Page 201 - Biofuels Refining and Performance

184 Chapter Six

TABLE 6.2 Comparison of Biodiesel Production by Acid, Alkali, and Enzyme
Acid-catalyzed Base-catalyzed Enzymatic
transesterification transesterification Transesterification

The glycerides and The glycerides and alcohol The alcohol needs to be
alcohol need not be must be substantially anhydrous
anhydrous anhydrous
Does not make a soap- Soap formation taking Does not make a soap-like
like product place during the reaction product
Easy to water wash Water washing is difficult Separation of the product is
for product separation due to soap emulsifier very easy; product is obtained
only by filtration
Recommended for any Recommended for low Recommended for any free-
free-fatty acid content free-fatty acid content of fatty acid content of
of vegetable oil vegetable oil vegetable oil
Converts free fatty acid Converts free fatty acid to Converts free fatty acid to
to ester soap ester
Product yield is high Product yield is com- Product yield depends on
paratively low different types of enzymes
used; reaction is selective
Is slower than alkali- Is faster comparatively Ezymatic reaction is slower
catalyzed transesteri- than acid and alkali-
fication catalyzed reaction
Percentage of conversion Percentage of conversion Percentage of conversion is
is low is high high
Under low water content Water content in the Under low water conditions,
in reactants (oil or reactant inhibits the the hydrolysis reaction is
alcohol), reaction rate reversible, i.e., the ester bond
transesterification is synthesized rather than
reaction is not hindered hydrolyzed. Lipases break
down the triacylglycerols into
free fatty acids and glycerol
that exhibit maximum activity
at the oil–water interface
Need high temperature Reacts even at room Conversion takes place at low
temperature temperature

heavy carbon deposits. But specific emissions and visible smoke char-
acteristics of diesel fuel and esterified cottonseed oil were comparable.
Ziejewski and Kaufman [55] conducted a long-term test using a 25–75%
blend of alkali-refined sunflower oil and diesel fuel in a diesel engine, and
compared the results with that of a baseline test on diesel fuel. Engine
power output over the tested speed range was slightly higher for this blend.
At 2300 rpm, the difference was 25%. At 1800 rpm, the gain in power was
6%. The smoke level increased at a higher engine speed from 1 to 2.2 and
decreased at a lower engine speed. Greater exhaust temperature was caused
by a higher intake air temperature. The major problems experienced were:

1. Abnormal carbon buildup in the injection nozzle tips.
2. Injector needle sticking.