How To Make Biodiesel With Sulfuric Acid

How To Make Biodiesel With Sulfuric Acid
by Norm McFarling, Nebraska BioPro

AE (acid esterification) of WVO (Waste Vegetable Oil) is a way of reducing the FFA (Free Fatty Acid) content to a level that produces more biodiesel and less byproduct.  I won’t bore you with all the why’s and where-to-fore’s of the process, just lead you through the steps that we need to take.  I will try to explain why we do each step.  Don’t try to take any short cuts in the process until you have tried it a few times and know how things are going to react with each other.  

Remember Safety First and Always
WVO makes a mess.  Sulfuric Acid can hurt you.  A lot. Be sure to wear a face shield or chemical safety goggles and chemical resistant gloves.

Getting Started
Start out by drying your WVO as much as possible.  You are going to make water as a byproduct of this process and, as we know, water is not our friend.  My processor can heat and stir the oil to drive off the suspended water.  A WVO drying tank similar to a circulating pump biodiesel drying tank is used by many people.  Some people heat the WVO past the boiling point of the water and hold it at this higher temperature for a time while stirring to get rid of the water. Settling the oil in a tote will also get the free water to drop out and heating it to 100° F or higher will facilitate this drop-out, but it will do nothing for the moisture that is suspended in the oil. You can test for the dryness of your oil by using a moisture test kit or by doing the hot pan test. If you use the hot pan test, make sure there are NO bubbles.

Keep a log of your processes to compare later when you are trying to modify your approach, or heaven forbid, trying to figure out why the results weren’t as expected. One thing AE experience will teach you is that this process is never predictable. One absolute rule will not work for every processor and every oil. Keeping notes will be a huge help in perfecting YOUR application.

Titrate Your Oil  
We need to know the titration number, which is our starting point. Almost any oil is a candidate for acid estrification, however, I usually do not pretreat WVO that titrates at less than 4, that oil goes directly to base processing with KOH.  

Calculating How Much Sulfuric Acid To Use
Once you have a titration number you can apply a simple formula to arrive at the amount of sulfuric acid to use.  Take the titration (T) and multiply it by 0.15.  That is the number of ml per L of WVO that is used in the batch.  

With a 190 liter batch (50 gallons) that titrates at 7 we would use 199.5 ml of sulfuric acid. Here’s the formula: 7 (T) x 0.15 (ratio) x 190 (volume) = 199.5 mL.

This needs to be considered the high side of the amount we want to use, too much sulfuric is NOT a good thing. Any acid not drained after the AE process will have to be neutralized with catalyst in the base process.  Also consider this like a game of horseshoes, being close counts–you don’t always need a ringer.  You are not going to get exact measurements using a disposable beaker with 20 ml graduations, so get it in the ballpark and err conservatively. 

Reacting The Oil With Sulfuric Acid
OK, so we know how much acid is required, now what?  Assuming that our processor is loaded with XX amount of dry WVO, lets get it heated to 130° to 135° F.  When the oil is ready, add 60% of the methanol you use for your normal base process and slowly pour in the measured amount of sulfuric acid while stirring or circulating the oil.  In our example above, with the 50 gallon load, we would normally use 10 gallons of methanol, so add 6 gallons of it now and save the 4 gallons for methoxide.  Continue mixing the brew for a half hour, then shut down the mixing but continue maintaining the heat if possible.  Heat is a very important factor in the AE process.  Maintaining 130° to 140° F will produce an efficient reaction.  Let it set for 24 hours for best results.

NOTE: As you refine your AE process you will find that oil titrating less than 12 KOH will react with less methanol. The total of 20% to 22% is still advised for the base phase, so don’t skimp on it. If you don’t add it in the AE phase, you must add it later. Also, do not add the methanol and sulfuric thru the same piping used for the methoxide. This will cause some or all of the acid to be neutralized and rendered ineffective.

Much of the chemical reaction is achieved during the initial half hour of stirring, this is not a long, drawn out process.  The main reason we shut down the agitation during the AE process is to allow the acid and water created as a by-product of the chemical reaction to drop to the bottom of the reactor.  Water and acid have an affinity for each other and the settling water will take any unused acid to the bottom of the reactor with it.  The “stuff” that settles to the bottom will look like thin milk, or a chocolate milkshake, gravy or water with or without some color.  It may also be in layers of the above.  It will be very aromatic, that’s a kind way of saying stinky.

The color of the drainage is an excellent indicator of the dryness of the oil at the start of the process. Dry oil will be darker, if there was water present the drainage will be a more caramel like color. Caramel drainage is more likely to need additional treatment to achieve the desired reduction in FFA’s.

If there is no apparent water or gunk present, drain a couple of liters anyway. The excess acid will be in the bottom of the processor and we don’t want it. Dispose of this mixture properly, it contains methanol and sulfuric acid as well as soap and oil. Whatever you drain will be highly metholated so add an amount of methanol back into the batch equal to the gunk you drained.  We don’t want to starve the base process.

After adding the methanol, turn the mixer (or circulation pump) back on for a few minutes to get a representative sample of the entire batch and titrate it.  If you have dropped the titration to an acceptable level you can proceed with base processing by mixing and introducing your methoxide, stir for an hour and let it settle out.  NOTE: Be sure to use the total volume of liquid in the processor, not the amount of oil you started with when you calculate your catalyst. Originally you had 50 gallons of oil, added 6 gallons of methanol and drained a couple in byproduct so your total volume is 54 gallons, not 50. Shorting the base will leave you with fuel that does not pass a 27/3 biodiesel conversion test.

If you are not satisfied with the titration there are several things you can do. The first thing I try is to let it set a few more hours. Bring it back up to temp if necessary.  It is usually advisable to perform titrations every couple hours to see if it is progressing.  If not you may want to do a second AE process by adding another 0.5 to 1 ml of acid per liter of oil along with a couple gallons of methanol. Be very careful because overdosing the sulfuric will drive the titration up 2.2 points for every excess ml of sulfuric acid used and it is next to impossible to get it down to realistic levels again.  You may also try just adding a couple gallons of methanol.  

Here is where keeping a log book will help you out.  If you put 190 ml in the last batch that titrated at 7 you can put 200 ml in this one and see what difference it makes.  Or put in 190 to see if you can duplicate the results to prove our system. Don’t forget to record the color of the drainage after AE. This will be very valuable information if troubleshooting is needed.

It is not recommended to premix the sulfuric and methanol before introducing them to the oil because the two cause a very exothermic reaction and the fumes are toxic. That means high heat generated along with spitting and boiling methanol and sulfuric acid spraying all over.  Add the methanol to the oil and then slowly add the sulfuric acid to that mix while stirring. It is much safer to handle a small beaker of acid than a couple gallons of acid in a carboy, especially if you have to climb up a ladder with it.

If you have a processor that must be circulating to pass the oil over the heat element, you are going to need to do a little experimenting to find out what works best for you. Oil holds the heat very well.  In a 70° F room the average uninsulated reactor will loose less than 20% of its heat in the first 12 hours.  So your 135° F oil will be about 110° F in the morning when you go to check on it. Less than 110° F if it is 40° F in the shed of course. At 100° F the reaction comes to a stop, simply reheating starts it up again, with no effect on the final outcome.

I had a 100 gallon load that had a starting T of 20, I treated it with 2.63 ml of sulfuric acid per liter, 12% methanol (by volume, 60% of normal base only amount) let it set for 24 hours and drained 4.5 gallons of watery crud.  T was 4, I added back in two gallons of methanol because I guessed that what I drained was half methanol because of the odor, stirred it for 10 minutes and let it set another 16 hours. Ending T was 2.7 and I went to the base reaction with 4200 grams of KOH. Settled off 20 gallons of glycerin, passed 27/3 testing at 65° F. Washed with misters and netted 97 gallons of fuel after drying. 97% conversion with oil that started with a titration of 20.  I would say that $3 worth of sulfuric acid and a little time was a pretty good investment.

Be sure to perform a 27/3 biodiesel conversion test to ensure you have a complete reaction. AE processed fuel will wash just like bio made from a base only reaction.  Be extra gentle with the first wash cycle.  Any time I got an emulsion in the first wash, it cleared with heat and settling in a relatively short period of time.

A Few Additional Points.
1) Never let the oil cool off once you have introduced the sulfuric acid.  Keep it warm until the base process has settled out. I do not know why and no one has ever been able to tell me, but if you let that oil go back to room temperature the titration goes high and no amount of tampering will get it back down.  
2) Don’t expect the next load of oil to react like this one did.  With a little luck, it will, but experience will help you recognize problems as they develop.
3) When you are working with WVO that titrates at 15 KOH or higher, reduce the acid in the formula to .09 per liter. Remember these are starting points only and you are required to adjust as needed for your situation. Using these numbers and following the above procedure will produce favorable and repeatable results. Every processor seems to be different and each may hold water or caustic from the last batch which will effect your outcome.
4) Higher titrating oils tend to have more bound water. Just as the name implies it is water that is bound with the oil and very hard if not impossible to remove. This is the one reason these oils need to be processed more than once. The water will use up some sulfuric acid and stop the reaction short of the desired outcome in the first AE reaction. That water is then gone (after draining) and the second AE can work on the FFA’s unimpeded.

Almost anything that goes wrong can be corrected if you maintain the heat and start with DRY WVO.  Start with wet oil and you may make glop.

Check out the video explaining this process from the
2012 Collective Biodiesel Conference

Download Norms presentation file

Link to video
Happy Homebrewing

Special Thanks
Thanks to the many and diverse personalities for contributing their wisdom and experimentation to further the AE cause.  Dave Kenny documented his findings and originated the formulation in what became known as the dkenny method a few years ago.  BioTom Affleck, Buckeye Ian Horvath and others refined the formula and procedures to where they are today.  I am certain that there are still things waiting to be discovered and the search continues.

Please feel free to contact the author at nmcfarling[at]gmail[dot]com with any questions or comments. 

Additional Resources:
How Using Sulfuric Acid Works When Making Biodiesel – by Graydon Blair

Want to give it a try? We carry high purity sulfuric acid pre-bottled & ready to use!
Click Here to see our selection!

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29 comments on “How To Make Biodiesel With Sulfuric Acid

  1. Would you mind some questions? It would be great if there were one right way to make biodiesel and every website agreed on the one and only correct method. This is obviously not the case. Whether you are doing a single or a double stage base reaction or doing an acid base reaction, there always seem to be different ideas out there on the correct method.

    I recently bought a bunch of chemical supplies from another home biodiesel brewer who was moving, which included a fair amount of sulphuric acid. So I’ve been exploring around looking at acid esterification processes. I have not tried one yet. I’ve always found the advice on this website to be good, so I’m seriously considering following your recipe here.

    The recipe you use here looks fairly similar to a couple of others I’ve read. The math and calculations are simple enough. I understand the need to get the water out of your oil. Why, though, do you consider it important to NOT allow the oil/acid/methanol mixture to cool off before starting the base stage? I have read other recipes that suggest allowing the mixture to cool and allowing the water formed in the acid stage to settle to the bottom and be drained. Could you explain why I should not allow the oil mixture to cool off? Should I drain off the water somehow or is this not important?

    One further question–how do you maintain your you oil at temperature without circulating it? This is impossible with the processer I use–the oil must be circulating or the heating element will short out. I assume you use some kind of immersion heater. I’ve never used one and would appreciate some suggestions on where to get one or how to make one. I use a 30-gallon processing tank.

    Hope you don’t mind posting some answers to my questions. I appreciate your help.

    • From what I understand, if the Biodiesel cools down, the titration tends to shoot back up. I’ve talked to a few people that have successfully been able to warm it back up & still get a good batch out of it, but it did require some extra work (if I remember right they did an 80/20 batch method on it). I’m not sure chemically why it can’t cool down though.

      Yes. You definitely want to drain off the water. The water will get in the way when you do the base method. Remember, Water + Oil + Catalyst = Soap. So getting rid of as much water as you can will help out your Biodiesel reactions quite a bit. Get the water out & AE works better and when you go to the base reaction less soap gets made too. We maintain our oil in a BioPro with the heaters on the bottom. In a machine that only heats with an inline heater, it may be difficult, if not darn near impossible, to do. You may want to consider doing acid esterification in a separate barrel or container that you can keep a band heater on.

      I’m not sure I’d recommend sticking an immersion heater into the processor with the methanol still in the mixture. It could be somewhat dangerous. However, they do make band heaters for poly tanks. Another thought would be to insulate the tanks as well as that will help keep the heat in. It might work well on a 30 gallon tank too.

      Great questions on the chemistry though. I’ll have to ask a chemist I know about the heat issue & see what they say.

  2. Thanks for your answers. I do have a number of steel drums and a band drum heater so if I try this method I’ll probably do the acid stage in one of them. I agree the immersion heater with methanol in the mixture sounds dangerous, and as you noticed maintaining the heat with a processor using an inline heater impossible.

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  4. Another issue is doing an acid reaction in a processor that has been used for a base reaction and has not be clean properly. I have read that even a bit of caustic can cause big products in an acid reaction.

  5. The end biodiesel I made using the acid/base method for the first time is very dark in color. It is clear, passes the 3/27 test, and passes the soap test. Is this product safe to use? Should the end product be the same color as when I use the base method?
    Thanks, John

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  7. Hey guys,

    I’ve been using the dkenny method for almost a year running 80 gallon batches, and three times I’ve run into an interesting problem–all goes well with the acid phase, but upon the intro of the base methoxide, the oil instantly turns into gel.

    On the most recent failed batch, I’ve verified there is no appreciable water in the original oil, which only titrated at 6 NAoH. I ran the acid reaction under heat and agitation (I have an inline heater) for some 16 hours, and the titration came in around 3 (I don’t remember specifically without my notes).

    But instantly upon adding the base methoxide, the oil turned into a gel. I instantly stopped adding base and shut down the processor. The floating gel was easy enough to fish out, but I’m loathe to toss the oil.

    The last batch that went like this, I added all the base catalyst, and the entire 96 gallons turned into gel by the time it was all over–ugh.

    I double checked the math, and we did the base calculations correctly…

    Any thoughts on what caused this? Anyone ever experience this?

    I have an old Mercedes I run on SVO–what would be danger of running this stuff through that after evaporating off the methanol? If I washed it with water, would it pull out any of the acid?



  8. Are you draining water, sludge and excess acid before going to base? If not, I suspect you are making soap with the NaOH.

    In my write up I caution people about letting the sulfuric / oil / methanol mix cool during the process. Several people have contacted me to inform me that they do that on a routine basis and have no problems. I don’t have to because my processor lets me continue with the heat when not mixing. Letting the mixture stand without mixing greatly improves the response time of the esterification because as the water is formed it can settle out and not interfere with the sulfuric. Water will render the sulfuric useless through a side reaction. The good news is that once it happens it is over, not a continuing process as both are used up. Your process is going to a satisfactory end point, so the only advantage would be that you could possibly get there sooner.

    I also recommend that potassium hydroxide (KOH) be used as the base catalyst after AE. If any soaps are formed they are of the liquid variety and can be washed out, although if there is enough it can form an emulsion, but emulsions can be broken even if they are stubborn sometimes.

    Let the treated oil settle out and drain off the water and sludge from the bottom. When draining, once you get to the oil layer, drain a couple more liters off as that will remove most of the excess acid which goes to the bottom because of weight. If this drainage is more than a gallon, add methanol equal to about half of the amount you drained off back into the mix so you don’t starve the base process for methanol. You may also find that this lowers the acid number because the leftover sulfuric is still acidic. Any extra sulfuric needs to be neutralized by the base before transesterification can begin.

    Then turn the heat and stir back on, bring it back to 135F and titrate again. Go to base and see if that works out better.

    Please feel free to contact me directly or thru Graydon if you need some help.


  9. Norman,

    Good stuff, thanks. We haven’t been settling/draining after acid because our oil generally titrates under 14 NAoH, but we will give it a shot–as you say, it’s got to be about the only thing that it can be. Funny how it only happens once in a while.

    I imagine this will also lower our ending titration–we never seem to get quite the same reduction in titration, so I suppose that would be the excess oil still swirling in the mix.

    Any thoughts about washing/drying/using the acid processed oil in my Mercedes? How acidic is the oil vs. your average WVO?



    • Nathan–

      Research by BioTom has shown that all batches should be drained after AE. If we only eliminate the excess acid and don’t get any separated water we are improving our chances for an easier wash phase.

      When you go to base the FAME will become PH neutral because the acid will be neutralized by the base and any excess base will be removed by the wash. There should be no difference between AE processed oil after it is transesterified with either Na or KOH.

      Let us know what you drain off the batch and how much sludge and water you find.


  10. Heated the oil, and it titrated out at 6 NaOH. Ran the acid phase as normal, only we let it settle out for roughly 24 hours after mixing.

    Drained off about two gallons–no real water evident, but there was a heavy smell of sulfur for the first gallon. The smell gradually changed to more methanol than anything else by the end of the second gallon.

    Following the drainage, we re-heated/circulated the oil. Titration came dead on 2 NaOH.

    This was positive–I’m sad to think of all the base I’ve wasted neutralizing the leftover acid in previous batches.

    Started base phase with no evidence of soap formation. Will update on final results/yield.

    Thanks again for the pointers.


  11. Processed the base phase for our standard two hours, and within ten minutes it was dropping out.

    Looking forward to seeing if the reduction in base due to the removal of excess acid results in appreciably less soap in the wash.


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  16. Hello Sir! Nice to meet You! i’m going to start a small biodiesel plant. I have some questions. If you would be so kind, please answer me.
    I’m going to make a biodiesel from old sunflower oil that titrates at 18. For AE i used formula (18-3) x 0.2(ratio) x amount of oil. So logicaly if the T increases, so ratio number increases too. in “A Few Additional Points” in third point written:
    ” When you are working with WVO that titrates at 15 KOH or higher, reduce the acid in the formula to .09 per liter.” – is it contradiction or what? Does it means that ratio must be reduced to .09?

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