Photoreactors

Immobilized catalyst photoreactor systems permit the continuous use of the photocatalyst, eliminating the need for post-process filtration coupled with particle recovery and catalyst regeneration. Such reactors may display a number of drawbacks 

Critical reviews of the designs described above emphasize the need for suitable irradiation, appropriate for given geometrical configurations (Borello et al., 1989 Cabrera et al., 1994 Robertson and Henderson, 1990). In addition, adequate mixing patterns and high water-photocatalyst interaction are identified as essential factors to achieve an optimized operation of these reactor units. 

COMPARING Ti02 SLURRY PHOTO REACTORS AND Ti02 IMMOBILIZED PHOTOREACTORS 

TABLE 2.1. Suspended versus immobilized photocatalytic systems 

Light may be considered as a highly specific and ecologically dean reagent . 

Photochemical methods offer less aggressive routes in chemical synthesis than thermal methods. 

Photochemical reactions may contribute to the safety of industrial processes, because they are generally performed at or below room temperature. 

Photochemically reacting organic compounds do not require any protecting groups. 

Many conventional syntheses can be shortened by inserting photochemical steps. 

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Irradiation of ethyleneimine (341,342) with light of short wavelength ia the gas phase has been carried out direcdy and with sensitization (343—349). Photolysis products found were hydrogen, nitrogen, ethylene, ammonium, saturated hydrocarbons (methane, ethane, propane, / -butane), and the dimer of the ethyleneimino radical. The nature and the amount of the reaction products is highly dependent on the conditions used. For example, the photoproducts identified ia a fast flow photoreactor iacluded hydrocyanic acid and acetonitrile (345), ia addition to those found ia a steady state system. The reaction of hydrogen radicals with ethyleneimine results ia the formation of hydrocyanic acid ia addition to methane (350). Important processes ia the photolysis of ethyleneimine are nitrene extmsion and homolysis of the N—H bond, as suggested and simulated by ab initio SCF calculations (351). The occurrence of ethyleneimine as an iatermediate ia the photolytic formation of hydrocyanic acid from acetylene and ammonia ia the atmosphere of the planet Jupiter has been postulated (352), but is disputed (353). 

Figure 13-2 Preparative Photoreactor Equipped for Filter Solutions. ...

Phenyl-2-oxa-3-azabicyclo[3.2.0]hepta-3,6-diene (11 0.200 g, 1.17 mmol) in hexane was irradiated in a quartz vessel using a Rayonet photoreactor to give the product as a yellow oil yield 0.160 g (SO %). The compound can be distilled below 100 C at 10 4Torr. It gradually resinifies when kept in air at 20 C. 

PCP presents a different picture from that of the lower chlorophenols and their derivatives. The corresponding dioxin shows much more stability to light than does TCDD, enough to permit its prolonged existence at low concentrations in a photoreactor. As a phenol it can directly yield dioxins, a process favored by its normal mode of application as the sodium salt. Although octachlorodibenzo-p-dioxin has much lower mammalian toxicity than TCDD (6), its formation, properties, and effects demand additional investigation. Technical preparations of PCP are frequently mixtures of tetra- and pentachlorophenols consequently, hepta-and possibly hexachlorodibenzo-p-dioxins might be expected as photolysis products in addition to the octachloro derivative. 

Photocatalytic oxidation tests were performed in a photoreactor [2]. The flat, rectangular stainless steel reactor has dimensions of 578 mm x 113 mm and inlet/outlet ports at the two ends. The aerogel catalyst placed in a recess located at the center of the reactor was uniformly irradiated by... 

Fig. 3 gives the conversions for acetic acid and ammonia decomposition over Ti02 and Al-Ti02 in a three-phase fluidized photoreactor. In the case of acetic acid decomposition (Inlet condition of 300 ppm), the conversion increased with alununum addition. In particular, the conversion to CO2 reached about 90% and then it was kept until 600 mins on Al-TiOa catalyst. On the other hand, in b), the anunonia removal (Inlet condition of 80ppm) also enhanced on Al-Ti02 compared to that conventional Ti02 catalyst the conversion to N2 reached above 95% in Al-Ti02. We have also observed that the ammonia conversion in a conventional batch type steady photoreactor could be obtained up to 70%. From this result, we could confirmed that... 

These results have confirmed that the VOC removal is more useful in a fluidized photoreactor than that in a steady photoreactor, and in particular when Al-Ti02 photocatalyst is used. 

Chaumont, C. et al., Scaling up a tubular photoreactor for continuous culture of Porphyridium cruentum from laboratory to pilot plant, m Algal Biotechnology, Stadler, T. et al., Eds., Elsevier, London, 1988, 199. 

Klotz B et al. (1998) Atmospheric oxidation of toluene in a large-volume outdoor photoreactor in situ determination of ring-retaining product yields. J Phys Chem A 102 10289-10299. 

In the vapor phase experiments, the photograftings are carried out in specially designed photoreactor constructed and built in our laboratory (Figure 1). The reactor is equipped with a 1 kW high pressure mercury UV lamp (HPM-15 from Philips) which can be moved to vary the distance to the substrate. The grafting takes place in an atmosphere of nitrogen in a thermostated chamber closed with a clear quartz window. Sensitizer and monomer evaporates from a solution of a volatile solvent in an open bucket which is shielded from the UV-irradiation with aluminium foil. 

Finally, a nice combination of a light-induced Wolff reaction of a diazoketone (e. g., 5-108) with a thermal Cope rearrangement, a light-induced Norrish type I cleavage and a recombination was developed by Stoltz and coworkers (Scheme 5.22) [38]. Here, irradiation of 5-108 at 254 nm in a photoreactor afforded the bicy-... 

There are new ideas and experiments on the rTCA cycle. A group from Harvard University studied some reaction steps in the rTCA cycle which were kept going by mineral photochemistry. The authors assumed that solar UV radiation can excite electrons in minerals, and that this energy is sufficient to initiate the corresponding reaction steps. In this photocatalytic process, semiconductor particles were suspended in water in the presence of a zinc sulphide colloid (sphalerite) the experiments were carried out in a 500 mL reaction vessel at 288 K. Irradiation involved a UV immersion lamp (200-410 nm) in the photoreactor. Five reactions out of a total of 11 in the rTCA cycle were chosen to check the hypothesis ... 

I,, = 0.7 h on silica gel, tA = 2.2 h on alumina and tA = 44 h on fly ash for different atmospheric particulate substrates determined in the rotary photoreactor (appr. 25 pg/g on substrate) (Behymer Hites 1985) direct photolysis tA = 9.08 h (predicted-QSPR) in atmospheric aerosol (Chen et al. 2001). Photodegradation k = 3 x 10-5 s in surface water during the summertime at mid-latitude (Fasnacht Blough 2002)... 

Fixed bed photoreactors, 19 99 Fixed-bed process technology benzene-based, 15 505—506 butane-based, 15 501-502 Fixed-bed reactors, in vinyl chloride manufacture by oxychlorination, 25 640... 

Photobleach mechanism, 19 203 Photobleach reversal grains, 19 201 Photocatalysis, 19 73-106. See also Photocatalysts Photoreactors aqueous pollutants eliminated and mineralized by, 19 89t catalyst modifications in, 19 94-95 catalysts in, 19 75-76 challenges in, 19 101-102 fate of photo-holes in titania, 19 82-85 in fine chemistry applications, 19 102 influence of oxygen pressure in, 19 82 ion doping in, 19 94-95 mass of catalyst in, 19 77-78 noble metal deposit in, 19 94 parameters governing kinetics in, 19 77-82... 

Solar-grade silicon, production of, 22 507-508 Solar heat control, use of gold in, 12 703 Solarization effect, 19 203 Solar photocatalysis, 23 23-24 Solar photocatalytic detoxification, 19 76 Solar photocatalytic processes, 19 100-101 Solar photocatalytic reactor, using deposited titania, 19 99 Solar photoreactors, 19 95-99 Solar salt harvesting, 22 802, 806-808 Solar spectrum, 23 2 Solar still, 26 89-92 Solar thermal converters, 23 10-13 Solar transmittance, for thin films, 23 19 Solatene, 24 558 Solder, 3 53... 

Fig. 2.37. Gradient LC separation of the retinoid solution components and retinoic acid isomers by (A) UV-DAD detection (350 nm) and (B) fluorescence detection with on-line photoreactor switched (a) off and (b) on with irradiation at 366 nm. Peak identification 1 = 13-civ retinoic acid 2 = 9-civ retinoic acid 3 = all-fraws retinoic acid 4 = vitamin A palmitate 5 = /1-carotene. Reprinted with permission from R. Gatti el al. [85].

N. Danesvar, M. Rabbani, N. Modirshahla and M.A. Behnajady, Photooxidative degradation of Acid Red 27 in a tubular continuous-flow photoreactor influence of operational parameters and mineralization products. J. Hazard, Mat. B, 118 (2005) 155-160. 

Photolytic. Fukuda et al. (1988) studied the photodegradation of acenaphthene and alkylated naphthalenes in distilled water and artificial seawater using a high-pressure mercury lamp. Based upon a rate constant of 0.23/h, the photolytic half-life of acenaphthene in water is 3 h. Behymer and Hites (1985) determined the effect of different substrates on the rate of photooxidation of acenaphthene using a rotary photoreactor equipped with a 450-W medium pressure mercury lamp (X = 300-410 nm). The photolytic half-lives of acenaphthene absorbed onto silica gel, alumina, and fly ash were 2.0, 2.2, and 44 h, respectively. The estimated photooxidation half-life of acenaphthene in the atmosphere via OH radicals is 0.879 to 8.79 h (Atkinson, 1987). 

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