External Mass‐Transfer Rate in Fixed‐Bed Adsorption

External mass‐transfer coefficients are measured with organic compounds, p‐nitrophenol and 2,4‐dichlorophenol, under conditions representative of fixed‐bed activated‐carbon adsorption for water treatment. The superficial velocity was in the range 1.3–8.0 mm/s and the Reynolds number range was 0.8–5. The observed external transfer coefficients are compared with predicted values from four mass‐transfer models; the relationship of Gnielinski best predicts the velocity dependence. The observed transfer coefficients in all cases exceeded the predictions based on ideal spherical geometry. Depending on the conditions and the choice of predictive model, the particle size correction factors ranged from 1.44–2.04. The data from previously published studies show deviations that are similar in direction, but smaller in magnitude.