Biodiesel obtained from the transesterification of vegetable oil
or animal fat is a promising renewable green alternative fuel for
compression ignition engines. Compression ignition engines are
particularly suitable for medium-to-large road, rail and marine
use. This is due to their excellent efficiency and longer operation
life which is about twice as much as that for spark ignition
engines. The replacement of conventional diesel fuel with biodiesel
fuel is an attractive solution since the latter is regarded as a
renewable, biodegradable, non-poisonous, and oxygenated fuel.
However, existing production technologies offer less competitive
prices than petroleum-derived diesel due to high input feed and
biodiesel purification costs. Non-edible vegetable oils such as
waste cooking oils may be used as cheaper substitutes to virgin
edible vegetable oils in the feed stream. Similarly, the
utilization of solid heterogeneous catalysts in biodiesel synthesis
instead of conventional homogeneous catalysts could diminish the
complexity of biodiesel processing. This is because they can be
easily separated from the reaction mixture and hence, improved
product purity. Therefore, the current study involves the
production of biodiesel fuel from waste cooking oil with ethanol in
presence of tri-potassium phosphate (K₃PO₄) as a solid base
heterogeneous catalyst. The principal variables for the present
study were, ethanol/oil molar ratio (6:1 to 12:1), catalyst
concentration (2-8 wt %), stirring speed (600-1800 r/min), reaction
temperature (30-70°C) and time (1-3 hours).
Runs were conducted following a 5-variable central composite
design of experiments in order to secure the optimal set of
variables combination. Liquid phase composition was determined from
GC-mass analysis. The effect of the operating variables on the
presence of different fatty acid ethyl esters was evaluated. Fuel
properties including iodine and saponification values were
practically calculated for each produced biodiesel sample. Higher
heating value and cetane number were also measured. Thus, the
effect of five examined operating variables on fuel properties was
also evaluated.
Results show that ethanol/oil molar ratio and catalyst
concentration posed the most positive impacts on ethyl ester yield
as well as all the evaluated fuel properties. However, the
influence of catalyst concentration was found to be more obvious.
Reaction temperature displayed a slightly lower effect, while
reaction time depicted the least influence. In contrast, stirring
speed showed a quite negative effect on ethyl ester yield. Optimal
ester yield of (95.2%) was achieved at 12/1 ethanol/oil molar
ratio, 8 wt% catalyst concentration, 600 rpm stirring speed, 70°C
reaction temperature after one hour reaction. Similarly, these
conditions were considered as the optimal route that produces best
fuel quality in terms of higher heating value and cetane
number.