Laughing Stock Farm, Freeport, Maine
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Laughing Stock Farm, Freeport, Maine |
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Alkaline
Catalysis Unused or
virgin soy oil, like other vegetable oils, consists of triglyceride molecules
and typically has very low acid values. Picture a triglyceride as the capital
letter E. The vertical line represents the glycerin backbone while the three
horizontal lines represent the attached fatty acids that contain the biodiesel.
The acid value is an indication of how complete the triglyceride molecules are
in the mixture. As oil ages or is heated during cooking, the fatty acids break
away from the glycerin backbone and become free fatty acids (FFAs). More FFAs
increase the acid value of the mixture. This is important because the alkaline
catalysis process, popular with small scale makers of biodiesel, and that will
make biodiesel from triglycerides with low acid values, such as unused vegetable
oil, will make very poor quality material from WVO or YG with high acid values.
These poor quality materials that do not meet the ASTM standard are NOT
biodiesel. In the
alkaline catalysis esterification process, hazardous materials such as methanol
(an alcohol) and an alkaline catalyst such as sodium hydroxide (lye) are
used to separate the ester portion of the fatty acids on the glycerin backbone
and replace the esters with alcohol to form glycerol. The ester, or biodiesel,
floats to the top, while the glycerol and most of the other impurities settle to
the bottom. This separation looks much like water poured into
a glass of cooking oil, where the water settles to the bottom and
there is a distinct line that shows the separation. This
same process is often erroneously recommended to make biodiesel from high acid
value feedstock such as WVO or yellow grease. Alkaline catalysis will make soap
from FFAs, especially if the feedstock contains moisture, as is most often true
with waste vegetable oil. You may be familiar with this saponification process;
it's the same one your grandmother used to make soap in the good old days. When
the alkaline is used up in making soap, it is no longer available to break the
esters free from the glycerin. Further, the soap will form stable emulsions of
glycerin, salts, soaps, water and other impurities in the ester that will resist
removal by the water wash. For this reason, the material that floats to the top
and looks like biodiesel actually contains large concentrations of impurities
that will damage diesel engines and produce harmful exhaust emissions. Several
processes including hydrolysis and acid catalysis discussed here, can overcome this problem, but all involve more steps, more
hazardous materials, and more cost.
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Send mail to lisa@laughingstockfarm.com with questions or comments about our farmSend to ralph@rturner.com with questions or comments about our energy project.Copyright 2008, Laughing Stock Farm
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