Solvent Evaporation

Quick Start

Solvent evaporation depends on parameters specific to each solvent, plus air temperature and velocity. A simple model gives an evaporation time for a chosen thickness of solvent, a more academic model lets you see how the quick model works.

Solvent Evaporation

Solvent
Airflow m/s
T °C
Thickness μm
tEvap

Now the more academic app which shows in more detail what is going on. The calculation method is the same, but for simplicity the quick model assumes that L=1000.

Flat Plate Evaporation

Airflow m/s
T °C
Length mm
Antoine A
Antoine B
Antoine C
VP mm/Hg
D cm²/s
ν cm²/s
Re
Sc
Sh
k m/s
mole/m.s
g/m.s

Flat plate evaporationWe often need to know the evaporation rate of a solvent on a flat plate with a steady air flow above it. This is rather difficult to calculate from first principles, but some well-known approximations are good enough given the uncertainties in, say, measuring the rate.

The calculation uses a set of dimensionless numbers. Based on the air velocity, U, the temperature T and length of the plate L, then basic parameters such as the kinematic viscosity of air, ν, diffusion coefficient of the solvent in air, D we have:

Reynolds NumberReUL/ν
Schmidt NumberScν/D
Sherwood NumberShkL/D

The key number is Sh because it contains the mass transfer coefficient, k. It is calculated from the other two via:

Sh = 0.664Re0.5Sc0.333

From k we can calculate the moles of solvent evaporated per unit width along the length L, given a molar concentration c at the surface and assuming that the air is solvent-free:

Evap = k.L.c

The molar concentration c depends on the vapour pressure, VP, the temperature T and the universal gas constant R:

c = VP/RT

The mass evaporated is simply the molar rate multiplied by the moelcular weight, Evap*MWt. Finally, we can calculate the vapour pressure if we know the Antoine Constants of the solvent:

log(VP) = AA-AB/(AC+T)

A few details remain. The air properties assume dry air - the calculated evaporation rates for moist air are not significantly different at this level of approximation. The kinematic viscosity of air changes with temperature; a simple polynomial is used to calculate it. The diffusion coefficient of the solvent in air can be calculated from T, MWt and the molar volume, MVol via a formula not shown here. The solvent values of MWt, MVol, AA, AB, AC are extracted from the values shown in the combobox for each solvent.