Ozone mass transfer in an ozone–water contacting process with Shirasu porous glass (SPG) membranes—A comparative study of hydrophilic and hydrophobic membranes

Ozone absorption into water in a bubble-free ozone–water contacting process was investigated with tubular hydrophilic and hydrophobized Shirasu porous glass (SPG) membranes having uniform pores to gain a better understanding of the membrane's role in the ozone transfer. The surface of SPG membranes was hydrophobized by chemical modification with an organosilane having a fluorocarbon functional group. At a water flow rate of 8 dm3 min−1, corresponding to a Reynolds number of 820, the overall mass transfer coefficient, KL based on water phase for the hydrophobized membrane with a mean pore diameter of 0.51 μm was 1.2 × 10−5 m s−1, which was much larger than the KL value of 2.1 × 10−6 m s−1 for the hydrophilic membrane with the same diameter. The main resistances to ozone transfer for the hydrophilic and hydrophobized membranes were in the membrane and in the water phase boundary layer, respectively. Under the same conditions, the mass transfer coefficient in the hydrophilic membrane, kM was 2.9 × 10−6 m s−1, which was four orders of magnitude lower than that of the hydrophobized membrane because of the presence of water in the pores. The higher KL values at water flow rates were ascribed to an enhanced turbulence on the membrane surface. Due to an increase in the length of the pores filled with water, the KL value for the hydrophilic membrane increased with an increase in the membrane thickness, while the KL value for the hydrophobized membrane was independent of the membrane thickness because of the gas-filled pores. The KL value was unaffected by the membrane pore diameter for both membranes.