Predicting vapour transport from semi-volatile organic compounds concealed within permeable packaging

Abstract

The vapour concentration present in enclosed spaces containing concealed semi-volatile organic compounds (SVOCs), such as explosives, is difficult to measure experimentally. Therefore, mathematical models play a key role in understanding the transport of these materials. Vapour transport has previously been modelled in a range of environments, from small emission cells to whole rooms, using both analytical and numerical approaches. These models typically include either a well-mixed air volume or a simple sorption model. This work has been extended by including a multi-layer vapour sorption/permeation model within a computational fluid dynamics (CFD) framework. This allows for vapour source terms from items concealed within permeable packaging to be considered. The CFD based permeation model includes sorption/desorption, using a linear isotherm at inner and outer surfaces and a blended wall function to account for the effects of near-wall turbulence. The model has been validated for the explosive SVOC, ethylene glycol dinitrate (EGDN). The model has been used to show how vapour concentrations around a cardboard box containing a SVOC vary when some of the key input parameters are changed. Changing the vapour source from EGDN to the much lower vapour pressure trinitrotoluene (TNT), had a significant effect, as expected, and this was most pronounced early on due to the difference in permeation lag times for the two materials. Conversely, changing the type of cardboard had only a small effect on the concentrations. This type of modelling approach can now be used to study a wide range of SVOC transport problems which would not previously have been possible.