Hi Michael >(Sorry to be a tad pedantic and maybe a bit >long-winded but it sort of goes with the patch as a >Visiting Professor here in Thailand!)
Not at all, not long-winded, nice clear explanation, I'm sure a lot of people will appreciate that. Actually I did understand this, having checked it out a bit and discussed it with a few people, but I understand it even better now - thanks! What puzzled me and mainly why I asked was what you said about methanol excesses. For the whole process, Aleks uses rather less methanol than you'd probably have to use for high-ffa feedstock by most other methods, especially single-stage base. Overall methanol use, not just for the esterification first stage, is about 67% excess, depending on the oil; your 9 to 12 times applies to the first stage only. Well, that's all clear now. By the way, there are tables showing the stoichiometric ratios for different oils here: http://journeytoforever.org/biodiesel_meth.html How much methanol? Re the condenser, that's one way, but it's hard to separate the glycerine/FFAs/catalyst in the glycerine layer with the methanol already removed. Maybe a better way is to drain off the glycerine layer first, then evaporate off the excess methanol in the biodiesel (it's about half-half in the biodiesel and the glycerine layer). After separating the glycerine layer (http://journeytoforever.org/biodiesel_glycsep.html), the rest of the excess methanol can be recovered from the purified glycerine (somewhat purified glycerine). I'd appreciate a picture and diagrams, if that's not too much trouble, thanks. So you are using the acid-base process as your main method, with both the 100-l and the 1-tonne reactors? Some people have been having trouble using it with crude palm oil (high FFA and high percentage of stearin), but not with olein palm oil. Thanks again, best wishes Keith >Hi Keith, > >In answer to your question: > >Lets start with a litre of oil. >Ours has a density of 910.9 grams/litre so a Free >Fatty Acid content of 5% on a weight basis means that >5% of that 910.9 grams is FFA > >So one litre of oil with a FFA content of 5% (weight >basis) contains 45.55 gms of FFA > >The chemical reaction of esterification is > > H+ >R-COOH + CH3OH <=> CH3-OOC-R + H2O > >In other words, 1 molecule of acid reacts with one >molecule of alcohol to give one molecule of ester plus >one molecule of water . > >Using the molecular weights and choosing palmitic acid >as the predominant fatty acid, we get that > >256.42 grams of palmitic acid needs 32.04 grams of >methanol to produce 18 grams of water and 270.5 grams >of methyl palmitate. > >so 45.55 grams will need 32.04 x 45.55/256.42 = 5.69 >grams of methanol > >Taking 791 grams/litre as the density of methanol, >this means that we need to have a minimum 5.69/791 >litres of methanol available for the reaction to go to >completion. > >This is only 0.007 litres! > >So why do we add 0.08 litres of methanol per litre of >oil as set out in "First stage para 4" of Aleksander >Kac's >http://journeytoforever.org/biodiesel_aleksnew.html? > >Well the <=> sign in the above equation is to show >that it is reversible. That is, if we take the ester >and add some water in the presence of a strong acid, >we get some free fatty acid formed and some alcohol. >(This method is actually used in the analysis of >unknown fats and oils ) > >The main reason we add excess methanol (about ten >times as much as the stoichiometric amount needed) is >to force the reaction further in the direction we want >it go. Removing the water has the same effect so using >concentrated sulfuric acid as the catalyst also helps >the reaction move over to the right because it is >really great at dehydrating things like sugar and >human flesh . . . . . . . . :-( > >But, as I suggested in my note, the excess methanol >also dilutes whatever water is left at the end of the >esterification reaction. (And we are going to need >that methanol for the transesterification stage >anyway). When we neutralise that sulfuric acid with >sodium hydroxide, we make sodium sulfate which does >not bond to water quite as strongly as the sulfuric >acid so, as a consequence, I suspect that the water >activity probably increases. > >Incidentally, you can make the same calculation for >any fatty acid found in vegetable or animal oils and >you will find that between 9 and 12 times the minimum >stoichiometric amount of methanol is used in the >Aleksander Kac esterification process. > >For similar reasons, the transesterification process >also uses excess methanol: 0.2 litres of >methanol/litre of oil is about 1.7 times the >stoichiometric amount needed and 0.25 litres of >methanol/litre of oil is about twice the minimum >amount needed. (Aleksander Kac's two methods). You get >slightly different results if you cover all the >possible mono-glycerides, di-glycerides, >tri-glycerides of all the fatty acids but the above >figures are quoted for glyceryl tri-palmitate which is >one of the major components in our oil feedstock. > >This is why I feel that a condenser on the outlet of >the reactor is a useful way of saving money. And >because chemical reactions go faster at higher >temperatures, we can carry out the reaction at a >temperature closer to the boiling point of methanol >(64.6C) by refluxing all the condensed methanol back >into the reactor until the reaction is finished. At >present we use a simple copper-pipe heat exchanger and >we totally recycle the cooling water through two x 200 >litre oil drums working as an evaporative cooler by >having a sprinkler over three perforated baking trays. >(I'll send you a picture and some plans if you like.) > >We have assessed the advantages of a condenser with >our 100 litre reactor and, as a result, we our >currently modifying the 1 tonne reactor used for >making biodiesel from stearin. (This 1 tonne unit is >the reactor which produces biodiesel for the railway >trials) > >(Sorry to be a tad pedantic and maybe a bit >long-winded but it sort of goes with the patch as a >Visiting Professor here in Thailand!) > >Best wishes > >Michael Allen > > >--- Keith Addison <[EMAIL PROTECTED]> wrote: > > Hello Michael > > > > <snip> > > > > >Incidentally, with respect to the high free fatty > > acid > > >feeds, the limit to the amount of FFA possible is > > >probably related to the formation of water in > > Alek's > > >first stage esterification using sulfuric acid as > > the > > >catalyst. Thus 5%FFA in the oil means that 4mls of > > >water are produced for every litre of oil. This > > water > > >increases soap formation in the second stage (which > > is > > >catalysed by sodium hydroxide). > > > > I know about the water, that'd be about right. But I > > don't know about this: > > > > >Alek Kak's method > > > >(http://journeytoforever.org/biodiesel_aleksnew.html) > > >reduces this impact by using 95% pure sulfuric acid > > >which has a strong affinity for water. He also > > >recommends about 9 times the amount of methanol > > >theoretically needed to react with the FFA. This > > also > > >has the effect of reducing the water concentration. > > > > It uses 8% methanol v/v oil for the acid stage, and > > 12% for the > > second stage. You're saying acid esterification of > > 5% FFAs needs less > > than 1% methanol? > > > > >So, for high(er) FFA, more methanol and more > > sulfuric > > >acid should be beneficial. However, that means more > > >sodium hydroxide to neutralise the acid and some > > form > > >of methanol recovery system. We use a simple > > condenser > > >connected to the reactor to grab the methanol. We > > can > > >either recycle this to the next batch or operate > > the > > >reactor under total reflux. This makes it possible > > to > > >operate the reactor at a higher temperature and so > > >accelerate the rate of the separate reactions. > > > > You're using Aleks's acid-base method this way? Or > > is that the > > one-stage transesterification you mentioned > > previously, for the > > train's B50? > > > > Best > > > > Keith ------------------------ Yahoo! Groups Sponsor ---------------------~--> Looking for a more powerful website? Try GeoCities for $8.95 per month. Register your domain name (http://your-name.com). More storage! 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