Removing fatty acids from Waste Vegetable Oils

Removing fatty acids from Oils:In commercial oil refining, this is done with NaOH. Use the titration amount of NaOH, 1 gram for 1 ml WVO, and mix it with 40 ml of water per liter of oil. It gets hot. Using a stainless steel container, it is mixed by stirring, Add the dissolved NaOH to the oil (room temperature), stir gently by hand until thoroughly mixed. Settle overnight. This leaves soap stock at the bottom. The water is apparently in the soap stock. Filter to remove the soap stock, no need for fine filtering, fine steel mesh will do (like a fine tea strainer). Now process as usual for virgin oil, 3.5 grams NaOH per liter of oil, but use 25% methanol, at 55o C,good and prolonged agitation as usual.

Production rate is around 80%. With oil like this, it is much easier process than the normal single-stage, and it is nice not to have to make such strong methoxide as a straight single-stage process would require with this oil, 13.1 grams of NaOH per liter of oil, or more like 13.6 grams (needs a bit of excess NaOH). You can add the soap stock to the glycerine layer after separation and neutralize as usual to separate catalyst, glycerine and FFAs. It is an alternative, better than straight single-stage base for oil like this, and while it would not get as a high a production rate as the acid-base method, and it uses more catalyst and gives you more co-products, it is very quick and simple. This is also useful if you are making ethyl esters biodiesel, using ethanol rather than methanol, the ethyl esters process does not work well with oils with more than about 2 ml titration.

The basic NaOH quantity of 3.5 grams:

This is the amount of NaOH, (sodium hydroxide) required as catalyst to trans-esterify 1 liter of virgin, uncooked oil. For used oils, titration determines the amount of lye needed to neutralize the Free Fatty Acid (FFA) content, and this quantity is added to the basic figure of 3.5 grams per liter. In fact 3.5 grams is an empirical measure, an average. Different oils have slightly different requirements, and even the same type of oil varies according to how and where it is grown. Other estimates are 3.1 gm, 3.4 gm. Different oils also require different amounts of methanol. For oils and fats requiring more methanol, coconut, palm kernel, as well as tallow, lard, butter, use more NaOH, up to 4.5 gm, even with new oils, and especially when it is used. Once again, do small test batches first.

How much methanol should you use?:

The stoichiometric quantity of methanol is the amount needed to convert triglycerides (fats and oils) into esters (biodiesel), the methyl portion of methyl esters. Excess of methanol is required to push the conversion process towards completion, without the excess the process runs out (reaches equilibrium) before all the triglycerides are converted to esters, resulting in poor fuel that does not combust well and can be corrosive. The excess methanol acts more like a catalyst, it encourages the process but does not become a part of the final product and can be recovered afterwards.

Stoichiometric quantity:

The stoichiometric quantity is usually said to be 12.5% methanol by volume, that is, 125 milliliters of methanol per liter of oil. Some manufacturers use it at 13%, or 13.5%, or even as low as 8%. In fact it depends on the amounts of the various fatty acids in the oil, and varies from one oil to another.

Excess Methanol:

How much excess is needed, to achieve 98% conversion? It depends on several different factors like the type of oil, its condition, the type, size and shape of the processor, the type and duration of agitation, the temperature of the process. However, excess is usually between 60% and 100% of the stoichiometric amount. So if the stoichiometric ratio of the oil you are using is 12.5%, that is 125 ml of methanol per liter of oil, the excess would range between 75 ml and 125 ml, for a total amount of methanol of 200-250 ml per liter of oil.

Oils with higher stoichiometric ratios seem to need higher excesses. So, for fresh oils, you can try 60%, though 67% or more would be better. For palm kernel or coconut, closer to 100% excess would be better. For tallow and lard, use higher excesses. For used oil, WVO, waste vegetable oil, often contains animal fats from the cooking, use 67% minimum excess. For heavily used oils with high titration levels, use higher excesses, up to 100%.

If you do not know what kind of oil your WVO is, try using 25% methanol, 250 ml methanol to 1 liter of oil. If you’ve taken care with the titration, used accurate measurements and followed the instructions carefully, you should get a good, clean split, with esters on top and the glycerine and free fatty acids cleanly separated at the bottom. If you have trouble washing it, with a lot of frothing, that could be because the process did not go far enough and unconverted material is forming emulsions, try using more methanol next time. If everything works well, try using less methanol. You will soon figure out what’s best for you. With the acid-base two-stage method, don’t worry about it.

Ethyl esters:

The same principles apply for making ethyl esters instead of methyl esters, using ethanol rather than methanol, with some differences. Use 1.4 times more ethanol than methanol. It would not work if there is any water in the ethanol or the oil. It works much better with some methanol added, up to 3:1 ethanol: methanol. Virgin oil is better, with waste oil (WVO) it would not work with FFA content much more than 1ml by titration.

Reclaiming excess methanol

Depending on the kind of oil you use, it takes from 110-160 milliliters of methanol per liter of oil to form the methyl esters molecule. But you also need to use an excess of methanol to push the conversion process towards completion, the total used is usually 20% and more of the volume of oil used, 200 ml per liter or more. Much of the excess methanol can be recovered after the process for reuse, simply by boiling it off in a closed container with an outlet leading to a simple condenser. Methanol boils at 64.7o C, though of course it starts vaporizing well before it reaches boiling point. Unlike ethanol, methanol does not form an azeotrope with water and relatively pure methanol can be recovered, pure enough to reuse in the next batch.

Methanol can be recovered at the end of the process, or just from the glycerine by product layer, since most of the excess methanol collects in the by-product and it is that much less material to heat. Start at 65 to 70o C, as the proportion of methanol left in the by-product mixture decreases, the boiling point will increase, so you will have to raise the temperature to keep the methanol vaporizing, perhaps to as high as 100o C or more, though the bulk of it should have been recovered by then. If you are planning to separate the glycerine from the soaps (FFAs) and catalyst it is best to leave methanol recovery to later as the glycerine probably won’t separate without the methanol. The excess methanol can then be recovered from the separated glycerine layer. To keep costs down, biodiesel producers try to salvage the unreacted methanol.There are two major methods to do this: heat extraction and vacuum/heat extraction.

Heat Extraction:

Heat the second-stage product to 70oC in a sealed vessel and lead the fumes into a condenser. Intercept the condensed methanol in a liquid trap. Take great care because methanol is highly flammable and the fumes are explosive.

Vacuum / Heat extraction: This is basically the same as heat extraction, but it requires less energy. The drawback of this method is that you need a special vessel and equipment to do this. When building your reactor it may be a good idea to take one step at a time. Build the reactor, get confident with the process and eventually upgrade to methanol recovery. At least a quarter of the methanol used can be recovered, that is, 50+ ml per liter of oil / fat. Mix it with fresh methanol for preparing the next batch of methoxide.

Glycerine: There is no set amount of by-product, such as 100 ml per liter, and there is no rule that the by-product must be solid at room temperature. What is much more important is that in each of the cases above, the test batch produced a good split, the glycerine separated and settled to the bottom, and, if the directions are followed carefully, the rest would have been good biodiesel, needing no more than settling and washing. The rule of thumb is 79 milliliters of glycerine for every liter of oil used (7.9%). And crude glycerine is not solid at room temperature. But the so-called glycerine layer is not just glycerine, it is a variable mixture of glycerine, soaps, excess methanol, and the catalyst NaOH. The quantity varies according to the oil used (more with heavily-used oil), the process used (less with the acid-base two-stage method), the amount of excess methanol used (most of the excess methanol ends up in the by product layer).

It is mainly the soaps combined with the glycerine that can cause it to solidify. Soaps made from saturated fats such as stearin are harder than those made from unsaturated fats such as olein, so the type of oil used makes a difference. More important is how much soap there is, the more soap, the more likely the by-product layer will solidify, no matter what oil you used. Other factors are Excess methanol which makes the by-product layer thinner, Too much NaOH creates excess soap, Potassium hydroxide (KOH) makes the by-product slightly thinner than sodium hydroxide (NaOH).