Biodiesel and SVO Forums Home
Need for a separate vessel ?
February 10, 2016, 06:32 AMJock
Need for a separate vessel ?
I have read on the forum that when doing acid esterification that some recommend using a separate vessel for the acid part of the process because of corrosion and neutralisation of the acid by the alkali.Is this due to residue of KOH or Naoh in the processor from previous batches.If so I would have thought that the one would neutralise the other.If this is going to affect the acid part of the process in the next batch would one have to up the acid to counteract this neutralisation and if so by how much?
February 11, 2016, 07:55 PMWesleyB
That's a simple question. I know how to do that calculation. You know the amount of KOH you used initially and its' purity. So what you might do is neutralize the KOH that's present prior to acid esterification. You produce a salt, potassium sulfate when you neutralize. Then you ignore the salt that's present as if it's not there. That's called spectator ions. They are there but they don't participate in the reaction. Acid + Base makes a salt. You add exactly the molar equivalent of the potassium hydroxide that you previously added to neutralize, then you ingnore the salt produced and proceed with acid esterification. BioTom can recomend to you adequate amounts of concentrated sulfuric acid to use for a figured titration. I don't remember how much I used for 100% free fatty acids. But a problem in fixing this screw-up is that when you neutralize the potassium hydroxide that's present it produces some water into your solution. The acid esterification reaction depends on being dry. It does not have to be 100% dry, but at some point too much water will cause acid esterification to not work. I did a series of calculations, if the potassium hydroxide is 100% pure (it's not) then about 4.749938461 milliliters of 100% pure sulfuric acid will neutralize the caustic and will produce about 3.2 milliliters of water. If you can do math you can calculate how much sulfuric acid to add to neutralize the potassium hydroxide to produce the salt potassium sulfate which will not stop the Fisher acid esterification from occurring. But so much water might be produced from the neutralization reaction that acid esterification might not work. Tilly is wrong to say it wiil not work. We actually don't know, it might work. If you have questions I will watch the forum and would probably answer. I had a government job once upon a time tudoring freshman chemistry. My student passed.
February 11, 2016, 11:56 PMBiotom
Jock the problem is no two reactors are the same and therefore do not harbor the same amount of caustic material after the base stage is removed, and if you don't use exactly the same amount of caustic in each base process the residual concentration of caustic material will differ batch to batch. The remaining caustic material has a huge negative effect on the acid esterification reaction. You can up the amount of acid used to compensate for this in your reactor vessel by trial and error process. This is really a hit and miss situation and one of the main reasons so many people have a problem when it comes achieving repeatable results using the acid esterification. The second problem is wet oil!!!
" I don't know what I don't know until I know"
1994 GMC 6.5 Tubo 2005 Dodge ram 3500, 3 VW's 2000, 2002, 2005.
February 12, 2016, 02:18 PMJock
Thank you Wesley and Tom for your input. if you do AE consistently using the same processor this problem must arise after each batch because you are doing the base process last.How do you overcome this.Although I suppose you want your biodiesel as close to neutral as possible.Is this what paves the way for the next batch?
February 12, 2016, 07:28 PMTilly
If you made your reactor so it was easy to remove the top and drain all hoses so that you could wash everything out and dry it after every reaction like I do then there would be no concern about left over water and caustic
February 13, 2016, 11:23 AMJock
Thank you all for your valuable input. My processor is made from a 210 litre drum with 1.5 mm thick walls ,a clamp on lid and a cone bottom with a drain right at the apex of the cone.Easy to wash out.So if as Tilly says to wash it out after each batch that should solve the problem for furture batches and to slow corrosion of the tank.
February 13, 2016, 08:05 PMWesleyB
It is not particularly significant. When a hydrogen is bonded to an oxygen as it is in alcohol for a percentage of time the hydrogen detaches and floats around as a charged ion. That's an acid. So, methyl alcohol is slightly acidic and as best I understand it due to oxygen pulling electrons towards it biodiesel should tend towards being acidic rather than base.I was not able to do the math calculations on the pH of methyl fatty acid esters (biodiesel) but maybe a chemist might comment if he could read this. So the pH of perfectly pure biodiesel is probably below 7.
February 14, 2016, 04:19 AMTilly
Originally posted by WesleyB:
It is not particularly significant. When a hydrogen is bonded to an oxygen as it is in alcohol for a percentage of time the hydrogen detaches and floats around as a charged ion. That's an acid. So, methyl alcohol is slightly acidic and as best I understand it due to oxygen pulling electrons towards it biodiesel should tend towards being acidic rather than base.
He was talking about the H2SO4 that is used in the AE stage.
was not able to do the math calculations on the pH of methyl fatty acid esters (biodiesel) but maybe a chemist might comment if he could read this. So the pH of perfectly pure biodiesel is probably below 7.
Biodiesel is an oil."pH is an index of the concentration of Hydrogen Ion (H +) in water.
Since oil is not an ionizing solvent, it has no free hydrogen ions and therefore, it does not have a pH per se.
If the oil contains materials which when mixed with water supply hydrogen ions to the water phase, then these will register when the pH of the water phase is measured."