Hello Mark
>I have to say I'm starting to change my mind about acid in the wash,
>but I think it has to be used a bit more scientifically than just
>'adding some' which is how homebrewers seem to use it.
Is that how they use it? I thought the usual way is to add acid until
the wash water reads near neutral pH, or 7.5, or whatever, not just
"adding some".
Aleks Kac specifies a set amount to use for the acid-base method,
which makes sense because you're using a set amount of catalyst in
the first place.
That's the reason for using acid, after all, to neutralise the
catalyst. Maybe the ones who just "add some" are the same ones who
use it to "make washing easier" - they don't seem to be trying to
neutralise the catalyst so much as cracking a galloping emulsion
problem, they start bubblewashing and it froths over. I suppose the
"some" that they add would be enough to stop it frothing. Adding acid
may cure the symptom, but they need to get the process right (as
you've said previously).
Prof. Michael Allen said this on the Biofuels-biz list recntly:
"The purpose of the acid added to the wash-water after the
transesterification is to break down any methoxide left in the ester
and to neutralise it. Any strong acid would do for this. We use
sulphuric acid to bring the wash water in contact with the methyl
ester to a pH of not more than 8. If you don't add acid, you may not
break down all the methoxide and there is a good chance that this
will cause extra wear in an engine.
"Water will also break down sodium or potassium methoxide but it will
not neutralise it. Maybe your teacher is right because your washing
process is very efficient at removing excess catalyst. We found that
this "efficiency" can only be achieved if we use much more water.
Perhaps you can give us some details of your washing process..."
As for how much soap will be reconverted to FFA by adding acid to the
wash, to dissolve back into your biodiesel, Mark Schofield just gave
a rather precise measure of it:
"But remember the soap will return back to FFA. The pysical volume
with respect to the final bio-diesel is very small indeed, maybe
0.001% v/v."
I asked him how he'd arrived at that figure, but he hasn't responded.
Michael Allen also said this on the Biofuels-biz list:
"The acid increases its [the wash'water's] capacity to wash out
excess soap formed by the break-down of excess methoxide. But while
the something acid may be taking out the soap as a sodium something
salt, some free fatty acid is formed as well. Maybe not too much if
we keep the wash water at a pH of 7 to 8 because these fatty acids
are very weak (poorly ionised). Some of this FFA will probably
disperse in the soapy wash water anyway and be removed (but some
won't!). The particular fatty acids in a particular oil may have
some effect on this too: For example, unsaturated fatty acids could
perhaps be made water-soluble by the acid-washing process (I'm
thinking of some of those sulphonates used in making shampoos here)."
Too many variables for a precise figure? Anyway, very little, if you
do it right - well within the limits, and a worthwhile trade-off for
the advantages. We seldom do it, but I don't think there are valid
reasons not to use acid in the wash. But yes, please, let's be a bit
scientific about it.
>I"ve messed up
>a batch while experimenting that way not too long ago, overdid it on
>the HCL and a serious mess happened (it looked like emulsion and it
>had a high acid number on a titration (1.5) which wouldn't wash out. I
>eventually washed the hell out of it, and diluted it with a very high
>percentage of good fuel before using). So I still don't recommend
>acidulating as a matter of course, unless you know what you're doing.
>It can be done right if you do a titration for soap/catalyst first to
>find out how much acid to use (that HCL/ bromophenol blue indicator
>titration that Juan described a week or so ago).
Why do a titration? Adding it slowly while monitoring the pH should do.
>Industry does it, but
>they also don't make particularly soapy biodiesel in the first place
>(because of using new oil or using acid-base ffa pretreatment), so the
>amount of ffa that is released when they acidulate isn't as large as
>it could be in a really problematic batch made by one of us...
>
>I want to make a comment on acid-base biodiesel, though- the one thing
>about it is that is different than singlestage biodiesel, is that you
>absolutely, positively must wash it. Think what you want about washing
>in general but for this method it's not an option not to wash. Fuel
>made with acid pretreatment contains water-soluble sodium sulfate
>formed by the neutralising of the sulfuric acid by some of the
>catalyst, and until you wash that stuff out, it's sulfur in your
>tailpipe emissions. For those wondering about how much of a danger it
>is that some sulfur might be left (I hear this question all the time)-
>well, the commercial guys who make fuel this way, pass the ASTM test
>for sulfur, and for various reasons they don't wash their fuel quite
>as thoroughly as homebrewers, so I assume it must wash out quite well.
One of the major advantages of biodiesel is that it contains no
sulphur, unlike petro-diesel - no sulphur emissions and no sulphur to
poison catalytic converters. So I checked this out long ago, before
we uploaded the acid-base method. As you say it's water-soluble and
washes out, should all be gone in the first wash (especially with a
counter-current washing system).
Do biodieselers who don't wash the stuff not wash it because they
think it's better not to wash it or because they couldn't be
bothered? They seem to be aware that what not washing leaves in the
fuel can damage the engine, or they should be aware of it, and if
they don't care about that are they likely to care about emissions?
If they don't care about damaging the engine or about emissions, then
why make biodiesel in the first place? Just to save a bit of money?
Anyway, I don't think it's a valid option not to wash, no matter
which method you use. But of course you're right, the sulphur is an
added reason with the foolproof method. The US ASTM standard limit
for sulphur is 0.050% by mass, max. (other national standards are
lower), and 1ml of 95% H2SO4 per litre of oil is nearly twice that
(though I don't know how much of the sulphur goes out with the
glycerine layer, if any).
By the way, regarding wash-water treatment, sulphur is an essential
plant nutrient, one of the six "macronutrients" (nitrogen,
phosphorus, potassium, calcium, magnesium, sulphur). The sulphur
cycle is quite similar to the nitrogen cycle.
Regards
Keith
> > Note also that you don't have to wash biodiesel made by the
>Foolproof
> > method any special way. That's how Aleks does it, and that's fine,
> > but once it's settled and the by-product ("glycerine") layer at the
> > bottom removed, biodiesel is biodiesel and washing it is washing it,
> > no matter what process you used. So you can decide for yourself
> > what's best fo
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