The Efficiency Increase of Equipments for Treatment in High Intense Electric Fields with Abundant Ozone Generation

The Efficiency Increase of Equipments for Treatment in High Intense Electric Fields with Abundant Ozone Generation

Ilie Suran1, Sorin Budu1, Octavian Daniel Oros1, Roman Morar1, Simona Ghizdavu Pellascio2, Letiia Ghizdavu3, Ioan Ovidiu Muntean4

1 Technical University of Cluj-Napoca, 400020, 15 C. Daicoviciu Street, Cluj-Napoca, Romania; 2 French College, 8 Debarcadere Street, 2502 Bienne, Suisse; 3 - Babe-Bolyai University of Cluj-Napoca, 1 Koglniceanu Street, Romania; 4 University of Petroani, Romania. Email: ilie_suarasan@yahoo.com; Abstract: The paper presents some results of experimental research work referring to the possibility of increasing the ozone electrosynthesis efficiency in liquid solutions; direct treatment cells using high intense electric fields were developed. Directly corona or corona-electrostatic type treatment cells, provided with punctiform or multiple thin wires corona discharging electrodes, feeded by A.C. and D.C. power supplies were analyzed. Evaluation of ozone electrosynthesis efficiency analyze by the VA characteristics and corona discharging current, was strongly correlated with the configurations of corona discharging elements able to generate the most uniform and abundant ionizations of the gaseous discharging gap. Measurement of the ozone quantity and concentrations generated by different corona configurations was made by iodometric method upon the present ozone in aqueous solution treated by corona discharging. Ozone quantity depends by: average and polarity of the applied high voltage, exposure period, width of the liquid layer and treatment cells configuration, using different length of corona discharging gap and different distances between the corona discharging elements. Increased efficiency of the treatment using the remanent ozone, directly applied in aqueous solutions among with the presence of free hydroxyl radicals.

Key-words: Efficiency of ozone generation; high intense electric fields.

1. Introduction

The ozone is, after fluorine, the most powerful oxidant element known; hes used as disinfectant in many industrial applications. Ozone is considered as an ideal disinfectant choice for an ecological treatment of polluted environments because hes use is neither generating toxic residues either wastes. Generally, the ozone production at industrial scale is performed through electrical discharges in Siemens ozonators.Ozonization by direct exposure of liquids in high intense electric fields present some remarcable advantages: flexibility and large diversity of the application field; major investments are not needed, such as for Siemens ozonization equipments; amazing simplicity of the application; very low energy consumption; presence of some free radicals which are effective factors in the pollutants oxidation/neutralization processes and also to the biostimulation of useful processes inside the living environment, etc.The paper aim is to obtain the increase of direct treatment cells in high intense electric fields efficiency; with abundant ozone generation by studying and analyzing of: current voltage characteristics, current density distribution and residual ozone concentrations in the treated liquids. Electric parameters which influence the ozone concentration in the treated aqueous environments are the level and frequency of the high voltage, exposure time and the width of the treated liquids layer. In order to obtain abundant and continuous ozonization, the corone current at the surface of the treated liquid must have high values, able to determine the ozone electrosynthesis and an uniform distribution of it. Coulombian electrostatic forces also influence the corone current distribution at the surface of treated liquid. 2. Material, methods and equipments

Elementary cells for direct treatment of liquids are composed by a high voltage supply, a Petri dish and two electrodes, (Figure 1).

a. The cell provided with an active electrode, having one, two, or three corona discharge elements peaks.b. The cell with multiple punctiform discharge, provided with a "brush type" active electrode.

Figure 1. The constructive elements of corona discharge cells; 1 - A. C. high voltage power supply; 2 - Petri dish containing the liquid; 3 - active electrode; 4 - liquid; 5 - metallic piece for the contact between the liquid and the grounded electrode; 6 plate electrode connected to the earth.

2.1. The stand for the current-voltage characteristics

From the plotting of the experimental V - A characteristics of the directly ozonizing liquids cells, it has been provided the electrical scheme for the high - voltage, respectively of the current, (Figure 2) and in the generally conditions - presented in Table 1, specific to each characteristic.Figure 2. The electric scheme used to realize the V-A characteristics of the directly ozonizing liquids cells; P1, P2 - multimeters; R1 additional resistor; R2, R3 - shunts; F1, F2, F4, F5 - gas spark gaps; F3, F6 - mechanical spark gaps.Table 1. The experimental conditions of the performed tests.

Condition

Value

Voltage applied to electrodes

0 - 20 kV

Type of active electrodes

Fix corona discharge points

Thickness of disruptive interspace

s = 20 mm, adjustable

Nr. of corona discharge points

1 - 2 -3 - and brush type

Steps of the discharge points

p = (1 - 2,5 - 5 - 7,5 - 10) mm

Duration of the corona discharge

(30 - 60) sec.

Type of discharge

With, without dielectric barrier

Absorbent liquid value

70 ml, normal drinking water

2.2. The stand used for the study of the corona current density distribution

The covering degree of the liquid surface with charge carriers resulting from ionic bombing, carriers that are determining the ozone electrosynthesis process during the treatment of different liquids inside direct treatment cells, will be shown in the experimental stand (Figure 3, 4) and in specific measurement conditions. Figure 3. Experimental stand for determining the electric field density distribution at the treated liquid surface.Figure 4. Measurement scheme of the current density distribution at the treated liquid surface; P1 multimeter, type MAVO 35; P2 ballistic galvanometer Tesla, type DG 20; R1, R2 shunts; F1, F2, F4, F5 gas spark gaps; F3, F6 mechanical spark gaps; V1, V2 rectifier monophased bridges; V3 V6 protecting diodes; C1, C2 - protection capacitors.

2.3. Ozone concentration determination in different corona cells configurations

In order to analyze and to compare the generated ozone quantity in corona discharges, tests were performed within the cells presented in Figure 1, under the general experimental conditions presented in Table 1, but specifically adapted to each test type.

The potassium iodide titration method, used to determine the generated ozone quantity through corona discharges is further described:

70 ml of a 2% potassium iodide solution is introduced in a Petri dish;

Discharges were produced at the liquid surface, within the following experimental conditions: different types of discharge cells; variable maintenance time; variable intensities for the corona discharge field; different solution thickness; The lode in iodide solution is acidified (5 ml H2SO4, 2N) and is titrated with a 0.001N Na2S2O3 solution up to yellow reaction, 3-4 drops of a 0.5% starch solution is added and the titration is continued up to complete loss of colour.

The calculation is performed according to the employed Na2S2O3 volume.

3. Results and discussion

3.1. Current-voltage characteristics

The corone current deppends by the level of the high voltage, presence or absence of the dielectric barrier, (Figure 7) and by the average of the steps between the corone discharging elements, (Figure 9).

Figure 7. The current - voltage characteristic of the cell with active electrode provided with three corona discharging peeks. Experimental conditions: the discharging interspace s = 20 mm; the distance of the punctiform discharges p = 10 mm and p = 20 mm.Figure 9. The evolution of the corona discharging current in dependence to the punctiform discharging step in the directly ozonizing liquids cell. Experimental conditions: active electrode with three corona peeks; the discharging interspace s = 20 mm; high - voltage U = 20 kV = ct.

The volt - ampermetric characteristic of the directly ozonizing liquid cells, equipped with a high - voltage active electrode, containing two or three punctiform corona discharging elements, where the ratio U/s had been preserved constant, is depicted in Figure 10, and in the Figure 11 the current voltage characteristic attached to an active multiple brush type electrode.

The presence of a large number of variables that influence the corona discharge current like: the external factors (pressure, temperature, humidity, air composition, etc.), the work parameters (level, frequency, voltage form, etc.), as well as the constructive cell differences (cell type, different discharge interspaces, etc.) increase the sensibility and the difficulty of interpreting the current-voltage characteristics. The direct liquid treatment cell current-voltage characteristics are similar to those of the Siemens ozonizers with dielectrically barrier and present a similar integration () shape.

Figure 10. The current - voltage characteristic of the directly ozonizing liquids cell. Experimental conditions with an active electrode provided with two, respectively three punctiform corona discharging peeks; the discharging interspaces s = 20 mm; the distance of the punctiform discharges p = 10 mm and p = 20 mm; the report U/s = 10 kV/cm = ct.Figure 11. The current - voltage characteristic of the directly ozonizing of the stationary liquids in the cell equipped with the high - voltage active electrode with multiple punctiform discharges, brush type. Experimental conditions: discharging interspace s = 30 mm, without dielectric barrier.

For the liquid direct treatment cells it is of interest only the situation comprises between the air ionization advent inside the gaseous interspace and the corona discharge degeneracy into sparks or electrical arch.

The strongly nonlinear nature of the gaseous interspace is emphasized by the afferent current-voltage characteristics of the corona discharge cells provided with two or three punctiform element, where, despite the fact that the U/s ratio = 10 kV / cm, (Figures 8 and 9) was kept constant, the current showed significant variation.

3.2. Corona current density distribution

The difference between the theoretical aspect given by the Wartburg formula and the practical current density, in account with the distance x -perpendicular is presented in Figure 13, and in the Figure 17 on the longitudinal y axis.

Figure 13. Current density variation with x distance, (1 punctiform corona element, perpendicular direction).Figure 17. Current density variation with y distance, (1 punctiform corona element, longitudinal direction, continuous current with negative semialternancy, with dielectrically barrier).

The strongly nonlinear shape of the corona discharge is emphasized by the quick weakening of the charge carriers (shown by the current density variation) when moving off of the punctiform vertical.

3.3. Ozone concentration in various corona cell configurations

Ozone generation in electrical discharges is a complex mechanism that is, until now, not fully understood, explored and studied; there are many chemical reactions that can lead to ozone generation. Figure 18 presents the generated ozone concentration inside a punctiform, single corona discharging cell, as a function of the level of the high voltage (a) and of the exposure time in the current (b).

a. b.

Figure 18. The effect of the high field level on the generated ozone concentration (a) and of the exposure time in the corona discharge field (b). Experimental conditions: 1 corona punctiform discharge element, s = 20 mm, exposure time: 60 sec (only for a), U = 20 kV (only for b).

The variation of ozone concentration in punctiform corona discharge cells depending of the density of the emissive tops is shown in Figure 19.

a. Corona discharge exposure time texposure = 30 s.b. Corona discharge exposure time texposure = 60 s

Figure 19. Direct relationship between the generated ozone concentrations in the corona discharge field as a function of the emissive punctiform density. Experimental conditions: 2 or 3 corona punctiform elements, discharge interspace s = 20 mm, with or without dielectrically barrier, U=20 kV.

If the average intensity of the discharge corona field is kept constant, "Emed" = U / s = 10 [kV / cm], the ozone concentration variation as a function of the high voltage applied to the ozonization cell would have the shape represented in Figure 22. Figure 23 presents the residual ozone variation in the potassium iodide solution, treated in the corona discharge cell containing three punctiform elements placed at p = 10 mm and with an s = 20 mm interspace, with dielectrically barrier, alimented at 20 kV as a function of the width of the exposed liquid layer. The direct relation between the treated liquid volume (QKI) and the layer width (h) is presented in Table 3.

Figure 22. Direct relation between the generated ozone concentrations at high voltage applied to the ozonization cell. Experimental conditions: three element punctiform corona electrode, with step p = 10 mm, with dielectrically barrier, texposure= 60 sec., U / s = 10 [kV / cm].Figure 23. Correspondence between the generated ozone concentrations, as a function of the ozone exposed liquid. Experimental conditions: three element corona punctiform discharge electrode, with p = 10 mm, s = 20 mm, with dielectrically barrier, texposure= 60 sec., U = 20 kV.

The aqueous solution remanent ozone dependency as a function of the corona element steps, corresponding to a constant corona discharge interspace, is shown in Figure 24.

Figure 24. Dependence of the generated ozone concentration on the variation of the punctiform corona element steps. Experimental conditions: electrode with two punctiform corona discharge elements, with s = 20 mm, with or without dielectrically barrier, texposure= 60 sec., U = 20 kV, 50 Hz.

4. Conclusion

Low intensities of the corona discharge electric fields leads to weak air ionization processes and thorough to low generated ozone quantities.

In punctiform corona discharge, the generated ozone quantity depends on the length of the exposure period and on the level of high voltage applied to the direct treatment cell. The main factor in the increase of the ozone electrosynthesis efficiency is the number of charge carriers, hence the corona discharge current.

At identical alimentation voltage values applied to the direct treatment cells, we observe that the absence of the dielectrically barrier leads to high remanent ozone quantities in the treated solution. Due to the dielectrically barrier, the corona discharge gains a discrete character, as long as the barrier dielectrically rigidity is not over passed. The barrier has also a standardizing role in distribution of the current density at the liquid surface.

The maximal current density at the liquid surface is aligned with the axe of the punctiform discharge element and depends on the voltage value, on the corona discharge gap as well as on the dielectrically barrier existence. The fast reduction in current density at the liquid surface, in case of the corona punctiform discharge, shows the strong nonlinear character of the gaseous gap/interspace.

The overlap between the electrostatic field due to the metallic support of the active electrode and the corona field of the punctiform elements leads to additional changes in the current density when compared with the theoretical Wartburg formula. So the ozone electrosynthesis efficiency increase is obtained through standardization at high end density values of the corona current due to the use of an electrostatic support, with the shape of a metallic disc and of the same size as the Petri dish.

For a given surface, the density increase of the corona discharge punctiform elements leads to an insignificant increase of the generated ozone concentration. For low density corona discharge punctiform surface, at places where the interaction between the generated charges from the neighbouring elements is weak, the ozone generation will show a significant increase. This ozone generation increase, when cumulated with liquid short exposure times will provide a maximal efficiency of the ozone electrosynthesis, at a proportion between the discharge interspace size over the corona element step equal with s / p = 1, , 2; the same behaviour was observed for layer flow cells.

The treated liquids show an inversely proportion residual ozone concentration with the width of the treated liquid layer, thus a thin layer treatment of the highly contaminated liquid is recommended.

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