WO1997032812A1

WO1997032812A1 - Method for the production of hydrogen peroxide by direct synthesis of hydrogen and oxygen in contact with a catalyst

Info

Publication number: WO1997032812A1
Application number: PCT/SE1997/000381
Authority: WO
Inventors: Erik Bengtsson
Original assignee: Erik Bengtsson Process Design
Priority date: 1996-03-07
Filing date: 1997-03-06
Publication date: 1997-09-12
Also published as: SE9600885L; AU1950297A; SE9600885D0

Abstract

The invention relates to a method for the production of hydrogen peroxide by direct synthesis of hydrogen and oxygen in contact with a catalyst. It is characterized in that the hydrogen and the oxygen are supplied to the catalyst in gaseous phase and that the reaction is carried out on the catalyst surface in this phase without the presence of any solvent.

Description

METHOD FOR THE PRODUCTION OF HYDROGENPEROXIDE BY DIRECT SYNTHESIS OF HYDROGEN AND OXYGEN IN CONTACT WITH A CATALYST

TECHNICAL FIELD:

The present invention relates to a method for the production of hydrogen peroxide, whereby water solutions of peroxide having different concentrations are obtained. These hydrogen peroxide solutions are used for different chemical purposes, especially for bleaching of cellulose.

PRIOR ART:

Different methods for the production of hydrogen peroxide are already known. A method which has been used for a long time comprises use of a complex anthraquinone solution which is initially hydrated and thereafter oxidised with air for the production of hydrogen peroxide, which thereafter is extracted in a water phase. However, this method is costly and in recent time it has become usual to produce hydrogen peroxide by direct reaction between hydrogen and oxygen in the presence of, for example, a palladium catalyst and water. Small additions of sulphuric acid and phosphoric acid and halogen ions promote the formation of hydrogen peroxide.

Various catalysts have been made for the direct synthesis of hydrogen peroxide by reaction of hydrogen and oxygen. They are usually based on the use of a noble metal. Palladium and platina or mixtures thereof are especially useful. Such catalysts are described, for example, in the US patents 4 681 751, 4 772 458, 5 082 458 and 5 338 531.

Direct synthesis of hydrogen peroxide through reaction between the starting materials may also be carried out in water or in water together with different other solvents. These solvents may be organic and have sometimes a limited solubility in water.

During recent times it has also been proposed to produce hydrogen peroxide by direct synthesis of hydrogen and oxygen in the presence of carbon dioxide under supercritical conditions. The carbon dioxide is then in a liquid-like phase and the carbon dioxide is therefore an inorganic solvent.

TECHNICAL PROBLEM:

A substantial disadvantage of the production method of hydrogen peroxide by oxidation of anthraquinone is that this method is complicated and requires large investment costs. This is one of the reasons that production of hydrogen peroxide by direct synthesis in water-based reaction media has been tried. However, with such a method the reaction velocity is comparatively low and the hydrogen peroxide is obtained in relatively low concentrations. The reason for this is that both reactants, hydrogen and oxygen, have low solubility in water. A second disadvantage is that contaminating additions of acid and halogenides cannot be avoided if hydrogen peroxide in at least moderately high yield is to be obtained. In spite of these additions, it is scarcely possible to produce more concentrated hydrogen peroxide than 15-20 weight percent. In addition, the side reaction of water forming instead of hydrogen peroxide forming will be very great. A costly purifying and concentrating after the reactor will therefore be inevitable.

Due to the fact that water is a less convenient solvent for direct synthesis of hydrogen peroxide, the mass transport will be slow and this makes it necessary to use a large reactor with high pressure. Only 10-20 kg of peroxide per hour can be produced per cubic metre reactor at 100 bar. The reaction mixture is also corrosive, which leads to increased costs of the process equipment and the mixture can be so aggressive that it can even dissolve the catalyst metal.

It has therefore long been a desire to be able to produce hydrogen peroxide by direct synthesis of hydrogen and oxygen in a simple and cheap way which gives hydrogen peroxide in high concentrations and which does not include large plants which take up a large amount of space and resulting high investment costs.

SOLUTIO :

According to the invention, a method has therefore been brought about which is not burdened with the above- mentioned disadvantages, which method, which relates to the production of hydrogen peroxide by direct synthesis of hydrogen and oxygen in contact with a catalyst, is characterized in that the hydrogen and the oxygen are fed to the catalyst in gas phase and that the reaction is carried out in this phase on the catalyst surface without presence of any solvent.

The method according to the invention comprises that the oxygen is suitably fed in the form of air, whereby the hydrogen-oxygen mixture also contains the other components of air and possibly other gases.

According to the invention, the content of hydrogen in the gas mixture which is to be reacted should be at the most 5 mol per cent, preferably 3 mol per cent, so that the gas mixture will not be explosive, whereas the content of oxygen may be 5-98 mol per cent.

According to the invention, it is suitable that the reacting gas mixture also contains acid compounds such as S0₂, S0₃ or N0₂ in an amount of, for example, 0.1-1 mol per cent of the gas mixture.

According to the invention, the gas mixture can also contain halogenides, preferably in the form of HCl, HBr or HJ in a concentration of for example 1-25 mol-ppm.

According to the invention, the catalyst can consist of a solid particle bed or monolith of a noble metal, preferably palladium, platina or mixtures thereof on a hydrophobic carrier of, for example, carbon, silicic acid, aluminium oxide or polymer materials.

According to the invention, it is suitable that a catalyst carrier is made hydrophobic by fluorinating or silanising. Catalyst carriers which contain acid groups are also useable.

According to the invention, the reaction mixture should during the reaction be unsaturated of water and hydrogen peroxide vapours.

According to the invention, the reaction between hydrogen and oxygen occurs preferably at a pressure from a sub- pressure up to several hundred bar, preferably between atmospheric pressure up to 20 bar, and a temperature up to 100°C, preferably 20-80°C.

The method according to the invention may be continuous, whereby formed peroxide is isolated from the reaction mixture by condensation or adsorption.

FIGURE DESCRIPTION:

The invention will now be described more in detail with reference to the attached drawing, which is a flow sheet of a preferred embodiment of the invention. DETAILED DESCRIPTION:

The figure shows a reactor 1 which contains a particle bed or a monolith of a catalyst. The reaction gases are fed to the reactor 1 through a pipe 2 from above and are taken out from the catalyst through the pipe conduit 3 at the bottom. In the present case, the method is continuous and a part of the reacting gases will therefore be returned to the reactor 1. Besides hydrogen peroxide and water, the reaction gases in the pipe 3 also contain unreacted oxygen and hydrogen. The hydrogen, which preferably has a concentration of about 3 mol per cent at the inlet in the reactor 1, has about 2.7 per cent at the outlet.

According to this embodiment, air is taken in through a side pipe 4 to the pipe 3 for replacement of consumed oxygen. By arranging a fan or compressor 5 after the supply pipe 4 causing sub-pressure in the pipe 3, the air through the pipe 4 can simply be added by opening a valve.

The pump 5 conducts the gases into a cooling scrubber 6 where the pressure is raised to for example 2.5 bar. The temperature in the gas is allowed to sink from 45°C at the inlet down to 20° at the outlet. In the scrubber a water hydrogen peroxide solution which is cooled in a water cooler 7 is circulated. The hydrogen peroxide mixture which is taken out at the lower part of the scrubber is conducted to an outlet for finished product through the manifold 8 as well as to the cooler . Those gases which are not condensed in the cooling scrubber will leave this at its upper end through the pipe 2 whereupon they again will be fed into the reactor 1. A side pipe 9 to the pipe 2 has been arranged for replacement of consumed hydrogen. Through this side pipe 9 hydrogen gas may be pumped or sucked into the reactor 1. Since air is introduced into the system via the pipe 4, that part of the air which is not consumed must be let out somewhere in the system. This occurs through an exhaust pipe 10 at the top of the cooling scrubber. Some hydrogen is also lost through this pipe. If only oxygen is introduced through the pipe 4 this exhaust pipe 10 will be unnecessary since no unnecessary gases which do not participate in the reaction will be introduced into the system.

The yield of hydrogen peroxide from hydrogen can in this embodiment be between 30-90 per cent. The pressure in the reactor can be regulated to about 0,5 bar. Further arrangements for regulating the temperature of the incoming gases can also be present. The method according to this embodiment is continuous and feeding of reactants, pressure and temperature as well as feeding out of finished products and waste gases can therefore be regulated automatically so that a stationary state will arise in the plant.

Via the fact that the reaction occurs in a gaseous medium the solubility for the reactants hydrogen and oxygen is unlimited and their concentration at the surface of the catalyst will be appreciably higher than in a water-based medium at the same time as the mass transport resistance from a gaseous phase to a liquid phase disappears. In this way the reaction velocity increases and it will be possible to obtain a high production in a comparatively small and simple reactor. The catalyst can be present in the form of a solid particle bed or a monolith, whereby the need for a complicated process equipment which is necessary with suspended catalysts disappears as do filters and stirrers.

Gases such as hydrogen and oxygen and air have always a certain ability to dissolve water vapour and hydrogen peroxide vapour. The limit of solubility is defined as the dew point. By allowing a sufficiently large flow of dry gas mixture to pass the reactor, it will consequently be possible to get formed hydrogen peroxide and water to vaporise and to dissolve in the gas mixture as soon as they are formed on the catalyst surface. The time they remain on the catalyst surface will therefore be very short, which means that the undesired decomposition of hydrogen peroxide to water and oxygen gas which always occurs on the catalyst surface will be small.

After the reaction, which is exothermic, the gas mixture can be cooled and/or the pressure can be increased, which means that the dew point for hydrogen peroxide and water is exceeded and these compounds can in a simple way be isolated as a concentrated liquid. This is a great advantage compared to earlier known methods where the hydrogen peroxide is usually obtained as a diluted solution which must be subjected to an expensive concentration step to become saleable.

According to the preferred embodiment, the gas mixture is circulated in the system and in the reactor the gas mixture will be unsaturated of water and hydrogen peroxide vapour and will therefore dry the formed reaction products from the catalyst surface. As shown above, condensation of water and hydrogen peroxide vapour in a separator will occur through cooling combined with a rise in pressure. At the return inlet to the reactor the gas mixture is warmed due to the fact that the reaction is exothermic. The pressure is lowered so that the gas mixture again will be unsaturated with regard to water and hydrogen peroxide vapour.

According to another embodiment, the separator consists of an adsorption drying device according to known technique.

The drying device is regenerated regularly with, for example, a heated gas mixture whereby the hydrogen peroxide can be condensed out from the regenerating flow.

It is not critical which catalyst is used but it is preferable to use a catalyst having an active surface of a noble metal especially palladium, platina or mixtures thereof with, for example, a weight per cent of palladium of approximately 90-100 weight per cent and up to 10 weight per cent platina. The catalyst carrier consists suitably of carbon, silicic acid, aluminium oxide or polymer materials which carrier should be hydrophobic and contain acid groups. Examples of hydrophobic carrier materials are carbon or silicic acid which have been rendered hydrophobic by fluorinating or silanising.

The reason that hydrophobic carrying materials work especially well in this production method is that the reaction product is easier to remove from the surface after the reaction so that it does not have time to decompose or hinder continued reaction. The saturation vapour pressure of hydrogen peroxide is higher above a hydrophobic carrier than above a hydrophilic carrier and the carrier is therefore easier to dry.

The content of both reactants H₂ and 0₂ in the gas mixture can vary within wide limits. To minimise the explosion risk it is preferred that the hydrogen gas content is kept below the so-called lower explosion limit, which is about 5 mol per cent. It is also preferable to have a relatively high oxygen content, 5-98 mol per cent, which improves the selectivity of the reaction, i.e. the desired hydrogen peroxide formation is promoted over undesired water formation. The gas mixture can without any disadvantage contain other gases than the reactants, such as for instance nitrogen and argon from air. Normally, the required gas flow through the reactor for transporting away the hydrogen peroxide is so great that only a small amount of the reactants have enough time to react at each passage of the reactor and accordingly the contents at the inlet and outlet do not differ very much.

The pressure in the gas mixture can as stated above vary within very wide limits, for example from sub-pressure up to several hundred bar. For practical and economical reasons a relatively low pressure or a pressure of between atmospheric pressure and up to about 20 bar is preferred since the cost for compressing and further equipment then will be low and the reaction velocity will nevertheless be high. It is also preferred to have somewhat higher pressure in the separator than in the reactor since this promotes the drying in the reactor and condensation in the separator.

The temperature can also vary within wide limits, for example from the freezing point of the reaction mixture and up to above 100°C. However, it is preferred that in the reactor part the temperature lies within the area of 20- 80°C since the reaction will then be more selective within this interval and the reaction velocity is sufficiently high while the gas has a sufficiently dew point that the formed reaction products can be dissolved. Since the reaction is exothermic the heat of the reaction contributes to maintaining the temperature of the gas mixture at a high level without any need to supply external heat.

The temperature in the separation step is suitably kept lower than that in the reactor so that the hydrogen peroxide can be cooled out and obtained as a condensate, for example 5-25°C. If the hydrogen peroxide is separated by adsorption in an adsorption plant it is, however, not necessary to have any temperature difference between the reactor and the separation step. The effectivity of the reaction can be improved by creating an acid environment on the catalyst surface. This can be done by addition of one or more acid compounds to the gas mixture, for example S0₂, S0₃ or N0₂. An advantage of the method according to the present invention is that these additions are easy to accumulate on the catalyst surface and will not follow the outgoing products as an impurity, which often is the case with earlier known methods when the reaction is carried out in a water environment. The size of the addition can be, for example, between 0.1-1 mol per cent of the gas mixture.

It is an advantage to add one or more halogenides to the reaction medium, such as chloride, bromide or iodide then preferably in the form of HCl, HBr or HJ so that the gas mixture contains, for example, between 1-25 mol-ppm halogenide. Compared with earlier known methods when a water solution is used, these halogen additions are also easier to depose on the catalyst surface without leaving the reactor and becoming an impurity. Organic halogenides can also be deposed on the catalyst surface.

Through the present invention a method has thus been created which requires low investment costs which gives a high yield of the desired product and which gives a cleaner product compared with those produced according to earlier known methods.

The invention is not limited to the above-described example but can be varied in different ways within the scope of the claims.

Claims

CLAIMS :

1. Method for the production of hydrogen peroxide by direct synthesis of hydrogen and oxygen in contact with a catalyst, c h a r a c t e r i z e d i n that the hydrogen and the oxygen are supplied to the catalyst in gaseous phase and that the reaction is carried out on the catalyst surface in this phase without the presence of any solvent.

2. Method according to claim 1, c h a r a c t e r i z e d i n that the oxygen is supplied in the form of air, whereby the hydrogen-oxygen mixture also contains the other elements of the air and possibly other gases.

3. Method according to claim 1, c h a r a c t e r i z e d i n that the content of hydrogen in the gas mixture is at most 5 mol per cent, preferably 3 mol per cent, and the content of oxygen 5-98 mol per cent.

4. Method according to any of claims 1-3, c h a r a c t e r i z e d i n that the reacting gas mixture also contains acid compounds such as S0₂, S0₃ or N0₂, in an amount of, for example, 0.1-1 mol per cent of the gas mixture.

5. Method according to any of claims 1-4, c h a r a c t e r i z e d i n that the gas mixture contains halogenides, preferably HCl, HBr or HJ, in a concentration of, for example, 1-25 mol-ppm.

6. Method according to any of claims 1-5, c h a r a c t e r i z e d i n that the catalyst consists of a solid particle bed or a monolith of a noble metal. preferably palladium, platina or mixtures thereof on a hydrophobic carrier of, for example, carbon, silicic acid, aluminium oxide or polymer materials.

7. Method according to claim 6, c h a r a c t e r i z e d i n that the catalyst carrier has been made hydrophobic by fluorinating or silanising.

8. Method according to claim 6, c h a r a c t e r i z e d i n that the catalyst carrier contains acid groups.

9. Method according to any of claims 1-8, c h a r a c t e r i z e d i n that the reaction mixture is unsaturated of water and hydrogen peroxide vapours.

10. Method according to any of claims 1-9, c h a r a c t e r i z e d i n that the reaction of hydrogen and oxygen occurs at a pressure of from sub- pressure to several hundred bar, preferably between atmospheric pressure to 20 bar, and a temperature of up to 100°C, preferably 20-80°C.

11. Method according to any of claims 1-10, c h a r a c t e r i z e d i n that it is performed continuously, whereby formed peroxide is separated from the reaction mixture by condensation or adsorption.