Ultrasound-Assisted Synthesis of Biodiesel from Palm Fatty Acid Distillate

Abstract

The present work aims at exploiting palm fatty acid distillate as a starting raw material for synthesis of biodiesel with the aim of reducing the cost of production. Esterification of palm fatty acid distillate (PFAD) with methanol in the presence of concentrated H2SO4 as a catalyst has been investigated in the presence of ultrasonic irradiations at 22 kHz frequency and 120 W as supplied power dissipation. The effect of different operating parameters such as molar ratio of reactants, catalyst quantity, and operating temperature have been studied with the aim of process optimization. It has been observed that ultrasound significantly enhances the rate of reaction and also the extent of equilibrium conversion. A certain degree of conversion attributed to heterogeneity of the system, which adds to mass transfer resistances under conventional approach, appears to get eliminated due to ultrasound. Conversion levels of more than 90% have been observed with the use of ultrasound in about 150 min under ambient operating conditions. microwaves, supercritical fluids, ultrasound, or fluid energy. 5-9 The most popular method of producing biodiesel is the transesterification of vegetable oils. 5,6 Biodiesel obtained by the transesterification process can be a mixture of monoalkyl esters of higher fatty acids. The substantially higher costs of vegetable oil makes pure vegetable oil an unfavorable starting raw material. However, if waste cooking oil or a mixture of off- specification oil or fatty acid cuts is used in a conventional approach of synthesis, significantly lower rates of reaction are observed with the requirement of severe operating conditions. The conventional techniques based on use of stirring typically utilize temperatures in the range of 70-200 °C, pressures in the range of 6-10 atm, and reaction times of up to 70 h for achieving conversions in the range of 90-95 mol % based on the type of raw material used, the type and concentration of the catalyst, and the percentage excess of alcohol. 10-12 The lower rates of synthesis have been typically attributed to mass transfer limitations due to heterogeneous conditions existing during the reaction. Use of cavitational reactors can favor the reaction chemistry and propagation by way of enhanced mass transfer and interphase mixing between the phases and also can lower the requirement of the severity of the operating conditions in terms of temperature and pressure. 13 Indeed, there have been some earlier illustrations referring to the use of cavitational reactors for intensification of biodiesel synthesis 5,7,14-16 but mainly these have been based on the use of pure vegetable oils as a raw material for the synthesis. A recent study 4 by our group reported the use of a fatty acid cut as an alternative, cheaper starting material. In continuation of our objective of utilizing cheaper raw materials for the synthesis of biodiesel using cavitational reactors, the present work reports the use of palm fatty acid distillate (PFAD), which is generated as a byproduct during the refining of palm oil and has a comparatively much lower value as compared to the palm oil. The esterification reaction between PFAD and methanol in the presence of sulfuric acid as a catalyst and ultrasonic irradiations as a source for generation of cavitating conditions has been carried out. 2. Materials and Methods 2.1. Reaction Scheme. The reaction considered for the present study is a typical esterification reaction of oil and alcohol in the presence of acid catalyst producing methyl esters and water. The reaction can be schematically represented as