A comparison of the experimentally determined values of the mass transfer coefficients and the values of the mass transfer coefficients according to the relations mentioned in literature has been made. It appeared that not internal mass transfer (sample bed diffusion, estimated according to Bischoff [13]), but external mass transfer is (partially) rate limiting at the experimental conditions applied. Furthermore, it appeared that the experimentally determined values for the external mass transfer coefficient kdiff agree reasonably well (within 20%) with the values obtained using the well-known theoretical Ranz–Marshall relation (Eq. (20)) as given by Ranz and Marshall [14]: