2.6. Kinetics of transesterification reaction

Based on transesterification reaction, according to stoichiometric ratio, 1:3 is required between triglyceride and methanol for reaction to reach completion. Therefore, the rate equation can be expressed as in Eq. (9). Since the reaction temperature and time are the major constraint factors in reaction kinetic of transesterification, hence, the model kinetic was performed at optimum parameter of transesterification process with setting temperature at 55, 60, 65, 70, and 75 °C.

where r is the rate of reaction (transesterification), k is the rate constant, [TG] is the molar concentration of triglycerides, and [MOH] is the molar concentration of methanol.

To avoid low biodiesel yield and a reversible reaction to take place based on stoichiometric ratio of 1:3 of the transesterification process, stoichiometric ratio, 1:3, a large excess of methanol was added [17], usually twice ratio (1:6) of methanol-oil molar ratio. Hence, the concentration of methanol is assumed a constant value. The reaction can then be a pseudo first order [37].

Rearranging the rate equation and integrate, to obtain Eq. (10):

With k as rate constant measure in min⁻¹, XFAME stands for FAME conversion at interval time (t). From Eq. (10), the graph of −ln|1−XFAME| vs t was plotted, and the slope of the graph was determined. The slope of the graph represents the rate constant k.

Furthermore, energy must be added to the reactants to overcome the energy barrier, which is recovered when products formation occur. This energy is known has E_a, the activation energy which can be obtained from Arrhenius Eq. (11):

Where k is reaction rate, A is pre-exponential factor, E_a is the activation energy, R is the gas constant, and T is the temperature usually measure in kelvin.

On logarithmic expression, Eq. (11) become Eq. (12):

The graph of ln k is plotted against 1/T, the slope (m) of the graph estimated stand for (−EaR), and the activation energy Ea was obtained using Eq. (13):