We apply a two-fluid model (TFM) from the open-source code Multiphase Flow with Interphase exchanges (MFIX) to investigate hydrodynamics in a gas-solids fluidized bed with immersed vertical tubes. The cut-cell method implemented in MFIX is used to fully resolve the flow around vertical U-tube banks. Simulations are performed in a bed diameter of 0.145 m with square and triangular tube arrangements, for inlet gas velocities of U-0/U-mf=2.3, 4.5 and 6.8. Simulation results are compared with experimental results from the literature and show very good agreement for the bubble size. The efficiency of vertical tubes in reducing bubble size depends upon inlet gas velocity and tube arrangement. Reduction in bubble size is due to the vertical tubes preventing bubble coalescence and promoting bubble splitting. In-bed vertical tubes result in uniform distribution of bubbles within the bed with increase in bubble frequency. The bubble frequency is higher within the bed for square tube arrangements. For a bed with vertical tubes, the bubble shape is generally elongated, which results in high bubble rise velocity. Axial solid velocity and solids circulation patterns are significantly affected by the vertical tubes, where triangular tube arrangements rarely show any solids circulating zone. (C) 2015 Elsevier B.V. All rights reserved.