RU2211800C2 - Ozone generation process and apparatus - Google Patents

Abstract

FIELD: chemical engineering. SUBSTANCE: process and apparatus are designed for generation of ozone form oxygen-containing gas. The latter is affected by pulse volume corona discharge having following characteristics: 0,014≤i_m≤i_s, where i_m is current amplitude and i_s maximum amplitude of pulse corona discharge current; 5×10³B≤U_m<U_s, where U_m is voltage amplitude and U_s voltage amplitude of irreversible passage to spark discharge within discharge space with pulse corona discharge; t_i≤3×10^-4s, where t_i is pulse duration provided that shape and duration of voltage and current pulses in discharge space may differ; dU/dt≥10⁸W/s, where dU/dt is voltage growth rate; and 0,01s^-1f_i≤0,3/t_i, where f_i is pulse succession frequency. Apparatus comprises high-voltage pulse generator and working chamber connected thereto accommodating electrode system in the form of coaxial electrode couple consisting of external low-voltage tubular electrode and internal high-voltage rod-shaped electrode. On the rod of internal electrode, in planes perpendicular to its longitudinal axis, conducting plates are additionally fixed. Distance L from the border of each plate to internal surface of tubular electrode follows the relationship: U_s/E_a<L<3×U_s/E_a, where E_a is average voltage of electric field in discharge space at voltage amplitude U_s, distance d among adjoining plates follows the relationship d_min<d<2×L with d_min being shielding distance; transverse dimension of plate D follows the relationship: ≤2×D_r<D = (D_T-2×L), where D_r is rod width and D_T transverse dimension of external electrode; and bending radius of plate border r follows the relationship: r<L, r≤δ/2 wherein δ is plate thickness. EFFECT: increased ozone generation efficiency. 8 cl, 1 dwg

Description

 The group of inventions relates to methods and devices for the physical generation of ozone from oxygen-containing gas, in particular to electric-discharge ozone generators, and can be used for bactericidal treatment of equipment, air in domestic and industrial premises, in agriculture for presowing treatment of seeds, for bactericidal treatment during long-term storage various agricultural products, including grain, seeds, vegetables, fruits, etc., water in utilities, for example, in installations for the preparation of drinking water and water swimming pools, etc.

 Known methods for producing ozone, based on the decomposition of oxygen-containing gas in discharge systems with a dielectric barrier, are described, for example, in SU 941276 A1 (C 01 B 13/11, 07/07/1982), SU 1774585 A1 (C 01 B 13/11, 15.05 .1994), SU 17753535 A1 (С 01 В 13/11, 11/15/1992). In accordance with them, the oxygen-containing gas is purified, drained, cooled and passed through the discharge zone, in which, when a high pulse voltage is applied, a high voltage pulse discharge is initiated.

 The ozone generators with a dielectric barrier are widely known (see ibid.). The main nodes of these devices are a high-voltage pulse generator and a working chamber connected to it with electrodes, one of which is covered with a dielectric layer.

Their common disadvantages include:
- the presence of a barrier - the element of ozonation installations most often damaged in operation during the transition of a barrier discharge to a spark discharge, and this transition is detrimental to an ozone generator, because a spark irreversibly violates the integrity of the barrier;
- the dependence of the electrical characteristics of barriers on moisture and pollution, their change during operation, a strong effect on gas discharge processes and ozone productivity; and therefore, the need for mandatory purification of the working gas in the discharge gap from dust and moisture;
- small discharge area and performance due to the low density of the discharge current;
- the need for cooling, because in narrow discharge gaps there is a strong dependence on temperature (with increasing temperature, ozone decomposes or its production decreases);
- the probability of refrigerant leakage into the environment;
- the need for high-precision manufacture of electrodes (their relative position and application of barrier insulation), special requirements for their surface reduce the manufacturability and increase the cost of it;
- the need to purge air through a narrow gap between the electrodes increases the cost of energy for maintenance and reduces efficiency;
- bulkiness.

Closest to the invention of the known methods for generating ozone is the method described in USSR author's certificate 941276. It consists in cleaning, moisture separation, cooling of atmospheric air and passing it through the discharge zone, in which, when a high pulse voltage is applied, a high voltage pulse discharge between the electrodes is initiated. The impact is produced by a high-voltage pulse discharge with a voltage amplitude of 10-600 kV, a pulse duration of (1.2-200) • 10 ^-6 s, a voltage rise rate of 10-10 ⁵ kV / μs and a duty cycle between pulses of 0.9-10 ^-3 - 1.12 s. In the description of the prototype is also mentioned the heterogeneity of the field as a drawback of a technical solution from a source of information taken into account during the examination, from which we can conclude that the decision on A. with. USSR 941276 field is uniform.

 The disadvantages of the prototype should be attributed primarily to the fact that with the parameters of the pulses of the high-voltage discharge indicated in the solution description, not only effective, but also any significant generation of ozone is not necessary, since these parameters are characteristic of a corona discharge with a low (initial) surface tension corona electrode, and for spark discharges. Spark discharges for producing ozone are inefficient; in them a certain amount of ozone (very small) can be generated only at the stage of spark discharge formation, when streamers, leaders, at whose heads the electric field is sharply inhomogeneous, grow to a highly conducting channel stage. However, a pulsed discharge in a non-uniform field is considered by the authors of the prototype to be ineffective for generating ozone.

 The considered method also does not allow to obtain acceptable performance due to the long pulse durations, namely, short durations (1 microsecond or less) are most effective for producing ozone.

The implementation of this method is also associated with significant difficulties associated with the fact that it requires a preliminary air preparation, including cleaning, moisture separation and cooling, to achieve concentrations of 10-20 g / m ^{3 of} ozone. At the same time, part of the energy expended is spent on auxiliary operations, which increases the cost of ozone produced.

 The closest known ozone generators is the device described in an article by Amirov R.Kh. and others. "Ozone synthesis initiated by a nanosecond corona in air" (see. "High Energy Chemistry", vol. 26, 1, 1992, p. 76).

The device contains a high-voltage pulse generator and a working chamber connected to it with an electrode system in the form of coaxial external low-voltage tubular electrode (steel pipe with a diameter of 20 cm, length 105 cm) and an internal high-voltage electrode in the form of a conductive rod (wire with a diameter of 0.25-1, 55 mm.)
The described device allows to increase the specific productivity of ozone in comparison with analogues by increasing the active discharge power. The latter possibility is provided by the exclusion of the barrier and the danger of its breakdown. However, a further increase in the productivity of the device runs into a technical contradiction, namely: on the one hand, it is necessary to increase the surface of the corona electrode, on the other hand, to maintain a sharp field inhomogeneity, and hence a sharp inequality of the radii of the electrodes. In addition, a decrease in the characteristic transverse size of the corona electrode is limited by its mechanical strength.

 The basis of the group of inventions is the task of creating a method and device to increase the efficiency of ozone production by initiating a volume overvoltage pulsed corona discharge in the discharge barrier-free gap.

The problem is solved in that in the method for generating ozone, including the effect of a high voltage pulse discharge on an oxygen-containing gas, in accordance with the invention, the effect is produced by a volume pulsed corona discharge with the following characteristics:
the amplitude of the current i _m 0,01A≤i _m ≤i _s , where i _s is the largest current amplitude of the pulse corona discharge,
voltage amplitude U _m 5x10 ³ V≤U _m <U _s ,
pulse durations t _and t _and <3x10 ^-4 s, and the shape and duration of voltage and current pulses in the discharge gap may differ,
voltage slew rate dU / dt 10 ⁸ V / C≤dU / dt,
the pulse repetition frequency f _cl 0.01 s ^-1 ≤f _slab ≤0,3 / t _u.

 The impact can be carried out on an oxygen-containing gas under pressure above atmospheric.

 Objects to be treated with ozone can be placed inside the working chamber.

The problem is also solved by the fact that in the device for generating ozone containing a high-voltage pulse generator and a working chamber connected to it with an electrode system in the form of coaxial external low-voltage tubular electrode and an internal high-voltage electrode in the form of a conductive rod, according to the invention, on an internal electrode rod in planes perpendicular to its longitudinal axis, conductive plates are additionally installed, the distance L from the edge of each plate to the inner surface and the tubular electrode satisfies the ratio
U _s / _Ea <L <3U _s / E _and,
the distance d between adjacent plates - the ratio
d _min <d <2xL,
characteristic lateral plate size D - relation
1 mm <2xD _with <D = (D _T -2xL),
the characteristic radius r of the rounded edges of the plates - relations
r << L and r≤δ / 2,
where D _c is the characteristic transverse size of the rod, D _T is the characteristic transverse size of the external electrode, d _min is the screening distance, U _s is the voltage amplitude of the irreversible transition to the spark discharge in the discharge gap with a pulsed corona discharge, E _a is the average electric field strength in discharge gap with a voltage amplitude of U _s , δ is the plate thickness.

 It is advisable to additionally introduce a high-voltage switch located between the high-voltage pulse generator and the working chamber.

 The plates can be made in the form of disks.

 Each of the plates may contain two external and one internal disk located between them, while the diameter of the internal disk is larger than the diameter of the external.

 At least one pair of coaxial electrodes can be additionally introduced into the electrode system of the device.

 The method of ozone generation is based on a new type of corona discharge - volume overvoltage pulsed corona discharge. Its characteristic feature is the development of luminescence in the entire volume of the working chamber between the high-voltage (corona) and low-voltage (external) electrodes, and not only near the corona electrode. In this case, the luminescence volume itself is significantly increased due to the parameters of the pulses, and the gas is processed inside the “cover” of the corona, which occupies the entire discharge gap. The discharge may contain many (incomplete) streamer channels (up to a quarter of the length of the discharge gap). A similar discharge arises as a result of the combined action of a number of factors.

 The above parameters of the initiating pulses are selected from the following considerations.

The amplitude of corona current pulses is a central parameter for ozone production. It (production) grows the more, the more current. In this case, the current amplitude is limited from above by an irreversible transition to spark and arc discharges, since after the formation of an irreversible spark, and then the arc, all the pulse energy is concentrated in the spark and arc channels, and the production of ozone practically ceases. It should be noted the possibility of reversible spark discharges during the implementation of an overstressed pulsed corona discharge with a pulse repetition rate f _SL . Reversible spark discharges indicate the proximity of the current amplitude i _m in the overstressed corona to the limiting value i _m = i _s .

Thus, the characteristics indicated in the prototype are not enough for the efficient production of ozone. It is believed that (see Reiser Yu.P. "Gas Discharge Physics", Moscow, "Nauka", 1987, pp. 511-512) with current amplitudes greater than 0.01 A, an irreversible transition to spark and arc discharges occurs, however in the invention, this limit is pushed back to areas previously unattainable (the peak current in the corona discharge according to the invention can significantly exceed 0.01 A), primarily due to the use of the pulse mode. Moreover, such currents are not only fundamentally possible, but also the most effective, ceteris paribus. Below the indicated amplitudes, the efficiency of ozone production drops sharply (experimental data). In the case of a large length of the electrode system along its longitudinal axis (more than 1 m), the role of the current amplitude i _m can be played by the current amplitude per 1 linear meter along this axis, i.e. current amplitude per unit length.

The amplitude of the voltage U _m selected 5x10 ³ V≤U _m <U _s .

With increasing amplitude of the voltage of the incident pulse, the volume of the corona discharge increases, the distance L between the edge of the corona electrode and the surface of the (internal) low-voltage electrode, the energy absorbed in the corona discharge, and, consequently, the output (amount) of generated ozone increases. The upper limit of the voltage amplitude is limited only by the currently existing technical capabilities, but does not reach the voltage U _s at which an irreversible transition to spark and arc discharges occurs. The lower boundary of the voltage amplitude corresponds to extremely small discharge gaps (L≈1 mm), at which it becomes difficult to obtain a sharply inhomogeneous field and corona discharge.

The lower limit of the pulse duration (currently ≈10 ^-9 ÷ 10 ^-10 s≤t _and ) is due to the current capabilities of switching devices and may change with their change, and the upper one - a decrease in the efficiency of ozone production.

The selected pulse durations and the rate of rise allow the corona discharge to develop over the entire length of the discharge gap, but do not lead to irreversible sparking and arcing, and also do not allow to reduce the current amplitude in the corona discharge and, therefore, do not allow to reduce the efficiency of ozone production. From the literature it is known (Amirov R.Kh., Asinovsky E.I., Samoilov I.S., Shepelin A.V. "Ozone synthesis initiated by a nanosecond corona in air", "High-energy chemistry", v.26, 1, 1992, p. 76), that a decrease in the duration of the voltage pulse and an increase in the steepness of the front increase the dissociation rate and, consequently, the concentration of ozone, therefore, by forming the pulse duration, they prevent the corona discharge from developing into a plasma channel (contracted) spark stage. The upper limit of the slew rate is limited by the current level of technology and is currently ≈10 ¹⁵ V / s, and the lower limit is a significant decrease in the efficiency of ozone production.

 For short pulse fronts, wave processes should be taken into account. In this case, the working chamber is a load with distributed parameters.

 The expansion of the pulse repetition frequency range was carried out in comparison with the prototype. From the side of high repetition frequencies, the range is limited by the transition of the corona discharge into the spark and arc ones, and from the side of the lower ones, by a significant decrease in ozone production.

 Exposure to an oxygen-containing gas at a pressure higher than atmospheric allows an additional increase in operating voltage. The increasing probability of a transition to a spark discharge is compensated by the short pulses.

Due to the possible large value of the discharge gap, which depends only on the ratio U _s / Е _a , it becomes possible to place the objects to be treated with ozone inside the working chamber and not only make better use of the obtained ozone itself, but also include the whole complex of factors in the processing: field, radiation, corona discharge plasma and products generated in this plasma, including ozone.

The presence of additional conductive plates on the internal high-voltage electrode, the method of their installation and the ratio of characteristic sizes can increase the effective length of the corona electrode without increasing, on the one hand, its length along the longitudinal axis of the electrode system (device dimensions) and, on the other hand, the radius of curvature edges of the corona electrode (and therefore, without thereby reducing field inhomogeneities.)
The length of the discharge gap (the distance L from the edge of each plate to the inner surface of the outer tubular electrode) satisfies the relation
U _s / _Ea <L <3U _s / _Ea
i.e., it depends only on the ratio of the amplitude U _{s of the} pulse voltage at which the irreversible transition to the spark and arc discharges occurs, and the average electric field strength E _a in the discharge gap, which means that it can be significantly increased in comparison with the prototype. The limits are due to: the lower - the need to maintain the stability of the corona discharge, to prevent the transition to an irreversible spark and then to the arc, the upper - the low efficiency of ozone production.

The condition for the distances d between adjacent disk plates is introduced for reasons:
the upper limit of these distances d = d _max <2xL is due to a decrease in the efficiency of ozone production due to a decrease in the total length of the sharp edge of the corona electrode and a decrease in the life of this electrode, the lower limit d = d _min is due to the screening of neighboring plates by each other, i.e. a decrease in the degree of local field heterogeneity and, consequently, a decrease in the efficiency of ozone production. d _min the less, the shorter the duration of the pulse front.

The ratio of the characteristic transverse dimensions of 1 mm≤2xD _with <D = (D _T -2xL) is due to the need to create optimal conditions for a pulsed overstressed corona discharge.

 The ratios for the characteristic curve radius r: r << L and r≤δ / 2 provide a sharply inhomogeneous electric field and an effective corona discharge in the discharge gap.

 The implementation of the plates in the form of disks makes it possible, due to the same curvature, to exclude the concentration of the corona discharge in the places of the greatest field heterogeneity, which means to create optimal conditions for the uniform distribution of the discharge in the cross section of the working chamber.

 The design of each disk allows to overcome the technical contradiction, consisting in the need to reduce the thickness of the corona electrode (which provides a sharp heterogeneity of the field) and at the same time maintain its mechanical strength. The presence of two external disks provides the required rigidity, and the location and size of the internal one - the possibility of manufacturing it with a minimum edge thickness. The latter acts as a point between the disks due to a slightly larger diameter.

 The introduction of a high-voltage switch between the high-voltage generator and the electrode system makes it possible to exacerbate the pulse front and, due to this, increase the number of effective electrons and the maximum current amplitude in the initial stage of the corona discharge, increase the dissociation rate, and, consequently, the ozone concentration. At the same time, spark gaps have by far the shortest switching times and are therefore preferred as commutators that sharpen the pulse front. Without them, the crown in the working chamber more easily passes into an arc. Due to the short switching times, they allow you to more effectively avoid space charge. Thus, the presence of a spark gap optimizes the corona discharge and allows one to obtain short pulse fronts and their optimal duration. Both with an additional sharpening switch and without it in a pulsed overstressed corona discharge, pulsed periodically repeating currents are possible during each pulsed corona discharge according to the type of current pulses detected by Trichel and Kip (1938) [Yu. P. Ryzer "Physics of a gas discharge", M., "Science", 1987, p.511-512], but with a significantly larger amplitude.

 In the presence of a spark gap, the packing density of the plates, and hence their total perimeter (total length of the corona edge of the corona internal electrode), may be greater, which, in turn, allows for more ozone. Without a spark gap, in places where the plates are located close to each other, they shield each other.

 With an increase in the discharge power, the thermal regime of the working chamber changes, which affects the kinetic constants of ozone electrosynthesis. However, due to the large width of the discharge gap, it is possible to avoid heating and, accordingly, reduce the production of ozone.

 The introduction of additional electrodes into the electrode system will further increase the efficiency of ozone production.

 The applicant does not know examples of the use of an overstretched volume corona discharge.

 The technical solutions underlying the method and device are the implementation of one and the same inventive concept, are aimed at solving the same problem and cannot be used without one another.

 An example of the implementation of this group of inventions is illustrated in the drawing, which shows a General view of the ozone generator (version with a switch). A detailed description of the group of inventions is combined with an example of their specific implementation.

 A device for generating ozone contains a high-voltage pulse generator 1 and a working chamber 2 connected to it with an electrode system in the form of a coaxial external low-voltage tubular electrode 3 and an internal high-voltage electrode 4 in the form of a conductive rod 5. On the rod 5 of the internal electrode 4 in planes perpendicular to its longitudinal axis, additionally installed conductive plates 6. Between the generator 1 of the high-voltage pulses and the working chamber 2 can be installed high-voltage switch 7, and in the case of it is switched on in parallel with a mutator of a disconnecting type, and in series in the case of a closing (high-voltage spark gap).

 The device operates as follows.

The air (or other oxygen-containing gas) to be treated is forcibly supplied to the working chamber 2. Passing through the interelectrode gap 3-4 (L), it is exposed to an electric field of high pulse voltage supplied from the generator 1, under the action of which in the working chamber 2 electrical discharge occurs. In a high-voltage pulsed discharge, conditions are provided for the formation of a large number of free electrons with significant energy, leading to the destruction of oxygen molecules to atoms, ions and, thus, the conditions for the formation of ozone O _{3 are created} .

 The advantages of the group of inventions include a sharp increase in the efficiency and productivity of ozone production due to the action of a whole indissoluble complex of factors.

 First of all, it should be noted that, other things being equal, it is the pulsed corona discharge that is most effective for ozone generation, since it allows you to get the maximum current amplitude without going to the contracted (spark) stage, and its presence (corona discharge current) in the entire working volume this efficiency is many times higher. The transition of a discharge into a spark is the limit for increasing the concentration of ozone in the corona discharge. The group of inventions described above has reached this limit.

 High efficiency and productivity of the ozone generator is also achieved by the fact that the transformer power supply (with high efficiency) operates at an ohmic-capacitive load (working chamber).

 One of the ways to increase the efficiency and productivity of ozonizers is to increase the length of the discharge gap, which makes it possible to increase the active discharge power and, accordingly, the amount of ozone produced. It should also be noted that in the invention, the discharge gaps can be very large (≈0.5-1 m and even more). This allows the objects to be treated with ozone to be placed inside the working chamber and not only make better use of the ozone itself, but also include the whole range of influences in the processing process: field, radiation, corona discharge plasma and products obtained in this plasma, including ozone . At the same time, long discharge gaps exclude heating of the discharge gap, which is harmful to ozone formation, and also reduce dynamic resistance to air flow. The latter reduces the energy costs of maintaining the installation and also increases the efficiency.

 The pulsed nature of the discharge in the absence of a barrier makes it possible to reduce the dependence on humidity, as well as to achieve high currents and voltages, and, consequently, large amounts of ozone (voltage boundary is the occurrence of a spark discharge with a constant discharge gap).

The use of a special shape of the electrodes makes it possible:
- increase the discharge area, efficiency and productivity;
- to increase the resource and maintainability during corrosion of the working surfaces of the electrodes in the environment of ozone.

 The selected operating mode, in addition to the advantages already indicated, allows you to work continuously for a sufficiently long time (for hours) at room temperature.

The absence of a dielectric barrier between the electrodes leads to the following advantages:
- increase ozone productivity by providing the possibility of increasing the density of the discharge current in the gap, because the risk of breakdown of the barrier is excluded, while the presence of separate (reversible) spark discharges is not scary for the integrity of the working chamber (and practically does not affect the efficiency of ozone production);
- increase the operational reliability of the device;
- increase manufacturability, because there is no need to apply barrier insulation; high requirements for the electrical properties of the barrier layer have been removed;
- the possibility of eliminating the system of mandatory preliminary air preparation (it has become optional), which is necessary both in analogues (where the lack of clean and dry air can lead to irreversible breakdowns), and in the prototype (to achieve concentrations of 10-20 g / m ³ ozone) . This reduces the energy costs of auxiliary operations and, accordingly, the cost of ozone produced. Even without preliminary air preparation, ozone concentrations of ≈10 g / m ^{3 were} achieved in the invention. When using the air pre-treatment system, the efficiency of ozone production increases.

 It should also be noted the simplicity of manufacturing the device: a successful technological solution was found that allows simple means to satisfy the requirement of high-precision manufacturing of electrodes (three-layer electrode). There are no special requirements for precision manufacturing of parts (tolerances are not strict).

Claims (7)

1. A method of generating ozone, including the effect of a high voltage pulse discharge on an oxygen-containing gas, characterized in that the effect is produced by a pulsed volume corona discharge with the following characteristics:
0.01A≤i _m ≤i _s ,
where i _m is the current amplitude;
i _s is the largest current amplitude of a pulsed corona discharge,
5x10 ³ V≤U _m <U _s ,
where U _m is the voltage amplitude;
U _s is the voltage amplitude of the irreversible transition to the spark discharge in the discharge gap with a pulsed corona discharge,
t _u ≤ Зх10 ^-4 s,
where t _u - pulse duration,
moreover, the shape and duration of the voltage and current pulses in the discharge gap may differ,
dU / dt≥10 ⁸ B / s,
where dU / dt is the slew rate
0.01s ^-1 ≤f _cl ≤0.3 / t _u ,
where f _SL - pulse repetition rate.  2. The method according to p. 1, characterized in that the effect is carried out on an oxygen-containing gas under pressure above atmospheric. 3. A device for generating ozone containing a high-voltage pulse generator and a working chamber connected to it with an electrode system in the form of a pair of coaxial external low-voltage tubular electrodes and an internal high-voltage electrode in the form of a conductive rod, characterized in that on the internal electrode rod in planes perpendicular to it longitudinal axis, conductive plates are additionally installed, while the distance L from the edge of each plate to the inner surface of the tubular electrode satisfies a relation
U _s / E _a <L <3xU _s / E _a ,
the distance d between adjacent plates - the ratio
d _min <d <2xL,
transverse plate size D - ratio
1 mm≤2xD _c <D = (D _T -2xL),
the radius r of the curve of the edges of the plates - to the relations
r << L and r≤δ / 2,
where D _c is the transverse dimension of the rod;
D _T is the transverse dimension of the outer electrode;
d _min is the screening distance;
U _s is the voltage amplitude of the irreversible transition to the spark discharge in the discharge gap with a pulsed corona discharge;
E _a is the average electric field strength in the discharge gap at a voltage amplitude of U _s ;
δ is the plate thickness.  4 The device according to p. 3, characterized in that it additionally introduced a high-voltage switch located between the high-voltage pulse generator and the working chamber.  5. The device according to p. 3 or 4, characterized in that the plates are made in the form of disks.  6. The device according to one of paragraphs. 3-5, characterized in that each of the plates contains two external and one internal disk located between them, while the diameter of the internal disk is larger than the diameter of the external.  7. The device according to p. 4, characterized in that the switch is made in the form of a spark gap.  8. The device according to claim 3, characterized in that at least one pair of coaxial electrodes is additionally introduced into the electrode system.