A disc-type Pd-Au membrane module was considered, and a computational fluid dynamics (CFD) model was developed to describe the actual flow dynamics and the distribution of H2 flux over the membrane. When the membrane size was increased to develop a module with a large separation capacity, the feed flow rate per unit membrane area decreased, indicating loss of utilization of the membrane area. To increase the utilization, the sizes of the feed inlet tube and retentate tube were varied (cases 1 and 2, respectively). The CFD simulation showed that the feed flow rates per unit membrane area increased by ca. 8% and 10%, respectively, whereas a change in the geometry from circular to a rectangle with rounded edges (case 3) resulted in an increase of approximately 19%. A change in the ratio of the edges (case 4) had a slight influence on the separation performance. The distribution of H2 flux where the geometries in cases 1-3 were combined clearly revealed that most of the membrane area was used to permeate H2; as a result, the number of membranes decreased by approximately 88% upon increasing their size, while the total membrane area remained the same. This indicated improved utilization of the membrane. The proposed approach is expected to be useful for acquiring valuable information on the design of a membrane module with a large separation capacity.