Transactions of the ASABE Vol. 53(6): 2010 American Society of Agricultural and Biological Engineers ISSN 2151-0032 1 A MASS TRANSFER MODEL OF ETHANOL EMISSION FROM THIN LAYERS OF CORN SILAGE H. M. El‐Mashad, R. Zhang, T. Rumsey, S. Hafner, F. Montes, C. Alan Rotz, V. Arteaga, Y. Zhao, F. M. Mitloehner ABSTRACT. A mass transfer model was developed and validated to predict ethanol emission from thin layers of corn silage. The model was developed using experimental data collected from silage placed in a wind tunnel under different temperatures and air velocities. Data from the wind tunnel experiments were used to derive a multiple regression equation that related the overall mass transfer coefficient of ethanol to temperature and air velocity. Evaluation of the model was done using data collected from experiments conducted in a controlled environmental chamber. Ethanol emission was determined from the ventilation rate of the environmental chamber and ethanol concentration in the chamber exhaust over a 24 h period, measured using a photoacoustic gas analyzer. Ethanol concentration in the silage was also monitored throughout the duration of each experiment. Predicted ethanol emission rates were strongly correlated (R 2 = 0.94) with values measured in the environmental chamber. A high correlation (R 2 = 0.96) was also found between predicted and measured ethanol concentrations in the silage. The model was used to estimate ethanol emission rates from thin layers of lightly packed silage on a dairy farm in California. Model predictions indicate that most of the ethanol contained in the silage could be emitted in the first 10 h after exposing the silage to ambient air temperature (18°C to 35°C) and air velocity (0.1 to 2.0 m s ‐1 ). Keywords. Corn silage, Dairy farms, Ethanol emissions, Mass transfer coefficient, Modeling, VOC emissions. airy farms are a significant source of volatile or‐ ganic compound (VOC) emissions. Feed, ma‐ nure, and cattle themselves are the specific emission sources. Many VOCs have been identi‐ fied as air pollutants emitted from dairy farms, including al‐ cohols, organic acids, aldehydes, ketones, and esters (Filipy et al., 2006; Shaw et al., 2007; Sun et al., 2008; Ngwabie et al., 2008), with silage being a major source (Alanis et al., 2008; Montes et al., 2009; Howard et al., 2010). The major public health concerns with VOCs are odor and the contribu‐ tion of these compounds to the formation of ground‐level ozone (NRC, 2008). Submitted for review in May 2010 as manuscript number SE 8565; approved for publication by the Structures & Environment Division of ASABE in October 2010. The authors are Hamed M. El‐Mashad, Post‐Doctoral Scholar, Department of Biological and Agricultural Engineering, University of California‐Davis, Davis, California, and Associate Professor, Department of Agricultural Engineering, Mansoura University, El‐Mansoura, Egypt; Ruihong Zhang, ASABE Member, Professor, and Tom R. Rumsey, Professor Emeritus, Department of Biological and Agricultural Engineering, University of California‐Davis, Davis, California; Sasha Hafner, ASABE Member, Agricultural Engineer , Felipe Montes, ASABE Member, Agricultural Engineer, and C. Alan Rotz, ASABE Fellow, Agricultural Engineer, USDA‐ARS Pasture Systems and Watershed Management Research Unit, University Park, Pennsylvania; Veronica Arteaga, ASABE Member, Staff Research Associate, Yongjing Zhao, Project Scientist, and Frank M. Mitloehner, ASABE Member, Associate Professor and Cooperative Extension Specialist, Department of Animal Science, University of California‐Davis, Davis, California. Corresponding author: Ruihong Zhang, Department of Biological and Agricultural Engineering, University of California‐Davis, One Shields Avenue, Davis, CA 95616; phone: 530‐754‐9530; fax: 530‐752‐2640; e‐mail: rhzhang@ucdavis.edu. Ethanol is produced in silage as a result of fermentation. Ethanol concentrations in silage vary with the characteristics of the raw material and the conditions under which the fer‐ mentation occurs (Pahlow et al., 2003). Sheperd and Kung (1996) reported that ethanol concentrations in corn silage ranged from 0.5% to 3% of the dry matter content. Ethanol emissions from silage are much greater than those of other compounds (Schmidt and Card, 2009; Mitloehner et al., 2009). According to Krauter et al. (2009), ethanol accounts for about 75% of total VOC emissions from silage sources. Based on model calculations, Howard et al. (2010) found that ethanol and other alcohols accounted for more than 50% of the ozone formation potential of VOC emissions from animal feeds in the San Joaquin Valley, California. To estimate VOC emissions from various farm sources, generalized emission factors have been produced based on data derived from laboratory and farm measurements. Al‐ though it is known that emission rates vary with time, source characteristics, and environmental conditions (Zhang et al., 2009), generalized emission factors do not account for these sources of variation. When used to estimate VOC emissions, process‐based models improve upon emission factors be‐ cause they account for additional aspects that influence emis‐ sion rates (NRC, 2003; Zhang et al., 2009). Hafner et al. (2009) developed a mathematical model for predicting VOC emission from the exposed surface of stored silage. Their model simulated diffusion in gas and aqueous phases through pores and silage particles, and convection from an exposed surface. Relationships used for predicting diffusion and mass transfer coefficients were based on correlations for smooth surfaces or relationships derived from work on soils (diffu‐ sion coefficients). These relationships have not been tested for silage. D