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CHAPTER 1
INTRODUCTION
1.1
BACKGROUND
Ozone (O3), or trioxide, is a triatomic molecule, consisting of three
oxygen atoms. It is an allotrope of oxygen that is much less stable than the
diatomic allotrope (O2). The ozone is an active modification of the oxygen. It
has a clear blue colour and a spicy odour, is more soluble in water and much
more active than oxygen. It is considered the most powerful oxidant on the
earth and it has a lot of applications.
Ozone, the triatomic allotrope of oxygen, was first identified as a
new chemical compound by SchoFilchnbein in the year 1839 (Kogelschatz
et al 1995). It is a powerful oxidizer; its electronegative oxidation potential
exceeded only by that of fluorine. The extreme reactivity of ozone causes it to
be very effective in several forms of water treatment. The ability of ozone to
disinfect water was recognized as early as 1886 by de Meritens
(Brink Deborah et al 1991). The first major application of ozone in water
treatment began in the city of Nice, France, in 1906.
Ozone application in water treatment has continued to grow since
the discovery of its usefulness, but its growth has been severely curtailed by
the development of inexpensive chlorine. Disinfection was ozone's first use in
water treatment, but it was soon realized that ozone is effective in improving
2
the taste and removing odour of water. It is also capable of oxidizing iron and
manganese, thus enabling their precipitation from the water supply;
coagulating particulates; removing colour, controlling algal growth; and
destroying dissolved pesticides. More recently, ozone has been used for the
control of disinfection by-products (e.g., some harmful by-products of
chlorination), as well as for biological stabilization (i.e., reduction of the
microbiological growth potential of water) (Bablon Guy et al 1991).
Most of the early applications of ozone in water treatment occurred
in Western Europe, especially in France. Today, ozone usage has spread to
many parts of the world, including North America, the Middle East, the Far
East and Africa. Interest in the use of ozone is increasing, due not only to its
effectiveness in a wide variety of water treatment applications, but also
because of concerns about the by-products of chlorination. Disinfection using
chlorine is known to produce large numbers of by-products, some of which
have been proved to be carcinogenic (eg., chloroform and other
trihalomethanes) and mutagenic (e.g., chlorohydroxy furanones). Although
ozonization produces many by-products, it is commonly known that these
products are less toxic than those arising from chlorine treatment.
1.2
COMMERCIAL OZONE GENERATORS
1.2.1
Methods of Production
As a commercially demanded treatment, there have been years of
research and development put into the methods of ozone production.
Presently there are four recognised methods:
1. Corona Discharge (CD)
2. Ultraviolet Radiation
3. Electrolysis
4. Radiochemical method
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Any method of generating ozone relies on applied energy to break
the bonds holding the oxygen atoms in a molecular form, allowing them to
dissociate and then re-form as ozone (Rip G Rice et al 1982).
The applied energy is random in its action, resulting in a high level
of friction in the reaction process. For this reason ozone production is
inefficient and is accompanied by a large percentage of waste heat.
1.2.1.1
Corona discharge (CD)
Corona discharge is the condition created when a high voltage
passes through an air gap. In the case of ozone production, this high voltage
transfers energy for the breaking of the O2 molecule, allowing the formation
of a 3-atom oxygen molecule-ozone (Jae Seok Park et al 2001). This method
is most widely used presently for commercial ozone production.
1.2.1.2
Ultraviolet radiation
The formation of ozone from oxygen is endothermic. When
exposed to light, an oxygen molecule in a ground state will absorb the light
energy and dissociate to a degree depending on the energy and the particular
wavelength of the absorbed light. The oxygen atoms then react with other
oxygen molecules to form ozone.
Each wavelength of light favours different reactions and their
quantum yield. The breakdown of the oxygen molecule has a higher yield at
wavelengths less than 200nm (Koudriavtsev et al 2000).
However just as oxygen absorbs light, so does ozone. The
dissociation or photolysis of ozone has its greatest yield between 200 and
308nm. Figure 1.1 highlights the fact that the wavelength of UV light used for
4
specific disinfection (254nm) is in the peak wavelength range for ozone
destruction.
Figure 1.1 UV Destruction of Ozone
For effective ozone production it is therefore necessary to utilize a
short wavelength ~185nm. In theory, the yield of O3 from 185nm UV light is
130g/kWh of light. As lamp efficiencies are very low (~1%), the production
per kWh from the power source is greatly reduced.
In practice, with the present state of development, UV lamps can
only produce about 20g O3/kWh of ozone when using oxygen as the feed gas
(Koudriavtsev et al 2002). However, they are more commonly used with
vacuum injection systems drawing atmospheric air over a UV lamp tube, and
generate 1-2g O3/kWh in concentrations of 0.1% w/w of air. These are very
simple in design, require no air preparation and are ideal for small
applications such as small fishponds, laboratory work, and odour elimination.
1.2.1.3
Electrolysis
Electrolysis is the process in which an electric current is passed
through a liquid, causing a chemical reaction, resulting in the liberation of
gases as shown in Figure 1.2. In relation to ozone production, water can be
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used as the electrolyte leading to direct diffusion or special electrolytes such
as H2SO4 can be used and ozone gas drawn off and diffused and contacted by
the usual methods.
Figure 1.2 Electrolytic Ozone Generator Cell Design
Work has been done with different electrolytes and anode materials
to improve the efficiency of production and minimize the corrosive reactions
on the anode surface. The concentration of ozone produced is determined by
the current density acting on the cell (Yanzhou Sun et al 2009).
With the use of an ozone gas evolving cell as depicted in this
diagram, high concentrations of ozone, at a minimum of 10% can be
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achieved. The use of electrolysis for ozone production is presently limited to
small units used for applications that require high concentrations of ozone,
and in-situ diffusion of ozone into ultra pure water. Currently, whilst their
capital cost is favourable compared to corona discharge units, the operating
costs are significantly higher. Further development needs to be done on the
composition of electrolytes and cathode/anode manufacture before they
become a commercially viable production method.
1.2.1.4
Radiochemical
High energy irradiation of oxygen by radioactive rays can promote
the formation of ozone. Whilst high yields have been achieved under specific
conditions using oxygen, the best results from an air flow through system at
atmospheric pressure, has been ~ 3-4 mg/m3(Alonso et al 2004). The process
is fraught with complications in filtering harmful isotopes and it is not viewed
with potential use in commercial applications.
1.2.2
Ozone Production by Corona Discharge
Ozone production by electrical discharges is a common occurrence
in photocopiers, faulty light switches, motor brushes and power transmission
lines. The use of electrical power to generate ozone by corona discharge has
been, and remains, the most commercially viable method. Essentially a
corona is characterised by a low current electrical discharge across a gas-filled
gap at a relatively high voltage gradient. This results in the gas becoming
partially ionized, and taking on a diffused bluish glow when pure oxygen is
used as the feed gas (the colour is more mauve when using air). As a contrast,
an arc discharge is characterized by a high current density, causing a highly
ionized gas and a low voltage gradient across the gap (Bruno Langais et al
1991).
7
In essence the configuration of a typical cell is as illustrated in
Figure 1.3.
Figure 1.3 Corona Discharge Cell Configuration
Ozone is produced in the corona as a direct result of power
dissipation in the corona. Electrons are accelerated across an air gap so as to
give them sufficient energy to split the oxygen double bond, thereby
producing atomic oxygen. These oxygen atoms then react with other diatomic
oxygen molecules to form ozone.
3O2 + ENERGY = 2O3
The amount of ozone produced together with the efficiency and
reliability of that production are directly related to a number of key factors the
main ones being:
1. Feed gas quality
2. Power input i.e., voltage and frequency
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3. Generation module construction
4. Temperature.
1.2.2.1
Feed gas quality
The amount of ozone produced in a given ozonator design is
relative to the concentration of oxygen in the gas feeding the corona.
Basically, the more oxygen in, the more ozone will be produced. In general,
ozone concentrations of 1-3% using air, and 3-10% using oxygen can be
obtained (Wei Linsheng et al 2010). There are, however, other complex
considerations that need to be accounted for, such as air preparation.
1.2.2.2
Power input
The amount of energy applied to the gas gap between the electrodes
is critical to the concentration of ozone produced. It is a combination of the
voltage and frequency that results in a given energy input. Typically, voltages
between 7 to 30 kV are used with frequencies ranging from the mains supply
of 50 or 60 Hz, medium up to 1000 Hz, and high up to 4000 Hz. Until
recently the most common design was to use mains frequency and vary the
voltage (Rabinowitz 2000).
Limitations to this method include: (a) high peak voltages increase
the stress on the dielectric resulting in more frequent failures, and (b) the
ozone output is not linear to the change in applied voltage (Dimitriou 1990).
Better technology has led to the use of frequency control to vary the
power input and thus ozone output as shown in Figure 1.4. By using higher
frequencies and operating at lower voltages the dielectric stress is minimised.
Other benefits include an increase in generator efficiency, a linear control
relationship and a greater turndown capability.
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Figure 1.4 Typical Relationship between Frequency and Ozone Output
When designed correctly, using modern power electronics, the
generator efficiency and capabilities are maximised by manipulation of all
parameters such as voltage, frequency, current and waveform.
1.2.2.3
Generation module construction
The design of a corona discharge cell is critical to ensure maximum
ozone output from given operating conditions such as power input and gas
feed whilst maintaining reliable operation and long service life.
Both the materials used for module construction and the geometry
in which they are configured are paramount to generator performance. It is
critical that the energy dissipates evenly across and through the entire cell gap
and dielectric material to prevent any ‘hot spots’ and premature failure.
There are two basic geometric designs; parallel flat plates and
concentric tubes. The flat plate generator is of two main design
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configurations. The Otto Plate consists of flat hollow blocks separated by two
glass plates and a gas space as shown in Figure 1.5. The cooling water flows
through the hollow blocks that serve as both the electrodes and heat
dissipaters. These units are designed to operate at below atmospheric pressure
and therefore restricted to use with negative pressure dissolution systems.
Figure 1.5 Otto Plate Ozone Generator Design
The Luther plate is characterised by the use of a ceramic dielectric
coating on the electrodes with air being forced through aluminum heat sinks
as the cooling system.
These units are designed to operate at slightly positive pressure
~ 100 kPa. The concentric tube design is the most common, and is categorised
by being either a vertical or horizontal configuration. Each of these again has
various configurations of tube design, airflow and electrical discharge path. In
essence, the dielectric is a glass tube and the high voltage electrode is either a
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conductor inserted within the tube or a metallic film coating the internal
surface of the tube. Typically the vertical tubes use an inner high voltage alloy
electrode with an air gap to the glass dielectric, which is in direct contact with
the cooling water, serving as a ground electrode. This can be as a two pass or
as a flow through design as shown in Figure 1.6.
Figure 1.6 CD Vertical Tube Flow Within and Return Design
12
High voltage
electrode
Air Feed
Cooling
water feed
Dielectric
Cooling
water jacket
Dielectric
Cooling
water return
Ozone
Figure 1.7 CD Vertical Tube Flow Through Design
These units may be designed for both vacuum or pressure feed, and
are most suitable for medium ozone outputs, up to 1.5 kg/h. The large
capacity generators are of the horizontal tube type. These consist of a metallic
film on the inside surface of the glass dielectric. In this case, the electrical
current travels through the dielectric first, before transverse the air gap to a
stainless steel water jacket serving as the ground electrode as shown in
Figure 1.7. The stress on the dielectric is greater, as it is not directly cooled as
in the case with the vertical design. However, should a tube fail, a simple
fusing system will allow the generator to continue operation on the remaining
tubes. Failure of a vertical tube cell will flood the module with water, making
the generator inoperative due to a direct electrical short. Whilst a fusing
system is possible, its complications are not justified (Moras et al 1993).
13
The horizontal tubes are arranged in a honeycomb configuration
into what is commonly called the ‘Iron Lung’ style of design as shown in
Figure 1.8. The largest of these units is available with a capacity over
100 kg/h.
Figure 1.8 CD Horizontal Tube Design
There are other generator designs using a ceramic dielectric
impregnated with the high voltage electrode. These usually operate at high
frequencies and have often proved to be unreliable (Filchev et al 2008).
Material selection is critical to cell performance and reliability.
Typically the dielectric material of either glass or ceramic is chosen for its
high dielectric strength (V/mm) and its high dielectric constant. A good HV
electrode is chosen for its ability to handle a high current density (W/cm2), as
characterised by its ability to conduct a high current per unit surface area with
minimal heating. In selecting materials and their physical dimensions, it is the
balance between a highly power efficient fragile construction and a less
efficient construction of robust quality that will give long term reliability.
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1.2.2.4
Temperature
As mentioned earlier, ozone generation is an inefficient process
whereby about 80% of the applied electrical energy is wasted as heat. It is
essential to remove this heat so because not only does it increase stress on
ozonator components but ozone is also destroyed at elevated temperatures
(Alonso et al 2009). The quality and temperature of the cooling water has a
major influence on the output efficiency and reliability of an ozonator.
Because the vertical tube design directly cools the dielectric, it is a
more efficient cooling system than that of the horizontal type. Cooling flows
of vertical designs are in the order of 1,400 L/h/kg of ozone produced at 20°C,
whereas horizontal designs require 2 to 3 times this flow.
Double cooled vertical tube designs as shown in Figure 1.9, provide
the best cooling, but they pose significant maintenance problems for sealing
and isolation of both electrode systems (Philip J. Barlow 1994). It is therefore
not a common design in practical applications.
Figure 1.9 CD Vertical Tube Double Cooled Generator
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1.2.2.5
Electric discharge or silent discharge
The most commonly employed type electrical discharge in
commercial ozone generators are the silent or dielectric barrier discharge.
This type of discharge is often referred as a form of corona discharge.
The silent discharge occurs in a gas-filled gap between two
electrodes separated by a dielectric. When an alternating voltage, high enough
to cause electrical breakdown of the gas in the gap, is applied across the
electrodes, a series of discrete discharges occurs in the gap. These discrete
discharges are termed micro discharges, and are actually current filaments
consisting of thin cylindrical conductive plasma columns. As current flows
through a micro discharge, the charge accumulates on the dielectric (ValdiviaBarrientos et al 2006). This accumulated charge causes an electric field which
opposes the applied field. Typically after a few nanoseconds, sufficient charge
has accumulated on the dielectric to reduce the net electric field in that
particular region of the gap to a level that is less than the field required for
gas breakdown. Additional micro discharges will form at different locations
throughout the gap. Thus the dielectric serves to limit the intensity of the
individual discharges, and causes the discharges to occur at many locations
throughout the gap. When the polarity of the applied voltage is reversed, a
similar scenario takes place, but the directions of charge motion are reversed.
The low energy of such micro discharges results in relatively cool
temperatures which are desirable for efficient production of ozone. If the
electron density in an electrical discharge is very high, the average electron
energy may drop such that oxygen-molecule dissociation and hence ozone
production, declines (Murata Takaaki et al 1995). This may be the case when
relatively bright micro discharges occur. Water vapour can affect the
functioning of the dielectric, and can cause extremely strong micro
discharges.
16
In spite of the relatively high efficiency of ozone production using
the silent discharge, only about one-tenth of the supplied energy is used to
make ozone. The majority of the supplied energy is lost as light, sound, and
primarily heat. Since high temperatures accelerate ozone decomposition, as
well as dielectric breakdown, ozone generators require some form of cooling
system (Rosen Haniey et al 1972). Ozone yield increases as the power
dissipated in the discharge is increased however, it must be kept in mind that
ozone production efficiency, ie., the net mass of ozone produced by a given
energy input, will decrease if the temperature and/or ozone concentration
increase substantially. This term “efficiency” was chosen to conform to the
efficiency term often used in ozone generator technology and industrial
literature (Castle et al 1969). The power measurement (leading to the energy
value associated with the efficiency term) is the real AC power supplied to the
ozone generator electrodes.
Ozone acts by direct or indirect oxidation, by ozonolysis, and by
catalysis. The three major action pathways occur as follows:
1. Direct oxidation reactions of ozone, resulting from the action of
an atom of oxygen, are typical first order, high redox potential
reactions.
2. In indirect oxidation reactions of ozone, the ozone molecule
decomposes to form free radicals or it reacts quickly to oxidize
organic and inorganic compounds.
3. Ozone may also act by ozonolysis, by fixing the complete
molecule double linked atoms, producing two simple molecules
with differing properties and molecular characteristics.
17
The main purpose of ozonation is of dual nature. Ozone is expected
to perform both oxidation and disinfection (Oxidation is to remove organic
and inorganic contaminants and disinfection to kill bacteria, etc). Irrespective
of the amount of ozone generated per hour, a minimum concentration of at
least 1% is required for both oxidation and disinfection (Zoran Falkenstein
et al 1998). UV ozone generators cannot generate ozone at this concentration
to perform simultaneous oxidation and disinfection.
The most common method of ozone generation is to produce an AC
corona discharge in a gap bounded by metallic electrodes and containing at
least one solid dielectric barrier. A typical arrangement is shown in
Figure 1.10.
Figure 1.10 Cylindrical Ozonisers Cross Section
Ozone generators can produce ozone at a concentration ranging
from 1 to 16% w/w compared to 0.1 to 0.001% w/w by UV ozone. This is
mentioned as 10 to 1000 times less when compared to CD ozone generators.
The amount of air required for UV ozone generators is 10 times more than
that required for low ozone concentrations.
18
Moreover, the remaining ozone after ozone treatment converts
naturally to oxygen, (Cheng Zhang et al 2010) so secondary environmental
pollution does not occur. The most common method of ozone generation is to
produce an alternative current corona discharge in a gap bounded by metallic
electrodes and containing at least one solid dielectric barrier. This procedure
was first proposed by Siemens in 1857 and has been considerably studied
later (Fukawa et al 2006). In practical application of ozonisers, the
concentration and yield level is most important for evaluation of the ozonisers
performance. However these are not independent of each other and cannot be
considered separately. The selection must be made as to which one of these
quantities should be maximized, depending on the individual application
encountered. Increasing the concentration of the ozone is important to reduce
its cost.
1.2.2.6
Ozone formation and decomposition
The formation of ozone from molecular oxygen can be represented
by the equation
3O2 + 2O3
(1.1)
which, if the O2 and O3 molecules are in their fundamental energy states,
requires an energy input of 142.3 kJ per mole of ozone produced
(Masschelein et al 1982). Given that one mole of ozone has a mass of 48 gm,
the theoretical energy requirement for ozone formation from oxygen is
0.82 Whr per gram of ozone, or 1220 gm of ozone per kilowatt-hour
(Goldman et al 1982).
The required energy for ozone production is usually provided by
atomic oxygen, oxygen containing free radicals, excited oxygen molecules or
oxygen ions. The energy required for production of these species can be
19
furnished by several means, including ultraviolet light, electrical discharges,
electrolysis, and radioactive sources (Horvath et al 1985).
The process most widely used for ozone generation is the passage
of an oxygen bearing gas through an electrical discharge. Although many
pathways are possible, the major reactions which result in the formation of
ozone in an electrical discharge are:
e-+ O2
2 O +e-
O+O2+M
(1.2)
O3+M
(1.3)
where, the third body KI is needed to absorb the excess energy of the reaction.
The initial dissociation reaction requires an energy of 5.1 eV per molecule of
oxygen, which is supplied by the bombarding electron (Peyrous et al 1990).
The
ozone
forming
reaction
releases
1.08eV
per
molecule
of
ozone produced, so the net energy input for reactions (1.2) and (1.3) is
5.1 × 112 - 1.08 = 1.47eV per molecule of ozone formed, which is equivalent
to 0.82 Whr per gram of ozone.
Several mechanisms are also available for the decomposition of
ozone. Ozone is thermally unstable; its half-life decreases significantly as
temperature increases.
Bombardment by electrons can break ozone molecules apart, e.g.
O3 +eO3 + e-
O 2 + OO2 + O + e -
(1.4)
(1.5)
Ozone molecules can combine with each other, as well as with
oxygen molecules and oxygen atoms:
20
O3 + O3
O3 + O
(1.6)
O3 + O2
O 2 + O2 + O
(1.7)
O3 + O
O2 + O2
(1.8)
O3+M
O2+O+M
(1.9)
If a high concentration of oxygen atoms is produced, recombination
of these atoms to form oxygen molecules, instead of ozone, becomes a
significant detriment to ozone production (Peyrous et al 1990). Many particles
other than oxygen may be present in the gas, and consideration of all the
possible reactions would be extremely complex. One possible contaminant
that can have a large impact on ozone production is water vapour. Water
vapour can cause a substantial decrease in net ozone production by absorbing
electronic energy that could otherwise be used in the ozone formation
process:
H2 O + e -
H + OH + e-
(1.10)
the hydroxyl radicals can be formed from water vapour, and consequently
following reactions :
OH + O3
HO2 + O2
(1.11)
HO2 + O3
OH + O2 + O2
(1.12)
H+ O3
OH + O2
(1.13)
H+O3
HO2+O
(1.14)
In addition, oxygen atoms can combine with water vapour and its
products.
21
1.2.2.7
Corona discharge
Commercially, ozone is generated by producing a high-voltage
corona, in purified air/oxygen as feed gas. The ozone is then contacted with
the water or wastewater, the treated effluent is discharged and the feed gas is
recycled or discharged.
Ozone's high reactivity and instability, as well as serious obstacles
in producing concentrations in excess of 6 percent, preclude central
production and distribution with its associated economies of scale (Cobine
et al 1958). The requirement for on-site generation and application of ozone
must yield a cost-efficient, low maintenance operation in order to be useful.
The feed gas employed in ozonation systems is either air, oxygen or oxygenenhanced air. The particular selection of feed gas for each application is based
on economics and depends on several factors.
The ozone generator design depends on the total quantity of ozone
required, desired concentration of ozone, the feed gas type and rate (recycle or
discharge) of the feed gas. For given ozone generator with a specified power
input and gas flow, two to three times as much ozone may be generated from
oxygen as from air. The maximum concentration economically produced from
air is about 2 percent, while that generated from pure oxygen is approximately
6 percent (Kirk-Othmer Encyclopaedia of Chemical Technology, Vol 17,
1996).
The use of higher concentrations of ozone provides two advantages:
capital and operating costs per pound of ozone produced are substantially
reduced, and a greater concentration gradient for mass transfer of ozone is
provided in the contacting step, yielding increased ozone-utilization
efficiency (Moras et al 1993). These advantages, however, must be weighed
against the increased cost of oxygen production. Air is generally employed in
22
those applications requiring less than 22.68 kg/day of low concentration
ozone. If air is the feed gas, it must be dried and cooled to reduce
accumulation of corrosive nitric acid and nitrogen oxides that occur as
by-products when the dew point is above 40oC (Cobine et al 1958).
Ozone may be produced by electrical discharge in an oxygencontaining feed gas or by photochemical action using ultraviolet light. For
large-scale applications, only the electric-discharge method is practical since
the use of ultraviolet energy produces only low-volume, low-concentration
ozone. In the electric-discharge (or corona) method, an alternating current is
imposed across a discharge gap with voltages between 5 and 10 kV and a
portion of the oxygen is converted to ozone. A pair of large-area electrodes is
separated by a dielectric and an air gap (approximately 3 mm).
Only about 10 percent of the input energy is effectively used to
produce the ozone. Inefficiencies arise primarily from heat production and, to
a lesser extent, from light and sound. Since ozone decomposition is highly
temperature dependent, efficient heat removal techniques are essential to the
proper operation of the generator (Gallo et al 1978).
The mechanism for ozone generation is the excitation and
acceleration of stray electrons within the high-voltage field. The alternating
current causes the electrons to be attracted first to one electrode and then the
other. As the electrons attain sufficient velocity, they become capable of
splitting some O2, molecules into free radical oxygen atoms. These atoms may
then combine with O2 molecules to form ozone.
Under optimum operating conditions (efficient heat removal and
proper feed gas flow), the production of ozone in corona-discharge generators
23
is represented by the following relationships, showing the factors
to be considered in the design of these generators (Nicholas P Cheremisinoff
et al 2002):
where,
V
pg
(1.15)
Y
A
V2 f
d
(1.16)
Y/A - ozone yield per unit area of electrode surface
V
- applied voltage
p
- gas pressure in the discharge gap
g
- discharge-gap width
f
- frequency of applied voltage
- dielectric constant
d
- thickness of the dielectric
The following requirements will facilitate optimization of the ozone
yield:
The pressure/gap combination should be constructed so the
voltage can be kept relatively low while maintaining reasonable
operating pressures. Low voltage protects the dielectric and
electrode surfaces. Operating pressures of 10 - 15 pounds per
square inch gauge (psig) are applicable to many waste treatment
uses.
For high-yield efficiency, a thin dielectric with a high-dielectric
constant should be used. Glass is the most practical material.
High dielectric strength is required to minimize puncture, while
minimal thickness maximizes yield and facilitates heat removal.
24
For reduced maintenance problems and prolonged equipment
life, high frequency alternating current should be used. High
frequency is less damaging to dielectric surfaces than high
voltage.
Heat removal should be as efficient as possible.
1.3
ESTIMATION
OF
OZONE
CONCENTRATION
BY
WET-CHEMISTRY
Wet-chemistry test is an ozone concentration measurement method
for the ultimate process of determining the ozone production rate of a
commercial ozone generator. In this regard, the measured ozone concentration
is combined with the measured gas flow rate to calculate the ozone yield
(Kerwin Rakness et al 1996).
1.3.1
Methodology
To standardize the 0.1N sodium thiosulphate titrant, take 250 mL of
distilled water in a 250 mL Erlenmeyer flask, add with constant stirring,
1.0 mL of concentrated H2SO4, 20 mL of 0.1N K2Cr2O7 and 2g of KI. Titrate
with the approximate 0.1N Na2S2O3 until the yellow colour is almost gone.
Add 1.0 mL of starch indicator solution and continue titrating carefully until
the blue colour just disappears as seen in Figure 1.11, 1.12 and 1.13
respectively.
1. Fill 50 mL of burette with the Na2S2O3 titrant that is
standardized. Fill the burette just prior to adding ozone to the
gas washing bottle.
2. Add 400 ml of 2% KI solution to each gas washing bottle.
3. Level the wet test meter.
25
4. Bubble ozone through the KI solution and initiate recording the
volume bubble through the KI solution by allowing the nonozonized gas within the sample line to bubble. Begin recording
the wet test meter as soon as the yellow colour is noticed at the
point of entry of the gas in the washing bottle.
5. After bubbling has stopped, quickly add about 10 mL of 2N
H2SO4 to each gas washing bottle to lower the pH of the
solution below 2.
6. Read the initial volume of Na2S2O3 titrant in burette. Titrate
with Na2S2O3 until the solution becomes pale yellow colour.
7. Add 5 mL of starch solution to the flask. A bluish colour will be
formed. Carefully continue the titration, drop by drop until the
blue colour just disappears and solution is clear.
26
Figure 1.11 Reagents Prepared for Wet Chemistry Test
Figure 1.12 Solution before Passing Ozone Gas
27
Figure 1.13 Reaction after Passing Ozone Gas
1.3.2
Calculation of Ozone Concentration and Generation Rate
The concentration of ozone is calculated by KI wet-chemistry
method. It is based on the principle that iodide ion is oxidized by ozone to
form iodine as the ozone gas is bubbled through two bubblers containing
250 mL of 2% KI solution. This is used to trap the ozone in off gas, when
bubbling is stopped. The liberated iodine is titrated with standardized sodium
thiosulphate (0.1N) and starch (indicator). Then, a sodium thiosulphate
titration procedure is performed to measure the ozone concentration trapped
in the KI solutions (Kerwin Rakness et al 1996). To find the concentration of
ozone in mg/LNTP (PPM) the mass and VNTP have to be calculated. Thus the
temperature/pressure corrected gas volume is calculated by
VNTP
Va
Pa
Pv Pm
PNTP
TNTP
Ta
(1.17)
28
where,
VNTP -
Gas volume in liters referenced to normal temperature
and pressure conditions
Va
-
uncorrected gas volume, in liters, as measured
PNTP -
normal, referenced standard pressure
TNTP -
normal, reference standard temperature,
Pa
Barometric pressure in kPa.
-
The mass of ozone trapped in KI is given by
Mass =24 x Vt x Nt
where,
(1.18)
24 is the conversion factor (24000 me/L per 1000 mL/L)
Vt
-
volume of sodium thiosulphate used in mL.
Nt
-
normality of sodium thiosulphate in mg/me. Hence ozone
concentration is calculated from equation (1.19)
Ozone concentration =
Mass from equation (1.18)
VNTP
(1.19)
The ozone production also has obvious relation with flow rate. By
using the flow rate of oxygen (LPM) and the concentration of ozone,
determines the production of ozone as given below.
Ozone production (gm/hr) = LPM x 0.001 x 60 x14.3 x %O3
29
1.4
OBJECTIVES OF THE THESIS
Variable ozone concentrations were required for different
applications in industries. For e.g.: lather industry utilities various advanced
oxidization techniques for process of leather and its waste water treatment,
where in different ozone concentration are required. To meet the
requirements, a novel attempt is made to study the effect of various
parameters scientifically in improving ozone concentration.
The main objectives of the thesis are to study and improve the
concentration of ozone by varying the electrical parameters and develop
method for optimal design based on application. The objectives are:
(i)
Study of ozone, ozone generation methods and calculation of
ozone concentration using analytical methods.
(ii) Study of conventional silent corona discharge method using
pulse control constant high voltage.
(iii) To design a PWM circuit using MATLAB/Simulink, for
generation of variable frequency and high voltage using fly
back transformer and monitoring frequency by PIC16F877
microcontroller.
(iv) Analysis and design of a Ferrite core transformer using
SOLIDWORKS and study of various parameters like
magnetic flux, applied current density, etc.
(v) To analysis ozone concentration and yield by varying
frequency, voltage, flow rate, gas type and effect of
temperature.
30
1.5
CONTRIBUTIONS
1.
Design of a PWM circuit for obtaining variable frequency,
monitoring with PIC 16F877 Microcontroller.
2.
Design of high voltage and high frequency ferrite core
transformer for ozone generator.
3.
Information on ozone yield, concentration with variable
voltage, variable frequency ,flow rate, gas type and effect of
temperature.
1.6
OUTLINE OF THE THESIS
The thesis is organized into eight chapters. Each chapter highlights
the significance / results with respect to study / investigations.
Chapter 1 presents the general introduction about the ozone,
commercial ozone generators, analytical methods for estimation of ozone
concentration and silent corona discharge parameter for ozone yield, followed
by thesis objectives, contributions and outline of the thesis.
Chapter 2 reviews the research work and studies that had been done
by different authors and published in journals. It presents some of the work
related to applied voltage frequency, resonant converters, electrical strength,
gap width, dielectric constant, high voltage transformers and materials used
for DBD tube to improve the concentration of ozone.
Chapter 3 describes the development of conventional small ozone
generators by corona discharge using pulse control method and ignition coil.
While the dry air feed gas gives lower concentration, oxygen as feed gas
giving high concentration as shown in experiment. The conventional ozone
31
generator uses 0-5 kHz, and constant 5 kV to produce ozone concentration
with constant feed gas flow rate.
Chapter 4 discusses the enhancement of ozone gas concentration
using a PWM circuit and high voltage transformer monitored by PIC16F877
Microcontroller. The idea is to improve the concentration by using Pulse
Width Modulation (PWM) technique by varying frequency starting from 1.5
kHz to 25 kHz and with voltage 0 kV to 10 kV using fly back transformer. A
series of simulation studies has been carried out with Proteus software to
estimate the frequency and voltage parameters for ozone generation. The
experimental result shows a fine increase in concentration with PWM
technique compared to conventional method.
Chapter 5 describes the design of a ferrite core transformer of small
physical size, least expensive with minimum loss for high frequency design
using SOLIDWORKS. The main contribution is to design a 5 kV, 5 kHz
ferrite core transformer of 720 VA.
Chapter 6 describes the effect of temperature, gas type over the
ozone concentration and yield.
Chapter 7 draws the result analysis and discussion of the
experimental methods for various ozone concentration and yield, carried out
with variations in voltage, frequency, flow rate, gas type and effect of
temperature on concentration of ozone.
Chapter 8 discusses the conclusions arrived due to this research
work and it also sets further directions for research possibility in the future
and highlights some industrial applications.