The term "miscible" means (for liquids) two liquids will mix with each other in any proportion. For instance, methanol is miscible in water.

The term "soluble" means two liquids will mix with each other. For instance methanol is soluble in biodiesel. There is limit to how much of liquid A will dissolve in liquid B.

What does this really mean?

Here is an example that helps me to understand the difference.

Gasoline containing 10% ethanol is sold in Illinois. Since there is a tax break involved, both the departments of revenue and agriculture want to be sure there is a minimum of 10% ethanol in gasoline advertised as "gasohol". The state uses a simple test.

A graduated cylinder with 110 equal marks is filled up to the 90th mark with gasohol. Then water is added to bring the total volume up to the 110th mark. The mixture is then gently agitated and allowed to settle/separate. After just a minute or so, two separate liquids become visible in the cylinder. The lower mixture comes up to about the 17th or 18th mark in the cylinder, while the second fluid extends from there to the top.

The lower mixture is alcohol and water, the upper mixture is gasoline.

Ethanol is soluble in gasoline, so once mixed to form gasohol they do not separate as long as they are the only two ingredients in the mix.

Water has a very low solubility in gasoline, but is MISCIBLE with the ethanol. Given the choice between gasoline and water, the ethanol chooses the water over the gasoline. The combined mixture is heavier than gasoline, so settles rapidly.

You would think the ethanol and water would register on the 20th mark on the cylinder. They don't because, at least in part, the ethanol/water mixture is somewhat soluble in gasoline.

How does this phenomenon affect methanol recovery from biodiesel?

Methanol is soluble in biodiesel, but it is miscible in water. When wash water is added to finished biodiesel, the excess methanol chooses the water over the biodiesel and drops out with the water.

Additional washes remove additional soap and glycerol. But, for the most part, the first wash removes the methanol.

However, as in the gasohol example, a small portion of the water/methanol mixture is soluble in biodiesel. We dry biodiesel to remove the dissolved water. In so doing a small amount of residual methanol is also removed.

That is an awesome explanation...so what prompted you to explain it so well?

From what I recall...miscible means the molecules literally are intermixing with each other. Soluble means "chunks" of one fluid can mix into the other fluid. But being "chunks", its still possible to separate fluid 1 back out of fluid 2 under the right conditions.

Nice thread, producer!

We can also bring the 27/3 into this discussion, as it involves the differing solubilities of mono, di and tri glycerides in methanol.

And, it also brings up the subject of temperature as being an important factor in solubility and saturation levels, because you may see dropout of perfectly good biodiesel if the test is done cold, but with no dropout if done warm.

70F is about the temperature most folks find the 27/3 gives reliably useful pass/fail results.

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The same is also true of water in biodiesel.

Water will dissolve in biodiesel, and be invisible above a certain temperature, when the solution is not saturated.

But if you allow the biodiesel to cool, the solution becomes saturated, water droplets are expelled, and the biodiesel takes on a cloudy appearance.

A common cause of disappointment, when you've washed and "dried" your warm biodiesel, but find it goes cloudy later, when cool - just a sign that you didn't properly dry it.

Very interesting...

So, do you think that 100% pure FAME would be miscible in 100% methanol, or just have a very high solubility?

Good question!

I suspect fame and methanol are miscible, as is ethanol and water. Miscible implies that saturation doesn't occur. But we haven't quoted temperatures.

Here are 2 extremes ...

0.1% fame solution in 99.9% methanol is feasible at 20C
0.1% methanol solution in 99.9% fame is feasible at 20C

Does that imply universal miscibility for this pairing? I don't know enough to be able to answer that one.

If not miscible at 20C, is there a higher temperature for miscibility of these two?

Is there a temperature band at which saturation occurs for this pair? Is it symmetrical as one becomes more concentrated than the other and vice versa? I don't know that either.

quote:

Originally posted by producer:
Gasoline containing 10% ethanol is sold in Illinois. Since there is a tax break involved, both the departments of revenue and agriculture want to be sure there is a minimum of 10% ethanol in gasoline advertised as "gasohol". The state uses a simple test.

A graduated cylinder with 110 equal marks is filled up to the 90th mark with gasohol. Then water is added to bring the total volume up to the 110th mark. The mixture is then gently agitated and allowed to settle/separate. After just a minute or so, two separate liquids become visible in the cylinder. The lower mixture comes up to about the 17th or 18th mark in the cylinder, while the second fluid extends from there to the top.

8><---

You would think the ethanol and water would register on the 20th mark on the cylinder. They don't because, at least in part, the ethanol/water mixture is somewhat soluble in gasoline.

If you put 10 ml of alcohol in a graduated cylinder and then add 5 ml of pre-measured water to it, the total volume will be less than 15 ml. It takes more than 5 ml of water to make the level rise to the 15 ml mark. I think what you describe seeing in the gasohol test is due to the water being absorbed by the alcohol as the gasoline is forced out. I did a very similar test with E85 here.

Ken

Bunk, thanks for the clarification.

The employee from the department of agriculture that explained the ethanol/gasohol test to me said part of the reason that a full 20% did not separate was due to solubility. However, he said solubility was not the only reason, but couldn't say what other factors were at play.

When I said "They don't because, at least in part, the ethanol/water mixture . . . " I included the qualifying phrase "at least in part" because I did not understand the whole reason either.

Your explanation has cleared this up for me quite nicely. Thank you.

Ok, how would you go about determining what percentage prewash would be enough to absorb any left over methanol, do we think 5% is sufficient? I believe there is a lot of methanol left in my bio with a 5% prewash.

fabricator,

That is an excellent question. The only test I can think of that would work would be flash point testing. or GC testing for methanol.

A couple of interesting threads on the 5% prewash and Solubility:

http://biodiesel.infopop.cc/eve/forums/a/tpc/f/719605551/m/888605061

http://biodiesel.infopop.cc/eve/forums/a/tpc/f/419605551/m/514605951

Interesting thread. I recently ran two identical batches to see if I should continue the prewash. The prewash was 5% and the distillation on the BD was run for 3hours at 194F. With the prewash I got 3l of methanol at 93% purity, without I got 4.8l at 98% purity. It would seem then that 5% is not enough to pull all of the excess methanol out. No?

Tony

Sorry all, forgot the particulars.

150l batch
Soy oil titrating at 2.0 naoh
33l methanol
5.5g base
GL processor
Methanol injected over 30 min by venturi
Processed for 2 hours
Passes 3/27

Tony

Tony,

That's actually pretty impressive. It turns out to a 40% reduction of methanol in the biodiesel. In the early tests in the threads I linked to above, they were mostly interested in how much biodiesel was released from the glycerin. It would be interesting to see how increasing the volume of water effects how much methanol is retained in the biodiesel.

Maximizing the effectiveness of methanol removal by pre-wash brings up another question. What is the maximum amount of water that you can add to the reactor without creating an emulsion? Would soap concentration play a part? You might get away with a 20% pre-wash with a feedstock source titration of 2 but make mayonnaise with a 15% pre-wash out of biodiesel with a source titration of 8.

Ken

quote:

Originally posted by fabricator:
Ok, how would you go about determining what percentage prewash would be enough to absorb any left over methanol, do we think 5% is sufficient? I believe there is a lot of methanol left in my bio with a 5% prewash.

I purposely did not try to quantify how much water is required to remove so much methanol. In the gasohol test things are straight forward. You have two chemicals, gasoline and ethanol, so the test is uncomplicated.

Unwashed biodiesel contains, at a minimum, unreacted oil, methyl ester, mono- and di-glycerides, excess catalyst, soap, water, and methanol. All of these ingredients make it difficult to predict the volume of prewash water needed to remove all methanol. Trial and error works better.

Also, keep in mind, in a 5% prewash we are not separating into a water/methanol fraction vs a biodiesel fraction. The reactants still separate into a glycerol layer and a biodiesel layer. This glycerol has a higher percentage of water than we would have if a prewash not been used.

quote:

Originally posted by Bunk:
What is the maximum amount of water that you can add to the reactor without creating an emulsion?

Bunk, do a search on this topic. I recall a discussion on this very topic. People reported on using a range of wash percentages. Some went as high as 15% or so. Others tested at 10%, and lower down to 2% and 3%. All things considered 5% was arrived at by consensus. You may be able to use a higher percentage, but you should test this on your BD, if it works, then good.

Keep in mind using more water in the prewash removes more contaminants, but it also greatly increases the risk of forming an emulsion.

I don't even do the 5% pre-wash most of the time. I never have trouble washing.

Ken