WO1998046336A1

WO1998046336A1 - Process for decomposing halogenated hydrocarbons with a catalyst, its production and installation for carrying out the process

Info

Publication number: WO1998046336A1
Application number: PCT/EP1998/002088
Authority: WO
Inventors: Erhard Kemnitz
Original assignee: Adelmann Ag
Priority date: 1997-04-12
Filing date: 1998-04-11
Publication date: 1998-10-22
Also published as: AU7642598A

Abstract

A process for decomposing halogenated hydrocarbons has the following steps: (a) water is added to the halogenated hydrocarbon, in a stoichiometric proportion 2 to 5 times, preferably approximately 3.5 times higher than that of the halogens; (b) the mixture is heated up to at least 400 °C, preferably 400 to 550 °C; (c) a catalyst having the following composition is used: (aa) acid zirconium oxide containing up to 5 % hafnium oxide; (bb) Brönstedt centres located or formed at the surface during the reactions; (d) the dwelling time is sufficient for the reaction to take place to the desired extent.

Description

Process for the degradation of halogenated hydrocarbons with a catalyst, its production and plant for carrying out the

Procedure

The invention relates to a process for the degradation of gaseous and / or halogenated hydrocarbons, in particular chlorofluorocarbons, with a catalyst, its production and plants for carrying it out.

The term "halohydrocarbons" denotes all aliphatic hydrocarbon compounds in which H atoms have been replaced by halogens. In their short form, they are also called freons or halons and are used extensively as refrigerants, blowing agents, fire extinguishing agents and solvents. In this context, examples are the brominated hydrocarbons, which have found widespread use as effective fire extinguishing agents in fires in closed rooms and electrical systems. This group of compounds also includes chlorofluorocarbons, in which the H atoms are substituted by chlorine and fluorine atoms. They are often referred to as chlorofluorocarbons - CFCs. The incorporation of fluorine gives these hydrocarbon compounds high chemical stability, so that under normal conditions they cannot be caused to undergo chemical reactions either chemically or by any other energy supply, so that they are neither flammable nor flammable, nor explosive or toxic. These compounds have recently been discredited because they disadvantageously degrade and weaken the stratospheric ozone layer through the following reaction mechanisms. Due to their inertia, they are not broken down at all in the atmosphere and reach the stratosphere, where they are exposed to hard UV radiation (frequency <300 nm), which leads to a radiation-chemical cleavage of the C-Cl bond, with the result that that the chlorine radicals released in this process now considerably deplete this ozone layer, its weakening, in several thousand cycles per atom As a result, it can only insufficiently fulfill its function as a UV radiation protection screen, so that hard UV radiation reaches the surface of the earth largely unfiltered and can cause increased skin cancer there. For these reasons, the major industrialized nations introduced a production stop of freons (CFCs) and halons at the beginning of this decade. On the other hand, hundreds of millions of tons of freons and halons have been produced in the past decades and put into circulation in various forms. They were used in aircraft and motor vehicles; CFCs were used in almost all refrigerators, both as a coolant and as an insulating gas that is enclosed in the closed pores of the insulating foam. The older air conditioning systems are all operated with Freon. The extensive use has the consequence that due to improperly disposed refrigerators, polyurethane foams, solvent residues and the like, a free content of up to 40 ppm is measured in exhaust gases from large landfills. In addition, the ozone-depleting compounds have only stopped producing in the industrialized nations, while developing countries continue to offer halogenated hydrocarbon production on the market and close the supply gaps. There is therefore a considerable need to eliminate the halogenated hydrocarbons, in particular CFCs, by means of suitable industrially applicable processes.

It is known, for example, to decompose the halogenated hydrocarbons in a hydrogen flame at temperatures around 1300 ° C., the halogens being obtained in the form of hydrogen halides and the carbon in the form of carbon monoxide. A subsequent absorption section separates the product components and leads them to further processing. This procedure is only suitable for large product flows and in the presence of a local composite, as is the case with large chemical plants. In the case of smaller quantities, the raw material must be collected and transported. In addition, it had to be found that even at these temperatures of around 1000 ° C, not all CFC types were completely broken down and dismantled. Better Results have been achieved when the halogenated hydrocarbons are introduced in a high-temperature flame of approximately 2000 to 2600 ° C., by means of which a complete decomposition of all the compounds introduced can be achieved. The high temperature is generated here by a classic oxyhydrogen mixture of hydrogen and oxygen, whereby halogen compounds (HCI and HF) are produced, which must be disposed of appropriately. Although a system with a compact structure and simultaneous mobility is possible, the energy costs are also unacceptably high.

Proceeding from this, the invention has set itself the task of creating a process which can be carried out on an industrial scale, in which the halogenated hydrocarbons and in particular the fluorocarbons are broken down into components which are harmless to ozone.

This object is achieved by using the process steps specified in claim 1:

Water is added to the starting material in a higher than stoichiometric ratio to the halogens and heated to a temperature of at least 400 ° C., advantageously to 400 to 550 ° C. The gas mixture flows through the reactor charged with the catalyst. In order to achieve complete conversion of the starting material, a minimum residence time in the catalyst is required. The current speed must be set accordingly. The decisive factor is the catalyst, which initiates the decomposition of the halogenated hydrocarbons at low temperatures and therefore with the lowest energy consumption. With regard to its chemical composition, it consists of a pure or modified zirconium oxide. The surface of the catalyst has Brönstedt acid centers, which are permanently present or can regenerate during the reaction. In the presence of the catalyst according to the invention, the halogenated hydrocarbons, in particular chlorofluorocarbons, break down into hydrogen halide, in particular hydrogen fluoride (HF) and in particular hydrogen chloride (HCl) and carbon dioxide (C0 ₂ ).

Series of tests were conducted with dichlorodifluoromethane as one of the most stable, industrially produced freons. The measurements showed a decomposition of up to 99% under reaction conditions of T = 450 ° C and a contact time of 0.5 seconds.

In a preferred embodiment, sulfur compounds in the form of sulfates S0 and / or sulfites S0 _{3 are} additionally introduced on the surface of the catalyst. They ensure that the production of Brönstedt centers is promoted and, because of their exposed position on the surface, they intensify the reactions taking place, so that as a result better conversions occur and a higher yield occurs. An extension of the service life is also achieved. Here, sulfur compounds with a sulfur content of 3 to 10 atom%, in particular 7 to 8 atom%, are to be regarded as particularly preferred.

The prerequisite for the catalytic reaction to take place is the presence of Brönstedt centers, it being irrelevant whether the Brönstedt centers are permanently present or - if necessary temporarily - only develop under the reaction conditions. One of the possible measures for generation under reaction conditions is to provide Lewis centers, preferably on the surface of the catalyst. In the presence of water, the Lewis centers produce the Brönstedt centers in the desired manner, which are solely responsible for the catalytic decomposition.

From the point of view of reaction kinetics, the surface area of the catalyst should be chosen to be as large as possible, since the Response times are the lowest. Experimental investigations have shown that satisfactory reaction rates first occur when the surface measured by the BET method is between 120 and 180 m ² / g. Regarding the BET process, reference is made to the article in the Journal American Chemical Society (J. Am. Chem. Soc. 60 (1938,309)).

Brönstedt centers which desorb previously adsorbed ammonia above 350 ° C. to characterize their strength are particularly preferred. There are then medium-strong acidic Brönstedt centers, which can be detected by the temperature-programmed desorption TPD of the ammonia.

Last but not least, in order to achieve the largest possible surface area, the catalyst should have a porous structure. The diameter of the pores is an optimal compromise, on the one hand, between a large radius of the pores, which is necessary to prevent the pores from clogging early and thus a short service life, and, on the other hand, a small pore diameter, which allows a higher pore density per unit area and thus also to achieve a higher surface. The optimal solution of these conflicting points of view leads to the fact that the pores are located in the area between mesopores and micropores, that is to say they have a pore radius which, in its most common distribution, is in the range between 1.9 and 5 nm, preferably 1.9 to 3 nm.

The splitting of the halogenated hydrocarbons into carbon dioxide and the corresponding halogenated hydrocarbons allow the extraction of valuable acids. Thus, hydrofluoric acid can be obtained from hydrogen fluoride under the influence of water and hydrochloric acid from hydrochloric acid and thus economically interesting products.

The decisive advantage of the method according to the invention consists in low procedural costs, which are based on a departure from the state of the art Technically necessary high decomposition temperatures establish, since the decomposition reaction is initiated catalytically.

Modifications in which the zirconium oxide is cubic or X-ray amorphous are particularly active and should therefore be preferred.

Processes for the preparation of the catalyst for use in the process according to the invention are described below. For this purpose, a homogenized mixture of solid zirconium oxide hydrate and solid (NH) ₂ S0 _{4 is} treated according to a multi-stage calcination program up to 600 ° C. The mass ratio of zirconium oxide hydrate to ammonium sulfate can vary between 50/50 to 95/05, but is advantageously in the range 75/25 to 90/10 mass%. Sulfuric acid serves as the sulfate source, the amount of which varies between 2 to 20, preferably 2 to 4,% by mass. The sulfation with SF ₄ takes place in such a way that the transition metal oxide hydrate is treated at temperatures between 120 and 180 ° C., preferably between 140 and 160 ° C., for 30-90 minutes using gaseous sulfur tetrafluoride.

The sulfated oxide formed by treatment with sulfates is crystalline and can be detected by X-ray diffraction.

The preparation of the catalyst used for the process according to the invention is described using the examples explained in detail below:

Example 1 Preparation of a zirconium catalyst with SF ₄ as the sulfate source

Zirconyl chloride hydrate is hydrolyzed in an aqueous ammonia solution until a pH of 8 is reached. The precipitate is separated off, washed chlorine-free with water and added for 6 to 10 hours Calcined temperatures between 90 and 120 ° C. The zirconium oxide hydrate thus obtained is treated at temperatures between 120 and 180 ° C, preferably between 140 and 160 ° C, for 30 to 90 minutes using gaseous sulfur tetrafluoride. A purging is then carried out using inert gas to remove the absorbed SF ₄ . The resulting dark red to black product is then subjected to a temperature program to generate the special catalytic properties. For this purpose, the material is first annealed at 150 ° C for 90 minutes; This is followed by continuous heating over 45 minutes to temperatures between 550 and 650 ° C, preferably 590 to 610 ° C. The material is left isothermal at this temperature for 3 hours and then cooled to 110 ° C. within 60 minutes. Here the material has reached its high activity.

Example 2:

Preparation of a zirconium catalyst with solid ammonium sulfate as

Source of sulfate

Solid ammonium sulfate is added to the zirconium oxide hydrate obtained by hydrolysis of zirconium chloride and already calcined at 110 ° C. The

Mass ratio of zirconium oxide hydrate to ammonium sulfate can be between

50/50 to 95/05 vary, but is advantageously in the range 25/75 to

90/10 mass%. After intensive mixing of both components, it will

Mixture also subjected to an annealing program. For this purpose, the reaction mixture is initially left isothermal at 150 ° C. for 90 minutes. Thereafter, the temperature is raised to 400 to 600 ° C., preferably to 480 to 520 ° C. within 45 minutes and the reaction mixture at this temperature for

Held for 180 minutes. It is then cooled to 110 ° C within 60 minutes. The solid obtained in this way has full catalytic activity. Example 3:

Manufacture of a supported zirconium catalyst with a solid

Ammonium sulfate as a source of sulfate

Activated carbon is added to an aqueous solution of zirconyl chloride until these grains are loaded with the solution. The activated carbon is then hydrolyzed with zirconyl chloride loading at pH = 8, as described in Example 1, calcined at 110 ° C. and finally solid ammonium sulfate is added. The mass ratio of zirconium oxide hydrate to ammonium sulfate can vary between 50/50 to 95/05, but is advantageously in the range 75/25 to 90/10 mass%. The zirconium content, based on the activated carbon component, can be between 1 and 30% by mass, but advantageously in the range from 2 to 8% by mass. After the two components have been mixed, the reaction mixture is also subjected to an annealing program. For this purpose, the reaction mixture is first left at 150 ° C. for 90 minutes. The temperature is then raised to 400 to 600 ° C., preferably to 450 to 500 ° C. within 45 minutes, and the reaction mixture is kept at this temperature for 180 minutes. It is then cooled to a temperature of 110 ° C. within 60 minutes. This gives the desired catalyst.

Example 4: Use of the catalyst for the degradation of dichlorodifluoromethane

The decomposition of dichlorodifluoromethane takes place under normal pressure conditions in a continuously operated flow reactor in which the catalyst is located. The reactor itself is made of corrosion-resistant material and is heated by means of an adjustable electric furnace. The catalyst prepared according to Example 2 was used in the form of pressed pellets with a particle diameter of 250 to 500 μm. After one hour Calcination of the catalyst at 500 ° C, a constant gas flow of dichlorodifluoromethane and water vapor was passed over the catalyst at a reactor temperature of 480 ° C. The ratio of water to dichlorodifluoromethane can be varied within the limits 2 to 4, but should advantageously be between 2.1 and 2.4. The temperature varies between 400 and 550 ° C, is preferably in the range between 450 and 500 ° C. The average residence time of the reaction gas can vary within wide limits between 0.1 and 3 seconds, but is selected between 0.2 and 0.5 seconds to optimize the process.

The following table shows the results for different temperatures with the same residence time and the same water / dichlorodifluoromethane ratio.

Temp. ° C residence time H ₂ 0 / CFC-12 rest CFC-12 see Zr0 ₂ / (NH ₄ ) ₂ S0 500 3 2 less than 0.3

Zr0 ₂ / (NH ₄ ) ₂ S0 ₄ 450 3 2 0

Zr0 ₂ / (NH ₄ ) ₂ S0 ₄ 400 3 2 under 7

Zr0 ₂ / (NH ₄ ) ₂ S0 ₄ 350 3 2 under 46

Zr0 ₂ /7% La0 ₂ 500 3 2 0

Activated carbon / 2% Zr0 ₂ 500 3 2 under 14

Example 5: Decomposition of dichlorodifluoromethane with the aid of the invention

Catalyst

The test results for the decomposition of CFC-11 are given below with the corresponding catalysts, temperatures, residence times and mixtures.

Temp. ° C dwell time H ₂ 0 / CFC-11 rest CFC-11 see.

Zr0 ₂ / (NH ₄ ) ₂ S0 ₄ 500 0.5 2 under 1

Zr0 ₂ / (NH ₄ ) ₂ S0 ₄ 450 0.5 2 less than 0.5

Activated carbon / 2% Zr0 ₂ 500 0.7 2 under 2

Further features, details, advantages and effects of the invention are described in more detail with reference to the drawing. This shows in:

Fig. 1 shows an inventive plant for the degradation of gaseous halogenated hydrocarbons, and

Fig. 2 shows another embodiment of the invention for the degradation of liquid halogenated hydrocarbons

In plant 1 according to FIG. 1, the halogenated hydrocarbon gas to be mined is cleaned of dust particles and preheated in a first stage 2. Water 3 is then added to the gas 4. In accordance with the main direction indicated by arrows 5, the gas-water mixture passes through a heating stage 6 to the flow-through reactor 7, in the interior of which the catalyst 8 is located. The desired reaction temperature is maintained using a temperature control device. Due to the properties of the catalyst, it is broken down into hydrogen halide and carbon dioxide. Controlled cooling 9 ensures a gas temperature of less than 600 ° C. It is then washed out in a gas scrubber 10, for example by adding calcium hydroxide, calcium chloride and calcium fluoride, so that pure air can leave the gas scrubber 10, the degree of purity of which can be checked with a measuring device 11.

In the embodiment 12 according to FIG. 2, in contrast to the system described above, liquid halogenated hydrocarbon 13 is split. This liquid CFC is converted into the gaseous state by mixing 14 with air, preheated 15 and then enriched with water 16. The further process sequence corresponds to that according to FIG. 1.

Claims

PATENT CLAIMS

1. A process for breaking down gaseous and / or liquid halogenated hydrocarbons, in particular chlorofluorocarbons (CFCs), with the following process steps: a) adding water to the halogenated hydrocarbon in 2 to 5 times, preferably about 3.5 times the stoichiometric ratio to the halogens , b) heating to at least 400 ° C, preferably 400 to 550 ° C, c) application of a catalyst of the following composition: aa) acidic zonium oxide, which contains up to 5% hafnium oxide, bb) on the surface there are Brönstedt centers or form during the reactions, d) sufficient residence time to achieve the desired scope of the reaction.

2. The method according to claim 1, characterized in that the catalytically active component is applied to a suitable large-area material.

3. The method according to claim 1 or 2, characterized in that the Zr0 _{2 is} cubic or in X-ray amorphous form.

4. The method according to any one of claims 1 to 3, characterized in that the surface is porous and the most frequent radius is between 1, 9 and 5 nm, preferably 1, 9 - 3 nm.

5. The method according to any one of claims 1 to 4, characterized in that the surface of the catalyst according to the BET method is between 120 and 180 m ² / g or above.

6. The method according to any one of the preceding claims, characterized in that the catalyst has additional sulfur compounds in the form of sulfates and / or sulfites in the surface.

7. The method according to any one of the preceding claims, characterized by a sulfur content of 3 - 10 at%, preferably 7 - 8 at%.

8. The method according to any one of the preceding claims, characterized in that Lewis centers producing Brönstedt centers are present in the presence of water.

9. The method according to any one of the preceding claims, characterized in that the Brönstedt centers of the catalyst adsorb ammonia only above 350 ° C.

10. Preparation of a catalyst for use in a process according to one of claims 1 to 9, characterized in that a homogenized mixture of solid zirconium oxide hydrate and a sulfate component is thermally treated according to a multi-stage calcination program, which up to a temperature of 600 ° C. he follows.

11. The method according to claim 10, characterized in that the mass ratio of zirconium oxide hydrate to ammonium sulfate varies between 50/50 to 95/05 and is advantageously in the range 75/25 to 90/10 mass%.

12. Preparation of the catalyst for a process according to one of claims 1 to 9, characterized in that solid zirconium hydrate is added sulfuric acid as a sulfate source, which varies between 2 to 20, preferably between 2 and 4% by mass.

13. Preparation of a catalyst for use in a process according to one of claims 1 to 9, characterized in that solid zirconium oxide hydrate for sulfation at temperatures between 120 and 180 ° C, preferably between 140 and 160 ° C for 30 to 90 minutes with gaseous sulfur tetrafluoride is treated or reacted.

14. Plant for performing the method according to one of claims 1 to 10, characterized in that a heating stage for heating the halogenated hydrocarbon as well as a feed station for water is connected upstream, a flow reactor with heating and a catalyst is connected and a cooling unit is connected on the output side .

15. Plant according to claim 14, characterized in that an alkaline gas scrubber is connected downstream, in which organic salts, in particular

BaF ₂ , BeF ₂ , KF, NaF, LiF, MgF ₂ , CaF ₂ , FeCI ₃ , NaCI, MgCI ₂ , CaCI _{2 can} be obtained as valuable substances.