Module 3 – Biodiesel Quality and Production
Learning Objectives
After completing this Module, you should:
- 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.
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 ASTM-certified 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.
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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.
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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.
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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.
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