Production of biofuels, limonene and pectin from citrus wastes

Abstract

Production of ethanol, biogas, pectin and limonene from citrus wastes (CWs) by an integrated process was investigated. CWs were hydrolyzed by dilute-acid process in a pilot plant reactor equipped with an explosive drainage. Hydrolysis variables including temperature and residence time were optimized by applying a central composite rotatable experimental design (CCRD). The best sugar yield (0.41 g/g of the total dry CWs) was obtained by dilute-acid hydrolysis at 150 °C and 6 min residence time. At this condition, high solubilization of pectin present in the CWs was obtained, and 77.6% of total pectin content of CWs could be recovered by solvent recovery. Degree of esterification and ash content of produced pectin were 63.7% and 4.23%, respectively. In addition, the limonene of the CWs was effectively removed through flashing of the hydrolyzates into an expansion tank. The sugars present in the hydrolyzates were converted to ethanol using baker’s yeast, while an ethanol yield of 0.43 g/g of the fermentable sugars was obtained. Then, the stillage and the remaining solid materials of the hydrolyzed CWs were anaerobically digested to obtain biogas. In summary, one ton of CWs with 20% dry weight resulted in 39.64 l ethanol, 45 m³ methane, 8.9 l limonene, and 38.8 kg pectin.

Introduction

World production of citrus fruits is over 88 million tons per year (Marin et al., 2007). Almost half of these fruits is squeezed to juice, and the remainder including peel, segment membranes and other by-products is considered as citrus wastes (CWs) (Wilkins et al., 2007a). These CWs can be dried and used as raw material for pectin extraction or pelletized for animal feed (Mamma et al., 2008). However, a large fraction of CWs is still deposited every year. This deposition is not favored due to both economic and environmental arguments such as high transportation costs, lack of disposal sites, and the land-filling material having high organic content (Tripodo et al., 2004).

CWs contain different carbohydrate polymers, which makes them interesting sources for production of biogas and ethanol (Gunaseelan, 2004, Mizuki et al., 1990, Pourbafrani et al., 2007, Wilkins et al., 2007b). The main obstacle to using CWs as a substrate for biogas production is the presence of limonene in CWs. This component is very toxic for digesting microorganisms and decreases the biogas yield (Mizuki et al., 1990). Limonene is also a strong inhibitor for microorganisms in ethanol production (Pourbafrani et al., 2007, Wilkins et al., 2007b). Therefore, this component should be separated from the CWs prior to digestion or fermentation steps. It should also be noticed that digesting bacteria can hydrolyze the carbohydrate polymers in CWs and convert them finally to biogas, while hydrolysis by enzymes or chemicals is necessary to convert these polymers to sugars and then ferment the sugars by e.g. baker’s yeast to ethanol.

Two alternative processes for production of ethanol from CWs based on enzymatic hydrolysis have been previously introduced (Stewart et al., 2005, Wilkins et al., 2007b). In the first alternative (Stewart et al., 2005), CWs were hydrolyzed using a mixture of enzymes (cellulose, pectinase and β-glucosidase). Then, limonene was removed from hydrolyzate by filtration and ethanol was produced from fermentable sugars. In the second alternative (Wilkins et al., 2007b), limonene was partly released using steam stripping. Then, carbohydrate polymers were hydrolyzed and fermented to ethanol through a simultaneous saccharification and fermentation (SSF) process. In both alternatives, non-fermentable sugars and the residue of solid polymers were dried to be used as cattle feed (Stewart et al., 2005). Application of these alternatives is hampered by the high cost of enzyme and the slow rate of hydrolysis reactions (Grohmann et al., 1995). In addition, mechanical pretreatment of biomass, and high demand for energy in distillation and drying processes, might considerably increase the cost of the process.

A process based on dilute-acid hydrolysis of CWs can be considered as another alternative (Grohmann et al., 1995, Talebnia et al., 2008). However, the data are limited to the lab-scale experiments and it is difficult to scale up the process to industrial scale. In both studies (Grohmann et al., 1995, Talebnia et al., 2008), mechanical pretreatment was used before hydrolysis and the experiments were carried out in low solid/liquid ratio (Grohmann et al., 1995). Furthermore, dilute-acid hydrolysis showed low yields of sugars from the carbohydrate polymers.

The aim of the current work was to introduce a new process for production of ethanol and biogas from CWs. The CW was pretreated with dilute-acid explosion process to hydrolyze the CWs and also to get rid of limonene. The resultant slurry was then centrifuged, and the liquid part was fermented to ethanol and distilled. The stillage from the distillation column and the remained solids were mixed and digested to biogas. An industrial configuration was suggested for this process.