Predicting first-order evaporation rate constant alpha (α) from small spills of organic solvents in a controlled environment

Abstract

Exposures to vapors generated by small spills of organic solvents are common in the occupational hygiene practice. In these scenarios, contaminant mass release is exponentially decreasing, driven by an evaporation rate constant alpha (α). Knowing α is fundamental for adequately modeling peak concentrations and/or short-term exposures that occur and for achieving efficient occupational risk analysis and management. The purpose of this study was to measure alpha experimentally using a gravimetric approach in a controlled environment during solvent evaporation tests designed to simulate small spills of solvents. The effects of several factors on α were evaluated. Equations based on regression models derived from the experimental data were proposed for predicting α. Predictions were externally validated against experimental data. A total of 183 tests was performed. Data analyses found that alpha (α) values increased with vapor pressure, spill surface area-to-spill volume ratio, and air speed across the spill. Larger α were associated with petri dish containers compared to watch glasses. Three regression models were created for predicting α. They had four variables in common, namely vapor pressure, molecular weight, air speed above the liquid, and surface tension of the liquid. The fifth variable was either spill volume, spill surface area, or spill surface area-to-spill volume ratio. The R² of the regression models were equal to 0.98. External validation showed mean relative errors of −32.9, −32.0, and −25.5%, respectively, with associated standard deviations of the relative errors of 17.7, 33.3, and 26.0%, respectively, and associated R² of 0.92, 0.65, and 0.87, respectively. The proposed equations can be used for estimating α in exposure scenarios similar to those evaluated in this study. Moreover, these models constitute a step further in the improvement of knowledge on estimating evaporation rates for small spills of organic solvents.

Keywords:

• Decreasing emission models
• evaporation rate
• exposure assessment
• model validation
• small spills
• solvents

Acknowledgments

We would like to extend our special appreciation and thanks to Professors Chris Keil and Mark Nicas for generously providing us the experimental data used to develop the predictive equation for evaporation rate constant alpha in their article titled “Predicting Room Vapor Concentrations Due to Spills of Organic Solvents” published in 2003 in the AIHA Journal. Those raw data were very helpful as they were used for external validation of predictive equations (for evaporation rate constant alpha) developed based on experimental data from the present study. The Keil and Nicas experimental data are published in an online supplementary appendix with the permission granted by the authors. We would also like to thank Carlos Castro Ruiz and Jean Mahe for their technical support.

Data availability statement

The data underlying this article are available in the article and in its online supplementary materials.

Additional information

Funding

This project was financially supported by the Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST #2012-0044). S.F.A received a scholarship from the Institut de recherche Robert-Sauvé en santé et en sécurité du travail.