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Separating glycerine/FFAs

Separation
How much acid?
Glycerine
Free fatty acids
The lye
Using other acids
Example
Discussion
Economics

Most of the lye and most of the excess methanol used in the biodiesel process collect in the glycerine layer that settles out at the bottom, along with the soap formed when Free Fatty Acids (FFAs) are neutralized by the excess lye.

The proportions of each depend on the oil you used and your process -- the two-stage acid-base process will give different results to a single-stage base process.

Adding phosphoric acid (H₃PO₄) converts the soap back to FFAs and separates it all into three distinct layers, with catalyst-phosphorus on the bottom, glycerine-methanol in the middle, and FFAs on the top.

The methanol can then be recovered from the glycerine in the middle layer by heating to above 65 deg C (150 deg F) in a closed container fitted with an outlet into a simple condenser.

Some home brewers recover the excess methanol at the end of the processing step while the brew is still warm, before settling and separating the biodiesel from the glycerine layer. But treating the glycerine layer to separate the catalyst and FFAs is more difficult if the methanol has already been recovered.

Separation
Add the phosphoric acid and mix it thoroughly. The mixture gets a bit hot from the reaction. A translucent glycerine layer will form under a dark FFA layer, and not long afterwards the catalyst precipitates out at the bottom -- sodium phosphates if you used NaOH as the catalyst, or potassium phosphates with KOH (in which case separation takes a bit longer).

The catalyst layer is yellowish, the glycerine layer is translucent and the color of sherry, the FFA layer much darker. There's a clear separation between each.

Then the three layers can be separated and the methanol can be removed from the glycerine.

How much acid?
How much phosphoric acid to use? You have to neutralize all the catalyst -- ie, the basic 3.5 gm NaOH or KOH equivalent per liter of oil for transesterification plus whatever titration indicated was needed to neutralize the FFAs. Traces of catalyst will remain in the biodiesel rather than collecting in the glycerine at the bottom, but it's better to allow for the full amount of lye used.

Concentrated phosphoric acid is usually 85% strength, the other 15% is mostly water. We found that it takes about 1.5 to 1.7 ml of 85% phosphoric acid for each gram of sodium lye used in the process.

So, if the WVO you used titrated at 3 ml, you'd have used 3 + 3.5 = 6.5 grams of lye per liter of oil in the process. To treat the glycerine layer, add 6.5 x 1.5 = 9.75 ml of 85% phosphoric acid per liter of oil used.

You can try it, but feedstocks, methods and techniques vary so widely that it might not work for everybody. Also it's worth trying to use less phosphoric acid, it's expensive stuff.

You can buy concentrated 85%-strength phosphoric acid by the litre online from DudaDiesel Biodiesel Supply: http://dudadiesel.com/

This is a better way, as recommended by Todd Swearingen of Appal Energy at the Biofuel mailing list.

"As the glycerine/FFAs from all batches accumulate, it's easier for us to conduct a titration with phosphoric acid rather than trying to assess an ideal quantity to use.

"We start at 10 ml of 85% phosphoric acid per 250 ml of glycerine/FFAs. Put 250 ml of the glycerine layer in a jar, add 10 ml of phosphoric acid, shake vigorously. The first hint of glycerine separation should be visible within 5 minutes. If not, then add another 5 ml; work your way up in 5 ml increments until it starts to separate.

"This is too much acid, so we then go into reverse with further titrations. Start with 2 ml less phosphoric acid than the sample that started to separate, and another 2 ml less in each titration after that.

"Come back 24 hours later, observe the 'break point' where samples do and don't separate out. We then use that level of phosphoric acid on 45-gallon batches of glycerine/FFAs."

See Discussion below.

Glycerine
The crude glycerine layer in the middle is acidified in the process and should be neutralized, with a dilute lye solution, or baking soda (sodium bicarbonate, NaHCO₃). This leaves you with industrial-grade glycerine of about 90% purity. It's a much more attractive product for glycerine refiners than prior to FFA recovery, and you might be able to sell it to a refiner, if not for much, or at least give it away rather than having to dispose of it. Getting it purer than this takes a great deal of energy and isn't worth it for anything less than a large-scale operation.

Separated crude glycerine is also easier to compost than the whole by-product cocktail -- in fact it's been found that it speeds up the composting process. See:
http://snipurl.com/rq1z
Re: [Biofuel] Crude Glycerin and Hot Compost
30 May 2006

Free fatty acids
The FFA burns well, but it doesn't climb a wick very well. It can be used as heating fuel. Like the by-product cocktail, it can be composted (hot compost is best). It also makes an effective weedkiller, and should biodegrade quickly once it hits the soil.

The lye
The lye catalyst reacts with the phosphoric acid to form sodium phosphates (with NaOH) or potassium phosphates (with KOH), and either can safely be added to the compost pile. Industrial-scale biodiesel operations use potassium hydroxide, KOH, as the catalyst, and after separating the by-product with phosphoric acid the potassium phosphate left is sold as a fertilizer.

If you use sodium hydroxide, NaOH, it's been said that the sodium phosphate you're left with is nothing but a "useless salt".

Not quite true.

First, we're not interested in chemical fertilizers, and we add any plant nutrients to the compost pile rather than applying it directly to the soil.

In fact sodium is also a plant nutrient, and almost as much of it is required as potassium. The common idea that more fertilizer is better is dangerous -- too much of either sodium or potassium, or of any nutrient, can cause soil imbalances that can ruin the soil structure and leave other nutrients unavailable to plants. As a minor constituent in a compost pile, this won't happen, any excesses are buffered.

However, farmers who use chemical fertilizers don't know much about this, as their advice mostly stems from the chemical fertilizer companies. And they will pay for potassium fertilizer, but not for sodium.

More about composting the salts:
http://snipurl.com/rq20
Re: [Biofuel] more goofy questions [composting the glycerine cocktail]
31 May 2006

Using other acids
Hydrochloric acid (HCl) or sulphuric acid (H₂SO₄) can also be used for separation. Sulphuric acid will yield sodium sulphate or potassium sulphate salts. Sodium sulphate is used in the pulp and paper industry, potassium sulphate is used as a fertiliser. Either can be added to the compost pile in small proportions.

With hydrochloric acid the salts will be sodium chloride (common table salt) or potassium chloride, also known as muriate of potash, which is sold as a fertiliser but has a very harsh effect on the soil microlife. Potassium chloride can safely be composted, and so can a little sodium chloride, though not too much.

NaOH lye with hydrochloric acid used for separation is the poorest choice for environmental recycling.

However, the sodium chloride that results can be turned back into sodium hydroxide, by electrolysis, for re-use as biodiesel catalyst. Here's one method:
http://www.mail-archive.com/biofuel@sustainablelists.org/msg28376.html
"... producing Chlorine, Hydrogen, and Lye from table salt, using PET
bottles, flashlight batteries, glue and stuff."

But we don't know of anyone who has done this yet.

To use hydrochloric acid or sulphuric acid, do some small tests first to find out how much is required for separation.

Photograph by Todd Swearingen The jar on the left shows what was the glycerine layer separated out of single-stage base reactions. The jar on the right shows the glycerine layer from two acid/base reactions (the Foolproof method) and two single-stage base reactions. Both were treated with 85% phosphoric acid, cracking the soaps back to FFAs. The precipitate on the bottom is potassium phosphate -- the catalyst neutralized by the phosphoric acid. We use potassium hydroxide as the base catalyst. If you use sodium hydroxide it will be sodium phosphate. The center layer is a blend of crude glycerin, water and excess phosphoric acid. The top layer is the recovered FFAs. The jar on the left is 16 fluid ounces and the one on the right 24 fluid ounces, but the important point here is the ratios of recovered co-products. The left jar (single-stage) has a much higher proportion of FFAs and catalyst than the right jar (half acid/base and half single-stage). This is one of the benefits of the acid/base process -- FFAs are converted to biodiesel in the acid stage rather than saponified. The acid stage also reduces the amount of base catalyst required, so less phosphoric acid is needed for FFA recovery -- more biodiesel, lower volumes of co-products. A third jar on the right using the glycerine layer from strictly acid/base reactions would show even smaller ratios of FFAs and catalyst. -- Todd Swearingen, Appal Energy

Discussion

• -- from Tom Kelly, Biofuel mailing list, 10 Apr 2006
  
  I have many 4.5 gal (17.7L) cubies full of the glycerine coproduct. I've begun the task of splitting it. I would appreciate comments on my initial results.
  
  The oil I use titrates between 1.0 and 1.5g/L. Rough calculations lead me to believe that I would need at least a Liter of 85% H₃PO₄ [phosphoric acid] and more likely 1.5 L for each cubie. I added 1L to each cubie ---- no split. I removed 0.5 gal from one of the cubies and added 50ml H3PO4, shook it and waited, --- no split. Added another 50ml, shook it and waited  --- split. I then added 500 ml acid to the 4 gal in the cubie --- no split. Another 100ml and yet another ---- split.
  
  No question; three distinct layers. Here's my concern: the mineral precipitate appears to be sandwiched between a dark top layer and a lighter bottom layer.
  
  I assume the top layer is FFA, the bottom layer is glycerine + methanol. Isn't the mineral layer supposed to be on the bottom?