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.
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.
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.
How Using Sulfuric Acid Works When Making Biodiesel – by Graydon Blair
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