Direct observation of single- and two-phase flows in spacer filled membrane modules

Spacers are introduced in membrane modules as turbulence promoter to enhance mass transfer while the detailed hydrodynamics inside the flow channel is hard to recognize. The situation becomes more complicated when air bubbles are incorporated as second phase, i.e. air sparging processes. In this work, transparent modules with precise slit accommodating for spacer were fabricated to visualize the behavior of single- and two-phase flows. For single-phase flow, dye injection method and pressure drop measurement were conducted in channel with and without net-type spacer. The results show that with the incorporation of the spacer the critical Reynolds number decreases from 1000 to 100 approximately which is lower than channel with ladder-type spacer always adopted by researchers. For two-phase flow consisted of liquid and air bubbles, the behavior of bubbles in coalescence system (water-air) and non-coalescence system (ethanol-water-air) were investigated respectively. With stationary liquid in tubular module, the bubble size increases accordingly with the increase of the gas flow rate while the size of the bubbles is obviously shrunken in non-coalescence systems. In empty rectangular channel, the smallest bubble we found (2 mm approximately) in non-coalescence system is larger than the channel height. Therefore bubbles of both systems could keep contact with channel wall which equips them with ability to clean fouling. Three different flow behaviors of two-phase flow are classified from the flow visualization in channel with diamond-like spacer D1. For both systems, working conditions in region (III) is optimal since the flow distribution is well managed. The threshold of region (III) is higher in non-coalescence system, because higher liquid and gas flow rate is required to sweep stagnant bubbles away. Higher porosity contributes less to the energy saving therefore porosity between 0.887 and 0.917 for the test spacer geometry is appropriate. Ladder-type spacers could avoid maldistribution of bubbles, which are suitable for conditions required lower gas and liquid flow rate.