|
Within the past few years biofuels have captured a considerable amount of interest among the public. Biodiesel fueled cars and trucks have toured the country and been featured in many stories in both the print and broadcast news media. The production and use of biodiesel have increased exponentially over the last decade. Recently three papers have appeared in this Journal (1–3) discussing the properties of these fuels and their production from biological oils, either new or used. With fresh oils the procedures found in the papers cited above and those given on several web sites (see ref 4 and links posted there) yield clean biodiesel samples. Environmental issues suggest the use of used fryer oils in making biodiesel. However, extensively used oils can cause problems with emulsion formation. Students in our laboratories occasionally encounter a single, stable emulsion where two phases are expected after the transesterification reaction. In the second run of a small biodiesel production plant at our university, a significant emulsion layer formed that required many washings to remove and significantly reduced the yield. Used fryer oil typically contains both water and free fatty acids at levels that depend on how heavily this oil has been used. Commercially these fatty acids are removed by acid-catalyzed esterification (5) early in the process. On smaller scales making biodiesel from used oil is typically preceded by a titration to determine the amount of fatty acids present and an appropriate correction is made to the amount of base added. When the base is added the fatty acids are neutralized and produce natural soap. Water may also compete with methanol in the transesterificaiton reaction and make additional soap. While I have never sampled the emulsions produced by my students to determine the emulsifying agent, it is my belief that this soap is that agent. Supporting this assertion is the fact that the emulsion produced by our university’s reactor could be separated by washing with diluted vinegar. This protonates the fatty acids, removing their surfactant capabilities and destabilizing the emulsion. We will frustrate students if we do not discuss why laboratory scale productions of biodiesel from used oil may produce significant amounts of emulsion. I intend to have my next environmental chemistry class develop a modified procedure for producing biodiesel involving vinegar washings where necessary. Literature Cited- Clarke, N. R; Casey, J. P; Brown, E. D; Oneyma, E; Donaghy, K. J. J. Chem. Educ. 2006, 83, 257.
- Akers, S. M; Conkle, J. L; Thomas, S. N; Rider, K. B. J. Chem. Educ. 2006, 83, 260–262.
- Bucholtz, E. C. J Chem. Educ. 2007, 84, 296–298.
- Alternative Fuels Data Center: Biodiesel (accessed Aug 2007).
- Ascendant Partners (accessed Aug 2007).
|
