This work is aimed at investigating the impact of the fluid flow features on the behavior of gas separation membrane modules. The investigation is carried out by particle image velocimetry (PIV) experiments and computational fluid dynamics (CFD) simulations. The experimental velocity data collected in a laboratory scale membrane module confirm the reliability of the gas velocity held calculated numerically solving the Navier-Stokes equations by a general-purpose finite volume CFD code, and the effect of the observed nonideal flow distribution on the module separation performances is discussed. Afterward, the case of a pilot scale module equipped with three Pd-Ag membranes, which are well-suited for the separation of from methane steam reforming gas mixtures, is considered. The appropriateness of the selected CH) method for the design and the geometric optimization of membrane modules of pilot size is demonstrated.