WO1995028350A1

WO1995028350A1 - A method for producing chemicals which can be used for bleaching of pulp to the paper industry

Info

Publication number: WO1995028350A1
Application number: PCT/NO1995/000064
Authority: WO
Inventors: Process Systems A.S Kværner
Original assignee: Unum, Arnfinn; Casen, Aslak
Priority date: 1994-04-15
Filing date: 1995-04-11
Publication date: 1995-10-26
Also published as: NO941388D0; AU2376195A

Abstract

A method for producing hydrogen peroxide by cyclic hydrogenation and oxidation of a working solution is described. By the use of low workin temperatures, a working solution containing an "all-tetra" hydroanthraquinone compound and free oxygen in the oxidation stage, a high yield of hydrogen peroxide and minimal formation of by-products are obtained.

Description

A method for producing chemicals which can be used for bleaching of pulp to the paper industry.

The invention concerns a method for the production of hydrogen peroxide (H₂0₂ ). In more specific terms the invention concerns a cyclic process wherein a working solution containing anthraquinone or a derivative thereof is alternately hydrogenated and oxidized for the production of H₂0₂.

In recent times it has become increasingly obvious that development within the cellulose pulp industry is moving in the direction of chlorine-free pulp production. As such, hydrogen peroxide is a good alternative to chlorine for bleaching of pulp.

Cyclic processes are known which employ a so-called working solution (WS), which is alternately hydrogenated and oxidized in order to form hydrogen peroxide which is subsequently removed from the working solution. For many years after this principle was adopted, all industrial processes were operated with a degree of hydrogenation corresponding to 9 g H₂0₂ per litre of working solution. A method of increasing the degree of hydrogenation is described in GB-PS 1 080 133, wherein the working solution consists of a tertiary anthraquinone mixture in which THAQ (tetrahydroanthraquinone) forms the third AQ component. The maximum practical degree of hydrogenation for this working solution is approximately 12 g/1, since higher hydrogenation causes precipitation of hydroquinone which results in blocking.

In DE-A 2 018 686 there is described the use of a new solvent which in practice permitted operation with a 13 g/1 working solution. This solvent was a substituted urea which replaced the hydroquinone solvent, TOF, which had been used previously.

A further method for increasing the degree of " 'Ogenation is proposed in GB-PS 1 390 408, in which amyl-AQ is used ir. z, nuclearly hydrogenated form. In practice amylanthraquinone (AAQ) is ed to a standard working solution consisting of ethyl- AQ, which increases the degree of hydrogenation to 12 g per litre of working solution.

These examples are representative of the two lines of thought involved with regard to the technique for increasing the capacity of the working solution, i.e. the choice of solvent and the use of other anthraquinones besides ethyl- anthraquinone.

The formation of working solutions containing tetra derivates of the antraquinones for the production of hydrogen peroxide is described in a number of publications.

US-PS 4 514 376 describes a method for increasing the tetra content in a first solution which is then added to the working solution in the presence of a Pd catalyst.

US-PS 4 539 196 similarly employs a palladium catalyst in order to prepare an all-tetra amyl working solution.

DE 40 13 090 Al describes a method for preparation of a working solution containing tetra by means of a nickel catalyst.

SE 306 923 concerns a method wherein the content of THAAQ in the working solution is higher than 150 g/1 and wherein a nickel catalyst is employed.

At the present time all industrial processes use a Pd catalyst. Many different forms of these catalysts are used and they are described in the literature.

A number of other hydroquinone solvents are employed, such as caprolactames, carboxylic acid amides and pyrrolidones. The common feature of all of these is that they have a relatively high density which limits their use in the working solution due to separation problems in the extraction process.

The use of amylanthraquinone, which is quite simply added to a normal working solution, also has the disadvantage of causing an increase in the density and that the working solution will contain a mixture of AAQ and

THAAQ (tetrahydroamylanthraquinone) due to the regeneration method which is employed in this process.

The method according to the invention uses a so-called all-tetra working solution which principally (i.e. at least 50%) consists of nuclearly hydrogenated anthraquinone of eth-yl and/or amylanthraquinone, which makes maximum use of the superior solubility of amyl-AQ, which in turn makes it possible to operate the process with the highest possible degree of hydrogenation and with a minimal formation of by-products. This method is therefore described as a high yield process.

The high yield process according to the invention will now be described in more detail with reference to the accompanying drawing.

Fig. 1 illustrates a method according to the invention. The working solution from the regeneration is fed to the hydrogenation step which comprises a reactor, preferably a continuously stirred tank reactor (CSTR) and a filtering system, preferably consisting of a primary filter and a safety filter. The reaction is performed at an overpressure of 2-3 bar in the presence of a palladium catalyst.

The working solution thereby hydrogenated is removed from the reactor via the primary filter which is subjected to backflushing at certain intervals, thus ensuring that the catalyst is kept in the reactor. The catalyst-free working solution is passed to a reciever from which it is pumped to an oxidation apparatus via a safety filter system in order to ensure that only the catalyst- free working solution is oxidized.

The hydroanthraquinone is oxidized with the use of pure oxygen, leading to the formation of H₂0₂. The oxidized solution is removed from the top of the column and passed to a reciever from which it is pumped to an extraction apparatus preferably in the form of a column.

The use of pure oxygen rather than air in the oxidation stage is described in a number of patent publications, e.g.:

British patent publication 1 434 518 deals with the use of pure oxygen in an atmospheric column which is free of packings.

Finnish patent application 894502 concerns a reactor for oxidation by the use of pure oxygen. In the extraction column, which is preferably a sieve tray column, water is fed in at the top of the column and flows downwards through the column.

The working solution is fed into the bottom of the column and, due to the difference in density between the water and the working solution, will move upwards through the column.

A virtually peroxide-free working solution is removed from the top of the column and is passed to a reciever via a coalescer in order to separate any water which may be left in the working solution after the extraction. The peroxide extracted in the water phase is removed from the bottom of the extraction column and cleaned in a washing column after which it is passed on for storage, for further distillation or to be used in its existing form.

The working solution then passes to a regeneration stage for removal of by_¬ products and also in order to regenerate inactive anthraquinone back into active anthraquinones again. This is achieved by means of an alkaline wash and/or by means of activated alumina. This stage is preferably conducted in a secondary flow but may also be conducted in such a manner that the entire flow of the working solution is treated.

The regeneration of a working solution is known from a number of publications.

DE 1 273 499 concerns the regeneration of epoxies in which the reduced or partially reduced working solution is treated with catalytic alkaline material.

SE 7613869-2 represents a further development in relation to the method indicated in the above-mentioned patent publication and concerns the use of A1₂0₃ containing metallic oxide or hydroxide.

FI 79 079 is very similar to the two above-mentioned patent publications and also utilizes alumina activated with 1-5% (w/w) Na₂0 for regeneration of by_¬ products.

GB 3 912 766 describes the treatment of the working solution in both the reduced and the oxidized form with different alkaline agents. For the working solution the method according to the invention uses a mixture of two solvents, e.g. in a volume ratio of 50:50. As a solvent for anthraquinones, aromatic, aliphatic or naphthenic hydrocarbons or mixtures of these are used. For anthrahydroquinones, solvents such as octanol-2, diisobutylcarbinol, caprolactames, amides, pyrrolidones, trioctylphosphate (TOF) or tetrabutylurea (TBU) are used.

Thus the process according to the invention comprises the following features:

1) Preparation method for the working solution

When the working solution is put into operation, a prolonged hydro- genation is carried out in order to convert the anthraquinones in the working solution to the form of tetraanthraquinones. A high content of tetrahydroanthraquinones (hydrogenated tetraanthraquinone) and a high production capacity is thereby obtained.

2) Oxidation with oxygen

When using oxygen the oxidation can be performed at a temperature of

40 - 45°C, thus reducing the formation of by-products as compared to oxidation with air, where a temperature of 60 - 70°C is used.

3) Regeneration with aluminium oxide and alkaline solution or alternatively with aluminium oxide only or alkaline solution only at a temperature of 40 - 50°C

The regeneration converts by-products from the oxidation back to tetraanthraquinones. With air oxidation substantial quantities of by¬ products are formed, and thus in a process of this kind regeneration is performed at a temperature of approximately 70°C. Due to the high temperature tetraanthraquinone is dehydrogenated to anthraquinone, and the benefit gained from the preparation of the working solution is lost. With a combination of the above-mentioned features a process is obtained which has:

* A working solution capacity of over 15 g/1. Existing commercial processes have a capacity of 13 g/1. * Low consumption of anthraquinone.

* A product with a low content of by-products. This is an advantage from the point of view of the use of undistilled hydrogen peroxide for bleaching.

This represents an improvement compared with the method which is described in GB-PS 1 434 518, in which a working solution is used consisting of three different anthraquinones with a typical composition as follows: EAQ 72 g/1, THEAQ 90 g/1, THAQ 31 g/1. The distinctive feature of this working solution was that it made use of an extra tetra- AQ, viz. the unsubstituted anthraquinone THAQ. This was done in order to obtain a higher degree of hydrogenation. The said composition gives approximately 12 g H₂0₂ per litre of working solution.

The limiting factor in the above-mentioned process is the solubility of THEAHQ which for the solvent system used, viz. aromatic solvent plus octanol-2 in a ratio of 50:50, is approximately 60 g/1, which gives a hydrogenation yield of approximately 9 g of peroxide per litre. With the use of the compound THAQ this was increased by more than 30%.

The problem with THAQ, however, was that it had a limited solubility, and if it were to be dehydrogenated the product would be anthraquinone which is completely insoluble.

The idea therefore occurred to the applicants that they could replace THAQ with THAAQ which had been described in the literature and which was said to show very high resistance to decomposition. Moreover, amylanthra- quinones have a very high degree of solubility. This led the applicants to initiate a development programme with the intention of studying the hydrogenation of amyl-AQ.

An all-tetra working solution can be produced, e.g. as indicated in US-PS 2 485 229. According to this method an aromatic solvent was used for antraquinones and an aliphatic alcohol was used as a hydroquinone solvent. The percentage of alcohol in the solvent mixture can be from 1 to 100%. Fig. 2 illustrates the course of tetra formation during a hydrogenation which was conducted at a hydrogen pressure of 3 atm. and a temperature of 60°C.

Set out below are some typical compositions for the working solution (WS) produced according to the above-mentioned method.

Content (g/1) WS I WS II WS III

EAQ 5 5

THEAQ - 60 70

AAQ 10 10 10

THAAQ 200 100 150

The degree of hydrogenation expressed as the percentage of anthraquinone which was hydrogenated was 70%. The yield expressed as the content of

H,0₂ in WS after oxidation was as follows:

WS I WS II WS III

Yield (g/1) 16,8 15,4 19,6

An advantage of the above-mentioned working solutions WS II and WS III is that the anthraquinone costs will be lower than for WS I since ethyl- AQ is much cheaper than amyl-AQ.

However, the consumption of AQ will be higher for THEAQ than for THAAQ due to a lower stability. Another reason may be that the degree of hydrogenation will be higher for THEAQ.

An increase in the THAAQ content of up to 150 g/1 will give approximately 19.6 g/1 with a 70% degree of hydrogenation. It is, of course, possible to operate the system with a higher degree of hydrogenation but this will result in a slight increase in the consumption of anthraquinone.

The most commonly used catalyst at present is Pd on a carrier. There are various carriers in use, but the most commonly used is A1₂0₃. The particle size is in the range of 50-200 μm. It is possible to increase the selectivity of the Pd catalyst by the choice of carrier as well as by special treatment of the catalyst, e.g. by treating it with hydrogen at a high temperature.

The method according to the invention employs a Pd catalyst on a carrier.

OXIDATION WITH PURE OXYGEN

The high yield process contains pure oxygen in the oxidation step, which has many advantages compared with air which is used in most other processes.

The most important advantage of using pure oxygen is that the volume of the oxidation tower will be substantially reduced which in turn means that the inventory stock of working solution is smaller, i.e. less investment cost.

Air oxidation is performed in very large packed columns at an overpressure of 4-5 bar with a large quantity of waste gas to be treated. Oxidation with air also requires a high temperature, which leads to the formation of large quantities of by products such as epoxies.

The use of pure oxygen reduces the waste gases to just a few percent of the oxygen feed and this is easy to handle.

Another advantage of pure oxygen is the possibility of performing the oxidation at a lower temperature, which in turn reduces the formation of by products in this stage, e.g. epoxies and carboxylic acids.

The use of pure oxygen permits a very high degree of oxidation to be achieved, defined as the molar ratio between the amount of H₂0₂ which is obtained in the oxidation and the total amount of hydrogenated antraquinones fed to the oxidation column.

Due to the small volumes involved in the oxidation it is possible to employ columns made of aluminium which is inert to hydrogen peroxide and does not cause decomposition as stainless steel does. EXTRACTION

The transfer of hydrogen peroxide which is dissolved in the working solution into an aqueous solution takes place in a sieve tray column.

The working solution is fed to the column at the bottom and is collected under each tray where it is forced through the holes in the tray, thereby forming small drops. This phase is therefore called the dispersed phase. The extraction water is fed in at the top of the column and flows downwards to constitute the continuous phase.

The drops raising from the tray below will accumulate under the tray and form a coalesced layer which separates into a clear, non-dispersed layer. This is a function of the rate at which the working solution passes through the holes.

It is vital that the construction material of the sieve tray does not moisten the dispersed phase. In order to prevent this some of the holes are stamped out in such a manner that an edge is formed, i.e. "jet type" holes.

The drop formation has a strong influence on the efficiency of the tray and the mechanism is extremely complicated. Ideally the drop should be formed from the top of a jet stream.

Another factor which is critical for the efficiency of the stage is the rate at which the coalesced layer is separated into a homogeneous layer under the tray. If this does not occur the coalesced layer will break through to the next tray (backflow).

Factors which will influence the coalescence are surface- active agents which can be formed in the working solution due to improper regeneration.

The rate at which the working solution passes through the holes will be in the range of 0.1-0.5, preferably 0.2-0.3 m/s for a high yield process. The first of the above-mentioned previously known working solutions which demonstrate high ability of coalescence allows a hole rate of 0.4-0.5 m/s. The installation of a hydrophobic net under each tray will further improve the coalescence.

YIELD AND PROCESS PERFORMANCE

Oxidation

The degree of oxidation for the high yield process which employs pure oxygen will be 98-99% which is a measure of the amount of hydrogenated AQ which has been oxidized. In order to calculate the total yield of the oxidation the decomposition of the peroxide formed must also be taken into consideration, and this is in the order of 2-3% based on the amount of hydro- AQ (in g H₂0₂/1 working solution) which is oxidized.

The total oxidation yield at 99% degree of oxidation and 2% decomposition is:

0.99 x(l - 0.02) = 0.97

The alkalinity of the working solution reduces the stability of the hydrogen peroxide, and by reducing the pH value by the addition of an acid it is possible to reduce the decomposition.

Furthermore, small quantities of catalyst particles which are carried into the oxidation step will also increase the decomposition rate, thus making it important to have rigorous filtration.

Extraction

As described earlier the efficiency of the extraction is principally a question of the correct design of the sieve tray colum. The decomposition of peroxide is insignificant due to the low pH value of the extraction water, approximately 2, and also due to the addition of stabilizing agents such as pyrophosphates to the water.

The total yield of the process calculated as the yield of extracted hydrogen peroxide from hydrogenated working solution in relation to the oxidation is as follows (assuming the same conditions as above): YIELD = 0.97 x 0.98 = 0.95 Example 1

In order to examine the stability of various anthraquinone systems a so-called accelerated test was conducted, in which repeated hydrogenations, oxidations and extractions (reaction cycles) were performed.

In the table below the result is shown of such a test conducted with a working solution which has a composition corresponding to WSII in the previously specified example.

Number of reaction cvcles 5 10 15 20

AQ composition (g/1)

EAQ 3 - - -

THEAQ 60 58 55 53

AAQ 7 5 3 -

THAAQ 100 99 98 98

The hydrogenations were conducted with a Pd catalyst at 50°C and a hydro¬ gen pressure of 1 atm. In all cases the degree of hydrogenation was 90-100%. The oxidation was performed with pure oxygen at 40°C .

The example shows the extremely high stability of tetraanthraquinone, which applies particularly to THAAQ.

Example 2

The following example illustrates the use of an AQ system in a version of the process according to the invention.

A working solution was produced according to the previous composition WS II and was continuously subjected to reaction cycles consisting of hydrogenation, oxidation, extraction and regeneration in a plant.

The hydrogenation was performed with a Pd catalyst on a carrier at a temperature of 50°C and a hydrogen pressure of 2 bar (g).

The oxidation was performed with pure oxygen at 40°C After 20 days circulation the working solution's AQ compound had retained its original composition within the tolerances indicated by the AQ analysis used (±2%).

The content of by-products (ballast) in the working solution was measured at 25 g/1 after the conclusion of the test period. The average yield during the period was 15 g H₂0₂ l\. During the period no anthraquinone was added externally to the circulating working solution.

BY-PRODUCTS AND REGENERATION

In the course of the various reactions decomposition products of AQ are also formed and these must be removed from the working solution or regenerated into active anthraquinone. This is carried out during the regeneration stage of the process.

The formation of the two most important by-products, i.e. epoxies and anthrones are described below.

In standard working solution systems which contain a large amount of anthra¬ quinone (non-tetraform) large quantities of anthrone are formed. Moreover, air oxidation gives rise to a high rate of formation of epoxies. In order to convert these by-products into active anthraquinones the working solution must be treated with a suitable regeneration catalyst.

Active aluminium oxide can be used as a suitable regeneration agent, but aluminium silicates are also used. This regeneration stage is provided after the extraction stage but before the oxidation stage and either the entire working solution stream or a part thereof can be treated.

The treatment with aluminium oxide in standard working solutions and anthraquinone processes takes place at a high temperature, preferably over 70°C, in order to achieve an efficient regeneration, which is essential due to the high rate at which both anthrones and epoxides are formed.

In contrast to the above-described high rate of formation of by-products which is observed in standard working solution systems and processes, the high yield process exhibits a very low formation of decomposition products, due to the all-tetra working solution together with the use of pure oxygen gas in the oxidation stage with mild reaction conditions.

The low formation of by-products also means that the regeneration in the high yield process can be performed at a low temperature, 40-50°C, preferably 40-45°C.

The low regeneration temperature means that dehydrogenation of tetra¬ anthraquinones does not take place, whereas this is the case at the tempera¬ ture used in a conventional anthraquinone process. This dehydrogenation means that the working solution will contain a large amount of the non-tetra form of the anthraquinone, which especially increases the formation of the by-product anthrone.

Finally the regeneration stage in the high yield process can be supplemented with an alkaline wash (e.g. with a potash solution), which further reinforces the regeneration effect. This step is provided preferably before the step with active aluminium oxide. In this step too the entire working solution stream or only a part thereof can be treated.

Thus the use of an all-tetra working solution in combination with pure oxygen oxidation at a low temperature gives rise to the formation of very small quantities of by-products. This in turn makes it possible to operate the regeneration at such a low temperature that the dehydrogenation of the tetraanthraquinone does not take place, which in turn almost completely eliminates the formation of anthrone, thereby giving a very low consumption of anthraquinone as make-up.

In order to compensate for loss/consumption of anthraquinones in the working solution, in the high yield process the tetra form of the anthraquinone used is added.

In addition to these most important by-products there will also be formed carboxylic acids which must be removed from the working solution since they can poison the catalyst and also interfere with the operation by the formation of surface-active compounds. The removal is effected by means of activated alumina, as well as by the use of an alkaline washing of the working solution. A number of other by products will also be formed during the continuous operation of the process, but usually in innocuous and insignificant amounts.

The regeneration of epoxies on activated alumina is preferably performed in a secondary stream comprising approximately 15% of the total stream of the working solution, between the hydrogenation and oxidation stages.

The regeneration of epoxies can also be performed by means of an alkaline treatment, e.g. a solution of sodium hydroxide or potash.

It should be understood that the above-mentioned examples are only intended to illustrate the invention, and should not be interpreted to indicate any limitation thereof. Thus other types of equipment, e.g., and other types of catalysts can be used without deviating from the scope and spirit of the invention.

Claims

PATENT CLAIMS

1. A method for producing hydrogen peroxide (H₂0₂) with a high yield and a minimum of by-products, wherein a working solution containing an anthra¬ quinone compound is successively hydrogenated and oxidized, characterized in that there are used:

(a) low working temperatures,

(b) a working solution containing substantially "all-tetra" hydro- anthraquinone compounds, and

(c) a gas containing 50-100% oxygen in the oxidation stage,

whereby a high yield of hydrogen peroxide and little decomposition of the tetrahydroanthraquinone compound to an anthraquinone compound and minimal formation of by-products in the working solution are achieved.

2. A method according to claim 1, characterized in that it comprises the following steps:

(a) a regenerated working solution containing an anthraquinone compound is fed to a reactor where the solution is hydrogenated by supplying hydro¬ gen in the presence of a catalyst,

(b) the thereby hydrogenated working solution is passed from the reactor through a filter, possibly to a reciever, and on to an oxidation stage,

(c) the hydrogenated working solution is oxidized in the oxidation stage with a gas containing free oxygen in an oxidation apparatus, preferably in the form of a column, in countercurrent or co-current for the formation of hydrogen peroxide,

(d) the oxidized solution containing hydrogen peroxide is removed from the column and passed, possibly via a reciever, to an extraction apparatus, preferably in the form of a column, where the working solution containing hydrogen peroxide is fed into the column's lower section and water is fed into the column's upper section in order to provide a top stream in the form of a working solution substantially free of H₂0₂ and a hydrous bottom stream containing H₂0₂, (e) the substantially H₂0₂-free working solution is passed to a water separation apparatus, preferably a coalescer for removal of remaining water,

(f) the hydrous bottom stream containing H₂0₂ is cleaned in a washing column before being passed to storage or further processing or to be used as such, and

(g) the working solution is subjected to regeneration for the removal of by¬ products which may be formed during the process, and also in order to regenerate inactive anthraquinone into active anthraquinone.

3. A method according to claim 2, characterized in that the regeneration of the working solution in step (g) com¬ prises:

(i) washing with an alkaline solution only

(ii) washing with an alkaline solution and treatment with activated alumina (iii) treatment with activated alumina only.

4. A method according to claim 3, characterized in that the regeneration is performed on the entire stream of the working solution.

5. A method acording to claim 3, characterized in that the regeneration is performed on a part of the working solution in the form of a secondary stream.

6. A method according to claim 2, characterized in that the catalyst which is used in step (a) consists of palladi¬ um, preferably in the form of palladium on a carrier.

7. A method according to one of the preceding claims, characterized in that the working solution consists of substantially alkyltetra- hydroanthraquinone compounds in an organic solvent.

8. A method according to claim 7, characterized in that the alkyltetraanthraquinone compounds are tetrahydroamylanthraquinone (THAAQ) and/or tetrahydroethylanthraquinone

(THEAQ).

9. A method according to claim 8, characterized in that the working solution can also contain ethylanthraquinone (EAQ) and amylanthraquinone (AAQ).

10. A method according to one of the preceding claims, characterized in that before being put to use the working solution is hydrogenated by means of so-called prolonged hydrogenation, thus ensuring that the anthraquinone compound exists substantially in its tetra form.

11. A method according to claim 2, characterized in that the oxidation with free oxygen in the oxidation column takes place at a temperature of 30-60°C, preferably 40-45°C.

12. A method according to claim 2, characterized in that the regeneration of the working solution takes place at a temperature of 30-60°C, preferably 35-45°C .

13. A method according to claim 2, characterized in that an extraction column of the sieve tray type is used as extraction apparatus in step (d).

14. A method according to claim 13, characterized in that the holes in the column are of a special design ("jet- type"), that the rate at which the working solution passes through the holes is in the range of 0.1-0.5 m/s and that the extraction of hydrogen peroxide is performed at an efficiency of at least 95%.

15. A method according to claim 10, characterized in that the degree of hydrogenation of the working solution is 60-80%, preferably 70-80%.

16. A method according to one of the preceding claims, characterized in that only alkyltetrahydroanthraquinone is used for make-up in order to replace lost working solution.

17. A method according to claim 9, characterized in that the content of by-products in the working solution which is used is less than 50 g/1, preferably less than 30 g/1.