The basics of biodiesel
WHAT IS BIODIESEL?
Biodiesel is a natural, renewable and alternative fuel for any diesel engine. It can be made from any plant based oil. In South Africa, biodiesel is commonly made from Sunflower Oil and Palm Oil and less commonly from Canola Oil and Soy Oil. Biodiesel contains no petroleum, is non-hazardous, non-flammable, non-toxic and is biodegradable.
Biodiesel is known by the chemical name Fatty Acid Methyl Ester (FAME). All it means is that it is a simple molecule made from vegetable oil which can be extracted from a variety of nuts and seeds. Vegetable oil is a triglyceride i.e. it has three hydrocarbons chains attached to the same glycerine molecule.
The production of Biodiesel is a relatively simple chemical process. Biodiesel is easy to use, handle, store and blend (mix with regular diesel).
- It costs less than regular diesel
- It contains no or little sulphur
- It has a higher Cetane rating that improves the efficiency of compression ignition
- NO engine modifications are necessary to use biodiesel on any diesel engine
- It is non-flammable, non-hazardous, non-toxic & environmentally friendly
- It has a flash point greater than 120°C while fossil diesel is approx. 70°C
- Users can switch between biodiesel and regular diesel as they wish
- It has superior cleaning properties over all other regular diesel
- It provides much better engine lubrication than regular diesel
- It produces 75% less exhaust smoke emissions
- It produces 80% less carbon dioxide emissions
- It is made from new or used cooking oil
- It extends engine life by up to 200%
- It mixes very well with regular diesel
- It is safe to store and handle
- It is a renewable fuel
- It is a “green” fuel
HOW IS BIODIESEL PRODUCED?
Biodiesel can be produced from new or used vegetable oil. New oil can be mixed with used oil and different vegetable oils can be mixed together. In used oil, the higher the Free Fatty Acid (FFA) content, the lower the yield of biodiesel. To increase the yield from used oil with a high FFA content, the oil has first to be “pre-treated” in a process called esterification. New oil and low FFA used oil can go straight to the transesterification process.
Transesterification is carried out in presence of methanol and a catalyst; which yields raw biodiesel (methyl ester) and glycerol as principal products, along with soap, excess catalyst and excess methanol.
Typical catalysts used are potassium hydroxide (KOH) or sodium hydroxide (NaOH). When the catalyst is mixed with methanol, a chemical reaction occurs, heat is generated and the liquid is referred to as Methoxide. A premixed liquid is available from chemical supply companies and is referred to as Methylate.
The ratio of methanol to oil is about 1:5 or 20%. The oil is heated to between 50ºC and 55ºC. Then the methoxide is slowly added to the hot oil. It is then vigorously mixed to allow complete conversion of the oil. The mixing allows the catalyst to break off each hydrocarbon chain one by one and bond with a floating methanol molecule to form biodiesel. The result of the chemical reaction is biodiesel and glycerol. The liquid is allowed to settle and separate for a minimum of 4 hours. The glycerol molecule is denser than the biodiesel and sinks to the bottom of the tank where it is drained off. Glycerol will account for about 20% of the total mixed volume.
The separated biodiesel is then washed with water or with an ion exchange resin to remove impurities like glycerol, soaps and traces of catalyst and methanol, along with ionic salts to produce pure clean biodiesel.
After the water washing process, the biodiesel has to be dried. Several methods are used to dry biodiesel which must not be confused with Dry Washing (see below). Drying biodiesel is the process of removing residual water from the biodiesel. This can be done by Flash Evaporation, Spraying or Bubbling dry air through the biodiesel.
– Mist Washing
Clean warm water (50ºC) is sprayed through mist nozzles in the top of the tank above the biodiesel. Water is more dense than biodiesel so the tiny water droplets travel through the biodiesel to the bottom of the tank. On their way down, the water droplets collect the impurities (glycerol, soap, residual catalyst, residual methanol and ionic salts) left behind in the biodiesel after the reaction. The “dirty water” then accumulates in the bottom of the tank and is automatically drained off.
– Bubble Washing
About 10% by volume of water is added to the biodiesel which settles at the bottom of the tank. An air stone connected to an air pump via an air line is introduced to the bottom of the tank. Millions of air bubbles, the same as in a fish tank, rise up through the water taking with them a layer of water on the outside of the bubble. When the bubble reaches the top of the biodiesel, it “pops” and the microscopic water droplets travel back down to the bottom of the tank, collecting the impurities in the biodiesel (as described above). The “dirty water” in the bottom of the tank has to be changed regularly in order to wash the biodiesel.
– Dry Washing
Biodiesel is pumped through one or two columns (pressure vessels) of resin beads such as Amberlite BD10DRY, Purolite PD206, Thermax Tulsion T-45 BD MP or Eco2Pure which absorb the impurities in the biodiesel. The dry wash resin is effective in removing impurities, is less time consuming and less labour intensive. The other advantage is that no water is added which means no water has to be removed.
Feedstock is a term that refers any type of vegetable oil needed to produce biodiesel. Examples of crops grown in South Africa that are suitable for feedstock are: Sunflower, Canola and Soy. In addition to these vegetable oils, South Africa imports large volumes of new palm oil for the catering industry. Generally palm oil is cheaper than Sunflower, Canola or Soy.
The following are some feedstock’s that can be used for biodiesel production:
A large percentage of biodiesel produced in South Africa comes from used vegetable oil recovered from the catering industry. The two main oils used in the catering industry are Sunflower Oil and Palm Oil.
Glycerol is the by-product of the biodiesel process. Glycerol contains residual chemicals from the biodiesel process. To purify glycerol will require a distillation column from which only 40% glycerine can be recovered. Glycerol accounts 16-25% of the total volume of liquid in a conventional stirred biodiesel processor.
There are products that can be made out of glycerol, most commonly soaps, degreaser, hand cleaners, floor cleaners etc. As long as the methanol has been removed, glycerol can be composted or fed to an anaerobic digester.
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