on 3/15/04 6:34 AM, Leif Forer at [EMAIL PROTECTED] wrote: > > > Ideal vegetable oil is triglycerides. > Due to factors....the triglycerides can break > into mono and di-glycerides, also known as free > fatty acids and enemy of transesterification.
First some basics -- alcohols (glycerol, methanol, ethanol, etc.) can "condense" with organic acids (acetic, oleic, lysergic, etc.). In the process, a water molecule is released and an "ester" is formed (ethyl lysergate, methyl oleate, etc.) In the case of glycerol, three alcohol groups (OH) are present in the one molecule, so this esterif- ication can happen one, two, or three times. As you know, a triester of glycerol with three fatty acid molecules is called a triglyceride, and is what we call a "vegetable oil" . If any fatty acids are present in the oil that AREN't condensed with a glycerol molecule, they're called FREE fatty acids. > > We add a base catalyst to facilitate the cracking > of the triglyceride. I really wish everyone would stop using that term "cracking" for transesterification -- it really conveys the wrong idea. It's a petroleum term meaning to break a long carbon chain into shorter chains, and it causes a lot of confusion. What you do for biodiesel is break the ester links between the glycerol and the fatty acids in the original oil, and create a new ester link between each separated fatty acid and a short alcohol like methanol or ethanol. In other words, you break (and re-esterify) a single molecule of, say, glyceryl trioleate into a molecule of glycerol and three molecules of methyl oleate. > Aren't mono and di-glycerides formed when the > triglyceride is cracked? They are formed when the triglyceride is attacked by WATER, which also releases one of the fatty acid tails as a free fatty acid. > If so, why can't the FFA's bond with the added > methyl alcohol to form methyl esters? This reaction occurs in the presence of acid catalyst, but not base catalyst. That's why a first step of acid catalysis can be used to methylate any free fatty acids prior to the usual base-catalyzed step. I'm home sick today -- plenty 'o' time fer chem questions..... :-) Biofuel at Journey to Forever: http://journeytoforever.org/biofuel.html Biofuels list archives: http://infoarchive.net/sgroup/biofuel/ Please do NOT send Unsubscribe messages to the list address. To unsubscribe, send an email to: [EMAIL PROTECTED] Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/biofuel/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
