Electrolytical ozone generator

Table 2-3 Characteristic operating parameters of electrical discharge and electrolytic ozone generators.

In the electrolytic ozone generator (ELOG) ozone is produced from the electrolysis of high purity water (Figure 2-4). In the electrolytic cell, water is split into molecular hydrogen H2... 

Figure 2-4 Working principle of the Electrolytic Ozone Generator (ELOG) (after Fischer, 1997).

Electrolytic ozone generators are supplied by several producers. The ozone production capacity of one cell is between 1-4 g03 h , but several cells can be combined in one generator. The principle dependency of the ozone production capacity (of one cell) on the voltage, the current and temperature is shown in Figure 2-5. 

Figure 2-5 Dependency of the ozone production capacity in an electrolytic ozone generator (ELOG) on the applied voltage and current (with cell temperature as a parameter) (from Fischer, 1997).

M. Katoh, Y. Nishiki and S. Nakamatsu, Polym. electrolyte-type electrochemical ozone generator with oxygen cathode, J. Appl. Electrochem., 1994, 24, 489-494 ... 

An electrolyte-free system for ozone generation using heavily boron-doped diamond electrodes. Diamond Relat. Mater., 40, 7-11. 

Kraft A, Stadelmann M, Wiinsche M et al (2006) Electrochemical ozone generation using diamond anodes and a solid polymer electrolyte. Electrochem Comm 8 883-886... 

In practice, such ozone cells are extensively automated to control the process cycle (Fig. 5.14(b)) and may incorporate ultraviolet facilities. Current applications include those sectors of the pharmaceutical and fine-chemicals industry which require ultra-pure water for critical synthetic steps or to meet stringent purity limits. In such applications, the electrolytic route to ultra-pure water, via ozone generation, is competing against the more traditional methods of sterile filtration (e.g. ultra-filtration), ultraviolet irradiation and the use of conventional air-phase corona-discharge ozonizers. 

Fig. 22.2. Effect of electrolyte concentration and current density on ozone generation characteristics.

Fig. 22.3. Effect of electrolyte temperatiure on ozone generation in 0.1 M H2SO4 solution at a current density of 1.3 A cm. ...

Generally, lead dioxide (Pb02) and platinum (Pt) electrodes are used as electrocatalysts for ozone generation [2-4]. The electrolytic cell consists of a porous anode, a porous cathode and a solid-state polymer electrolyte membrane instead of an electrolyte solution these are stacked, as shown schematically in Fig. 24.1(a). Pure water, or tap water without additives as an electrolyte, is directly supplied to the anode compartment, the electrolysis of water occurs, and the electrolyzed water containing dissolved ozone is directly drained. Electrolytic ozonizers based on this system have already become available on the market. 

Fig. 24.1. Schematic diagrams of (a) an electrolytic ceU for ozone generation and (b) a flow system to generate ozone-water using the electrolytic cell with a Pb02 electrode.

The application of diamond electrodes to ozone generation has already been reported, for work in which a conventional one-compartment electrolytic cell was used with sulfuric acid solutions as the electrolyte [5,6]. Recently, an ozone generation system with a diamond electrode set in a thin-layer electrolytic cell was developed in this cell, the anode and cathode lie in parallel, the electrolyte solutions flow between them, and the electrogenerated ozone gas is collected [5]. 

Fig. 24.6 represents the results of the durability test of this electrolytic ozone-water generation system with the diamond electrode. Even though data points are not represented continuously in the figure, the water electrolysis was performed continuously. The concentrations of ozone-water were ca. 1.0, ca. 1.8 and ca. 3.0 ppm at applied currents of 6, 10 and 15 A, respectively. Practically usable electrolyzed ozone-water with sufficient concentration and volume was continuously produced with this system. 

Another significant reaction on BDD anodes is the generation of ozone from OH (Babak et al. 1994 Michaud et al. 2003). Reduced solubility and high reactivity are responsible for the rare detection of ozone in the liquid phase. If ozone quickly disappears, radical formation from H202 and ozone in the bulk electrolyte [(7.52) and (7.53)] is negligible. 

Based on these early studies, two current approaches have evolved for generating ozone electrolytically. One of them, shown in Fig. 13, uses glassy carbon anodes, a specialized electrolyte, tetrafluoboric acid (HBF4), and an air cathode [66]. At a current density of 400 mA/cm, 35 vol Vo of ozone was obtained from 48 wt.% HBF4 maintained at -5 to 0°C, with a cell potential of 3.2-3.4 V vs. NHE. The ozone gas formed within the cell was immediately diluted with air to lower the ozone concentration to 15 wt.%, a value below explosion limits. The electrodes were not attacked at these current densities and in the presence of this electrolyte. 

Ozone was first discovered accidentally by electrolysis of sulfuric acid in 1840 . Since then electrolytic generation of ozone has developed rather slowly, owing to the relatively low current efficiencies that have been observed at practical operating temperatures. 

Asea Brown Boveri of Switzerland has commercialized the MEMBREL process for the electrolytic generation of ozone. The synthesis takes place in a cell whose anodic section is made of titanium and the cathodic section of stainless steel. A Nafion membrane (Du Pont) acts not only as the electrolyte for the system, but also as the separator. This membrane is sandwiched between the anode (lead dioxide) and the cathode (platinum). Electrolysis takes place at a total cell voltage of 3-5 V with a current density of 0.5 - 2.0 A/cml The corresponding cathodic reaction is hydrogen evolution. 

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