The use of moving granular bed filters (MGBFs), which hold an important role in the gasification or combustion of coal and biomass, has been growing. Furthermore, these filters have great potential to be developed for the high-temperature gas cleanup of advanced power generation systems. However, the creation of a stagnant zone due to defective designs in MGBFs can cause serious issues such as plugging. Therefore, in order to minimize these problems, flow-corrective inserts for filter vessels have been researched, most of which use mono-sized filter granules. This study proposed a new method that introduced two granule sizes in one filter vessel, called the two-stage filtration mode. Nevertheless, the design theory of a mass flow vessel by Johanson could not be satisfied by using two sizes of filter granules to diminish the stagnant zone. The flow patterns in a two-dimensional and cross-flow moving granular bed were evaluated in six test configurations based on different geometric designs in a filter. The two kinds of filter granules consisted of coarse and fine silica sands. The findings revealed that the flow patterns of filter granules were influenced by the vessel geometry. The optimal design for two-stage filtration diminished the stagnant zone in 165 minutes.