Sironi S., Eusebio L., Capelli L., Boiardi E., Del Rosso R., 2014, Odour Sample Diffusion Through Double NalophanTM Bags, Chemical Engineering Transactions, 40, 169-174.
This paper describes the studies regarding ammonia losses through NalophanTM bags. Ammonia was chosen as target compound in order to be representative of odorous molecules of small dimensions. The losses observed for storage conditions and times as allowed by the reference standard for dynamic olfactometry (EN 13725:2003) indicate that odour concentration values due to the presence of small molecules may be significantly underestimated if samples are not analysed immediately after sampling. The diffusion coefficient of ammonia through the NalophanTM film was evaluated using the Fick’s law, andit turned out to be equal to 2.38E-12 (m2/s). The study also discusses the effect of the exchange surfaceon diffusion, by highlighting to which extent the surface-to-volume ratio affects the diffusion rate. For a given material, to which corresponds a given diffusion coefficient D, one way to reduce diffusion over time is trying to reduce the surface-to-volume ratio. As a consequence, diffusion phenomena can be reduced by using bigger bags, filled to their maximum capacity. For cylindrical bags obtained from tubular NalophanTM, the surface-to-volume ratio can be minimized by realizing bags with a slenderness ratio (h/D) equal to 1. Moreover, the effect of a double barrier on diffusion was studied by realizing “double bags”. Double bags are concentric bags obtained starting from two different tubular films cut in different lengths. The experimental data show a reduction of ammonia losses due to the effect of the external multi barrier. The expedient of the double bag loaded both internally and externally with the same gas mixture allows to reduce the diffusion of ammonia into the internal bag. The diffusion through the internal bag is comparable to that obtained with a bag of triple thickness than the one used.
