[Double D]

Greetings Ladies and Gents.
This is my first post here on the forum. After lurking in the background, it appears this is a great knowledge base for chemistry so I thought you may be willing to answer a/some questions.
Thanks in advance.

I'm a weekend home biodiesel producer.
The byproduct after transesterification contains methanol, water, biodiesel, excess caustic, glycerol, and soap.
I'm wondering if the excess caustic can be converted into Calcium hydroxide through the introduction of Gypsum.

CaSO4.2H2O + 2NaOH ->  2NaSO4 + Ca(OH)2 +2 H2O

There is generally a small excess of caustic left over (if any) that hasn't been converted into soap especially following distillaion of the excess methanol.
This is not a necessarily a feasibility question but just to satisfy my curiosity.
Any takers?

[Borek]

CaSO4.2H2O + 2NaOH ->  2NaSO4 + Ca(OH)2 +2 H2O

This idea won't work, equilibrium for this reaction is far to the left. Calcium sulfate is the least soluble of all substances involved.

Note: it is Na₂SO₄.

[Double D]

Borek,
You say Calcium sulfate is the least soluble. My thought was that the CaSO4 is soluble in Glycerol (per wiki), which is soluble in water.
Is that how you determine that the equilibrium will shift to the left, solubility?
Could there be any Calcium Hydroxide that is formed? Not necessarily ending up with the balanced equation as written?
How about if an excess of CaSO4 were used? Would the equalibrium shift to the right, provided you could dissovle the CaSO4?

I was mainly wondering if there could even be an ion exchange between the CaSO4 and the NaOH and if so what would happen the water that was attached to the CaSO4 as a dihydrate.

Please don't take these questions as challenging the accuracy of your answer.
I would just like to understand how your determination was made in order that I may be able to come to a better understanding.

[Borek]

You say Calcium sulfate is the least soluble. My thought was that the CaSO4 is soluble in Glycerol (per wiki), which is soluble in water.

I am not sure about solubility of calcium sulfate in glycerol, I don't think it can be really high. Glycerol is much less polar than water.

Is that how you determine that the equilibrium will shift to the left, solubility?

Depends, there is no one easy rule. That's what chemistry is about. But in this particular case it is solubility that counts.

Could there be any Calcium Hydroxide that is formed?

Not sure what you are asking about. There can be some hydroxide present, it has low solubility as well, but it won't dominate.

How about if an excess of CaSO4 were used? Would the equalibrium shift to the right, provided you could dissovle the CaSO4?

First - you can't dissolve it, second - amount of solid doesn't matter.

I was mainly wondering if there could even be an ion exchange between the CaSO4 and the NaOH and if so what would happen the water that was attached to the CaSO4 as a dihydrate.

Water goes into solution when CaSO₄ gets dissolved, but only in stoichiometrci amount (that is 2 moles of water per each mole of dissolved sulfate). As there will be not much sulfate dissolved, there will not much additional water in solution.

[Double D]

Thank you Borek,
I do believe your wisdom has satified my curiosity.

[AWK]

http://www.dow.com/PublishedLiterature/dh_0032/0901b8038003228b.pdf?filepath=glycerine/pdfs/noreg/115-00668.pdf&fromPage=GetDoc

[Double D]

http://www.dow.com/PublishedLiterature/dh_0032/0901b8038003228b.pdf?filepath=glycerine/pdfs/noreg/115-00668.pdf&fromPage=GetDoc

Thanks AWK.
Your chart aroused my curiosity again.
According to your chart ~5 parts CaSO4 will dissolve in 100 parts Glycerol. I'm guessing this is by weight.
The solubility of Ca(OH)2 is ~1 part per 100 parts glycerol (per the chart).
The solubility of Ca(OH)2 is .173g/ ml water (wiki).
The solubility of NaOH is 111g/ml water (wiki).
The solubility of CaSO4 is ~1 part per 100 parts glycerol (per the chart).
the solubility of CaSO4(dihydrate) is .24g/ml water (wiki).
NaOH is soluble in water and Ca(OH)2 is almost insoluble. Caso4 is almost insoluble in water.
I was thinking that since there is water present in our byproduct, the NaOH would stay in solution in the water and CaSO4 would stay in solution in the glycerol. Since glycerol and water are miscible, my thought was any Ca(OH)2 created would precipitate out. I was thinking that any NaSO4 created would precipitate out upon cooling but it looks like there is so little that could be created, It would not precipitate out given the wiki solubility in water is listed as 4.76g/ 100ml at 0C.


This is the equation I'm guessing may occur. I have no idea if this is even possible.
I'm thinking that since NaOH is soluble in water and there is glycerol and water present,
CaSO4 + 2NaOH -> Na2SO4 + Ca(OH)2

MW  CaSO4 = 136g, mol
MW NaOH = 40g, mol
MW NaSO4 = 142g, mol
MW Ca(OH)2 = 74g, mol

5g  CaSO4 = 5g/136g= .0367mol CaSO4
mols CaSO4*2 = .0367mol *2= .0734mol *40g = 1.496g NaOH needed.

products,
.0367 mol NaSO4 = 5.2g
.0367 mol Ca(OH)2 = 2.7g

Any thoughts?