Process design and economic analysis of a citrus waste biorefinery with biofuels and limonene as products Mehdi Lohrasbi a,c , Mohammad Pourbafrani b,c, * , Claes Niklasson b , Mohammad J. Taherzadeh c a Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, OH 44325, USA b Chemical Reaction Engineering, Chalmers University of Technology, 412 96 Göteborg, Sweden c School of Engineering, University of Borås, 501 90 Borås, Sweden article info Article history: Received 21 January 2010 Received in revised form 21 April 2010 Accepted 24 April 2010 Available online 21 May 2010 Keywords: Citrus wastes Ethanol Biogas Limonene Economic analysis abstract Process design and economic analysis of a biorefinery for the treatment of citrus wastes (CW) at different capacities was carried out. The CW is hydrolyzed using dilute sulfuric acid and then further processed to produce limonene, ethanol and biogas. The total cost of ethanol for base case process with 100,000 tons/ year CW capacity was calculated as 0.91 USD/L, assuming 10 USD/ton handling and transportation cost of CW to the plant. However, this price is sensitive to the plant capacity. With constant price of methane and limonene, changing the plant capacity from 25,000 to 400,000 tons CW per year results in reducing eth- anol costs from 2.55 to 0.46 USD/L in an economically feasible process. In addition, the ethanol produc- tion cost is sensitive to the transportation cost of CW. Increasing this cost from 10 to 30 USD/ton for the base case results in increasing the ethanol costs from 0.91 to 1.42 USD/L. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Citrus fruits are among the most abundant crops in the world with an annual production of over 88 million tons (Marin et al., 2007). Almost 33% of the crops, including oranges, lemons, grape- fruit and mandarins, are industrially processed for juice produc- tion, where about half of the processed citrus including peels, segment membrane and seeds ends up as wastes (Wilkins et al., 2007a). These solid residues are referred to as citrus wastes (CWs) with estimated worldwide production of 15 million tons per year (Marin et al., 2007). Currently, parts of the CWs are dried and marketed as low-protein cattle feed called ‘‘citrus pulp pellets” and the rest are disposed in landfills, constituting severe economic and environmental problems (Tripodo et al., 2004). CW consists of various soluble and non-soluble carbohydrate polymers which make it an ideal feedstock for biological conver- sion to biofuels such as ethanol and biogas (Grohmann et al., 1994, 1995; Naparaju and Rintala, 2006; Wilkins et al., 2007b). In addition to carbohydrates, CW contains an extractive which is re- ferred to as peel oil. The major component of peel oil is D-limonene which is extremely toxic for biological activity and must be re- moved prior to fermentation or anaerobic digestion processes (Lane, 1983; Mizuki et al., 1990; Pourbafrani et al., 2007). A new process for production of ethanol and biogas was devel- oped in our previous study (Pourbafrani et al., 2010). In this process, the CW is mixed with dilute sulfuric acid and then steam-exploded to hydrolyze the CW and also get rid of limonene. The resultant slurry is then centrifuged, where the liquid part is fermented to ethanol and distilled. The stillage from the distillation and solids from the centrifuge were mixed and digested to biogas (methane). This biogas can partly be utilized to produce steam required for the process, such as steam explosion and distillation. The aim of this work was to investigate the economic feasibility of an industrial process of ethanol and biogas from CW (Pourbafrani et al., 2010). The process was simulated using Aspen Plus Ò (Aspen Tech, Massachusetts, USA) based on experimental data in lab and pilot plants (Pourbafrani et al., 2010). The process was then analyzed economically to investigate the effect of major parameters including capacity of CW and transportation cost on the price of fuel produced. 2. Methods 2.1. Process description An overview of the proposed ethanol and biogas process is illus- trated in Fig. 1. The process was assumed to work 8000 h per year. The CW contains 20% dry matter, where its major components are 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.04.078 * Corresponding author at: Chemical Reaction Engineering, Chalmers University of Technology, 412 96 Göteborg, Sweden. Tel.: +46 33 435 4361; fax: +46 33 436 4008. E-mail address: pour@chalmers.se (M. Pourbafrani). Bioresource Technology 101 (2010) 7382–7388 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech