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Thread: Formation of soap during biodiesel manufacture

  1. #1
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    Formation of soap during biodiesel manufacture

    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

    • 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:


    Last edited by Dr Mark; 12th March 2019 at 01:12 AM.

  2. #2
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    Re: Formation of soap during biodiesel manufacture

    I calculated the pH of ethyl alcohol years ago. I don't remember what it was rather that it was a very weak acid. Similarly glycerol is probably a very weak acid, being a polyol / poly alcohol. So writing something like, the potassium salt of glycerol seems incorrect to me. The formula above for lipids giving C6 seems odd to me, glycerol has a 3 carbon chain not 6 carbon chain. H5C3(OCOR)3 .
    Last edited by WesleyB; 11th March 2019 at 01:52 AM.

  3. #3
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    Re: Formation of soap during biodiesel manufacture

    Mark,
    Are you saying that Methanol and KOH when mixed together make Potassium methoxide.
    How much Potassium methoxide is created when you add KOH to Methanol?
    What is the Stoichiometric ratio of the Methoxide reaction?
    Is this process temperature dependent (I know it is an exothermic reaction)?
    Is all of the KOH converted to Potassium methoxide? If not all, will some / all of the remaining KOH be converted after a period of time?
    It is likely that there will be an excess of Methanol in the process also. This does not appear in your reactions above. What part does the excess Methanol play a part in the transesterification process?

    So many questions. I hope you can answer them all for me.
    Life is a journey, with problems to solve, lessons to learn, but most of all, experiences to enjoy.

    Current Vehicles in stable:
    2000 Ford Courier Crew Cab 2.5L Turbodiesel on Blended veggie oil.
    '2014 Toyota Prius (on ULP)


    Previous Vehicles:
    '90 Mazda Capella. (2000 - 2003) My first Fatmobile. Converted to fun on veggie oil with a 2 tank setup. Died when supercharger stuck at max boost for weeks. Stretched head bolts.
    '80 Mercedes 300D. 2 tank conversion [Sold]
    '84 Mercedes 300D. 1 tank, no conversion. Replaced engine with rebuilt OM617A turbodiesel engine. Finally had good power. Donor for current Fatmobile coupe. (body parted out and carcass sold for scrap.)
    '99 Mercedes W202 C250 Turbodiesel (my darling Wife's car)[sold]
    '98 Mercedes W202 C250 Turbodiesel (my car)[sold]
    Parts Car C220 1993 SOLD.
    '85 Mercedes Benz W123 300CD Turbodiesel single tank using 95% used cooking oil and 5% to 10% misfuel (where someone had filled diesel vehicle with petrol).
    '06 Musso Sports Crew Cab. Running on used cooking oil with 5% to 10% misfuel. [Head gasket blew!]


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  4. #4
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    Re: Formation of soap during biodiesel manufacture

    Hi Mark,

    Quote Originally Posted by Mark View Post
    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.

    The principle is this:
    He sounds like a real bounder to me!!

    Is he that chap who did the experiment that demonstrated there is nothing special about making biodiesel at room temperature, or is he the fellow who performed the tests that showed this foolproof method was not foolproof?



  5. #5
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    Re: Formation of soap during biodiesel manufacture

    Quote Originally Posted by WesleyB View Post
    I calculated the pH of ethyl alcohol years ago. I don't remember what it was rather that it was a very weak acid. Similarly glycerol is probably a very weak acid, being a polyol / poly alcohol. So writing something like, the potassium salt of glycerol seems incorrect to me. The formula above for lipids giving C6 seems odd to me, glycerol has a 3 carbon chain not 6 carbon chain. H5C3(OCOR)3 .
    Sorry yes - glycerol formula corrected. Well spotted

    Ethanol, methanol, and glycerol are all very weak acids, as they are all alcohols. The only one of those that I've been able to find any data on is MeOH, which is over 15, so very weak. Consequently its conjugate base, the methoxide ion, is a very strong base.

    In the transesterification reaction, because it is so negative it looks for a positive charge very aggressively, and this makes it a (very strong) nucleophile. It therefore attacks the relatively positively charged ester carbon (due to the electron withdrawing effect of the oxygens bound to it) at the ether linkage by an SN2 mechanism. This cleaves the ether linkage, and the two electrons in the sigma bond transfer to the oxygen, thus cleaving the bond and creating a glycerol trianion. The only cation available in the mixture to form an ion pair with this is the potassium from the methoxide salt.

    Thus, the potassium salt of the glycerol is not formed by deprotonation by the methoxide, but as a reaction product of the transesterification reaction. It remains as the potassium salt as I have removed water from the mixture.

    Edit: this explanation leaves out a very important factor - the regeneration of the catalyst. In other words, if this is all that's happening, then the product yield will be determined by the concentration of the methoxide, not the methanol.

    So here's what I now think is happening (and it must be right): The methoxide ion adds onto the cleaved lipid chain from the triglyceride, forming the methyl ester. This leaves the potassium salt of the methoxide.

    But then, it reacts with the MeOH to make more methoxide, thus regenerating the catalyst:

    C3H5O3K3 + 3MeOH <-> C3H8O3 + 3MeOK

    After the MeOH is consumed (or at least mostly depleted by formation of the methyl ester), this equilibrium will shift back towards the left, thus generating a substantial amount of the potassium salt of the glycerol
    S
    Last edited by Dr Mark; 12th March 2019 at 01:09 AM.

  6. #6
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    Re: Formation of soap during biodiesel manufacture

    Quote Originally Posted by Tony From West Oz View Post
    Mark,
    Are you saying that Methanol and KOH when mixed together make Potassium methoxide.
    How much Potassium methoxide is created when you add KOH to Methanol?
    What is the Stoichiometric ratio of the Methoxide reaction?
    Is this process temperature dependent (I know it is an exothermic reaction)?
    Is all of the KOH converted to Potassium methoxide? If not all, will some / all of the remaining KOH be converted after a period of time?
    It is likely that there will be an excess of Methanol in the process also. This does not appear in your reactions above. What part does the excess Methanol play a part in the transesterification process?

    So many questions. I hope you can answer them all for me.
    Quote Originally Posted by Tony From West Oz View Post
    Mark,
    Are you saying that Methanol and KOH when mixed together make Potassium methoxide.
    How much Potassium methoxide is created when you add KOH to Methanol?
    What is the Stoichiometric ratio of the Methoxide reaction?
    Is this process temperature dependent (I know it is an exothermic reaction)?
    Is all of the KOH converted to Potassium methoxide? If not all, will some / all of the remaining KOH be converted after a period of time?
    It is likely that there will be an excess of Methanol in the process also. This does not appear in your reactions above. What part does the excess Methanol play a part in the transesterification process?

    So many questions. I hope you can answer them all for me.
    Sure thing Tony. Happy to help

    1. Yes. KOH + MeOH <-> MeOK + H2O. One molecule of potassium hydroxide reacts with one molecule of methanol to form one molecule of potassium methoxide and one molecule of water. It is a dynamic equilibrium, which means that at any one time the rate of the forward reaction is the same as the back reaction. So if we took a snapshot of the solution at any point in time, all four chemicals would be present.

    If the equilibrium is disturbed, it'll adjust so as to attempt to restore the equilibrium (Le Chateleir's Principle). In this case, with my method, I remove water from the mixture with a drying agent. This leaves a vacancy on the RHS so the equilibrium attempts to replace the water by reacting to the right. In other words, under these circumstances, the forward reaction increases in rate, and the backwards reaction drops away to nothing. As I remove (virtually) all water from the mixture, the result is that the forward reaction will proceed until all the KOH is consumed, and the mixture is now composed entirely of potassium methoxide and methanol.

    2. See above. All KOH is converted to potassium methoxide

    3. See above

    4. Some equilibria are temperature dependent. That is, when a dynamic equilbrium has been established, if there is a delta H (keyboard doesn't have the right symbol) then temperature changes can cause the equilibrium to shift. I'm not sure whether this one is, but it doesn't matter. By removing the water I'm pulling it all the way to the right anyway

    5. Yes - all converted (essentially)

    6. OK, this is easily the most interesting question of the lot. When I first thought up this method, I just tossed it all together and the bio popped out. Didn't give too much thought to the mechanistic details (other than the obvious one of the effect of dehydrating the methoxide solution). So I've had to, at various times, mostly in response to questions like this, or from WesleyB, sit down and work out some stoichiometries and reaction mechanisms.

    So here's what's happening:

    The methoxide is the catalyst. After it cleaves the ester linkage and adds on to form the methyl ester, we have left over the potassium salt of the glycerol. This then reacts with the MeOH to generate more methoxide, and so on. In other words, as the definition of a catalyst is that it is regenerated and not consumed, it is continually regenerated, until all the triglycerides have reacted, and we have leftover methanol.

    I'm going to update the reply to Wes above.

    Also, later on I'm going to post a separate post to explain le Chateleir's principle in layman's terms, as it is the principle upon which my method is based

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