Biodiesel From Plant Oils

8/13/2019 Biodiesel From Plant Oils

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Biodiesel refers to a vegetable oil or animal fat-based diesel fuel

consisting of long-chain monoalkyl esters or fatty acid methyl ester.

Biodiesel is typically made by chemically reacting lipids such asvegetable oil with an alcohol such as methanol, producing fatty acidesters.

Biodiesel can be used as a fuel for vehicles in its pure form, but itis usually blended with petrodiesel to reduce levels of particulates,carbon monoxide, and hydrocarbons from diesel-powered vehicles.

Biodiesel is the most common biofuel in Europe.


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Plant oils are oils derived from plant sources, as opposed to animal fatsor petroleum.

Plant oils are basically mixtures of varying amounts of triglycerides.When oils are split into glycerine and fatty acids, the resulting mixturecontain three molecules of fatty acid for each molecule of glycerine.

While there are a large number of triglycerides in nature, thetriglycerides of seven acids, namely, lauric, myristic, palmitic,stearic, oleic, linoleic, and linolenic, constitute the majority of plantoils.

The properties of plant oils, therefore, depend on the characteristics ofthe triglycerides present.


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There are two distinct types of plant oils:

Fixed oils such as coconut and castor oils, which do not readily evaporateon exposure to air.

Essential oils such as citronella and cinnamon oils, which readilyevaporate on exposure.

Fixed oils are usually extracted by:

Crushing and pressure

Boiling Chemical solvents

Essential oils are almost always extracted by distillation, many of them fromflowers such as ilang-ilangoil.

Some fixed oils that are liquid at relatively high temperature become solid inambient temperatures. These fixed oils from plants are used for diesel fuel

production.

While essential oils are used in the production of pharmaceuticals, cosmeticsand perfumes.


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The oil from oil-producing plants is usually concentrated in the fruits (fleshor pulp) and seeds (mainly embryo). The roots, stalks, branches, and leaves

may also contain some oil but, in very small amounts.

The oil yield per unit weight of material being processed is an importantparameter in assessing the cost of extraction required to produce the desiredproduct, which is the oil.

This table presents the typical amounts of oil that can be extracted from someplants or crops:


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When a molecule of the fatty acid contains the maximum amount ofhydrogen, the acid is called a saturated fatty acid.

Myristic, lauric, palmitic and stearic acids are saturated fattyacids. These fatty acids are usually solid at ambient temperaturesuch as palm oil (shown below).

When the molecule of the fatty acid does not contain the maximumamount of hydrogen, the acid is called an unsaturated fatty acid.

Oleic, linoleic and linolenic acids are unsaturated fatty acids.These fatty acids are usually liquid at ambient temperature.


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Extraction of oils is the first step in producing biodiesel. This stepmay involve several different types of operations or processes dependingon the kind of feedstock.

There are two main processes for extracting oil from seed feedstock. Theyare:

mechanical press extraction

solvent extraction

In mechanical press extraction, the oil seed is first heated to about43C. The oil seed is then crushed in a screw press releasing the oil.

The solvent process extracts more of the oil contained in the oil seedbut requires more costly equipment. The process uses a solvent todissolve the oil. After extraction, a distillation process separates theoil from the solvent.

Solvent extraction produces oil with a higher degree of purity than themechanical press process.


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Biodiesel is produced by transesterifying the vegetable oil to achieve

a viscosity close to that of petrodiesel. These oils as alternative

fuels are extremely viscous; with viscosities ranging from 10 to 17

times greater than that of petrodiesel. The purpose of the

transesterification process is to lower the viscosity of the oil.

During the process of transesterification, an alcohol reacts with the

triglyceride oils (plant oils) to form fatty acid alkyl esters

(biodiesel) and glycerine.

The most common form of this process uses methanol to produce methylesters (commonly referred to as fatty acid methyl ester or FAME) as it

is the cheapest alcohol available, although ethanol can be used to

produce an ethyl ester and higher alcohols such as isopropanol and

butanol have also been used.


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The reaction requires heat and a strong base catalyst such as sodium hydroxide

or potassium hydroxide. This is the simplified chemical reaction whichproduces biodiesel:


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The complete process for the production of methyl ester (biodiesel) from plant

oil basically involves five steps: acid esterification, transesterification,methanol recovery, biodiesel refining, and glycerine refining.

This is simplified process flow diagram:


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Acid Esterification

The oil feedstock containing more than 4% free fatty acids is first filtered

and then pre-processed to remove water and unwanted solids. The pretreated oil

is then fed to the acid esterification process. Sulphuric acid (catalyst),

which is dissolved in methanol, is then mixed with the pretreated oil and

heated. The free fatty acids are thereby converted to biodiesel. Once the

reaction is complete, it is dewatered and then fed to the transesterification

process.

Transesterification

The plant oil, which contains less than 4% free fatty acids, is first filtered

and then pre-processed to remove water and contaminants. The pretreated oil is

then fed directly to the transesterification process along with any products

of the acid esterification process. Potassium hydroxide (catalyst), isdissolved in methanol and then mixed with the pretreated oil. Once the

reaction is complete, the major co-products, biodiesel and glycerine, are

separated into two layers.


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Methanol recovery

The methanol is usually removed immediately after the biodiesel and glycerine

have been separated. This is done to prevent the reaction from reversingitself. The recovered methanol is cleaned and recycled back to the beginningof the process.

Biodiesel refining

Once separated from the glycerine, the biodiesel goes through a series ofpurification steps to remove excess alcohol, residual catalyst and soaps.These consist of multistage washings with clean water. The product biodieselis then dried and sent to storage.

Glycerine refining

The crude glycerine contains unreacted catalyst and soaps that must beneutralized with an acid. The water and alcohol are also removed to produce50%-80% crude glycerine. Further purification produces glycerine with a purityof 99% or higher which is suitable for use in the pharmaceutical and cosmeticindustries.


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Some advantages of biodiesel as fuel are:

renewability

lubricating qualities

higher combustion efficiency

lower sulphur content

higher biodegradability

Biodiesel is the only alternative fuel that allows for low concentration

biodieseldiesel blends to run on conventional unmodified engines. It can be

stored anywhere where petroleum diesel fuel is stored.

Biodiesel is much safer to handle and transport due to its high flash point.

The higher heating values (HHVs) of biodiesels are relatively high. The HHVs

of biodiesels (3941 MJ/kg) are slightly lower than that of gasoline (46

MJ/kg), petrodiesel (43 MJ/kg) or petroleum (42 MJ/kg), but higher than coal

(3237 MJ/kg).


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Some disadvantages of biodiesel are:

higher viscosity

lower engine speed and power

engine compatibility

higher cost

Other disadvantages of biodiesel in comparison withpetrodiesel are the lower energy content and fuel pumpingdifficulty from higher viscosity. This increases fuelconsumption when biodiesel is used in comparison to pure

petrodiesel.


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The first generation biofuels are those that are characterized by wellunderstood technologies.

The development and use of first generation biofuels are encouraged bythe desire to reduce oil and coal imports, support the rural economy, andto decrease the accumulation of greenhouse gases.

The second generation biofuels address many of the problems and concerns

associated with first generation biofuels.

To develop second generation biofuels, research and development work hasbeen directed towards advanced technologies such as:

biodiesel enzymes

higher carbon fixation in roots

improved oil recovery

Through advances in genetic engineering, it has become possible todevelop crops that are:

disease-resistant

viable even in degraded lands

require much lower inputs of chemicals and water


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Biodiesel From Plant Oils

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