A Photocatalytic Reactor for Water Purification
Semiconductor photocatalysis has received increasing attention for water purification due to its advantages. Low energy UV-light is used to generate holes and electrons, which oxidizes toxic organic pollutants to environmentally harmless components. Degussa P25
TiO2 is widely used as photocatalyst since it is inexpensive, biologically and chemically inert, and nontoxic. Moreover,
TiO2 requires low energy UV-A light (l< 380 nm), but the efficiency is low as the holes and electrons may recombine within
TiO2 and dissipate energy as heat.
There are factors that impede photocatalytic reactor design. Catalyst illumination is the most important factor, in addition to other conventional reactor design factors. The high degree of interaction among the various transport processes lead to a strong coupling of physiochemical phenomena and a barrier for scale-up. Photocatalytic reactions occur on the catalyst surface and the catalyst can either be dispersed in liquid or can be immobilized on surfaces. However, the use of a suspension requires the troublesome recovery and recycleing of ultrafine particles. Moreover, UV-light penetration depth is limited due to absorption by both catalyst particles and pollutants. To avoid this, reactors can be designed, in which a solid catalyst is immobilized, but catalyst immobilization creates its own problems. The accessibility of the catalytic surface to photons and pollutants significantly influences the degradation rate. Usually the pollutant concentration is low, and there is an increased diffusional length of pollutant from the solution to the surface. In designing fixed bed reactors, one must address uniform distribution of light and mass transfer. Our group has studied reactors containing catalyst-coated tube bundles, catalyst-coated extremely narrow diameter immersion-type lamps, and catalyst-coated rotating tube bundles. The reaction occurs at the interface and the rate is limited by the pollutant transfer to the surface. We have enhanced mass transfer by increasing mixing of pollutants through turbulence and baffles. Rate increase is achieved via flow instability; unsteady Taylor-Couette flow is created in between two co-axial cylinders where an inner cylinder coated with TiO2 is rotated at different speeds.
Time dependent Laminar Taylor Vortex Flow (LTVF) was observed when the rotation speed of the inner cylinder exceeds 2.2 rpm (Reynolds number (Re) > 111). The vortex starts at the bottom at the critical Reynolds number and progresses toward the centre.
At steady state (16 minutes), about 21 counter-rotating vortex pairs were observed and the dimension of the each pair was twice the gap of the annular space. When Re was increased beyond the critical value, the Taylor vortex flow takes the shape of azimuthally traveling waves when Re/Rec is approximately 2.05. This wavy vortex flow changes to turbulent vortex flow at higher Re (Figure 1).
|Taylor Vortex Flow (Re = 177)||Wavy Vortex Flow (Re = 505)|
|Weakly Turbulent Vortex Flow (Re = 3027)||Turbulent Vortex Flow (Re = 8072)|
Figure 1: Photograph of flow patterns at different Reynolds Numbers.
Figure 2 shows photocatalytic dye degradation when experiments were conducted at different Re. Rate increases with an increase in Re, demonstrating external mass transfer control. In the laminar region, mixing is realized, but it is not significant enough for complete removal of external mass transfer resistance. In turbulent region, appreciable enhancement of mixing within the vortices is achieved. The results of a Taylor vortex photocatalytic reactor shows that it could be excellent for water purification.
Figure 2: Dye degradation at different Reynolds numbers: Co= 25 ppm, I = 1.3 mW/cm2.
Collaborators for the Taylor vortex reactor are A/P TT Lim (Department of Mechanical Engineering, NUS) and Visiting Professor TK Sengupta (IIT, Kanpur), while for other aspects the team consists of Prof. Gregory Yablonsky (Washington University, St. Louis), Prof. Virender Sharma (Florida Institute of Technology), A/P SO Pehkonen (ChEE, NUS) and Dr. A Dey (Chemitreat).
Assoc Prof AK Ray