25 / 36
23
Chemical Technology • May 2016
that there may be an equilibrium limit and simulate the
limit by using a computer programme where the properties
of the volatile are used and the solid is assumed to have
properties equivalent to the least volatile component in
the computer programme data base. While this approach
may
provide an approximation of equilibrium, it does not
allow for the non-ideality between the volatile and the solid.
Thus reliance on this approach may cause one to believe
that this estimated equilibrium can be reached in a very
short time period.
If one is to avoid the potential product, process, envi-
ronment and safety problems related to devolatilisation, it
is imperative that both equilibrium and mass transfer be
considered more carefully than described above. In devel-
oping the theoretically correct approach, I have chosen to
use a purge bin ‘plug flow’ dryer that is shown in Figure 1.
From a practical point of view, the same theory can be
used for any counter-current flow dryer. It can also be used
for a co-current flow dryer or fluid bed dryer with some
modifications in the equations.
Background and theory
In a purge bin, gas and solids flow counter-current to each
other. The bed is designed to promote ‘plug flow’ character-
istics for both the gas and solids. In true ‘plug flow’, each
solid particle has an identical residence time. In addition,
the
equilibrium
concentration of volatiles in the exiting
solids is a function of
inlet gas concentration
. This is in
contrast with a single stage fluidised bed where the equi-
librium concentration of volatiles in the exiting solids is a
function of
outlet gas concentration
. A well-designed purge
bin is an excellent approach to reducing volatiles to a very
MINERALS PROCESSING
& METALLURGY