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.