A population balance model is developed for the growth of pre-formed agglomerates in a fluidized bed with several agglomeration mechanisms considered. Primary agglomeration takes place when dry particles collide with the wet surface of the original agglomerate. Secondary agglomeration occurs either by wet particles colliding with the dry surface of agglomerates, or by liquid migration to the top layer of an agglomerate and subsequent collisions with dry particles. The properties of the liquid binder determine the dominant mechanism. The new agglomeration model for high-and low-viscosity liquid binders is implemented in ANSYS FLUENT commercial software. Model predictions agree reasonably well with available experimental results without altering the previously published parameters of the sub-models employed. It is found that the viscosity of the liquid binder and surface asperities play notable roles in the agglomeration. Surface asperities are especially important when asperity heights are of the same order as the thickness of the liquid film. (C) 2017 Published by Elsevier B.V.