Bubbles in spacers: Direct observation of bubble behavior in spacer filled membrane channels

Abstract

Air sparging is a means to prevent biofouling and scaling in hollow fibers and tubular membranes. Little is known for the case of bubbles flowing in spacer filled channels. We first demonstrate that the flow of bubbles in feed channels prevents biofouling. Then we describe a method to quantify the hydrodynamics of bubbles for various spacers, liquid and gas velocities. The bubble size at a given Reynolds number is similar for the six spacers under investigation. At low liquid velocities (<0.15 m/s) the bubbles are elongated in the direction of flow. With increasing liquid velocity, bubble sizes become smaller and the bubbles are more spherical. The bubble diameter remains large enough to be in contact with both walls, which is required for efficient fouling reduction. The membrane area coverage of bubbles from a single source shows a maximum at intermediate liquid velocities: at low velocity the bubbles follow a single path dictated by spacer geometry and presence of stagnant bubbles; at high speeds the bubbles follow a straight path from the inlet to the outlet. At intermediate speeds, less stagnant bubbles are present and the moving bubbles deviate from the single path followed at low liquid velocities, which increases the membrane area coverage.