Hi,

I just joined and have been searching through back posts to find an answer to my problem, but please respond if you can assist:

We recently processed a 60 liter batch of biodiesel. The first rxn. ran with 20% MeOH/8.5 g NaOH per liter. Glycerine was drained off and totalled to a couple liters greater than expected. Next, to ensure complete reacton, we reprocessed the BD with 5 % MeOH/1 g NaOH per liter. Everything seemed fine, though during the last few moments of the 2nd rxn. it seemed that the BD passing through the tubing became either somewhat aerated or soapy, as the appearance went from golden and translucent to golden and very slightly cloudy. A very small small amount of glycerine was drained off after settling for 9 hours. At this point we commenced methanol recovery. We use a 40 gallon water heater and reflux still. Assuming a recovery rate of 2% recovered methanol to original volume, we expected a couple hundred mL of methanol, but we recovered nearly 3 liters. We were surprised by the very large amount of methanol recovered and then were also surprised to find that the final BD product drained from the reaction vessel was a gel-like substance with some soapy residue. So...some questions:

1. Was the gel-like substance produced as a result of a reverse reaction during methanol recovery? If so, how to prevent this? Can we simply recover MeOH for a shorter duration and/or at a lower temperature?

2. The veg. oil was titrated at almost 5mL NaOH solution. Could the gel be a result of high FFA concentration rather than reverse reaction (and are these two related)?

I've considered two solutions: a) heating the final product more gently (165 F instead of 180) for a shorter period of time during methanol recovery; and b) pre-treating the oil feedstock with H2SO4 to reduce FFA conc.

Please respond with any input. Thanks.

Jeremy

you might want to drop that temperature to about 130F (55C), otherwise you'll lose loads of methanol by evaporation. methanol boils at 150F (65C).

My guess, in short, is that you have carried out Tilly's re-process test. The title is a bit of a mis-nomer as what it means is "re-process (part of) your batch to see if it turns to soap. If so, your initial batch had complete conversion."

So in fact it appears you have over-processed your oil. You can't have a reverse reaction if you've removed all the glycerine. The gel will most likely be soap, particularly as you have used NaOH. 25% methanol as you have used is getting to be excessive and will hinder separation, especially on a re-process.

Water washing at this stage will be fraught with emulsion danger. Does the gel melt with heating? If so, you could try air treatment to drive off ALL remaining methanol and then let stand to see if soap/glycerine drops to the bottom as per dry-wash technique.

You could try acid treatment to break the soap up, but this is beyond my experience/knowledge.

Thanks for your replies....it's interesting, I let the gel sit and it separated into a dark layer (top), a golden-brown layer (middle) and a whitish layer (bottom). I am supposing that the top layer is some sort of glycerine/soap/triglyceride residue and the bottom a water/soap residue. Beyond that I'm not quite sure of the identity of those layers. I am hoping that the middle is biodiesel, though have no clue what purity it might be.

What is interesting, Paulus, is your suggestion that I "overprocessed" the 1st stage biodiesel. I don't know if I want to put the gel back into the processor to drive off excess methanol as I am concerned about the possibility of soap/gel residue remaining on the inside of the processor tank.

I have had some thoughts after your suggestion: I agree with you that I likely overprocessed the biodiesel, though I gather that the problem is not excess methanol per se. It seems that the first stage reaction went to a great degree of completion (perhaps even >98%). Thus, the quantities of sodium methoxide during the 2nd reaction were in excess of the amount needed to react with any remaining, unreacted glycerides. Since there would be excess sodium methoxide , the lye would preferentially react with any glycerides to form soap, and hence the gel-like emulsion. If anyone has further ideas please chip in.

Take a look around en.wikipedia.org where there is a huge amount of valuable information on chemistry. In particular look at the bottom of http://en.wikipedia.org/wiki/Methyl_ester.

If your reaction went to near completion and glycerine was properly settled and drained off, it will not be a factor in the second reaction.

Soap is a chemical salt. Salt is the result of mixing an alkali and acid - here, the Na+ ion and carboxylic free fatty acid. At the same time, a hydroxyl group and a proton combine to form water. Glycerine is not part of the equation.

So, correcting your second-to-last sentence, the excess lye reacts not with glycerides to form soap, but rips apart your methyl ester (bio) to form soap, and you suffer a loss of yield.

Look at that wikipedia link above. Notice it says a product of the saponification of an ester is alcohol. You said you were surprised how much methanol you distilled off?

How thick are the layers? The dark stuff at the top will probably be soap. You need to remove all remaining methanol, and I think it should sink to the bottom. Do you use a separate settling tank? You should.

Join the Yahoo group Biodieselhouston and ask Del about shop vacs, his aeration cross and dry washing.

Also, note that you are not really using sodium methoxide. You are using a mixture of sodium hydroxide in methanol. Different thing. Look up Methoxide in wikipedia.

Could be wrong about all this, of course, but it seems the best fit.

Paul