Two Stage Acid Base Recipes

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The Two Stage Acid/Base method is used in commercial biodiesel production to increase yield and reduce soap production. In commercial settings it is used on oil titrating from 2-6 (NaOH). It can be used to make biodiesel from high and very high FFA oils. The process creates water as a byproduct. As the concentration of water increases the reaction is slowed. To combat the slowing of the reaction for high FFA oils, the Acid stage is often performed in steps with the water drained between steps. The most commonly used variation of the Acid/Base method in homebrewing calls for one Acid step followed by one Base step without draining the water between.

Precautions

WarningAcids attack steel. If you intend to use the Acid Eserification process repeatedly, you will need to make your processor from Stainless Steel. Processor made from mild steel and black pipe plumbing will literally be eaten away from the inside out.

Acids react violently with bases. You will need to take special care to keep your caustics away from the acids. Always add acid to water. Adding water to acids can cause the acids to splash.

FATTA Recipe

The internet has a large number of acid/base recipes. The original FATTA recipe seems to be the most commonly cited on homebrew sites.

First stage acid catalyst (to esterify the FFA)

Oil 1000ml (1 liter)
CH3OH 100ml (Methanol)
H2SO4 (conc. 98% 18M) 1-2ml
React at 48- 54C until titration falls to a minimum hopefully 2.6 ml could take longer than 2 hours. Greater levels of acid will raise the titration end point.

Second stage alkali catalyst (transesterification of the oil)

100ml methanol
6.4grams of NaOH
(3.5grams for catalyst + 2.9 grams for acid neutralization if titration of 2.6ml achieved)
React at 48- 54C for 1 hour.
Settle and separate


DKenny's Method

Dkenny is a regular on the biodiesel.infopop.cc forum who regularly uses acid/base techniques. This is his recipe. The thing that makes his recipe stand out from the others is that it has a formula to determine how much acid to use. Throughout he uses KOH titration levels. If you use NaOH to perform your titrations you will need to adjust the formulas accordingly. The acid used is sulfuric acid H2SO4 93% or more pure.

When performing the acid stage, we reduce our titration number by converting FFA into biodiesel. If we are successful, we can reduce the FFA to zero. However, since there is acid left over at the end of the stage, we can never get a titration of zero. DKenny's formula calculates the acid needed to hit a predetermined target titration number. His formula subtracts the target titration from the initial titration and multiplies the result by 0.2 to come up with the milliliters of acid needed for one liter of oil.

    Milliliters of H2SO4 per liter of oil = (Initial Titration - Target Titration) * 0.2

It should be noted that we do not need to add any extra KOH for our base to neutralize the acid. The KOH needed to neutralize the acid is included in the titration portion of our base stage.

This method has been successful on oils titrating up to 55ml using KOH titrating solutions. As the titration goes up, there are extra steps added. If your original titration was 18 or over, it becomes mandatory to settle and drain some of the acid water. An amount equal to about half the methanol you added. If your original titration was under 14 do not settle and drain. In between 18 and 14 it becomes optional. When adding the methanol for the base stage, increase it from 10% to 16% to compensate for the methanol lost during draining.

 

If your original titration was over 40, it is mandatory to split the acid stage into two steps draining off some of the acid water in between the two acid stages. Again, we will drain off an amount equal to about half the methanol originally added. DKenny has used this two step approach several times and the formula has held true. That is, after letting the first stage go as far as it would go, he hit the target titration within a few milliliters on each attempt.

Acid Stage

  1. Titrate the oil.
  2. Use this formula to determine the amount of H2SO4 needed:. (Titration - 3) * 0.2 = ml of acid needed per liter of oil
  3. Figure out 12% of the oil volume. This is the amount methanol needed.
  4. Heat the oil to 140F to 150F. 60C to 65C.
  5. Pour the acid into the methanol. SLOWLY is best, because it'll boil like mad.
  6. Add the methanol/acid mix to the oil.
  7. Stir for 2 to 12 hrs. The easy way to determine when it's done is to titrate the mix as it processing. It'll reach 3 or so when it's done, if all went well.

Base Stage

Titrate your oil and use the standard 8g + Titration (KOH) to determine your KOH amount for the base stage.

If your initial titration was below 18, you will add methanol equal to 10% of the initial oil volume. That would give you 22% total methanol when you add the methanol from the acid stage to the methanol added in the base stage. Note that if your initial titration was below 14, the acid water will float to the surface and you will not be able to settle and drain it.

If your initial titration was above 18, then drain half of the methanol you added for the acid stage before proceeding with the base stage. You will now add methanol equal to 16% of the initial oil volume. The extra 6% is to compensate for the methanol drained. Note that with initial titrations between 14 and 18 settling and draining acid water is optional.

If you drained, remix and recheck the titration. Normally it drops from the ending titration

Math Example

Oil titrates at 25 KOH
60 gallons of oil or 227 liters
25-3 = 22 * 0.2 = 4.4 ml of H2SO4(acid)
4.4 * 227 = 998.8 ml total acid. HANDLE WITH CARE!!!!
12% of 60 = 7.2 gallons of methanol or 27.25 liters
In this case the titration might only drop to 7.
Let it settle
drain 3.6 gallons out
Remix and titrate.
lets say it reached 3 for the titration after draining
for the base stage using KOH
8g (base) + 3 (titration) = 11g per liter
11 * 227 = 2497 grams of KOH total
10% of 60 = 6 gallons of methanol or 22.7 liters
If you drained at the end of the acid stage. Add that quantity in additional methanol. In this example since we drained, we would use 6 +3.6 = 9.6 gallons in the base stage
mix the KOH in the methanol. Use a sturdy container. it will get HOT!!
Add the methanol/KOH to the oil. This is same as a single base only stage.
Test for 27/3. Reprocess as needed...

Related Links

Single Stage Base Recipes
Two Stage Base Base Biodiesel Recipes

 

Biodiesel Production from Oils and Fats with High Free Fatty Acids
 
Canakci, M. and J. Van Gerpen
A.S.A.E. 2001

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