Article pubs.acs.org/EF
Evaluation of Different Operational Strategies for Biodiesel Production by Direct Transesterification of Microalgal Biomass Pamela Hidalgo,† Claudio Toro,‡ Gustavo Ciudad,†,∥ Sigurd Schober,§ Martin Mittelbach,§ and Rodrigo Navia*,†,∥ †
Scientific and Technological Bioresources Nucleus, Universidad de La Frontera, Casilla 54-D, Temuco, Chile Centro de Investigación en Polímeros Avanzados (CIPA), Beltrán Mathieu 224 piso 2, Concepción, Chile § Institute of Chemistry, Working Group Chemistry and Technology of Renewable Resources, University of Graz, Heinrichstraße 28, A-8010 Graz, Austria ∥ Department of Chemical Engineering, Universidad de La Frontera, Casilla 54-D, Temuco, Chile ‡
ABSTRACT: In this study, different operational strategies for biodiesel production by direct transesterification of microalgal biomass (Botryococcus braunii) were evaluated. These operational strategies include the use of different acyl acceptors and the application of different catalysts and solvent mixtures. All these strategies were performed in two reaction systems: a conventional batch reactor (CBR) and a reflux extraction reactor (RER). The highest biodiesel production yields (80.6 wt %) were obtained in the RER using methanol as acyl acceptor and H2SO4 as catalyst. On the opposite, the lowest biodiesel production yield (64.5 wt %) was observed in the CBR system. Moreover, when a low proportion of cosolvent (i.e 3:1 v/v solvent/cosolvent) was incorporated in the reaction, an increase in biodiesel production yields was observed. A higher cosolvent content in the reaction mixture provoked however a diminishment in FAAE (fatty acid alkyl esters) yield in both systems, due to a drastically reduction of alcohol−lipids molar ratio.
1. INTRODUCTION Microalgae are receiving increasing attention worldwide as an alternative and renewable source for energy production. The microalgae have higher lipids production yields, which have been reported between 58 000 L/ha to 136 000 L/ha, besides have much faster growth rates than terrestrial crops.1 Instead, oil from oilseeds such as rapeseed or soybean present oil yields of 1190 L/ha and 446 L/ha, respectively. Besides, microalgae can grow in wastewater with high organic matter content, as a wastewater of carpet mill effluents,2,3 or even in brackish water and use nonarable land, also requiring less land extensions for their cultivation. Additionally, microalgae can produce different types of lipids and hydrocarbons depending on the species of microalgae. Despite the several advantages of using microalgae for biofuels production compared to oil crops, its production at industrial scale still faces relevant problems, mainly due to the high costs of biomass production and fuel conversion routes. Microalgal biofuels are 4−10 times more expensive than petroleum-derived fuels or first generation biodiesel.4 Key technologies for biofuels production are culture conditions for high oil productivity, development of effective and economical microalgae cultivation systems, as well as separation and harvesting of microalgal biomass. Besides, cost-effective routes for biofuel production including biomass drying, lipids extraction, and added-value products recovery (e.g., proteins, carbohydrates, and pigments) as well as biodiesel production and refining processes should be also optimized.5 The main critical points are biomass drying, with an energy consumption near 80%, and lipids transesterification, with a 10% of the total energy consumption of biodiesel production from microalgae.6 © XXXX American Chemical Society
The reduction of the energy consumption in these limiting steps is fundamental for a possible industrial scale up. With the purpose of eliminating the biomass drying process, the implementation of high temperature and pressure transesterification of wet biomass has been tested. In this process, direct conversion under supercritical methanol condition appears as a great alternative because it is a one-step process for direct liquefaction and conversion of wet algal biomass into biodiesel. This one-step process enables simultaneous extraction and transesterification of lipids.7 Patil et al.7 reached 90 wt % of biodiesel yield obtained by supercritical transesterification using a wet microalgal biomass/ methanol ratio of 1:9 wt/v, a reaction time of 25 min, and a temperature of 255 °C. Moreover, Levine et al.8 reported a process that combines two steps, lipids hydrolysis (at 250 °C) using subcritical water and wet biomass, where an easily filterable cells conglomerate (hydrochar) retains the lipids, followed by a supercritical transesterification of the hydrochar (at temperatures between 275 and 325 °C). On the other hand, high conversion yields have been achieved in conventional direct transesterification processes at moderate temperatures (
