Understanding of "A review on biodiesel production using transesterification" article with respect to FFA.

22 September 2018

Issue

Depletion of world's petroleum reserves resulting in demand for alternative sources of fuel.

Solution

Replacing diesel with biodiesel.

What is biodiesel?

Biodiesel is a clean renewable fuel, which can be used in any compression ignition engine without modification.  it is typically made from nontoxic biological resources such as vegetable oil.

Chemical composition of biodiesel

Biodiesel is a mixture of methyl esters with long-chain fatty acids.

Biodiesel from edible an non-edible vegetable oils

More than 95% of biodiesel is manufacture from edible oils. However, this causes competition with the edible oil market, and increase deforestation in countries. Thus research is carried out on production of biodiesel using non-edible oils.

Pros and cons of non-edible oil

Pros

• Non-edible oil crops can be grown in waste lands.
• Lower cost of cultivation.

Cons

• Non-edible oils contain high free fatty acids (FFA) resulting in multiple chemical steps to produce biodiesel. This increases production cost.
• Lower ester yield of biodiesel.

Biodiesel from Used cooking oils (UFO)

Using UFO as feedstock for biodiesel production significantly reduces cost of Biodiesel production. However, quality of UFO depends on the contents of fresh cooking oil. UFO may contain many impurities such as FFA and water. Yield and properties of biodiesel from different feedstocks would differ significantly from one another, thus the choice of feedstock is important.

Advantages of biodiesel over diesel

• Higher combustion efficiency
• Biodegradable
• Lower emission of toxic gas

4 Primary methods of biodiesel production

• Direct use and blending of raw oils
• Micro emulsions
• Thermal cracking
• Transesterifictaion

Catalyzed transesterification

Common vegetable oils or animal fats are esters called triglycerides. Transesterification is the process in which triglycerides are reacted with alcohol in the presence of catalyst to produce glycerol and fatty acid esters. In alkali-catalyzed transesterification, the alkali catalyst will react with small amounts of FFA within vegetable oils or animal fats to form soap and water. This is undesirable as

1. Soap lowers the yield of biodiesel.
2. Soap inhibits separation of esters from glycerol.
3. Soap binds with catalyst, increasing amount of catalyst required.
4. Water retards the transesterification by hydrolysis of triglycerides to diglycerides and forming             more FFA.

Acid-catalyzed esterification

This reaction is useful as it allows FFA to react with alcohol to form fatty acid ester (Biodiesel).
Thus it is useful for handling feedstock with high FFA. However acid-catalyzed esterification has not gained much attention due to

1. Slow reaction rate.
2. High alcohol to oil ratio required.

Raw material treatment

• Hydration process to reduce solubility of phospholipids allowing it to separate from oil by natural settlement.
• Citric acid and phosphoric acid added to oil at 70°C to remove non-hydratable phospholipids.
• Refining with supercritical CO2 allows removal of FFA and peroxidation products in crude oil. High pressure is required in this process.
• Deodorization. Steam at 1-6 mm Hg pressure is injected to the oil at 490-550K to eliminate FFA, aldehydes, unsaturated hydrocarbons and ketones.
• Titration is used to determine percentage of FFA in oil if percentage of FFA is over 2.5%, pretreatment has to be performed to reduce FFA content.

Pretreatment of acidic feedstocks

Steam distillation
-Requires high temperature
-Low efficiency

Extraction by alcohol
-Requires large amount of solvent due to limited solubility of FFA in alcohol
-very complicated

Thus esterification by acid-catalyst makes the best use of FFA. Alternatively iodine can be used as a catalyst. It can be recycled after esterification reaction.

Glycerol can also be added to the acidic feedstock and heated to 200 this causes glycerol to react with FFA to form monoglycerides and diglycerides, thus reducing FFA level in oil.

Pros

• No alcohol required
• Water formed can be immediately vaporized and vented 

Cons

• Slow reaction rate
• High temperature requirement