Energy and exergy analysis during drying in rotary dryers from finite control volumes: Applications to the drying of olive stone

Abstract

• An exergy and energy analysis during drying is performed from finites control volumes. • A big thermal shock is produced in the first third of the trommel due to evaporation. • Heat loss and exergy destruction is very high at the beginning of the drying process. • The dryer works in optimum conditions at low-medium air flows and low-medium temperatures. • This study will help improve aspects like design and control in rotary dryers. Nowadays, the design, control and development of rotary dryers are based on the study of the complete equipment. Rotary dryers are treated as a black box where only inlet and outlet parameters are known. This implies the need to know the main interactions of mass, energy and exergy throughout the trommel to help improve the drying process. In this paper, we carried out an analysis of energy and exergy during drying of olive stone from finites control volumes in an experimental rotary dryer. Mass, energy and exergy balances are applied to each control volume in the drying air. The design of experiments is based on three initial drying air temperatures (210 °C, 180 °C and 150 °C) and three drying air flows (576 kg/h, 425 kg/h and 280 kg/h), with a by-product mass flow of 40 kg/h and a rotational speed of 5.5 rpm. The results indicated that olive stone moisture content was reduced to less than half in the first third of the trommel where a big thermal shock is produced due to the interaction between the by-product and the drying air flow at high moisture contents and high temperatures, respectively. This fact led to the highest exergy destruction values, thereby diminishing the flow exergies for the following control volumes. Heat losses were considerable in all equipment, especially in the first two control volumes. Furthermore, drying process was analyzed from the energetic, exergetic and drying efficiencies and the unit energy consumption.