Effect of Vibrating Fluidized Bed (VFBD) and continuous microwave drying on drying characteristic of green tea

Abstract

In industrial green tea processing, people usually use endless chain pressure (ECP) dryers and fluidized bed dryers. ECP dryers have disadvantages including case hardening, uneven dryness, high energy consumption and long drying time, while fluidized bed dryers still have disadvantages of long drying time and high hot air capacity to ensure tea leaves can be fluidized perfectly. Therefore, another alternative dryer is needed that can overcome the problems in the ECP dryer and fluidized bed dryer. This study aims to determine the comparison of drying characteristics between vibrating fluidized bed dryer (VFBD) that have been developed previously with microwave dryers. Drying condition on both dryer was set at 80oC, the airflow velocity at VFBD was 1.09 m/s. Result shows the time needed by the microwave at temperature to dry 500 grams of tea leaves to reach a constant weight is 24 minutes, whereas if using VFBD it takes 85 minute. In VFBD, the constant rate period takes 25% of total drying time, and the rest is critical and falling rate period. While in microwave drying, around 50% of total drying time is constant rate period and the rest is critical and falling rate period. The falling rate period in VFBD requires longer time, which is up to 75% of the total drying time. Highest drying rate in VFBD dryer was 0.08 gr water/gr solid.min, while microwave dryer was 0.14 gr water/gr solid.min.