DE102004006868A1 - Electrolytic double diaphragm cell is used for displacement of acidic or basic gases from aqueous solution e.g. regeneration of diethanolamine in natural gas scrubbing units - Google Patents

Abstract

Liquor containing dissolved/chemically reacted, acid or basic gas, e.g. from a scrubbing unit, is circulated through anode, intermediate, and cathode chambers defined by a pair of selective diaphragms in an electrolytic cell to liberate the dissolved gas.

Description

The The invention relates to an electrochemical process accordingly the preamble of claim I.

Should From gas mixtures gas components are separated, which in aqueous solution acidic or basic, these are reacted with basic or acidic Washed detergents because their solubility due to the corresponding Protolysis equilibria substantially from the pH of the solvent depends. An example for one such method is the separation of traces of hydrogen sulfide from natural Methane gas with diethanolamine as basic detergent. This is e.g. necessary if the methane gas for the production of hydrogen used with the aid of hydrogen sulfide-sensitive catalysts should be (reforming).

• – Waschchemikalien werden verbraucht (Basen oder Säuren)
• – Es fallen Abfallstoffe an („verbrauchte" Waschchemikalien) bzw. die verwendeten Waschchemikalien müssen durch den Einsatz von Säuren oder Basen regeneriert werden wodurch Abfallstoffe entstehen
• – Die abgetrennten Gase werden nicht gleichzeitig separat aus dem bestehenden Verfahren ausgeschieden sondern können nur durch ein zweites Verfahren unter der Verwendung von Säuren oder Basen aus den verwendeten Waschchemikalien ausgetrieben werden
• – Das bestehende Verfahren ist nicht wartungsfrei da die Waschchemikalien regelmäßig ausgewechselt werden müssen. Um eine vollständige Abtrennung zu gewährleisten kann die Aufnahmekapazität der Waschchemikalien oft nicht vollständig genutzt werden.

The disadvantages of this technology are:

• - Washing chemicals are consumed (bases or acids)
• - Waste materials are generated ("spent" washing chemicals) or the washing chemicals used must be regenerated by the use of acids or bases, which produces waste
• - The separated gases are not excreted separately from the existing process simultaneously but can only be driven out by a second method using acids or bases from the washing chemicals used
• - The existing process is not maintenance-free because the washing chemicals have to be changed regularly. To ensure complete separation, the uptake capacity of the laundry chemicals often can not be fully utilized.

task The invention is the continuous and operational during operation maintenance-free separation of the abovementioned gas components from a flowing Gas mixture and the simultaneous separate deposition of these gas components in high purity.

These Task is through a special electrochemical cell and it coupled device solved with the features of claim I.

• – Die Abtrennung der oben genannten Gasbestandteile erfolgt kontinuierlich
• – Die Abtrennung erfolgt praktisch vollständig
• – Die abgetrennten Gasbestandteile werden ohne aufwendige Aufarbeitungsprozesse separat und in hoher Reinheit wieder aus dem Verfahren ausgeschieden
• – Es werden keine Waschchemikalien verbraucht
• – Es fallen keine Abfallstoffe an
• – Das Maß der Abtrennung lässt sich durch Veränderung des Elektrodenstroms kontrollieren
• – Es entsteht reiner Wasserstoff als Verfahrensnebenprodukt. Dieser lässt sich z.B. als sekundärer Energieträger verwenden
• – Es entsteht reiner Sauerstoff als Verfahrensnebenprodukt

The advantages of this method are:

• - The separation of the above gas components is carried out continuously
• - The separation is almost complete
• - The separated gas components are eliminated without costly work-up processes separately and in high purity again from the process
• - No washing chemicals are consumed
• - There are no waste materials
• - The degree of separation can be controlled by changing the electrode current
• - There is pure hydrogen as a process by-product. This can be used, for example, as a secondary energy source
• - Pure oxygen is produced as a process by-product

In the anodic reactions of water electrolysis, H ⁺ ions are formed as redox by-products and OH ^- ions in the cathode reaction. If the anode compartment is separated from the cathode compartment of a water electrolyzer by a diaphragm (eg made of sintered glass or clay), the ion exchange between the two compartments is eliminated by convection and the ions mainly diffuse through the electrical field of the electrodes through the diaphragm (migration). , Since the diffusion rate of the OH ^- and H ⁺ ions through such a diaphragm of their concentration, the nature of the diaphragm, the geometry of the electrochemical cell (distances of the electrodes, thickness of the diaphragm, possible gap between two or more diaphragms of the anodes and cathode spaces is delimited), the voltage at the electrodes, the electrode current and the temperature is dependent adjusts depending on the construction and operating parameters after some time a constant pH gradient between the anode and cathode space. This can be very big. It amounted in tests to ca pH1 (anode space) and pH13 (cathode space). Once this state of equilibrium has been reached, exactly the same number of OH ^- or H ⁺ ions migrate through the diaphragm in the direction of the oppositely charged electrode as they are produced on the other electrodes and "neutralize" mainly in or on the diaphragm to water ,

If a water electrolysis cell is constructed which has a double diaphragm (cf. A , the cell is shown in cross-section) and, for example, CO ₂ (which is acidic in aqueous solution) is introduced into the cathode compartment of the cell ( 1 ) are formed there depending on the pH predominantly hydrogen carbonate ions ( 2 , schematic reaction of CO ₂ with OH ^- . In fact, the resulting carbonic acid protolyses to bicarbonate or carbonate ions.

These negatively charged ions diffuse in the electric field of the electrodes in the direction of the anode ( 3 ).

The simultaneously formed in the anode compartment H ⁺ ions diffuse accordingly in the direction of the cathode ( 4 ).

Since both types of ions diffuse in this way into the gap of the double diaphragm and this forms its own reaction space, there the hydrogen carbonate ions are protonated again to the unstable carbonic acid, which decomposes to water and CO ₂ ( 5 ).

If the diaphragm between the anode and central space is replaced by a cation exchange membrane, the bicarbonate ions are completely prevented from diffusing to the anode space and reacting there to CO ₂ and thus to mix with the anode gas oxygen (in the experiment were already without cation exchange membrane 97 , 8% of the bicarbonate ions in the middle of the cell excreted). An anion exchange membrane instead of the diaphragm between the cathode and central space can additionally maximize the overall transport efficiency, measured at the electrode flow (in the experiment a transport efficiency of almost 100% could already be achieved without this device).

This principle also works analogously for SO ₂ , SH ₂ , HCN and all other gases which react acidically in aqueous solution.

NH ₃ , volatile amines, PH ₃ and all other gases which react basicly in aqueous solution can also dissolve in the acidic anodic electrolyte using this principle and are transported to the central space where they are excreted again.

Are introduced in place of the pure gases gas mixtures in the cathode or anode electrolyte containing the above-mentioned gases, this principle can be used technically to separate these gas components and excrete separately in the central space of the cell again. For this purpose, an apparatus is provided in which the electrolyte is pumped through the three-chamber cell whereby a separate separation of the redox by-products hydrogen and oxygen is possible (see. B , the cell is shown in cross-section. The case example is the separation of CO ₂ from a CO ₂ -containing "inert gas"). The cell ( 6 ) of this apparatus consists of an acid- and base-resistant cuboidal housing which through a rectangular cation exchange membrane ( 7 ) and a rectangular anion exchange membrane ( 8th ) is separated into three reaction spaces. The distance between the membranes should be only a few millimeters. The electrodes are used as wire nets, lie directly on the membranes and consist of an electrochemically "inert" material (eg platinum) and have low hydrogen or oxygen overvoltages.The housing has corresponding supply lines for the electrodes ( 9 ), one inlet and drain per reaction space and is otherwise tight.

The electrolyte (eg sodium sulfate solution) is divided into three cycles ( 10 . 11 . 12 ) of pumps ( 19 ) pumped through the cell.

The gases produced in the individual compartments of the cell are stored in gas separators ( 13 . 14 . 15 ) and in the case of the cathode cycle ( 12 ), the basic electrolyte is replaced by a membrane contactor ( 18 , shown here in simplified form as a gas washing bottle). The gas mixture to be liberated from the base-soluble gas constituents ( 16 ) is also passed in countercurrent through this membrane contactor whereby the gas released from these gas constituents exits again ( 17 ).

The gas components dissolve in the cathode electrolyte ( 12 ), diffuse through the anion-exchange membrane to the central space of the cell and finally ( 13 ) again out of the apparatus.

Hydrogen ( 15 ) and oxygen ( 14 ) are also deposited separately.

Become the electric poles of the electrodes can be reversed the same apparatus for the separation of gas components in aqueous Solution basic use react. The exit points of oxygen and hydrogen are also reversed.

A Non-industrial application of this method is the use as electronic recirculation diver device. The diver breathers the membrane contactor in a counterlung. This is the carbon dioxide the spent air is separated and directly into the sea water promoted which flows through the central space of the cell. The at the maintenance of the pH gradient resulting electrolytic oxygen is also in the counterlung passed around the freed of carbon dioxide air with oxygen to enrich. The air regenerated in this way can again be inhaled, which closes the cycle.

Claims (4)

A method for the separation of acidic or basic in aqueous solution gas components from gas mixtures, characterized in that an electrochemical cell for water electrolysis is used which is divided by a double diaphragm into three separate reaction spaces (anode space with anode, central space, and cathode space with cathode) and that the electrolyte with which the gas is to be washed, is pumped in the circuit to extract it by a gas separator, the electrode gas (anode gas or cathode gas) and then by to conduct a gas scrubber in which the appropriate gas can be washed. A method according to claim I, characterized in that the Double diaphragm through a cation exchange membrane (between Anode and central space) and anion exchange membrane (between Cathode and middle space) is replaced. A method according to claim I or II, characterized in that in place of the scrubber a membrane contactor is used around the acid or base soluble Gas components in countercurrent to the acidic or basic electrolyte to be able to record better. Method according to one of the preceding claims, characterized that it is used as electronic recirculation diver device.