Exergy analysis of a pistachio roasting system

Abstract

Roasting is one the most widely used unit operations for the processing of pistachio nuts. The operation is aimed at improving their appeal to consumers. The process is energy-intensive, requiring considerable amounts of electrical and thermal energy. Accordingly, this study is aimed at determining the key points in an industrial-scale pistachio roasting plant for energy improvements using the concept of exergy. The system is comprised of two principal subunits: i.e. roasting and drying units. Exergetic indicators for all components of each of the subunits are individually formulated and then computed based on actual operating data. The amounts of exergy utilized and destroyed during processing a given amount of pistachio nuts are determined using the mass allocation approach. A sensitivity analysis is used to show how the exergetic indicators of the process change if the reference state temperature is modified. The heater of the drying unit (114 kW) and the furnace of roasting unit (78.3 kW) are responsible for the majority of the exergy destruction rate (due to irreversibilities) of the overall plant. The exergy utilization and destruction per kilogram of unshelled pistachio are, respectively, found to be 5030 and 4120 kJ. The contributions of the roasting and drying units to the exergy utilization for 1 kg unshelled pistachio are found to be 20.3 and 79.7%, while their contributions to the exergy destruction for 1 kg unshelled pistachio are computed as 19.6 and 80.4%, respectively. The product/fuel exergetic efficiencies of the overall roasting and drying units are found to be 1.58 and 0.44%, while their output/input exergetic efficiencies are determined to be 9.96 and 8.65%, respectively. The investigated exergetic parameters vary as the reference state temperature changes. More specifically, the product/fuel exergy efficiencies of the overall roasting and drying units are almost constant with increasing reference state temperature, while their output/input exergy efficiencies decrease linearly as reference state temperature increases. The output/input exergy efficiencies of the overall roasting and drying units decrease, respectively, up to 8.4 and 12.2% compared with the base reference state temperature (15 °C).