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
>
>
> >Hope this gives you some useful ideas
> >
> >Regards
> >
> >Michael Allen
> > >
> > > --- In [EMAIL PROTECTED], Michael Allen
> > > <[EMAIL PROTECTED]> wrote:
> > > > Dear David,
> > > >
> > > > We did extensive work last year on heated
> Straight
> > > > Palm Oil (SPO) in two-wheeled tractors and
> fishing
> > > > boat motors. We had field trials by local
> farmers
> > > of
> > > > four commercial tractors and did test-bed work
> > > with
> > > > three others. Crude palm oil caused erosion of
> the
> > > > pistons by late ignition but refined palm-oil
> (of
> > > the
> > > > grade used for cooking oil) worked well. But
> we
> > > never
> > > > got around to using it in a locomotive.
> > > >
> > > > This year we have been trying a range of
> reactor
> > > > designs to optimise methyl ester production
> from
> > > > refined oil. We are now moving back through
> > > various
> > > > forms of oil "refinement" towards the crude
> palm
> > > oil
> > > > (CPO). And yes, we are currently using the
> Aleks
> > > Kak
> > > > two-stage process. And yes it is currently at
> > > > atmospheric pressure (although the reactor was
> > > > designed to handle 200 kPa mainly as a safety
> > > feature.
> > > > Even so, some enthusiastic welders have
> > > > "overpressurised" it twice now through
> forgetting
> > > to
> > > > flood (and then drain) it with water before
> > > modifying
> > > > the unit).
> > > >
> > > > The locomotive I mentioned is running on a B50
> > > blend:
> > > > It uses esters from a one stage
> > > trans-esterification
> > > > reaction of methanol with the stearin and
> palmitin
> > > > which has separated from the CPO. This waxy
> stuff
> > > is
> > > > probably quite comparable with the good
> Scottish
> > > lard.
> > > >
> > > > Lots of luck!
> > > >
> > > > Michael Allen
> > > >
> > > > --- Keith Addison <[EMAIL PROTECTED]> wrote:
> > > > > >Thanks for your response, prof. Allen.
> I'll
> > > > > formulate an inquiry to
> > > > > >Mohammed Farid as you suggest. You
> mentioned
> > > Thai
> > > > > railway application. I
> > > > > >saw somewhere that German railways are
> using
> > > SVO in
> > > > > some of their shunting
> > > > > >engines.
> > > > >
> > > > >
> > > >
> > >
>
>http://www.wired.com/news/technology/0,1282,53591,00.html
> > > > > Choo-Choo Trains on Energy Crunch
> > > > >
> > > > > >The encouraging part of your message is you
> are
> > > > > reacting at 60C and that
> > > > > >this is near methanol boiling point. That
> > > implies
> > > > > you are succeeding at
> > > > > >atmospheric pressure. Do you use conc.
> > > sulphuric
> > > > > acid first stage? I agree
> > > > > >meth recovery is so simple that using
> excess is
> > > not
> > > > > really a problem.
> > > > > >
> > > > > >David T.
>
>
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