Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman Fermentative ethanol production from Madhuca indica flowers using immobilized yeast cells coupled with solar driven direct contact membrane distillation with commercial hydrophobic membranes Ramesh Kumar a , Alak Kumar Ghosh a, ⁎ , Parimal Pal b, ⁎ a Department of Chemistry, The University of Burdwan, 713104, India b Environment and Membrane Technology Laboratory, Department of Chemical Engineering, National Institute of Technology Durgapur, 713209, India ARTICLE INFO Keywords: Bioethanol Mahua flowers Immobilized fermentation Solar driven membrane distillation Hydrophobic membranes Clean biofuel ABSTRACT The objective of the present investigation is to develop and evaluate economic production of biofuels (ethanol) using immobilized yeast cells (Saccharomyces cerevisiae NCIM 3095) from renewable carbon source Madhuca indica (Mahua flowers). The entrapment of yeast cells in calcium alginate-polyvinyl alcohol (CA-PVA) film were found most suitable immobilizing matrix as a protective carrier to increase the stability and tolerance limit during continuous fermentation. Under similar fermentation conditions, immobilized yeast cells produced higher ethanol yield (∼0.48 g/g) and productivity (∼28 g/L/h) as compared to traditional suspended free cells system in continuous fermentation with dilution rate of 0.4/h. Further, the fermenter was coupled with solar driven membrane distillation (SDMD) process for separation and concentration of ethanol after fermentation. The new system does not require centrifugation/microfiltration for separation and/or recycle of biomass as the cells were immobilized and could be easily separated by simple filtration. The SDMD with cross flow module in rectangular shape and counter-current flow of streams (hot and cold) ensured higher ethanol flux (23 kg EtOH/ m 2 /24 h) in comparison to existing literature. The new process design is compact yet flexible, eco-friendly, energy intensive and sustainable process for the development of economic biofuel production. 1. Introduction The gradual depletion of crude petroleum reserve and growing global environmental concern need to exploration of renewable and eco-friendly sources of energy like biofuels derived from microbial fermentation of renewable resources [1]. Excessive use of fossils fuels (> 80%) is not only leading to increase in greenhouse gas emission (GHGs) which causes global warming but also total exhaustion of oil reserve [2,3]. Thus there is an increasing concern about the clean and renewable biofuels which may be the direct substitute of the conven- tional petroleum fuels in transportation as an immediate alternative [4]. Bioethanol derived from renewable resources may offer partial substitute for the fossil fuels and could reduce 32% of the global ga- soline utilization in transportation with reduced (GHGs) emission [5,6]. Currently, ethanol blending in gasoline is varies from 5 to 27% in dif- ferent countries like India, USA and Brazil. Globally, the production of bioethanol has seen sharp increase from 74 to 100 billion litres over the period 2009 to 2014. 90% of world’s total bioethanol production is done in USA (corn), Brazil (sugarcane juice) and European Union (wheat), whereas in India, this is only 2% using molasses. Now, variety of feed stocks are available in Mother Nature like sugar, lignocellulose and algal based raw materials like sugarcane, Mahua flowers, cheese whey, lactose, maize, cassava, corn, agro-in- dustrial waste, pre-treated rice straw, microalgae for preparing the biofuels which may be categorized as 1st, 2nd and 3rd generation of biomass [7]. Lignocellulosic and algal-based feedstock is cheap carbon source but these substrates demand additional enzymatic and/or che- mical pre-treatment such as liquefaction and saccharification to release the fermentable sugar before fermentation. Mahua flowers on the other hand have been tried very little though it could be very promising carbon source being clean, relatively cheap and renewable. Mahua flowers are traditionally used for the preparation of country liquor called “mahuli” widely available in central, northern plains and de- ciduous forest of India in two main species Madhuca indica and Madhuca latifolia. Mahua flowers having 1.5 cm long and droopy fleshy off white colour are dense fascicles near end of branches which contain mostly reducing sugars (66–72%). Mahua tree is relatively unexploited species which can adapt to grow in arid climate and provide cheaper source of https://doi.org/10.1016/j.enconman.2018.12.050 Received 20 September 2018; Accepted 18 December 2018 ⁎ Corresponding authors. E-mail addresses: alakghosh2002@yahoo.co.in (A.K. Ghosh), parimalpal2000@yahoo.com, ppal.nitdgp@gmail.com (P. Pal). Energy Conversion and Management 181 (2019) 593–607 0196-8904/ © 2018 Elsevier Ltd. All rights reserved. T