Handbook of Solvents - George Wypych - ChemTech - Ventech!

404 Seung Su Kim, Jae Chun Hyun
solvent wasn’t decreased by enhancement of airflow rate. From these results we can infer
that the drying rate is controlled by internal mass transfer above this airflow rate.
In a case of PMMA-toluene system, the drying rate at the airflow rate of 72 m/sec is
lower than that of airflow rate of 36 m/sec (Figure 7.3.20). These are the contrary results to
what we expected. It shows that this system exhibits ‘trapping skinning’ at the high airflow
rate. Thus the residual solvent of final dried coating may be minimized at middle airflow as
shown in the Figure 7.3.20. As shown in these HADES experiments, it is often the solution
to lower the airflow rate or temperature of drying air when trapping skinning is suspected to
occur.
7.3.3.4 MEASUREMENT OF DRYING RATE IN THE PRODUCTION SCALE
DRYER
Numerous drying rate data have been obtained in laboratory drying experiments using
thermo-gravimetric analyzers as shown in the previous chapters. However it was difficult to
know the actual drying rate or solvent concentration profiles in the pilot or production scale
dryer from these experimental results.
Figure 7.3.21. Construction of material balance to calculate the solvent concentration of the coating at each zone
end from the measured solvent concentration of exit gas. [After reference 30].