Investigation of fuel consumption and essential oil content in drying process of lemon verbena leaves using a continuous flow dryer equipped with a solar pre-heating system

Fuel consumption and the drying behavior of lemon verbena leaves were determined in a continuous flow dryer equipped with a solar pre-heating system. In order to reduce the share of fossil fuels and their negative impact to the environment, the total energy during the drying process was supplied using the combination of solar energy (solar water heater) and natural gas. The experiments were carried out at three levels of drying air temperature (30, 40, and 50 °C) and three levels of air velocity (1, 1.5 and 2 m/s). Two working conditions were considered for the drying system: a) solar-gas drying (SGD) mode and b) gas drying (GD) mode. According to the results, in SGD mode, the fuel consumption at drying temperatures of 30 and 40 °C was zero at all levels of the air velocity, while in the GD mode the fuel consumption increased with increasing the inlet air temperature. The highest and lowest essential oil (EO) content was obtained at 40 °C and 2 m/s by 0.60% and at 50 °C and 1 m/s by 0.33%, respectively. The central composite design-Face Centered technique with response surface methodology was used for the optimization of the drying process. The optimum conditions for drying lemon verbena leaves were obtained at drying temperature of 39.54 °C and an air velocity of 2 m/s. In this mode, the fuel consumption was zero, drying time was 131.48 min and EO content was 0.52%.