Hi All,
I've been doing a lot of reading about acid esterification and the more I read the more conflicting information I get. I've been working with some oil that titrates at 14+ NaOH and had mixed results. I've read that higher temperature (60c) works better or faster than say 48C to 54C and that it could take more than two hours but I've never read what is too long and what happens if you process too long. I understand that the temperature has to be maintained for the entire process but does the mixture have to be stirred the entire time. Is stirring better than circulating? I have a couple other questions as well. I'm sure I'm not the only one with these questions and I bet others would appreciate the advice of those who use the acid esterification method on a regular basis and can give good answers and stay on topic so this is a condensed pool of information. Thanks

I think most of us have read to heat wvo to 48C and to add between 1 and 2 ml of sulfuric acid to 100ml methanol/1liter of wvo. Combine with wvo in a sealed vessel and agitate for what could take more than 2 hours. Keep it hot the whole time.


Summery of questions:

I've read to use between 1 and 2 ml of sulfuric acid. That's a 50% difference. How do we know when to use 1 ml verses 2 ml?

Is a 60C operating temp better than 48C? If so, what is the advantage? What is too hot? If your processor is enclosed the methanol should stay?

Does the mixture (wvo, methanol and sulfuric acid) need to be mixed the entire time or can it be intermittently mixed? Say with a timer to come on every 15 minutes for a period of 5 minutes. I'm just throwing numbers out there but the idea is to conserve electricity.

Mixing...is agitating with a pump as good as mechanical stirring?

How much processing time is considered to be normal? How much processing time is too long?

If you haven't achieved the desired drop in FFA after say 4 hours do you stop and add more acid or ???


I've read some drain off the watery substance after the acid cycle while other don't. Which is better?


There are so many different oils out there that you can't cover every one but I'm hoping to come up with a general description of a normal process and perhaps a couple "What to do if this happens".

I start with 1 ml acid. If at the end of the second stage I did not achieve proper seperation, I add 1 ml acid per gal. mix for 5-10 min then add 50% original methoxide and mix with heat for several Hrs. set over nite. Has been working for me .

quote:

Originally posted by mcguyver:
Hi All,
I've been doing a lot of reading about acid esterification and the more I read the more conflicting information I get. I've been working with some oil that titrates at 14+ NaOH and had mixed results. I've read that higher temperature (60c) works better or faster than say 48C to 54C and that it could take more than two hours but I've never read what is too long and what happens if you process too long.

Acid esterification proceeds at a MUCH slower rate on triglycerides than base transesterification - approx. 2000x slower, IIRC. It does, however, convert FFAs into esters (ie - biodiesel) fairly quickly which is all it typically is used for in making biodiesel. Therefore, the amount of time to run acid-catalysis depends on the amount of fatty acid in the oil! In practice, this means titrating samples of the reaction at, say, 15 minute intervals until the amount of NaOH to neutralize the FFAs drops below, say, 0.5g. The total amount of NaOH required to neutralize the acid-catalysis mixture, plus the standard 5g/L needed for base-catalysis, equals the amount of NaOH to add (don't forget additional MeOH).

quote:

I understand that the temperature has to be maintained for the entire process but does the mixture have to be stirred the entire time. Is stirring better than circulating? I have a couple other questions as well.

The higher the temperature, the faster the reaction runs. However, keep in mind that acid-catalyzed esterification and transesterification are two different reactions. The former runs fairly quickly; it is the latter that is slow as Christmas. The ideal temperature, from a practical standpoint, is 65C if using a reflux condenser on the reactor. Otherwise, run it at the same temp as you would base-catalysis (55C).

Because the reaction is not homogenous - ie, it's a mixture - agitation of the reactants is critical both to speed the reaction along and to minimize the formation of byproducts (what? I dunno - this is a rule of thumb). What is best: stirring or circulating? Difficult question to answer. In general, stirring is better than circulating, but there are situations where this would not be true. For example, if setup "A" used the infamous HF clear water pump to circulate the reactants in a 30 gal. tank while setup "B" used a gallon-can-size paint stirrer hooked up to a drill to stir the contents of a 55 gal. barrel, well, I'd put my money on setup "A" working a lot better. That said, it is probably easier for the backyard biodieseler to make circulation work better than mechanical stirring, especially if one can't weld.

This is what I feel best able to contribute at this time - I'll defer to others with more experience for the rest!

definitive answer #1 - if you acid react too long the biodiesel becomes black. and its not just protein remnants,the acid actually burns the freed glycerol whether made by hydrolisis or esterification. ive never made black biodiesel clear again- i cant see to do any seperations.

definitive answer #2 - the higher your ffa content the more esterification of the triglycerides will occur and the more hydrolisis will occur. this makes the amount of base to use for subsequent base reaction very indefinate, with same problems of using wrong amount of base in "normal" oil and additional problem of already having reletively large percentage of monos and dis (and associated water) present in the beginning of base reaction. typically no seperation of glycerol occurs and washing must be hotter, vigorouser, longer and plentier(excuse my english - im not english major but physics minor). best to get ffa% below 1/2 % before trying any base. this is most easily accomplished by seperating to eliminate water/methanol and rereacting with more methanol/acid.

definitive answer #3 - acid catalyed reactions are very fast(on ffa) faster than base on triglycerides and faster still with lower amounts of acid catalyst (although depth of reaction higher with more acid), and dont need much stirring - ffa's and methanol form single phase.

definitive technique #1 for oil titrating at or around 14(besides tried and true "find a different oil") - castic strip (with methanol/lye not water/lye(unless you want to wait for emulsion). about 2/3 of your oil will be usuable <2 titration in normal base reaction. the other 1/3 you acidulate,use what you can for methanol recovery fuel/home heating fuel and dispose the rest (now 20+% ffa)(not into a resturant container whose oil youd like to get free at some point in the future, since this (even in small quanities)would very quickly make it unsuitable for yellow grease, and is probly against the law)

Found this thread and it seemed appropriate to bring back up.

Today I finally had a chance to play with some acid esterification. I had oil that titrates about 9-10. Warmed a 300ml sample in a flask to about 135*F, added about 15% Methanol with about 0.5ml of sulfuric. I noticed the sulfuric had a visible reaction in the meth--it was as though they wanted to separate and needed a little agitation to make them stay together.

I left the flask on the hotplate with the heat off, and added a magnetic stirring device (I'm working in a lab with a hot plate/stirrer, t-stats, etc which is VERY helpful). The magnet provides some gentle vortex action, stirring the acid and oil and meth around.

After about an hour, the titration had actually risen to about 11-12. Guess it needed more time to work?

After an hour, I had to head home. I left the agitation on overnight.

Any bets as to what I'll find when I return tomorrow? Does it just take longer to make the titration drop? Should I have heated the oil hotter? Or did I miss something?

Hey progunprogressive,
I've never checked the titration so quickly after starting the acid step. It's not at all uncommon for it to take 24 hours to get a good drop in the titration. You can't heat the oil much hotter than you have before the methanol tries to boil out but the hotter the better until the methanol starts to boil. I think it is important to keep the process hot the whole time. I'm going to take WAG (wild a$$ed guess) at it and say your titration will be .3 less than you started at.

I'd like to know the results
.

-mcguyver

Hmm, not much of a drop if that's what I find. I'm going to head to the lab here in a bit; I'll post up what I find.

The problem is I'm working in a flask on a hot plate stirrer, and I didn't want to leave this concoction on heat over night. The heat radiates out pretty quickly from the glass, and I'm using a small volume. If I had 200 gallons of yellow grease in a steel reactor, the heat would stay in a long, long time. But 300ml in a glass flask tends to drop heat fast.

I might try adding a bit more methanol and reheating it to see how it goes...

Titration had dropped about 1, to about 8 from about 9.

I have a feeling heat might be a variable to work with. I'm not going to add any methanol or sulfuric, I'm just going to put some heat on it and circulate it for a few hours and see what happens.

That means my WAG was only about .7 off. What do I win??
I wouldn't leave the flask unattended either. I bought a heated stir plate just for testing the acid process. I could never get the heat just right so the temp of the little batch was maintained. It was always going up or down.

-mcguyver

Seems that the hot plate I'm using is marked so I can keep it right around 125-130*F. My biodiesel partner's been using it a long time and knows how to keep it where he wants it.

From reading the acid esterification forum, I gather that the % of methanol isn't so important, but getting the right amount of sulfuric and keeping it at the right temperature is. The flask does tend to cool off to quickly if you take the heat off.

After a couple more hours of heat, it had dropped to about 6.

So I'm making progress...but I need to refine this a bit before I can confidently take a big vat of high titrating yellow grease and make it esterify.

Somewhere I've read that you want H2SO4 to be about 5% of FFA content, and there's a formula for translating titration into the percentage of FFA. Anyone know where to find that? EDIT: Found it. According to neutral, "FFA% = homebrew titration (ml) x 0.766 with NaOH". Not sure the derivation of that formula, but from what I gather it's on the money, or at least wasn't rebutted in the thread I found it in.

Alright, so I figure I know how to get the right amount of acid. Looks like controlling temperature and keeping it high enough is the key.

Hey, real quick like, somebody check my math.

Titration of the oil was 9. So 9 times .766 is 6.9% FFA content. So my FFA in a 300ml sample are about 20.7ml. I want 5% of that to be sulfuric acid, so I wanted about 1ml of sulfuric. Looks like I didn't have enough.

Thoughts?

Hey progunprogressive,
That translates to about 3.3 ml of sulfuric acid per liter. That's a little higher than what I usually use but not out of line. Girl Mark says that the more acid you use in the beginning the higher the titration will rise at the end. I'm not really sure what she means by that but I have had some wvo go down to a titration of 3.5 NaOH just to have it raise again while I was waiting for it to cool enough to add methoxide. I've had to go up 1.5 titration NaOH. Usually 2.75 ml of sulfuric acid/liter of wvo is the most I've used. I'd like to here your results with the higher acid. It might be better if you start with a new sample of wvo.


-mcguyver

Well, I rechecked, and my titration had dropped to six. I figured it'd be good enough, and tried doing a Dr. Pepper batch. I added 6 plus base 7 of KOH 100% pure plus a tad extra (about 13.5g) within about 100ml of methanol to about 500ml of WVO...which is actually way more catalyst than I should have needed. About double...but I wanted to make sure I got separation.

But I barely got any separation at all. After about an hour, I realized that A) the separation I did get was gone and the bd looked like Coca Cola, and B) the Dr. Pepper bottle I used was leaking!

Did I still have sulfuric acid running around in my bd?

It was really foamy and bubbly on top as well, seemed way thinner and less viscous than BD should be. No glycerine, no separation. And a big mess.

What the heck?

Anyway, too many variables went into that batch. I kept heating it and removing heat and adding methanol and the like...too many variables.

Got another flask full on the burner now, going to try to keep it at temperature for a couple hours reliably and accurately and see how the titration fares. It's about 0.579lbs or about 263g of oil. I titrated at 9, so that's about 6.9% FFA, or about 18g. 5% of 18g was about 0.9g of sulfuric. So I mixed .9g of sulfuric into about 50ml of methanol, poured it into the oil at about 125*F and left it on the burner.

Is it possible I had too much H2SO4 in the first batch and it didn't get consumed and attacked my bottle? I'm wondering...but the last thing I want is fuel that acidic eating through my gas tank .

Hello mcguyver
The H2SO4 used in the acid stage is detected in the titration as well as FFA's.
Every ml H2SO4 used in a litre of WVO increases it's titration by about 1.4 NaOH.
If you use 3ml H2SO4 in a litre of WVO the lowest titration you could expect at the end of the acid stage is about 1.4 X 3= 4.2.

quote:

If you use 3ml H2SO4 in a litre of WVO the lowest titration you could expect at the end of the acid stage is about 1.4 X 3= 4.2.

Hey Tilly,
Your reply makes me ask, if we know how much of the titration value is actually just acid then how much NaOH/KOH does it take to neutralize 1 ml of acid. I know that it seems I have to use slightly more KOH on batches that have been run through an acid stage.
Should we adjust our KOH for the "acid part" of the titration??

-mcguyver

quote:

But I barely got any separation at all. After about an hour, I realized that A) the separation I did get was gone and the bd looked like Coca Cola, and B) the Dr. Pepper bottle I used was leaking!

I'm not sure what's going on there but I know that my acid/base batches always take longer to separate than my normal base processes. We're going to have figure how much KOH goes into neutralizing the acid to try to make a determination of what happened to your batch.
-mcguyver

Hi Mcguyver

I tried the acid-base method a few times and I have a question....after the acid stage do you let the mix to separate?? so you can eliminate the water that's a product of the reaction??....In my experience most of the acid goes with the watery layer....does this happen to anyone else??.....

After I eliminate the water layer I continue with the tranesterification...

quote:

tried the acid-base method a few times and I have a question....after the acid stage do you let the mix to separate?? so you can eliminate the water that's a product of the reaction??....In my experience most of the acid goes with the watery layer....does this happen to anyone else??.....

After I eliminate the water layer I continue with the tranesterification...

quemala,
I tried to separate the watery layer but all I really wound up with was a lot of garbage to throw away.
I do the acid process at a higher temp (around 150 to 155F) with around 20 psi pressure in the tank. Once I've reached the desired titration I turn the PID off (which controls the heating element) and let the pump continue pumping until the temp drops to a point that I can add the methoxide. Once the temp is around 138 F or so then I release the pressure in the tank through the condenser. I get a little methanol from the condenser and pour it back into the batch.
I then add the methoxide, turn the PID back on and set it to 140F. Set the timer and let it go. I really don't have much trouble with soap (the usual product of water in the batch). Every now and then I get a soapy batch but most of the time it's not bad at all.

I've had pretty good luck this way. All my oil is pretty rough stuff and I've made some great BD. I'm current mixing my BD with 10% strait regular diesel and it's running fine down at 7F. I consider that to be pretty good.


-mcguyver

Hello mcguyver

If 1g NaOH is the equivelent of 1.4g KOH then 1.4g NaOH is the equivelent of 1.4 X 1.4= 1.96g KOH.

So each ml of H2SO4 should increase the KOH titration by about 2.

Tilly,
I agree with you most of the time, but this time I don't. I made several acid/base batches over the summer when I have bad oil ( titration over 14KOH). in every batch, except 1, I got the titration to end at 2.5 KOH. all of the batches used 2ml or more ( some were 2.5ml) of H2SO4 per liter. I don't understand why there is a difference but there is.

I hope to get more 'bad' oil in the future to repeat the results.

the one batch that didn't end right was a dumb operator error. using the same methanol/KOH container for the acid stage and the base stage isn't a good idea. I now use separate containers.

-dkenny