284253 Utilization of Citrus Processing Waste As Renewable Feedstock for Biorefinery Applications: Technical Advances, and Analysis of Engineering Challenges for the Pretreatment and Enzymatic Hydrolysis Steps
Citrus processing waste (CPW) is one of the major wastes generated worldwide through the industrial processing of fruits for juice and canned products. Over the three season’s period 2008-2011, almost 10.5 million of metric tons of wet CPW, composed of peel, pulp, seeds, essential oils, and with a water content of 75-80%, were generated in the U.S.A. from the processing of oranges and grapefruits.
These CPW materials are traditionally treated with a drying and extrusion process, and used in the formulation of a low-value cattle feed for final disposal. Environmental issues arising from the drying process include air emissions estimated in 10-20,000 tons/year of d-limonene as part of volatile organic compounds (VOCs), with basis on Federal Clean Air Act Amendments of 1990, the Environmental Protection Agency (EPA) required citrus juice processors to file a Title V permit if limonene emissions exceed 100 tons/year.
Additional issues due to regulated emissions of carbon monoxide (CO), particulate matter (PM), sulfur dioxide (SO2), nitrogen oxides (NOx) and hazardous air pollutants (HAP) (primarily methanol and formaldehyde), and carbon dioxide (CO2) from the burning of natural gas and other fuels used in the drying process combined with the lost revenue from un-recovered limonene oil, estimated in 20-40 million USD/year, and the high treatment costs from the consumption of gas and electricity, have led many citrus juice processors searching for more sustainable alternative methods for treatment and disposal. An alternative and more sustainable utilization of CPW could provide a renewable source of pectin-rich- and low lignin content cellulosic biomass for product conversions and biofuels.
Research on CPW as cellulosic biomass has been ongoing by USDA-ARS researchers over the last 25 years. This effort has resulted in the pilot development of one pretreatment process using water steam explosion under relatively high pressure and vacuum for the recovery of d-limonene as co-product, and the production of slurry of biomass containing cellulose, pectin, flavonoids and free sugars. There has been a lack of published information for relevant scientific and engineering parameters in designing the pretreatment and enzymatic hydrolysis steps of a proposed CPW biorefinery. Data gaps exist for the impact of physico-chemical parameters such as structural polysaccharides composition, particle size, surface area and morphology of treated- and untreated-CPW on its bioprocessing. Furthermore, there is a lack of information on relevant kinetic parameters for engineering scale up including mass and energy balances, and environmental concerns. All of these parameters are needed for reliable design, modeling, and scale up of a CPW biorefinery at both pilot and commercial scales.
A review of recent technical advances on pretreatment processes and its effects on enzymatic hydrolysis as the main steps of a proposed citrus processing waste (CPW) biorefinery is presented. Engineering challenges and relevant gaps in scientific and technical information for reliable design, modeling, and scale up of a CPW biorefinery are also assessed. Some integrated physico-chemical pretreatment is proposed for testing for CPW, including high speed knife-grinding and simultaneous alkali addition. These new proposed processes and the effect of parameters such as particle size, surface area and morphology, pore volume, and chemical composition of the diverse fractions resulting from pretreatment and enzymatic hydrolysis need to be evaluated and compared for pre-treated and untreated samples of grapefruit processing waste (GPW). This review suggests the potential for filling the data gaps, and preliminary results demonstrate that the reduction of particle size and the increased surface area for the CPW will result in higher reaction rates and monosaccharides yields, for the pre-treated waste material.