EP0324754B1 - Process for dehalogenation of hydrocarbons - Google Patents

Abstract

In order to effect dehalogenation of hydrocarbons, in particular polychlorinated aromatic compounds, which are also found in oils, oil sludge or in sea ooze or in the earth, said compounds are dispersed by the DCR process, and the finely-dispersed reaction product thus obtained is made to react in the presence of a nucleophile reaction body until dehalogenation is achieved.

Description

Halogenated hydrocarbons can cause environmental problems. This applies in particular to compounds such as polychlorinated biphenyls, dibenzodioxins, dibenzofurans and other polycyclic aromatics. The compounds can be in bulk, as an impurity in mineral oils, in bulk or dissolved in mineral oil in the soil, e.g. in the form of seepage oils or, together with water, as seepage water from contaminated sites or as contamination in compact, especially bituminous phases.

It is known that compounds of this type are used at high temperature, e.g. can burn without damage above 1200 ° C, provided that this temperature can be safely maintained over a longer distance in the combustion process. If this is not the case, if suitable starting materials are present, on the contrary, halogenated dioxins or dibenzofurans can be produced under the usual conditions of combustion, so that an additional pollution of the environment occurs.

It is also known that halogenated hydrocarbons can be dehalogenated with metals such as sodium and potassium. The reaction is carried out at elevated temperature, for example with molten sodium in the form of a suspension.

Halogenated hydrocarbons that are present in the soil as impurities can be driven off in rotary kilns. The gas stream is then fed to a high-temperature combustion device or condensed.

The halogenated hydrocarbons present in the condensed phase can then be subjected to a conventional dehalogenation reaction.

A number of dehalogenation reactions are described in the organic chemical literature. They all only work on the condition that they are purified substances. For example, pure halogenated hydrocarbons can be dehalogenated in a relatively simple manner by treating them with hypophosphorous acid in the presence of palladium catalysts. This method fails immediately if the smallest contaminants are present in the medium. It can therefore be seen that these processes cannot be used in the field of environmental protection because they are mixtures with constituents which poison the required catalysts after a short time.

A method with the features of the preamble of claim 1 is described by EP-A-0 188718. Here, in the known process, the halogenated hydrocarbons are split chemically / thermally using a dehalogenation catalyst, the known catalyst being a mixed catalyst composed of calcium oxide and / or calcium hydroxide and iron oxide. The known process requires reaction temperatures in the range between 600 ° C and 800 ° C because of the pyrolytic decomposition of the halogenated hydrocarbons.

The methods mentioned have considerable disadvantages. If the dehalogenation consists in the fact that the halogenated hydrocarbon molecule as a whole is oxidatively destroyed, that is to say destroyed by combustion, then unusually high temperatures are required. Processes of this type are therefore very expensive and there is a risk that highly toxic substances are produced if the framework conditions mentioned above are not observed. In chemical methods outside of oxidative destruction, reactants and, if necessary, catalysts are always required that fail in heavily contaminated mixtures or in water-containing systems, such as the aforementioned dehalogenation of chemically pure substances using molten sodium metal, sodium alcoholates or catalysts is.

Furthermore, there is always a risk in the known process that toxic secondary products are formed from the halogenated hydrocarbons originally present at the relatively high temperatures, e.g. Dioxins or dibenzofurans from halogenated aromatics.

The present invention has for its object to provide a method of the type specified, in which the formation of toxic secondary products from the originally present halogenated hydrocarbons is excluded.

This object is achieved according to the invention by a method having the characterizing features of patent claim 1.

The process according to the invention is based on the basic idea of dehalogenating the halogenated hydrocarbons at low temperatures by chemical means alone. To achieve this, it is proposed in the process according to the invention to first disperse the halogenated hydrocarbons by chemical means. This ensures that the halogenated hydrocarbons to be degraded are freely accessible for the subsequent dehalogenation, which increases their chemical reactivity in such a way that they can be broken down chemically at very low reaction temperatures. In this case, temperatures are used in the claimed process, which are between the ambient temperature and a maximum of 510 ° C, so that on average, the required reaction temperatures are halved compared to the prior art listed above. This in turn means that no undesirable, highly toxic secondary products can form in the claimed process.

In particular, from numerous publications by the inventor, it is known that substances and mixtures of substances can be dispersed chemically. The dispersion through chemical reaction (DCR method for short) is a simple method for distributing liquid substances and solutions of solid or liquid substances in the course of the formation of large surfaces by chemical reaction and is the subject of German patents 20 53 627, 2328777, 2328778, 2520999, 2533789, 25 33 790, 25 33 791 and their foreign equivalents. In particular, DE-AS 25 33 790 describes a process for the chemical dispersion of a compound forming a hydroxide in mineral oils, mineral oil-like or mineral oil-containing substances. However, there is no indication in this specification that halogenated hydrocarbons can also be used for such a process.

Among the numerous chemical reactions which satisfy the condition for an increase in surface area in the sense described and which can therefore be used for chemical distribution, the reaction of calcium oxide with water to calcium hydroxide and the hydrolysis of aluminum alcoholates to aluminum hydroxide are particularly noteworthy. The compounds present in the resulting finely dispersed solid preparations have a particularly high chemical reactivity. One of the advantages of the chemical dispersing process is that one can co-disperse the reactants required to perform certain chemical reactions with the dispersed substances. In this way, both reactants are side by side in a highly reactive form.

For the dehalogenation of halogenated hydrocarbons, nucleophilic reactants are advantageously used. If calcium oxide is used as starting material (starting material) to carry out the dispersing reaction, a solid preparation with calcium hydroxide as carrier material is obtained. If the dispersing reactions are carried out with the introduction of halogenated hydrocarbons, the calcium hydroxide formed contains the halogen compound homogeneously.

The halogen compound and calcium hydroxide are therefore in a highly reactive form. Hydroxyl ions are nucleophilic reactants. When the solid preparation is heated in a closed system, the halogenated hydrocarbon is dehalogenated. For this, a reaction temperature of only about 500 ° C. is required with a residence time of about one hour. By co-dispersing nucleophilic reactants of higher reactivity, e.g. Alcoholates, depending on the structure of the alcoholate, the temperature drops to about 300 ° C. If potassium hydroxide is metered into the water which is required for the reaction of the calcium oxide to calcium hydroxide, the reaction temperature drops to about 400 ° C. under otherwise identical conditions. In this context, the relationships known in chemistry between residence time, reaction temperature and nucleophilic reactivity and, last but not least, the solvent apply. The person skilled in the art is able, through simple preliminary tests, to optimize the conditions for the halogenated hydrocarbon to be completely dehalogenated in conjunction with the cheapest starting material and a particularly effective nucleophilic reaction partner in such a way that the dehalogenation can be completed at the lowest possible temperature in the shortest possible time . It is therefore also quite possible for the finely dispersed reaction product to be allowed to react completely in the presence of the nucleophilic reaction partner at ambient temperature until dehalogenation. If the nucleophilic reactant does not already arise from the starting material of the dispersing chemical reaction, such as e.g. in the case of calcium or magnesium hydroxide, the nucleophilic reaction partner is added and expediently incorporated in the dispersion by chemical reaction. Particularly suitable nucleophilic reactants are, in addition to the alkali hydroxides and alcoholates already mentioned, alcohols or amines. If alcohols are also present in addition to KOH, e.g. Diethylene glycol, alcohol ions are formed in equilibrium which have a high nucleophilic reactivity.

If heating is necessary for the dehalogenation of the halogenated hydrocarbons in the finely dispersed reaction product of the DCR method used in the presence of the nucleophilic reaction partner, this is expediently carried out in a closed reaction space in order to prevent the halogenated hydrocarbons from escaping before they are completely dehalogenated. In order to ensure a complete reaction of the possibly gaseous halogenated hydrocarbons with the solid reaction partners, it is advisable to keep the gas volume as small as possible so that the reaction space is filled as completely as possible by the finely dispersed reaction product and the nucleophilic reaction partners. However, the dehalogenation can also be carried out continuously in a fluidized bed or a fixed bed reactor, the hydrocarbons which are formed being returned, if appropriate, to the fluidized bed or the bed of the solids as long as they still contain halogenated components.

With the method according to the invention, highly toxic substances which contain halogens can be dehalogenated and converted into substances with low hazard potential in the simplest and most reliable manner, i.e. for example tetrachlorodibenzodioxin in dibenzodioxin, which, in contrast to the first compound, is not an ultra poison at all, but a relatively harmless compound. However, in order to render the secondary products formed after the dehalogenation harmless, the fully reacted product, i.e. After dehalogenation, which is still in powder form, bring the product to normal combustion, i.e. a combustion device that works at about 800 ° C.

Since the secondary products always burn easily, the environmental hazard described above can no longer occur.

For the purposes of the invention, calcium oxide is in the form of commercially available quicklime, e.g. white fine lime, preferred, but also coarse grains can be used in many cases. The quicklime can contain up to 18% by weight magnesium oxide or other foreign components.

With the help of hydrophobized calcium oxide, halogenated pollutants can be "collected" in soils. This adsorption or pre-distribution can still be improved by adding bituminous substances or mineral oils, which are preferably used in the form of waste materials.

In the following examples, all quantities are parts by weight.

example 1

14 parts of polychlorinated biphenyls (PCB) in the form of an industrially produced isomer mixture are stirred with 28 parts of CaO. The addition of 10 parts of water results in a dry powdery preparation in an exothermic dispersing chemical reaction, which is heated to 510 ° C. in a closed tube and held at this temperature for 30 minutes. 99.996% of the dehalogenation takes place under these conditions.

Example 2

14 parts of a mineral oil contaminated with 10,000 ppm PCB are chemically dispersed with 28 parts CaO and 10 parts water in which 1.4 parts KOH have been dissolved. After a dwell time of the reaction product of 30 minutes at 350 ° C, only 0.98 ppm PCB are detectable.

Example 3

14 parts of a mineral oil contaminated with 10,000 ppm PCB are chemically dispersed with 28 parts CaO and 10 parts water in which 1.4 parts KOH and 2 parts of a polyethylene glycol with an average molecular weight of 400 were dissolved. After a dwell time of the reaction product of 30 minutes at 300 ° C, 1.4 ppm PCB are still detectable.

Example 4

14.9 parts of a mineral oil-containing residue with 0.14 parts PCB and 0.9 parts tetrachloroethane from a storage tank are mixed with 28 parts CaO and 10 parts water, in which 1.4 parts KOH and 2 parts of a polyethylene glycol with an average molecular weight of 400 were dissolved, dispersed chemically. After a dwell time of 30 minutes at 350 ° C in the autoclave, no tetrachloroethane is detectable and PCB only 0.9 ppm.

Example 5

14 parts of a waste material contaminated with 10,000 ppm of a 50% solution of PCB in trichlorobenzenes are chemically dispersed with 28 parts of CaO and 10 parts of water in which 1 part of NaOH has been dissolved. After a dwell time of 30 minutes at 350 ° C in a fixed bed tractor, neither PCB nor trichlorobenzenes were detectable under otherwise identical analytical conditions.

Example 6

56 parts of a cohesive soil contaminated with 100,000 ppb of a mixture of chlorine-containing waste materials with more than 50% hexachlorobenzene from the production of crop protection agents were intimately mixed in a screw reactor with 50 parts of hydrophobic CaO in the presence of 18 parts of water. The organic impurities absorbed by the hydrophobic CaO in the mixing process can thus be dispersed chemically. With a residence time of the reaction mixture at 350 ° C of 30 minutes, 5.5 ppb halogenated hydrocarbons can still be detected.

If you add 1 part KOH to this approach, e.g. in the form of an aqueous solution, the degree of dehalogenation is increased to 99.9973% and only 1.1 ppb of halogenated hydrocarbons are detectable under otherwise identical analytical conditions.

Example 7

56 parts of a sandy soil that was contaminated with 2,000,000 ppb of a mixture of chlorine-containing waste materials with more than 50% hexachlorobenzene in addition to pentachlorobenzene as well as tetra- and trichlorobenzenes, differently chlorinated naphthalene and small proportions of differently chlorinated dibenzodioxins from the production of crop protection products were used in a screw reactor 50 parts of hydrophobic CaO intimately mixed in the presence of 10 parts of a waste mineral oil. The organic impurities absorbed by the waste mineral oil in the mixing process can be chemically dispersed after the addition of 18 parts of water. The reaction mixture is kept at 350 ° C. for 30 minutes. Thereafter, 2.0 ppb of halogenated hydrocarbons can be detected.

An almost identical result can be achieved if first waste mineral oils, bituminous waste materials, used bleaching earth loaded with low-volatile organic substances, or other substances that take the halogenated hydrocarbons in solution, or, like the hydrophobic CaO, bind adsorptively and thus optimally the nucleophilic reaction partners make accessible, chemically dispersed and the hydrophobic reaction product of the dispersing chemical reaction subsequently mixed with the contaminated soil, sand or other contaminated substances and brought to the appropriate temperature for the appropriate time. Soil, sand, etc. must be almost dry.

In general, the procedural According to decontaminated soils and sands, use them as soil bodies, e.g. as paved subsoil, since they no longer release the remaining but contaminated traces of pollutants into the environment. This applies in particular if the floors are installed with compaction.

Claims (13)

1. Process for the dehalogenation of halogenated hydrocarbons in the presence of a nucleophilic reactant, characterised in that the halogenated hydrocarbons are chemically dispersed and the resulting finely-dispersed reaction product is dehalogenated solely by a chemical reaction with the nucleophilic reactant at reaction temperatures which are between ambient temperature and 510°C.

2. Process according to Claim 1, characterised in that the halogenated hydrocarbons are chemically dispersed in the presence of the nucleophilic reactant.

3. Process according to Claim 1 or 2, characterised in that the nucleophilic reactant is produced from the educt of the dispersing chemical reaction.

4. Process according to Claim 3, characterised in that calcium oxide is used as the nucleophilic reactant.

5. Process according to one of Claims 1 to 3, characterised in that alkali hydroxides, alkali alcoholates, calcium hydroxide or magnesium hydroxide, alcohols or amines are used as the nucleophilic reactant.

6. Process according to one of the preceding Claims, characterised in that the finely-dispersed reaction product is heated to just the necessary dehalogenation temperature in as short a time as possible in the presence of the nucleophilic reactant.

7. Process according to one of the preceding Claims, characterised in that the heating is carried out in a closed reaction chamber.

8. Process according to Claim 7, characterised in that the reaction chamber is filled up as completely as possible with the finely-dispersed reaction product and the nucleophilic reactant.

9. Process according to one of the preceding Claims, characterised in that the dehalogenation is carried out discontinuously in an autoclave.

10. Process according to one of Claims 1 to 8, characterised in that the dehalogenation is carried out continuously in a fixed bed reactor or in a fluidised bed.

11. Process according to one of the preceding Claims, characterised in that the dispersal is carried out by chemical reaction and the dehalogenation is carried out as a one-stage process.

12. Process according to one of the preceding Claims, characterised in that polychlorinated aromatics, in particular chlorinated biphenyls and dioxines, are used as the halogenated hydrocarbons.

13. Process according to one of the preceding Claims, characterised in that halogenated hydrocarbons are used which are present as impurities in oils, oil sludge, in the earth or in muds.