A model based on force balance was proposed to estimate the equilibrium size of agglomerates formed in the fluidization process of nanoparticles. This model was derived based on the balance between drag, collision, and gravity forces as separation forces, and the van der Waals force as an adhesion force. The effects of permeability of agglomerates and addition of isopropanol to the gas-solid suspension on the agglomerate size were investigated for the first time. Results showed that the average size of agglomerates decreased when increasing the gas velocity. In addition, the average agglomerate sizes decreased in the presence of isopropanol due to the weakening of van der Waals adhesion force. An overall agreement with an average error of 20% was observed between the estimated agglomerate sizes and the experimental data.