Handbook of Solvents - George Wypych - ChemTech - Ventech!

388 Seung Su Kim, Jae Chun Hyun
rate steadily falls. This period is called as falling drying rate period (FDRP). Generally the
diffusion coefficient of solvent in a polymeric solutions is a strong function of solvent concentration,
temperature and molecular size. 53-55,60 The binary diffusion coefficient can be estimated
by the free volume theory of Vrentas and Duda 17 and can be measured by the NMR.
However, there are many difficulties to measure or estimate the ternary diffusion coefficient.
The evaporated solvents are swept away by the hot air and are exhausted to the outside
of the dryer. The vapor concentration of solvent of a zone (oven) must be lower than the
lower explosive level (LEL) of solvents to meet the safety. Thus enough fresh air should be
supplied to each zone to meet this LEL safety. 10,28 However, too much supply of fresh air
brings about too much energy consumption to heat the fresh air, and it also increases the
VOC containing waste gas which should be treated by VOC emission control unit. The LEL
of a zone can be controlled within appropriate LEL ranges by measuring and predicting of
drying rate. 28
We are going to deal with some important features of drying a liquid coated film in the
subsequent sections.
First, we will introduce theory for the drying and method of modeling of the drying
process. A drying process of liquid coated film can be simulated by setting up heat and mass
transfer equations 8,9,40,59 and vapor liquid equilibrium equations 19,39 with appropriate heat
and mass transfer coefficient. 37,45
Secondly, we will try to give examples of drying rate measurements in laboratory experiments
and in a pilot or production scale dryer. Traditionally, the rate of drying was obtained
by measuring weight loss using a balance under the controlled experimental
conditions of air velocity and temperature. 5,40,51 However, it is difficult to obtain such data in
the high airflow rate experiments, for the balance is disturbed by the high airflow motion.
Therefore the alternative way of measuring was proposed in which hydrocarbon analyzers 57
or FT-IR 47,50 was used instead of balance. Moreover, the weight changes of each solvent in a
multi-solvent coating system could be found by applying rapid scanning FT-IR method. 50
However, rare data has been given about measuring the actual drying rate and solvent
concentration profile along the dryer while operating. 28 The drying rate of coated film in an
industrial dryer is measured by analyzing dryer exit gas, and we will illustrate how the solvent
concentration profiles of coating at the exit of each zone are estimated from the experimentally
measured drying rate data. 28
In third, we will try to illustrate the dependence of dried coating structure on the drying
path by means of the basic phenomena, phase equilibrium. 29,35,38,41,42,56,58 We will also briefly
introduce the mechanisms behind the formation of microstructure of various kind of functional
films. 44,51
Finally, we will categorize the drying related defects according to the origin of defects,
and we will show some examples of defects and the cause and curing of them. 8,15,21,25
7.3.2 THEORY FOR THE DRYING
7.3.2.1 Simultaneous heat and mass transfer
Because evaporation is an endothermic process, heat must be delivered to the system, either
through convection, conduction, radiation, or a combination of these methods. The solvents
are evaporated from the coating surface and at the same time the latent heat of solvent cool