Ozone generation

Since ozone is an unstable molecule, ozone has to be generated on-site. The various methods of ozone production differ in their working principles and ozone sources. The methods and their differences are summarized in Table 2-2. 

Method of Ozone generation Working Principle Ozone Source Field of Application 

Electrical Electrical discharge (ED) air or Oz Common standard from laboratory to full-scale 

Electrochemical Electrolysis (EL) water (highly purified) Predominately for pure water applications, laboratory to small industrial scale 

Photochemical p. 185 nm) Irradiation (abstraction of electrons) 02 (air), water (drinking water quality or highly purified) New technology, laboratory to full-scale 

The electro-generation of ozone has recently been reviewed [20, 21]. Ozone is electro-generated by the anodic oxidation of water  

An electrolytic technology for the preparation of ozone was based on glassy carbon as the anode material in concentrated fluoroboric acid (HBF ) [22-24]. The cells also employed a Pt-catalyzed, oxygen reduction cathode (gas diffusion electrode (GDE)) to lower the cell voltage (and hence the energy consumption) and to avoid the need to handle H2 off-gas from the cathode. It was necessary to operate with cooling, and the preferred temperature was 268 K. 

Temperature 268 K. Current density 0.4 A cm Data taken from [23]. 

Doped diamond is the alternative carbon material to glassy carbon for ozone generation [20,21], but the technology is much less developed. Boron-doped diamond is prepared by chemical vapor deposition and has been used as an anode when (i) deposited as a thin film on substrates such as silicon, titanium, or niobium or (ii) fabricated as a bulk plate - for example. Element Six supplies Diafilm EP as a free-standing polycrystalline plate capable of supporting current densities of 1 Acm [25]. Typically, the boron-doping concentration is 10 °-10 atoms cm when its resistivity is of the order of 0.05 cm. These boron-doped 

A cell with a boron-doped diamond film on an n-Si(lll) plate has also been used to generate ozone in the gas phase [29]. A simple undivided, parallel plate cell with sulfuric acid electrolyte was employed. The diamond anode was shown to give more stable performance than Pb02. When operated at 2.3 A cm , it gave a cell off-gas with 0.8% ozone for a period of 3000 h. 

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O ne. Air pollution (qv) levels are commonly estimated by determining ozone through its chemiluminescent reaction with ethylene. A relatively simple photoelectric device is used for rapid routine measurements. The device is caHbrated with ozone from an ozone generator, which in turn is caHbrated by the reaction of ozone with potassium iodide (308). Detection limits are 6—9 ppb with commercially available instmmentation (309). 

Commercial production and utilization of ozone by silent electric discharge consists of five basic unit operations gas preparation, electrical power supply, ozone generation, contacting (ie, ozone dissolution in water), and destmction of ozone in contactor off-gases (Fig. 1). 

Ozone Generation from Oxygen. Oxygen is dissociated iato atoms by iaelastic coUisions with energetic electrons (6—7 eV) (89,90). 

Ozone can be destroyed thermally, by electron impact, by reaction with oxygen atoms, and by reaction with electronically and vibrationaHy excited oxygen molecules (90). Rate constants for these reactions are given ia References 11 and 93. Processes involving ions such as 0/, 0/, 0 , 0 , and 0/ are of minor importance. The reaction O3 + 0( P) — 2 O2, is exothermic and can contribute significantly to heat evolution. Efftcientiy cooled ozone generators with typical short residence times (seconds) can operate near ambient temperature where thermal decomposition is small. 

Ozone Generator Design. A better understanding of discharge physics and the chemistry of ozone formation has led to improvements in power density, efficiency, and ozone concentration, initiating a trend toward downsizing. 

Ozone. Ozone generators are based on uv or silent discharge. Although uv ozone generators are marketed, they are not effective for treating... 

The mechanism for ozone generation is the exeitation and aeceleration of stray eleetrons within the high-voltage field. The alternating eurrent causes the electron to be attracted first to one electrode and then to the other. As the electrons attain sufficient velocity, they beeome capable of splitting some oxygen moleeules into free radical oxygen atoms. These atoms may then eombine with Oj moleeules to form O3. 

The mechanism for ozone generation is the excitation and acceleration of stray... 

Contactor design is important in order to maximize the ozone-transfer efficiency and to minimize the net cost for treatment. The three major obstacles to efficient ozone utilization are ozone s relatively low solubility in water, the low concentrations and amounts of ozone produced from ozone generators, and the instability of ozone. Several contacting devices are currently in use including positive-pressure injectors, diffusers, and venturi units. Specific contact systems must be designed for each different application of ozone to wastewater. Further development in this area of gas-liquid contacting needs to be done despite its importance in waste treatment applications. In order to define the appropriate contactor, the following should be specified ... 

A typical ozone system consists of 100 g/hr at a concentration of 1.0 percent to 1.5 percent in air fed to the bottom of bleach collection tanks through ceramic spargers (pore size of approximately 100 t). The system contains air compression and drying equipment, automatic control features, and a flat-plate, air-cooled ozone generator. Regeneration of bleach wastes totaling about 10,000 gallons a year, and recovery of other chemicals can also be cost effective. 

Ozonation systems are comprised of four main parts, including a gas-preparation unit, an electrical power unit, an ozone generator, and a contactor which includes an off-gas treatment stage. Ancillary equipment includes instruments and controls, safety equipment and equipment housing, and structural supports. The four major components of the ozonation process are illustrated in Figure 8. 

Electrical power supply units vary considerably among manufacturers. Power consumption and ozone-generation capacity are proportional to both voltage and frequency. There are two methods to control the output of an ozone generator vary voltage or vary frequency. Three common electrical power supply configurations are used in commercial equipment ... 

Figure 9 shows the details of a typical horizontal tube-type ozone generator. This unit is preferred for larger systems. Water-cooled plate units are often used in smaller operations. However, these require considerably more floor space per unit of output than the tube-type units. The air-cooled Lowther plate type is a relatively new design. It has the potential for simplifying the use of ozone-generating equipment. However, it has had only limited operating experience in water treatment facilities. 

Temperature, pressure, flow rate, and ozone concentration of the ozonecontaining gas being discharged from all the ozone generators. This is the only effective method by which ozone dosage and the ozone production capacity of the ozone generator can be determined. 

Power supplied to the ozone generators. The parameters measured include amperage, voltage, power, and frequency, if this is a controllable variable. 

Flow rate and temperature of the cooling water to all water-cooled ozone generators. Reliable cooling is important to maintain constant ozone production and to protect the dielectrics in the generation equipment. 

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