RESEARCH ARTICLE Optimization of wet microalgal FAME production from Nannochloropsis sp. under the synergistic microwave and ultrasound effect Edith Martinez‐Guerra 1 | Md Shamim Howlader 2 | Sara Shields‐Menard 3 | W. Todd French 2 | Veera Gnaneswar Gude 1 1 Civil and Environmental Engineering Department, Mississippi State University, Mississippi State, MS 39762, USA 2 Dave C. Swalm School of Chemical Engineering, Mississippi State University, Mississippi State, MS 39762, USA 3 Department of Biological Sciences, Nicholls State University, Thibodaux, LA 70301, USA Correspondence Veera Gnaneswar Gude, Civil and Environmental Engineering Department, Mississippi State University, Mississippi State, MS 39762, USA. Email: gude@cee.msstate.edu Funding information United States Environmental Protection Agency (USEPA), Grant/Award Number: SU835519 Summary The synergistic effect of microwave and ultrasound irradiations was evaluated for biodiesel production from microalgae biomass (Nannochloropsis sp.) as raw material. A response surface methodology technique based on central com- posite design was used to understand the process parametric interdependence and optimize the process reaction variables. Reaction kinetics of algal fatty acid methyl ester (FAME) production was also studied. The optimum reaction con- ditions were determined as wet algal biomass to methanol ratio of 20 g to 30 mL, 1 wt% catalyst concentration, and 7‐minute reaction time at 140 W of microwave power and 140 W of ultrasound power. The estimated activation energy was 17,298 J/mol -1 K -1 for a first‐order reaction kinetics. This study revealed that microwave energy dissipation at a low rate of 140 W combined with 140 W of ultrasound intensity is adequate to produce FAMEs at a maxi- mum yield of 48.2%. Results from this optimization study suggest that a more detailed and mechanistic energy optimization study is critical to increase the FAME yield and maximize energy benefits. KEYWORDS kinetics, microwaves, RSM, synergy, ultrasound, wet algae 1 | INTRODUCTION Microalgal lipids are an attractive feedstock for biodiesel production due to their high energy density when com- pared with petro diesel and suitable for use in diesel vehi- cles with insignificant modifications to the engines. 1 However, the lipid recovery can be difficult when using wet biomass because water around algal cells generates a hydrated shell that acts as a barrier for both energy and mass transfer. 2,3 Despite this fact, with the use of appropri- ate technologies, the direct extraction from wet microalgal biomass is expected to reduce the overall biodiesel produc- tion cost significantly. Microalgae can accumulate lipids up to 50% dry cell weight in certain species, 4 which is approximately 40 times more oil per acre than other plants used for biofuel production. 5 However, the estimated cost of biodiesel production from microalgae was reported to be $4.92 per gallon in 2011, 6,7 which is currently not com- petitive with petroleum diesel or gasoline. One of the major challenges for commercializing microalgal biodiesel production is the high cost of dewatering and biomass drying prior to lipid extraction. The energy requirements for this step can account for up to 59% of the total energy consumption in biodiesel production. 8,9 The lack of efficiency in conventional processes to extract and transesterify microalgal lipids has led to Received: 9 June 2017 Revised: 3 November 2017 Accepted: 13 December 2017 DOI: 10.1002/er.3989 Int J Energy Res. 2018;1–16. Copyright © 2018 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/er 1