A three-dimensional computational fluid dynamics (CFD) model of flow through cyclones was developed to obtain the pressure drop and flow profiles. The CFD results agreed very well with the experimental data obtained by Bohnet (Chem. Eng. Process. 1995, 34, 151) and Boysan et al. (Inst. Chem. Eng. Symp. Ser. 1983, 69, 305). CFD simulations were also performed at various flow rates, temperatures, and pressures. A fictitious fluid was used to observe the separate effects of density and viscosity on the flow pattern. The influence of density on the pressure profiles in the cyclone is significant, but that of viscosity is negligible. The results indicate that decreasing gas density and increasing gas viscosity can similarly reduce the magnitude of the tangential velocity in the region of the outer vortex at the same inlet volumetric flow rate. A first-order logarithmic equation was derived to correlate the ratio of the maximum tangential velocity to the inlet velocity with the Reynolds number for predicting the tangential velocity at high temperatures and high pressures.