiesel Quality and Production
Learning Objectives
After completing this Module, you should:
1.
2.
3.
4.
5.
6.
Understand the quality concerns with WVO and how we detect and solve these problems.
Understand the general steps in our biodiesel production method at ISU.
Understand the purpose and chemistry of acid esterification.
Understand the purpose of the "methanol test" and "wash test."
Understand basic waste glycerin management.
Understand basic wastewater management..
Quality of Fats and Oils
There are many potential quality issues with fats and oils that can either create problems in the
reaction or in the quality of the biodiesel produced. Since we are dealing with waste vegetable oil
that has been used in food preparation, there are three quality issues we are most concerned
about: debris, water, and free fatty acids (FFAs).
Debris is simply food particles that have remained in the oil after frying. Debris can interfere
with the reaction to make biodiesel. As a result, most people filter out the debris before using the
oil. However, maintaining the filters is a very messy and labor-intensive process, so some people
simply allow sufficient time for the debris to settle. This is the approach we use at ISU. You will
learn to perform a WVO clarity test to assure that food debris has been removed.
Water can be a problem because it inhibits the biodiesel reaction. If much water is present in the
oil, the biodiesel reaction will not reach completion and quality will be poor. Water gets into the
waste vegetable oil either from the foods that have been cooked or from contamination with
water during storage (typically from rain entering outside oil storage containers). The best way to
minimize the amount of water in waste vegetable oil is to prevent water from entering during
storage. In addition, the settling process that removes food debris will also remove most of the
water since water separates from oil over time and sinks. You can use a "flash test" to detect
excessive water in WVO, but because we've never had a problem with water we generally skip
this test.
Free fatty acids (FFAs) are the major quality concern with waste vegetable oil. During cooking,
some of the fatty acid tails can become separated from the triglyceride molecules. These free
fatty acids become a problem during the biodiesel reaction because the catalyst can react with
them and produce soap. The soap inhibits the separation of biodiesel and glycerin, making the
final product unusable. Because of this problem, waste vegetable oils are checked for the amount
of FFAs present. If FFA is too high, the oil is not used. However, if it is under 4%, the oil can
often be used either by adding extra catalyst (a base) or by first doing an "acid esterification"
reaction. When you make production-scale biodiesel with the BioPro 190, you will be using acid
esterification (described below).
FFA levels in waste vegetable oil are checked with the acid titration test. You will learn how to
do this titration in day one of production.
Acid Esterification
Free fatty acids (FFAs) in the trans-esterification reaction produce soap in the biodiesel - a bad
thing. The equipment we use to make biodiesel - the BioPro 190 - uses an acid esterification
reaction to convert FFAs into biodiesel so the remaining oil can then undergo trans-esterification.
Acid esterification uses sulfuric acid as a catalyst in the following reaction:
The acid forces the FFA and MeOH to react, producing fatty acid methyl ester (biodiesel) and
water. If this reaction is relatively complete, little or no FFA remains to produce soap in the
transesterification reaction. However, there are two downsides to acid esterification. One is that
it produces water, which inhibits trans-esterification. The other problem is that the sulfuric acid
catalyst will consume some of the base catalyst (depleting base catalyst and producing water)
when trans-esterification is performed. However, as long as the initial FFA level was about 4%
or less, these problems are not too large.
Watch the video below to understand the steps involved with biodiesel production at ISU.
Although the reactor used in the video is the same model we have, some of the equipment we
actually use for adding and removing products will vary. The video provides an excellent
introduction to what you will be doing during your runs. Be sure you understand the steps, since
the quiz will cover video content.
http://www.youtube.com/watch?v=cfmECYprs1s
Inputs and Products
As you have seen in the video, we use waste vegetable oil, acid, potassium hydroxide catalyst,
methanol, and water to produce biodiesel. In addition to the biodiesel, we have to handle glycerin
and wastewater byproducts of the process.
Biodiesel Quality
The qualities of biodiesel required for retail sale are defined by the American Society for Testing
and Materials, or ASTM. The table below lists just a subset of the total biodiesel requirements
under ASTM-6751, the standard for biodiesel. Vehicle manufacturers require that only ASTMcertified biodiesel be used, or the warranty for the vehicle is considered void. In the summer of
2010, the ISU Student Biodiesel Project submitted a sample of their biodiesel from the BioPro
190 for ASTM testing. We passed! This allows our biodiesel to be used in any ISU diesel
vehicles, even those under warranty.
Selected criteria for ASTM 6751 Certification
Full ASTM testing costs about $1,000, so we'd rather not repeat it. Fortunately, there are really
only two major quality concerns, and we can test for these ourselves rather easily.
Incomplete reaction is our biggest concern. In this case, not all the fatty acid tails have been
removed from the triglyceride molecule. This could leave a molecule with one, two, or all three
fatty acid tails remaining. These molecules and their names are provided below. The presence of
these molecules is easily detected with a "methanol test", which you will learn to perform. By
mixing our fuel with methanol, we can quickly determine if the reaction failed: droplets of oil
settle out of the solution. A good methanol test will be clear after shaking, since B-100 and
methanol are fully miscible. If we see droplets form, then we know we still have unreacted oil in
our fuel, and the batch will need further treatment.
Another quality problem is excess methanol and catalyst left in the biodiesel. After the
reaction, most of the catalyst and unreacted methanol are attracted to the glycerin (since they are
all polar) and are removed once the settled glycerin is removed. However, some remains in the
biodiesel. This residual catalyst and methanol can typically be removed by "washing" the
biodiesel with water. If done too vigorously the water and catalyst can form an emulsion - not a
good thing. However, if done gently, the water will attract the methanol and catalyst and sink to
the bottom, where it can be removed. The BioPro 190 does three washes at the end of the
production process. Sometimes, to aid washing, we add some acid to the water. We test for
adequate removal of catalyst and methanol with the "wash test," which you will learn to perform.
First, water is mixed with the fuel and shaken vigorously. Then, the water and fuel separate. If
the water appears cloudy after separation, this indicates that excess catalyst and methanol are still
present in the fuel. If these are still present, then the wash test "failed," and this indicates that
another wash cycle must be run on the fuel to remove the contaminants.
Waste Glycerin
Glycerin is produced in the trans-esterification reaction. It settles to the bottom of the reactor and
can be easily removed. The glycerin will contain most of the catalyst and unreacted methanol, as
well as most trace impurities from the waste vegetable oil. Pure glycerin is a valuable product,
but removing all the other contaminants is typically too expensive.
An alternative is to make soap from the glycerin waste. To do this, only the methanol need be
removed. At ISU, we are attempting to distill the glycerin to recapture methanol and make the
glycerin usable for other purposes. Although you will not be participating in this part, you are
welcome to ask the teaching assistants about the process.
Wastewater
Watewater from washing the biodiesel will contain significant amounts of catalyst, unreacted
methanol, and some biodiesel carried over from the reactor. Of these, only catalyst is present at
concentrations considered a possible problem for discharge to sewer: very basic (high pH)
wastewater can corrode drain pipes. As you will learn, we test the pH of wastewater before we
discharge it. If necessary, we neutralize with acid.
Quiz Me…
After a production run, the finished biodiesel had excessive amounts of di- and monoglycerides. This
would be discovered by
a. Acid titration.
b. Methanol test
c. Wash test.
d. Reaction test.
After a production run, the finished biodiesel had excessive catalyst present. This is most likely due to
a. Insufficient washing.
b. Incomplete reaction.
c. Insufficient methanol.
d. Excessive reaction time.
What must be done to waste glycerin before it can be used to make soap?
a. It must be neutralized.
b. Excess catalyst must be removed.
c. It must be washed.
d. Methanol must be removed.
If WVO contains excessive FFA and undergoes trans-esterification, what is the likely outcome?
a. Soap.
b. Excessive unreacted MeOH.
c. Unreacted WVO.
d. Failing the methanol test.