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Biodiesel is a renewable fuel which is produced from vegetable oil or animal fat through a chemical process and can be used as either direct substitute, extender or as an additve to fossil diesel fuel in compresion ignition engines. The most promising feature of biodiesel is that it can be utilized in existing design of diesel engine with no or very little modifications. It has positive energy balance and it is enviornmtally sound.
Biodiesel is a natural and renewable domestic fuel alternative for diesel engines made from vegetable oils, mostly soy and corn. It contains no petroleum, is nontoxic and biodegradable.Biodiesel burns clean, which results in a significant reduction of the types of pollutants that contribute to smog and global warming and emits up to 85% fewer cancer-causing agents. It is the only alternate fuel approved by the Environmental Protection Agency (EPA), has passed every Heath-Effects Test of the Clean Air Act and meets the requirements of the California Air Resources Board (CARB).
Biodiesel is made using an alcohol like methanol and a chemical process that separates glycerine and methyl esters (biodiesel) from fats or vegetable oils. Glycerine is used in many common products including soap and is highly marketable; therefore there is little waste in the process. That said, growing crops requires time and significant investment, and the fuel must be made and shipped to a local station. For these reasons biodiesel is more expensive than petroleum, gallon for gallon. This must be considered against the many economic advantages, however, that arise from a domestic form of fuel, a cleaner environment, an improvement in air quality, and a reduction of cancer-causing agents.
Biodiesel is one of the environmentally friendly alternative liquid biofuels that has proven itself commercially, with international standards all around the world. Industrial and scientific studies on reducing biodiesel production costs are one of the major contributions that have strengthened the position of biodiesel commercially. The type of vegetable oil used for biodiesel production is the parameter that has the greatest effect on biodiesel production cost. For this reason, investigations on the types of no-to-low-cost vegetable oils become crucial. In this study, the optimum conditions for biodiesel production from restaurant-originated used cooking oil (which is composed primarily of oleic and linoleic acids) and the refining procedure were investigated. A refining method of "washing with hot water" was used for biodiesel refinement. One of the properties of biodiesel that has an influence on biodiesel purity is glycerin content. In the refining studies, the effects of glycerin amount, used washing water amount, and the number of washing steps were discussed and biodiesel that meets EN 14214 standards was produced.
The major steps required to synthesize biodiesel are as follows:
If waste vegetable oil (WVO) is used, it is filtered to remove dirt, charred food, and other non-oil material often found. Water is removed because its presence causes the triglycerides to hydrolyze, giving salts of the fatty acids (soaps) instead of undergoing transesterification to give biodiesel.
Determination and treatment of free fatty acids
A sample of the cleaned feedstock oil is titrated with a standardized base solution in order to determine the concentration of free fatty acids (carboxylic acids) present in the waste vegetable oil sample. These acids are then either esterified into biodiesel, esterified into bound glycerides, or removed, typically through neutralization.
While adding the base, a slight excess is factored in to provide the catalyst for the transesterification. The calculated quantity of base (usually sodium hydroxide) is added slowly to the alcohol and it is stirred until it dissolves. Sufficient alcohol is added to make up three full equivalents of the triglyceride, and an excess of usually six parts alcohol to one part triglyceride is added to drive the reaction to completion.
Products of the reaction include not only biodiesel, but also byproducts, soap, glycerin, excess alcohol, and trace amounts of water. All of these byproducts must be removed, though the order of removal is process-dependent.
The density of glycerin is greater than that of biodiesel, and this property difference is exploited to separate the bulk of the glycerin byproduct. Residual methanol is typically removed through distillation and reused, though it can be washed out (with water) as a waste. Soaps can be removed or converted into acids. Any residual water must be removed from the fuel.
Triglycerides (1) are reacted with an alcohol such as ethanol (2) to give ethyl esters of fatty acids (3) and glycerol (4):
Animal and plant fats and oils are typically made of triglycerides which are esters containing three free fatty acids and the trihydric alcohol,glycerol. In the transesterification process, the alcohol is deprotonated with a base to make it a stronger nucleophile. Commonly, ethanol or methanol are used. As can be seen, the reaction has no other inputs than the triglyceride and the alcohol.
Normally, this reaction will proceed either exceedingly slowly or not at all. Heat, as well as an acid or base are used to help the reactionproceed more quickly. It is important to note that the acid or base are not consumed by the transesterification reaction, thus they are not reactants but catalysts.
Almost all biodiesel is produced from virgin vegetable oils using the base-catalyzed technique as it is the most economical process for treating virgin vegetable oils, requiring only low temperatures and pressures and producing over 98% conversion yield (provided the starting oil is low in moisture and free fatty acids). However, biodiesel produced from other sources or by other methods may require acid catalysis which is much slower. Since it is the predominant method for commercial-scale production, only the base-catalyzed transesterification process will be described below.
An example of the transesterification reaction equation, shown in skeletal formulas:
Since natural oils are typically used in this process, the alkyl groups of the triglyceride are not necessarily the same. Therefore, distinguishing these different alkyl groups, we have a more accurate depiction of the reaction:
R1, R2, R3 : Alkyl group.
During the esterification process, the triglyceride is reacted with alcohol in the presence of a catalyst, usually a strong alkali (NaOH, KOH, orAlkoxides). The main reason for doing a titration to produce biodiesel, is to find out how much alkaline is needed to completely neutralize any free fatty acids present, thus ensuring a complete transesterification. Empirically 6.25 g / L NaOH produces a very usable fuel. One uses about 6 g NaOH when the WVO is light in colour and about 7 g NaOH when it is dark in colour.
The alcohol reacts with the fatty acids to form the mono-alkyl ester (or biodiesel) and crude glycerol. The reaction between the biolipid (fat or oil) and the alcohol is a reversible reaction so the alcohol must be added in excess to drive the reaction towards the right and ensure complete conversion.
There are three basic methods to biodiesel production:
Base catalyzed transesterification of the oil
Acid catalyzed transesterification of the oil
Conversion of the oil to its fatty acids and then to biodiesel.
Most of the biodiesel produced today is done with the base catalyzed reaction. This catalyst splits the oil into glycerine and biodiesel.
The catalyst is typically sodium hydroxide or potassium hydroxide, which is dissolved in methyl alcohol. The reaction mix of oil and catalyst is kept just above the boiling point of the alcohol to speed up the reaction. Recommended reaction time varies between 1 to 8 hours. Excess alcohol is normally used to ensure total conversion of the fat or oil to its esters. After separation of the glycerol and biodiesel phases, the excess alcohol is removed with a evaporation process or by distillation.
Degussa catalyst for biodiesel production
Degussa has developed a special catalyst: sodium methylate. During the production process a 30 percent sodium methylate solution in methanol controls the reaction of the oil to biodiesel and glycerol. One tonne of raw material requires only about 17 to 18 kilograms of this catalyst. At the end of the process, the methyl ester and glycerol are separated. The glycerol can be sold to the chemical or pharmaceutical industry, after purification.
Degussa markets these catalyst solutions in a ready-to-use form. The catalyst can be added directly from the storage tank to the production process and are mainly used in large facilities with an annual capacity of 50,000 to 100,000 tonnes.
One advantage of the Degussa catalyst is that it produces a high glycerol yield of high quality. Around two thirds of the large biodiesel facilities are designed around this catalyst.
Another related catalyst, potassium methylate, is used to make biodiesel from old cooking fat. Biodiesel production with the alkoxide catalysts offer significant commercial advantages over the alternatives sodium and potassium hydroxide.
MOST USED METHOD IS THE BASE CATALYSED METHOD:
Base catalyzed transesterification of the oil
Nowadays, the most common process used to create biodiesel is through the base catalyzed reaction. The process has a number of advantages, among them includes the fact that the process involves low temperature and pressure, chances of converting the oils into fuel are high with minimal side reactions and reaction time, the process involves no intermediate compounds and does not require the use of other materials that can be highly toxic.
To achieve the production of biodiesel through base catalyzed production, one needs to mix alcohol and catalyst together. The catalyst is dissolved in the alcohol using an agitator or mixer which is one of most basic of equipment needed when creating biodiesel. The resulting mix of alcohol and catalyst is then charged in a closed reaction container.
The oil or fat is added is then added into the container together with the mix. Everything should be done in a sealed container to keep the alcohol from evaporating.
It takes about 1 to 8 hours of reaction time before the biodiesel is produced. Some companies prefer mixing the components together in room temperature while others do it in temperatures slightly above the boiling point of alcohol. During the whole process, tight monitoring of alcohol and water level is being monitored to maintain the right proportions.
After the reaction has taken place, biodiesel needed to be separated from excess glycerin and methanol which are by-products of the whole process.