Handbook of Solvents - George Wypych - ChemTech - Ventech!

414 Seung Su Kim, Jae Chun Hyun
cause to flow to the edges, giving fat edges. The surfactants are often helpful to eliminate
the fat edges.
7.3.4.2.3 Defects caused by air motion and others
Dryer bands
At the early stages of drying, where the coating has enough mobility to flow, the coating
layer is apt to being disturbed by the motion of drying air. 21 This defect is observed in the
dryer which uses the arrays of round jet nozzles rather than the slot nozzles. Often it looks
like a lane running in the machine direction and the width of bands is nearly equal to the
nozzle diameter. When the air comes out of slots, bands are less likely if the flow across the
slot is uniform. 10,21,25 The bands are more likely to be developed with low viscosity solution,
thick coating and high air velocity. Thus we can reduce the bands by reducing air velocity
(but this also decreases the drying rate) and by increasing the initial solid content so as to increase
the viscosity of solution and to coat thinner layer with concentrated solution.
The geometry of arrays of round jet nozzles should be designed to avoid this defect by
applying nozzles of larger diameters in the first part of zones. Large amount of fresh air is
needed in the first part of zones of the multiple zone-dryer, because in the first zone the
amount of evaporated solvent vapor is large, hence one should supply enough fresh air to
ensure the dryer is operated below the lower explosive level (LEL) of a given solvent.
Therefore the diameters of round jet nozzles are gradually decreased along the dryer length.
Skinning
If the drying rate of solvent is extremely high, then the solvent concentration at the surface
of coating falls rapidly, for the rate of solvent evaporation is much higher than that of
diffusion of solvent within the coating. This rapid decrease of solvent concentration at the
surface causes to form a thin solid layer near the coating surface. This phenomena called as
‘skinning’, and it retards the drying of coating. 4,5,8-10 Skinning can be reduced by using solvent
laden gas as a drying air, 4,8,10 but it isn’t applicable to the conventional drying process in
which uses the volatile organic compound as a solvent. Most of the solvent has the possibility
of explosion above a certain concentration in the air. Thus solvent laden drying gas
should be applied in a inert environment, in which no oxygen is present. If the skinning occurs
in a aqueous coating system, it is possible to use humid air to reduce the skinning.
7.3.4.3 Control of lower explosive level (LEL) in a multiple zone dryer
If the solvent concentration of a zone exceeds certain level, then the system becomes to be in
danger of explosion due to the flammability of organic solvent vapor. This level of solvent
concentration is called as lower explosive level (LEL), and the dryer should be operated below
the LEL prior to any constraints. Thus each zone needs sufficient airflow rate to meet
these needs. However, too much airflow rate results in waste of energy to heat the excess air,
and it increases the load of waste gas facilities (e.g., VOC emission control units). Therefore
the airflow rate of each zone and the ratio of recycled to fresh air should be optimized. The
typical airflow system for a multiple zone dryer is shown in Figure 7.3.32. The supplying air
of each zone is consisted of fresh and recycled (used or returned from the exit gas) air as
shown in the figure. LEL can be calculated if we know the evaporation rate of solvent of a
zone. The evaporation rate of solvent can be measured or calculated as we explained in previous
sections. Thus we can distribute the fresh air to each zone to meet the LEL safety without
substantial increase in the total exhaust air.