Handbook of Solvents - George Wypych - ChemTech - Ventech!

7.3 Drying of coated film 393
Figure 7.3.5. Calculated heat transfer coefficient of industrial dryer
with varying air velocity.
cess, magnetic particulate dispersed
solution is coated on the
PET (polyethlylene terephthalate)
film. The wet coating thickness is
about 5 to 6 μm and the thickness
of substrate is 14 to 15 μm. The
line speed is normally 400 m/min
to 1000 m/min. Usually several
kinds of solvents are used for the
coating solution, here we used
three kinds of solvents - toluene,
methyl ethyl ketone (MEK) and
cyclohexanone (CYC). Equations
7.3.1 and 7.3.3 are applied to each
solvent component. The temperatures
of dryer zones were 50 to
130 o C, and the air velocities at the
nozzle exit were 10 to 20 m/sec.
The average heat transfer coefficient
of a zone could be found by
applying Martin’s correlation for
the slot nozzles, however in this case we had the empirical coefficient for the Equation 7.3.7
which was supplied by the dryer nozzle manufacturer. And we obtained average heat transfer
coefficients which were ranging from 80 to 140 J/m 2 s o C according to the air velocities of
the zones.
The coupled and non-linear set of equations is solved by standard numerical method
(such as Runge-Kutta-Gill method). The temperature and concentration of coating at time 0
was given as the initial concentration of coating. The concentration of coating at the next
time step was calculated by solving Equation 7.3.1 with assuming that there was no significant
temperature change of coating. This is a plausible assumption if the time step is small
enough. Then the result of concentration of coating was substituted to the Equation 7.3.2
and the temperature of coating at
that time step was calculated.
The calculated concentration
and temperature profile of
coating are given as in the Figure
7.3.6. The concentration of solvent
gradually decreases along
with the dryer length. The slope of
solvent concentration profile at
zone 1 was nearly constant, but
the slop was changed within the
zone 1 though there were no
changes in the drying conditions.
Figure 7.3.6. Predicted residual solvent and temperature profile
along with the dryer length.
It was due to the fact that the less
volatile solvent (cyclohexanone)