Largescale Applications

Pilot- and full-scale studies using solar illumination and reactor volumes of 100-300 gal have been investigated by the National Renewable Energy Laboratory (NREL) in Golden, Colorado. Pacheco and Holms [94] describe engineering-scale experiments for the detoxification of solvent-contaminated groundwaters. Two reactors were used: a parabolic concentrating trough (7 ft by 120 ft with a 51-fold concentration efficiency) and a falling film reactor. PCO of a model compound, salicylic acid, was examined. The light intensity ranged from 1032 to 484 W/m2 for clear to cloudy days, respectively. Results for the two reactors are presented in Figure 6, where the exposure time is given as the total time multiplied by the ratio of light-exposed water volume to total water volume. Thus less than 3 min exposure to illumination was required for effective removal of this compound. The temperature of the suspension rose from 7 to 53°C as a result of the applied concentrated solar illumination, which may have enhanced the reaction rate.

A field experiment using concentrating trough reactors is described by Mehos et al. [95] and Turchi and Mehos [3], Two sets of parabolic reactors, 120 ft long, were used for treating deionized water spiked with TCE and, subsequently, contaminated groundwater at Livermore, California. A TiOz loading of 0.8-0.9 g/L was used. A flow rate of 4 gpm allowed a 10-min exposure time and heated the water to 140°F. Results from the treatment of the contaminated

Exposure Time (min)

Figure 6 Salicylic acid destruction using full-scale photocatalytic reactor with solar illumination. (Reprinted with permission from Pacheco and Holms [94]. Copyright 1990 American Chemical Society.)

Exposure Time (min)

Figure 6 Salicylic acid destruction using full-scale photocatalytic reactor with solar illumination. (Reprinted with permission from Pacheco and Holms [94]. Copyright 1990 American Chemical Society.)

groundwater showed a decrease in TCE from 107 ppb to 10 ppb. However, by reducing the water pH to 5.6 to decrease the effect of bicarbonate ions, the effluent TCE concentration was reduced to <0.5 ppb.

Recent work at NREL [3,95] found that, on a basis normalized by the photoreactor surface area, PCO employing nonconcentrating reactors is more efficient than concentrating (i.e., parabolic) reactors. This is attributed to (1) the high light reflection losses in large-factor concentrating reactors; (2) the importance of capturing diffuse UV radiation, which is not collected in concentrating reactors; and (3) the decrease in PCO efficiency due to radical recombination processes at high light intensities, as discussed in Section II.C.

Turchi et al. [96] present a detailed analysis for the treatment of TCE-contaminated groundwater using a full-scale photocatalytic reactor. Models of solar irradiation are used in conjunction with calculations of weather conditions to estimate available solar flux for the PCO reaction. In cases where bicarbonate concentrations are high, pre- and post-treatment steps consisting of, respectively, a lowering and neutralizing of pH must be employed in the system for efficient PCO. A strainer and filter are used to remove any particulate matter that may inhibit light transmission through the contaminated water.

For a 100,000-gpd PCO treatment system, land use for solar reactors ranges from 0.7 to 1.5 acres [96], Solar sensors are used to adjust flow rates through the reactor system; higher flow rates are used during periods of high solar intensity. The photocatalytic reactor would operate at its maximum rate during midday, slow down as solar intensity decreases during the evening, and shut off completely overnight. Surge tanks would be used to store the water both before and after the phototreatment. This would allow continuous operation of groundwater pumping wells as well as the pre- and post-treatment processes.

PCO reactor schemes evaluated include parabolic reflector flow reactors as well as sequencing batch reactors and a continuous serpentine reactor, each approximately 1 m deep.

A detailed cost analysis using PCO reactors for a 100,000-gpd contaminated groundwater cleanup facility in Livermore, California yields costs of $5.00-$6.00 per 1000 gal treated. In comparison, activated carbon treatment was estimated at $6.20/1000 gal and UV/H202 oxidation as $4.40/1000 gal, thus indicating that PCO can economically compete with these other common groundwater remediation processes in an area that has a favorable solar flux.

A major concern in full-scale PCO processes is the separation of the photocatalyst from the water stream after the completion of the reaction. Most commercially available Ti02 particles are on the order of 1-10 pm in diameter, making efficient removal difficult. An active research area incorporates the fixation of Ti02 particles on various supports to allow easy separation. For example, the PCO of several organic dyes using Ti02-coated sand has been demonstrated [97], However, rates are apparently limited by mass transfer of the substrate to the immobilized photocatalyst.

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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