Microwave fluidized bed drying of celery: drying kinetics and exergy analysis

Experiments were conducted to study the drying kinetics of celery pieces in a microwave assisted fluid bed dryer and to investigate the energy and exergy efficiency of the process. In the first part, the drying kinetics of celery was studied using a microwave assisted fluidized bed dryer at different drying air temperatures (45°C, 55°C and 65°C) and at different initial microwave power densities (0W/g,1W/g and 2W/g). A central composite design was used to set the factor combinations. Dried product quality, product mass, air temperature, air relative humidity, and electric power consumption were used to monitor the performance of the drying process. The temperature and relative humidity of the air at the exhaust of the dryer were used to estimate the loss of drying air potential. The results showed that the Midilli-Kucuk equation model was best in predicting the moisture ratio as a function of time. At any given temperature, the utilization of the microwave energy reduced the drying time by more than 50%. The electric power consumption decreased with an increase in air temperature and microwave power density. The fastest drying time was achieved at 65°C with microwave power density of 2W/g. Drying trials performed at a power density of 2W/g resulted in partial browning of cut celery, whereas no adverse effect was seen on the final product quality at 0W/g and 1W/g. Drying conditions did not affect the rehydration properties of the final product. In the second part, an energy and exergy analysis were performed on drying celery in a microwave assisted fluidized bed dryer at different recirculation ratios (0, 0.5 and 1) and at different initial microwave power densities (0W/g,0.5 W/g and 1W/g). A central composite design was used to set the factor combinations. Product mass, air temperature, air relative humidity, air velocity and electric power consumption were used to monitor the energy performance of the dryer. The drying time reduced with the addition of microwaves whereas recirculation ratio had no significant effect on it. Total power consumption and specific energy consumption of the drying process decreased with increasing initial microwave power density and increasing amount of exhaust air recycled into the system. Recirculation of exhaust air resulted in higher enthalpy of inlet air and lowered energy consumption. Exergy of evaporation was affected by the application of microwaves but remained unaffected by the recirculation ratio. The maximum consumption of available energy was achieved for the 100% recirculation of exhaust air. Thus, higher amount of recirculation of exhaust air resulted in a better sustainable drying system.

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