Ozone Generation

Ozone is produced at the site of use because it is unstable and cannot be stored.

The ozone-generating system is selected according to the requirements on site,

including the ozone bleaching technology (medium- or high-consistency), the

source of oxygen (cryogenic or adsorption), the temperature of cooling water, and

the possibilities to recycle the vent gas for oxygen delignification. Figure 7.80 illustrates

the principal elements of an ozone bleaching system, including the oxygen

source, the ozone-generating system, the ozone delivery system with an ozone

compressor in the case of medium-consistency ozone bleaching technology, the

mixer or reactor, the off-gas destruction system and the vent gas recovery and

recycle loops.

Oxygen

Source

Ozone

generator

Ozone

compressor

Mixer /

Reactor

Ozone

Destruction

Cooling

water

O2

O2/O3

Pulp O3 treated Pulp

Vent gas

O2 gas to recycle

or reuse

Fig. 7.80 Principal course of ozone in a pulp

bleaching system (according to [14]).

Ozone is produced from oxygen-containing gases in ozone generators by means

of silent electrical discharge in the so-called “corona discharge process”. To date,

in bleaching operations only oxygen gas is used to achieve a high ozone concentration

and to avoid the formation of reactive byproducts such as nitric acid. Oxygen

is passed through two electrodes which are separated from each other by a

dielectric and two discharge chambers (Fig. 7.81). When a high voltage is applied

between two concentrically arranged electrodes, and the voltage exceeds the ionization

potential of the dielectric material, then electrons flow across the gap and

782 7Pulp Bleaching

provide energy for the dissociation of oxygen molecules; these then combine with

oxygen molecules to form ozone. The key element of a corona discharge ozone

system is the dielectric. The electrical charge is diffused over this dielectric surface,

creating an electrical field where high-energy electrons bombard gas molecules

so that they are ionized and a light-emitting gaseous plasma is formed,

which is commonly referred to as a “corona”. Many different materials in a variety

of configurations are used for the dielectric, including scientific-grade glass (e.g.,

borosilicate) and nonglass materials such as silicone rubber. The quantity of

ozone produced is related to a number of factors, such as the voltage and frequency

of the alternating current applied to the corona discharge cells, the cooling

system, and the design of the ozone generator.

O2

O2/O3

Outer ground

electrode

Discharge

gap

HV electrode

Cooling

water

dielectric tube

Fig. 7.81 Schematic diagram of an ozone generation system.

The generally accepted technologies can be divided into three types: low-frequency

(50–100 Hz); medium-frequency (100–1000 Hz); and high-frequency

(1000+ Hz). Medium-frequency ozonators are now favored as they provide many

benefits over the older low-frequency technology. An example of this is a greater

ozone production with less electrode surface area, so that the equipment can be

smaller for a given ozone output, and the power consumption per kg ozone produced

is also reduced.

Since ozone generation by corona discharge is an exothermic physico-chemical

reaction, and ozone decomposition increases as the gas temperature and ozone

concentration increase, correct cooling is an important factor in generator design.

Moreover, oxygen entering the ozone generator must be very dry (minimum

65 °C), because the presence of moisture affects ozone production and leads to the

formation of nitric acid. Nitric acid is highly corrosive to critical internal parts of a

corona discharge generator, and this can lead to premature failure and a significant

increase in the frequency of maintenance. Besides the destruction of the

ozone generator itself, transition metal ions are released from the stainless steel

electrodes, and this can be very harmful to the pulp during the course of ozone

bleaching. Depending on the strength of the electric field, cooling and the design

of the ozone generator, ozone yields of up to 16% by weight (~240 g m–3) can be

7.5 Ozone Delignification 783

achieved in the production gas. The specific energy consumption for the production

of 1kg ozone is usually between 6 and 10 kWh, depending on the desired

concentration. The efficiency of medium-consistency ozone bleaching is limited

by a certain gas void fraction, X g (according to Bennington, the upper operating

limit is reached at X g = 0.13 [15], and according to industrial experience at X g ~0.25

[16]). The gas void fraction is defined by Eq. (88):

Xg _

Vg

Vg _ VL

XR _

Vg

VL

Xg _

VR

1 _ VR

_ XR _

Xg

1 _ Xg

_88_

where:

Vg,T,P= Vg_T0_P0 _ 101_3 _ T

P _ 273_15 is the volumeof the gas fraction, with P in kPa and T in K;

VL= Prod

_con _ qsusp_

is the volume of the aqueous pulp suspension;

XR is the volume ratio;

Prod is the standardized pulp production (e.g., 1odt pulp);

qsusp = 1

con

1_53 _ _1 _ con_

qliquid _

is the density of the pulp suspension;

con is the pulp consistency, expressed as a fraction; and

qliquid ~1is the density of the liquid.

Both high-concentration ozone feed and compression of the feed gas are required

to ensure an efficient ozone consumption rate in medium-consistency

ozone bleaching. Compression is exclusively carried out isothermally by means of

liquid-ring compressors to avoid ozone destruction. The influence of ozone concentration

in the feed gas to the compressor on the ozone charge being efficiently

consumed in a medium-consistency mixer at a constant pressure of 8 bar at typical

industrial conditions (T = 50 °C, Xg,max = 0.25) is shown in Tab. 7.36.

The data in Tab. 7.36 indicate that the ozone charge in a medium-consistency

ozone mixer is limited to 3.2–4.2 kg odt–1. Clearly, the efficiency of medium-consistency

ozone bleaching also depends on the specific energy dissipation, e, and

on the retention time (see Section 7.5.5.2, Mixing). However, in the case of a modern

medium-consistency mixer the addition of higher ozone charges is connected

with decreasing amounts of ozone consumption rates (see Fig. 7.107).

784 7Pulp Bleaching

Table 7.36 Effect of ozone concentration in oxygen gas prior and

after compression to 0.8 MPa on the limit of ozone charge in a

medium consistency ozone mixer.

Ozone concentration in oxygen X g

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