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Thread: The chemistry of the glycerol (and soap) phase

  1. #1
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    The chemistry of the glycerol (and soap) phase

    When making biodiesel, we're mostly concerned with the biodiesel phase, and the thick black syrup is just this annoying stuff that we have to dispose of somehow.

    My method overturns this, however, as it produces a glycerol phase that is so thin that it can be tipped down the sink like water. See the videos in my method.

    So I thought I'd post up an explanation of this phase, and the location of the various components in the system.

    Let's start at the beginning. The state of a substance (gas, liquid, solid) is determined by the intermolecular forces - that is, the degree and type of interaction between molecules of the same type. For our purposes, the two relevant ones are hydrogen bonding and Van der Waals forces. These are the two weakest types of forces between molecules, with VDW being the weakest. Hydrogen bonding occurs when an organic molecule contains and oxhgen-rich atom such as oxygen, nitrogen or sulfor (but mostly O), with whose electron clouds tha naked hydrogen proton can interact.

    Consequently, the first four carbon hydrocarbons are gases - methane, ethane, propane and butane. It's not until you get to C5 - pentane, that we get a liquid at RTP. This is simply because the weak VDW forces are not strong enough to overcome the thermal energy at RTP.

    Let's now look at propane. C3H8. Let's now add a single oxygen, so we have C3H8O. This is propanol. This is now a liquid at RTP, simply because the oxygen provides a site at which hydrogen-bonding can occur. Add a further two oxygens and we have glycerol. Now with three oxygens, we get three sites per molecule at which hydrogen bonding can occur, and we now have a thick, viscous liquid.

    This stuff is not easy to dispose of. It's so thick that it can clog plumbing if you tip it down the sink or toilet, and in any case the foam can cause problems.

    So how is my method different? How is it that it is so thin that it can be tipped down the sink, and doesn't foam?

    Let's look at the chemistry of the transesterification process:

    1. Methoxide solution:

    Eq 1: MeOH + KOH <-> KMeO + H2O

    2. Transesterification reaction

    Eq 2: C6O6H5R3 + 3KMeO + 3H2O -> C3O3H5K3 (potassium salt of glycerol) + 3(MeOCOR) + 3H2O

    3. Regeneration of catalyst

    Eq 3: C3O3H5K3 + 3H2O -> C3H8O3 (glycerol) + 3KOH

    So the final step in the process results in the regeneration of the catalyst and protonation of the potassium salt of the glycerol to form glycerol.

    My method differs in that it removes water from the process, which means that the final state of the glycerol is the potassium salt of the glycerol. Since the hydrogen bonding in glycerol is almost entirely due to the electron-poor hydroxy proton, its absence means that there is less interaction between the molecules. What bonding there is, is almost entirely due to the protons on the carbon backbone, somewhat ameliorated by the fact that the highly electronegative potassium cation will have a substantial electron withdrawing effect on the nonbonding electron pair on the oxygen.

    Here is my process:

    Methoxide solution:

    Eq 1: MeOH + KOH <-> KMeO + H2O

    Drying of methoxide solution:

    Eq 2: H2O + CaO -> Ca(OH)2

    Adding Eq 1 and Eq 2:

    Eq 4: MeOH + KOH + CaO -> KMeO + Ca(OH)2

    Transesterification reaction:

    Eq 5: C6O6H5R3 + 3KMeO + 3H2O -> C3O3H5K3 (potassium salt of glycerol) + 3(MeOCOR) + 3H2O

    And this is the final reaction. As the water has been removed, the KOH is not regenerated, and the final state of the glycerol is the potassium salt, which is much thinner than the glycerol and therefore more easily disposed of.

    One more issue - what happens to the soap?

    When the highly alkaline methoxide is added to the WVO it will obviously first react with the Free Fatty Acids:

    KMeO + RCOOH -> RCOOK (soap) + MeOH

    In other words, the methoxide reacts with the FFA to form soap + methanol.

    So what happens to the soap?

    Soap, of course, is a surfactant. It will then, obviously, look for interfaces. In this case, it will be the interface between the hydrophilic glycerol phase and the hydrophobic phase.

    So the upper phase will be the biodiesel. Below this will be the soap, and underneath that will be the glycerol phase.

    And this certainly aligns with my observations. When I leave the raw material to clarify (by bubbling air), when it has done so (by removing all the excess MeOH) I see a light brown layer settling on top of the dark brown glycerol. And, as I would expect, this light brown material is somewhat gelatinous and clumpy in nature, settling out on top of the liquid glycerol.

    So that's it. Happy to answer any genuine questions from any genuine people. As with all my posts, I will ignore all contributions from a certain individual, a bloke who sees himself as an expert on biodiesel but who has no qualifications in chemistry, whose understanding of chemistry could be written on the back of a postage stamp, and whose posts are nothing more than noise.

  2. #2
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    Re: The chemistry of the glycerol (and soap) phase

    "it will obviously react first with the Free Fatty Acids" . There is reaction rate to consider. I made soap from fairly pure hot/soft octadecanoic acid (present in vegetable oils and animal fats). The soap cures for days or weeks. It's not a fast inorganic reaction like muriatic acid and sodium hydroxide. I expect the transesterification reaction rate might exceed the neutralisation reaction rate especially in very dry conditions. So biodiesel might form as fast as soap forms. Tell me where that's wrong.

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    Re: The chemistry of the glycerol (and soap) phase

    Quote Originally Posted by WesleyB View Post
    "it will obviously react first with the Free Fatty Acids" . There is reaction rate to consider. I made soap from fairly pure hot/soft octadecanoic acid (present in vegetable oils and animal fats). The soap cures for days or weeks. It's not a fast inorganic reaction like muriatic acid and sodium hydroxide. I expect the transesterification reaction rate might exceed the neutralisation reaction rate especially in very dry conditions. So biodiesel might form as fast as soap forms. Tell me where that's wrong.
    The reaction with the FFA is a simple acid-base reaction and is all but instantaneous. In years of teaching and doing analytical chemistry I've not encountered one occasion where an acid-base process wasn't instantaneous. Kinetic data for acid-base reactions simply doesn't exist. And of course we know that because if it wasn't instantaneous, you wouldn't be able to titrate the FFAs with an instant result, as many people on this forum do.

    The transesterification process, OTOH, is a Nucleophilic Substitution reaction with an SN2 mechanism (I think) and is therefore subject to kinetic considerations. We know, for example, that the Free Energy chart has an Activation Energy, which is why it needs a catalyst.

    But even if it wasn't instantaneous it's a moot point, as both reactions go to completion anyway

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    Re: The chemistry of the glycerol (and soap) phase

    Hi Mark,

    Quote Originally Posted by Dr Mark View Post
    When making biodiesel, we're mostly concerned with the biodiesel phase, and the thick black syrup is just this annoying stuff that we have to dispose of somehow.

    My method overturns this, however, as it produces a glycerol phase that is so thin that it can be tipped down the sink like water. See the videos in my method.

    So I thought I'd post up an explanation of this phase, and the location of the various components in the system.
    Interesting.
    However, the more likely reason that your by-product (which you call glycerol) is so thin is that because the largest component of your by-product is probably Biodiesel instead of glycerol.

    You have posted that you have a large by-product layer.
    By using such a huge excess of KOH (20g! KOH per litre of WVO) in the reaction instead of performing a simple 5 minute titration to accurately determine the amount of KOH required, you are likely to be producing an excess of soap.
    The soap, along with about twice it's volume of biodiesel, becomes part of the by-product phase
    My calculations suggest that your by-product composition is probably about 36% biodiesel and only about 30% glycerol.

    I do urge that in the future you perform a simple titration to determine the amount of KOH actually required for the reaction to prevent pouring so much biodiesel down the drain.


    Last edited by tillyfromparadise; 8th March 2019 at 12:18 PM.

  5. #5
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    Re: The chemistry of the glycerol (and soap) phase

    Dr Mark; I believe in the formulas you have not accounted for wetness in waste vegetable oil. You described the neutralization reaction (acid+base--> salt + water) but you did not describe saponification reaction where trifatty acid glyceryl + caustic in the presence of water --> soap + glycerol +regenerated water (saponification). Some of the error in making biodioesel stems from wetness of vegetable oil. Your no titration fool proof method doesn't consider wet oil or very high titration oil. For Example a titration of 20 oil. add the suggested amount of caustic, the instantaneous neutralization reaction consumes the caustic and there's no caustic to form methoxide plus the water formed by the neutralization reaction. Beautiful weather here today sir.

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    Re: The chemistry of the glycerol (and soap) phase

    Quote Originally Posted by WesleyB View Post
    Dr Mark; I believe in the formulas you have not accounted for wetness in waste vegetable oil. You described the neutralization reaction (acid+base--> salt + water) but you did not describe saponification reaction where trifatty acid glyceryl + caustic in the presence of water --> soap + glycerol +regenerated water (saponification). Some of the error in making biodioesel stems from wetness of vegetable oil. Your no titration fool proof method doesn't consider wet oil or very high titration oil. For Example a titration of 20 oil. add the suggested amount of caustic, the instantaneous neutralization reaction consumes the caustic and there's no caustic to form methoxide plus the water formed by the neutralization reaction. Beautiful weather here today sir.
    Correct. I didn't bother with the saponification process. I'm going to do that in a subsequent post devoted to the generation and fate of soap.

    The amount of water in clear, free-phase oil is negligible. For about the millionth time, those of you that use the dr Pepper method (or any method without the drying step) add water to your mixture from the methoxide solution, and given the 1:1 stoichiometric ratio with the methoxide, the amount of water will easily swamp whatever small amount may be present in the oil:

    KOH + MeOH <-> KMeO + H2O

    And as for "high titration" oil, yes, in principle a high concentration of FFAs will neutralise the methoxide solution. But I have been using this method now for about ten years with not a single problem. And if I wasn't going to have any problems at a SVO/MeOH ratio of 100:15 then I'm not going to have any problems with a ratio of 100:20

    Also, unless I'm very much mistaken, the amount of caustic I use in my method is much higher than is used by other methods, so if the conc of FFAs do not inhibit the catalysis in these methods, it certainly isn't going to in mine.

    By way of example, the equivalent concentration of KOH in my mix is about 0.3M.

    But even if this is wrong - even if the amount of caustic in my mixture is less than the Dr Pepper method, my method doesn't have the interfering saponification reaction to consume catalyst.

    The simplicity of my method means that I can toss in as much KOH as I want, and there are never any interfering reactions to get in the way.

    And the water sure makes a difference. I know this as I once had a batch where I was lazy when transferring the WVO and some free-phase water transferred across. She'll be right, I thought and went ahead anyway. When I started the reaction, it wasn't going - not a nice situation to be in when you make 1000L at a time. It was the middle of winter and it was about 14 degrees I think. So I pit my heater in and kept it recirculating overnight. When it got to about 23 the reaction kicked off.

    So the great key to my method is that it is anhydrous and the interfering saponification reaction is eliminated. I'm composing a post about soap at the moment, and this just occurred to me, so I thought I'd better update it
    Last edited by Dr Mark; 9th March 2019 at 10:50 PM.

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    Re: The chemistry of the glycerol (and soap) phase

    Hi Mark,

    Quote Originally Posted by Dr Mark View Post
    For about the millionth time, those of you that use the dr Pepper method (or any method without the drying step) add water to your mixture from the methoxide solution, and given the 1:1 stoichiometric ratio with the methoxide, the amount of water will easily swamp whatever small amount may be present in the oil:

    KOH + MeOH <-> KMeO + H2O
    You seem to keep forgetting, so for about the millionth time I will remind you that water is only produced when you actually Produce methoxide.
    You claim that without your drying procedure, there is very little methoxide produced. That means VERY LITTLE WATER is produced.

    The Chemist Neutral used to point out that when you add KOH to Methanol you are mostly just dissolving KOH in methanol.
    Dissolving KOH in methanol does not produce water.
    It is only when you actually produce a tiny bit of methoxide that an equivalent tiny bit of water is produced.
    That means that those of us who use the Dr Pepper Method
    (or any method without the drying step) are putting Very Little Water in the reaction.

    Last edited by tillyfromparadise; 9th March 2019 at 05:41 PM.

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