DE2901577C2 - - Google Patents

Description

The invention relates to that described in claims 1 to 6 Process for removing gaseous, electrochemical oxidizable and / or reducible impurities from gases and that described in claims 7 to 9 Device for carrying it out.

All previously known absorption devices suitable for cleaning gases such as packed columns, tray columns, Sprayers and film washers are characterized in that the gas to be cleaned via the largest possible exchange surface is brought into contact with the washing liquid. Through physical or chemical bonding in gas scrubbing gas components to be removed from the washing liquid the desired cleaning takes place. Almost everyone on absorption based gas cleaning processes become gas and washing solution performed in countercurrent, but in some cases also in Direct current or cross current.

For a given decrease in the concentration of the gas to be washed out, a given height of the absorption apparatus is required for a given exchange surface, constant gas and liquid throughput and constant mass exchange rate. This required apparatus height is greater, the more the concentration of the gas component to be washed out in the washing liquid varies from the initial value x ₁ to the final value x ₂ (cf. Ullmann's Encyclopedia of Technical Chemistry, 4th edition, Verlag Chemie, Weinheim / Bergstr., 1973, Vol 2, p. 582). This is equivalent to the fact that the concentration gradient between gas and liquid determines the rate of mass transfer for the component to be washed out. The required apparatus height and thus the apparatus volume required for a given cleaning task can therefore be reduced if it is possible to keep the concentration gradient between gas and liquid in the entire apparatus as large as possible for the gas component to be washed out. It has therefore long been known from absorption technology that the effectiveness of absorption apparatus can be increased considerably if the stationary concentration of the gas to be removed in the detergent is kept at a low level by chemical reaction with the washing liquid. A well-known example of this gas scrubbing improved by chemical reaction is the absorption of chlorine in sodium hydroxide solution instead of in water. The reaction runs in water

off, their balance far to the left and thus at high chlorine concentrations in the detergent. Used instead you use caustic soda as a detergent the balance of the reaction

on the right side and therefore at low chlorine concentrations.

To accomplish the task of concentrating the wash-out To keep gas components in the washing liquid low, it is known that redox reactions can also be used. So in a VDI guideline for exhaust gas purification oxidizing gas scrubbing (VDI guideline 2443, draft, VDI-Verlag GmbH, Düsseldorf, 1977) as suitable oxidizing agents Oxidants based on oxygen, oxidants based on oxygen compounds higher quality metals and oxidants Based on chlorine. Just like oxidizable gas impurities  can also be removed by oxidation reducible gas impurities through suitable reducing agents be removed. In all of these procedures occurs as Disadvantage the fact that chemicals are required for operation will. In many cases, this leads to the loaded Washing liquids are not regenerable and thus the waste gas problem becomes a waste water problem. The selection suitable and at the same time inexpensive oxidation or reducing agent is due to the normal potential of the limited oxidizing or reducing gas component.

DE-OS 18 14 027 describes a method for removing gaseous, electrochemically oxidizable and / or reducible Impurities from gases by dissolving the impurities in an electrolytically conductive washing liquid in one Absorption column and electrochemical oxidation and / or Reduction of the impurities dissolved in the washing liquid known in which the dissolution of the impurities in the electrolytically conductive washing liquid in an absorption or washing tower of conventional construction, which with the Contaminated washing liquid then from the Wash tower is pulled off and eventually the contaminants outside the wash tower in an external circuit that contains an electrolytic cell, are implemented electrochemically.

On the other hand, J. S. Newman et al. a., Advances in Electrochemistry and Electrochemical Engineering, Vol. 11, John Wiley & Sons, New York, 1978; G. Kreysa, Chem.-Ing.-Tech. 48 (1976), No. 10, pp. 852 to 854; G. Kreysa, Chem.-Ing.-Techn. 50 (1978), No. 5, pp. 332 to 337 and DE-OS 26 22 497 the principle fixed bed electrolysis known. The well-known fixed bed electrolysis However, it was only used to separate heavy metals dissolved as ions in metallic form, e.g. B. of dissolved metals from waste water. In these known applications the fixed bed electrolysis was only the electrode bed Flows through electrolytes.

The invention has for its object a method of to provide in the manner described in DE-OS 18 14 027, where the concentration gradient between gas and washing liquid as large as possible in the entire absorption column is kept and a lower one with the same effectiveness Equipment volume required, works reliably and easily can be automated, and a device for performing to provide this procedure.

According to the invention, this object is achieved for the method solved that one is solving the impurities in the electrolytic conductive washing liquid and the electrochemical Carries out oxidation and / or reduction in an absorption column, which is electrically conductive, at least one fixed bed electrode forming fillers and / or electrode internals contains.

For the device for performing the method, the Object achieved according to the invention in that the absorption column is formed as the electrochemical cell, wherein at least one of the two electrode spaces with packing Made of metal, graphite or one with an electrically conductive Coated non-conductor is filled, which is a monopolar Form fixed bed electrode by washing liquid and gas is flowed through.

In contrast to conventional absorption processes with chemical Oxidation or reduction works the method according to the invention chemical-free, the concentration of which is to be washed out Gas component in the washing liquid through the electrochemical Reaction is kept low.

Compared to the so-called known from DE-OS 18 14 027 The process according to the invention provides an outer cell process so-called inner cell process, in which an integration absorption step and electrochemical conversion, where instead of the usual packing in absorption towers conductive packing can be used simultaneously with  their entire surface act as an electrode. Doing so on the one hand by constantly shifting the balance the absorption capacity is increased, and on the other hand, as a result the large electrochemically active surface of the packing achieved high electrochemical sales speeds that are significantly higher than those of conventional electrolysis cells. Since in the application of the known principle according to the invention fixed bed electrolysis not only the bed electrode from the electrolyte, but from a two-phase gas-liquid mixture is flowed through, which has a much lower conductivity when the pure electrolyte has, it was surprising determine that in this case too the electrochemical Effectiveness of the bed electrode is fully guaranteed.

In the inventive method using the Principle of fixed bed electrolysis space-time yields achieved, and the required equipment volume kept as small as possible. The electrode bed will in preferred arrangements from above with the washing liquid as electrolytes and from below with the gas to be cleaned acted upon. Gas and washing liquid can also in the Direct current or cross current can be conducted, or the gas can Pearl from bottom to top through a resting electrolyte. As electrode material from which the packing or internals the absorption column, metals, coal, graphite, Semiconductors or conductive or semi-conductive coated Non-conductor can be used. The geometric shape of each other touching packing is arbitrary. In preferred arrangements are made of graphite and are as Balls of 1-15 mm in diameter are formed. These for fixed bed electrodes quite large diameters have to be used in order to achieve the low effective electrolyte resistance is reduced by the gas loading of the fat bed Reduction of the effective bed depth (see DE-OS 26 22 497) counteract.

The method according to the invention and the preferred configurations the device for performing the method explained in more detail using the examples and figures.

Example 1

Fig. 1 shows a process scheme for the removal of reducible gas contaminants. Halogens are examples of reducible gases.

Cl2 + 2e ^- → 2 Cl ^- ε ₀ = +1.360 V (3)
Br₂ + 2e ^- → 2 Br ^- ε ₀ = +1.087 V (4)
J₂ + 2e ^- → 2 J ^- ε ₀ = +0.621 V (5)

The absorption apparatus consists of a cathode chamber 1 filled with the fixed bed, a diaphragm 3 and the anode chamber 4 with the anode 5 . The fixed bed electrode 1 is supplied with current via a contact electrode 2 , which may consist of graphite, metal or a non-conductor coated with a conductor. The diaphragm 3 can consist of a mechanical separation system (plastic or ceramic) or of an ion exchange membrane. A mechanical separation system is suitable if the detergent is to be loaded, for example, during halogen washing only at low hydrogen halide concentrations. If higher hydrogen halide concentrations are aimed for, a cation exchange membrane is expediently used. The anode 5 can consist of graphite, metal or a conductive non-conductor in a known manner. The detergent circuit 6 is filled in batches or continuously with fresh detergent. In the simplest case, water is used as the detergent; however, an electrolysis solution can also be used. Suitable electrolyte solutions are, for example, hydrogen halide solutions or halide salt solutions which correspond to the halogen to be removed, or dilute alkali metal solutions. The anolyte is also renewed in batches or continuously. Water or an electrolyte solution is used as the anolyte. The raw gas to be cleaned enters the apparatus at the gas inlet 7 , flows through the electrode bed in countercurrent and exits the gas outlet 8 as a cleaned gas.

Example 2

Fig. 2 shows a process scheme for the removal of oxidizable gas impurities. Examples of this are the following reaction equations:

NO + 2 H₂O → NO₃ ^- + 4 H⁺ + 3e ^- ε ₀ = +0.957 V (6)
N₂O₄ + 2 H₂O → 2 NO₃ ^- + 4 H⁺ + 2e ^- ε ₀ = +0.803 V (7)
SO₂ + 2 H₂O → SO₄ ^2- + 4 H⁺ + 2e ^- ε ₀ = +0.138 V (8)

The method arrangement in FIG. 2 corresponds analogously to Example 1. However, an electrode bed 9 is connected via a contact electrode 10 as an anode, and a counter electrode 12 in a counter electrode space 11 as a cathode.

Example 3

Fig. 3 shows a process scheme for the removal of oxidizable and reducible gas contaminants. The method is characterized by the use of two cells, one having a fixed bed cathode 13 with detergent circuit 14 and the other having a fixed bed anode 18 with detergent circuit 19 . This method is particularly easy to use if either the oxidizable component is not attacked when the reducible component is removed or if the reducible component is not reduced when the oxidizable component is removed. FIG. 3 shows the first of these two cases, ie the raw gas passes through an inlet 15 into the first absorption apparatus with the cathodic fixed bed electrode 13 . The gas freed from the reducible component leaves the apparatus at an outlet 16 , enters the second apparatus with an anodic fixed bed electrode 18 via an inlet 17 , and the gas now also freed from the oxidizable component leaves the apparatus at an outlet 20 . A technical simplification of this process which lowers the investment costs is that the two electrochemical cells acting as absorption apparatus are combined to form a bipolar filter press unit, as shown in FIG. 4. An anodic bed 21 and a cathodic bed 22 are separated by diaphragms 24 from a bipolar counter electrode 23 . The gas and electrolyte connections can be made analogously to FIG. 3.

Example 4

An embodiment of the device suitable for carrying out the method according to the invention is shown in FIGS. 5 and 6. This is a fixed bed absorption column for the cathodic purification of a gas, as described in Example 1. The washing liquid serving as the catholyte passes via an inflow 25 into a sprinkling ring 26 , flows from top to bottom through an electrode fixed bed 27 consisting of graphite balls and leaves the cylindrical apparatus through the drain opening 29 located in an outer wall 28 . The electrode fixed bed is supplied with current by a contact electrode 30 located on its outer lateral surface. The gas to be cleaned enters the apparatus at a gas inlet 31 , flows through the electrode bed 27 in countercurrent to the catholyte from bottom to top and leaves the apparatus as a cleaned gas through a gas outlet 32 . The anode space is separated from the electrode bed 27 by a cation exchange membrane 33 . The anolyte flows into the anode compartment via an inlet 34 and leaves it via an outlet 35 . A rod anode 36 is located in the middle of the anode compartment. A section through the device shown in FIG. 5 is shown in FIG. 6. An advantageous embodiment of the anode is shown in FIG. 7. An anolyte feed pipe 37 is led to the lower end of the anode space and also serves as an anode. For this purpose, it is made of graphite, metal or a non-conductor with a conductive coating.

Example 5

Since the thickness of the electrode bed parallel to the current flow direction is limited by the effective bed depth, the cross section of the device described in Example 4 cannot be increased arbitrarily. This difficulty, which is characteristic of fixed and fluidized bed electrodes, can be overcome in the construction of the fixed bed absorption column according to the invention in an analogous manner, as is described in DE-PS 22 27 084 for an electrochemical cell with a fluidized bed electrode. As the section shown in FIG. 8 shows, a plurality of anode space tubes 39 , which may for example consist of a cation exchange membrane, are arranged at regular intervals in a fixed bed electrode 38 . The distances are usually between 0.5 and 5 cm. Anode rods 40 , which are electrically short-circuited, are located in the center of these anode compartments. Since the cathodically operating bed electrode 38 is a fixed bed electrode, in contrast to DE-PS 22 27 084, the use of a plurality of contact electrodes in the bed electrode can be dispensed with. Instead, the bed electrode is contacted via a cylinder-shaped contact electrode 41 analogous to Example 4.

Example 6

The fixed bed electrode absorption column according to the invention described in Example 4 for carrying out the method according to the invention can also be constructed in a manner which is characterized by a rectangular cross section and an anode space and a fixed bed electrode lying opposite one another instead of having a circular cross section and concentric structure. The connections for gas and electrolyte described in Example 4 are retained in a completely analogous manner; only a plate-shaped or net-shaped anode is to be used instead of a rod-shaped anode. FIGS. 9 and 10 show the manner in which Festbettelekrodenabsorptionskolonnen rectangular cross section can be joined together to form larger units. The cathode chamber is filled with a fixed bed electrode 42 , which is supplied with current via a contact electrode 43 . The anode compartment with an anode 43 is separated from the bed electrode by a diaphragm 44 . All contact electrodes 43 and all anodes 45 are electrically short-circuited. The variant shown in FIG. 9 is characterized in that each fixed bed electrode is assigned an anode space and an electrolyte-impermeable and non-conductive partition wall 46 is attached between non-assigned but abutting anode spaces and bed electrodes. In the variant shown in FIG. 10, each bed electrode 47 is surrounded on two sides by an anode space and is delimited against it by a diaphragm 49 . A contact electrode 48 is arranged in the middle of the bed electrode between the diaphragms. The contact electrodes 48 and anodes 50 are electrically short-circuited.

Claims (9)

1. A process for removing gaseous, electrochemically oxidizable and / or reducible impurities from gases by dissolving the impurities in an electro-optically conductive washing liquid in an absorption column and electrochemically oxidizing and / or reducing the impurities dissolved in the washing liquid, characterized in that the dissolving carries out the impurities in the electrolytically conductive washing liquid and the electrochemical oxidation and / or reduction in an absorption column which contains electrically conductive packing elements and / or electrode internals which form at least one fixed bed electrode. 2. The method according to claim 1, characterized in that one to remove one or more electrochemically reducible impurities from the gas an absorption column used the one connected as a cathode Contains fixed bed electrode. 3. The method according to claim 1, characterized in that one to remove one or more electrochemically oxidizable impurities from the gas an absorption column used a fixed bed electrode connected as an anode contains. 4. The method according to claim 1, characterized in that to remove electrochemically oxidizable and reducible impurities the gas to be cleaned one after the other by at least two absorption columns, each one Contain fixed bed electrode, or by an absorption column, which contains at least two fixed bed electrodes, can flow, the two passing one after the other Fixed bed electrodes are connected as anode and cathode. 5. The method according to any one of claims 1 to 4, characterized in that you wash the washing liquid in batches circulates and usable electrolyte solutions, especially acids, in a manner known per se.   6. The method according to any one of claims 1 to 4, characterized in that the used washing liquid Wastewater feeds. 7. Device for performing the method according to one of claims 1 to 6, containing an absorption column and an anode space with anode, a diaphragm and a cathode space with cathode containing electrochemical cell, characterized in that the absorption column is designed as the electrochemical cell, wherein at least one of the two electrode chambers with packings made of metal, graphite or provided with an electrically conductive coating non-conductor is filled, the monopolar fixed-bed electrode (1, 9, 13, 21, 27, 42, 47) form, which washing liquid (6 14, 25 ) and gas ( 7, 15, 31 ) is flowed through. 8. The device according to claim 7, characterized in that the absorption column has a fixed bed electrode ( 38 ) in which a plurality of diaphragm ( 39 ) separated counter electrode spaces with counter electrodes ( 40 ) are arranged. 9. The device according to claim 7, characterized in that the absorption column has a plurality of fixed bed electrodes ( 42, 47 ) and counter electrode spaces combined in a filter press-like manner to form a block.