The formation of soap during biodiesel manufacture is a common subject of discussion on this and other biofuels forums.
Unfortunately, however, as this site has many armchair experts (one in particular) who don't know any chemistry, a great deal of the comments (and therefore advice) are wrong.
I'll now explain it.
Soap is nothing more than the alkali metal salt of a Free Fatty Acid. We may represent the FFA as RCOOH, with R being the alkyl chain and COOH of course being the carboxylic acid group. Thus the soap (if potassium is the cation) is RCOOK.
How is it formed? Well, for our purposes, there are two mechanisms. I'll illustrate it by outlining the reactions that take place in both my FRT method, and the Dr Pepper method.
FRT Method.
Before the Reaction:
When the methoxide is added to the WVO, present in the mixture is
Notable by its absence is water. The methoxide solution is dried (see hyperlink) and the water that is normally added with the methoxide solution is removed.
There are two reactions:
Reaction 1. Neutralization of FFAs
The first reaction that happens is the reaction of the highly alkaline methoxide with the FFA. As methanol has a pKa of 15, it is obviously the case that its conjugate base (the methoxide ion) is highly alkaline.
Thus the reaction is as follows: RCOOH + MeOK -> RCOOK (soap) + MeOH (methanol).
So the product of the reaction of the methoxide ion with the FFA is soap and methanol.
Reaction 2: Transesterification:
The second reaction that occurs is the reaction of the leftover methoxide ion with the WVO:
C6O6H5R3 + 3KMeO -> C3O3H5K3 (potassium salt of glycerol) + 3(MeOCOR)
After the Reaction:
Present in the mixture after the reaction is:
What, then, is the fate of these four components?
The first thing that happens is that is separates into two phases - the glycerol and the biodiesel.
The excess methanol is present in both phases. I don't know the partition coefficient, so I can't comment on the ratio between the phases, but it's certainly present in both phases.
What of the soap? What happens to that? Well, we can see what happens to the soap if we look its HLB (Hydrophilic Lipohilic Balance):
As we use soap to wash ourselves, it an oil in water (o/w) emulsifier, with a HLB between about 10 and 16. This means that it's insoluble in hydrophobic material (oils), but water soluble. So in the case of our mixture, it will be held in solution by the methanol, as methanol is obviously hydrophilic in nature (like water).
Because my method is anhydrous, I don't wash with water, but wash with air. As the air bubbles through the mixture it gradually evaporates the methanol. Eventually, when it is all gone, the soap simply falls out of the solution (as it is insoluble in the biodiesel) as a gel-like, clumpy, light brown material that sits on top of the glycerol.
I have a batch currently clarifying, and I'll post some pics when it's ready.
So as soon as I see the soap falling out of solution, I therefore know that the batch is ready for use (although I still do a QC test on it). I've actually been observing this phenomenon for some time now, but didn't actually sit down to look at the chemistry of it until WesleyB asked some questions about it.
The Dr Pepper Method
Before the Reaction:
When the methoxide is added to the WVO, present in the mixture is
One of the weirdest things about the discussions that take place on this forum is the time that people waste ensuring their oil is dry, only to add water to the mixture with the methoxide solution:
Eq1: KOH + MeOH <-> MeOK + H2O
In other words, for every molecule of methoxide you are adding a molecule of water
There are three reactions:
Reaction 1. The neutralisation of the FFAs.
With the water present this is not quite as straightforward as it is with my FFR method. There are two ways that the FFA can be neutralised:
RCOOH + KOH -> RCOOK + H2O (pulls Eq1 to the left)
RCOOH + MeOK -> RCOOK + MeOH (pulls Eq1 to the right).
Thus, there are competing influences in terms of the effect on the equilibrium
Reaction 2: Transesterification:
The second reaction that occurs is the reaction of the leftover methoxide ion with the WVO:
C6O6H5R3 + 3KMeO -> C3O3H5K3 (potassium salt of glycerol) + 3(MeOCOR).
This will pull the equilibrium (Eq1) to the right, thus generating more water
Reaction 3: Saponification:
C6O6H5R3 + 3KOH -> C3H8O3 + 3(RCO2K)
Obviously if you want to make biodiesel, you want to discriminate against the 3rd reaction and promote the 2nd reaction. This is a common approach in organic synthesis - designing the reaction conditions to favour one process over another. This is a complex question from a theoretical viewpoint, as it involves issues such as the reaction mechanism (pretty sure the transesterification is SN2, not sure about the saponification), the activation energies (Ea) of the respective processes, and the order of the reaction (usually determined by the mechanism, but I suspect both are second order reactions).
In this case this is done by strictly controlling the amount of catalyst, and the reason is obvious - you need enough catalyst to achieve the Ea at the selected temperature (55 degrees I think), but without putting in so much so that you generate enough water to promote the saponification reaction (consistent with a 2nd Order reaction). In other words, the kinetics of both reactions are probably pretty similar, but the transesterification reaction dominates purely because there is more methanol than water. But both reactions are certainly occurring.
And now we come to a significant difference between the transesterification and saponification reactions:
In the transesterification reaction, the KOH is a catalyst
In the saponification reaction, the KOH is a reactant, and water is (probably) the catalyst
That is, in the transesterification reaction the KOH is not consumed, but in the saponification process it is.
The implications of this ought to be obvious;
Firstly, with this method you need enough KOH to neutralise the FFAs, and have enough left over to initiate the transesterification reaction. But if you put in too much KOH, then you generate more water. More water (assuming 2nd order reaction kinetics) means the rate of the saponification increases. Also, the more water in the system (remembering it is not consumed in any reactions) means there is a greater chance of forming an emulsion, and there are plenty of posts on this site that have done just that.
Secondly, as the KOH participates in two reactions (and not one as with the FFR method), more KOH may be required than with the FRT method. Never have used the Dr Pepper method myself except right at the outset I have no idea what typical values are, but the beauty of the FRT method is that there are no interfering reactions, so all the methoxide can be used to catalyse the transesterification.
So what happens with the soap in this method. Well, it's going to be dissolved in the hydrophilic phase, in this case the methanol and water. So in principle, with repeated water washings, it will be washed out. A word of warning though - if you have a high enough concentration of FFAs in your WVO you'll finish up with a high concentration of soap in your final product, and the result could be an emulsion that you'll never break. I think that's what's happened here.
And the water sure makes a difference. Back when I was using a 100:15 ratio I once got some free-phase water into the reaction vessel. I considered draining it off, but decided I clouldn't be bothered. So I started the process, and after a while it became apparent that the reaction wasn't working. It was the middle of winter, and I think it was about 14 deg. So I put a heater in there, and let it run overnight. When it got to about 23 degrees the reaction finally kicked off. Lesson learned - keep water away from the process. So if you're using the Dr Pepper method, and are actually adding water with your methoxide, no wonder you have to heat it to 55 deg!
And note this - water has very low solubility in vege oil. If the oil is visibly clear, you can take it that there isn't enough water in the oil to interfere with the reaction. I once got about 200L of WVO from a bloke in Capel. It had been stored outside for years and was full of water. I was only able to use about a 1/3 of it. But even if there was free-phase water in the bottom of the drum, if the oil on top was clear it was OK.
I've never done a titration on my WVO, so I have no idea what the concentration of FFAs is. But when I first developed the method, I settled on a WVO:MeOH ratio of 100:15 and a KOH concentration of 10% in the MeOH. It worked immediately, and I used this for years, but then I (eventually) did some calculations, and worked out that this was close to the stoichiometric ratio. Then I actually measured the amount of meOH that was being consumed by the drying agent, and as a consequence I have now upped the ratio to 100:20.
Conclusions:
FRT method produces soap by direct reaction of the methoxide salt with the FFA. It is insoluble in the biodiesel phase, and drops out of solution when all the MeOH evaporates. For WVO high in FFA, the concentration can easily be increased if required with no adverse effects, as there are no interfering reactions. In 10y of using this method, this has never been required.
Dr Pepper method produces soap by both direct reaction of the methoxide salt with the FFAs, as well as saponification of the triglyceride by the catalyst (KOH or NaOH). This is an interfering reaction that reduces the amount of catalyst available for transesterification. For WVO high in FFA, the result is more water in the mixture, potentially enough to generate a reversible emulsion, and the loss of the batch.
Ok that's it. As before I'm happy to answer any genuine questions.
Those that have read my posts before will know that I neither read, nor respond to, a certain individual who bombards all my posts with bold text, but whose knowledge of chemistry could be written on the back of a postage stamp.
As a matter of indisputable fact, he will reply to this post with some typically childish drivel
The principle is this:
Unfortunately, however, as this site has many armchair experts (one in particular) who don't know any chemistry, a great deal of the comments (and therefore advice) are wrong.
I'll now explain it.
Soap is nothing more than the alkali metal salt of a Free Fatty Acid. We may represent the FFA as RCOOH, with R being the alkyl chain and COOH of course being the carboxylic acid group. Thus the soap (if potassium is the cation) is RCOOK.
How is it formed? Well, for our purposes, there are two mechanisms. I'll illustrate it by outlining the reactions that take place in both my FRT method, and the Dr Pepper method.
FRT Method.
Before the Reaction:
When the methoxide is added to the WVO, present in the mixture is
- Waste Vegetable Oil (WVO)
- Free Fatty Acids (FFA)
- Potassium Methoxide (MeOK)
- Methanol (MeOH)
Notable by its absence is water. The methoxide solution is dried (see hyperlink) and the water that is normally added with the methoxide solution is removed.
There are two reactions:
Reaction 1. Neutralization of FFAs
The first reaction that happens is the reaction of the highly alkaline methoxide with the FFA. As methanol has a pKa of 15, it is obviously the case that its conjugate base (the methoxide ion) is highly alkaline.
Thus the reaction is as follows: RCOOH + MeOK -> RCOOK (soap) + MeOH (methanol).
So the product of the reaction of the methoxide ion with the FFA is soap and methanol.
Reaction 2: Transesterification:
The second reaction that occurs is the reaction of the leftover methoxide ion with the WVO:
C6O6H5R3 + 3KMeO -> C3O3H5K3 (potassium salt of glycerol) + 3(MeOCOR)
After the Reaction:
Present in the mixture after the reaction is:
- Methyl ester of the triglyceride (biodiesel)
- Potassium salt of the glycerol
- Soap
- Methanol
What, then, is the fate of these four components?
The first thing that happens is that is separates into two phases - the glycerol and the biodiesel.
The excess methanol is present in both phases. I don't know the partition coefficient, so I can't comment on the ratio between the phases, but it's certainly present in both phases.
What of the soap? What happens to that? Well, we can see what happens to the soap if we look its HLB (Hydrophilic Lipohilic Balance):
As we use soap to wash ourselves, it an oil in water (o/w) emulsifier, with a HLB between about 10 and 16. This means that it's insoluble in hydrophobic material (oils), but water soluble. So in the case of our mixture, it will be held in solution by the methanol, as methanol is obviously hydrophilic in nature (like water).
Because my method is anhydrous, I don't wash with water, but wash with air. As the air bubbles through the mixture it gradually evaporates the methanol. Eventually, when it is all gone, the soap simply falls out of the solution (as it is insoluble in the biodiesel) as a gel-like, clumpy, light brown material that sits on top of the glycerol.
I have a batch currently clarifying, and I'll post some pics when it's ready.
So as soon as I see the soap falling out of solution, I therefore know that the batch is ready for use (although I still do a QC test on it). I've actually been observing this phenomenon for some time now, but didn't actually sit down to look at the chemistry of it until WesleyB asked some questions about it.
The Dr Pepper Method
Before the Reaction:
When the methoxide is added to the WVO, present in the mixture is
- Waste Vegetable Oil (WVO)
- Free Fatty Acids (FFA)
- Potassium Methoxide (MeOK)
- Methanol (MeOH)
- Water
One of the weirdest things about the discussions that take place on this forum is the time that people waste ensuring their oil is dry, only to add water to the mixture with the methoxide solution:
Eq1: KOH + MeOH <-> MeOK + H2O
In other words, for every molecule of methoxide you are adding a molecule of water
There are three reactions:
Reaction 1. The neutralisation of the FFAs.
With the water present this is not quite as straightforward as it is with my FFR method. There are two ways that the FFA can be neutralised:
RCOOH + KOH -> RCOOK + H2O (pulls Eq1 to the left)
RCOOH + MeOK -> RCOOK + MeOH (pulls Eq1 to the right).
Thus, there are competing influences in terms of the effect on the equilibrium
Reaction 2: Transesterification:
The second reaction that occurs is the reaction of the leftover methoxide ion with the WVO:
C6O6H5R3 + 3KMeO -> C3O3H5K3 (potassium salt of glycerol) + 3(MeOCOR).
This will pull the equilibrium (Eq1) to the right, thus generating more water
Reaction 3: Saponification:
C6O6H5R3 + 3KOH -> C3H8O3 + 3(RCO2K)
Obviously if you want to make biodiesel, you want to discriminate against the 3rd reaction and promote the 2nd reaction. This is a common approach in organic synthesis - designing the reaction conditions to favour one process over another. This is a complex question from a theoretical viewpoint, as it involves issues such as the reaction mechanism (pretty sure the transesterification is SN2, not sure about the saponification), the activation energies (Ea) of the respective processes, and the order of the reaction (usually determined by the mechanism, but I suspect both are second order reactions).
In this case this is done by strictly controlling the amount of catalyst, and the reason is obvious - you need enough catalyst to achieve the Ea at the selected temperature (55 degrees I think), but without putting in so much so that you generate enough water to promote the saponification reaction (consistent with a 2nd Order reaction). In other words, the kinetics of both reactions are probably pretty similar, but the transesterification reaction dominates purely because there is more methanol than water. But both reactions are certainly occurring.
And now we come to a significant difference between the transesterification and saponification reactions:
In the transesterification reaction, the KOH is a catalyst
In the saponification reaction, the KOH is a reactant, and water is (probably) the catalyst
That is, in the transesterification reaction the KOH is not consumed, but in the saponification process it is.
The implications of this ought to be obvious;
Firstly, with this method you need enough KOH to neutralise the FFAs, and have enough left over to initiate the transesterification reaction. But if you put in too much KOH, then you generate more water. More water (assuming 2nd order reaction kinetics) means the rate of the saponification increases. Also, the more water in the system (remembering it is not consumed in any reactions) means there is a greater chance of forming an emulsion, and there are plenty of posts on this site that have done just that.
Secondly, as the KOH participates in two reactions (and not one as with the FFR method), more KOH may be required than with the FRT method. Never have used the Dr Pepper method myself except right at the outset I have no idea what typical values are, but the beauty of the FRT method is that there are no interfering reactions, so all the methoxide can be used to catalyse the transesterification.
So what happens with the soap in this method. Well, it's going to be dissolved in the hydrophilic phase, in this case the methanol and water. So in principle, with repeated water washings, it will be washed out. A word of warning though - if you have a high enough concentration of FFAs in your WVO you'll finish up with a high concentration of soap in your final product, and the result could be an emulsion that you'll never break. I think that's what's happened here.
And the water sure makes a difference. Back when I was using a 100:15 ratio I once got some free-phase water into the reaction vessel. I considered draining it off, but decided I clouldn't be bothered. So I started the process, and after a while it became apparent that the reaction wasn't working. It was the middle of winter, and I think it was about 14 deg. So I put a heater in there, and let it run overnight. When it got to about 23 degrees the reaction finally kicked off. Lesson learned - keep water away from the process. So if you're using the Dr Pepper method, and are actually adding water with your methoxide, no wonder you have to heat it to 55 deg!
And note this - water has very low solubility in vege oil. If the oil is visibly clear, you can take it that there isn't enough water in the oil to interfere with the reaction. I once got about 200L of WVO from a bloke in Capel. It had been stored outside for years and was full of water. I was only able to use about a 1/3 of it. But even if there was free-phase water in the bottom of the drum, if the oil on top was clear it was OK.
I've never done a titration on my WVO, so I have no idea what the concentration of FFAs is. But when I first developed the method, I settled on a WVO:MeOH ratio of 100:15 and a KOH concentration of 10% in the MeOH. It worked immediately, and I used this for years, but then I (eventually) did some calculations, and worked out that this was close to the stoichiometric ratio. Then I actually measured the amount of meOH that was being consumed by the drying agent, and as a consequence I have now upped the ratio to 100:20.
Conclusions:
FRT method produces soap by direct reaction of the methoxide salt with the FFA. It is insoluble in the biodiesel phase, and drops out of solution when all the MeOH evaporates. For WVO high in FFA, the concentration can easily be increased if required with no adverse effects, as there are no interfering reactions. In 10y of using this method, this has never been required.
Dr Pepper method produces soap by both direct reaction of the methoxide salt with the FFAs, as well as saponification of the triglyceride by the catalyst (KOH or NaOH). This is an interfering reaction that reduces the amount of catalyst available for transesterification. For WVO high in FFA, the result is more water in the mixture, potentially enough to generate a reversible emulsion, and the loss of the batch.
Ok that's it. As before I'm happy to answer any genuine questions.
Those that have read my posts before will know that I neither read, nor respond to, a certain individual who bombards all my posts with bold text, but whose knowledge of chemistry could be written on the back of a postage stamp.
As a matter of indisputable fact, he will reply to this post with some typically childish drivel
The principle is this:
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