Storage / Handling
Used Cooking Oil
Trans-Esterification BioDiesel Process
Reaction raw materials :
- Used Cooking Oil or Palm Stearin or Animal Tallow
- Methanol (CH3OH) 99%+ pure
- Potassium hydroxide (must be dry)
Materials for Titration :
- Isopropyl alcohol 99%+ pure
- Distilled water
- Phenolphthalein solution (not more than a year old, kept protected from strong light)
Materials for Washing :
- Magnesium Silicate
BioDiesel Manufacturing Process
- If Used Cooking oil is raw material, it is filtered to remove any solid particles.
- Only used Cooking oil is heated to remove any water content (optional).
- Titration is done to determine how much catalyst is needed.
- Exact quantity of Potassium Hydroxide is then thoroughly mixed in Methanol till Potassium Hydroxide dissolves completely to get potassium methoxide.
- Used Cooking oil or Palm Stearin or Animal Tallow, is heated if required (during winter), and mixed in the potassium methoxide with agitation.
- Biodiesel formed is then allowed to settle and glycerine is removed from bottom.
- BioDiesel fraction is then dry washed.
- BioDiesel produced is then checked for quality.
In trans-esterification, KOH and methanol are mixed to create potassium methoxide (K+ CH3O-). When mixed with the oil this strong polar-bonded chemical breaks the oil into glycerine and ester chains (Biodiesel), along with some soap if you are not careful. The esters become methyl esters. They would be ethyl esters if reacted with ethanol instead of methanol.
BIODIESEL manufacturing PROCESS in DETAIL
Filtering : Filter the oil in case of used cooking oil to remove solid particles. You may have to warm up, used cooking oil a bit first to get used cooking oil to run freely. Heating to 35oC should be enough. A Cartridge filter is used for the same.
Removing the water : Heat the used cooking oil first to remove any water content. Used cooking oil will probably contain water, which can slow down the reaction and cause soap formation. The less amount of water in the used cooking oil the better. Raise the temperature to 100oC, hold used cooking oil at that temperature and allow any water to boil off. Start agitation to avoid steam pockets forming below the used cooking oil surface and exploding to splash hot oil. Or drain water puddles out from the bottom as they form, you can save any used cooking oil that comes out with the water later. When boiling slows, raise the temperature to 130oC for 10 minutes. Remove heat and allow to cool.
Regular source of Vergin oil or Palm Stearin or Animal Tallow, does not need to have the water boiled off. Boiling means extra energy and time.
Basic titration : Dissolve 1 gram of KOH in 1 liter of distilled or de-ionized water (0.1% KOH solution). You can use phenolphthalein solution to get end point. In a smaller beaker, dissolve 1 ml of dewatered oil in 10 ml of pure isopropyl alcohol. Warm the beaker gently by standing it in hot water, stir until all the oil dissolves in the alcohol and the mixture turns clear. Add 2 drops of phenolphthalein solution.
Using a burette, add 0.1% KOH solution, drop by drop to the oil alcohol phenolphthalein solution, stirring all the time, until the colour of solution, turns pink (magenta) and stays pink (magenta) for 10 seconds. To the number of milliliters of 0.1% KOH solution you added, add 5. (If colour change is after adding 5 ml, add 5 to it. You will need, 10 gms of KOH per liter of oil.) This is the number of grams of KOH you will need per liter of oil.
Test batches : The first few times you do this process, it is a good practice to first try out your KOH amounts on a 1 liter batch. This works really well and you do not need to heat up the oil too much, just enough to spin well. Add 200 ml of methanol and add the exact quantity of KOH to be dissolved in 200 ml of methanol. First make sure that vessels used are dry. Forming of the exothermal potassium methoxide polar molecule will heat up the vessel a bit. Keep mixing until all the KOH has been dissolved.
Once the potassium methoxide is prepared, add to 1 liter of oil. Make certain all your weights and volumes are precise. If you are unsure of the titration result then use 5.0 grams of KOH per liter of oil. Smaller batches need only be run for about 15 to 20 minutes for separation to be completed before switching off. The settling takes some time to complete. The solution can be poured from the vessel into another container right after switching off the agitator. It is good to do a few batches with varying amounts of KOH recorded, so later when checking results one can choose the KOH quantity that did the best job. When too much KOH is used the result can be a troublesome gel that is tough to do anything with. When not enough KOH is used the reaction does not go far enough so some unreacted oil will be mixed with the biodiesel and glycerine. This will form three levels with biodiesel on top above unreacted oil with glycerine on the bottom. If there is too much water in the oil, it will form soaps and settle right above the glycerine forming a fourth level in the container. This layer is not too easy to separate from the unreacted oil and glycerine layers.
Preparing the potassium methoxide: Generally the amount of methanol needed is 20% of the Used Cooking oil or Palm Stearin or Animal Tallow, by mass. The densities of these two liquids are fairly close, so 20% of methanol by volume should be right. To be completely sure, measure out a half liter of both liquids, weigh, and calculate exactly what 20% by mass is. Different oils can have different densities depending on what type of oil it originally was.
When trans-esterifying 100 liters of Used Cooking oil or Palm Stearin or Animal Tallow, use 20 liters of methanol. The methanol is mixed into a solution with the KOH, creating potassium methoxide in an exothermic reaction (it gets warm due to bonds formation). Keep all equipments that KOH comes in contact with, as dry as possible.
CAUTION : Treat potassium methoxide with extreme caution! Do not inhale any vapors! If any potassium methoxide gets splashed on your skin, it will burn you without feeling it (killing the nerves). Wash immediately with lots of water. Always have a hose running when working with potassium methoxide. Potassium methoxide is also very corrosive to paints. KOH reacts with aluminum, tin and zinc. Use glass, enamel or stainless steel containers, stainless steel is best.
Heating and mixing : Pre-heat Used Cooking oil or Palm Stearin or Animal Tallow, to 48 to 54oC. In lab, a low speed propeller coupled to an electric motor works fine as a mixer. Too much agitation causes splashing and bubbles through vortexing and reduces mix efficiency. There should be a vortex just appearing on the surface. Adjust the speed, or the size of the stirrer to get the right effect. Alternately an electric pump plumbed to form a mixing loop for stirring the oil would do a nice job. Mount the pump above the level that glycerine will gel at, to prevent clogging up the pump.
Add the potassium methoxide to the oil while stirring, stir the mixture for 50 minutes to an hour. The reaction is often complete in 30 minutes, but longer is better. The trans-esterification process separates the methyl esters from the glycerine. The CH3O of the methanol then caps off the ester chains and OH from the KOH stabilizes the glycerine.
Settling and Separation : Allow the solution to settle and cool for at least eight hours, preferably longer. The methyl esters (Biodiesel) will be floating on top while the denser glycerine will have congealed on the bottom of the container forming a hard gelatinous mass (the suction of mixing pump must be mounted above this level). An alternative method is to allow the reactants to settle for at least an hour after mixing while keeping the mixture above 38oC, which keeps the glycerine semi-liquid (Glycerine solidifies below 38oC). Then carefully decant the biodiesel. This can be done by draining the reactants out of the bottom of the container through a transparent hose. The semi-liquid glycerine has a dark brown color and the Biodiesel is honey-colored. Keep a watch on what flows through the sight tube. When the light colored Biodiesel appears, divert biodiesel to a separate container. If any Biodiesel stays with the glycerine, it is easy to retrieve biodiesel later once the glycerine has solidified. If you left the mixture in the tank until the glycerine gelled, reheat the tank just enough to liquify the glycerine again. Do not stir it. Then decant it out as above.
Glycerine: The glycerine side stream typically contains a mixture of glycerine, methanol, water, inorganic salts (catalyst residue) free fatty acids, unreacted mono-, di-, and triglycerides, methyl esters, and a variety of other materials, in varying quantities. The glycerine from oil is brown and usually turns to a solid below about 38oC. Glycerine from fresh oil often stays a liquid at lower temperatures. Reclaimed glycerine is composted, after heating mixture to 66oC, to boil off any methanol content (the boiling point of methanol is 64.7oC). The excess methanol is recovered for reuse when boiled off, when you run the vapors through a condenser. Another way of disposing of the glycerine, though a great bit more complicated, would be to separate its components, mostly methanol, pure glycerine (a valuable product for medicines, hand lotions, dried plant arrangements and many other uses) and wax. This is often accomplished by distilling mixture, but glycerine has a high boiling point even under high vacuum so this method is difficult. Other idea for disposing of the glycerine is breaking glycerine down to usable methane gas, with a Bio Gas methane digester.
Soap residue : Suspended in the Biodiesel will also be some soapy residues. These are the result of K+ ions from the KOH reacting with water created when the methanol bonds with the ester chains along with any water that was suspended in the oil. The reaction produces more than the usual amount of soap, if KOH comes into contact with water before KOH has a chance to react with the oil. In this case the excess water should have been boiled off first.
The part of the process where it is vital to keep all water out of the reaction, is when making the potassium methoxide. Keep the vessels where KOH comes in contact with, as dry as possible. The chances of a good clean splitting of ester from glycerine with little soap by-product are much better on a warm dry summer day than on a damp winter day.
Washing : The Biodiesel from this stage can be used to the fuel tanks of vehicles. Mixture is left to let it settle for a while (about 2 days), allowing the majority of the soap residues to settle before running the biodiesel through a filtration system then into the vehicle fuel tank. Another method is to dry wash the soaps out of the fuel with Magnesium Silicate, one or more times.
Quantity of methanol to be used
The stoichiometric quantity of methanol is the amount needed to convert triclycerides (Used Cooking Oil, Palm Stearin or Animal Tallow) into esters (Biodiesel), the methyl portion of methyl esters. You also need an excess of methanol 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. Excess methanol 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, 125 ml of methanol per liter of oil. In fact Stoichiometric Quantity depends on the amounts of the various fatty acids in the oil, and these vary from one oil to another. Calculate the average proportions of the different fatty acids in each of the more common oils, calculate their total molecular weights, and from this calculate the stoichiometric amount of methanol required to convert them. The amount varies from 11.3% for rapeseed oil (canola) to 16.3% for coconut oil. These figures are averages, fatty acid quantities vary somewhat when oil crops are grown in different conditions in different parts of the world. But they are close enough for our purposes, and a lot more accurate than the general figure of 12.5%. If you have an analysis of the fatty acid content of your oil, you can calculate the exact stoichiometric ratio.
Excess : How much excess is needed depends on several different factors: 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, and it does not make much sense anyway if the stoichiometric ratio is wrong in the first place. However, excess is usually between 60% and 100% of the stoichiometric amount. The stoichiometric ratio of Used cooking oil or Palm Stearin or Animal Tallow, 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 soy or canola, you can try 60%, though 67% or more would be better. For palm kernel or coconut, closer to 100% excess would be better. If you do not know what kind of oil it is, try using 25% methanol, 250 ml methanol to 1 liter of oil. If you have 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 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.
Glycerine (glycerin, glycerol) is the main by-product of manufacture of Biodiesel. The rule of thumb is 79 ml of glycerine is formed, per liter of used cooking oil or Palm Stearin or Animal Tallow used, or 7.9%. The glycerine by-product burns well, but unless glycerine is properly combusted at high temperatures, glycerine will release acrolein, which is toxic. What sinks to the bottom of the Biodiesel processor during the settling stage is a mixture of glycerine, methanol, soaps and the KOH catalyst. Most of the excess methanol and most of the catalyst remains in this layer. Once separated from the Biodiesel, adding phosphoric acid to the glycerine layer precipitates the catalyst out and also converts the soaps back to free fatty acids (FFAs), which float on top. You are left with a light-colored precipitate on the bottom, glycerine / methanol / water in the middle, and FFA on top. The methanol is typically stripped from this stream and reused, leaving behind, after neutralization, what is known as crude glycerine. The glycerine will be approx. 95% pure, a much more attractive product to sell to refiners. In raw form, this crude glycerine has high salt and free fatty acid content and substantial color (yellow to dark brown). Consequently, crude glycerine has few direct uses due to the presence of the salts and others, and its fuel value is also marginal. The Biodiesel industry generates millions of tons of crude glycerine waste each year, and the amount produced is growing rapidly along with the dramatic growth of Biodiesel production.