The quasi-lognormal distribution (Q-LND) approximation was used to predict breakthrough curves in fixed-bed adsorbers for a linear adsorption system with axial dispersion, external film diffusion resistance, and intraparticle diffusion resistance for slab–, cylindrical–, and spherical-particle geometries. The exact solution and parabolic profile approximation were also obtained for different particle geometries. Numerical results show that the Q-LND approximation is a simple and handy solution. It predicts breakthrough curves with an accuracy comparable to the parabolic-profile approximation over a wide range of parameters; compared with the latter, it only takes less than one hundredth the computation time and does not have a convergence problem in numerical calculations. A criterion for the applicability of the Q-LND approximation is suggested. The effect of particle geometries on the breakthrough curves is discussed. A criterion is also provided for the Q-LND approximation to explore the conditions where one should consider this effect on breakthrough curves.