2.2. Drying experiments in a single rotary drum dryer
Drum drying is a continuous indirect heating process. A single rotary drum dryer consisting of a hollow cylinder made up of stainless steel was mounted horizontally on a supporting frame and rotated continuously. The speed of the drum could be regulated by a variable-speed drive. The diameter and length of the cylindrical drum were 48.5 and 45.2 cm, respectively. During the operation, thermal oil filled in the storage tank was heated up to 100 ± 2 °C which was pumped into the hollow cylindrical drum for heating. The closed system of the drum did not allow the sample to be in contact with the oil, but provided uniform temperature distribution over the drum surface, which ensrured consistent product quality.
The microalgal slurry was uniformly attached to the drum surface from the algal reservoir through continuous rotation. The heated drum slowly dried the wet microalgal slurry on the outside of the drum surface and was finally scrapped by the doctor blade (scraper device). The exposure of the microalgal slurry to a high temperature for a short period reduces the risk of damage to the dried-biomass. The residence time of the product on the drum surface depends upon the speed of the drum ranging from 0.1 revolution per minute (rpm) to several rpm. In the present investigation, the microalgal slurry was dried at varying drying temperature from 70, 80, 90, and 100 °C at eight different rpm, i.e., 0.1, 0.2, 0.3, 0.4, 0.5, 1.0, 1.5, and 2.0 rpm.