Dispersion of VOC vapours in the surface treatment workspace: influence of variability in diffusivity, mass transfer and air velocity.

Wherever solvents are allowed to disperse into the workspace it is necessary to be able to predict and determine their concentration and the effect of air velocity variations. Models developed to predict dispersion for assessing ventilation efficiency and worker exposure are validated against measured data with varying success. In numerical convection–dispersion models, including computational fluid dynamics methods, the transport coefficient effective diffusivity is used as a turbulence closure parameter and air velocities are used to define convective mass transport. This study shows how transport coefficient values, empirically estimated from airborne volatile organic carbon vapour concentrations from a solvent source, vary in a ventilated workspace. Variability in effective diffusivity values demonstrates non-Fickian dispersion from the source along the length of a one-dimensional axis. An important finding was that a correlation between air velocity and vapour transport data was not found. This suggests that air velocity should not be used a priori to represent mass transport in the determination of vapour dispersion in the workplace.