Potential Applications of Concentrated Solar Energy Proceedings of a Workshop (1991) / Chapter Skim
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1 SESSION 1 - WATER TREATMENT: ADVANCED PHOTOOXIDATION PROCESSES
1 SESSION 1 - WATER TREATMENT: ADVANCED PHOTOOXIDATION PROCESSES
Pages 1-34
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Some scientists and engineers have referred to all of these treatment procedure" as Advanced oxidation technologies.. Hydroxyl radicals are involved as intermediates in most of these procedures.
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The average light absorption rate in a system is proportional to the incident light intensity and can be reduced by light attenuation within the system. In the various water treatment systems discussed in the workshop, ozone and hydrogen peroxide absorb W radiation strongly in the far W region (<280 nary)
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Moreover, in heterogeneous systems, particularly those with immobilized catalysts, mass transport to the catalyst surface can be rate limiting. As discussed by Turchi, mass transport limitations can lead to a lack of rate dependence on light intensity in some reactor systems, obviously an important consideration in designing concentrated solar treatment systems.
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The cession included several discussions of the mechanism for heterogeneous photocatalytic oxidations. Arthur Nozik presented mechanistic studies in electrochemical slurry cells that provided support to the hypothesis that the photocatalyzed oxidations involve hydroxyl radicals formed via oxidation of hydroxide on the catalyst surface.
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usage of concentrated solar photons for W /ozone, W /hydrogen peroxide, combined photocatalytic/ozone/hydrogen peroxide treatments; (7) development of improved photocataly~tm that use solar photons more efficiently and are less susceptible to poisoning.
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have been defined as ambient temperature processes which involve the generation of highly reactive oxyradicals, especially the hydroxyl radical. These processes show promise for the destruction of hazardous organic substances in municipal and industrial wastes and in drinking water.
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1988. Advanced Oxidation Processes for Treating Groundwater Contaminated with TCE and PCE: Laboratory Studier.
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1988. Advanced Oxidation PRocesses for Treating Groundwater Contaminated with TCE and POE: Pilot-Scale Evaluations.
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Turner, and A.J. Nozik Solar Energy Research Institute Golden, Colorado The general photo-oxidizing and photocatalytic properties of suspensions of "emiconducting TiO2 particles are well established in the literature.
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generates the anodic photocurrent and reaction (7) generates the cathodic photocurrent in the PEC slurry cell.
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One important point concerns the dependence of the rate of photodecomposition on the light intensity. If we take reactions (1)
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2. At high light intensity, we would see a square root dependence, while at low intensity, we would see a linear dependence.
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13 o to a, .
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The intensity dependence affects how diffuse or concentrated one wishes the radiation to be. If reaction rate increases with light intensity to some power greater than unity, a concentrated light beam is advantageous.
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using a mechanism teamed on hydroxyl radical recombination rather than electron-hole recombination. Id Data In general, available data concerning photocatalytic reaction as a function of light intensity support the theory of low-flux direct proportionality and high-flux square root dependence, yielding the behavior depicted in Figure 1.
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Both experimental data and theory based on photoexcitation of semiconductor" suggest that the photocatalytic reaction rate will vary directly with light intensity at low fluxes and with the square root of intensity at high f luxes . Although the region of this transition is likely to vary with photocatalyst and other system properties, laboratory data imply that this transition occurs near or below the equivalent of one sun of ultraviolet flux.
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1990. Solar-Assisted Photocataly~i" for Water Purification: Issues, Data, Question" presented 8th International Solar Energy Conversion Conference, Palermo, Italy, July.
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Secondorder reaction rate constants in the 107-10~° M-~- range indicate that hydroxyl is a powerful, indiscriminate oxidant. The Advanced Oxidation Processes (AOPs)
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3. Define limits of water treatment imposed by the use of solar photon=, in terms of milligrams per liter of contaminant removed from a Q gallon/minute stream using a collector of area A with semiconductor of bandgag Eg with charge separation efficiency eS and a solar intensity of I einsteins/m in different regions of the country.
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The application of thin technology is steadily expanding, however, a" it offers a means of solving many of the problems created by the toxic water soluble organic chemicals that are found today in groundwater, wastewaters, leachate, and drinking water supplies. More conventional or better-known unit processes and operations such as liquid/nolids Reparation, reverse osmosis, air stripping, biotreatment, or granular activated carbon can remove many of the toxic organics encountered today.
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Within the reactor, the water flows from stage to stage in a sinusoidal path using gravity flow. When the reactor uses ozone, the residual ozone in the offings i" decomposed back to oxygen by the use of a fixed-bed catalytic unit operating at 150°F (66°C)
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24 TABLE 1 Direct Operating and Maintenance Costs for W -Oxidation at Industrial Installations Volume Contaminant Discharge Treated Direct O&M Type of Water Contaminants Concentration toper Day Cost Range Wood treating Pentachloro- 150 ppm POTW 30,000 $1.25-$1.35 wastewater phenol and per 1000 gal phenol Wood treating Pentachloro- 5 ppm POTW 86,400 So. go-Sl.oO groundwater phenol and per 1000 gal phenol Fume Hydrazine, 5 ppm Biotreat- 600-1500 S0.086 scrubber monomethylhydrazine ment plant per gal water unsymmetrical on-site dimethylhydrazine Contaminated TCE, trann 5 ppm Surface 300,000 S0-47 groundwater DCE, MeCl2 water per 1000 gal Contaminated TCE, TCA, 600 ppb POTW 72,000 S0.33 groundwater DCA, PCE, per 1000 gal MeCl2, ViC1 Contaminated THF 1 ppm Ground 216,000 S0.39 Groundwater per 1000 gal Wastewater Phenol 90 ppm POTW 4,300 S6.48 per 1000 gal
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Through literature searches and government and industry contact=, SERI ha" identified three potential markets: groundwater remediation, industrial wantewater treatment, and drinking water purification. While the nation is presently supporting efforts to reduce pollution, even the Environmental Protection Agency has difficulty in estimating the potential economic impact of its new regulations.
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Technologies that Livermore staff identified as providing the desired treatment capabilities at the lowest cost are granular activated carbon and lamp-driven advanced oxidation. The lamp system features a commercial unit that combines ultraviolet lamps and hydrogen peroxide to oxidize the contaminant" online.
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chin Thin discussion has provided a brief overview of work which has led to the identification of likely niche markets for early application of solar photocatalytic processes. Although estimates of solar systems using present technology are higher than those of competing technologies, system improvements have been identified that should lead to cost competitiveness within a few years.
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Theme are effective in removing many organic pollutants from water streams, but they exhibit a major drawback. Both carbon adsorption and air stripping merely transfer the organic pollutants from one medium to the other.
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. 3 4 5 6 7 8 9 10 Irradiated Time /min Figure 1 Photocatalytic degradation of a mixture containing phenol derivatives (10 ppm each)
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ORS Iris Mark S Nehos Solar Energy Research Institute Golden, Colorado The use of solar concentrating systems as an economically viable alternative for detoxifying contaminated groundwater will require an inexpensive means by which a titanium dioxide photocatalyst can be immobilized inside a photoreactor.
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These problems are currently being investigated by in-house researcher" at the Solar Energy Research Institute and Sandia and will soon be investigated through industry/university subcontract=. Resolution of these issues will increase the likelihood of the competitiveness of solar detoxification systems.
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solar radiation data for a number of sites to estimate processing rates for solar detoxification systems. We also did a series of experiments to measure roan race or LEA a ~ one and in the presence or TCE at concentrations of 2 COO ppb .
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Figure 1 Sandia's trough facility used for solar detoxification of water experiments. Reprinted With Permission From: Sandia National Laboratories/ Solar Thermal Collector Technology
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