46
Alternative Fuels for Transportation
the alcohol, producing an alkoxide and a protonated catalyst. The nucleophilic attack of the alkoxide at the carbonyl group of the triglyceride generates
a tetrahedral intermediate, from which the alkyl ester and the corresponding
anion of the diglyceride are formed. Similar process converts diglycerides
into monoglycerides and monoglycerides into glycerol (Demirbas 2009).
The vegetable oil charged in the reactor and heated to about 60–70°C
with the moderate stirring. Meanwhile, about 0.5–1.0% (w/w) of anhydrous
alkaline catalyst (sodium hydroxide or potassium hydroxide) is dissolved
in 10–15% (w/w) of methanol. This sodium hydroxide–alcohol solution is
mixed with the oil, and heating and stirring are continued. After 25–45 minutes, the reaction is stopped and the products are allowed to settle into two
phases. The upper phase consists of esters and the lower phase consists of
glycerol and impurities. The mixture of KOH and methanol settles at the
bottom of the funnel because of its higher density compared with biodiesel
whereas a small amount of catalyst, excess methanol, and glycerol are in the
upper biodiesel layer. Washing is a purification process to remove entrained
glycerol, catalyst, soap, and excess methanol in the upper layer. The excess
methanol in biodiesel corrodes the fuel injection system and should be separated from the biodiesel. The ester layer is washed with water several times
until the washing becomes clear.
3.2.1.1 Purification of Biodiesel
The transesterified vegetable oils (i.e., biodiesel/esters) have reduced viscosity and increased volatility relative to the triglycerides present in vegetable
oils. A dark viscous liquid (rich in glycerol) is the by-product of the transesterification process. Traces of the methanol, catalyst, and free fatty acids
(FFAs) in the glycerol phase can be processed in one or two stages depending
upon the level of purity required. Distillation column recovers the excess
alcohol and it can be recycled. The by-product glycerol needs to be recovered because of its value as an industrial chemical such as CP glycerol, USP
glycerol, and dynamite glycerol. The glycerol is then removed by gravity
separation and the remaining ester is mixed with hot water for separation of
catalyst. By using silica gel moisture it can be removed.
The triglycerides should have lower acid value and all material used in
the process should be substantially anhydrous. The addition of more base
catalyst to compensate the higher acidity of vegetable oils results in formation of soap and the increase in viscosity or formation of gels that interferes
in the transesterification reaction as well as with the separation of glycerol.
Saponification reaction also takes place simultaneously along with the transesterification process but soap formation is not a major problem if presence
of water is less than 1%. Prolonged contact with air will diminish the effectiveness of these catalysts through interaction with moisture and carbon
dioxide. When the reaction conditions do not meet the above requirements,
ester yields are significantly reduced.
Alternative Fuels for Transportation
the alcohol, producing an alkoxide and a protonated catalyst. The nucleophilic attack of the alkoxide at the carbonyl group of the triglyceride generates
a tetrahedral intermediate, from which the alkyl ester and the corresponding
anion of the diglyceride are formed. Similar process converts diglycerides
into monoglycerides and monoglycerides into glycerol (Demirbas 2009).
The vegetable oil charged in the reactor and heated to about 60–70°C
with the moderate stirring. Meanwhile, about 0.5–1.0% (w/w) of anhydrous
alkaline catalyst (sodium hydroxide or potassium hydroxide) is dissolved
in 10–15% (w/w) of methanol. This sodium hydroxide–alcohol solution is
mixed with the oil, and heating and stirring are continued. After 25–45 minutes, the reaction is stopped and the products are allowed to settle into two
phases. The upper phase consists of esters and the lower phase consists of
glycerol and impurities. The mixture of KOH and methanol settles at the
bottom of the funnel because of its higher density compared with biodiesel
whereas a small amount of catalyst, excess methanol, and glycerol are in the
upper biodiesel layer. Washing is a purification process to remove entrained
glycerol, catalyst, soap, and excess methanol in the upper layer. The excess
methanol in biodiesel corrodes the fuel injection system and should be separated from the biodiesel. The ester layer is washed with water several times
until the washing becomes clear.
3.2.1.1 Purification of Biodiesel
The transesterified vegetable oils (i.e., biodiesel/esters) have reduced viscosity and increased volatility relative to the triglycerides present in vegetable
oils. A dark viscous liquid (rich in glycerol) is the by-product of the transesterification process. Traces of the methanol, catalyst, and free fatty acids
(FFAs) in the glycerol phase can be processed in one or two stages depending
upon the level of purity required. Distillation column recovers the excess
alcohol and it can be recycled. The by-product glycerol needs to be recovered because of its value as an industrial chemical such as CP glycerol, USP
glycerol, and dynamite glycerol. The glycerol is then removed by gravity
separation and the remaining ester is mixed with hot water for separation of
catalyst. By using silica gel moisture it can be removed.
The triglycerides should have lower acid value and all material used in
the process should be substantially anhydrous. The addition of more base
catalyst to compensate the higher acidity of vegetable oils results in formation of soap and the increase in viscosity or formation of gels that interferes
in the transesterification reaction as well as with the separation of glycerol.
Saponification reaction also takes place simultaneously along with the transesterification process but soap formation is not a major problem if presence
of water is less than 1%. Prolonged contact with air will diminish the effectiveness of these catalysts through interaction with moisture and carbon
dioxide. When the reaction conditions do not meet the above requirements,
ester yields are significantly reduced.