High-Consistency Ozone Treatment

High-consistency bleaching requires a press to be utilized before the stage of efficient

pulp dewatering. A plain dewatering press is preferable as it achieves higher

consistencies (up to 40%) than a wash press. It is necessary to adjust the desired

pH in the press feed, because there is no means by which sulfuric acid can be

mixed in between the press and the reactor. The fiber mat leaving the press nip

must be thoroughly disintegrated in order to ensure good accessibility for the

bleaching chemicals to all fiber surfaces.

The Metso ZeTrac ozone delignification system is illustrated in Fig. 7.100. Although

former HC systems required a screw feeder and pulp fluffer, the Metso

ZeTrac does not. Instead, the specially designed shredder screw of the press delivers

well-fluffed pulp which falls into the horizontal reactor and is brought into

contact with the ozone/oxygen gas mixture [100]. Paddles keep the pulp in motion

as it travels concurrently with the gas through the reactor. The reactor operates at

a slight vacuum, thus ensuring that no gas can escape to the ambient air [101].

The delignified pulp is discharged into a dilution screw conveyor, where it is

alkalized. The medium-consistency slurry then falls into a tank before being

pumped to the subsequent stage.

Fig. 7.100 The Metso ZeTrac high-consistency (HC) ozone bleaching system [100].

7.5 Ozone Delignification 827

7.5.6.3 Ozone/Oxygen Gas Management

The oxygen gas needed for the generation of ozone can be supplied from a liquid

oxygen storage tank, or it can be produced on site by oxygen plants using either a

pressure swing adsorption (PSA) or vacuum swing adsorption (VSA) process.

Oxygen is by far the predominant gas in any commercially generated ozone/

oxygen gas mixture. When entering the pulp delignification process, each kilogram

of ozone is accompanied by 6–9 kg of oxygen. This amount of oxygen, together

with oxygen which has been created by the decomposition of ozone, forms

the major part of the offgas leaving the delignification process.

The flow scheme of the common once-through gas system is shown in

Fig. 7.101. Oxygen is fed to the ozone generator and, if used for medium-consistency

delignification, is subsequently compressed. The compression step can be

omitted in the case of high-consistency delignification. After addition to the pulp,

ozone is partly consumed during the delignification reaction and partly decomposed.

Only small concentrations of ozone are left in the offgas, and these are

destroyed in a dedicated destruction unit. Catalytic destruction is the most popular

approach, followed by thermal destruction. The residual gas after destruction is available

for other applications which normally require its re-compression. In the bleaching

plant there are several points where residual oxygen from ozone delignification

can be re-used. These include oxygen delignification, oxygen-reinforced extraction or

peroxide bleaching, and white liquor oxidation. Other opportunities for re-use may

exist in other areas of the mill.

Offgas

separation

Ozone

generation

(Ozone

compression)

Ozone

delignification

Ozone

destruction

Pulp Pulp

Oxygen

compression

Oxygen to oxygen

delignification and

Oxygen other utilisation

(Excess oxygen

to atmosphere)

Fig. 7.101 Once-through gas management.

If the oxygen supplied with the offgas exceeds the mill’s demands, the most appropriate

option is to exhaust some offgas into the atmosphere. Although at first sight

this may not be the most economic solution, the ambivalent operational experiences

fromthe so-called long loop system(Fig. 7.102) seembarely worth the effort. The long

loop contains an additional purification step, where gas components known to interfere

with ozone generation (including moisture) are eliminated as far as possible.

Since the ozone generators are very sensitive to inappropriate gas feed, the perfect

function of gas purification is vital for the availability of ozone from a long loop

system.

828 7Pulp Bleaching

Oxygen to oxygen

delignification and

other utilisation

Offgas

separation

Ozone

generation

(Ozone

compression)

Ozone

delignification

Ozone

destruction

Pulp Pulp

Oxygen

compression

Gas

purification

Oxygen

Fig. 7.102 Long loop gas management.

With modern ozone generators delivering 14% by weight of ozone, the oxygen

supply and demand in the mill is balanced at ozone charges between 4 and

6 kg ton–1 of pulp. Even at lower ozone concentrations in the feed gas, the majority

of ozone delignification applications features specific charges below the balanced

maximum. As a result, once-through gas management has become the system of

choice over the past years.

7.5.7

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