I have various acids available, though not any Sulfuric. Will other acids behave similarly? What about Phosphoric, HCl, Nitric? I have some HF but I don't even like looking at that drum much less using any!

What about the glycerine? Is it any different quality than the glycerine from the caustic-only blend?

I can see where the Acid Esterfication process is effective on oils with high fatty acid levels but what about more average oils? I guess my question is the cost comparison of doing the acid route. How much more biodiesel does it produce compared to standard biodiesel blending? And how much acid does it take to achieve that (on average).

I know we are dealing with a lot of variables but am curious to some general rules of thumb.

Thanks in advance.

Todd T

I think you know better than I do since you're the chemist, but I think one should avoid nitric acid in our chemistry!!!!!

In the patent literature it says somewhere that this process works with phosphoric. I can't quote which patent, as I just don't remember when I saw it- I assume a 1940's one.

Mark

Oh and depending on the acid, you'll get different salts to wash out of your biodiesel when you wash it, or different salts contaminating the glycerol I guess.
(ie NaOH or KOH plus various acid= carious salts)

Mark

quote:

(ie NaOH or KOH plus various acid= carious salts)

Meaning "varoius" or "precarious?" Or, is my vocabulary failing me? Not picking on what is probably a typo, just don't want to be uninformed when the day comes that my BD has to do some acid.

various. Sorry. I'm a terrible typist.

Mark

Todd T

I think from your post you were not talking about acidifying the glycerine but acid esterification. To get the esterification to work you need not only an acid catalyst but a dehydrating agent. Nobody seems to realize the importance of this.

Concentrated sulphuric acid is a powerful dehydrating agent and can be used in dessicators. It works. Hydrochloric acid cannot be obtained as a liquid at greater than about 32% and cannot perform the reaction. It might work a bit if used as a dry gas but to get it to go properly a dehydrating agent would be needed with it. Phosphoric acid is usually obtained at about 80%. It won't work for the same reason.

If you really want to see the reaction go like clockwork use thionyl chloride. It is a powerful dehydrating agent and generates HCl gas as a byproduct which acts as the catalyst. It also generates SO2 gas. The product obtained from the base reaction afterwards is as good as from new oil. It does not have settling problems which sulphuric acid seems to cause and for that reason I believe sulphuric acid does some damage to the oil. It is still however the only reagent cheap enough to use.

Regarding yield, you should be able to get over 90% with bad oil giving you 85% with base only. You can increase the yield further by acidifying the glycerine, recovering the oils released, and adding them to the next batch. This option is not available with base only.

The amount of sulphuric to use varies with the oil FFA%. Idaho Uni recommends 5% of the FFA content and it seems to work.

HF? Don't touch it! One drop on the finger and you are in serious trouble. If you have it in a can it seems a worry. What happens if it corrodes?

neutral, I always thought that the separation problems with esterification was due to the fact that there is little or no glycerin produced making the byproduct a mix of water and methanol with a specific gravity very similar to the feedstock.

Also, how would one set about experimenting with thionyl chloride? Would the reactor design need to be different to deal with the gases?

What about this: "A fuming sulfuric acid. Sulfur trioxide (SO3) and 100 percent sulfuric acid (H2SO4) are combined to form oleum."

Dropout, you are quite right about the difficulty of getting anything to separate out after the acid stage. I was not talking about this however, but separation after the base stage. I never get such good separation as I do with base only.

With base only, after the settling has proceded for a while, you will see a sharply defined surface, rising gradually.

With acid/base you will see a rough surface moving down, with beads of ester layer in it, so that the higher levels are paler. It takes a long time for these to settle and I don't think they ever quite do. Yields are never quite as high as you think they should be, given the titration achieved before the base stage.

Has anyone else noticed the same thing?

Regarding use of thionyl chloride, it is too expensive to consider in a production batch. The gases would finish up in the glycerine as common salt and sodium sulphide.

Regarding sulphuric acid, I think conc sulphuric is bad enough. If you use oleum you would probably get more damage to the oil. There could be a case however for adding only a small part of the conc sulphuric at the start and slowly adding oleum as the reaction progressed. It is after all "drier" than conc sulphuric. I don't know what it costs.

Thanks neutral.

Are you saying that the thionyl chloride removes the water as the HCl gas is generated? Does that mean that the thionyl chloride disappears during esterification? If so, would less NaOH (or whatever) be needed with this than with conc. sulphuric?

You say that it is too expensive to use, but isn't there a point where better water control, possibly using less base catalyst, better separation, easier washing and getting higher yields may tilt the balance the other way.

Yes the thionyl chloride disappears. It has a voracious appetite for water and will dehydrate the veg oil for a start. Have a look at the equation.

SOCl2 + H2O -> 2 HCl + SO2

I suppose you could strip out the HCl and SO2, being gases, after the acid stage and so would not need to use so much NaOH, but what would you do with them then? I suspect a commercial user would be required to scrub his effluent gases and what would he use? NaOH. Back where we started.
Do some research on the cost.

thionyl chloride, VWR# 101189-368, 50 gram, $75.60.

VWR# MK852204, 500 grams, $202.65

The feeling of pride after proving it works,

Priceless!

Acetyl chloride also works, and might be cheaper. Needs about 4g per litre of oil. It boils at 51deg C so you would have to keep the reaction temp down a bit. A reflux condenser would be a help.

neutral, how is the 4g per litre related to titration or ffa?

I would try 0.5 x homebrew titration (NaOH) for acetyl choride. With sulphuric acid I would use 0.1 x homebrew titration. Answers in g per litre of oil.

Will sulfamic acid work? I can get bags of the stuff locally.

quote:

Originally posted by neutral:

If you really want to see the reaction go like clockwork use thionyl chloride. It is a powerful dehydrating agent and generates HCl gas as a byproduct which acts as the catalyst.

neutral, does this mean that the feedstock needs a certain % of water for this to work?

No. These reagents work mainly by turning the free fatty acids into acid chlorides. Acid chlorides attack the OH group of alcohols. This leads to the intriguing observation that they could be used without methanol. In this situation they would cause the FFA to be reattached to the glycerol that they previously fell off. The released HCl aids the reaction so less reagent is needed but free water destroys the reagent so more is required.

Thanks neutral

In a conversation with a chemical engineer today the suggestion of using hydroiodic acid instead of sulfuric acid came up. Would this work?

Hydroiodic acid is idiotic. It is a solution in water just like conc hydrochloric acid. Your engineer apparently didn't know that water was a problem.

Hydroiodic acid is also illegal as hell, its used to make methamphetamine...

Phosphorus Pentoxide would be good acid choice if it would work...