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Abstract
To assess the magnitude and speed of desorption of adsorbed vapors from commercial carbon beds used in laboratory hoods, known quantities of organic solvents were evaporated in one ducted (i.e., exhausted to the outside) and two nonducted (i.e., recirculating) laboratory fume hoods. The air downstream of the filters was then sampled and analyzed.
Benzene (BZ), carbon tetrachloride (CT), and ethyl acetate (EA) were individually volatilized at room temperature in a ducted chemical fume hood at 6, 4, and 5 ppm, respectively. BZ and EA were released twice; CT was released four times. The total amount of vapor desorbed, as a percentage of the mass released, ranged from 10 percent for the first BZ release to 100 percent for the fourth CT release.
BZ, CT, and n-hexane (HX) were simultaneously volatilized in nonducted laboratory fume hoods at 10, 50, and 125 ppm, respectively, over six 4-hour periods, with each 4-hour release followed by 4 hours of vapor-free air (88 hours of vapor-free air after the sixth release). In that time period, about 15 percent of the BZ, 50 percent of the HX, and 75 percent of the CT desorbed.
A theoretical solution of the desorption phenomenon indicated that the logarithm of the concentration of the desorbing vapor should be inversely proportional to the time after adsorption. The regression coefficients for the experimental data ranged from 0.62 to 0.97, with 6 of 12 equal to or greater than 0.90 and 10 of 12 equal to or greater than 0.85. The experimental results were consistent with theory. Rapid and substantial desorption from previously unused carbon beds was observed.