Ultrasonic transesterification of Jatropha curcas L. oil to biodiesel by a two-step process

Abstract

Transesterification of high free fatty acid content Jatropha oil with methanol to biodiesel catalyzed directly by NaOH and high-concentrated H₂SO₄ or by two-step process were studied in an ultrasonic reactor at 60 °C. If NaOH was used as catalyst, biodiesel yield was only 47.2% with saponification problem. With H₂SO₄ as catalyst, biodiesel yield was increased to 92.8%. However, longer reaction time (4 h) was needed and the biodiesel was not stable. A two-step, acid-esterification and base-transesterification process was further used for biodiesel production. It was found that after the first-step pretreatment with H₂SO₄ for 1 h, the acid value of Jatropha oil was reduced from 10.45 to 1.2 mg KOH/g, and subsequently, NaOH was used for the second-step transesterification. Stable and clear yellowish biodiesel was obtained with 96.4% yield after reaction for 0.5 h. The total production time was only 1.5 h that is just half of the previous reported. The two-step process with ultrasonic radiation is effective and time-saving for biodiesel production from Jatropha oil.

Introduction

Biodiesel is a fuel obtained from renewable biomass feedstock that can be used in diesel engines as neat fuel or blended at various proportions with conventional fossil diesel fuel [1], [2], [3], [4]. It consists of mono-alkyl esters usually produced by transesterification of vegetable oils, animal fats and cooking oils with low-molecular weight alcohols, most commonly methanol or ethanol. Biodiesel is an excellent substitute for conventional diesel fuel because of being renewable, nontoxic and biodegradable. The energy content, cetane number and viscosity of biodiesel are similar to those of petroleum-based diesel fuel. Moreover, it is essentially sulfur-free and emits significantly fewer particulates, unburnt hydrocarbons and less carbon oxides as compared with conventional fossil fuels [5], [6], [7], [8], [9], [10], [11], [12], [13].

At present, the high cost of biodiesel is the major obstacle for its commercialization. Approximately 70% of biodiesel cost is attributed to raw feedstocks [14], [15], [16]. So using cheap and non-edible vegetable oils, animal fats and waste oils as raw feedstocks for biodiesel production is an effective way to reduce the cost particularly in developing countries. Jatropha curcas L. oil is a potential cheap feedstock for biodiesel production as compared with refined and edible-grade oils such as rapeseed oil, soybean oil and sunflower oil that are common feedstocks in USA and Europe. The fatty acid composition of Jatropha oil is similar to edible oils but the presence of some anti-nutritional factors such as curcin render this oil unsuitable for cooking purposes [17], [18], [19]. The oil content of Jatropha seed ranges from 25% to 40% and the kernel from 45% to 60% by weight. It was suggested that Jatropha–palm oils as biodiesel raw materials is an optimum mix for Asia [19]. But, palm tree is not grown well in China. Nowadays, only Jatropha as an alternative biodiesel tree is widely cultivated in Southwest of China such as Yunnan, Sichuan, and Guangxi provinces. In the near future, it can supply part of raw material for biodiesel production in China.

The conventional industrial production of biodiesel is via transesterification of crude oil with a homogeneous strong base catalyst (e.g., NaOH, KOH or NaOCH₃) or acid catalyst (e.g., H₂SO₄) [5], [18]. Other methods such as transesterification with solid catalysts, biocatalysts and non-catalytic supercritical methanol were studied extensively [1], [2], [3], [4], [5], [16], [17], [20], [21], [22], [23], [24], [25]. Jatropha oil with high free fatty acids (FFAs) as crude feedstock with KNO₃/Al₂O₃ solid catalyst, 84% biodiesel yield was obtained at 70 °C for 6 h [16]. The maximum biodiesel yield of 94% with immobilized lipase catalyst at 55 °C for 48 h was achieved [23]. Transesterification of Jatropha oil using supercritical methanol without catalyst, 100% biodiesel yield could be obtained at 320 °C and 8.4 MPa [22]. The above methods need either long reaction times (up to 48 h) or high temperatures and pressures (e.g., 320 °C and 8.4 MPa) that will increase cost and consume a lot of energy. The objective of this work is to study the production of biodiesel from Jatropha oil by conventional methods catalyzed with homogeneous NaOH and H₂SO₄. Since Jatropha oil contains high FFAs (up to 15%) [28], when a base homogeneous catalyst is used, FFAs react with the catalyst to produce emulsified soap that will inhibit biodiesel production. If an acid catalyst is applied, despite the saponification phenomenon is avoided but the acid has a less catalytic effect on the transesterification thus causes slower reaction rate [26]. Therefore, a two-step process, acid esterification to remove FFAs followed by base transesterification, was used for the study of Jatropha oil transesterification. The two-step process was studied extensively before [27], [28]. In the previous work [27], FFA was reduced from 14% to less than 1% at 60 °C with H₂SO₄ for 88 min reaction time at the first step. Subsequently, more than 99% biodiesel yield was obtained at 60 °C with KOH for 24 min. In the later work [28], only 1 h was needed to reduce FFA from 15% to less than 1% at 50 °C at the first pretreatment step. Two hours were needed to produce only 90% biodiesel at 65 °C at the second step. Their results are very different. At the pretreatment step, Tiwari et al. used more acid catalyst (2.92 vs. 1 wt.% H₂SO₄) and higher temperature (60 °C vs. 50 °C) but longer reaction time (88 min vs. 1 h) was needed to reduce FFA to less than 1%. Surprisingly, at transesterification step, Tiwari et al. obtained higher biodiesel yield (99% vs. 90%) with a shorter reaction time (24 min vs. 2 h) and less base catalyst (11/35 mol ratio of KOH/NaOH) at the similar reaction temperature. According to their own predicted model [27], using 24 min reaction time, only 71% biodiesel was achieved. Probably, 24 min was a wrong datum, and longer reaction time was needed at the second step to reach 90–99% yield. In order to verify the very different results, further experiments are needed for the two-step process. Since ultrasonic radiation can effectively promote transesterification reaction of crude oil and enormously reduce the reaction times [3], all experiments were conducted in an ultrasonic reactor.