Influence of flow orientation on mass transfer in a mini-channel

Bubble train flow (BTF) constitutes a very attractive flow pattern for catalytic multiphase monolith reactors because of its excellent mass transfer properties and it consists of a sequence of bubbles. Multiphase monolith reactors can be operated in co-current upward or co-current downward flow. Recent experimental investigations of BTF in a square mini-channel suggest that mass transfer between the slug and the channel wall may be more efficient in upward than in downward flow. This is attributed to the lower recirculation time, ?cir.In this study the recirculation time in bubble train flow through a rectangular channel is investigated analytically. By assuming a fully developed laminar liquid slug, a relation ?cir = ?cir (?) is obtained, where ? = UB/J and UB and J are the magnitude of the bubble velocity and the total superficial velocity, respectively. The relation ?cir (?) shows that ?cir is smaller in downward than in upward flow, indicating that mass transfer in downward flow may be more efficient. This analytical result is in contrast to some results in literature and indicates that the measurements may be erroneous because of small dimensions. The present analytical results are confirmed by 3D numerical simulations of upward and downward bubble train flow in a square mini-channel with the volume-of-fluid method based CFD code (TURBIT), which show that mass transfer between the wall and the liquid slug is indeed more efficient for downward flow.