Moisture removal at f = 1.5 kHz and SPL = 150 dB was higher
and faster than the other investigated sound wave parameters.
Shorter time of drying was observed at a higher acoustic energy
thus increased drying rate. This increase is because of the enhancement
by improving contact area between gas and lignite which
favors the migration of water from the surface of lignite to the
gas. In addition, acoustic waves produce pressure variations, oscillating
velocities and micro-streaming; these mechanisms reduce
the boundary layer thickness and increase the water from the
inside to the surface transmission from lignite. The drying rate
remarkably increased with the increasing sound frequency from
1 kHz to 1.25 kHz and decreased with increasing sound frequency
from 2 kHz to 4 kHz. Sound pressure level is one of most important
variables while attempting to manipulate drying rate. Higher