GB1600055A

GB1600055A - Process and apparatus for manufacturing ozone

Info

Publication number: GB1600055A
Application number: GB9801/78A
Authority: GB
Original assignee: Societe National Elf Aquitaine
Current assignee: Societe National Elf Aquitaine
Priority date: 1977-03-11
Filing date: 1978-03-13
Publication date: 1981-10-14
Also published as: CA1097258A; ES467770A1; BE864745A; CH628596A5; DE2810530A1; NL7802666A; FR2383123A1; BR7801485A; JPS549188A; FR2383123B1; IT1093654B; IT7821001A0; AR216312A1

Abstract

The device used comprises a cylindrical pipe (1) of circular cross-section connected by one end (1a) to a means for supplying air and, by the other end (1b), to a discharge pipe, this cylindrical pipe being connected to earth, and a straight metal wire (4), having the same axis as the cylindrical pipe and brought to a voltage chosen in the range from 1000/50 000 volts, a direct voltage stabilised at virtually ten per cent, this wire being entirely covered with a porous and air-permeable insulating material (5). The use of this device makes possible the production of ozone under output conditions such that the decontamination of various effluents, especially water, can be carried out economically. <IMAGE>

Description

(54) PROCESS AND APPARATUS FOR MANUFACTURING OZONE (71) We, SOCIETE NATIONALE ELF AQUITAINE, a French Body Corporate, of Tour Aquitaine, 92400 Courbevoie, France, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention concerns apparatus for producing ozone, which allows the ozone to be produced very economically on an industrial scale, whereby it then becomes feasible to consider using it to treat effluents and to solve problems of industrial oxidation.

Various types of ozone generators exist, in which an alternating voltage is applied between two electrodes, thereby creating an alternating electrical field. This field acts on the oxygen molecules in an airstream, possibly oxygen-enriched, which passes through the gap between the electrodes.

Between the two electrodes there is usually an air gap, and also a sheet of insulating material.

When the peak voltage at the electrodes is below starting level, the ozone generator acts as an air capacitance in series with a dlelectric capacitance.

When the peak voltage is above starting level, electrical discharges occur in the air during each positive and negative half-wave of the voltage, and while these discharges last the generator acts approximately as a variable resistance in series with the dielectric capacitance.

Existing industrial ozone generators generally operate in this way. They consist of a combination of several cells connected together in parallel between two terminals supplied with high-voltage alternating current. They are constructed from flat sheets or concentric tubes.

In the various kinds of ozone generators, the formation of electric arcs has to be avoided, since they reduce the efficiency of the equipment, by interrupting its operation, as well as creating conditions wherein any ozone already formed is destroyed. To prevent arcing, the gas, air, oxygen or oxygen-enriched air, used to feed the equipment has to be specially treated.

This gas is absolutely clean, and special precautions must be taken to remove any dust, which could start arcs, and traces of oil, which could settle on the surfaces of the electrodes and the dielectric.

Said gas also must be very dry, since water vapour encourages the passage of arcs, and the resulting ionization of the vapour absorbs energy, without contributing to the production of ozone.

The gas further must not be at a temperature above atmospheric temperature, since high temperatures tends to decompose the ozone.

The electrical power of the discharges depends on the peak voltage, which however is limited by the arc-striking threshold.

Power also increases in direct proportion to frequency, and complex, expensive equlpment is needed to obtain very high frequencies, such as 500 hz, instead of 50 hz.

The new apparatus according to this invention overcomes these drawbacks, by creating a state of stable, direct current electric discharges in an ozone-generating tube, thereby removing the risk of arcing.

The new ozone-production apparatus according to the invention comprises a tube of an electrically conductive material the inside surface of which forms a first boundary surface, one end of which tube opens into a device for providing a stream of air through the tube, and the other end opens into a delivery duct, this tube being earthed to zero potential, and the apparatus further comprising a body of an electricallyconductive material the outside surface of which forms a second boundary surface and which is fully coated with porous insulating material which body is located inside the tube and connected to a high-voltage electric source of positive polarity so that it can be raised to a positive potential of 1,000 to 50,000 volts, the arrangement being such that when said potential is applied to the body, a potential gradient is maintained between the boundary surfaces having an average value of 500 to 5,000 volts per centimeter.

In some embodiments, the body comprises metals which have oxides of low conductance, and is entirely covered with a porous insulating material of such oxides. In this case, the oxides may be formed by contact of the metal with the atmosphere.

In some embodiments, the porous insulating material of low conductance which entirely covers the body (which may be in the form of a wire) consists of oxides of the metals composing the conducting wire.

In the various embodiments, the potential of the high-voltage source of positive polarity is selected within the range 1,000 to 50,000 volts, stabilized to within ten per cent, the diameter of the tube of circular section is selected within the range 2 to 50 centimetres, and the diameter of the body conductor is selected within the range 0.05 to 1 millimetre.

In such embodiments, the average potential gradient between the body and the tube is usually between 1,000 and 5,000 volts per centimetre.

The potential of the high-voltage positive polarity source is preferably selected within the range 5,000 to 20,000 volts, stabilized to within ten per cent, the diameter of the tube is selected within the range 2 to 10 centimetres, and the diameter of the body (which in this case is a wire) is selected within the range 0.1 to 0.5 millimetres.

In such embodiments, the average potential gradient between the wire and the tube is usually between between 1,000 and 4,000 volts per centimetre.

It will be easier to understand the invention from the following description of one of the many possible forms of an apparatus designed to perform this new process, illustrated by the accompanying figures: Figure 1: Basic diagram of a cylindrical tube with coaxial wire.

Figure 2: Sectional view of the coaxial wire forming the positive electrode.

Figure 3: Perspective view of an industrial apparatus.

Figure 4: Perspective view of the positive electrodes in the apparatus shown in figure 3.

Figure 5: Electrical diagram.

Figure 6: Single-phase version.

Figure 7: Three-phase version.

Figure 1 shows a cylindrical tube 1 of an electrically-conductive material constructed from a sheet of iron or light-weight metal such as aluminium. One end la of this tube leads to an air-supply device (not shown here), and the other end 1b to a delivery duct (not shown here). Tube 1 is connected by a wire conductor 2 to a zero-potential mass 3, such as earth.

A straight wire 4 is stretched coaxially right through the tube 1, supported at the ends by insulated brackets (not shown here). This wire is connected to a directcurrent power supply with positive polarity.

As shown in Figure 2, which is a crosssectional view of wire 4, it is entirely covered by a porous insulating material 5 of low conductance.

Figure 3 shows a view in perspective of an industrial-scale apparatus, consisting of an assembly of sixteen basic tubes 1, as illustrated in figure 1, mounted in parallel.

These tubes open at one end into an inlet chamber 6 and at the other end into an outlet chamber 7. The outer side of each chamber, facing the tube ends, contains a single opening. The opening in chamber 6 leads to an air-supply system 8, and the opening in chamber 7 to a delivery conduct 9.

All these tubes are connected by a wire conductor 2 to a zero-potential mass 3.

Figure 4 shows a perspective view of all the positive electrodes in the apparatus illustrated in figure 3. These electrodes consist of the same number of wires 4, as illustrated in figure 1, as there are basic tubes.

At each end, these wires are attached to the points of intersection of orthogonal lattices 10 and 11, made from wire of the same conducting material as the electrodes themselves.

One end lattice 10 is connected by a wire conductor 12 to the positive terminal of a high-voltage d.c. generator (not shown here .

Withe wires 4 and the end lattices 10 and 11 are entirely covered by a porous, airpermeable insulating material. The end lattices 10 and 11 are attached inside the chambers 6 and 7 by means of insulating brackets (not shown here).

Figure 5 shows the electrical diagram for a direct-current generator which can be used to supply the positive electrodes of an ozone generator of this kind.

A transformer 13, supplied with alternating current 14, produces an alternating current 15 of higher potential. A diode 16 produces a rectified current 17, and a condenser 18 placed between the diode output and the corresponding transformer output results in a certain smoothing of the curve representing the output voltage 19. This provides a standard example of the means of supplying a direct current voltage stabilized to within 10% of the desired voltage.

Figure 6 shows a diagram of a full wave rectifier generator which can replace the device illustrated in Figure 5.

After the power input there is variable auto-transformer 20, which immediately precedes the voltage step-up transformer 13. The variable autotransformer 20 is used to change from current 14 to current 14'.

Instead of a single diode, this device contains a diode bridge 16. A low-value resis tance 21 is placed in series at the bridge output, preceding the filter condenser 18.

Stabilization of the supply by this known method is generally to within 10% on either side of the desired value.

To improve filtering still further, a threephase power supply may be chosen, and this may be fully rectified. Figure 7 shows a diagram of this type of generating system.

ibis diagram contains components similar to those in figure 6: a variable ratio threephase autotransformer 20, a voltage step-up transformer 13, a diode bridge or Graetz bridge 16, a low value resistance 21 and a filter condenser 5.

The advantage of the three-phase version is that better filtering is obtained by full wave rectification of all three phases.

In an industrial appliance of the type illustrated in figures 3 and 4, the diameter of the tubes 1 is selected within the range 2 to 10 centimetres, and the diameter of the wires 4 within the range 0.1 to 0.5 millimetres.

When the wires 4 are connected to the positive polarity terminal of a direct-current generator stabilized to within ten per cent above or below the desired output, the average potential gradient at any point within the space inside the tubes 1 is between 1,000 and 4,000 volts per centimetre.

Under these conditions, a system of direct current electrical discharges is created between the wires 4 and the inside surface of the tubes 1.

Various experiments performed using different models of cylindrical tubes with a coaxial wire, as defined above, have led to the following observations.

The diameter of the cylindrical tube determines the maximum potential that can be applied, as well as the optimum value of the potential to be applied to the coaxial wire.

When the diameter of the tube is between 2 and 10 cm, the optimum potential is between 1,000 and 20,000 volts.

The diameter of the wire must be large enough to endow it with enough mechanical strength and a long useful life, with regard to the particular corrosion conditions of an oxidizing medium. On the other hand, it must not be too large, or the current density on the surface of the wire as would be undesirably low. A high current density is desirable in order to enhance starting of the discharges. Usually the diameter is from 0.1 to 0.5 mm.

With a bare wire, a slight increase in the potential beyond the level needed to start discharges causes arcing, particularly when the air contains moisture and dust. Under such conditions, with a bare metal wire, the output of ozone remains small.

With a wire covered with a continuous layer of oxide, for example an oxide of the wire metal itself, and provided that this oxide is porous, the current intensity can be raised considerably for the same potential.

Under these conditions, at around 10,000 volts for a tube 6 cm in diameter, 100 cm long, 600 mg ozone an hour is produced, for a power consumption of 10 watts, equivalent to a yield of approximately of 60 mg ozone per watt/hour. The current passing through the cell is 1,000 microamps.

Tubes of the same diameter, containing a wire with the same characteristics, raised to the same potential, produce ozone, measured in milligrammes per hour, in amounts roughly proportional to their length, up to a point beyond which the increase becomes less than proportional. This is because the electrical charge destroys the ozone at a speed that apparently increases in relation to the ozone content of the air.

With a tube 6 cm in diameter, containing a wire 0.3 mm in diameter for the conducting part, it has been found that the production of ozone ceases to be proportional to the length of the tube beyond approximately 100cm.

In many applications, particularly for small amounts, increased yields can be obtained by installing two or more tubes in series. For large scale ozone-production installations, yields are more likely to be improved by parallel installations, as illustrated in figures 3 and 4.

For an apparatus ot this type, the potential to which the wire is to be raised is chosen to suit the desired hourly output of ozone. Choice of this parameter involves determining the current intensity that is to pass through the apparatus.

For high potentials, having regard to the diameter of the tube, a high air-flow velocity allows the onset of arcing to be delayed, but causes greater dilution of the ozone, since the air-velocity inside the tube, or the airflow rate, does not have any significant effect on output.

With wires coated with a continuous layer of porous, insulating oxide of low conductance (oxides of iron and a large number of iron-based alloys comply with these criteria, but not oxides of aluminium), it is found that the operation of the apparatus is not affected by dust in the air supply. The same applies to artificial coverings which meet the requirements of insulation, porosity and low conductance and which do not contain oxides of the metal used for the wire itself.

This new process for producing ozone offers the following advantages over any others so far known: - use of a d.c. voltage which can be produced without any particular precautions to regulate it, from the three-phase main power supply; - very simple technology, involving very light-weight, inexpensive equipment, thereby offering a considerable reduction in installation costs for a given performance; - higher energy efficiency than the existing industrial apparatuses, for which any comparative assessment must take into account the energy consumption in various ancillary mechanical installations, such as equipment to increase the oxygen content of the air supply, or to dry and remove dust from it; - negligible maintenance.

These advantages make it possible to use ozone economically to treat water, depollute many biological effluents, and for gaseous oxidation processes in industry.

WHAT WE CLAIM IS:- 1. An apparatus for producing ozone, comprising a tube of an electrically conductive material the inside surface of which forms a first boundary surface, one end of which tube opens into a device for providing a stream of air through the tube, and the other end opens into a delivery duct, this tube being earthed to zero potential, and the apparatus further comprising a body of an electrically-conductive material the outside surface of which forms a second boundary surface and which is fully coated with porous insulating material which body is located inside the tube and connected to a high-voltage electric source of positive polarity so that it can be raised to a positive potential of 1,000 to 50,000 volts, the arrangement being such that when said potential is applied to the bodya potential gradient is maintained between the boundary surfaces having an average value of 500 to 5,000 volts per centimeter.

2. An apparatus according to claim 1, wherein the tube is a cylinder of revolution, and the body inside it is a stretched wire, located along the centre-line of the cylinder forming the tube.

3. An apparatus according to claim 2, wherein the wire comprises at least one metal having a porous insulating oxide, and is entirely covered with a layer of such oxide.

4. An apparatus according to claim 3, wherein the oxide is formed by contact of the metal with the atmosphere.

5. An apparatus according to claim 1, wherein the potential of the high-voltage source of positive polarity is selected within the range 1,000 to 50,000 volts, stabilized to within ten per cent, the diameter of the cylindrical metal tube of circular section is selected within the range 2 to 50 centimeters, and the diameter of the metal wire conductor is selected within the range 0.05 to 1 millimeter.

6. An apparatus according to claims 1 or 2, wherein the potential of the high-voltage source of positive polarity is selected within the range of 5,000 to 20,000 volts, stabilized to within ten per cent, the diameter of said cylindrical tube is selected within the range 2 to 10 centimeters, and the diameter of said wire is selected within the range 0.1 to 0.5 mm.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. light-weight, inexpensive equipment, thereby offering a considerable reduction in installation costs for a given performance; - higher energy efficiency than the existing industrial apparatuses, for which any comparative assessment must take into account the energy consumption in various ancillary mechanical installations, such as equipment to increase the oxygen content of the air supply, or to dry and remove dust from it; - negligible maintenance. These advantages make it possible to use ozone economically to treat water, depollute many biological effluents, and for gaseous oxidation processes in industry. WHAT WE CLAIM IS:-

1. An apparatus for producing ozone, comprising a tube of an electrically conductive material the inside surface of which forms a first boundary surface, one end of which tube opens into a device for providing a stream of air through the tube, and the other end opens into a delivery duct, this tube being earthed to zero potential, and the apparatus further comprising a body of an electrically-conductive material the outside surface of which forms a second boundary surface and which is fully coated with porous insulating material which body is located inside the tube and connected to a high-voltage electric source of positive polarity so that it can be raised to a positive potential of 1,000 to 50,000 volts, the arrangement being such that when said potential is applied to the bodya potential gradient is maintained between the boundary surfaces having an average value of 500 to 5,000 volts per centimeter.