I've read somewhere here on infopop about the idea of using sodium bicarbonate (baking soda, pool pH adjuster) to neutralize sulfuric acid after the acid stage, but there were no direct conclusions reached, and the practice did not seem to catch on.

As I understand it, sodium bicarbonate + H2SO4 would produce sodium sulfate + CO2, and maybe some water? However, the sulfate should bind a lot of the water, and adding NaOH produces a lot more water - the net effect should be much less water produced with the bicarbonate.

I'm willing and interested to try it in a small batch (5-10 gallons), but would appreciate some guidance as to how much I should use per mL of H2SO4, and how to add it to the mixture. Does it dissolve in methanol, or can I just add it as a powder?

Thoughts and experiences with it welcome

--- David

I researched and experimented with this some, and wanted to note the results for future reference.

The reaction is:

2 NaHCO3 + H2SO4 -> 2 CO2 + 2 H2O + Na2SO4

According to http://theodoregray.com/PeriodicTable/MSP/BalanceReactions
we'd need 1.7 g bicarbonate per gram of H2SO4.

With standard lye, the reaction would be:

2 NaOH + H2SO4 -> 2 H2O + Na2SO4

and we'd need .815 g NaOH per gram H2SO4.

Since the price of pool grade bicarbonate is more than half the price of lye on a per-pound basis, the given assumption that it's less expensive is just plain wrong.

In real terms, it's a lousy way to neutralize anything. I tried it in a small, safe, way and got no reaction at all. I tried adding it to a 45 liter test batch with no effect at all, even after running the circulator pump for 6 hours.

If it *did* work as intended it would release nearly a gram of CO2 for each gram of acid; that can't be good.

So my conclusion is that this is a bad idea from both an economic and technical perspective.

--- David

What are your titration numbers, from what I read most seem to think the 2-step acid/base process is not practical until you get VERY high titrations, upwards of 15, There have been several discussions on both the biodiesel and the SVO sides of the forum about caustic stripping, reducing 12-15 titrating oil down to 4-5. You will loose some of the original oil volume as soap but the base process will use MUCH less lye and produce very little soap, washing will be easier and quicker.

Unless you only have very bad oil available, and in limited quantities, most accept the soap loss and move on to processing the next batch.
Link to FFA removal discussion

Link to lye stripping discussion

I don't make biodiesel but use the oil directly, I use baking soda to convert excess FFA into soap and also to neutralize any water soluable acids. I use a flash evaporator for dewatering so need to flush all the water soluable acids out of the oil to insure they are not concentrated during the evaporation dewateing step.

I use baking soda dissolved in the wash water as the first step in cleaning all my collected oil. From using the BS for a couple years now I find that it WILL NOT dissolve directly into even wet oil if stirred in as a powder. I suspect the BS granuals get coated with oil before they contact enough water to dissolve. I do a 10% water wash on my collected oil anyway so I dissolve one pound of BS in a cubee of water, pour this in to a 50 gallon barrel, then fill the barrel with oil to within about 4 inches of the top (room for soap scum), then I stir the water into the oil. I do a very thorough job of mixing in the BS/water, stirring with a drill powered paint paddle til I get a well mixed peanutbutter colored fluid, only takes about a minute. this then settles for a couple weeks. There will be CO2 bubbles rising from about the second day, then for several more days, as the reaction progresses, it may even bubble for the entire 2 weeks, depends a lot on how warm the oil is, cooler oil bubbles slower but longer. The water ends up on the bottom, soap and wet particulate floating in a seperate layer above the water, then fairly clean oil above that. I have not titrated the oil but sometimes I get a lot of soap, sometimes almost none, depends totally on the quality of the particular barrel full of oil. Since I do get CO2 and soap the BS is definitely able to react with any FFA in the oil, but only if first dissolved in water, never tried dissolving it in any alcohol. Since some soap is always produced, the titration number has to be reduced, the mild BS will only react with already free fatty acids, it won't break any additional FFA out of the trigliceride, the soap has to be being produced from reducing only the FFA's that are already in the oil, reducing the amount of FFA's has to reduce the titration number.

Don't have a clue what happens when there are other acids present ?

Hi David,

If neutralising FFA oil, you need to be sure to use enough water for the BS to dissolve, or it will just sit around in granular state and do nothing for you..

Only 7g of BS will dissolve in 1 litre of water at 20C, adding any more won't give you any added return.

Thanks Graham!

I definitely don't want to be adding water to the oil between acid and base stages, so this is one more reason to forget about baking soda.

I recently ran across the thread on "Reuse of Catalyst" (http://biodiesel.infopop.cc/eve/forums/a/tpc/f/719605551/m/8711085612/p/1)
and thought it would be perfect.

If I have ~4 grams/liter of NaOH catalyst left in the by-product it's enough to neutralize 4/1.45 ml of H2SO4, or 2.75 ml of H2SO4. In addition, it HAS to be neutralized before acidulation of the byproduct, so it's a perfect match.

This makes my fast-food processing cycle look like this:

1) Add 2.5 ml H2SO4 and 60 ml or so of recovered methanol (or x ml of glycerine) to each liter being processed

2) Circulate at high temps for X time units until the FFA's are esterified

3) Add the last batch's byproduct in to neutralize the acid, and let the glycerine pick up the water

4) Drain the glycerine/water/salt/methanol. This byproduct is close to neutral, has some extra water, and is acidulated

5) Continue with normal base transesterification. The byproduct from this stage(s) is (are) used to neutralize the acid phase of the next batch.

It's still conceptual at this point; the thing I'm sure about is that I don't want to use any baking soda.

--- David
The bypr

Out of curiosity, where did the idea that using NaOH instead of NaHCO3 will produce more water come from? It will be the same.

It is common practice to use saturated sodium bicarbonate to neutralise solutions in organic chemistry (lab scale). Using the crude glycerol layer/catalyst has advantages, but I wonder if perhaps it might promote some saponification of the triglycerides as well, as it is a much stronger base than sodium carbonate. One advantage of sodium carbonate is that the evolution of gas (Carbon dioxide) can be used as a qualitative indication of the level of remaining FFA. The CO2 could perhaps also be captured and fed to an algae pond or greenhouse if you have such a setup.

Using a saturated solution minimises the risk of adding water to the oil, however it may still occur to a small extent.

quote:

Out of curiosity, where did the idea that using NaOH instead of NaHCO3 will produce more water come from? It will be the same.

I guess that should have been obvious, since NaOH + CO2 = NaHCO3. At least from an elemental perspective, it's the same number of hydrogens and oxygens.

quote:

It is common practice to use saturated sodium bicarbonate to neutralise solutions in organic chemistry (lab scale). Using the crude glycerol layer/catalyst has advantages, but I wonder if perhaps it might promote some saponification of the triglycerides as well,

Well, the general idea is that if the acid stage is carried to it's conclusion there shouldn't be very many FFA's left available. Also that the H2SO4 will quickly neutralize most of the leftover NaOH. So there won't be much leftover lye to saponify anything.

Once the acid is neutralized and the byproduct + salts are drained off the remaining oil "should be" neutral to slightly basic, dry, and ready for a first base stage.

I'll be trying it in the next few days, I can let everyone know how it turns out.

quote:

One advantage of sodium carbonate is that the evolution of gas (Carbon dioxide) can be used as a qualitative indication of the level of remaining FFA.

If the acid stage has worked there should be far more H2SO4 molecules around than FFA's. The CO2 would give an indication of how complete the overall neutralization process is.

But I still think baking soda is not optimal at all. It's at least as expensive as lye, apparently impossible to apply without adding water at a point we don't want to add water, and produces CO2 - potentially creating a very hazardous situation.

--- David

Sorry about the FFA and saponification comments, I got mixed up between this thread and the "Experiments in oil pre-treatments". Thats what you get for reading two threads at once. lol.

In spite of that, it is doubtful you will esterify all of the FFA, unless you remove the water as you process, and add a decent excess of methanol.

quote:

Well, the general idea is that if the acid stage is carried to it's conclusion there shouldn't be very many FFA's left available. Also that the H2SO4 will quickly neutralize most of the leftover NaOH. So there won't be much leftover lye to saponify anything.

If you are using base stage byproduct to neutralise the sulfuric acid, you don't really have any idea of how much NaOH remains (unless you do a titration).

If you have more NaOH than sulfuric, then you risk saponifying some of the oil, if you have more sulfuric than NaOH, you will likely find the excess acid protonates the soap to form FFA's again.

Good luck with your testing.