WO2002025008A1

WO2002025008A1 - Process and system for bleaching pulp with ozone wherein oxygen-rich gas is recycled

Info

Publication number: WO2002025008A1
Application number: PCT/SE2001/002002
Authority: WO
Inventors: Monica BOKSTRÖM; Roland EDSTRÖM
Original assignee: Metso Paper Inc.
Priority date: 2000-09-22
Filing date: 2001-09-19
Publication date: 2002-03-28
Also published as: AU2001290420A1; SE0003391D0; SE0003391L

Abstract

The present invention relates to a method and a system for bleaching of pulp with ozone wherein oxygen is recycled and concentrated. The system comprises an ozone generator (2) for the providing of ozone-containing oxygen; a reactor (3) for bleaching of the pulp; an ozone destruction unit (5) for the destruction of ozone; and an oxygen concentration unit (5) for concentrating oxygen-rich gas, which concentration unit makes use of vacuum- and/or pressure swing adsorption technology and/or membrane technology.

Description

Process and system for bleaching pulp with ozone wherein oxygen-rich gas is recycled

Technical field

The present invention relates to a method and a system for bleaching with ozone wherein oxygen-rich gas is recycled and concentrated to give concentrated oxygen.

Technical background

During ozone bleaching or ozone treatment of cellulosic pulps the carrier gas for the ozone (O₃) is oxygen (oxygen, O₂). The bleaching process as to the rest for achieving an end product contains usually additional other bleaching steps wherein among other things chlorine dioxide or chlorine gas may be used. After the ozone reaction with the pulp the remaining gas contains small smounts of the surplus ozone which usually is destroyed. The oxygen does not react with the pulp and is therefore accessible to other application areas.

As there usually occurs a certain amount of dilution of the oxygen in the system, through nitrogen leakage from surrounding air or nitrogen input through the pulp, it is not always possible to use the remaining gas for methods that require pure oxygen. Oxygen which therefore cannot be used in other methods thus is a loss or waste and is added to the cost for the ozone production.

As the oxygen consumption in total for a pulp plant which uses ozone bleaching/treatment or oxygen delignification is high it has become more and more interesting for the production (manufacturing) of oxygen in situ. The most common system is to use vacuum and or pressure swing adsorption technology for oxygen production, which gives an oxygen content of approximately 93% in the produced gas. The air is used as raw material for the oxygen production. The oxygen produced in such a unit has approximately atmospheric pressure. The cost for the produced oxygen depends of course on the size of the production unit which, in turn, depends on the total gas flows.

The problem of the technology of today is that large amounts of oxygen with low purity, for instance remaining gas from an ozone bleaching plant is loss and can not be used fully in other methods. Through EP 0588704 and EP 0971066 is known the use of re-circulating remaining gas after ozone bleaching for re-use in ozone bleaching again. Further the remaining gas is preferably dried so that water is removed when guiding the remaining gas, which may have an oxygen content of 90% (or even 89.3% as set out in EP 0588704) to the ozone generator. Water may further disturb the ozone production.

Through US 6,059,925 a further method is known wherein a pulp is ozone bleached by using a gas mixture of oxygen and ozone, wherein the remaining gas after bleaching is regenerated through the removal of carbon dioxide. The regenerated gas may then be guided to oxygen delignification step and/or a peroxide bleaching step.

The problem in the three above mentioned publications is, however, that the gas which is recirculated does not contain a sufficiently high level of oxygen in order to keep the efficiency of the methods, containing the recirculated oxygen, on a sufficiently high level, that is it is not economical to recirculate the remaining gas and use it as it is, in the methods described in the above-mentioned publications. Concentration of the recirculated oxygen is not mentioned in the above documents. Further, there is a high risk for lower efficiencies in the methods above through variations in oxygen purity of the remaining gas which may range from 70 to 90%. Oxygen delignification is further not suitable by using 70% oxygen.

Summary of the invention

The present invention solves the above problem through enhancing the efficiency during ozone bleaching comprising a re-circulation of remaining gas through concentrating the remaining gas with respect to oxygen before re-use. The remaining gas containing oxygen, for instance from ozone bleaching or ozone treatment, is thus used as a raw material for oxygen production. The oxygen content of the remaining gas from the ozone step is much higher than the oxygen content in air. Through this the production unit for oxygen may achieve a much higher capacity and the oxygen which will be used in other methods may be taken directly from the oxygen production unit (in which the oxygen purity usually ranges from 91 to 96 %). In this way there will be no waste oxygen which ends up as a loss, which would further add on the chemical costs and you may thus not use oxygen with lower purity in other methods, which in turn minimizes the risk for lower efficiency in these methods through variations in oxygen purity. Further the recycled oxygen may be used in for instance oxygen delignification step.

The present invention relates to a method for bleaching of pulp with ozone during recycling of oxygen rich gas, comprising: i) providing to a pulp ozone-containing oxygen; ii) recycling of oxygen-rich gas after bleaching; m / bh U . I U U 0

iii) reduction (destruction), preferably essentially total reduction (destruction), of ozone in the oxygen-rich gas; and iv) concentration of oxygen from the oxygen-rich gas; wherein the oxygen content of the oxygen-rich gas is from 20 to 89.2%, Further the present invention relates to a system for bleaching of pulp with ozone during recycling and concentration of oxygen-rich gas comprising: a) an ozone generator for providing of ozone-containing oxygen; b) a reactor for bleaching of pulp; c) an ozone destruction unit for destruction of ozone; and d) an oxygen concentration unit for concentrating of oxygen-rich gas, wherein the oxygen concentration unit makes use of vacuum and/or pressure swing adsorption technology and/or membrane technology.

Detailed description of the invention The expression "pulp" in the present invention is intended to embrace softwood pulp, hardwood pulp or mixtures thereof which also may contain water. The pulp, which may be used in the method according to the present invention, is preferably chemical pulp where the fibre freeing has been achieved chemically, for instance for the production of bleached pulp from both soft wood (softwood) and pulp from hard wood (hardwood) or from annual plants. The pulp may derive from a mixture of different type of woods. The pulp may further already be oxygen delignified and/or alkali extracted when it is bleached according to the method according to the present invention. The pulp may further have been produced before through sulphate- or sulphite cooking. The pulp may further have low, medium or high consistency, preferably medium or high consistency. The pulp which is intended to be ozone bleached/ozone treated may further be oxygen delignified, washed and/or acidified, before the bleaching/treatment, to a pH of 1 to 5 which may have been achieved using for instance sulphuric acid or oxalic acid.

The expression "oxygen rich gas" in the present description is intended to embrace gases and gas mixtures comprising oxygen which is produced and/or remains after bleaching. The oxygen content is as said above from 20 to 89.2%, and may preferably be from 60 to 89.0%, most preferred 65 to 85%. The oxygen rich gas may further contain unused ozone, air, nitrogen, carbon dioxide, carbon monoxide and hydrocarbons. Preferably the oxygen rich gas contains, after passage through the ozone destruction unit, essentially no ozone. This passage may preferably comprise a passage through fibre scrubber before the ozone destruction unit.

The expression "ozone-containing oxygen" in the present description is intended to embrace gas mixtures of oxygen and ozone that are suited for ozone bleaching. The ozone may be generated from dry oxygen with high purity, that is essentially free from carbon dioxide, carbon monoxide and volatile hydrocarbons, but also oxygen from an oxygen concentration unit which is suitable for generating of ozone. Preferably oxygen comes from an oxygen concentration unit. The ozone generator typically generates from oxygen an ozone containing oxygen which comprises from 1 to 20%, preferably from 5 to 15%, most preferred from 6 to 14% (weight) ozone. The bleaching with ozone may be performed in one or more bleaching units to which the ozone containing oxygen is added with up to 20% atmospheres pressure (pounds per square inch).

The expression "ozone destruction unit" in the present invention is intended to embrace any known ozone destruction unit wherein ozone is converted to oxygen. A such unit may allow the passage of remaining gas, comprising ozone, over catalytic bedding of aluminium doped with manganese oxide and copper oxide or such a unit may comprise a pipe which contains an electrically heated element wherein the remaining gas is guided along the pipe.

The method according to the present invention may preferably additionally comprise a step v): production of ozone, preferably ozone containing oxygen. The method according to the present invention further has concentrating of the oxygen in step iv) wherein this preferably may be achieved using vacuum and/or pressure swing adsorption technology and/or membrane technology.

In the method according to the present invention the oxygen rich gas i step i) or after step iii) may preferably contain from 20 to 89.2% oxygen, preferably 60 to 89% oxygen, most preferred 65 to 85% oxygen.

The method according to the present invention may preferably comprise that the oxygen produced through iv) is guided to an oxygen consuming process. This oxygen consuming process may be:

• black liquor oxidation • oxygen delignification

• _. white liquor oxidation

• polysulphide forming from white liquor • air enhancement during lime reburning

• oxygen extraction processes

• lime oxidation

• ozone generation The bleaching in the method according to the present invention may further be adapted, which as such is known to a person skilled in the art, for either LC, MC or HC-ozone bleaching according to known technology. During ozone bleaching of LC-pulp (pulp with low consistency) the pulp concentration normally ranges from 1 to 8 weight-%. Further, during ozone bleaching of MC-pulp (pulp with medium consistency) the pulp concentration normally ranges from 8 to 20 weight-% and during ozone bleaching of HC-pulp (pulp with high consistency) the pulp concentration normally ranges from 20 to 50 weight-%.

The system according to the present invention may preferably additionally comprise a scrubber for cleaning of remaining gas, preferably placed between the ozone reactor and the ozone destruction unit. The scrubber may be any known scrubber, preferably a fibre scrubber. The ozone generator in the method and the system according to the present invention may further be any which is suited for ozone production.

The oxygen concentration unit in the system according to the present invention may preferably additionally comprise also an oxygen production unit for oxygen production in situ.

The oxygen concentration unit in the system according to the present invention may preferably make use of vacuum and/or pressure swing adsorption technology (that is NSA, PSA or VPSA technology) and/or membrane technology, wherein the oxygen concentration unit most preferred comprises zeolite beddings and/or membrane. Normally the oxygen produced using any of these technologies contains from 90 to 96% O₂.

We will now more in detail describe the present invention with the help of a figure, but this shall only be seen as illustrating and not in any way limiting for the invention as set out in the appended claims.

Description of figure

Fig. 1 shows a principle sketch of an example where the method and system according to the invention is illuminated in the form of a system for ozone bleaching, wherein oxygen rich gas is re-circulated and oxygen is concentrated. Explanations to the numerals in the figure: 1. oxygen concentration unit 2. ozone generator

3. reactor

4. scrubber

5. ozone destruction unit 6. other oxygen consuming processes

The lines drawn between the boxes symbolise gas flows (and the gas mixture flows) and the lines (with no connection to another box) coming in and from box 3 symbolise pulp flows (incoming to the reactor and outgoing, from the reactor, ozone bleached/ozone treated pulp). Between the components 1 and 2 essentially O is present during the method and between 2 and 3 essentially ozone-containing oxygen is present. Between 3 and 4 essentially remaining gas (oxygen rich gas) is present. Between 5 and 1 essentially oxygen rich gas is present, preferably without ozone, O .

Claims

1. A method for bleaching of pulp with ozone during recycling of oxygen-rich gas comprising: i) providing to a pulp ozone-containing oxygen; ii) recycling of oxygen-rich gas after bleaching; iii) reduction of ozone in the oxygen-rich gas; and iv) concentration of oxygen from the oxygen-rich gas; wherein the oxygen-rich gas in step ii) or after step iii) contains from 20 to 89.2% oxygen.

2. A method according to claim 1 additionally comprising step v): production of ozone, preferably ozone-containing oxygen.

3. A method according to claim 1 wherein the concentration of oxygen in step iv) is achieved by using vacuum and/or pressure-swing adsorption technology and/or membrane technology.

4. A method according to claim 1 wherein the oxygen-rich gas in step ii) or after step iii) contains from 60 to 89% oxygen, preferably 65 to 85% oxygen.

5. A method according to claim 1 wherein the oxygen produced through step iv) is guided to an oxygen consuming process.

6. A system for bleaching of pulp with ozone during recycling and concentration of oxygen- rich gas, comprising: a) an ozone generator for the providing of ozone-containing oxygen; b) a reactor for bleaching of the pulp ; c) an ozone destruction unit for destruction of ozone; and d) an oxygen concentration unit for concentrating oxygen-rich gas, characterized by that the oxygen concentration unit makes use of vacuum- and/or pressure swing adsorption technology and/or membrane technology.

7. A system according to claim 6 further comprising a scrubber, preferably a fibre scrubber, placed between the reactor and the ozone destruction unit.

8. A system according to claim 6, characterized by that the oxygen concentration unit comprises also an oxygen production unit for oxygen production in situ.

9. A system according to claim 6 or 7, characterized by that the oxygen concentration unit comprises zeolite beddings and/or membrane.