WO2007078247A1

WO2007078247A1 - A process for the manufacture of peracetic acid and a method to bleach chemical pulp with peracetic acid

Info

Publication number: WO2007078247A1
Application number: PCT/SE2006/050607
Authority: WO
Inventors: Stefan Backa; Erik Nilsson; Martin Ragnar
Original assignee: Kvaerner Pulping Ab
Priority date: 2006-01-04
Filing date: 2006-12-20
Publication date: 2007-07-12
Also published as: SE0600022L

Abstract

The invention concerns a process for the production of peracetic acid (CH3COOOH) for use in association with the bleaching or delignification of cellulose pulp and a method for the bleaching or delignification of cellulose pulp in which at least one treatment stage is used in which peracetic acid is batchwise added as the principal bleaching agent. The peracetic acid is formed according to the invention from a reaction between hydrogen peroxide H2O2 and cellulose acetate AC in alkali conditions in the cellulose pulp. The complete reaction mixture is subsequently acidified and allowed to react with the cellulose pulp in a Paa stage. A reduced COD load is achieved through the invention, and equally good or better delignification and bleaching results are achieved as those achieved with the use of peracetic acid produced by another method. Residual products from the production process allow also the possibilities for modification of the properties of the fibres, through increased pulp strength, and increased yield.

Description

A process for the manufacture of peracetic acid and a method for bleaching cellulose pulp with peracetic acid

The present invention concerns a process for the manufacture of peracetic acid for use in association with the bleaching of cellulose pulp according to the introduction to claim 1 , and a method for bleaching cellulose in which bleaching at least one bleaching step is used in which peracetic acid is added in batches directly or indirectly as the principal bleaching agent according to the introduction to claim 9.

The Prior Art

A chemically pure, often distilled, peracetic acid is often used in the bleaching and delignification of cellulose pulp, which peracetic acid has been manufactured at another site than the pulp mill, to which delivery of the peracetic acid takes place by tanker. This is a relatively complex and risk-filled operation from the point of view of the environment and of safety. Peracetic acid can often be produced in the pulp mill in an equilibrium reaction between hydrogen peroxide, acetic acid and a strong acid, and the complete equilibrium mixture of peracetic acid is used as such. This method of producing peracetic acid, however, is rare.

The peracetic acid has also a certain tendency to disintegrate, and many process have for this reason been developed to be able to produce stable peracetic acid that does not disintegrate, with or without the use of additives.

US 6.677.477 reveals a process for the manufacture of peracetic acid in which this is manufactured with the aid of a catalyst that is neutralised after the reaction. The peracetic acid must be distilled from the reaction medium, after which the distillate is mixed with the neutralised reaction medium.

US 5.886.216 also reveals a process for the manufacture of peracetic acid in which the peracetic acid formed is distilled off from the reaction product. Other variants of the manufacture of peracetic acid in special electrolytic processes are revealed in WO 0110215 and WO 0146519, in which the peracetic acid produced can be used in a Paa stage in which the pulp is bleached with peracetic acid as the principal bleaching agent. The article "Formation ofperacetic acid from hydrogen peroxide and pentaacetyl glucose to active oxygen delignification", by GJ. Kang, A. Malekian and Y. Ni published in Tappi Journal 2004, Vol. 3, Pages 19-22, reveals a further process in which peracetic acid is formed after a reaction between hydrogen peroxide and pentaacetyl glucose, the latter also being denoted by the acronym "PAG". A relatively high degree of conversion of hydrogen peroxide is obtained with this process, well over 80%, and in the article specified as 85.4%. Glucose is formed, in addition to peracetic acid, and it is less desirable that this should enter the bleaching process, since it increases the chemical oxygen demand (COD) in the waste output from the bleaching process. It would be most preferable to separate out the glucose before the peracetic acid is added in batches to the cellulose pulp, but this would be expensive.

Aim and Purpose of the Invention

The present invention concerns the allowance of a manufacture of peracetic acid from safer and more easily handled substances, which process can advantageously be carried out at the pulp mill and in direct association with the bleaching process in cellulose pulp that has been given an alkali pH. The use of peracetic acid manufactured on site removes completely the environmentally harmful and risk-filled operations of the manufacture and transport of peracetic acid that is transported from distant factories.

A further aim is that the complete reaction product from the formation of the peracetic acid, including the base reagents from which the peracetic acid is formed, can be added directly in batches to the cellulose pulp without requiring additives or treatments that increase the cost. All that is required is acidification, preferably with sulphuric acid, which is conventionally used at the mill.

A further aim is to produce a peracetic acid in which also the residual products from the production contribute in a positive manner to the properties of the cellulose pulp in a delignification or bleaching stage, not only with respect to improved bleaching power, but also with respect to increased yield. Hydrogen peroxide that has not reacted, together with the peracetic acid, together give an acceptable bleaching effect on the cellulose pulp. A bleaching stage that involves peracetic acid in normally known as a "Paa stage" or a "T stage". It has surprisingly turned out to be the case that a Paa bleaching stage for the cellulose pulp using peracetic acid generated in a reaction between hydrogen peroxide and cellulose acetate gives a brightness that is as high, or higher, than that obtained when industrially manufactured peracetic acid is used, and when a peracetic acid manufactured from PAG is used.

A further aim is to reduce the COD from the bleaching plant, a demand that increases if peracetic acid manufactured from PAG is used, due to its content of water-soluble glucose and residual PAG in the peracetic acid that is added in batches to the pulp that is to be delignified and bleached. The COD remains at the same low level as it is during the use of pure peracetic acid if cellulose acetate is instead used during the manufacture of peracetic acid.

A further aim is that of obtaining the possibility of influencing the properties of the fibres, and principally the strength of the cellulose pulp, when cellulose formed from the cellulose acetate precipitates onto the cellulose pulp that is to be bleached and changes the properties of the bleached cellulose pulp. The degree to which the properties of the cellulose pulp can be modified can be influenced by the fraction of remaining cellulose acetate or other functional groups on the cellulose in the bleaching charge with peracetic acid.

Description of Drawings

Figure 1 shows comparative samples with different peracetic acids and their effects on the reduction in kappa value;

Figure 2 shows comparative samples with peracetic acid generated in different ways and their effects on the increase in brightness (measured as

ISO brightness);

Figure 3 shows the invention implemented between a peroxide stage (P stage) and a peracetic acid stage (Paa stage); and

Figure 4 shows a variant of Figure 3 in which "in situ" generation of the peracetic acid takes place in a subsidiary flow of the cellulose pulp. Detailed Description of Preferred Embodiments

The present invention concerns a process for the manufacture of peracetic acid (CH₃COOOH) for use in association with the bleaching of cellulose pulp in which process hydrogen peroxide (H₂O₂) is mixed and allowed to react with acetyl groups under alkali conditions in the cellulose pulp. The acetyl group is contained in a polymer carbohydrate such as acetylated cellulose (cellulose acetate), acetylated hemicellulose or acetylated starch. The molecular weights of these polymers are at least 10 times greater than the molecular weight of, for example, pentaacetyl glucose, and typically more than 1000 times greater. A carbohydrate polymer is referred to here with a degree of polymerisation (DP) that is much greater than 50, and typically greater than 500. Through this use of polymeric carbohydrate, this will precipitate to a significant degree onto the fibre in the cellulose pulp that is to be treated, and considerably more than 50% of the polymeric carbohydrate will precipitate onto the cellulose pulp. This is a large difference compared to that obtained with peracetic acid generated from PAG, in which the glucose that is formed dissolves in the fluid and essentially accompanies the filtrate in the subsequent wash. According to the invention, it is preferable to allow the hydrogen peroxide to react with cellulose acetate in alkali conditions in cellulose pulp that has been given an alkali pH.

The alkali conditions are established with the addition of a batch of alkali, preferably in the form of sodium hydroxide (NaOH), white liquor or oxidised white liquor, in order to maintain a pH between 10 and 12 during the reaction. The batches of hydrogen peroxide (H₂O₂) and of cellulose acetate that are added are adapted such that a batch of peracetic acid is obtained that lies between 1 and 20 kg peracetic acid/tonne (air dry weight) of pulp, preferably between 3 and 8 kg peracetic acid/tonne (air dry weight).

In order to obtain favourable reaction conditions, the reaction takes place under severe agitation of the cellulose pulp for at least 0.5 minutes, preferably at least 2-10 minutes. Once the amount of peracetic acid desired has been formed, the pH is lowered to the interval 3-9. It is preferable that the mixture is acidified to a pH under 6.5, and more preferably to a pH of approximately 5. The temperature may lie between 10 and 100 ⁰C, preferably 30-50 ⁰C.

Peracetic acid was generated in a laboratory experiment from cellulose acetate under the following reaction conditions, where the mixture was diluted with deionised water to a final volume of 100 ml:

• Batch size of hydrogen peroxide, H₂O₂, corresponding to 29.4 mmol

• Batch size of sodium hydroxide, NaOH, corresponding to 32.2 mmol

• Batch size of chelating agent, DTPA, 0.2 g/100 ml

• Vigorous stirring for 10 minutes

• Temperature, approximately 20 ⁰C.

This gave the following results from the reaction, comparing the use of cellulose acetate with that of PAG.

The cellulose acetate that was used was a commercially available quality that was delivered in the form of dried flakes. A higher degree of conversion can be obtained if different reaction conditions are selected, such as:

• A different formulation of cellulose acetate, either finely divided or soluble form

• Cellulose acetate that contains other functional groups (such as acidic groups) in addition to the acetate groups

• Longer or more vigorous agitation

• Higher temperature

• Additives that dissolve the cellulose acetate.

The degree of conversion for hydrogen peroxide for the reaction with PAG was compatible with that specified by Kang et al. The degree of conversion of hydrogen peroxide for the reaction with cellulose acetate was considerably lower, 42%, when using cellulose acetate, than the 82% when using PAG. In the light of these figures, it would be easy to believe that the process using cellulose acetate is less useful. But, approximately half of the hydrogen peroxide remained unaltered. The total amounts of hydrogen peroxide and peracetic acid, however, were higher in the case using cellulose acetate.

This may depend, to a certain extent, on the fact that the cellulose acetate was added in the form of flakes that were difficult to dissolve, where the hydrogen peroxide was only able to react with the surface of the cellulose acetate.

PAG, in contrast, was added in an equivalent manner in a powder form, although in this case the powder dissolves and thus erodes the PAG particles such that the hydrogen peroxide obtains access and can react with the bulk of the acetyl groups.

When using the peracetic acid formed for delignification and bleaching, the pH is adjusted from 10.0-10.5 down to a typical value of pH in the interval 5-6 by the addition of an acidifier. The peracetic acid formed is most effective as a delignification agent in this weakly acidic region of pH.

It is preferable that the acidifier is sulphuric acid (H₂SO₄), which is frequently used as an acidifier in the bleaching lines of the pulp mill. Bleaching and delignification of cellulose pulp was carried out in a comparative test using peracetic acid produced in different ways: • Chemically pure distilled peracetic acid (CH₃COOOH in Figure 1)

• peracetic acid produced from PAG (PAG in Figure 1)

• Peracetic acid produced from cellulose acetate (Cell. Ac in Figure 1).

The reductions in kappa value obtained during the bleaching and delignification of the cellulose pulp using these three peracetic acid preparations are shown in Figure 1. The cellulose pulp is dissolved in a suspension at a concentration of approximately 12%, this is known as pulp of medium consistency. The invention can, however, be used also in other regions of consistency. The cellulose pulp was a pre-bleached pulp from conifer wood that had been subjected to oxygen gas delignification to give a kappa value of 10.7, with an ISO brightness of 41.8% and the concentration of peracetic acid was corresponding to that given in the table above for peracetic acid formed from PAG, for that formed from cellulose acetate, and for the chemically pure peracetic acid.

The figure makes it clear that the reduction in kappa value decreased to the same extent independently of the peracetic acid that was used, and this shows that the residual lignin in the pulp was degraded to the same extent.

Figure 2, however, shows another, surprising, correlation, where the ISO brightness of the pulp increased more when peracetic acid that had been formed from the reaction of hydrogen peroxide and cellulose acetate was used, than it increased when using another peracetic acid. A major part of the positive effect on the improvement in brightness may probably be that cellulose from the cellulose acetate has precipitated onto the fibres of the cellulose pulp. Another part may result from the excess of hydrogen peroxide that accompanies the peracetic acid and that has not reacted with the cellulose acetate.

The experiments show that peracetic acid for the bleaching of cellulose pulp can be formed from hydrogen peroxide and cellulose acetate, where the degree of delignification and the improvement in brightness are essentially equal to or better than those obtained when using peracetic acid generated in another way.

The degree of conversion of the cellulose acetate to peracetic acid can be further influenced by modification of the reaction process, since a degree of conversion of only 42% was obtained in these experiments.

The invention allows an improved method of bleaching cellulose pulp in a bleaching process with one or several bleaching stages, where at least one bleaching stage is used in which peracetic acid is added as a batch as the principal bleaching agent. It is appropriate that the peracetic acid is generated at the bleaching stage in cellulose pulp that has been given an alkali pH, while the hydrogen peroxide is allowed to react with the cellulose acetate under alkali conditions, at a pH of approximately 10-11. The peracetic acid that is formed under these conditions is contained within a reaction product, where the reaction product in its entirety reacts with cellulose in a weakly acidic bleaching stage where the cellulose pulp is bleached with the reaction product.

The production of peracetic acid may take place either in a special process that is arranged beside the bleaching line, in which peracetic acid and polymeric carbohydrate are either added in batches in the bleaching line where the pH is approximately 5-6, or the peracetic acid is first acidified before the reaction product is added to the cellulose pulp.

An alternative is to produce the peracetic acid in situ in the cellulose pulp in the treatment stage, where the reaction product is formed in the cellulose pulp under alkali conditions and in the presence of hydrogen peroxide and cellulose acetate, after which the reaction mixture is acidified. The alkali stage in this case may be a pressurised peroxide stage, known as a (PO) stage, where peroxide and oxygen gas are added in batches to the pulp, and where the process of formation for the peracetic acid uses the residual hydrogen peroxide that may remain after the (PO) stage, or with the addition of a small amount of hydrogen peroxide, where the cellulose acetate is mixed into the final phase of the (PO) stage. The mixing in of the cellulose acetate may take place at the outlet valve of a (PO) reactor, followed by repeated mixing in one or several mixers, and possibly, also a feed pump or pressurisation pump that passes the pulp onwards to a Paa stage, where the pulp is acidified to a pH of 5-6 before the Paa stage. The mixing in of the cellulose acetate may require a certain mixing time, the duration of which is adapted to the reactability of the cellulose acetate used for the hydrogen peroxide, where the mixing may either be continuous in a special mixing tank or in the form of several mixing positions in a sequence along the pulp line. The reaction mixture containing the peracetic acid may also be added after a washing stage if there is one between the (PO) stage and the Paa stage.

A Paa stage with peracetic acid generated according to the invention may be used also in association with a chlorine dioxide stage, known as a D stage, either as a combination in which the peracetic acid is added subsequent to the D stage, or where a D stage is fully replaced by a Paa stage. Also other types of acidic bleaching stages, such as, for example, an ozone stage, can be combined with a Paa stage.

Figure 3 shows a variant in which a peroxide stage, P, carried out in a reactor 1 is followed by in situ generation of peracetic acid in a flow of cellulose pulp out from the P stage. It is normally the case here that residual peroxide is present in the pulp, but in order to establish the correct reaction conditions for the in situ generation, alkali, NaOH, and peroxide, H₂O₂, are added to the flow of pulp, after which the acetyl group, AC, is added before a vigorous mixing operation in a mixer 11. Generation of the peracetic acid subsequently takes place in the flow of cellulose pulp, and it is preferable that a small intermediate reactor 20 is present in order to establish the reaction time required. After the peracetic acid has been formed in the pulp, the pH of which has been made alkali, an acidifier, appropriately H₂SO₄, is mixed into the pulp in order to establish the optimal reaction conditions for the Paa stage, which takes place in the reactor 2. This solution with two reactors in sequence can be placed under pressure by a single pump 10 located before the first reactor 1. A further pump may be placed between the reactors, which pump gives an additional mixing effect in the flow of pulp. A wash between the P and the Paa stages is not necessarily required in this embodiment, since the residual peroxide that is present in the pulp after the P stage is, to a certain extent, used. A wash may, sometimes, however, be advantageous, if, for example, the stage in the reactor 1 is an extraction stage, an E stage, possibly with reinforcing reagents such as oxygen gas or peroxide, or both, in stages known as an EO stage and an EOP stage. A wash may be justified in cases in which the COD content in the fluid phase in the cellulose pulp suspension before the in situ generation lies above 20-50 kg COD per tonne of pulp. COD is an acronym for "chemical oxygen demand" and is a measure of the amount of oxidisable material in the filtrate or washing phase.

The first stage 1 also may be a delignification stage with oxygen gas in one or several stages, O or OO stages, with or without peroxide as reinforcement, i.e. an OP stage. Figure 4 shows another variant in which the components that have equivalent function have the same reference numbers. The difference here is that in situ generation of peracetic acid takes place in a subsidiary flow to the flow of cellulose pulp out from the P stage. It is appropriate that this subsidiary flow is of an order of magnitude 5-10% of the main flow out from the preceding reactor 1. Thus, 90-95% of the flow from the reactor passes directly to the mixer 1 1 , while the reagents that are required for in situ generation are added to the subsidiary flow of 5-10%. The generation is completed in the intermediate reactor 20. These subsidiary flows are then mixed together in the mixer 11.

A vigorous mixing, for example, can in this case take place also before or in the intermediate reactor 20, using an MC mixer in the line into the intermediate reactor or with a stirrer in the intermediate reactor.

It is a characteristic of the invention that the production process can advantageously be installed on site at the pulp mill. The complete reaction product from the production process, not only the peracetic acid which is primarily formed and the polymeric carbohydrate that is formed, but also partially or fully reacted cellulose acetate and residual hydrogen peroxide, all contribute positively to the delignification and the bleaching, such that no special distillation, filtration, or separation of the other reaction products is either required or desirable. The complete reaction product can be produced in situ in the cellulose pulp. This makes the process particularly interesting. Peracetic acid is primarily known as an efficient delignification agent, and thus it can be used early in the bleaching process, where the pulp typically has a kappa value greater than 8. This corresponds in modern bleaching lines to a position immediately after the oxygen gas delignification. The invention has demonstrated, however, improved potential for improved bleaching effect, and for this reason the use of peracetic acid as bleaching agent at a later location in the bleaching line, for example, in association with (PO) stages that lie after the oxygen gas delignification, may be interesting,

The use of cellulose acetate as source of acetate is particularly interesting in the later part of the bleaching line. The bleaching filtrate from a late Paa stage normally must be cleaned, and it is normal for this reason that one attempts to achieve a low COD level in these filtrates. If PAG is used as acetate source, the COD level increases markedly, and a greater cleaning capacity must be installed. If cellulose acetate is used, the requirement for cleaning is not higher than that experienced with pure peracetic acid.

The invention can be modified in a number of ways within the scope of the attached patent claims.

Claims

1. A process for the manufacture of peracetic acid (CH3C000H) for use in association with the bleaching of cellulose pulp in which process hydrogen peroxide (H₂O₂) is mixed and allowed to react with acetyl groups under alkali conditions, characterised in that the acetyl group is contained in a polymeric carbohydrate, for example cellulose, hemicellulose, or starch, and in that the acetyl group is added to cellulose pulp that has been given an alkali pH, which cellulose pulp is subsequently to be bleached with the peracetic acid that has been produced by the process.

2. The process according to claim 1 , characterised in that the acetyl group is bound into a cellulose acetate.

3. The process according to claim 1 or 2, characterised in that the alkali conditions are established with the addition of a batch of alkali, preferably sodium hydroxide (NaOH), which establishes a pH in the interval 9.5-12.

4. The process according to claim 3, characterised in that the batch of hydrogen peroxide (H₂O₂) and the batch of acetyl group are adapted to give a batch of peracetic acid of 1-20 kg peracetic acid/tonne of cellulose pulp, preferably 3-8 kg peracetic acid/tonne of cellulose pulp.

5. The process according to claim 4, characterised in that the reaction takes place under vigorous agitation of the cellulose pulp for at least 0.5 minutes, preferably at least 2-10 minutes.

6. The process according to claim 5, characterised in that the pH of the cellulose pulp is reduced once the reaction is complete, by the addition of an acidifier, to a pH in the interval 3-9, preferably an acidification to a pH under 6.5, and more preferably to a pH around 5.

7. The process according to claim 6, characterised in that more than 50% of the polymeric carbohydrate precipitates out onto the cellulose pulp.

8. The process according to claim 6 or 7, characterised in that the acidifier is sulphuric acid (H₂SO₄).

9. A method for the bleaching of cellulose pulp in a bleaching process with one or more stages in which peracetic acid is added batchwise as principal bleaching agent in at least one stage, characterised in that the peracetic acid is generated on site at the bleaching stage in the cellulose pulp while hydrogen peroxide is allowed to react with an acetyl group that is contained in a polymeric carbohydrate, for example cellulose, hemicellulose, or starch, at alkali conditions, in order to form peracetic acid in a reaction mixture in the cellulose pulp, and where the reaction mixture in its entirety is added in an acidic treatment stage where the cellulose pulp is delignified or bleached, or both, with the reaction mixture.

10. The method of bleaching cellulose pulp according to claim 9, characterised in that the acetyl group is a part of cellulose acetate.

11.The method of bleaching cellulose pulp according to claim 9, characterised in that the reaction product, together with the cellulose pulp, is first acidified before the reaction product is added to the bleaching stage for the cellulose pulp, in which bleaching stage the peracetic acid is the principal bleaching agent.

12. The method of bleaching cellulose pulp according to claim 9, characterised in that the reaction product is formed in the cellulose pulp under alkali conditions and in the presence of hydrogen peroxide, after which the cellulose pulp is acidified.