CA2579364A1 - Apparatus and process for electrochemical chlorine recovery - Google Patents

Abstract

A process and apparatus for reducing the concentration of chlorine gas in a first gaseous mixture further comprising at least one inert gas, said process comprising (a) treating said gaseous mixture with a first aqueous solution comprising a mediator compound in a first oxidation state operably substantially reducible by said chlorine to a higher and second oxidation state mediator compound to produce (i) a second aqueous solution comprising said mediator compound in said second oxidation state and chloride anion, and (ii) a second gaseous mixture of lower chlorine concentration than said first gaseous mixture;
(b) treating said second solution in an electrochemical cell having an anode, cathode, and anion exchange membrane between said anode and cathode to provide an anolyte chamber and a catholyte chamber, to effect (i) substantial reduction of said mediator compound in said second oxidation state to said first oxidation state at said cathode in said catholyte chamber; (ii) passage of said chloride anion from said catholyte chamber through said anion exchange membrane into said anolyte chamber; and (iii) oxidizing said chloride anion at said anode to produce chlorine gas; and recovery of said chlorine gas.
Preferably, the mediator is a metal ionic species, particularly of iron or copper. The process provides an efficacious and effective recovery of chlorine from chlorine--contaminated gaseous mixtures, particularly, vent or tail gas from chlor-alkali production plants.

Description

APPARATUS AND PROCESS FOR
ELECTROCHEMICAL CHLORINE RECOVERY
BACKGROUND TO THE INVENTION

Chlorine is a valuable chemical commodity used in many industrial and domestic applications. Chlorine is primarily produced by the electrolysis of an alkali metal chloride salt in aqueous solution, or brine, with co-produced alkali metal hydroxide in aqueous solution and also with co-produced hydrogen gas, and this process forms the main basis of the chlor-alkali industry. The electrolysis of an aqueous salt solution is also used to produce other halogen gases such as bromine.
Molten salt electrolysis is commonly used to produce chlorine and a metal product such as magnesium from magnesium chloride, or sodium from sodium chloride.
Molten salt electrolysis is also used in the production of fluorine. There are also processes described for the production of chlorine from hydrogen chloride, including electrolysis of the anhydrous gas or electrolysis of the aqueous hydrochloric acid solution. Hydrogen chloride may also be reacted with oxygen to produce chlorine and water in a number of variations of the thermal oxidation process known generically as Deacon process. More generally, electrolysis or thermal oxidation of hydrohalic acids or anhydrous hydrogen halides can be used in appropriate process schemes to produce halogen gases. In the most general case, a halogen gas will be produced as a component of a gaseous mixture and after primary separation methods to obtain a relatively pure halogen product stream, there is a residual gas stream that still contains some amount of the valuable component gas.

The invention herein disclosed is described for particular application to the recovery of chlorine from a mixed gas stream but the concept with appropriate chemical substitutions may be applied to the recovery of any other valuable gas, notably a halogen gas, from a mixture of gases. Furthermore, the invention is not limited to any particular method for the production of the chlorine or other gas to be recovered; nor is the invention limited to the gas source being a production facility for the gas of interest. For example, chlorine is typically transported as a liquid form in pressurized containers. The larger capacity containers such as rail or barge tankers are usually off-loaded with pressurized air or nitrogen leaving a gas mixture containing chlorine that is displaced during refilling of the container and this invention may be applied to the recovery of chlorine from the displaced gas mixture.
Other potential applications of this invention would be for the recovery of chlorine from process gas streams in sodium chlorate plants or integrated chlorine dioxide plants. Sodium chlorate cells have a single gas stream, primarily comprised of hydrogen but due to small inefficiencies, the gas stream contains up to about 0.5 volume percent chlorine on a dry basis.

However, a larger use of the present invention is likely to be in chlor-alkali plants with electrolysis of sodium chloride brine. For better operating performance and energy efficiency of electrolytic cells (either mercury, diaphragm, or membrane), calcium and magnesium impurities arising from natural contamination of the sodium chloride source are removed to low levels by using carbonate and hydroxide additions in excess, to precipitate calcium carbonate and magnesium hydroxide, respectively.
Clarification and filtration remove the precipitated solids. Excess carbonate in the brine solution passed to the electrolytic cell results in the formation of carbon dioxide gas that exits with produced chlorine gas. A small amount of oxygen gas is added to the chlorine gas due to anodic oxidation of water and excess hydroxide.
Hydrogen gas may also be added to the produced chlorine by different mechanisms according to the type of electrolytic cell employed. In all plants, significant efforts are made to minimize contamination of the chlorine gas but with particular emphasis on minimizing hydrogen contamination, which can cause unsafe conditions in downstream process steps. Intentional or accidental ingress of air or other gas such as nitrogen into plant equipment can also add to the amount of impurities in the chlorine gas mixture.

The chlorine gas stream flowing from brine electrolysis cells is a water vapor saturated gas mixture. The typical chlorine processing steps are cooling/chilling for water condensation followed by contacting with sulfuric acid as a desiccant.
Subsequently, the dry chlorine gas is then safely handled in steel equipment that compresses and further chills the gas mixture to condense chlorine. The condensation step to produce a chlorine liquid product stream is referred to as liquefaction. The typical impurities of the chlorine gas mixture have boiling points much lower than chlorine and mostly pass through the liquefaction equipment to form a residual gas stream referred to as tail-gas. The tail gas stream is saturated with chlorine in

2 accordance with vapor-liquid equilibrium properties. Liquefaction process schemes may include, at added capital and operating expense, two or more stages of liquefaction and/or compression to minimize losses of chlorine in the tail gas stream.
Such schemes are often said to provide "deep chlorine recovery". The elaborate liquefaction schemes are difficult to justify for small capacity plants having reduced "economy of scale".
In practice, the liquefaction tail-gas stream will always have some chlorine content and is commonly scrubbed with caustic to remove chlorine to very small trace levels before the remaining non-volatile gas mixture is discharged to the atmosphere.
The reaction of caustic and chlorine produces a hypochlorite solution that may be sold for use as a bleaching agent or for sterilization or disinfecting. Any carbon dioxide in the tail gas will also react with caustic forming carbonate compounds, usually with some precipitation of carbonate species resulting in cloudy product that is undesirable for the more lucrative domestic bleach market thus necessitating additional processing, usually filtration. The carbon dioxide may be removed from the tail gas stream using a zeolite molecular sieve as described by Moore et al. in US
6,709,485, allowing for subsequent production of a hypochlorite solution essentially free of carbonate. However, a small sideline business in the sale of hypochlorite solution is not typically viable. The hypochlorite may be reduced or decomposed to salt for recycle or for disposal, which represents an absolute loss in chlorine production.
Several technologies are described for the recovery of chlorine from the liquefaction tail gas mixture or from other gas mixtures containing chlorine.
Some technologies involve selectively absorbing the chlorine in water, carbon tetrachloride, or onto solid absorbents. The chlorine can also be reacted with sulfur to produce sulfur mono-chloride, with hydrogen to produce hydrogen chloride, or with ice or water to produce chlorine hydrate (Zeller et al., U.S. patent No. 5,985,226).
Permselective membranes can separate the chlorine from the other gases (Pinnau et al., U.S. patent No. 5,538,535). There are various problems with the different technologies, including high capital costs, corrosion of equipment, poor efficiency, and serious environmental concerns such as ozone depletion by carbon tetrachloride.
To avoid the environmental issues associated with carbon tetrachloride, alternative absorbent compounds have been proposed such as dichlorotoluene (Orosz et al., U.S.
patent No. 6,063,162) and chlorinated derivatives of benzotrifluoride (Rowe, U.S.
patent No.5,308,383), but carbon tetrachloride is less expensive.

3 United States patent No. 6,203,692 describes an electrolytic process for separating chlorine gas from a mixture with other gases. Chlorine gas, either as an impure gas or as dissolved gas in hydrochloric acid, is reduced at the cathode of an electrochemical cell to form chloride ions, which are discharged at the anode of the same cell as pure chlorine gas. However, in the case of reduction of the gaseous chlorine directly at the cathode, a complete conversion of chlorine gas to chloride ions at the cathode is impractical, particularly if the chlorine gas is impure since the other components of the impure gas increase mass transfer resistance. The alternative process involves absorbing the chlorine into a hydrochloric acid solution, but the solubility is low and results in a large acid flow rate. Furthermore, low chlorine solubility in the hydrochloric acid solution reduces the mass transfer driving force, and increases the necessary contact area and theoretical stages such that the absorption equipment can be quite large. The mass transfer driving force is further restricted when the gas mixture contains very low amounts of chlorine. An example of reduced chlorine concentration arises in those cases of chloralkali plant liquefaction, where the hydrogen contamination of the chlorine gas may be significant. Subsequently, inert gas, nitrogen or air is added, further reducing chlorine concentration, to ensure the residual tail gas, after removal of the chlorine component, remains in the non-explosive region of H2 - 02 gas mixture concentrations.
As yet, no technology has been generally accepted as a successful chlorine recovery method. Thus, new plants are most often designed to scrub chlorine from the tail gas with caustic, decompose or reduce active chlorine species, and dispose of the effluent in accordance with regulatory provisions.

There, thus, remains a need for an improved process of completely recovering chlorine gas contained in a gaseous mixture, in an efficacious cost effective manner.
SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved chorine recovery apparatus and process having improved efficacy and effectiveness, particularly for recovery of chlorine from vent or tail gas of chlor-alkali production plants.
However, the invention can also be applied in other facilities having dilute chlorine gas mixtures.

4 It is a further object to provide said chloralkali plants with replacement of commonly installed secondary liquefaction systems for "deep chlorine recovery", while providing better chlorine recovery.
We have found that chlorine in vent gas, also referred to as tail-gas or sniff gas, contains significant amount of volatile, "inert" gas component such as oxygen, nitrogen, carbon dioxide, and hydrogen of a chlorine liquefaction system can be recovered very effectively by scrubbing the gas with an aqueous solution containing a reduced "mediator" compound as hereinafter defined. For example, reduced metal cation chloride compound e.g. ferrous chloride, cuprous chloride, mixtures of such compounds, herein denoted, for example, as MC1n wherein chlorine oxidizes the metal ions quickly to a higher valence state (denoted as MC1õ+1) according to the reaction, Clz + 2 MCIn ) 2 MCln+l 9 and the scrubbing solution is then passed to an electrochemical cell employing a separator comprising an anion exchange membrane dividing the cell into an anode chamber and a cathode chamber, wherein the higher valence metal ions of the scrubbing solution are reduced to lower valence metal ions at the cathode and chloride ions are transported across the anion exchange membrane to the anode where the chloride ions are oxidized to chlorine gas, which is substantially free of volatile gas components; and the resulting chlorine gas is recycled to the main chlorine processing system of the chloralkali plant. It is understood that neither the chemical oxidation of the metal ion mediator nor the electrochemical reduction need not be complete, however substantial conversion is typically desired. In another words, to ensure complete removal of chlorine from the treated gas stream an excess of the mediator solution will typically be employed.

Accordingly, in one aspect, the invention provides a process of reducing the concentration of chlorine gas in a first gaseous mixture further comprising at least one inert gas, said process comprising (a) treating said gaseous mixture with a first aqueous solution comprising a mediator compound in a first oxidation state operably oxidisable by said chlorine to a higher and second oxidation state form to produce (i) a second aqueous solution comprising said mediator compound in said second oxidation state and chloride anion, and (ii) a second gaseous mixture of lower chlorine concentration than said first gaseous mixture;

5 (b) treating said second solution in an electrochemical cell having an anode, cathode, and an anion exchange membrane between said anode and cathode to provide an anolyte chamber and a catholyte chamber, to effect (i) reduction of said mediator compound in said second oxidation state to said first oxidation state at said cathode in said catholyte chamber; (ii) passage of said chloride anion from said catholyte chamber through said anion exchange membrane into said anolyte chamber; and (iii) oxidation of said chloride anion at said anode to produce chlorine gas; and (c) recovering of said chlorine gas.
By the term "mediator compound" in this specification and claims is meant a "redox" material which can provide species in an aqueous solution in a first or lower oxidation (valence) state operably oxidisable by chlorine to a second or higher oxidation state. The material may be organic or inorganic, and includes suitable metal ions.

The present invention uses an absorbing solution that reacts quickly with chlorine to facilitate mass transfer such that the concentration of chlorine in the impure gas stream is of relatively low significance, even if the chlorine concentration has to be reduced with additional inert gas to ensure a safe, non-explosive scrubbed residual gas. Furthermore, the absorbing solution is easily regenerated in an electrochemical cell wherein chloride ions are readily transported to an anode for oxidation to "pure" chlorine. With the use of a mediator comprising the metal ion solution for absorbing chlorine, an additional benefit with respect to reduced degradation of cell components is found.

Thus, an important object of the invention is to provide an essentially complete removal of the chlorine from a stream having significant levels of other volatile gas impurities. A solution containing metal ions, such as ferrous ions that are easily oxidized by chlorine satisfy this important object.
The metal ion solution is, preferably, an acidified aqueous solution having a dissolved metal chloride salt, such as ferrous chloride dissolved in a hydrochloric acid solution. Alternative metal chloride salts, for example, are cuprous chloride, also dissolved in hydrochloric acid solution. Reducible metal ions include, but are not limited to, chromium (III), cobalt (III), silver (II), cerium (IV), and gold (III), while preferred practical choices are iron and copper because of availability, cost, solubility, and toxicity. Iron is the most preferred metal ion over copper, since copper is known

6 to form anionic complexes with chloride more readily than iron. Anionic or neutral complexes of the above metals could also be employed as mediators, although in case of anionic complexes there could be a potential detriment to current efficiency when using an anion exchange membrane.
A mixture of different, perhaps several, metal chlorides may be considered useful as mediators. For example, a useful mixture consists of iron chloride salt and another metal chloride that suppresses oxidation by oxygen. Other additives may be contemplated for such a purpose.

Examples of other categories of reduction-oxidation compounds as mediators are those that potentially enhance the process by further limiting unwanted membrane transfers, reducing cell voltage, and the like. Ferri-cyanide, phenantroline and similar metal complexes are examples. One category of organic mediators are those based on substituted quinones or naphtoquinones, whereby the substituent groups are used to adjust the redox potential and/or impart water solubility to the basic molecule.
Another category comprises aromatic amines, such as tri-phenyl amine of the type shown herein below. Upon oxidation with chlorine the tri-phenyl amine forms reversibly, a corresponding cationic radical.
x x 0 }( 't 112 Cl2 ~ + ~:I
~

Y y where: X = Br, -CH3, F, etc.
Y = -SO3H, -COOH

It is understood that many other organic-based electrochemical reversible redox mediators could be employed, providing they have sufficient solubility (>

7 0.1 M) in the catholyte and are not irreversibly degraded chemically by contact with halogen or hydrochloric acid (HCI). In general, the use of organic electrochemical mediators as defined herein, is not widely known. Such mediators offer good possibility for fine-tuning their redox potential, solubility and chemical stability.
In a further aspect, the invention provides apparatus for recovering chlorine gas from a first gaseous mixture further comprising at least one inert gas said apparatus comprising (a) an electrochemical cell having an anode, a cathode, an anion exchange membrane between said anode and said cathode to provide an anode chamber and a cathode chamber;
(b) a first aqueous solution comprising a mediator compound in a first oxidation state operably oxidisable to a higher and second oxidation state by chlorine;
(c) means for treating said first gaseous mixture with said first aqueous solution to effect production of said mediator compound in said second oxidation state and chloride anion in a second aqueous solution; and a chlorine-depleted second gaseous mixture;

(d) means for transferring said second aqueous solution to said catholyte chamber to effect (i) reduction of said mediator compound in said second oxidation state to said first oxidation state at said anode; (ii) passage of said chloride anion through said anion exchange membrane and (iii) oxidation of said chloride anion at said anode to produce chlorine; and (e) means for recovering said chlorine.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be better understood, preferred embodiments will now be described, by way of example only, with reference to the accompanying drawings wherein Fig. 1 is a schematic flowsheet of an electrochemical chlorine recovery apparatus and process with an external anolyte circulation system according to the invention.

8 DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

With reference to Fig. 1, this shows generally as 100 apparatus according to the invention comprising 1. Tail-gas stream from chlorine liquefaction vent.
2. Scrubber vessel with integral pump tank.
3. Scrubber packing.
4. Residual gas stream, scrubbed of chlorine.
5. Scrubbing solution containing redox metal chloride 6. Pump 7. Heat Exchanger 10. Electrochemical cell.
11. Cathode compartment.
12. Anode compartment.
13. Anion exchange membrane.

20. Anode feed stream comprised of aqueous solution.
21. Anode exit stream comprising aqueous solution and wet chlorine gas.
22. Separated wet chlorine gas stream.
23. Anolyte Tank 24. Anolyte Pump 25. Heat Exchanger 31. Bleed stream to catholyte.
32. Make-up streams to anolyte.

The process employs a scrubbing column 2, which has internal packing 3 to facilitate gas-liquid contact for improved mass transfer. Impure chlorine gas 1 is contacted by a solution containing Fe++ ions that quickly react with the chlorine which is reduced to chloride ions, such that the dissolved chlorine concentration is essentially zero in catholyte compartment 11.

9 The process includes at least one circulation loop for the catholyte solution wherein pump, 6, forces flow, from scrubbing vessel/tank, 2, through heat exchanger, 7, then through cathode compartment, 11, of electrochemical cell, 10, and back to the scrubbing vessel/column. The flowrate of solution through cell 10 is large enough to provide sufficient quantity of reducible Fe++ ion for the current applied to the cell and in sufficient excess to reduce the over-voltage of the cathodic reduction reaction to a practical minimum. The flowrate of solution to the cell must also be of sufficient quantity to remove heat generated by cell resistances maintaining a cell temperature as high as possible for reduced cell voltage but limited to temperatures below those causing unacceptable degradation of cell components.
Using presently available anion exchange membranes, the maximum operating temperature of the cell is less than -100 C. However, lower temperatures are desirable to limit corrosion of other cell components or to limit the amount of vapor components in exiting scrubbed gas stream, 4. The vapor components, such as water and hydrogen chloride are substantially reduced in exiting scrubbed gas stream, 4, by passing the gas through a downstream heat exchanger (not shown) to cool the gas and condense water and hydrogen chloride.
A secondary circulation of solution in the catholyte system is preferred in alternative embodiments for greater removal of heat due to the exothermic metal ion oxidation reaction in scrubber vessel, 2; or a larger flowrate through the scrubber vessel may be necessary for improved mass transfer characteristics.
Solution flowrates for the two circulation loops may be controlled by appropriate valve(s). Alternatively, two separate circulation loops may be used with the scrubber vessel common to both loops, with the loops having separate pumps and heat exchangers. Various permutations of pump(s) and heat exchanger(s) may be utilized.

Fig. I shows an anolyte circulation loop wherein anolyte inlet stream, 20, is fed to anode compartment, 12, of electrochemical cell 10 employing an anion exchange membrane separator, 13. The anolyte solution is an aqueous solution of hydrochloric acid (-4 to -36% w/w HCI, most preferably about 20% w/w HCl), which provides a high chloride ion activity that suppresses anodic water oxidation resulting in oxygen evolution.

Chloride ions migrating from cathode compartment, 11, through anion exchange membrane, 13, are oxidized at the anode to chlorine gas that exits cell 10 with anolyte solution as anolyte exit stream, 21. Gas-liquid separation may be effected in the piping of stream 21, or by the use of a dedicated separator vessel (not shown), such that a stream of chlorine gas, essentially free of volatile impurities (02, C02, H2, N2), exits the process as stream 22. Product chlorine stream, 22, has vapor components of water and hydrogen chloride that are substantially removed by a downstream cooling/condensing equipment (not shown). Chlorine gas stream, 22, is added to the main stream chlorine production stream of, say, a chloralkali plant upstream (not shown) of the main chlorine cooling/chilling/drying process steps.
Cell anolyte exit stream, 21, may continue as a two-phase flow into anolyte tank, 23, where chlorine gas separates from the anolyte solution.
Subsequently, product chlorine gas stream, 22, exits from the anolyte tank through a separate tank outlet (not shown).

Stream 32 represents make-up water and/or hydrogen chloride added to the anolyte circulation loop. The make-up components are most preferred to replenish losses of these components from the embodied process. The primary losses occur due to vapor losses but may be recovered as condensate(s).

Membrane transfers of water, hydrochloric acid, and metal chlorides occurs.
Electro-osmotic water transfer (chloride ion hydration shell) predominates and requires a bleed of anolyte into the catholyte loop as indicated by stream 31.
Different source points, endpoints, and configurations of the make-up and bleed streams allows suitable containment of membrane transfers and appropriate control of component concentrations in the electrolyte solutions.

A variation of the above process embodiment involves the use of aqueous sodium chloride solution (brine) as the anolyte solution. The brine solution preferably contains a high concentration of sodium chloride to minimize anodic water oxidation but limited to concentrations that would avoid crystallization. Some hydrochloric acid concentration in the anolyte brine solution further reduces anodic water oxidation.
However, sodium chloride brine anolyte is beneficial to improved current efficiency of anion exchange membranes that are known to have poor proton rejection.
Sodium cations in brine have a reduced "back-migration" through the anion exchange membrane compared to hydrogen cations.

Other advantages of a sodium chloride anolyte solution are availability in a chloralkali plant, and the opportunity to circulate from/to the plant brine circuit is possible, provided that the brine exiting the chlorine recovery cell has manageable impurity contamination. A small iron contamination into the main brine circuit is acceptable and may even be beneficial for removing aluminum in the main brine circuit.
Other brine solutions may be used for the anolyte such as calcium chloride which has high solubility in water and hydrochloric acid solutions. The higher valency of the calcium ion further increases the selectivity for chloride ions.
A further variation of the above process embodiment involves the use of polyvalent metal halide solutions as the anolyte solution. Aqueous metal chloride anolyte solutions are convenient since they are also used as catholyte solution in the invention. In aqueous solution, metal halides will form ion complexes that are expected to be detrimental to obtaining high selectivity, especially complexes of the reduced ions such those formed with ferrous ions produced in the catholyte chamber, e.g. FeC13-. Ion complexes of the reduced metal ion having excess negative charge can transfer through the membrane to the anolyte where the oxidation of the reduced metal ion occurs thus resulting in reduced chloride selectivity. Cationic complexes formed in the anolyte compartment with ferric ions such- as FeC12+ will also be available for back-migration to the catholyte; such charge transfer reduces the transfer of chloride ion. Thus, it was surprising to obtain very high selectivity with metal halide anolyte solutions and therefore the use of polyvalent metal halide solutions such as ferric chloride is a preferred embodiment of the invention.
Another process embodiment involves recovery of chlorine from large discharges of gases during upset plant conditions or from chlorine transport containers, by the use of inventoried reduced ion solution. The chlorine transport containers are filled on a periodic basis over a short time which causes a large rate of chlorine-containing gas which is conveniently accommodated by maintaining sufficient inventory of reduced ion solution to absorb such chlorine discharges. The inventoried solution of oxidized ion solution can be treated by the invention over a longer period of time between filling of other transport containers; e.g.
filling operations are often during day shifts that would deplete inventory of reduced ion solution but the invention can run continuously over 24 hour period with little operator attention and replenish the reduced ion solution over night.
Therefore, the capacity of the invention does not have to be sized for large instantaneous rates of chlorine discharges. Other electrolytic solutions such as hydrochloric acid to absorb chlorine for recovery by use of AEM electrolysis may be contemplated but the capacity of such solutions to absorb chlorine is much lower and the large amounts of such solutions required means large and expensive storage vessels and increased risk of chlorine release into the environment, or the capacity of the electrolytic equipment must be overly sized to allow for large instantaneous rates.
An inventory of the reduced ion solution produced by this invention may be used for recovery of chlorine from different sources In a chloralkali plant, in addition to liquefaction tail-gas, miscellaneous venting of chlorine occurs such as during equipment maintenance procedures, sampling and chemical analysis, during start-up and shut-down of the main chloralkali celiroom, and possibly during an emergency shut-down. The reduced ion solution can replace caustic used in conventional vent scrubbing systems for chloralkali plants which reduces loss of a valued product while also allowing for the recovery of chlorine by the use of the electrolytic AEM
cells of this invention. Furthermore, the invention can reduce waste into the environment since the sodium hypochlorite product solution of the conventional vent scrubbing system using caustic solution must occasionally be disposed when there is no economical use; the hypochlorite is decomposed to salt and oxygen by various means and discharged as effluent from the plant. All of the gas vents of a chloralkali plant could be continuously scrubbed to remove the chlorine component into an inventory of reduced ion solution produced by the electrolytic AEM cells of the invention. The cells can be operated continuously or may only have to be operated intermittently to replenish the inventory of reduced ion solution.

Electrochemical cells of conventional design can be employed in the process according to the invention. Alternate cell design incorporating 3D
electrode(s) such as described in PCT/CA03/01569, filed 16 October 2003 in the name of Aker Kvaerner Canada Inc. can be employed to provide a useful reduction of cathodic and/or anodic over-voltages.

Perfluorinated, oxidation-resistant anion-exchange membranes, such as Tosflex manufactured by Tosoh Corp. (Japan) or the FAP anion-exchange membrane produced by Fuma Tech GmbH (Germany), are preferred.
Since the electrochemical cell of this invention ideally consumes, via mediator, only chlorine on the cathode and produces only chlorine at the anode, an alternative embodiment of the invention provides the elimination of the electrolyte circulation loops. The impure chlorine gas is sparged into a redox metal, preferably also an ion, chloride solution in the cathode compartment of the cell and chlorine gas is produced at the anode into an anolyte solution of hydrochloric acid or sodium chloride brine. The volatile components of the impure chlorine gas stream provide mixing in the catholyte compartment and evolved chlorine gas provides mixing in the anode compartment. The mixing in the electrode chambers can be augmented by incorporating baffles and/or other elements that facilitate internal circulation using gas-lift principles as is commonly done in most of the modern large-scale chloralkali electrolyzers.

Heat generated by cell resistances and by reactions can be removed from the electrochemical cells by incorporating an additional chamber with a flow through of a suitable cooling fluid. A suitable cooling fluid is most preferably non-conductive like deionized water or a commercial heat exchange fluid, typically an organic compound(s).

In an electrolyzer that is a series of unit cells, the additional cooling chambers are between unit cells. In a bipolar electrolyzer, the additional chamber is formed as the interior of bipolar elements constructed of conductive material that could be inexpensive steel but faced with materials suitable for electrodes, such as titanium, graphite, graphite/thermo-plastic composite, or the like. A non-conductive material such as, for example, a thermally stable plastics material can be used in the design of the cooling chambers for a mono-polar electrolyzer although steel could also be used.

The anode and cathode compartments have inlet and exit ports to allow make-up solution to be injected as necessary or to allow any solution build-up to overflow. A inlet gas sparge distributor would be used for feeding impure chlorine gas into the cathode compartments.

A third embodiment of the invention also comprises an electrolyzer that incorporates internal electrolyte mixing induced by gas-lift principles.
However, one of the compartments also has external circulation of the electrolyte, which provides cooling for the electrolyzer and the electrolyzer is simpler/lower cost.

Preferably, for greatest simplification, only the catholyte is externally circulated and the external circulation loop includes scrubbing equipment as previously described.

A fourth embodiment of the invention is similar to the above-mentioned third embodiment including the preferred external catholyte circulation with a scrubber vessel for the contact between the mediator solution and the impure chlorine gas. However, in this embodiment, the anode chamber is operated with no anolyte solution, which is potentially feasible, when the anode is in intimate contact with and/or embedded into the surface of the anion exchange membrane. An anode may be constructed of carbon cloth coated with electro-catalytic material such as ruthenium oxide (or other well-known electro-catalysts, or mixtures of electro-catalysts), and the anode cloth is pressed against the anion exchange membrane. Alternatively, the anode can be formed on a face of the anion exchange membrane by depositing electrocatalyst by various means known in the art. The latter structure is commonly referred to as a membrane-electrode-assembly or MEA (most commonly a MEA is constructed with anode and cathode faces but only the anode face is required for this invention).

The advantages of this embodiment are significant with respect to managing membrane transfer and also in potentially reducing current efficiency losses due to leakage of cations, particularly hydrogen ion, through the anion exchange membrane.

Although this disclosure has described and illustrated certain preferred embodiments of the invention, it is to be understood that the invention is not restricted to those particular embodiments. Rather, the invention includes all embodiments which are functional or mechanical equivalents of the specific embodiments and features that have been described and illustrated.

Claims (26)

1. Claims A process of reducing the concentration of chlorine gas in a first gaseous mixture further comprising at least one inert gas, said process comprising (a) treating said gaseous mixture with a first aqueous solution comprising a mediator compound in a first oxidation state operably substantially oxidisable by said chlorine to a higher and second oxidation state mediator compound to produce (i) a second aqueous solution comprising said mediator compound in said second oxidation state and chloride anion, and (ii) a second gaseous mixture of lower chlorine concentration than said first gaseous mixture;
   (b) treating said second solution in an electrochemical cell having an anode, cathode, and an anion exchange membrane between said anode and cathode to provide an anolyte chamber and a catholyte chamber, to effect (i) substantial reduction of said mediator compound in said second oxidation state to said first oxidation state at said cathode in said catholyte chamber;
   (ii) passage of said chloride anion from said catholyte chamber through said anion exchange membrane into said anolyte chamber; and (iii) oxidation of said chloride anion at said anode to produce chlorine gas; and (c ) recovering said chlorine gas.
2. 2. A process as defined in claim 1, wherein said mediator compound is a metal cation.
   
   3. A process as defined in claim 1, wherein said mediator compound is a substituted amine or substituted quinone organic compound.
3. 3. A process as defined in claim 1 or claim 2 wherein said one inert gas is selected from oxygen, nitrogen, carbon dioxide and hydrogen.
4. 4. A process as defined in claim 1 or claim 2 wherein said first gaseous mixture is vent gas, tail gas, sniff gas and the like.
5. 5. A process as defined in any one of claims 1 to 4 wherein said metal is selected from iron, copper, chromium, cobalt, cerium, silver and gold.
6. 6. A process as defined in any one of claims 1 to 5 wherein said first aqueous solution comprises hydrochloric acid.
7. 7. A process as defined in any one of claims 1 to 6 wherein said second aqueous solution subsequent to said reduction of said mediator compound in step (b) (i) is recycled to step (a) as to reconstitute said first aqueous solution.
8. 8. A process as defined in any one of claims 1 to 7 wherein said circulation of said mediator compound comprises a closed-loop recycle process.
9. 9. A process as defined in any one of claims 1 to 8 further comprising passing said recovered chlorine to a chlor-alkali production plant.
10. 10. A process as defined in any one of claims 1 to 9 comprising contacting said first gaseous mixture with said aqueous solution in a scrubbing chamber to produce said second aqueous solution; passing said second aqueous solution to said catholyte chamber containing catholyte solution to effect reduction of said mediator compound in said second oxidation state and produce said mediator compound in said first oxidation state in said catholyte solution, and passing said catholyte solution containing said mediator compound in said first oxidation state to said scrubbing chamber as to constitute an essentially closed-loop mediator cation recycle process.
11. 11. A process as claimed in any one of claims 1 to 10 wherein said anolyte chamber contains polyvalent metal cations.
12. 12. A process as claimed in claim 11 wherein said polyvalent cation is calcium.
13. 13. A process as claimed in any one of claims 11 to 12 wherein said analyte chamber contains chloride anions.
14. 14. Apparatus for recovering chlorine gas from a first gaseous mixture further comprising at least one inert gas said apparatus comprising (a) an electrochemical cell having an anode, a cathode, an anion exchange membrane between said anode and said cathode to provide an anode chamber and a cathode chamber;
    
    (b) a first aqueous solution comprising a mediator compound in a first oxidation state operably substantially oxidisable to a higher and second oxidation state by chlorine;
    
    (c) means for treating said first gaseous mixture with said first aqueous solution to effect production of said mediator compound in said second oxidation state and chloride anion in a second aqueous solution; and a chlorine-depleted second gaseous mixture;
    
    (d) means for transferring said second aqueous solution to said catholyte chamber to effect (i) substantial reduction of said mediator compound in said second oxidation state to said first oxidation state at said anode; (ii) passage of said chloride anion through said anion exchange membrane and (iii) oxidation of said chloride anion at said anode to produce chlorine; and (e) means for recovering said chlorine.
15. 15. Apparatus as defined in claim 14 wherein said means for treating said first gaseous mixture with said first aqueous solution comprises a scrubbing tower.
16. 16. Apparatus as defined in claim 14 or claim 15 further comprising means for recycling said mediator compound in said first oxidation state from said catholyte chamber to said first aqueous solution.
17. 17. Apparatus as defined in any one of claims 14 to 16 wherein said mediator compound in said first aqueous solution is a metal cation.
18. 18. Apparatus as defined in claim 17 wherein said metal is selected from iron, copper, chromium, cobalt cerium, silver and gold.
19. 19. Apparatus as defined in any one of claims 14 to 16 wherein said mediator compound is substituted amine or substituted quinone organic compound.
20. 20. Apparatus as defined in claim 18 wherein said metal is selected from iron and copper.
21. 21. Apparatus as defined in any one of claims 14 to 19 wherein said anolyte chamber contains hydrochloric acid solution.
22. 22. Apparatus as defined in any one of claims 14 to 21 wherein said anolyte chamber contains sodium chloride solution.
23. 23. Apparatus as defined in any one of claims 14 to 22 further comprising means to recycle chloride-depleted anolyte solution back to said anolyte chamber.
24. 24. Apparatus as claimed in any one of claims 14 to 23 wherein said anode chamber contains a polyvalent metal cation.
25. 25. Apparatus as claimed in claim 24 wherein said polyvalent metal is calcium.
26. 26. Apparatus as claimed in claim 25 wherein said anode chamber contains chloride anions.