WO2009087265A2

WO2009087265A2 - Treatment of ozone-containing gas in connection with bleaching of pulp

Info

Publication number: WO2009087265A2
Application number: PCT/FI2009/000002
Authority: WO
Inventors: Janne Vehmaa
Original assignee: Andritz Oy
Current assignee: Andritz Oy
Priority date: 2008-01-08
Filing date: 2009-01-08
Publication date: 2009-07-16
Anticipated expiration: 2010-07-08
Also published as: FI20080008L; WO2009087265A3; FI20080008A0; FI20080008A7

Abstract

The present invention relates to a method of and apparatus for decreasing the ozone content of gas in connection with an ozone bleaching process for pulp in a chemical pulp mill plant provided with or connected to an ECF-bleaching plant for pulp and provided with at least production of a mixture of ozone and carrier gas, ozone bleaching equipment, discharge line for mixture of residual ozone and carrier gas from the ozone bleaching equipment and emergency discharge line for ozone-containing gas bypassing the ozone bleaching equipment and after these a connected first gas scrubber for treating the ozone-containing gas. An essential feature of the invention is that the last treatment stage for ozone-containing gas for removing ozone is carried out in the second gas scrubber, wherein both residual gas from ECF bleaching and residual gas of the ozone stage are treated with reductants. Thereby the elimination of residual ozone can be ensured, especially in disturbance situations.

Description

TREATMENT OF OZONE-CONTAINING GAS IN CONNECTION WITH BLEACHING OF PULP

The present invention relates to a method of decreasing the ozone-content of gas in connection with an ozone bleaching process of pulp at a chemical pulp mill plant.

Ozone gas is considered a detrimental, emission-producing gas in situations where it is released into the atmosphere from e.g. processes of the chemical pulp industry, chemical in- dustry or water purification. Therefore, processes using ozone have always been provided with both ozone production apparatuses and apparatuses relating to the destruction of residual ozone of waste ozone mixture.

These destruction-related apparatuses have typically been apparatuses based on either thermal or catalytic degradation of ozone. Other methods have also been described, but manufacturers of ozone plants have not began to develop them into industrial applications.

Ozone is an oxidant in the same way as other active bleaching chemicals, such as chlorine, chlorine dioxide, peroxide etc. Of these, active chlorine can be reduced using e.g. a sodium bisulfite solution under alkaline conditions. Many other reductants are also suitable for this purpose. Thus, it has been noticed that a chemical reduction method can be used for both chlorine-containing chemicals and peroxide, by means of which method the oxidizing potential is chemically eliminated. Vent gases containing active chlorine can in some cases be eliminated also by using alkaline gas scrubbing without a separate reductant, but in that case the amount of active bleaching chemical, such as chlorine, in the vent gases is small and the retention time in the scrubbing apparatus is long.

Elimination of active chlorine chemical by means of a reductant is technically effected so that gas containing residual chlorine dioxide is introduced into conditions where it is passed in contact with a bisulfite solution. As the volume of the gas is great and the reductant is in liquid form, apparatuses are used, in which the gas is led into a gas scrubber. Thereby a reacting reductant is pulverized or atomized into the gas being led to the gas scrubber so that the pulverized or atomized liquid has so large a reacting surface size that the reducing reaction can take place in a time that is short enough. In practice, this kind of apparatuses are e.g. gas scrubbers that may comprise a venturi part, wherein a very efficient reaction surface is formed with pressure, or either spray- or plate -type gas scrubbers, where the liquid is atomized, forming a reaction surface with a size as large as possible. Additionally, various adsorption columns are also suitable apparatuses, in which the reacting liquid surface can be increased e.g. by means of packing elements and thus the reaction between the liquid and the gas can be promoted.

Especially in relation to ozone it can be adequate that the reaction between the reductant and the ozone-containing gas takes place for a very short time. In that case, for the reducing reaction a simplified gas scrubber is adequate, e.g. a reduction nozzle in a pipe enlargement or a very small gas scrubbing device, because the reactivity of ozone is very high.

In ozone bleaching, the chemical pulp is led into contact with an ozone-containing gas. Ozo- ne is highly reactive, and therefore, when being led to a bleaching stage the ozone is typically in form of a mixture of approximately 10-12 weight per cent with a carrier gas, such as oxygen or another gas. In different applications the concentration of the gas may, however, vary e.g. between 4 and 18 weight per cent. The consistency of the pulp varies depending on the applications. In low-consistency ozonization, the consistency of the pulp is between 0.1-6 %, whereby normal process pumps, mixers and reactors are used in the processes. Medium- consistency ozonization is carried out at a pulp consistency range of 6-18 %, whereby e.g. MC-pumps used for pumping high-consistency pulp are needed, and mixing devices designed for this consistency range. Today these systems are apparatuses having a pressure of 2-20 bar overpressure, and the mixing takes place in a closed space. The temperature of the pulp may vary depending on the application between 20-95 ⁰C, but the most common temperature range applied in mill conditions is 50-75 ⁰C. The pulp is most usually acid, i.e. the pH is below 7. As ozone reacts most efficiently at a pH level below 3.5, the pH in practical applications is most usually between 2 and 4, most preferably 2.5-3.5.

High-consistency ozonization takes place under conditions where the consistency of the pulp is raised above 18 %, and the most commonly reported consistency range is 25-40 %. In technical sense, the consistency could be raised even up to a level of 50 %, whereby ozonization would be carried out at this consistency range. Pulp at a high consistency cannot any- more be pumped or mixed according to the same principles as medium-consistency pulp, but the reaction of pulp and ozone is arranged such that the pulp is decomposed to slush, which is mixed in a specially designed reactor. The reactor is provided with a gas space for ozone- containing gas, wherefrom the ozone is mixed into the slush and reacts on the surface of fi- bers and fiber bundles. The temperature of the pulp in high-consistency ozonization is lower than in medium-consistency ozonization, so that in industrial applications temperatures of 30- 55 ⁰C are preferred, but applications exist, where the temperature has for various reasons been raised up to a level of 75 ⁰C. The pH level of the pulp is the same as in medium- consistency ozonization.

In industrially effected ozone bleachings the reported ozone dose has been 1-15 kg/adt, most typically 2-7 kg/adt. Experimentally, it has been possible to try ozone doses up to 20 kg/adt in laboratories, and on the other hand in these special applications small doses of 0.5-1 kg/adt have been experimented.

In all these applications of ozone bleaching, the ozone is fed together with said carrier gas to react with the pulp. In the present solutions, the carrier gas is usually oxygen, whereby ozone reacts from the carrier gas with the pulp. In these ozone bleaching conditions the carrier gas is practically inert. However, not all the ozone is consumed for treating pulp during ozone bleaching, but the efficiency of ozone bleaching is considered good, when more than 95 % of the ozone has been consumed during the bleaching stage. Also such ozone bleachings have been reported where only 85-95 % of the ozone has been consumed. The rest of the ozone gas remains in the mixture of carrier gas and ozone. This residual gas is regarded as detrimental environmental emission, which as such must not be led into the atmosphere. Further, if the carrier gas is oxygen, there may be a desire to utilize it in other process stages of the mill, such as in oxygen delignification of pulp, whereby passing of ozone to other departments of the mill, or even via a process stage into the atmosphere is to be prevented.

The molecular structure of ozone is highly instable, so that at the simplest it can be decom- posed thermally. This kind of devices based on elimination of ozone are commonly used. Additionally, decomposing the molecular structure of ozone can also be effected by means of a catalyst, under the effect of which ozone is decomposed back to oxygen. Both methods require optimal process conditions in order to function and their own separate apparatuses just for the elimination of ozone. The apparatuses are secure, but still they require maintenance and consume energy.

In chemical technique it is obvious that a strong oxidant, such as ozone, can be chemically reduced by means of reductants so that its oxidizing potential is eliminated. In industrial sense, the ozone production technique was primarily developed for e.g. cleaning plants for drinking water, in which plants ozone was the only chemical that was used. Ozone producers also had to develop treatment systems for residual ozone, so that it was natural that the system that was used was a thermal or catalytic destruction method operating without additional chemicals.

In a chemical pulp mill, chemical elimination of chlorine-containing oxidants from a gaseous mixture has been known technique for years, and therefore suitable apparatuses are found in existing industrial plants.

In view of the volume of material flows being treated, unit operations carried out in an industrial bleaching plant are in a totally different size range than a process or process simulation carried out in a laboratory or a pilot-equipment. At present, in industrial sense the smallest operative units are of a size of approximately 100 adt pulp/d, but some special pulps are still produced in production lines with a production rate of approximately 50 adt/d. No upper limit can be set for industrial production, i.e. it has been possible to increase the production in new plants up to 3000 adt/d, and at the moment there seems to be no individual restriction to the productivity increase of industrial plants in pace with development.

It is appropriate to restrict the lowest limit of chemical pulp industry to 100 adt/d, whereby it can be seen how the material flow amounts significantly exceed those used in laboratory experiments, where the sample amounts of pulp being tested are typically limited to a few kilograms maximum. If 100 adt/d of pulp per day is bleached with ozone so that the amount of ozone is quite the usual, i.e. 4 kg/adt, the production of ozone per day is 400 kg. Because the concentration of ozone is 12 weight per cent, the total amount of gas led into ozone bleaching is 3330 kg per day. The design consumption of gas consumed from the ozone is 95 %, so that the bleaching plant is to treat 20 kg of ozone per day to a harmless form. As the releasing ozone is mixed in the carrier gas, 20 kg of ozone gas is mixed in 2950 kg of oxygen gas. The amounts seem to be large when given in kilograms, but when approximately 3000 kg of oxygen-ozone mixture is converted to nominal cubic meters, a total volume flow of 2100 nominal cubic meters is achieved. This is according to the lowest pulp production alternative. When the production is higher, e.g. 2000 adt/d, the volume flow of gas at a chemical pulp mill with the same efficiency and dose exceeds 20000 nominal cubic meters per day and the amount of ozone to be destructed is 400 kg/d. All the above presented dosing alternatives are possible, but these examples show that industrial gas treatment creates a specific problem of its own.

Andritz Oy has developed for the treatment of chlorine dioxide a gas scrubber system for chlorine-containing chemicals, where the gas is first led via a venturi part into a gas scrubber. In the venturi part, a mixture of sulfur dioxide (SO₂) and sodium hydroxide solution is pumped at high pressure into the gas, whereby the reducing active ingredient is sodium bisulfite solution. As the reduction of ozone is effected with the same chemical, the reduction of ozone can also be effected using the same gas scrubber technique. In this application, such a large re- action surface is formed between the gas and the reducing liquid by means of the venturi part that the reduction of ozone succeeds. In the gas scrubber part, the gas at large volume further contacts the reacting solution, and simultaneously solid particles, such as residual fibers, are washed off the gas being discharged.

Because in many ozone bleaching applications more than 98 % of the dosed gas is consumed, the ozone reduction reaction does not need even a venturi part, but scrubbing of the gas with a reductant-containing solution is then an adequate measure. The use of a reductant for eliminating the oxidizing capability of ozone is disclosed in publication WO 93/18226, where the residual gas of the ozone stage is washed in a scrubber with a liquid originating from the chemical pulp mill for removing fibers and ozone. Suitable scrubbing liquids are e.g. a sodium bisulfite stream or a filtrate from an alkaline bleaching stage. The scrubbed gas stream can further be taken to oxygen recovery without a separate thermal or catalytic destruction treatment for ozone, or at least the need of the latter is minimized. It has been noticed in the publication that by means of a suitable scrubbing liquid it is possible, in addition to removing the fibers, also to destroy the ozone. US patent 6210527 presents that residual ozone gas is scrubbed with white liquor to form an oxygen-containing flow, whereby no catalytic ozone destructor is needed. WO-publication 9706305 presents that residual ozone gas can be scrubbed in a gas scrubber with an alkaline solution, such as white liquor, weak black liquor or a filtrate of E₀ stage. If needed, the ozone gas flow can bypass the ozone bleaching tower, and fresh gas is led into said ozone stage gas scrubber.

However, elimination in one step only is in case of ozone not adequate for guaranteeing ozone security and adequate elimination of ozone emissions in environmental point of view. During normal use this is adequate, but controlling disturbance situations creates a problem.

In some prior art cases the disturbance control has been carried out by stepping reductive gas destruction and thermal or catalytic elimination. This solution does not allow decreasing the number of gas treatment apparatuses, whereby the mill has the previous amount of service and maintenance objects.

An object of the present invention is to provide a simpler method for destroying residual ozone gas so that the number of apparatuses required is smaller than in known methods. Especially the object of the invention is to provide a process, where the destruction of ozone can be carried out efficiently and reliably also in disturbance situations.

For reaching these objectives, the present invention relates to a method of decreasing the ozone content of gas in connection with an ozone bleaching process for pulp in a chemical pulp mill plant provided with or connected to an ECF-bleaching equipment for pulp and provided with at least production of a mixture of ozone and carrier gas, ozone bleaching equipment, discharge line for mixture of residual ozone and carrier gas from the ozone bleaching equipment and emergency discharge line for ozone-containing gas passing the ozone bleaching equipment and after these a connected first gas scrubber for treating the ozone- containing gas. An essential feature of the invention is that the last treatment stage for ozone- containing gas for removing ozone is carried out in a second gas scrubber, wherein both residual gas from ECF bleaching and residual gas of the ozone stage are treated with reduc- tants.

The treatment of ozone-containing waste gas according to the invention is suitable for all chemical pulp mills using ozone, where ECF bleaching is used and where one bleaching chemical is chlorine dioxide. (ECF-bleaching covers all such bleaching sequences, which have at least one chlorine dioxide stage and which do not use elemental chlorine in any bleaching stage.) Then fractions that contain elemental chlorine are led into a dedicated gas scrubber and reduced using the same chemicals as here presented for use in case of ozone. The basic idea of the invention guarantees in the sense of security an acceptable solution, by means of which elimination of ozone can be assured both in a normal stage and in case of disturbance. In the gas scrubbing stage the pH of the scrubbing liquid exceeds 7. The reduc- tant used is bisulfite, mixture of SO2-water and NaOH or sodium sulfite.

Ozone bleaching at medium consistency is carried out so that the medium consistency pulp is pressurized to a pressure of 4-40 bar and led to a mixer. By means of the mixer, a mixture of ozone and carrier gas is introduced into the pulp, and thereafter the pulp and the gas mixture are mixed in order to generate a reaction. After the first mixer, a second mixer can be provided for intensifying the bleaching reaction, where after the pulp is taken into a small reac- tion vessel. The reaction vessel is provided with a discharge device and degassing, after which the pressure is released and the pulp is led into a blow tank. Also, one embodiment of low consistency ozonization is of the same type, but in high consistency ozonization the treatment of pulp is different. Nevertheless, all embodiments comprise the reaction of gas with the pulp and the separation of carrier gas and ozone from the process and their treatment for re- use or for releasing them into the atmosphere.

Because the ozone is produced on the spot and not stored anywhere, in a disturbance situation the bleaching plant has to be bypassed and all ozone that is pressurized and remaining in the piping has to be ventilated off the pipings and eliminated usually by means of a sepa- rate ozone eliminator.

After the ozone bleaching equipment there is most usually a first gas scrubbing stage, where water is fed to the venturi part of the gas scrubber. The function of this first scrubbing device is first of all to wash off the fibers from the ozone-containing gas flow and to regulate the tem- perature of the exhaust gas. This gas scrubber operates mainly with water, but the scrubbing liquid can also be arranged to comprise reducing conditions for destroying ozone. Disturbance-free operation of this first scrubbing device allows carrying out different gas treatment methods. In a normal situation the ozone is led into the bleaching equipment, wherefrom the mixture of carrier gas and unreacted ozone is taken to a first gas scrubber, where the fibers are separated and preferably the unreacted ozone is reduced. After the reduction the gas is taken e.g. to a compressor, wherefrom the gas is led e.g. to an oxygen stage, an EOP-stage or oxidizing of white liquor. If the gas mixture is used in the oxygen stage so that no residual ozone has been eliminated, then it is possible - in case of a water ring compressor - to add to the water ring water a small amount of reducing solution in order to avoid corrosion further in the piping.

A disturbance situation questions the capability of this known system for eliminating ozone. In the arrangement according to the invention for ECF-bleaching the gas scrubber operating at the mill acts as a safety system. When the flow of the ozone-containing gas in the process is disturbed, i.e. in situations where the gas is not led to re-use or the gas bypasses the ozone bleaching plant, the mixture of ozone and carrier gas is first led into a normal gas scrubber of the system, which gas scrubber is preferably provided with a reductant. After this, the gas mixture is not led to a compressor, but directly to a second treatment stage to a gas scrubber of ECF-bleaching together with a gas flow containing residual active chlorine from the chlorine dioxide stage.Tthis second gas scrubber usually is bigger than the first one both in view of volume and capacity, its operation guarantees adequate elimination of ozone in view of safety and the environment, before the waste gas of the ozone bleaching stage is released into the atmosphere. The scrubbing device is preferably a venturi scrubber, a plate scrubber or a packed bed scrubber.

Ozone was first regarded as a technique for TCF-bleaching only. Then the reduction in a gas scrubber as a precaution was out of question, because chlorine-containing chemicals were not used and therefore the plant was not provided with a scrubber operating with reducing chemicals. The only way has been to use a separate ozone eliminator, the operation of which is based on either thermal or catalytic destruction of the molecular structure of ozone. Be- cause the use of a gas scrubber is inevitable in ECF-bleaching due to environmental reasons, the coupling according to the present invention is in view of investments and operational expenses superior compared with existing techniques. Because the biggest problem in a disturbance situation is instantaneous high-volume flow of gas not used in the process, difficulties resulting from that can be alleviated by means of simple design details. Because in MC and LC -ozonization the pulp is led into the process at high pressure, impediments caused by a sudden emission of gas to the operation of gas scrub- bers can be alleviated e.g. by using pressure accumulators or larger pipe diameters, whereby the gas expanding in the piping flows more calmly to the gas scrubber and the reduction takes place efficiently.

If the elimination of ozone has to be done during remarkable process disturbance situations, in which ozone-containing gas cannot be led to the bleaching plant any more, ozone remains in the piping after closing of the valves. Usually the ozone has then been led directly to an ozone eliminator bypassing ozone bleaching. Because the situation in the system according to the invention is the same, i.e. ozone-containing gas flow bypasses the bleaching reactor, the situation in case of the invention does not differ at all in view of distrurbance situations. On the other hand, the gas elimination system according to the invention also allows back-up elimination. While in a normal situation after gas treatment following the ozone reactor the gas would be re-pressurized for other process stages, in a disturbance situation when the gas mixture bypasses the ozone bleaching reaction(s), it is inevitable to make sure that no ozone is released from the process, so that after the first gas scrubber the gas is preferably led into the vent gas system of ECF-bleaching in accordance with the present invention.

This is possible because chlorine dioxide bleaching is provided with a gas elimination process for residual active chlorine, whereby in accordance with the invention the elimination of gas in a disturbance situation takes place in two steps, i.e. in an ozone gas scrubber and a gas scrubber of ECF-bleaching. Thus it is possible to make sure that the equipment really is safe to operate and no ozone is released into the atmosphere.

Although chemical reduction of ozone is known, literature and industrial reports do not mention that residual ozone could industrially be controlled by means of reductants in gas scrub- bing devices. If the scrubbing device is adequately big and the amount of chemical is sufficient, acceptable level of elimination is possible. However, because the carrier gas of the ozone stage has other objects of application than being released into the atmosphere, e.g. an oxygen stage, the two-stage elimination system according to the invention is preferable due to sudden, but short high loading of the gas elimination devices.

When applying the invention, an essential feature e.g. when using NaOH- and SO₂ -solutions is the pH, which should be between 8-13, preferably 9-11. SO₂ -solution is dosed in propor- tion to the NaOH-amount (SO₂ + NaOH -> NaHSO₃) so that there continuously is an adequate amount of reductant in the elimination system.

For improving the control of disturbance situations, it is preferable in designing the system to allocate sufficient volume to the gas scrubber either to the first unit or the unit of ECF- bleaching, and to ensure that the venturi part is sufficiently large. It is also possible that the high-pressure pump of the venturi is provided with rotational speed control for disturbance situations. Then, in a disturbance situation, the elimination efficiency can be intensified by increasing the rotational speed of the pump and thus the efficiency of the venturi.

According to a feature of the invention it is preferable, if in the ozone production system the volume of the gas and thus the amount of the gas is kept as small as possible.

According to a preferred embodiment of the invention, the ozone generator is located as close as possible to the compressor that is located as close as possible to the ozone bleaching equipment. After the gas discharge point of the bleaching equipment, the distance to the ozone eliminator is short.

The novel method according to the invention is suitable for all kinds of ozone bleaching processes treating chemical pulp. Thus the pressure during ozone bleaching or in the reactor can be 0.1-50 bar absolute pressure and the consistency of the pulp 0.1-50 % and the reaction time 0.1-3600 s. The temperature can be 1-99°C. It is noticed from said conditions that the gas elimination system is totally independent on whether it is used with low, medium or high consistency ozone bleachings.

Because the method according to the invention is clearly an industrial method, wherein the elimination of an oxidant is carried out industrially in a different way than before, the use of the method can be restricted to the minimum limit of industrially treatable gas amount, where the flow of ozone-containing gas is at least 1000 nm3/d in a plant complex according to the invention. This restriction separates laboratory experiments and an industrial installation in a way that shows the essence of the invention, i.e. that by means of the liquid mixture of a re- ductant it is possible to eliminate dangerous amounts of residual ozone into a harmless form and after that to utilize the carrier gas elsewhere in the process. Further, it is to be noted that in the system the reductant is in liquid form that reacts with ozone, reducing it. Thus, e.g. a catalytic or thermal reductant operates according to a different principle than the system presented herein.

An advantage of the invention can be seen in the simplifying of the systems, because therein the functionality of the gas scrubber already being delivered to ozone bleachings is significantly increased. In industrial applications where large gas flows are treated at the gas scrubber part, so far only fiber separation from the gas flow and decreasing the moisture of the gas have been effected there. In the system herein presented, a third unit process critical in view of the mill, can be included in the same apparatus, wherein the residual ozone is eli- minated from the gas flow. This allows totally removing the ozone eliminator, which is troublesome for the mill, whereby the ozone bleaching system will be significantly simplified.

The invention is described in more detail with reference to the accompanying figures, of which Fig. 1 illustrates schematically a preferred embodiment for implementing the method according to the invention.

Fig. 2 illustrates schematically another preferred embodiment for implementing the method according to the invention. Fig. 3 illustrates schematically a third preferred embodiment for implementing the method according to the invention.

Fig. 4 illustrates schematically a preferred method for arranging washing liquid circulations for the gas scrubbers.

Fig. 5 illustrates schematically another preferred method for arranging washing liquid circulations for the gas scrubbers. Fig. 6 illustrates schematically a third preferred method for arranging washing liquid circulations for the gas scrubbers.

The embodiment of Figure 1 illustrates an arrangement for removing ozone gas residuals, which arrangement is simple in view of the invention.

The pulp is introduced via line 2 into the ozone bleaching stage. A mixture of ozone and carrier gas, most usually oxygen, is added into the pulp via line 3. The gas is mixed with the pulp in two mixers 4 installed in line 2. The pulp is taken into an upflow reactor 5, wherein the reactions with ozone mainly take place. After the bleaching, the pulp is discharged from the reactor 5 via line 6 into an ozone stage blow tank 7, from the lower part of which the ozone- bleached pulp is taken via line 8 into a following process stage. In the blow tank, due to decreased pressure, the gas is separated from the pulp. The unpurified mixture of residual ozone and carrier gas is led via line 9 into a first gas scrubber 10. In the ozone gas scrubber, water and, if need also reductant, such as bisulfite, is added into the gas via line 11 , whereby fibers are washed off the gas and ozone is destructed by reduction. The need for reduction depends on future use of the gas and the ozone amount. The gas is taken via line 12 for use preferably in the oxygen stage of the pulp, wherein oxygen gas that acted as carrier gas can be utilized.

If the ozone gas cannot be fed to the bleaching reactor, the gas is led via bypass line 3a and line 9 directly to scrubber 10. In accordance with the invention the ozone gas scrubber is connected to a gas scrubber 13 for chlorine-containing gases, which receives chloride and/or chlorine -containing waste gas from the chlorine dioxide bleaching stage for pulp via line 14 by means of a fan 15. Reductant, such as bisulfite solution, is added from line 16. In a disturbance situation, if the flow of gas into the oxygen stage or ozone stage is prevented, the reduction of residual ozone is assured by leading it via line 17 together with chlorine-containing gases to a gas scrubber 13. The purified gas is removed via line 18.

The arrangement of Figure 2 is corresponding, but in this case the oxygen gas that acted as carrier gas is not reused in the process, but it is led continuously via 17 to a gas scrubber 13 for chlorine-containing gases. In this arrangement, a disturbance situation does not cause any changes, as the connection (line 17) provides continuous preparation for that.

Figure 3 illustrates a solution provided after an ozone gas scrubber with a known per se separate ozone eliminator 19, wherein the ozone is destructed either thermally or catalytically in a way known per se. As a precaution means for disturbance situations, in accordance with the invention a scrubber 13 for chlorine-containing gases in provided after the ozone eliminator. With this solution, essentially complete elimination of ozone can be ensured. In the basic solution of Figure 3, the oxygen gas that acted as carrier gas is not reused in the process, but it is led continuously via line 17 to a gas scrubber 13 for chlorine-containing gases. If the waste gas from the ozone stage is used in the oxygen stage or elsewhere as oxygen gas, the oxygen gas is naturally obtained before the last gas scrubbing device via line 20, but it can also be separated upstream of the oxygen eliminator 19 via line 21.

If after the ozone stage gas is led into an oxygen stage process and this is prevented in a disturbance situation, all the gas can be led to a gas scrubber of bleaching. Because the last gas scrubber acts in the removal of residual ozone gas as a backup system only, no special arrangements are needed. If it is inevitable to prepare to use the plant for long periods so that no waste gas can be led into the oxygen stage, then also the first gas scrubbing device is preferably provided with both pH regulation and a possibility to add a reductant. If the ozone dose is low, all the ozone is in practice consumed in the process and then just an ECF- bleaching scrubber is sufficient or if the ozone-containing gas is taken into the oxygen stage, a small amount of ozone does not harm.

As the gas scrubbing devices with ozone gas have to be resistant to very wide loading fluc- tuations, the solution has to be such that during a peak load there always is sufficiently of reducing chemical for eliminating the ozone in typically both scrubbers. Therefore, the use of an ECF-bleaching gas scrubber is preferable, because its volume compared with the ozone- stage gas scrubbing device is large.

Because at present there is no clear regulation method for regulating the amount of reducing chemical, the dosing of chemical is excessive. Thus, in an ozone stage gas scrubber, where the amount of ozone gas to be eliminated is very small in a normal stage, a remarkable amount of reducing chemical could be wasted. Thus, in a preferred embodiment the reducing chemical solution is led to a gas scrubbing device of the ozone stage, wherefrom the over- flowing liquid containing reducing chemical is led to a gas scrubbing device of ECF-bleaching for chlorine-containing gases. As in a normal state, reducing chemical is practically not consumed in the first scrubbing device, it will be sufficient for reducing also the chlorine- containing gases. Additionally, such an amount of chemical is "stored" in the liquid volumes of the gas scrubbing devices, that it is well sufficient in situations where high-volume ozone gas emissions are to be reduced. This solution works when the oxygen is reused in the process, and no chloride residuals or chlorine residuals in any chemical form are let to enter the oxygen gas.

If the chloride content of the chlorine-containing gases is low and a small chloride-content is accepted in the gas (in line 12) being led into the oxygen stage, the connection of Figure 4 is advantageous. There the reducing chemical is added to scrubber 13 for chlorine-containing gases in line 16 and pumped therefrom (pumps 31, 34) to both scrubbing devices, into ozone stage gas scrubber 10 and scrubber 13, which preferably are venturi scrubbers (venturi parts 30 and 33). Overflow is preferably executed via an ozone scrubber, whereby fibers entering the ozone scrubber are allowed to exit the system. Fresh water is introduced into the system to the nozzle part of the scrubbers for chlorine-containing gases, wherefrom it is passed via the bottom both to the ozone scrubber and the scrubber for chlorine-containing gases. In this connection, the carrier gas for ozone gets into contact with chlorides, but on the other hand, the chloride-concentrations are small. In order to control the liquid balance without problems, the gas scrubbing devices can be installed at a same height and a connection pipe 35 installed there between. Then the gas scrubbers become communicating vessels and it is thus ensured that the overflow will correspond to the amount of introduced fresh water. Because the flow direction is mainly from the ozone scrubber to the scrubber for chlorine-containing gases, the connection pipe can be provided with a filter (not shown) for preventing the passing of fibers.

If the carrier gas, such as oxygen, is not reused in the process, it is continuously led via line 17 and line 14 into the venturi 33 of gas scrubber 13 and further into the gas scrubber itself. In this kind of case the liquid circulation of both gas scrubbing devices can be combined, such as Fig. 5 illustrates. Gas scrubbers 10 and 13 are connected to each other via line 43. Thus, if the gas scrubbing device is a venturi scrubber, one and the same circulation pump 40 can be used for pumping reducing chemical solution to the venturi part 30 and 33 of both scrub- bing devices via lines 42, 44 and 45. Because the liquid contains small residues of chlorine- containing substances, it might be risky to use after the first scrubbing device 10 said gas in an oxygen stage, and corrosion might appear especially in gas compressors. Because gas may in acute disturbances in ozone bleaching be discharged with high pressure into the gas scrubbing system, major flow changes are to be taken into account when designing the piping. Therefore it is advantageous, if the pipe (such as pipe 43) between the gas scrubbing devices is sufficiently big or if the scrubbing devices are located side by side, whe- reby it is possible to arrange between the scrubbing devices either a container acting as a pressure accumulator or a distinct pipe enlargement.

Figure 6 illustrates an arrangement, where both the ozone gas scrubber 10 and the scrubber 13 for chlorine-containing gas have separate scrubbing liquid circulation systems. Pump 31 of scrubber 10 and pump 40 of scrubber 13 feed and circulate washing liquid from lines 11 and 16 to venturi parts 30 and 33 and further into the actual scrubbers.

When designing the arrangement, it must be made sure that ozone-containing gas after the first gas scrubbing stage or after the gas scrubbing stage and the ozone eliminator is not al- lowed to flow in the piping back towards the bleaching equipment. This problem can be solved by arranging in the gas discharge lines fans so that passing of the gas to a wrong direction is prevented. Because an ozone bleaching plant usually is sealed, the fan is preferably installed for vent gases coming from the area of other bleaching and the waste gas coming from the ozone bleaching plant is connected to the pressure side of the fan in a place up- stream of the venturi or gas scrubber independent on the type of the gas scrubber.

If it is desired to make sure that chlorine-containing vent gas cannot in any conditions pass towards the ozone bleaching plant backwards, a fan can be installed also after the gas scrubber of the ozone bleaching plant before connecting to the gases being treated of an- other bleaching plant.

Because plants exist, where on one and the same industrial area various different bleaching lines are located and these bleaching lines use different bleaching sequences for producing pulp, it is possible that two parallel sequences produce both TCF and ECF chemical pulp us- ing an ozone stage of TCF-pulping. Also in that case, the invention is advantageous because the gas containing ozone residual from TCF-bleaching can be led to a gas scrubber of the adjacent ECF-bleaching line for final elimination. Also in this case, the gas scrubbing device of ECF-bleaching provides a synergy advantage, by means of which a separate ozone gas eliminator can be removed from the line and reducing conditions of the gas scrubber of ECF- bleaching can be utilized.

As can be noticed from the above, the present invention allows decreasing the ozone emissions from a chemical pulp mill to absolute minimum also in disturbance situations. Although the above description relates to an embodiment that is in the light of present knowledge considered the most preferable, it is clear to a person skilled in the art that the invention can be modified in many different ways within the broadest possible scope defined by the appended claims alone.

Claims

CLAIMS:

1. A method of decreasing the ozone-content of gas in connection with an ozone bleaching process for pulp at a chemical pulp mill plant provided with or connected to ECF-bleaching plant for pulp and provided with at least production of a mixture of ozone and carrier gas, ozone bleaching equipment, discharge line for mixture of residual ozone and carrier gas from the ozone bleaching equipment and emergency discharge line for ozone-containing gas bypassing the ozone bleaching equipment and after these a connected first gas scrubber for treating the ozone-containing gas, characterized in that the last treatment of ozone-containing gas for removing ozone is effected in a second gas scrubber, where residual gas from ECF- bleaching and residual gas from the ozone stage are treated with reductants.

2. A method according to claim 1 , characterized in that in the first so-called gas scrubbing stage the fibers are removed and if needed, the normal ozone residual in the carrier gas is eliminated for reuse of the gas and that for a disturbance situation the gas discharge line is connected to the second gas scrubbing stage to the gas scrubber of the ECF-bleaching plant so that in situations where the gas is not led to reuse or the gas bypasses the ozone bleaching plant the reduction of the gas takes place in two gas scrubbing stages.

3. A method according to claim 1 , 2 or 3, characterized in that the second gas scrubbing stage is common to gases coming from other bleaching stages.

4. A method according to any one of the preceding claims, characterized in that in the second gas scrubbing stage the pH of the scrubbing liquid is higher than 7.

5. A method according to any one of the preceding claims, characterized in that the scrubbing devices of the first and second gas scrubbing stages are connected so that reductant is added into the first scrubber only, wherefrom it is led into the second, latter scrubber.

6. A method according to any one of the preceding claims 1-4, characterized in that the scrubbing devices of the first and second gas scrubbing stages are connected so that reductant is added into the second scrubber only, wherefrom it is led also into the first scrubber.

7. A method according to any one of the preceding claims, characterized in that the mixture of ozone and carrier gas is continuously led from the first gas scrubbing stage to the second scrubbing stage for destroying residual ozone without recovering the carrier gas for further use.

8. An arrangement for decreasing the ozone-content of gas in connection with an ozone bleaching process for pulp at a chemical pulp mill plant, said arrangement comprising at least an ECF-bleaching plant, production of a mixture of ozone and carrier gas, an ozone bleaching plant, discharge of the mixture of residual ozone and carrier gas from the process and an emergency discharge line for ozone-containing gas bypassing the bleaching plant, where after a gas scrubber is connected, characterized in that the arrangement comprises a second gas scrubber, that is connected to the first gas scrubber and in the second gas scrubber residual gas from both ECF-bleaching and the ozone stage are treated with reductants.

9. An arrangement according to claim 8, characterized in that the gas scrubbing device is a venturi scrubber.