WO2010020345A1

WO2010020345A1 - Catalytic oxidation of hydrogen chloride with oxygen in non-thermal plasma

Info

Publication number: WO2010020345A1
Application number: PCT/EP2009/005683
Authority: WO
Inventors: Eberhard Stelter; Frank-Dieter Kopinke; Ulf Roland; Robert Köhler; Frank Holzer
Original assignee: Bayer Materialscience Ag
Priority date: 2008-08-16
Filing date: 2009-08-06
Publication date: 2010-02-25
Also published as: DE102008038032A1

Abstract

The invention relates to a method for producing chlorine by the catalytic oxidation of gaseous hydrogen chloride exposed to the action of non-thermal plasma.

Description

Catalytic oxidation of hydrogen chloride with oxygen in non-thermal plasma

The invention relates to a process for the production of chlorine by catalytic oxidation of gaseous hydrogen chloride under the action of a non-thermal plasma. The combined effect of catalyst and plasma results in higher levels of HCl conversion than either of the two activation methods taken alone. By lowering the reaction temperature in the plasma reactor in comparison to the catalyzed Deacon reaction very high degrees of HCl conversion are possible without significant thermodynamic limitation.

In the chlorine-processing chemical industry, the recycling of hydrogen chloride (HCl) to chlorine (Cl ₂ ) plays an essential role (see Ulimann Encyclopedia of Industrial Chemistry, Verlag Chemie, Weinheim, 4th Edition 1975, Vol 9, p 357 ff. or Winnacker - Küchler: Chemical Technology, Processes and Products, Wiley-VCH Verlag, 5th Edition 2005, Vol. 3, pp. 512 ff). A distinction is made between gaseous HCl and its aqueous solution, hydrochloric acid. HCl recycling processes according to the prior art are the electrolysis of hydrochloric acid and the oxidation of hydrogen chloride with oxygen (Deacon reaction) according to the reaction equation:

4 HCl + O ₂ <-> 2 Cl ₂ + 2 H ₂ O

The Deacon reaction can be carried out purely thermally at high temperatures (> 700 ° C) or catalyzed at lower temperatures (300 to 500 ° C). Suitable catalysts are described, for example, in DE 1 567 788 A1. Older methods use as the catalytically active component usually doped copper chloride (for example, Shell-Chlorine method, Dow-Hercules method), while more modern methods work with precious metals or their oxides and chlorides.

The Deacon reaction is exothermic with a standard reaction enthalpy of -57 kJ / mol chlorine. It is reversible. For thermodynamic reasons, low reaction temperatures and excess oxygen are desirable in order to shift the equilibrium position as far as possible in the direction of product formation. This is offset by the desire for the highest possible, technically usable reaction rates, which require temperatures above 300 ⁰ C on the known catalysts. At these temperatures, the Deacon equilibrium is no longer completely on the product side. From the known thermodynamic data of the Deacon reaction can be calculated that at 350 ⁰ C reaction temperature, 0.1 MPa gas pressure and a stoichiometric Eduktverhältnis of four parts HCl to one part of oxygen, a maximum HCl conversion rate of 85% can be achieved. This value can be increased by an excess of oxygen. However, then increases the product processing to be supplied gas flow. Accordingly, the Effort for the chlorine separation. However, one could lower the reaction temperature to 100 ⁰ C, an equilibrium conversion of 99% would be possible at stoichiometric starting materials.

A disadvantage of catalyzed HCl oxidation, especially with noble metal catalysts, is that the catalyst used for the reaction is sensitive to contaminants in the feedstock. These can lead to significant loss of activity and thus to a reduction in the total plant capacity.

In order to lower the reaction temperatures down to the thermodynamically favorable range (<300 ° C), so-called non-thermal excitation sources were investigated. Such process principles are described, for example, in Stiller (W. Stiller: Non-thermally Activated Chemistry, Birkhäuser Verlag, Basel, Boston, 1987, pp. 33-34, pp. 45-49, pp. 122-124, pp. 138-145). Non-thermal activation HCl oxidation processes are described in JP 5 907 3405, RU A 2253 607, DD 88 309, SU A 180 1943, Cooper et al. Ind. Eng. Chem. Fundam., 7 (3), 400-409 (1968).), Van Drumpt (JD van Drumpt: Oxidation of. (WW Cooper, HS Mickley, RF Baddour: Oxidation of hydrogen chloride in a microwave discharge Hydrogen Chloride with Molecular Oxygen in a Silent Electrical Discharge, Ind. Eng. Chem., Fundam., 22 (4), 594-595 (1972)) and DE 10 2006 022 761 A1. They are based predominantly on the photochemical activation of one of the two reaction partners, HCl or O ₂ .

Initiation of the Deacon reaction in cold plasma is mentioned only in a few documents:

SU A 1 801 943 describes a process for HCl oxidation with air in an electrical pulse discharge (10 to 40 kV peak voltage) at 20 to 30 ^° C. With a residence time of up to 140 s in the discharge zone, HCl conversion rates up to 74% are achieved. The patent does not contain information on the discharge arrangement used and the specific energy consumption. The inventors emphasize the absence of a catalyst as an essential feature of their process. From the embodiments of the invention it appears that the dew point of the water of reaction in the product mixture is exceeded. This inevitably leads to the formation of water films on the reactor walls, which ultimately lead to an unstable discharge behavior (sparkover). How the condensation of the water of reaction in the discharge reactor at 20 to 30 ⁰ C can be avoided is not described. Since in the methods according to the above-mentioned prior art it comes to the formation of shielding water films, it remains unclear how the claimed degrees of conversion could be achieved. In our own experiments, no comparable yields were achieved. EP 1914199 describes a process for the production of chlorine from a gaseous hydrogen chloride and oxygen-containing mixture under the action of ultraviolet radiation, at a wavelength in the range of 165 nm to 270 nm at a density of ultraviolet radiation of (10-40) 10 -4 W / cm 3 and a pressure of not more than 0.1 MPa. Alternatively, the oxygen may be activated upon exposure to a current of accelerated electrons having an energy of 100 keV to 2 MeV. The oxidation of the hydrogen chloride is carried out by the activated oxygen to form the target product. Almost quantitative degrees of implementation are asserted without there being any heating of the product mixture. It is not mentioned which energy expenditure is necessary for the excitation sources (UV or electrons) to achieve the stated degrees of conversion. Furthermore, there is no information on the residence time of the reactants or the size of the reaction apparatus used. According to the application, the main reaction path is via excited (individual) oxygen atoms, ozone is only mentioned as a by-product of the oxygen excitation.

WO2008002197 describes a method for the production of chlorine, in which a gaseous hydrogen chloride and oxygen mixture is transferred at a pressure of 1.1 to 5.0 bar under the action of electrical pulse discharge in a low-temperature plasma, in which hydrogen chloride is oxidized by oxygen. The oxidation product is separated after condensation of the water contained.

Van Drumpt et al. (1972) describe HCl oxidation in a modified Siemens tube at atmospheric pressure. It was possible to convert up to 6% of hydrogen chloride into chlorine. These are by far too low degrees of implementation for a sensible technical application.

Cooper et al. (1968) describes the HCl oxidation in a high-energy microwave plasma. In this case, low gas pressures of 13 to 27 mbar were used and temperatures of 500 to 700 ⁰ C occurred. However, these temperatures far exceed the thermodynamically favored range by far. Furthermore, the low operating pressures of the microwave plasma are not very attractive for technical implementation.

EP1914199 describes a process for the production of chlorine from a gaseous hydrogen chloride and oxygen-containing mixture under the action of ultraviolet radiation, at a wavelength in the range from 165 nm to 270 nm at a density of ultraviolet radiation of (10-40) 10 -4 W / cm 3 and a pressure of not more than 0.1 MPa. Alternatively, the oxygen may be activated upon exposure to a current of accelerated electrons having an energy of 100 keV to 2 MeV. The oxidation of the hydrogen chloride is carried out by the activated oxygen to form the target product. It Almost quantitative degrees of implementation are asserted without there being any heating of the product mixture. It is not mentioned which energy expenditure is necessary for the excitation sources (UV or electrons) to achieve the stated degrees of conversion. Furthermore, there is no information on the residence time of the reactants or the size of the reaction apparatus used. According to the application, the main reaction path is via excited (individual) oxygen atoms, ozone is only mentioned as a by-product of the oxygen excitation.

The document DE 10 2006 022 761 A1 describes an integrated process for the preparation of isocyanates from phosgene and at least one amine and oxidation of the resulting hydrogen chloride with oxygen to chlorine, wherein the chlorine is recycled to produce the phosgene. In particular, it is assumed that processes for the production of chlorine by non-thermally activated reaction of hydrogen chloride with oxygen, in which from the resulting gas mixture in the reaction consisting at least of the target products of chlorine and water, unreacted hydrogen chloride and oxygen and optionally other minor components such as carbon dioxide and Nitrogen chlorine is removed and recycled into the phosgene production.

The main disadvantage of the few plasma-assisted, non-thermal HCl oxidation processes described so far is an unsatisfactory energy efficiency. In no case very high degrees of HCl conversion (> 90%) have been achieved, as they are desirable for a technical application with simple gas workup.

A particular disadvantage of the methods that operate using electron excitation is that stable operation of HCl oxidation is not possible with condensation of water. In the case of the use of the UV excitation is expected to be a continuous reduction in sales for the duration of the reaction.

The object of the present invention is therefore to provide a method for recycling hydrogen chloride to chlorine, which can be operated simply, safely and energy-efficiently. In particular, very high degrees of hydrogen chloride conversion should be achieved, so as to simplify the work-up of the reaction products. These objects are achieved by a heterogeneously catalyzed oxidation of hydrogen chloride in combination with a non-thermal plasma.

The invention relates to a process for the heterogeneously catalyzed oxidation of hydrogen chloride by means of oxygen-containing gases, characterized in that a gas mixture consisting at least of hydrogen chloride and oxygen flows through a suitable solid catalyst and simultaneously exposed to the action of a non-thermal plasma.

Preferably, a method is used in which the non-thermal plasma is generated by silent electrical discharge.

In a particularly preferred embodiment of the new method, the silent electric discharge is carried out in a barrier reactor with one or more current-limiting dielectric barriers.

Another particularly preferred embodiment is based on a tube bundle reactor. The individual reactor tubes are designed, for example, as plasma reactors with a cylindrical discharge gap, similar to the Siemens tubes used for ozone generation. The discharge gap is filled with a suitable catalyst in the form of a fixed bed and is continuously flowed through by the reaction mixture (hydrogen chloride, oxygen, chlorine, water vapor). Any number of these reactors can be connected in parallel to produce sufficient plant capacity. After igniting the non-thermal plasma in the discharge tubes by applying a high voltage, the hydrogen chloride is oxidized by synergistic interaction of catalyst and plasma to chlorine. The reaction temperature can be varied within a wide range. It should preferably be kept above the dew point of the product mixture at the reactor outlet (about 100 ° C.) in order to avoid condensate formation in the reactor.

A further preferred embodiment of the new method is therefore characterized in that the plasma-assisted HCl oxidation is carried out in a reactor having a tube with inner and outer electrodes. Such a reactor is also referred to as a plasma reactor, wherein the electrodes are designed corrosion-protected, in particular with respect to the process gases are.

Particularly preferred is a variant of the method in which the catalyst is introduced as a packed bed in the discharge gap of a barrier reactor. A barrier reactor is a plasma reactor in which the plasma is generated by a barrier discharge. A further embodiment variant of the novel process is characterized in that the plasma-assisted HCl oxidation is carried out in a plurality of parallel-connected reactors in a tube bundle arrangement or parallel plate reactors.

The plasma-assisted HCl oxidation is carried out in particular at elevated pressure, preferably at a pressure of up to 25 bar, more preferably 5 to 25 bar.

The reaction temperature in the plasma-activated HCl oxidation can be set below about 300 ⁰ C. In this case, the HCl conversion is determined by the plasma-chemical activation of the reactants and their synergistic reaction on the catalyst. At the same time, the homogeneous plasma-activated HCl oxidation in the gas space of the catalyst bed contributes to the total sales. The total conversion in the presence of the catalyst is always higher than that in the empty discharge tubes under the same external discharge conditions, ie the same applied high voltage.

Preference is therefore given to a variant of the new process, which is characterized in that the reaction temperature of the HCl oxidation in the plasma in a range up to 300 ⁰ C, preferably from 120 to 250 ⁰ C, preferably 130 to 200 ⁰ C is set.

An alternative to this is the operation of the plasma reactor at a catalyst temperature above 300 ^° C. In this case, the catalyzed and the plasma-activated HCl oxidation overlap. Even under these reaction conditions, the combination according to the invention under certain boundary conditions leads to higher overall HCl conversions than either of the two forms of activation individually considered. However, the maximum achievable HCl conversion is limited by the thermodynamic boundary conditions. The catalyst present catalyses the back and forth reaction in the Deacon equilibrium.

Therefore, a method is also preferred in which the plasma-assisted HCl oxidation is combined with a catalyzed HCl oxidation reaction and is carried out in a reaction space.

In a particularly preferred embodiment of the novel process, the reaction temperature of the combined plasma-supported and catalyzed HCl oxidation is set in a range of at least 300 ^° C., preferably from 320 to 450 ^° C., preferably 350 to 400 ^° C.

It is possible to overcome the thermodynamic conversion limits specified in the catalyzed process by using reactors connected in series, which operate on different activation principles. First, in a catalyzed process at high temperatures (eg, 38O ⁰ C), a certain HCl conversion> 50% is achieved. The product mixture leaving the process is released Temperatures below 300 ⁰ C cooled and passed through a plasma reactor. In this reaction procedure by connecting different reactors in series, it was surprisingly found that even with low HCl contents and high chlorine contents in the influx of the plasma reactor no chlorine destruction, but always a significant increase in the chlorine content (HCl conversion) occurred. Very high total HCl conversion levels can be achieved in the manner described, the catalyzed activation contributing the major part to the HCl conversion, while the plasma activation brings about the desired, very high overall conversion up to almost 100%.

In another variant of the new process, the plasma-assisted HCl oxidation is combined with a catalyzed HCl oxidation reaction and is carried out in separate reaction spaces, wherein the plasma-assisted HCl oxidation is followed by the catalyzed HCl oxidation reaction.

A particularly preferred variant of the aforementioned method is characterized in that first the main portion of the HCl oxidation is achieved in particular up to a HCl conversion of at least 70%, preferably at least 80% in the catalyzed oxidation process, which is followed by the plasma-assisted HCl oxidation.

Different discharge arrangements are possible for the production of the nonthermal plasma. In a preferred embodiment, a tubular reactor with cylindrically symmetrical inner and outer electrodes is used. Between them there is at least one barrier (e.g., ceramic material) which prevents a direct breakdown of electrical discharge. There may also be multiple barriers, e.g. used in front of each electrode. Between both electrodes, a discharge gap (dielectric, plasma space) with a typical thickness of a few millimeters is formed, in which the catalyst is arranged in an optimum particle size as a packed bed. To generate the plasma, an alternating electrical voltage (for example 5 to 50 kV) is applied between the two electrodes. This high voltage can be applied as a low frequency sinusoidal voltage (e.g., 50 Hz) or as a short pulse duration pulsed voltage (e.g., in the micro or nanosecond range) and higher in frequency (e.g., in the kHz or MHz range). The packed bed between the high-voltage electrodes is not only catalytically active, it can also significantly influence the discharge characteristics by its dielectric properties (plasma physical effect).

The following parameters can be used as electrical manipulated variables for generating optimal plasma discharges:

o the applied high voltage, in a range of 5 to 50 kV, preferably 20 to 30 kV o the active power density, in a range of 50 to 5000 J / l, preferably in the range 200 to 1000 J / l

o the type of applied AC voltage, either in sinusoidal form with a frequency of 50 or 60 Hz or as a pulsed high voltage with a pulse frequency in the range of 50 Hz to 50 kHz, preferably 1 to 5 kHz, and a pulse rise time of 5 to 5000 ns, preferably from 100 to 1000 ns

o the reduced field strength (measured in Townsend [Td]) should be adjusted in a range of 200 to 600 Td, preferably 350 to 500 Td.

The discharge gap should have a thickness of 1 to 50 mm, preferably 3 to 10 mm.

Discharge arrangements are possible with both one-sided and double barrier.

As a barrier material ceramic materials and glasses are used. High Dielectric Constant Barrier Materials, e.g. Ferroelectric ceramics, the generation of very high field strengths.

The thickness of the barriers is in the range of about 0.1 to 5 mm, preferably in the range between 0.5 and 2 mm.

A particular embodiment uses solids with ferroelectric properties (eg barium titanate) as constituents of the packed bed or as a catalyst support. As a result, increasingly high-energy species are generated in the plasma, which can contribute to increased conversions in the plasma zone. Further suitable ferroelectrics are other ionic crystals with perovskite structure such as lead zirconate titanate Pb (Zr _x Ti _x ) O ₃ and the following substances: strontium bismuth tantalate SrBi ₂ Ta ₂ O ₉ , bismuth titanate Bi ₄ Ti ₃ Oi ₂ , Bismuth lanthanum titanate Bi _4-x La _x Ti ₃ O ₁₂ , bismuth titanate niobate Bi ₃ TiNbO ₉ , strontium titanate SrTiO ₃ , barium strontium titanate Ba _x Sr _1x TiO ₃ , sodium nitrite NaNO ₂ , hexagonal manganates RMnO ₃ with R = Y, Sc, In, Ho, Er, Tm, Yb, Lu.

In a further advantageous embodiment of a plasma reactor, the electrical barrier or the electrical barriers are simultaneously used as a catalyst carrier. For this purpose, a porous or rough barrier material is coated with the catalytically active components. A particular advantage of this embodiment lies in the fact that compared to the flow through the catalyst bed a lower flow resistance. This allows smaller gap widths and thus higher electric field strengths in the plasma zone. In another embodiment of the new method, the silent electric discharge is carried out in a barrier reactor having one or more current-limiting dielectric barriers whose barrier or barriers simultaneously serve as carriers for the catalyst.

Such a method is particularly preferred when the silent electrical discharge in a barrier reactor with a ferroelectric barrier material, in particular based on

Barium titanate is performed. However, other ferroelectrics can also be used.

As heterogeneous catalysts for the plasma enhanced Deacon reaction, a variety of solids have been found to be effective. Depending on the temperature range used, they preferably contain oxides, oxide chlorides or chlorides of the metals of Groups 1, 7 and 8 of the Periodic Table which are dispersed in finely divided form on porous support materials, e.g. Titanium dioxide, are applied. Another effective catalyst consists of these catalytically active compounds on ferroelectric support materials, whereby several mechanisms of action - the chemical catalysis and a plasma-physical effect (the amplification of the plasma intensity) - can be advantageously combined.

The catalyst which is preferably used in the new process, has as catalytic active components metals of the 1st, 7th or 8th subgroup of the Periodic Table of the Elements or oxides, oxide chlorides or chlorides of the metals of the 1st, 7th or 8th subgroup, or Mixtures of these metals or metal compounds.

The catalyst is particularly preferably selected from the series: ruthenium oxide, ruthenium chloride, ruthenium oxychloride or other ruthenium compounds on silica, alumina, titania, tin dioxide or zirconium dioxide as support.

The novel plasma-assisted HCl oxidation is particularly preferably used in combination with the catalyzed HCl oxidation with molecular oxygen which is also known as the Deacon process and which is operated in an upstream step or simultaneously. Here, hydrogen chloride is oxidized with oxygen in an exothermic equilibrium reaction to chlorine, whereby water vapor is obtained. The reaction temperature is usually 150 to 500 ⁰ C, the usual reaction pressure is 1 to 25 bar. Since it is an equilibrium reaction, it is expedient to work at the lowest possible temperatures at which the catalyst still has sufficient activity. Furthermore, it is expedient to use oxygen in excess of stoichiometric amounts of hydrogen chloride. For example, a two- to four-fold excess of oxygen is customary. Since no loss of selectivity is to be feared, it may be economically advantageous to work at relatively high pressure and, accordingly, longer residence time than normal pressure. Suitable preferred catalysts for the Deacon process include ruthenium oxide, ruthenium chloride, ruthenium oxide chloride or other ruthenium compounds supported on silica, alumina, titania, tin dioxide or zirconia. Suitable catalysts can be obtained, for example, by applying ruthenium chloride to the support and then drying or drying and calcining. Suitable catalysts may, in addition to or instead of a ruthenium compound, also contain compounds of other noble metals, for example gold, palladium, platinum, osmium, iridium, silver, copper or rhenium. Suitable catalysts may further contain chromium (III) oxide.

The catalyzed hydrogen chloride oxidation can either adiabatic or isothermal or approximately isothermal, discontinuous, but preferably continuously as flow or

Fixed bed process, preferably as a fixed bed process, particularly preferably in tube bundle reactors of heterogeneous catalysts at a reactor temperature of 180 to 500 ⁰ C, preferably 200 to

400 ⁰ C, more preferably 220 to 350 ⁰ C and a pressure of 1 to 25 bar (1000 to 25000 hPa), preferably 1.2 to 20 bar, more preferably 1.5 to 17 bar and in particular 2.0 to 15 bar be performed.

Typical reactors in which the catalyzed hydrogen chloride oxidation is carried out are fixed bed or fluidized bed reactors. The catalyzed hydrogen chloride oxidation can preferably also be carried out in multiple stages.

In the case of the adiabatic mode of operation, it is also possible to use a plurality of reactors with intermediate cooling, in particular 2 to 10, preferably 4 to 8, particularly preferably 5 to 7 reactors connected in series. In the isothermal or approximately isothermal driving can also several, ie 2 to

10, preferably 2 to 6, particularly preferably 2 to 5, in particular 2 to 3, connected in series

Reactors with additional intermediate cooling can be used. The hydrogen chloride can be added either completely together with the oxygen before the first reactor or distributed over the various reactors. This arrangement of individual reactors can also be combined in one apparatus.

A further preferred embodiment of a device suitable for the method consists in using a structured catalyst bed in which the catalyst activity increases in the flow direction. Such structuring of the catalyst bed can be done by different impregnation of the catalyst support with active material or by different dilution of the catalyst with an inert material. As an inert material, for example, rings, cylinders or balls of titanium dioxide, tin dioxide, zirconium dioxide or mixtures thereof, alumina, steatite, ceramic, glass, graphite or stainless steel can be used. In the preferred use of shaped catalyst bodies, the inert material should preferably have similar external dimensions.

Suitable shaped catalyst bodies, even as shaped inert bodies, are shaped bodies of any shapes, preference being given to tablets, rings, cylinders, stars, carriage wheels or spheres, rings, cylinders or star strands being particularly preferred as the form.

Ruthenium compounds or copper compounds on support materials, which may also be doped, are particularly suitable as heterogeneous catalysts, preference being given to optionally doped ruthenium catalysts. Examples of suitable carrier materials are silicon dioxide, graphite, rutile or anatase titanium dioxide, tin dioxide, zirconium dioxide, aluminum oxide or mixtures thereof, preferably titanium dioxide, tin dioxide, zirconium dioxide, aluminum oxide or mixtures thereof, particularly preferably γ- or δ-aluminum oxide or mixtures thereof ,

The copper or ruthenium-supported catalysts can be obtained, for example, by impregnation of the support material with aqueous solutions of CuCl ₂ or RuCl ₃ and optionally a promoter for doping, preferably in the form of their chlorides. The shaping of the catalyst can take place after or preferably before the impregnation of the support material.

For doping the catalysts are suitable as promoters alkali metals such as lithium, sodium, potassium, rubidium and cesium, preferably lithium, sodium and potassium, more preferably potassium, alkaline earth metals such as magnesium, calcium, strontium and barium, preferably magnesium and calcium, particularly preferably magnesium, Rare earth metals such as scandium, yttrium, lanthanum, cerium, praseodymium and neodymium, preferably scandium, yttrium, lanthanum and cerium, more preferably lanthanum and cerium, or mixtures thereof.

The moldings can then be dried at a temperature of 100 to 400 ⁰ C, preferably 100 to 300 ⁰ C, for example, under a nitrogen, argon or air atmosphere and optionally calcined. First C, the shaped bodies are preferably dried and subsequently calcined at 200 to 400 ⁰ C at 100 to 15O ^0th

The aforementioned catalyst systems known for the Deacon process can also be used for the plasma-assisted HCl oxidation.

The conversion of hydrogen chloride in a single pass may preferably be limited to 15 to 90%, preferably 40 to 85%, particularly preferably 50 to 70%. Unreacted minor hydrogen chloride residues can be partially or completely recycled to the catalyzed hydrogen chloride oxidation after separation. The volume ratio of Hydrogen chloride to oxygen at the reactor inlet is preferably 1: 1 to 20: 1, preferably 1: 1 to 8: 1, particularly preferably 1: 1 to 5: 1.

In a last step, the chlorine formed is separated off. The separation step usually comprises several stages, namely the separation and optionally the recycling of small residues of unreacted hydrogen chloride from the product gas stream of hydrogen chloride oxidation, the drying of the resulting, essentially chlorine, oxygen and water-containing stream and the separation of chlorine from the dried Electricity.

The chlorine produced by the new process is further used in production processes for the production of polymers such as polyurethanes and PVC, which as a by-product provide the hydrogen chloride for the new oxidation process.

Another object of the invention is a barrier reactor with one or more current-limiting dielectric barriers, in particular for the oxidation of hydrogen chloride by means of oxygen-containing gases, characterized in that in the discharge gap of the barrier reactor catalyst is introduced as a fixed bed.

Preference is given to a barrier reactor, characterized in that the reactor is designed as a tubular reactor with a tube having inner and outer electrodes, wherein in particular the electrodes are designed to be protected against corrosion by process gases.

Also preferred is a barrier reactor in which the reactor is designed in the form of a tube bundle arrangement or several parallel plate reactors.

The invention is explained in more detail below by the examples, which, however, do not represent a limitation of the invention.

Examples

example 1

The experimental apparatus consisted of a glass tube reactor (d _Au SEN _r oiir x di _nn e _m -ear x length = 15 mm x 10 mm x 300 mm) as well as measuring and metering devices. This tubular reactor consisted of a heatable outer tube and an inner tube. The wall thickness of outer and inner tube was about 1 mm. The outer tube was steamed with a metal mirror. The inner tube was cohesively filled by a copper tube. The metal sheath functioning as an outer electrode was connected to the ground potential of a high voltage generator. The copper inner tube was connected to the high voltage output of the generator. Between both electrodes a pulsed high voltage with variable amplitude and pulse frequency was applied. Typically, AC pulses were applied with a peak voltage of 30 kV, a frequency of 600 Hz and a pulse duration of approximately 10 μs. The gap between outer and inner glass tube (gap width 3 mm) was filled with rod-shaped catalyst (about 10 cm ³ , L xd (the rods) "2 mm x 1 mm, 1 wt .-% RuO ₂ on TiO ₂ ). The reactor temperature was adjusted to 150 ⁰ C. At the reactor exit was a cooled condensate separator.

The catalyst-filled annular gap was flowed through by 50 ml / min of a gas stream which consisted in equal parts of hydrogen chloride and oxygen. At the outlet of the reactor gas samples were taken and their chlorine content (absorption at 330 nm) was analyzed photometrically. When the high voltage was switched off, no significant chlorine content (<0.2% by volume) in the product gas could be measured. After switching on the high voltage and stabilizing the plasma, 20% by volume of chlorine in the product gas was measured. This concentration corresponds to a degree of HCl conversion of 63%.

After removing the catalyst filling, the same experimental conditions were readjusted. The absence of the solid lengthened the residence time of the gas stream in the annular gap from about 4 s to 8 s. The chlorine content in the product stream after switching on the plasma was 5.5% by volume. This value corresponds to a HCl conversion of only 20%.

The example shows that even under the experimental conditions used, a homogeneous plasma leads to significant HCl oxidation. The introduction of a catalyst significantly enhances this effect. The catalyst without plasma is not active under the reaction conditions used (150 ⁰ C). Example 2

The experimental conditions described in Example 1 were changed so that the plasma reactor was operated at a temperature of 350 ^° C. Under these conditions, the catalyzed process is already activated. When the plasma voltage was switched off, a chlorine concentration of 28.5% by volume was measured in the cooled product gas. This value corresponds to 80% HCl conversion. After switching on and stabilizing the plasma, the chlorine concentration increased to 34% by volume, which corresponds to an HCl conversion rate of about 90%.

The example shows that under the conditions of a catalyzed HCl oxidation, a significant increase in the chlorine yield by switching on the plasma is possible.

Example 3

The experimental setup described in Example 1 was extended by connecting two reactors of the same type in series. The first reactor was operated under the experimental conditions described in Example 2 (350 ° C, without condensate) without plasma assistance. The second reactor was operated under the conditions described in Example 1 (15O ⁰ C) with the product gas from reactor 1 as input gas. Both reactors were connected by a heated to 120 ⁰ C PTFE tube. First, reactor 2 was operated without plasma. In the product gas, a chlorine concentration of 28.5% by volume was measured (corresponds to 80% HCl conversion). After switching on the plasma, the chlorine concentration increased to 38% by volume, which corresponds to an HCl conversion rate of about 97%.

The example shows that very high levels of HCl conversion can be achieved by connecting catalyzed and plasma-activated oxidation in series.

Example 4

The experimental setup described in Example 1 was modified such that the discharge gap, instead of rod-shaped catalyst (about 1% by weight of RuO ₂ on TiO ₂ ), with a fine-grained mixture of this catalyst having dielectric properties and barium titanate particles ferroelectric material which is catalytically inactive for the HCl oxidation was filled. Both components were mixed as sieved grain fraction (0.5 to 1 mm) in a mass ratio of 1: 1 and filled in the annular gap. Among the in Example 1 described experimentally measured 26 vol .-% chlorine in the cooled reaction gas. This corresponds to a degree of HCl conversion of 75%.

The example shows that a mixture of catalytically active dielectric and catalytically inactive ferroelectric particles has a positive overall effect on the plasma-activated oxidation of HCl.

Example 5

The experiment described in Example 4 was modified such that the annular gap of the reactor was filled solely with ferroelectric material which was used as carrier for a catalytically active component. For this purpose, barium titanate (particle fraction 0.5 to 1 mm) by wet impregnation with ruthenium acetate coated (about 0.5 wt .-% Ru), calcined at 400 ⁰ C in an air stream and then installed as a packed bed in the reactor. Under the experimental conditions described in Example 1, 13.5% by volume of chlorine in the product stream were measured, which corresponds to an HCl conversion rate of approximately 45%.

The example shows that the special plasma-physical properties of ferroelectric materials can be exploited for plasma-activated HCl oxidation.

Example 6

The inner electrode of the reactor described in Example 1 was modified in the following manner: The smooth glass tube functioning as an inner barrier was replaced by a tube of porous, ceramic material with an outer diameter of 12 mm and a wall thickness of 2 mm. The free gap width thereby reduced to about 1 mm. The outer wall of the ceramic tube was wet-impregnated with ruthenium acetate (about 0.3 mg Ru per cm ^{2 of} the geometric surface). After calcination of the catalyst layer at 400 ^° C. in a stream of air, the barrier tube was installed in the reactor and the experimental conditions described in Example 1 were established. Without active plasma, no significant chlorine concentrations (<0.2% by volume) were measured in the product gas. After switching on and stabilizing the plasma, 11.8% by volume of chlorine was measured. This value corresponds to an HCl conversion rate of around 40%.

The example shows that a significant plasma effect can also be achieved with a catalyst layer on porous barrier material.

Claims

claims

1. A process for the heterogeneously catalyzed oxidation of hydrogen chloride by means of oxygen-containing gases, characterized in that a gas mixture consisting at least of hydrogen chloride and oxygen flows through a suitable solid catalyst and simultaneously exposed to the action of a non-thermal plasma.

2. The method according to claim 1, characterized in that the catalyst as catalytic active components metals of the 1st, 7th or 8th subgroup of the Periodic Table of the Elements or oxides, oxide chlorides or chlorides of the metals of the 1st, 7th or 8th subgroup, or mixtures of these metals or metal compounds.

3. The method according to claim 1 or 2, characterized in that the catalyst is selected from the series: ruthenium oxide, ruthenium chloride, ruthenium or other ruthenium compounds on silica, alumina, titania, tin dioxide or zirconia as a carrier.

4. The method according to any one of claims 1 to 3, characterized in that the non-thermal plasma is generated by silent electrical discharge.

5. The method according to any one of claims 1 to 4, characterized in that the silent electric discharge is carried out in a barrier reactor with one or more current-limiting dielectric barriers.

6. The method according to any one of claims 1 to 5, characterized in that the plasma-assisted HCl oxidation in a reactor with a tube with inner and

Outside electrode is performed, wherein the electrodes are designed corrosion-protected in particular with respect to the process gases.

7. The method according to any one of claims 1 to 6, characterized in that the catalyst is introduced as a packed bed in the discharge gap of a barrier reactor.

8. The method according to any one of claims 1 to 7, characterized in that the plasma-assisted HCl oxidation is carried out in several parallel-connected reactors in a tube bundle arrangement or parallel plate reactors.

9. The method according to any one of claims 1 to 8, characterized in that the plasma-assisted HCl oxidation at elevated pressure, in particular a pressure of up to 25 bar, preferably 5 to 25 bar is performed.

10. The method according to any one of claims 1 to 9, characterized in that the reaction temperature of the HCl oxidation in the plasma in a range up to 300 ° C, preferably from 120 to 250 ⁰ C, preferably 130 to 200 ⁰ C is set.

11. The method according to any one of claims 1 to 9, characterized in that the plasma-assisted HCl oxidation is combined with a catalyzed HCl oxidation reaction and is carried out in a reaction space.

12. The method according to claim 11, characterized in that the reaction temperature of the combined plasma-supported and catalyzed HCl oxidation in a range of at least 300 ⁰ C, preferably from 320 to 450 ⁰ C, preferably 350 to 400 ⁰ C is set.

13. The method according to any one of claims 1 to 10, characterized in that the plasma-assisted HCl oxidation is combined with a catalyzed HCl oxidation reaction and is carried out in separate reaction chambers, wherein the plasma-assisted HCl oxidation of the catalyzed HCl oxidation reaction is connected downstream.

14. The method according to any one of claims 1 to 13, characterized in that first the main portion of the HCl oxidation is achieved in particular up to a HCl conversion of at least 70%, preferably at least 80% in the catalyzed oxidation process, the plasma-assisted HCl oxidation is downstream.

15. The method according to any one of claims 5 to 14, characterized in that the silent electric discharge is carried out in a barrier reactor with one or more current-limiting dielectric barriers whose barrier or barriers serve as a support for the catalyst at the same time.

16. The method according to any one of claims 5 to 14, characterized in that the silent electrical discharge is carried out in a barrier reactor with a ferroelectric barrier material, in particular based on barium titanate.

17. Barrier reactor with one or more current-limiting dielectric barriers, in particular for the oxidation of hydrogen chloride by means of oxygen-containing gases, characterized in that in the discharge gap of the barrier reactor catalyst is introduced as a fixed bed.

18. Barrier reactor according to claim 17, characterized in that the reactor is designed as a tubular reactor with a tube with inner and outer electrodes, wherein in particular the electrodes are carried out against corrosion of process gases.

19. Barrier reactor according to one of claims 17 or 18, characterized in that the reactor is designed in the form of a tube bundle arrangement or a plurality of parallel plate reactors.