Handbook of Solvents - George Wypych - ChemTech - Ventech!

18.2 Prediction of organic solvents emission 1231
18.2.7 RELATIONSHIPS BETWEEN EMISSION AND TECHNOLOGICAL
PARAMETERS
Emission of solvents depends on the evaporation rate of the solvent in the process. The
evaporation rate from the surface depends on the concentration in the layer on the surface
and the coefficient of mass transfer on the air-side. This relation is approximately true for
degreasing operations using both liquid and vapors of organic solvents.
The concentration of solvent in a saturated vapor layer depends on temperature and
vapor pressure. The coefficient of mass transfer on the air-side depends on the air velocity in
the layer on the surface and Schmidt�s number (includes dynamic vapor viscosity, vapor
density, and diffusion coefficient). Emissions are measured in mass unit per unit of time and
the amount depends on surface area and the rate of evaporation, which, in turn, depends on
temperature, air velocity over the surface of solvent and the mass of solvent carried out on
the wetted parts which have been degreased.
18.2.7.1 Laboratory test stand
A thin-metal, flat dish 6 cm in diameter was filled with solvent up to 2-3 mm from the upper
edge and placed on a laboratory balance (Figure 18.2.3). The amount of evaporated solvent
was measured with (precision 0.1 mg) as the difference between the mass of the dish and
solvent at the start of the test and the mass of the dish with solvent after pre-determined period
of time. The test was repeated under different conditions of temperature and air velocity
near the surface of the solvent. The results were reported as the evaporated mass per 1
minute.
18.2.7.2 The influence of temperature on emission
Temperature was measured with a mercury thermometer with a range from 0 to 30 o C. Air
velocity was 0.3 to 0.4 m/s in this temperature region. The relationship is linear and the
equations expressing emission [g/m 3 /h] relative to temperature [ o C] for different solvents
are:
trichloroethylene E= 63.6 × t + 699.6
naphtha solvent E= 9.3 × t + 4.9
naphtha anti-corrosive E= 0.55 × t + 5.65
1-butanol E= 6.29 × t + 16.9
1,1,1- trichloroethane E= 94.7 × t + 805.6
extraction naphtha E= 49.5 × t + 1147
petroleum(mineral) spirits E= 11.9 × t + 76.7
wood preservatives E= 2.0 × t+42
Figure 18.2.3. Test laboratory stand.