Modeling Diffusion-Controlled Emissions of Volatile Organic Compounds From Layered Building Materials
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Building materials are a major source of indoor air contaminants. Volatile organic compounds (VOCs) are an important class of contaminants prevalent in indoor air. Attempts have been made to model the emission of VOCs from building materials. Diffusion has been shown to control the rate of mass transfer within certain types of building materials. The primary objective of this research is to develop a fundamental diffusion-based model for single and double layer building materials. The single-layer model considers a slab of material located on the floor of a chamber or room with the material acting either as a source or a sink for VOCs. The behavior of the model is governed by the material phase diffusion coefficient (D), the material/air partition coefficient (K), the concentration of VOC in the influent air stream, and the initial concentration within the material phase. The single-layer model extends a previously developed version, incorporating the non-uniform initial concentration inside the building material and a transient influent concentration. Experimental work is performed to check the validity of the model. A steel chamber housing a piece of vinyl flooring is used to simulate building material within a room. D and K values for two representative VOCs, n-dodecane and phenol, are available from earlier experiments. These parameters are used in the model to predict the VOC concentration inside the chamber. The predicted values compare very well to the observed experimental data. A double layer version of the model is developed and studied from a theoretical perspective. The model also permits a time dependent influent concentration and a non-uniform initial concentration profile within each of the two layers. A parametric analysis is performed varying the ratio of the diffusion coefficients, the partition coefficients and the thickness of the two layers. Three cases of practical interest are studied using the double-layer model. The use of a thin low-permeability barrier layer placed on top of a building material is shown to hold considerable promise for reducing the emission rate of VOCs into indoor air.
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