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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 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 (it 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 lighter-colored biodiesel appears divert it to a separate container. If any biodiesel stays with the glycerine it is easy to retrieve it 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 matter organic non-glycerol (MONG) 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 being vented for three weeks to allow residual methanol to evaporate off or after heating it to 66oC to boil off any methanol content (the boiling point of methanol is 64.7oC). The excess methanol can be recovered for re-use when boiled off if 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, tinctures, hand lotions, dried plant arrangements and many other uses) and wax. This is often accomplished by distilling it, 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 it 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 other water that was suspended in the oil. If the reaction produces more than the usual amount of soap, this happens when KOH comes into contact with water before it 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 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 and drying : The biodiesel from this stage can be used to the fuel tanks of vehicles. It is 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 wash the soaps out of the fuel with water, one or more times. When washing biodiesel the first time it is best to add a small amount of dilute acetic acid before adding the water. The acetic acid brings the pH of the solution closer to neutral because it neutralizes and drops out any KOH suspended in the biodiesel.
A simple way of washing is using a PVC container with a valve 100 mm from bottom. Fill with water until it is halfway between the container's bottom and the valve, then fill up with the biodiesel to be washed. After a gentle stirring (keep it gentle, do not agitate up soaps) followed by 12-24 hours of settling, the oil and water will separate, the cleaned oil can be decanted out the valve, leaving the denser soapy water to be drained out the bottom.
This process might have to be repeated two or three times to remove close to 100% of soaps. The second and third washings can be done with water alone. After the third washing any remaining water gets removed by re-heating the oil slowly, the water and other impurities sink to bottom. The finished product should have a pH of 7, checked with litmus paper or with a digital pH tester.
The water from the third wash can be used for the first or second washes for the next batch. The impurities can be left in the re-heater for the next batch and removed when it accumulates. Transesterified and washed biodiesel will become clearer over time as any remaining soaps drop out of the solution.
Quantity of methanol to be used
The stoichiometric quantity of methanol is the amount needed to convert triclycerides (oils) 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. 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 ml of methanol per litre of oil. In fact it depends on the amounts of the various fatty acids in the oil, and varies 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 correct 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 Jatropha oil is 12.5%, that is 125 ml of methanol per litre of oil, the excess would range between 75 ml and 125 ml, for a total amount of methanol of 200-250 ml per litre of oil. oils with higher stoichiometric ratios seem to need higher excesses. So, for fresh soya 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 litre 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.
Glycerine
Glycerine (glycerin, glycerol) is the main by-product of making biodiesel. The rule of thumb is 79 ml of glycerine per liter of oil used, 7.9%. The glycerine by-product burns well, but unless it's properly combusted at high temperatures it 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 other species, 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 |
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