J
Electrochemical Oxidation

TECHNOLOGY DESCRIPTION

Electrochemical oxidations have been developed for many of organic chemical syntheses. The electrochemical cells used are based in general on designs that have been developed for the chlor-alkali industry or for Monsanto's acrylonitrile/adiponitrile process. Processes have been developed for carrying the oxidation to completion, with all of the carbon in the original hydrocarbon converted to CO2. The complete oxidation process appears to be best carried out by using a combination of electrolysis and chemical reaction.

The mediated electrochemical oxidation (MEO) process was developed by AEA Technology (formerly the Atomic Energy Authority of Great Britain). An electrolysis cell is used to generate an active metal ion, Ag2+ in the MEO process. The metal ion is the active chemical agent; it may react directly with the organic material to be destroyed or it may first react with water to form hydroxyl radicals, which in turn oxidize the material.

The electrolytic cells used have two compartments usually with a permeable membrane between; the cathode is in one compartment, the anode in the other. Highly engineered cells have been developed that, for example, incorporate a large electrode surface area and minimize electrical resistance heating losses. The use of silver as the mediating ion requires a cation selective membrane (see Eq. 1 for the electrode reactions).

Cathode1:

The Ag2+ ion is very reactive. Water can be dissociated and organic material reacted with oxygen from the water. The Ag2+ ion is reduced to Ag+. A typical hydrocarbon oxidation might appear as follows:

1  

This is the preferred cathode reaction, but see below.



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Alternative Technologies for the Destruction of Chemical Agents and Munitions J Electrochemical Oxidation TECHNOLOGY DESCRIPTION Electrochemical oxidations have been developed for many of organic chemical syntheses. The electrochemical cells used are based in general on designs that have been developed for the chlor-alkali industry or for Monsanto's acrylonitrile/adiponitrile process. Processes have been developed for carrying the oxidation to completion, with all of the carbon in the original hydrocarbon converted to CO2. The complete oxidation process appears to be best carried out by using a combination of electrolysis and chemical reaction. The mediated electrochemical oxidation (MEO) process was developed by AEA Technology (formerly the Atomic Energy Authority of Great Britain). An electrolysis cell is used to generate an active metal ion, Ag2+ in the MEO process. The metal ion is the active chemical agent; it may react directly with the organic material to be destroyed or it may first react with water to form hydroxyl radicals, which in turn oxidize the material. The electrolytic cells used have two compartments usually with a permeable membrane between; the cathode is in one compartment, the anode in the other. Highly engineered cells have been developed that, for example, incorporate a large electrode surface area and minimize electrical resistance heating losses. The use of silver as the mediating ion requires a cation selective membrane (see Eq. 1 for the electrode reactions). Cathode1: The Ag2+ ion is very reactive. Water can be dissociated and organic material reacted with oxygen from the water. The Ag2+ ion is reduced to Ag+. A typical hydrocarbon oxidation might appear as follows: 1   This is the preferred cathode reaction, but see below.

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Alternative Technologies for the Destruction of Chemical Agents and Munitions The H+ ions created in the process migrate to the cathode possibly to form H2 gas; the Ag+ ions are reoxidized at the anode to Ag2+. In principle the only products of a hydrocarbon oxidation will be CO2 and H2. Heteroatoms in organic compounds are also oxidized: sulfur to sulfate, phosphorus to phosphate, nitrogen atoms to molecular nitrogen or N-oxides, chlorine to molecular chlorine, and fluorine to fluoride and molecular fluorine. Gaseous products (CO2, N2, NO, Cl2) are withdrawn; the other materials remain in the electrolyte and must be recovered from the solution. The two components of the process, electrolysis and chemical reaction, may be combined in one vessel or separated into two. That is, the organic may be added directly to the oxidizing fluid of the anode compartment. Alternatively, the oxidizing anode fluid may be pumped to a separate vessel to contact the organic material to be destroyed. STATUS AND DATABASE The electrolytic cells required are well developed. The cell size required (e.g., area of anode) would be large but in the range of industrial experience. There has been a substantial research effort on electrolytic decontamination processes, generally for small flow rates as for air purification. Various mediating ion couples have been used: cerium (HI/IV), iron (II/III), or chromium (III/VI). The AEA Technology process using silver as the mediating ion has been developed to at least pilot plant scale of a few hundred pounds per day. It has been used to oxidize uranium during separations processing of plutonium from uranium. It has been tested at small scale for the destruction of solvents. The metal ions differ in their reactivity and ability to oxidize. Silver is very reactive at moderate temperature(less than 100°C). It has been used with nitric acid (10 percent by weight) as the electrolyte. The primary cathode reaction is then reduction of nitrate ion instead of H2 production: This has the advantage of reducing the voltage required (reducing power); but it has the disadvantage of an additional chemical step to reoxidize the nitrous acid or recover NOx in the cathode gas. Other metal ions are less reactive; they require a longer time to complete the oxidation or higher temperature. An experimental process using iron, for example, is reported to operate at 180°C; the vessel pressure would have to exceed 10 bars to prevent boiling.

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Alternative Technologies for the Destruction of Chemical Agents and Munitions Alternative nonaqueous electrolytes could be considered, such as propylene carbonate, or dimethyl sulfoxide (DMSO). APPLICATION TO CHEMICAL WEAPONS DESTRUCTION The MEO process has not been tested with chemical agents. Related compounds have been oxidized in small scale test work: two phosphate esters, tributyl phosphate and tritolyl phosphate; and a compound related to mustard, 2-chloroethyl ethyl sulfide. High conversions were obtained. Complete oxidation of organic compounds requires the transfer of a large number of electrons, commonly four for every carbon atom for example. The overall chemical reaction for oxidation of GB would be (in principle): The transfer of 26 electrons per mole of GB destroyed (Eq. 4) translates into a very large electric current flow and power consumption. For the destruction of 1 ton per day of GB: A current flow of about 200,000 amps for a 24-hour period would be required. The electrode potential of the Ag+/Ag2+ couple is 1.98 V; with allowance for electrical heating losses and electrode over voltages, the cell voltage would be expected in the range of 4 to 5 V. The power consumption for a 4 V cell would then be 800 kW for 24 hours. The usual electrode current density is limited to 200 A/square foot; thus the anode area requirement would be 1,000 square feet. The usual industrial arrangement is to operate several cells in series. In that way the current flow can be reduced (which is preferable) while the overall voltage is increased; the power consumption stays the same. No data are available on the rates of the chemical reactions. Very high conversion levels are required, so that the concentration of chemical agent in the reacting mixture must be very low. The size of reactor needed to meet the destruction rate required under these conditions is not known.

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Alternative Technologies for the Destruction of Chemical Agents and Munitions ADVANTAGES AND DISADVANTAGES The MEO process operates at a temperature of less than 100°C, and at atmospheric pressure. It is stable in operation over long time periods; perturbations of flow, temperature, or electrolyte composition do not cause adverse reactions. The reactivity of the chemical process can be controlled to a degree by control of the temperature. The process can be stopped by terminating power input to the cell. The amount of organic material held up in the cell or the chemical reactor at any time is very small. The process is continuous and agent would be destroyed as fast as it is fed; the concentration in the exit stream would be very low, corresponding to 99.9999 percent destruction. The process is energy intensive. The estimated electric energy requirement (800 kW for 24 hours to destroy 1 ton of GB) is several-fold larger than the heat of combustion of the agent. Part of this energy goes into the production of H2 (or HNO2), but most of it must be removed as heat. This large heat flow must be removed at low temperature-less than 100°C-to prevent the electrolyte from boiling. The electric energy is provided as a very large current flow, at low voltage. This power will require a substantial substation of transformers, rectifiers, and large busbars to carry the necessary current, which are standard electrolysis equipment. The presence of heteroatoms, such as fluorine, sulfur, or phosphorus, complicates the operation because they remain in the electrolyte solution as fluoride, sulfate, or phosphate ions. For a steady-state operation they must be removed continuously. In the process, the metal mediating ion, (e.g., silver) also would be withdrawn and lost to the reactor. It would be separated and recovered from the other materials of the spent electrolyte. The amount of mediating ion to be recovered will depend on its concentration in the solution, which will in turn depend on important system variables the chemical reaction rate and the concentration of oxidizing ion required to achieve the organic destruction and the flow past the anode to re-oxidize the ion (i.e., Ag+ going to Ag2+). This ion loss cannot be predicted but could be substantial: several hundred pounds of silver per ton of GB destroyed, for example. The process would be applicable only to agent and not to metal parts, energetics, or dunnage. SPECIAL CONSIDERATIONS Problems can develop that set operating limits on the processes: The anode requires a reasonably uniform concentration of the

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Alternative Technologies for the Destruction of Chemical Agents and Munitions reduced ion species; this is usually supplied by a high flow rate of electrolyte. The organic being decomposed may go through a polymerization resulting in insoluble materials; the membrane may be plugged as one of the undesirable consequences. The organic concentration must be kept low. Miscellaneous materials in the feed also may cause membrane fouling, e.g., alkaline earth elements or thickening compounds sometimes present in nerve agent. The chlorine present in mustard may precipitate silver (as silver chloride) unless concentrations are kept very low. A mediating ion other than silver may be necessary for this case. Cobalt has been suggested as a probable substitute. The very large destruction efficiency needed may be difficult to achieve in a completely mixed reactor. Reaction rate information is needed. The differing solubilities in aqueous medium of GB, VX, and HD may or may not require different reactor configurations. Although HD has low solubility, the amount in solution must in any case be kept low. Test work will be needed to determine whether HD destruction would require a two-phase reactor. (Most organic oxidations have had two-phases.) BY-PRODUCTS AND WASTE STREAMS The principal gas streams produced are carbon dioxide and hydrogen. Some contamination with nitrogen oxides, chlorine, or carbon monoxide should be expected. This would be particularly true with silver as the mediating ion. An electrolyte solution of strong acid, containing most of the heteroatoms in the feed as well as mediating metal ion, will be produced. Suitable recovery for all these materials must be provided. DEVELOPMENT NEEDS The nature and size of the chemical reactor will need to be determined through development. Electrolytic reactions are generally limited by transport and other physical properties, such as solubility. They resemble combustion reactions in this regard. Reaction rate data do not appear to be available; destruction levels of 99.9999 percent will need rate data for reactor design. The choices of mediating ion and reaction conditions such as temperature, ion concentration, and organic species concentration, will need to be developed. A substantial research and development program should be anticipated.